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Humidification: Challenged by humidity

When heating and cooling are provided by systems that have no humidity control, it's important to deal with this separately. Mike Slattery of Walter Meier (Climate UK) explains how this can be done cost-effectively and with minimum energy consumption.
Humidification: Challenged by humidity
In situations where split, multi-split or variable refrigerant flow (VRF) heat pump systems are used to provide heating and cooling, there is a danger the humidity aspects of environmental control will be overlooked, to the detriment of the health and comfort of the people in the workplace. So it's important for building services engineers to recognise this problem and make a conscious effort to include humidity control in their designs.

Failing to do so can cause serious problems, particularly in the winter when relative humidity (RH) is inclined to fall, in parallel with the lower RH of outside air. While high RH (>60 per cent) can make people feel hot and sweaty and more inclined to turn up the comfort cooling and waste energy, low humidity (<40 per cent) can have more serious repercussions.
These include dry and itchy eyes, particularly for contact lens wearers, and drying out of the respiratory surfaces. The latter problem can have far-reaching health implications as it leaves the body more vulnerable to airborne infections. Nor do office machines fare any better, as low RH will increase the risk of a build up of static electricity.

There are also environmental implications, as people feel colder at a lower RH and turn the heating up, thus using more energy and increasing carbon emissions. Ironically, increasing the temperature also lowers the RH even further, thus exacerbating the problem. Consequently, many organisations that are trying to reduce energy consumption and carbon emissions need to take a look at their humidity control for this reason as well. And it's up to the building services engineer to help them do this.

So the key is to establish the right balance of humidity control and energy consumption and there are various options available. First, however, it's worth looking at how such problems can arise.

The key point to make about split heat pump systems, such as variable refrigerant flow (VRF), is that they only take care of heating and cooling, they don't deal with the fresh air aspects of the workplace. So this is left to a separate ventilation system - which may then be linked to the indoor fan coil units. And increasingly, as modern buildings get tighter, the ventilation system is the only way for significant volumes of outdoor air to enter the building.

However, all of this depends on the outdoor air having suitable moisture content, and in winter the RH of outdoor air can fall below 25 per cent.

One option would be to increase greatly the amount of outdoor air being introduced but the need to temper this air, not to mention the increased fan power required to move it around, would greatly increase energy consumption.

An alternative is to introduce separate humidification to compensate for the low RH of the outdoor air. Clearly, this needs to be done in the most energy efficient manner possible and, in most workspaces, needs to take account of low ceilings. This latter consideration eliminates some options and points to direct room injection, which can be supplied in several different ways.

Atmospheric steam generators with fan boxes on top represent one option, as are ultrasonic humidifiers, but both are located in the perimeter of the area being humidified, taking up space and doing nothing for the aesthetics. Wetted media placed above the ceiling are out of sight but will require extra ductwork and diffusers, thus adding to the cost.

There are also other considerations. For example, all cold water systems will have an evaporative cooling effect, which would increase the heating load, albeit only marginally. Legionella must be also considered, but controls and maintenance regimes can be put in place to control this risk. Steam systems take care of the legionella risk within themselves.

In our experience, as a company that offers most humidification systems, a pressurised water fogging system using fan enhanced multi-directional nozzles is the ideal solution in many cases. The fan assisted nozzles, which ensure the water is atomised and absorbed within 1.5 metres of the nozzle, are no larger than a CCTV camera, so they are quite discreet and can be used with ceiling heights as low as 2.4 m. They are served by a high pressure, demineralised water ring main that uses mechanical joints so no fire certification is required. And because cold water is used there is no additional heating energy.

In taking these steps to gain effective control over RH in winter, building occupiers don't just get a healthy and comfortable workplace and reduce absenteeism, they are also able to turn the temperature set point down to reduce energy usage. So, when the full picture is taken into account, humidity control makes perfect sense.
8 July 2010

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