Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Refrigerants 91

In the 400 series, R-401A and R-401B are acceptable as substitutes for R-11, R-12, R-500, and

R-502 in the following end uses:

• new and retroﬁtted reciprocating chillers,

• new industrial process refrigeration,

• new cold storage warehouses,

• new refrigerated transport,

• new retail food refrigeration,

• new commercial ice machines,

• new vending machines,

• new water coolers,

• new air coolers,

• new household refrigerators,

• new household freezers, and

• new residential dehumidiﬁers.

R-401A and R-401B appear to be acceptable as a substitute for R-400 ((60/40)% by weight)

and R-114 in retroﬁtted industrial process air conditioning. Note that different temperature regimes

may affect the applicability of the above substitutes within these end uses.

R-404A is acceptable as a substitute for R-12 in new household refrigerators. None of this

blend’s constituents contains chlorine, and thus this blend poses no threat to stratospheric ozone.

However, R-125 and R-143a have very high GWPs, and the GWP of HFC-134a is somewhat high. In

addition, Pearson (1991) suggested R-410A to replace R-22 in high-pressure unitary air-conditioning

applications.

2.9.4 Azeotropic Mixtures

During the past several years, a number of new azeotropic refrigerant mixtures have been proposed

as substitutes to replace harmful CFCs, for example, R-22 in air-conditioning systems and R-12,

R-22, and R-502 in household and industrial refrigeration systems. Table 2.6 lists some common

azeotropic refrigeration mixtures in the 500 series. Among these, R-507 is acceptable as a substitute

for R-12 in new household refrigerators and for R-502 in cold store plants. It is an azeotropic blend

by weight of 53% R-125 and 47% R-134a at atmospheric pressure. At lower temperatures, the

azeotropic range of the blend may vary from 40 to 60% R-134a by weight (Konig, 1996). Note

Table 2. 6 Some common azeotropic mixtures in the 500 series.

R-number Substances GWP Concentration (weight %)

R-502 CFC-115/HCFC-22 5,576 51/49

R-507 HFC-143a/HFC-125 3,300 50/50

R-508A HFC-23/PFC-116 10,175 39/61

R-508B HFC-23/PFC-116 10,350 46/54

R-509A HCFC-22/PFC-218 4,668 44/56

Source: Pederson (2001).

92 Refrigeration Systems and Applications

that neither R-125 nor R-134a contains any chlorine, and ester oils can be used for lubrication.

In terms of material compatibility (i.e., corrosiveness) with seals and metals including copper and

aluminum, R-507 is comparable to R-502 and retroﬁtting R-502 refrigeration plants for R-507

can easily be carried out in accordance with the already familiar procedures for conversion from

R-12 to R-134a. None of this blend’s constituents contains chlorine, and thus this blend poses no

threat to stratospheric ozone. However, R-125 and R-143a have very high GWPs. In many countries,

recycling and reclamation of this blend is strongly recommended to reduce its direct global warming

impact. Although R-143a is ﬂammable, the blend is not. Leak testing has demonstrated that its

composition never becomes ﬂammable.

It is important to mention that substituting HFC refrigerants for CFC (or HCFC) refrigerants in

household refrigerator and freezers, auto air conditioners, and residential air-conditioning systems

with compressors located below the evaporator and no liquid receivers has a high probability of

success with the original mineral or alkyl benzene oil (Kramer, 1999). The application of HFC

refrigerants to receiver-based refrigeration systems and systems with suction risers has the greatest

probability of success where reduced viscosity alkyl benzene or mineral oils are used along with

effective oil separators, with the suction riser design implementing appropriate refrigerant vapor

velocity.

2.9.5 Ammonia (R-717)

This has been the most widely used one among the classic alternative refrigerants. Two character-

istics of R-717, the saturation pressure–temperature relationship and the volume ﬂow rate per unit

refrigeration capacity, are quite similar to those of R-22 and R-502. On the other hand, R-717 has

some advantages over R-22 and R-502, such as lower cost, better cycle efﬁciency, higher heat-

transfer coefﬁcients, higher critical temperature, greater detectability in the event of leaks, lower

liquid pumping costs for liquid recirculation systems, more tolerance of water contamination, more

favorable behavior with oil, zero ODP and GWP, and smaller refrigerant piping.

After 120 years of extensive usage, a tremendous amount of practical experience exists with

this refrigerant. There is no doubt about its excellent thermodynamic and transfer properties, much

superior to those of the halocarbons, and its important practical advantages such as tolerance to

normal lubricating oils and limited pollution with water, easy leak detection, and low price. All these

factors contribute to its sustained popularity and wide application, in spite of the often expressed

doubts about its safety.

It is true that ammonia is poisonous and can burn with air, although it is quite difﬁcult to ignite

and will hardly sustain a ﬂame by itself. The risk is strongly counteracted by the fact that it has

an extremely strong odor and that it is much lighter than air. A leak is easily detected by smell at

a concentration far below a dangerous level, and a massive escape of ammonia rapidly disappears

upward in the atmosphere. Accidents are therefore extremely rare.

It has not been possible to ﬁnd any reliable statistics on refrigeration accidents. Serious cases

are reported from time to time, and it is the impression that they are fairly evenly divided between

ammonia and halocarbon plant (Lorentzen, 1993). A recent effort to survey the situation in Norway

has brought to attention a considerable number of incidents, some of them quite serious. Over a 20-

year period, altogether four people were killed by refrigerants, two each by ammonia and R-22. One

further died in a nitrogen-cooled truck. Great caution and sound professionalism are clearly needed

in design, erection, and operation of large refrigeration systems, regardless of the refrigerant used.

Whenever there is a need to avoid any risk of ammonia smell, this can easily be arranged by

enclosing the plant in a reasonably gastight room or casing, absorbing any fumes in a water spray

or venting them over the roof in a safe place. A secondary refrigerant (brine) has to be used for

distribution outside the enclosure. In this way it is possible to use this excellent medium safely in

nearly all practical applications.

Refrigerants 93

Ammonia has been used successfully for generations and has demonstrated its reliability and

superior thermodynamic and transport properties. It can be applied to advantage over practically

the whole ﬁeld of medium and large refrigeration systems, when the necessary safety requirements

are taken care of by simple and obvious means. The total consumption for this purpose will only

be a tiny fraction of the ammonia production for other uses.

2.9.6 Propane (R-290)

Propane has been used as a working medium in large refrigeration plants for many years, notably

in the petrochemical processing industry. It has excellent thermodynamic properties approaching

those of ammonia, but the explosion and ﬁre hazard are much more severe. This will certainly

limit its application in the normal refrigeration ﬁeld, although the risk should not be overesti-

mated. Combustible gases are commonplace in many technical applications and do not cause many

problems when simple precautions are observed. Propane has a special advantage for use in turbo

compressors because of its near ideal molar mass.

Another area where propane or mixtures of HCs may have a future is as a substitute for R-12 in

household refrigerators and freezers, and also perhaps in small air-conditioning units. The charge

in a normal refrigerator need not be more than about 50 g, propane being a much lighter ﬂuid than

R-12, and this is less than what may be used as the drive gas in a hair spray. Recent extensive

studies have testiﬁed that the risk involved is negligible (Lorentzen, 1993).

The few hundred grams needed in a small air conditioner would also seem to be perfectly

acceptable with proper design. A propane–butane mixture may be advantageous in order to achieve

a temperature glide to match the limited air volume ﬂow of the evaporator and condenser.

In a study undertaken by James and Missenden (1992), the implications of using propane in

domestic refrigerators were examined in relation to energy consumption, compressor lubrication,

costs, availability, environmental factors, and safety, and compared with the results obtained for

R-12, R-22, and R-134a. From the experiments they found that propane can substitute for R-12 with

a similar performance at a lower charge and concluded that propane is an attractive and environ-

mentally friendly alternative to ODSs. Although propane is very stable and meets the refrigeration

requirements (e.g., COP, pressure ratio, comparative discharge, etc.) easily, the only concern is its

reactive characteristics such as combustion and halogenation.

2.9.7 CO

(R-744)

Carbon dioxide was a commonly used refrigerant from the late nineteenth and well into the twentieth

century. Owing to its complete harmlessness it was the generally preferred choice for usage onboard

ships, while ammonia was more common in stationary applications. With the advent of the “Freons”

and R-12 in the ﬁrst place, CO

was rapidly abandoned, and it has nearly been forgotten in the

course of the last 40–50 years. The main reasons for this development were certainly the rapid

loss of capacity at high cooling water temperatures in the tropics, and, not the least, the failure of

the manufacturers to follow modern trends in compressor design toward more compact and price-

effective high-speed types. Time is now ripe for a reassessment of this refrigerant for application

with present-day technology.

is naturally present everywhere in our environment. Air contains about 0.35 parts per

thousand of it, in total nearly 300 billion tons for the whole atmosphere, and several hundred

billion tons per year circulate in the living biosphere. No complicated and time-consuming research

is needed to ascertain its complete harmlessness.

One may possibly object that CO

is also a greenhouse gas, and this is of course correct, although

its GWP is deﬁned as 1, and the GWPs of other refrigerants are indexed to it. But in reality, gas will

94 Refrigeration Systems and Applications

be used, which is already available as a waste product in unlimited quantity from other activities.

What we do is just to postpone its release. This is in principle good for the environment, like

planting a tree to bind carbon for a period of time.

With regard to personal safety, CO

is at least as good as the best of halocarbons. It is nontoxic

and incombustible, of course. On release from the liquid form about half will evaporate while the

remainder becomes solid in the form of snow and can be removed with broom and dustpan, or

just left to sublimate. Most people are already familiar with the handling of “dry ice.” In the case

of accidental loss of a large quantity, a good ventilation system is required to eliminate any risk of

suffocation, in particular, in spaces below ground level. In this respect, the situation is the same as

for any large halocarbon plant.

It is sometimes asserted that the high pressure of CO

could constitute a special danger in the

case of accidental rupture. Actually this is not so since the volume is so small. The explosion

energy, similar to the product P × V, is approximately the same for all systems with the same

capacity, regardless of the refrigerant used.

also has a number of further advantages such as the following:

• nonﬂammable, nonexplosive, nontoxic,

• low cost and good availability,

• having zero ODP and 1 as GWP,

• thermal stability,

• pressure close to the economically optimal level,

• greatly reduced compression ratio compared to conventional refrigerants,

• complete compatibility to normal lubricants and common machine construction materials,

• easy availability everywhere, independent of any supply monopoly, and

• simple operation and service, no “recycling” required, very low price.

Its only technical disadvantage is the high triple-point temperature and the low critical tempera-

ture. Therefore, CO

as a pure substance cannot be an alternative refrigerant.

is a refrigerant with a great potential for development of energy- and cost-effective systems.

Examples have demonstrated this for some applications and appropriate technology for other ﬁelds

will certainly be found. This substance comes very close to the ideal refrigerant and a rapid revival

of this popularity for usage over a wide ﬁeld can be expected. CO

now appears to be a substitute

for the following:

• R-13, R-13B1, and R-503 in very low-temperature refrigeration (retroﬁt and new),

• R-13, R-13B1, and R-503 in industrial process refrigeration (retroﬁt and new), and

• R-11, R-12, R-113, R-114, and R-115 in nonmechanical systems (retroﬁt and new).

2.10 Selection of Refrigerants

In the selection of an appropriate refrigerant for use in a refrigeration or heat pump system, there

are many criteria to be considered. Brieﬂy, the refrigerants are expected to meet the following

conditions (Dincer, 1997):

• ozone- and environment friendly,

• low boiling temperature,

• low volume of ﬂow rate per unit capacity,

• vaporization pressure lower than atmospheric pressure,

• high heat of vaporization,

• nonﬂammable and nonexplosive,

Refrigerants 95

• noncorrosive and nontoxic,

• nonreactive and nondepletive with the lubricating oils of the compressor,

• nonacidic in case of a mixture with water or air,

• chemically stable,

• suitable thermal and physical properties (e.g., thermal conductivity, viscosity),

• commercially available,

• easily detectable in case of leakage, and

• low cost.

When selecting the refrigerant, the saturation properties of the refrigerant should be taken into

account. To have heat transfer at a reasonable rate, a temperature difference of 5 to 10

◦

C should

be maintained between the refrigerant and the medium with which it is exchanging heat. If a

refrigerated space is to be maintained at 0

◦

C, for example, the temperature of the refrigerant

should remain at about −10

◦

C while it absorbs heat in the evaporator. The lowest pressure in

a refrigeration cycle occurs in the evaporator, and this pressure should be above atmospheric

pressure to prevent any air leakage into the refrigeration system. Therefore, a refrigerant should

have a saturation pressure of 1 atm or higher at −10

◦

C in this particular case. Ammonia and

R-134a are two such substances. Also, the temperature (and thus the pressure) of the refrigerant on

the condenser side depends on the medium to which heat is rejected. Lower temperatures in the

condenser (thus higher COPs) can be maintained if the refrigerant is cooled by a lower temperature

medium such as liquid water (Cengel and Boles, 2008).

Example 2.1

A refrigerator using R-134a is used to maintain a space at −10

◦

C while rejecting heat to a reservoir

at 20

◦

C. If a temperature difference of 10

◦

C is desired, which evaporating and condensing pressures

should be used?

Solution

The evaporating temperature should be (−10) − (10) = −20

◦

C and the condensing temperature

should be 20 + 10 = 30

◦

C. Then from Table B.3,

sat

@ −20

◦

= 132.8 kPa (evaporator pressure)

sat

@30

◦

= 770.6 kPa (condenser pressure)

Tables 2.7 through 2.10 tabulate the percentage compositions of refrigerant blends (for R-12,

R-502, R-22, R-113, R-13B1, and R-503) found acceptable subject to narrowed use limits or

acceptable subject to use conditions by the Clean Air Act of the United States, along with their

trade names and ASHRAE numbers.

2.11 Thermophysical Properties of Refrigerants

It is essential to have sufﬁcient knowledge and information on the thermophysical properties of the

refrigerants and their mixtures in order to provide optimum system design and optimum operating

conditions. As is obvious, an effective and efﬁcient use of ecologically and environmentally friendly

refrigerants can only be achieved if, among others, their thermophysical properties are known to a

sufﬁciently high degree of accuracy.

96 Refrigeration Systems and Applications

Table 2. 7 Percentage compositions of substitutes for R-12.

Trade Name ASHRAE Number HCFCs HFCs HCs

22 124 142b 134a 152a 227ea Butane Isobutane

(R-600) (R-600a)

MP-39 401A 53% 34% – – 13% – – –

MP-66 401B 61% 28% – – 11% – – –

MP-52 401C 33% 52% – – 15% – – –

GHG 406A 55% – 41% – – – – 4%

FX-56 409A 60% 25% 15% – – – – –

FRIGC FR-12 – – 39% – 59% – – 2% –

GHG-HP – 65% – 31% – – – – 4%

Hot Shot 414B 50% 39% 9.5% – – – – 1.5%

GHG-X4 414A 51% 28.5% 16.5% – – – – 4%

Freeze 12 – – – 20% 80% – – – –

GHG-X5 – 41% – 15% – – 40% – 4%

Source: US Environmental Protection Agency, EPA (2009).

Table 2. 8 Percentage compositions of substitutes for R-502.

Trade Name ASHRAE Number HCFC-22 HFCs HCs

32 125 134a 143a 152a Propane Propylene

HP-80 402A 38% – 60% – – – 2% –

HP-81 402B 60% – 38% – – – 2% –

HP-62, FX-70 404A – – 44% 4% 52% – – –

KLEA 407A 407A – 20% 40% 40% – – – –

KLEA 407B 407B – 10% 70% 20% – – – –

FX-10 408A 46% – 7% – 47% – – –

R-411A 411A 87.5% – – – – 11% – 1.5%

R-411B 411B 94% – – – – 3% – 3%

G2018C – 95.5% – – – – 1.5% – 4%

AZ-50 507 – – 50% – 50% – – –

Source: US Environmental Protection Agency, EPA (2009).

Refrigerants 97

Table 2. 9 Percentage compositions of substitutes for R-22.

Trade Name ASHRAE Number HFC-32 HFC-125 HCFC-134a

KLEA 407C, AC9000 407C 23% 25% 52%

AZ-20, Puron, Suva 9100 410A 50% 50% –

AC9100 410B 45% 55% –

Source: US Environmental Protection Agency, EPA (2009).

Table 2. 10 Percentage compositions of substitutes for CFC-113, R-13B1, and R-503.

Trade Name ASHRAE HCFC-22 HFC-23 HFC-152a Propane PFC-116 PFC-218

Number (Perﬂuoro-ethane) (Perﬂuoro-propane)

R-403B 403B 56% – – 5% – 39%

KLEA 5R3 508A – 39% – – 61% –

Suva97 508B – 46% – – 54% –

NARM-502 – 90% 5% 5% – – –

Source: US Environmental Protection Agency, EPA (2009).

In general, thermophysical properties are conventionally classiﬁed into three categories such as

equilibrium or thermodynamic properties, nonequilibrium or transport properties, and other miscel-

laneous properties including radiation, optical, and electrical properties. However, Watanabe and

Sato (1990) provide an excellent categorized and prioritized grouping of thermophysical property

data needed to permit an assessment of the environmentally acceptable refrigerants as follows:

• Group Zero. Normal boiling point, molecular structure;

• Group I. Vapor pressure, critical parameters, saturated liquid density, vapor-phase pressure–

volume–temperature (PVT) properties, ideal gas heat capacity;

• Group II. Viscosity and thermal conductivity at saturated states, liquid-phase PVT properties,

surface tension, dielectric strength;

• Group III. Liquid-phase heat capacity including saturated liquid, more extensive PVT properties

in single-phase region, miscellaneous molecular properties such as dipole moment;

• Group IV. More extensive viscosity and thermal conductivity in single-phase region, velocity of

sound, second virial coefﬁcient, vapor-phase heat capacity, additional transport properties such

as thermal diffusivity.

In recent years, increasing attention has been paid to research activities related to dynamic

simulation of refrigeration systems which require a substantial amount of thermodynamic property

evaluations. Cleland (1986) stated that many refrigeration design and simulation works mainly need

the following:

• boiling (saturation) temperature from vapor pressure,

• vapor pressure from boiling (saturation) temperature,

• liquid refrigerant enthalpy from saturation temperature and liquid subcooling,

98 Refrigeration Systems and Applications

• vaporized refrigerant enthalpy from saturation temperature and vapor superheating,

• speciﬁc volume of vapor from saturation temperature and vapor superheating, and

• enthalpy change due to isentropic compression from vapor superheating prior to compression

and suction and discharge pressures (or other equivalent saturation temperature).

In an experimental work undertaken by Heide and Lippold (1990), thermophysical properties of

the refrigerants R-134a and R-152a in terms of dynamic viscosity, thermal conductivity, and surface

tension were determined experimentally depending on the saturation temperature and pressure, and

the following correlations were developed with high accuracy (i.e., average relative derivation less

than 1.5%):

• For dynamics viscosity:

ln η = 2.7847 − 0.1576T + 3.5324 × 10

−6

+ 2.5718 × 10

−9

for R-134a and

ln η = 12.119 − 0.1215T + 3.8074 × 10

−4

− 4.3428 × 10

−7

for R-152a

• For thermal conductivity:

ln λ = 194.6 − 0.3626T for R-134a and

ln λ = 241.0 − 0.4523T for R-152a

• For surface tension:

σ = 58.96(1 − T/374.26)

1.276

for R-134a and

σ = 59.23(1 − T/386.65)

1.235

for R-152a

where η is dynamic viscosity, mPa·s; λ is thermal conductivity, mW/mK; σ is surface tension,

mN/m; and T is temperature, K.

2.12 Lubricating Oils and Their Effects

It is known that the lubricating oil contained in the crankcase of the compressor is generally in

contact with the refrigerant. When oil dissolves in the refrigerant, it affects the thermodynamic

properties of the refrigerant. The main effect is the reduction of the vapor pressure by the amount

depending on the nature of the oil and the refrigerant and on how much oil dissolves. It is important

to state that the refrigerants are expected to be chemically and physically stable in the presence of

oil, so that neither the refrigerant nor the oil is adversely affected by the relationship.

For example, in ammonia systems the amount of oil in the solution with liquid ammonia is

extremely small to cause any effect. However, with HC refrigerants the amount of oil in the

solution is much larger and some HC refrigerants therefore react with the oils to some extent.

The magnitude of the effect is dependent upon the operating conditions – at normal operating

conditions with high-quality oil in dry and clean system the reaction becomes minor to cause any

effect. However, if contaminants such as air and moisture are present in the system with low-quality

oil, various problems may appear including decomposition of the oil and formation of corrosive

acids and sludges. The other aspect is that high discharge temperatures accelerate such causes

tremendously.

As far as the lubricating oil and refrigerant relationship are concerned, one of the differentiating

characteristics for various refrigerants is the oil miscibility, which is deﬁned as the ability of the

Refrigerants 99

refrigerant to be dissolved into the oil and vice versa. With reference to oil miscibility, refrigerants

may be divided into three groups as follows (Dossat, 1997):

• those that are miscible with oil in all proportions under the conditions found in the refrigerating

system,

• those that are miscible under the conditions normally found in the condensing section, but separate

from the oil under the conditions normally found in the evaporator section, and

• those that are not miscible with oil at all (or only very slightly) under the conditions found in

the system.

The viscosity of the lubricating oil is also one of the signiﬁcant thermophysical aspects and

should be maintained within certain limits in order to form a protective ﬁlm between the various

rubbing surfaces and to keep them separated. For example, if the viscosity is too low, the oil cannot

do so; if it is too high, the oil cannot have enough ﬂuidity to make the necessary penetration. In

both cases, the lubrication of the compressor is not adequate. It is important to mention that in order

to minimize the circulation of oil in the refrigerant, an oil separator or trap is sometimes installed

in the discharge line of the compressor.

One of the more recent works (Kramer, 1999) addresses the fact that HFC refrigerants and blends

are not miscible with mineral oil. It also explores factors that favor using or retaining mineral oil

when retroﬁtting existing systems by discussing the potential problems arising from such use or

retention. In practice, one of the big questions is why do we retain mineral oil? The following

factors favor retaining or using mineral oil in the systems with HFC refrigerants:

• lower lubricant cost,

• direct refrigerant replacement,

• lower refrigerant solubility,

• improved working viscosity,

• reduced refrigerant charge,

• faster refrigeration on start,

• reduced slugging and oil carry-over on start (less need for pump down cycles or heaters),

• less distortion of the composition of refrigerant blends,

• reduced oil separator ﬂooding,

• reduced hygroscopicity,

• reduced chemical reactivity,

• reduced electrical resistivity, and

• reduced dirt transfer.

The utilization of mineral oils with HFCs is preferred over the polyol ester lubricants because

of several advantages including (Kramer, 1999)

• nonsticking suction need,

• better visual detection,

• water solubility and less environmental impact, and

• improved foaming characteristics (to promote bearing lubrication and reduce compressor noise).

2.13 Concluding Remarks

In this chapter, a large number of aspects related to refrigerants, alternative refrigerants, and their

environmental impact have been studied for refrigeration systems and applications. Various technical

criteria for selecting and evaluating alternative refrigerants are also presented.

100 Refrigeration Systems and Applications

Study Problems

Introduction

2.1 Are CFCs greenhouse gases? What is the effect of CFCs on greenhouse effect in comparison

with carbon dioxide?

2.2 What is a refrigerant? What are the application areas of refrigerants?

Classiﬁcation of Refrigerants

2.3 What are the main classiﬁcations of refrigerants?

2.4 What is a halocarbon? What are the most commonly used halocarbons in refrigeration

applications?

2.5 What are the most commonly used CFCs?

2.6 What are the health effects of CFCs?

2.7 What are the advantages of HCs as alternative refrigerants? Which HCs are suitable as

refrigerants?

2.8 What are the three most commonly used inorganic compounds as refrigerants? Which refrig-

erant has the highest COP and which one has the lowest COP? Which inorganic compound

is used as refrigerant in aircraft air conditioning?

2.9 What is an azeotropic refrigerant? What is the most common azeotropic refrigerant?

2.10 What is a nonazeotropic refrigerant? Is there any other name for it?

Preﬁxes and Decoding of Refrigerants

2.11 What are the advantages of using preﬁxes and decoding of refrigerants?

2.12 What is the meaning of each letter in CFC and HCFC?

2.13 Are CFC-12 and R-12 the same refrigerant? What does “R” stand for?

2.14 Determine the number of atoms for each substance in HCFC-124 and HFC-152a.

2.15 Determine the number of atoms for each substance in CFC-12 and Halon 1301.

Secondary Refrigerants

2.16 What is a secondary refrigerant? What are the commonly used secondary refrigerants?

Refrigerant–Absorbent Combinations

2.17 Which refrigerant–absorber combinations are used in absorption refrigeration systems?