Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSFIXED QUANTITY OF OUTDOOR AIR CONTROLFunctional DescriptionRAExample, calculate the mixed air temperature of a 10,000 cfmfan with 25% OA at 5°F. RA is 75°F.MAT = 75 + 2500 (5 - 75) = 57.5°F10,000or:MAT = (75 x .75) + (.25 x 5) = 57.5°FOAMASASPECIFICATIONSOPEN2CONTROLPROGRAM1ONM10445The operation of the OA damper shall be two-position. TheOA damper shall open when the fan is on and close when thefan is off.ItemNo.Function1 Control system energizes when fan isturned on.2 Damper opens on fan startup, closes on fanshutdown.FEATURESPSYCHROMETRIC ASPECTSThe proportions of OA and RA admitted are constant at allOA temperatures.In the following chart it is assumed that:1. One-third OA and two-thirds RA are admitted.2. RA condition is 78°F DB and 62.5°F WB.3. OA condition is 40°F DB and 35°F WB.4. MA = (2/3 x 78) + (1/3 x 40) = 65.3°F.1. A fixed quantity of OA is admitted when the fan isoperating.2. This system is composed of a minimum of ventilationand control equipment.CONDITIONS FOR SUCCESSFUL OPERATION1. The system provides the desired proportions of OA andRA (a manual RA damper, shown, is usually required).2. The system is designed so that coils are not subject tofreezing.OA40°F DB, 35°F WBMAThe following results are obtained:12RA 78°F DB, 62.5°F WBC2544-1LIMITATIONSItemNo.ExplanationThe MA temperature varies as a function of OA temperature.The equation is:MAT = RAT +OA cfmTotal cfm(OAT - RAT)or:MAT = (RAT x %RA) + (OAT x %OA)1 MA condition at 40°F DB OA condition.2 As OA temperature rises, the MA temperaturemoves to the right of the initial MAtemperature.ENGINEERING MANUAL OF AUTOMATIC CONTROL194