**2.5. Slide valve capacity control**

capacity reduces. A throttling valve is needed before the compressor to reduce the mass flow rate through the compressor. It was found that the cylinder-unloading method was the best

In another study, Yaqub and Zubair [23] investigated three different capacity control schemes. He compared cylinder unloading scheme with hot gas bypass and compressor suction throttling. These schemes are investigated for HFC-134a by considering finite size of the components that are used in the refrigeration systems. A comparative study was performed among these schemes in terms of the system coefficient of performance (COP), the operating temperatures, and percentage of refrigerant mass fraction as a function of the percentage of full-load system capacity. The models consider the finite-temperature difference in the heat exchangers, thus allowing the variations in the condenser and evaporator temperatures with respect

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

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

and had the highest COP and minimum irreversible losses at any system capacity.

to capacity and external fluid inlet temperatures.

protects the compressor from liquid flood back [25].

**2.4. Hot gas bypass capacity control**

pressure side.

128 Refrigeration

The slider control allows an adaptation of the compressor displacement to the power requirement by shifting the start of the compression process through an axial slide of the control slider. At the same time, the outlet window is adapted to the newly developing displacement in this series. Rotary-screw compressors use slide valves to adjust the necessary refrigeration capacity at partial loads by permitting the equipment to reduce the total volume of refrigerant compressed within the housing. The slide valve modulates compressor capacity between 25% to 100% range by 25% steps. The suction gas flow is measured in order to get the cooling capacity, and the volume ratio of the compressor is defined by the position of the suction contour and the size of the discharge port [31]. According to Reindl [32] one of the most common capacity control methods is capacity control slide valve for screw compressor. As the volume of refrigerant vapor to undergo compression is reduced, the compressor's capacity is reduced, and the effective volume ratio of the compressor also decreases.
