Humidification Basics Part 10: Types of Humidifiers

By Chad Edmondson (JMP) and Norman Hall (RLD)

Now that we know there are two processes (adiabatic or isothermal) by which we can achieve humidification, let’s talk about the various types of humidifying equipment within these process categories.

Starting with isothermal, there are several types of humidifiers and configurations to choose from. The common denominator is that we are creating steam from an outside source and are using it to either directly or indirectly humidify the space.

The following products are available for an isothermal approach.

Boiler steam humidifiers (direct injection or steam-to-steam). Steam humidifiers can be used to put steam directly into the duct, air handler or the actual space.  Assuming there is an onsite boiler, steam generated from this source can be used for direct dispersion into the duct system/space or as an indirect source for generating humidification steam.  The latter would be used whenever there is a concern about boiler chemicals being directly injected into the air.

Steam-to-steam humidifier

In either case, humidification components are available in a variety of configurations, including single or multiple dispersion tubes or panel/coil type humidifiers that keep humidification steam separate from boiler steam. Steam-to-steam systems route boiler steam through a heat exchanger located in a boiling chamber filled with clean water and create clean humidification steam that doesn't contain boiler chemicals.

Gas steam-to-steam humidifiers.  These are self-contained humidification systems that rely on natural or LP gas to generate steam inside a coil which transfers heat to water inside an evaporator chamber.  These systems require flue venting and possibly a sealed combustion air supply.

Electric steam-to-steam humidifiers. These humidifiers generate a separate steam supply for dispersion into a duct or space just like gas steam-to-steam humidifiers, but instead of gas they use electricity, thus they can be installed anywhere electric power is available. Electric humidification systems provide tight RH control, usually with an electric resistive element steam generator, using deionized (DI) or reverse osmosis (RO) treated water.

Electric Humidifier

Adiabatic Humidifiers

As we learned before, humidification doesn’t have to involve steam generation. Adiabatic humidifiers introduce unheated water into air, either by dispersing water into the air as a fine mist, or by allowing water to evaporate from a wetted media. This causes the relative humidity (RH) levels to increase and air temperature (dry bulb) to decrease. These systems rely on the latent heat energy within the air to change the water into a vapor. When this occurs, cooling also results.

Adiabatic Humidifier

Adiabatic humidifiers work especially well in regions where the air is consistently warm and dry, or in spaces where there is an additional heat load such as from equipment in a computer room.