01The heat-duty equation
For air with no phase change, P (kW) = ṁ (kg/s) × cp (kJ/kg·K) × ΔT (K). Around ordinary conditions, cp for dry air is approximately 1.0 kJ/kg·K. If mass flow is given in kg/h, divide by 3600. This equation estimates the heat transferred to the air; it does not include casing losses or every process load.
For example, 1.0 kg/s of air heated by 100 K requires about 100 kW of ideal sensible heat. The real heater duty may be higher after accounting for heat loss, leakage, equipment warm-up and the required recovery time.
02Convert airflow to mass flow
If airflow is stated in m³/s or m³/h, use density at the stated reference conditions: ṁ = ρ × Q. Do not mix actual cubic metres with normal or standard cubic metres without checking the reference temperature, pressure and humidity. Air density changes as the air heats, so be clear whether the flow is specified at inlet, outlet or standard conditions.
For a rough estimate using normal cubic metres, the reference density and conversion basis must be stated. A vendor should confirm the calculation basis before equipment selection, especially for high-temperature air, pressurized gas or altitude-sensitive installations.
03Add the loads the simple equation misses
- Heat loss through duct walls and leakage to the surroundings
- Heating of the product, fixtures or equipment carried by the air
- Fresh-air replacement and exhaust flow
- Startup recovery time and cold equipment mass
- Control range, operating schedule and ambient extremes
A large blanket percentage is not a substitute for identifying the real loads. Calculate each important contribution and document assumptions. If the process has strong transient or batch behavior, check both warm-up and steady-state duty.
04Check heater face velocity and controls
After duty is known, review the heater cross-section and face velocity. Excessive velocity or poor element spacing can create pressure drop, vibration or uneven outlet temperature. Confirm fan capability at the added system resistance, and provide airflow proving, staged or modulating control, and independent over-temperature protection as required by the process.
For an accurate quotation, send airflow and its reference condition, inlet and target outlet temperature, pressure, humidity if relevant, duct dimensions, allowable pressure drop, supply voltage, control method and installation environment.