In finned-tube heat exchangers the thin aluminium fins arranged along the tubes enlarge the heat transfer surface. The distance between them is the fin spacing. Close fins (3–4 mm) give more surface and more capacity in the same volume; wide fins (6–12 mm) give less surface but more room for frost build-up and easier air passage.
The choice is made by room temperature. In rooms above 0 °C (fruit and vegetables, delicatessen, dairy) water condenses on the evaporator surface but there is little icing; 4 mm fins are used. In rooms between 0 °C and −20 °C ice forms, so 6 mm is used to keep the fins from blocking between defrosts; in blast freezing and −30 °C rooms 8–12 mm fins are chosen for heavy frost and high air flow. An evaporator with the wrong (close) fin spacing in a frozen room ices up within a few hours and loses its capacity.
In condensers the fin spacing is 2–2.5 mm; since there is no icing, closer fins add capacity, but they need regular cleaning against dust and fluff blockage. In our catalog the evaporator models are divided into series by fin spacing; the 4, 6 and 8 mm versions of the same body give different capacities.