The freezer defrost sequence is as follows: 1. Defrost is initiated, or started, according to the time setting on the defrost clock. 2. The compressor and the evaporator fan(s) stop. 3. The electric heater (or hot gas) starts warming the coil. 4. When the coil reaches a temperature high enough to have melted all the [...]
Hot gas defrost is primarily used in supermarket applications and ice machines because it is quick and efficient. During defrost, hot gas enters the evaporator downstream of the TEV. The hot refrigerant warms the tubing inside the evaporator, which is more effective than electric heaters that warm the fins outside the tubing. Installing a hot [...]
Low-temperature evaporators require a time clock, controls, and a source of heat to melt their normal accumulation of frost. Also, because frost is produced easily when the coil temperature is below 0°F, the fin spacing must be wide enough to prevent frost bridging between fins for at least 4–6 hours of normal operation. AC coils [...]
Sometimes the standard off cycle is not long enough to defrost the evaporator. This problem usually occurs when the box temperature is maintained between 34°F and 36°F. The evaporator can also develop too much frost when the box receives very heavy usage from warm product, bad door gaskets, or just excessive door openings. Technicians have [...]
Medium-temperature commercial refrigeration usually operates at box temperatures between 34°F to 40°F with evaporator temperatures about 15°F to 25°F. Because the coil temperature is well below freezing, it is normal for frost to build up on the evaporator fins during the “on cycle” when the compressor is running. When the tstat is satisfied, the compressor shuts [...]
A starving evaporator is one in which the refrigerant is boiling off too soon. The refrigerant does not fill the evaporator sufficiently; therefore, it picks up more sensible heat than normal. A starving evaporator has high superheat. EXAMPLE: A system with a normal superheat of 10°F is considered as starving if its superheat is above [...]
A flooding evaporator is not boiling off enough of its refrigerant to prevent liquid from leaving the evaporator. Refrigerant that has not totally boiled off cannot pick up any sensible heat. A flooded system has no superheat. However, flooding is sometimes used to describe an evaporator that has a superheat much lower than normal. EXAMPLE: [...]
Measuring the amount of superheat allows the technician to determine if all refrigerant has boiled off in the evaporator and to measure approximately the efficiency of coil operation. It can also be a measurement of the margin of safety against flooding the compressor with liquid refrigerant. The four steps in calculating superheat are as follows: [...]
It is important that technicians understand what happens inside an evaporator and how different evaporators affect different conditions. Following is a sample of the evaporator type and proper operating conditions a technician could expect in a medium-temperature walk-in refrigerator. A standard walk-in refrigerator designed for a 35°F box temperature would use a fan-coil unit designed [...]
The speed of air moving across an evaporator or the rate of liquid flow through tubing can change the rate of heat exchange. Readers familiar with AC are well aware of the effects air volume has on the system. In a standard AC system the evaporator absorbs about 12,000 Btu with approximately 400 cubic feet per [...]