Optimizing Defrost Cycles

Aug. 1, 2008
by Kala Ikegami On most systems using electric heaters or hot gas defrost as the means of frost removal, the main objective of a standard defrost control

by Kala Ikegami

On most systems using electric heaters or hot gas defrost as the means of frost removal, the main objective of a standard defrost control is to keep frost accumulation on the system’s evaporator coil to a minimum.

When warm, moist air enters a refrigerated space and freezing on the coil, without any intervention the coil would become frosted over and unable to function properly. A standard schedule or forced defrost scheme works by shutting the evaporator off while defrost heaters are turned on. This activity is initiated by a timer and melts accumulated frost off the evaporator coil surfaces while warming the drain pan to allow the defrost condensate to exit down the drain line without refreezing in the pan.

Once the defrost cycle is terminated, there may still be some water on the coil. To avoid this from ending up on the product or floor, evaporator fans may need to remain off until this remaining water has refrozen on the coil.

A standard defrost control gives the user limited management over the system’s defrost cycles. The entire cycle is initiated based only on time and does not consider any variance in the load on the system. To determine the best defrost settings for the system, it’s critical to understand how the refrigerated space will be used. Take note of factors such as busy times, product loading, product conditions, and door activity. Observe the system operation in general. The best number of defrosts, best length of defrosts, and best fan delay after defrosts must be determined.

How Many Defrost Cycles Are Needed?
This number must be balanced as to minimize heat gain or space temperature variations while still meeting the needs of the system. Most applications find two to four defrosts cycles per day sufficient, while others with high product temperature pull-down loads, such as blast coolers and freezers, may need more.

Other considerations when determining number of cycles include loading schedules and periods of heavy use in which doors will be opening often allowing in warm, moist air to accumulate and freeze on the coil.

How Long Should a Defrost Cycle Last?
It’s important to keep the length of a defrost cycle to a minimum. The goal is to melt away any accumulated frost on the coil while minimizing heat gain to the space. It’s also important to keep it short enough to prevent steaming that causes water and ice droplets to form on the ceiling and possibly the floor of the refrigerated area. This can be a serious safety hazard for the end user.

Temperature-termination controls with a failsafe time backup can be used to better regulate the length of a defrost cycle. If set correctly, using a temperature-termination control provides significant energy savings to the end user.

How Much Evaporator Fan Delay is Needed?
The length of time between defrost heaters being turned off and evaporators fans being turned on should be sufficient enough to allow any remaining moisture to refreeze on the coil. It is very important to prevent water blow off for safety and product quality. Typically, fans are set to turn on after a completed defrost cycle once the coil temperature has dropped below freezing.

A typical defrost system does not take into consideration the actual amount of frost accumulation at the pre-scheduled defrost cycle time. Because of this, a system may go through unnecessary defrosts translating into wasted energy consumption, less consistent box temperatures and increased stress on the system. With recent technological advances, optimized defrost cycle controls are available. Using this new technology optimizes the system by better regulating defrost cycles and skipping the cycle automatically if it’s deemed unnecessary based on frost accumulation.

Kala Ikegami is a product manager for Heatcraft Refrigeration Products LLC, Stone Mountain, GA. Contact [email protected] or call 800/321-1881 for additional information.