When humidification is required, does it make sense to provide airside economizer controls given that they use more outside air than systems without economizers? Should only waterside economizers be used with humidification? Let's evaluate the different economizer options for an AHU with an isothermal (steam) humidifier. Last month, we made an operational cost comparison between airside and waterside economizer systems in different climates. Using the same model, let's assume it's desired to maintain a minimum space rh of 50% using a steam humidifier.
If we add an airside economizer, the outside air volume can vary from the minimum of 10% to as much as 100% of the total flow as the control dampers modulate to maintain setpoint whenever outside air conditions allow. If we simply assume the humidification load is based on 100% OA at zero humidity, the load would be 50,000 cfm x 65.4/7,000 x 0.075 x 60 = 2,102 lb/hr.