Pressure drop analysis of evaporator using refrigerants R-22, R-404A and R-407C

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1 drop analysis of evaporator using refrigerants R-22, R-404A and R-407C #1 Pallavi Sawkar, #2 Dr. Pradeep Patil #12 Department of Mechanical Engineering, JSPM s JSCOE, Savitribai Phule Pune University, India Abstract: The frictional pressure drop across the evaporator is experimentally investigated using refrigerants R- 22, R-404A and R-407C. Experiments presented in the paper are performed for various condensing temperatures and evaporating temperatures. The test runs are performed for each average saturated condensing temperatures ranging from 38.6 ᵒC to 45.6 ᵒC for which evaporating temperatures are varied. The experimental results were studied for effect of mass flux on frictional pressure drop and friction factor. It indicates that the frictional pressure drop increases whereas friction factor decreases with increase in mass flux. It is observed that frictional pressure drop for R-404A is more than R-22 and R-407C. Keywords: drop, friction factor, condensing temperature, evaporating temperature, mass flux I. INTRODUCTION Recently refrigeration and air-conditioning is becoming increasingly aware of efforts for environmental protection. Such awareness is resulting into a demand for environmental friendly refrigerants in smaller evaporators (Pamitran et al., ). In the refrigeration and air-conditioning industry a very basic understanding of multiphase flow and heat transfer with boiling and condensation of refrigerants for small channels is important. In particular, there is performed on evaporation process of refrigerants, mostly through experimental studies but, in the literature, comparatively less attention has been given to the model simulation of evaporation of the refrigerant in horizontal tube (Sripattrapan & Wongwises, 2005). To maximize the performances of the refrigerating and air conditioning plants, a detailed analysis of each component is necessary. Among the important components that influence the cycle efficiency are requirement of developing a validated design correlation for two-phase pressure drop which will help for design and optimization of compact heat exchangers (Tran et al., 2000). Hydro-fluorocarbon (HFC) refrigerants, like R134a and zero-tropic refrigerant mixtures like R407C have been introduced in order to replace R12 and R22, respectively. As per the agreement done in The Montreal Protocol, efforts towards the replacement of these refrigerants are under progress. For the optimum performance, an accurate design technique is essential for the prediction of refrigerant pressure-drops and flow-patterns through the evaporator, condenser and other heat exchangers (Smith et al., 2001). A lot of research has been carried-out on flow-boiling of pure refrigerants, mixed refrigerants and of refrigerant lubricating-oil mixtures. In most of the flow boiling tests, local heat-transfer coefficients of boiling are expressed as a function of vapour quality instead of mean values over a whole tube with a large change in vapor quality or even in its exit superheating (Greco & Vanoli, 2004). Lot of research has been transfer coefficients and pressure drops is fundamental. For sure, it can reduce costs by avoiding over design of the heat exchangers, and therefore, it is a prerequisite to increase the cycle performance through correct sizing of each plant component (Aprea et al., 2008). The calculation of the pressure drop in any two phase flow system is very important in the design of steam-power and petrochemical plants as well as in refrigeration and air-conditioning systems. Information about the two phase frictional characteristics is important as it would certainly improve the accuracy of the design of a heat transfer system (Chen et al., ). In other words, if an evaporator is inaccurately designed with a two-phase pressure drop, with only one-half the real value, then the system efficiency will suffer accordingly from the larger than expected fall in saturation temperature and pressure through the evaporator (Didi et al., 2000). Hence, accurate design of the system plays a very important role. In the present article the behavior of frictional pressure drop and the friction factor were studied for two phase flow across evaporator using R-22, R-404A and R-407C as refrigerants.

2 II. EXPERIMENTAL APPARATUS AND METHOD The setup used for the experiments is a Compressor-Calorimeter test rig, as shown in fig. 1, which can also be used to analyze Vapor Compression Cycle (VCC). The refrigerants are charged at the suction port of the rig. Once the compressor starts, it pumps up the refrigerant to the higher pressure. Oil Separator is provided to separates the oil that is probably carried along with refrigerants during the compression stroke. The outlet of the compressor has two thermocouples to measure the compressor top shell temperature and discharge temperature of the compressor. Fig.1 Experimental test rig Fig.2 Evaporator cabin (Sealed tank) From the oil separator the refrigerant passes to the shell and tube condenser, where in the cooling water supplied by the cooling tower at the ambient condition exchanges heat in the shell and tube frame work of the condenser. The refrigerant coming out of the condenser expected to be in the saturated state is made to the drier, to get wiped out of any moisture content present within. The receiver following next to the drier is just a storage unit where in it stores the refrigerant. Mass flow meter provided measures the amount of refrigerant that is been used by the compressor for particular given condition. Then the refrigerant is passed to the evaporator, where actually the cycle is theoretically believed as the starting point. The evaporator cabin (Sealed tank) as shown in fig.2 incorporates evaporator coil, stirrer and glycol heater. The glycol mixture is used as anti-freezing agent in the cabin to see to that the temperature does fall beyond the set value during the experimentation. There are 7 temperature sensors fixed in the circuit lines to measure the temperature condition of the refrigerant and the cooling water that runs in the system during cycle operation. The suction pressure and the discharge pressure of the compressor is been regulated by the pressure switch, the magnitudes of the same is measured by the transducer fixed at the inlet and outlet of the compressor. There are two more pressure transducer to sense the pressure in water supply lines to the condenser and sub cooler. A proportional-integral-derivative controller (PID controller) is a control loop feedback mechanism (controller) used in this system 3 in numbers that is been used to set the discharge pressure of the compressor, desired sub cooling temperature of the refrigerant and the suction temperature of the refrigerant before it enters the compressor. This is merely the complete working of the setup.

3 R22, R404A, R407C are charged in the system one by one. For each condensing temperature, evaporating temperature is varied. This completes one trial. Four such trials are taken for each refrigerant. III. DATA REDUCTION Total pressure drop of the fluid consists of pressure drop due to the variation of potential energy, kinetic energy and friction on channel walls: As evaporator tubes are horizontal, there is no change in static head. Hence, value of pstatic= 0. For analysis of evaporator attention should even be paid towards the superheating provided before the entry of the refrigerant to the compressor. Frictional pressure drop in superheating region should be considered. 3.1 Experimental Frictional pressure drop For evaporation in horizontal tubes, while measuring two phase pressure drop the frictional pressure drop is obtained by subtracting momentum pressure drop and pressure drop caused due to superheating from total pressure drop.

4 IV. RESULTS AND DISCUSSION Present experiment was performed for various condensing temperatures ranging from 38.6 ᵒC to 45.6ᵒC on the test apparatus shown in fig. 1. For each condensing temperature evaporating temperature was varied from 2 ᵒC to 10 ᵒC and corresponding readings of total pressure drop, mass flow rate and refrigerant temperature after sub-cooling were noted down. The values of the ptot observed from the experiment were used to calculate frictional pressure drop, momentum pressure drop and friction factor for refrigerants R-22, R-404A, R-407C using equations (2), (4) and (12) respectively. The graphs were plotted to check the trend of variation of pressure drop and friction factor with respect to the variation of mass flux. From calculations done it was observed that frictional pressure drop values were much higher than momentum pressure drop values. Hence the frictional pressure drop plays a vital role in design of evaporator. Physical properties such as density, surface tension, viscosity and pressure have significant effect on pressure drop.

5 Frictio n Factor (Pa) Frictional Dro p (Pa) n Factor Friction Pre ssu re Factor (Pa) International Engineering Research Journal (IERJ) Special Issue Page , June 2016, ISSN Variation of Different s Total Frictional Momentum (Total ) (Frictional ) Fig.7 Variation of different pressure drops with mass flux for R-407C Effect of Mass Flux on Friction Factor friction factor (friction factor ) (Momentum ) Fig.5 Variation of different pressure drops with mass flux for R-404A Effect of Mass Flux on Friction Factor 0.06 Friction 0.05 Factor (Friction 0.02 Factor) Fig.6 Variation of friction factor with mass flux for R-404A Fig. 9 shows the comparative frictional pressure drop analysis with respect to mass flux for all the three refrigerants. Only frictional pressure drop is considered for analysis as it accounts major part of total pressure drop. It can be seen that R-22, R-407C and R-404A is the increasing order of frictional pressure drop with respect to mass flux. Variation of Different s Total Frictional Momentum (Total ) (Frictional ) (Momentum ) Fig.8 Variation of friction factor with mass flux for R-407C Fig. 4, fig. 6 and fig. 8 show the variation of friction factor with mass flux for R-22, R-404A and R-407C respectively. As the mass flux increases there is decrease in friction factor. The same trend is observed for all the refrigerants. This decrease is 0.3 % to 2.5 %, 0.1% to 5%, 0.7 % to 2% with respect to mass flux for R-22, R-404A and R-407C respectively Effect of Mass flux on Frictional pressure drop R R404a R407c (R22) (R404a) (R407c ) Fig.9 Comparative analysis for variation of different pressure drops with mass flux Effect of Mass Flux on Friction factor R R404a 0.07 R407c (R22) (R404a) (R407c) Fig.10 Comparative analysis for variation of friction factor with mass flux

6 Fig 10 shows the comparative analysis for variation of friction factor with mass flux for all the three refrigerants. V. CONCLUSION drop and friction factor for refrigerants R-22, R-404A, R -407C were investigated across evaporator. Effect of mass flux on frictional pressure drop and friction factor was found out for each refrigerant and even comparative analysis is also done. drop is higher for higher mass fluxes where as friction factor is lowest for higher mass fluxes. Frictional pressure drop values play significant role for design purpose as it accounts major part of total pressure drop. Frictional pressure drop for R-404A is found to be larger than R-404A and R-22. R-22 and R-404A shows more or less same frictional pressure drop, but R-22 is not environment friendly refrigerant. Hence R-404A can be considered as best amongst these three refrigerants from the experiment performed. Friction factor correlation can also be developed from current data and can exist as future scope of the same experiment. REFERENCES Y.M. Lie, F.Q. Su, R.L. Lai, T.F. Lin, (2006), Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes, International Journal of Heat and Mass Transfer, Y.M. Lie, F.Q. Su, R.L. Lai, T.F. Lin, (2008), Experimental study of evaporation pressure drop characteristics of refrigerants R-134a and R-407C in horizontal small tubes, International Journal of Heat and Mass Transfer, C. Aprea, A. Greco, A. Rosato, Comparison of R407C and R417A heat transfer coefficients and pressure drops during flow boiling in a horizontal smooth tube, Energy Conversion and Management, A.S. Pamitran, Kwang-Il Choi, Jong-Taek Oh, Pega Hrnjak, (2010) Characteristics of two-phase flow pattern transitions and pressure drop of five refrigerants in horizontal circular small tubes, International journal of refrigeration, Pradeep A. Patil, S.N. Sapali, (2011), Condensation pressure drop of HFC-134a and R-404A in a smooth and micro-fin U-tube, Experimental Thermal and Fluid Science, T.N. Trana, M.-C. Chyub, M.W. Wambsganssa, D.M. Francec, (2000), Two-phase pressure drop of refrigerants during flow boiling in small channels: an experimental investigation and correlation development, International Journal of Multiphase Flow, 1739±1754. S.J. Smith, L. Shao, S.B. Riffat, (2001), drop of HFC refrigerants inside evaporator and condenser coils as determined by CFD, Applied Energy, M.B. Ould Didi, N. Kattan, J.R. (2002), Prediction of twophase pressure gradients of refrigerants in horizontal tubes, International Journal of Refrigeration, Adriana Greco, Giuseppe Peter Vanoli, (2004), Evaporation of refrigerants in a smooth horizontal tube: prediction of R22 and R507 heat transfer coefficients and pressure drop, Wisis Sripattrapan, Somchai Wongwises, (2005), Twophase flow of refrigerants during evaporation under constant heat flux in a horizontal tube, International Communications in Heat and Mass Transfer,