The question we receive most often runs like this: "I have this many machines — how many horsepower of chiller do I need?" It is a reasonable question, but it cannot be answered directly. Horsepower rates compressor power, whereas what has to be established first is how much heat your process needs to reject.
Start from the heat rejected, not from the unit
Cooling load in process applications is generally calculated from three things: the mass or volume of water circulated, the temperature difference between water leaving the machine and water returned by the chiller, and how long the process runs. With those three figures, the required capacity can be calculated rather than guessed.
As a rough illustration: water has a specific heat of about 4.2 kJ per kilogram per degree Celsius. Lowering the temperature of 1,000 litres of water by 5 °C therefore requires rejecting roughly 21,000 kJ. Divide by the time available and you have the load in kilowatts.
What calculations commonly miss
- Heat entering from the surroundings through uninsulated pipework.
- Heat from the circulation pump itself, all of which ends up in the water.
- Peak load when every machine runs at once, not the daily average.
- Ambient air temperature around the condenser — air cooled chillers lose significant capacity in a hot room.
That last point is often behind the complaint "the chiller is new but it is not cooling". Nothing is wrong with the unit; it has simply been installed in an enclosed space with nowhere to reject heat, so the condenser cannot shed heat anywhere.
Do not oversize by too much
There is a habit of specifying capacity well above the calculation "to be safe". The problem is that an oversized chiller reaches temperature too quickly and stops, then restarts minutes later. Short-cycling shortens compressor life and raises electricity consumption rather than lowering it. A sensible margin beats a generous one.
In short
Before requesting a chiller quotation, have ready: the type and number of machines being cooled, the water temperature required, the flow rate the machines demand, the approximate pipe run length, and the conditions of the space where the unit will sit. With those five, the calculation can begin and the quotation has a basis you can trace.