Sensible heat load from airflow: the kilowatt formula
Convert airflow and supply-to-return temperature difference into mass flow and sensible heating or cooling load in kilowatts, with three worked zones.
The sensible load carried by an air stream equals mass flow times the specific heat of air times the temperature difference. It captures the heat that changes air temperature, not the moisture that changes humidity.
How do you convert airflow to mass flow?
Airflow is usually quoted in cubic meters per hour. Divide by 3,600 for cubic meters per second, then multiply by air density, about 1.2 kilograms per cubic meter near sea level at room temperature.
For a fictional conference zone moving 1,800 cubic meters per hour: =B11/3600*C11 gives 1800/3600*1.2 = 0.6 kg/s.
How do you get the load in kilowatts?
With specific heat of 1.005 kJ/kg·K, supply air at 14 °C and return air at 24 °C, the temperature difference is 10 K:
=H11*D11*G11 gives 0.6*1.005*10 = 6.03 kW.
Because a kilojoule per second is a kilowatt, no extra conversion factor appears.
How do three zones compare?
| Zone | Airflow m³/h | Supply | Return | Mass flow | Load |
|---|---|---|---|---|---|
| Conference, cooling | 1,800 | 14 °C | 24 °C | 0.60 kg/s | 6.03 kW |
| Open office, cooling | 3,600 | 15 °C | 24 °C | 1.20 kg/s | 10.85 kW |
| Perimeter, heating | 1,200 | 32 °C | 20 °C | 0.40 kg/s | 4.82 kW |
Open office: 1.2*1.005*9 = 10.854. Perimeter: 0.4*1.005*12 = 4.824. The workbook takes the absolute temperature difference, so heating and cooling both report positive loads.
What does this formula leave out?
- Latent load from people, cooking and outside air moisture.
- Duct leakage, fan heat and heat gain through duct walls.
- Density changes at altitude or high temperature; enter the actual density if it departs from 1.2.
The sensible heat load calculator handles each zone as a row. For the fan or pump side of the system, estimate shaft power with the pump hydraulic power calculator, and price running hours with the electrical energy cost calculator.