Engineering Manual o.. - HVAC.Amickracing

AIR HANDLING SYSTEM CONTROL APPLICATIONSFIXED QUANTITY OF OUTDOOR AIR CONTROLFunctional DescriptionExample, calculate the mixed air temperature of a 4.720 m 3 /sfan with 25% OA at –15°C. RA is 24°C.2 As OA temperature rises, the MA temperatureMAT = OA (m3 /s)Total (m 3 /s) (OAT - RAT) moves to the right of the initial MARAMAT = 24 + 1.180 [(-15) - 24] = 14.25°C4.720or:MAT = (24 x 0.75) + (-15x 0.25) = 14.25°COAMASAOPEN21 ONSPECIFICATIONSThe operation of the OA damper shall be two-position. TheCONTROLOA damper shall open when the fan is on and close when thePROGRAMM10445 fan is off.ItemNo. FunctionPSYCHROMETRIC ASPECTS1The proportions of OA and RA admitted are constant at allControl system energizes when fan isOA temperatures.turned on.2 Damper opens on fan startup, closes on fanshutdown.In the following chart it is assumed that:1. One-third OA and two-thirds RA are admitted.2. RA condition is 26°C DB and 17°C WB.FEATURES3. OA condition is 4°C DB and 1.5°C WB.4. MA = (2/3 x 26) + (1/3 x 4) = 18.3°C.1. A fixed quantity of OA is admitted when the fan isoperating.2. This system is composed of a minimum of ventilationand control equipment.18.3°C DBCONDITIONS FOR SUCCESSFUL OPERATIONRA 26°C DB, 17°C WBMA 21. The system provides the desired proportions of OA and1RA (a manual RA damper, shown, is usually required).OA2. The system is designed so that coils are not subject to4°C DB, 1.5°C WBfreezing.C3235The following results are obtained:LIMITATIONSThe MA temperature varies as a function of OA temperature.The equation is:ItemNo.1ExplanationMA condition at 4°C DB OA condition.temperature.or:MAT = (RAT x %RA) + (OAT x %OA)ENGINEERING MANUAL OF AUTOMATIC CONTROL211