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Controlling Walk-in Coolers and FreezersIntroduction Control of walk-in coolers and freezers can make a major contribution to energy cost savings. Coolers and freezers can be controlled to respond to utility rates by decreasing peak demand or shifting energy use. The reasons walk-in coolers and freezers make ideal loads to control include:
The benefits can only be realized if walk-in coolers and freezers are controlled properly. The Dencor Energy Administration System includes all the features needed for effective control of walk-in coolers and freezers. A typical record provided by the Dencor system is shown in Figure 1. The figure shows the temperature variations for two coolers in a convenience store operating during a day without control. Monitoring temperature and energy use is an important step in gathering information for determining an effective control strategy. Similar records during operation using a control strategy help assure successful cost savings. Control Considerations Several important points to consider when controlling refrigeration equipment: Control the Thermostat Circuit When controlling thermostatically controlled devices it is always best to interrupt the thermostat circuit. This makes control very similar to the normal operation of the cooler or freezer. All safety devices are allowed to operate normally. Prevent Accumulation of Liquid Refrigerant Walk-in coolers or freezers often have very long refrigerant lines. Special procedures may be needed to prevent the accumulation of liquid refrigerant that might be returned to the compressor in liquid form and damage the compressor. To prevent the accumulation of refrigerant the thermostat controls a liquid-line solenoid rather than directly controlling the compressor contactor. When the thermostat is satisfied it activates the liquid line solenoid which stops the flow of refrigerant. The compressor then continues to run until it has pumped the refrigerant back to the compressor and the pressure drops until the compressor stops on low-pressure cutout. This may take up to 2 minutes. This process will keep the refrigerant in the compressor and not let it condense in some cold part of the system. Return the Lubricating Oil The lubricating oil often travels with the refrigerant, especially during turn-on. It may take up to 7 minutes of continuous operation for the oil separator to become effective and return oil to the crankcase that had been carried with the refrigerant. Short periods of operation should be avoided, especially several short periods in succession.
Allow Completion of Defrost Cycles It is important to permit proper defrost cycles to keep frost from plugging the evaporator coils. Several methods are used for defrosting. For coolers it is often enough to simply turn off the compressor long enough for the ice to melt. Freezers need heat added for defrosting. It is also necessary to be sure the water will drain before it freezes. Use Independent Override Thermostat Where product spoilage is a consideration, an independent thermostat must be used to eliminate control if the temperature is beyond required limits. The override device must be a "make on temperature rise" thermostat which is connected to close the circuit the Dencor controller opens when it turns a load off. Learn the Characteristics of the Controlled Equipment It is most important to discuss the control of refrigeration equipment with a knowledgeable person. Determine if the equipment is functioning properly before control is applied. Find out if the equipment can meet the current needs. Common sense tells us that control should not be active during periods the equipment cannot handle the cooling load without control. The high temperature peaks for cooler B shown in Figure 1 indicates a defrost cycle every 6 hours. This cooler has a defrost timer that simply turns the compressor off for about 45 minutes and allows the temperature to rise which creates the higher temperature variations shown by the blue line. In this specific application the customer desires to decrease demand starting at 7:00 am. and ending at 7:00 pm. In this case the Dencor Energy Administration System shows the defrost timer could be set for a much more useful time. Shifting the defrost period to start at 7:00 am instead of ending at 6:30 am would enable the defrost cycle to occur when the controller is keeping the cooler compressor off to help reduce demand. The Dencor Energy Administration System provides a variety of control procedures to implement the strategies needed to reduce utility costs by responding to the best available energy rates. The control procedures include timers that can be used to schedule defrost periods to reduce demand as well as provide the necessary frost removal. It also includes minimum on and off timers to specify run times long enough to facilitate return of lubricating oil. Proper application of the many control features of the Dencor Energy Administration System permits safe control of virtually any refrigeration system. The Dencor Energy Administration System provides a permanent record of minute by minute operation of a facility. These records are very important for maximizing savings as well as assuring proper equipment operation.
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