Humidifiers may be installed as part of the central station airhandling unit, or in terminals at the point of use, or both. Where close humidity control of selected spaces is required, the entire supply airstream may be humidified to a lower humidity level in the air handler, with terminal humidifiers located in the supply ducts serving just those selected spaces bringing them up to their required humidity levels. For comfort installations not requiring close control, moisture can be added to the air by mechanical atomizers or point-of-use electric or ultrasonic humidifiers. Proper location of this equipment prevents stratification of moist air in the system.
In this application, the heat of evaporation should be replaced by heating the recirculated water, rather than by increasing the size of the preheat coil. Steam grid humidifiers with dew-point control usually are used for accurate humidity control. It is not possible to add moisture to saturated air, even with a steam grid humidifier. Air in a laboratory or other application that requires close humidity control must be reheated after leaving a cooling coil before moisture can be added. The capacity of the humidifying equipment should not exceed the expected peak load by more than 10%. If the humidity is controlled from the room or the return air, a limiting humidistat and fan interlock may be needed in the supply duct. This prevents condensation and mold or mildew growth in the ductwork when temperature controls call for cooler air. Humidifiers add some sensible heat that should be accounted for in the psychrometric evaluation.