Troubleshooting A Frozen Evaporator Coil

Walk-in freezers can present service technicians with a variety of problems. This article describes how to troubleshoot a frozen evaporator coil.

BY JOE MARCHESE

One common problem you may encounter when called to troubleshoot a walk-in freezer is a frozen evaporator coil. Although there are several possible reasons for this problem, a common cause is a faulty defrost system.

It is important to fix this problem. An evaporator coil that is severely frozen will reduce the refrigerating ability of the system. The ice built up on the coil will act as an insulator restricting the transfer of heat energy from the product to the refrigerant.

This can cause several problems. First, the temperature of the product will increase, which could cause the product to defrost, if the problem is left unresolved. Second, because there is no heat load on the evaporator, liquid refrigerant could exit the evaporator and possibly return to the compressor. This could lead to premature compressor failure.

For some reason, the system might not properly defrost the evaporator's coil on a regular basis. To effectively troubleshoot this problem, a technician first must understand the design and operation of the defrost systems typically employed for walk-in freezers.

One popular defrost system used on walk-in freezers is an electric defrost system. An electric defrost system is comprised of several components: resistive heater(s), defrost timer, defrost termination switch, fan-cycling control and drain-line heater.

This method involves positioning an electric resistance heater(s) on the outer surface of the evaporator's coils. When energized these heaters will supply enough heat to completely defrost the coils.

The resistive heaters used on a typical electric defrost system are sized to provide sufficient heat to effectively defrost the coil's surface. Their capacities normally are rated in watts per foot. They are shaped to fit snugly on the coil surface, creating an efficient heat transfer during defrost.

Most heaters are manufactured for a specific coil, and when replacing these heaters it is best to obtain the original equipment manufacturer (OEM) replacement unit. Universal defrost heaters are available, but matching their wattage and shape may be difficult at times.

The defrost timer is used to control the entire defrost operation. It initiates the defrost cycle, controls the operation of the compressor and defrost heaters, and is part of the defrost termination. Defrost timers can be adjusted to initiate a defrost just once a day or several times a day. The actual number of defrosts per day will depend on the location of the walk-in.

Walk-in freezers usually are designed to defrost once or twice a day. The more humid and warm a location is, the more defrosts it will need. If a system needs to be defrosted more frequently, add only one additional defrost period at a time and monitor the results. Avoid adding too many defrost periods because it may not benefit the system or the customer.

Figure 1 above shows a common wiring diagram for a time-initiated, temperature-terminated electric defrost system. As the diagram indicates, the time motor continuously is energized. The normally closed contacts of the defrost timer numbered 2-4 are wired in series with the compressor and the evaporator fan motor. The normally open contacts numbered 1-3 are wired in series with the electric defrost heaters and the timer release solenoid.

The timer motor controls the operation of contacts 2-4 and 1-3. They work opposite each other. When contacts 2-4 are closed, 1-3 are opened. When 2-4 are opened, 1-3 are closed. When the timer motor initiates a defrost cycle, contacts 2-4 will open and 1-3 will close. This will stop the compressor and evaporator fan motor, and energize the defrost heaters.

Terminating defrost cycles

There are three methods by which a defrost cycle can be terminated: temperature, pressure or time. These three methods commonly are referred to as time-temperature (defrost cycle initiated by time, terminated by a temperature switch), time-pressure (defrost cycle initiated by time, terminated by a pressure switch) and time-time (defrost cycle initiated by time, terminated by the defrost timer).

Most defrost timers also will have a fail-safe time, which you can set to terminate the defrost based on
time. On systems terminated by a pressure or temperature switch, if the termination switch fails, the defrost will terminate by time.

Set the fail-safe time long enough to allow the system to terminate by the temperature or pressure switch, and short enough to prevent the system from over defrosting or creating a hazardous condition by having the heaters energized all the time. Usually the fail-safe time is set to between 35 and 45 minutes.

The most popular method is terminating by temperature. A temperature control is used as the defrost termination switch. It is installed on the evaporator at a location where the design engineers feel that frost will leave the coil last.

At a specified temperature the defrost termination switch will close and energize the time-release solenoid, switching the system back into the refrigerating mode. The time-release solenoid is an electrical solenoid located in the defrost timer.

When the solenoid is energized it will mechanically switch the timer contacts; 2-4 will close and 1-3 will open. The temperature at which the defrost termination switch closes will vary from design to design. It is best
to check with the manufacturer of the system to determine their temperature setting. Some defrost termination switches will have their setting stamped onto the body of the device. One typical temperature cut-in used on many systems is 60 F.

You can terminate the defrost cycle with a pressure switch. In this method a pressure control is used as the defrost termination switch. It is connected to sense the pressure of the refrigerant in the evaporator. When it senses a pressure that will ensure that all ice is removed from the evaporator, it will close and terminate defrost.

Many times the pressure control and the defrost timer are combined in one unit. The pressure at which the pressure switch closes will depend on the type of refrigerant used in the system. Check with the manufacturer for their recommended pressure setting.

You can use the defrost timer to terminate defrost by time, although this method is not popular. The time required to defrost an evaporator will vary depending on how much frost has developed on the coil. The ambient humidity level and the usage of the walk-in will be a factor in how much frost develops on the coil.

If the walk-in is in a humid location with heavy usage, a heavy accumulation of frost will develop on the coil. If the walk-in is installed in a location where the humidity level is low and has very little usage, the frost developed on the coil will be less during the same time span.

The fan-cycling control normally is used to control the operation of the evaporator fan motor(s) after the termination of the defrost cycle. The fan-cycling control will not allow the evaporator fan to turn on until the evaporator's coil temperature has reached 20 F. This prevents water vapor from being blown off the evaporator coil
and re-freezing within the case.

Some manufacturers combine the fan-cycling control and the defrost termination switch into one assembly, using the same control for both functions. This usually
is done with a set of normally open and normally closed contacts.

A drain-line heater is used to prevent the defrost water from re-freezing in the drain line. It normally is wrapped around the drain line within the case. Some systems also will utilize a heater installed in the drain pan to prevent the water from re-freezing in the pan.

It is recommended that copper or stainless steel pipe be used on drain lines on walk-in freezers, as they tend to conduct heat more effectively than plastic pipe.

Over- or under-defrosted systems

Defrost systems can run into two problems: the system is over- or under-defrosted. Over-defrosting occurs when the defrost heaters remain energized too long, causing the box temperature to rise too high during defrost causing the product to melt and re-freeze. You can identify this problem by monitoring the box temperature during defrost or by examining the products in the case. Ice crystals forming on frozen products may be a sign that the product began to melt and re-froze.

Many products such as vegetables are frozen at the processing factory so the individual pieces will not freeze together. If these packages begin to defrost and then re-freeze, they will freeze solid.

Over-defrosting normally is caused by a malfunction in the defrost cycle termination process. With time-time defrost systems, either the defrost time is set too long or the time clock is defective. With time-pressure systems, either the terminating pressure control is set too high, the pressure control is defective or the solenoid coil in the defrost timer is incorrectly wired or defective. A malfunction with a time-temperature system usually will involve a temperature control set too high, a defective control or a defective solenoid in the defrost timer.

Systems that are under-defrosting will result in a frozen evaporator. The frost that normally develops on the evaporator coil will continue to build until the entire surface of the evaporator is covered with ice. A defective or incorrectly set time clock that does not initiate a defrost cycle can cause this. Or an open heater element or a defective defrost termination switch that continually terminates the defrost upon initiation of a new cycle can cause this.

A defective fan control also will cause a system problem. If the fan control is electrically open, the evaporator fans will not operate. This also will cause the evaporator to freeze. However, this is not an under-defrosting issue; it is the result of a lack of airflow.

A fan control that turns on the evaporator fans too soon will cause the water vapor to blow off the coil. It usually will re-freeze on the ceiling of the walk-in close to the evaporator. To verify that this is happening, monitor the temperature of the evaporator coil. The fan(s) should not cycle on until it reaches the temperature designed by the manufacturer (typically 20 F).

When an evaporator ices over, it should be defrosted. You can manually initiate a defrost cycle with a mechanical defrost timer by rotating the inner knob on the timer in a clockwise direction.

Many times, the technician needs to manually defrost the coil. Take extreme care when doing this. Do not use sharp objects to chip away the ice from the coil because this easily could cause a refrigerant leak.

Using water is the best method, but is not always practical since the water will need to be drained away. If you cannot easily use water, a heat gun usually works well. Do not rush when defrosting a coil. A careless mistake eventually could become a costly one.

Joe Marchese, CMS, is owner of Coldtronics in Pittsburgh