ISSN 2348 2370 Vol.08,Issue.21, November-2016, Pages:4164-4170 www.ijatir.org Thermal Analysis of Vapour Compression Cycle for Different Refrigerants ALI MOHAMMED ABDOULHA MASSOUD 1, DR. MOHAMMAD TARIQ 2 1 Research Scholar, Dept of Mechanical Engineering, SSET, SHIATS-DU, Naini, Allahabad, India, E-mail: ahuwiada360@gmail.com. 2 Assistant Professor, Dept of Mechanical Engineering, SSET, SHIATS-DU, Naini, Allahabad, India, E-mail: mohammad.tariq@shiats.edu.in Abstract: Vapour compression cycle is an improved type of air refrigeration cycle in which a suitable working substance, termed as refrigerant, is used. The refrigerants generally used for this purpose are ammonia (NH3), carbon dioxide (CO2) and sulphur-dioxide (SO2). The refrigerant used, does not leave the system, but is circulated throughout the system alternately condensing and evaporating. In evaporating, the refrigerant absorbs its latent heat from the solution which is used for circulating it around the cold chamber and in condensing; it gives out its latent heat to the circulating water of the cooler. The vapour compression cycle which is used in vapour compression refrigeration system is now-adays used for all purpose refrigeration. It is used for all industrial purposes from a small domestic refrigerator to a big air conditioning plant. The present analysis has been worked out for the evaporator temperature 00C and Condenser temperature 350C while the duty of the unit has been taken as 352 kw. Two refrigerants with specified conditions at evaporator and condenser have given a designer for more options for optimization. The optimization of the vapour compression cycle with superheating at evaporator by 5deg and sub cooling a the outlet of condenser by 5 deg of both the refrigerants proven that the results of refrigerating effect for the refrigerants R134a and R717 with their conditions have least difference for their dryness fraction and mass flow rate of refrigerants. Work required by compressor is very less for R134a for without condition and with condition as compared to ammonia. As far as COP of the cycle is concerned, the refrigerant R134a with prevailing conditions have maximum value as compared to ammonia. But Ammonia have maximum refrigerating effect. Keywords: Refrigerants, Vapour Compression Cycle, COP, Air Conditioning. I. INTRODUCTION A. Air Conditioning Air conditioning (often referred to as AC, A.C., or A/C) is the process of altering the condition of air by removing heat and humidity to achieve a more comfortable interior environment, typically with the aim of distributing the conditioned air to an occupied space such as a building or a vehicle to improve thermal comfort and indoor air quality. In common use, an air conditioner is a device that removes heat Copyright @ 2016 IJATIR. All rights reserved. from the air inside a building or vehicle, thus lowering the air temperature. The cooling is typically achieved through a refrigeration cycle, but sometimes evaporation or free cooling is used. Air conditioning systems can also be made based on desiccants [5]. In the most general sense, air conditioning can refer to any form of technology that modifies the condition of air (heating, cooling, (de- )humidification, cleaning, ventilation, or air movement). However, in construction, such a complete system of heating, ventilation, and air conditioning is referred to as heating, ventilation, and air conditioning [6]. System, in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, largescale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems [7]. 1. Description of the Vapour Compression Refrigeration System Figure.1 Vapour compression refrigeration The vaporcompression uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. All such systems have four components: a compressor, a condenser, a thermal expansion valve (also called a throttle valve or metering device), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapour [8] and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with either cooling water or cooling air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by either the water or the air (whichever may be the case). The condensed liquid refrigerant, in the
ALI MOHAMMED ABDOULHA MASSOUD, DR. MOHAMMAD TARIQ thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporator of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. Fig.1. A Fictitious Pressure Volume Diagram for a Typical Refrigeration Cycle Then all the lubricating oil from the system should be removed and the maximum amount of oil allowed to be remained inside the system is 5% of the total amount of oil present in the system. The mineral oil should be replaced with ester based synthetic oil. The drier and the oil filter also should be replaced. The amount of R134a required in the system is about 90 to 95% of R12. Labels should be placed in the systems that have been retrofitted with R134a describing the new refrigerant and the lubricating oil. II. MATERIALS AND METHODS The liquefaction of the refrigerant is accomplished in the condenser, which is, essentially, a container cooled externally by air or water. The hot high-pressure refrigerant gas from the compressor is conveyed to the condenser and liquefies therein. Since there is a high gas pressure in the condenser, and the liquid refrigerant there is under the same pressure, it is easy to complete the cycle by providing a needle valve or other regulating device for injecting liquid into the evaporator. This essential component of a refrigerant plant is called the expansion valve. The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser. To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor. 2. R12 to R134a Conversion The process of retrofitting R12 system with R134a is quite an easy process. First of all, complete R12 should be removed from the system and recovered in the container. TABLE I. Properties of R-134a S. No. Properties R-134a 1 Boiling Point -14.9 F or -26.1 C 2 Auto-Ignition 1418 F or 770 C Temperature 3 Ozone Depletion Level 0 4 Solubility In Water 0.11% by weight at 77 F or 25 C 5 Critical Temperature 252 F or 122 C 6 Cylinder Color Code Light Blue 7 Global Warming Potential (GWP) 1200 Fig.2. Basic Vapour Compression Cycle Applied to a Water Chiller. Fig.3. Simple Saturation Refrigeration Cycle on a Pressure-Enthalpy Diagram.
Thermal Analysis of Vapour Compression Cycle for Different Refrigerants This basic vapour compression refrigeration cycle is illustrated in figure 2 where it is shown applied to a waterchilling set. Figure 3 illustrates the changes in the state of the refrigerant as the simple, basic vapour compression cycle takes place on p-h diagram. In order to explain the cycle and describe the diagram, it is first necessary to consider the thermodynamics of the present analysis. G. Power Needed for Compression- The product of work done in compression with the mass flow rate of refrigerant handled (yields the power needed for compression,, in Watt or kw. In the figure 3, state 1 is known because, with the simplifying assumptions made in the present analysis, it is that of the dry saturated vapour leaving the evaporator. A. Thermodynamics and Refrigeration- The subject of thermodynamics referred to the physics of air-water vapour mixtures. As it refers to the behaviour of refrigerants in vapour compression cycles of refrigeration, in order to deal with such cycles quantitatively. As a preamble to this, some principles and definitions must be reviewed. B. Wet Vapour- This is a mixture of saturated liquid and saturated vapour. The quality of the mixture is expressed in terms of its dryness fraction, defined as the mass of saturated vapour divided by the total mass of saturated vapour and saturated liquid. The point 4 in figure3 represents wet vapour. Wet vapour is a mixture of saturated liquid at state 4' and dry saturated vapour at state 1. The state of the wet vapour entering the evaporator can be defined in terms of the relevant enthalpies and its dryness fraction (D) at state 4, is given by C. Refrigerating Effect- In the basic vapour compression cycle of refrigeration shown in figure 3 a mixture of relatively cold, saturated liquid and saturated vapour enters the evaporator. The liquid part of the mixture is boiled to a saturated vapour and leaves the evaporator at state 1 as shown in figure 3. The refrigerating effect () is the enthalpy change across the evaporator. Using the notation of figure 3, this is expressed by D. Mass Flow Rate- The mass flow rate of refrigerant ( is calculated by is the refrigeration duty in kw. H. Heat Rejected at the Condenser- All the heat removed at the evaporator together with all the energy provided by the compressor must be rejected from the system. This heat is rejected at the condenser and is expressed as follows; The rate of heat rejection at the condenser is determined by I. Coefficient of Performance- The coefficient of performance of a refrigeration machine is the ratio of the energy removed at the evaporator (refrigerating effect) to the energy supplied to the compressor. Thus, using the notation of figure 3., the COP can be calculated as follows: COP (Carnot) = J. Actual Vapour-Compression Cycle- An ideal, simple, reversible vapour-compression cycle is not a practical proposition. The departure from reversibility arises from the irreversible nature of the throttling expansion process, pressure losses in the evaporator, condenser and pipelines, heat transfer through finite temperature differences and a measure of irreversibility in the compression process. Figure3. represents two of these irreversibility the throttling expansion (3--4) through the expansion valve and the desuperheating process (2-2') during the vapour first enters the condenser. The condensation process (2'-3) and the evaporation process (4-1), on the other hand, are reversible. Liquid refrigerant is incompressible and if it entered the suction valve of a reciprocating compressor it could damage the cylinder head or the valve plates at top dead centre. E. Volumetric Flow Rate- In the present analysis, assuming that there is no pressure drop in the suction line and that there are no heat exchanges between the suction line and its surroundings, the state entering the compressor is the same as that leaving the evaporator, namely, 0 0 C saturated. The volumetric flow rate at the suction state will be given by F. Compressor Work- In the ideal case, where there is no heat lost from the cylinders and no friction, the process of compression is adiabatic, reversible and isentropic. This is shown in figure 3 by the line joining the points1 and 2. It is assumed that there is no pressure drop in the pipeline between the outlet from the evaporator and the suction port on the compressor and that no heat gains or losses occur between the suction line and its environment. The compressor sucks dry saturated vapour from the evaporator and power must be provided to affect this process. The work done in compression () for this ideal case is given by the enthalpy increase across the compressor. Fig.4. Simple Saturation Refrigeration Cycle on a Temperature-Entropy Diagram To avoid this risk it is arranged that evaporation is completed before the vapour leaves the evaporator and, consequently, the state then entering the suction valve has several degrees of superheat. It is often also arranged that there is more than enough heat transfer surface in the
ALI MOHAMMED ABDOULHA MASSOUD, DR. MOHAMMAD TARIQ condenser to change the refrigerant from a superheated vapour to a saturated liquid. In this case the liquid is subcooled to a temperature less than its saturated temperature for the prevailing pressure. A consequence of this is that any loss of position head (because the condenser might be at a lower level than the expansion valve), or any frictional pressure drop in the liquid line, is less likely to cause the liquid to flash to gas before it reaches the expansion valve. Figure 4 shows a pressure-enthalpy diagram of a simple cycle with superheat at the outlet from the evaporator and sub-cooling at the outlet from the condenser. For the calculation of the properties, reference [15] has been used for this work. 2. Dryness Fraction For saturated liquid at a temperature of 0 0 C and a pressure of 292.69kPa, h 4 =200.00 kj/kg. For dry saturated vapour at 0 0 C and 292.69 kpa, h 1 = 398.68 kj/kg. Hence, the dryness fraction will be calculated by using equation Thus 25 per cent, by weight, of the liquid refrigerant entering the expansion valve flashes to vapour as the pressure drop through the valve occurs. Since the volume of the vapour is much greater than that of the liquid, the space occupied by the vapour is significantly large. 3. Refrigerating Effect = (398.68-248.94) = 149.74 kj/kg 4. The Mass Flow Rate kg/s Volumetric flow rate TABLE III. Table for Interpolation Fig.5. Pressure-Enthalpy Diagram of a Simple, Actual, Vapour-Compression Cycle Showing Superheat at Evaporator Outlet and Sub-Cooling at Condenser Outlet Pressure drops through the piping, the evaporator and the condenser are ignored and isentropic compression is assumed. It can be seen that the dryness fraction is reduced and the refrigerating effect is improved, compared with the simple plant as shown in figure5. 1. Assumption Following assumption have been made for the present analysis Compression should be isentropic. Pressure drops in the piping, evaporator and condenser are ignored Specific heat does not alter very much for the small temperature change of 5 K at constant pressure. TABLE II. Input Data Table Refrigerant Properties Values Refrigerant Evaporator temperature 0 0 C 134a (T e ) Condenser Temperature 35 0 C (T c ) Duty of the plant 352 kw Refrigerant Superheat at the 5 K 134a evaporator outlet Sub-cooling at the outlet 5 K from the condenser Refrigerant Superheat at the 5 K R717 evaporator outlet (Ammonia) TABLE IV. Table for Interpolation Pressure Entropy (s) Enthalpy (p) kpa kj/kgk (h 2 ) kj/kg 1 1000 1.7274 424.30 43.95 2 887.11 1.7274 421.64 39.00 3 800 1.7274 419.58 35.18 Sub-cooling at the outlet 5 K Mass flow rate of refrigerant = 2.180 kg/s from the condenser Volumetric flow rate of the refrigerant S. No. Work Done in Compression W r = (421.64-398.68) = 22.96 kj/kg Compression Power P = 2.351 x 22.96 = 53.98 kw Heat rejected at the condenser q c = (421.64-248.94) = 172.7 kj/kg Rate of heat rejection = 2.351 X 172.7 = 406 kw Coefficient of performance = 6.52 Temperature (t 2 ) 0 C Carnot Coefficient of performance COP (Carnot) = 7.8 Refrigerant R134a R134a with 5 K of superheat at the evaporator outlet and 5 K of sub-cooling at the outlet from the condenser Dryness fraction = 0.21 Refrigerating effect Q r = (h 1 h 4 ) = (h 1 h 3 ) = (403.10-241.64)= 161.46 kj/kg
Thermal Analysis of Vapour Compression Cycle for Different Refrigerants = 0.069 35 (273 + 5)/(273 + 0) = 0.070 62 m 3 /kg Work done in compression W r W r = (425.43-403.10) = 22.33 kj/kg Power absorbed by the compressor=2.180x22.33=48.68 kw Heat Rejected at the condenser = 183.79 kw/kg Rate of heat rejection at the condenser Qc = 2.180 x (425.43-241.64) = 400.7 kw 352 + 48.68 = 400.7 kw COP of the cycle 161.46/22.33 = 7.23 Carnot COP COP (Carnot) = (273 + 35)/(35-0) = 8.8 Refrigerant R717 (ammonia) Refrigerant R717 with 5 K of superheat at the evaporator outlet and 5 K of sub-cooling at the condenser outlet. Dryness fraction = 0.11 Refrigerating effect Q r = (h 1 h 4 ) = (h 1 h 3 ) Q r = (1456.5-322.8) = 1133.7 kj/kg Mass flow rate Volumetric flow rate v = 0.3105 x 0.2966 = 0.0921 m 3 /s At compressor discharge the entropy, s 2 =s 1 = 5.3842 kj/kg K. Interpolating for this entropy in table 3.4 at a pressure of 1350 kpa yields: TABLE V. Values of Properties at Interpolated Temperature Entropy (s) Temperature (T- T s ) Enthalpy (h) 5.3632 50 0 C 1615.1 5.3842 52.9 0 C 1622.7 5.3995 55 0 C 1628.2 III. RESULTS AND DISCUSSION The results of the present work have been discussed in the following paragraphs in the form of table and graphs. Graphs are plotted using origin 50 software. Sub cooling and superheating of the refrigerants have been selected same for both the refrigerants as suggested by [21]. Results are well within the published values. In the following graphs, the results are presented for variation of the refrigerating effect with different thermal parameters of the cycle. The refrigerants have been chosen as 134a (1,1,1,2 Tetrafluorethane) and R717 (Ammonia). Figure 6 shows the variation of refrigerating effect of different refrigerants with dryness fraction at their respective conditions. It has been observed from the figure that the refrigerant R134a produces maximum refrigerating effect and minimum dryness fraction under given conditions. The conditions of sub cooling and superheating as mentioned in the table V, has imposed to both the refrigerants decreases the refrigerating effect as the dryness fraction increases. Figure6 represents the refrigerating effect with mass flow rate of the refrigerants. It has been found that, the refrigerating effect is higher and mass flow rate is minimum for refrigerant 134a as compared to other two conditions. Whence h 2 = 1622.7 kj/kg Work done in compression W r = (1622.7-1456.5) = 166.2 kj/kg Fig.6. Variation of the Different Refrigerants for Refrigerating Effect at Dryness Fraction Compressor power P = 0.3105 x 166.2 = 51.60 kw Heat rejection at the condenser Rate of heat rejection = 0.3105 x (1622.7-322.8) = 403.6 kw COP of the cycle COP = (1456.5-322.8)/(1622.7-1456.5) = 6.82 Carnot COP COP (Carnot) = Carnot COP = (0 + 273)/[(35 + 273) - (0 + 273)] = 7.8 Fig.7. Variation of Refrigerating Effect with Mass Flow Rate
ALI MOHAMMED ABDOULHA MASSOUD, DR. MOHAMMAD TARIQ Fig.8. Variation of Refrigerating Effect with Volumetric Flow Rate effect with work required by compressor. The work required by the compressor is maximum with the refrigerant ammonia with its prevailing conditions as compared to the other refrigerant. Figure 10 shows the variation of het rejected at the condenser for all the refrigerants with their prevailing conditions. It has been found that the refrigerant ammonia with their conditions have highest refrigerating effect as compared to 134a. The refrigerating effect is also maximum for the ammonia. Figure 11 represents the variation of refrigerating effect with coefficient of performance of the cycle for both the refrigerants with their conditions. The COP of the cycle is maximum for R134a with given conditions as compared to other two refrigerants. Though, it gives less refrigerating effect as compared to the ammonia. Figure11 represents the variation of combined graph of three parameters namely; refrigerating effect, work required by compressor and heat rejected at condenser with dryness fraction. The comparative study reflects that the value of heat rejected at the condenser is maximum as compared to other two values. On the other hand, the value of work required in compressor is minimum. Fig.9. Variation of Refrigerating Effect with Work Required By Compressor Fig.11. Variation of Refrigerating Effect Coefficient of Performance of Cycle Fig.10. Variation of Refrigerating Effect with Heat Rejected At Condenser Figure10 represents the refrigerating effect of the refrigerants with volumetric flow rate of the refrigerants. The refrigerating effect is highest for 134a but volume flow rate is minimum for refrigerant 134a with their conditions as given. On the other hand, the ammonia with the prevailing conditions shows the maximum volume flow rate of the refrigerant. Figure 10 represents the variation of refrigerating Fig.12. Variation of Different Parameters with Dryness Fraction.
Thermal Analysis of Vapour Compression Cycle for Different Refrigerants IV. CONCLUSION The present analysis has been worked out for the evaporator temperature 0 0 C and Condenser temperature 35 0 C while the duty of the unit has been taken as 352 kw. Two refrigerants with specified conditions at evaporator and condenser have given a designer for more options for optimizatiom. The optimization of the vapour compression cycle with superheating at evaporator by 5 deg and sub cooling a the outlet of condenser by 5 deg of both the refrigerants proven that the results of refrigerating effect for the refrigerants R134a and R717 with their conditions have least difference for their dryness fraction and mass flow rate of refrigerants. Work required by compressor is very less for R134a for without condition and with condition as compared to ammonia. As far as COP of the cycle is concerned, the refrigerant R134a with prevailing conditions have maximum value as compared to ammonia. But Ammonia has maximum refrigerating effect. Saved power in compressor for optimum designed and matched systems is expected to be higher than 2.27% in respect to that of the Simple Cycle. The bleed system for condensate subcooling can be used to upgrade already existing plants. Some increase of the cool power, depending on the Refrigeration plant features, can be obtained, and a COP increase of some 9.8 %, is expected by using R143a with prevailing conditions. V. REFERENCES [1]Daou, K; Wang, Xia (2005). Desiccant cooling air conditioning: a review, Renewable and Sustainable Energy Reviews. 10 (2): 55 77. [2]McDowall, Robert (2006). Fundamentals of HVAC Systems. Elsevier. p. 3. [3]Franklin,Benjamin(June17,1758). Letter to John Lining, 6 August 2014. [4] Early University Benefactors, 2012-11-08. [5] Unsung Engineering Heros: Robert Sherman, 2015-06- 10. [6]Mate, John "Making a Difference: A Case Study of the Greenpeace Ozone Campaign" RECIEL 10:2 2001. [7]Y.V.C.Rao(2003).AnIntroductionto thermodynamics (2nd ed.). Universities Press. [8]Simarpreet Singh et al., Comparative study of cycle modification strategies for trans-critical CO2 refrigeration cycle for warm climatic conditions, Case Studies in Thermal Engineering 7(2016)78 91 [9]Book: Principles of Refrigeration by Roy J. Dossat, fourth edition, Prentice Hall [10]Daou, K; Wang, Xia (2005). Desiccant cooling air conditioning: a review, Renewable and Sustainable Energy Reviews. 10 (2): 55 77. [11]McDowall, Robert (2006). Fundamentals of HVAC Systems. Elsevier. p. 3. [12]ranklin, Benjamin (June 17, 1758). Letter to John Lining, 6 August 2014. [13] A brief history of Air conditioning, Popular Mechanics. 2015-01-01. [14] Early University Benefactors, 2012-11-08. [15]American Society of Heating, Refrigerating and Air- Conditioning Engineers from the (1997) ASHRAE Handbook. [16]Shailendra Singh Chauhan and S.P.S. Rajput, Parametric analysis of a combined dew point evaporativevapour compression based air conditioning system, Alexandria Engineering Journal (2016) PP 1-12