ASHRAE Journal - August 2009 - 38

suction through the defrost relief regulator. Aljuwayhel6 collected data on a penthousemounted evaporator during both cooling mode and defrost mode of operation. For the evaporator defrost control as-found, the hot-gas dwell period was 40 minutes. Figure 5 shows model-predicted and field-measured average air temperatures within the penthouse during the hot-gas and subsequent bleed periods of the defrost sequence for two cases. The first case allowed the evaporator to operate for 24 hours before initiating a defrost cycle. Once hot gas flowed to the coil, all the frost had melted in a period of less than seven minutes. The second case allowed the evaporator to operate for 48 hours before initiating a defrost cycle. In this situation, the coil was completely cleared of accumulated frost in less than 11 minutes during the hot-gas supply. This suggested that a 40 minute hot-gas dwell period was excessive. Within 15 minutes of the main hot-gas valve opening, the average penthouse air temperature reached a balmy 68°F (20°C) and that temperature was maintained for 25 of the 40 minutes, which suggests that the continued supply of hot-gas to the coil was not resulting in the full condensing of the refrigerant vapor. Rather, a significant portion of the hot-gas was flowing back to suction and creating a parasitic load (internal) on the compressors. The parasitic effect of excessive hot-gas dwell periods presents an opportunity for improving the system’s energy efficiency by simply reducing the scheduled hot-gas dwell period. Step 4: Bleed Solenoid Plot Pressure Regulator [Open] Bleed Hand Valve Suction Stop Pilot Solenoid [Closed] Mode Valve(s) Suction Stop Valve Position Open Closed Open Open Closed Closed Suction Stop Valve [Open] Wet Suction Return Liquid Feed Solenoid [Open] Re-Chill Suction Stop Pilot Solenoid Bleed Solenoid Liquid Feed Solenoid Soft-Gas Solenoid Soft-Gas Solenoid [Closed] Pumped Liquid Supply Hot-Gas Solenoid [Closed] Hot-Gas Solenoid [Evaporator Fans – Off ] Regulated Hot Gas Defrost Return (Medium Pressure) Defrost Condensate Defrost Relief Regulator Recirculated Liquid/ Vapor Return Recirculated Liquid Supply Defrost Hot-Gas Supply Eva p t ora or s Fan ff ] [O n Pa Figure 7: Valve positions and fan operation during re-chill period for typical liquid overfed coil. A D Coil Initial Condition (No Frost) Evaporator Capacity Coil Capacity Decreases As Frost Continues to Form Coil Capacity Drops Rapidly as Refrigerant Flow is Stopped and the “Pump Out” Process Proceeds, Preparing the Coil for Defrost Time Parasitic Energy is Attributed to Warming the Coil Mass and Both Sensible and Latent Losses to the Space B Hot-Gas Defrost Terminates and Coil Begins to Cool Down Coil Transitions from a Temperature Warmer Than the Space to a Temperature Cooler Than the Space, So Useful C Refrigeration is Now Restored At the conclusion of the hot-gas dwell Figure 8: An illustration of the time-dependent energy flows for cooling mode and defrost period, a bleed or equalize sequence is mode of operation (note: this graphic is not to scale in either capacity or time).9 initiated. During the bleed period, the hot-gas solenoid valve (and soft-gas solenoid if open) is closed for hydraulic hammering to the coil and the connected suction and a small bleed solenoid valve opens to slowly depressurize piping. The bleed period also prevents rapid swings in suction the coil by relieving the pressure in the coil back to suction. The pressure and compressor loading that would normally result as bleed solenoid valve is typically three to four sizes smaller than the engine room responds to maintain a constant suction presthe main suction stop valve but not less than 0.5 in. (13 mm).7 sure. The duration of the bleed period is installation-dependent An optional hand valve in the bleed line can be used to field and should be adjusted so no audible hammering occurs and the adjust the rate of coil depressurization as shown in Figure 6. time is sufficient to decrease the coil pressure to within 5 to 10 The bleed period is necessary, particularly on large coils psid (0.3 to 0.6 bar) of the normal cooling mode evaporator pres(with coil volumes greater than 8 ft3 (0.23 m3) or suction pip- sure.4 Generally, the bleed period will last five to 10 minutes. ing greater than 2½ in. (65 mm),2 to prevent what would be a At the conclusion of the bleed period, the suction stop pilot very rapid depressurization of the coil when the suction stop solenoid is de-energized allowing the main valve to open. As valve opens. Rapid coil depressurization increases the potential configured in the evaporator schematics, the pilot pressure 38 ASHRAE Journal ashrae.org August 2009
ASHRAE Journal - August 2009

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