Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSFEATURES1. Chilled water is supplied to the coil at a constanttemperature and varying volume.2. A reasonably constant flow is maintained through theentire piping system.CONDITION FOR SUCCESSFUL OPERATION2. Design OA temperature is 35°C DB and 24°C WB.3. Air entering the system is from the ECONOMIZERCYCLE DECISION application. This system operateson 35 percent OA during the cooling cycle.4. Coil discharge temperature is 13°C.The following results are obtained:The water must be supplied at a reasonably constant pressure.LIMITATIONSModulating water flow through a constant air volume chilledwater coil usually causes a rise in space RH because the coilleaving water temperature rises significantly.SA 13°C DB12MARA 25°C DB,50% RHOA 35°C DB,24°C WBC3249SPECIFICATIONSThe flow of chilled water through the cooling coil shall becontrolled by a three-way valve modulated by a spacetemperature PI control loop. The valve shall close to the coilupon fan shutdown and open to the coil upon loss of actuatormotive force.PSYCHROMETRIC ASPECTSThe temperature, and often moisture content, of leaving airincreases as the sensible cooling load lightens.In the following chart it is assumed that:1. Desired space and RA condition is 25°C DB and 50%RH (18°C WB).ItemNo.Explanation1 Mixed air temperature at cooling designcondition.2 Air entering the coil is cooled along a line ofconstant moisture content until saturation isapproached. Near saturation the moisturecontent is reduced as the air is cooled. Thisprocess involves both latent and sensiblecooling.NOTE: Condition of coil leaving air will change with thecooling load in the space. As the cooling loaddecreases, the three-way valve will provide less chilledwater flow to the coil and the discharge air temperaturewill rise (approximately along Line 2).ENGINEERING MANUAL OF AUTOMATIC CONTROL237