For many operators and designers of low temperature refrigeration systems, defrosting an evaporator may be somewhat of an afterthought. But the truth is, the design of a defrost system can have a tremendous impact on the energy consumption at your facility. Each time you defrost, you not only lose the cooling capacity of that evaporator, but you also introduce heat into the space.
When an evaporator coil operates at freezing temperatures, and moisture is present in the air, frost will accumulate on the coil surface. This frost has a detrimental effect on the refrigeration performance, both impeding the airflow through the coil and reducing heat transfer between the air and the refrigerant, thus reducing cooling capacity. These effects tax the refrigeration system, demanding greater energy consumption. As the frost layer grows, so does the refrigeration demand because despite a steady flow of refrigerant, the reduced capacity of the evaporators can allow the room to warm. The frost is cleared by one of (or a combination of) several defrost methods. Several factors should be considered when deciding on a method or methods.
Defrost Cycle Time
Many facilities initiate defrost at a specific time interval. Ideally, these cycles are defined and adjusted based on observed need, but sometimes they are defined by “rules of thumb.” The length of the defrost cycle is a factor that works best when finely tuned so refrigeration begins again as soon as the frost has cleared. Demand defrost systems can optimize your defrost cycles using infrared technology to monitor frost accumulation, provide defrost only when necessary, and shorten the defrost cycle to only what is necessary. Savings associated with demand defrost can be upwards of 80% of the total defrost energy cost, but this varies depending on your baseline of comparison.
Pressure Regulators and Float Valves
Hot gas defrost (HGD) systems introduce high stage discharge refrigerant (compressor discharge) to the evaporator. Compressing this gas to the high defrost temperature is costly and compressing this gas to high pressure for refrigeration does not make it free for defrost. Typically, this hot gas—which is controlled by a pressure regulator—is blown through the evaporator warming the coil so the ice and frost can melt off. But utilizing a float valve greatly reduces the amount of gas that is used as it only lets condensed liquid drain out, as well as venting a small amount of bleed gas. Incorporating a float valve into your defrost system requires certain design considerations to provide proper function, such as a bleed for flash gas on your liquid drain. But it can pay off. One study showed savings associated with float valves to be around 15% of the overall defrost cost.
Other Defrost Methods
Electric resistance can be used to defrost your evaporator coils, but this is generally expensive from an energy standpoint. The energy cost of electrical resistance increases with colder spaces. Some defrost systems utilize water sprayed on the coil surface to induce defrost. Water defrost can be attractive, in that the low cost of water compared to hot gas can produce energy cost savings and the external application of water can greatly reduce defrost time; however, these systems can be complicated and expensive to incorporate. Some applications, particularly in food processing or where hot gas is not available, require the use of water defrost or a combination of defrosting methods.
Defrosting your low temperature evaporators is an important design consideration and has significant impact on refrigeration performance and energy use. A little bit of enhanced design can be well worth the time and effort, leading to great savings and improve your environmental impact.