Evaporator coil frosting is a consequence of the condensation of humid air inside the fridge, cooler or cold room due to the expulsion of cold air through the evaporator. This condensation of water on the evaporator surface occurs when the refrigerated display reaches the dew temperature. If the evaporator surface is above zero degrees, dripping water is generated as we see in air conditioning; but if the temperature is below zero, then ice in the cooling coil is generated.
From the moment the cooler starts and once the temperature inside reaches the dew temperature, two types of power are generated:
- Sensible power or heat; this is the power that lowers the temperature between the inlet and outlet of the evaporator.
- Latent heat or power, which precipitates the relative humidity in the gas to liquid and solid phase on the surface of the evaporator fins.
The ice on the cooling coil slows down the heat transfer from the cooler space to the refrigerant gas so that the evaporating temperature of the refrigerant gas becomes lower and lower. Consequently, it becomes more and more difficult to absorb the heat from the display, and therefore the cooling capacity is progressively reduced, so regular and periodic defrosting is necessary to prevent the formation of this frost.
Defrost evaporator coil
The most common way of defrosting is to stop the refrigerated display for 15 to 30 minutes whenever the working temperature of it is above zero degrees Celsius. This stop can be manual or automatic with a timer and is sufficient time for the ice to melt but insufficient to break the cold chain.
Regular defrosting can also be carried out by installing an electrical resistance through the fins of the coil. This resistor must provide sufficient watts of energy to melt the ice layer that can be expected to form so that when configuring the evaporator, it is necessary to calculate the power of the resistor based on the delta T and temperature of the refrigerant with which the evaporator is going to work.
Another way is hot gas defrosting, i.e. applying heat from inside the evaporator tube to the outside, i.e. making the evaporator act as a condenser.
If you see an evaporator with ice on it, you should be wary of the quality of the food.
Static evaporator configuration to avoid frost: Fin separation.
In addition to the installation of a heating element across the fins of the heat exchanger, it is necessary that the fin spacing of the heat exchanger is sufficient to prevent a thin layer of ice from blocking the passage of air, as this would cause the refrigeration system to collapse. At Sereva we recommend a minimum fin spacing of 5 mm to work at negative temperatures.