Heating coils/Heater batteries sizing.

[cookie85]

Can anyone please size a heater battery for me? On a supply and extract heat recovery air handling unit. (50% efficient for heat recovery)

Outside temperature -2 degrees C
Room temp/Return air - 21 degrees C
Air voume - 1.3 m3/sec
Boiler flow and return - 80-60 degrees C.

I have been told i need to use Q=M.Cp.DT. Is this right and how would i do this? I have no idea.

I know i need to use the Cp of air, 1.02 and the DT of the on/off coil temperatures. I need some guidance on this please.

Fill explination would be appreciated. Thanks.

 

[Navin A S]

Yes, you are correct in using Q=M.Cp.DT to size your heater battery. The formula works like this: Q = Heat Load (kW) M = Mass Flow Rate (kg/s) Cp = Specific Heat Capacity (kJ/kgK) DT = Temperature Difference (K) In order to calculate the heat load, you need to know the mass flow rate of air flowing through the system, and the temperature difference between the outside air and the return air. The mass flow rate of air can be calculated with the equation: M = Air Volume (m3/s) x Density (kg/m3) Assuming standard atmospheric pressure and a temperature of 0°C, the density of air is 1.2 kg/m3. So in your case, the mass flow rate is 1.3 m3/s x 1.2 kg/m3, or 1.56 kg/s. The temperature difference is the difference between the outside air and the return air. In your case, that would be -2°C - 21°C, or -23°C. Once you have these values, you can plug them into the Q=M.Cp.DT formula and calculate the heat load. Q = 1.56 kg/s x 1.02 kJ/kgK x (-23°C) = -35.75 kW This is the total heat load required for the heater battery. You will need to find a heater battery that can handle this kind of load. Hope this helps!