Head-pressure control. Condenser fans taken off on a cool day to hold a minimum condensing temperature — a constraint an expansion valve needs, not an economy. Head-pressure control.
On a cold day an air-cooled condenser rejects heat too well: the condensing pressure falls, the expansion valve loses the pressure difference it meters on, and the circuit starves. Head-pressure control holds the condensing temperature at a floor by taking condenser air away — fan cycling, a fan bank staged down, or a speed-controlled fan — the least air that keeps the head where the valve needs it.
It is a constraint, not an economy: at full output the staged point draws more than the free-floating one, because a higher condensing temperature costs compressor power. HVAC Builder settles the head before a stage is judged, walking the fan bank's ladder down until the running condensing temperature reaches the setpoint, and re-settles it at the part load the machine actually runs at.
Open the builder — the examples in the Design tab's sources list include a machine of this kind, and the Simulate tab runs it against a year of weather.
DOAS — dedicated outside-air system · SZVAV — single-zone variable air volume · VAV terminal box · Economiser · Waterside economiser · Demand-controlled ventilation · Face-and-bypass · Humidity ceiling and floor · Cycling and modulation · Coupling · Priority · Category · Nested system · Design point and TMY year