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.
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3. Simple Vapor Compression Refrigeration System
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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 :
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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^)
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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.
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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.
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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.
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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.
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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.
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