SUMMARY
This article provides information as to how to limit the minimum evaporating pressure in air conditioning and refrigeration systems during periods of low load.

RESOLUTION
On many air conditioning and refrigeration systems it is desirable to limit the minimum evaporating pres-sure during periods of low load either to prevent coilicing or to avoid operating the compressor at a lower suction pressure than it was designed to operate.

Various methods have been used to achieve this result— integral cylinder unloading, gas engines with vari-able speed control, or multiple smaller systems.Compressor cylinder unloading is used extensively onlarger systems but is too costly on small equipment,usually 10 hp and below.

Cycling the compressor with a low pressure cutout control is a method often used but is being re-evaluated for three reasons.

1.On-off control on air conditioning systems is uncomfortable and does a poor job of humidity and mold control.

2.Compressor cycling reduces equipment life.

3.In most cases, compressor cycling is not economicalbecause of peak load demand charges.

One method that offers a practical and economical solution to the problem, is to bypass a portion of the hot discharge gas directly into the low side. This is done by amodulating control valve — commonly called aDischarge Bypass Valve (DBV). This valve, which openson a decrease in suction pressure, can be set to auto-matically maintain a desired minimum evaporating pressure regardless of the decrease in evaporator load.

See attached PDF articles for additional information.