Chilled-Water VAV Systems - HVAC.Amickracing

Overview of a Chilled-Water VAV SystemThe VAV air-handling unit can be located either outdoors, typically on the roofof the building, or indoors, typically in a penthouse or mechanical equipmentroom in the basement or on one of the occupied floors of the building. Abuilding may use a single air-handling unit or several units, depending on itssize, load characteristics, and function. Return air from inside the building isdrawn back to the air-handling unit. Some of this air is exhausted while therest enters the air-handling unit through a return-air damper to be mixed withoutdoor air that enters through a separate damper. This mixed air typicallypasses through a filter, a heating coil, a chilled-water cooling coil, and asupply fan before it is discharged from the unit (Figure 2).Figure 2. Typical air-handling unit used in a VAV systemdischargeplenumfilterreturn-airdampersupplyfanchilled-watercooling coilhot-waterheating coiloutdoor-airdamperThe supply air is distributed through ductwork that is typically located in theceiling plenum above each floor (Figure 1). The supply ductwork delivers airto each of the VAV terminal units, then this air is introduced into the zonesthrough supply-air diffusers. Each independently controlled zone has a VAVterminal unit that varies the quantity of air delivered to maintain the desiredtemperature in that zone. Air typically returns from the zones through ceilingmountedreturn-air grilles and travels through the open ceiling plenum to acentral return duct that directs this return air back to the air-handling unit.The chilled water for cooling is provided by a chilled-water system, whichincludes one or more water chillers with associated water distribution pumpsand heat rejection equipment (cooling towers for water-cooled chillers,condenser fans for air-cooled chillers).Heating can be accomplished in several ways. One approach uses a heatingcoil (hot water, steam, or electric) or gas-fired burner inside the air-handlingunit. In this configuration, the air-handling unit can warm the supply airduring cold weather or during a morning warm-up period. A secondapproach uses individual heating coils (hot water or electric) installed in theVAV terminal units. Each coil is controlled to warm the supply air whennecessary. A third approach uses perimeter baseboard radiant heat withinthose zones that require heat. The baseboard heaters can be controlledseparately or by the controller on the VAV terminal unit.2 Chilled-Water VAV Systems SYS-APM008-EN