01The standard formula
Air (and most flue or process gases) has a specific heat of about 1.01 kJ/(kg·°C). The duty formula is: P = ṁ × c × ΔT / 3600, where ṁ is mass flow in kg/h, c = 1.01 kJ/(kg·°C), ΔT the temperature rise in °C, and 3600 converts kJ/h into kW.
A handy mental constant: at standard conditions, every 1,000 m³/h of air needs roughly 0.33 kW per °C of temperature rise. With that number you can sanity-check any quotation on the spot.
02The classic mistake: airflow units
Process documents usually quote actual (operating) m³/h — volume at the real duct temperature. The formula needs mass flow, so convert first: ṁ = qv × ρ, with ρ taken at the actual temperature. 10,000 m³/h of hot air can carry 30% less mass than the same number at ambient — mixing up the two skews the result by exactly that much.
The second trap is leakage and fluctuation: negative-pressure sections pull in extra air, and fan flow shifts with system resistance. If the airflow number is wrong, no amount of formula precision saves the answer — verify against the fan's actual operating point.
03How much margin
Add 10–20% on top of the theoretical duty: the high end for poor insulation, frequent starts or fluctuating inlet temperatures, the low end for stable continuous duty with well-defined airflow. Oversizing is not free — an oversized heater running at low turndown controls worse and wastes both capital and transformer capacity.
For large airflows and high temperature rises, split the heater into 2–4 staged banks: energize sequentially at start-up to avoid grid inrush, then combine stages as needed for steady, precise outlet control.
04Example 1: process drying air
Duty: 8,000 m³/h recirculating at 80°C (ρ ≈ 1.0 kg/m³), returning air at 60°C, supply at 120°C → ΔT = 60°C. Mass flow = 8,000 × 1.0 = 8,000 kg/h. P = 8,000 × 1.01 × 60 / 3600 ≈ 134.7 kW. With 15% margin, specify roughly 155 kW in three staged banks.
05Example 2: exhaust preheat ahead of RTO
Duty: 20,000 m³/h exhaust at 100°C (ρ ≈ 0.95 kg/m³), preheated from 25°C to 320°C → ΔT = 295°C. Mass flow = 19,000 kg/h. P = 19,000 × 1.01 × 295 / 3600 ≈ 1,570 kW. At this scale, pure electric duty deserves a grid-capacity review — staged multi-level heating or a primary heat source (steam/thermal oil) with electric trim is often the better architecture. With VOC-laden streams, also verify element surface temperatures against the medium's safety margin.