Choosing too small a heater means the tank takes forever to reach temperature. Choosing too large a heater wastes money and can shorten heater life. Fortunately, the calculation is simple.
The basic formula
The energy needed to raise the temperature of a liquid is:
Energy (kWh) = Mass (kg) × Specific heat (kJ/kg·°C) × Temperature rise (°C) ÷ 3600
To find the heater power, divide by the heat-up time you want:
Power (kW) = Energy (kWh) ÷ Heat-up time (hours)
Then add an allowance for heat losses (explained below).
Useful values
| Liquid | Density (approx.) | Specific heat (approx.) |
|---|---|---|
| Water | 1.0 kg/litre | 4.19 kJ/kg·°C |
| Mineral / thermal oil | 0.85–0.9 kg/litre | about 1.9–2.1 kJ/kg·°C |
(Values vary with the exact liquid and temperature – check your liquid’s data sheet for accurate numbers.)
Example 1: Heating water
A tank holds 500 litres of water. You want to heat it from 25 °C to 75 °C in 2 hours.
- Mass = 500 litres × 1.0 = 500 kg
- Temperature rise = 75 − 25 = 50 °C
- Energy = 500 × 4.19 × 50 ÷ 3600 = 29.1 kWh
- Power = 29.1 ÷ 2 = 14.5 kW
Example 2: Heating oil
A tank holds 300 litres of oil (density 0.87, specific heat 2.0). Heat from 30 °C to 150 °C in 3 hours.
- Mass = 300 × 0.87 = 261 kg
- Temperature rise = 120 °C
- Energy = 261 × 2.0 × 120 ÷ 3600 = 17.4 kWh
- Power = 17.4 ÷ 3 = 5.8 kW
Remember: oil needs a low watt density heater to avoid carbon build-up, so the elements will be longer or more in number than for water at the same kW.
Don’t forget heat losses
The formula above only covers heating the liquid. In practice, heat is also lost:
- from the tank walls and open surface,
- to the tank material itself during heat-up,
- through any liquid that is drawn off and replaced.
An open, uninsulated tank can lose a lot of heat, especially at higher temperatures. A common practical approach is to add a safety margin of around 20–30 % for an insulated, covered tank, and more for open or uninsulated tanks. For critical applications, the losses should be calculated properly.
For Example 1, adding 25 % gives about 18 kW – so an 18 kW three-phase immersion heater would be a sensible choice.
Maintaining temperature
Once the liquid is hot, the heater only needs to replace the losses (plus any heat taken by product or fresh liquid). A temperature controller switches the heater on and off to hold the set point.
Let us size it for you
Send us the tank size, liquid, start and final temperature, heat-up time and whether the tank is insulated or open. Thermosys Technologies will recommend the right immersion heater – screw-plug, flanged or over-the-side – and manufacture it in Pune.