Humidification Water: Method

Steam and Adiabatic Humidification

Dry winter air heated to room temperature falls to 10 or 20 percent relative humidity, and raising it needs water added to the air stream: ṁ_a(W₂ − W₁), the dry-air mass flow times the rise in humidity ratio. How the water enters sets the leaving temperature and the energy.

Steam injects vapor already evaporated by a boiler or electrode humidifier; the air gains the vapor's enthalpy, about 2,675 kJ/kg (1,150 Btu/lb) for atmospheric steam, so its dry bulb rises slightly (a degree or two) and the process on the chart is nearly vertical. The heat to make the steam is the water rate times the steam enthalpy.

Adiabatic humidifiers (evaporative pads, atomizers, ultrasonic) put liquid water into the air and let the air evaporate it with its own sensible heat. The enthalpy stays constant, the process follows the constant-wet-bulb line down and to the right, and the air leaves cooler; the target relative humidity is reached at a lower dry bulb, and if the room needs the original temperature a reheat coil supplies the difference, ṁ_a c_p (t₁ − t₂). Adiabatic humidification is cheap in energy where cooling is welcome and costs the reheat where it is not.

Dehumidification is the reverse balance, done with a cooling coil below the dew point (the Mixing and Coil tool) or a desiccant. Temperatures are entered as plain numbers in the units of the selected system.