Calculate the cooling capacity of a water or glycol circuit from the flow rate and the temperature difference, or the flow needed for a given capacity, with fluid properties at the actual temperature.
Cooling capacity from flow and temperature difference
Fluid properties at the mean temperature, from CoolProp. Glycol concentrations are by volume and approximate.
The formula
The capacity carried by a water or glycol circuit follows from a heat balance:
Q = V x rho x cp x dT
Q is the capacity in kW, V the volume flow in m³/s, rho the density in kg/m³, cp the specific heat in kJ/(kg K) and dT the difference between return and supply temperature in kelvin. For water at around 10 °C, rho x cp is about 4,190 kJ/(m³ K). This gives the field version that many technicians know by heart: capacity in kW equals flow in m³/h times dT times 1.163. A chiller delivering 17.2 m³/h of water with 7/12 °C works at about 100 kW.
The calculator uses density and specific heat at the mean fluid temperature, so it also works for glycol mixtures and for warmer process circuits, where the simple factor 1.163 no longer applies.
How to use the calculator
- Choose whether you want the capacity from a known flow, or the flow needed for a given capacity.
- Select the fluid. For glycol, select the concentration by volume as measured with a refractometer.
- Enter supply and return temperatures. On a chiller, supply is the cold water leaving the unit and return is the warm water coming back.
- Enter the flow in m³/h, l/s or l/min, or the required capacity in kW.
Glycol lowers the capacity per m³
Glycol mixtures have a lower specific heat than water. A 30 % ethylene glycol mixture carries roughly 9 % less heat per cubic metre and kelvin than water, a 40 % mixture about 13 % less. In practice this means a higher flow for the same capacity, a higher pressure drop because the mixture is more viscous, and a lower heat transfer coefficient in the evaporator. A chiller selected for water loses several per cent of capacity when it is filled with glycol, which must be checked when a system is converted.
| Fluid at 10 °C | Density (kg/m³) | Specific heat (kJ/(kg K)) | Freezing point (°C) |
|---|---|---|---|
| Water | 1000 | 4.19 | 0 |
| Ethylene glycol 20 % | 1030 | 3.84 | -9 |
| Ethylene glycol 30 % | 1045 | 3.64 | -16 |
| Ethylene glycol 40 % | 1060 | 3.42 | -25 |
| Propylene glycol 20 % | 1018 | 3.95 | -7 |
| Propylene glycol 30 % | 1029 | 3.82 | -13 |
| Propylene glycol 40 % | 1038 | 3.67 | -22 |
Typical values
Comfort cooling usually works with 7/12 °C or 6/12 °C, so a dT of 5 to 6 K. Process circuits run anywhere from 2 K for laser and machine tool cooling to 10 K for some industrial loads. Many chillers allow a dT between about 3 and 8 K across the evaporator. Outside that range, the flow is either so high that the pressure drop and erosion become a problem, or so low that the evaporator heat transfer drops and the freeze risk rises.
Using the calculation for diagnostics
The formula is the quickest way to check whether a chiller still delivers its capacity. Measure the water temperatures at the chiller connections with calibrated probes and take the flow from a flow meter, or from the pump curve and the measured pump differential pressure. If the calculated capacity is clearly below the selection data at the same conditions, look for a fouled heat exchanger, a refrigerant shortage or a compressor problem.
- Use the same thermometer for supply and return, or two that you have compared in the same water bath. A 0.3 K error on a 5 K difference is a 6 % capacity error.
- Make sure the unit runs at steady full load during the measurement, not while stages switch.
- Check the glycol concentration first. A wrong concentration skews both the flow and the capacity result.
- Compare the electrical input at the same time to see the real efficiency.
To check the pipe sizes for the calculated flow, use our chilled water pipe sizing tool. If you suspect the capacity has dropped, a technical inspection with measurements will show where it went.
Frequently asked questions
Why do I get a different result than with the factor 1.163?
The factor 1.163 is valid for water at around 10 to 20 °C. The calculator uses the real density and specific heat at the mean temperature, and for glycol mixtures the difference reaches 10 % or more.
Can I use the calculator for heating circuits?
Yes. The formula is the same for heating and cooling; the tool uses the absolute temperature difference. Properties are available up to about 60 °C.
How do I get the flow if there is no flow meter?
Measure the differential pressure across the pump and read the flow from the pump curve, or measure the pressure drop across the evaporator and compare with the manufacturer data. Both methods are approximate, but good enough to spot a large deviation.
What concentration should I enter if I do not know it?
Measure it. A refractometer for glycol costs little and gives a reading in a minute. Guessing the concentration makes both the capacity and the freeze protection uncertain.
Is your chiller losing capacity?
We measure capacity, efficiency and the refrigerant circuit on site and tell you what has changed.