TABLE OF CONTENTS Selection of supply fans for cold-air distribution systems is governed by the same considerations as for conventional designs. Guidance on fan selection is provided in the 7996 ASHRAE Hand-book HVAC Systems and Equipment (ASHRAE 1996), in Lorsch et al. (1993), and in Carpenter (1984).
An important fan selection consideration is the placement of the fan relative to the cooling coil in either a "blow-through" or a "draw-through" configuration. In a blow-through configuration, the fan is upstream of the cooling coil. A draw-through configuration places the fan downstream of the coil.
With a draw-through arrangement, the fan heat is transferred to the supply airstream, increasing the supply air temperature by 2 F to 4 F (1 C to 2 C) and increasing the air volume required to meet the load. With a blow-through arrangement, the fan heat is transferred from the airstream directly to the coil, allowing a lower final supply air temperature from the same coil.
The blow-through fan configuration has some limitations that may offset the advantage of the small decrease in supply air temperature. A blow-through arrangement can produce nonuniform air distribution across the coil, which may be more likely to cause moisture carryover, and which may detract from coil performance. The less desirable fan inlet and exit conditions may produce a system effect that results in diminished fan performance.
In addition, a blow-through unit may be more likely to experience condensation on downstream surfaces, particularly if there is any fluctuation in the air temperature leaving the coil. For...
