Chiller Capacity Explained: kW, Tons of Refrigeration and Water Flow
When you ask suppliers for a chiller, the first thing they ask is "how many tons?" or "how many kilowatts?". People use both, and mixing them up is a common source of error. This guide explains the two units, how to calculate the water flow that goes with a given capacity, and why a bigger chiller is not automatically a better one.
Kilowatts and tons of refrigeration
Cooling capacity is the rate at which the chiller removes heat. It is measured in kilowatts (kW) in most of the world and in tons of refrigeration (TR) in many HVAC catalogues. One ton of refrigeration equals about 3.517 kW, or 12,000 BTU per hour. To convert:
- kW to TR: divide by 3.517.
- TR to kW: multiply by 3.517.
Example: a 350 kW chiller is 350 ÷ 3.517 ≈ 99.5 TR, so about 100 tons.
Chilled-water flow
A water-cooled-coil system moves heat with water. The flow needed depends on the capacity and on the temperature difference between the water leaving and returning to the chiller (ΔT). For water, the flow in litres per second is approximately:
flow (L/s) = capacity (kW) ÷ (4.186 × ΔT in °C)
Example, with a 350 kW chiller and a design ΔT of 5 °C (for instance 7 °C supply and 12 °C return, a very common design point): 350 ÷ (4.186 × 5) ≈ 16.7 L/s, which is about 60 m³/h. If the design uses ΔT of 6 °C instead, the flow drops to about 13.9 L/s, or 50 m³/h. A larger ΔT means a smaller flow, which allows smaller pipes and pumps, but the coils and controls must be selected for it.
Capacity depends on the conditions
A chiller's rated capacity applies only at stated conditions: the chilled-water temperatures and, for an air-cooled chiller, the outdoor air temperature, or for a water-cooled chiller, the condenser-water temperature. When the outdoor temperature is higher than the rating point, the capacity of an air-cooled chiller drops. In a hot climate, always ask for the capacity at your real design conditions rather than the catalogue number. See our comparison of air-cooled and water-cooled chillers.
Why oversizing is a mistake
It is tempting to add a large safety margin. But a chiller that is much too large for the load switches on and off frequently, which wastes energy and wears the equipment, and it may control humidity poorly. Instead, size the chiller to a calculated load, add a reasonable allowance, and use several smaller chillers or a unit with good part-load control if the load varies a lot. If the system must keep running when one unit fails, specify redundancy, such as two chillers each sized for part of the load.
Part-load efficiency
Most buildings run at part load for most of the year, so the efficiency at part load can matter more than the full-load figure. Ask suppliers for part-load efficiency, not only the full-load rating.
What to send with your enquiry
Capacity in kW or TR and how it was calculated, the chilled-water temperatures, the outdoor design temperature, the electrical supply, space and noise limits, and the scope of supply. Our quote information checklist lists all of it.
Summary
Remember the three numbers: 1 TR is about 3.517 kW, flow in L/s is kW ÷ (4.186 × ΔT), and capacity always depends on conditions. Then size for the calculated load, not the largest number available. You can use the HVAC engineering advisor for a first estimate, see our chiller systems, or request a quote. For hall projects, read HVAC for cinemas and auditoriums.
