Reading chiller performance: approach, head pressure and kW/TR
Three readings that tell you what a chiller is doing, and what a drift in each of them usually means.
A chiller that is running is not necessarily a chiller that is well. Three readings, logged consistently, will tell you more than any amount of inspection.
Approach temperature
The difference between the refrigerant saturation temperature and the water leaving that heat exchanger. It is a direct measure of how well the surface is transferring heat. A rising approach means the surface is becoming less effective — fouling, scale, or on the evaporator side sometimes a refrigerant charge issue. Approach is the single most useful trend to keep, because it moves long before anything trips.
Head pressure
Condensing pressure, and by extension condensing temperature. Above design it means heat is not leaving the machine fast enough. On air-cooled plant the usual cause is the coil: dust, debris, damaged fins, a failed fan, or air recirculating because the machine is boxed in. On water-cooled plant it points at condenser water flow or temperature, which points at the tower.
kW per tonne of refrigeration
Electrical input divided by cooling output. This is the number that shows up on the electricity bill. It varies with load and ambient, so a single figure means little; the trend at comparable conditions means a great deal. A machine drifting from its commissioned kW/TR is costing money every hour it runs, without any fault being present.
Why the trend matters more than the value
Design figures vary by machine, refrigerant and application, so an absolute reading is hard to judge. The same machine compared against its own commissioning record is unambiguous. This is the practical reason to insist on readings in every service report: without them there is no baseline, and every diagnosis afterwards starts from nothing.