Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSThe dotted line of F igure 10 shows a modification of this str ategywher ein the constant v olume r equir ement is r elaxed a little topr ovide some VAV cooling contr ol pr ior to g oing into the hea ting/mixing por tion of contr ol. This allows a dead band and a littleVAV cooling contr ol. This uses the same har dware (and cost) andallows an easil y adjusta ble minim um airf low, up to the maxim um.In this conf igur ation, the r oom will oper ate at the hea ting setpointmost of the time and the lo wer the minim um airf low is set, themore time control will be in the deadband range or at the coolingsetpoint. Rooms ma y have either high minim um airf low setpointsor low minim um airf low setpoints.Dual Duct Pressure IndependentConstant Volume ATUFigur e 11 shows a constant v olume pr essur e independent dualduct ATU. Modula ting the hot duct damper , up to a total f lownot to exceed the maximum airflow setpoint maintains the roomtemper atur e. A constant total airf low is nor mally maintainedby modula ting the cold duct damper . With this conf igur ation,there is no deadband and comfort is maintained by mixing hotand cold air a t a constant total airf low. Hot and cold ducttemper atur e load r eset is very impor tant in all dual duct systemsto minimiz e ener gy waste.25.5HOTDUCT13COLDDUCTWALL MODULETEMPERATURECOOLINGSETPOINT00 770.4550.45524 24DAMPERPERCENTOPENCURRENTAIRFLOW(m 3 /s)AIRFLOWSETPOINT(m 3 /s)CONSTANTAIRFLOWSETPOINTCOLD DUCTAIRFLOWAIRFLOW(m 3 /s)HOT DUCTAIRFLOWZEROMAXHEATINGLOADMAXCOOLINGLOADM15312Fig. 11. Dual-Duct Constant Volume Air Terminal Unit.ENGINEERING MANUAL OF AUTOMATIC CONTROL 407