In these units, the evaporator coil is automatically defrosted in regular intervals every day. A fan is mounted on the evaporator to draw air from one side, move it over the evaporator coil, and force it into the freezer and fresh-food compartments through a damper (see figs. 6, 8, and 9). A thermostat controls the operation of the compressor to maintain food at desired temperatures. It is regulated by a manually adjustable knob. A baffle control knob inside the cabinet of the unit manually controls the temperature of the fresh-food compartment by increasing or reducing the flow of cold air into that compartment.
There are two defrosting systems used in residential frost-free refrigerators and freezers:
1. Electric heater system
2. Hot refrigerant vapor system (hot gas system)
Electric-defrost system. A timer (see figs. 7 and 11a-11e) automatically de-energizes the compressor and the evaporator fan motor, and at the same time it energizes the evaporator defrost heater every six, eight, or twelve hours (depending on the type of timer being used) to de-ice the evaporator plate. The power from the timer to the defrost heater flows through a defrost thermostat (also referred to as a “defrost termination switch”), which is clipped to the evaporator coil. When the ice buildup on the evaporator coil is melted, the temperature of the evaporator coil rises to about 50°F ±6°F (10°C ±3°C). The rise in temperature causes the bimetal within the termination switch to open its contacts, thus, de-energizing the defrost electric heating element to end the process of defrosting even with the timer still in its defrost cycle. Contacts within defrost thermostats close at 20°F (-7°C). The whole defrosting process takes between eight and thirty minutes. Over 90% of the frost-free residential refrigeration units of today are equipped with this type of defrosting system.
Hot gas, automatic defrost system. In a hot gas defrost system, a bypass line connects the compressor discharge line to the evaporator inlet (between the capillary tube and the evaporator inlet [see figs. 16 and 16a]). When the unit is taken into the defrost mode, a solenoid defrost valve becomes energized, allowing discharged hot refrigerant to circulate directly through the evaporator and quickly defrost it. It is then drawn back into the compressor through the suction line. In this way, the condenser and the capillary tube are bypassed. In the normal cooling cycle, the valve is de-energized (its defrost port is closed) allowing refrigerant to flow through the condenser coil, capillary tube, evaporator, and then back into the compressor suction side. Figure 129 shows a different defrost system in which hot vapor refrigerant is circulated through a second coil adjacent to the evaporator.
Normally, a frost-free unit is equipped with a fan-forced condenser, which is mounted at the bottom rear of the unit.
A fan is mounted next to the condenser and moves ambient air through the condenser to help it dissipate heat.
NOTE: The most common problem in this type of unit is related to a dirty or lint-filled condenser. A dirty condenser loses its ability to transfer heat causing the unit to run continuously but not cool sufficiently. The condenser in a frost-free refrigerator or freezer requires annual cleaning.
In side-by-side refrigerators, the evaporator is mounted at the back of the freezer compartment. A damper regulated by the consumer controls flow of cold air from the freezer to the fresh-food compartment. This cold air forces the warmer air back toward the evaporator cold coil through the bottom of the fresh-food compartment. (See fig. 9)
In units with the freezer compartment on top, the evaporator is placed either in the back or on the bottom of the freezer. An evaporator fan forces cold air to the fresh-food compartment through a damper. In frost-free refrigerators, the freezer air temperature runs between 0°F and 10°F, and the fresh-food compartment temperature ranges between 38°F and 45°F (see fig. 3, 6 and 9).