a review of gas-gas heat recovery systems

7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS

http://slidepdf.com/reader/full/a-review-of-gas-gas-heat-recovery-systems 1/39

eat Recorery Systems

V o l 1 p p 3 t o 4 1

P e r g a m o n P r e s s L t d 1 9 80 P r i n t e d i n G r e a t B r i t a i n

A R E V I E W O F G A S - G A S H E A T R E C O V E R Y S Y S T E M S

D.A. REAY

International Research & Developmen t Co Ltd (UK ), Fossway,

Newcastle-upon-Tyne, NE6 2YD , U.K.

Abstract--Gas-gas hea t recovery systems are probably the mo st prolific in all application areas.

and are certainly of considerable interest for rap id 'retro-fitting on industrial plant as an energy

saving measure. Th ey are of growing importance in the ho me , and their use in larg e HVA C

systems is now mand atory in Sweden. A m ajor development effort in equipment design to meet

the requirements of mo re arduous env ironments and economic eonstraints is evident in many

countrieS.

This paper reviews this field, giving descriptions of equipm ent available an d, for the more

impo rtant system s, factors to b e taken into accou nt in their selection.

i I N T R O D U C T I O N

I T IS i n t h e f i el d o f h e a t r e c o v e r y f r o m e x h a u s t g a s a n d a i r s t r e a m s t h a t i n m o s t c a s e s t h e

l a r g e st b e n e fi t s f r o m i n v e s t m e n t in e n e r g y c o n s e r v a t i o n e q u i p m e n t c a n b e r e a li z ed . T h e r e

a r e a c o n s i d e r a b l e n u m b e r o f u s e s t o w h i c h t h i s w a s t e h e a t c a n b e p u t , a n d t h e se d e p e n d

t o a l a r g e e x t e n t o n t h e te m p e r a t u r e a n d c o n d i t i o n o f th e e x h a u s t g a s e s o r a i r, t h e h e a t

r e c o v e r y e q u i p m e n t u s ed , a n d t h e e c o n o m i c a s s e s s m e n t o f th e o v e r a l l s y s t e m p e r f o r m -

a n c e. W i t h r e g a r d t o t h e u s es w h i c h d e t e r m i n e t h e t y p e o f h e a t e x c h a n g e r t o b e u s e d . w e

m a y i d e n t i f y t h r e e m a i n a r e a s , t h e s e b e i n g t h e h e a t i n g o f l i q u id , s t e a m r a i s in g , a n d a i r

h e a t i n g . T h i s p a p e r i s c o n c e r n e d w i t h t h e l a s t o f t h e s e c a t e g o r i e s , t h e r e c o v e r y o f h e a t

f r o m e x h a u s t g a s o r a i r s t r e a m s f o r p r e h e a t i n g o f o t h e r a i r o r g a s s tr e a m s .

W i t h i n t h i s c a t e g o r y i t i s p o s s i b l e t o i d e n t i f y t h r e e m a i n a p p l i c a t i o n a r e a s f o r t h e w a s t e

h e a t r e c o v e r y e q u i p m e n t :

( i) U s e o f p r o c e s s w a s t e h e a t f o r p r e h e a t i n g p r o c e s s s u p p l y a i r .

(ii) U s e o f p r o c e s s w a s t e h e a t f o r s p a c e h e a t i n g a n d a i r c o n d i t i o n i n g .

(iii) R e c o v e r y o f e x h a u s t h e a t f r o m a n a i r c o n d i t i o n i n g s y s t e m i n a c o m m e r c i a l o r

d o m e s t i c b u i ld i n g f o r p r e h e a t i n g s u p p l y a i r. ( N o t e t h a t i n s u m m e r s u c h a h e a t

e x c h a n g e r m a y a l s o b e u s e d f o r p r e c o o l i n g i n c o m i n g a i r, e ff e c ti n g s a v i n g s o n t h e

r e f r i g e r a t i o n l o a d . )

O n e c a n a l s o id e n t i f y a f e w m o r e s p e c i a l i s e d a r e a s o f a p p l i c a t i o n , i n v o l v i n g f o r

e x a m p l e p o l l u t i o n c o n t r o l , r e c o v e r y o f h e a t f r o m p r i m e m o v e r s , a n d t h e u s e o f i n c in e r a -

t o r w a s t e h e a t .

M o s t i t e m s o f e q u i p m e n t d e s c r i b e d h e r e c a n b e u s e d w h e n t h e w a s t e h e a t i s a t a

s u f fi c ie n t ly h ig h t e m p e r a t u r e n o t t o r e q u i r e ' u p g r a d i n g ' f o r r eu s e . H o w e v e r , h e a t p u m p s ,

w h i c h c a n u s e o u t s i d e a i r o r p r o c e s s a ir a s a h e a t s o u r c e , a r e a l s o a v a i l a b l e . T h e h e a t

p u m p , w h i c h is a b l e to u p g r a d e w a s t e h e a t , m a y a l s o b e e f f e c ti v e ly u s e d i n s o m e d r y i n g

p r o c e s s e s , a n d i s d e a l t w i t h h e r e i n t h i s c o n t e x t .

A p o i n t o f te n o v e r l o o k e d w h e n s e l e c ti n g h e a t r e c o v e r y e q u i p m e n t , w h a t e v e r t h e t y p e,

i s t h a t w h e n i n s t a l le d i n a n e w b u i l d i n g o r p r o c e s s , it c a n g e n e r a l l y r e s u l t i n a r e d u c t i o n

i n s iz e o f , s a y , b o i l e r o r r e f r i g e r a t i o n p l a n t . H o w e v e r , w h e n r e t r o f i t te d , t h e e q u i p m e n t ,

w h i l e r e l i e v i n g t h e l o a d o n s t e a m r a i s i n g p l a n t , f o r e x a m p l e , m a y a s a r e s u l t c a u s e i t t o

o p e r a t e a t a l o w e r ef fi ci en c y. S u c h c h a n g e s s h o u l d b e t a k e n i n t o a c c o u n t w h e n c a r r y i n g

o u t a n e c o n o m i c a s s e s s m e n t , b u t , o f c o u r s e , s u c h a l o ss in e f f ic i e n c y i s u n l i k e l y t o r e s u l t

i n a n y d r a m a t i c r e d u c t i o n i n t h e e f f e c ti v e n e s s- b f h e a t r e c o v e r y e q u i p m e n t a s a n e n e r g y -

s a v i n g t e c h n iq u e . ( I n t h e s a m e w a y , o f c o u r se , e x t r a f a n p o w e r n e e d e d t o o v e r c o m e t h e

p r e s s u r e d r o p t h r o u g h a w a s t e h e a t r e c o v e r y u n it s h o u l d a l s o b e in c l u d e d i n a n o p e r a t i n g

c o s t b a l a n c e s h e e t . )



4 D.A. REAY

Three other points of importance should be taken into account when considering the

installation of waste heat recovery equipment, particularly applicable to heat recovery

from exhaust gas streams. Firstly, while recovering heat obviously saves energy, it may

result in such a reduction in fuel consumption that the burners used in the process to

which the heat exchanger is being applied may not have a sufficiently large operating

range to cope with reduced fuel requirements. In such a case it will of course be necessary

to replace the existing burners, and this may have a serious adverse effect on the econ-

omic analysis. Obviously this will only happen if the recovered heat is being used to

preheat the combustion air, and if burner turn-down range is insufficient, an al ternative

use may be found for the waste heat, i.e. for space heating or replacing calorifiers.

Secondly, the use to which the waste heat is put can have a significant effect on the

total installed cost of the heat recovery installation. Taking the example above, where

preheating of combust ion air may be required, a complex oven used for food baking may

have several burners along a length of oven approaching 50 m. Such an oven will also

have possibly four exhaust gas flues situated along its length. Ducting of preheating air to

each burner depending upon the type of gas-gas heat recovery system used) can be an

expensive exercise, and in any heat recovery installation under study, it is extremely

important to look in detail at all the associated costs, such as ductwork, fans. controls,

and additional thermal insulation, in arriving at a realistic cost. It is worth pointing out

here that the installed cost of gas-gas heat recovery system can be anything between 1.5

and 4 times the cost of the basic heat exchanger itself, depending upon the system

complexity and operating temperature. Installation costs increase with operating tem-

perature, because of the need to use more expensive materials, etc.)

2. TYPES OF GAS-GAS HEAT RECOVERY EQUIPMENT

Of the techniques for waste heat recovery, it is in the area of gas-gas heat exchange

that the widest variety exist. However, commercially available systems may be broadly

grouped into two classifications, recuperators and regenerators. The recuperator func-

tions in such a way that the heat flows steadily and continuously from one fluid to

another through a separating wall. In a regenerator, however, the flow of heat is intermit-

tent, and is typified by rotary systems such as the heat wheel. In this unit a matrix of

metal comes into contact alternate ly with the hot and cold fluid, first absorbing heat and

then rejecting it. The run-a round coil or liquid-coupled heat exchanger) unit, which

incorporates a pumped liquid circuit carrying heat between two gas-liquid heat

exchangers, can also be classified as a regenerator.

From the point of view of the user of the heat recovery equipment, the distinction

between recuperative and regenerative heat exchange may be regarded as largely aca-

demic, as each type of heat exchanger in both categories has its own merits and draw-

backs, discussed later. A much more important distinction in the selection of heat recov-

ery equipment is the operating temperature range. Temperature of course can have

adverse effects on heat exchanger materials at both ends of the scale. If temperatures are

too low, brought about by the removal of too much heat from the exhaust gases, the dew

point may be reached and condensation can result in corrosive products affecting heat

exchanger materials. Freezing in air conditioning heat recovery systems can also be a

serious problem, and it will be noted that defrosting systems are offered as optional

equipment on many heat recovery devices. The life of heat exchangers used at high

temperatures can be seriously reduced if incorrect materials are selected. Even ceramics

are not immune in exhausts from heavy oil-fired equipment. As discussed in more detail

later, the fluids used in heat recovery systems such as heat pipes and run-around coils

may also be subject to thermal degradat ion if temperature limits are exceeded. Safety

spects in such pressurized systems should also not be neglected, and use of toxic work-

ing fluids should be communicated to the customer--this is mandatory under some

health and safety regulations, but is by no means universal.



A review of gas-gas heat recovery systems 5

A n o t h e r a s p e c t o f p a r t i c u l a r i m p o r t a n c e i n p r o c e ss a p p l i c a t i o n s o f g a s - g a s h e a t r e c o v -

e r y e q u i p m e n t is fo u l in g . T h e a c c u m u l a t io n o f m a t t e r o n t h e h e a t e x c h a n g e s u r f a ce

a f f ec t s b o t h p r e s s u r e d r o p a n d h e a t t r a n sf e r , a n d d i s p o s a b l e h e a t e x c h a n g e r e l e m e n t s

( d e s c r ib e d e l se w h e r e in t h is J o u r n a l ) a r e o f f e r ed b y s o m e m a n u f a c t u r e r s a s a n a l t e r n a t i v e

t o r e g u l a r c l ea n i n g , w h i c h m a y b e d i f f ic u l t o r n e c e s s i ta t e p r o c e s s s h u t - d o w n f o r e x t e n d e d

p e r i o d s . T h e u s e o f d i s p o s a b l e u n i t s c a n g e n e r a l l y o n l y b e e n t e r t a i n e d i f t h e c o s t is l o w ,

h o w e v e r .

T h e d e s i g n e r o f g a s - g a s h e a t r e c o v e r y s y s t e m s m u s t , b e c a u s e, a t l e as t i n t h e c a s e o f

i n d u s t r i a l i n s t a l la t i o n s , th e m a j o r i t y o f u n i t s a re s t il l r e t r o f i t te d t o p l a n t , m a k e s u r e t h a t

t h e i n s t a l la t i o n is e as y t o i m p l e m e n t , a n d t h e u n i t h a s a m i n i m u m e f fe c t o n p l a n t

o p e r a t i o n . I t m a y b e s a i d t h a t p r a c t i c a li t ie s a s s o c i a t e d w i t h i n s t a l la t i o n , o p e r a t i o n , m a i n -

t e n a n c e a n d , o f c o u r s e , e c o n o m i c s , t a k e p r e c e d e n c e o v e r t h e r m a l d e s i g n . A fe w p o i n t s

i m p r o v e m e n t i n t h e r m a l e f f ic i en c y is m e a n i n g l e s s to t h e p r o c e s s p l a n t m a n a g e r i f t h e

d e s i g n d o e s n o t m e e t h i s m a n y o t h e r r e q u i r e m e n t s .

T h i s p a p e r r e v ie w s t h e f o l lo w i n g t y p e s o f ~ q u i p m e n t :

R o t a t i n g r e g e n e r a to r s

S t a t i c r e g e n e r a t o r s

P l a t e h e a t e x c h a n g e r s

R u n - a r o u n d c o i l s

C o n v e c t i o n r e c u p e r a t o rs

R a d i a t i o n r e c u p e r a t o r s

R e c u p e r a t i v e b u r n e r s

T h e r m o s y p h o n a n d h e a t p i p e h e a t e x c h a n g e r s

M u l t ip l e t o w e r e n t h a l p y e x c h a n g e r s

G a s - g a s h e a t p u m p s

T h e a b o v e s y s t e m s re p r e s e n t t h e v a s t m a j o r i t y o f c o m m e r c i a l l y a v a i l a b l e g a s - g a s u n i t s ,

a n d m o r e u n u s u a l o r s p e c u l a t i v e d e s i g n s w i l l b e d i s c u s s e d i n s e p a r a t e p a p e r s i n f u t u r e

i s su e s ( as w i ll d e v e l o p m e n t s a n d a p p l i c a t i o n s o f th e m o r e c o m m o n t y pe s ).

3. ROT ATING REGENERATORS

T h e r o t a t i n g r e g e n e r a t o r v a r i o u s l y k n o w n a s t h e h e a t w h e e l , r o t a r y a i r p r e - h e a t e r ,

M u n t e r w h e e l, o r L j u n g s t r o m w h e e l a ft e r i ts D a n i s h i n v e n t o r ,* h a s b e e n u s e d o v e r a

p e r i o d o f a b o u t 5 0 y e a r s fo r h e a t r e c o v e r y in l ar g e p o w e r p l a n t c o m b u s t i o n p r o c e s se s [ 2 ] .

I t h a s a ls o b e e n w i d e l y u s e d in a i r c o n d i t i o n i n g a n d a v a r i e t y o f i n d u s t r i a l p r o c e s s h e a t

r e c o v e ry a p p l i c a t i o n s - - i n 1 9 7 5 i t w a s e s ti m a t e d t h a t in E u r o p e a l o n e u p w a r d s o f 1 5 0 0 0

r o t a t i n g r e g e n e r a t o r s w e r e i n u s e [ 3 ] .

T h e o p e r a t i o n o f t h e r o t a t i n g r e g e n e r a t o r i s s h o w n i n F i g . 1. I n c o m m o n w i t h p l a t e

a n d h e a t e x c h a n g e r s , t h e r e g e n e r a t o r w h e e l s p a n s t w o a d j a c e n t d u c t s , o n e c a r r y i n g

e x h a u s t g a s a n d t h e o t h e r c o n t a i n i n g t h e g a s f l o w w h i c h i t i s r e q u i r e d t o h e a t . T h e g a s

f l o w s a r e c o u n t e r - c u r r e n t . A s t h e w h e e l r o ta t e s , it a b s o r b s h e a t f r o m t h e h o t g a s p a s s i n g

t h r o u g h i t a n d t r a n s f e r s t h e h e a t t o t h e c o o l e r g a s f l o w . A l a t e r d e v e l o p m e n t , t h e h y g r o -

s c o p i c w h e e l , i s a b le t o t r a n s f e r m o i s t u r e a s w e l l a s s e n s ib l e h e a t b e t w e e n t h e t w o d u c t s .

R o t a t i n g r e g e n e r at o r s, i n c o m m o n w i t h m a n y o t h e r h e a t r e c o v e r y s ys te m s , c a n b e u s e d

i n h o t c l i m a t e s f o r p r e - c o o l i n g a i r u s e d f o r c o n d i t i o n i n g l a r g e b u i l d i n g s , a n d t h e w h e e l

w o r k s e f fe c ti v e ly i n a p p l i c a t i o n s w h e r e t h e t e m p e r a t u r e d i f fe r e n c es b e t w e e n h o t a n d c o l d

a i r s t r e a m s a r e t o o l o w f o r e f f ec t iv e u s e o f r e c u p e r a t iv e h e a t e x c h a n g e r s .

M a n y m a n u f a c t u r e r s p r o d u c e t h r e e d if f er e n t t y p e s o f r o t a t i n g r e g e n e r a to r . T h e m o s t

c o m m o n f o r m u t i l i z e s a w h e e l m a d e u p f r o m a k n i t t e d a l u m i n i u m o r s t a i n l e s s s t e e l w i r e

ma t r i x . Th i s ma t r i x i s c he a p , a nd t he he a t t r a ns fe r e f f i c ie nc y i s h i gh a s t he a i r f l ow i s

e x p o s e d t o a l a r g e a m o u n t o f s u r f a c e a s it ~ p a ss e s t h r o u g h t h e w h e e l . H o w e v e r , t h e

* Some power generating plants use a second typ e of rotating regenerator, the Rothemuhle design, which

incorporates a station ary matrix with rotating hoo ds to d istribute the gas and air flows, as sho wn in Fig. 2.

However, few m anufacturersoffer hese commercially o r process plant app lications[1].



6 D A R E AY

W a r m H o t

ai r g a s

ou r I I Jn

" ~ / ,..._ _ ~ ) 1 R o ta tin g

hme~:~eXChange

' t : : : 5 / /

C i r c u m f e r e n t i a l

s e a l s

C o l d Cool

,d I I ? o,

i n ou t

F i g 1 Schemat ic of rotat ing regenerator .

p r e s su r e d r o p t h r o u g h t h i s ty p e o f m a t r i x c a n b e r e l a ti v el y h i g h a n d t h e f o u l i n g o f t h e

m a t r i x t e n d s t o b e m o r e s e v e r e t h a n o n o t h e r t y p e s .

D e v e l o p m e n t o f l a m i n a r f l o w w h e e l s i n w h i c h t h e m a t r i x is c o rr u g a t e d r e s e m b l i n g a

s m a l l p o r e h o n e y c o m b h a s a ll e v ia t e d t h e p r e s s u r e d r o p a n d f o u l i n g p r o b l e m s o f t h e

m es h m a tr i x a nd th i s ty pe o f w h ee l i s ea s i er to c lea n f ibres et c. t end i ng to co l l ec t o n the

fa ce o f the m a tr i x . I n t erm s o f therm a l e f f i c i ency the per fo rm a nce o f a m eta l l i c co rrug a ted

m a tr i x w hee l i s s i m i l a r to tha t o f the m es h ty pe .

A t h ir d fo r m o f m a t r ix u s e d i n r o t a t i n g r e g e n e r a t o r s i s n o n - m e t a l l ic . K n o w n a s t h e

hy g ro s co p i c w hee l th i s ty pe ca n tra nsfer m o i s ture a s w e l l a s s ens i b l e hea t a nd i s pa r t i cu-

l a r ly us e fu l i n hea t i ng a n d v en t i l a t ing a ppl i ca t i o ns . Th e s t ruc ture i s s i m i l a r to tha t o f the

C o l l a r s e a l s

W a r m ir

o u t

f

H o t

gos

in

S t a t i o n a r y

h e a t - e x c h c

m a t r i x

g a s

hoods

o u t

C o l d a i r

in

F i g 2 Operat ion of Rothemuhle regenerator .



A review of gas gas heat recovery systems 7

metallic laminar flow wheel. While the hygroscopic wheel is likely to be up to 35Yo more

expensive than the metallic type, the increased capital cost is generally more than offset

by the increased heat transfer attributable to latent heat recovery. The latent heat content

can vary considerably from one application to another, and should be carefully assessed

before settling for a particular unit.

The use of a stainless steel matrix in the regenerator permits operation in exhaust gases

at temperatures in excess of 800°C. In some processes regeneration is required at even

higher temperatures for example in gas turbines and in steel-making plant). Glass-

ceramics and silicone nitrides have been used as core materials 14]. During the last 2-3

years, however, difficulties have been encountered with material degeneration and crack-

ing in some ceramic regenerators.

3.1. Performance and operation

In determining the performance and operating characteristics of a rotating regenerator,

a number of factors must be taken into account, and these include the following:

Operating temperature.

As discussed above, the type of matrix used, be it metallic or

ceramic, depends upon the operating temperature range likely to be encountered in the

application.

Operating pressure.

In general it is desirable to operate this type of regenerator in

situations where the exhaust and supply gas streams have similar pressures. If knit ted

mesh is used for the matrix, there is a potentially large flow path available through the

mesh if pressures are not equalised. In wheels of the laminar flow type, where the gas is

restricted to movement in the axial direction, some pressure differential can be supported

if suitable seals are fitted to the unit. Matrices of this type are used on regenerators in

large boilers up to 660 MW size in the United Kingdom) and leakages of up to 10~o can

occur, however. To minimise carry-over of contamination, it is preferable to operate the

exhaust stream at a marginally lower pressure than the supply stream.

Cross contamination. The possibility that contamination of clean supply gas could

occur .with this type of regenerator has led to the incorpora tion by manufacturers of

purge sections. Here a proportion of the supply air is used to scavenge the matrix section

leaving the exhaust duct, and the contamination or residual exhaust gas is blown back

into the exhaust duct. Proper purge section operation depends on correct fan location so

that the pressure on the supply side is higher than on the exhaust side, and in cases where

this requirement can be met, cross-contamination can be as low as 0.04~ by volume, and

particle carry-over less than 2~. Some manufacturers specify a carry-over of less than

0.1 vol.yo in their literature, and where this factor is critical, their advice should be

sought [5]. Tests in the U.S.A. [6] have shown that, in general, the use of purge sections

on wheels having mesh or randomly orientated) . media will reduce carry-over to less

than tYo by volume, and to less than 0.2yo where directionally orientated media are used.

Purging is not obtained without sacrificing efficiency. The sizing of system fans may be

increased by up to 10~o of rated volume to handle the greater gas flow requirements, and

if the correct fan arrangement cannot be installed because of practical difficulties, seals

can be incorporated at each radial partition, but these are not nearly as effective, cross-

contamination rates possibly then approaching 8 vol.Yo.

On some of the higher temperature metallic regenerators, a separate fan specifically for

purging may be fitted. These may use up to 4 kW in electrical energy, and, together with

the much lower motor power for rotating the heat exchanger, should be accounted for in

any cost analysis.

Very ,high temperature ceramic regenerators also suffer from cross-contamination, but

in the .case of gas turbine units, for a 4:1 engine compression ratio transverse flow

accounts for only about 0.3~o of engine air flow at full speed conditions.

Unequal f low rates . Potential users will find that most manufacturers quote perform-

ance figures based on equal supply and exhaust flow rates. If the flows are unequal,

correction factors must be applied. It is normal practice to size the generator for the

maximum flow rate and to select the efficiency on the basis of the lower flow.



8 D A REAY

Mixer

Exhaust air Exhaust air

from BLDG. to weather

/ r l /...

Supply

air

from

wheel

By pass air

Outside air

Fig 3 Com pens at ion for unequ al f lows in a rota t ing regenerator

In s i t ua t ions w here the in l e t gas f low ra t e is m uch l a rger t han the exh aus t f low, say by

a f ac t o r o f 4 , th e ro t a t i n g r eg en e ra t o r m ay b e s e l ect ed o n t h e b a s i s o f t h e ex h au s t f l ow ,

an d u sed t o h ea t o n l y a p ro p o r t i o n o f t h e su p p l y a i r, s ay 2 5 ~ . T h e a r r an g em en t fo r th is ,

sho wn in F ig . 3 , perm i t s t he ba l ance of t he supply a i r t o b ypass t h e regen era tor , a f t e r

which i t i s mixed wi th the prehea ted a i r .

Pressure drop The

p re s su re d ro p t h ro u g h t h e h ea t ex ch an g e r is a fu n c t i o n o f g a s

v e l o c it y an d m a t r i x d e s ig n . S o m e m an u fac t u re r s o f f e r t w o t y p es o f m a t r i x , o n e d e s i g n ed

fo r o p t i m u m h ea t t r an s fe r c ap ab il it y w h i ch , b ecau se o f i t s g r ea t e r a m o u n t o f su rf ace, h a s

a p re s su re d ro p w h i ch m ay b e u n accep t ab l e i n so m e ap p l ic a t io n s . B y co m p ro m i s i n g o n

t h e rm a l e ff ic ien cy, a l o w er p re s su re d ro p u n i t h av i n g l a rg e r p o re s m ay b e ad o p t ed .

Typ ica l ly a h igh e f f i c i ency uni t (81% a t a ve loc i ty of 4 m/ s) wi ll ha ve a pressure dro p of

ap p ro x i m a t e l y 3 5 N / m 2, w h e rea s t h e co r r e sp o n d i n g l o w p re s su re d ro p sy s t 0 m w ill o p e r ,

a t e a t 7 6~ ef f ic i ency a t t he sam e ve loc i ty , resu l t i ng in a pressure loss of on ly 17 N /m 2 . In

co m p ara t i v e a s se s sm en t s o f w h ee l m a t r ix m a t e ri a ls , r an d o m l y o r i en t a t ed m ed i a h av e

pressure drops propor t iona l t o ( face ve loc i ty) ' ~ , whi l e t he pressure drop in d i rec t iona l ly

or i en t a t ed media i s p ropor t iona l t o ( face ve loc i ty)1 °7s.

Control M o s t ro t a t in g r eg en e ra t o r s (ex c l u d in g h i g h t em p e ra t u re ce ram i c t y p es) a r e

d r i v en b y e l ec t r ic m o t o r s , t h e p o w er o f w h i ch o n t h e l a rg e s t w h ee l s (4 m d i am e t e r )

ap p ro a ch es 0 .5 k W. F o r o p t i m u m h ea t t r an s fe r , r o t a t i o n a l sp eed i s t y p i ca ll y 1 0 -20 rev/

min. As th i s speed reduces , t he therm al e f f ic i ency i s l owered , and hence s pee d con t ro l ca n

b e u sed t o m ee t r ed u ce d h ea t l o ad d u t ie s . A l te rn a t i v e ly t h e su p p l y a i r m ay b e r eg u l a t ed

u s i n g d am p er s an d / o r a b y p as s sy s t em . In m o s t i n d u s t r i a l p ro ce s se s w h e re a co n s t an t

d u t y i s r eq u i r ed , an y fo rm o f m o d u l a t i o n w i ll b e u n n eces sa ry , an d a co n s t an t sp eed

m o t o r is r e c o m m e n d e d .

3.2.

Applications

In co m m o n w i th m an y o t h e r t y p es o f g a s -g a s h ea t r e co v e ry sy s tem s , t h e ro t a t in g

reg en e ra t o r is eq u a l l y ap p l ic ab l e i n h ea t i n g , v en t il a ti n g an d a i r co n d i t i o n i n g (H V A C ) an d

p ro ces s h ea t r e co v e ry , t h e m a i n d i f f er en ces b e i n g i n t h e t y p e o f w h ee l u sed (h y g ro sco p i c

or sens ib le hea t on ly) .

I t i s n o w p o ss ib l e t o o b t a i n m e t a l l i c m a t r i x m a t e r i a l f o r H V A C reg en e ra t o r s , t h e co re

o f o n e su ch u n i t b e i n g o f a l u m i n i u m fo i l , ag a i n fo rm ed i n t o a h o n ey co m b . T h e h y g ro -

sco p i c p ro p e r t ie s i n t h is i n s t an ce a r e en su red b y t r e a t i n g t h e w h ee l a f te r f ab r i ca ti o n so

t h a t a m i c ro -p o ro u s l ay e r o f a l u m i n i u m o x i d e is fo rm ed o n t h e su rf ace . T h i s l ay e r h a s t h e

sam e p ro p e r t i e s a s a d ry d e s i ccan t , ab so rb i n g w a t e r v ap o u r i n t o t h e p o re s .

T h e n ee d t o u se a h y g ro sco p i c w h ee l , i n sp it e o f t h e f ac t t h a t i t i s m o re ex p en s i v e o n

f i rs t cos t t han a s im ple sens ib l e hea t m eta l l ic wheel , can be judg ed f rom the fac t t ha t t h e

l a t en t h ea t co n t en t o f a i r ex h au s t ed T ro m b u il d i n g s can r each a l m o s t t h e s am e m ag n i t u d e

as t h e s en s i b l e h ea t co n t r i b u t i o n . In ad d i t i o n t o t h e r eco v e ry o f t h e h ea t , h o w ev e r , an

ex ch an g e o f h u m i d i t y i ts e lf is r eq u i r ed i n o rd e r e i t h e r t o d eh u m i d i fy th e su p p l y a i r i n

su m m er , o r t o i n c rease t h e h u m i d i t y in w i n t e r [7 ] . R e cu p e ra t i v e h ea t r e co v e ry sy s tem s ,



A r e v ie w o f g a s g a s h e a t r e c o v e r y s y s t e m s 9

by v i r tue of the f ac t tha t a so l id wal l s epara tes the two a i r s t r eams , a r e unab le to t r ans fer

mo is ture , and can on ly a f f ec t the t em pera tu re o f the incom ing a i r.

K r u s e an d V au t h , fo l l o w in g o n t h e r e s ea rch o f a n u m b e r o f o t h e r w o r k e r s , i n c lu d i n g

S p ah n G n i e l in s k i [ 8 ] , R u t h

e t a l

[ 9 ] co m p a r ed t h e p e r f o r m an ce o f an a s b es t o s /l i th i u m

chlor ide (hygroscopic) wheel and an a lum in ium kni t t ed w i re (non-hy groscop ic) wheel

u n d e r c l i m a t i c co n d i t i o n s ap p r o p r i a t e t o H V A C i n G e r m an y . O f p a r t i cu l a r i n t e r e s t w as

t h e ir w o r k o n t h e f o r m a t i o n o f f r o st o n t h e m a t r i ce s u n d e r ad v e r s e am b i en t co n d i t i o n s .

Thei r r esu l t s ind ica te tha t hyg rosco pic wheel s i ce up a t low er t em pera tu res ( typ ica lly

-20°C ambien t in the i r t es t s ) than meta l l i c s ens ib le hea t wheel s ( i c ing commenced a t

-10°C . This was a t t r ibu ted to the f ac t tha t in the l a t t e r case , a l l mois tu re was in l iqu id

form and inv ar iab ly forms f ros t when the m ean ef f ec tive ro to r t em pera ture f al ls be low

t h e i ce p o in t . T h ey a l s o r eco m m en d ed t h a t t h e p e r f o r m an ce o f h y g r o s co p i c w h ee l s

s h o u l d b e p r e s en ted b y m an u f ac t u r e r s i n te r m s o f b o t h t em p e r a t u r e an d h u m i d i ty .

Un l ike the ma jor i ty o f p roces s p lan t hea t r ecov ery opera t ions , where , once s e t up , the

proces s ope ra tes con t inuou s ly w i th l it t le change in exhaus t gas con di t ions , hea t

exchanger s used in a i r condi t ion ing p lan t r equ i r e , in genera l , some form of modula t ion .

O n r o t a t in g r eg en e r a t o r s t h e p r i m a r y co n t r o l t e ch n i q u e i s to v a r y t h e s p eed o f r o ta t i o n ,

and F ig . 4 show s how a typ ica l r egene ra tor i s a f f ec ted , in t e rm s of e ff ic iency , as the speed

of ro ta t ion i s a lt e r ed .

T h e r o t a t i n g r eg en e r a t o r m ay a l s o b e u s ed i n d o m es t i c h ea t r e co v e r y , i n co n j u n c t i o n

wi th a fo rced ven t i la t ion sys tem. A l thoug h l es s co m m on a t p resen t than the p la te hea t

exchange r d i s cus sed l a t e r, i t s h igh e f fi ci ency sho uld m ake i t wor th y o f fu r ther exp lo i t a -

t ion in th i s smal l, bu t r ap id ly growing area o f use .

P r o c e s s h e a t r e c o v e r y

P r o ces s h ea t r e co v e r y ap p l i c a t io n s o f r o ta t i n g r eg en e r a t o r s a r e

m any and var i ed , r ang ing f rom sm al l un i t s o f 0 .5 -1 m in d iameter on dryer s , to the l a rge

wheel s in 660 M W pow er s t a t ion bo i l e rs , where d iam eter s o f 15 m are typ ica l . Opera t ing

t em p e r a t u r e r an g es v a r y f r o m j u s t ab o v e am b i en t t o 1 00 0° C , an d a w i d e r an g e o f

mater i a l s a r e ava i l ab le to meet cor ros io n and therma l s t r es s r equ i r em ents in these pro-

c e s s e s .

At the top end o f the t emp era ture s ca le , the Br it i sh S tee l Co rpo ra t io n has been w ork-

ing on a ceramic ro ta t ing r egenera tor fo r use on l a rge r ehea t fu rnaces . F lex ib le s ea l s a r e

used to min imise gas l eakage be twee n the tw o f low reg imes [10] . The r egene ra tor w i ll

p r o v i d e a co n s t an t a ir p r eh ea t w i t h o u t an y o f t h e t h e r m a l d o w n g r ad i n g o r s w i t ch in g l o s s

° °

7 0

: -

~ 6c

5C

4

30

20

I0

I I I I I I I

A i r v e l o c i t y 2 m / s

3 m l s

4 m l s

I I ~ I I I I l I

0 I 2 3 4 5 6 7 8 9 I0

R o t o r s p e e d ,

r e v / m i n

F i g . 4 . E f f i c i e n c y o f a F l a k t r o t a t i n g r e g e n e r a t o r a s a f u n c t i o n o f r o t a t i o n a l s p e e d .



10

I

D A R E A Y

Gloss tank

A I

5 0 0 =

Chequer

~ b r i c k w o r k

7 ~

J

Flue gas

ta s tack

F i g 5 A d d i t i o n a l h e a t re c o v e r y o n a f l a t g l a s s t a n k

of s t a t i c regenera tors ( such as t he type used in g l ass mel t i ng furnaces) and wi l l ach ieve

ef f ic i enci es i n t he reg ion of 90 . Because the l eve l o f re f rac tory u t i li sa t ion i s very h igh

(abo ut 2000 W /kg c om pa red w i th less t han 100 W /kg for a s t a t i c regenera tor ) , the un i t

w ill b e m u ch m o re co m p ac t t h an t h e eq u i v a l en t s ta t ic u n i t.

I t has been es t imated by the Br i t ish S tee l Co rpo ra t ion tha t i f appl i ed to a 150 t / h

r eh ea t fu rn ace , a ro t a t i n g r eg en e ra t o r o f t h i s t y p e co u l d s av e £ 5 0 0 0 0 0 p e r an n u m b y

ra i s ing the a i r p rehea t t empera ture f rom 400 to 900°C.

H o w d en , o n e o f t h e m a j o r m an u fac t u re r s o f l a rg e ro t a t i n g r eg e n e ra t o r s fo r p ro ce s s u se

i n th e U n i t ed K i n g d o m , h av e e s t i m a t ed [1 1 ] t h a t t h e t o t a l an n u a l v a l u e o f fu el sav in g s

poten t i a l l y ava i l ab l e f rom the recovery of f l ue gas hea t i n t h ree indus t r i a l sec tors , (o i l

re f in ing , s t ee l and a luminium, and packaged boi l e rs ) i s £286 mi l l i on . Heat wheel s have

been dev e loped for use in t hese and o the r energ y- in t ens ive processes such as g l ass-

mak ing . F igure 5 shows the layo ut of an ins t a ll a t ion , us ing a s t a in l ess s tee l hea t recove ry

u n i t, d o w n s t r eam o f th e co n v en t i o n a l b r ick s t a t ic r eg en e ra t o r s , o n a g l as s m e l t in g fu r -

nace . By redu cing the exhaus t gas t em per a ture f rom 700°C do wn to 400°C, fue l sav ings

of 20 a re ach ievable . W ork , a l so us ing a s t a in l ess st ee l un i t , has been car r i ed ou t us ing

su p p o r t f ro m t h e E u ro p ean E co n o m i c C o m m u n i t y , w i t h a v i ew t o d em o n s t r a t i n g i t s

su i tab i li ty i n t h e a l u m i n i u m m an u fac t u r i n g i n d u s t ry .

A n o t h e r a r ea o f ap p l i c a t io n w h e re a h ea t w h ee l c an b e u sed is i n t h e p r i n t in g i n d u s t ry .

In t h i s i n s t an ce a ro t a t i n g r eg en e ra t o r w as i n s t a l l ed i n t h e ex h au s t f ro m co a t i n g an d

g rav u re m ach i n es a t H a r r i so n & S o n s L t d . T h ese m ach i n es u se l a rg e q u an t i t ie s o f h o t a i r

fo r ev ap o ra t i n g o f f so l v en t s i n t h e i n k s an d co a t i n g m a t e ri a ls . H ea t i s f r eq u en t ly w as t ed

b y d i r ec t l y ex h au s t i n g t h e a i r t o a t m o sp h e re fo l l o w i n g p a s sag e t h ro u g h t h e p r i n t i n g

machine , a l t hough in some cases so lvent recovery i s p rac t i sed [12] .

In t h i s par t i cu l a r fac tory , t he a i r supply to t he coa t ing and gravure ins t a l l a t i on i s

h ea t ed b y c i r cu la t in g h ea t t r an s fe r o i l t h ro u g h f i n n ed t u b es o v e r w h i ch t h e a i r f lo w s . T h e

m ax i m u m h ea t e r o u t p u t is 1 27 00 M J / h , a t w h i ch r a t e co n su m p t i o n is 0 .3 9 m 3 / h o f 3 50 0 s

fue l o il . By recover ing 70 ~ of t he hea t i n t he exhau s t a i r , t he fue l b i ll can be red uced by

£600 per week , based o n cu rren t fue l o i l p r ices . Tw o regen era tors a re u sed in t he p l an t ,

an d t h e l o ca t io n i n o n e m ac h i n e is sh o w n i n F ig , 6. T h e y h av e d i am e t e r s o f 2 m an d a r e

280 m m th i ck .

Th e to t a l ins t a l l a t ion cos t was £42000, o f which £11000 i s a t t r i bu tab le t o t herm al

w h ee l co s ts . M u ch o f th e d u c t w o rk u sed , w h i ch is i n c l u d ed i n t h e ab o v e co s t. w o u l d b e



A re~ie~ of gas-gas heat recovery systems l I

~x h~us t c ) l r t '0

HmOt o I r C

H e a t e d o z r

1 o m a c h i n e B 2 * C

Fig. 6. Rotating regenerator used to recover heat from a coating and gravure plant.

n e c e s s a ry t o r e m o v e e x h a u s t e v e n i f h e a t r e c o v e r y w a s n o t u s ed . I n t h i s c a s e t h e p a y b a c k

p e r i o d o n t h e i n v e s t m e n t i s s u b s t a n t i a l l y l e s s t h a n t w o y e a r s .

Static regenerators

T h e s t a t ic r e g e n e r a t o r t h e o p e r a t i n g p r i n c i p l e o f w h i c h i s i ll u s-

t r a t e d i n F i g . 7 t a k e s s e v e r a l f o r m s a n d i s c o m m o n l y a s s o c i a t e d w i t h h i g h t e m p e r a t u r e

p r o c e s s e s s u c h a s g l a s s m e l t i n g f u r n a c e s . S t a t i c r e g e n e r a t i v e h e a t e x c h a n g e r s a r e i n e f f e c t

h e a t s t o r a g e d ev i ce s t h e h e a t s t o re o f te n b e in g r e q u i r e d t o f u n c t i o n o v e r a c o m p a r a t i v e l y

s h o r t c y c l e ti m e .

B y s w i t c h i n g t h e g a s f l o w s t h r o u g h t h e r e g e n e r a t o r c o r e t h e c o r e i s a l t e r n a t e l y e x p o s e d

t o t h e h o t e x h a u s t a n d c o l d s u p p l y a i r s t r e a m s . T h e e x h a u s t g a s h e a t s u p t h e c o r e

m a t e r i a l w h i c h m a y b e a s t a c k o f b r i c k s a s i n g l a ss m e l t i n g f u r n a c e r e g e n e r a t o r s a n d

t h e n t h e s u p p l y a ir i s d i r e c t e d t h r o u g h t h e c o r e p i c k i n g u p h e a t r e t a i n e d b y t h e c o r e

b e f o r e p a s s i n g t o t h e p r o c e s s w h e r e i t i s t o b e u s e d .

A c o m p a r a t i v e l y r e c e n t d e v e l o p m e n t h a s b e e n t h e d e s i g n a n d a p p l i c a t i o n o f s t a ti c

r e g e n e r a t i v e s y s t e m s i n d o m e s t i c h e a t r e c o v e r y u n i t s. W i t h t h i s e x c e p t i o n p r o g r e s s i n t h e

f ie l d o f r e g e n e r a t i v e h e a t e x c h a n g e r s o f t h i s t y p e a w a i t s f u r t h e r a p p l i c a t i o n o f h e a t

s t o r a g e s y s t e m s u n d e r d e v e l o p m e n t f o r l o n g e r t e rm s t o r a g e a n d m o r e s o p h i s t ic a t e d

c o n t r o l te c h n i q u e s p o s s i b ly m i c r o p r o c e s s o r - b a s e d .

4 P L A T E H E A T E X C H A N G E R S

T h e g a s - to - g a s p l a t e h e a t e x c h a n g e r ta k e s s e v e r a l f o r m s t w o o f w h i c h a r e i l l u s t r a t e d i n

F i g s 8 a n d 9 . T h e y n o r m a l l y in v o l v e a c r o ss - fl o w c o n f ig u r a t i o n a l t h o u g h s o m e a p p r o a c h

F l o w ~ - -

control

v o l v e s . ~ _

H e a t e d

supply

ai r A

1

Regenerator

core

[

t

Cooled

exhaust

e

i

G a s

f low

/

I

H o t SupPly

exhaust a i r

Fig. 7. Op eration of a static regenerator.

- -~ F low

c ont ro l

/ v a l v e s



12 D .A. ReAY

Fig. 8. A conventional plate h eat exchang er module.

c o u n t e r f l o w , a n d a r e a v a i l a b l e i n a v a r i e t y o f m a t e r i a l s t o c o v e r H V A C a n d p r o c e s s

o p e r a t i n g c o n d i t i o n s . A s w i th r e g e n e r a t i v e a n d h e a t p i p e t y p e s, th e e x h a u s t a n d i n le t

d u c t s m u s t b e b r o u g h t t o g e t h e r a t t h e l o c a t i o n o f th e h e a t e x c h a n g e r .

T h e m a t e r i a ls o f c o n s t r u c t i o n o f t h e p l a t e s u se d , d e p e n d i n g u p o n t h e a p p l i c a t i o n a n d

o p e r a t i n g t e m p e r a t u r e r a n g e, r a n g e f r o m p a p e r t o m e t a l, g la s s a n d c e r am i c s , I m p r e g -

n a t e d p a p e r h e a t e x c h a n g e r e l e m e n t s a r e u s e d i n H V A C a p p l i c a t i o n s , o n e J a p a n e s e u n i t ,

t h e L o s s n a y h e a t e x c h a n g e r , b e i n g a v a i l a b le f o r u se a s a r o o m a i r c o n d i t i o n i n g s y s t e m f o r

h o u s e s , w h e r e f lo w s a s l o w a s 1 0 0 m 3 / h m a y e x i s t.

T h e o p e r a t i n g p r i n c i p l e o f t h e L o s s n a y p l a te h e a t e x c h a n g e r e l e m e n t , w h i c h is a

c r o s s - fl o w u n i t, d if fe r s f u n d a m e n t a l l y f r o m t h a t o f t h e o t h e r p l a t e h e a t e x c h a n g e r s d i s-

c u s s e d b e l o w in t h a t m o i s t u r e t r a n s f e r c a n o c c u r v i a c a p i l l a r y a c t io n a n d / o r o s m o s i s i n

t h e t r e a t e d p a p e r w h i c h f o r m s t h e b a r r i e r b e t w e e n t h e t w o a i r s t r e a m s .

I t i s p o s s i b l e t o q u a l i t a t i v e l y e x a m i n e t h e p e r f o r m a n c e d i f f e r e n c e s b e t w e e n a s e n s i b l e

h e a t u n i t a n d t o t a l h e a t L o s s n a y u n i t b y s t u d y i n g t h e p s y c h o m e t r i c c h a r t i n F i g. 1 0.

P o i n t A o n t h e c h a r t r e p r e s e n t s t h e c o n d i t i o n o f a i r s u p p l i e d t o a r o o m u s in g s e n s i b le

h e a t u n i ts , a n d p o i n t S r e p r e s e n t s c o n d i t i o n s o b t a i n e d u s i n g t h e L o s s n a y u n i t . tN o t e t h a t

suf f ices O = o u t d o o r a i r, R = r e t u r n a ir , a n d S = s u p p l y a i r. ) T h u s a i r i s d e h u m i d i -

f ie d i n s u m m e r a n d h u m i d i f ie d i n w i n te r , in a d d i t i o n t o b e i n g c h a n g e d i n t e m p e r a t u r e . *

T h e im p l i c a t i o n s o f t h is i n te r m s o f h e a t t r a n s p o r t c a p a b i l i t y a n d h u m i d i t y c h a n g e s

m a y b e s e e n w i t h r e f e r e n c e t o T a b l e 1, f o r w i n t e r c o n d i t i o n s .

M o v i n g u p t o t h e t e m p e r a t u r e s ca l e t o e n c o m p a s s i n d u s t r i a l p r o c e s s h e a t r e c o v e r y , a

w i d e v a r i e t y o f m e t a l l i c p l a te h e a t e x c h a n g e r s e x i st . U t i l iz i n g a l u m i n i u m , m i l d o r s t a i n -

l es s s te e l, o r c o a t e d m e t a l s (t h e c o a t i n g b e i n g c o m m o n l y p l a s t ic - b a s e d a s a n a n t i - c o r r o -

* Note that in any heat exchanger in a gas stream the recover? of latent he at is possible ~ here condensation

occurs, but, exce pt in the case of the hygroscopic plate or rotating regenerator , this do es not involve an

exchang e of mo isture.



A r e v i e w o f g a s g a s h e a t r e c o v e r y s y s t e m s 13

. . . . . . . .

Fig. 9. A p late heat exchanger emp loyingglass p lates.

s i o n p r o t e c t i o n ) , t h e p l a t e s m a y b e c o r r u g a t e d t o i n c r e a s e t h e h e a t t r a n s f e r s u r f a c e p e r

u n i t v o l u m e o f h e a t e x c h a n g e r , o r t o p r o m o t e t u r b u l e n c e . T h e s u r f a c e o f t h e M u n t e r s

E c o n o v e n t E X se r ie s o r E c o n o p l a t e i n N o r t h A m e r i c a ), a m o d u l e o f w h i c h i s i l l u s tr a t e d

i n F i g . 11 , i s t yp i c a l o f t he fo rm use d , t he p l a t e s i n t h i s c a se be i ng a l umi n i um. The

c o r r u g a t i o n s a l s o s e r v e a s p a t h w a y s f o r c o n d e n s a t e t r a n s p o r t a n d r e m o v a l t o t h e d r a i n

p a n a t t h e l o w e r a p e x o f t h e h e a t e x c h a n g e r , a n d p r e v e n t e n t r a i n m e n t o f t h e c o n d e n s a t e .

I t i s i n t e r e s t i n g t o n o t e t h a t t h e m o d u l e s m a y b e m o u n t e d i n p a r a l l e l t o c a t e r f o r h i g h

f l o w ra t es , o n e m o d u l e h a n d l i n g u p t o 1 .5 m 3 / s. M o d u l e s a r e a l s o r e m o v a b l e f o r c l e a n i n g .

A n o v e l f e a t u r e o f t h e M u n t e r s s y s t e m , w h i c h m a y a l s o b e a p p l i e d i n H V A C s y s t e m s , is a

f re e z e p ro t e c t i on de v i c e . I l l us t ra t e d i n F i g . 12 , t h i s sma l l l i d i s t r a ve rse d by a mot or

a c r o s s t h e o u t d o o r a i r s id e o f t h e h e a t e x c h a n g e r c o r e , t e m p o r a r i l y i n h i b i t i n g t h e c o o l i n g

e f fec t o f t he f re sh a ir , t hus pe rm i t t i ng t he c o re o f t he e xc h a ng e r t o i nc re a se i n t e m pe ra -

ture .

T h e s e l ec t io n o f p l at e h e a t e x c h a n g er s in c o m m o n w i t h m a n y o t h e r t y p e s o f h e a t

r e c o v e r y u n i t ) i s b e c o m i n g r e l a t i v e l y e a s y i n s o f a r t h a t m a n y m a n u f a c t u r e r s n o w p r o d u c e

s e l ec t io n c h a r ts , p a r t i c u l a r ly f o r u se a t t h e H V A C e n d o f t h e t e m p e r a t u r e s c a le . T h e

m o d u l a r c o n s t r u c t i o n o f m o s t u n i t s s im p l i fi e s t h i s p r o c e d u r e s ti ll f u r t h e r . F i g u r e 1 3

s h o w s a c a p a c i t y c h a r t f o r t h e M u n t e r s E c o n o v e n t E X 1 0 /1 1 p l a t e h e a t e x c h a n g e r . W i t h

t h e a s s is t a n c e o f a p s y c h o m e t r i c c h a r t , a n d k n o w l e d g e o f f lo w s , t e m p e r a t u r e s a n d h u m i -

d i t ie s o f t h e a m b i e n t a n d e x h a u s t a i r, it is p o s s i b l e t o s e l e ct t h e n u m b e r o f m o d u l e s

r e q u i r e d , p r e d i c t s u p p l y a n d e x h a u s t s i d e p r e s s u r e d r o p s , a n d t h e s u p p l y a i r c o n d i t i o n s

H.R.S. J I B



1 4 D . A . R E a Y

P

/ . . . . . . o / z , , k c

Outs ide air / ~ O//~ ,

condit ion ~ -? g

in winter ~

- - - 4 ° / ~ -

I, / Y

~-~_.'~__~/ /

r, IAir cond ition at th e

Iout le t o f Lossnoy in to

~ . l t h e r o om t , '

~,

/ , i 9 ::o

I Room air condit ion

J in winter

¢

¢

% % . .

Room air condihon /

in summ er /

' r ~ - L X , u ~ . ~ I

outle t o f Lossnay into ~ - ~ | /

t he r oo m ~ W O

Outside air

condit on n summ er

Absolute hum idi ty , kg/k g '

F ig . 1 0 . Ps yc h o me tr ic c h a r t s h o win g L o s s n a y u n i t p e r fo rma n c e .

af te r the hea t exchanger , toge ther w i th hea t excha nger e f fi ciency. I t mus t o f cour se be

em p h as i s ed th a t t h e i n p u t d a t a s h o u l d b e accu r a t e - - a f a ct s o m e t i m e s o v e r l o o k e d w h en

asses s ing the po ten t i a l benef i t s o f hea t r ecovery sys tems . )

A p p l i cab l e i n b o t h H V A C an d p r o ce s s p l an t s , a l t h o u g h m o r e a t t r a c t i v e a t p r e s en t i n

the l a t t e r because of cap i t a l cos t s , is the g las s p la te hea t exch anger . The use of g las s in

h ea t ex ch an g e r s i s n o t n ew , b u t f o u l i n g an d co r r o s i o n p r o b l em s a s s o c i a t ed w i t h m an y

meta l l i c hea t r ec overy un i t s have l ed to increased in te res t in g las s p la te and tub ular )

r ecupera tor s . D i f fe r ing l i t t l e in des ign f rom i t s meta l l i c counterpar t s , the g las s p la te hea t

ex ch an g e r h a s f o u n d ap p l i c a t i o n i n t h e t ex t i l e i n d u s t r y an d i n t h e p ap e r i n d u s t r y o n

d r y i n g p lan t . W h e r e p o w d e r s an d o t h e r p a r t i cu la t e m a t t e r a r e b e i n g d ri ed , t h e e a s e w i th

w h i ch gl a ss h ea t ex ch an g e r s c an b e c l ean ed i s an ad v an t ag e . A ce r ta i n am o u n t o f c a r e i s

need ed in ensur ing tha t the d i f f e ren t i a l p res sure be tween in le t and e xh au s t duc t s i s no t

ex cess iv e , an d t h e b en e f i ts o f c le an ab i l it y an d l a ck o f co r r o s i o n m u s t b e w e i g h e d ag a i n s t

the s ize ex tended sur faces a re no t used on g lass ) and cos t .

T a b l e 1. P e r f o r m a n c e o f L o s s n a y u n i t

A i r i nt o r o o m

O u t s i d e a i r S e n s i b l e h e a t e x c h . L o s s n a y

D r y b u l b t e m p . C ) 0 1 5 t 5

A b s o l u t e h u m i d i t y g / k g ) 1 .9 1 .9 5 .4

Rel a t ive hu mi d i t y o~, ) 50 l g 52

E n t h a l p y k c a l / k g ) 1 .1 4 7 6 . 9

H e a t r e c o v e r e d k c a l / h ) 4 3 3 6 8 9



A r e v i e w o f ga s - ga s he a t r e c ove r y s y s t e ms 15

E x h a u s t

fly

Condensat~

Fi g . 11 . Ec onove n t EX p l a t e he a t e xc ha nge r modu l e .

I n o r d e r t o r ed u ce t h e v o l u m e o f t h e h ea t ex ch ang e r , co n s i d e rab l e am o u n t s o f

ex tende d sur face ma y be in corpo ra ted be twe en the p la tes, as i s v i s ib le in F ig . 14, which i s

a cu t - aw ay v i ew o f t h e B e l t ran C o o p e r a l u m i n i u m p l a t e h ea t ex ch an g e r. I n t h is

contex t , i t i s in te res t ing to no te tha t [13] , in compar ing the four major types o f gas -gas

hea t r ecove ry sys tems , show ed tha t the ' compa ctnes s ' o f the p la te type was l es s than tha t

of the o th er th ree , as fo l lows :

H e a t e x c h a n g e r C o m p a c t n e s s

W/ m3/ ° C)

He a t p i pe 7200

He at wheel 5400

R u n - a r o u n d c o i l 4 6 8 0

P l a t e 4140

T h e d a t a w as o b t a i n e d f r o m an an a ly s i s o f co m m er c i a ll y av a i lab l e u n i ts f o r o p e r a t i o n

i n t h e t em p e r a t u r e r an g e 3 0 - 3 0 0 ° C .

The use of meta l l i c p la te he a t exchan ger s in indus t r ia l p roces ses has in so me ins tances

been l imi ted by the techniqu e used for sea l ing the gap b e twe en the p la tes an d the

s u p p o r t i n g s t r u c tu r e . E p o x y - b as ed re s in s an d o t h e r c em en t s h av e b een u s ed , an d t h e

ap p l i c a ti o n o f c e r am i c b a s ed cem en t s i s n o w o v e r co m i n g t h is t em p e r a t u r e l im i t a ti o n

p r o b l em . C e r am i c co r e s c ap ab l e o f o p e r a t i n g i n ex h au s t g a s s t re am s o f 1 3 50 °C a r e n o w

avai lable.

4.1.

pplications and economics

S o m e ap p l i c a t io n a r ea s o f p l a t e h ea t ex ch an g e r s h av e a l re ad y b een m en t i o n ed . U s es

d i ff e r l i tt l e f rom those o f the o the r types o f gas -g as h ea t r eco very un i t s l i s ted abo ve , bu t

Fi g . 12. F r e e z e p r o t e c t i on de v i c e on t he Ec o nove n t un i t .



16 D A REAY

% % supp ly o ir ef f ic iency~

8C

7C ~

6 6 * / , - -

50

40

m3/~ air f low q~

q t ~

q f m

(~ I I ' 1 11 I I I I j I I I I I ) I I I I I

P o p r e s s u re d r o p A p

5 o o F I

~

4 o o t - I I /

F i g 1 3 C a p a c i t y c h a r t f o r E c o n o v e n t E X I 0 / I I unit.

A T

W e , q e x l . 2 x ~ k d / s, C

A t

A~ is token from process line

in psychrometic chort

He a t c a p a c ity f lo w W s , k J ls , °C

I * ~ l , , i f ] I I * ~ t l l I I I I I " I ' ' H t I ' [

I 2 3 5 I 0 2 0

w , I

, / / / /

g

I

I I ~ 1 I , I I I I I ~ l l l

3 . 6 I 2 3 4 5 1 0 2 0

Ex h o u s t a i r f lo w q e , m 3 /s

a d v a n t a g e s o f , i n g e n e r a l , z e r o l e a k a g e a n d s i m p l i c i t y n o m o v i n g p a r t s) m u s t b e w e i g h e d

a g a i n s t t h e n e e d t o b r i n g d u c t s t o g e t h e r a t t h e h e a t e x c h a n g e r l o c a t i o n . T h e u s e o f g l a s s

p l a t e s , w h i l e i n t r o d u c i n g i t s o w n l i m i t a t i o n s , o f f e r s s c o p e i n f o u l e d a n d c o r r o s i v e g a s

Fi g 14 I nc o r po r a t i o n o f f i n s i n the Beltran Cooper plate heat exchanger.



A review of gas-gas heat recovery systems 17

s t r eams , and the low cos t o f mo dules for the dom es t i c s ec tor has a t t r ac ted severa l s tud ies

of the i r po ten t i a l ( and tha t o f o ther sys tems) even in UK c l imat ic con di t ions . A 'd i spos -

ab le ' p la te module , no t descr ibed in th i s paper bu t d i scussed e l sewhere in the Journa l , i s

one answer to hea t r ecovery in s i tua t ions where c lean ing i s d i f f i cu l t o r imprac t i ca l .

H V A C ap p l i ca ti o n s o f p l a te h ea t ex ch an g e r s a r e m an y , i n c l u d in g co m m er c i a l b u i l d -

ings , hosp i t a l s , and swimm ing pool s . An exam ple of the econo mic s of the g las s p la te hea t

exchanger i s g iven by the A i r Froe l i ch da ta for a un i t ins ta l l ed in a sw imming pool in

Swi tzer l and . An indoor swimming complex conta in ing two ba ths was s tud ied , and vent i -

l a t ion equ ipm ent s i zed on the bas i s o f the fo l lowing da ta :

Maximum num ber of swimmers 370

Roo m temperature 30°C

Room humidity 60%

W ater temperature 26°C

Total surface of water 427 mz

Quantity of air: Intake air/exhaust air: 32800/

21000 m3/h; F resh air/exhaust air: Variable (depending

on the hu midity of the fresh air).

By ins ta l l ing a g las s p la te hea t exchanger a t the ou t se t , the hea t ing bo i l e r spec i f i ed

could be r educed in s i ze , as could the hea ter ba t t e ry in the a i r condi t ion ing equipment .

Thi s amo unts to a reduc t ion of 120000 kca l /h in the hea t ing r equi remen t , l ead ing to a

reduc t ion in running cos t s o f Swis s Franc s 17700 a year a t a fue l p r i ce of app roxim ate ly

Swiss Fran cs 18/100 kg (pr ices in 1973) .

T h e p ay - b ac k p e r io d o f t h e ad d i ti o n a l h ea t r e co v e r y eq u i p m en t w o u l d b e ap p r o x i -

m ately 3.3 years , and the sa vings ov er the l i fe of the p oo l are i l lus t rate d in Fig. 15.

Indus t r i a l appl i ca t ion s of p la te hea t exchang er s have con cen t ra ted l a rge ly on dry ing

p l an t , p a r t l y b ecau s e o f l im i t a ti o n s o n t h e m ax i m u m t em p e r a t u r e o f s o m e o f th e u nit s.

H o w ev e r , p r o ces s - t o - p r o ces s h ea t r e co v e r y ap p l i ca t i o n s a r e b eco m i n g m o r e w i d es p r ead .

The O ak Me dica l p lan t in the U .S.A . is typ ica l o f the oven appl i ca t ions of p la te hea t

ex ch an g e r s w h e r e h ea t m ay b e r eu s ed i n th e p r o ces s . T h i s co m p an y m an u f ac t u r e s s u r g i-

ca l p las t i c g loves , and the product ion process inc ludes a cur ing s t age in an oven . Pr ior to

ins ta l la t ion of was te hea t r eco very equipmen t , m ore than 11 500 m3/h o f a ir a t 150°C w as

being d i scharged to a tmosph ere . S ince f it t ing the hea t r ec overy un i t o n o ne of the o ven

ex h au s t s , p l a t e ex ch an g e r m an u f ac t u r ed b y U n i t ed A i r S p ec i a l i s t s an d m ar k e t ed u n d e r

the nam e 'Temp -X-Cha nger ' , t he incoming a i r to the process , a t a f low ra te o f

10000 m3/h , i s be ing hea ted f rom am bien t (21°C) to 93°C , s av ing app roxim ate ly 8 therms

per hour . The ann ual s av ing in the f i rs t year was 9600 for an ins ta l l ed cos t o f 12400,

and the success of th i s p ro jec t has l ed to fur ther inve s tmen t in hea t r ecovery by th is

c o m p a n y .

1 0 0 0 , 0 0 0

9 0 0 , 0 0 0

8 0 0 , 0 0 0

7 0 0 , 0 0 0

6 0 0 , 0 0 0

8

. _ ~ 5 0 0 , 0 0 0

c 4 0 0 , 0 0 0

a : 3 0 0 , 0 0 0

P o y b 0 c k pe r io d

R u n n i n g c o s t s w i t h o u t

h e a t r e c o v e r y

R u n n i n g

costs

w i t h h e a t

recovery

o o I

~ , I i I f , i , J , I , I I t , l , l , t , I , I t l i l f i , l , , , I I

0 1 2 3 4 5 6 7 8 9 I0 I I 12 13 14 15 16 17 18 t9 20

Yeors of se rv ice

Fig. 15. Econom ics of A ir Froehl ich heat exchanger.



1 8 D . A . R E v

The recovery of process heat for space heating is also a viable proposition, as can be

shown by the following example of a plant recently installed in a heat treatment furnace:

Objective To recover heat from the exhaust stack of a continuous Heat Treatment

Conveyor Tunnel Furnace and use it for industrial warm air space heating, to

provide basic background heating over a wide area, thereby gaining maxi-

mum utilisation of the plant.

Factors 1. Stack temperature Max. 95°C Min. 45°C verage65°C (from recorder).

2. Stack gas flow rate average 3.14 m3/s (at temperature).

3. Workshop temperature 20°C.

4. Util isat ion: Furnace runs for 120 h per week.

Heated areas occupied for 80 h per week.

1. Total stack loss based on an internal tempera ture of 20°C = 1455 kW.

2. Using 70% heat recovery efficiency = 101 kW heating input.

3. Based on current fuel costs and overall plant efficiency 75,% cost to

generate 1 kW = 0.76 p/h.

Cost saving = 76.5 p/h.

4. Based on 80 h utili sation per week.

Cost return rate = £61.00 per week.

5 Heat exchanger --Laminai re 2-336.

Ex works cost £2000. Approximate cost returned in 33 weeks use.

This data was prepared by Laminaire Products. Note that the heat exchanger cost is

for the basic unit, and in order to do a more realistic payback analysis, installation costs

should be known. In this case the operating temperature is not sufficiently h i ~ to inflate

installation costs, and the only unknown is the availability of ductwork to transfer heat

from the furnace flue to the areas to be space heated.

System

5. R U N - A R O U N D C O I L S

The run-around coil system, sometimes referred to as a lipid-coupled indi rec t heat

exchanger network, or a coil heat recovery loop, can be a highly effective and readily

installed waste heat recovery technique, particularly when it is desirable to locate a

system without the need to reroute ductwork. This makes it particularly attractive for

'retrofit' installations and sets it apart from the other types of heat recovery units dis-

~

. ~ . 4 E x t r a c t a i r 2 7 ° C .

. i t t o ) l ' c _ t

I r ~ _ : . t

n o o n e

system.- D u c t

mounte coi ls .

F ' A . - 5 " C .

(F r e s h a i r i n p u t . )

Fi g . 16 . Run - a r ound c o i l l a you t .




cussed in this paper. In its simplest form, used for HVAC and low temperature process

waste heat recovery, it consists of standard extended surface finned tube water coils, used

with a circulating pump. As illustrated in Fig. 16, one coil is located in the exhaust gas

stream, and the other is located in the duct through which the air to be preheated is

flowing. The pump is used to circulate water, an anti-freeze solution, or a higher tem-

perature heat transfer fluid such as Dowtherm or a heat transfer mineral oil through the

two coils. The liquid picks up heat from the exhaust gas as it passes through the coil in

that duct, and subsequently rejects the heat to the incoming air stream before being

pumped back to the exhaust gas coil.

Run-around coils cater for air precooling in Summer in HVAC applications, and, in

common with other types of air-air recovery systems, can be incorporated by many

manufacturers in a package , together with fans, filters, cont rol circuitry, etc.

One of the principal advantages of the run-around coil system is its ability to operate

effectively without having to reroute ductwork. Because the intermediate heat transfer

medium is a liquid, the coils in the exhaust and inlet air ducts can be many meters apart,

even on different floors of a building, and the interconnecting pipework takes up little

space and is much cheaper to install than air ducting. This has obvious benefits when the

system is to be retrofitted to a HVAC installation or item of process plant, but can also

lead to cost savings when incorporated at the design stage.

Run-around coils rely on sensible heat recovery, at least in their basic form, although

the occurrence of condensation on the coil in the hot exhaust duct may result in the

capture of some latent heat content also. Maximum efficiencies are about 70~, but

operation in the efficiency range of 40-60~ is more typical.

Two other merits of the run-around coil may be cited. Because an intermediate heat

transfer fluid is used in a sealed system, no cross-contamination between the two gas

streams can occur. Also, the coils used in the ducts are widely used on their own as

heaters, evaporators, and condensers in most HVAC plant, and are therefore available

from a large number of manufacturers and at very competitive prices.

The pump may introduce a reliability factor into the system, being the only moving

part, but for water/glycol and water intermediate heat transfer fluids problems should

occur very infrequently. Pumps for high temperature heat transfer fluids can, however, be

expensive.

An excellent analysis of the practical engineer of a high temperature liquid-coupled

heat exchanger loop is given by Boyen 113]. In considering both fired heaters and sys-

tems employing waste heat from, for example, gas turbines or incinerators, Boyen points

out that two-phase, as well as single phase systems may be used. Charging of the system

with Dowtherm A, one of several working fluids which may be used in both vapour and

liquid phases, offers advantages in a number of applications. In processes where heat

uniformity is important, condensing vapour offers precise temperature control. If a single

phase system was used, extreme flow rates would be necessary in order to maintain a

similar degree of precision.

In designing single and two-phase run-around coil systems utilising high temperature

organic fluids as the intermediate heat transfer medium, it is important to have an

accurate knowledge of the maximum film temperature, which the fluid is likely to

achieve. This has a critical bearing on the life of the fluid and the successful long-term

operation of the unit at peak efficiency. (Manufacturers of the various fluids are able to

advise on the limiting values and degradation rates.)

Coils for use at high temperatures (above 200°C) may differ significantly in design and

arrangement to those used in HVAC and low temperature process applications. The

finned tubes may, for instance, be arranged in a spiral around the inner wall of the duct

carrying the hot exhaust gas, as shown in Fig. 17. Individually finned tubes used in such

a configuration may prove easier to clean in cases where the exhaust gas contamination

is high. At these higher temperatures the sensible heat effectiveness of the units is gener-

ally about 10--15~ less than those using water as the working fluid, due to the reduced

heat transfer capabilities of these thermal fluids.



2 0 D . A . R EA Y

F i g . 1 7. S p i r a l g a s - l i q u i d c o i l a s u s e d o n s o m e r u n - a r o u n d s y s t e m s .

T h e ap p l i c a ti o n a rea s fo r ru n -a ro u n d co il s di ff e r l it tl e f ro m t h o se o f o t h e r g a s -g a s h ea t

reco very uni t s. How eve r the i r use in h igh t em pera ture processes is l imi t ed by the ava i l -

ab i l i ty of su i t ab le work in g f lu ids . Mo st o f t he orga nic f lu ids used shou ld no t be t aken

ab o v e 3 5 0 °C an d a s y e t a p u m p ed l o o p u s i n g l iq u i d m e t a l s h a s n o t r e ce iv ed se r io u s

cons idera t ion .

W h ere d u c t l o ca t io n o r t h e ex p en se i n v o lv ed in r e ro u t i n g o f d u c t p ro h i b it s t h e u se o f

o t h e r h ea t r e co v e ry sy s tem s t h e ru n -a ro u n d co il is t h e o n l y co m m o n l y av a il ab le u n i t

par t i cu la r ly for HV AC app l i ca tions . I t i s a lso how ever par t i cu la r ly e f fect ive in proces s

to space and process to process hea t rec ove ry appli ca t ions . T he sys t em show n in F ig . 18

i n s ta l led b y F l ak t i n S w ed en is a co m p l ex a r r an g e m e n t o f co i ls u sed t o r eco v e r h ea t

p ro ces s ex h au s t s i n a s au sag e sk i n m an u fac t u r i n g p l an t. T h e r eco v e red h ea t is th en u sed

t o p reh ea t c l e an in co m i n g p ro ces s m ak e -u p a ir . I n t h is i n s t an ce t h e t o t a l i n v es t m en t o f

£70000 was recov ered in l ess than o ne year and i t is i n t e res t ing to no te tha t t he use of

l iqu id-coupled hea t exc hang ers fac il i ta t e hea t t ran sfer be tw een d i f fe ren t f loors wi th in the

sam e bui ld ing .

R u n -a ro u n d co i l s o f b o t h t h e s in g le an d t w o -p h ase t y p e h av e b een ap p l i ed i n k i ln s i n

m a l t i n g p lan t a l t h o u g h t h e fo rm er h av e b een t h e m o s t su ccess fu l t o d a t e. D ep en d i n g

u p o n t h e f i r i n g sy s t em u sed an d t h e t y p e o f p o l l u t i o n an t i c i p a t ed i n t h e ex h au s t g a s

s t ream the co il i n th i s s t ream m ay be of a d i f fe ren t ma ter i a l and f in p i tch ing than tha t

u sed to h ea t t h e m ak e -u p a i r. I n o n e m a l t in g p l an t i n s ta l la t io n fo r ex am p l e t h e ex h au s t

gas co i l i s o f a l l-copper cons t ruc t ion coa ted wi th a pro tec t ive p l as ti c fi lm where as the

other co i l i s cons t ruc ted a long l ines iden t i ca l t o tha t used in most HVAC uni t s - - -copper

tubes expanded in to a luminium pla t e f ins on a c lose p i t ch .

A n e w a re a o f co n s i d e rab l e i n te r e s t t o s t a t i c h ea t e x ch an g e r m an u fac t u re r s is t h e f ie ld

o f b o i le r a ir p r eh ea t e rs . A s w i th t h e h ea t p i p e h e a t r e co v e ry u ni t t h e ru n -a ro u n d co il

~ Min-14 C

35 38%

<

.

R e c o v e r y f

l r

i / |

I

i g . 1 8. A r u n - a r o u n d c o i l c o m p l e x u s e d f o r h e a t r e c o v e r y i n a s a u s a g e s k i n fa c t o r y .



A revie,x of gas-gas heat recoverysystems 21

m a y b e u s e d o n l a r g e st e a m - ra i s in g p l a n t m u l t i -M W s iz e) t o r e c o v e r h e a t f r o m t h e

e x h a u s t g a s e s f o r b o i l e r a i r p r e h e a t i n g . T r a d i t i o n a l l y t h e l a r g e b o i l e r a i r p r e h e a t e r m a r k e t

h a s b e e n d o m i n a t e d b y r e g e n e r a ti v e h ea t e x c h a n g e rs , p ri n c i p a ll y t h e r o t a t i n g r e g e n e r a t o r

b u t . t o a l e s s e r e x t e n t , t h e R o t h e m u l e r e g e n e r a t o r . R e g e n e r a t i v e u n i t s a r e n o w r e c e i v i n g

c o m p e t i t i o n f r o m t h e r u n - a r o u n d c o i l, w h i c h, u s in g e c o n o m i z e r t e c h n o l o g y , c a n b e s u c -

c e s s fu l l y a p p l i e d i n t h is a p p l i c a t i o n . T h e l a r g e s t i n s t a l l a t i o n t o d a t e i n t h e U K is a t

S t a n l o w r e f i n e r y .

6. CONVECTION TUBULAR) RECUPERATORS

T h e t u b u l a r c o n v e c t i o n r ec u p e r a t o r c om e s i n m a n y f o rm s , d e p e n d i n g u p o n t h e o p e r a t -

i n g t e m p e r a t u r e r a n g e , p l a n t s i ze . a n d t h e t y p e o f f o u l i n g i n th e e x h a u s t g a s s t r e a m . I n

t h i s s e c t i o n a n u m b e r o f d i f f er e n t t y p e s a r e d e s c r i b e d a n d a p p l i c a t i o n s l is t ed . I t i s d i ff i c u lt

t o g e n e r a l i s e o n t h e r a t e o f r e t u r n f o l l o w i n g i n v e s t m e n t i n t h e s e u n i t s b e c a u s e o f t h e w i d e

v a r i e t y o f a p p l i c a t i o n s , b u t d a t a f r o m A i r F ro t ,~ q i ch , t a b u l a t e d l a t e r i n t h e s e c t i o n , g i v es

a n i d e a o f t h e p o t e n t i a l f o r th e i r p a r t i c u l a r u n i t i n s e v e r a l p r o c e s s i n d u s t r i es .

6 1 L o w t e m p e r a t u r e u n i ts

A s w i t h a n y t y p e o f h e at r e c o v e r y u n it . th e t e c h n o l o g i c a l p r o b l e m s t o b e o v e r c o m e i n

t h e d e s ig n a n d o p e r a t i o n o f t u b u l a r r e c u p e r a t o r s i n c re a s e w i th i n c r e a s i n g p r o c e s s e x h a u s t

g a s t e m p e r a t u r e s , a n d a t t e m p e r a t u r e s b e l o w a b o u t 2 5 0 ° C , s e v e r a l i n t e r e s t i n g t y p e s o f

t u b u l a r r e c u p e r a t o r e x i st w h ic h b e a r l i tt le r es e m b l a n c e t o t h e i r h i g h e r t e m p e r a t u r e

c o u n t e r p a r t s .

T h e m o s t i n t e r e s t i n g o f t h e s e is t h e g la s s t u b e r e c u p e r a t o r , a n e x a m p l e o f w h i c h ,

m a n u f a c t u r e d b y A i r F r o e h l i c h , is s h o w n i n F i g . 1 9. A g la s s t u b u l a r h e a t e x c h a n g e r l ik e

t h e g la s s p l a t e h e a t e x c h a n g e r d e s c r i b e d i n a n o t h e r s e c t i o n ) is v e r y e a s y t o c l e a n a n d h a s

e x c e l le n t r e s is t a n c e to c o r r o s i o n . I n c o m m o n w i t h al l o t h e r t u b u l a r r e c u p e r a t o r s , o n e g a s

s t r e a m f l o w s a c ro s s t h e tu b e s , w h i c h in t h is c a s e a r e n o t f i n n e d , a n d t h e o t h e r s t r e a m

p a s s e s t h r o u g h t h e t u b e s , e ff e c ti n g a c r o s s - fl o w a r r a n g e m e n t . O f c o u r s e , t h e t w o g a s

s t r e am s a r e c o m p l e t e l y s e a le d f ro m o n e a n o t h e r , t h u s n o c r o s s - c o n t a m i n a t i o n c a n o c c u r .

I n s o m e t u b u l a r r e c u p e r a t o r s , a n u m b e r o f p a s s e s o n t h e t u b e s i d e a r e u s e d , a n d t h i s is

i l l u s t r a te d l a t e r in t h e c a s e o f a h i g h t e m p e r a t u r e u n i t . I t i s a l s o p o s s i b l e , a l t h o u g h n o t

c o m m o n , t o p u t t h e d ir t y g a s s t re a m t h r o u g h t h e i ns i de s o f t h e t u b e s , w h i c h m a y , w i t h

s o m e t y p e s o f d e p o s i t s , b e e a s i e r t o c l e a n t h a n t h e o u t s i d e o f t h e t u b e s .

Fig. 19. A glass tube recuperator.



T

e

2

A

c

o

a

e

m

i

c

d

a

o

g

a

u

o

A

F

o

c

I

n

y

T

e

f

n

n

C

m

i

c

F

C

m

i

c

B

w

n

P

m

p

a

p

F

z

F W

o

P

o

d

p

o

H

g

e

m

p

u

a

d

y

n

o

p

n

e

t

e

e

H

g

e

m

p

u

t

h

m

o

x

o

D

y

n

a

g

a

n

w

h

s

d

y

P

w

z

o

o

m

i

k

c

p

a

o

e

c

w

h

s

d

y

P

w

z

o

o

c

o

n

a

K

n

d

y

n

o

m

a

P

p

a

p

d

y

n

D

y

n

o

a

m

a

d

G

r

d

n

w

h

c

n

P

d

c

p

d

y

n

E

a

A

a

A

y

y

t

e

m

p

u

v

u

m

e

E

a

o

n

m

e

C

)

k

g

/

h

)

i

m

p

e

h

)

7

1

5

2

D

u

u

b

2

1

2

u

o

1

F

b

n

4

5

1

5

5

C

m

i

c

p

w

c

m

b

o

m

p

e

6

6

1

u

o

2

P

w

8

1

2

P

w

4

2

7

3

2

D

c

m

b

o

m

p

e

4

6

1

4

2

P

m

p

n

b

5

9

1

5

2

D

p

a

y

m

o

6

9

1

5

5

D

g

e

3

1

1

2

5

W

o

d

n

4

R

e

e

g

p

y

w

h

l

O

k

h

a

v

u

m

e

G

4

I

1

7

1

5

1

5 5 3

6

P

p

o

f

o

a

o

i

n

m

e

y

0

4

0

6

0

1

8

2

2

0

9

0

8

1

2

0

6

2

3



A r e v i e w o f ga s ga s he a t r e c ove r y s y s t e ms 23

-20 C

9 0 0 0 m~/ h

Fresh air I~

G loss

tube~ ' ' j ~

recuperotor

< 1

9 0 0 0 m 3 / h

3 5 C

Exhaust

Fi g . 20 . G l a s s t ube r e c up e r a t o r dow ns t r e a m o f a d r ye r hood .

T ab l e 2 g iv e s d a t a o n t h e h ea t r e co v e r y cap ac i t y an d t h e r e s u l ti n g p ay - b ack p e r i o d f o r

g las s tube r ecu pera to r s in a num ber o f indus t r i a l p roces ses . As w i th o ther hea t

ex ch an g er s , t h e h ea t r e co v e r y cap ab i l i ty d ep en d s o n t h e t em p e r a t u r e d i ff e ren ce b e t w een

the ho t and co ld gas s t r eams , and th i s i s to some ex ten t r e f l ec ted in the pay-back f igures ,

when r e la t ed to the exhaus t t empera tures in the th i rd co lumn. In mos t cases these un i t s

a r e d e s ig n ed t o r e co v e r b e t w een 6 0 an d 7 0 ~ o f t h e h ea t i n t h e ex h au s t g a s s tr e am .

As an exam ple of a p lan t in opera t io n fo r severa l years , cons ider an ins ta l la t ion in a

t ex t i l e f ac tory . In a t ex t i l e f in i sh ing machine w i th cy l inder d ryer s , fumes and vapour a re

ex t r ac t ed t h r o u g h h o o d s o v e r t h e d r y e r s ec t i o n an d t h en ex h au s t ed t o t h e a t m o s p h e r e .

Thi s exhaus t a i r has to be r ep laced by ou t s ide a i r p rehea ted to a t em pera ture o f 20--25°C

for work ing sp ace ven t i l a t ion . As il lus t r a t ed in F ig . 20 com ple te hea t ing of ou t s ide a i r i s

accom pl i shed w i thout any ex terna l energy , i. e . no hea t ing co i l s a r e ins ta l led in the ven t il -

l a t ion p lan t . Even i f the ou t s ide a i r tem pera ture i s as low as - 20°C , a l l energy neces sary

to r a i s e the t em pera ture to the r equ i r ed l evel is r ecove red f rom the e xhaus t a i r which i s a t

3 0 - 3 5 ° C . I n o r d e r n o t t o o v e r h ea t t h e t ex t il e p l an t d u r i n g t h e w a r m er p e r i o d s o f t h e y ea r ,

supply a i r t empera ture i s con t ro l l ed by a bypass in the exhaus t a i r duc t .

In th i s ins ta l l a t ion the fou l ing in the exhaus t a i r was the pr imary r eason for s e lec t ing a

g las s hea t exhan ger . Thi s cons i s t s main ly of dus t , fib res and s t i cky com po nen t s f rom the

f in ish ing proces s , and se t tl es on the in le t side of the he a t exchan ger . F ro m t ime to t ime

the r es idues a re f lushed away w i th a permanent ly ins ta l l ed spr ink ler nozz le . No depo-

s i t ions cou ld be found in the un i t i t s e l f , main ly due to the f ine sur face s t ruc ture o f g las s

and the h igh a i r ve loc i ty a long the sur face .

O p e r a t i n g s av i n g s o n t h i s u n i t am o u n t ed t o ab o u t 6 0 0 0 S w i s s F r an cs p e r an n u m . A

n u m b er o f si m p l e tu b u l a r r e cu p e r a t o r s u s in g m e t a l t u b es a r e a l s o av a i l ab le f o r u s e a t l o w

gas tem pera tures . I f fou l ing i s no t a p rob lem , the use of ex te rna l and in som e cases

preferab ly w i th in te rna l) tu be f inn ing i s adv antag eou s in h e lp ing to keep th e s i ze o f the

h ea t ex ch an g e r w i t h i n r e a s o n ab l e p r o p o r t i o n s .

6.2. igh temperature recuperators

M et a l l i c co n v ec t i o n r ecu p e r a t o r s c an b e u s ed w h e r e g a s ap p r o ach t em p e r a t u r e s a r e

less than 1000°C, a l thou gh h igher t em pera tu res can be perm i t t ed i f spec ia l mater i a l s and

co n s t r u c t i o n t e ch n i q u es a r e u s ed . O r i g i n a l l y a l l r e cu p e r a t o r s w e r e m ad e f r o m ce r am i c

mater i a l s , bu t these un i t s suf fe red f rom ser ious l eakage prob lems , an d have tod ay bee n

large ly super sed ed by m eta l li c r ecupera tor s , excep t in some spec ia l cases s ee be low) .

T h e r e a r e t w o b a s i c t y p es o f co n v ec t i o n r ecu p e r a t o r , t h o s e w h i ch u s e ca s t t u b es an d

t h o s e u s i n g d r aw n t u b es w h i ch a r e a s s em b l ed i n b u n d l e s , i n co m m o n w i t h m an y o t h e r

t y p es o f h ea t ex ch an g e r . T h e u s e o f c a s t t u b es i s n o r m a l l y r e co m m e n d ed f o r l o w p r e s s u r e

appl i ca t ions , where l eakage i s un l ike ly to be a s ign i f i can t r ep lacement i s r e l a t ive ly

easy [14] .

The tubes used a re ava i l ab le e i ther p la in o r w i th a w ide var i e ty o f ex tended sur face

conf igura t ions . Wide p i t ch ing o f sur face pro jec t ion s is used w hen the ex haus t gas f low

m a y b e h eav i ly co n t am i n a t ed . I n s o m e ca s es , t h e o u t s i d e o f t h e t u b es m a y b e le ft co m -

p l e te l y b a r e . H o w ev e r t h e o v e ra l l h ea t t ran s f er t h r o u g h a t u b e o f th i s t y p e m ay b e



2 4 D . A . R E AY

I P r e h e o te d L

a i r C o l d a i r

i n l e t _ _

~ L ~ ~

L l g 1 1 IL o

~ ~

~/~;;, ; , ; :; ; ;~,

F i g . 2 1 . A 4 - p a s s c a st c o m p o s i t e t u b u l a r r e c u p e r a t o r i n s t a l la t i o n .

Waste

g a s

o u t l e t

m ai n t a i n ed a t an accep t ab l e leve l b y r e t a i n in g t h e ex t en d e d su r face s o n t h e i n s i d e o f t h e

tube , t h rough which the a i r t o be hea t ed i s passed .

A n a l t e rn a ti v e m e t h o d o f lo ca t i o n u sed o n ca s t t u b e r ec u p e ra t o r s i n v o lv e s t h e u se o f

end f l anges wi th in t egra l ly cas t s tee l expan s ion jo in t s . Thi s p ermi t s t he tub es to be

w e l d ed t o g e t h e r to fo rm t u b e b an k s o f an y s iz e. A 4 -p as s h o r i zo n t a l f l u e ca se c o m p o s i t e

t u b e r ecu p e ra t o r d e s i g n ed b y T h e rm a l E f f i c ien cy L t d i s il lu s t r at ed i n F i g . 2 L I t c an b e

seen tha t bo th bare and ex terna l ly f i nned tubes a re used in t h i s i ns t a l l a t i on . These rows

o f p l a in sp u n a l lo y s t ee l t u b es co n t a i n i n g a p e rcen t ag e o f ch ro m i u m an d n i cke l , a r e

a r r an g ed i n f ro n t o f t h e co m p o s i t e t u b es t o s a f eg u a rd t h e l a t t e r f ro m l o ca l is ed h ea t in g ,

c r ea t ed b y n o n -u n i fo rm an d ex ces s iv e r ad i a ti o n . T h e ch ro m i u m p ro v i d e s a r e s is t an ce t o

o x i d a t i o n a t h i g h t em p e ra t u re s , t h e t h e n i ck e l co n t en t i m p ro v ed d u c t i li t y i n a r ea s w h e re

h igh thermal s t resses a re l i ke ly to be encountered . Typica l uses of t hese un i t s i nc lude

so ak i n g p i t an d r e -h ea t su r face r ecu p e ra t i o n , w h e re an ad d i t io n a l r e q u i r e m e n t i s f o r

r e s is t an ce t o ab ra s i v e an d s i n t e r ed d u s t l ad en g a ses . A l t h o u g h t u b e r ep l acem en t i s n o t a s

co n v en i en t l y ca r r i ed o u t a s w i th b o l t ed u n i ts , e ach t u b e can b e r em o v ed f ro m t h e b u n d l e

o n ce t h e w e l d b ead s h av e b een g ro u n d o f f.

C o m p o s i t e t u b e r ecu p e ra t o r s a r e u sed ex c l us i v el y a s co n v ec t i o n t y p e h ea t ex ch an g e r s

w i t h w as te g as t em p e ra t u re s o f u p t o 9 50 °C . U s i n g t h e p l a in sp u n t u b e sy s t em d esc r i b ed

ab o v e , th e t em p e ra t u re r an g e can b e s l ig h t ly ex ten d ed . T h e u n i t i n F i g . 2 2 c an o p e ra t e i n

g ase s a t 1 10 0° C . D ra w n s t ee l t u b e r ecu p e ra t o r s a r e av a i lab l e i n m an y fo rm s . E ach t u b e

b u n d l e i s a t t a ch ed t o h ead e r b o x es , an d t h e co n s t ru c t i o n t e ch n i q u e u sed a l lo w s t h e t u b es,

an d i n d i v i d u a l t u b e b u n d l e s , t o ex p an d r e l a t i v e t o o n e an o t h e r . I n so m e sy s t em s t h e

tubes a re a l so bent a t t he i r mid poin t t o min imise s t resses a r i s ing f rom thermal expan-

s i o n . T h ese r ecu p e ra t o r s a r e o f t en u sed w h e re i t i s r eq u i r ed t o r eco v e r a co n s i d e rab l e

p ro p o r t i o n o f t h e r ad i a t i o n h ea t, an d t h e t u b es a r e g en e ra l ly n o t f in n ed . H o w ev e r co n -

d u c t i o n t h ro u g h t h e w a l l i s en h an ced b y t h e f ac t t h a t w h e rea s ca s t r e cu p e ra t o r t u b es

h av e w a l l t h ick n es se s o f t h e o rd e r o f 8 r am , d raw n t u b es m a y h av e a t h ick n es s o f o n l y

3 ram.

I t h a s b een s t a t ed ab o v e t h a t m e t a l li c r e cu p e ra t o r s h av e l a rg e l y su p e r sed ed t h e r e f rac -

t o ry t y p e . H o w ev e r , h i g h p re s su re / h i g h t em p e ra t u re ce ram i c r ecu p e ra t o r s h av e b een

o v e rc o m e u s i n g a c e ram i c f ib r e p ack i n g ~ d o n a l u m i n i u m silic ate . A n ex am p l e o f su ch

a r ecu p e ra t o r , w i t h t u b es o f ' C a rb o f r ax ' an d R e f rax ' p ro d u ced b y t h e C a rb o ru n d u m

C o m p an y , co n n ec t ed d ir ec t ly t o t h e o u t l e t o f a h i g h t em p e ra t u re k i ln , i s d e s i g n ed t o

accep t 0 .86 m3/ s of gas a t 1800°C, g iv ing an a i r p re-hea t t emp era tu re o f 1200°C and a

hea t t ransfer a t e of 104 kW .

T h e C o rp o ra t e E n g i n ee r i n g L ab o ra t o r i e s o f th e B r it ish S t eel C o rp o ra t i o n (BS C )

w o r k e d f o r a n u m b e r o f y e a rs o n c e r a m i c h i g h t e m p e r a t u r e r e c u p e r a to r s a n d r e g e n e r -




Fig 22 Spun cast flue type recuperator

ators for steel plant applications. The ceramic recuperator, which can preheat combus-

tion air to 650=C is illustrated in Fig. 23. A prototype system commenced operation in

November 1973, and its performance is considerably better than metallic recuperators.

particularly as far as restrictions on operating temperating temperature are concerned.

Leakage problems are also minimised by the use of flexible ceramic seals.

A design problem area with tubular recuperators, particularly at high temperatures.

has been the effect of differential expansion on the tube-to-tube plate joints. In some

instances tubes have been bent, in the form of a U, to join a common header, as a means

of overcoming this difficulty. Where ceramic tubes have been used in conjunction with

metallic headers, ceramic fibre rope has been employed in an attempt to obtain a flexible

seal [10], but leakages in excess of 15 led to the abandonment of this system in favour

of a new seal design. High nickel/chrome steels are preferred to ceramics by some, and

preheats of 550-600'C are possible in aggressive environments using metallic tubular

recuperators, and hybrid ceramic/metallic recuperators have provided preheats of 700C

in the steel industry.

7 RADIATION RECUPERATORS

Radiation recuperators take two basic forms. They may consist of two concentric

cylinders, the air to be heated normally flowing through the outer annulus while the



2 6 D . A . R EA Y

\ ~ G l a z e d

- ,

c e r a m i c

t u b e

\

\ x H e Q d e r

~ o x

\ .

\

\ \

F i g . 2 3 . Brit ish Steel ceramic tube recuperator

exhaust gas f low through the central duct . Al ternative ly the uni t may be bui lt u p with

tubes between two headers .

Co mp ared w ith the con vection recuperator the radiation type offers very low resi st-

ance to gas f low and in most instances never needs c leaning . The dirt iest o f exhaust gases

can be permitted through i t and by i ts nature this type of recuperator can a lso act as

part of the chimney or f lue .

Th e s ize of radiation recuperator can vary considerably the largest uni ts being ab out

50 m long and 3 m in diameter . Ra diation recup erators can be con structed using tubes to

G a s

in

F i g . 2 4 . C om b i n ed rad i an t t u b e h ea t er an d recu p era tor




Fig. 25. lnka Radiation recuperator.

separate the exhaust gas and air, rather than a single annulus. These are used in instances

when preheat temperatures in excess of 600°C are required. It is preferable to use parallel

flow of air and gas in these, as the tubes tend to be subject to near equal temperatures

along their whole length, thus keeping temperature-induced stresses to a minimum.

The radiation recuperator is generally regarded as being the most reliable of the two

main types available, and has the longest life.

As well as applications involving heat recovery from boiler and furnace exhausts, the

radiation recuperator may be used in conjunction with a radiant tube heater, forming

self-contained radiant tube heater and recuperator unit, as shown in Fig. 24. The recuper-

ator replaces the normal exhaust stack on individual radiant tubes, absorbing heat which

is then used to preheat combustion air for the burner.

One of the main application areas for tubular radiation recuperators is in glass melting

furnaces. Such a unit, manufactured by Johnson Construction Company AB consists of a

bundle of tubes hanging freely in a vertical refractory shaft, as shown in Fig. 25. The

bundle of tubes, which serves as the heating surface, is made of high alloy heat-resistant

steel and welded as one unit. The inlet and outlet are placed on two ring manifolds,

between which the tubes are welded.

One of the main advantages of such a recuperator over conventional ceramic regener-

ators used on glass furnaces is the weight reduction. Metals in glass furnace exhaust gas

streams have suffered from corrosion and severe fouling, but with the correct selection of

materials, and reliance on radiation as the mode of heat transfer, these difficulties can be

overcome and lives equivalent to tha t of the furnace lining 5-10 years) can be achieved

without repairs.

8 R E C U P E R A T I V E B U R N E R S

Descriptions have already been given of several types of recuperators, many of which

are applied for the preheating of combustion air. These include the radiant tube recuper-

ator described above, which may be directly linked to a burner to provide this combus-

tion air preheat. However, it is also possible to integrate the recuperator within the body

of the burner.

One of the most successful recuperative burners is that developed by the Midlands

Research Station of British Gas, and their design is illustrated in Fig. 26. The unit

consists of a high velocity nozzle-mixing burner, in itself an effective burner because of

the good heat transfer obtained by directing high velocity hot gases over the furnace

load, surrounded by a counter-flow heat exchanger. This heat exchanger supplies hot



28 D A REACt

t

combust ion products

. . products

c o m O u s , ,o n . , ,, , e , o . o , u s t ,o n

Fig 26 Recuperative burner schemat ic

combustion air to the burner nozzle. The heat exchanger consists of a series of concentric

tubes which act as interfaces between the combustion gases and the air to be preheated,

and is made from a heat resisting steel.

In operation, the combustion air enters the burner at a manifold, which totally

encloses the flue annuli. It then passes forward along the air annulus, thus keeping the

external surfaces of the recuperator cool. Flue products are extracted in a counterflow

direction around the outside of the air annulus and thus preheat the air. At the front of

the recuperator the air flow doubles back and enters the burner nozzle. The gas and

preheated air mix at the nozzle, on the face of which the flame stabilises, and combustion

is essentially complete before the gases pass into the furnace chamber. Exit velocities

from the burner tunnel considerably exceed 50m/s. Normally all the combustion

products are extracted through the recuperator, assisted by the use of an air-driven

eductor mounted on the flue of the burner. By controlling the amount of eductor air the

furnace pressure can be maintained at the desired level.

9. HEAT PIPE HEAT EXCHANGERS

The heat pipe heat exchanger used for gas-gas heat recovery is essentially a bundle of

finned heat pipes assembled like a conventional air-cooled heat exchanger, Because the

heat pipe is a comparatively recent development in the waste heat recovery field, it will

be discussed in some detail in this context. It is in many ways, however, similar to the

Perkins tube, a two-phase thermosyphon which was in regular use until the t960 s as a

heat transfer device in bread ovens, and which was invented in the 19th century.

9 1 The heat pipe

A heat pipe [15] is basically a sealed container, normally in the form of a tube,

containing a wick lining the inside wall. The purpose of the wick is to transport a

working fluid, contained within the heat pipe, from one end to the other by capillary

action. The full operating cycle may be described with reference to Fig. 27. Heat applied

externally to the evaporator section of the heat pipe causes the working fluid contained

within the wick to evaporate, and the increase in pressure causes the vapour to flow

along the central vapour space to the slightly cooler condenser section, where it con-

denses, giving up its latent heat of condensation. This heat is then rejected from the outer

surface of the condenser, and the condensate is pumped back to the evaporator by the

capillary action in the wick. Because of the reliance on capillary action to return the



A r e v ie w o f g a s g a s h e a t r e c o v e r y s y s t e m s

o u o i

5 H e a t o u t

F i g . 2 7 . T h e h e a t p i p e .

29

c onde nsa t e t o t he he a t i npu t s e c t i on , t he he a t p i pe i s pa r t i c u l a r l y se ns i t i ve t o t he e f fe c t s

o f g r a v i t y , a n d h e n c e i ts i n c l i n a t i o n t o t h e h o r i z o n t a l . D e p e n d i n g o f c o u r s e o n t h e t y p e o f

w i c k u s e d a n d i t s p o r e s i z e , a h e a t p i p e o p e r a t i n g w i t h t h e e v a p o r a t o r b e l o w t h e c o n -

d e n s e r m a y b e c a p a b l e o f t r a n s p o r t i n g s e v e r a l t i m e s a s m u c h a s o n e h a v i n g t h e e v a p o r -

a t o r a b o v e t h e c o n d e n s e r . T h i s i s p a r t i c u l a r l y i m p o r t a n t i n l o n g h e a t p i p e s ( m o r e t h a n

500 mm i n l e ng t h) , a nd i t s i mpl i c a t i ons wi t h re spe c t t o he a t p i pe e xc ha nge rs a re d i sc usse d

be l ow.

I n c a s es w h e r e g r a v i t y a i d s r e t u r n o f t h e c o n d e n s a t e t o t h e e v a p o r a t o r s e c ti o n , i t is

p o s s i b l e t o o m i t t h e w i ck , e i t h e r i n w h o l e o r i n p a r t . T h e d e v i c e t h e n b e c o m e s a s i m p l e

t h e r m o s y p h o n . I t s h o u l d b e n o t e d h e r e t h a n s o m e m a n u f a c t u r e r s u se t h e t e r m ' t h e r m o s y -

p h o n ' to d e s c r i b e t h e b a s i s o f th e i r h e a t r e c o v e r y e q u i p m e n t , w h i le o t h e r s u s e ' h e a t p i pe

o r ' g r a v i t y - a s s is t e d h e a t p ip e '. ( I t i s b e c o m i n g i n c r e a s in g l y c o m m o n t o u s e s o m e f o r m o f

i n t e r n a l h e a t f l u x - e n h a n c i n g t o i m p r o v e i n t e r n a l h e a t tr a n s f e r c o e ff ic i en t s . L o n g i t u d i n a l

a n d c i r c u m f e r e n t i a l g r o o v e s , m e s h e s , a n d r o u g h e n e d s u r f a c es a r e d i sc u s s e d i n m o r e d e t a il

i n Dr . Gro l l ' s pa pe r i n t h i s i s sue . )

9.2. eneral description of the hea t exchanger

I n a g a s - g a s h e a t p i p e h e a t e x c h a n g e r t h e e v a p o r a t o r s e c t io n s o f t h e h e a t p i p e s s p a n

t h e d u c t c a r r y i n g t h e h o t e x h a u s t g a s , a n d t h e c o n d e n s e r s a r e l o c a t e d i n t h e d u c t t h r o u g h

whi c h t he a i r r e qu i r i ng p re -he a t i ng i s pa s s i ng , a s shown i n F i g . 28 .

S p l i t t e r p l a t e

Recoven

nas . l I c

Finned heat p ipes

F i g . 2 8 . L a y o u t o f h e a t p i p e h e a t e x c h a n g e r .



30 D.A . REA~

A s m e n t i o n e d a b o v e , h e a t p i p e s a r e i n f l u e n c e d i n t h e i r p e r f o r m a n c e b ~ t h e a n g l e o f

o p e r a t i o n . I n t h e h e a t p i p e h e a t e x c h a n g e r , t h e t u b e b u n d l e m a y b e h o r i z o n t a l , o r t i l t e d

w i t h t h e e v a p o r a t o r s e c t i o n s b e l o w t h e c o n d e n s e r s . B e c a u s e o f t h i s s e n s i t i v i t y , t h e a n g l e

o f t h e h e a t p i p e s m a y b e a d j u s t e d

n s tu

a s a m e a n s o f c o n t r o l l i n g t h e h e a t t r a n s p o r t .

T h i s i s a u s e f u l f e a t u r e i n a i r c o n d i t i o n i n g a p p l i c a t i o n s , a n d a n u m b e r o f p r o p r i e t a r y

u n i t s i n c o r p o r a t e t i l t c o n t r o l m e c h a n i s m s w h i c h , e i t h e r m a n u a l l y o r a u t o m a t i c a l l y , c a n

b e a d j u s t e d t o c a t e r f o r c h a n g e s i n h e a t t r a n s p o r t r e q u i r e m e n t s [ 1 6 ] .

T h e b a s i c f i n n e d h e a t p i p e h e a t e x c h a n g e r e x t e r n a l l y r e s e m b l e s a n a i r - c o o l e d c o n -

d e n s e r c o i l, c o m p l e t e w i t h f l a n g e d c a s in g a n d c o v e r s p r o t e c t i n g t h e e n d s o f th e t u b e s . T h e

m a i n e x t e rn a l d i f f e re nc e i s t he i nc orp or a t i o n o f a sp l i t te r p l a t e , v i si b le i n F i g . 28 , whi c h i s

use d pa r t l y t o supp or t t he h e a t p i pe s, whi c h c a n be se ve ra l me t re s i n le ng t h , bu t p r i ma r -

i ly t o p r e v e n t c r o s s - f l o w b e t w e e n t h e t w o a i r - s tr e a m s , e f fe c ti v e ly s e a l in g t h e m f r o m o n e

a n o t h e r . I n c o m m o n w i t h o t h e r a i r - c o o l e d h e a t e x c h a n g e r s , t h e f i n n i n g m a ~ b e a p p l i e d

i n d i v i d u a l l y to e a c h t u b e , u s i n g i n t eg r a l o r h e l i c a ll y w o u n d f i n s, o r m a y b e i n t h e f o r m o f

p l a t e s i n t o w h i c h t h e t u b e s a r e e x p a n d e d . I n t h e l a t t e r c a s e t h e c o n t a c t b e t w e e n t h e t u b e

a n d f i n r e s u l t s i n a h i g h e r t h e r m a l r e s i s t a n c e b e t w e e n t h e m , b u t t h e c o s t o f s u c h t u b e

b u n d l e s is g e n e r a l l y l o w e r . T u b e s a r e n o r m a l l y s t ag g e r e d , a n d t h e n u m b e r o f t u b e r o w s i n

t h e d i r e c t io n o f fl o w is t y p i c a l l y b e tw e e n f o u r a n d t en . { Tw o o t h e r c o n f i g u r a t i o n s w i ll b e

de sc r i be d l a t e r . )

M a t e r i a l s e l e ct i o n t b r th e h e a t p i p e s d e p e n d s u p o n t h e w o r k i n g f l u id c o n t a i n e d w i t h i n

t h e m , a s w e l l a s t h e e x t e r n a l e n v i r o n m e n t . W o r k i n g f l u i d s u s e d i n h e a t p i p e h e a t

e x c h a n g e rs r a n g e f r o m f l u o r o c a r b o n s a n d w a t e r t o h i g h t e m p e r a t u r e o r g a n i c f l u id s a n d ,

f o r s p e c ia l a p p l i c a t i o n s , li q u i d m e t a l s s u c h a s m e r c u r y a n d s o d i u m . D e p e n d i n g u p o n t h e

f l u id u s ed , a l u m i n i u m , c o p p e r a n d s t a in l e s s st ee l a r e s u i ta b l e c o n t a i n e r m a t e r i a ls , a n d

i d e n t i c a l m a t e r i a l s ar e u s e d f o r t h e f in s a l t h o u g h i n s o m e u n i t s a l u m i n i u m f in s m a y b e

a p p l i e d o n c o p p e r h e a t p ip es ). E n v i r o n m e n t a l f a c t o r s a ff e c ti n g t h e s e l ec t io n o f t h e t u b e

a n d f in m a t e r ia l a r e c o m m o n t o a ll o th e r t y p e s o f h ea t e x c h a n g e r - - t e m p e r a t u r e , c o r-

ros i on , e ros i on , e tc . S i mi l a r l y , fi n p i t c h i ng a nd t he sha pe o f t he f i n m a y be d i c t a t e d by

p r e s s u r e d r o p o r f o u l i n g c o n s i d e r a t i o n s .

W o r k r e p o r t e d i n J a p a n [- 17 ] d i r e c t e d a t a p p l y i n g h e a t p i p e s i n a g g r e s si v e e n v i r o n -

m e n t s s u c h a s a r e f o u n d i n s t e e l - w o r k s , h a s b r o u g h t t o g e t h e r t h e l o n g i t u d i n a l g r o o v e s

m e n t i o n e d e ar l ie r , a n d p r o t e c t i o n i n c o r r o s i v e e n v i r o n m e n t s . T h e m a i n f e a t u r e s o f t h is

d e v e l o p m e n t w o r k a r e a s f o l l o w s :

i) P r o d u c t i o n o f l o n g i t u d i n a l l y - f i n n e d h e a t p i p es i n t e rn a l s t r u c t u r e t u s i n g a h y d r o s t a t i c

e x t r u s io n p r o c es s w h i c h p r o m i s e s e c o n o m i c m a s s p r o d u c t i o n o f lo n g h e a t p i p e s h a v -

i n g s u p e r i o r h e a t t r a n s f e r c a p a b i li t ie s .

i i )

D e v e l o p m e n t o f e n v e l o p e m a t e r i a ls w h i c h c a n b e u s e d a s e x t e r n a l c l a d d i n g o n w a t e r -

f i ll e d c op pe r he a t p i pe s .

A s w i ll b e e m p h a s i s e d e l se w h e r e , w a t e r i s a g o o d w o r k i n g f l ui d , h a v i n g a ll t h e d e s i r a b l e

p r o p e r t i e s , i n c l u d i n g a h i g h l a t e n t h e a t . H o w e v e r . i t i s n o t c o m p a t i b l e w i t h m i l d o r

s t a i n l e s s s t e e l , a s n o n - c o n d e n s i b l e g a s e s c a n b e g e n e r a t e d w i t h i n t h e h e a t p i p e , a n d

t h e r e f o r e m u s t b e u s e d i n c o n j u n c t i o n w i t h a c o p p e r c o n t a i n m e n t v e s s e l . C o p p e r h o w e v e r

b e c o m e s r a p i d l y a n n e a l e d a b o v e 2 0 0 C , a n d t h e r e f o r e c a n n o t w i t h s t a n d h i g h i n t e r n a l

pre s sure s . I t a l so ha s a l ow re s i s t a nc e t o a t t a c k i n a c or ros i ve a t mosphe re .

B y c la d d i n g a c o p p e r t u b e w i t h a s t a i n l e ss s t ee l t u b e , f o r e x a m p l e , w i t h f i n s a p p l i e d t o

t h e s t e e l o u t e r s u r f a c e , a h e a t p i p e o r t h e r m o s y p h o n m a y b e c o n s t r u c t e d w h i c h r e t a i n s

t h e b e n e f it s o f w a t e r a n d c o p p e r / w a t e r c o m p a t i b i l i t y , w h i l e o f f e r in g t h e h i g h s t r e n g t h a n d

c o r r o s i o n - r e s i s t a n c e o f s t a in l e s s s te e l. A g a i n t h e p r o g r a m m e i n J a p a n i s d i r e c t e d a t u s i n g

a h y d r o s t a t i c e x t r u s i o n p r o c e s s f o r m a n u f a c t u r i n g t h e c l a d t u b e .

U n i t s iz e v ar ie s w i t h t h e a i r f lo w , a v e l o c i t y o f a b o u t 2 - 4 m / s b e i n g g e n e r a l l y a c c e p t e d

t o k e e p t h e p r e s s u r e d r o p t h r o u g h t h e t u b e b u n d l e t o a r e a s o n a b l e l e v e l . S m a l l u n i t s

ha v i ng a f a c e si z e o f 0 .6 m { l e ng th) x 0 .3 m {he i gh t) a re a v a i l a b l e , a n d t he l a rge s t s i ng l e

u n i t s e m p l o y h e a t p i p e s h a v i n g l e n g t h s i n e x c es s o f 5 m . a s u s e d i n l a r g e b o i l e r a i r

p r e h e a t e r s . A w i d e r f in p i t c h m a y b e u s e d o n t h e e v a p o r a t o r s e c ti o n s , w h e r e c o n t a m i -




n a t e d e x h a u s t g a s d i c t a t e e a s y c le a n i n g , w h i le r e t a i n i n g a p i t c h o f 2 m m o r l es s o n t h e

c o n d e n s e r s l o c a t e d i n t h e s u p p l y a i r d u c t . S o m e m a n u f a c t u r e r s o f f e r t h i s o p t i o n .

9.3. p p l i c a t i o n c t r e a s

A l t h o u g h t h e h e a t p ip e e x c h a n g e r h a s b e e n i n p r o d u c t i o n f o r a p p r o x i m a t e l y a d e c a d e ,

a r e v ie w o f m a n u f a c t u r e r s l i te r a t u r e a n d p u b l i s h e d p a p e r s o n t h e i r d e v e l o p m e n t a n d u s e

i n d i c a t e a v e r y w i d e r a n g e o f a p p l i c a t i o n s i n in d u s t r y a n d c o m m e r c i a l a n d m u n i c i p a l

buildings [16, 18, 19-t .

T h e g e n e r a l a p p l i c a t i o n a r e a s f o r h e a t p i p e h e a t r e c o v e r y u n i t s a r e , i n c o m m o n w i t h

m o s t o t h e r g a s - g a s s y s t e m s , c o v e r e d b y t h e t h r e e c a te g o r i e s l i s te d i n t h e I n t r o d u c t i o n .

H e a t p i p e h e a t e x c h a n g e r s h a v e l o w e r e f fi ci en c ie s t h a n s o m e o t h e r g a s - g a s h e a t r e c o v e r y

s y s t em s , n o t a b l y t h e h e a t w h e e l, b u t i n c o m m o n w i t h t u b u l a r r e c u p e r a t o r s a n d p l a te

h a v e th e a d v a n t a g e s o f z e r o c r o s s - c o n t a m i n a t i o n , b r o u g h t a b o u t b y t h e p r e se n c e o f a

sp l i t te r p l a t e whi c h e f fe c t ive l y se a ls t he i n l e t a nd e xh a us t d uc t s , a nd t he fa c t t ha t t he re a re

n o m o v i n g p a r t s , i n c l u d i n g p u m p s . P e r f o r m a n c e m o d u l a t i o n a n d f u l l r e v e r s i b i l i t y o f t h e

un i t , t he l a t te r f e a t u re be i ng of be ne f i t in a i r c on di t i o n i n g sys t e ms , a re o t he r s e l l ing

poi n t s .

O n e o f t h e m a i n l i m i t at i o n s o f h e a t p i pe h e a t e x c h an g e r s, i n c o m m o n w i t h m a n y o t h e r

t y p e s, is th e m a x i m u m o p e r a t i n g t e m p e r a t u r e , d i s c u s s e d in m o r e d e t a i l l at e r. H i s t o r i c a ll y ,

h e a t p i p e h e a t e x c h a n g e r s w e r e d e v e l o p e d i n i t i a l l y t o m e e t t h e r e q u i r e m e n t s o f t h e

h e a t i n g , v e n t i l a ti n g a n d a i r c o n d i t i o n i n g ( H V A C ) i n d u s t r y , w h e r e t e m p e r a t u r e s a r e o f

c o u r s e l i tt le i n e x ce s s o f a m b i e n t . D u r i n g t h e p a s t f e w y e a r s , h o w e v e r , t h e y h a v e b e c o m e

i nc re a s i ng l y popul a r i n p roc e ss - spa c e he a t i ng . Suc h a un i t , unde r t e s t , i s shown i n F i g . 29 .

M o r e r e c e n t a p p l i c a t i o n a r e a s a n d h e a t p i p e h e a t e x c h a n g e r c o n c e p t s a r e o f c o n s i d e r -

able inte res t .

F o s t e r W h e e l e r a r e n o w m a r k e t i n g a h e a t p i p e u n i t f o r u se a s a n a l te r n a t i v e t o r o t a t i n g

r e g e n e r a t o r s ( a n d o t h e r s y s t e m s ) a s b o i le r a i r p r e h e a te r s , u s i n g f lu e g a s d o w n s t r e a m o f

t h e e c o n o m i z e r ( if f it te d ) t o p r e h e a t c o m b u s t i o n a ir . S u c h a s y s t e m h a s b e e n i n s t a l l e d b y

A s h l a n d O i l C o m p a n y o n a f i r e d h e a t e r i n t h e U . S . A . I l l u s t r a t e d i n F i g . 3 0 , w h e r e t h e

o v e r a l l i n s t a l la t i o n a n d a d e t a i l o f th e h e a t p i p e h e a t e x c h a n g e r i s s h o w n , t h e m a i n

p a r a m e t e r s a r e g i v e n in T a b l e 3 .

Fig. 29 . Simple heat p ipe heat exchanger in tes t r ig .



32 D. A. REAY

s t c k t

A i r ~ I I 2 5 0 A i r

i n l e t " ~ . _ .~ ~ ~ preheoter

_ _ ( ~ _ _ ~ . _ - -- - - - -

Convection

i section

F D . A i r

f a n ~ ~ F lu e g as

S e a l p l a t e ~

Support strut

Support plate - -

Fig. 30. Fors ter Wheeler heat pipe a i r preheater .

B a r r a tt a n d H e n d e r s o n [2 0- 1 s t a te t h a t t h e r e q u i r e d p e r f o r m a n c e c o u l d b e o b t a i n e d

w h e n t h e h e a t p i p e s w e r e t il te d a t a n a n g l e o f 1 0 : t o t h e h o r i z o n t a l { e v a p o r a t o r b e l o w

co nden s er ) . W el d ed f in s t ee l tub i ng w a s u s ed fo r the hea t p i pes , a nd i t i s no t i cea b l e f ro m

F i g . 3 0 th a t t h e c o n d e n s e r s e c t i o n s a r e c o n s i d e r a b l y s h o r t e r t h a n t h e e v a p o r a t o r s e c t io n s .

T h i s i s d o n e t o m i n i m i s e t h e p r e s su r e d r o p o n t h e f l u e g a s s id e , a n d a l s o r e s u l ts f r o m t h e

fa c t tha t the a ir - s ide co nde ns er ) s ec t i o ns a re m o re e f f ec t i ve per un i t v o l u m e bec a us e they

a re a b l e to a cco m m o da te a h i g her f i n dens i ty , a s fo u l i ng i s un l i ke l y to o ccur .

The fue l f o r the f i red hea ter i s na tura l g a s , but pro v i s i o n i s m a de i n the i ns ta l l a t i o n fo r

s o o t b l o w e r s s h o u l d t h e f u e l b e c h a n g e d t o o i l a t s o m e f u t u r e t i m e .

R e f e r en c e h a s a l r e ad y b e e n m a d e t o w o r k i n J a p a n a t th e N a t i o n a l C h e m i c a l L a b o r a -

t o r y f o r I n d u s t r y [ 1 7 ] o n h e a t p i p e m a n u f a c t u r i n g t e c h n i q u e s . T w o n e w h e a t p i p e h e a t

e x c h a n g e r c o n c e p t s a r e b e i n g st u d i e d a t th i s l a b o r a t o r y f o r a p p l i c a t i o n s w h e r e a ) o p e r -

a t i o n i n h ig h l y f o u l e d e n v i r o n m e n t s a n d b ) h i g h e r p e r f o r m a n c e i s r eq u i re d .

W i t h r e g a r d t o o p e r a t i o n i n f o u l e d e n v i r o n m e n t s , a s w o u l d p r e d o m i n a t e i n a l m o s t a l l

e q u i p m e n t e m p l o y e d i n s te e l - m a k i n g p l a n t , a u n i t h a s b e e n d e s i g n e d w h i c h , i t i s c l a im e d .

c o u l d b e o p e r a t e d f re e fr o m p r o b l e m s o f d u s t a d h e s i o n . I l lu s t r a te d in F i g . 3 1 . t h e e v a p o r -

Table 3 . Boi l er a i r preheater data

Heat t ransported

F l u e g a s m a s s f l o w

Inlet a i r mass f low

F l u e g a s i n l e t t e m p .

F l u e g a s o u t l e t t e m p .

Air in l et t emp.

Air out l et t emp.

W o r k i n g f l u i d t em p .

M a x i m u m h e a t t r an s fe r p e r h e a t p i p e

N u m b e r o f h e a t p i p e s

H e a t p i p e l e n g t h

H e a t p i p e d i a m e t e r

F i n p i t c h

Pres sure drop on f lue gas s ide

Pres sure drop on air s ide

938 kW

36 700 kg/h

34500 kg/h

260°C

177:C

27°C

121°C

108 198°C

6.74 kW

144

4.57 m

5.1 c m

2.4/cm on air s ide

1.2/ cm on f lue gas s ide

57 Pa

249 Pa




H e a

ra

Con

V i h

old gas

\

~ C o n t a i n e r

f o r

granular media

ucket e l e v a t o r

J

f

Fig. 31. M oving granu lar bed heat p ipe heat exchanger .

a t o r s ec t i o n s o f t h e h ea t p i p e s o r t h e r m o s y p h o n s , w h i ch a r e ex p o s ed t o t h e f o u l ed

exha us t gases , a r e imme rsed in a bed of g ranular mater i a l . The be d of g ranules i s in

cont inuous , bu t s low , mot ion , a ided by grav i ty which car r i es the par t i c l es down the

sur face of the tubes , p revent ing adhes ion by fou l ng mat t e r . Un de r the base o f the

conta iner fo r the granules , a v ibra t ing screen c leans them, r emoving any powder , and

buc ket e l eva tor then r e turns th em to the top o f the con ta iner fo r r ecycl ing . T here i s o f

cour se no need to have such precau t ions on the supply a i r s ide .

Th e sec ond un i t wh ich is the sub jec t o f r esearch i s a ro ta ry hea t p ipe he a t exchanger .

I l lus t r a t ed in F ig . 32 , one concep t unde r cons idera t ion inc orpo ra tes an annu lar hea t p ipe

hea t exchang er , f ixed to a tube p la te an d ro ta ted w i th in the cas ing . The c i r cumference of

the tube p la te i s s ea led a t the cen t r e o f the cas ing . The two d i f f e ren t gases to be hea t

ex ch an g ed a r e f ed i n t o t h e cen t r e an d l e av e t h e p e r ip h e r y o f th e h ea t ex ch an g e r t h r o u g h

the hea t p ipe bund le on the i r r espec t ive s ides o f the tub e p la te . A co-cur ren t f low

ar r an g em en t i s ad o p t ed , a l t h o u g h t h i s m ay b e m ad e co u n t e r - cu r r en t i n o t h e r co n cep t s .

Th e pu rpo se of th i s r esearch i s to ach ieve hea t t r ans fer more than 15 0~ h igher pe r un i t

s u r f ace a r ea t h an t h a t n o w b e i n g ach i ev ed w i t h s t a t i c h ea t p i p e h ea t ex ch an g e r s . W h en

the hea t exchanger i s fu l ly deve loped , i t i s in tended for app l i ca t ion in cor ros ive and

dus t - l ad en f lue gases a t re l a t ive ly low tem pera ture s (200°C) a r i s ing in the i ron and s t ee l

indus t ry .

The improved hea t t r ans fer c r ea ted by ro ta t ion , which should a r i s e f rom benef i t s bo th

ex terna l and ins ide the hea t p ipes , mus t be weighed aga ins t the power needed for ro ta -

t ion . Th e d i s t r ibu t ion of work ing f lu id w i th in the hea t p ipes , whi l e in some w ays a ided by

cent r i fuga l fo rces , ma y no t a lways f avour the hea t t r ansp or t r eq u i r ements , a nd b as ic

w o r k i s u n d e r w ay t o ex am i n e t h e b eh av i o u r .

Ceramic heat pipe heat exchangers

Moving c lose to the top of the indus t r i a l -p roces s

ex h au s t g a s t em p e r a t u r e r an g e , w o r k i s p r o g r e s s i n g a t L o s A l am o s L ab o r a t o r y , N ew



3 4 D . A . REAY

I T 0 e l

j 0 , o , e

Casino I ~ ~

\ , ,

- I

\

A

Heat pipe bundle

~ 1 . l , lit,, ~ .

Exhaust 0as Air

Fig. 32. Rotary heat pipe heat exchanger,

/~ i i

otor

M ex i co , o n h ea t p i p e h ea t ex ch an g e r s f o r u s e i n ex h au s t s o f u p t o 1 5 0 0 ~ C . U n l i k e

h i g h - t em p e r a t u r e ap p l i ca t i o n s f o r u s e in s p ace , w h e r e co s t co n s t r a i n t s a r e n o t s o s ev e re ,

en ab l i n g r e fr ac t o r y m e t a l s s u ch a s n i o b i u m o r t an t a l u m t o b e u s ed , th e ad o p t i o n o f s u ch

sys tems for p roce ss hea t r eco very necess i t a t es c lose exam inat ion of the un i t cos t - e f f ec tive-

ness [21] .

An a l t e rna t ive to the r e f r ac tory meta l s i s to cons t ruc t the hea t p ipes f rom ceramic

t u b i n g . C e r am i cs s u ch a s s i l i co n e ca r b i d e an d a l u m i n a h av e ex ce l l en t co r r o s i o n an d

eros ion r es i s tance a t these h igh t em pera tures , and are no t excess ive ly expens ive .

O n e m a j o r p r o b l em ex i s t s w i t h t h e s e l ec t i o n o f a c e r am i c a s t h e co n t a i n e r m a t e r i a l ,

h o w ev e r . O f th e t w o w o r k i n g f l u id s av a i l ab le f o r u s e in t h e h ea t p i p e s a t t h e s e t em p e r a -

tures , namely sodium and l i th ium, the l a t t e r i s par t i cu lar ly r eac t ive w i th ceramics , and i f

u s ed i n co n j u n c t i o n w i t h a c e r am i c t u b e , s o m e f o r m o f p r o t ec t i v e co a t i n g m u s t b e

ap p l i ed i n s i d e t h e t u b e . T h e t e ch n i q u e f o r o v e r co m i n g t h i s p r o b l em s e l ec t ed a t L o s

A l am o s i s t o d ep o s i t a th i n l ay e r o f r e f r ac to r y m e t a l o n t h e i n s i d e o f th e t u b e b y a

m e t h o d k n o w n a s ch em i ca l - v ap o u r d ep o s i t i o n . T h i s w i l l p r o v i d e an i m p e r v i o u s b a r r i e r

be tween the work ing f lu id and the ceramic wal l .

T h e p r i n c i p a l ap p l i c a t i o n a r ea s f o r h ea t r e co v e r y u s i n g ce r am i c h ea t p i p e h ea t

exchan ger s i s in indus t r i a l fu rnaces . In the USA, for exam ple , i t has bee n es t imate d by

Essenhigh [22] ( c i t ed in [21] ) tha t these acco unt for 12% of the gross na t iona l en ergy

u s ag e , o r ab o u t 9 x 1 0 1 8 J p e r an n u m . A p p r o x i m a t e l y 2 0 % o f th i s en e r g y i s l o s t i n t h e

form o f hea t , in the s t ack gases . A poten t i a l ann ual s av ing , in f inanc ia l t e rms , $4 x 109 i s

therefore poss ib le .

W h i l e c e r am i c r eeu p e r a t o r s an d r eg en e r a t o r s a r e n o t n ew , m o r e co n v en t i o n a l s y s t em s

us ing th i s mater i a l h ave suf fered bad ly in the p as t f rom therm al s t r es ses and v ibra t ion ,

t h e l a r g e n u m b er s o f j o i n t s b e i n g u n ab l e t o w i t h s t an d ex ces s st r e ss cy c li n g . T h e h ea t p i p e

u n i t , h av i n g i n d i v i d u a l t u b es a s s em b l ed a s s h o w n i n F i g . 3 3 i s n o t s o s u ~ p t i b l ¢ t o s u ch

w e a r .

E co n o m i c an a l y s e s h av e b een ca r r i ed o u t o n ce r am i c h ea t p i p e h ea t ex ch an g e r s o f t h i s

t y p e . B as ed o n p l a i n t u b es , p ay b ack s o f ab o u t 5 y ea r s u s in g s i li co n e ca r b i d e an d 2 y ea r s

u s i n g m u l li t e m ay b e p o s s i b le , a s s u m i n g t h a t t h e r a t i o o f t h e co s t o f e ach f i n is h ed h ea t

p i p e t o t h e t u b e m a t e r i a l is . 5 an d t h e co s t r a t io o f t h e fi n is h ed r ecu p e r a t o r t o t h a t o f t h e

h ea t p i p e s i s 1 .3 3 . F i n n i n g o f t h e ce r am i c t u b es ( w i t h ce r am i c f in s) w o u l d r ed u ce h ea t

exchanger s i ze , a l though the cos t r educt ion i s l es s easy to pred ic t . An a l tm 'na t ive t ech-

n ique for r a i s ing the hea t t r ans fer would be to loca te the un i t in a f lu id ized bed .

De ta i l ed cos t e f f ec t i venes s ana lys is .

Ser ious a t t em pts a t cos t - e f fec t iveness s tud ies on any

t y p e o f w as t e h ea t r e co v e r y eq u i p m en t a r e v e r y d i ff icu lt to f in d. I n m an y i n s tan ces t h e



A review of gas-gas heai recov ery systems

H e o t

F lue gos obsorbing

e x i t s t r e o m

Refrac

p o r t i t ~

3 5

ol

e$

/ 1 I / i U i?

- g - J

l u e g a s P r e h e o t e d

i n l e t s t r e o r n

Fig 33. Conce pt of ceramic heat p ipe heat exchanger .

cost data obtained through a manufacturer s literature, when it is available, relates solely

to the capital cost of the heat exchanger and, particularly in the case of high-temperature

heat recovery systems, bears little relationship to the installed cost, which may be up to

four times as high as the basic unit price.

It is possible, although it can be an extensive exercise, to approach as many manufac-

turers as possible with requests for quotations for a range of heat exchangers. However,

analysis of the data obtained, taking into account differences in pressure drop (hence

operating costs), etc. will be the most time-consuming component.

Possibly because the heat pipe heat exchanger has grown up in an environment where

serious academic interest is present (being the subject of much research and development

in universities and other research laboratories), the development of computer optimiza-

tion programs, both technical and economic, as an aid to unit selection, has become

almost routine. One such study carried out at the University of New Mexico was recently

published by Lu and Feldman [23]. The results were based on data supplied by the

major heat pipe heat exchanger manufacturer in the U.S.A., Q-Dot Corporation.

Cost data for a variety of heat pipe heat exchangers, covering operating temperature

ranges from HVAC to process at up to 400°C, is given in Fig. 34. The initial equipment

cost includes materials, labour and overheads, and makes allowance for the fact that the

units will be part of a production run, rather than specials . Installation cost includes

ducting, controls and labour. The results are presented in such a way as to show the

relationship between the cost per unit surface area of the heat exchanger~ and the total

size of the heat exchanger. As one would expect, the cost per square metre of surface

decreases as the heat pipe heat exchanger increases in size. Other interesting points may

be noted from the graph. First the aluminium heat pipe heat exchanger has the lowest

capital cost, followed by copper, then carbon steel. Second, the installation cost of a

copper unit in a HVAC application is considerably less than the cost needed to install an

identical heat exchanger in a process. Economies of scale are particularly noticeable on

the carbon steel high-temperature heat exchangers.

Based on the cost data, the staff of the University of New Mexico designed a heat pipe

heat exchanger to recover heat from the exhaust flue of the site boilers for air preheating.

With an exhaust gas temperature of 316°C (flow of 8534 m3/min), a carbon-steel unit

with Dowtherm A as the working fluid was selected.



3 6 D . A . RE A Y

%

.

g

o

IO0 ~ Equipment cost

. . . . Total in i t ia l cost

90 (includ ing equipment cost

and installation cost)

8 0 - \

\

7 0 -- \

\

6 0 - \

\

\ \

5 0 . - \ \ \ . Ca rb on s te e l ( in du s t r ia l )

I % \ . . / . . . . . . .

30 J- - ~ x - /~ . , -- -Coppe r ( in du s t r ial )

J ~ ~ _ . ~ A l u m i n i u r n ( H V A C)

0 2 4 6 8 I0 1 2

3

H eat transfer surface area, m 2 x l 0 2

F i g . 3 4. C o s t s o f h e a t p i p e h e a t e x c h a n g e r s a s a f u n c t i o n o f s i z e a n d c o n s t r u c t i o n m a t e r i a l s .

R eco v e r i n g ap p r o x i m a t e l y 1 .5 M W , t h e u n i t w o u l d s av e f u el v a l u e s a t S 4 5 0 0 0 p e r

annu m, a t a cos t per ann um of $8000 (amo r t i zed over 10 year s a t 10% in teres t ). Inc lud ed

i n t h e co s t s w e r e a l l o w an ces f or fan p o w er an d m a i n t en an ce , t o t a ll i n g $ I 7 0 0 p e r an n u m .

T h e lif e o f t h e u n i t w as e s t im a t ed t o b e 1 5 y ea r s. C o s t i n g w as b a s ed o n o p e r a t i o n f o r

24 hou rs per day , 355 day s per year , and the e f f ec t iveness of the he a t exch anger was 60%.

10. G A S G A S H E A T P U M P S

T h e h ea t p u m p can i n m a n y i n s t an ces b e r eg a r d ed a s a h ea t r e co v e r y d ev i ce , an d t h is

i s par t i cu lar ly t rue when i t i s used as an a id to increased ef f i c i ency in dryer s . Whi le

i n d u s tr i a l ap p l i c a t i o n s o f h ea t p u m p s h av e b een i n ex i s ten ce si n ce th e I 9 4 0 's , t h e t o t a l

n u m b er o f i n s ta l l a ti o n s r em a i n s co m p ar a t i v e l y f ew . H o w ev e r , t h ei r ap p l i c a t i o n i n d r y e r

technolog y , which has be en dem ons t ra ted , a lbe i t on a smal l s ca le , to of fe r subs tan t i a l

p e r cen t ag e r ed u c t i o n s in d r y e r en e r g y co n s u m p t i o n , i s li k e ly t o b e o n e o f th e m a j o r

g r o w t h a r ea s o v e r t h e n ex t d ecad e .

H e a t p u m p s a r e m o r e w i d el y k n o w n t h r o u g h t he ir d o m e s t i c a n d c o m m e r c ia l a p p l i ,

c a t io n s , f o r h ea t i n g b u i l d i n g s u s i n g f r ee h ea t i n t h e a t m o s p h e r e o r t h e g r o u n d , f o r

red i s t r ibu t ing w as te hea t ava i l ab le w i th in bu i ld ings , and for us ing was te hea t f rom ref ri ,

g e r a t i o n s y s tem s in l a rg e o l d s t o r e s o r i c e r in k s . T h e h ea t p u m p w o u l d n o t b e i m m e d i -

a t e l y i n c l u d ed b y ev e r y o n e in a r ev i ew o f g a s - g as h ea t r e co v e r y d ev i ce s, b u t i ts i m p o r t -

ance jus t i f ies a s ec t ion here .

10.1.

peratin cycle

T h e r e a r e s ev e r al h e a t p u m p o p e r a t i n g c y cl es , t h e t w o m o s t c o m m o n b e i n g th e v a p o u r

co m p r es s i o n an d ab s o r p t i o n cy c le s . I n d r y i n g p r o ces s e s , i n v o l v in g g a s - g as h ea t t ran s fe r ,

t h e v ap o u r co m p r es s i o n cy c l e h a s p r ed o m i n a t ed t o d a t e , an d i t i s t h is o n w h i ch I w il l

co n cen t r a t e h e r e .

Th e hea t pum p cy c le i s i l lus tr a ted on a p res sure- -en tha lpy d iagram in F ig . 35 , and the

p r i n c i p a l co m p o n en t s a r e s h o w n i n t h e c i r cu i t d i ag r am i n F i g . 3 6 . T h e l o w g r ad e h ea t

i n p u t o ccu r s a t t h e ev ap o r a t o r , t h i s c au s i n g t h e h ea t p u m p w o r k i n g f l u i d ( t y p i ca l l y a



Pressure

A review o f gas g as heat recovery systems

~

~ , , ~ ~ C on s t on t entropy

~ ~ X ~ ~ i Constant tem perature

Enthalpy

37

Fig. 35. Hea t pump p ressure enthalp y diagram.

f luor ina ted hyd roca rbon ) to evapo ra te . The va t , our pas ses to the com pres sor , whe re i t i s

r a i s ed in p res sure ( and hence t empe ra ture) b y the appl i ca t ion of ex te rna l wo rk of com -

pres s ion . Thi s h igher t empera ture vapour then pas ses to the condenser , where usefu l hea t

i s r e j ec ted as the vapour condenses , g iv ing up i t s l a t en t hea t . Th i s l iqu id i s then r educed

i n p r e s su r e b y p a s s i n g t h r o u g h an ex p an s i o n v a l v e b e f o r e en t e r in g t h e ev ap o r a t o r .

The e f f ic i ency of a hea t p um p i s expres sed in a d i f f e ren t way to tha t o f convent ion a l

hea t r ecovery dev ices , and i s normal ly expres sed in t e rms of a 'Coef f i c i en t o f Per form-

ance ' (COP) .

T h e C O P is th e r a t i o o f h ea t d e l iv e r ed a t th e h i gh t em p e r a t u r e t o w o r k s u p p l i ed b y t h e

co m p r es s o r . T h eo r e t i c a l m ax i m u m f i g u r e w h i ch a f u l l y r ev e r s i b l e t h e r m o d y n am i ca l l y

idea l hea t pump can g ive i s g iven by :

Tcon

COPmax =

Tcon

c v a p

w h er e

T e a n

= co n d en s i n g t em p e r a t u r e

T c , , ~ p = ev ap o r a t i n g tem p e r a t u r e .

In p rac t i ce th i s i s no t very usefu l , and i t i s much more en l igh ten ing to s tudy the cyc le

on the pres sure-en tha lpy (p-h) d iagram. Thi s type of p resen ta t ion i s used exc lus ive ly in

ref r igera t ion /a i r condi t ion ing /hea t pump equipment and i s very usefu l in ca lcu la t ing

CO P. Thi s i s because the en tha lpy chan ge h3 - h , i s the ou tpu t hea t , whi le the en tha lpy

h3 - h2 r epresen t s the input wor k so

C O P - h3 - h4

ha - h2

No te tha t i s en t rop ic comp res s ion i s no t ach ieved , an d we have as sum ed 709/0 i s en t rop ic

e f f i c ien cy f o r t h e co m p r e s s o r

ha - h2 (isentro pic)

= 709/o.

e.g. h3 _ h2

I f the p-h d iagram i s s tud ied for d if f e ren t pos s ib le work ing f lu ids, then one can r ap id ly

iden t i fy su i t ab le and u nsu i t ab le f lu ids fo r any g iven se t o f opera t ing t empe ra tures . Thi s i s

High emperature Low temperature

T c o o

l / C°mpress°r l /

Heat output Heat input

Expansion a l v e

Condenser Evaporator

Fig. 36. H eat pum p circuit showing ma jor components.



38 D A REAY

Air flow

C r

V 3

Drying

b e d

L t .

Compressor

Fig 37 Closed cycle hea t pum p dryer

b ecau s e t h e f l u i d s m u s t b e s e l ec t ed s u ch t h a t t h ey a r e w o r k i n g a t r e a s o n ab l e p r e s s u r e s

an d t em p e r a t u r e s , t o s u i t t h e co m p r e s s o r b e i n g u s ed .

Th e theore t i ca l coef f i c i en t o f per fo rm ance decrea ses as the d i f f e rence be twe en hea t

s o u r ce an d h ea t si n k tem p e r a t u r e g r o w s , an d i n o r d e r t o o p e r a t e e co n o m i ca l l y , t h e C O P

for indus t r i a l app l i ca t ions should genera l ly be wel l in exces s o f 3 .0 .

10.2. eat pumps in drying

O t h e r s ec t i o n s h av e m en t i o n ed t h e u s e o f co n v e n t i o n a l g a s - g a s h ea t r e co y e r y u n i ts i n

d r y i n g ap p l i c at i o n s , b u t s u ch h ea t ex ch an g e r s d o n o t p e r m i t h i g h l y e f fi c ien t o p e r a t i o n o f

c losed c i r cu i t d ryer s. The h ea t p um p can ful fi ll t h is r equ i r em ent .

In a con ven t iona l d ryer on ly par t o f the exhaus t a i r can be r ec i r cu la ted , as increases in

h u m i d i t y w o u l d q u i ck l y r ed u ce i t s d r y i n g cap ac i t y . T h e h ea t p u m p , h o w ev e r , m ay b e

used m os t e f f ec tive ly in d ryer s as a mea ns o f dehu mid i fy ing the exh aus t a i r so tha t i t may

be r ec i r cu la ted in s ign i fi can t ly g rea te r quant i t i es . The ex haus t a i r l eav ing the pro duc t

b e i n g d r ied i s p a s s ed o v e r t h e ev a p o r a t o r co i l o f t h e h ea t p u m p , w h e r e i t is co o l ed . A s

a r e s u l t o f t h is co o l in g , s o m e o f th e m o i s t u r e co n t a i n ed i n t h e ex h au s t a i r co n d en s e s

o u t , an d i s d r a i n ed aw ay . T h e r ec i r cu l a t ed a i r w h i ch h a s b een co o l ed t h en h a s t o b e

r a i sed i n t em p e r a t u r e b e f o r e r e -en t e r in g t h e p r o d u c t t o b e d r i ed . T h i s i s d o n e b y p a s s i n g

the a i r over the condenser , where i t p i cks up bo th l a t en t and sens ib le hea t r ecovered f rom

t h e ev ap o r a t o r , an d t h e h ea t a s s o c i a t ed w i th t h e w o r k o f co m p r e s s io n , a s i l l u st ra t ed i n

Fig. 37.

I f r ec i r cu la t ion i s no t pos s ib le because , fo r exam ple , the fou l ing in the exhau s t s t r eam

is d i ff icu l t t o r em o v e , an o p en cy c l e h ea t p u m p d r y e r c an b e u s ed a s s h o w n i n F i g . 3 8. I n

t h is c a s e a p r o p o r t i o n o f th e l a t en t h ea t co n t en t o n t h e ex h au s t c an s ti ll b e r e co v e r ed , a s

can so m e of the s ens ib le hea t , bu t the e f f i c i ency w i ll no t be as h igh as in a c losed cyc le

sys tem.

T h e e f fi ci ency o f t h e s y s t em is s u b jec t t o t h e s am e co n s i d e r a t i o n s a s an y o t h e r h ea t

p u m p ap p l i c a t i o n - - t h e C O P b e i n g a f u n c t io n o f t h e di f fe r en ce b e t w een ev ap o r a t i n g an d

co n d en s i n g t em p e r a t u r e s . A r ed u c t i o n i n t em p e r a t u r e d i ff e ren ce is co m m o n l y ach i ev ed i n

t h e d r y e r b y i n co r p o r a t i n g a p l en u m ch am b er w h e r e ch i ll ed s a t u r a t ed a i r i s m i x ed w i t h

unchi l l ed a i r before pas s ing over the condenser .

I t m ay b e a r g u ed t h a t o n m an y d r y e r s , p a r t i cu l a r l y t h o s e i n v o l v i n g o n l y t h e ev ap o r -

a t i o n o f w a t e r , t h e h ea t p u m p co u l d w e l l s u r p l an t co n v e n t i o n a l g a s - g a s w as t e h ea t

r ecov ery sys tem s as , d i s cus sed l a te r , t he econ om ics a re a t t r ac t ive . Th i s con cept has a l so

b een ap p l i ed t o t h e d o m es t i c t u m b l e r d r y e r .

~ Evaporator

-

g 1

Compressor

Drying bed A

U Condenser

Internal ~ External

Fig 38 Op en cycle hea t pum p dryer




M o r e u n co n v en t i o n a l t y p es o f h ea t p u m p s m ay b e u s ed i n t h is c a t eg o r y o f cy cl e us e s

superh ea ted s t eam as the hea t t r ans fer medium. In a p rac t i ca l a r r an gem ent the exhaus t

a i r f rom the dryer , would be sp l i t i n to two s t r eams , one s t r eam be ing compres sed ad iaba-

t ica lly, thu s r a i sing i ts t empe ra ture and pres sure . A t the h igher p res sure the l a t en t hea t o f

con dens a t ion i s t r ans fer r ed v ia a hea t exchanger a t h igh t em pera ture to the s econd

s tr eam . H ea t ed an d d r i ed i n t h is m an n e r , t h e s eco n d s t r e am m a y b e r ec i r cu la t ed t h r o u g h

the dryer . The f i r s t s t r eam i s r e j ec ted as condensa te .

Becau se the use of r e f rigeran t s , which a re l i ab le to therm al deg rada t io n a t h igh (grea te r

than 120°C) t empera tures , i s no t r equ i r ed in such a cyc le , there i s po ten t i a l fo r opera t ing

such a sys tem a t very h igh t empera tures . Rep or t ing in 1976 , the E lec t ri c i ty Co unci l

R es ea r ch C en t r e a t C ap en h u r s t i n th e U n i t ed K i n g d o m s u g g es ted t h a t C O P ' s o f 3 to 3 .5

should be a t t a inab le fo r a 150°C outpu t t empera ture , w i th a compres sor p res sure r a t io o f

b e t w een 4 an d 4 . 5 . I t w as h o w ev e r p o i n t ed o u t t h a t t h e p e r f o r m an ce o f t he s y s t em a t

of f -des ign cond i t ions an d un der pa r t load ma y be unsa t i s f ac tory [24] .

The appl i ca t ion of hea t pu m ps in the f i eld o f d ry ing and dehu midi f i ca t ion i s l ikely to

be a subs tan t i a l g rowth a rea , and commerc ia l sys tems for th i s purpose a re ava i l ab le on

t h e m ar k e t . M o s t o f t h e w o r k i n t h e U . K . o n t h e u s e o f h ea t p u m p s i n d r y i n g an d

dehum idi f i ca t ion has been car r i ed ou t a t Cape nhu rs t ( s ee abov e) over a per iod o f s evera l

year s , where i t i s con t inu ing . Ho dge t t [25] has ana lysed the s t a t e o f the a r t in d ryer

t e ch n o lo g y , an d a l s o ca lcu l a t ed t h e am o u n t o f w a t e r r em o v ed i n i n d u s tr i a l p r o ce s s e s in

the Uni ted K ingdom. I t i s es t imated tha t in the U .K . be tween 20 and 30 mi l l ion tonnes

o f w a t e r a r e ev ap o r a t ed b y i n d u s t r y p e r an n u m . T ak i n g a v a l u e o f 3 0 m i ll io n t o n n es, t h e

energy r equi r ed to ev apo ra te th i s would be 74 x l06 G J , and , as sum ing an overa l l d ryer

eff ic iency of 50 , t he to ta l energy c ons um pt ion in these proces ses w ould be

148 x 106 G J.

On e r ecen t app l i ca t ion o f a hea t p um p deh um idi f i e r has been i t s use in ceramics

dry ing . Typica l o f the ins ta l l a t ions pos s ib le in th i s indu s t ry i s tha t a t P or tace l L td in

Kent , wh o man ufac ture ceramic f il te rs fo r water pur i f ica t ion . H ig h s t anda rds o f p rodu c-

t ion cont ro l m us t be exerc is ed , and the hea t pu m p sys tem has p rove d benef ic i a l in

improving product ion e f f i c i ency and th roughput .

The ceramic e lements a re cu t f rom a cont inuous cy l indr ica l ex t rus ion to l eng ths o f

b e t w een 1 2 5 an d 2 5 0 m m . T h es e e lem en t s a r e t h en d r i ed b e f o r e b e i n g k i ln ed u n d e r

cont ro l l ed condi t ions . Fo l low ing f ina l com ple t ion and t es t ing in water , ano ther d ry ing

s tage i s r equ i r ed . A Wes ta i r Dynamics dehumidi f i e r i s used to d ry the f i l t e r s p r io r to

ki lning and fol lowing tes t ing.

The en ergy cos t s o f the two dry ing proces ses when gas - an d o i l -f i red had been near ly

£ 6 5 0 0 p e r y ea r. T h es e co s t s h av e b een r ed u ced t o u n d e r £ 3 0 0 0 p e r an n u m w i t h t h e n ew

elec t ri c dehum idi fy ing sys tem. The sav ing of m ore tha n £3500 p .a . p rov id ed a pay -bac k

per iod o f l ess than 3 year s on the p lan t and ins ta l l a tion cos t s o f und er £10000 . Equ al ly

i m p o r t an t t o t h e co m p an y , h o w ev e r , is t h e i m p r o v em en t i n w o r k f l o w an d t h e av a i l-

ab i li ty o f ad d i t i o n a l d r y i n g cap a c i t y t o m a t ch a p l an n ed i n c r ea s e i n o u t p u t . B ecau s e o f

t h e s i g n i f i c an t r ed u c t i o n i n d r y i n g t i m e , p r o d u c t i o n b o t t l en eck s a r e av o i d ed an d m o r e

e f fec ti v e u s e m ad e o f fa c t o r y s p ace an d m an p o w er .

11. M U L T I P L E T O W E R H E A T R E C O V E R Y U N I T S

T h e m u l t i p le t o w er ex ch an g e co n cep t is ap p l i ed in H V A C co m f o r t co n d i t io n i n g , an d ,

l ike the hygroscopic ro ta t ing r egenera tor in s imi la r app l i ca t ions , i s capable o f t r ans fer -

r ing bo th s ens ib le hea t and mois ture f rom warmer to coo ler a i r s t r eams . The f low d ia-

gram for a mul t ip le tower exchanger , i l lus t r a t ed in F ig . 39, resem bles tha t o f a run-

around co i l sys tem, excep t fo r the f ac t tha t the c i r cu la t ing l iqu id i s b rought in to d i r ec t

contac t w i th the exhaus t and supply a i r , r a ther than be ing r e ta ined in a f inned co i l hea t

exchanger .

A c i r cu la t ing l iqu id , known as the sorben t so lu t ion , i s sprayed on to the contac t sur f ace

which spans the warm exhaus t a i r duc t . Th i s con tac t sur f ace i s normal ly non-meta l l i c , o f



40 D A REAY

II r l

/ / ' / P - ~ M P C O N T A C T O R

T O W E R S

Fig 39 M ul t ip le tower heat exchanger

t h e t y p e u sed i n m o d e rn co o l i n g t o w er s. S p ray i n g i s g en e ra l l y d i r ec t ed co u n t e r f l o w t o t h e

a i r s t r e am , an d a d em i s t e r p ad i s l o ca t ed d o w n s t r eam o f t h e co n t r ac t o r t o p r ev en t

d ro p l e t c a rry o v e r . H av i n g ex t r ac t ed b o t h h ea t an d m o i s t u re f ro m t h e ex h au s t a i r , th e

so rb en t i s p u m p ed t o an i d en ti c a l i n s ta l la t io n i n t h e su p p l y a i r d u c t , w h e re t h e h ea t a n d

mois ture i s re j ec t ed to t he a i r , aga in by spray ing . The cooled sorbent i s t hen re turned to

the exhaus t a i r duc t .

T h e so rb en t so l u t i o n i s t y p i ca l ly a m i x t u re o f l it h iu m ch l o r i d e an d w a t e r , h a l o g en s a l t

so l u ti o n s b e i n g t h e m o s t co m m o n . T h e o t h e r so l u t i o n r eg u l a r l y u sed is c a l c iu m ch l o r i d e

in water .

T h e t r an s fe r o f h ea t an d m o i s t u re i s r ev e r s i b l e i n t h a t t h e sy s t em can b e u sed i n

su m m e r fo r p r eco o l i n g an d d eh u m i d i fy i n g a su p p l y a i r s t r e am t o an a i r - co n d i ti o n ed

bui ld ing . Al though as i l l us t ra t ed in F ig . 39 , t he l i qu id and a i r a re countercurren t , i t i s

poss ib l e , as may of t en be d i c t a t ed by duc twork conf igura t ions , t o l oca t e t he towers wi th

t h e a i r p a s s in g h o r i zo n t a l l y th ro u g h t h em , t h e sp ray b e i n g i n t ro d u ce d f ro m t h e t o p i n a

cross- f low mode . Thi s l eads to a s l i gh tly l ower un i t e f f ic i ency , how ever .

Cro ss-co ntam ina t ion i s a poss ib i li ty wi th mul t ip l e t ower ex cha nge r sys t ems, and

g aseo u s c ro s s - co n t am i n a t i o n is m o s t co m m o n . In a H V A C i n s ta l la t io n , su ch c ro s s - co n -

t am i n a t i o n , d u e t o so l u b i li ty o f t h e g a se s in t h e so rb en t , h a s b een m easu red t o av e rag e

o n l y ab o u t 0 .0 25 b y v o l u m e . I f ch em i ca l fu m es o r o t h e r s ev e re co n t am i n a t i o n is p r e sen t

in t he exhaus t , however , carefu l se l ec t ion of t he sorbent may ass i s t t o prevent excess ive

t r an s fer t o t h e i n co m i n g ai r. M o s t o f t h e w o rk i n g f l u id s u sed a r e b ac t e ri o s ta t ic , an d t h e

t o w er s , b y t h e i r n a t u re , h av e b een sh o w n t o b e e f f ect iv e ' s c ru b b e r s ' o f m i c ro -o rg an i sm s .

75

70

6 5

~

bd

5 5

5¢

i

1.5 2 0

F a c e v e l o c it y , m / s

Fig 40



A review of gas-g as heat recovery systems 41

11.1. erformance characteristics

The ef fec t iveness of a mul t ip l e tower exchanger as a func t ion of t he face ve loc i ty i s

show n in F ig . 40 . I t i s genera l ly h igher (of the ord er of 709/o) in typ ica l sum m er opera t ing

cond i t ions in N or th Am erica , and of t he ord er of 60 in win ter . Fo r a g iven face

v e lo c it y , t h e p re s su re d ro p t h ro u g h a t y p i cal to w er t en d s t o b e so m ew h a t h i g h e r t h an fo r

devices which t ransfer sens ib l e hea t on ly .

C ap ac i t y co n t ro l i s ef f ec t ed b y au t o m a t i c ad d i t i o n o f m a k e -u p w a t e r, t h u s m a i n t a i n i n g

a f ixed concent ra t ion of sorbent so lu t ion and hence enabl ing a cons tan t de l ivery a i r

humidi ty to be assured . An auxi l i a ry hea te r i ncorpora t ed in the so lu t ion supply l i ne

l ead ing to the tower th rough which the a i r t o be prehea ted i s pass ing , cont ro l l ed by a

thermosta t , se rves as a t empera ture cont ro l dev ice .

R E F E R E N C E S

1 B

C hojnows k i and

P E

Chew, Get t ing the bes t out of ro tary a i r hea ters , CEGB Research No. 7,

14 21

(1978).

2 . G . Applegate, H eat ing a fac tory for nothing, Heating Ven t. Engng 44, 21-2 4 (1970).

3 . J . Sohlberg, K l imatechn ishe Tenderizer in schwedischen K rank enh ausb au, Heizung Lufiung Klimatech. 27

(3) (19761

4. J . Mort im er, Exch anging heat to save fueL The Engineer 34-37, 2-9 August (1973).

5 . G . Applegate , Heat regenera t ion by thermal w heel, in Prec. Waste Heat Recovery Conf., Ins t . Plant Engng,

Lon don , 25-26 Sept. (1974).

6 . Ano n, Energy recovery equipm ent and sys tems, Publ . Sheet Meta l Air Co ndi t io ning Con trac tors

Na tio nal A ssn., Virginia, U .S.A. (1978).

7 . H. Kruse and R. Vra th , Ap pl ica t ion l imits and performances of hygroscopic and non-hyg roscopic regener-

ative rotary heat exchangers, Sci. Te ch. Froid. 1, 705-715 (1976).

8 . H. Spah n and V. Gnie l inski , Heat and m ass transfer perform ance of hygroscopic r ota t ing regenera tor un der

s umm er cond i t ions , Verfahrestechnik 5, 143 (1971).

9 . D. W. Ruth et a l . , Inves t iga t ion of fros t ing in rotary a i r- to-a i r hea t exchangers , T ra ns . A S H R A E 81, Pt. 2,

410--417 (1975).

10. D. R. Dugwell, Waste heat recovery and use in the British Steel Corporation, Steel Times Ann. Rev.

784--793 (1977).

11. N. B. Orr , Flue gas --a n in-hou se energy source , and ways of put t in g i t to work, in Energy for Industry

(edited by P. W. O 'Cal laghan) . Perg am on Press , Oxford (1979).

12. P. But ler , Us ing the M untor wheel to recover your hea t and mak e big savings . The Engineer 20, 20 Nov.

(1975).

13. J. L. Boyen, Practical Heat Recovery. John Wiley, New York (1975).

14. H. K ay, Recupera tors --- the ir use and abuse ,

Iron and Steel Int.

231-240, June (1973).

15. P. D. Du nn an d D. A. Reay, Heat Pipes. 2nd Edn. Pergamon Press , Oxford (1978).

16. D: A. Reay, The heat p ipe heat exch ang er--a techn ique for was te h ea t recovery, Heating Vent. Engng 50,

NO. 593 (1977).

17. T . Hak uta , W aste hea t u t i l i sa t ion technology, Technocrat 11, No. 3, 11-17 (1978).

18. A. Basiulis and M. P lost, W aste heat utiliz ation through the use of heat pipes, Trans. Am. Soc. Mech. Engrs

ASME Paper 75-WA/HT-48 (1975).

19. K. ,T. Feld ma n, Simplified design of hea t pipe heat exchangers, P rec. 2 nd In t. Heat p ipe Conf., Bologna,

ESA Report SP3112 (1976).

20. R. O. Ba rran and G . O. H enderson , The heat p ipe a i r preheater , Prec . Am erican Power Conf., Chicago,

Illinois, Apr il (1979).

21. W . A. Ran ken and L. B. Lund berg, High tem pera ture hea t p ipes for te rres tr ia l appl ica tions , , Prec . 3rd Int .

HraI p ipe Conf. , Pa l e Alto , U.S.A. AIAA P aper 78--435 (1978).

22. R. H. EssenhiglL Energy use , e ff ic iency and conservat ion in indus try , Prec . Symp. on Environment

Energy C onservat ion, Denv er , Colorado, p . 161 (1975).

23. D . C. Lu and K , T, Fe ld man , Cos t-effect iveness s tudy of hea t p ipe heat exchangers , P rec . AS ME W inter

An nua l Meet ing, ASM E Pap er 77-W /HT-5 (1977).

24 . Anon , UK Wo rks hop on Hea t Pumps , 30 J une - 2 J u ly 1976 , Ene rgy Techno logy Suppor t Un i t R epor t

HL77/683, Harwell, U.K. (1977).

25. D. L., Hodget t , Eff ic ient drying us ing heat pum ps , Chem. Engr 510-512, July/August (1976).

a review of gas-gas heat recovery systems

Documents