Air-operated humidifying units operate in the same manner as electrical units, except that they utilize a pneumatic hygrostat as a humidity controller and an air operator to open or close the steam valve (see Fig. 3-17). Adecrease in relative humidity increases the air pressure under a springloaded diaphragm to open the steam valve wider. An increase in relative humidity reduces the pressure under the diaphragm and allows the valve to restrict the steam flow. In a humidifier operation of this type, the steam supply is taken off the top of the header (see Fig. 3-18). Any condensate formed in the supply line is knocked down to the humidifier drain by a baffle inside the inlet of the humidifier-separating chamber.
Any droplets of condensation picked up by the stream as it flows through the humidifier cap when the steam valve opens will be thrown to the bottom of the reevaporating chamber. Pressure in this chamber is essentially atmospheric. Since it is surrounded by steam at supply pressure and temperature, any water is reevaporated to provide dry steam at the outlet. The humidifier outlet is also surrounded by steam at supply pressure to ensure that there will be no condensation or drip at this point. Aclamp-on temperature switch is attached to the condensate drain line to prevent the electric or pneumatic operator from opening the steam valve until the humidifier is up to steam temperature.