VOL. 16 No. 4, pp. 105-140 OCTOBER 1954

Philips Technical ReviewDEALING WITH TECHNICAL PROBLEMS

RELATING TO THE PRODUCTS, PROCESSES AND INVESTIGATIONS OFTHE pmLIPS INDUSTRIES

EDITED BY THE RESEARCH LABORATORY OF N.V. PHILIPS' GLOEILAMPENFABRIEKEN, EINDHOVEN, NETHERLANDS

CONSTRUCTION OF A GAS REFRIGERATING MACHINE

by J. W. L. KÖHLER and C. O. JONKERS.

A theoretical study of the gas refrigeration cycle, recently published in this Review, was basedon research in the Philips laboratories in Eindhoven closely parallel to work on the hot-gasengine. The present article deals with the application of this cycle and describes a practicalmachine for its realization. The results achieved with this machine will be discussed,especially with a view to possible applications of this new refrigeration cycle.

621.573

For the practical application of the gas refrigera-tion cycle - the principles of which were discussedin the previous issue of this Review 1) - a numberof problems have first to be solved. Before discussingthese problems it may be useful to recapitulatebriefly the principle of the gas refrigeration cycle.

1) J. W. L. Köhler and C.O. Jonkers, Fundamentals ofthe gasrefrigerating machine, Philips tech. Rev. 16, 69-78,1954/55(No. 3), hereafter referred to as I.

A quantity of gas is compressed at room tempera-ture, after which it is cooled to a low temperature.At this temperature it is permitted to expand and thecold thus produced is utilized. After the expansion,the gas is re-heated to room temperature, and thecycle is completed. The cooling and re-heating of thegas takes place by an exchange of heat in a regenera-tor. The working fluid is at all times in the gaseousstate and will be considered here as a perfect gas.

106 PHJLIPS TECHNICAL REVIEW VOL. 16, No. 4

As mentioned in I, it has been found in the courseof research that machines based on this principlehave the most favourable properties when used toproduce cold between -80°C and -200°C. Theexistence of this optimum range, however, cannotbe derived from the discussion of the idealizedprocess in I. This follows from the fact that theefficiency of the ideal cycle (no losses) is at alltemperatures equal to that of a Carnot cycle workingwithin the same temperature limits; as regardsefficiency therefore, the ideal process cannot be surpassedat any temperosure. The provision that the workingfluid must behave as a perfect gas to a sufficientapproximation, does not constitute a restrietioneven at -200°C, for it can be satisfied by usinghydrogen or helium even at considerably lowertemperatures.

Limitations of the process

It has been found that the limitation of theprocess to the above temperature range for efficientworking is due to deviations from the ideal cyclewhich occur in practice. They result in an increaseof the required shaft power and a decrease in therefrigerating capacity; the smaller the values of thesequantities, the more serious are the relative effects ofthe losses. Fig. 1 shows that we may expect thatincrease of the required shaft power limits the processat high temperature while decrease of the refrigeratingcapacity limits it at low temperature. In this way wearrive at the aforementioned temperature range, i.e.

p

t

Fig. 1. Graphs of the shaft power P and the refrigeratingcapacity qE of the gas refrigerating machine as functions of thetemperature ratio 'I: = TelT E (Tc = temperature of the coolerF:::l 300 oK, TE = the desired low temperature). The fulllinesapply to the ideal machine (see fig. 10 in I), the dottedline to a practical machine subject to various losses.

the optimum working range, in which the actualefficiency differs least from the ideal efficiency -see fig. 2. The limits of this range are not fixedprecisely, since they greatly depend on present and

future technological possibilities. We shall nowexamine in more detail these deviations from idealbehavior which give rise to the optimum range.

o 100 200-Te:

Fig. 2. The "figure of merit" 'T]1'T]e of the gas refrigeratingmachine as a function of''thc temperature (1) = efficiencyof theactual machine, 'T]e=Carnot-efficiency, cf. I). The diagramshowshowthe influence of the losses limits the useful tempera-ture range of the machine.

82127

Losses increasing the shaft power

The drive of every machine is subject to frictionallosses, so that a certain amount of extra pow~r isrequired merely to keep the machine going. In thegas refrigeraring machine the absolute value of this"mechanicalloss" is practically independent of thefreezing temperature 2), as may be seen from thedotted curve in fig. la; the relative effect of thisloss is therefore greatest at a high temperature.Another effect of a similar character to the mecha-

nical loss is what is termed the "adiabatic loss".In I it was assumed that in the cylinders the heattransfer between the gas and the surrounding wallsis so complete that the heat of compression andthe cold of expansion can be discharged duringevery phase pof the process. This, however,is very difficult to realize. For this reason heatexchangers have been incorporated between thecylinders and the regenerator (a "cooler" at thecompression side and a "freezer" at the expansionside), which establish the thermal contact betweenthe interior of the machine and the outside. In theseheat exchangers the refrigerant gas is caused toflow through narrow channels, so that a goodthermal contact with the walls is attained.If we assume the thermal contact in these heat

2) The pressure ratio, which determines the forces and hencethe losses set up in the drive, is in fact, only slightlyinfluenced by the freezing temperature. Incidentally it isto bc noted in this connection that in the compressionrefrigerator the mechanical losses will increase as theevaporator temperature decreases.

OCTOBER 1954 GAS REFRIGERATING :MACHINE 107

exchangers to he perfect, then we obtain a situationas shown infig. 3, in which the temperatures of thevarious parts of the machine have heen indicatedschematically. The temperature of the cooler isdetermined by the temperature of the coolingwater, and the freezer temperature hy the tempera-ture at which the cold is to he utilized, e.g. thehoiling point of air. The gas enters the cylinders atthe temperature of the heat exchangers; after thisthe gas temperature varies adiabatically with thepressure inside the machine. This means that theaverage temperature TCn in the compressioncylinder will he higher than the temperature Tc ofthe cooler (due to compression of the gas), whilstthe average temperature TEn of the expansioncylinder will he lower than TE'

Tca

~::::::::::.::::::::::Q~Î, Reg I

.. 80066

Fig. 3. Temperature distribution in the gas refrigeratingmaehine. The machine is equipped with two heat exchangerseffecting the thermal contact between the refrigerant and theambient atmosphere. The first (the "frèezer") is at the lowtempéruture TE' the second (the "cooler") is at room tempera-ture Tc. Between the two, in the regenerator (Reg), there is agradual transition between the two values. The two arrowsindicate the direction of flow of heat in the two cases.Owing to the adiabatic behaviour of the refrigerant in both

the expansion and the compression cylinder, the averagetemperature TEn in the former is slightly lower than TE' whilstthe average temperature Ten in the latter is slightly higherthan Tc. This is the cause of the "adiabatic loss".

We then see that Tcn/TEn = Tn > T, whichmeans that the machine works internally at a greatertemperature ratio than externally. From fig. la itcan he concluded that the shaft power is thengreater than in the isothermal case, It can heshown very simply that the value of this additionalshaft power is very little influenced hy the freezingtemperature, so that the effect of this loss, as forfrictional losses in the drive, is' relatively greaterat high temperatures.

By analogy with the expression derived for the shaft powerin the isothermal process, Pi,o = qE (r-I), (cf. J, eq. 14),we can put:

The value of the refrigerating capacity qE is found to be praetic-ally the same for both processes, owing to the fact that thedecrease due to the greater temperature ratio, as would beexpected according to fig. Ib, is compensated by the greaterpressure ratio inherent in the adiabatic process.The additionalshaft power thus amounts to:

The ratio 7:u/,. depends very little on the freezing températureTE' because the pressure ratio changes very little with thetemperature; we can therefore put:

where p is independent of TE' Consequently

This makes it clear that the relative effectof adiabatic loss isgreatest when TE differs least from Tc.

The adiabatic loss is approximately proportionalto the pressure ratio. It can therefore he limited hyreducing the pressure ratio. This, however, impairsthe refrigerating capacity, so that it must not hecarried too far.

Losses reducing the refrigerating capacity

The cold parts ofthe machine can never he com-pletely protected against loss of cold (or influx ofheat) through conduction. The lower the freezingtemperature, the greater becomes the influence ofthis "insulation loss".If G is the thermal conductivity of the particular

section causing the loss, then the loss of cold perunit time amounts to;

(1)

This loss should he compared with the refrigeratingcapacity qE' which is approximately proportional toTE, i.e. qE R:i CTE. Hence

The relative influence of the insulation loss at verylow temperatures is therefore practically inverselyproportional to the freezing temperature TE (cf.the dotted line in fig. lb); this effect is mainly dueto the decrease of the refrigerating capacity with TE'The insulation losses through conduction play

only a minor part in the gas refrigeration machine,as the cold parts can he huilt compactly. The losses

108 PHILlPS TECHNICAL REVIEW VOL. 16, No. 4

caused by a non-perfect regenerator, however, willalso have the character of insulation losses, and itis these losses that are more difficult to combat.This "regeneration loss" is the major limitation of .the refrigerating capacity at very low temperaturesIt can be roughly estimated as follows:

The quantity of heat Qr which the gas has todischarge on its way from the compression spaceto the expansion space, is stored in the mass of theregenerator. For this a loosely packed mass of thinwire is used. This mass of wire shows a continuoustemperature change in the direction of the gas flow;the form of this temperature distribution is roughlyrepresented in fig. 3.

The value of Qr is given by the equation

where Wg represents the thermal capacity of the gasflowing through the regenerator per half cycle. Inpractice, however, slightly less than the full amountof heat Qr is transferred to the regenerator mass,viz. 'YJrQr, 'YJrbeing the efficiency of the regenerator('YJr = 1 for ideal regeneration). The differencebetween Qr and 'YJrQr, which is denoted by LIQr.is the regeneration loss:

or, per second:

where wg represents nWg/60, and n is the numberof r.p.m. of the machine.

This regeneration loss wastes part of the coldproduced and thus reduces the refrigerating capacity.If we compare (4a) with (1), we notice that theregeneration loss has formally the character of aninsulation loss if (1-'YJr)wg is considered as thethermal conductivity of the regenerator.

The relative regeneration loss now becomes:

Llqr wg Tc- TE 10g- ~ - (1-'YJr) -- =- (1-'YJr) (1"-1). (5)qE C TE C

Wg/C is very little dependent on TE; its value isapproximately 7. The following table, computed fora value of 'YJr= 0.99 (which can actually be obtainedin practice), shows the influence of the regenerationloss in various cases).

Temperature range TE in OK Ll s«, Of<-In 0qE

liquid air 75 21

liquid hydrogen 20 98

hot-gas engine 900 4.7

At the boiling point of air the loss (21%) isacceptable. At 20° K, however, the loss has risen to98%, so t~at the ·refrigerating capacity is almostcompletely vitiated by the regeneration loss. Forcomparison, the value for the hot-gas engine' hasalso been given; this shows that there the regenera-tion, problem is less important than in therefrigerator.

There are two main factors which cause the efficiency ofregenerators to deviate from unity, viz, imperfect heat transferbetween the gas and the regenerator mass and the finiteheat capacity of this mass. Because of the imperfect heattransfer the gas will not exactly follow the temperature of theregenerator mass, so that it is not cooled down or warmedup to the correct temperature. Because of the finite' heatcapacity of the regenerator, its temperature changes while thegas flowsthrough it; this has the same effect.The relationship between heat transfer, heat capacity and

the efficiency of regenerators has been dealt with by Hausen 3).

(4a)

Miscellaneou$ losses: dimensioning of the gasrefrigerating .machine

Apart from the losses already dealt with, whichlimit the field of application of the gas refrigerationcycle, there are some other causes that impair theefficiency in practice, which we shall briefly discussnow.

In fig. 3 it is explicitly assumed that the coolerand the freezer are ideal. If this is not the case, thentemperature differences occur, both at the insideand at the outside of the heat exchangers, and inaddition, a temperature difference arises betweeninside and outside, owing to the thermal resistanceof the materialof the heat exchanger. In this waythe gas temperature in the cooler becomes higherand that in the freezer lower than required, so thatthe machine has to operate internally at a highertemperature ratio. This loss is somewhat similar tothe adiabatic loss, though of less importance 4).

Consequently, in designing a gas refrigeratingmachine, the aim will be to attain the largest possibletransfer of heat in cooler, regenerator and freezer.The attainment ofthe highest heat-transfer however,involves new difficulties, since the componentsmentioned each constitute a resistance to the gasflow from the compression space to the expansionspace and back. To overcome this flow-resistancea certain difference in pressure between the twospaces is required, for which additional shaft powerhas to be supplied, and which results in a reductionof the refrigerating capacity. This "flow loss" is

3) H. Hausen, Z. angew. Math. Mech., 9, 173-200,1929.4) Contrary to the adiabatic loss, however, this loss does

impair the refrigerating capacity.

OCTOBER 1954 GAS REFRIGERATING MACHINE 10~

intimately connected with the extent of the heattransfer and with the size (total diameter) of theducts. It will be clear that in practice a compromise

. is necessary, aiming at a reasonable heat transfer atacceptable values of the flow loss and the deadspace.

It is not easy to give clean-cut directions fordetermining the values of the parameters relevantto the refrigerating machine; moreover, the choice is,as always, influenced by the designer's personalpreference. We must, therefore, confine ourselvesto some general observations.

In practice the machine must be designed tosatisfy the demand for a certain refrigeratingcapacity at a prescribed freezing temperature. Therefrigerating capacity, which is the output of coldper second, has been given by formula (13) in I,which reads:

Ö nqE = 5.136]i Vo 2 sin e-- watt,1+ Vl-ö 1000where

w sin cptan e = - __ __:__1: +w cos cp

In the expression (6) only the variables ]i (averagepressure), Vo (volume of the expansion space) and-n (number ofr.p.m.) can be chosen without restric-tion. 1: is prescribed by the freezing temperature,whilst 15, cp, and ware more or less fixed for a well-designed refrigerator, viz. 0.3 < ö < 0.4,6.00 <sp <1200, and w has the order of magnitude 1.In view of the fact that the refrigerator should

preferably be as small as possible for a given capacity(and hence Vo should be small),]i and n have to beselected as high as possible. In raising n one ishandicapped by increasing losses (mechanical lossand flow loss) and by criteria concerning the opera-ting life of the machine. In practice, therefore, itis only the value of ]i that can be freely varied.Increasing the pressure level, is indeed a most effectivemeans of drastically reducing the size of the gasrefrigerating machine, contrary to the evaporationrefrigerator, where this cannot be done. The raisingof the working pressure is Iimited by mechanicalconsiderations, such as the required strength of thepartition walls and the load applied to the drive.

Still another factor precludes undue raising of the pressurelevel. According to formula (6) the refrigerating capacityshould increase linearly with rising pressure. In practice,however, losses occur which cause the refrigerating capacityto increase less than linearly. The higher the pressure, thegreater the relative influence of the loss, until finally therefrigerating capacity will. even decrease if the pressure ismade still higher (cf. jig. 4). It was because of this effect that

our experimental machines were initially unable to reachtemperatures below -160°C (the lossincreases at lowerfreezingtemperature), a behaviour which baffled us for a long time. Wefinally came to the conclusion that the interaction between theregenerator and the rest of the machine is responsible for thisbehaviour, As already mentioned, the average temperature of

(6)

80061

//

/

//

//

/

//

//

///

//

//

//

//

////

p

Fig. 4. Variation of the refrigerating capacity qE with thepressure p in the refrigerator. According to the theoreticalformula (equation) (13), part I) the dotted straight line wouldbe expected. The measured curve deviates from this straight-line as shown by the full ciurve. Clearly there is a surprisingdecline of the refrigerating capacity at increasing pressure inthe high-pressure range.

the regenerator mass varies periodically as a result of thcperiodic accumulation and discharge of heat. This affects thenormal pressure cycle of the machine and results in a smallerrefrigerating capacity (as well as decreased shaft power).This effect is governed by the ratio of the thermal capacityof the regenerator mass to that of the gas contained in it; thesmaller the value of this ratio, the more is the linearity impair-ed. The ratio becomes smaller as the denominator increases,which happens if the pressure level in the machine is raisedor the freezing temperature lowered: in both cases the quantityof gas in the regenerator is increased. These considerationsadequately explain the non-linear behaviour, and calculationsof the effect agreed well with the actual measurements.According to this explanation, the effect can be minimized bychoosing the highest possible thermal capacity per cm3 for theregenerator mass. This requirement is in addition to thatof classical regenerator theory (cf. footnote 3), according towhich the total thermal capacity of the regenerator mustexceed a certain prescribed value.

It is now convenient to deal with the actualrefrigerating machine developed in the Eindhovenlaboratories. The way in. which the volume varia-tions are effected in this machine, is somewhatdifferent from what has been described in I.This type of machine is termed a "displacermachine"; its working principle is outlined below.

110 PHILJPS TECHNICAL REVIEW VOL. 16, No. 4

The displacer machineIt has been found that the displacer-type of

mechanism, which was used in some of the oldhot-air engines, has certain partir ular advantagesfor the refrigerator. The principle is illustrated infig. 5. The volume variations are no longer obtainedwith the aid of two equivalent pistons, butby means of a main piston and an auxiliarypiston, which is termed the "displacer" . Themain piston 1 moves in a cylinder 2 and variesthe volume of the entire working space. Thisworking space is divided by the displacer 3, whichlike the main piston, has a harmonic motion:there is thus a space 4 between main piston anddisplacer, and a space 5 above the displacer, bothspaces varying harmonically. The displacer motionis such that space 4 is lagging in phase with respectto space 5, so that space 4 is the compression spaceand space 5 is the expansion space (cf. I); this isillustrated in the graph infig. 6.

The spaces 4 and 5 are in open communicationwith one another via the annular heat exchangersurrounding the displacer. The gas pressures aboveand below the displacer are nearly the same (hence

Fig. 5. Principle of the displacer machine. 1 = main piston,moving in cylinder 2. 3 = displacer, causing a periodic Howof gas back and forth between the spaces 4 and 5. The gasthen flows through the annular heat exchangers surroundingthe displacer.

the name "displacer") . Owing to this, there is onlya slight leakage of gas from the expansion spaceso that the accompanying loss of cold is veryslight, which is a substantial advantage of the

--a 80069

Fig. 6. Varia tions of the volumes of spaces 4 and 5 of fig. 5as functions of the shaft angle a. The constant volumes takenup by the displacer body (3) and by the body of the mainpiston (1) are shown in grey. The fixed head of the cylinderis shown shaded. In the lower part of the diagram the volumeof space bctween displacer and master piston has been plottedseparately, in order to demonstrate its sinusoidal variationand the phase shift with respect to space 5.

displacer-type machine. In a machine having twonormal pistons this loss of cold is considerablygreater, due to the far greater pressure differences.Another advantage of the displacer machine is thefar smaller mechanical loss, thanks to the smallfrictionalloss of the displacer.

Description of the gas refrigerating machine

The machine is shown in fig. 7. The componentsalready mentioned are marked by the same numbers.The main piston is driven, via two parallel con-necting rods 6, by the cranks 7 of the crankshaft 8.The displacer rod 9 passes through the centre of themain piston to the crankcase, where it is coupled,via the connecting rod 10, to a third crank 11 of thecrankshaft. The angle between the cranks 7 and 11has been so chosen that the motion of the displacerhas the desired phase difference with respect tothat of the main piston. The gas flows out of thecompression space through the ports 12 to the

OCTOBER 1954 GAS REFRIGERATING MACHINE

space contammg the cooler, the regenerator andthe freezer; the upper end of the freezer com-municates with the expansion space.

The displacer consists of a piston body 16 and the"cap" 17. The piston body carries piston rings andfits in the cylinder liner as a normal piston, and has

This construction, the principle of which was applied in theearliest hot-air engines, provides an elegant solution of theproblem of how to seal a cold space by means of a movingpiston at the ambient temperature, without loss of cold. Itis remarkable that the designers of expansion engines for theliquefaction of air have overlooked this solution, which couldhave saved them many a difficulty.

80070

Fig. 7. Simplifiedcross-section of a gas refrigerating machine designed for the liquefactionof air. The figures1 - 5 have the same meaning as in fig. 5. Further figures represent: 6 =two parallel connecting rods with cranks 7 of the main piston. 8 = crankshaft. 9 = dis-placer rod, linked to connecting rod la and crank 11 of the displacer. 12 = ports. 13 =cooler.14 = regenerator.1S = freezer.16 = displacerpiston and 17= cap.18 = condenserfor the air to be liquefied, with annular channel19, tapping pipe (goose-neck)20, insulatingscreening cover 21, and mantle 22. 23 = aperture for entry of air. 24 = plates of the iceseparator, joined by the tubular structure 25 to the freezer (15). 26 = gas-tight shaftseal. 27 = gas cylinder supplying refrigerant. 28 = supply pipe with one-way valve 29.

about the same temperature as the liner, which issurrounded by cooling water. The cap is madeof a heat-insulating material and is filled with aloose woolly substance in order to preclude gascirculation within the cap. The cap has a slightlysmaller diameter than the piston body, so that itdoes not touch the cylinder. All these provisionsconsiderably reduce the loss of cold through con-duction to the warm parts of the machine.

The outside of the freezer forms the "condenser"18, against which the air can condense. The liquifiedair is collected in an annular channel19 and can betapped via a pipe 20. The condenser is surroundedby an insulating screening cover 21 and a mantle22. Fresh air can enter through the aperture 23 andflow through holes in the plates 24, which are ther-mally connected via the tubular structure 25, to thefreezer. The water vapour and the carbon dio:xide

III

112 PHJLIPS TECHNICAL REVIEW VOL. 16, No. 4

Fig. 8. Photograph of the gas refrigerating machine.

contained in the air will precipitate as a frost onthese cold plates (termed the "ice separator"), sothat the condenser 18 is not contaminated by ice orsolid CO2,

The complete refrigerator is shown infig. 8. Fig. 9is a photograph of the cooler, the regenerator andthe cylinder head. We shall now proceed to discusssome of the details of the machine.

Constructional features

The crankcase is closed and contains the gasserving as the refrigerant; the gas pressure isapproximately equal to Pmin in the working space.

The crankshaft is led out via a shaft seal (fig. 7,26).The filling gas is supplied from the cylinder 27 tothe crankcase; from there, pipe 28 with one-wayvalve 29 leads to the working space.

Whenever leakage round the main piston (whichreduces the pressure level in the working space)causes Pmin to drop below the pressure in the crank-case, the gas will flow back to the working spacethrough 28. In principle the machine is gas-tight;and only if incidental leakage has caused the gaspressure to fall below the necessary level is themachine replenished from the supply cylinder.The crankcase has to be of comparatively heavy

OCTOBER 1954 GAS REFRIGERATING MACHINE 113

Fig. 9. Left to right: cooler, regenerator and cylinder head (of which the freezer andthe condenser form integral parts) of the gas refrigerating machine.

Thc cooler is built-up of a large number of thin pipes, parallel to the axis of the cylinder.The refrigerant gas flows through these pipes, which are surrounded by the cooling water;this water also cools that part of the cylinder wall along which the displacer moves (seefigs. 5 and 7).

The regenerator, like that of the hot-air engine, consists of a mass of extremely fine metalwire. Several rings of this material (one ring is shown on the right) are stacked together andkept in shape by two concentric rings of heat-insulating material (in this case nylon). Theouter wal! of the machine at the location of the regenerator should also be a poor con-ductor of heat in the axial direction to avoid "short-circuiting" of the regenerator. It hastherefore been made of a steel of great strength but poor thermal conductivity, which,although extremely thin, is capable of withstanding the pressure inside the refrigerator.

The freezer is formed hy the massive cylinder head, in which a large number of very fineslots have been cut. The outside of the cylinder head (i.e. the condenser) is also providedwith a large number of slots in order to improve the heat exchange with the air to beliquefied. The cylinder head is made of copper to ensure the slightest possible temperaturedifference between the exterior and interior of the refrigerator.

construction to cope with the high gas pressure ap-plied. A pressurized crankcase offers some substantialadvantages over a crankcase at atmospheric pres-sure. With a pressure Pmin in the crankcase, thepressure difference exerted on the main piston is

far smaller (varying between 0 and (Pmax - Pmin)),than in case of operation under atmospheric pres-sure, when the difference would vary between Pminand Pmax' The forces acting upon the drive andmoving parts are thus smaller and so are the frict-ional losses. The gas leakage round the piston islikewise less; also the amount of gas thus lostis automatically returned to the working space,

which would otherwise not be possible. Finally itprovides the means of drastically curbing leakageof gas from the machine since, unlike a piston, arotating shaft can be provided with a perfectly gas-tight seal.

A particularly tricky problem was the fact thatthe working space had to be completely free of oil,since this would freeze in the colder parts of theregenerator and clog them up. After extensiveresearch it was found possible to give the piston sucha shape that despite ample lubrication of all partssubject to friction, no oil can enter the working space.This subject will be dealt with in a separate article.

>

114 PHILJPS TECHNICAL REVIEW' VOL. 16, No. 4

The pipe from which the liquid is tapped off formsa goose-neck, thus providing a liquid seal. Non-purified air is, therefore, prevented from enteringalong this pipe and contaminating the condenser.Liquid air can be tapped off notwithstanding thefact that the gas pressure around the condenser isslightly lower than that of the ambient atmospherebecause of the flow resistance between the inletopening and the condenser; the liquid in pipe 20simply assumes a level above the highest point ofthe goose-neck. As a consequence of this, freshair is, as it were, sucked into the machine atjust the rate at which it can be condensed (i.e.in accordance with the refrigerating capacity).No special means are thus required to supply thecondenser with air; the refrigerator itself sucks in therequired quantity. As a result of this, the tempera-ture of the condenser is fixed at the condensationpoint of atmospheric air, viz. - 194°C. Thisclearly demonstrates the essential simplicity of theinstallation, owing to the fact that the air is con-densed at atmospheric pressure.

Some data and resultsThe following list provides some data on the

machine:Cylinder bore 70 mmPiston stroke 52 mmCrankshaft speed 1440 r.p.m.Pmax 35 kg/cm2

Pmin 16 kg/cm2

Pmax/Pmin 2.2 .The refrigerant used is hydrogen or helium; air

is obviously out of the question in view of the factthat at any excess pressure it would be liquefiedabove the normal boiling point (at 35 kg/cm2 airliquefies at -144°C).

In the laboratory, the following measurementswere made when using cooling-water at a temper-ature of 15°C:Yield \ with dry air 5.8 kg/h

( with moist air 4.8-5.8 kg/hShaft power . . . . 5.8 kWSpecific shaft power 5) 1.0 kWh/kg of airStarting-up period approx. 13 minutesPeriod of contin- ~dry, CO2-free air several daysuous operation ? moist air 6) 20-30 hours

5) For liquefiers, kWh/kg is a familiar measure of the "effi-ciency" of the installation; in order to avoid ambiguityconcerning the word "efficiency", however, we haveintroduced the designation "specific shaft power" for thisquantity (which may be read as kW per kg air/hour).It applies to the liquefaction of dryair.

G) Thc period of continuous operation depends on the volumeof the ice separator. Whenever this is clogged up, it hasto be defrosted, which takes, inclusive of the new starting-up period, approximatelyI hour.

Fig. 10 shows the measurements for higher freezingtemperatures; the diagram also shows the figure ofmerit curve (cf. fig. 2).

7kW 0,7II

... II

.KI <,I: ...-- ~- -,

i 'I/'IV'", F.....r-. "

I '" ,Iy .... r-, ,

/ r-,~

;'

i _...Vl..--~»>1

l.J.-%i

4 0,,"

3 0,3

2 0,2

0,1

oo-2fJO-19*

-175 -150 -125 -100 -75 -s: oe-Te 82128

Fig. 10. Measured shaft power P and refrigerating capacityqE of the gas refrigerating machine plotted as functions of thcfreezing temperature TE' The figure of merit 1J/ric (cf. fig. 2)derived from this, has also been plotted.

The value of the specific shaft power, which is ameasure of the coefficient of performance ("effi-ciency"), requires some further explanation. Thevalues given in the literature for the specific shaftpower of various liquefiers vary within wide limits.With very large installations (producing 100-1000 kg/h), using expansion turbines, a specificshaft power of approximately 0.7 kWh/kg can beobtained; for smaller installations (producing a fewkg/h) the figures lie between 1.5 and 3 kWh/kg.We see, therefore, that the efficiency of the gasrefrigerating machine, in spite of its small capacity,approaches very nearly that of the large installa-tions.

The scope of this article does not allowan analysis of theeause of this relatively high efficiency; this would require acomparison of the gas refrigeration cycle with other systems,which are quite different. It may suffice here to mention twopoints in this connection. First of all thc pressure ratio (approx.2.2) is small, so that the adiabatic losses (which play a part inthe compressors of the other systems) are small. In the secondplace there is the circumstance that the power released uponexpansion, is recovered by extremely simple and thereforeefficient means, viz. by the gas pressure acting upon the piston.

The efficiency of the process, moreover, can. beimproved a great deal. No less than half the coldproduced by the machine is spent in cooling the air;the remainder is used for condensation. By means

OCTOBER 1954 GAS REFRIGERATIN.G MACHINE _ 115---

of a second refrigerator the air may he pre-cooled toan intermediate temperature, so that part of thenecessary cold is ohtained with a higher efficiency:consequently the overall efficiency is improved.It has heen calculated that by means of an inter-mediate stage of this kind the specific shaft powercan he reduced helow 0.8 kWh/kg.

The gas refrigerating machine compares favoura-hly with the conventional types not only in effi-ciency hut also hy its simplicity of operation andhy the fact that it is not suhject to contaminationby dirt and dust (no expansion valves, etc.). Anadditional advantage is that the liquid air is com-pletely free of oil, owing to the fact that the airneed not he compressed.

Moreover, the refrigerating capacity can he variedhoth hy changing the speed of rotation and hyvarying the gas pressure in the working space:hence a very simple control is achieved.

We shall conclude this article hy giving a roughsurvey of the possihle applications of the gasrefrigerating machine. It will already he clear thatthe machine can he of great use in lahoratories andfactories for the production of liquid air. For use inlahoratories particularly it "will he of great valuethat other gases, such as nitrogen, argon, oxygenor methane can also he conveniently liquefied hythis refrigerator. In this way liquid hathswith a well defined hoiling point can he madereadily availahle. Preliminary tests have revealedthat in certain cases the machine can he success-fully used for gas separation, e.g. the fractionationof air. Finally, it can he employed as a pre-cooler for

hydrogen that is to he liquefied on a small scalewiththe conventional type Linde-liquefier *).The optimum working range of the machine, as

previously mentioned, lies between -80 oe and-200 oe, where it can serve as a source of cold forany desired purpose. The machine has a goodefficiency (cf. fig. 10) and is considerahly less com-piicated than the cascade machine used up to now inthis field. A further great advantage is that anytemperature in this range is ohtained with theone machine in a single stage. The simplicity andconvenience of this refrigerating machine willundouhtedly act as a stimulus in low temperatureresearch.

*) See this issue, p. 116. - Ed.

Summary. In its practical form the gas refrigerating machine,is subject to certain losses. Some of these Iosscs increasethe shaft power (mechanical loss and adiabatic loss), otherscause a reduction of the refrigerating power (insulationloss and regeneration loss). The former group of losses definethe upper limit and the latter group the lower limit of theuseful temperature range of the gas refrigerating machine(-80°C to -200 °C). After a brief consideration of these andother losses and their quantitative significance, this articledeals with the factors that influence the design of such arefrigerator. Of considerable practical importance is the factthat the volume of a refrigerating machine of given refrigera-ting capacity can be drastically reduced by using a highpressure level. A description is given of a small machine inwhich the pressure varies between 16 and 3,5 kgfcm2, at acrankshaft speed of 1440 r.p.m, This machine is builton the displacer principle, which was applied in some ofthe early hot-air engines, and is used for liquefying air. Ownigtothe fact that the air does not re_quirea preliminary compres-sion, this liquefier is ofvery simple designand is easily operated.It has an output of approx. 5.5 kg liquid air per hour and the"specific shaft power" amounts to 1.0 kWh per kg of liquidair - a value which is extremely low for such a small andsimple machine, and which may be even further reducedwithout undue difficulty. In conclusion the article deals withthe properties and further potential applications of the gasrefrigerating machine.

116 PHILJPS TECHNICAL REVIEW VOL. 16, No. 4.

LIQUEF ACTION OF HYDROGEN

Photo \Valter Nürnberg

The investigation of the solid state has made remarkable advances in the last two decades. This work has beenparticularly fruitful in clectronic and electrotechnological applications (ferromagnetic materials, semiconductors, dielectrics,luminescent substances, etc.).

In these investigations it is sometimes important to work at very low ternperatures, e.g. when it is required tominimize thermal agitation of atoms anel molecules, which may otherwise complicate or mask a phenomenon. In thisconnection, a simple hydrogen liquefier using a liquid nitrogen pre· cooler is inst.alled in the Philips Research Laboratories.Liquid hydrogen has a boiling point of 20.4 OK at standard atmospheric pressure. The wbite tubes shown in thephotograph contain the equipment for the cleaning and pro-cooling of the gas and also the Iiquefier proper. Theoperator is in the process of syphoning some liquid hydrogen into a Dewar flask.