Dryers and Hot Air: Heat for Removing Moisture
Drying is one of the most energy-hungry jobs in industry, because evaporating water needs its full latent heat. Most dryers blow hot air over or through the product; the air is heated by steam, thermic fluid, or directly by a burner or hot air generator.
You'll learn
- Calculate the minimum heat to evaporate moisture
- Describe steam-heated, indirect-fired and direct-fired hot air
- Understand dryer thermal efficiency and its losses
- Choose a heat source for a dryer
Before this: Latent heat, Choosing a heating medium
Drying is evaporation
Removing moisture from a product means turning that moisture into vapor, and every kilogram needs its latent heat — over 2,300 kJ at typical drying temperatures — plus the heat to warm the product and the air. That is why dryers in rice mills, textile stenters, food plants and chemical works are among the largest heat users on a site.
Most dryers use hot air: it carries heat to the product and carries the vapor away. The air can be heated through a steam or thermic fluid coil, by an indirect hot air generator that keeps combustion gases separate, or directly by mixing in hot combustion gases.
The minimum heat, and the real heat
- Moisture evaporates at about 60 °C in the dryer, where its latent heat is 2,357.7 kJ/kg.
- Minimum heat:
- A convective dryer at an illustrative 50% thermal efficiency needs 655 kW.
| Source | Air temperature | Contact with product |
|---|---|---|
| Steam coil air heater | Up to a little below steam temperature | Clean air |
| Thermic fluid coil | Higher, to about 250 °C | Clean air |
| Indirect hot air generator | High | Clean air; combustion gases kept separate |
| Direct-fired | High | Combustion gases mix with the air |
Where dryer heat goes
- Exhaust air: the largest loss. Air leaves warm and only partly saturated with moisture; recirculating part of it, within the product’s humidity limit, saves heat.
- Heating the product and carriers: unavoidable but can be recovered from hot product in some processes.
- Casing and leakage: insulation and seals matter as much as on a boiler.
- Over-drying: drying past the target moisture wastes heat and can damage the product. Moisture control saves both.
Mechanical dewatering first — pressing, centrifuging, vacuum — removes water for a fraction of the energy that evaporating it costs. A dryer is the most expensive way to remove water; it should only remove what cheaper methods cannot.
Key takeaways
- Drying needs the latent heat of every kilogram of water evaporated, plus losses.
- 500 kg/h of water needs at least 327 kW; a real dryer needs far more.
- Hot air comes from steam or thermic fluid coils, indirect hot air generators, or direct firing.
- Exhaust air is the biggest loss; dewater mechanically first and avoid over-drying.