Dinc Ibrahim. Refrigeration systems and applications 2th edition

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General Aspects of Thermodynamics, Fluid Flow and Heat Transfer 61

1.26 A 50-liter rigid tank contains oxygen at 52

◦

C and 170 kPa. Now the oxygen is heated until

the temperature reaches 77

◦

C. What is the amount of heat transfer during this process?

1.27 A 50-liter rigid tank contains oxygen at 52

◦

C and 170 kPa. Now the oxygen is heated until

the temperature reaches 77

◦

C. What is the entropy change during this process?

1.28 A rigid tank contains 2.5 kg oxygen at 52

◦

C and 170 kPa. Now the oxygen is heated in an

isentropic process until the temperature reaches 77

◦

C. What is the pressure at ﬁnal state?

What is the work interaction during this process?

1.29 A piston-cylinder device contains 2.5 kg oxygen at 52

◦

C and 170 kPa. Now the oxygen is

heated until the temperature reaches 77

◦

C. Determine the work done and the amount of

heat transfer during this process?

Refrigerators, Heat Pumps, and the Carnot Refrigeration Cycle

1.30 Show that for given values Q

and Q

the COPs of a refrigerator and a heat pump are

related to each other by COP

= COP

+ 1.

1.31 How can the COP of a Carnot refrigerator be increased?

1.32 A Carnot refrigerator is used to keep a space at 18

◦

C by rejecting heat to a reservoir at

◦

C. If the heat removal from the cooled space is 12,000 kJ/h, determine the COP of the

refrigerator and the power input in kW.

1.33 A Carnot refrigerator is used to keep a space at 65

◦

F by rejecting heat to a reservoir at

◦

F. If the heat removal from the cooled space is 10,500 Btu/h, determine the COP of the

refrigerator and the power input in kW.

1.34 A Carnot refrigerator is used to keep a space at −20

◦

C. If the COP of the refrigerator is

7.5, what is the temperature of the reservoir to which heat is rejected? For a power input

of 3.7 kW, what is the rate of heat rejected to high-temperature reservoir?

Exergy

1.35 What are the two main reasons that cause irreversibility?

1.36 What happens to the parameters mass, energy, entropy, and exergy during an irreversible

process: conserved, decreases, or increases?

1.37 How does an exergy analysis help the goal of more efﬁcient energy resource use? What are

the advantageous of using an exergy analysis?

Psychrometrics

1.38 What is the difference between humidity ratio and relative humidity?

1.39 Why is heating usually accompanied by humidiﬁcation and cooling by dehumidiﬁcation?

1.40 Consider moist air at 24

◦

C at sea level with a relative humidity of 70%. Using psychrometric

chart, determine humidity ratio, wet-bulb temperature, and enthalpy of moist air.

1.41 Consider moist air at 25

◦

C at sea level with a relative humidity of 40%. What is the partial

pressure of water vapor in the air? The saturation pressure of water at 25

◦

C is 3.17 kPa.

62 Refrigeration Systems and Applications

1.42 Consider moist air at 80

◦

F at sea level with a relative humidity of 40%. What is the partial

pressure of water vapor in the air? The saturation pressure of water at 80

◦

F is 0.507 psia.

1.43 Air at 1 atm and 32

◦

C with a relative humidity of 20% enters an evaporative cooling section

whose effectiveness is 80%. What is the air temperature at the exit of the evaporative cooler?

General Aspects of Fluid Flow

1.44 What is the physical meaning of the Reynolds number? What makes the ﬂow laminar and

what makes it turbulent?

1.45 What is viscosity? How does viscosity change with temperature for gases and for liquids?

General Aspects of Heat Transfer

1.46 What are the modes of heat transfer? Explain mechanism of each mode.

1.47 A 20-cm thick wall of a house made of brick (k = 0.72W/m ·

◦

C) is subjected to inside

air at 22

◦

C with a convection heat-transfer coefﬁcient of 15 W/m

◦

C. The inner surface

temperature of the wall is 18

◦

C and the outside air temperature is −1

◦

C. Determine the

outer surface temperature of the wall and the heat-transfer coefﬁcient at the outer surface.

1.48 A satellite is subjected to solar energy at a rate of 300 W/m

. The absorptivity of the surface

is 0.75 and its emissivity is 0.60. Determine the equilibrium temperature of the satellite.

1.49 A satellite is subjected to solar energy at a rate of 95 Btu/h · ft

. The absorptivity of the

surface is 0.80 and its emissivity is 0.65. Determine the equilibrium temperature of the

satellite.

1.50 An 80-cm-diameter spherical tank made of steel contains liqueﬁed natural gas (LNG) at

−160

◦

C. The tank is insulated with 4-cm-thick insulation (k = 0.015 W/m ·

◦

C). The tank is

subjected to ambient air at 18

◦

C with a convection heat-transfer coefﬁcient of 20 W/m

◦

How long will it take for the temperature of the LNG to drop to −150

◦

C. Neglect the thermal

resistance of the steel tank. The density and the speciﬁc heat of LNG are 425 kg/m

and

3.475 kJ/kg ·

◦

C, respectively.

References

Borgnakke, C. and Sonntag, R. (2008) Fundamentals of Thermodynamics, 7th edn, John Wiley & Sons, Ltd.,

New York.

Cengel, Y.A. and Boles, M.A. (2008) Thermodynamics: An Engineering Approach, 6th edn, McGraw Hill,

New York..

Dincer, I. (2002) The role of exergy in energy policy making. Energy Policy, 30, 137–149.

Dincer, I. (2003) Refrigeration Systems and Applications, 1st edn, John Wiley & Sons, Ltd., New York.

Dincer, I. and Rosen, M.A. (2005) Thermodynamic aspects of renewables and sustainable development. Renew-

able and Sustainable Energy Reviews , 9, 169–189.

Klein, S.A. (2006) Engineering equation solver (EES), F-Chart Software, www.fChart.com.

Marquand, C. and Croft, D. (1997) Thermoﬂuids – An Integrated Approach to Thermodynamics and Fluid

Mechanics Principles, John Wiley & Sons, Ltd., New York.

Moran, M.J. and Shapiro, H.N. (2007) Fundamentals of Engineering Thermodynamics, 6th edn, John Wiley &

Sons, Ltd., New York.

Raznjevic, K. (1995) Handbook of Thermodynamic Tables, 2nd edn, Begell House, New York.

Refrigerants

2.1 Introduction

The ﬁrst designers of refrigeration machines, Jacob Perkins in 1834, and others later in the nineteenth

century, used ethyl ether (R-610) as the ﬁrst commercial refrigerant. The reason is easy to understand

if one has ever spilt this liquid on the hand and felt the effect. It was not particularly suitable

to the purpose however, being dangerous as well as requiring an excessive compressor volume.

Other and more appropriate refrigerants, for example, ammonia (R-717), carbon dioxide (R-744),

ethyl chloride (R-160), isobutane (R-600a), methyl chloride (R-40), methylene chloride (R-30), and

sulfur dioxide (R-764), were soon introduced, including air (R-729). Three of these refrigerants

became very popular, that is, ammonia and sulfur dioxide for refrigerators and other small units

and carbon dioxide preferably for ships’ refrigeration. A large number of substances were tried

over the following years, with varying success.

In the early 1930s, the introduction of chloroﬂuorocarbons (CFCs) was revolutionary as

compared with the natural substances. In addition to their use as refrigerants in refrigeration and

air-conditioning systems, CFCs were utilized as foam-blowing agents, aerosol propellants, and

cleaning solvents since 1950. The main arguments put forward in their favor were complete safety

and harmlessness to the environment. Both these claims were proved wrong. Many accidents have

occurred because of suffocation in the heavy gas, without warning, in below threshold spaces.

It was evident that CFCs and related compounds contribute tremendously to the destruction of

the stratospheric ozone layer and to the greenhouse effect (i.e., global climate change), which

are considered among the most signiﬁcant environmental problems. In fact, CFCs are greenhouse

gases that give a combined contribution to incremental global warming of the same magnitude.

The most abundant greenhouse gas is CO

and the others are CH

O, CFCs, and so on. The

effect of CFCs on global climate change was assumed to vary considerably, roughly contributing

in the range 15–20% as compared to 50% for CO

. The interesting point is that in order to

minimize global climate change, making reductions in CFC utilization seemed to be easier than

reducing fossil fuel use. Therefore, a full ban on these substances was essential.

Almost a decade ago, CFCs were banned worldwide as a result of their alleged effect on the

stratospheric ozone layer and global climate change, despite the fact that CFCs were among the

most useful chemical substances ever developed. During the past decade, research activities

have been expanded tremendously to conduct ozone level measurements using various types of

ground-based or airborne equipment; of course, more recently satellite technology has become a

prominent technique providing more accurate ﬁndings about the ozone levels in different locations.

It is well known that the stratospheric ozone layer acts as a shield against harmful ultraviolet

(UV) solar radiation. More than two decades ago, researchers discovered that chlorine released from

Refrigeration Systems and Applications

Ibrahim Dinc¸er and Mehmet Kano

glu

 2010 John Wiley & Sons, Ltd

64 Refrigeration Systems and Applications

synthetic CFCs migrates to the stratosphere and destroys ozone molecules and hence the ozone

layer, which was recognized as ozone layer depletion, known as one of the biggest environmental

problems. In 1987, 24 nations and the European Economic Community signed the Montreal Protocol

to regulate the production and trade of ozone-depleting substances. This was considered as a

landmark in refrigeration history.

This is by no means a unique experience. Similar predicaments have occurred following release

to the environment of many other new chemicals. The extensive use of more new compounds is

one of the big problems of our time. In this situation, it does not seem very sensible to replace the

CFC/hydrochloroﬂuorocarbons (HCFCs) with a new family of related halocarbons, equally foreign

to nature, to be used in quantities of hundreds of thousands of tons every year.

2.1.1 Refrigerants

In general, refrigerants are well known as the ﬂuids absorbing heat during evaporation. These refrig-

erants, which provide a cooling effect during the phase change from liquid to vapor, are commonly

used in refrigeration, air conditioning, and heat pump systems, as well as process systems.

2.2 Classiﬁcation of Refrigerants

This section is focused only on the primary refrigerants, which can be classiﬁed into the following

ﬁve main groups (Dincer, 2003):

• halocarbons,

• hydrocarbons (HCs),

• inorganic compounds,

• azeotropic mixtures, and

• nonazeotropic mixtures.

2.2.1 Halocarbons

The halocarbons contain one or more of the three halogens – chlorine, ﬂuorine, or bromine – and

are widely used in refrigeration and air-conditioning systems as refrigerants. These are more com-

monly known by their trade names, such as Freon, Arcton, Genetron, Isotron, and Uron. Numerical

indication is preferable in practice.

In this group, the halocarbons, consisting of chlorine, ﬂuorine, and carbon, were the most com-

monly used refrigerants (so-called chloroﬂuorocarbons, CFCs). CFCs were commonly used as

refrigerants, solvents, and foam-blowing agents. The most common CFCs have been CFC-11 or

R-11, CFC-12 or R-12, CFC-113 or R-113, CFC-114 or R-114, and CFC-115 or R-115.

Although CFCs such as R-11, R-12, R-22, R-113, and R-114 were very common refrigerants

in refrigeration and air-conditioning equipment, they were used in several industries as aerosols,

foams, solvents, etc. Their use rapidly decreased, because of their environmental impact. In the

past decade CFC phaseout in refrigeration became a primary political issue as well as, technically

speaking, a more and more difﬁcult problem. In addition to ozone layer depletion, the refrigeration

and air-conditioning industry faces another problem – the increase in the greenhouse effect, which

will be explained later.

It is well known that CFCs are odorless, nontoxic, and heavier than air, as well as dangerous

if not handled properly. Inhalation of high concentrations is not detectable by human senses and

can prove fatal because of oxygen exclusion caused by CFC leakages in an enclosed area. The

combustion products of CFCs include phosgene, hydrogen ﬂuoride, and hydrogen chloride, which

Refrigerants 65

are all highly poisonous if inhaled. Although these CFCs are not identical in performance and

composition, they are part of the same basic family of chemicals.

In this family, there are some other components such as halons, carbon tetrachlorides, and per-

ﬂuorocarbons (PFCs). Halons are the compounds consisting of bromine, ﬂuorine, and carbon. The

halons (i.e., halon 1301 and halon 1211) are used as ﬁre extinguishing agents, both in built-in sys-

tems and in handheld portable ﬁre extinguishers. Halon production was banned in many countries;

for example, in the United States it ended on December 31, 1993 because of the contribution of

halons to ozone depletion. They cause ozone depletion because they contain bromine. Bromine

is many times more effective at destroying ozone than chlorine. Carbon tetrachloride (CCl

)isa

compound consisting of one carbon atom and four chlorine atoms. Carbon tetrachloride was widely

used as a raw material in many industrial applications, including the production of CFCs, and as a

solvent. Solvent use ended when it was discovered to be carcinogenic. It is also used as a catalyst

to deliver chlorine ions to certain processes. PFC is a compound consisting of carbon and ﬂuorine.

PFCs have an extremely high effect on global climate change and very long lifetimes. However,

they do not deplete stratospheric ozone; but the concern is about their impact on global warming.

2.2.2 Hydrocarbons

HCs are the compounds that mainly consist of carbon and hydrogen. HCs include methane, ethane,

propane, cyclopropane, butane, and cyclopentane. Although HCs are highly ﬂammable, they may

offer advantages as alternative refrigerants because they are inexpensive to produce and have zero

ozone depletion potential (ODP), very low global warming potential (GWP), and low toxicity.

There are several types of HC families such as the following:

• Hydrobromoﬂuorocarbons (HBFCs) are the compounds that consist of hydrogen, bromine, ﬂuo-

rine, and carbon.

• HCFCs are the compounds that consist of hydrogen, chlorine, ﬂuorine, and carbon. The HCFCs

are one class of chemicals being used to replace the CFCs. They contain chlorine and thus deplete

stratospheric ozone, but to a much lesser extent than CFCs. HCFCs have ODPs ranging from

0.01 to 0.1. Production of HCFCs with the highest ODPs will be phased out ﬁrst, followed by

other HCFCs.

• Hydroﬂuorocarbons (HFCs) are the compounds that consist of hydrogen, ﬂuorine, and carbon.

These are considered a class of replacements for CFCs, because of the fact that they do not

contain chlorine or bromine and do not deplete the ozone layer. All HFCs have an ODP of 0.

Some HFCs have high GWPs. HFCs are numbered according to a standard scheme.

• Methyl bromide (CH

Br) is a compound consisting of carbon, hydrogen, and bromine. It is

an effective pesticide and is used to fumigate soil and many agricultural products. Because it

contains bromine, it depletes stratospheric ozone and has an ODP of 0.6. Its production is banned

in several countries, for example, in the United States since the end of December 2000.

• Methyl chloroform (CH

CCl

) is a compound consisting of carbon, hydrogen, and chlorine. It is

used as an industrial solvent. Its ODP is 0.11.

For refrigeration applications, a number of HCs such as methane (R-50), ethane (R-170), propane

(R-290), n-butane (R-600), and isobutane (R-600a) that are suitable as refrigerants can be used.

2.2.3 Inorganic Compounds

In spite of the early invention of many inorganic compounds, today they are still used in many

refrigeration, air conditioning, and heat pump applications as refrigerants. Some examples are

66 Refrigeration Systems and Applications

ammonia (NH

), water (H

O), air (0.21O

+ 0.78N

+ 0.01Ar), carbon dioxide (CO

), and sulfur

dioxide (SO

). Among these compounds, ammonia has received the greatest attention for practical

applications and, even today, is of interest. Below, we will brieﬂy focus on three compounds of

this family – ammonia, carbon dioxide, and air.

2.2.3.1 Ammonia (R-717)

Ammonia is a colorless gas with a strong pungent odor which may be detected at low levels

(e.g., 0.05 ppm). Liquid ammonia boils at atmospheric pressure at −33

◦

C. The gas is lighter than

air and very soluble in water. Despite its high thermal capability to provide cooling, it may cause

several technical and health problems including the following (Dincer, 1997):

• Gaseous ammonia is irritating to the eyes, throat, nasal passages, and skin. Although workers

apparently develop a tolerance to ammonia, exposure to levels in the range of 5 to 30 ppm may

cause eye irritation.

• Exposure to levels of 2500 ppm causes permanent eye damage, breathing difﬁculties, and asth-

matic spasm and chest pain.

• Potentially fatal accumulation of ﬂuid in the lung may develop some hours after the exposure.

Nonfatal poisoning may lead to the development of bronchitis, pneumonia, and an impaired lung

function.

• Skin exposure to gaseous ammonia at very high levels causes skin irritation, skin burns, and the

formation of ﬂuid-ﬁlled blisters.

• Eye contact with liquid ammonia may lead to blindness and skin contact may lead to potentially

fatal chemical burns.

• Ammonia is a ﬂammable gas and forms potentially explosive mixtures in the range of 16 to 25%

with air. Ammonia which is dissolved in water is not ﬂammable.

• Ammonia reacts or produces explosive products with ﬂuorine, chlorine, bromine, iodine, and

some other related chemical compounds.

• Ammonia reacts with acids and produces some heat.

• Ammonia vapors react with the vapors of acid (e.g., HCl) to produce an irritating white

smoke.

• Ammonia and ammonia-contaminated oil must be disposed of in a proper way approved by local

regulatory agencies.

Despite its disadvantages, Lorentzen (1988) considered that ammonia is an excellent refrigerant

and indicated that these possible disadvantages can be eliminated with proper design and control

of the refrigeration system.

2.2.3.2 Carbon Dioxide (R-744)

Carbon dioxide is one of the oldest inorganic refrigerants. It is a colorless, odorless, nontoxic,

nonﬂammable, and nonexplosive refrigerant and can be used in cascade refrigeration systems and

in dry-ice production, as well as in food freezing applications.

2.2.3.3 Air (R-729)

Air is generally used in aircraft air conditioning and refrigeration systems. Its coefﬁcient of perfor-

mance (COP) is low because of the light weight of the air system. In some refrigeration plants, it

may be used in the quick freezing of food products.

Refrigerants 67

2.2.4 Azeotropic Mixtures

An azeotropic refrigerant mixture consists of two substances having different properties but behav-

ing as a single substance. The two substances cannot be separated by distillation. The most common

azeotropic refrigerant is R-502, which contains 48.8% R-22 and 51.2% R-115. Its COP is higher

than that of R-22 and its lesser toxicity provides an opportunity to use this refrigerant in household

refrigeration systems and the food refrigeration industry. Some other examples of azeotropic mix-

tures are R-500 (73.8% R-12 + 26.2% R-152a), R-503 (59.9% R-13 + 40.1% R-23), and R-504

(48.2% R-32 + 51.8% R-115).

2.2.5 Nonazeotropic Mixtures

Nonazeotropic mixture is a ﬂuid consisting of multiple components of different volatiles that, when

used in refrigeration cycles, change composition during evaporation (boiling) or condensation.

Recently, nonazeotropic mixtures have been called zeotropic mixtures or blends. The application

of nonazeotropic mixtures as refrigerants in refrigeration systems has been proposed since the

beginning of the twentieth century. A great deal of research on these systems with nonazeotropic

mixtures and on their thermophysical properties has been done since that time. Great interest

has been shown in nonazeotropic mixtures, especially for heat pumps, because their adaptable

composition offers a new dimension in the layout and design of vapor-compression systems. Much

work has been done since the ﬁrst proposal to use these ﬂuids in heat pumps. Through the energy

crises in the 1970s, nonazeotropic mixtures became more attractive in research and development

on advanced vapor-compression heat pump systems. They offered the following advantages:

• energy improvement and saving,

• capacity control, and

• adaptation of hardware components regarding capacity and applications limits.

In the past, studies showed that widely used refrigerants such as R-11, R-12, R-22, and R-114

became most popular for the pure components of the nonazeotropic mixtures. Although many

nonazeotropic mixtures (e.g., R-11 + R-12, R-12 + R-22, R-12 + R-114, R-13B1 + R-152a,

R-22 + R-114, and R-114 + R-152a, etc.) were well known, a decade ago research and develop-

ment mainly focused on three mixtures, R-12 + R-114, R-22 + R-114, and R-13B1 + R-152a. It

is clear that the heat-transfer phenomena during the phase change of nonazeotropic mixtures are

more complicated than with single-component refrigerants.

2.3 Preﬁxes and Decoding of Refrigerants

Various refrigerants (e.g., CFCs, HCs, HCFCs, HBFCs, HFCs, PFCs, and halons) are numbered

according to a system devised several decades ago and now used worldwide. Although it may

seem confusing, in fact it provides very complex information about molecular structure and also

easily distinguishes among various classes of chemicals. In practice, it is of great importance to

ﬁrst understand the preﬁxes of refrigerants and their meanings, as well as decoding for them. In

this section, we have three subsections as preﬁxes, decoding the number, and isomers. Further

information on these aspects is found in EPA (2009).

2.3.1 Preﬁxes

Some of the most common refrigerants’ preﬁxes are CFC, HCFC, HFC, PFC, and Halon, respec-

tively. In CFCs and HCFCs, the ﬁrst “C” is for chlorine (Cl), and in all of them, “F” is for

68 Refrigeration Systems and Applications

Table 2. 1 The preﬁxes and atoms in refrigerants.

Name Preﬁx Atoms Contained

Chloroﬂuorocarbon CFC Cl, F, C

Hydrochloroﬂuorocarbon HCFC H, Cl, F, C

Hydrobromoﬂuorocarbon HBFC H, Br, F, C

Hydroﬂuorocarbon HFC H, F, C

Hydrocarbon HC H, C

Perﬂuorocarbon PFC F, C

Halon Halon Br, Cl (in some), F, H (in some), C

ﬂuorine (F), “H” is for hydrogen (H), and the ﬁnal “C” is for carbon (C). PFC is a special preﬁx

meaning perﬂuorocarbon. “Per” means “all,” so PFCs have all bonds occupied by ﬂuorine atoms.

Consequently, halons are a general term for compounds that contain C, F, Cl, H, and bromine

(atomic symbol: Br). Halon numbers are different from the others and will be discussed later. For

example, an HFC contains no chlorine, so your results should not show any Cl atoms. Table 2.1

summarizes the preﬁxes and atoms contained in each refrigeration commodity.

Compounds used as refrigerants may be described using either the appropriate preﬁx as given

in the table or with the preﬁxes “R-” or “Refrigerant.” For example, CFC-12 may also be written

as R-12 or Refrigerant 12.

Blends of refrigerants are assigned numbers serially, with the ﬁrst zeotropic blend numbered

R-400 and the ﬁrst azeotropic blend numbered R-500. Blends that contain the same components

but in differing percentages are distinguished by capital letters. For example, R-401A contains

53% HCFC-22, 13% HFC-152a, and 34% HCFC-124, but R-401B contains 61% HCFC-22, 11%

HFC-152a, and 28% HCFC-124.

2.3.2 Decoding the Number

The preﬁx describes the kinds of atoms in a particular molecule, and the next step is to calculate

the number of each type of atom. The key to the code is to add 90 to the number; the result

shows the number of C, H, and F atoms. For HCFC-141b:

141 + 90 = 2 (#C) 3 (#H) 1(#F)

Additional information is needed to decipher the number of Cl atoms. All these chemicals are

saturated, so that they contain only single bonds. The number of bonds available in a carbon-based

molecule is 2C + 2. Thus, for HCFC-141b, which has two-C atoms, there are six bonds. Cl atoms

occupy bonds remaining after the F and H atoms. So HCFC-141b has two C, three H, one F, and

two Cl atoms.

HCFC-141b = C

FCl

where the HCFC designation is a good double-check on the decoding, containing H, Cl, F, and C.

The “b” at the end describes how these atoms are arranged; different “isomers” contain the same

atoms, but they are arranged differently.

Refrigerants 69

Let us see this time HFC-134a as an example.

134 + 90 = 2 (#C) 2 (#H) 4(#F)

where there are six bonds. But in this case, there are no bonds leftover after F and H, so there are

no Cl atoms. Thus,

HFC-134a = C

where the preﬁx is accurate: this is an HFC, so it contains only H, F, and C, but no Cl.

Note that any molecule with only one C (e.g., CFC-12) will have a two-digit number, while

those with two C or three C will have a three-digit number.

Halon numbers directly show the number of C, F, Cl, and Br atoms. The numbering scheme

above does not give a direct number for the number of Cl atoms, but that can be calculated.

Similarly, Halon numbers do not specify the number of H atoms directly and there is no need to

add anything to decode the number.

Halon = 1 (#C) 2 (#F) 1(#Cl) 1(#Br)

For this molecule, there are 2 × 1 + 2 = 4 bonds, all of which are taken by Cl, F, and Br, leaving

no room for any H atoms. Thus,

Halon 1211 = CF

ClBr

2.3.3 Isomers

Isomers of a given compound contain the same atoms but they are arranged differently. Isomers

usually have different properties; only one isomer may be useful. Since all of the compounds

under discussion are based on carbon chains (1–3 carbon atoms attached in a line of single bonds:

e.g., C—C—C), the naming system is based on how H, F, Cl, and Br atoms are attached to that

chain. A single C atom can only bond with four other atoms in one way, so there are no isomers of

those compounds. For two-C molecules, a single lowercase letter following the number designates

the isomer. For three-C molecules, a lowercase two-letter code serves this purpose.

Consider two-C molecules, for example, HCFC-141, HCFC-141a, and HCFC-141b in which all

have the same atoms (two carbon, three hydrogen, one ﬂourine, and two chlorine) but are organized

differently. To determine the letter, total the atomic weights of the atoms bonded to each of the

carbon atoms. The arrangement that most evenly distributes atomic weights has no letter. The next

most even distribution is the “a” isomer, the next is “b,” and so on, until no more isomers are

possible. A common way of writing isomers’ structure is to group atoms according to the carbon

atom with which they bond. Thus, the isomers of HCFC-141 are as follows:

• HCFC-141:

• HCFC-141a:

• HCFC-141b:

CHFCl—CH

Cl (atomic weights on the 2C = 37.5 and 55.5)

CHCl

—CH

F (atomic weights on the 2C = 21 and 72)

CFCl

—CH

(atomic weights on the 2C = 3 and 90)

For HFC-134, the isomers are as follows:

• HFC-134:

• HFC-134a:

CHF

—CHF

—CH

In order to specify the chemical structures for each of the Cl, F, and Br atoms, we use the

ordinal number of the C to which they are bonded and numerical preﬁxes (i.e., 2 = di, 3 = tri,

70 Refrigeration Systems and Applications

4 = tetra, etc.) to specify the total number of each kind of atom. The sufﬁx for the molecular name

is dependent on the number of carbons. Molecules with one C end in “methane” (since there are

no isomers of methane-derived molecules, they have no letter designation), two-C end in “ethane,”

and three-C end in “propane.” It is assumed that any bonds not occupied by Cl, F, or Br are

occupied by H, so H atoms are not speciﬁed. So, the isomers of HCFC-141 can be written in the

following way:

• HCFC-141:

• HCFC-141a:

• HCFC-141b:

CHFCl—CH

Cl: 1,2-dichloro-1-ﬂuoroethane

CHCl

—CH

F: 1,1-dichloro-2-ﬂuoroethane

CFCl

—CH

: 1,1-dichloro-1-ﬂuoroethane

For HFC-134, the isomers are as follows:

• HFC-134:

• HFC-134a:

CHF

—CHF

: 1,1,2,2-tetraﬂuoroethane

—CH

F: 1,1,1,2-tetraﬂuoroethane

CFC-12 does not have any isomers, since it contains only one C. In addition, there is no need

to number the carbons, even in a case such as diﬂuorodichloromethane.

Molecules with three-C atoms are more complicated to name. The ﬁrst letter designates the atoms

attached to the middle C atom, and the second letter designates decreasing symmetry in atomic

weights of atoms attached to the outside C atoms. Unlike 2C chains, however, the most symmetric

distribution is the “a” isomer, instead of omitting the letter entirely. The code letters for the atoms

on middle C are a for Cl

, b for Cl and F, c for F

, d for Cl and H, e for H and F, and f for H

;

for example,

HCFC-225ca: C

(3C = 8 bonds), CF

—CF

—CHCl

, and

1, 1,1,2,2-pentaﬂuoro-3,3-dichloropropane

When no isomers are possible, no letters are used. For example, there is only one way to arrange

three C and eight F, so it is written as PFC-218 and not PFC-218ca.

2.4 Secondary Refrigerants

Secondary refrigerants play a role in carrying heat from an object or a space being cooled to the

primary refrigerant or the evaporator of a refrigeration system. During this process, the secondary

refrigerant has no phase change. In the past, the most common secondary refrigerants were brines,

which are water–salt (e.g., sodium chloride and calcium chloride) solutions, and even today they are

still used in spite of their corrosive effects. Also, the antifreezes, which are solutions of water and

ethylene glycol, propylene glycol, or calcium chloride, are widely used as secondary refrigerants.

Of these ﬂuids, propylene glycol has the unique feature of being safe when in contact with food

products. A decade ago dichloromethane (CH

), trichloroethylene (C

HCl

), alcohol solutions,

and acetone were also used in some special applications. The following features are considered as

main criteria in the selection of a proper secondary refrigerant (Dincer, 2003).

• satisfactory thermal and physical properties,

• stability,

• noncorrosiveness,

• nontoxicity,

• low cost, and

• usability.