Frosting
Also known as: frost formation, frost build-up
Short definitionMoisture building up as thin frost on evaporator fins and cold pipes that are below zero. A little is in the nature of the job; when it grows, capacity drops and a fault is looked for behind it.
In a room whose door is opened often, the evaporator is seen to turn white quickly. The cause is the moisture in the air that gets in: when this air touches a surface below both zero and its own dew point, its moisture becomes frost, and the coldest surface in the room is the fins. The frost layer cuts heat transfer, blocks the gaps between the fins and reduces airflow. Thick frost can take 20–30% off the evaporator’s output. Worn gaskets, wet unwrapped product and goods loaded warm increase the frost.
There are two settings in design: fin spacing chosen for the room temperature, and the number of defrosts. In most rooms three or four defrosts a day are enough. Strip curtains, air curtains and a cooled loading area reduce the moisture getting in. Where the frost collects is also informative. If only the inlet tubes are white, the evaporator is starved; the charge is short or the valve is throttled. If the suction line is frosted all the way to the compressor, superheat is low and there is a risk of liquid returning.