This article describes the arrangement of compressors in the package air conditioners with multiple compressors. It also describes the capacity control system in these units.
Compressors and Control System in the Packaged Air Conditioners
The packaged air conditioners are used for the low to medium tonnage of air conditioning load ranging from 5 to 20 tons also higher. For smaller loads up to 5 tons single compressor is used. For the higher load of more than 5 tons two compressor are used in the package air conditioning system forming two independent refrigeration systems. Though the two systems are independent they are housed in the same supporting framework. The evaporator systems of these two systems are entwined together.
With the two compressors installed in the system there is facility for the capacity control of the air conditioning plant thus one can run the plant fully or partially as per the heating load or as per the number of rooms to be cooled. At full capacity both the compressors can be kept running, while at the partial load one of the compressors can be kept off. The capacity control can also be done automatically by thermostat. The two compressors also ensure that the air conditioning plant won't stop completely when one of the compressor breaks down, for the system can still keep on running though on partial capacity.
The water cooled condenser used with the two compressor system can be single shell but partitioned inside it for different passages for the refrigerant from the two compressors. There can also be two different water cooled condensers for two different systems. If the condenser is of air cooled type, there would be two condensers either placed side-by-side or entwined together just like the evaporator of the system.
Thermostat Setting for Two Compressor Systems
For controlling the operation of the two compressor packaged air conditioning system, two different thermostats can be connected to the two cooling systems. This will allow for the capacity control of both the compressors to ensure optimum running of the plant.
In many cases the thermostat is connected to the compressor of one cooling system only. In this case one compressors keeps on running at full capacity and the other compressor remains shut or runs at partial or full capacity depending on the cooling load. To ensure that one compressor won't wear and tear at the fast rate, the compressor running at the full capacity is interchanged from time-to-time.
The two compressor system is used for the cooling loads between 5 to tons 15 tons, for higher loads more than two units can be installed as per the requirements. There can be multiple air handling units in these systems sending the chilled air to different parts of the building. The compressor and the condenser units are housed in the same location of the building thus there is one common place for the return air. If these units have water cooled condenser, the hot water coming out from them is cooled in the single cooling tower.
During the full load all the compressors run at the full capacity, and when the load reduces one or more compressors are stopped manually or automatically, which makes the other compressors to run at the full load. When the compressor runs at full capacity it runs more efficiently. Whether the packaged air conditioner consists of single compressor or the multiple compressors, most of the components are assembled at the factory site, so the installation of the package AC is quite easy.
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Reference
1) Book: Basic Refrigeration and Air Conditioning by P. N. Ananthanarayanan, Second Edition, Tata Mc-Graw-Hill Publishing Company Limited