01What the heater has to supply
Sensible heat of the makeup gas
Dry air or nitrogen heated from its inlet temperature to the zone set point: kW = ṁ × cp × ΔT ÷ 3,600, with cp about 1.04 kJ/kg·K for nitrogen and 1.01 for air. Use Nm³/h × 1.25 (air) or × 1.16 (nitrogen) to get kg/h.
Latent heat of the removed solvent
NMP evaporating into the gas stream costs roughly 0.55-0.65 kWh per kg evaporated, water about 0.75 kWh per kg. On a line pulling 60 kg/h of NMP that is 35-40 kW of pure latent load — invisible to anyone sizing from airflow alone.
Strip and foil heat
Aluminium foil and copper current collectors are light, but the coating carries water or NMP that must come out at 80-150°C. The ramp is fast; the plateau is solvent-limited.
Envelope and duct losses
Long dryers with many zones leak heat through doors, viewports and the return duct. Treat the return duct as part of the heater load if it is not insulated — it usually is not.
The consequence for heater design: the biggest kW demand is in the first zones, where the coating is wet and the air must be both hot and dry. Later zones run cooler and drier and need far less power, but they need tighter control, because over-drying embrittles the binder.
02Zone arithmetic, worked example
Gas per zone
4,000 Nm³/h of dry air per zone at 1.25 kg/Nm³ = 5,000 kg/h. Heating it from 25°C to 120°C costs 5,000 × 1.01 × 95 ÷ 3,600 ≈ 133 kW before losses and efficiency.
Dew point cost
To keep the zone at a −30°C dew point at atmospheric pressure, the air must hold only a few grams of water per kilogram. Everything the coating gives up beyond that has to be swept out, which is what fixes the flow rate.
Zone profile
A typical eight-zone line puts the heaviest duty in zones 2-4 (wettest coating, highest evaporation) and the tightest tolerance in the last two zones, where the target is a few degrees, not ten.
Installed power
Zone kW is not additive to a peak: with staggered ramp-up and independent controllers, the connected load can be 1.5-2× the running demand. Declare both figures when you order a transformer.
These are illustrative numbers for the method. Your line builder owns the coating speed, solids content and airflow; the useful habit is to ask for the per-zone mass flow and the target dew point, then run the two formulas above yourself.
03Heater construction in dry air and nitrogen
Watt density
Air or nitrogen at good velocity tolerates 3-5 W/cm². In zones where the duct widens and the velocity drops below 2 m/s, take 1-2 W/cm² and lengthen the heated section instead of raising the load.
Sheath and fins
Smooth tubular elements in a duct waste surface area; finned elements multiply it. Fins must match the gas: aluminium fins are fine in dry air, stainless in nitrogen zones with any hydrocarbon carry-over.
Interlock
The fan must prove airflow before the bank energises, and lose the bank if it stops. A flow switch at the heater inlet is not enough on a multi-zone line — interlock each zone with its own proving.
Control
SCR or zero-cross power control per zone holds ±2°C; stepped contactor control gives ±8-10°C and is usually rejected for the finish zones. Binder chemistry is sensitive to overshoot, not only to the set point.
Nitrogen lines deserve one extra note: the atmosphere is inert, so the fire risk from solvent is lower, but the asphyxiation risk to people is not. Heater access doors and purge sequences belong in the safety review.
04The failure list to design around
Cold spots at the duct wall condense solvent, which then drips onto the strip. Insulate the duct, not only the heater, and keep the element outlet temperature away from the dew point of the gas stream.