The condenser is selected for the hottest summer day. When the outside air cools, the same condenser rejects more heat than necessary and the condensing pressure falls. Up to a point this is good; the compressor draws less electricity. But if the pressure falls too far, the pressure difference across the expansion valve is not enough, the evaporator is starved and capacity falls. Fan speed control keeps the condensing pressure above the set lower limit by reducing the air flow.
Four methods are used. Step control: the fans are switched on and off in sequence by pressure switches or the controller; simple, but the pressure fluctuates. Phase-cut (triac) control: lowers the speed of single-phase fans by chopping the voltage; it is cheap, but at low speed the motor heats up and hums. Frequency drive: drives all the three-phase fans together. EC fan: stepless with a 0–10 V signal and the most efficient solution. The choice is made according to the fan type, the size of the condenser and the budget.
The control works from a signal taken from a pressure sensor in the discharge line or from the liquid line temperature. If, instead of a fixed condensing temperature setting (for example 40 °C), a setting that shifts with the outside air temperature is used, the condensing temperature is tracked 8–12 K above the air and compressor consumption falls; every 1 K reduction in condensing temperature cuts compressor consumption by about 2–3%. This is called floating condensing. The lower limit is set by the pressure difference the expansion valve needs.
In the evaporator, speed control is done for different purposes: slowing the fan when the compressor has stopped reduces the fan heat entering the room; in fruit and vegetable rooms low speed limits the product’s water loss; in processing areas where people work it lowers the draught and the noise. Note: when the fan speed falls the evaporator capacity falls too, the evaporating temperature drops and frosting may increase. In places with a night-time noise limit a separate night speed limit is defined for the condenser fans. The setting is found by trial, according to the needs of the product and the room.