4 Alternative Cycle

8/12/2019 4 Alternative Cycle

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4 Alternative Cycles *A Introduction

4.01 are a number of possible cycles for refrigeration besides thevapor compressioncycledescribed in detail in theprecedingchapteL Thefirstpartof this chapter \1 ill dealwith heat operated cycles. TheCarrecytle will be treated and brief commentswill bemade on different "sorption" cycles. Followingthis, we \\ discuss other options forrefrigeration:ExpansioncyclessuchastheJoule("coldaircycle")or Stirlingcyclesandfinallyalso\vewillbrieflyreviewthePeltiercycle-

B n heat operated andmechanically driven cycles4.02 The designof heat operatedsystems canbe basedon differentprinciples. astarting

point \ve canuse the combination of a (heat operated) power station + an electricallyoperated vaporcompressionsystemforrefngerationorairconditioning, seeFig402a whole this combinationrepresentsaheat operated theprimaryenergysourceforoperationis theheatsuppliedinthe power station.

CJr:10ressor J U{ Hr:e,

Steam

4.02. A vapor compression refrigerating plant is equivalent to a heatoperated :: ystem i the system boundary is extended to include a thermal)pOl-Fer generating plant.

From the refrigerating process"separated from via

4.02 it isobviousthat there is one generatingprocess"these two

and t\V iselectricgenerator,transferiinesc and

one

i Author: Granryd

4

..


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REFRIGEPI.ATLVG

combination (from Fig 4.02) is toor to integrate processes to one

a loop based onhvo Rankine cycles. the most COIlli '110n example on a heat operated system isthe absorption based on the pnnciple of Carre. The only moving pa rt such asystem is (in principle) a liquid pUInp Another interesting development is the Platenand l0 unters refrigerator-a unit where the pump IS avoided so that it operates withoutny moving parts. Also there are possibilities to upgrade waste heat to highertemperatures by using a ''heat transformer . There are still other heat operated cycles to

create and maintain lower temperatures than the environment: intermittent absorptionand ejector cycles and we will in this chapter briefly describe principles behind these.The systems mentioned will later on be described in more detail. As an introduction,however, some general comments are given on the coefficient ofperformance, COPh, ofheat operated systems. We can defme the COPh as the ratio between the achievedrefrigeration capacity, Q2, and the necessary heating energy for its operation, Qh,compare Fig 4.01:

4.02a

Notice that this relation differs from what is used for the definition of the COP2 of theregular vapor compression cycle by the fact that it defines the ratio between two termsexpressing heat energies . Returning to the example defined in 4.01, the COP!:includes not only the refrigerating system but also the efficiency of the power station.Let us use the symbol lt for the overall power station thermal efficiency fu'1d the symbolCOP2 for the coefficient ofperforrnance of the refrigerating system. t is obyious that thefollowing relations must prevail:

COP =_Ql 4.02bn O~

The COPf.: of a heat operated system can thus always be expected to be smaller than theCOP2 of a system which for the operation is based on mechanical (electric) energy.This observation also implies that, for a given cooling capacity, the heat operated systemmust be equipped with heat exchangers of larger capacities tha l1 a mechanically operatedsystem. This is also n indication that a heat operated system may become more costlyto build than a.. 1 electrical system of the same capacity (although this comparison issensitive for the ratio between the cost of an electric motorcompressor in comparisonto the cost of heat exchangers). Some further comments on economic comparisons aregiven n point 4.27.


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CH4PTER 4. AL1ERNATlVE

C The Carre absorption cycleC1 Principle o operation

4.03 The most common principle of operation for simple absorption systems is based onprinciples first developed by the French scientist Ferdinand Carre (about 1860). Ascheme of the system is shown in Figure 4.03. More details on absorption systems aregiven by Nibergall (1959), Backstrom (1970) and ASHRAE (1996) (References aregiven at the end of the chapter.)

absorption system of this type consists, just as a vapor compression system, of thefonowing components (numbers refer to Fig 4.03):

condenser, (l) where the refrigerant is condensed to liquidexpansion device, (2) where the pressure of the refrigerant liquid is reduced

from condensing pressure to the pressure theevaporator

evaporator, (3) where the refrigerant evaporates, for which process heatis supplied-from the object to be cooled

Instead of the mechanically operated compressor in the vapor compression cycle theabsorption system has a thermal compressor involving the following components:

absorber, (4) where the refrigerant vapor is absorbed, and forms aliquid solution. DtIrhig this process heat is released,which means that the absorber must be cooled. The heatreleased is equivalent to the heat of condensation plus aheat of solution (or mixing) pertinent to the combinationof refrigerant and the absorption medium.

pump, (5) by which the solution leaving the absorber is given apressure equivalent to the pressurcin the generator (sfu '1leas in the condenser).

generator, (6) where the refrigerant is separated from the solution by adistillation process-which in the simplest case is donejust by "'boiling" off the refrigerant from the solution.Heat (equal to the operating energy of the system) mustbe supplied for this process.

regulating valve, (7) where the poor solution from the generator is passingon its way from the generator to the absorber. The valveis necessfu-Y since the pressures in these two componentsare different.

to decrease heal, thus to the itis beneficial to use a:

heatfrom the absorber before it enters the


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ammonia being the

valve

;\\: . ~1lJt

eat exchanger

Absorber

Figure 4 03 Simple scheme ofan absorption system by Carre4 04 In a system the refrigerant formed in the evaporator is absorbed (by

chemical affinity) to liquid form. The refrigerant is thus transferred to the condenserpressure in liquid form, simply by means of a pump. The power demand for operationthis pump is only i fraction of that which would be neceSSGlY for a cOn1pressor in avapor compression system (since the liquid volume is much smaller than the vaporvolume). t remains to separate the refrigerant from the liquid solution. This is, in theabsorption system, accomplished by a "distillation" accomplished in the generator. Forthis process high temperature is necessary. From the generator the refrigerant-

solution is returned to the absorber and the process is repeated. In a Carre systemthere are two closed circuit loops in which the two working media are circulated-onefor the refrigerant nd one for the absorption solutionC2 orking media

the preceding it is obvious that we need a pair of media for the Carre cycle.traditional combination is the pair of arruTIonia and water; therefrigerant and water the absorption medium. The essential reasons why thissuitable to use are the fact that ammonia is a good refrigerant nd that thevapor pressure of ammonia in water solutions is considerably lower than that

Onein the generator contains not only ammonia, also to certain ex1:ent water

water content inlarge plants the

ammonia. This is illustrated in 405.The pair of amIIlonia properties: -- Both

good heat is that the vapororder to decrease separator", ainstalled after the is designed as acolumn.

44


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p 20 [ 7 / / / / / 1 / / V /Vv / .Iv / /1 bar //V/j///V ( y // V VV- - / / , ./ / '// 1/17/ /7 ' ~ _ ~ 9 r 7 . 1 " / .I V ( ?1V / /rf-- -f-.'1----1--" I '..- . , . , . . ,- , - .

- v /l7 -;f+t7-----r .f 1.. -1[-..-.-. 0 - 0 ,--- -,-1- . ---i - - - ' - - . --I---f--t-- 1--1

- - - - - -1- 1--1,. - -,I - ' I ~ - - - - - ' -.. ~ ~ H- ' ~ j-f-H

-f-- j_ ~ . _ ~ _ i~ - - H H -I-f-,I - - lf ~ f - - --' - ~ - - ' - f ~ - - -

H- - i - ---',-i----'.."i-- ,-1

I

.I .I 7 ~ r - / I / ~ ; Z; - - - ~ . = = - = ~ v / , ; ; ' /V./ /_ / / / / '7 /' ./.1 7 /

. V . / / / v/ / / / ' / I / /- J/ . / . / . / / ./ 7 -7 1 / / . 1 . ,..-- 7 / / . / . / .I" 7 . - , , ' ~ . ........, .---'- ./ 7 ----- -.-- .. 7/ / . " /3 . / /' V/ / r7://' . V/ Z / - / . // / iL / .-- ~ ' 7 ' - / ... '-' r '_ ~ ~ y / 1/ V/ ./ V . / ; S ~ l Z2 )- ~ ~ v} 7[7 h V ~ V / G--r- () -- -

IVapor pressure f o r ~Inixtures of' ....1arnUlonUl auewater

+ , , ~ __ .. oI I,

1.0 -l i----;

--,----1- ~ ~ l ~ ....-t r - - z -.1 - 7 - 7 ' - ~ - ~ /;7 7/ ' L__ / '.1 . / / - /

./ l/ /

J ~ . / G ( ) ; J Y ~ ~ i j ' b 1 , r 7 9 ~ ~ ; / : T - - - -1 L Z L / /-r:: f -. .. --,L .I -7 ,' 1/ . / L---r= /--- .. ----tv / / / - 7 7 7 ' / . . ~ Z-7 :: /-- ~ - ~ V - - - ~ ' 1 - - 'Z -L. Z / ' .' / ' / ' 7'1--7'1_ Z 7 -- .. ,--L _

" / 1/ / / / i/ V [ / - - 7 - 1 / ~ /- ',,'u ::> .....L :-T- - . .L _.__. L - - - ., . / '7' / / ' . I " ' ~ r -- /. / ' / 1- / / / . 1 / ./ / / L / ... L. _L _ - - 7 - [ 7 - ' / / / 7 /

..

, _ /,;7 / ' / ' / - -7 " 7 .../ / / // / / V ; / / / / . 7 _L . ~ - -- 7// // 7 ~ _ - L i /-/ / / v ~ - / 7 ~ 7Op 2 - __ l7 /L(L, . / ' / - ' / / /" )7--/, / 7_/ /

W/ / / V / /7 / V 11- 0 ,1 -LrC .-.....L 7 7 . / ~ . - - / / - ' . ~ - I - ,r " 7 ' - ./ 7 ..L . _L 1- . -.-. / 7 7 - 7 -.- - ~ -/ 7 [ ' L / ~ / - [:I.. .-.. /" : / - / ~ V ~ 7 - / / j- / / / :/ /I/,/ ~ L / / ./ / / .. - ~ - - . - - / h - - - ~ - _ _. ' T - / ' - - ' - + -/-- -- '/ -r- / - . . _ L . - --- -- - - - ] '.---/ ___ .L __ __0 . . / . / 7._ / ' 7 .1'./ / . - Z ~ - - - '/ / - // / / / L--L. . / / -/ / 7 = : Z / . - --- .- - ....,.- 0 , / -; / 1/ 7 7 " / / / " - - 7 . ' - - - . --.--'--..' ... - - 7 / / '. / 7 ' V / -l.L-- - - .-- - - ' - - - , - - i ' - 'o 02 JL....L- - /1 / ' ----1 I - _. f- - ___. .L__.60 -50 _IW -30 -20 .10 20 40 60 80 iou 150 DC ~ : o o- a, t .1if

,


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REFRIGERA TIXG

most common are thoseconditioning ammonia is not regarded as asuitable safety conside;:ations. working mediacOITLTf cercial air conditioning absorption on the US marketscale around 1 -- is instead }v ter and lithium bromide (H20 - LiBr), water isthe refrigerant lithium bromide is a hygroscopic which easily absorbs water.Since water has a very low saturation pressure' will expect that the ~ 1 1absorption of this type will be extremely low. This affects the way the plant willbe designed-the low pressure means that volumes of water vapor must be

t r ~ n s f e r r e d the evaporator and the absorber. Naturally the operation is limitedto evaporating temperatures higher than O T-limited by the freezing temperature of, , ~ \ a t e r . The of the LiBr-salt the saturation vapor of thesolution so that over pure waler. The saturationpressure ofH2 LiEr solutions are given in the diagram in Figure 4 06Due to the fact that LiBr is a salt (with a vapor pressure almost zero) vapor fannedin the generator contains only vater vapor. Of this reason the generator not have tobe equipped with any rectification to remove LiBr-vapor. This simplifies the design(compared t that in a NH3 - H20 HO\vever the pair H20 C ~ l exhibit aproblem of crystallization: This is to the fact that the salt crystals may fonn insolutions high salt concentration relatively low temperatures. limits \vheresalt will form are indicated in 4.06.The concentrations and temperatures where crystallization can occur must be avoided ina real The critical part of the is the process in exchanger whererefrigerant-poor (salt-rich) solution is circulated from the n the heat

e x c h ~ g e r this sohition is cooled and the crystallization may occur at a point in the heatexchanger where the temperature has lowered so that the conditions fall in the areawhere crystallization my occur-compare the diagram in Figure 4.06. If this occur theheat exchanger may be blocked so that the circulation is stopped. (Restarting the

involves heating the exchanger, from outside, so the salt crystalsdissolved in the solution. is a time consuming may t ~ t c e a long

to a unit running again.)Solutions of H20 - LiBr in concentrations that prevails in an absorption system isquite corrosive and this may be a problem especially in the Materials as steel

copper, preferable used in bundles for heating the solution in the generator, areto corrosion. Corrosion inhibitors must be used cromate has been used

successfuJly--although the use of cromates are subject to due to healthif persons are exposed.)

n


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--

P 1( bar(::.: 0,5

0 , )0,2

0,1

0,0,0

,

o 0')

+-

Saturation vapor pressureor (1120 - LiBr) -solutions.

-,'-k f- .-- ~ - - . - - - + - - . - , ~ ' - - - - - - f - - - . - -T I

II / / / ~ ~ / / l / / / V L ~ f - I c. / v / /v / /VT/v V V/ 7 / ' V/ ~ --' 1----+-----+

o . / . / 7 ;/h./ / ~ - , - - - + - - ~ - , - , - , ~ , - 1--:-- 'p ./ / / / / / 77 '7 '7 [7 go --------- .. , - .--1---..-.. / / /' /" / / -7-717277/ ' ' ' ' ' ( , - ---r -_ ...

.. / / 7 ./ / / ' / ' / / ' / j /' l? . .v - -- . --'.. / / - / / / /7 / 77/ / /. -- - 0, .;./ / / . / 7-7-/ L / / ' - / ' / ' / ' 7 7 /' / ' /' /' / ' - _ ' - ' -1

y /' / '.

~ -7'-T ' - ,7/ / / ' / / /V/ . / / iLZ - -. .. . 7 ./ '7 v / 1/ /V / -7 /7 / - - - - - . / ./ ./ / - 7'7 ~ ~ ./ > :_..../.. 1 -- .__ . -> - - - -/ 1/ 7 / / / ./'/ / ' 1/ ---.r------7- / / r/;- . --7/ 7 7 ./ 7 ~ r / 7 V / - - ' ~ - ' - . ~ - I - . ~ - -- - - 777 / / / h / V / . / ~ / 7 r 7 - . ._

/ . / / 17 / / /. :......L / 7 777// ///0, - / / / / / 7 7 /' 7; V / /// / / / ,V ooO _ O O l W ~ ~ ~ % S " ~ < i ~ ~ ~ ~ - - l

.J::.-.l -20 -10 0 +10 +20 +)0 +40

1

v 77/ 7 r7i'//I/Y ~ /' / / /7/7/ / / / ~ ~ ~ ll' 0 '\ ~ ~ ) ( o / [ 7 7 r 4 ~ ~ c~ ~ 3r / q ~ V / -vo// / /a/ / V / ~ ~ ~ / O/ / Z / // /' 7 lZ 7 - - : = f - f = =/ ' /' /' i/ / ' / . / ~.L L-/ / /.///V/ \----1

- ~-_ . .-. ~ -1--..

. _ - - - t = 1 ~ t J 1 r-f-tj -1-

+50 +60 +80 +100 +120 +140 +16t

Ij. '-'I

- - - - - - 'I'


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C3 he cycle in a vapor pressure diagram4 pressure as and the absorption can

traced a way demonstrated in 1407 The scheme in diagram gives us apossibility to estimate which temperatures that can be obtained, with a given temperaturefor the heat supplied to the generator (or vice versa which generator temperatures arerequired).

Pressure

, Temperature, vCEvaporating Condenser andtemperature,t 2 absorbertemperature t ; tabs Generatortemperature, tgenFigure 4 07. 7 e absorptlOn cycle represented in a saturation vapor diagram j r

nzedia p ir usedThe cycle can be traced in the following way: The desired condensing and evaporatortemperatures defines the conditions for the pure refiigerant in the condenser andevaporator as given in the picture by points 1 and 2 This defines the pressures in thecondenser and evaporator, and P

pressure m absorber is the same as that in evaporator (disregardinglosses) he temperature for the solution the absorber,

then 3 ofThis is the highest concentration

limitedexchange areas.

4:8


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1

CHAPTER 4.

4,possible concentration,;p the refrigerant-poor the

and m the

t is obviously necessary that refrigerant concentration is higher in the soiutionleaving the absorber than the solution entering. With symbols this can be written as r

The smaller the difference in conce 1tration (;. .; p , the larger mass flow of liquidmust be circulated between the generator and the absorber. To describe this theconcept of a pumping factor" can be used. Let us use the symbol y for the pumpingfactoL This factor is important in order to define the capacity of the pump in theFigure 4.03) as well as the load of the heat exchanger in the Figure 4.03).

pumping jactor, y, tells us the mass (number of kg) of mixture that must becirculated between the generator and absorber in order to have 1 kg of refrigerantabsorbed If the concentrations ;r and ;p are knov:n the factor y can be estimated by thefollowing simple relations based on a mass balance for the absorber:

_------"'1 y -1 kg solution, refrigerant cone sp1 kg refrigerant - bsorberfrom evaporator Y kg sol t o r ~ refrigerantconc. -

FIgure 4.07. IVfedia balance of the absorberRefrigerant mass balance gives: 1 + (y-l) ;p Y ;r from which the following relation isobtained:

Example: Assume a H20 - LiEr-system where the temperatures in the condenser andthe absorber are tJ = 35C, in the evaporator t - -SOC and thegenerator, 90C.The condensing and evaporating temperatures will correspond to pressures ;::;0,056 and P2:;::; 0,0087 bar. The diagram in Fig 4.06 gives with these data:

;:: 0, nd ;.. 0y = (I-O, 43-0,34) 7,3.

This is the minimum pumping factor for ideaJ processes in the absorberthe conditions. In reality it is necessary to circulate more than

kg refrigerant due to the fact that we to allow certainconcentration differences. A pumping factor is favorable, since it gives aon the and to

..

4.07


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C4. oefficient o performanceof one gIve a

isequivalent to r2 more precisely - (J )'2

Xir: vapor quality of the refrigerant after the throttling). For stationaryconditions, 1 kg of refrigerant must be delivered from the generator to the condenser foreach kg fed to the evaporator. This gives usa relation to calculate the necessary heatinput to the generator. Energy must be supplied equivalent to the latent heat ofvaporization of the refrigerant at generator conditions (approx. rh plus allY heat ofsolution L). From this simple reasoning we can deduce that the COPh rougt y can beestimated as

where r =the latent heat of vaporization of the refrigerant (index 2 for conditions inthe evaporator, index h for conditions in the generator)Xlr: =the vapor content in the refrigerant entering the evaporator from thecondenserL - heat of mLxing for 1 of the refrigerarlt in absorber solution.

Since the heat of mixing is positive for all knOV.l1 substances suitable as working pairsfor absorption systems, and since the variation in latent heat is small for most mediaused r2 ' it is easy to deduce that the COPh for the simple Carre cycle cannotexceed 1.

Example: The heat of mixing is about 460 kJ/(kg H20) for H20 LiBr systems withwater concentrations of 35 to 40 (as given by Niebergall (959)). With thelatent heat of vaporization for water at about +5C of r2 2500 kJikg and at+90C (assumed generator temperature) about rh 2290 kJ/kg and with XiI'0,05 (1- Xm) 0,95 we will, from equation 4.08 get an ideal COPf; 0,86.

For a real system there are, however" several losses that will decrease the performance tolower values than the ideal ones. One of the most prominent influence is the loss due toincomplete heat exchange between the rich and poor solution in the heat exchanger. Forlarge absorption systems using f120 LiBr as working pair normal values are COP h0,7 0,75. In systems using NH3 H20 there are also additional losses occurring tothe influence of water vapor leaving the generator (incomplete rectification).

4.09 The fol lowing table give some numbers which may be considered as normal for good(single effect) systems:

Temperatures, COP;,Pair Generator Cooling water Evaporator

to 30 .,5 to 0 65H3-H20 120 to 1105 to 140 to 25 -5 to -10 0 to 0,62105 to 140 to iO 0,160 to 1 15 to 18 AD to -

to 1 5 to 8 to 5530 to 2 0, 5 lO 0,7

: 1

4.08


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CHAPTER 4~ ~ .t is of interest to L 1at COPh absorption system does not seem todecrease as rapidly the evaporating tenJpe:rat as is the case a mechanicalt vapor compression system.CS xamples designs

4.10 The practical designs of absorption systems are quite different for different workingmedia pair NH3 - lhO and H20-LiEr . The season for this is mainly the differences inpressures and the consequences of this in terms of vapor volumes.Figure 4.1 Oa shows a scheme of a NH3 H2 - system and Figure 4.10b gives a photo,of a system with a capacity of about Q 2 = 580 kW cooling capacity for an evaporationtemperature of -55C. There are plants of this type built for quite large capacities andalso for low temperatures.For H20 - LiBr -systems the scheme is quite different. Figure 4.10c illustrates acommon way of design of such systems used for air conditioning applications. Asmentioned earlier the pressures of the system will be very low: For an evaporatortemperature of the pressure in the evaporator and absorber will be about P20,0087 bar (abs), see diagram in Figure 4.06.Also in the generator and condenser the pressures will be low if condensingtemperature is -35C the pressure will be Pi 0,056 bar (abs). The pressure differencebet\veen the high and low pressure sides are only about 0,047 bar equivalent to about 0,5ll '.'later coltmm!

Comment: Thanks to the small pressure difference there 1S actually a possibility to avoida mechanical pump and h 1stead rely on natural' circulation based on a"thermosiphon" pump (utilizing the difference .in density of the liquid in a"downward"-tube and of liquid with vapor bubbles in a "riser"-tube). Suchsystems have been built but mainly for relatively small capacity systems.

Due to the fact that the pressures are so extremely small there are huge vapor volumesthat must flow between the evaporator and the absorber. (The evaporation of 1 kg waterat +5C will create about 147 m3 vapor giving a cooling capacity of slightly less than2500 kJ For a cooling capacity of 100 k the vapor flow is hence about 6 m3/sec.)With this in mind it is easv to understand why it is beneficial with a design (as the one.. "" '"-' ,illustrated in Figure 4.l0c) where the evaporator and absorber are combined in onepressure vessel (vacuum vessel). Similarly the generator and the condenser arecombined in ~ n o t h r vesseL Figure 4.10 d shows a photo of such a plant. There are alsodesigns where only one cy'lindricai enclosure is used for an four components (where aninternal wall is used to separate the n;vo pressure levels from each other).One of the most crucial components of an absorption system is the absorber. Toa large area for the vapor absorption process the liquid is sprayed in a fashion thatsmall droplets are created. However it is also important to have an efficient heatexch3..tlge so the temperature of the liquid is as low as possiblecapability to absorb refrigerant. The cooling is 2.ccomplished

lOco

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4.10a. Schematic layout of nabsorption system using H20 by York).A: system; B: Vapor separator; C: Distillation D: Condenser; E: Ammoniareceiver; F: Evaporator; G: Absorber (three parallel); H: Solution receiver, : Solution pump(with motor, K: Heat L: Liquid level control valve.

4.10b. of nabsorption system NH3 H2 (Borsig).Cooling cajDa :;z bout 58 at an evaporating temperature ofabout


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------- ALTERNATTflE~ ~ ~ ~ ~ ~'. .

c2.::0a

Heatexchanger

onaer.serWater

Steam fer

Evaporator

pump

2:;


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ENGIlvEERING

generator

The condenser and absorber cooling of most plants is arranged by means of coolingwater. A cooling tower is usually used for water cooling if lake or river \V-ater are notavailable_ absorption plant has about twice the cooling water demand compared tovapor compression plants_ (The cooling load of the condenser is approximately equal tothe evaporator cooling capacity-and in addition to this we have the cooling demand forthe absorber which is about the same as the heat L 1put to the generator.) A result of thisis the cooling tower must designed to handle larger capacity than for vaporcompression plants_ Backstrom (1970) recommends that it is suitable to use about 1,7times larger temperature differences on the condenser side w-hen using absorptionsystems compared to vapor compression systems. Since the heat transfer coefficient in acooling tower increases with temperature difference the net result is that it appearseconomic to use a 30 to 40 larger cooling tower than for a vapor compression pla.. 1t of

same cooling capacity_The pressure in lL. 1its with H20-LiBr is . If is notsealed and tight, outside will leak into it If this occurs the capacity will deterioraterapidly_ The oxygen in the air will also increase the risk of corrosion and there is a riskof crystallization of the LiBr salt since the rate of absorption is decreased and thesolution circulating between the generator and the absorber contains less and less water.One of the problems in manufacturing LiBr absorption systems is to make them trulyair tight. n spite of the fact that the solution purnps have hermetical designs (with theelectric motorenc3sed) and that great efforts are devoted to all joi;1ts tight one mustdesign for the case that a certain leakage may occur. In order to remove noncondensable gases in the system (which also may be products'from internal corrosion)units are normally equipped with a purging system In principle the purging systemconsist of vacuum pumps, sometimes of piston types, but also other types are used.n order to enhance the process in absorber it is found that certam additives have a

beneficial effect by which for instance the surface tension can be influenced. This is afield that recently has attracted the interest of several researchers. Examples are Rush etaL 1994) a.. 1d Nordgren and Settervall (1996). extensive overview is given byZiegler and Grossmarm (1996)- Improvement in heat and mass transfer coefficients areexplained due to different phenomena_ One such is by virtue of concentrationinstabil ities in a liquid film creating a Marangoni convection _Absorption systems with H20 - UBr are normally manufactured as factory madecomplete package units. This is the case for - H20 units - althoughlarge such systems are built on place.C6 apacity modulation and system oper tion

4 1 1 used toapplications

4: 14


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>CH4PTER 4. LTERS TIVE CYCLES

.t to change the generator temperature decreasing the flow of hot water or throttlingr the steam supply)r by means of a shunt circuit by whlch a certain portion of the rich soiution leaving the

absorber is mixed with the flow of the poor solution coming from the generator. Inthis way the amount of refrigerant that can be absorbed in the absorber is decreased

as is the amount of refrigerant coming to the generator. A similar fLlIlction can bereached by reducing the pump capacity.:

The latter methods are the most economical and seem to give quite an effective modulation all the way down to 10 % of nominal capacity, with reasonably well maintainedCOPh

4.12 In systems with H20-LiBr the risk of crystallization must be observed-in operation aswell as when the system is shut down. The solution leaving the generator often has awater concentration of only 35 - 40 . Salt crystals will, with such salt-rich solutions,start to form in the solution as soon as the temperature of the liquid is approachingbelow 30 to 15 cC (see the diagram in Fig 4.06). Before the plant is shut off it isimportant that this solution is diluted with more water, especially if the plant is locatedin a surrounding where the temperature may drop to the temperatures mentioned instandstill. Crystallization in the heat exchanger may also occur during operation if, forinstance, the generator temperature is increased. It is easy to understand that the effect ofsuch a temperature increase in the generator may be that the solution leaving thegenerator becomes too poor in water - creating conditions for salt crystals to be formedin the heat exchanger where the solution is cooled.Modem absorption units are equipped with automatic centrols for aVOlc1mg theproblems of crystallization. On older plants it is a time consuming task to get the saltdissolved if crystallization has occurred. The method is simply to try to heat the partswhere one suspects that the salt crystals have formed so that they eventually dissolve.

D Multiple effect cycles4.13 There are a large number of different multistage cycles for absorption systems described

in the literature. Many of these alternatives was described by Altenkirch (1954), \vhodeveloped an ingenious methodology for different schemes of principles for multistagesystems.The aim ofmultistage arrangement call be different:

l to improve the COP of the system to make it possible to increase the temperature "lift" of the cycle (increase t: - t2) or to maintain a desired temperamre lift bat using low temperature (waste) heat input

for operating the system.are a nU1Tlber of differeflt svsterrJ, alternatives beSIdes the single effect s:/stern

described so far: double effi:ct" or "triple effect"; "absorber heat exchanger" (AHX)and "generator absorber heat excharrger" G , ~ . : X . ) cycles to mention a fe'N b general theyachieve better performaIlce by adding components like absorbers 3...l1d heat exchangers

4: 15

....


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an tolso cost and complexity. Figure 4.1this (from Sum,.'uerer, 1996)

achieve improvements in COP the prinGiple is to arrange heat exchangers so thatcan be used at different temperature levels and that the same heat energy is used severaltimes To increase the temperature lift two (or multi-) stage arrangements can be usedin the cooling cycle in a similar way as in vapor compression systems. Figure 4.13bshows for such a case a temperature, pressure- diagram of a cycle for a twoevaporator system at 1:\1, 0 temperature levels. We will not go into details here - theinterested reader is referred to for instance Niebergall (1959), Ziegler and Alefe1dt(1987) or Cheung et al (1996).

OJ2oosLo)

usual

0 15

double effect

singleeftect

0.25 0 30 035te;ta area of heat exchangers per k N cooling capacity [ m 2 f k \ f I ; ~

Figure 4.13a. Realistic COPh for single nd double effect Systems in AC-applications v,'ith H20-Libr-systems versus total heat exchanger area (fromSummerer (1996).

1: generator;2: condenser;3: medium pressure

evaporator;4 low pressure evaporator,5. low pressure absorber:6 medium pressure

absorber.

F ,?u.re 4. 13b. Temperature, pressure-diagram of ('tIO evaporalOrs lvorking t d?Uerent temperature levels,Niebergall, 1959)

r

416

I
http:///reader/full/F(,?u.rehttp:///reader/full/F(,?u.re


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--

-Absorber

Solution PUr.lP

Con.denser-

CHAPTER 4 ALTERNATIVE CYCLES

414 We here only gIve one more example of a double effect cycle: A lrhough theprinciples for mUltiple effect systems for increased COP h have been known for a longtime it is only very recently that unfts have been introduced to the market. Figure 4.14ashow a scheme of a unit (from Meacham et al 1990) and Figure 4.14b give the cycleprinciples in a temperature, pressure - diagram (from Niebergall 1959).In the first effect generator (1) in Figure 4.14a, operating at high temperature, vapor isdriven out from the solution. The vap'or flows to a heat exchanger (6) where it iscondensed while the heat released is used to drive off vapor from the solution forming asecond effect generator (6). This generator is hence working at an intermediate

temperature and pressure (lower than what is prevailing in the first effect generator) andthe solution leaving the second generator (6) is very weak on refrigerant (as shown in thediagram to the right). The refrigerant vapor is condensed in the condenser (3), andmerged with the liquid from the second generator, before the liquid is fed via thethrottling valve to the evaporator (4). The vapor formed in the evaporator is absorbed inabsorber (5). The solution is pumped to the first effect generator via the two heatexchangers, HXl and HX2. The remaining part of the scheme should be easy to follow

1st effectgenerator-

-2[ IC effect

IiI I .1

i Ig PII

pli-Figure 4 4a nd 4.14b. Scheme ofdouble effect absorption unit to the right; and

the cycle illustrated in a temperature pressure diagram to the leftIn this scheme ex1ernal heat for driving the operation is introduced to the first effect

.. l ' . rl . , h h 1 rl 1 1generator anu tp.JS energy 15 use, t\:V1C-e Slnce tHe Heat re:.cease J vvne:1 tne V2 PQf iscondensed furnishing heat mput to the second generator. Hence, since we obviously makeuse of the external heat input twice, the limit of a maximum theoretical COP h should be 2- instead of 1 as for the single effect system. In practice the COP h is more modest, butvalues approaching and somev,:hat abOVe COP h = 1 can be reached (compare for instanceFigure 4.13a).

417

bt


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E The Platen unters cycle4,15 t\\'o inventorsBaltzar vonPlaten and Carl Munters to develop a

ny solution pump in theCarre ~ s o r p t i o n system, They developed a classicalunit 'withoutanymoving partswhichhastheintriguingcapabilityto performthe taskofmovingheat from alowtemperatureto ahigher- by using (hightemperature) heatasoperatingenergy,The original work begun in the year 1 as a graduation thesis work at theRoyalInstituteof Teclmology(KTH)Stockholm, at thedepartmentwhichtodayisthedivisionofAppliedThermodynamicsandRefrigeration,Lateron,thedevelopmentwascontinuedand commercializedbyElectrolux- andsometimesthe system is called the Electro)uxrefrigerator. This scheme this scheme has attracted many scientists, As a matter ofcuriosity itmight be mentioned that, in 1926, thefamous scientistsAlbert Einstein andLeo Szilard filed a patent on a competing scheme, An interesting recent descriptionaboutthedevelopmentis publishedbyDannen(1997),E1 Principle operation4,16 he secretof thePlaten-Muntersunjtis thatthe (total) pressure is thesametrsoughoutthe whole unit while, in the process the partial pressure of the refrigerant varies indifferent parts of the cycle, The principle of operation is shown in Figure 4.16 (fromBackstrom, 1970), Thesystem permitsthepartial pressureof the refrigerant to beat asuitably low level in the evaporatorto achieve thedesiredlowtemperature for coolingThis is achievedbymeansof an inert gas- athird componentbesides the t , vo mediaused a conventional absorption system. As working media, ammonia is used asrefrigerant, w fter as absorbing medium nd hydrogen is used as an mert gas. Thesefluidswere chosenfrom thebeginningbyPlatenandMunters andno betterhas beenfound later on l C12 CO,idenser

vapcr" '.:;:,;: .: .., 1 removal

E X : : l m p l e ~ Tela pressure i 2 har: , , - + , , ~ _ - T : - ; e n T i O S f D h : J n - p u m p5

0 f20ar t)iguids) \ RiCh

4

!: 0: 0 + f = j 2b r- 0 = 1:: Dcr

1 : taT1. 5 2

4, Schernatic Bdckstrom J9

4:18


19/54

\Vith 4.16 thef o l l o v ~ s . generator driving C l l L U l l ' J H J ; lwater solution. vapor (after passing a water separator) iscondenser where off. From the condenser ~ t r u n o n i a inflo\l/ing (by gravity) evaporator section of the unit. n thecondenser determines the :lvstem. Assuming theto be 30C then will be Il,/; bar (abs) \vhlch is theammonia at that (or about 12 bar as for simplicity IS indicated for the

-

in the lower part of the generator\ \ 'hen they are raising they drag with

level in the generator. The arruTlonia

bet\;veen the absorber

as m a Carre-system. The system isabsorber will flow into the generator

When heat is applied toammonia yapor) is formed in theliquid, which thus is pumped

water solution \vi11 then be able to

stCHAPTER 4 ALTERNA

~ - - - - -

example in Figure 4.16).\Vhen the ammonia liquid comes into the evaporator it is exposed to an ofhydrogen. The partial of ammonia becomes suddenly quite low, this willmake the ammoma evaporate into the hydrogen atmosphere at quite 10Vi ' , , ~ . l H . jLet us assume that the partial of the hydrogen in the first part of the evaporator(where the hydrogen gas is ammonia-free) is 11 bar. This will then apartial pressure for the ammoma of only bar (for the total pressure of 12 bar


20/54

the

are ordinary steel which, for normal temperatures , is compatiblealnmopja (and water). At the temperatures in the generator, however, a corrosioninhibitor is necessary in order to protect,the materials. One effective substance ispotassium chromate, which is added to the water solution (2,8% is reported to be usedSuch an inhibitor has been proven very effective. Units of this type seem to be able tooperate without problems for many decades.In theof'j the Platen Munters refrigerators are limited by similar rules as the Carreabsorption systems for attainable COPo. However in practice COPh in the order of 0,2 toare achieved for the small units which are commonly built ,:vith 25 to 100 W coolingcapacity. It seems to be difficult to build units of the Platen-Munters type for largercapacities.xamples of developments

4,17 Figure 4.17 show the scheme of an Electrolux refrigerator unit working according to theprinciple described in a design as manufactured in recent years.

lOW TEMPER TUREEVAPORATOR

eak ~ ~ ~ n ~ SOhJtfOI".S ~ t : 1 9 ;&r.-ttnon:a seh.ltlo('

_ llQI..i d amrnoni._ AmmQnI2; vaz:.o..ttL nyC roger . as

H ) ' e r ~ g l n .1mmcni.a v < t p ~ v r

Figure 4.17. Scheme o an absorption unit or small refrigerarors Courtesy4 18 figure 4. 18 an advanced scheme is shown a

heat the streams(1968). The figure indicate a design where more or less only ' 'IJ'


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HAPTER 4 ALTERNAITVEthis design a heat

atnrTIorua to the condenser and the two streamspoor water solutions flmving e t w ~ e n the absorber and the generator.exchanger was introduced in modern designs by the Swiss inventor Stierlin, although thefirst patent describing the principle was issued to S A.S. Dahlgren already 1931Swedish patent no 74 727).

large number o improvements on details have been patented. Very good descriptionsare given by Matts Backstrom 1970).

hEAiNPUT

Figure 4.18. Simple schematic diagram a heat driven refrigerating unit Reistad,1968)

4


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F Sorption units o different typesF1 Discontinuous operating units

4. 9 A simple discontinuous operating unit for achieving refrigeration is described already byCarre. The unit is hennetically closed and contains a given amount of refrigerant andabsorbing medium.Figure 4.19 illustrates the operation: In th irst time sequence the charge of the unit isregenerated - heat is applied to so that the refrigerant and absorbent is separated

(Figure 4.19a). In the scheme shown ammonia and water are used. Thus during theregeneration phase ammonia is distilled out of the solution and condensing in the otherpart of the unit. In the next phase refrigeration is achieved by using the liquid arrunoniaas refrigerant. In this phase the ammonia vapor is absorbed in the water solution (Figure419b). Hence the four component present in the C a m ~ s y s t e m can be seen also in thisscheme although it is accomplished by two components shifting function in differenttime sequences (the condenser becomes the evaporator and the generator turns into theabsorber).

Cooling water

Liquid N

a bFigure 4.19. Schematic diagram a/intermittent operating units Backstrom 1970);

Systems of this type have also in recent time been built. One such scheme use forinstance water as refrigerant and a hygroscopic salt (or a zeolite) for absorbing the \vatervapor. Interesting applications may be to utilize solar energy for cooling purposes.Regeneration of the salt (or the zeolite) is achieved by solar heat during the day-and

the refrigeration can be used in the night to create a storage of ice. Figure 4.20 illustratetwo alternatives for such a scheme. The scheme to the left is directly an analogy to figure4.19 but with a zeolite material to adsorb the \vater vapor. There are such units on themarket for refrigerated boxes for use on e.g. sailing boats. The other scheme of Figureis also an intermittent operating unit, but in this case the condenser and theevaporator does not change place. The refrigerant vapor (water) from the evaporator isadsorbed in the zeolite during the adsorption phase, when refrigeration is accomplished

phase can continue until the zeolite is saturated. lJler that it must be regenerated,v;hich is done by heating the zeolite up to relatively temperatures. The vapor

ashe zeolite condenses inon \ as

capacitiesnamed

energy storage system (using a salt (NaS) as abssorptionA


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waIer-vapo:- so veheMen:ry, tt '12tt:-le i e 2 . ~ of e v a p c ~ a t i o r t the ie s: Ce\.iapOianng water freezes to Ice.

Phase a): Adsorption = Refrigeration

Phase a) Phase b)Refrigeration

INare, va;:)Cy' tree: ': c;:)r:censes

hase b): Desorption = zeolite regenerationFigure 4.20. Thefigures to the right, show an example ofa solar powered refrigerator

for instance suitable for a sailing bOff t.j. The figures to the left show analternatIve scheme, y9ith separate heat exchangers as evaporator and condenser.A check valve controls the flow direction of the vapor.

material and \vater as "refrigerant"). In summertime the salt storage is regenerated("dried") by means of for instance solar heat In wintertime, (or vihen there is a heatdemand) water vapor from an evaporator (from for instance in the ground) is absorbed inthe salt. During the absorption process heat is released in the salt, and this heat energycan be used for heating purposes.F2. Open systerns desiccant cooling

421 A.Il Interesting open system for air conditioning was first described Carl Munters andwas called the "Lizzy" system. Figure 4.21 shows the principle. In principle water is therefrigerant in an open system where the ambient air plays a similar role as the inert(hydrogen) in a Platen-r.'1unters refrigerating unit. The cycle involves one drying process,one heat exchange and one h u m i d ~ f y i n g process In figure 1are carried out in a (with a hygroscopic salt or a

in "saturator pads '.

\ ,;'aler apor

ZeO(ite Jedwa:e: v p o ~ out Jf

423

....


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at wet air ,\-vaspad it is even cooler after (in

conditioned space, air" isa Condition (6) has the same enthalpy: lower temperature but

higher humidity than in This low temperature air is now used to cool theexchanger wheel After passing this the "return air" has conditions as indicated in point7 A port ion the is heated (for instance by means of a gas burner) and is used toregenerate drying wheel air leaving the drying wheel to the outside carnesaway also the humidity that was adsorbed from air in the process (1) to (2) indrying wheel.This cycle has attracted considerable interest world \vide and many research projectshave devoted to development of such so called "dessicant cooling" systems.

r WATERII~ ~ \

r .i ;-;::;\3 I 1 c n ~ ~ i j vU ..... v \I I \:::J ) SATHOT I d COOL i I Ai R

Uia:::DRY d ~ : 1 _ 1 f - ~ _ l . . . \ - ~ - i - ~ O - ; - ~ - 8n l ~ I ~ l i l51 QZ. ' I r ,W, :; Iv 1 I\ I cr I ' Z I W ... \WET Z '8 I 0 I HOT WARM ~ c o o ~ i


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G Heat transformersG1. General

4 22 In some applications there are available ample amounts of waste heat at atemperature leveL the same time there is a demand for heat at a higher temperature.The absorption also can, operated in an inverse way, used for such applicationsThis is called an Absorption Heat Transformer (sometimes shonened ART).

illustrated in Figure 4.22a heat is applied at an intermediate temperature level Onepart of the heat input is permitted to fall in temperature and is rejected from the systemat a low level The exergy that can be generated during this process is used to pumpanother part of the waste heat input to a higher temperature level, at which it is deliveredfrom the process. Perhaps it IS easier (for a mechanical engineer) to understand amechanical arrangement of cycles based on vapor compression processes as illustrated InFigure 4.22a. In practice, however, only systems based on an absorption cycle have sofar been built.

Heat ~ p uII..... Heat transformer

Heat t amble;)

echanical analogy

1tT

Figure 4.22a. he pUlpose ofa heat transformer is to "upgrade" waste heat to atemperature level where it can be use for instance in an industrial process.

A scheme a heat transfo rmer system is given in Figure 4 The same componentsare used as in the conventional Carre absorption system, but the tlow is backwards .The operation of each component is hence reversed compared to \.vhat \:ve are used to in

conventional absorption system: generator becomes the absorber, the condenserbecomes the evaporator etc.Let with reference to Figure 4.22b, follow the operation more in detail of a heattransformer based on a reversed Carre process. The system operates at two pressure-- - the generator and the condenser are here at a iow pressure and the evaporatorand the at a higher

is applied to tne generator as well as to the

rnbient

Allother part Qe:)The vapor

b


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v por in meets

is led to the where v por isby means of w ste he t input t intermediate temperature level -- and now weare b ck to the point where we started

Fig dre 4.22b. A heat transformer based on the reversed Carre process.bar

0 2 l ...;..

Fig clre 4.22c, Heat transformer cycle in a t,p-diagramjor H20 LiBrin a conventional bsorption the

gener tor via a heat exchanger and aflow round

ISat a high

temperature

4


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Cli PTER 4 LTERN TTVE CYG ES

Theoreticallyca.'1 be

for4.22c an example. In order to reach about 120 C in the absorber a heat

input of 65 to about 80C is required t the generator and evaporator aIld an fu 1bientlo\\' temperature of about 30::C for heat rejection in the condenser.A limited number of installations have 'been built, especially in Japan (about 15 plantswere in operation 1990 with capacities in the range 500 to 3000 kW). These use 1120LiBr as \vorking fluid. The experiences are good, however the corrOSlon with H20-LiBris presentL.T1g a difficult problem at temperatures of 140CC or higher. In Sweden one heattransformer plant has been built with a capacity about 150 k\V

G2. Cold transformers4.23 A similar function but at different temperamre levels is also possible to arrange order

to "multiply" a given amount of "cold" As proposed by 0:olken


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Alcohol COriee

I-SO r +20 t C

Figure 4.23. Cold transformer as proposed Nolken and lvfaiuri (fromBackstrom 1970). The p,t-diagram to the right indicates schematically the cycle.Notice that the pressure indicated is the partiai pressure of the ammoma) thetotal pressure is constant as in the Platen \funters scheme.

Thus for each cycle the scheme indicates that the refrigerant (ammonia) is evaporatedtwice (first in FI and then n F2 ) at temperatures where we want to create cold

it is absorbed once in A ) and condensed once (in K ). In this way it is possibleto multiply the energy released when the CO2-ice is sublimating at thetemperature - and \-'ie can achieve twice as much cold at an intenuediatetemperatureIt is not known to the author if this scheme has used in practice- itmight some ideas as to possibilities to use temperature energy in an effective, decreasing the irreversibility of a process - and saving costs. .

4


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t

H Ejector cyclesH1 General

4 4 another heat operated cycle for refrigeration or comfort cooling isby means of an ejector cycle This type of system has however, not

wide use, in of considerable development efforts. However, for smalltifts", should be possibilities'for interesting applications in practice, wherelow heat is available at lov cost.

principle is indicated in Figure 4.24. The cycle is very similar to the one used in aconventional compression refrigerating but the compressor is nmv in thefonn of an ejector. In this device the vapor from the Imv temperature evaporator is'induced' into a high velocity vapor stream. The velocity oftne fluid must be highenough so that the deceleration in the diffuser after the mixing section mcrease

from evaporator to condenser leveL

Generator

Condenser

vaporafor

Figure 4 Scheme oj nejector air conditioning or refrigerationdriving fluid operating the ejector is high vapor in the

The ejector same timction as an arrangement of a turbine driving a compressorsuch a the turbine is driven by high vapor, and pmver

generated the turbine is used to drive the compressor. vapor at exit of theturbine as well as compressor flows to a common condenser. With this as a

an ejector efficiency as the productIt seems as the

4


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proposed as medium,as

good performance. Normally same fluid is used in both cycies - theweIl as refrigeratring cycle - but proposals are to use

different fluids. the ejector cycle it appears that it would be advantageous to use afluid with a higher molecular weight in the power cycle than in the cooling cycle.

s ex mple can be mentioned a patent by Neumann and Lustwerk proposing to usemercury in the power cycle and water in the COOlLl g cycle. In the condenser these t\Vmedia w I be easy to separate since they do not mix in liquid form. Of course to usemercury in a common machine would not be permitted today, due to tile healilihazards wlth mercury.

H2 Simple theory for ejectors4.25 A few simple equations for a first degree analysis can be established to describe the

phenomenon in an ejector. For a more in depth analysis reference is made to thetreatment by Wahl (1966).igure 4.25 gives a scheme of pressures prevailing in an ejector. The operating vapor isexpanding in the nozzle from the high pressure in the generator to a pressure lower than

that in the evaporator. This makes it possible to entrain the vapor from the generator intothe high velocity stream. The entrainment is occurring in the mixing section of the ejectorafter which the t\V streams are assumed to be perfectly mixed_ The stream of iscompressed to condenser pressure in the following diffuser section where the streams aredecelerated

T

Vapo of l Wpressure. p2

m n the right re

:0, 2

s

t.


31/54


32/54

Entrainment ratio versus enthaply ratioin the

th , , ' rT fH theas

4.

~ - - - - j . - + I - - M I - y L - - - , , , , - - + - / - - - + - m ~ g o nosilr Jil7f.l /,

Ross taflt

o 8

Wahl 1966to In used here similarly G


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successfi.tliy Figure 4.27

systems are assumed tolines } Three sets0 I and 0,2 SEK/(KV/h,

...

-I Comments o the economy o heat operated systems versuselectrically driven

for an absorption system is lower than the COP for a vaporprice of the heat energy used operation must be lower thanmechanical) energy in order absorption system to have arelations that mmt prevail

for heating and electric energy for achieving a situation where thewill be equal for the two systems. The data shmvn are based on assumptions

for the n.evo systems a indicated.Of the diagram it can be seen that the potential for the absorption systems is somewhat

for low temperature applications. The reason for this is as has beenmentioned - that the COP2 of a compression system is decreasing more rapidly when theevaporating temperature is lowered than the COPh for absorption ' ,,' ,, ''''

0,5lJ..J 0,4C/):::;2 0.3Q)r --)

...... 0,2ct:iQ).c: 0,10Q)lt : .Q

;

0 0,5 1, 1 2Price o electricity SEKlk1lv17

Figure 4 2 . Approximate relations betweenwhich operating costs arecompreSS IOn systems. Elr?ctric operatedandCOP2h 1parameter

Thea correcr econorruc compansGn one [nust not

a


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more and more interest in order to avoid the peak costmay be

where peakhigh.

Relarions: Fora simple economic comparison let us consider the total annual cost,of a refrigerating system with a given cooling capacity 2 which can be written.

demand

\vhere a annuity of investment, interest and depreciation, [1 investment [SEK]r= am Ual operating time [h/year]

price ofoperating energy input [SEKlkWh]COP; Coefficient of PerformanceIe sum of constant costs of system operation (maintenance etc (SEK]

Equivalent expressions can be set up for different types of systemsTo find conditions for equal costs of heat operated systems (index and electricallyoperated (index er) we can write:

4.27bwhich can be simplified to the follmving conditions in terms oj energy prices for havingthe total cost of a heat operated system lower than that of a electrically operated:

- 2ith x afln-where t IS assumed that the 02 a and a r e equal for both types of systems. Theparameter X signifies the difference in capital and maintenance costs bet\veen an electricand a heat operated system in SEKlk\vn cooling capacity.The diagram in Figure 4.27 exemplifies the influence of the parameter X (the values0 0,1 and 0,2 are used The curves for x 0 correspond to a case where the investmentand fixed costs for the operation are equaL


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PRiME MOVERS

J Expansion cyclesJ1 General

4 28 different types cycles exist using expansion of a as the phenomenon for achieving low temperatures. Of a speciai interest is course to use air as aworking substance, since this can permit cycle to be open to the atmosphere also potentially make it possible to bmit one the heat exchangers the cycleT\vo cycles will especiaily be mentioned in this chapter the Joule or louIe-Braytoncycle (which with minor modifications also is equivalent to the Ericsson cycle) and theStirling cycle.All these cycles their counterparts as power generating cycles. The Joule cycle canbe characterized as a reversed gas turbine cycle. For refrigerating applications it issometimes called a cold air cycle n . Similarly the Stirling cycle can be as an

However its applications refrigerating engineering for cryogenic applicationsactually developed into a much larger market than engine-applications for Stirling

machines. Stirling refrigerator is suited for large temperature hence forlow

Generally the expansion have more successfhl handling large temperaturethan vapor compression cycles in its different forms. An illustration to this is shown

in Figure 41.0 ---------------------------------------

u0:-

ruz 0.5wQ RANKINE 6::w HZ I TEMPERATUREI AT WHICH HEAT\ l is REJE::TED0Z /::


36/54

4

J2 The oule cyclethis case

Coder md \ L;J c

machine Compressor,- E

air has a

a simple arrangement a Joule cycle a cold airigure 4.29.The Joule cycle consists theoretically four processes; t\..vo(compression expansion) and two' (cooling and heating)

.be described as follows \vith reference to the in Figure 429:

to be cooled is sucked into the compressor. l\fier compressor thepressure, but also a higher temperature. In the heat (cooler)

heat is from the high pressure air to an exiemal heat sink (i.e. air). Theprocess air IS expanded in the expansion machine (or turbine '') to pressure in thespace to and internal energy of air in this expansion is transferredto work and the air temperature drops. work extracted is used to supplement thedrive of the compressor. The net power to operate the system is the betweenthe power the compressor and theThe cycle is attractive several points of view. Of course is a greatadvantage to use air as working substance in an open arrangement so the air ofspace to cooled can be used in the For an ideal, case the COP ofcycle is identical to that of a cycle (see point 2.13 . , the COP dropsrapidly as soon as the irreversibilities of practical machinery are introduced Tl-lis is easyto understand if we consider that the net work to operate process being the

between the work of and that extracted in expansion. The workin the turbine are proportional to the gas volume

to absolute

on th same If:stanceinternal heat

:3


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pressureusing thisits inlet to the

USl g a heat eXCha lger r l p r lU F F ncoid the compressor Inlet to

A scheme of this type is shovm inIt is also possible to design the system with a closed loop for such a case the room

cold figure 4.29 would replaced \vith a heat exchangeranoLl-Jer medium on the 10\,,/ temperature side Closing the loop make it possible to

the pressure level (by higher pressures volume flow rates can be decreased)and to use other gases than air (e.g. \\1th better heat transfer properties)

430 Figure 4.30 shows a schematic s,T-diagram ofthe simple cycle, \vhere the ideal cycle isrepresented by the dotted lines ofisentropic compression and expansion.

Heatsource

Figure 4. The simple louIecycle in a s T-diagram Entropy s

Thefollowing equations can be derived as a basis for further analysis ofthe cycle:capacity of the cycle ishem. Cr' (T T 4.30ar D "

where m S the mass flow ofair in the cycle= thespecific heat ofthe air

1 0 and are temperatures ofthe cycle as shown inNoticethat temperature can be determined if'iNe knov, 1d and the power extractedin the expansion machine, as:

m

ofthe compressor asIS

Hl

43

...


38/54


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4 ALTEP,NA T/VE CYCLES

3,02 52 0

0JQo 1 5)

1 00,50 0

- -1 00- -0 9 8-L'-0,950 90---... ..-0,85- - 0 8 0-0-0 75- - 0 7 01 2 3 4 5 6

Pressure ratio p1/p2

Figure 43Gb. Estimated Coefficient of Performance, COP2 a simpleoperating with K J,4 and for temperctures to 3 G ~ C and td 20 GC, seedefinitions in figure 4.30a. Curves are given Jor different isentropic efficiencies thecompressor and the expansion machine, assumed to equal, 7K


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The HiIsch tube givethe expansion was done in a

EVGLVEERING

J3 The Hilsch tube / Ranque vortex tube4J 1 unconventional method

it isycle - should also bemore correct to name it the vortex tube for expanding a one can separatea stream of compressed gas into one hot and one cold stream a device withoutany moving parts I This sounds and when the principle \vas presented by

in 1933 many were skeptical to the phenomena However the scientistdemonstrated a working model during the second world war. scheme of

a is shown in Figure 4is furpjshed to a radial in a tube and the flow will create a strong

vortex in the tube. t is shmvn that the temperature

it is easy to achieve- woe on the

in the center is theperiphery In the vortex tube cold alr the center of this vortex ISthrough an orifice \'lhile the rest of the air exits the other way, as shown. With

air (typically 7 bar and aton the hot side

the efficiency ofth the processestemperature drop that

turbine, extracting work fromtube has come to use special applications, such as local

cool ing objects, especially' in an environment where there is pressurizedavailable at low cost Other examples of application are for cooling suits ofhot environments (like in a steel mill) or for the cabin of a largemachine latter is a simple but energy AC- unit

Compr .ssed ai rIt

NPUT

l Liflque vOrtex

4


41/54

:; ALTERNA.

J4 The Gifford McMahon refrigerator4 a special be mentioned, at extremelyThe Gifford-McManon refrigerator is a unit developed coolinginfrared detectors This unit is actually an expansion device for a pressurized gas ) ;-ith a

built in regenerator arrangement so that very low temperatures can be achieved I asimple way. A scheme such an expansion device is shO\vn in Figure 4.32.

The unit consists of a relatively long thinwalled cyiinder sleeve (1) forming acold end (sometimes called a coldfinger ) Inside the cylinder is a piston(5) with a regenerator (2), throughwhich gas can enter and exit thecylinder. The unit operates as follO\vs:Let us foIl 0 ,v the operation starting in aposition when the space in the cylinder isfilled with gas of a high pressure andwhen the piston is at a top position.Inlet and outlet valves are closed. \Vhenthe piston moves down in the cylinderthe gas 1S expanding, and thetemperature the gas drops. achosen position piston the exitvalve opens and gas flm.vs out (externalpressure is lower than that in thecylinder). The temperature of thehas dropped during its expansion. Thecylinder is in the next step emptiedwhen the piston is moving up to itstop position in the cylinder In theFigure 4.32. Schematlc of a Gifford- next phase, just before the piston hasA1dviahon expansion unit reached its top position, the inlet valveCold en opens and the high pressure2 Regenerator into the cvlinder through the regenerator- -' 'through piston matrix The material in the regeneratorInlet valve wiil now pre-cool the gas during itsvalve pa3sage to the top the cylinder A) Piston with on warm end new cycle starts and the processes arerepeated.

3

room temperature and

:41

b


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4

j

The cycle into acompressIon

320

280

240

200

160

12Q

80

J5. The Linde nd Claude cycles for liquefaction ofgases.the of air. It is based on so

effect as a consequence certam circumstances)the process of throttling a from a high pressure to a lower will causetemperamre the gas to drop. An (irreversible) throttling process is as we kI10Wthennodynamics; characterized by the fact that the enthalpies; h of the fluid before

the process are the same For a perfect idea! the enthalpy is independent of thepressure and for such cases the Thomson-Joule is zero. However, for real gases atr.igh pressures the enthalpy is a function of as can be seenFigure 4 33 giving a h,T-diagram for air.

Example: Consider throttling a stream of air from a state 200 bar and 27(300 K) down to atmospheric pressures l bar). From Figure 4.33 we seefollowing a line of equal enthalpy from inlet to outlet conditions, 'Nil upwith an outlet temperature about 35C lower or about 265K). This is theeffect that is utilized in the Linde-cycle.

T K

100 200 350h

Figure 4 33 Enthalpy-Temperaturediagram air ]rom Pierre

Figure 4 34simple

.__ ~ ~ 0 1 1


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larger isas can be seen in the diagra;m of in steady state operation

air at point reaches the wet region a certain fraction of the air of condition c') willbe in liquid state. liquid can be drained off from the vessel as indicated in Figure434.

4.35 interesting figure of merit IS what fraction of the compressed air that can be drainedoff in liquid form Let us use the symbol y for tris ratio. simple energy balance forstationary conditions will give the necessary reiations to estimate the fraction yEnclosing the heat exchanger, the throttiing device and the vessel in theconsidered we can write:

nJ:ha = mch: ni mc)hd 435a'Nhere ill is the mass flow through the compressor and

n1c is the air mass flow drained offin liquid fonnha hd and he are the enthalpies in points "a", "d" and

Introducing y = me/Ii] we find from 5.3 5a thathd - hY= 4,35bs hd he

an ideal isothermal compression at ambient temperature the compression isby the following equation (the exergy difference of the air after and before the

compressor):4.35c

and the work to condense one mass unit of air, Eri/ y Iil), can now be estimated eq4.35b and c.t is obvious that the compressor work per mass unit of air is a function of the pressure

ratio, Pc/Pc. \Vith po ; ; 1 bar, experience indicate that p 200 bar is close to optimal4 36 relations are sufficient to estimate how much air that has to compressed andthe work needed to produce 1 kg ofliquid air.Example: -/ fu 1 ideal b = 200 bar high pressure n ambient

conditions pc produce y liquid aIr kgcompressed air and 7 ideal (isothermal)work per kg liquid air which is equivalent to 1,44 kV/hlkg .

See 9.24

4

b


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components) for(with

example 170 k:."hn/kg). t15 obvious thatimprovements.

is a large potential for

7 Improvements of the Linde-cycle involves closing thecycle using a two stage throttling ("high pressurecirculation") with pre-cooling by a conventionalrefrigerating machine at a mid section of the heatexchanger. A process scheme such a cycle is givenin Figure 4.37. By such a the energy demand perkg liquid air can be decreased to about 113 of that forthe simple scheme data fonn Pierre, 1982)

438 main irreversibility of the Linde-cycle is thethrottling of gas with large pressure differentials. t\vould natural to instead use an expansion turbineas in a Joule-cycle, The expansion of a to

low tempera tures required for liquef'):ing theho\vever practical problems several

reasons - which of course was the background to thesuccess of the simple Linde-cycle

arises

Conven!;onalrefn -geratlngur t

Figure 4.37Arnproved cycle 11mthrottling and pre cooling

the Claude process a combination of an expansionmachine (as in the Joule cycle) and a throttling (as inthe Linde cycle) is used A scheme of the Claudecycle is shown in Figure 4.38. A typical high pressureis about 40 bar, hence considerably lower than for theLinde-process (Still another cycle, the ClaudeHeylandt process, use about the same pressures asLinde-process). Originally Claude pistonexpansion machines - which he with ingenuitymanaged to to function well even at extremely lowtemperatures Kapitza, in a development first described

used turbines for the expansion of the air.is the prevaiiing technique large plants.

performance of components

perthe Linde process

4.

... ... ---- _ _.....


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p

has systems capable to achieve extremely temperatures: :>UI.UUv capacities for i q u e f Y i n ~ Hydrogen as weB as Helium. Expansion machines

are of type, but arranged Nith long "sleeves" so that the expansion takes place inthe "cold" end a long cylinder, with seals on the warm side (compare GiffordMacMahon principle, Figure 4.32).J6 he Stirling cycle

4.39 The Stirling cycle has been paid a large attention and interest by scientists and use forengines as well as for low temperature refrigeration has been studied extensively. Anumber of different schemes have been explored for different purposes and applications,electrically driven as well as heat driven. We will here only very briefly touch upon theprinciple of operation and the use as a cryogenic refrigerator. he interested reader isreferred to Lundqvist (1993).A few of the different possible mechanical arrangements are shown in igure 4. (fromLundqvist, 1993). The working medium is a gas, normally at an elevated pressure

is enclosed in a madline comprising pistons for volume changes and heat exchangersfor transferring heat into and out of the gas. An important heat exchanging device is alsothe regenerator by which an internal heat exchange and heat storage between phasesof the cycle takes place.

H HeaterC , Cooler

!-crm Beta FormP;stcnwin PistonStirling engines r refrigerators

II

Gar 1cna Ferrn~ s t o r -

mwhen the unit continues to rotate m the same

take heat the heat exchanger temperature onmakes the to operate as an engine the temperature is not


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the heat exchanger indicated

anin the

now pump heaUrom aand reject it at a temperature439 It is thus operating as a refrigerating unitThe cycle of an ideal Stirling cycle consist offourbasic processes:

1 isothermal expansion2 heating the gas in the cycle at constant volume3. isothennal compression4. cooling the in the at constam volume.

The heat exchange during the processes 2 and 4 are perfonned by means of aregenerator in which heat energy is stored over cycle External heat is exchanged toand from the gas in the process 1 and 3 In Figure 4.40 a schematic description isstep by The cylinder arrangement is simplified. The working is enclosed in the

between two opposing pistons and with a regenerator matrix in the middle

2. Heating the gas in the at constantv

Isothermal expansion PIston P 1 moves to volume Both pistons PI and P2 movesthe and the enclosed gas expands. Heat simultaneously and the gas is transferredQ (refrigeration) IS applied so that the gas through the regenerator matrix (where it ismaintains its temperature heated)

v4Piston P2 moves to

and l1eejecred at

at constant

n40 Jllustrations rom


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Small refrigerators have also been developed for other spec:ial purposes. TheA,mencan company Sunpower has developed a design based on pistons driven by aelectric motor and with a free oscillating displacer in order to avoid complicatedmechamcal arrangements and lubrication. Figure 4.42b shows an externai view such adesign and also some performance data.

Figure 4.42a Design a 5 tirling refrigerator y Philips for liquefying air (CourtesyPhilips Company)1: Pistons; 4: Compression space; 5: EX'Pansion space; 13: Water cooled Heat Exchanger;14: 15. Cold side heat exchanger ith fins (18) for condensing 16+17for collecting 20 Outlet of air.U J : V J , , ~ I ; O J

00'111

4

P


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4 ALTERNA

K lectric and magnetic cyclesK1. eneral

4.43 Two phenomena possible to utilize for refrigeration were mentioned in Chapter 2Peltier effect and phenomena in lvfagnetic processes. The latter one of these has itsapplications in extremely low temperatures, although some experiments have been donerecently also for more moderate temperature levels using extremely strong magneticfields (requiring supraconducting magnets). We v :i11 not deal more with the magneticprocesses in this tex1The Peltier process, however, has merits for normal temperatures and it has foundpractical applications in special niches. One of its advantages is that it has no movingparts. The development of sern.jconductors, especially in the 60ties gave great hopes for abreakthrough for the technology, but it seems as if there are lirn.jts for the possibilities totailor the properties of semiconductors, and further improvement seem difficult toachieve A short description and simple relations will be given.K2.. The Peltier process

4.44 In a thennocouple there is, as we know, an electric potential difference if the cold" andthe hot Junctions of the thennocouples are at different temperatures. Peltier, a Frenchwatchmaker detected (already 1834) that also the reverse phenomenon occurs; if anelectric current is forced through the conductors the junctions v ill strive for differenttemperatures This is the so called Peltier effect. Figure 4.44 gives a schematic picture ofhow a Peltier refrigerator can be arranged. The perfoffilance of a Peltier cooler is as wewill see, strongly depending on the properties of the materials of the Peltier pairs,indicated N and P in the figure.

~ e a r2iecting surface Hot side

eat absorbing sLr:ace

4 1 2 3 4.

t


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IlN ENGIA EERING

in a thermocouple a differenceThe Seebeck coefficient the dimension VoltiK (in SI-

units).Materials suitableforPeltierelementsshouldpossess a high Seebeck coefficient butit iseasytounderstandthattheyalso must have

alow thermal conductivity, } inordertoavoidlargelossesduetoheat conductionbetvveenthehotand cold

a low electric resistance, Pei in orderto avoidlarge electric losses or in otherwords theyshouldbe good electric conductors.

Most electric conductingmaterials are also good thermalconductors, and versa(which for pure metals follows from theWiedeman-Franzlaw, and ina more generalform by the Lorenz' law (from 1881), which states that 1, Pel is proportional to theabsolute temperature, T. It is hence difficult to findsuitable materials. Those selectedarealwayscompromisesand themost interestingmatenalsaresemiconductors. It canbeshown a, 1 import.aIlt paran1eter how well amateria! meets thedemandsfora Peltier element is theso calledJoffes parameter orJoffes number after aRussianscientist. The maximum achievableCOP (later thetextdenoted of aPeltierelement that canbeobtainedis afunctionof theJoffesnumber.It isdefi.l1ed as

Z 4.45aRKwhere e istheSeebeckcoefficientof pair[InSl-.:units: voltslK or ];

R istheelectricresistanceof apair[ohm,n=volt/ampere] andKis theconductivityof apair[W/C].

Inserting thedimenSIOns of e R and K (noticingthatvolt ampere isequivalenttom Tf) we findthat'ZhasthedimensionofThe most interesting materials for use in Peltier coolers arc, as already mentioned,semiconductors. For useat normal temperatures (aroundroomtemperature)thesearebased on Bi2Te3 with small additions (of Br and eu order to have the matenal doped so that and "P"-materialsare formed, suitableforabeen possible to develop materials ""'ith in theThis was reachedin themid 60-ties and Il spite of considerable research efforts one

'10.3

have not been able to increase the Z-value more than to 41 [1scientistsbelievethatthereisatheoreticallimit that1.5 l/Tm (with rn denoting the average benveen the hot and cold side absolute

1to

It can current

4:


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IS can be

possiblein order tom would compete successfJlly with regular vapor compression

systems in tenus energy efficiency compare Figure 2.1 6). This is at least six timeshigher Z-values than what the best available materials have today.

5

OA

0.3Q.0UtUG 0.2

0.1

0.0

----t11t2=40/ '-10Q C i ~ ~ ~0 4 0 / 0 Ct t 40/-1 0 C

0 0.005 001 0.015 002 0025Assumed value for Z, 1f C

4 0

3 000C \I 2 0Q00

1 0

0 00 0 005 0 01 0 015 o 0 025

Asumed value for Z,

51


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thermal, ,v11uu'v.

REFPJGERATlNG ENGINEERING

4.46 and np l c tn r rn

L'-'UCUu' we assume and P thesame properties in tem1S

electric resisitvity: p [0.m]them1al conductivity: I. [ff?(m'oC)]a...'1d together they "Vi ill form a thermocouple with theSeebeck coefficient: e

We assume that the bars of the thermocouple each have an cross sectional area of A[m 2] a.'1d a length of L [m]. Hence for each pair (of two bars) we have:

The electric resistance: R=2pL/A [D.} 4.46aTotal thennal conductance: K 2 AlL [W;CCj 4.46bWe can here also estimate Joffe number from Z = K) = 4.46c

A set of n thermo-pairs operating betvieen temperatures and Tj and with anelectric current of I will require:

Electric power input, [W]: E n R ] 4.46dwhere the first term in the parenthesis is the necessary to overcomepotential difference due to the Seebeck effect for one and the second oneis that due to the electric resistance.

Cooling capacity, [\V]:Seebeck effect for one pair, the second one is the heat caused by the electricresistance in the thermopairs of which half is conducted to the cold side, andfinally the third term is the loss caused bv the thermal conduction

Finally the heating capacity" \\'ill be4.46ff E h =nlelTJ f

and the COPz of the unit operating as a refrigerator is achieved as021E

The current, I, can be chosen and the capacity as well as the P l ISdepending on which value is used. The dependence is illustrated in 4.46.Nerice that if a material could be found with R 1 0 and K 1 0 the theelectnc

bet:xeen the ternperatures Tl andalSO nave

4.46ewhere the first term in the parenthesis is the ideal capacity due to the


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CHAPTER 4 TERNA TlV CYCLES=+40C;

L =O -Q05 .AlL '2 =1

C \iQ0u

10.09.0 t

1.00.80.60.40.20.0

8.07 06.05.04.03.02.01 00.0

0

o

10

10 20 30Electric current, I A

t1 = +40 c C;L=O,005 m; AlL 2=1

20 30Electric

40 50

4 50

Figure 4 46 Performance pair for an example vith hotavailable materials

(giving Z 0COP2 is


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References,F.

Altenkirch, E.: Absorptionskaltemaschinen, (VEB-Vedag) Berlin, 1954.Backstrom, M.: "Kylteknikern", p 795 - 805, 848 - 849, Stockholm, 1970.Cheung, K. et a1: "Perfonnance assessment of multistage absorption cycles",

International Journal of Refrigeration, Vol 19, No 7, P 473 -481,1996.Dunnen, G.: "The Einstein-Szilard Refrigerators", Scientific A.J. nerican, p 74-79,

January, 1997Electrolux - ServelInc.: The miracle of Ice from Heat" Brochure, New York 1Ekroth, 1 and Gra.. lIYd, : Tillampad tennodynamik, Institutionen for Energiteknik,

KTH, Stockholm, 1994Lundqvist, P.: , Doctoral AppliedThennodynamics and Refrigeration, KTH Stockholm, 1993.Meacham, H.C. et a1: Status the Double Effect Absorption Pump (DEARP),

lEA Heat Pump Conference, 1990Niebergall, W: Sorptions-Kiiltemaschinen, Ha..'ldboeh der Kaltetechnik Bd VII, Berlin

1959.Nordgren, M. a. ld Setterwall, . e x p e r i m e n t ~ l s t u y of the effects surfactant on a

flalling liquid film", International Journal ofRefrigeration, Vol 19, No 5, P 310316, 1996.

Pierre, B.:Kylteknik, allman kuTS, Instituttionen for Mekanisk viinneteori oeh kylteknik,KTH, Stockholm 1982

Reistad, B.: "Thenna1 conditions in heat driven refrigerating units for domesticKylteknisk Tidskrift, p 43, 1968.

Summerer , : "Evaluation of absorption cy-cles with respect toInternational Journal 19, No 1, p I - 24,1996"

Wahl, L: "Dampfstrahl-Kaltemaschinen", Handbuch der KaItetechnik band , p432, Berlin, 1966, F. and Grossmann G.: transfer additives", InternationalJournal Refrigeration, Vol 19, No 5, P 301 309,1996.

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