ASHRAE Journal - March 2009 - (Page 46)

ASHRAE 19 ambient air temperatures or condensing pressures. It also serves to hold the suction pressure below an excessive amount. The electrical controls for the heat bank system are similar to the other systems discussed in that a timer must assure that the compressor is running, stop the evaporator fans, and open the hot gas solenoid valve. Normal defrosting with this system requires 3 periods of 10 min each which includes a 3 min drainage period. The system shown in Fig. 6 employs an unusual re-evaporator since it is an inner tube within the evaporator coiling. During defrost the hot gas solenoid valve opens to allow the compressed vapor to flow through this inner tube, which acts as a condenser. The heat of condensation originally derived from the heat of compression is the source of heat used to melt the frost. The liquid and vapor then flow through the check valve and constant pressure valve which acts as an automatic expansion valve to maintain a refrigerant temperature above 32°F and yet introduce enough pressure drop to cause any condensed liquid to gasify before leaving the coil. The refrigerant flowing from the constant pressure valve passes into the evaporator which serves as a re-evaporator. The actual heat used for defrosting the coil is the heat of compression of the compressor. This system is controlled with a time clock as are the other systems. Still another system is a reverse cycle defrost method whereby a four-way solenoid valve actually reverses the flow of refrigerant during defrost to let the condenser serve as an evaporator while 46 ASHRAE Journal 50 59–2009 YE A R S JOURNAL the evaporator acts as a condenser using the melting ice as a cooling means. Summarizing the hot gas systems with their specific application to supermarket refrigerators, it has been established that while this method of defrosting is a good one for use on walk-in evaporators, it is too costly for use on remote display cases. The systems depicted in Figs. 3, 4 and 6 are the simplest types and overcome some of the disadvantages mentioned for the basic hot gas system. The system shown in Fig. 5 while overcoming more of the disadvantages is more complex in structure as well as in installation. Electric Defrost, like the hot gas system, is especially applicable to low temperature refrigerators although it is frequently used in meat and dairy refrigerators to give a rapid defrost period. Electric defrost systems in most instances have heat applied externally as compared with hot gas systems where the heat is applied internally. Because of this factor and because of the limitations on the amount of electrical heat that can be applied safely, the electric defrost requires a longer interval, usually one and one-half or more times that of hot gas defrost systems. However, systems have been developed which apply electric heat from within, similar to the hot gas system shown in Fig. 6, to give rapid defrost. In any defrost system the quantity of heat required can be stated as Q d = HF + HA + HS, where Q d, is the total quantity of heat required; HF is the heat necessary to a s h r a e. o rg melt the frost; HA is the heat lost to the air; and HS is the heat necessary to warm the coil surfaces and walls to at least 34°F. Considering first HF , the heat necessary to melt the frost is in direct proportion to the weight of frost and its temperature at the time defrost is instigated. A given volume of frost does not always contain the same weight in water. A comparatively light frost load at 20°F to 25°F would be considerably heavier than the same volume of frost at –20°F or –30°F. For an ice cream display fixture with frost at –40°F at the time defrost is instigated, HF can be calculated to be 198 Btu required to melt one pound of frost and raise its temperature to 50°F as it drips from the drain. It is good practice to allow as a rule of thumb 200 Btu/lb of frost to be melted for all evaporators, both low and high temperature, as this allows a factor of safety and is an aid in the ever-important post defrost drainage period. The exact weight of frost is a variable depending upon the conditions at which the evaporator is operating. Approximate weights of frost for any given refrigerator operating under normal conditions can be obtained experimentally in the laboratory. Under normal operating conditions it has been determined experimentally that a typical 12 ft frozen food open top display cabinet employing a finned evaporator coil with forced air circulation will collect from one to two pounds of frost per day. A similar display case for fresh meats would collect from three to five pounds of frost per day whereas a similar forced air type dairy refrigerator would collect from five to 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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