Effect of compressor suction pressure in Vapor Compression Refrigeration System

here in this presentation we will be discussing about Effect of compressor suction pressure in Vapor Compression Refrigeration System under Suction Discharge and Evaporator

1. Refrigeration and Air Conditioning Effect of compressor suction pressure in VCRS ​ ​

2. Introduction Discharge pressure (also called high side pressure or head pressure) is the pressure generated on the output side of a gas compressor in a refrigeration or air conditioning system. An extremely high discharge pressure coupled with an extremely low suction pressure is an indicator of a refrigerant restriction. Your desired suction pressure is 33-48 degrees or 58-80 psi depending on the indoor temperature. The hot or warm air from a hot house traveling across the evaporator coil will warm the refrigerant faster than normal causing a higher suction pressure. 2

3. Simple Vapor Compression Refrigeration System 3

4. Theoretical Vapor Compression Cycle with Dry Saturated Refrigeration Cycle Vapor after Compression A vapor compression cycle with dry saturated vapor after compression is shown on T-s diagrams in Figures 2.2(a) and (b) respectively. At point 1, let T1, p1 and s1 be the temperature, pressure and entropy of the vapor refrigerant respectively. The four processes of the cycle are as follows : 4

5. Effect of Suction Pressure The suction pressure (or evaporator pressure) decreases due to the frictional resistance of flow of the refrigerant. Let us consider a theoretical vapor compression cycle 1-2-3-4 when the suction pressure decreases from PS to PS^ as shown on p-h diagram. It may be noted that the decrease in suction pressure. Decreases the refrigerating effect from (h1- h4) to (h1^ - h4^) Increases the work required for compression from (h2 – h1) to (h2^-h1^) 5

6. Effect of Suction Pressure Since the C.O.P, of the system is the ratio of refrigerating effect to the work done, therefore with the decrease in suction pressure, the net effect is to decrease the C.O.P. of the refrigerating system for the same refrigerant flow. Hence with the decrease in suction pressure the refrigerating capacity of the system decreases and the refrigeration cost increases. 6

7. Effect of Dis-Charge Pressure In actual practice, the discharge pressure (or condenser pressure) increases due to frictional resistance of flow of the refrigerant. Let us consider a theoretical vapor compression cycle l-2-3-4 when the discharge pressure increases from pD to pD ‟ as shown on p-h diagram in Figure 2.4 resulting in increased compressor work and reduced refrigeration effect. 7

8. Effect of Evaporator Pressure Consider a simple saturation cycle 1-2-3-4 with Freon 12 as the refrigerant as shown in Figure 2.5 for operating conditions of tk = 40°C and t = – 5°C. Now consider a change in the evaporator pressure corresponding to a decrease in the evaporator temperature to – 10°C. The changed cycle is shown as 1^-2^-3^-4^. It is therefore, seen that a drop in evaporator pressure corresponding to a drop of 5°C in saturated suction temperature increases the volume of suction vapor and hence decreases the capacity of a reciprocating compressor and increases the power consumption per unit refrigeration. 8

9. Effect of Condenser Pressure An increase in condenser pressure, similarly results in a decrease in the refrigerating capacity and an increase in power consumption, as seen from the changed cycle 1^-2^-3^ -4^ for tk = 45°C in Figure 2.6. The decrease in refrigerating capacity is due to a decrease in the refrigerating effect and volumetric efficiency. The increase in power consumption is due to increased mass flow (due to decreased refrigerating effect) and an increase In specific work (due to increased pressure ratio), although the isentropic line remains unchanged. 9

10. Advantages of VCRS Used in domestic and commercial refrigerators, large-scale 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. Cascade refrigeration systems may also be implemented using 2 compressors. 10

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