Modern adiabatic humidification systems offer a number of benefits, including reduced energy consumption. Michael Whitehead of Walter Meier (Climate UK) explains why it’s time to take a fresh look
There have been many examples of a particular technology falling short of expectations and gaining a poor reputation, despite issues later being resolved.

This is certainly the case with old adiabatic humidifiers, which caused concern about the spread of micro-organisms, dust and allergens. Modern versions, however, incorporate fail-safe treatment mechanisms to eliminate this risk, while offering energy benefits.

And these energy benefits are considerable. They use cold water, so there are none of the heating costs associated with other types of humidification. They also offer some free cooling from the evaporation of the water into a mist.

There are several ways of addressing the hygiene issue. Adding biocides to the water is one option, but an expensive one. Irradiating the water with ultra-violet light can be effective, but only in the areas that are fully exposed to the radiation. Every system has edges, corners or other areas that the radiation will not reach - and UV radiation will not prevent bacterial growth in non-irradiated areas.

Clearly, therefore, if these risks are going to be eliminated in a cost-effective way, a more comprehensive approach is required that addresses every aspect while also providing low cost of ownership.

The obvious place to start is with the water, and using fully demineralised water will ensure a high standard of hygiene right from the start. But other mechanisms - such as silver ionisation - also need to be included to maintain those high hygiene levels. At the same time, demineralised water has a higher cost than mains water, so any such unit should minimise total water consumption.

Water vaporisation also needs to be highly efficient so that no water aerosol droplets are allowed to accumulate in the system and create potentially hazardous pools of water.

Some modern adiabatic humidifiers address this requirement by using molecular, low-pressure (4-6 bar) nozzles with ceramic orifices, arranged on a mesh grid. This creates a homogenous mist that immediately starts to evaporate in the air flow. Unlike classic nozzle systems, which require a long absorption distance and a droplet separator, this arrangement functions very efficiently with a short absorption distance, which increases efficiency and reduces water consumption.

Further to increase the efficiency of the evaporation, the air flow then passes over highly porous, adjacently arranged 50mm-thick ceramic plates located 800mm downstream.

Any remaining spray mist is deposited on the plates, and then evaporated by the energy of the passing air. This arrangement make maximum use of the water and raises efficiency levels to more than 90%. It also enables a rapid drying effect when spraying is switched off - a further safeguard against hygiene hazards.

As an extra precaution, the system should automatically drain humidifying water out of the lines every 12 hours during a system shutdown - with additional system purging every time the system is reactivated.

As noted earlier, any such system also requires supplementary measures against micro-organisms. And one of the most effective methods has proved to be silver ionisation. Silver ions have been used for purifying drinking water for many years, and they are also used widely in the textile industry. When the ions come into contact with micro-organisms, they enter the cells, block key metabolic processes and kill them.

These principles can also be applied to an adiabatic humidifier by adding silver ions to the humidification water by electrolysis, at a rate that is proportional to the flow rate of the water. As the silver is then included in the aerosols, it subsequently accumulates on the ceramic plates, creating an additional layer of protection within the system.

It is by combining all of these measures that adiabatic humidifiers can be brought into most situations, even the most vulnerable. At a hospital in Zurich, for example, engineers tested the Condair Dual 2 adiabatic humidifier and found bacteria levels well below the 100 bacteria per cubic metre specified for humidified supply air.

As a result, they are now using these units for humidifying operating theatres.

In terms of specifying adiabatic humidifiers, therefore, it is important to get confirmation from the manufacturer that their units meet the necessary standards. One of the highest standards to look for is a long-term certificate from respected bodies such as the SGS Fresenius Institute.

The significance for the long-term is that testing will have verified that the humidifier will remain hygienic in operation for many years, assuming it is correctly maintained.