It is possible to run two or more indoor units from a single condensing unit, with economies in the number and costs of components. Such systems are referred to as multisplits, and several different types of circuit will be encountered. The usual split package air-conditioner comprises one condensing unit connected by pipes to one evaporator unit.
Twin condensing units are made to save on outdoor casings and reduce the number of pieces on a roof or wall. Such twins will be connected in the usual way to two separate indoor units. Units having single-speed compressors will require some automatic method of shedding the excess cooling capacity when some of the fan-coil units do not call for cooling. Liquid from the condenser coil passes directly through an expansion valve, and the resulting mixture of cold liquid and flash gas is distributed to each of the fancoil units on the circuit.
On–off control of the cold liquid to each room is effected by a solenoid valve within each indoor unit, which will be switched by the room thermostat. Returned refrigerant gas, sometimes with unevaporated liquid present, is caught in a suction trap before entering the compressor, or the liquid is boiled off with a suction/liquid heat exchanger. Both the outgoing and return refrigerant pipes to each fan-coil unit must be carefully insulated. If any of the rooms does not require cooling, then the excess compressor capacity is taken up by injecting hot gas directly from the compressor discharge into the return.
Under conditions of light load the head pressure will fall, and this pressure must be maintained by slowing the condenser fan. It may also be necessary to inject some liquid into the return pipe, if the bypass gas makes it too hot.
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