Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsFigure 11. Impact of coil face velocity on AHU footprint (see Table 4)Size 25Size 35Size 30Note that in order to deliver equivalent capacity, the cooling coil in thesmaller-sized AHU requires more fins than the coil in the larger-sized AHU.This slightly increases the cost of the coil and, along with the smaller coil facearea, increases the airside-pressure drop—0.69 in H 2O (173 Pa) for size 25unit versus 0.50 in H 2O (125 Pa) for the size 30 unit. This does impact fanenergy use. However, in a VAV air-handling unit, the airside-pressure dropdecreases quickly as supply airflow is reduced at part load, so the actualimpact on annual fan energy use is lessened. Finally, the smaller coil reducesthe fluid pressure drop—12.1 ft H 2O (36.3 kPa) for size 25 unit versus 13.6 ftH 2O (40.6 kPa) for the size 30 unit—which may decrease pumping energy.If the goal for a project is to minimize footprint and/or equipment cost,consider specifying that the face velocity at design airflow should not exceed90 percent (for example) of the manufacturer’s tested and published limit toprevent moisture carryover, rather than specifying an arbitrary maximum coilface velocity of 500 fpm (2.5 m/s). For example, if the manufacturer’spublished limit is 600 fpm (3.0 m/s), specify that the coil face velocity notexceed 540 fpm (2.7 m/s) at design airflow.However, if the goal for a project is to reduce energy use, consider selecting aslightly larger air-handling unit. The example in Table 4 also shows aselection for a size 35 unit. With more surface area, the cooling coil candeliver equivalent capacity with only four rows of tubes, rather than six.Fewer rows, along with the larger coil face area, decreases the airsidepressuredrop to 0.36 in H 2O (90 Pa) and decreases the fluid pressure drop to9.1 ft H 2O (27.1 kPa). The result is reduced fan and pumping energy. However,this does increase the cost, footprint (Figure 11), and weight of the airhandlingunit.Freeze preventionAs discussed in “Ventilation,” p. 101, at part-load conditions a properlycontrolled VAV air-handling unit typically brings in a high percentage ofoutdoor air. During cold weather it is difficult to mix the outdoor air andrecirculated return air when the two air streams are at widely differing18 Chilled-Water VAV Systems SYS-APM008-EN