Unlike wet
humidifiers, steam humidifiers do not chill the airstream during the
humidification process. This is because the moisture is delivered to the
airstream already in the vapour state (as steam) having been created by a
heating element.
Electrode-Boiler Humidifiers (below figure) are the most widely used type of
steam humidifier in direct and indirect
humidification due to their low cost and ease of installation.
The core element is
a small boiler comprised of a non-conductive polypropylene tank fitted with
three or more bare steel electrodes. When the tank is filled with water the
electrodes become immersed. Electrical connections are made to the electrodes
and current flows directly through the water causing it to heat up and boil.
Output of the unit is controlled by varying the depth of water in the tank.
Continual boiling of the water causes the concentration of minerals in the tank
to build up. To avoid this there is an automatic cycle of emptying and re-filling
the tank with fresh water. When the boiler eventually scales up it is simply
replaced or opened and de scaled. The primary disadvantage of this system is
high running costs and the need to regularly replace boilers.
Resistive Element Humidifiers (below
figure) are like small kettles boiling the water within them using an electric element.
Regular drain and refill cycles prevent excessive scale build up. Switching off
individual elements and modulating the power supply provides very close control
of steam output, making them the preferred choice for close control
applications.
Gas-Fired Steam Humidifiers use a gas heater to boil water and create
steam. Gas is approximately four times cheaper per unit of energy than
electricity. As a consequence the running costs of gas fired steam humidifiers
are low, making them increasingly popular for humidification. Their structure
is similar to the gas boilers except that the water is heated to boiling point
by the burner.
