This article compares three schemes of fan control used on large air conditioning and refrigeration condensers. The first scheme is step control; the second is a hybrid with a VFD on only one bank of fans, while the rest remain step controlled. The final, and most efficient, scheme is VFD control of all fans, at all times.

A basic HVAC system shows the application of the Second Law of Thermodynamics. The compressor adds energy to the refrigerant, and it becomes hot, just the way a hand operated tire pump does. Since the compressed refrigerant is hotter than the air blowing across the condenser, the heat will flow to the cooler air. This is a transfer of heat energy out of the refrigerant.

Thermodynamics, as with most concepts in science and physics, is most accurately described using mathematical equations. However, simple explanations, although not perfectly accurate, are sufficient to convey the information needed to understand everyday experiences. This article is an attempt to simplify these concepts.