ASHRAE Journal - March 2009 - (Page 44)

ASHRAE 19 50 59–2009 YE A R S JOURNAL f i g . 5 Hot gas defrost using a heat bank. f i g . 6 Hot gas defrost using a re-evaporator. come. The orifice also serves to hold the suction pressure below an excessive amount. Controls for this system include a time clock which accomplishes three periods of time during each complete defrost period. The first interval (usually two to three min) starts the compressor and stops the evaporator fans to assure an adequate supply of hot gas. During the second period, the clock opens the hot gas line solenoid valve and energizes the electric drain pan heater. This period is usually five to 10 min and occurs approximately four times per day. The last period is a delay period during which the hot gas solenoid valve opens and evaporator fans remain stopped to allow water to drip from the coil surface and to allow the evaporator to reach a lower temperature before resuming air circulation. This period requires a setting of three min. Fig. 4 shows a hot gas system using a vapor pot. This system uses an accumulator in the suction line to trap the liquid refrigerant and, by means of a bleed tube, allows small amounts of liquid refrigerant to flow back to the 44 ASHRAE Journal compressor along with the suction gas. The bleed tube is sized to overcome the liquid refrigerant slug-back disadvantage but allows enough to pass to pick up the latent heat of vaporization in compression. This latent heat of vaporization is the source of heat utilized to melt the evaporator frost. The accumulator or vapor pot has a built-in heat exchanger which is incidental and, although helpful during the refrigeration cycle, offers no help to the defrost process. It is controlled by a time clock which must have a switching mechanism designed to open the hot gas solenoid valve, stop the evaporator fans and start the compressor (Fig. 3). Defrost period is terminated either by the timer or, as is done sometimes, by a thermostat which measures the temperature of the evaporator and at a pre-set point energizes a solenoid coil in the time clock which mechanically reverses the switching mechanism to the refrigeration cycle. The heat bank method of utilizing hot gas as a defrosting means schematically depicted in Fig. 5 actually uses a re-evaporator coil immersed in an insulated tank of water (heat bank). The a s h r a e. o rg re-evaporator overcomes the refrigerant liquid slug back problem. The unusual feature of the heat bank principle is that a heating coil from the discharge line of the compressor also immersed in the tank of water builds up heat and stores it during the refrigeration cycle. This heating coil also serves to prevent flood back during the refrigeration cycle by warming the suction gases in the re-evaporator. The heating coil is provided with a bypass which allows the discharge gas to go directly to the condenser during the refrigeration cycle as the temperature rises in the heat bank. During the defrost period the hot gas flows to the solenoid valve, through the frosted evaporator coil, and then through the heat bank reevaporator where heat is picked up from the hot water. The heat bank is designed with a holdback valve which maintains a low suction pressure in the re-evaporator and is set low enough to actually freeze the water. This allows the gas to not only pick up sensible heat but to pick up the latent heat of fusion given up by the freezing water. This method provides a continuous heat source not dependent upon the March 2009 http://www.ashrae.org

Table of Contents Feed for the Digital Edition of ASHRAE Journal - March 2009

Contents Commentary Industry News Letters Meetings and Shows Special Section 2009 ASHRAE Technology Awards Feature Articles Heat Recovery for Office Tower Air Motion Control in the Hospital Operating Room Anniversary Feature: Five Defrost Methods for Commercial Refrigeration Groundwater Issues: Commercial Open Loop Heat Pump Systems Used Filters and Indoor Air Quality Building Sciences Emerging Technologies Washington Report Products Special Products People Classified Advertising Advertising Index

ASHRAE Journal - March 2009

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