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Humidification Water

Water or steam needed to take an airflow from its entering state to a target humidity, by the two ways it is done: steam injection, which adds moisture at nearly constant dry bulb and needs the steam's enthalpy as heat, and adiabatic (evaporative or atomizing) humidification, which adds moisture along a line of constant wet bulb and cools the air as it does. For the target relative humidity the tool gives the leaving state of each method, the water mass rate and its volume per hour, the steam heat, and the reheat an adiabatic humidifier needs to restore the dry bulb; dehumidification by a cooling coil is the same balance run the other way. Temperatures are plain numbers in °F (USCS) or °C (SI).

  • Steam humidification: water rate, steam heat, leaving dry bulb

  • Adiabatic humidification: leaving state along the constant-wet-bulb line, water rate, reheat to the entering dry bulb

  • Psychrometric chart with both process lines

Humidification Water

Unit System: /

Entering Air (temperatures °F for USCS, °C for SI)





Target



Steam and Adiabatic Humidification | Method page
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Humidification Water

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: ṁa(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, ṁa cp (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.



References

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2020). ASHRAE Handbook: HVAC Systems and Equipment. ASHRAE, Peachtree Corners, GA.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (2021). ASHRAE Handbook: Fundamentals. ASHRAE, Peachtree Corners, GA.

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