Learning Objective: Understand and recognize the basic types of commercial and domestic refrigeration equipment.
Refrigeration equipment can be classified as either self-contained or remote units. Self-contained equipment houses both the insulated storage compartments (refrigerated), in which the evaporator is located, and an uninsulated compartment (non-refrigerated), in which the condensing unit is located, in the same cabinet. This type of equipment can be designed with a hermetically sealed, semi-sealed, or an open condensing unit. These units are completely assembled and charged at the factory and come ready for use with little or no installation work. Self-contained refrigerating equipment includes such equipment as domestic refrigerators and freezers, water coolers, reach-in and walk-in refrigerators, small cold-storage plants, and ice plants.
Remote refrigerating equipment has the condensing unit installed in a remote location from the main unit. These types of units are used where the heat liberated from the condenser cannot enter the space where the unit is installed or space is limited for installation.
Reach-in refrigerators have a storage capacity of 15 cubic feet or greater. They are mainly used for storing perishable foods, but they can be found in medical facilities for storing biologicals, serums, and other medical supplies requiring temperatures between 30°F and 45°F. Standard-size units most frequently used are those with storage capacities between 15 and 85 cubic feet. Figure 6-34 shows a typical reach-in refrigerator with a remote (detached) condensing unit.
Figure 6-34.—A reach-in refrigerator with a remote condensing unit.
Exterior finishes for reach-in refrigerators are usually of stainless steel, aluminum, or vinyl, while the interior finishes are usually metal or plastic, and the refrigerator cabinet is insulated with board or batten type polystyrene or urethane. Reach-in refrigerators are normally self-contained, with an air-cooled condenser, but in larger refrigerators, with remote condensers, water-cooled condensers are sometimes used. A typical self-contained unit is shown in Figure 6-35. The evaporator is mounted in the center of the upper portion of the food compartment. In operation, warm air is drawn by the fan into the upper part of the unit cooler, where it passes over the evaporator coils, is cooled, and then is discharged at the bottom of the cooler. The air then passes up through the interior and around the contents of the refrigerator. The cycle is completed when the air again enters the evaporator.
Figure 6-35.—A self-contained reach-in refrigerator.
The low-pressure control is set to operate the evaporator on a self-defrosting cycle, and temperature is thus control led. Another type of control system uses both temperature and low-pressure control or defrost on each cycle. The evaporator fan is wired for continuous operation within the cabinet.
Evaporators in reach-in refrigerators are generally the unit cooler type with dry coils (Fig. 6-36). In smaller capacity refrigerators, ice-making coils, similar to those used in domestic refrigerators, are often used as well as straight gravity coils. R-12 and R-502 are normally used in these units.
Figure 6-36.—A. Unit cooler in a reach-in refrigerator; B. Dome cooler in a reach-in refrigerator.
Walk-in refrigerators are normally larger than reach-in types and are either built-in or prefabricated sectional walk-in units. They are made in two types—one for bulk storage of fresh meats, dairy products, vegetables, and fruits requiring a temperature from 35°F to 38°F and the other for the storage of frozen food at temperatures of 10 o F or below. The 35°F to 38°F refrigerators are built and shipped in sections and assembled at the location they are installed. They can be taken apart, moved, and reassembled in another area if needed. Standard-size coolers can be from 24 square feet up to 120 square feet in floor area. A walk-in refrigerator with reach-in doors is shown in figure 6-37.
Figure 6-37.—A walk-in refrigerator with reach-in doors.
The exterior and interiors of these units are normally galvanized steel or aluminum. Vinyl, porcelain, and stainless steel are also used. Most walk-in refrigerators use rigid polyurethane board, batten, or foamed insulation between the inner and outer walls. For storage temperatures between 35°F to 40°F, 3 to 4 inches of insulation is generally used. For low-temperature applications, 5 inches or more of insulation is used. These refrigerators are equipped with meat racks and hooks to store meat carcasses.
Walk-in refrigerators also have a lighting system inside the refrigerator compartment. Most systems have the compressor and condenser outside the main structure and use either a wall-mounted forced-air or gravity-type evaporator that is separated from the main part of the cabinet interior by a vertical baffle.
The operation of the walk-in refrigerator is similar to that of the reach-in units. The evaporator must have sufficient capacity (Btu per hour) to handle the heat load from infiltration and product load.
Most domestic refrigerators are of two types—either a single door fresh food refrigerator or a two-door refrigerator-freezer combination, with the freezer compartment on the top portion of the cabinet, or a vertically split cabinet (side-by-side), with the freezer compartment on the left side of the cabinet. They are completely self-contained units and are easy to install. Most refrigerators use R-22 refrigerant, normally maintaining temperatures of 0°F in the freezer compartment and about 35°F to 45°F in the refrigerator compartment. The technician must be able to perform various duties in the maintenance and repair of domestic refrigerators, water coolers, and ice machines. This section provides information to aid you in handling some of the more common types of troubles. But let us remind you that the information given here is intended as a general guide and should, therefore, be used with the manufacturer's detailed instructions.
Single Door Fresh Food Refrigerator
A single door fresh food refrigerator (fig. 6-38) consists of an evaporator placed either across the top or in one of the upper corners of the cabinet. The condenser is on the back of the cabinet or in the bottom of the cabinet below the hermetic compressor. During operation, the cold air from the evaporator flows by natural circulation through the refrigerated space. The shelves inside the cabinet are constructed so air can circulate freely past the ends and sides, eliminating the need for a fan. This refrigerator has a manual defrost, which requires that the refrigerator be turned off periodically (usually overnight) to enable the buildup of frost on the evaporator to melt. Both the outside and inside finish is usually baked-on enamel. Porcelain enamel is found on steel cabinet liners. The interior of the unit contains the shelves, lights, thermostats, and temperature controls.
Figure 6-38.—Single door fresh food refrigerator.
Two-Door Refrigerator-Freezer Combination
The two-door refrigerator-freezer combination is the most popular type of refrigerator. It is similar to the fresh food refrigerators in construction and the location of components except it sometimes has an evaporator for both the freezer compartment and the refrigerator compartment. Also, if it is a frost-free unit, the evaporators are on the outside of the cabinet.
Because of the two separate compartments (refrigerator-freezer) and the larger capacity, these types of refrigerators use forced air (fans) to circulate the air through the inside of both compartments. The two-door refrigerator also has one of the following three types of evaporator defrost systems: manual defrost, automatic defrost, or frost-free.
There are two types of automatic defrosting: the hot gas system or the electric heater system. The hot gas system, through the use of solenoid valves, uses the heat in the vapor from the compressor discharge line and the condenser to defrost the evaporator. The other system uses electric heaters to melt the ice on the evaporator surface.
A frost-free refrigerator-freezer (fig. 6-39) has the evaporator located outside the refrigerated compartment. On the running part of the cycle, air is drawn over the evaporator and is forced into the freezer and refrigerator compartments by a fan. On the off part of the cycle, the evaporators automatically defrost.
Figure 6-39.—Frost-free refrigerator airflow diagram.
Refrigerator-freezer cabinets are made of pressed steel with a vinyl or plastic lining on the interior wall surfaces and a lacquer exterior finish. Most domestic refrigerators have urethane foam or fiber glass insulation in the cabinet walls. The side-by-side refrigerator-freezer arrangement has a number of features not found in other refrigerators. In addition to the automatic icemaker in the freezer compartment, it has an option for a cold water dispenser, a cube or crushed ice dispenser, and a liquid dispenser built into the door.
Water Coolers and Ice Machines
Water coolers provide water for drinking at a temperature under 50°F. Two types of water coolers are instantaneous and storage. The instantaneous type only cools water when it is being drawn; the storage type maintains a reservoir of cooled water. One instantaneous method used places coils in a flooded evaporator through which the water flows. A second instantaneous method uses double coils with water flowing through the inner coil with refrigerant flowing in the space between the inner coil and the outer coil. A third instantaneous method is to coil the tubing in a water storage tank. This allows refrigerant to flow through it (fig. 6-40).
Figure 6-40.—Storage type of water cooler.
Water coolers are of two basic designs—wall mounted or floor mounted. Both types are the same in construction and operation; the only difference is in the method of installation. Water cooler cabinets have a sheet metal housing attached to a steel framework. The condenser and hermetic compressor are located in the housing base, and the evaporator is located in the cabinet depending on its type of evaporator, but normally under the drain basin. Most water coolers use a heat exchanger or precooler, which precools the fresh water line to the evaporator, reducing cooling requirements for the evaporator. A thermostat, which is manually set and adjusted, is located in the cooler housing close to the evaporator.
Automatic ice machines, similar to the units shown in figures 6-41 and 6-42, are often used in restaurants, schools, hotels, stores and other public areas.
Figure 6-41.—Flake or chipped-ice machine.
Figure 6-42.—A cutaway view of an automatic ice machine.
Ice machines are self-contained, automatic machines, ranging from a small unit producing 50 pounds of ice per day to a commercial unit producing 2,400 pounds of ice per day. The primary difference in the design of these machines is the evaporator. They automatically control water feed to the evaporator, freeze the water in an ice cube mold, heat the mold and empty the ice into a storage bin, and shut down when the storage bin is full.
Floats and solenoids control water flow, and switches operate the storing action when ice is made. Electrical heating elements, hot water, hot gas defrosting, or mechanical devices remove the ice from the freezing surfaces depending on the unit. Figures 6-43 and 6-44 show the freezing and defrost cycle of a typical ice cube machine. In recent years, many companies have begun to manufacture their units to use HFC R-404a refrigerant instead of HCFC R-22.
Figure 6-43.—An ice cube machine refrigeration cycle during the freezing process
Figure 6-44.—The defrost cycle of an ice cube machine