The air entering a cold room carries moisture. When this air touches a surface whose temperature is below both 0 °C and its own dew point, the moisture turns directly into hoar frost. Because the evaporator fins are the coldest surface in the room, the frost collects there. Every evaporator running below 0 °C frosts up; this is not a fault, it is the nature of the job. This is exactly why defrost exists, and why the fin spacing is chosen according to the temperature of the room.
The harm done by frost is twofold: with its porous structure it acts like an insulator and reduces heat transfer, and it narrows the gaps between the fins and lowers the air flow. As a result the evaporating temperature and suction pressure fall and the compressor runs longer. An evaporator carrying thick frost can lose 20–30% of its capacity. How much frost builds up is determined by the moisture entering the room: the frequency of door openings, the door gaskets, unpackaged and wet product, goods loaded warm. Consumption rises and the room temperature starts to fluctuate.
The distribution of the frost is valuable for diagnosis. On a healthy evaporator the frost spreads evenly over the whole surface. If only the inlet tubes and the area around the expansion valve frost up while the rest stays bare, this shows that the evaporator is starved of refrigerant: low charge, a blocked filter drier or a throttled valve. Frosting of the suction line all the way to the compressor, on the other hand, suggests the opposite, that the superheat is very low and there is a danger of liquid returning to the compressor; no decision is made without measuring the superheat.
The ways of reducing frosting are well known: an air curtain or PVC strip curtain at the door, replacement of worn gaskets, shortening the time the door stays open, cooling the loading area. The number of defrosts is set according to the amount of frost; in most rooms 3–4 defrosts a day are enough. If the frost closes off the fins between defrosts, or if residue is left after every defrost, the situation is turning from frosting into icing. In our catalog the evaporators are divided into series by fin spacing. When choosing the spacing suitable for your room, take the room temperature as the basis.