FLUOROCARBON
`
`REFRIGERANTS
`
`HANDBOOK
`
`RALPH C. DOWNING
`
`

`
`Library of Congress Cataloging-in-Publication Data
`
`Downing, Ralph C.
`Fluorocarbon refrigerants handbook.
`
`Bibliography: p.
`Includes index.
`
`I. Refrigerants.
`'I'P492.7.D63
`1988
`ISBN 0-13-322504—6
`
`2. Fluorocarbons.
`62l.5’64
`
`I. Title.
`87-14572
`
`Editorial/production supervision and
`interior design: Theresa A. Soler
`Cover design: Edsal Enterprises
`Manufacturing buyer: Peter Havens
`
`© 1988 by Prentice-Hall, Inc.
`A Division of Simon & Schuster
`
`Englewood Clifis, New Jersey 07632
`
`All rights reserved. No part of this book may be
`reproduced, in any form or by any means,
`without permission in writing from the publisher.
`
`Printed in the United States of America
`
`1098765432]
`
`ISBN D-1.3-331'-.'5E|'-I-in
`
`Prentice-Hall International (UK) Limited, London
`Prentice-Hall of Australia Pty. Limited, Sydney
`Prentice-Hall Canada Inc., Toronto
`Prentice-Hall I-lispanoamericana, S.A., Mexico
`Prentice-Hall of India Private Limited, New Delhi
`Prentice-Hall of Japan, Inc. , Tokyo
`Simon & Schuster Southeast Asia Pte. Ltd., Singapore
`Editora Prentice-Hall do Brasil, Ltda., Rio de Janeiro
`
`Page 2 of 34
`
`Page 2 of 34
`
`

`
`PREFA CE
`
`1 FLUOROCARBON REFRIGERANTS
`
`History
`
`1
`
`Nomenclature
`
`5
`
`Manufacture
`
`8
`
`APPLICA TIONS
`
`Industrial Cooling and Large-Tonnage Air Conditioning
`
`11
`
`Rcfrigerants
`
`12
`
`REFRIGERA TION
`
`Isothermal Change
`
`19
`
`Adiabatic Change
`
`20
`
`Charts and Tables
`
`21
`
`Refrigerants
`
`33
`
`Page 3 of 34
`
`Page 3 of 34
`
`

`
`4 REFRIGERANT PROPERTIES
`
`Chemical Formula and Molecular Weight
`Liquid Density 45
`'
`
`45
`
`Vapor Density
`
`57
`
`Boiling Point
`
`58
`
`Pressure
`
`58
`
`Velocity of Sound 61
`
`Pressure Drop and Velocity
`
`68
`
`Heat of Fusion
`
`82
`
`Heat of Formation
`
`82
`
`. Refrigerant Hydrates
`
`84
`
`Absorption Spectra
`
`89
`
`Refractive Index
`
`89
`
`Electrical Properties
`
`92
`
`TRANSPORT PROPERTIES
`
`Thermal Conductivity and Viscosity
`
`110
`
`Heat Capacity
`
`110
`
`REFRIGERANT MIXTURES AND AZEOTROPES
`
`Mixtures
`
`130
`
`Azeotropes
`
`139
`
`WATER
`
`Solubility
`
`162
`
`Distribution of Water between Liquid and Vapor
`
`174
`
`Moisture Indicators
`
`175
`
`Driers
`
`176
`
`Field Drying
`
`178
`
`Condensation
`
`179
`
`Liquid-Phase Separation
`
`180
`
`Page 4 of 34
`
`Page 4 of 34
`
`

`
`Water Entering Refrigeration Systems
`
`180
`
`Hydrolysis
`
`182
`
`SOLUBILITY OF AIR AND OTHER GASES
`
`Air
`
`190
`
`Nitrogen
`
`201
`
`Other Results and Comments
`
`202
`
`OIL RELATIONSHIPS
`
`Oil Problems
`
`206
`
`Oil Return
`
`208
`
`Oil Properties
`
`210
`
`Solubility Relationships
`
`215
`
`Refrigerant Migration
`
`240
`
`Viscosity
`
`247
`
`EFFECT ON POLYMERS
`
`Elastomers
`
`272
`
`Plastics
`
`280
`
`LEAK DETECTION
`
`Refrigerant Leakage
`
`287
`
`Methods of Leak Detection
`
`287
`
`Monitoring Fluorocarbons in the Atmosphere
`
`292
`
`Sensitivity
`
`294
`
`Comparative Leak Rate
`
`296
`
`STABILITY
`
`Inherent Stability
`
`299
`
`Sealed-Tube Tests
`
`300
`
`Effect of Air
`
`301
`
`Page 5 of 34
`
`1
`
`

`
`Effect of Metals
`
`302
`
`Effect of Oil
`
`303
`
`Acidity in Refrigeration Systems
`
`303
`
`Stability of R-12 and R~22
`
`306
`
`SOLAR APPLICATIONS
`
`CONVERSION FACTORS AND SHIPPING
`
`Units
`
`3 17
`
`Color
`
`323
`
`Shipping
`
`324
`
`SAFETY
`
`Underwriters’ Laboratories
`
`331
`
`Acute Toxicity
`
`336
`
`Chronic Toxicity
`
`336
`
`Oral Toxicity
`
`337
`
`Effect on the Skin
`
`337
`
`Human Exposure
`
`338
`
`Asphyxiation
`
`339
`
`Water Solutions
`
`340
`
`Cardiac Arrhythmia
`
`344
`
`Mutagenicity
`
`344
`
`Teratogenicity
`
`345
`
`Retention in Body
`
`346
`
`Comparisons of Toxicity
`
`346
`
`Safety Code for Mechanical Refrigeration
`
`346
`
`Threshold Limit Values
`
`348
`
`Material Safety Data Sheets
`
`350
`
`Flammability
`
`350
`
`Page 6 of 34
`
`

`
`Food Freezing
`
`365
`
`Reuse of Cylinders
`
`366
`
`16 THERMODYNAMIC PROPERTIES
`
`Equations
`
`371
`
`INDEX
`
`Contents
`
`Page 7 of 34
`
`

`
`|——*’— 3
`REFRIGERATION
`
`In refrigeration it is the duty of the refrigerant to carry heat from some place where
`it is not wanted to a place where it can be discarded. This cycle consists of compres-
`sion of the refrigerant gas, condensation to a liquid, and evaporation at a lower
`pressure. While all of these steps are important,
`the compression step is a very
`crucial part of the cycle and some relationships describing what happens during
`compression are briefly reviewed here. The following equations are from Dodge
`[3—1]. For a more complete discussion of gas relationships, see Dodge or another
`reference work on thermodynamics, such as the discussion by Suttle [3—2].
`An ideal gas is defined as one obeying the following equation:
`
`Differentiation gives
`
`PV = RT
`
`(for 1 mol)
`
`(3-1)
`
`(3-2)
`
`(3-3)
`
`(3-4)
`
`(3-5)
`
`Page 8 of 34
`
`

`
`Other relationships include the following:
`
`db? = C,, dT
`
`dH = Cp dT
`
`H = E + PV
`
`dC
`
`(dV)r
`
`V
`
`:0
`
`‘U’ T
`
`g—q=R
`
`T2
`
`AH=Q=J<;fl
`
`T:
`
`dT
`dV
`dS=C,,7-l-R‘-V‘
`
`dS
`
`Cp T
`
`R P
`
`where P = pressure, psia
`V = volume, ft3
`T = temperature, °F + 459.67
`R = gas constant (lO.7318 when P is psia, V is ft3, and T is °R
`E = internal energy, Btu/lb
`H = enthalpy or heat content, Btu/lb
`S = entropy, Btu/lb-°R
`C, = heat capacity at constant volume, Btu/lb-°F
`Cp = heat capacity at constant pressure, Btu/lb-°F
`Q = amount of heat transferred
`W = work
`For perfect gases, the entropy, enthalpy, and heat capacity at constant volume
`and at constant pressure are all functions of temperature. Change in pressure or
`volume at constant temperature will not change their values.
`
`Tables of thermodynamic properties often list values for entha1P}’ (H), specific
`volume, and entropy (S) at constant pressure at several temperatures for the super-
`heated gas. The change in enthalpy with temperature can easily be obtained from
`such tables. Since the tables are at constant pressure, values for PV can also be
`obtained (with some interpolation) and a value for E can be calculated from equation
`(3-8). Values of H and E are not absolute, but that is of little consequence since it
`is the changes in H and E that are of interest in refrigeration.
`On the other hand, Q, the heat transferred, and W, the work done on or by a
`gas, are definite quantities, although the value depends on how the process is operated
`(i.e., isothermally, adiabatically, etc.).
`Dodge lists the following general equations for changes of state for any fluid:
`
`Refrigeration
`
`Chap. 3
`
`Page 9 of 34
`
`

`
`dQ=TdS=C,,dT+T(d—P-) a'V
`
`dT 1,
`
`dQ=TdS=CpzII‘—T€-d¥)
`
`dP
`
`P
`
`dQ=TdS=C,,(§-V) 4v+C,,(%;) dP
`
`p
`
`‘U
`
`(3-15)
`
`(3-16)
`
`(3-17)
`
`r the special case of an ideal gas and using the equation of state, PV =
`mole), these equations reduce to
`
`TdS= C,,dT+PdV
`
`RTdP
`TdS = CPJT — T
`
`T
`
`=ds
`
`c,,T W + C,,T dP
`V
`P
`
`(3-18)
`
`(3-19)
`
`(3-20)
`
`3/-IANGE
`
`sotherrnal (constant temperature) change, the work done with an ideal gas
`
`V
`
`V
`
`P
`
`W=J2PdV=RT[ IL‘:/=‘RTI zjdfi
`
`VI
`
`PI P
`
`VI
`
`(3-21)
`
`(3-22)
`
`W = -RT in 23 = -2.3026RT log 5% (per mole)
`
`1
`
`1
`
`r a nonideal gas, the work can be calculated in the same way if an equation
`is known, by substituting it for the pressure. This calculation can be simple
`t equation of state is available, but as a rule equations for the fluorocarbon
`nts tend to be more complicated,
`in an effort to better fit experimental
`ue of the earliest attempts to represent the behavior of gases by an equation
`Ian der Waals in about 1873. He proposed the following equation:
`
`(3-23)
`
`Page 10 of 34
`
`Ial Change
`
`

`
`Isothermal work is the absolute minimum that must be used to compress a
`gas over a given pressure range. However, it is usually not practical to remove the
`heat fast enough to approach isothermal conditions. Rather, only a relatively small
`amount of heat is lost and compression is nearly adiabatic.
`
`ADIABA TIC CHANGE
`
`In adiabatic compression there is no change in entropy and
`
`TdS = 0
`
`All of the heat developed in compressing the gas is retained and goes into
`raising the temperature of the gas. Although compression is not exactly at constant
`entropy, it is usually close. Some heat is lost by conduction through the walls of
`the compressor and some is gained from mechanical friction and the passage of
`the gas through small valve openings. If there is a departure from constant entropy,
`it is often in the direction of more heat lost than gained, so real gas discharge
`temperatures may be a bit lower than calculated assuming constant entropy.
`The work of adiabatic compression can be calculated with the following rela-
`tionships:
`
`_ P] V] (
`
`j l__..__
`k‘-1
`Pl
`
`P2)“-l)Ik
`
`P2 (It-I)/k
`RT
`j: I _ _
`
`(3-26)
`
`(3-27)
`
`In these equations, k is the heat capacity ratio, C},/C,,, and is assumed to be
`constant during the compression. Although the ratio does vary with conditions, the
`change is usually quite small and an average value can be used.
`The increase in temperature during compression can be calculated as follows:
`
`P (k-l)/k
`
`T2 = T, (-3)
`
`Pl
`
`(3-23)
`
`It should be noted that the work of compression depends on the pressure
`ratio and not on the pressure level. For example, the work will be the same whether
`compressing from 100 Psia to 1000 psia or from 0.1 psia to 1 psia.
`The relationships above are based on the ideal gas law, PV = RT. Actual
`performance of a given refrigerant will be somewhat different and can be determined
`exactly only by measurement. For the ideal gas, PV/RT = 1, but for a real gas,
`PV/RT = 2, where Z is Called the compressibility factor. The ideal quantities are
`useful as standards of performance and approximations of the real quantities. They
`can be converted to actual Values by using experimental correction factors such as
`the compressibility.
`
`Refrigeration
`
`Chap. 3
`
`Page 11 of 34
`
`

`
`cmnrs AND TABLES 13.3]
`
`When evaluating a refrigerant,’ many general properties must be considered, such
`as toxicity, flammability, stability, relationships with water and oil, electrical proper-
`ties, and so on. All of these properties have a direct bearing on the suitability of a
`fluid for use as a refrigerant. However, from the standpoint of performance, five
`physical or thermodynamic properties are especially important: temperature, volume,
`pressure, enthalpy, and entropy. Since the refrigeration cycle involves a change of
`state from liquid to gas and back to liquid, knowledge of these properties for both
`liquid and gas is important. “Vapor” and “gas” are sometimes used interchangeably
`to describe the same physical state. At other times, “vapor” is reserved for “gas”
`that is in equilibrium with a liquid phase, and “gas” is used for conditions where
`a liquid phase is not possible.
`Temperature is a measure of how hot or cold an object is, but is not a measure
`of how much heat the object will hold or how much heat is needed to change from
`one temperature to another. Two temperature scales are in general use. The Celsius
`(formerly Centigrade) scale is used in many parts of the world and in most ‘ ‘scientific’ ’
`work. It is the official temperature unit in the SI system of measurement and in
`earlier practice with metric units. It is based on 0 degrees at the freezing point of
`water and 100 degrees at the atmospheric boiling point of water. The Fahrenheit
`scale is named after the German chemist Gabriel Fahrenheit and is based on 0
`
`degrees for the coldest temperature then available with a mixture of salt and water
`and 100 degrees for normal body temperature. However, he must have had a slight
`fever on the day he fixed the upper temperature since normal body temperature is
`98.6°F. It is perhaps fortuitous that a Celsius degree is exactly 1.8 times larger
`than a Fahrenheit degree.
`Volume is a measure of the space occupied by a specific amount of liquid or
`gas with units of cubic feet per pound, cubic meters per kilogram, and so on. It is
`the reciprocal of the density.
`Three different types of pressure may be encountered in refrigeration applica-
`tions: vapor, gas, and hydrostatic. Vapor pressure is exerted when both liquid and
`gas are present and in equilibrium. It is affected only by changes in temperature,
`not by the amounts of liquid and gas present. It is thus said to have one degree of
`freedom. For a particular refrigerant there can be only one value for the vapor
`pressure at a given temperature. In a refrigeration system the pressure is determined
`by the temperature of a liquid phase somewhere. For example, the temperature of
`the liquid in the condenser governs the pressure in the compressor discharge lines
`subject to somepressure drop in the piping. Liquid temperatures in the evaporator
`determine the pressure on the suction side of the compressor.
`When no liquid is present, the gas is said to have two degrees of freedom
`and both temperature and volume affect the pressure. At a given temperature gas
`pressure may have several different values, depending on the specific volume of
`the gas.
`When a cylinder, container, pipe, or any enclosed space becomes completely
`filled with liquid, hydrostatic pressure develops. This pressure changes rapidly with
`
`Page 12 “.34 Charts and Tables
`
`

`
`temperature and can be extremely hazardous. For example, with R‘- 12 the hydrostatic
`pressure increases about 40 psi for each 1-degree rise in temperature at room tempera-
`ture. Great care should be used to avoid overfilling cylinders, trapping liquid between
`two shutoff valves in a line, or other conditions where a liquid-full condition could
`develop.
`Enthalpy or heat content is a measure of how much heat or cold a fluid can
`hold and how much heat is needed to change the temperature. Absolute values of
`enthalpy are not usually known and are not necessary since only changes in enthalpy
`are important in the refrigeration cycle. An arbitrary value for the enthalpy is assigned
`at a given temperature—often a value of zero for the liquid at —40°F. Changes in
`enthalpy with changes in temperature, pressure, and volume can be calculated with
`thermodymically exact equations. One problem with a -40-degree reference point
`is the presence of negative enthalpies at temperatures lower than —40°F. This concept
`does not affect the mathematical solution of refrigeration problems but does add a
`little confusion to the understanding of refrigeration. In more recent tables of thermo-
`dynamic properties, the reference point has been selected to avoid the use of negative
`numbers.
`
`Entropy is more difficult to define. It can be regarded as a measure of that
`portion of the heat energy transferred which is unavailable for work [3-1]. It is the
`ratio of the heat transferred to the absolute temperature. Entropy stays the same
`during compression if heat is not added to or taken from the gas. The compression
`is called adiabatic when the entropy is constant. Absolute values for the entropy
`are not precisely known and for refrigeration purposes are unnecessary. An arbitrary
`reference value is selected—usually zero for the liquid at -40 degrees and values
`at other conditions are calculated using thermodynamically developed equations. In
`order to eliminate negative numbers at temperatures below -40 degrees, current
`practice is to assign a value for entropy at a reference point so that negative numbers
`are not involved.
`
`these five properties of fluids are provided in tables
`For refrigeration use,
`and charts as illustrated in Chapter 16.
`Richard Mollier, a German professor after whom charts of this type are named,
`had a great interest in displaying as much information as possible on one chart.
`Most of his work was with the properties of steam. He is best known for an arrange-
`ment with enthalpy and entropy shown along the axes and other properties as families
`of curves, although his name is often associated with other, similar arrangements.
`The chart most often used in refrigeration work has pressure on the vertical
`axis and enthalpy on the horizontal axis, as shown in Figure 3-1. This chart is for
`R-12 and will be used to illustrate the information that can be obtained from it.
`
`The curved line at the left in Figure 3-1 represents liquid at the saturation temperature.
`The vapor pressure can be read on the vertical scale and the heat content or enthalpy
`on the bottom scale at a number of different temperatures. The curve at the right
`represents saturated vapor in equilibrium with the liquid, such as in a cylinder, in
`a condenser, or in a flooded evaporator. All of the area to the right of this curve is
`for superheated gas where no liquid can be present.
`Use of the chart to obtain the compression ratio is illustrated in Figure 3-1.
`An evaporation temperature of 0°F and a condensing temperature of l20°F are as-
`
`22
`
`Page 13 of 34
`
`Refrigeration
`
`Chap. 3
`
`‘I:1-}
`
`

`
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`sumed. The pressures corresponding to these temperatures can be read on the vertical
`scale. Dividing the higher pressure by the lower pressure will give the compression
`ratio. Note that the pressures must be absolute.
`In this case the ratio is a little
`more than 7 and can be handled easily by reciprocating compressors. Compression
`ratios of 12 to 15 and higher will be found with reciprocating compressors, especially
`in small units where operating costs may not be a large factor. In larger equipment,
`when ratios higher than about 10 are called for, a second stage of compression is
`often recommended. With rotary compressors, compression ratios up to about 5
`are used and with centrifugal compressors up to about 3.
`A typical refrigeration cycle is drawn on a pressure—enthalpy chart in Figure
`3—2. An evaporating temperature of 0°F and condensing temperature of 120°F have
`been used in this illustration and refer to actual refrigerant temperatures rather than
`temperatures in the vicinity of the condensor or evaporator.
`The latent heat or heat of vaporization is the amount of heat needed to change
`a liquid to a vapor at the same temperature. It is the difference between the enthalpy
`of the saturated vapor and the saturated liquid. These numbers can be read from
`the chart or with greater accuracy from tables of thermodynamic properties. In
`refrigeration the change from liquid to gas takes place in the evaporator and is the
`only place where any cooling occurs. All of the rest of the equipment is needed to
`turn the gas back into a liquid and get it back into the evaporator. In Figure 3-2
`the latent heat at 0°F for R-12 is the enthalpy change represented by the line AC
`and amounts to 68.8 Btu/lb. It can be seen that when evaporation takes place at
`higher temperatures the latent heat becomes smaller and vanishes completely at the
`critical temperature of 233.6°F, where the liquid line and vapor line come together.
`No liquid phase can exist above the critical temperature.
`In the area above the
`critical temperature,
`to the right of the saturated vapor line and to the left of the
`saturated liquid line, only one phase exists. At higher pressures, higher than the
`critical pressure, the refrigerant could be called a light liquid or a dense gas. It
`does not really make any difference what the fluid is called—especially in refrigera-
`tion, where pressures never get that high (or at least should not).
`Not all of the latent heat of vaporization is available for cooling the food in
`the refrigerator or the air in the air conditioner. Part of it must be used to reduce
`the temperature of the hot liquid coming from the condenser. Cooling of the liquid
`is represented by EB in Figure 3—2. Of course, heat changes in the evaporator do
`not occur in such neat, consecutive steps. The net change is found by the heat
`content of the vapor leaving the evaporator at C minus the heat content of the
`liquid entering the evaporator at E or leaving the condenser, assuming no heat
`change in the liquid line. The amount of heat to be extracted from the liquid to
`cool it from 120°F to 0°F is shown by the segment AB, or 27.5 Btu/lb. The rest of
`the latent heat, BC, or 41.3 Btu/lb, is available for cooling work and is called the
`net Vefrigerating effect.
`'
`Lines of constant volume for the gas are shown on the chart as nearly horizontal
`lines. Point C is shown as the outlet of the evaporator and the inlet to the compressor.
`In a real machine the temperature of the gas entering the compressor would be
`considerably higher than shown at C. However, the pressure would be substantially
`constant and would be the vapor pressure at 0°F. The volume of the gas actually
`
`24
`
`Page 15 of 34
`
`Refrigeration
`
`Chap. 3
`
`

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`entering the cylinder of the compressor depends on the temperature of the 835 at
`that point. This temperature is difficult to measure since it is in the interim Of the
`compressor and is considerably higher than the temperature at the entrance to the
`compressor shell. However, the volume change is small for small changes in tempera-
`ture. The compressor inlet temperature would probably be between 80 and l00°F,
`but for this simplified cycle it will be considered as at point C. The volume of the
`saturated vapor at 0°}: is about 1.6 Btu/lb.
`The specific volume of the refrigerant gas entering the compressor cylinder
`can be used to calculate the load if the compressor displacement is known or the
`compressor size needed for a given capacity. For example, suppose that it is desired
`to develop 1 ton of refrigeration—or 12,000 Btu/hr or 200 Btu/min. The amount
`of cooling available is 41.3 Btu/lb, so to get 200 Btu/min, the refrigerant must go
`around the system at a rate of about 4.8 lb/min. With a specific volume for the
`refrigerant of 1.6 ft3/lb,
`the volumetric displacement of the compressor must be
`about 7.8 ft?‘/min or more.
`
`Assuming that the refrigerant gas enters the cylinder of the compressor at C
`and that the compression is adiabatic or at constant entropy, the increase in enthalpy
`of the gas is represented by CD and is 15.2 Btu/lb. The point D is located by the
`pressure in the condenser and is the vapor pressure of the liquid at 120°F. The
`intersection of the constant-pressure line and the constant-entropy line fixes point
`D. This analysis ignores pressure drop in the discharge line from the compressor.
`A correction can be made by measuring the pressure at the compressor outlet or
`by calculating the pressure drop as described elsewhere. A correction can be made
`for departure from constant entropy by measuring the temperature of the gas at the
`compressor discharge. Without these corrections the temperature of the gas as it
`leaves the compressor at point D is 140°F.
`The heat added to the gas during compression is given in Btu per pound in
`Figure 3-2. To convert to power, a time factor must be added. Multiplying the
`compressor heat in Btu/lb by the refrigerant circulated in lb/min, the power required
`to compress the gas is found to be 73.6 Btu/min. Some other units for expressing
`power are shown below. These values are the theoretical power needed to produce
`1 ton of refrigeration with R-12 at 0°F evaporating and 120°F condensing tempera-
`tures.
`
`73.6 Btu/min
`
`1.73 hp
`
`1293 W
`
`954 ft-lb/sec
`
`the
`During the condensing of the refrigerant, shown in Figure 3-2 as DE,
`pressure remains at the vapor pressure of the liquid at 120°F. Some heat must be
`removed from the gas before condensation begins; in the example, this is 3.8 Btul
`lb. As more heat is removed, more and more liquid is formed until the liquid line
`is reached at E. The latent heat of condensation is 52.6 Btu/lb. The amount of
`
`Page 18 of 34
`
`Charts and Tables
`
`27
`
`

`
`superheat in the gas going to the condenser is important because heat transfer from
`a gas is much poorer than with a condensing liquid. Several times as much condenser
`surface must be provided for removing superheat as for condensing liquid. If the
`condenser is on the small side, the condensing temperature and pressure will be
`higher and more power will be required to compress the refrigerant.
`One of the uses of the pressure—entha1py chart is to compare different operating
`conditions. Suppose that the condenser is at I00°F instead of at 120°F, as illustrated
`in Figure 3-3. Perhaps the condenser has been cleaned or the ambient air temperature
`is lower or a larger condenser is used. What will happen to the capacity and the
`horsepower needed for compression? Although the latent heat of evaporation remains
`the same,
`the net refrigeration available is increased because the liquid entering
`the evaporator requires less cooling. Less compressor heat and power are needed
`because the compression ratio is lower; The discharge temperature would be lower
`and the amount of superheat in the gas would be slightly less. The total amount of
`heat removed in the condenser, however, would be a little greater.
`The effect of subcooling the liquid before it enters the evaporator is illustrated
`in Figure 3-4. Subcooling means removing heat from the liquid without a change
`in pressure. If the condenser is large enough, subcooling may occur in the bottom
`rows of the condenser. In order to be helpful, the heat must be removed by some
`means other than evaporation of the liquid. In the example in Figure 3-4, reducing
`the temperature of the liquid by 20°F will increase the net refrigeration and also
`the capacity by more than 10%. The compression power, however, will stay the
`same since the pressure in the condenser will remain at the vapor pressure at l20°F.
`Earlier it was assumed that saturated refrigerant vapor left the evaporator and
`immediately entered the compressor cylinder. However,
`this is not ordinarily a
`very realistic or desirable condition. Some superheat is needed to ensure that liquid
`refrigerant does not enter the compressor. The effect of superheat is illustrated in
`Figure 3-5. Assume that the refrigerant is superheated 65°F—in this case the gas
`will be at 65°F since the evaporating temperature is O°F. The pressure does not
`change because it is controlled by the vapor pressure of the evaporating liquid.
`The increase in enthalpy is represented by CK in Figure 3-5. If all of the enthalpy
`change occurs outside the evaporator, there will be no change in the net refrigerating
`effect and the capacity will remain the same. However, if part of the increase in
`enthalpy takes place inside the evaporator, that amount of enthalpy would be added
`to the net effect and the capacity would be increased proportionally. In either case,
`the gas entering the compressor will be at a different entropy and somewhat more
`heat and power will be needed for compression, as illustrated in Figure 3-5. The
`discharge temperature will also be considerably higher. Often, some heat is taken
`from the hot liquid and added to the cold suction vapor with direct benefit from
`subcooling the liquid. This can be done with a formal heat exchanger or by soldering
`together the liquid and suction lines for a short distance. Of course, adding heat to
`the suction gas leads to an increase in power requirements and higher discharge
`temperatures, so the gain must be balanced against the loss. The nature of the
`refrigerant is also important. For example, more will be gained by using a liquid-
`vapor heat exchanger with R-502 than with R-12 or R-22.
`Pressure—enthalpy charts are tools to use in better understanding refrigeration
`
`28
`
`Page 19 of 34
`
`Refrigeration
`
`Chap. 3
`
`

`
`
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`cycles, for judging whether the operation is normal, and for estimating the effect
`of changes in operation. Learning how to read the charts and to do the calculations
`involved will pay rich dividends in all phases of refrigeration work. Examples of
`refrigeration calculations are shown in Table 3—1.
`
`TABLE 3-1 REFRIGERATION CALCULATIONS
`
`.
`.
`. Net refrigerating
`efiect (Btu/lb)
`
`_
`
`heat content of
`vapor leaving
`evaporator (Btullb)
`
`_
`
`heat content of
`liquid entering
`evaporator (Btullb)
`
`. Net refrigerating
`effect (Btu/lb)
`
`[momheatof]
`
`vaporintion
`'
`(Btu/lb)
`
`changeinheatcontent
`
`of liquid from condens-
`ing to evaporating
`temperature (Btu/lb)
`
`. Net refrigerating
`Cffect (Btu/lb)
`
`capacity (Btu/min)
`refrigerant circulated (lb/min)
`
`. Refrigerant circulated
`(lb/min)
`
`load or capacity (Btu/min)
`net refrigerating effect (Btu/lb)
`
`. Compressor
`displacement (ft3/min)
`
`refrigerant
`circulated
`(lb/min)
`
`volume of gas
`entering com-
`pressor (ft3/lb)
`
`X
`
`. Compressor
`displacement (ft3/min)
`
`_
`_
`
`.
`
`volume of gas
`
`[capacity ] X [enteringcom- J
`(Btu/mm)
`pressor (ft3/lb)
`net refrigerating effect (Btu/lb)
`
`. Heat of
`compression (Btu/lb)
`
`_ heatcontentof
`
`vapor leaving
`compressor (Btu/lb)
`
`J
`
`[heatcontentof
`
`vapor entering
`compressor (Btu/lb)
`
`J
`
`. Heat of
`compression (Btu/lb)
`
`= (42.4l8 Btu/min) (compression horsepower)
`refrigerant circulated (lb/min)
`
`. Compression work
`(Btu/min)
`
`: heat of com-
`[pression (Btu/lb)]
`
`refrigerant cir-
`[culated (lb/min)]
`
`. Compression
`horsepower
`
`. Compression
`
`horsepower
`
`. Compression
`horsepower
`
`_
`
`compression work (Btu/min)
`conversion factor (42.41 8 Btu/min)
`
`heat of com-
`[pression (Btu/lb)]
`
`X capacity
`[(Btu/min)]
`
`(42,418 Btu/min) X
`
`net refriger-
`ating effect
`(Btu/lb)
`
`capacity (Btu/min)
`
`(42.4l8 Btu/min) >< [gfpfggfiance]
`
`32
`
`Page 23 of 34
`
`Refrigeration
`
`Chap. 3
`
`

`
`TABLE 3-1
`
`(Continued)
`
`13. Compression
`horsepower per ton
`
`4.715
`_
`* coefficient of perfonnance
`
`= I:
`
`compression horse-
`power per ton
`
`] x 745.7
`
`__ net refrigerating effect (Btu/lb)
`
`heat of compression (Btu/lb)
`
`refrigerant
`= circulated
`(lb/rnin)
`
`net
`X refrigerating
`effect (Btu/lb)
`
`net
`compressor
`refrigerating
`dis3placement
`effect {Btu/lb}
`_ {ft /min}
`_
`[volume of gas entering compressor (ft )]
`
`X
`
`. Power (W)
`
`. Coefficient of
`
`performance
`
`. Capacity
`(Btu/min)
`
`. Capacity
`(Btu/min)
`
`. Capacity
`(Btu/min
`
`REFRIGERANT8
`
`N: compression ] X [42.418 :1 X [net refrigerating]
`‘
`heat of compression (Btu/lb)
`
`horsepower
`
`Btu/min
`
`effect (Btu/lb)
`
`The operation of a refrigeration system depends on two principal parts: the compressor
`and the refrigerant. Other parts of the system, such as the condenser, evaporator,
`receiver, oil separator, and connecting piping, are also important and must be properly
`sized, located, and installed for best results. The compressor is the real heart of
`the system and the only part where mechanical movement occurs—other than auto-
`matic expansion valves. Compressor design must consider clearance volume,‘ heat
`losses by radiation and other means, compression ratios that might be encountered,
`expected life, cost, and similar factors, including the nature of the refrigerant.
`For many applications, any of two or more refrigerants may be suitable. The
`decision regarding the refrigerant may be based on precedent, stability, toxicity,
`oil and water solubility, flammability, cost, and so on, and such criteria are indeed
`important and must be satisfied by the final refrigerant selection. However, it is
`also interesting, entertaining, and occasionally enlightening to compare refrigerants
`strictly on the basis of performance.
`W. A. Pennington at the University of Maryland reviewed some refrigerant
`comparisons [3—5] and suggested a plot of evaporator pressure versus capacity as a
`correlating device. A straight line is obtained with a number of refrigerants, assuming
`the same volumetric displacement and the same evaporating and condensing tempera-
`tures. Such a comparison is shown in Figure 3-6. In this comparison both the
`capacity and evaporator pressure are given as percentages of R-12 since a direct
`
`‘In a reciprocating compressor the space between the top of the cylinder and the top of the piston at
`the apex of its stroke is called the clearance volume. Gas in this volume is compressed but not expelled
`from the cylinder, so compression work is required without any gain in refrigeration.
`
`Refrigera nts
`
`Page 24 of 34
`
`

`
`E?
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`100
`
`150
`
`200
`
`Capacity (% of R-12)
`
`Figure 3-6 Capacity versus evaporator pressure related to R-12.
`
`comparison would produce a different curve for each different refrigeration cycle.
`The line in Figure 3-6 is larg