On many air-conditioning and refrigeration systems it is desirable to limit the minimum evaporating pressure. This is so especially during periods of low load either to prevent coil icing or to avoid operating the compressor at lower suction pressure than it was designed for. Various methods of operation have been designed to achieve the result—integral cylinder unloading, gas engines with variable speed control, or multiple smaller systems. Compressor cylinder unloading is used extensively on larger systems. However, it is too costly on small equipment, usually 10 hp or below. Cycling the compressor with a low pressure-cutout control has had widespread usage, but is being reevaluated for three reasons:
? On-off control on air-conditioning systems is uncomfortable and does a poor job of humidity control.
? Compressor cycling reduces equipment life.
? In most cases, compressor cycling is uneconomical because of peak load demand charges.
One solution to the problem is to bypass a portion of the hot discharge gas directly into the low side. This is done by the modulating-control valve—commonly called a discharge-bypass valve (DBV). This valve, which opens on a decrease in suction pressure, can be set to maintain automatically a desired minimum evaporating pressure, regardless of the decrease in evaporator load.
