Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSINLETPREROTATIONVANESIMPELLERVOLUTEReciprocating chiller capacity is controlled in stages (steps).Methods of capacity control include the following:– Unloading cylinders– On-off cycling of multiple compressors– Hot-gas bypass– Hot-gas through evaporatorM11417Reprinted by permission: The Trane Company,LaCrosse, WI 54601Fig. 5. Cutaway of Single Stage Centrifugal Compressor.Centrifugal compressors are driven by turbines, electric motors,or internal combustion engines. Inlet vane control or speed controlvaries the capacity. Each method has different performancecharacteristics. A combination of speed and inlet vane controlprovides the highest operating efficiency. Multiple stage directdrive type compressors are available in many configurations.Refrigerant pressure is the pressure difference between thecompressor inlet and outlet and is the primary factor affectingchiller efficiency. For a given load, reducing refrigerant pressureimproves efficiency. Evaporation and condensationtemperatures establish these pressures and are determined bychilled water temperature and condenser water temperature.Refrigerant pressure is reduced by the following:– Reducing condenser water temperature.– Raising chilled water temperature.– Reducing load.– Decreasing design differential temperature of evaporatorand condenser heat exchangers by increasing the size ofthe heat exchangers.Cylinder unloading or multiple compressor on-off cycling issequenced by automatic controls. The cylinder inlet valves areheld open so no compression takes place during cylinderunloading. Capacity control mechanisms and controls areusually packaged with the chiller. Step capacity control ofrefrigeration must provide a compromise between to frequentcycling and to wide temperature swings. Use of chilled waterreturn temperature as controlling variable lengthens thecompressor on and off cycles. When cylinder unloading is used,the minimum of time after the compressor is cycled off on lowload, is normally less than for multiple compressors. Off timeis critical because the refrigeration system must have time toequalize the pressure between high and low sides so that thestarting load will not be too great for the motor.SCREW COMPRESSORA screw compressor is a positive displacement device whichuses two meshed helical rotors to provide compression. It isalso known as a helical rotary compressor. Basic constructionof a helical rotary twin screw compressor is shown in Figure 6.The capacity of a screw compressor can be modulated by speedcontrol or a sliding valve that varies the length of compressionarea of the helical screws and bypasses some gas back to theinlet of the compressor.INLETMAX MINABThe load for maximum chiller efficiency varies with chillersand chiller manufacturers. It is often 70 to 80 percent, but canbe 100 percent.VIEW A-APISTONCYLINDERPORT AREASABYPASSBOUTLETVIEW B-BSLIDING VALVEM10507RECIPROCATING COMPRESSORThe reciprocating compressor is a positive displacementdevice consisting of several cylinders and piston .The crankshaftis driven by a motor or engine. Spring loaded valves allow lowpressure refrigerant vapor to enter the cylinder on thedownstroke and high pressure refrigerant vapor to exit on theupstroke. Because the compressor is a positive displacementdevice its capacity is not greatly influenced by refrigerantpressure. However, power required per unit of cooling is directlyrelated to refrigerant pressure. Keeping condenser temperatureas low as possible also reduces energy requirements, therefore,compressors with water cooled condensers use less power thanair cooled condensers. However, condenser water temperaturemust not be allowed to go too low or there will not be enoughpressure difference to circulate the refrigerant.Fig. 6. Helical Rotary Twin Screw Compressor.ABSORPTION REFRIGERATIONABSORPTION CYCLEThe absorption cycle uses a fluid called an absorbent to absorbevaporated refrigerant vapor in an “absorber” section. Theresulting combination of fluid and refrigerant is moved into a“generator” section where heat is used to evaporate therefrigerant from the absorbent.In the absorber (Fig. 7) the absorbent, also called strongabsorbent at this point, assimilates the refrigerant vapor whensprayed through it. The resulting weak absorbent is pumped bythe generator pump through the heat exchanger, where it picksup some of the heat of the strong absorbent, then into theENGINEERING MANUAL OF AUTOMATIC CONTROL299