Outside air condition = -4.0C, 100% RH
Temperature after frost coil = 5.0C
Supply air temperature = 18.0C
Room condition = 22.0C, 50% RH
Return air temperature = 24.0C
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.
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.
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. Below is a summary of the year round savings based on weather data for Kew – west London with the example above. The data is for 24/7 operation and shows a total saving of 262.5 MWh.
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