x = humidity ratio (kg h2o /kg dry_air )
If water is added at the same temperature as the air the enthalpy change is zero.
An airflow of 3000 m 3 /h at 25 o C and 10% relative humidity (A) is humidified to 60% relative humidity (C) by adding water through spray nozzles.
Using the Mollier diagram following the constant enthalpy line 30 kJ/kg from A to 60% relative humidity, the state at C can be found at 14.7 o C .
The humidity ratio at (A) is 0.002 kg/kg and at (C) 0.0062 kg/kg .
The amount of water:
m w = ((3000 m 3 /h) / (3600 s/h)) (1.184 kg/m 3 ) ( (0.0062 kg/kg) - (0.002 kg/kg) )
= 0.0041 kg/s
= 14.9 kg/h
If steam is added to the air the state will change along a constant dh/dx - line for steam as indicated in process A to B in the Mollier diagram 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 1 o C . 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 estimated by using the Mollier diagram or a psyhrometric chart.
An airflow of 3000 m 3 /h at 25 o C 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 25 o C to 60% relative humidity and approximately 25.5 o C (a temperature increase less than 1 o C ).
Note! - by adding enough steam the state of air (B) is moved to the saturation line where the air is fully saturated. By adding even more steam the process starts following the saturation line - moisture content and dry bulb temperature will increase - and droplets or fog may be introduced in the moist air.
The humidity ratio at (A) is 0.002 kg/kg and at (B) 0.012 kg/kg .
The amount of water added can be calculated as:
m w = ((3000 m 3 /h) / (3600 s/h)) (1.184 kg/m 3 ) ( (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 m 3 /h) / (3600 s/h)) (1.184 kg/m 3 ) (55 kJ/kg - 30 kJ/kg)
= 24.7 (kJ/s, kW)
The chart below indicates a similar process of humidifying air with steam in the psychrometric chart.
Steam required for humidification of indoor space at 21°C (70°F), 2 air-changes per hour and 70% outdoor relative humidity:
Moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content.
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Online calculator, figures and tables showing density, specific weight and thermal expansion coefficients of air at temperatures ranging -100 to 1600 °C (-140 to 2900 °F) at atmospheric and higher pressure - Imperial and SI Units.
Basic air changing state processes - heating, cooling, mixing, humidifying and dehumidifying by adding steam or water - psychometric diagrams and the Mollier charts.
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Psychrometric chart for air at barometric pressure 29.921 inches of Mercury and temperature ranging 20oF to 120oF.
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Latent and sensible cooling and heating equations - imperial units.
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Sensible and latent heat of moist air.
The Mollier diagram is a graphic representation of the relationship between air temperature, moisture content and enthalpy - and is a basic design tool for building engineers and designers.
The relationship between the psycrometric chart and the Mollier diagram.
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Calculating the amount of steam in non-flow batch and continuous flow heating processes.
An introduction to vapor and steam.
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