Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSItemNo.Function1 Control system energizes when fan is turnedon (See FAN SYSTEM START-STOPCONTROL).2 Space temperature inputs to control program.3 Setpoint determines refrigerant on mode.4 Refrigerant solenoid valve closes whenfan is off.5 Relay controls liquid line solenoid valve.6 Relay enables compressor start controlsystem.7 Temperature differential determinescompressor off mode.8 Compressor minimum on and minimum offtimes prevent short cycling.9 Control program coordinates cooling, safety,and fan interlock control.FEATURES1. The refrigerant liquid line solenoid valve is closed andthe compressor cannot be energized when the supply fanis off.2. Software time delays for compressor protection.On a rise in space temperature to the setpoint, the refrigerantliquid line valve shall open and a relay shall enable thecompressor to start under it’s controls.When the space temperature drops to a value equal to thespace temperature setpoint minus a differential, the liquid linesolenoid valve shall close, and the compressor shall continueto operate until shut down by it’s low-pressure cut-out.Refrigerant system control and interlock wiring shall be asrecommended by the compressor manufacturer.PSYCHROMETRIC ASPECTSWith on-off control, either cooled air or uncooled air issupplied into the space.In the following chart it is assumed that:1. Desired space and RA condition is 78°F DB and 50%RH (65°F WB).2. Design OA temperature is 95°F DB and 75°F WB.3. Air entering the system is from the ECONOMIZERCYCLE DECISION application. The system operates ona minimum of 35 percent OA during the cooling cycle.4. Coil leaving air temperature is 55°F.CONDITIONS FOR SUCCESSFUL OPERATIONThe differential timers are set wide enough and the softwaretimers are set high enough to prevent short cycling of thecompressor under light load.355°F DB2RA 78°F DB,50% RHSA41MAOA95°F DB,75°F WBLIMITATIONSC2559-11. Direct expansion coils are difficult to control from leavingair due to the large and rapid temperature drop whenenergized.2. Compressor operating and safety controls must beincorporated into the control system.SPECIFICATIONSSee FAN SYSTEM START-STOP CONTROL.The DX control system shall be enabled anytime the fanoperates.Cooling system shall be cycled by a temperature control loopwith a 2.5 degree (adjustable) differential to maintain the spacetemperature setpoint. When the system is commanded on bythe control program, it shall remain on at least eight minutes,and when it is commanded off (or drops off during powerinterruptions) it shall remain off at least ten minutes.The following results are obtained: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.3 When the space thermostat has the DXcooling energized, coil leaving air is at thisvalue.4 Air supplied to the space will alternatebetween Point 1 and Point 3 as determined bythe space thermostat.ENGINEERING MANUAL OF AUTOMATIC CONTROL221