**2.4. Hot gas bypass capacity control**

Hot gas bypass is a method that modulates refrigerant flow by bypassing some of the high pressure refrigerant gas (hot gas) discharged from the compressor back to the suction line without going through the evaporator, and the gas does any cooling. The capacity of reciprocating and centrifugal compressors can also be controlled by this method. Some applications use two or more methods for smoother switching and better control such as unloading in conjunction with hot gas bypass. Extra valves and piping are required for this capacity control, and capacity can be quickly adjusted by opening or closing a valve, but the number of capacity steps is finite. It may not prove precise and smooth temperature control [8, 24]. Suction pressures below the compressor designed limit are prevented because low suction densities result in poor compressor cooling. Therefore, hot gas bypasses into the systems' low pressure side.

Hot gas can be injected into different locations: the first one is the evaporator inlet after the distributor nozzle but before the distributor tubes, and the second one is the suction line [25, 26].

Bypass into evaporator inlet: on single evaporator and close connected systems, it is generally possible to introduce the hot gas into the evaporator inlet immediately after the expansion valve. Bypassing at the evaporator inlet results an artificial cooling load. Since the thermostatic expansion valve meters required refrigerant feed to maintain preset superheating value, the refrigerant returns to the compressor at normal operating temperatures and prevents motor heating problem. High flow velocity helps the oil returning in the evaporator [25].

Bypass into suction line: in this method, multiple evaporators are connected to a compressor, or if the condensing unit is remote from the evaporator, it may be necessary to bypass hot gas into the refrigerant suction line. Suction pressures can be controlled with this method. To meter liquid refrigerant into the suction line is required in order to keep the temperature of the refrigerant gas entering to the compressor within allowable limits. If this method is used by passed hot gas and liquid refrigerant must be mixed at correct amount in order to provide mixed gas into the compressor at desired temperature. For this purpose a mixing chamber is recommended. A suction line accumulator can serve as a mixing chamber and also protects the compressor from liquid flood back [25].

The first study related with this method is investigated by Yaqub et al. [27], and in this study, an automatic hot gas bypass technique is applied to reduce the capacity of refrigeration and air-conditioning systems at part-load conditions. Hot gas bypass valve sends high pressure refrigerant into the suction port. They discussed three hot-gas bypass schemes for HFC-134a and analyzed on the basis of the first and second laws of thermodynamics. Second-law-based thermodynamic analysis indicated that the total irreversible losses of the bypass valve increase substantially, as the capacity decreases. In another study, Yaqub et al. [28] investigated capacity control of a vapor-compression refrigeration system by injecting hot gas and liquid refrigerant into the suction side of the compressor. It was demonstrated that the compressor discharge temperatures increase significantly, when the hot gas from the compressor discharge is extracted and injected (without any liquid injection) directly into the suction side of the compressor.

Besides, Tso et al. [29] compared hot gas bypass control and suction modulation in refrigerated shipping containers by using mathematical model. They studied to analyze compressor power draw, coefficient of performance, and the sensible heat factor of evaporator against the container load. They resulted that suction modulation method is more energy efficient than hot gas bypass.

The performance of the showcase refrigeration system with three evaporators was measured during on/off cycling and hot gas bypass defrost by Cho et al. [30]. Based on the test results, the effects of off-period in the on/off cycling and electronic expansion valve (EEV) opening in the hot gas bypass defrosting cycle on the performance of showcase system were analyzed.
