The fan motor moves the air the heat exchanger needs in order to work. An evaporator or condenser with no air passing through it delivers only a small part of its capacity, however large its surface. Axial (propeller) fans are the most widely used in refrigeration; the motor is mostly the external rotor type built into the hub of the impeller. In this way the motor sits in the air stream, cools itself, and the unit fits into a short, compact casing.
Diameters range from 200–350 mm in small evaporators to 400–910 mm in industrial evaporators and condensers. Power varies from a few tens of watts to 2–3 kW; small diameters run on single-phase 230 V, large ones on three-phase 400 V. Three types of motor are found: low-efficiency shaded-pole motors (in small cabinets), capacitor-run or three-phase asynchronous motors, and EC motors. The protection class is usually IP54; for frozen storage, grease and bearings suitable for −40 °C are required.
In selection the air flow alone is not enough; what counts is the flow the fan delivers against the resistance of the fins and of any duct. As the fin spacing narrows and the number of tube rows increases the resistance grows, and the same fan delivers less air. All the electricity drawn by the evaporator fan returns to the room as heat; four 150 W fans running twenty-four hours a day mean 14 kWh of extra load per day. The cooling load calculator takes this share into account.
The faults seen most often on site are a weakening capacitor (the fan turns slowly and runs hot), bearing noise and, in frozen rooms, the impeller rubbing on ice. The direction of rotation and the direction of air throw of a replaced fan are always checked; a three-phase fan running backwards appears to deliver air but its flow falls sharply. In our catalog the fan diameter, number, air flow, current and sound level are stated for every evaporator and condenser model. A fan whose bearings have started to make noise is replaced without waiting; a seized motor burns out its winding.