With the aid of a suitable diagram, describe the operation and construction of a reciprocating compressor employed in a VCR system (30 marks).
A compressor is the most important and often the costliest component of any vapour compression refrigeration system (VCRS). The function of a compressor in a VCRS is to continuously draw the refrigerant vapour from the evaporator, so that a low pressure and low temperature can be maintained in the evaporator at which the refrigerant can boil extracting heat from the refrigerated space. The compressor then has to raise the pressure of the refrigerant to a level at which it can condense by rejecting heat to the cooling medium ...view middle of the document...
The pressure in the inlet manifold is equal to or slightly less than the evaporator pressure. Similarly the pressure in the outlet manifold is equal to or slightly greater than the condenser pressure. The purpose of the manifolds is to provide stable inlet and outlet pressures for the smooth operation of the valves and also provide a space for mounting the valves.
The valves used are of reed or plate type, which are either floating or clamped. Usually, backstops are provided to limit the valve displacement and springs may be provided for smooth return after opening or closing. The piston speed is decided by valve type. Too high a speed will give excessive vapour velocities that will decrease the volumetric efficiency and the throttling loss will decrease the compression efficiency.
Performance of reciprocating compressors
For a given evaporator and condenser pressures, the important performance parameters of a refrigerant compressor are:
a) The mass flow rate (m) of the compressor for a given displacement rate
b) Power consumption of the compressor (WC)
c) Temperature of the refrigerant at compressor exit, Td, and
d) Performance under part load conditions
The mass flow rate decides the refrigeration capacity of the system and for a given compressor inlet condition, it depends on the volumetric efficiency of the compressor. The volumetric efficiency, ηV is defined as the ratio of volumetric flow rate of refrigerant to the maximum possible volumetric flow rate, which is equal to the compressor displacement rate, i.e.,
For a given evaporator and condenser temperatures, one can also use the volumetric refrigeration capacity (kW/m3) to indicate the volumetric efficiency of the compressor. The actual volumetric efficiency (or volumetric capacity) of the compressor depends on the operating conditions and the design of the compressor.
The power consumption (kW) or alternately the power input per unit refrigeration capacity (kW/kW) depends on the compressor efficiency (ηC), efficiency of the mechanical drive (ηmech) and the motor efficiency (ηmotor). For a refrigerant compressor, the power input (WC) is given by:
Where Wideal is the power input to an ideal compressor.
The temperature at the exit of the compressor (discharge compressor) depends on the type of refrigerant used and the type of compressor cooling. This parameter has a bearing on the life of the compressor.
The performance of the compressor under part load conditions depends on the type and design of the compressor.
a) Ideal reciprocating compressor:
An ideal reciprocating compressor is one in which:
i. The clearance volume is zero, i.e., at the end of discharge process, the volume of refrigerant inside the cylinder is zero.
ii. No pressure drops during suction and compression
iii. Suction, compression and discharge are reversible and adiabatic
Figure 2 shows the schematic of an ideal compression process on pressure-volume and...