Humidifying Air by Adding Steam or Water

Air can be humidified by adding water or steam

Air can be humidified by

psychrometrics changing conditions steam water

Humidifying Air by adding Water

If water is added to air without any heat supply the state of air change adiabatic along a constant enthalpy line - h - in the Mollier or psychrometric chart. The dry temperature of the air decreases as shown in the Mollier diagram above.

The amount of added water can be expressed as

mw = v ρ (xC - xA)         (1)

where

mw = mass of added water (kg/s)

v = volume flow of air (m3/s)

ρ = density of air - vary with temperature, 1.293 kg/m3 at 20oC (kg/m3)

x = specific humidity of air (kg/kg)

If the water added is at the same temperature as the air, the change in enthalpy is zero.

Example - Humidifying Air by adding Water

An airflow of 3000 m3/h at 25oC and 10% relative humidity (A) is humidified to 60% relative humidity (C) by adding water through spray nozzles.

Using the Mollier diagram and following the constant enthalpy line 30 kJ/kg from A to 60% relative humidity, the state at C can be found at 14.7oC.

The specific humidity at (A) is 0.002 kg/kg and at (C) 0.0062 kg/kg.

The amount of water added can be calculated as:

mw = ((3000 m3/h) / (3600 s/h)) (1.184 kg/m3) ( (0.0062 kg/kg) - (0.002 kg/kg) )

= 0.0041 kg/s

= 14.9 kg/h

Humidifying Air by adding Steam

If steam is added to the air the state of air will change along a constant dh/dx line for steam as shown in the figure above.

When adding saturated steam at atmospheric pressure the constant line dh/dx = 2502 kJ/kg (the evaporation heat of water at atmospheric pressure). When adding saturated steam at atmospheric pressure the temperature rise is very small - in general less than 1oC. For practical purposes the process of adding saturated steam at atmospheric pressure approximates the horizontal temperature line.

The water vapor added can be calculated using (1).

The enthalpy added can be found using a Mollier diagram or a psyhrometric chart.

Example - Humidifying Air by adding Steam

An airflow of 3000 m3/h at 25oC and 10% relative humidity (A) is humidified to 60% relative humidity (B) by adding saturated steam at atmospheric pressure.

Using the Mollier diagram the process from (A) to (B) can be approximated by following the constant temperature line 25oC to 60% relative humidity and approximately 25.5oC (a temperature increase less than 1oC).

The specific humidity at (A) is 0.002 kg/kg and at (B) 0.012 kg/kg.

The amount of water added can be calculated as:

mw = ((3000 m3/h) / (3600 s/h)) (1.184 kg/m3) ( (0.012 kg/kg) - (0.002 kg/kg) )

= 0.01 kg/s

= 35.5 kg/h

The enthalpy change can be estimated from the Mollier diagram. The enthaply at (A) is 30 kJ/kg and at (B) 55 kJ/kg. The enthalpy difference is

dh = (55 kJ/kg) - (30 kJ/kg)

= 25 kJ/kg

The total heat added by the steam can be calculated as:

q = ((3000 m3/h) / (3600 s/h)) (1.184 kg/m3) (55 kJ/kg - 30 kJ/kg)

= 24.7 (kJ/s, kW)

Related Topics

  • Air Psychrometrics - The study of moist and humid air - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more

Related Documents

Tag Search

  • en: humid air steam water

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