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Balancing relative humidity 

Maintaining acceptable relative humidity while minimising energy consumption means choosing the most suitable humidification system. Dean Ward of Walter Meier (Climate UK) explains 

One of the many balancing acts that is part and parcel of managing energy in buildings is the need to maintain comfortable conditions for the occupants while keeping energy consumption to a minimum. And, increasingly, there is very little room for compromise. Occupants have higher expectations of comfort, while a host of energy initiatives all pile on the pressure to not just reduce energy consumption but also to achieve ongoing improvements in energy efficiency. 

When it comes to comfort the first things that spring to mind are usually heating and cooling but itís also vital to consider relative humidity (RH). Unfortunately, though, humidification is sometimes overlooked, or it may be included in the design at first and then dropped to save money. 

However, there are many good reasons for including, or adding, energy efficient humidification in a building, as maintaining a suitable RH makes a big difference to comfort and can also help to reduce the energy consumption of other systems. And there are now technologies that will enable very efficient humidification. 

The importance of RH 

The acceptable RH range for commercial workplaces is 40-60% and this suits both human and machine occupants. Consequently it is supported by the World Health Organisation, the National Association of Optometrists and the Humidity Group of the Hevac Association Ė and recommended by BS EN 29241 as the optimum for visual display terminals. 

When the RH is either side of this range, problems can occur. Below 40% there is a dehydrating effect as the body loses moisture to the air from respiratory surfaces, mucous membranes and surface of the eyes. This may be good news for the suppliers of water coolers but it also erodes the bodyís natural defences and leaves it more vulnerable to infection from airborne pathogens. Many people also suffer from dry and itchy eyes in low RH environments. 

In the context of energy, another important consideration is that people feel cold out of proportion to the actual temperature when the RH is low. As a result they tend to turn up the heating unnecessarily, thus wasting energy. And in raising the temperature, they dehumidify the air even more, exacerbating the problem. 

At the other end of the scale, an RH over 60% will make people listless so that productivity falls. This may also result in turning up the air conditioning or opening windows, which can also have an impact on energy consumption. 

In the UK though, itís far more common to experience problems with low RH, especially in the winter. This is because ventilation requirements are generally met by introducing fresh air and then either heating or cooling it in the air handling units. In the winter, the outdoor air will naturally have a low RH as the colder the air gets the less water vapour it holds. Then, as the air is introduced to the ventilation system it is heated, removing even more of the water vapour, so that many workplaces consistently have an RH below 25% during the winter. 

The effects of using cold outside air could be tempered by greatly increasing the volume of outdoor air, but that would consume even more energy because more heating would be required, and fan power consumption would increase to move it through the distribution system. 

Doing it efficiently 

In our experience the most effective and energy-efficient way to deal with low humidity, particularly when retrofitting to an existing building, is to introduce a separate humidification system that will inject water vapour directly into the space. And, as noted earlier, this system needs to be energy efficient so it needs to use cold water, avoiding the need for additional heating. In the majority of workplaces, it also needs to be a system that can be used in conjunction with relatively low ceilings. 

In the past, the traditional methods for introducing water vapour directly to the air have included atmospheric steam generators or ultrasonic humidifiers in the space, or wetted media placed above the ceiling and linked to the space by additional ductwork and diffusers. All of these have limitations, either because of the space they require and/or the services that need to be run to and from them. 

In addition, as cold water is being used, there needs to be an integral method for controlling Legionella Ė again without heating the water. Adding biocides to the water is one option, but a costly 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 and other areas the radiation will not reach, and UV radiation will not prevent bacterial growth in non-irradiated areas. 

Bearing all of these issues in mind it is clear the best solution will be discreet and not take up any floor space while incorporating effective, inexpensive water treatment. It should also be easy to retrofit. 

One system that fulfils these criteria is a pressurised water fogging system using fan assisted nozzles, a design that has been available for a relatively short period of time compared to other systems and one that has been fine-tuned in the light of experience. 

So, for example, the use of fan-assisted nozzles ensures that water is atomised and absorbed within 1.5 metres of the nozzle, making them suitable for spaces with low ceilings. The nozzles themselves are about the same size as a CCTV camera and are connected to a high pressure water ring main so that installation is simple Ė using mechanical joints so no fire certification is required. 

This design of system also facilitates centralised water treatment at the pumping station, and the most effective method has proved to be addition of silver ions by electrolysis, in proportion to the flow rate. The silver accumulates within the nozzles to create an additional layer of protection. Any such system should also automatically drain the lines every 12 hours as an extra precaution. 

So while itís not possible to maintain effective humidity control with zero energy consumption the choice of the right type of system will enable a comfortable environment to be maintained with the minimum use of energy. A sensible compromise for all.