lmprovin Industrial Refrigeration -!nergy Efficiency R efvigeration systems consume a large portion of the total energy used in food processing and food storage industries. There are a number of simple low-cost operation and maintenance measures for Cold storage and freeze tunnels that ill improve their energy efj4cieflcy and save substantial operating dollars without sacrificing product quality. Com m o n Ref r i g e rat i on S ys t e ms In a typical direct expansion (DX) refrigeration system (Figure 11, low-pressure refrigerant gas is compressed into a superheated gas by the compressor. The superheated gas is condensed to high-pressure liquid by removing heat at the condenser coils located outside of the building. The high-pressure liquid is then forced through an expansion valve that adiabatically reduces the pressure of the liquid before it flows through the evaporator coils. Heat is transferred from the room air to the liquid refrigerant in the evaporator, which causes the lowpressure liquid to boil and change state from liquid to gas. The low-pressure gas is then returned to the compressor. In a flooded system (Figure 2) the evaporator coil is kept full of liquid refrigerant by a low-pressure liquid receiver. The refrigerant level in the low-pressure liquid receiver is maintained between high and low levels by float switches that open and close a solenoid valve in the high-pressure liquid line. The pressure and temperature inside the evaporator coil and the low-pressure receiver are maintained by a thermostatically controlled pressure regulator located on the low-pressure side of the evaporator. p P e i i " r 0 Evaporator I 1 1 Toll-free Hotline 1-800-872-3568 FAX 1-800-872-3882 Electronic Bulletin Board 1-800-762-3319 Bonnevllle POER ADMINISTRATION
Energy Efficiency Tips AT ABSORBED The following suggestions are intended as general recommendations. System operators should consult a refrigeration specialist or service technician before implementing any recommendations. Optimize Defrost Cycle Set the defrost cycle based on seasonal, humidity, and load requirements. Defrost is generally initiated with a time clock that typically is set based on the experience of your refrigeration technician. However, actual defrost requirements vary with the season and humidity levels, the products being stored, the frequency and duration of door opening, and the air exchange rate within the storage area or freeze tunnel. To reduce the refrigeration load imposed by the defrost cycle, High-pressure liquid ELECTKICAL ORK INPUT CONDENSER HEAT REJECTED you can either manually reset the cycle frequency throughout the season or install automatic control. The optimum and more expensive solution is to control the defrost cycle automatically. It is important to maintain adequate defrost since freezing a coil can cause damage. Parameters you may use to determine the optimum defrost cycle are: Ambient humidity Case humidity Visual inspection (photo electric relay and fiber optic systems) of ice build-up on the evaporator coil Air velocity through the evaporator coil Air pressure drop across the evaporator coil Each of these alternatives has advantages and disadvantages; the ideal control type will depend on individual circumstances. Consult with a refrigeration service technician when making your decision. Net defrost requirements can be reduced in freeze tunnels by improving product dewatering prior to freezing, and by setting the temperature of the evapora- - tor coil in the initial section warmer than subsequent coils. A warmer temperature in the in- - itial section will disseminate the frost accumulation further into the tunnel, reducing the heavy frost buildup at the beginning. Once the defrost cycle is complete, start the refrigerant flow before the evaporator fans are cycled on. This allows the most effective removal of hot gas from the evaporator coil, especially in the case of a hot gas defrost method. You should also consider the actual temperature requirements of the product being stored. If your product does not need to be frozen or maintained below 34 F and the heat load is small relative to the evaporator capacity, defrost can be totally eliminated by running the evaporator at 32 F or higher. Purge Non-Condensables Remove non-condensables from the refrigeration system at each purge point. Non-condensables typically consist of Air that leaks into portions of the system operating below atmospheric pressure Gasses produced from the - breakdown of lubricating oils -~ Organic acids Salts Sludge ater - -
Air may leak though valve stem packing, gaskets, and shaft seals. Air may be introduced during repairs and service or when transferring liquid refrigerant from a truck or cylinder to a plant. The air collects in the coolest, lowest-velocity areas of the high-pressure components, such as condensers, receivers, and oil separators. The following table shows evaporator temperatures below which air may be drawn into the system: Refrigerant Evaporator Temperature (below which air can be drawn into the system through leaks) Ammonia -28.3"F R-12-21.6"F R-22-41.5"F R-502-49.9"F Air reduces the system's operating efficiency by acting as an insulator. This decreases heat transfer between the refrigerant and the condensing surface, causing increased condensing pressure requirements. ith each 10 psi increase in condensing pressure, there is an approximate 6 percent increase in kilowatts used per ton of refrigeration. In addition to wasting energy, a higher condensing pressure increases harmful effects such as bearing wear, lubricating oil breakdown, gasket and joint leakage, and cooling water usage in water-cooled condensers. Your condensers and pressure vessels should have connections for purge lines. Evaporative condensers usually have purge points at the top of the outlet header of each circuit. You can determine the amount of air in the system by monitoring the difference between the design condensing pressure and the actual condensing pressure. hen you observe abnormally high condensing pressures, it is important to expel air at each purge point to ensure that all non-condensables are removed from the system. Properly Sequence Compressors Manage compressors so that each operates at or near full capacity. Plants having two or more compressors can save energy by load management techniques. Fully load one compressor before starting the next compressor in the sequence. Determine which compressors are needed to operate at fixed loads and which are allowed to manage the load swing. Switch the cut-in and cutout points to make one compressor the primary compressor for one season or month. Then make the second compressor the primary for the following period. In so doing, your compressors will have an equivalent equipment life, while operating at peak efficiencies. Reduce Condensing Pressure Operate your refrigeration system at the lowest condensing pressure possible with existing condenser capacity. Lower condensing pressure requires less energy from your compressor while producing the same cooling effect. Use caution when reducing condenser pressure. Minimum pressure levels may be dictated by specifications of your equipment, for example: Screw compressors are constructed with a built-in volume ratio that is chosen to match the system pressure ratio. Lowering the discharge pressure significantly below design pressure can cause operational problems. Screw compressor injection oil cooling systems may require modification to operate at lower pressures. Direct expansion evaporators or inter coolers that may be part of the system could suffer a reduction in capacity due to insufficient pressure difference across the expansion valve. Maintain Cooling Tower Maintain cooling tower and related equipment, and treat cooling tower water. Proper cooling tower maintenance is essential to conserve both water and energy resources. Dirty cooling tower pipes or water will reduce the heat transfer and the overall system efficiency, decrease the life of system components, and lead to expensive repairs. Fouled condensing surfaces and clogged spray nozzles can raise condensing pressures. The following should be included as part of your regular cooling tower maintenance program: Periodically inspect mechanical and electrical equipment along with internal components such as the evaporative fill material. In addition, inspect both the hot and cold water basins in the cooling tower to ensure they are clean and well maintained. >
~~ ~ Electric Ideas Clearinghouse is a comprehensive informa tion source for commercial and industrial energy users. It is operated by the ashington State Energy Office and is part of the Electric Ideas technology transfer program sponsored by participating utilities and the Bonneville Power Administration. Neither the United States noy the Bonnmille Power Administra tion, the state of ashington, the ashington State Energy Office, nor any oftheir contractors, subcontractors, or their employees make any warran ty, expressed or implied, or assume any legal responsibility for the accuracy, completeness, or usefulness of any informat ion, apparatus, product, or process disclosed within the publication. Technology Update CH-7 Toll-free Hotline: 1-800-872-3568 Fax: 1-800-872-3882 Electronic Bulletin Board: 1-800-762-3319 Additional Reading ASHRAE. 1985 ASHRAE Fundamentals Handbook. 1985. ASHRAE. 1988 ASHRAE Equipment Handbook. 1988. The Boiler Efficiency Institute. HVAC Plant Improvement. Carroll, John F. "How to Install and Maintain Cooling Towers." Air Conditioning & Refrigeration Business. May 1980. Stoeker,.F. Industrial Refrigeration. Business News Publishing Company. Strong, Arthur P., P.E. "Hot Gas Defrost for Industrial Refrigera tion." Hea ting/piping/air Conditioning. July 1988. Yencho, John, P.E. "Purging Non Condensable Gases." Heating/Piping/Air Conditioning. February 1989. Reprinted from Energy Tips ~ O Y Industry written by the ashington State Energy Office. DOEIBP-39833-15 December 1991 1.5M
~~ ~ Periodically drain and clean wetted surfaces and areas of alternate wetting and drying. This prevents accumulation of dirt, scale, or biological organisms, such as algae and slime, in which bacteria may develop. Treat the circulating water for biological control and corrosion in accordance with accepted industry practice. During operation of a cooling tower, some of the water evaporates and is replaced with make-up water. The water has varying amounts of impurities called dissolved solids. It is important to control the concentration of these dissolved solids by bleeding off some of the water. Proper bleedoff will not waste water; rather, it will keep chemical water treatments to a minimum. Systematically document operation and maintenance functions. ith good documentation, you can evaluate the effectiveness of your maintenance policy and procedures. amount of insulation recommended when the facility was constructed may be insufficient considering today's energy costs. Check weatherstripping and door seals frequently; repair if needed. Replace all insulation damaged by moisture. The insulation value of moisture-damaged insulation is drastically reduced. Install motion sensors or train personnel to turn off lights in unoccupied refrigerated areas. Lights operating in unoccupied refrigerated areas add heat, which must be removed by the refrigera tion system. Shut down the refrigeration in rooms while extensive loading or unloading is occurring. Minimize opening entry doors. Additional refrigeration is needed to maintain proper cooling if entry doors are left open. See Figure 3. Use properly-maintained plastic strip curtains or automatic doors on all open cold storage entrances if your system must be operated with open doors. Plastic curtains and automatic doors can be up to 95 percent effective in reducing infiltration when the ~ doors are not in use. Air curtains are also reported to be effective, although not as effective as plastic curtains or automatic doors. Control Product Load Install temperature sensors to monitor the product temperature rather than the air temperature within your cold storage. Control your refrigeration system to operate only when the product needs cooling, and enable temperature sensors in each room to individually shut down the refrigerant flow and the evaporator fans for that room. Raise the evaporator temperature to the maximum level while meeting product requirements. Manage Loads Manage both supplementary and product loads. The total refrigeration load is a combination of supplementary and product loads. The supplementary load is caused by electric lights, motors, tools, and human beings. The product load consists of the heat contained within the product being stored or frozen. Following are several approaches you can take to manage these loads. Storage Reduce Supplementary Load Install sufficient levels of insulation in the ceilings, walls, and doors. The \a+- 4. "Iced" floor.._- P