Description of the Technology

Figure 1 presents the basic components of a commercial refrigeration system. Heat is removed from the product display cases and discharged to the outdoors.

The basic steps of the vapor compression cycle and it's primary components are:

• Evaporator: A heat exchanger inside the display case in the sales area. Heat removed from the conditioned space within the case causes the liquid refrigerant to boil away at a very low temperature, producing a low-temperature, low-pressure gas.
• Compressor: This unit pulls the refrigerant gas from the evaporator through refrigerant piping and compresses, or pumps it up, to a much higher pressure. Compression also results in a much higher temperature for the gas. Compressors are usually centrally located in a machine room at the back of the store.
• Condenser: Typically located outside, this is a heat exchanger which transfers the heat from the refrigerant to the outdoors. High-pressure refrigerant gas from the compressor is converted back into a liquid (i.e., condensed) as it is cooled inside the condenser. All the refrigerant vapor changes back into a liquid, but it is still at the same pressure as when it entered the condenser. A high-pressure, medium-temperature liquid refrigerant is now moving out of the condenser.
• Receiver: A vessel which stores the liquid refrigerant leaving the condenser until it is needed at the evaporators.
• Expansion Valve: This valve, also located in the refrigerated display case, releases the high-pressure liquid in a controlled fashion. When the pressure is removed from the liquid, it rapidly be becomes a vapor that is much colder than the liquid. Now a mixture of cold liquid and vapor is coming out of the expansion device, getting ready to enter the evaporator and start the cycle over.
• Refrigerants: Commercial refrigeration systems have traditionally used R-12, R-22, and R-502 as refrigerants. These are all chlorofluorocarbon (CFC) refrigerants and as of January 1, 1997, R-12 and R-502 are no longer being produced for U.S. consumption. The cost of existing quantities has risen sharply and their future availability is uncertain. Some stores are converting from R-502 to HFC-404A. New stores or major remodels are using HFC-404A for the low-temperature systems and HFC-134A for the medium-temperature systems. Another solution is to stay with R-22 designs until firm recommendations are available from chemical suppliers and compressor manufacturers.

Typical Applications

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Efficiency Opportunities

A variety of strategies can be used to reduce refrigeration costs. These include:

• Night covers for display cases: The covers should have small perforated holes to decrease moisture buildup.
• Strip curtains for walk-in boxes: Install on doors of walk-in cases or doorways of refrigerated warehouses.
• Glass or acrylic doors: Install on existing upright display cases.
• New refrigeration case with doors: Replace existing older refrigeration case doors.
• Humidistat controls: Install a device that senses the humidity in the air outside of the upright display case and turns off the glass door and frame anti-sweat heaters at low-humidity conditions. Dew-point or analog dew-point controllers are recommended.
• Case lighting electronic ballasts: Install a fully electronic ballast to control lights in a refrigerated display case.
• Insulate bare suction lines: These large diameter lines that run from the evaporator to the compressor lose heat to the environment.
• Cooler or freezer door gaskets: Replace worn gaskets.
• Auto-closers for coolers or freezers: Auto-closer must be able to firmly close a door that is within one inch of full closure.
• Heat recovery: Supermarkets can often take advantage of the large amount of heat rejected through the refrigeration system by using it for space heating or water heating. This is referred to as heat recovery or heat reclaim. Figure 2 shows a system with a heat recovery coil in the air handler. During normal operation, discharge gas from the compressors is piped directly to the condenser. When space heat is needed, a three-way valve directs the refrigerant gas to a coil in the air handler. Heat that would normally be rejected in the condenser is now rejected to the store's supply air. The refrigerant then passes to the condenser.

Tips for Efficient Operation and Maintenance

Ongoing:

• Move delivered product quickly into the walk-in coolers from the receiving dock. Allowing the product to warm up to outdoor temperatures will increase the pull down load on the refrigeration system.

• Clean the condenser coils to maintain optimum heat transfer performance.
• Check to be sure that display case air curtains, grilles, or registers are clean and free of obstructions to assure proper air flow across the evaporator coils.
• Check door gaskets for leaks and tears to minimize infiltration loads.
• Check the refrigerant charge; add more when needed.
• Check the defrost cycle time clocks to see that they are set and operating properly. The number and length of defrosts can be varied seasonally.

• Have a professional refrigeration contractor give the system a thorough check-up, paying special attention to the compressors and motors.
• Check and lubricate fans.
• Examine refrigerant lines for leaks.

In-Depth Technical Articles

• Efficiency Opportunities in Restaurants
• Efficiency Opportunities in Grocery Refrigeration
• Efficiency Opportunities in Industrial Refrigeration Systems