Frequently the effect on capacity of discharge line pressure drop is over-estimated since it is assumed the compressor discharge pressure and the condensing pressure are the same. In fact, there are two different pressure, the compressor discharge pressure being greater then the condensing pressure by the amount of the discharge line pressure drop. An increase in pressure drop in the discharge line increase the compressor discharge pressure materially, but have little effect on the condensing pressure. Although there is a slight increase in the heat of compression for an increase in head pressure, the volume of gas pumped is decreased slightly due to a decrease in volumetric efficiency of the compressor. Therefore the total heat to be dissipated through the condenser may be relatively unchanged, and the condensing temperature and pressure may be quite stable, even though the discharge line pressure drop and therefore the compressor discharge pressure might vary considerably.
The performance of a typical Copelametic compressor, operating at air conditioning condition with R-22 and an air cooled condenser indicates that for each 5 psi pressure drop in the discharge line,the compressor capacity is reduced is increased about 1%. On a typical low temperature Copelametic compressor operating with R-502 and on air cooled condenser, approximately 1% of compressor capacity will be lost each 5 psi pressure in power consumption.
As a general guide, for discharge line pressure drops up to 5 psi, the effect on system performance would be so small as to be difficult to measure. Pressure drops up to 10 psi would not be greatly detrimental to performance provided the condenser is sized to maintain reasonable condensing pressure.
Actually a reasonable pressure drop in the discharge line is often desirable to dampen compressor pulsation, and thereby reduce noise and vibration. Some discharge line mufflers actually derive much of their efficiency from pressure drop
No comments:
Post a Comment