Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSkPa0.3513PRIMARYAIR52PERCENTOPEN0.165 1.445 0.543 2424MINIMUMAIRFLOWSETPOINT(m 3 /s)CURRENTAIRFLOW(m 3 /s)MAXIMUMAIRFLOWSETPOINT(m 3 /s)CURRENT SPACETEMPERATURESETPOINTCURRENT SPACETEMPERATUREADJUSTABLEMAXIMUMAIRFLOWAIRFLOWADJUSTABLEMINIMUMAIRFLOWZEROLOWHIGHSPACE COOLING LOADM15306Fig. 5. Variable Air Volume Single-Duct Air Terminal Unit.Figure 5 displays the primary supply air temperature and staticpr essur e. These values ar e data points in the AHU digitalcontr oller and not ph ysical points a t the VAV box. They willvar y when measur ed at the VAV boxes. When a BMS user looksat a VAV box displa y, it is usuall y because a pr oblem exists.The fan system suppl y air temper atur e and sta tic pr essur e ar enecessary data for problem analysis.Variable Air Volume ATU with Electr ic ReheatThe VAV ATU with electr ic reheat in F igur e 6 adds r eheat toFigur e 5. Figur e 6 also shows four bo xes on one displa y. Thecooling PID varies the box airflow from maximum to minimumas the cooling load drops to maintain setpoint. If the coolingload dr ops fur ther, the space temper atur e dr ops thr ough a deadband until the heating setpoint is reached, at which point theheating unit is contr olled to maintain the hea ting setpoint. Whenelectric reheat is used, the heating PI may be modified withsoftwar e heat anticipa tion to ad vance the hea ter sta ges offcommand to minimize wide swings in space temperature whichwould result from simple temperature sensed control.Although F igur es 3, 4, 5, and 6 sho w several inputs, outputs,and setpoints, man y mor e par ameter s ar e available andnecessar y to allow technicians to ada pt the standar d pr ogramto a unique r oom and set of equipment. These ad ditionalparameters, usually addressable and commandable from theBMS or portable terminal, include the cooling loop PI gains,the heating loop PI gains, reheat airflow setpoint, unoccupiedspace temperature setpoint, standby temperature setpoint,temper atur e sensor calibr ation offset, bypass push-b utton sta tus,bypass push-b utton o verride time in min utes, smoke pur ge mode% of maximum airflow setpoint, smoke pressurization mode% of maximum airflow setpoint, and unoccupied mode % ofmaximum airflow setpoint.NOTE:On-off electr ic heat contr ol often r equir es a minim umair velocity above that of the minimum cooling airflowto pr event excessive heating element temper atur es.Constant Air Volume ATUsThe constant air v olume (CAV) ATUs ar e similar to thepr essur e independent VAV units, except the y contr ol at a singleair v olume setpoint. They usuall y include r eheat coils, anassociated room sensor that controls the heating, and are oftenfound connected to VAV AHUs which also ha ve VAV boxesand the associa ted var ying duct sta tic pr essur es. CAV units ar eused wher e the r oom suppl y air v olume m ust be maintained a ta constant v alue.Induction VAV ATUThe induction VAV ATU (Fig. 7), uses induced r etur n air asthe initial r eheat medium. Induction VAV ATUs ar e usuall yinstalled a bove the ceiling and dr aw retur n air fr om the plen umcreated b y a false ceiling. The contr oller r esets the pr imar y airairflow control setpoint in unison to opening the return airdamper s per the oper ation cycle shown in F igur e 7. As thedampers open, more return air is induced into the unit andrecirculated into the space.402ENGINEERING MANUAL OF AUTOMATIC CONTROL