SUCTION PIPING FOR MULTIPLEX SYSTEMS

  It is common practice in supermarket application to operate several fixtures, each with liquid line solenoid valve and expansion valve control, from a single compressor. Temperature control of individual fixtures is normally achieved by meas of a thermostat opening and closing the liquid line solenoid valve as necessary. this type of system, commonly called multiplexing, requires careful attention to design to avoid oil return problems and compressor overheating.

  Since the fixtures fed by each liquid line solenoid valve may be each liquid line solenoid valve may be controlled individually, and since the load on each fixture is relatively constant during operation, individual suction lines and risers are normally run from each fixture or group of fixtures controlled by a liquid lines solenoid valve for minimum pressure drop and maximum efficiency in oil return. This provides excellent control so long as the compressor is operating at design suction pressure, but there may be periods of light load when most or all of the liquid line solenoid are closed. Unless some means of controlling compressor capacity is provided, this can result in compressor short cycling or operation at excessively low suction pressure, which can result only in overheating the compressor,

  Because of the fluctuations in refrigeration load caused by closing of the individual liquid line solenoid valves, some means of compressor capacity control must be provided. In addition, the means of capacity control must be such that it will not allow extreme variations in the compressor suction pressure.

  Where multiple compressor are used, cycling of individual compressor provides satisfactory control. Where multiplexing is done with a single compressor, a hot gas bypass system has proven to be the most satisfactory means of capacity reduction, since this allows the compressor to operate continuously at a reasonably constant suction pressure while compressor cooling can be safely controlled by meas of a desuperheating expansion valve

  In all cases, the operation of the system under all possible combinations of heavy load, light load, defrost, and compressor capacity must be studied carefully to be certain that operating condition will be satisfactory