N°3 – Relation between flow rate, power and temperature difference – level 2 to 3

Flow rate, power and temperature difference of water are linked through the formula:
P = q_v × 1.16 × ΔT
With:
– P in [kW]
– q_v in [m³/h]
– 1.16: Volumetric heat of water in [kWh/m³ K]
– ∆T: Temperature difference gained or lost by water in [°C] (or [K])

This formula is logical:
The greater the water flow qv, the more the transmitted power (of heating up or cooling down) is increased.

But taken alone the flow amount is not enough to judge the distributed power (of heating up or cooling down). Hence we can send as much cold water as we like through a radiator, but it won’t get any warmer.
For there to be heating or cooling, the temperature of water must vary by ΔT.

When working on boilers or chillers, their power is generally calculated in [kW] from flow rates expressed in [m³/h]. When working on radiators or convector ventilators, the Watt and the [l/h] are more suitable. We therefore use them as units:

P = q_v× 1.16 × ΔT
With:
– P in [W]
– q_v in [l/h]
– 1.16: Volumetric heat of water in [kWh/m³ K]
– ∆T: Range of water temperature loss or gain in [°C] (or [K])