Engineering Manual o.. - HVAC.Amickracing

INDIVIDUAL ROOM CONTROL APPLICATIONSABBREVIATIONSThe following abbr eviations ar e used:BMS — Building Management SystemATU — Air Terminal UnitVAV — Var iable Air VolumeZEB — Zer o Ener gy BandDDC — Direct Digital ControlCAV — Constant Air VolumeAIR TERMINAL UNIT CONTROLAir ter minal units (A TUs) r egulate the quantity and/ortemper atur e of conditioned air deli vered to sa tisfy thetemper atur e requir ements of a r oom or space . ATUs ar eclassified by air handling system design and ar e available inman y configur ations. F or inf ormation on the air suppl y to ATUs,refer to the Air Handling System Contr ol Applica tions sectionand the Building Airflow System Contr ol Applica tions section.ATU contr ols can be as basic as a r oom sensor contr olling adamper or as comple x as a r oom sensor and an airf low sensoroper ating a damper and a r eheat coil valve. In all cases, anindividual r oom contr ol applica tion contr ols the environmentof the r oom or space . Figur e 1 shows basic indi vidual r oomcontr ol with an ATU.CONDITIONEDAIRTO OTHERSPACESVARIABLE AIR VOLUME ATUA var iable air v olume (VAV) ATU contr ols the cooling of aspace b y var ying the amount of conditioned air supplied r atherthan c hang ing the temper atur e of the conditioned air . Most VAVsystems pr ovide ener gy savings by reducing the load on thecentral fan when cooling loads are less than design and avoidreheating cooled air .A system designed to r educe overall system airf low as ATUsshut do wn offers the greatest ener gy savings because systemfans and f an motor s oper ate at lighter loads. These par agraphsdescr ibe the oper ation of VAV ATUs in single-duct and dualductair handling systems.PRESSURE-DEPENDENT ANDPRESSURE-INDEPENDENT ATUA Pressur e-dependent ATU is affected b y chang ing duct sta ticpr essur es. It ma y have mechanical minim um and maxim umairf low limits.A pr essur e-independent ATU automa tically adjusts to ductstatic pr essur e chang es because it contains airf low sensor s andcontr ollers to compensa te for pr essure chang es. When used withDDC systems, the pr essur e-independent ATU ma y beprogrammed to change the airflow delivered based on theoper ating mode . Table 1 descr ibes some modes with theassociated command and comments on the reasoning.FANATUDAMPERACTUATORATUCONTROLLERCONTROLLERGRILLESPACE 1WALLMODULEGRILLESPACE 2WALLMODULEC2399-1Fig. 1. Basic Individual Room Control.Table 1. AHU Operating Modes and Associated ATU Airflow Settings.Mode Command CommentsSmoke Purge Mode All associated boxes to 82%maximum airflow.Thermal or IAQ Pre-Purge All the boxes to 50% maximumairflow.Preoccupancy Cool Down Perimeter boxes to 50%maximum airflow and the interiorboxes to 80% maximum airflow.AHU was sized with diversity such that the total boxmaximum airflows exceed the fan design airflow.Take advantage of the fan motor power cube rootrelationship to airflow (fan runs twice as long at oneeighth the motor power).Reduce the fan energy per cubic meter of air deliveredby half, match the fan-to-chiller load for better chillerefficiency and for better dehumidification.ENGINEERING MANUAL OF AUTOMATIC CONTROL 399