Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsIn a draw-thru configuration, the fan draws air through a cooling coil locatedupstream. The heat generated by the fan and motor is added to the airdownstream of the cooling coil.Assuming equivalent supply airflows, in a draw-thru configuration the airmust leave the cooling coil at a colder temperature in order to achieve thesame supply-air temperature delivered down the duct. Because the fan heatgain is equal, the sensible cooling capacity required is the same for bothconfigurations (Figure 28). However, because the cooling coil in the drawthruconfiguration must make the air colder (since the fan heat is addeddownstream of the coil), it also makes the air drier (in non-arid climates). Thisincreases the latent cooling capacity. Therefore, achieving the same 55°F(13°C) supply-air temperature with the draw-thru configuration requiresslightly more total cooling capacity, but offers the benefit of slightly drier airdelivered to the zones.Figure 28. Effect of fan heat gain on cooling capacity (equal supply airflows)sensiblecoilcapacityblow-thrudraw-thrulatentcoilcapacityOf course, this colder air leaving the coil in a draw-thru configuration can alsobe of benefit. At equivalent supply-air temperatures, the draw-thruconfiguration delivers the supply air at a lower dew point (Figure 28), whichimproves dehumidification performance.In a blow-thru configuration, the air-handling unit typically needs to be longerto avoid uneven velocities as the air passes through the cooling coil.Alternatively, a diffuser (set of baffles) can be used to provide even airflowacross components downstream of the fan and minimize additional length. Inthe example shown in Figure 29, the diffuser section increases the overalllength of the air-handling unit by 1.5 ft. (0.4 m), or 8 percent.36 Chilled-Water VAV Systems SYS-APM008-EN