Refrigeration1.1. IntroductionRefrigeration may be defined as the process of achieving and maintaining atemperature below that of the surroundings, the aim being to cool some product or space tothe required temperature. One of the most important applications of refrigeration has been thepreservation of perishable food products by storing them at low temperatures. Refrigerationsystems are also used extensively for providing thermal comfort to human beings by meansof air conditioning. Air Conditioning refers to the treatment of air so as to simultaneouslycontrol its temperature, moisture content, cleanliness, odour and circulation, as required byoccupants, a process, or products in the space.Q: Which of the following can be called as a refrigeration process?a) Cooling of hot ingot from 1000oC to room temperatureb) Cooling of a pot of water by mixing it with a large block of icec) Cooling of human beings using a ceiling fand) Cooling of a hot cup of coffee by leaving it on a tablee) Cooling of hot water by mixing it with tap waterf) Cooling of water by creating vacuum over itAns: b) and f)1.2. Natural RefrigerationIn olden days refrigeration was achieved by natural means such as the use of ice orevaporative cooling. In earlier times, ice was either:1. Transported from colder regions,2. Harvested in winter and stored in ice houses for summer use or,3. Made during night by cooling of water by radiation to stratosphere1.2.1. Art of Ice making by Nocturnal Cooling:The art of making ice by nocturnal cooling was perfected in India. In this method icewas made by keeping a thin layer of water in a shallow earthen tray, and then exposing thetray to the night sky. Compacted hay of about 0.3 m thickness was used as insulation. Thewater looses heat by radiation to the stratosphere, which is at around -55C and by earlymorning hours the water in the trays freezes to ice. This method of ice production was verypopular in India.1.2.2. Evaporative Cooling:As the name indicates, evaporative cooling is the process of reducing the temperatureof a system by evaporation of water. Human beings perspire and dissipate their metabolicheat by evaporative cooling if the ambient temperature is more than skin temperature.Animals such as the hippopotamus and buffalo coat themselves with mud for evaporativecooling. Evaporative cooling has been used in India for centuries to obtain cold water insummer by storing the water in earthen pots. The water permeates through the pores ofearthen vessel to its outer surface where it evaporates to the surrounding, absorbing its latentheat in part from the vessel, which cools the water.Evaporative cooling by placing wet straw mats on the windows is alsovery common in India. The straw mat made from khus adds its inherent perfume also to theair. Now-a-days desert coolers are being used in hot and dry areas to provide cooling insummer.1.2.3. Cooling by Salt Solutions:Certain substances such as common salt, when added to water dissolve in water andabsorb its heat of solution from water (endothermic process). This reduces the temperature ofthe solution (water+salt). Sodium Chloride salt (NaCl) can yield temperatures up to -20Cand Calcium Chloride (CaCl2) up to - 50C in properly insulated containers. However, as it isthis process has limited application, as the dissolved salt has to be recovered from its solutionby heating.Q. The disadvantages of natural refrigeration methods are:a) They are expensiveb) They are uncertainc) They are not environment friendlyd) They are dependent on local conditionsAns: b) and d)Q. Evaporative cooling systems are ideal for:a) Hot and dry conditionsb) Hot and humid conditionsc) Cold and humid conditionsd) Moderately hot but humid conditionsAns: a)1.3. Artificial RefrigerationRefrigeration as it is known these days is produced by artificial means. Though it isvery difficult to make a clear demarcation between natural and artificial refrigeration, it isgenerally agreed that the history of artificial refrigeration began in the year 1755, when theScottish professor William Cullen made the first refrigerating machine, which could producea small quantity of ice in the laboratory. Based on the working principle, refrigerationsystems can be classified as vapour compression systems, vapour absorption systems, gascycle systems etc.1.3.1. Vapour Compression Refrigeration Systems:The basis of modern refrigeration is the ability of liquids to absorb enormousquantities of heat as they boil and evaporate. Professor William Cullen of the University ofEdinburgh demonstrated this in 1755 by placing some water in thermal contact with etherunder a receiver of a vacuum pump. The evaporation rate of ether increased due to thevacuum pump and water could be frozen. This process involves two thermodynamicconcepts, the vapour pressure and the latent heat. A liquid is in thermal equilibrium with itsown vapor at a pressure called the saturation pressure, which depends on the temperaturealone. If the pressure is increased for example in a pressure cooker, the water boils at highertemperature. The second concept is that the evaporation of liquid requires latent heat duringevaporation. If latent heat is extracted from the liquid, the liquid gets cooled. The temperatureof ether will remain constant as long as the vacuum pump maintains a pressure equal tosaturation pressure at the desired temperature.This requires the removal of all the vaporsformed due to vaporization. If a lower temperature is desired, then a lower saturation pressurewill have to be maintained by the vacuum pump. The component of the modern dayrefrigeration system where cooling is produced by this method is called evaporator.If this process of cooling is to be made continuous the vapors have to be recycled bycondensation to the liquid state. The condensation process requires heat rejection to thesurroundings. It can be condensed at atmospheric temperature by increasing its pressure. Hence, a compressor is required to maintain a high pressure so that the evaporatingvapours can condense at a temperature greater than that of the surroundings.The man responsible for making a practical vapor compression refrigeration systemwas James Harrison who took a patent in 1856 for a vapour compression system using ether,alcohol or ammonia. Charles Tellier of France patented in 1864, a refrigeration system usingdimethyl ether which has a normal boiling point of 23.6C.Carl von Linde in Munich introduced double acting ammonia compressor. It requiredpressures of more than 10 atmospheres in the condenser. Since the normal boiling point ofammonia is -33.3C, vacuum was not required on the low pressure side. Since then ammoniais used widely in large refrigeration plants.David Boyle, in fact made the first NH3 system in 1871 in San Francisco. JohnEnright had also developed a similar system in 1876 in Buffalo N.Y. Franz Windhausendeveloped carbon dioxide (CO2) based vapor compression system in Germany in 1886. Thecarbon dioxide compressor requires a pressure of about 80 atmospheres and therefore a veryheavy construction. Linde in 1882 and T.S.C. Lowe in 1887 tried similar systems in USA.The CO2 system is a very safe system and was used in ship refrigeration until 1960s. RaoulPictet used SO2 (NBP -10C) as refrigerant. Its lowest pressure was high enough to preventthe leakage of air into the system.Palmer used C2H5Cl in 1890 in a rotary compressor. He mixed it with C2H5Br toreduce its flammability. Edmund Copeland and Harry Edwards used iso-butane in 1920 insmall refrigerators. It disappeared by 1930 when it was replaced by CH3Cl. Dichloroethylene(Dielene or Dieline) was used by Carrier in centrifugal compressors in 1922-26.

Figure 1.3 shows the basic components of a vapour compression refrigeration system.As shown in the figure the basic system consists of an evaporator, compressor, condenser andan expansion valve. The refrigeration effect is obtained in the cold region as heat is extractedby the vaporization of refrigerant in the evaporator. The refrigerant vapour from theevaporator is compressed in the compressor to a high pressure at which its saturationtemperature is greater than the ambient or any other heat sink. Hence when the high pressure,high temperature refrigerant flows through the condenser, condensation of the vapour intoliquid takes place by heat rejection to the heat sink. To complete the cycle, the high pressureliquid is made to flow through an expansion valve. In the expansion valve the pressure andtemperature of the refrigerant decrease. This low pressure and low temperature refrigerantvapour evaporates in the evaporator taking heat from the cold region. It should be observedthat the system operates on a closed cycle. The system requires input in the form ofmechanical work. It extracts heat from a cold space and rejects heat to a high temperatureheat sink.

A refrigeration system can also be used as a heat pump, in which the useful output isthe high temperature heat rejected at the condenser. Alternatively, a refrigeration system canbe used for providing cooling in summer and heating in winter. Such systems have been builtand are available now.10.2. Vapour Compression Refrigeration SystemsAs mentioned, vapour compression refrigeration systems are the most commonly usedamong all refrigeration systems. As the name implies, these systems belong to the generalclass of vapour cycles, wherein the working fluid (refrigerant) undergoes phase change atleast during one process. In a vapour compression refrigeration system, refrigeration isobtained as the refrigerant evaporates at low temperatures. The input to the system is inthe form of mechanical energy required to run the compressor. Hence these systems arealso called as mechanical refrigeration systems. Vapour compression refrigeration systems are available to suit almost all applications with the refrigeration capacitiesranging from few Watts to few megawatts. A wide variety of refrigerants can be used inthese systems to suit different applications, capacities etc. The actual vapour compressioncycle is based on Evans-Perkins cycle, which is also called as reverse Rankine cycle.Before the actual cycle is discussed and analysed, it is essential to find the upper limit ofperformance of vapour compression cycles. This limit is set by a completely reversiblecycle.Carnot refrigeration cycle is a completely reversible cycle, hence is used as a model ofperfection for a refrigeration cycle operating between a constant temperature heat sourceand sink. It is used as reference against which the real cycles are compared. Figures 10.1(a) and (b) show the schematic of a Carnot vapour compression refrigeration system andthe operating cycle on T-s diagram.As shown in Fig.10.1(a), the basic Carnot refrigeration system for pure vapour consists offour components: compressor, condenser, turbine and evaporator. Refrigeration effect (q4-1 = qe) is obtained at the evaporator as the refrigerant undergoes the process ofvaporization (process 4-1) and extracts the latent heat from the low temperature heatsource. The low temperature, low pressure vapour is then compressed isentropically inthe compressor to the heat sink temperature Tc. The refrigerant pressure increases from Peto Pc during the compression process (process 1-2) and the exit vapour is saturated. Nextthe high pressure, high temperature saturated refrigerant undergoes the process ofcondensation in the condenser (process 2-3) as it rejects the heat of condensation (q2-3 =qc) to an external heat sink at Tc. The high pressure saturated liquid then flows throughthe turbine and undergoes isentropic expansion (process 3-4). During this process, thepressure and temperature fall from Pc,Tc to Pe, Te. Since a saturated liquid is expanded inthe turbine, some amount of liquid flashes into vapour and the exit condition lies in thetwo-phase region. This low temperature and low pressure liquid-vapour mixture thenenters the evaporator completing the cycle. Thus as shown in Fig.10.1(b), the cycleinvolves two isothermal heat transfer processes (processes 4-1 and 2-3) and twoisentropic work transfer processes (processes 1-2 and 3-4). Heat is extracted isothermallyat evaporator temperature Te during process 4-1, heat is rejected isothermally atcondenser temperature Tc during process 2-3. Work is supplied to the compressor duringthe isentropic compression (1-2) of refrigerant vapour from evaporator pressure Pe tocondenser pressure Pc, and work is produced by the system as refrigerant liquid expandsisentropically in the turbine from condenser pressure Pc to evaporator pressure Pe. All theprocesses are both internally as well as externally reversible, i.e., net entropy generationfor the system and environment is zero.Applying first and second laws of thermodynamics to the Carnot refrigeration cycle,

now for the reversible, isothermal heat transfer processes 2-3 and 4-1, we can write:

thus the COP of Carnot refrigeration cycle is a function of evaporator and condensertemperatures only and is independent of the nature of the working substance. This is thereason why exactly the same expression was obtained for air cycle refrigeration systemsoperating on Carnot cycle (Lesson 9). The Carnot COP sets an upper limit forrefrigeration systems operating between two constant temperature thermal reservoirs(heat source and sink). From Carnots theorems, for the same heat source and sinktemperatures, no irreversible cycle can have COP higher than that of Carnot COP.

It can be seen from the above expression that the COP of a Carnot refrigeration systemincreases as the evaporator temperature increases and condenser temperature decreases.This can be explained very easily with the help of the T-s diagram (Fig.10.2). As shownin the figure, COP is the ratio of area a-1-4-b to the area 1-2-3-4. For a fixed condensertemperature Tc, as the evaporator temperature Te increases, area a-1-4-b (qe) increasesand area 1-2-3-4 (wnet) decreases as a result, COP increases rapidly. Similarly for a fixedevaporator temperature Te, as the condensing temperature Tc increases, the net work input(area 1-2-3-4) increases, even though cooling output remains constant, as a result theCOP falls. Figure 10.3 shows the variation of Carnot COP with evaporator temperaturefor different condenser temperatures. It can be seen that the COP increases sharply withevaporator temperatures, particularly at high condensing temperatures. COP reduces asthe condenser temperature increases, but the effect becomes marginal at low evaporatortemperatures. It will be shown later that actual vapour compression refrigeration systemsalso behave in a manner similar to that of Carnot refrigeration systems as far as theperformance trends are concerned.

Practical difficulties with Carnot refrigeration system:It is difficult to build and operate a Carnot refrigeration system due to the followingpractical difficulties:i. During process 1-2, a mixture consisting of liquid and vapour have to be compressedisentropically in the compressor. Such a compression is known as wet compression due tothe presence of liquid. In practice, wet compression is very difficult especially withreciprocating compressors. This problem is particularly severe in case of high speedreciprocating compressors, which get damaged due to the presence of liquid droplets inthe vapour. Even though some types of compressors can tolerate the presence of liquid invapour, since reciprocating compressors are most widely is refrigeration, traditionally drycompression (compression of vapour only) is preferred to wet compression.ii. The second practical difficulty with Carnot cycle is that using a turbine and extractingwork from the system during the isentropic expansion of liquid refrigerant is noteconomically feasible, particularly in case of small capacity systems. This is due to thefact that the specific work output (per kilogram of refrigerant) from the turbine is givenby:

since the specific volume of liquid is much smaller compared to the specific volume of avapour/gas, the work output from the turbine in case of the liquid will be small. Inaddition, if one considers the inefficiencies of the turbine, then the net output will befurther reduced. As a result using a turbine for extracting the work from the high pressureliquid is not economically justified in most of the cases1.One way of achieving dry compression in Carnot refrigeration cycle is to have twocompressors one isentropic and one isothermal as shown in Fig.10.4.

As shown in Fig.10.4, the Carnot refrigeration system with dry compression consists ofone isentropic compression process (1-2) from evaporator pressure Pe to an intermediatepressure Pi and temperature Tc, followed by an isothermal compression process (2-3)from the intermediate pressure Pi to the condenser pressure Pc. Though with thismodification the problem of wet compression can be avoided, still this modified system isnot practical due to the difficulty in achieving true isothermal compression using highspeedcompressors. In addition, use of two compressors in place of one is noteconomically justified.From the above discussion, it is clear that from practical considerations, the Carnotrefrigeration system need to be modified. Dry compression with a single compressor ispossible if the isothermal heat rejection process is replaced by isobaric heat rejectionprocess. Similarly, the isentropic expansion process can be replaced by an isenthalpicthrottling process. A refrigeration system, which incorporates these two changes isknown as Evans-Perkins or reverse Rankine cycle. This is the theoretical cycle on whichthe actual vapour compression refrigeration systems are based.

10.4. Standard Vapour Compression Refrigeration System (VCRS)Figure 10.5 shows the schematic of a standard, saturated, single stage (SSS) vapourcompression refrigeration system and the operating cycle on a T s diagram. As shown inthe figure the standard single stage, saturated vapour compression refrigeration systemconsists of the following four processes:Process 1-2: Isentropic compression of saturated vapour in compressorProcess 2-3: Isobaric heat rejection in condenserProcess 3-4: Isenthalpic expansion of saturated liquid in expansion deviceProcess 4-1: Isobaric heat extraction in the evaporatorBy comparing with Carnot cycle, it can be seen that the standard vapour compressionrefrigeration cycle introduces two irreversibilities: 1) Irreversibility due to non-isothermalheat rejection (process 2-3) and 2) Irreversibility due to isenthalpic throttling (process 3-4). As a result, one would expect the theoretical COP of standard cycle to be smaller thanthat of a Carnot system for the same heat source and sink temperatures. Due to theseirreversibilities, the cooling effect reduces and work input increases, thus reducing thesystem COP. This can be explained easily with the help of the cycle diagrams on T scharts. Figure 10.6(a) shows comparison between Carnot and standard VCRS in terms ofrefrigeration effect.

thus there is a reduction in refrigeration effect when the isentropic expansion process ofCarnot cycle is replaced by isenthalpic throttling process of VCRS cycle, this reduction isequal to the area d-4-4-c-d (area A2) and is known as throttling loss. The throttling lossis equal to the enthalpy difference between state points 3 and 4, i.e,

It is easy to show that the loss in refrigeration effect increases as the evaporatortemperature decreases and/or condenser temperature increases. A practical consequenceof this is a requirement of higher refrigerant mass flow rate.The heat rejection in case of VCRS cycle also increases when compared to Carnot cycle.

As shown in Fig.10.6(b), the heat rejection in case of Carnot cycle (1-2-3-4) is givenby:

Hence the increase in heat rejection rate of VCRS compared to Carnot cycle is equal tothe area 2-2-2 (area A1). This region is known as superheat horn, and is due to thereplacement of isothermal heat rejection process of Carnot cycle by isobaric heatrejection in case of VCRS.Since the heat rejection increases and refrigeration effect reduces when the Carnot cycleis modified to standard VCRS cycle, the net work input to the VCRS increases comparedto Carnot cycle. The net work input in case of Carnot and VCRS cycles are given by:

To summarize the refrigeration effect and net work input of VCRS cycle are given by:

If we define the cycle efficiency, R as the ratio of COP of VCRS cycle to the COP ofCarnot cycle, then:

The cycle efficiency (also called as second law efficiency) is a good indication of thedeviation of the standard VCRS cycle from Carnot cycle. Unlike Carnot COP, the cycleefficiency depends very much on the shape of T s diagram, which in turn depends on thenature of the working fluid.If we assume that the potential and kinetic energy changes during isentropic compressionprocess 1-2 are negligible, then the work input w1-2 is given by:

Now as shown in Fig.10.7, if we further assume that the saturated liquid line 3-fcoincides with the constant pressure line Pc in the subcooled region (which is areasonably good assumption), then from the 2nd Tds relation;

Substituting these expressions in the expression for net work input, we obtain thecompressor work input to be equal to area 1-2-3-f-1. Now comparing this with the earlierexpression for work input (area 1-2-3-4-c-d-4-1), we conclude that area A2 is equal toarea A3.As mentioned before, the losses due to superheat (area A1) and throttling (area A2 A3)depend very much on the shape of the vapor dome (saturation liquid and vapour curves)on T s diagram. The shape of the saturation curves depends on the nature of refrigerant.Figure 10.8 shows T s diagrams for three different types of refrigerants.

Refrigerants such as ammonia, carbon di-oxide and water belong to Type 1. Theserefrigerants have symmetrical saturation curves (vapour dome), as a result both thesuperheat and throttling losses (areas A1 and A3) are significant. That means deviation ofVCRS cycle from Carnot cycle could be significant when these refrigerants are used asworking fluids. Refrigerants such as CFC11, CFC12, HFC134a belong to Type 2, theserefrigerants have small superheat losses (area A1) but large throttling losses (area A3).High molecular weight refrigerants such as CFC113, CFC114, CFC115, iso-butanebelonging to Type 3, do not have any superheat losses, i.e., when the compression inletcondition is saturated (point 1), then the exit condition will be in the 2-phase region, as aresult it is not necessary to superheat the refrigerant. However, these refrigerantsexperience significant throttling losses. Since the compressor exit condition of Type 3refrigerants may fall in the two-phase region, there is a danger of wet compressionleading to compressor damage. Hence for these refrigerants, the compressor inletcondition is chosen such that the exit condition does not fall in the two-phase region. Thisimplies that the refrigerant at the inlet to the compressor should be superheated, theextent of which depends on the refrigerant.Superheat and throttling losses:It can be observed from the discussions that the superheat loss is fundamentally differentfrom the throttling loss. The superheat loss increases only the work input to thecompressor, it does not effect the refrigeration effect. In heat pumps superheat is not aloss, but a part of the useful heating effect. However, the process of throttling isinherently irreversible, and it increases the work input and also reduces the refrigerationeffect.1Analysis of standard vapour compression refrigerationsystemA simple analysis of standard vapour compression refrigeration system can be carried outby assuming a) Steady flow; b) negligible kinetic and potential energy changes acrosseach component, and c) no heat transfer in connecting pipe lines. The steady flow energyequation is applied to each of the four components.Evaporator: Heat transfer rate at evaporator or refrigeration capacity is given by:

Where is the refrigerant mass flow rate in kg/s, h1 and h4 are the specific enthalpies (kJ/kg) at the exit and inlet to the evaporator, respectively. (h1 h4 ) is known as specific refrigeration effect or simply refrigeration effect, which is equal to the heat transferred atthe evaporator per kilogram of refrigerant. The evaporator pressure Pe is the saturationpressure corresponding to evaporator temperature Te, i.e.,

Compressor: Power input to the compressor, is given by:

where h2 and h1 are the specific enthalpies (kJ/kg) at the exit and inlet to the compressor,respectively. (h 2 h1 ) is known as specific work of compression or simply work ofcompression, which is equal to the work input to the compressor per kilogram ofrefrigerant.Condenser: Heat transfer rate at condenser, is given by:

where h3 and h2 are the specific enthalpies (kJ/kg) at the exit and inlet to the condenser,respectively.The condenser pressure Pc is the saturation pressure corresponding to evaporator Temperature, i.e.,

Expansion device: For the isenthalpic expansion process, the kinetic energy changeacross the expansion device could be considerable, however, if we take the control volume, well downstream of the expansion device, then the kinetic energy gets dissipateddue to viscous effects, and

The exit condition of the expansion device lies in the two-phase region, hence applyingthe definition of quality (or dryness fraction), we can write:

where x4 is the quality of refrigerant at point 4, hf,e, hg,e, hfg are the saturated liquidenthalpy, saturated vapour enthalpy and latent heat of vaporization at evaporatorpressure, respectively

The COP of the system is given by:

At any point in the cycle, the mass flow rate of refrigerant can be written in terms of volumetric flow rate and specific volume at that point, i.e.,

applying this equation to the inlet condition of the compressor,

where is the volumetric flow rate at compressor inlet and v1 is the specific volume at compressor inlet. At a given compressor speed, V1 is an indication of the size of thecompressor. We can also write, the refrigeration capacity in terms of volumetric flow rateas:

where is called as volumetric refrigeration effect (kJ3/m of refrigerant).

Generally, the type of refrigerant, required refrigeration capacity, evaporator temperatureand condenser temperature are known. Then from the evaporator and condenser temperature one can find the evaporator and condenser pressures and enthalpies at theexit of evaporator and condenser (saturated vapour enthalpy at evaporator pressure andsaturated liquid enthalpy at condenser pressure). Since the exit condition of thecompressor is in the superheated region, two independent properties are required to fixthe state of refrigerant at this point. One of these independent properties could be thecondenser pressure, which is already known. Since the compression process is isentropic,the entropy at the exit to the compressor is same as the entropy at the inlet, s1 which is thesaturated vapour entropy at evaporator pressure (known). Thus from the known pressureand entropy the exit state of the compressor could be fixed, i.e.,

The quality of refrigerant at the inlet to the evaporator (x4) could be obtained from theknown values of h3, hf,e and hg,e.Once all the state points are known, then from the required refrigeration capacity and various enthalpies one can obtain the required refrigerant mass flow rate, volumetric flow rate at compressor inlet, COP, cycle efficiency etc. Use of Pressure-enthalpy (P-h) charts:

Q. Compared to natural refrigeration methods, artificial refrigeration methods are:a) Continuousb) Reliablec) Environment friendlyd) Can work under almost all conditionsAns. a), b) and d)Q. In the evaporator of a vapour compression refrigeration system:a) A low temperature is maintained so that heat can flow from the external fluidb) Refrigeration effect is produced as the refrigerant liquid vaporizesc) A low pressure is maintained so that the compressor can rund) All of the aboveAns. a) and b)Q. The function of a compressor in a vapour compression refrigeration system is to:a) To maintain the required low-side pressure in the evaporatorb) To maintain the required high-side pressure in the condenserc) To circulate required amount of refrigerant through the systemd) To safeguard the refrigeration systemAns. a), b) and c)Q. In a vapour compression refrigeration system, a condenser is primarily required so that:a) A high pressure can be maintained in the systemb) The refrigerant evaporated in the evaporator can be recycledc) Performance of the system can be improvedd) Low temperatures can be producedAns. b)Q. The function of an expansion valve is to:a) Reduce the refrigerant pressureb) Maintain high and low side pressuresc) Protect evaporatord) All of the aboveAns. b)Q. In a domestic icebox type refrigerator, the ice block is kept at the top because:a) It is convenient to the userb) Disposal of water is easierc) Cold air can flow down due to buoyancy effectd) None of the aboveAns. c)Q. An air conditioning system employs a refrigeration system to:a) Cool and dehumidify air supplied to the conditioned spaceb) To heat and humidify the air supplied to the conditioned spacec) To circulate the air through the systemd) To purify the supply airAns. a)1.3.2. Vapour Absorption Refrigeration Systems:John Leslie in 1810 kept H2SO4 and water in two separate jars connected together.H2SO4 has very high affinity for water. It absorbs water vapour and this becomes theprinciple of removing the evaporated water vapour requiring no compressor or pump. H2SO4is an absorbent in this system that has to be recycled by heating to get rid of the absorbedwater vapour, for continuous operation. Windhausen in 1878 used this principle forabsorption refrigeration system, which worked on H2SO4. Ferdinand Carre invented aquaammoniaabsorption system in 1860. Water is a strong absorbent of NH3. If NH3 is kept in avessel that is exposed to another vessel containing water, the strong absorption potential ofwater will cause evaporation of NH3 requiring no compressor to drive the vapours. A liquidpump is used to increase the pressure of strong solution. The strong solution is then heated ina generator and passed through a rectification column to separate the water from ammonia.The ammonia vapour is then condensed and recycled. The pump power is negligible hence;the system runs virtually on low- grade energy used for heating the strong solution to separatethe water from ammonia. These systems were initially run on steam. Later on oil and naturalgas based systems were introduced. Figure 1.4 shows the essential components of a vapourabsorption refrigeration system. In 1922, Balzar von Platen and Carl Munters, two students atRoyal Institute of Technology, Stockholm invented a three fluid system that did not require apump. A heating based bubble pump was used for circulation of strong and weak solutionsand hydrogen was used as a non-condensable gas to reduce the partial pressure of NH3 in theevaporator. Geppert in 1899 gave this original idea but he was not successful since he wasusing air as non-condensable gas. The Platen-Munters refrigeration systems are still widelyused in certain niche applications such as hotel rooms etc. Figure 1.5 shows the schematic ofthe triple fluid vapour absorption refrigeration system.

Another variation of vapour absorption system is the one based on Lithium Bromide(LiBr)-water. This system is used for chilled water air-conditioning system. This is adescendent of Windhausens machine with LiBr replacing H2SO4. In this system LiBr is theabsorbent and water is the refrigerant. This system works at vacuum pressures. Thecondenser and the generator are housed in one cylindrical vessel and the evaporator and theabsorber are housed in second vessel. This also runs on low-grade energy requiring a boileror process steam.

1.3.3. Solar energy based refrigeration systems:Attempts have been made to run vapour absorption systems by solar energy withconcenerious consideration to solar refrigeration systems was given since 1965, due to thescarcitytermittent absorption systems based on solar energy have also been built andoperatetrating and flat plate solar collectors. Several small solar absorption refrigerationsystems have been made around 1950s in several countries. Professor G.O.G. Lf ofAmerica is one of the pioneers in the area of solar refrigeration using flat plate collectors. Asolar refrigeration system that could produce 250 kg of ice per day was installed in Tashkent,USSR in 1953. This system used a parabolic mirror of 10 m2 area for concentrating the solarradiation. F. Trombe installed an absorption machine with a cylindro-parabolic mirror of 20m2 at Montlouis, France, which produced 100 kg of ice per day.Sof fossil fuel based energy sources. LiBr-water based systems have been developedfor air conditioning purposes. The first solar air conditioning system was installed in anexperimental solar house in University of Queensland, Australia in 1966. After this severalsystems based on solar energy were built in many parts of the world including India. In 1976,there were about 500 solar absorption systems in USA alone. Almost all these were based onLiBr-water as these systems do not require very high heating temperatures. These systemswere mainly used for space air conditioning.Ind successfully. In these systems, the cooling effect is obtained during the nighttime,while the system gets charged during the day using solar energy. Though the efficiency ofthese systems is rather poor requiring solar collector area, they may find applications in remote and rural areas where space is not a constraint. In addition, these systems areenvironment friendly as they use eco-friendly refrigerants and run on clean and renewablesolar energy.Solar adsorption refrigeration system with ammoniacates, sodium thiocyanate,activate. Compared to the compression systems, vapour absorption refrigeration systems:d charcoal, zeolite as adsorbents and ammonia, alcohols or fluorocarbons asrefrigerants have also been in use since 1950s. These systems also do not require acompressor. The refrigerant vapour is driven by the adsorption potential of the adsorbentstored in an adsorbent bed. This bed is connected to an evaporator/condenser, which consistsof the pure refrigerant. In intermittent adsorption systems, during the night the refrigerantevaporates and is adsorbed in activated charcoal or zeolite providing cooling effect. Duringdaytime the adsorbent bed absorbs solar radiation and drives off the refrigerant stored in thebed. This refrigerant vapour condenses in the condenser and stored in a reservoir fornighttime use. Thus this simple system consists of an adsorbent bed and a heat exchanger,which acts as a condenser during the nighttime and, as an evaporator during the night. Pairsof such reactors can be used for obtaining a continuous coolingQ Compared to the compression systems, vapour absorption refrigeration systems:a) Are environment friendlyb) Use low-grade thermal energy for operationc) Cannot be used for large capacity refrigeration systemsd) Cannot be used for small capacity refrigeration systemsAns. a) and b)Q. In absorption refrigeration systems, the compressor of vapour compression systems isreplaced by:a) Absorberb) Generatorc) Pumpd) All of the aboveAns. d)Q. In a triple fluid vapour absorption refrigeration system, the hydrogen gas is used to:a) Improve system performanceb) Reduce the partial pressure of refrigerant in evaporatorc) Circulate the refrigerantd) Provide a vapour sealAns. b)Q. Solar energy based refrigeration systems are developed to:a) Reduce fossil fuel consumptionb) Provide refrigeration in remote areasc) Produce extremely low temperaturesd) Eliminate compressorsAns. a) and b)Q. Solar energy based refrigeration systems:a) Cannot be used for large capacity systemsb) Cannot be made continuousc) Are not environment friendlyd) None of the aboveAns. d)1.3.4. Gas Cycle Refrigeration:If air at high pressure expands and does work (say moves a piston or rotates aturbine), its temperature will decrease. This fact is known to man as early as the 18th century.Dalton and Gay Lusaac studied this in 1807. Sadi Carnot mentioned this as a well-knownphenomenon in 1824. However, Dr. John Gorrie a physician in Florida developed one suchmachine in 1844 to produce ice for the relief of his patients suffering from fever. Thismachine used compressed air at 2 atm. pressure and produced brine at a temperature of 7oC,which was then used to produce ice. Alexander Carnegie Kirk in 1862 made an air cyclecooling machine. This system used steam engine to run its compressor. Using a compressionratio of 6 to 8, Kirk could produce temperatures as low as 40oC. Paul Gifford in 1875perfected the open type of machine. This machine was further improved by T B Lightfoot, AHaslam, Henry Bell and James Coleman. This was the main method of marine refrigerationfor quite some time. Frank Allen in New York developed a closed cycle machine employinghigh pressures to reduce the volume flow rates. This was named dense air machine. Thesedays air cycle refrigeration is used only in aircrafts whose turbo compressor can handle largevolume flow rates. Figure 1.6 shows the schematic of an open type air cycle refrigerationsystem. The basic system shown here consists of a compressor, an expander and a heatexchanger. Air from the cold room is compressed in the compressor. The hot and highpressure air rejects heat to the heat sink (cooling water) in the heat exchanger. The warm buthigh pressure air expands in the expander. The cold air after expansion is sent to the coldroom for providing cooling. The work of expansion partly compensates the work ofcompression; hence both the expander and the compressor are mounted on a common shaft.

1.3.5. Steam Jet Refrigeration System:If water is sprayed into a chamber where a low pressure is maintained, a part of thewater will evaporate. The enthalpy of evaporation will cool the remaining water to itssaturation temperature at the pressure in the chamber. Obviously lower temperature willrequire lower pressure. Water freezes at 0oC hence temperature lower than 4oC cannot beobtained with water. In this system, high velocity steam is used to entrain the evaporatingwater vapour. High-pressure motive steam passes through either convergent or convergentdivergentnozzle where it acquires either sonic or supersonic velocity and low pressure of theorder of 0.009 kPa corresponding to an evaporator temperature of 4oC. The high momentumof motive steam entrains or carries along with it the water vapour evaporating from the flashchamber. Because of its high velocity it moves the vapours against the pressure gradient up tothe condenser where the pressure is 5.6-7.4 kPa corresponding to condenser temperature of35-45oC. The motive vapour and the evaporated vapour both are condensed and recycled.This system is known as steam jet refrigeration system. Figure 1.7 shows a schematic of thesystem. It can be seen that this system requires a good vacuum to be maintained. Sometimes,booster ejector is used for this purpose. This system is driven by low- grade energy that isprocess steam in chemical plants or a boiler.

In 1838, the Frenchman Pelletan was granted a patent for the compression of steam bymeans of a jet of motive steam. Around 1900, the Englishman Charles Parsons studied thepossibility of reduction of pressure by an entrainment effect from a steam jet. However, thecredit for constructing the steam jet refrigeration system goes to the French engineer, MauriceLeblanc who developed the system in 1907-08. In this system, ejectors were used to producea high velocity steam jet ( 1200 m/s). Based on Leblancs design the first commercialsystem was made by Westinghouse in 1909 in Paris. Even though the efficiency of the steamjet refrigeration system was low, it was still attractive as water is harmless and the system canrun using exhaust steam from a steam engine. From 1910 onwards, stem jet refrigeration systems were used mainly in breweries, chemical factories, warships etc. In 1926, the Frenchengineer Follain improved the machine by introducing multiple stages of vaporization andcondensation of the suction steam. Between 1928-1930, there was much interest in this typeof systems in USA. In USA they were mainly used for air conditioning of factories, cinematheatres, ships and even railway wagons. Several companies such as Westinghouse, IngersollRand and Carrier started commercial production of these systems from 1930. However,gradually these systems were replaced by more efficient vapour absorption systems usingLiBr-water. Still, some east European countries such as Czechoslovakia and Russiamanufactured these systems as late as 1960s. The ejector principle can also be used toprovide refrigeration using fluids other than water, i.e., refrigerants such as CFC-11, CFC-21,CFC-22, CFC-113, CFC-114 etc. The credit for first developing these closed vapour jetrefrigeration systems goes to the Russian engineer, I.S. Badylkes around 1955. Usingrefrigerants other than water, it is possible to achieve temperatures as low as 100oC with asingle stage of compression. The advantages cited for this type of systems are simplicity androbustness, while difficult design and economics are its chief disadvantages.1.3.6. Thermoelectric Refrigeration Systems:In 1821 the German physicist T.J. Seebeck reported that when two junctions ofdissimilar metals are kept at two different temperatures, an electro motive force (emf) isdeveloped, resulting in flow of electric current. The emf produced is found to be proportionalto temperature difference. In 1834, a Frenchmen, J. Peltier observed the reverse effect, i.e.,cooling and heating of two junctions of dissimilar materials when direct current is passedthrough them, the heat transfer rate being proportional to the current. In 1838, H.F.E. Lenz froze a drop of water by the Peltier effect using antimony and bismuth (it was later found thatLenz could freeze water as the materials used were not pure metals but had some impuritiesin them). In 1857, William Thomson (Lord Kelvin) proved by thermodynamic analysis thatSeebeck effect and Peltier effect are related and he discovered another effect called Thomsoneffect after his name. According to this when current flows through a conductor of athermocouple that has an initial temperature gradient in it, then heat transfer rate per unitlength is proportional to the product of current and the temperature. As the current flowthrough thermoelectric material it gets heated due to its electrical resistance. This is called theJoulean effect, further, conduction heat transfer from the hot junction to the cold junctiontransfers heat. Both these heat transfer rates have to be compensated by the Peltier Effect forsome useful cooling to be produced. For a long time, thermoelectric cooling based on thePeltier effect remained a laboratory curiosity as the temperature difference that could beobtained using pure metals was too small to be of any practical use. Insulating materials givepoor thermoelectric performance because of their small electrical conductivity while metalsare not good because of their large thermal conductivity. However, with the discovery ofsemiconductor materials in 1949-50, the available temperature drop could be increasedconsiderably, giving rise to commercialization of thermoelectric refrigeration systems. Figure1.8 shows the schematic of the thermoelectric refrigeration system based on semiconductormaterials. The Russian scientist, A. F. Ioffe is one of the pioneers in the area ofthermoelectric refrigeration systems using semiconductors. Several domestic refrigeratorsbased on thermoelectric effect were made in USSR as early as 1949. However, since 1960sthese systems are used mainly used for storing medicines, vaccines etc and in electroniccooling. Development also took place in many other countries. In USA domesticrefrigerators, air conditioners, water coolers, air conditioned diving suits etc. were made

using these effects. System capacities were typically small due to poor efficiency. Howeversome large refrigeration capacity systems such as a 3000 kcal/h air conditioner and a 6 tonnecapacity cold storage were also developed. By using multistaging temperatures as low as 145oC were obtained. These systems due to their limited performance (limited by thematerials) are now used only in certain niche applications such as electronic cooling, mobilecoolers etc. Efforts have also been made to club thermoelectric systems with photovoltaiccells with a view to develop solar thermoelectric refrigerators.1.3.7. Vortex tube systems:In 1931, the French engineer Georges Ranque (1898-1973) discovered an interestingphenomenon, which is called Ranque effect or vortex effect. The tangential injection ofair into a cylindrical tube induces to quote his words a giratory expansion withsimultaneous production of an escape of hot air and an escape of cold air. Ranque wasgranted a French patent in 1928 and a US patent in 1934 for this effect. However, thediscovery was neglected until after the second world war, when in 1945, Rudolph Hilsch, aGerman physicist, studied this effect and published a widely read scientific paper on thisdevice. Thus, the vortex tube has also been known as the "Ranque-Hilsch Tube. Though theefficiency of this system is quite low, it is very interesting due to its mechanical simplicityand instant cooling. It is convenient where there is a supply of compressed air. The present day vortex tube uses compressed air as a power source, it has no moving parts, and produceshot air from one end and cold air from the other. The volume and temperature of these twoairstreams are adjustable with a valve built into the hot air exhaust. Temperatures as low as46C and as high as 127C are possible. Compressed air is supplied to the vortex tube andpasses through nozzles that are tangential to an internal counter bore. These nozzles set theair in a vortex motion. This spinning stream of air turns 90 and passes down the hot tube inthe form of a spinning shell, similar to a tornado. A valve at one end of the tube allows someof the warmed air to escape. What does not escape, heads back down the tube as a secondvortex inside the low-pressure area of the larger vortex. This inner vortex loses heat andexhausts through the other end as cold air. Currently vortex tube is used for spot cooling ofmachine parts, in electronic cooling and also in cooling jackets for miners, firemen etc.

Q. In an air cycle refrigeration system, low temperatures are produced due to:a) Evaporation of liquid airb) Throttling of airc) Expansion of air in turbined) None of the aboveAns. c)Q. Air cycle refrigeration systems are most commonly used in:a) Domestic refrigeratorsb) Aircraft air conditioning systemsc) Cold storagesd) Car air conditioning systemsAns. b)Q. The required input to the steam jet refrigeration systems is in the form of:a) Mechanical energyb) Thermal energyc) High pressure, motive steamd) Both mechanical and thermal energyAns. c)Q. A nozzle is used in steam jet refrigeration systems to:a) To convert the high pressure motive steam into high velocity steamb) To reduce energy consumptionc) To improve safety aspectsd) All of the aboveAns. a)Q. The materials used in thermoelectric refrigeration systems should have:a) High electrical and thermal conductivityb) High electrical conductivity and low thermal conductivityc) Low electrical conductivity and high thermal conductivityc) Low electrical and thermal conductivityAns. b)Q. A thermoelectric refrigeration systems requires:a) A high voltage AC (alternating current) inputb) A low voltage AC inputc) A high voltage DC (direct current) inputd) A low voltage DC inputAns. d).

Questions and answers:1. In an ammonia-water system a rectification column is used mainly to:a) To improve the COP of the systemb) To reduce the operating pressuresc) To minimize the concentration of water in refrigeration circuitd) All of the aboveAns.: c)2. In a reflux condenser:a) Heat is extracted so that the vapour leaving is rich in ammoniab) Heat is supplied so that the vapour leaving is rich in ammoniac) Heat is extracted so that the vapour leaving is rich in waterd) Heat is supplied so that the vapour leaving is rich in ammoniaAns.: a)3. Due to the requirement of rectification:a) The required generator pressure increasesb) The required generator temperature increasesc) The required generator heat input increasesd) All of the aboveAns.: c)4. In pumpless vapour absorption refrigeration systems:a) The evaporation process is non-isothermalb) The system pressure is almost same everywherec) A pressure equalizing fluid is required to increase condenser pressured) A pressure equalizing fluid is required to increase evaporator pressureAns.: a), b) and d)5. Which of the following statements regarding pumpless systems are TRUE:a) Pumpless systems can use a wide variety of heat sourcesb) Pumpless systems are silent, reliable and ruggedc) Pumpless systems offer high COPsd) Pumpless systems operate at very low pressuresAns.: a) and b)6. Compared to compression systems, the performance of absorption systems:a) Is very sensitive to evaporator temperatureb) Is not sensitive to load variationsc) Does not depend very much on evaporator superheatd) All of the aboveAns.: b) and c)7. Compared to compression systems, absorption systems:a) Contain very few moving partsb) Require regular maintenancec) Offer less noise and vibrationd) Are compact for large capacitiesRefrigerants2.1. Introduction:The development of refrigeration and air conditioning industry depended to a largeextent on the development of refrigerants to suit various applications and the development ofvarious system components. At present the industry is dominated by the vapour compressionrefrigeration systems, even though the vapour absorption systems have also been developedcommercially. The success of vapour compression refrigeration systems owes a lot to thedevelopment of suitable refrigerants and compressors. The theoretical thermodynamicefficiency of a vapour compression system depends mainly on the operating temperatures.However, important practical issues such as the system design, size, initial and operatingcosts, safety, reliability, and serviceability etc. depend very much on the type of refrigerantand compressor selected for a given application. This lesson presents a brief history ofrefrigerants and compressors. The emphasis here is mainly on vapour compressionrefrigeration systems, as these are the most commonly used systems, and also refrigerants andcompressors play a critical role here. The other popular type of refrigeration system, namelythe vapour absorption type has seen fewer changes in terms of refrigerant development, andrelatively less number of problems exist in these systems as far as the refrigerants areconcerned.2.2. Refrigerant development a brief historyIn general a refrigerant may be defined as any body or substance that acts as acooling medium by extracting heat from another body or substance. Under this generaldefinition, many bodies or substances may be called as refrigerants, e.g. ice, cold water, coldair etc. In closed cycle vapour compression, absorption systems, air cycle refrigerationsystems the refrigerant is a working fluid that undergoes cyclic changes. In a thermoelectricsystem the current carrying electrons may be treated as a refrigerant. However, normally byrefrigerants we mean the working fluids that undergo condensation and evaporation as incompression and absorption systems. The history that we are talking about essentially refersto these substances. Since these substances have to evaporate and condense at requiredtemperatures (which may broadly lie in the range of 100oC to +100oC) at reasonablepressures, they have to be essentially volatile. Hence, the development of refrigerants startedwith the search for suitable, volatile substances. Historically the development of theserefrigerants can be divided into three distinct phases, namely:i. Refrigerants prior to the development of CFCsii. The synthetic fluorocarbon (FC) based refrigerantsiii. Refrigerants in the aftermath of stratospheric ozone layer depletion2.2.1. Refrigerants prior to the development of CFCsWater is one of the earliest substances to be used as a refrigerant, albeit not in a closedsystem. Production of cold by evaporation of water dates back to 3000 B.C. Archaeologicalfindings show pictures of Egyptian slaves waving fans in front of earthenware jars toaccelerate the evaporation of water from the porous surfaces of the pots, thereby producingcold water. Of course, the use of punkahs for body cooling in hot summer is very wellknown in countries like India. Production of ice by nocturnal cooling is also well known.People also had some knowledge of producing sub-zero temperatures by the use ofrefrigerant mixtures. It is believed that as early as 4th Century AD people in India wereusing mixtures of salts (sodium nitrate, sodium chloride etc) and water to producetemperatures as low as 20oC. However, these natural refrigeration systems working withwater have many limitations and hence were confined to a small number of applications.Water was the first refrigerant to be used in a continuous refrigeration system byWilliam Cullen (1710-1790) in 1755. William Cullen is also the first man to havescientifically observed the production of low temperatures by evaporation of ethyl ether in1748. Oliver Evans (1755-1819) proposed the use of a volatile fluid in a closed cycle toproduce ice from water. He described a practical system that uses ethyl ether as therefrigerant. As already mentioned the credit for building the first vapour compressionrefrigeration system goes to Jakob Perkins (1766-1849). Perkins used sulphuric (ethyl) etherobtained from India rubber as refrigerant. Early commercial refrigerating machinesdeveloped by James Harrison (1816-1893) also used ethyl ether as refrigerant. AlexanderTwining (1801-1884) also developed refrigerating machines using ethyl ether. After thesedevelopments, ethyl ether was used as refrigerant for several years for ice making, inbreweries etc. Ether machines were gradually replaced by ammonia and carbon dioxide basedmachines, even though they were used for a longer time in tropical countries such as India.Ethyl ether appeared to be a good refrigerant in the beginning, as it was easier to handle itsince it exists as a liquid at ordinary temperatures and atmospheric pressure. Ethyl ether has anormal boiling point (NBP) of 34.5oC, this indicates that in order to obtain low temperatures,the evaporator pressure must be lower than one atmosphere, i.e., operation in vacuum.Operation of a system in vacuum may lead to the danger of outside air leaking into thesystem resulting in the formation of a potentially explosive mixture. On the other hand arelatively high normal boiling point indicates lower pressures in the condenser, or for a givenpressure the condenser can be operated at higher condensing temperatures. This is the reasonbehind the longer use of ether in tropical countries with high ambient temperatures.Eventually due to the high NBP, toxicity and flammability problems ethyl ether was replacedby other refrigerants. Charles Tellier (1828-1913) introduced dimethyl ether (NBP =23.6oC) in 1864. However, this refrigerant did not become popular, as it is also toxic andinflammable.In 1866, the American T.S.C. Lowe (1832-1913) introduced carbon dioxidecompressor. However, it enjoyed commercial success only in 1880s due largely to the effortsof German scientists Franz Windhausen (1829-1904) and Carl von Linde (1842-1934).Carbon dioxide has excellent thermodynamic and thermophysical properties, however, it hasa low critical temperature (31.7oC) and very high operating pressures. Since it is nonflammableand non-toxic it found wide applications principally for marine refrigeration. Itwas also used for refrigeration applications on land. Carbon dioxide was used successfully forabout sixty years however, it was completely replaced by CFCs. It is ironic to note that eversince the problem of ozone layer depletion was found, carbon dioxide is steadily making acomeback by replacing the synthetic CFCs/HCFCs/HFCs etc.One of the landmark events in the history of refrigerants is the introduction ofammonia. The American David Boyle (1837-1891) was granted the first patent for ammoniacompressor in 1872. He made the first single acting vertical compressor in 1873. However,the credit for successfully commercializing ammonia systems goes to Carl von Linde (1842-1934) of Germany, who introduced these compressors in Munich in 1876. Linde is creditedwith perfecting the ammonia refrigeration technology and owing to his pioneering efforts;ammonia has become one of the most important refrigerants to be developed. Ammonia has aNBP of 33.3oC, hence, the operating pressures are much higher than atmospheric.Ammonia has excellent thermodynamic and thermophysical properties. It is easily availableand inexpensive. However, ammonia is toxic and has a strong smell and slight flammability.In addition, it is not compatible with some of the common materials of construction such ascopper. Though these are considered to be some of its disadvantages, ammonia has stood thetest of time and the onslaught of CFCs due to its excellent properties. At present ammonia isused in large refrigeration systems (both vapour compression and vapour absorption) and alsoin small absorption refrigerators (triple fluid vapour absorption).In 1874, Raoul Pictet (1846-1929) introduced sulphur dioxide (NBP=10.0oC).Sulphur dioxide was an important refrigerant and was widely used in small refrigerationsystems such as domestic refrigerators due to its small refrigerating effect. Sulphur dioxidehas the advantage of being an auto-lubricant. In addition it is not only non-flammable, butactually acts as a flame extinguisher. However, in the presence of water vapour it producessulphuric acid, which is highly corrosive. The problem of corrosion was overcome by anairtight sealed compressor (both motor and compressor are mounted in the same outer casing). However, after about sixty years of use in appliances such as domestic refrigerators,sulphur dioxide was replaced by CFCs.In addition to the above, other fluids such as methyl chloride, ethyl chloride, isobutane,propane, ethyl alcohol, methyl and ethyl amines, carbon tetra chloride, methylenechloride, gasoline etc. were tried but discarded due to one reason or other.2.2.2. The synthetic CFCs/HCFCs:Almost all the refrigerants used in the early stages of refrigeration suffered from oneproblem or other. Most of these problems were linked to safety issues such as toxicity,flammability, high operating pressures etc. As a result large-scale commercialization ofrefrigeration systems was hampered. Hence it was felt that refrigeration industry needs anew refrigerant if they expect to get anywhere. The task of finding a safe refrigerant wastaken up by the American Thomas Midgley, Jr., in 1928. Midgley was already famous for theinvention of tetra ethyl lead, an important anti-knock agent for petrol engines. Midgley alongwith his associates Albert L. Henne and Robert R. McNary at the Frigidaire Laboratories(Dayton, Ohio, USA) began a systematic study of the periodic table. From the periodic tablethey quickly eliminated all those substances yielding insufficient volatility. They theneliminated those elements resulting in unstable and toxic gases as well as the inert gases,based on their very low boiling points. They were finally left with eight elements: carbon,nitrogen, oxygen, sulphur, hydrogen, fluorine, chlorine and bromine. These eight elementsclustered at an intersecting row and column of the periodic table, with fluorine at theintersection. Midgley and his colleagues then made three interesting observations:i. Flammability decreases from left to right for the eight elementsii. Toxicity generally decreases from the heavy elements at the bottom to the lighterelements at the topiii. Every known refrigerant at that time was made from the combination of thoseeight Midgley elements.A look at the refrigerants discussed above shows that all of them are made up of sevenout of the eight elements identified by Midgley (fluorine was not used till then). Otherresearchers have repeated Midgleys search with more modern search methods and databases,but arrived at the same conclusions (almost all the currently used refrigerants are made up ofMidgley elements, only exception is Iodine, studies are being carried out on refrigerantscontaining iodine in addition to some of the Midgley elements). Based on their study,Midgely and his colleagues have developed a whole range of new refrigerants, which areobtained by partial replacement of hydrogen atoms in hydrocarbons by fluorine and chlorine.They have shown how fluorination and chlorination of hydrocarbons can be varied to obtaindesired boiling points (volatility) and also how properties such as toxicity, flammability areinfluenced by the composition. The first commercial refrigerant to come out of Midgleysstudy is Freon-12 in 1931. Freon-12 with a chemical formula CCl2F2, is obtained by replacingthe four atoms of hydrogen in methane (CH4) by two atoms of chlorine and two atoms offluorine. Freon-12 has a normal boiling point of 29.8oC, and is one of the most famous andpopular synthetic refrigerants. It was exclusively used in small domestic refrigerators, airconditioners, water coolers etc for almost sixty years. Freon-11 (CCl3F) used in largecentrifugal air conditioning systems was introduced in 1932. This is followed by Freon-22(CHClF2) and a whole series of synthetic refrigerants to suit a wide variety of applications.Due to the emergence of a large number of refrigerants in addition to the existence of theolder refrigerants, it has become essential to work out a numbering system for refrigerants.Thus all refrigerants were indicated with R followed by a unique number (thus Freon-12 ischanged to R12 etc). The numbering of refrigerants was done based on certain guidelines. Forall synthetic refrigerants the number (e.g. 11, 12, 22) denotes the chemical composition. Thenumber of all inorganic refrigerants begins with 7 followed by their molecular weight. ThusR-717 denotes ammonia (ammonia is inorganic and its molecular weight is 17), R-718denotes water etc.. Refrigerant mixtures begin with the number 4 (zeotropic) or 5(azeotropic), e.g. R-500, R-502 etc.The introduction of CFCs and related compounds has revolutionized the field ofrefrigeration and air conditioning. Most of the problems associated with early refrigerantssuch as toxicity, flammability, and material incompatibility were eliminated completely.Also, Freons are highly stable compounds. In addition, by cleverly manipulating thecomposition a whole range of refrigerants best suited for a particular application could beobtained. In addition to all this, a vigorous promotion of these refrigerants as wonder gasesand ideal refrigerants saw rapid growth of Freons and equally rapid exit of conventionalrefrigerants such as carbon dioxide, sulphur dioxide etc. Only ammonia among the olderrefrigerants survived the Freon magic. The Freons enjoyed complete domination for aboutfifty years, until the Ozone Layer Depletion issue was raised by Rowland and Molina in1974. Rowland and Molina in their now famous theory argued that the highly stablechlorofluorocarbons cause the depletion of stratospheric ozone layer. Subsequent studies andobservations confirmed Rowland and Molina theory on stratospheric ozone depletion bychlorine containing CFCs. In view of the seriousness of the problem on global scale, severalcountries have agreed to ban the harmful Ozone Depleting Substances, ODS (CFCs andothers) in a phase-wise manner under Montreal Protocol. Subsequently almost all countries ofthe world have agreed to the plan of CFC phase-out. In addition to the ozone layer depletion,the CFCs and related substances were also found to contribute significantly to the problem ofglobal warming. This once again brought the scientists back to the search for saferefrigerants. The safety now refers to not only the immediate personal safety issues such asflammability, toxicity etc., but also the long-term environmental issues such as ozone layerdepletion and global warming.2.2.3. Refrigerants in the aftermath of Ozone Layer Depletion:The most important requirement for refrigerants in the aftermath of ozone layerdepletion is that it should be a non-Ozone Depleting Substance (non-ODS). Out of thisrequirement two alternatives have emerged. The first one is to look for zero ODP syntheticrefrigerants and the second one is to look for natural substances. Introduction ofhydrofluorocarbons (HFCs) and their mixtures belong to the first route, while the reintroductionof carbon dioxide (in a supercritical cycle), water and various hydrocarbons andtheir mixtures belong to the second route. The increased use of ammonia and use of otherrefrigeration cycles such as air cycle refrigeration systems and absorption systems also comeunder the second route. Both these routes have found their proponents and opponents. HFC-134a (synthetic substance) and hydrocarbons (natural substances) have emerged asalternatives to Freon-12. No clear pure fluid alternative has been found as yet for the otherpopular refrigerant HCFC-22. However several mixtures consisting of synthetic and naturalrefrigerants are being used and suggested for future use. Table 2.1 shows the list ofrefrigerants being replaced and their alternatives. Mention must be made here about the otherenvironmental problem, global warming. In general the non-ODS synthetic refrigerants suchas HFC-134a have high global warming potential (GWP), hence they face an uncertainfuture. Since the global warming impact of a refrigerant also depends on the energyefficiency of the system using the refrigerant (indirect effect), the efficiency issue has becomeimportant in the design of new refrigeration systems. Though the issues of ozone layerdepletion and global warming has led to several problems, they have also had beneficialeffects of making people realize the importance of environmental friendliness oftechnologies. It is expected that with the greater awareness more responsible designs willemerge which will ultimately benefit the whole mankind.

Q. Ethyl ether was the first refrigerant to be used commercially, because:a) It exists as liquid at ambient conditionsb) It is safec) It is inexpensived) All of the aboveAns. a)Q. Ammonia is one of the oldest refrigerants, which is still used widely, because:a) It offers excellent performanceb) It is a natural refrigerantc) It is inexpensived) All of the aboveAns. d)Q. In the olden days Carbon dioxide was commonly used in marine applications as:a) It has low critical temperatureb) Its operating pressures are highc) It is non-toxic and non-flammabled) It is odorlessAns. c)Q. Sulphur dioxide was mainly used in small refrigeration systems, because:a) It is non-toxic and non-flammableb) It has small refrigeration effectc) It is expensived) It was easily availableAns. b)Q. Need for synthetic refrigerants was felt, as the available natural refrigerants:a) Were not environment friendlyb) Suffered from several perceived safety issuesc) Were expensived) Were inefficientAns. b)Q. The synthetic CFC based refrigerants were developed by:a) Partial replacement of hydrogen atoms in hydrocarbons by chlorine, fluorine etc.b) Modifying natural refrigerants such as carbon dioxide, ammoniac) Modifying inorganic compounds by adding carbon, fluorine and chlorined) Mixing various hydrocarbonsAns. a)Q. The synthetic refrigerants were extremely popular as they are:a) Environment friendlyb) Mostly non-toxic and non-flammablec) Chemically stabled) InexpensiveAns. b) and c)Q. CFC based refrigerants are being replaced as they are found to:a) Cause ozone layer depletionb) Consume more energyc) React with several materials of constructiond) ExpensiveAns. a)Psychrometry27.1. Introduction:Atmospheric air makes up the environment in almost every type of airconditioning system. Hence a thorough understanding of the properties ofatmospheric air and the ability to analyze various processes involving air isfundamental to air conditioning design.The study of the properties of moist air is known as psychrometry. The psychrometric properties(temperature, humidity ratio, relative humidity, enthalpy etc.) are normally available in the formof charts, known as psychrometric charts.

Psychrometry is the study of the properties of mixtures of air and watervapour.Atmospheric air is a mixture of many gases plus water vapour and a numberof pollutants (Fig.27.1). The amount of water vapour and pollutants vary from placeto place. The concentration of water vapour and pollutants decrease with altitude,and above an altitude of about 10 km, atmospheric air consists of only dry air. Thepollutants have to be filtered out before processing the air. Hence, what we processis essentially a mixture of various gases that constitute air and water vapour. Thismixture is known as moist air.The moist air can be thought of as a mixture of dry air and moisture. For allpractical purposes, the composition of dry air can be considered as constant. In1949, a standard composition of dry air was fixed by the International JointCommittee on Psychrometric data. It is given in Table 27.1.

Based on the above composition the molecular weight of dry air is found to be28.966 and the gas constant R is 287.035 J/kg.K.As mentioned before the air to be processed in air conditioning systems is amixture of dry air and water vapour. While the composition of dry air is constant, theamount of water vapour present in the air may vary from zero to a maximumdepending upon the temperature and pressure of the mixture (dry air + watervapour).At a given temperature and pressure the dry air can only hold a certainmaximum amount of moisture. When the moisture content is maximum, then the airis known as saturated air, which is established by a neutral equilibrium between themoist air and the liquid or solid phases of water.For calculation purposes, the molecular weight of water vapour is taken as 18.015and its gas constant is 461.52 J/kg.K.27.2. Methods for estimating properties of moist air:In order to perform air conditioning calculations, it is essential first to estimatevarious properties of air. It is difficult to estimate the exact property values of moistair as it is a mixture of several permanent gases and water vapour. However, moistair upto 3 atm. pressure is found to obey perfect gas law with accuracy sufficient forengineering calculations. For higher accuracy Goff and Gratch tables can be usedfor estimating moist air properties. These tables are obtained using mixture modelsbased on fundamental principles of statistical mechanics that take into account thereal gas behaviour of dry air and water vapour. However, these tables are valid for abarometric pressure of 1 atm. only. Even though the calculation procedure is quitecomplex, using the mixture models it is possible to estimate moist air properties at other pressures also. However, since in most cases the pressures involved are low,one can apply the perfect gas model to estimate psychrometric properties.27.2.1. Basic gas laws for moist air:According to the Gibbs-Dalton law for a mixture of perfect gases, the totalpressure exerted by the mixture is equal to the sum of partial pressures of theconstituent gases. According to this law, for a homogeneous perfect gas mixtureoccupying a volume V and at temperature T, each constituent gas behaves asthough the other gases are not present (i.e., there is no interaction between thegases). Each gas obeys perfect gas equation. Hence, the partial pressures exertedby each gas, p1,p2,p3 and the total pressure pt are given by:

where n1,n2,n3, are the number of moles of gases 1,2,3,Applying this equation to moist air.

27.2.2. Important psychrometric properties:Dry bulb temperature (DBT) is the temperature of the moist air as measured by astandard thermometer or other temperature measuring instruments.Saturated vapour pressure (psat) is the saturated partial pressure of water vapour atthe dry bulb temperature. This is readily available in thermodynamic tables andcharts. ASHRAE suggests the following regression equation for saturated vapourpressure of water, which is valid for 0 to 100oC.

Relative humidity () is defined as the ratio of the mole fraction of water vapour inmoist air to mole fraction of water vapour in saturated air at the same temperatureand pressure. Using perfect gas equation we can show that:

Relative humidity is normally expressed as a percentage. When is 100percent, the air is saturated.Humidity ratio (W): The humidity ratio (or specific humidity) W is the mass of waterassociated with each kilogram of dry air1. Assuming both water vapour and dry air tobe perfect gases2, the humidity ratio is given by:

Substituting the values of gas constants of water vapour and air Rv and Ra inthe above equation; the humidity ratio is given by:

For a given barometric pressure pt, given the DBT, we can find the saturated vapourpressure psat from the thermodynamic property tables on steam. Then using theabove equation, we can find the humidity ratio at saturated conditions, Wsat.It is to be noted that, W is a function of both total barometric pressure andvapor pressure of water.Dew-point temperature: If unsaturated moist air is cooled at constant pressure, thenthe temperature at which the moisture in the air begins to condense is known asdew-point temperature (DPT) of air. An approximate equation for dew-pointtemperature is given by:

where is the relative humidity (in fraction). DBT & DPT are in oC. Of course, sincefrom its definition, the dew point temperature is the saturation temperaturecorresponding to the vapour pressure of water vapour, it can be obtained from steamtables or using Eqn.(27.3).1 Properties such as humidity ratio, enthalpy and specific volume are based on 1 kg of dryair. This is useful as the total mass of moist air in a process varies by the addition/removal ofwater vapour, but the mass of dry air remains constant.2 Dry air is assumed to be a perfect gas as its temperature is high relative to its saturationtemperature, and water vapour is assumed to be a perfect gas because its pressure is lowrelative to its saturation pressure. These assumptions result in accuracies, that are, sufficientfor engineering calculations (less than 0.7 percent as shown by Threlkeld). However, moreaccurate results can be obtained by using the data developed by Goff and Gratch in 1945.Degree of saturation : The degree of saturation is the ratio of the humidity ratio W tothe humidity ratio of a saturated mixture Ws at the same temperature and pressure,i.e.,

Enthalpy: The enthalpy of moist air is the sum of the enthalpy of the dry air and theenthalpy of the water vapour. Enthalpy values are always based on some referencevalue. For moist air, the enthalpy of dry air is given a zero value at 0oC, and for watervapour the enthalpy of saturated water is taken as zero at 0oC.The enthalpy of moist air is given by:

The unit of h is kJ/kg of dry air. Substituting the approximate values of cp and hg, weobtain:

Humid specific heat: From the equation for enthalpy of moist air, the humid specificheat of moist air can be written as:

Since the second term in the above equation (w.cpw) is very small comparedto the first term, for all practical purposes, the humid specific heat of moist air, cpmcan be taken as 1.0216 kJ/kg dry air.KSpecific volume: The specific volume is defined as the number of cubic meters ofmoist air per kilogram of dry air. From perfect gas equation since the volumesoccupied by the individual substances are the same, the specific volume is alsoequal to the number of cubic meters of dry air per kilogram of dry air, i.e.,

27.2.3. Psychrometric chartA Psychrometric chart graphically represents the thermodynamic properties ofmoist air. Standard psychrometric charts are bounded by the dry-bulb temperatureline (abscissa) and the vapour pressure or humidity ratio (ordinate). The Left HandSide of the psychrometric chart is bounded by the saturation line. Figure 27.2 showsthe schematic of a psychrometric chart. Psychrometric charts are readily availablefor standard barometric pressure of 101.325 kPa at sea level and for normaltemperatures (0-50oC). ASHRAE has also developed psychrometric charts for othertemperatures and barometric pressures (for low temperatures: -40 to 10oC, hightemperatures 10 to 120oC and very high temperatures 100 to 120oC)

27.5. Psychrometer:Any instrument capable of measuring the psychrometric state of air is called apsychrometer.

1. Which of the following statements are TRUE?a) The maximum amount of moisture air can hold depends upon its temperature andbarometric pressureb) Perfect gas model can be applied to air-water mixtures when the total pressure ishighc) The minimum number of independent properties to be specified for fixing the stateof moist air is twod) The minimum number of independent properties to be specified for fixing the stateof moist air is threeAns.: a) and d)2. Which of the following statements are TRUE?a) Straight-line law is applicable to any fluid-air mixturesb) Straight-line law is applicable to any water-air mixtures onlyc) Straight-line holds good as long as the Prandtl number is close to unityd) Straight-line holds good as long as the Lewis number is close to unityAns.: b) and d)7. Moist air at 1 atm. pressure has a dry bulb temperature of 32oC and a wet bulbtemperature of 26oC. Calculate a) the partial pressure of water vapour, b) humidityratio, c) relative humidity, d) dew point temperature, e) density of dry air in themixture, f) density of water vapour in the mixture and g) enthalpy of moist air usingperfect gas law model and psychrometric equations.Ans.:a) Using modified Apjohn equation and the values of DBT, WBT and barometricpressure, the vapour pressure is found to be:pv = 2.956 kPa (Ans.)b) The humidity ratio W is given by:W = 0.622 x 2.956/(101.325-2.956) = 0.0187 kgw/kgda (Ans.)c) Relative humidity RH is given by:RH = (pv/ps) x 100 = (pv/saturation pressure at 32oC) x 100From steam tables, the saturation pressure of water at 32oC is 4.7552 kPa, hence,RH = (2.956/4.7552) x 100 = 62.16% (Ans.)d) Dew point temperature is the saturation temperature of steam at 2.956 kPa.Hence using steam tables we find that:DPT = Tsat(2.956 kPa) = 23.8oC (Ans.)e) Density of dry air and water vapourApplying perfect gas law to dry air:Density of dry air a =(pa/RaT)=(ptpv)/RaT = (101.3252.956)/(287.035 x 305)x103= 1.1236 kg/m3 of dry air (Ans.)f) Similarly the density of water vapour in air is obtained using perfect gas law as:Density of water vapour v = (pv/RvT) = 2.956 x 103/(461.52 x 305) = 0.021 kg/m3(Ans.)g) Enthalpy of moist air is found from the equation:h = 1.005 x t+W(2501+1.88 x t) = 1.005 x 32 + 0.0187(2501+1.88 X 32)h= 80.05 kJ/kg of dry air (Ans.)

1.3.1.2. Air conditioning systems:Refrigeration systems are also used for providing cooling and dehumidification insummer for personal comfort (air conditioning). The first air conditioning systems were usedfor industrial as well as comfort air conditioning. Eastman Kodak installed the first airconditioning system in 1891 in Rochester, At present comfort air conditioning is widely used in residences, offices, commercialbuildings, air ports, hospitals and in mobile applications such as rail coaches, automobiles, aircrafts etc. Industrial air conditioning is largely responsible for the growth of modernelectronic, pharmaceutical, chemical industries etc. Most of the present day air conditioningsystems use either a vapour compression refrigeration system or a vapour absorptionrefrigeration system. The capacities vary from few kilowatts to megawatts.Applications Of Refrigeration &Air Conditioning3.1. Introductionrefrigeration deals with cooling of bodies or fluids totemperatures lower than those of surroundings. This involves absorption of heat at alower temperature and rejection to higher temperature of the surroundings. In olden days,

the main purpose of refrigeration was to produce ice, which was used for coolingbeverages, food preservation and refrigerated transport etc. Now-a-days refrigeration andair conditioning find so many applications that they have become very essential formankind, and without refrigeration and air conditioning the basic fabric of the societywill be adversely affected. Refrigeration and air conditioning are generally treated in asingle subject due to the fact that one of the most important applications of refrigerationis in cooling and dehumidification as required for summer air conditioning. Of course,refrigeration is required for many applications other than air conditioning, and airconditioning also involves processes other than cooling and dehumidification. Figure 3.1shows the relation between refrigeration and air conditioning in a pictorial form.temperatures cryogenic systems are more economical. Now-a-days refrigerationhas become an essential part of food chain- from post harvest heat removal to processing,distribution and storage. Refrigeration has become essential for many chemical andprocessing industries to improve the standard, quality, precision and efficiency of manymanufacturing processes. Ever-new applications of refrigeration arise all the time. Somespecial applications require small capacities but are technically intriguing andchallenging.As mentioned before, air-conditioning is one of the major applications ofrefrigeration. Air-conditioning has made the living conditions more comfortable,hygienic and healthy in offices, work places and homes. As mentioned in Lesson 1, airconditioninginvolves control of temperature, humidity, cleanliness of air and itsdistribution to meet the comfort requirements of human beings and/or some industrialrequirements. Air-conditioning involves cooling and dehumidification in summermonths; this is essentially done by refrigeration. It also involves heating andhumidification in cold climates, which is conventionally done by a boiler unless a heatpump is used.The major applications of refrigeration can be grouped into following four majorequally important areas.1. Food processing, preservation and distribution2. Chemical and process industries3. Special Applications4. Comfort air-conditioning3.5 Application of air conditioning:Air-conditioning applications can be divided into two categories, namely,industrial and comfort air-conditioning.

IAQ stands for Indoor Air Quality and it refers to the ways and means of reducingand maintaining the pollutants inside the occupied space within tolerable levels. IAQinvolves specifying suitable levels of fresh air supply (ventilation), suitable air filters, useof proper materials of construction, furniture, carpets, draperies etc.

Q. Air conditioning involves:a) Control of temperatureb) Control of humidityc) Control of air motiond) Control of air puritye) All of the aboveAns.: e)Q. The purpose of industrial air conditioning is to:a) Provide suitable conditions for products and processesb) Provide at least a partial measure of comfort to workersc) Reduce energy consumptiond) All of the aboveAns.: a) and b)Q. Air conditioning is required in the manufacture of precision parts to:a) Achieve close tolerancesb) Prevent rust formationc) Provide clean environmentd) All of the aboveAns.: d)Q. Modern electronic equipment require cooling due to:a) Dissipation of relatively large amount of heat in small volumesb) To prevent erratic behaviourc) To improve lifed) All of the aboveAns.: d)Q. Human beings need air conditioning as:a) They continuously dissipate heat due to metabolic activityb) Body regulatory mechanisms need stable internal temperaturesc) Efficiency improves under controlled conditionsd) All of the aboveAns.: d)Q. Small residences and offices use:a) Window air conditionersb) Split air conditionersc) Central air conditioningd) All of the aboveAns.: a) and b)