Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSINTRODUCTIONThis section provides descriptions of and control informationabout water chillers, cooling towers, hot water boilers, steamboilers, and water, steam, and district heating distribution systems.It discusses various methods of controlling and distributing steamor water to heating and cooling coils, and methods of ensuringsafe and proper control of boilers, chillers, and converters. Samplesystem solutions are based upon digital control systems andnetworks. These digital control strategies may not be practicalwith other forms of control, and may not be possible with alldigital controllers or by all digital controller programmers. Manysolutions are portrayed as they may be specified to be displayedon a PC based BMCS color graphic monitor. The data pointsshown on the color graphics are still recommended for systemswithout color graphics, and may be specified via a points list orby publishing the graphic picture. The values (setpoints, timings,parameters, etc.) shown in the examples are arbitrary. Values forany given project must be specific to the project.ABBREVIATIONSAHU — Air Handling UnitBMCS — A Building Management and ControlSystem, including digital controllersand communication networks.CHW — Chilled WaterDB — Dry BulbDP — Differential PressureDelta T (∆T) — Differential TemperatureEPID — Enhanced PID. A PID algorithm whichincludes integral component wind-upprevention, user defined initial value,and user defined start-up ramp time.See CONTROL FUNDAMENTALSsection.HW — Hot WaterHX — Heat exchangerOA — Outside airPID — Proportional-Integral-DerivativeSG — Specific GravityVAV — Variable Air VolumeVSD — Variable Speed DriveWB — Wet BulbDEFINITIONSApproach:1. The temperature difference, in a cooling tower,between outdoor wet bulb and condenser waterleaving the tower.2. The temperature difference in an air cooledcondenser between the liquid refrigerant leaving thecondenser and the entering air dry bulb.3. The temperature difference in a conduction heatexchanger between the entering working fluid andthe leaving treated fluid.Central plant: An area or building where the chillers and boilersfor a building or group of buildings are located.Compressor: A mechanical device for increasing a gas pressure.• Centrifugal compressor: A device which uses centrifugalforce to compress refrigerant vapors in a vaporcompressioncycle chiller. This is not a positivedisplacement compressor.• Positive displacement compressor: A compressorthat reduces the volume of a compression chamber tocompress a gas.• Screw compressor: A positive displacement compressorwhich uses the rotary motion of two meshedhelical rotors to compress a gas.Constant speed pumping: A pumping system where the systempressure is maintained by a constant speed pump.Deadband: A range of the controlled variable in which nocorrective action is taken by the system and no energyis used.Diversity: A design method where building elements (such aschillers and pumps) are sized for the instantaneousbuilding peak load requirements (rather than the sum ofall the loads, which may individually peak at times otherthan when the building peaks). Diversity does not allowthe use of three-way AHU control valves.Double bundle Condenser: A chiller condenser having two coilsin the shell to allow the chiller to dissipate heat either tothe cooling tower or to a heating load.Non-symmetrical loading: Diversity in multiple load systems,where individual loads operate at different times orloading from the others.• Reciprocating compressor: A positive displacementcompressor which uses the reciprocating motion of oneor more pistons to compress a gas.ENGINEERING MANUAL OF AUTOMATIC CONTROL295