Evaporators and their operating principles
Evaporators vaporise and absorb
Evaporators vaporise the refrigerant and absorb heat from the environment. Natural circulation evaporators are used in cold rooms and warehouses. Forced circulation evaporators are used in cold stores and freezer spaces.
The operation of natural circulation evaporators
The operation is based on gravity. The air cooled in the evaporator, which is colder and denser than the ambient air, flows downwards, circulating the air in the space being cooled.
The evaporator block is made of copper pipes and aluminium fins at a spacing of 7–10 mm. This ensures sufficiently low flow resistance. A drain pan is located under the evaporator block. The pan is made of slats to ensure air flow through it. Ice formation on the evaporator surface is eliminated with air or electric defrosting.
Air defrosting is used in cold rooms where the temperature is > 2°C. When the cooling is stopped, the surrounding air melts the ice. Electric defrosting involves resistors installed in the evaporator block, which melt any accumulated ice in the block as necessary. This method is used in colder temperatures or in spaces with a high moisture load.
The operation of forced circulation evaporators
The operation of these evaporators is also based on gravity, but the fin spacing is 4–10 mm. Wider spacings are used if the moisture load in the room is high. In freezer rooms, the fin spacing is wider, 7–12 mm. This prevents excessive drops in air flow and cooling power. In cold rooms, forced circulation evaporators are usually defrosted with air, which enables the fans to be kept on to shorten the defrosting time.
Electric defrosting of freezer and cold rooms
Electric and hot gas defrosting are used for fast defrosting. The evaporator fans are not active during the defrosting process.
For defrosting freezer rooms, resistors are also installed in the drain pan and drain pipe. This prevents the water that drains from the evaporator from freezing so it can flow into the sewer system.
In hot gas defrosting, the evaporator block and drain pan are heated with the hot gas of the compressor.
Plate heat exchanger as an evaporator
Plate heat exchangers are often used as evaporators in indirect refrigeration and cooling systems. The evaporator consists of ribbed plates that have been soldered together with copper or nickel. Their content alternates between refrigerant and the fluid to be cooled. Evaporators of this kind are much smaller than multi-tube evaporators of equal power, both in terms of external dimensions and internal volume.
The refrigerant is led into the heat exchanger from the bottom, and the vapour exits through the top.
Correspondingly, the fluid to be cooled enters through the top section and exits through the bottom after cooling. The refrigerant side of the evaporator can include one or two circuits.
In multi-tube evaporators, the refrigerant flows in tubes located inside the casing. The water, in turn, flows inside the casing and is led into it through the couplings in the sides or top of the evaporator. The refrigerant is led into the evaporator through the couplings in the bottom section of the evaporator’s end. The steam is expelled through the top section of the evaporator’s end.
The refrigerant side of the evaporator can be divided into 1–4 circuits. The casing also contains spacer plates that support the pipes inside the evaporator and ensure that the water flows and twists inside the casing.
See also: Cold compressors
Sources: Kaappola, Hirveä, Jokela, Kianta 2014, Tytti Rentola, Pakastevaraston lauhdelämmön talteenotto ja lämpöpumpun valinta, Thesis 2014, JAMK University of Applied Sciences, Degree Programme in Energy Technology, Field of Technology and Transport.