The Professor: Defrost Termination/Fan Delay Controls

Figure 1. Defrost termination/fan delay control with remote sensing bulb. (Courtesy Ranco Controls Division.)

The defrost termination/fan delay control is a temperature-activated, single pole-double throw switch controlled with a remote sensing bulb (Fig. 1). The control can be an adjustable type. One example of the installation of an adjustable defrost termination/fan delay control is on a walk-in freezer’s evaporator (Fig. 2).

The control is wired into the refrigeration circuit. The control’s remote sensing bulb is located high on the evaporator where the frost is likely to clear last. The function of this temperature-activated switch is to terminate defrost when the evaporator coil has been defrosted, and to delay the evaporator fans from coming on immediately after defrost.

Defrost time clocks can be programmed for certain defrost duration periods. This is a time duration set at the time clock in minute increments. For example, a defrost time clock on a freezer could be programmed to defrost every six hours (four times daily), and have defrost durations of 40 minutes. However, there will be times throughout the year where the coil does not need the entire 40 minutes. These times could be from low usage of the freezer where the door openings are minimal, or when the humidity is low and not much frost accumulates on the coil. This is where the defrost termination part of the control comes into play.


Let’s assume that the system does not have a defrost termination/fan delay control.

Once the normally open (NO) contacts of the defrost time control have closed and the unit is in defrost, the defrost heaters will be emitting heat, and frost will be melting off the evaporator coil. Let’s say that it only took 10 minutes for all of the frost to leave the evaporator coil. However, there are still 30 minutes (40 minutes minus 10 minutes) left in the programmed defrost duration increment.

If the system has an optional heater safety switch - or sometimes referred to as a defrost limit control - in series with the defrost heater, this limit switch will open and take the defrost heaters out of the active circuit. However, the system’s defrost timer will still have 30 minutes left in the defrost mode. The system will simply sit idle, and the product load will suffer in temperature because of no refrigeration for 30 minutes.

After 30 minutes, the defrost timer will switch over to refrigeration and the fans will start immediately. The fans will blow the moist residual defrost heat through the refrigerated space and through the evaporator coil while the system is in refrigeration. This puts the system, thus the compressor, under an extremely high load from high suction pressures.

The compressor will see high suction pressures with dense vapors coming to its cylinders. This will cause high amperage draws and may overload the compressor to a point where its internal or external overload may open.

To prevent this long defrost period and the compressor overloading after defrost, a defrost termination/fan delay switch can be wired into the system. Now, once the normally open contacts of the defrost timer control have closed and the unit is in defrost, the defrost heaters will be emitting heat, and frost and ice will be melting off of the coil.

If it only takes 10 minutes for the ice and frost to leave the coil, the remote bulb of the defrost termination control will sense the defrost heat and contacts between 2 and 3 will be made on the control. This will energize a defrost termination solenoid (release solenoid) in the time clock which will mechanically put the system back into refrigeration. It does this by solenoid action and levers, and will mechanically close the normally closed (NC) contacts and open the normally open contacts of the defrost time control.

This action by the defrost termination solenoid (release solenoid) prevents the system from sitting idle for 30 minutes in defrost with the heaters off. This action actually ends the defrost mode and puts the refrigeration mode back into service.

Note: Often, if the system is microprocessor-controlled, when contacts 2 and 3 close on the defrost termination control, it will be a digital input to the microprocessor and defrost will end.

Figure 2. Control on walk-in freezer’s evaporator.


Now that the system is in refrigeration, the evaporator fans will be delayed from coming on. This happens until the contacts between 2 and 1 of the defrost termination/fan control close. This usually happens at about (20 to 30°F), and is sensed and controlled by the control’s remote bulb. This is an adjustable setting on most controls. This lets the evaporator coil pre-chill itself and get rid of some of the defrost heat still in the coil. Delaying the fans prevents the suction pressure from getting too high after defrost and overloading the compressor from high amp draws. It also prevents warm, moist air from being blown on the product load in the refrigerated space.

Publication Date: 04/06/2009

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