Which humidifier?

Boiling water to create steam is very hygienic and offers an accurate way to manage humidity. The most common steam humidifier incorporates electric heaters, of which there are two types.

The electrode boiler humidifier creates steam by passing current through water in a plastic boiling cylinder. These are easy to install, operate on mains water tend to have the lowest capital cost. They provide humidity to an AHU with a steam lance or directly to a room with a fan unit. The removal of limescale, which makes up the biggest service requirement, is just a matter of draining and replacing the cylinder with a new one. This is quick and has minimal downtime.

The control available with these units is quite wide at around ±5%RH. This is because fresh water needs to be added to the boiling chamber to manage mineral levels, which causes an occasional drop in steam output.

If tighter humidity control is needed, the electric resistive steam humidifier is a good choice. Rather than passing current through the water, it has heating elements a bit like a kettle. This means the steam cylinder can be made of stainless steel and the limescale is simply emptied, rather than the cylinder being replaced.

Cold water humidifiers can offer a lower maintenance and lower energy alternative to steam humidifiers, and potentially provide the additional benefit of evaporative cooling. Although this effect can be a double-edged sword.

Rather than boiling water to create steam, water is simply sprayed or evaporated into the air. For this process to occur, the air must be at a low enough humidity to absorb the moisture. To ensure the air can accept the moisture and offset the cooling effect, the air needs to be pre-heated before the humidifier. If waste heat is available for this, adiabatic humidification can be a very low energy alternative to electric humidifiers. If additional pre-heating is needed, the energy consumed in the overall process is the same as an equivalent steam humidifier.

Adiabatic models include evaporative humidifiers, where water is fed over an evaporative media through which air passes. These can sit inside an air handling unit and incorporate features such as flush cycles and ultraviolet sterilisation to control microbial growth and the need for servicing to remove limescale. Because of these cycles, control is around ±10%RH.

If closer control is needed in an AHU, hybrid humidifiers combine the dual function of spray and evaporative humidification. An array of low-pressure nozzles sprays water towards a ceramic droplet separator. Moisture is absorbed directly from the spray section and then again from the separator. Control of ±2%RH can be achieved but a slightly longer AHU section length is needed.