The pump-out period begins by de30 energizing (closing) the evaporator’s liquid feed solenoid valve while the 25 suction stop valve remains open, and the unit’s fans operate as shown in Figure 1. Heat from the fan motors and room 20 (or product) causes the residual liquid refrigerant within the coil to evaporate 05 with the refrigerant vapor returning to the engine room via the wet suction return (also referred to as recirculated 10 suction). The amount of time scheduled for 5 pump-out varies from an extremely short duration, more typical for gravity flooded recirculation and direct-expan0 0 2 4 6 8 10 12 14 16 18 20 sion unit designs (zero to five minutes), Pump-Out Dwell Time (min) to a longer period for liquid overfed unit designs (10 to 15 minutes 2). A short Figure 2: Coil capacity decrease during pump-out.6 pump-out period for a gravity flooded evaporator is made possible because the low refrigerant-side design requires a short pump-out period because its normal pressure drop of the coil allows any residual liquid refrigerant liquid refrigerant inventory within the unit during cooling (and liquid condensate) to be readily cleared when hot-gas mode operation is low. Liquid overfed coil designs require is supplied to the coil for defrost. The direct-expansion coil a longer pump-out period due to a combination of effects. Evaporator Capacity (ton) www.info.hotims.com/25207-35 34 ASHRAE Journal August 2009