Defrost technology manufacturers realize that food quality is of utmost importance when designing energy efficient controls, says John Wallace, director of product management for Emerson Climate Technologies, Retail Solutions Group.
According to Wallace, the FDA has specified that 41F is the safest maximum temperature for preserving non-frozen food items in refrigerated cases.
“The longer a product is above 41F, the more bacterial growth is found in the food,” Wallace says. “Therefore, you want to keep the product temperature low. Yet, you still have to perform the defrost to clear the coil, which helps eliminate mechanical problems. That's the tradeoff.”
Temperature terminations during the defrost cycle must be monitored with care due to temperature spikes caused by evaporator shutdown.
“If you graph evaporator coil temperature during defrost, while the ice is melting you see relatively constant temperature on the coil. But once the ice has melted off, there's a pronounced need for lower temperature on the coil,” Wallace explains.
To track coil temperatures, a defrost termination sensor can be mounted on the evaporator coil, and wrapped around it to ensure good surface contact. As ice forms on the coil, it also forms on the temperature sensor, which enables the service technician to monitor the coil temperature. When ice is present, the temperature is quite pronounced on the low end of the scale. As the ice melts off the coil, the temperature rises quickly. The trick is catching where the transition occurs, and terminating the defrost at that point.
To avoid extended periods of defrost, most newer controllers have been engineered with a maximum setting for defrost duration, after which there's a time-based termination.
To lower their electricity costs, many managers of supermarkets or food storage sites are opting for off-peak defrost schedules or defrosts that are based on real time.
“For example, some of the older technologies would run a defrost ‘every six hours,’ Wallace explains. “The newer technologies are staggering the defrost to minimize the peak loading of electric defrost. Or, they're shifting some defrost into off hours, when electricity rates are much lower. That's another key trend going forward.” emersonclimate.com
Tony Powell, regional account sales manager for the appliance/thermostat division of Danfoss, Baltimore, MD, agrees that defrost technology is at the forefront as an energy saver.
“What used to be very much an afterthought to keep the cabinet operating, is now an energy-saving feature. Both manufacturers and their customers are interested in improved defrost technology,” Powell says.
Danfoss provides “all-in-one” controls that manage not only defrost, but other refrigerated case operations.
“What used to be a mechanical defrost timer, accompanied by various temperature limit switches can be combined into one electronic control. We can do it more cost effectively as well. You also eliminate wiring and other potential issues that can come with multiple components,” Powell explains.
One of Danfoss's recent control innovations is the ETC1H. It was designed as something of a replacement to the mechanical thermostat, but given the flexibility provided by advanced electronics, Danfoss designers built in a defrost functionality and the ability to control the evaporator fan, defrost heater, or solenoid.
“The main relay on the ETC1H is capable of zero cross-switching,” Powell explains. “It can sense when the AC voltage crosses zero volts, and will change over the compressor relay at zero volts every time. This prevents arcing, and provides longer life to the control.”
Danfoss is also exploring more precise real-time defrost methods.
“We have several modes of defrost on our controls, depending on how advanced the customer wants it to be. Even in the most basic mode, which is a time-initiated defrost, it still counts only compressor run time, and not total time,” Powell explains.
“That gives you a much more accurate basis on which to initiate the defrost. If the cabinet is so lightly loaded that it's only running 5% of the time, it's not going to ice up very quickly. Whereas, if it's running 100% of the time, it could very well ice up quickly. Even with the simplest of timed defrosts, real-time offers a distinct advantage over a strict defrost that occurs every X number of hours.” danfoss.com
Improved defrost technologies are helping to eliminate guesswork, and enhance the professionalism of refrigeration technicians. Energy savings is improved, food quality is protected, and everyone saves time and money.
Sophistication in defrost technology is going a long way toward helping refrigeration contractors provide extra savings to a market sector that's always feeling the pinch of traditionally slim profit margins.
About the Author
Terry McIver
Content Director - CB
As director of content for Contracting Business, he produces daily content and feature articles for CB's 38,000 print subscribers and many more Internet visitors. He has written hundreds, if not two or three, pieces of news, features and contractor profile articles for CB's audience of quality HVACR contractors. He can also be found covering HVACR industry events or visiting with manufacturers and contractors. He also has significant experience in trade show planning.