Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSDUAL-DUCT ATUIn a dual-duct air handling system, suppl y air is di vided a tthe centr al fan and hot air and cold air f low thr ough separ ateducts thr oughout the b uilding . Two of the man y DDC dualductcontr ol configur ations ar e included her e.A typical or iginal dual-duct ATU dr aws hot and cold air fr omrespective ducts, mixes the air as dir ected b y a sensor in thecontr olled space , and disc har ges the mix ed air into the space .This basic dual-duct mixing system is not economical becauseair is cooled and hea ted m uch of the y ear. DDC systems ar estill sometimes used with digital hot and cold duct load reset tominimiz e the ener gy waste dur ing mixing per iods. Adaptingdual-duct systems to VAV contr ol has r esulted in signif icantener gy savings without discomf ort.Dual Duct Pr essure Independent VAV ATUFigure 10 shows a dual-duct pressure independent ATU. Theairflow sensor in the outlet of the ATU controls the cold ductinlet damper in the cooling mode from the cooling maximumairflow to the minimum ventilation airflow, through the deadband, and to the heating setpoint. As the heating load increases,the hot duct damper modulates open to control the supplyairflow from zero to the maximum heating value. This samesequence may be achieved with two airflow elements, one ineach primary duct.kPa0.3525.5HOTDUCT13COLDDUCTWALL MODULETEMPERATURECOOLINGSETPOINT0.165 0.543 0.443 00 77 0.445 24 24 -1.75MINIMUMAIRFLOWSETPOINT(m 3 /s)MAXIMUMCOOLINGAIRFLOWSETPOINT(m 3 /s)MAXIMUMHEATINGAIRFLOWSETPOINT(m 3 /s)DAMPERPERCENTOPENCURRENT(m 3 /s)HEATINGDEADBANDCOOLINGMAXAIRFLOWHEATINGMAXHOT DUCTAIRFLOWCOLD DUCTAIRFLOWMINZEROFULLHEATINGLOADHEATING SETPOINT0 0DEDBAND(ADJUSTABLE)COOLINGLOADCOOLING SETPOINTFig. 10. Dual-Duct Pressure Independent VAV Air Terminal Unit.FULLM15311406ENGINEERING MANUAL OF AUTOMATIC CONTROL