The performance of a compressor is the result of compromise:
· Physical limitations of the compressor (building materials) and the type of fluid
soda;
· Drive motor;
· Durability;
· Cost of manufacturing and operational;
· Operating range;
· Vibration level and noise.
The purpose of the compressor manufacturers is to get a value of COP (coefficient of
performance) that allows low power consumption and good efficiency
refrigeration or at least within the desired operating conditions.
The COP is the result of the relationship between capacity (cooling capacity) and the work
(Absorbed power). The power absorbed is related to the efficiency
(Yield) of the compressor.
COP = Power consumption
cooling capacity
In hermetic compressors (or semi-hermetic) is considered the power absorbed
by the electric motor. In compressors the absorbed power relates to potency
absorbed into the compressor shaft.
The cooling capacity or cooling capacity is a function of volume displaced
refrigerant per unit time.
In an ideal compressor the displaced volume comprises the theoretical geometric volume
displacement compressor (G).
Performance of the actual compressor
The actual performance of the compressor deviates from the ideal due to various losses
reduce the cooling efficiency and increase the power absorbed. In the compressor
positive displacement, causes influencing the performance are in accordance with
the type compressor, all or some given below:
1) Pressure loss in the compressor.
· By means of locking valves.
· Suction filter through.
· Engine Through (hermetic compressor or semi-hermetic).
· Through the suction and discharge manifolds.
· Through the valves and ports of the valves (suction and discharge).
2) Heat absorbed by coolant
· The internal electric motor (hermetic and semi-hermetic compressors).
· By friction of the compression rings and oil.
· Compression heat (heat absorbed from the evaporator plus work
compressor).
3) Inefficiency valve (mechanical imperfection)
4) Internal leakage of refrigerant gas
5) Oil Circulation
6) cylinder of gas remaining re-expansion (dead space)
7) Offset isotropic compression
8) On or subcompressão
It is difficult to individualize these losses. Generally they are grouped by categories
result in the following income values:
· Compression efficiency (h c): is the relationship between the work required for
isentropic compression and the work supplied to the gas refrigerant only in
Volumetric compression;
· Mechanical income (hm) is the grating between the working fluid supplied to the gas
refrigerant and the work absorbed by the compressor shaft;
· Volumetric Efficiency (hv) is the ratio of the gas volume of the fluid
refrigerant entering the compressor and the theoretical displacement volume;
· Income isentrópico (hi). It is the relationship between the necessary work for the
isentropic compression and the work absorbed by the compressor shaft.
The compressor COP is considered true when the relationship between the power
real cold and the power absorbed by the compressor is acceptable.
Some compressor manufacturers indicate, in catalogs, and cooling capacity
absorbed power for conditions of certain subcooling values of
liquid and overheating of the suction gas. These factors, which influence the
compressor efficiency, are not intrísecos compressor but the system.
It must take account of this situation, especially when comparing
equipment or systems.
Liquid subcooling is the temperature difference between the temperature
condensation and the temperature at which the liquid reaches the expansion device,
Overheating of the suction gas is the temperature difference between the temperature
evaporation and the vapor entering the compressor.
In the refrigerating capacity (also referred to as cooling capacity) and power
provided absorbed by the engine, another important quantity is the heat rejected in
condenser.
The heat rejected in the condenser is the sum of the cooling capacity (heat absorbed by
evaporator) and the power absorbed by the compressor (power supplied by the engine).
