**2.6. Multiple compressor capacity control**

Refrigeration system capacity can be modulated by using multiple refrigeration circuits or by using multiple compressors in single-circuit systems. Under partial load conditions, the compressors may be cycled in and out of service as required as well as providing a level of redundancy in the event one of the compressors should fail. Oil equalization is needed for these kinds of compressors. One of the least expensive forms of modulation, reliability, can be seen as advantages of the multiple compressor capacity control. On the other hand, multiple compressor capacity provides finite number of capacity steps and limited efficiency gains. For example, in a 40 hp system requiring 25 hp of output at a given time, the system must operate at 30 hp output. Also, precise and smooth temperature and humidity control may not obtainable, because of the capacity modulation step [7, 8].

Winandy and Cridtian [33] realized a study regarding multiple compressors in which the condensing units have tandem scroll compressors. The main drawback on this configuration is oil returning to the compressor which is a serious trouble especially at part load.

According to ASHRAE [34], the gas velocities and piping geometry are the most important matter in multiple compressor control method because of ensuring adequate oil return. When working at part load, some modifications may be needed to ensure proper oil return. In addition, it is recommended that to separate the refrigerant circuits while parallel operating of compressors, however, this configuration is not always possible and does not give the same operational advantage at part load.
