Refrigeration Defrost
This article is intended to introduce the defrosting methods in refrigeration equipment. More specifically in freezers. The majority of all freezers have a defrost cycle of some sort. In general the sequence of operation is the same, although components differ from unit to unit in many cases. We will explore the defrost cycle and the why it is necessary for freezers to have them. By the end of reading this you should a basic understanding of why defrost cycles are necessary, the primary methods of defrosting, as well as how they are initiated and how they are terminated.
Understanding Refrigeration Defrost Cycles – Why it’s Needed
The basic principle of refrigeration is to remove heat from the desired space or product, then get rid of the absorbed heat outside of the refrigerated area. The component responsible for absorbing the heat from the refrigerated area is called the evaporator coil. This component is nothing more than multiple loops of copper and aluminum with a fan motor to move air through it. The idea is to allow low temperature refrigerant, generally in the negative degrees, to absorb the heat from the air traveling through the evaporator coil. Along with air comes humidity, which is evaporated moisture in the air. This evaporated moisture will condense and collect on the cold surfaces of the coil, just like a cold soda can sitting outside (sweating). These water droplets will freeze on the evaporator. The continuous freezing of moisture will eventually lead to a build up of ice on the evaporator. As this ice begins building it will create a “blanket” covering the surface of the evaporator coil. This “blanket” of ice leads to the restriction of air flow allowed through the coil. The bottom line is this, if airflow cannot pass through the coil, the coil cannot absorb heat. Your refrigerated space and products begin to suffer. The ice has to be removed to regain efficient heat absorption. This is why defrost is needed. One last thing, coolers are not completely except, at least those that are operating near the freezing point of water. These units should also incorporate a defrost cycle of some sort to keep the coil clear of ice.
Defrosting Methods in Refrigeration – Hot Gas Defrost vs. Electric
Electric
The most common defrost method is using electric heating elements that are embedded into the evaporator coil. They are generally embedded lengthwise, along the back side, from top to bottom. There is also one underneath the coil in the drain pan, when equipped. When the electric heating elements are energized they heat up the entire evaporator coil melting any ice.
Hot Gas
The second method is using “hot gas.” In this method the hot discharge vapor leaving the compressor is routed into the evaporator coil(s). The discharge vapor is routed and enters into the evaporator using a solenoid coil and refrigerant valve. Once the hot discharge vapor enters the coil it heats up the entire evaporator coil, thus melting any ice accumulation. Use of this method requires more refrigerant piping and components, making it a more expensive to manufacturing, which means the consumer pays more.
Comparing the two defrost methods
“Hot gas” defrost is much faster and more efficient simply because the “hot gas” is heating the evaporator coil from the inside. Electric heating elements are only able to radiate heat from the outside of the coil and this process takes more time to reach all areas of ice buildup. Although these elements take a little longer, they still get the job done and are less expensive in manufacturing. Since hot gas in faster in removing the ice, it uses less electricity. The average length of an electric defrost is approximately 30 minutes. The average length of the hot gas method is approximately 10 minutes.
Commercial Freezer Defrost Cycle
The sequence of operation is fairly standard across most manufactures. With the advancement of technology more equipment are coming equipped with controllers. However, even with controllers the sequence is comparable to the previous technologies or methods. Having said this, we will discuss using the general sequence we see most.
A defrost cycle is started by using defrost timer. We find many defrost cycles occur four times per 24 hour period. There can more and sometimes less depending on the humidity and use of the equipment. These occurrences are typically spread apart every 6 hours. For example noon, 6pm, midnight, and 6am. Once the defrost time is reached, the defrost will start. At this point the defrost heating elements will be energized and begin melting any ice on the evaporator coil. At the same time the evaporator fan motors and the condensing unit will be de-energized and stop running. Once the ice on the coil is melted, the temperature of the coil will rise more rapidly, since there is no longer any ice to keep the temperature down. A defrost temperature control will then detect when the temperature of the coil is at the point where there cannot possibly be ice remaining (hopefully), let’s say 55*F (for example). Once this temperature is reached the heating elements will de-energized. If for any reason that temperature is not reached, defrost will end via time. This adds an additional (back-up) method of terminating defrost which is also good in case the defrost temperature control fails. At the same time the evaporator fan motors and condensing unit circuit will be re-energized. The refrigeration will resume as normal. Finally, once the evaporator temperature falls to a predetermined temperature (below freezing) the evaporator fan motors will resume their normal operation. Below is a summary of the sequence.
- Time initiates defrost cycle.
- Heating elements are supplied power and begin melting ice.
- Refrigeration system is turned off.
- Evaporator fans are turned off.
- Temperature or time terminates defrost cycle.
- Refrigeration system resumes normal operation.
- Evaporator fan motors resume, once coil temperature is below freezing.
Conclusion
Hopefully the article has provided you with an idea of why defrost cycles are needed and the general sequence of operation. As the site expands its articles you will find more detailed information on components and troubleshooting.
The videos that may give you further details and a better visual understanding of components.