ASHRAE Journal - August 2009 - 35

First, the liquid refrigerant inventory [Closed] Bleed Hand Solenoid Valve Suction Stop Pilot within the coil is higher compared to Solenoid [Open] Valve(s) Plot Pressure Mode Position a direct-expansion evaporator. Second, Regulator Suction Stop Valve Closed the refrigerant-side coil pressure drop Suction Stop Open Pilot Solenoid is relatively high due to the presence Suction Stop Valve [Closed] Wet Suction Return Bleed Solenoid Closed of button orifices located within each Liquid Feed Solenoid Soft-Gas Liquid Feed [Closed] Closed Solenoid circuit on the refrigerant feed-side of the Pumped Open Soft-Gas Solenoid coil (typical for mechanically pumped Soft-Gas Solenoid Liquid Supply Hot-Gas Solenoid Closed [Open] overfed designs). Hot-Gas Solenoid [Evaporator Fans – Off ] [Closed] Because a longer pump-out period is Regulated Hot Gas required for overfed coil designs, it is Defrost Return (Medium Pressure) natural to ask “how long of a pump-out period is sufficient?” The pump-out s Fan period should be long enough to evapoDefrost Condensate or rat po Defrost Relief rate the majority of residual liquid in Eva Recirculated Liquid/ Regulator Vapor Return the coil but not too long that parasitic ff ] Recirculated Liquid Supply heat load effects to the space become [O n Pa significant. The parasitic heat load efDefrost Hot-Gas Supply fects during pump-out arise because the supply of liquid refrigerant to the coil Figure 3: Valve positions and fan operation during soft-gas period for typical liquid overfed coil. has been interrupted; the evaporator’s fans continue to run; it is heat from [Closed] Bleed Hand Suction Stop Pilot Solenoid Valve the fans that are a parasitic space load. Solenoid [Open] Valve(s) Plot Pressure Mode Position In addition, longer pump-out periods Regulator Suction Stop Valve Closed extend the time the unit is unavailable Suction Stop Open Pilot Solenoid Suction Stop Valve [Closed] to meet space loads. Wet Suction Return Bleed Solenoid Closed Aljuwayhel, et al.,3 reported extenLiquid Feed Solenoid Hot Gas Liquid Feed [Closed] Closed Solenoid sive data collected on a field-installed Pumped Soft-Gas Solenoid Closed Soft-Gas Solenoid evaporator unit located in a penthouse Liquid Supply Hot-Gas Solenoid Open [Closed] for a low temperature holding freezer. Hot-Gas Solenoid [Evaporator Fans – Off ] [Open] The coil in this particular unit has a Regulated Hot Gas rated capacity of 37 tons (130 kWt) with Defrost Return (Medium Pressure) five fans that deliver 60,000 cfm (102 000 m3/h) of air during cooling mode ans rF Defrost Condensate operation, but that result in approxiato r po Defrost Relief Eva mately 5 tons (17.6 kWt) of parasitic Recirculated Liquid/ Regulator Vapor Return heat load during fan operation. Data ff ] Recirculated Liquid Supply [O were collected on the unit’s refrigeran Pa Defrost Hot-Gas Supply tion capacity during the pump-out period and the unit’s decrease in capacity over five separate pump-out cycles is Figure 4: Valve positions and fan operation during hot-gas period for typical liquid overfed coil. shown in Figure 2. At the end of the 20 minute pump-out period, the coil’s capacity has decreased Step 2: Soft-Gas to a level approaching a break-even capacity to just meet The use of a soft-gas step in the defrost sequence is recomthe fan heat gain. mended for evaporator coils with 15 tons (53 kWt) of capacity A pump-out period longer than 20 minutes is usually not or greater.2,4,5 The soft-gas period of the defrost sequence required. Shorter pump-out periods should be validated by begins by shutting off the evaporator fans and energizing the observing the frost melt pattern on the coil during the hot-gas pilot solenoid for the suction stop valve. The pilot solenoid supply period of the defrost sequence. Assuming the coil is applies hot-gas pressure to the top of the suction stop valve’s top-fed with hot-gas (typical), an adequate pump-out period is piston, forcing this normally open valve closed. likely established when the bottom rows of the coil completely With the coil now isolated from the system’s suction pressure, release their frost during the hot-gas dwell period and when no a small ported (e.g., 0.5 in. [13 mm]) soft-gas solenoid valve is audible effects of hydraulic hammering are observed on the coil opened to allow a low flow rate of hot-gas into the coil—usuand its connected piping during the early part of the hot-gas ally after flowing first through the drain pan warming circuit; supply period. slowly raising the pressure of refrigerant in the coil. The softAugust 2009 ASHRAE Journal 35
ASHRAE Journal - August 2009

Table of Contents for the Digital Edition of ASHRAE Journal - August 2009
