Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSPSYCHROMETRIC ASPECTSThe steam humidification process is almost isothermal.In the following chart it is assumed that:1. Space conditions are 72°F and 45% RH.2. Mixed air (entering the heating coil) is 60°F and 42°FDP.3. Additional moisture required to maintain space conditionsis 0.0014 lbs per lb of dry air.STEAM HUMIDIFICATION PROCESSDETERMINES SLOPE OF THIS LINEItemNo.Explanation1 RA and dry OA mix and enter a heating coil.2 Heating coil leaving air; air gainssensible heat.3 Humidifier leaving air; air gains mostlylatent heat.4 RA; cooled via heat loss and humidityreduced.ENTHALPYHUMIDITY RATIO =RA 72°F DB,45% RH43SA 77°F DB,60°F WBOA40°F DB, 35°F WBMA12COILDISCHARGEC2557-1COOLING CONTROL PROCESSESThe following are common control arrangements for cooling.CONTROL OF MODULATING CHILLED WATER COIL THREE-WAY VALVEFunctional DescriptionItemNo.Function808032MA5CONTROLPROGRAM504SUPPLYFANSA1ON1 Control system energizes when fan is turnedon (See FAN SYSTEM START-STOPCONTROL).2, 3 Space temperature PID control loopmodulates the three-way valve to maintainspace temperature setpoint.4 Chilled water valve directs flow through oraround coil as needed to furnish properamount of cooling.5 Control program coordinates temperaturecontrol and fan status.M10469FEATURES1. Chilled water is supplied to the coil at a constanttemperature and varying volume.2. A reasonably constant flow is maintained through theentire piping system.ENGINEERING MANUAL OF AUTOMATIC CONTROL219