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Steam or adiabatic humidification in mixed-air ventilation systems?

What type of humidifier should you use in your mixed-air ventilation system: steam or an adiabatic humidifier? One of the major considerations is the amount of energy needed to operate the humidifier, not only the direct energy input into the humidifier but the impact on the total energy used by the ventilation system. Is it possible to change the humidifier type in an existing installation? Let’s consider a typical mixed-air ventilation system to see what happens with steam humidifiers and modulating adiabatic humidifiers.

 

 

 

The specific example we'll look at is a mixed-air ventilation system with fairly typical values as follows:

  • Outside air condition = -4.0°C, 100% RH
  • Temperature after frost coil = 5.0°C
  • Supply air temperature = 18.0°C
  • Room condition = 22.0°C, 50% RH
  • Return air temperature = 24.0°C
  • Minimum fresh air content = 20.0%

The psychrometric chart below shows the processes needed to achieve the required room condition for both steam humidification and modulating adiabatic humidification.

 

 

Fresh air enters the ventilation system (point A). It is heated by the frost coil to a temperature of 5.0°C (point B). Return air at 24.0°C (point C) is mixed with the fresh air (20% fresh air, 80% recirculated air) (point D).

From this point, the air could be humidified adiabatically. The specific enthalpy (a measure of the energy in the air) of the air does not change with an adiabatic humidification process so the air is cooled to the required supply air temperature of 18.0°C as it reaches the required humidity (point E). The only energy required is that needed to operate the humidifier, a few hundred Watts. This process is indicated by the green arrow. 

The alternative is to use steam humidification. Humidification with a steam humidifier is an isothermal process, i.e. it does not change the air temperature. This implies that the air must be at the desired temperature before it is humidified or the air temperature corrected after humidification. In our example, heating and cooling is carried out before the humidifier. There are two possibilities here, the first being to cool the air from point D to point F or, to make use of more fresh air and lower the mixed-air temperature to point G. A combination of these approaches could also be used.

The energy required for the cooling and humidifying with steam is significant. In our example, the specific enthalpy of the air (kJ/kg) is the same for point D and point E which has a value of 39.9 kJ/kg. The enthalpy of the air at point F is 37.5 kJ/kg so, in order to move the condition of the air from point D to point F, 2.4 kJ for every kilogram of air must be removed. A supply air volume of 10.0 m3/s for example, would correspond to an air mass of 12.0 kg/s and would require heat to be removed at a rate of 12.0 x 2.4 = 28.8 kJ/s or 28.8 kW.

With point D and point E having the same enthalpy, moving from point F to point E will require a power input of 28.8 kW from the steam humidifier. Taking into account the performance of the cooling system and the efficiency of the steam humidifier, the total load on the building will be (9.6 kW cooling + 31.6 kW humidification) 41.2 kW.

The alternative would be to alter the mix of fresh / re-circulated to point G and humidify the air from point G to point E. The specific enthalpy of point G is 35.0 kJ/kg. The power required by the steam humidifier in our example would be (39.9 – 35.0) x 12.0 = 58.8 kW. Taking into account the efficiency of the humidifier, the load on the building would be 64.7 kW.

This compares with a building load of 0.3 kW for the Gibbons adiabatic humidifier, for example.

The example we have used here is fairly typical of the worst case winter conditions for London. If the same exercise is carried out for conditions in the autumn and spring using the right mix of fresh air and re-circulated air, the savings are repeated. It is also possible, with an adiabatic system, to make use of more fresh air, and get the benefits of free cooling in warmer conditions.

For more information, please do not hesitate to contact us on 01621 868138, email steve.rix@gibbonsgroup.co.uk or visit our Humidification division page.

 

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