adfa, p. 1, 2011.

© Springer-Verlag Berlin Heidelberg 2011

Performance enhancement of rotary desiccant wheel by

innovative designs of multiple desiccant layers

Selvaraji Muthu, Prabal Talukdar, Sanjeev Jain

Department of Mechanical Engineering, Indian Institute of Technology Delhi,

New-Delhi – 110016

muthu.selji@gmail.com

Abstract. Rotary Desiccant wheel in the desiccant air conditioning system is

used for the dehumidification of process air by adsorption of water vapour by

the desiccant (porous media) layer. And while the hot air which is heated up by

the heater (or waste heat recovered from other equipments) flows through re-

generation side, water is desorbed out from the desiccant and gets regenerated.

In a desiccant wheel, the process air enters the wheel at one end with high mois-

ture content and the moisture is continuously transferred from air to desiccant.

Since, the equilibrium adsorption capacity of desiccant material is directly pro-

portional to the relative humidity of air in contact, the rate of moisture transfer

decreases along the width of the wheel. This is considered to be a bottleneck for

the efficiency of a rotary desiccant wheel where improvements can be done.

The innovative design of rotary desiccant wheel presented in this paper en-

hances the moisture removal capacity by using multiple desiccant materials

over the width of the wheel and increases the moisture removal capacity and ef-

ficiency of the wheel without increasing the overall dimensions of rotary desic-

cant wheel. The numerical results of this multiple desiccant wheels (Hybrid) are

compared with conventional wheel made up of either molecular sieves or Silica

gel. The results show that the performance of the wheel can be enhanced by 10

to 25% by using this innovative design. The governing equations of heat and

moisture transfer are discretized using the finite volume method and the simula-

tions are carried out using a code developed in FORTRAN 95.

Keywords: Porous media, finite volume method, rotary desiccant wheel, Silica

Gel, Molecular Sieves

1 Introduction

Desiccant heating, ventilating and air conditioning systems (D-HVACs) consist of

rotary desiccant dehumidification wheel, rotary heat exchanger, evaporative cooler,

electric heater (or waste heat recovery system) and blowers for the process and regen-

eration air supply streams. The desiccant wheel consists of process air flow and re-generation air flow sections split by the clap board as shown in Fig.1. The ambient air

with high moisture content is passed through the process air section to dehumidify the

air by transferring the moisture from air to desiccant pores by adsorption. The ad-

sorbed moisture content is desorbed out of the desiccant pores in the regeneration

section by passing the regeneration air at high temperature.

The hybrid wheel is split into the 2 portions across the depth of the wheel in such a

way that the desiccants having the adsorption isotherm as per type-III is used in first

portion of the wheel that is the entry side of the process air and the remaining portion

is used with the desiccant having the adsorption isotherm having as per type-I , in the

exit of the process air side as shown in Fig.2.

The hybrid wheel can be made by the combination of silica gel wheel in the first por-tion and the second portion is of molecular sieves type with respective split ratio as

shown in Fig.2. But in this case, in order to avoid the channel blockage at the intersec-

tion of the both the sections, a mixing chamber is to be provided to have the uniform

airflow exit from first portion and entry in to second section of this hybrid wheel.

Also, the common drive motor can rotate both the wheel portions if they are con-

nected in to the common wheel case. In case if both the wheel portions are not joined

together, then separate drive motor is suggested to drive each of the wheel portions.

An innovative hybrid wheel can be made with common substrate with coating or im-

pregnating the desiccants having type-III isotherms on the first portion of the wheel

depth and remaining by using the desiccant having type-I isotherm as per IUPAC [5],

as shown in Fig.3 and in this case the air mix chamber is not required and the com-

mon motor can drive the wheel as the wheel is manufactured with single substrates.

Fig. 1. : Schematic of Rotary desiccant wheel

Process air inlet

moist air

Process air outlet

dehumidified air

Regeneration air inlet

Hot and dry air

Regeneration

air outlet

Adsorption section

(Moisture transfer from

air to desiccant)

Desorption section

(Moisture transfer

from desiccant to air)

0

180

0z

Lz

Fig. 2. : Schematic of Hybrid Rotary desiccant wheel with 2 different desiccant materials

Fig. 3. : Honeycomb channel of Hybrid desiccant wheel with 2 different desiccant materials

coated or impregnated over common substrate material, a=3.5 mm, b=1.75mm

In order to test the performance of the hybrid desiccant wheel, five different configu-

rations are considered for numerical simulations as presented in Fig. 4.

Portion of Desiccant-1 Fully SG 1/2 SG 1/3 SG 1/4 SG No SG

Portion of Desiccant-2 No MS 1/2 MS 2/3 MS 3/4 MS Fully MS

Hybrid Rotary Desiccant

wheel

SG - Silica Gels

MS - Molecular Sieves

Fig. 4. : Schematic of three types of hybrid desiccant wheels along with conventional SG and

conventional MS wheels

2 Mathematical model

Pesaran and Mills [1] used a PCP (Parabolic Concentration Profile Model) model for

simultaneous heat and mass transfer in a thin packed bed of silica gel particles.

Charoensupaya and Worek [2] established a 2-D GSS (Gas and Solid Side) model in

which the heat conduction and mass diffusion were considered. Ge et al. [5,7] devel-oped a model coupling solid side and gas side resistances and results show very good

correlation with the experimental data. Gao et al. [4] studied the effect of channel

shape and the desiccant layer thickness in the simulation. Narayanan et al. [8] showed

Type –I isotherm

Molecular Sieve

Type – III isotherm

Silica gel

Type –I isotherm

Molecular Sieve

Type –III isotherm

Silica gel

a

b

that the counter flow desiccant wheel is far more effective as compared to the parallel

flow. Zhai [6] modelled the effect of residual moisture in the desiccant wheel.

The mathematical model of rotary desiccant wheel presented in this work is based on

assumptions as per Ge et al. [7] and in addition to them, the simulation of counter

flow hybrid desiccant wheel is performed for periodic steady state with input condi-

tions which are independent of time.

2.1 Governing Equations

The governing equations are derived based on the conservation of moisture and con-

servation of energy for the air and desiccant medium.

The moisture conservation in the air and desiccant can be expressed as:

)( ady

aa

e YYKz

Yu

t

Yd aa

(1)

)( ady

d

e

d

s YYKz

YD

zt

Y

z

WD

zt

Wad

(2)

Energy conservation in the air and desiccant are given as:

))(()( adadypvad

aae

aaaapae TTYYKcTTh

z

Tk

zd

z

Tu

t

Tcd

(3)

))(()()( dadaypvstdaydad

dd

dpd TTYYKcqYYKTThz

Tk

zt

Tc

(4)

The generalized isotherm for different type of desiccant material is defined as [5]:

W

Wcc max1/

(5)

Fig. 5. Isotherm of Composite SG [7], Conventional SG [7] and Molecular sieves

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Equili

biriu

m A

dsorp

tion C

apacity,

W, k

g/k

g

RH

Composite at 25 C Composite at 30 C

Composite at 40 C Silicagel at 25 C

Silicagel at 30 C Silicagel at 40 C

Generalized Isotherm SG Generalized Isotherm MS

generalized isotherm composite SG

2.2 Initial and Boundary conditions

The initial values (at t = 0) of temperature, moisture content of air and desiccant and

equilibrium adsorption capacity of desiccant are defined as:

000

00

)0,( ,)0,( ,)0,(

)0,( , )0,(

WzWTzTTzT

YzYYzY

ddaa

ddaa

(6)

Inlet condition of air at process and regeneration sections

360)360( if ),( ,),(

)360(0 if ),0( , ),0(

,,

,,

inrainra

inpainpa

YtLYTtLT

YtYTtT (7)

Adiabatic and impermeable boundary conditions:

0 , 000

Lz

a

z

a

Lz

a

z

a

z

Y

z

Y

z

T

z

T (8)

The auxiliary conditions and properties of air is calculated and used in simulation as

per Selvaraji et al. [3].

2.3 Solution method

The general form of conservation equations can be written for Eqs. (1) to (4) as Eq.

(9) and discretized using finite volume method [4] with fully implicit scheme.

pS

zzzu

t

uS

(9)

Where represents for WTYT daa and ,, and represents for

se D and D ,, da KK

and pSuS represents the source terms as applicable for the each governing equa-

tions. A FORTRAN95 code is developed for the numerical simulations. A grid and time independency study is carried out and it is found that a control volume of size

0.005 m and a time step of 0.008 sec are sufficient to give accurate results.

3 Validation

The present simulated results of SG wheel are validated with the simulated results of

Narayanan et al. [8]. Also it is compared with experimental results of Brillhart as

referred by Narayanan et al. [8] as per the corresponding wheel parameters and oper-

ating conditions, shown in Fig. 6.

Fig. 6. Performance comparisons with simulated results by Narayanan et al. and Experimental

results by Brillhart

4 Results and Discussion

The average moisture content of process air at outlet of the wheel is calculated using

the equation (10) [7].

360

0

),(360

1dLYY aLza

(10)

In this work, the main parameters to investigate the system performance are the mois-

ture removal capacity (D), relative moisture removal efficiency ( ) and dehumidifi-

cation coefficient of performance (DCOP) and are defined as :

Lzaza YYD

0

(11)

0

0

za

Lzaza

Y

YY (12)

ambrinrr

Lzazafgp

hhm

YYhmDCOP

,,

.

0

.

(13)

Where, hfg is the latent heat of water vapour, hr,in and hr,amb are enthaly of renegration

air at inlet and ambient conditions in J/kg

It can be seen in Fig. 7, that the moisture in the air is transferred to desiccant as the

flow is travelling from entry of the wheel (z=0) towards the exit of the wheel (z=L) in

the adsorption section. Also, the heat of adsorption is released to the desiccant and by

convection, the air is heated up. Due to these two phenomena, the relative humidity of

the air is steeply reduced as it approaches the mid of the wheel from 40 % RH to 10%

RH and further it reaches to 5% RH. The silica gel desiccant is having comparatively

higher moisture adsorption capacity at relative humidity more than 40% and propor-

tionally reduced to negligible capacity if the relative humidity is less than 10%. Whereas the molecular sieves desiccant has considerably linear performance in terms

moisture adsorption capacity of 0.22 for the wide range of relative humidity from

0

10

20

30

40

Mois

ture

con

ten

t of

air,

g/k

g

Wheel Rotation angle, deg

Simulation Narayanan et al. Experiment (Brillhart)

Yp(0,t) = 14.2 g/kg

Yr(L,t) = 18.2 g/kg

Tp(0,t) = 35 °C

Tr(L,t) = 120 °C and

L = 0.2 m, = 180°

up=ur=2 m/s D=0.4 m

0180

360Adsorption Regeneration

10% and above, as shown in Fig. 5. Hence the drawback of silica gel wheel can be

greatly improved by using the Hybrid wheel which is made up of silica gel and Mo-

lecular sieves desiccants. The relative humidity of air is further reduced to 2% RH by

using the hybrid wheel designs, as shown in Fig. 7.

Fig. 7. Moisture content and relative humidity of air in mid of the process and regeneration

sections across the depth of the wheels at 120 °C of regeneration temperature

Fig. 8 shows that the hybrid wheels are having superior performance in terms of D

compared to MS wheel for the range of regeneration air temperature from 60 to 90 °C

and significantly higher performance for the range of regeneration air temperature

from 90 to 120 °C than SG wheel.

Fig. 8. : Moisture removal Capacity (MRC) of Hybrid wheels over SG and MS wheels vs

regeneration air inlet temperature

The SG wheel is having optimum DCOP of 0.294 at 60 to 70 °C, whereas the MS

wheel is 0.289 between 100 to 120 °C. But the hybrid wheel is having optimum per-

formance of 0.307 at 90 °C, as shown Fig 9. The relative DCOP compared the MS wheel at 120 °C as reference of 100% shows

that the hybrid is having 106% that is 6% higher performance and having steady per-

formance for the wide operating range of 70 to 110 °C, as shown in Fig 10. This ca-

pability of hybrid wheel is the remarkable benefit for the wide range of usage of waste

heat recovery available from variety of heat rejecting equipments in the industry.

0%

1%

10%

100%

0

10

20

30

40

rela

tive

hum

idit

y, R

H,

%

Mois

ture

con

ten

t of

air,

g/k

g

Width of the wheel

Ya - SG Ya- 1/2 SG Ya- 1/3 SG Ya- 1/4 SG Ya - MS

RH - SG RH- 1/2 SG RH- 1/3 SG RH- 1/4 SG RH - MS

1.00

10.00

60 70 80 90 100 110 120

Mois

ture

Rem

oval

C

apac

ity,

g//

kg

Inlet Temperature of regeneration air, C

SG 1/2 SG 1/3 SG 1/4 SG MS

z=0 z=L z=0

Fig. 9. : Dehumidification Co-efficient of Performance (DCOP).of Hybrid wheels over SG and

MS wheels vs regeneration air inlet temperature

A hybrid wheel design is proposed in this work and the performance of this wheel is

compared with conventional SG and MS wheels. . The optimum performance is

achieved in SG wheel at lower regeneration temperature of 60 °C with DCOP of

0.294 compared to MS wheel of 0.203 and at the same time, the MS wheel is opti-

mum at higher regeneration temperatures of 100 °C with DCOP of 0.304. Since the

hybrid wheels are combination of both SG and MS desiccant portions, the optimum

performance is achieved between 0.296 and 0.306 at wide regeneration temperature

from 70 to 110 °C by bring the positive aspects of both SG and MS desiccants.

5 CONCLUSIONS

Hybrid wheel designs are proposed in this work and the performance of these wheels

are compared with conventional SG and MS wheels. The moisture removal capacity

and relative moisture removal efficiency of hybrid wheels are enhanced by 35% at 60

°C compared to MS wheel and 45% at 120 °C compared to SG wheel and they out-

perform both SG and MS wheels between 70 to 100 °C.

The average DCOP of hybrid wheel is 5% higher compared MS wheel and 12 %

higher for the regeneration temperature range from 60 to 120 °C, The DCOP of SG

wheel is optimum at 60 to 70 °C and for all higher regeneration temperatures, it is inversely proportional and the MS wheel is optimum at 100 °C and it is proportional

to lower regeneration temperatures. But whereas, the hybrid wheels have a steady

performance for the entire range of regeneration temperatures, which is the unique

advantage of these hybrid wheel designs (patent application by Selvaraji et al. [9]).

NOMENCLATURE

Cp Specific heat at constant pressure (J/kg K)

D Moisture removal capacity (kg/kg)

DK Knudsen diffusivity (m2/s)

DO Ordinary or molecular diffusivity (m2/s)

DS Surface diffusivity (m2/s)

De Combined diffusivity (m2/s)

de Equivalent diameter of channel flow (m)

k Thermal conductivity (W/m K)

Ky

Convective mass transfer coefficient

(kg/m2 s)

0.170

0.200

0.230

0.260

0.290

0.320

60 70 80 90 100 110 120

DC

OP

Inlet Temperature of regeneration air, C

SG 1/2 SG 1/3 SG 1/4 SG MS

110%

100%

90%

80%

70%

60%

h

Convective heat transfer coefficient

(W/m2/K)

L Width of the wheel (m)

qst Heat of adsorption (J/kg desiccant)

T Temperature (K)

t Time (sec)

u Velocity (m/s)

W

Adsorption capacity in kg water va-

pour/kg desiccant

Y

Humidity ratio, kg water vapour/kg dry

air

z Axial direction

Greek symbols

Density (kg/m3)

Thickness of matrix and desiccant mate-

rial layer (m)

Porosity factor

Tortuosity factor

Relative moisture removal efficiency

Relative humidity (%)

Regeneration section angle (°)

Instantaneous angle of rotation (°)

Rotation speed (r/h)

Subscripts

0 initial state

i Inlet condition

a Air

d Desiccant

p process air

r regeneration air

sg silica gel desiccant

ms Molecular sieves desiccant

v Water vapour

REFERENCES

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study”, International Journal of Heat Mass Transfer 30(6):1037–49.

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published on 16th Jan 2015

10. 11.