NATIONAL BUREAU OF STANDARDS

'Cecliniccd 'fl-ote39

January 27, 1960

HELIUM REFRIGERATION AND LIQUEFACTION USING

A LIQUID HYDROGEN REFRIGERATOR FOR

PRECOOLING

by Accesion For

NTIS CRA&IDTIC TAB

D. B. Chelton Unannounced

J . W . D ea n ..i.tif.cat ---........-

T. R. Strobridge By.... .............Dist, ibution

B . W. Birmingham Availability Codes

D. B. Mann

NBS Technical Notes are designed to suppleme -the u-reau's regular publications program. They provide ameans for making available scientific data that are oftransient or limited interest. Technical Notes may belisted or referred to in the open literature. They are forsale by the Office of Technical Services, U. S. Depart-ment of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

TABLE OF CONTENTS

Abstract I

Introduction 2

General Discussion 2

Hydrogen Refrigerator 5

Helium Refrigerator 10

Helium Liquefier 21

Bibliography 31

Helium Refrigeration and Liquefaction Using

a Liquid Hydrogen Refrigerator for

Pre cooling

by

D. B. Chelton

J. W. Dean

T. R. Strobridge

B. W. Birmingham

D. B. Mann

ABSTRACT

Consideration is given to the use of a hydrogen refrigerator to

assist in the production of temperatures below those obtained with

hydrogen. A hydrogen refrigerator is used to maintain a precooling

evaporator at or near 210K in a helium gas circuit. The helium

circuit may be arranged to produce liquid for external use or to pro-

duce refrigeration between 21 K and 4.2 0K in a closed system.

Charts have been developed that show the requirements of the

composite helium - hydrogen system and the effect of heat exchanger

performance. The relative quantities of refrigeration (hydrogen and

helium) at various temperature levels have been determined.

INTRODUCTION

At the present time, a number of hydrogen refrigerators andliquefiers are being used to maintain experimental apparatus at lowtemperatures. For example, several Z2 -27 Khydrogen refrigeratorsare used for continuous refrigeration of liquid hydrogen bubble cham-bersl, 2 Refrigeration at this temperature level is also being plannedfor experimental apparatus associated with nuclear reactors andelectromagnets. Certain applications may require temperature belowthose obtainable with hydrogen.

Hydrogen refrigerators can be used for the production of lowertemperatures by using a hydrogen evaporator to precool a stream ofhelium gas in an additional circuit. In some instances, it may befeasible to convert a hydrogen liquefier to perform the function of arefrigerator for this precooling.

This paper discusses a relatively simple method of using ahydrogen refrigerator to assist in the production of temperaturesfrom 21 K to liquid helium temperature. With proper considerationsit should be possible to operate the hydrogen precooled helium systemat any temperature within these limits. Consideration in the text isalso given to the production of liquid helium for external use. Cycleinformation is presented with text only sufficient to define the parti-cular conditions or assumptions upon which the calculation is based.

GENERAL DISCUSSION

A schematic arrangement of the composite helium-hydrogenlow temperature apparatus is shown in Figure 1. Symbols indicatedon the schematic are used throughout the text. The automatic controlof such a unit has been discussed elsewhere 2 and will not be consideredhere. Both the helium and the oydrogen circuits use liquid nitrogenprecooling at 65 K, although 78 K is considered in a few instances.The liquid nitrogen required as precoolant is given in liters per hoursince, for most applications, it will be supplied as liquid from an ex-teraal source. It is further assumed that the nitrogen will be suppliedto the unit as saturated liquid at 2 atmospheres.

The hydrogen gas is further cooled, after liquid nitrogen pre-cooling, in a final heat exchanger and expanded at the hydrogen throttlevalve to produce liquid in the evaporator. The evaporator can be areservoir as shown, or a bundle of heat exchanger tubes. Hydrogenevaporator temperatures from 21 K (1. 23 atm) to 1 5K (0. 13 atm) arestudied.

3

N 0 0

x: z z : 2a: a: al: a:aCa-,,I -I ""

AT4 AT 4 AT5 AT5

E -E E E

___---- __ _,•.

AT LIQUID NITROGEN

EVAPORATORS(PRECOOLANT)

r.,,H2 Throttle Valve

LIQUID HYDROGENEVAPORATOR

(PRECOOLANT)

He Throttle Valve....

Q2 or-x-•o

HELIUM RESERVOIRor EVAPORATOR

FIGURE 1. SCHEMATIC ARRANGEMENT OF LOW

TEMPERATURE CYCLE

J==m= .nlmmmm a i ~ l i J n

4

The helium gas, after liquid nitrogen precooling, is passedthrough a counterflow heat exchanger and further cooled in the liquidhydrogen evaporator. The temperature of the gas is still furtherreduced in a final heat exchanger and expanded at the helium throttlevalve where liquid can be formed. Liquid formed in the expansion

process can be evaporated in the reservoir, as in a refrigerator, orcan be extracted from the reservoir, as in a helium liquefier. Ifrefrigeration is required at temperatures above liquid helium, thefinal heat exchanger is partially by-passed. The quantity of by-passedgas determines the operating temperature of the refrigerator.

If a hydrogen refrigerator is available, the method describedabove is a very simple way of producing temperatures lower than thatobtainable with hydrogen. An alternate method for the final stagecooling of the helium gas stream would utilize an expansion engine asindicated by the dotted lines of Figure 1. The expander inlet temp-erature could range from the temperature of the hydrogen evaporator

to some lower temperature. This method requires the use of an ex-pansion engine operating at low temperatures, but it substantiallyincreases the thermodynamic efficiency of the cycle. Additionalimprovement in the thermodynamic efficiency of the cycle can be

obtained by replacing the helium throttle valve with an expansionengine. Use of such an expander is limited to temperatures abovethat of liquid helium if condensation in the expander is to be avoided.

The performance of this alternate cycle must be computed for the

refrigeration temperature desired. These more extensive calcula-tions have not been considered in the present paper.

Small capacity refrigeration systems do not necessarilyrequire a high thermodynamic efficiency. Often the simplest cycle isdesirable for reliability and ease of operation. For any cycle,however, it is desirable to produce refrigeration with a minimum ex-

penditure of energy, since the cost of gas compression equipmentmaterially affects the total capital investment. Compression equip-ment may account for 20-40 percent of the total system cost.

Sufficient information is presented later to determine the per-formance of a practical helium refrigeration or liquefaction systembased on the cycle indicated by the solid lines in Figure 1. A conven-

ient reference is the theoretical performance based on a heat exchangerefficiency of 100 percent. For the final heat exchangers in the cycleunder consideration, this corresponds to AT3 = AT a 0. The designparameters considered are:

5

1) heat exchanger efficiency (presented asfunctions of AT through AT 5 )

2) liquid nitrogen consumption

3) mass flow

4) power required for compression

The power required reflects only that necessary for compression ofthe gas in each circuit. A 65 percent isothermal compressionefficiency is used for the calculations. With the aid of the designcharts presented, it is possible to determine the quantity of refrigera-tion (Q 1 ) required at liquid hydrogen temperature for a given quantityof refrigeration (Q ) at lower temperatures or for a given quantity ofhelium liquefied (x'.

All helium calculations are based on the thermodynamic pro-perties presented by Mann and Stewart 3 , Keesom4 and Zelmanov 5.The hydrogen calculations are based on the properties given byWoolley, Scott and Brickwedde 6 . The results of the calculations havebeen smoothed where necessary since they require greater accuracythan obtainable from the above properties.

HYDROGEN REFRIGERATOR

Hydrogen refrigerators usually operate from 21 0 K to 27 0 K.For efficient operation of the helium refrigerator or liquefier, it isdesirable to maintain the hydrogen evaporator as low in temperatureas possible. In fact, it would be desirable, on this basis, to operatethe evaporator at a reduced pressure. Practical considerationsusually limit the precooling temperature to about 150K (0.132 atm),slightly above the triple point of hydrogen. Operating the hydrogenevaporator at a reduced pressure results in a substantial increase inthe work of hydrogen compression although there is a considerablereduction in the work of compression for the total cycle in Figure 1.Figure 2 shows the work of hydrogen compression (kw) required perunit (kw) of refrigeration at the evaporator temperature for theore-tical performance (100 percent efficiency) of the final heat exchanger.Minimum work occurs for an inlet high pressure near 120 atmospheresand this pressure has been used throughout the remainder of the calcu-lations. The power required to compress 1 scfm (1 atm, 200C) ofhydrogen gas from 1 atmosphere to 120 atmospheres is equal to 0. 37kw. Compression from 0. 132 atm requires 0. 52 kw per scfm (I scfm=0. 04 gm/sec).

6

The refrigeration available as a function of heat exchangerperformance is shown in Figure 3. It is observed that the quantity ofrefrigeration is reduced because of finite temperature differences,by 5 percent per degree K for 65 0 K precooling and by 6.7 percent perdegree K for 78 0 K precooling.

The liquid nitrogen required in the precooling evaporator isaffected by both AT and AT 4 . A lower limit is imposed on the valueof AT 4 because of tae variation in the specific heat of hydrogen. Thislimit can be seen in a cooling curve analysis 2 . The quantity ofnitrogen is shown in Figure 4 in units of liters per hour per watt ofrefrigeration ( I) at the hydrogen evaporator. Both Figure 3 and 4are independent of the hydrogen evaporator temperature.

Since it is thermodynamically advantageous to reduce thetemperature (and pressure) of a hydrogen evaporator, it is ofinterest to determine the effect of a reduced pressure on the perfor-mance of a hydrogen refrigerator not specifically designed for suchoperation. The reduction in temperature (and pressure) will requirethat the heat exchangers operate under conditions other than theoriginal design. The important design parameters influenced byreduced pressure operation are the heat exchanger efficiency and thepressure drop across the heat exchangers. The heat exchangerefficiency is affected by both the quantity of heat transferred and theheat transfer coefficients within the heat exchanger.

By considering the basic equations of heat transfer, it can beshown that the heat transfer coefficients will remain approximatelythe same for a given flow rate regardless of pressure. The changein efficiency of a heat exchanger will therefore be determined by theadditional heat removed and its effect on the log mean temperaturedifference. However, for the case considered this effect has beencalculated and found to be insignificant. Thus, neither the change inthe heat transfer coefficients nor the quantity of heat transferred hasan appreciable effect on the heat exchanger efficiency.

The pressure drop across a heat exchanger, for a given flowrate, is proportional to the specific volume of thejprocess fluid. Fora given heat exchanger, the pressure drop in a 1 5-K hydrogen refrig-erator will be approximately 10 times the pressure drop in a 21 Krefrigerator. Therefore, the lowest attainable operating temperatureof the evaporator will usually be dictated by the heat exchangerpressure drop. However, the actual value must be determined foreach heat exchanger design and cannot be computed on a generalizedbasis.

T Ir I I I I II; III jIT 71

I 1 1 1 1 11 1rI

1 Q1111

I I I T 1 1 1

Ii 0

I I 1 1 1 1 1 . 1 1

1" 11 1

(0 to 1L I TI I III II 111.4 I)/) I N IS~d O ?H Io T T

E ~Precooling Temp. -

4--

78

wcn

L L L-

0 I2

AT3 0K

FIGURE 3. HYDROGEN REFRIGERATOR

REFRIG ERATION AVAILABLE

N2 Precool ing Temp.= 65 K

AT4 15- OKI--00 --LL4-0 J-

00

.01 L4 j---t 1 4

00.022

4 T= -TK-

FIUR 4.HYRGE RFIGRAOLIQUI NITRGEN CNSUMTIO

10

HELIUM REFRIGERATOR

The performance of a helium refrigerator with liquid hydrogenprecooling has been calculated on the basis of the assumptions setforth in the General Discussion. The design charts presented(Figures 5 through 1 2) are applicable to refrigeration temperaturesfrom the temperature of the liquid hydrogen evaporator to the temp-erature of liquid helium.

Figure 5 shows the work of compression (kw) required perunit (kw) of refrigeration at the helium evaporator temperature fortheoretical performance of the final heat exchanger, AT 2 - 0.Minimum work occurs for an inlet high pressure near 30 atmospheresand this pressure has been used throughout the remainder of the cal-culations. The compression of I scfm (1 atm, 20 0 C) of helium gasfrom 1 to 30 atmospheres requires 0. 274 kw (1 scfm = 0. 0835 gm/sec).

As previously indicated, the quantity of helium refrigeration(Q 2 ) available is substantially influenced by the hydrogen precoolingtemperature. Theoretical refrigeration as a function of the precoolingtemperature is shown in Figure 6. It can be seen that an increase inrefrigeration of 80 percent is achieved by reducing the precoolingtemperature from 21 0 K to 1 50 K when the return helium stream is ata pressure of one atmosphere. An increase in the low pressure ofthe system will decrease the available refrigeration but will alsodecrease the power of compression. A low pressure of 5 atmospheresresults in a decrease in the work of compression of 47 percent (30atmospheres high pressure) while the theoretical refrigeration avail-able is decreased by 20-25 percent. Thus, operation at a slightlyelevated low pressure may be beneficial if refrigeration temperaturesabove liquid helium are desired. If refrigeration is required nearthe normal boiling point of helium (4.2 0K), it is important to keep thispressure as low as practicable.

Applying the First Law of Thermodynamics to the processshown in the schematic of Figure 1 shows that AT affects the final2quantity of refrigeration (Q 2 ) and that AT affects the relationship ofQ to the quantity of refrigeration required in the hydrogen evaporator

Figures 7 and 8 indicate the ratio of refrigeration, Q,/Q., as al 0 0 /

function of both AT and AT for 21 K and 15 K precooling, respec-tively. The temperature diherence at the warm end of the final heliumheat exchanger (AT 2 ) has a profound influence on the refrigerationavailable - considerably greater than the influence of AT In fact,when AT a 1.9 K with 21 K precooling, no refrigeration is availablebelow the hydrogen evaporator temperature. The point of zero refrig-

eration with 15oK precooling occurs when AT 2 = 3. lOK.

11

The basic assumption of theoretical refrigeration, AT 2 = 0,requires further consideration for the helium refrigeration cycle. Bya cooling curve analysis 2 , for the conditions under consideration, aviolation of the Second Law of Thermodynamics is observed. Toavoid this difficulty, a modification to the schematic shown in Figure1 must be made for the portion of the helium circuit below the7hydrogen evaporator. This modification is shown in Figure la below.

High LowPressure Pressure

H2EVAPORATOR T 2

Figure la ModifiedSchematic for Helium

HX I Refrigerator

SECONDARYTHROTTLE

VALVE

SECONDARY HX frPRESSURE

12

The final helium heat exchanger of Figure 1 is subdividedinto two heat exchangers by the addition of a secondary throttle valve.The inlet temperature of this valve and the resulting down streampressure is selected to prevent a violation of the Second Law ofThermodynamics. A cooling curve for this arrangement is shown inFigure 9. For a 30 atmosphere high pressure and the precoolingtemperatures under consideration, a secondary pressure of 5-10atmospheres is sufficient.

To be consistent with the Second Law of Thermodynamics, ifprovisions for the two throttle valves are not made, the minimumvalues of AT are 0 . 5 Kfor 21 0 K precooling and 1.7 0 K for 15 0 Kprecooling. ?These temperature differences are sufficient to providea AT = 0 at the cold end of the final heat exchanger. The effect ofthis AT2 can be seen from Figures 7 and 8.

Additional design parameters are given in Figures 10 and 11.From these figures, it is possible to determine the helium flow rateand the consumption of liquid nitrogen for a helium refrigerator.The liquid nitrogen consumption shown in Figure 11 is independent ofthe hydrogen evaporator temperature.

The effect of eliminating the nitrogen evaporator in the heliumcircuit is shown in Figure 12 for a 21 0 K hydrogen evaporator. Theratio of the quantity of helium refrigeration to the quantity ofhydrogen refrigeration is greatly reduced, conclusively indicating thedesirability of this precooling in the helium circuit.

As indicated in the General Discussion, the installation of anexpansion engine across the final helium heat exchanger, as shown bythe dotted line in Figure 1, adds substantially to the thermodynamicefficiency of the cycle. Preliminary calculations indicate that theaddition of an expansion engine having a 210K inlet temperature andan 85 percent isentropic expansion efficiency will decrease the workof compression by at least a factor of 2 and will increase the avail-able refrigeration (Q 2 ) by a considerable factor. The optimum highpressure, based on work of compression, is reduced to about 15atmospheres.

13

Low Pressure =I atmAT2 0I'll

1+ tT-pT

6Wcoln~ep4)+ -

0It 1t rC,)

044-ýf+hA0) F

Prco Ii n Tmp

4000 10! 20 304

HIGH PRSUR-tFIGUE 5.HELIM RERIGEATO

WORK F HEIUM OMPRSSIO

14

I T 1 1 ~ .

T R0 - 'J L~-- t:iT- 2TT

Ft' -r- T4l

En I

-J4

I--4

+5 1+ 17 1: 19 04PRE 4OIN TEMPERATURE F-

FIUR 6.HLUMRFIERAO HOEIAREFRIGERATIO

15

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CY1 Wx z -44-

0

0 D

NO IV S2LL

16

4-4-

OD

717V. 0

I [ T i ll

j4 0

I -oo-i I f I I W IL

17

HIGH PRESSURE

I Secondary

LOw Throttle Valvew LOW-"*"T~,,.vovPRESSURE SECONDARY

PRESSURE

w0-

w •- HXI HXTT\E'-

"\I

'I

HEAT TRANSFERRED

FIGURE 9. HELIUM REFRIGERATOR -COOLING

CURVE FOR MODIFIED CYCLE

18

6 Precooling Temp. -

4--_44

0L

0

oL -t2

AT2 ,IKF #UE1.HLU ERGRTR-HLU

FLOWin RATE.

19

N? Precooling Temp. =65 OK

S.--

.08

I-4

CUD)

.0

0

00

ST1 I I

FIGRE I. ELIM RFRIERAOR LIUINIRGNCOSMTO

20

H2 Precooling Temp. =21 OKNo N2 Precooling

0T

I.-7

12 12.6 2

RAI of HeRFIoH E.(~Q

FIGUE 12 HELUM EFRIERATR -EFFET OFHEAEXCHNGERPERORMACE No NTROGN PECOOING

HELIUM LIQUEFIER

Essentially the same principles apply to the helium liquefierthat apply to the helium refrigerator discussed in the previous section.However, for the pressures and temperatures considered, there is nodanger of violations of the Second Law of Thermodynamics. Althoughthe work of helium compression in a helium liquefier differs fromthat of a refrigerator, the level of optimum high pressure remainsthe same. This is shown in Figure 13 where the work of helium com-pression per unit of helium liquefied is presented as a function of thehigh pressure. The liquid produced is calculated assuming theoreti-cal performance of the final heat exchanger, AT, - 0, and a lowpressure in the cycle of one atmosphere. The remainder of the cal-culations are based on the optimum high pressure of 30 atmospheres.The influence of the heat exchanger temperature difference (AT ) onthe quantity of liquid produced (fraction liquefied) is shown in Figure14. If liquid helium at one atmosphere is desired as an end product,increasing the low pressure will both reduce the fraction liquefiedand cause flashing losses when the liquid is transferred from theliquefier to an external storage container. It is particularly impor-tant therefore, to keep the low pressure as near one atmosphere aspossible. The pressure drop on the low pressure side of the heliumsystem must be carefully evaluated.

The ratio of helium liquefied to the quantity of hydrogen refrig-eration required is shown in Figures 15 and 16. The effect of heatexchanger performance can clearly be seen from these figures. With21 0 K precooling, no liquefaction occurs when AT - 1.9 0 K. With2 o15 0 K precooling, no liquefaction occurs when AT2 a 3. 1 K. Therequired helium flow rate and liquid nitrogen consumption are shownin Figures 17 through 20.

The effect of eliminating the liquid nitrogen evaporator in thehelium circuit is shown in Figure 21. The ratio of helium liquefiedto the quantity of hydrogen refrigeration required is greatly reduced,again indicating the desirability of this precooling in the heliumcircuit.

22

_11- -r-77

0~ I_

~ 200021 0K Precooling

(60

W 1200CL

80

0 4003: 0 10 20 30 40

HIGH PRESSURE , atm

FIGURE 13. HELIUM LIQUEFIER - WORK OF

HELIUM COMPRESSION.

23

20

LL~

.01-08l

ILL~

.04

0 2

AT2 , *K

FIGURE 14. HELIUM LIQUEFIER - FRACTION

LIQU EFI ED.

24

i+~~4 4+t-x4

4t+-~~ -4,+J4

00Iz

w 0LrLLUO

44- - LL a.+1;H0 4+4-1

e# vt#

4-J

0 0 44-.4+4 -44++

M I Ok

N 0Y-

>10' 'Lv 3

LL

25

44

4.0

LL~

IL 0LL-W

w -44L -a

-j

01ID-

14+4H>+- 1-44-

26

Precooling Temp. =2-1 OK

8011 1

U04

i f --

0

00

EjI l f I-

00-if

0Em I 2-T

FIGURE 17 HELIUM LIQUEFIER -FLOW

RATE (210 K PRECOOLING)

27

Precooling Temp. =15 K

10

4-r

I-4

0 I 2fI I

FIGURE ,OF 18 HEIU LIU1ERFORAE(50 PEOLN)

28

H2 Precooling Temp. a21 OKN2 Precooling Temp. c 65 OKtAT 5 : 100K

0.16 1 ri

E

0I,0.1

-Tz

0 0.102

.T1 ,K

CO0MTO (20K0RCOLN)

29

H2 Precooting Temp. =15 KN2 Precooling Temp. =65 OK

HG'T 02 AT* -

0~0.28O

44+

0 t 2

D '4T1 4 KFIGRE20.HEIU LIQUFE J- IQUKID NITOGECOZMTO (15 K-RCH~N)

30

~w(I II I fi l ; TIl il I 1 11 Uz

Il ;I 1[111 1 0 4- '8

ILI

NI L LId

wODWZ

i t~~ 1 1 ! 1 1 .

LW

0W (9 N 0

i.

31

BIBLIOGRAPHY

1. B. W. Birmingham, D. B. Chelton, D. B. Mann and H. P.

Hernandez, "Cryogenic Engineering of Hydrogen Bubble

Chambers", ASTM Bulletin, No. Z40, pp. 34-39 (1959).

2. D. B. Chelton and J. W. Dean, "Design and Construction ofa Liquid Hydrogen Temperature Refrigeration System",

National Bureau of Standards Technical Note No. 38 (1960).

3. D. B. Mann and R. B. Stewart, "Thermodynamic Propertiesof Helium at Low Temperatures and High Pressures",

National Bureau of Standards Technical Note No. 8 (1959).

4. W. H. Keesom, A. Bijl and L. A. J Monte, "Le Diagrammelog T, Sde l'helium, Leiden Communication, SupplementNo. 108c.

5. J. Zelmanov, "The Entropy Diagram for Helium at LowTemperatures", Journal of Physics (USSR), Volume 8,Number 3, (1944) pp. 135-141.

6. H. W. Woolley, R. B. Scott and F. G. Brickwedde,"Compilation of the Thermal Properties of Hydrogen in ItsVarious Isotopic and Ortho-Para Modifications", Journal of

Research, National Bureau of Standards, 41, 379 (1948)RP 1932.

7. I. L. Zelmanov, Comptes rendus (Doklady) Acad. Sci. URSS,2Z, 25 (1939).