Many uses can be found for portable freezers. For example, much of the ice cream sold during the summer months is sold from refrigerated trucks. It is essential that these trucks have reliable portable freezing units. See Figs. 13-22 and 13-23.
The compressor is run by 110/120-V, 60-Hz AC. It runs during the night when the truck is out of service. A small fan circulates the cold air. The fan runs on the truck battery during the day. At night, it is plugged into line current (120-V, AC).
This unit uses a hermetically sealed compressor designed for use with R-22. About 10 or 12 oz of R-22 are used for a full charge.
If the machine runs short of refrigerant, it should be allowed to warm to room temperature and checked for leaks with a halide torch. For finding small leaks, at least 90 lb of internal pressure are needed. It maybe necessary to add refrigerant to obtain this pressure. If so, connect the suction-line service opening to a drum of refrigerant (probably R-22), making sure the drum remains upright so that only gas will enter the unit.
Never connect in this manner a drum that is warmer than any part of the system. The gas will condense in the system, resulting in overcharge and waste of refrigerant.
If the unit is charged due to leaks, or any major repairs are made on the system, it is recommended that a new drier be installed. When replacing the original drier, be certain that the replacement drier has a good filter and strainer incorporated with the drying agent.
If a gas leak has allowed air to enter the system, the system must be evacuated or thoroughly cleaned with R-22. A new drier must be installed before charging. Air remaining in the system cannot be purged off. It permeates the complete system and is not trapped in the high side as in other systems using a liquid receiver.
Allow pressure to build to approximately 100 lb in the unit and shut off the charging valve immediately. After the unit is started, add refrigerant slowly until backpressure is between 10 and 16 lb, depending on the ambient temperature. (A high-ambient temperature will produce a higher head and back pressure.) The back pressure will then remain about the same until the eutectic (contents of the freezer) is completely frozen.
The charge should be checked again when the cabinet is around −15°F (−26°C) or colder. Then, with the condensing unit running, the suction line should frost out of the cabinet about 6 to 8 in. The desired frost line can be obtained by adding or purging of refrigerant (make sure the purged refrigerant is captured and reclaimed) a little at a time, allowing time for the system to equalize. Ifthe compressor will not start, but the condenser fan isrunning, check the head and back pressures. If the pressures are not equal, a capillary tube maybe clogged with moisture or foreign material. Heating the end of the capillary tube where it enters the cabinet will usually begin to equalize pressures if the restriction is due to moisture freezing. Evacuate the system, install a new drier, and recharge the system as described already. If the capillary tube is clogged with material other than frozen moisture, it should be replaced.
When the compressor does not start and the head and back pressures are approximately equal, check for trouble as follows.
Check the line voltage by holding the voltmeter leads on contacts of the motor base plug. Take a reading when the overload protector clicks in and the compressor is trying to start. This reading should be 100 V or more. If less, the trouble is probably in the supply line.
• Replace the capacitor, if the unit has one.
• Replace the relay and/or overload.
If, after these checks, the compressor will not start, the unit should be returned to the manufacturer.
Figure 13-24 shows an ice-cream vending unit. Most of the mechanical parts are located on top of the unit to prevent damage when the unit is handled frequently. This type of freezer, in various sizes, can be mounted in a variety of vehicles. The cabinet provides for economical operation that can pay for itself in dry ice savings alone. The unit is plugged in at night. The smaller units rely upon insulation to hold the cold air. Other units plug into the vehicle’s battery.
Small trucks can be fitted with portable freezers. These are useful to dairies servicing school cafeterias and other large food-dispensing operations where milk must be kept cool and ready for servicing at a specific time. Some units can handle between 400 and 700 bottles or cartons of milk. They can be rolled into a cafeteria line.