Capacity Control (Reciprocating Compressor)

Capacity Control (Reciprocating Compressor) Points : capacity control (recriprocating compressor) 1. On — Off control
2. Multi-speed compressor motors
3. Hot gas bypass
4. Cylinder cut-off method
5. Cylinder un-loader 1. On — Off control The simplest form of compressor control is found in the low pressure switch of the dual, pressure control furnished as standard equipment in most reciprocating compressors. This is nothing more than a simple pressure switch arranged to open an electric circuit at a predetermined low pressure. The tube of this switch is connected to the compressor suction manifold and the electrical contacts are wired in series with the holding coil of the compressor motor starter. Thus, on falling load when the pressure in the suction manifold drops below the desired low limit the switch opens and operates, the motor starter, to stop the compressor: When the pressure in the suction manifold rises to a predetermined level, the switch closes and energizes the motor starter and starts the compressor. When sufficient gas has been pumped to lower the suction pressure the machine again stops. 2. Multi- Speed Compressor Motor As the capacity of a compressor is proportional to the compressor speed, multi-speed compressor motors are sometimes used. These maybe used with a manual speed changing switch and constant speed motor starter arranged so that the starter stops and starts the machine at a single speed. This is satisfactory where the loads are seasonal and do not fluctuate widely. In spring and fall seasons when loads are light, the machine can be operated at low speed. In summer when the load is heavy the machine can operated at high speed. This system is more costly because of two low pressure switches and to speed magnetic starters. 3. Hot Gas Bypass Because multi-speed motors and their controls are usually too costly to use on the larger machines other means of capacity reduction have been devised. One of the early methods used to reduce capacity, without reduction of compressor speed takes the form of hot gas bypass in which the hot gas discharged by the compressor is returned to the suction side of the machine. The hot gas bypass method of capacity control is used quite widely with packaged air conditioning apparatus up to 10 tons cooling capacity. The compressor in these small capacity packages are built without un-loader, so that the hot gas can be used for capacity modulation. 4. Cylinder Cut Off Method The cylinder cut off method is less frequently encountered. In this method, a solenoid stop-valve is used to close the suction line to one bank of cylinders so no gas can be pumped by them. This is a poor practice. The valves of the cylinders to which the ‘flow of gas is stopped are permitted to operate. A relatively high vacuum is created in these cylinders and in the manifold behind’ the stop valve. This vacuum encourages oil pumping anyway, in case of a worn machine which causes serious trouble. 5. Cylinder Un-Loader Modern reciprocating compressors with capacities of approximately 10 Hp and larger are generally provided with cylinder un-loader for capacity control. Cylinder un-loader is usually accomplished by holding the suction valves open. This permits the piston to draw refrigerant gas from the suction manifold on the down stroke but instead of compressing the gas on the upstroke, the gas is allowed to return uncompressed to the suction line through the open suction valve.