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Taking control of humidity in buildings

Humidity control is increasingly important, so building services engineers have the challenge of delivering it within building performance and lifecycle cost criteria. John Barker considers the options
A few decades ago there was relatively little consideration given to humidity control in buildings, unless the relative humidity (RH) was critical to a process, such as printing, or for safeguarding the condition of items such as museum exhibits. Some buildings may have had an element of humidity control through the air conditioning system but in most cases the RH fluctuated in line with factors such as natural ventilation/leaky buildings, staff coming in with wet coats, operation of the heating system etc.

So while building occupants were very aware of temperature variations less thought was given to RH unless the air was very dry or mould was growing on the walls. A bit of an exaggeration perhaps, but not too far from reality.

More recently, improved building insulation and tighter, less 'leaky' building fabric has resulted in more effective separation of internal and external environments. This makes temperature control more energy efficient but also increases the need to take control of RH.

RH can be too low
The most commonly experienced problem with humidity control in commercial environments is that the RH is too low (<40 per cent), a situation that can lead to a range of problems. At low RH the eyes become dry and itchy, especially for contact lens wearers, and respiratory surfaces start to dry out, leading to dehydration. In addition, the mucous membranes that need to be moist to defend against airborne pathogens are deprived of moisture so we become more susceptible to infections.

At low RH we also feel colder than is justified by the actual temperature, so people often turn the heating up to compensate, adding to the running costs, energy consumption and carbon footprint of the building. Ironically, raising the temperature also lowers the RH even further, thus magnifying the problem.

As a consequence of all these factors, there are very good reasons for including humidification in new build projects and potentially retrofitting it to existing buildings. The most appropriate choice of humidifier will be determined by a number of factors specific to each building and there are a number of criteria to take into account.

Of course, the first consideration must be how much moisture, in the form of steam or cold water vapour, will be needed to achieve the required level of humidification. For example, resistance boilers will not supply more than 80kg/hr. of steam, while electrode boilers can provide up to 90kg/hr for a single unit. and modular gas-fired humidifiers will deliver up to 400kg/hr. The ability to use a modular configuration for gas-fired humidifiers (as with modular boilers) also means that the system can respond to varying humidification requirements very efficiently.

The most common humidification systems heat water to generate steam, which is then introduced to the ventilation system. As a rule of thumb, it takes 0.73kW of heat to produce 1kg of steam, though distribution and other losses might add as much as a further 20 per cent. In today's energy- and carbon-conscious world, therefore, the method of heating is an important consideration. In most cases, gas-fired humidifiers will offer higher efficiency with a lower carbon footprint, compared to mains electricity.

Gas fired units may also prove less costly to install because electric humidifiers have heavy cabling requirements. A 400kg/h electric unit, for example, will require a power supply of 280kW, whereas a gas fired unit will simply require a gas pipe to deliver the same duty. When the steam is being injected into ductwork, gas-fired systems using a multi-lance configuration will also reduce the length of the ductwork required for high duties.

Cost of ownership must also be considered and there are several issues to take into account. For instance, in hard water areas the potential for limescale formation cannot be ignored and demineralised water is often recommended. In some projects, therefore, overall costs may be increased by installing demineralisation or reverse osmosis plant to treat the water.

Location of plant
Another consideration is where to locate the humidification plant - this is particularly important in retrofit situations but plant room space is also at a premium in many new build applications. Gas-fired units tend to have a relatively small footprint and can be supplied in skidmounted configurations for installation outside the building. They are also lighter than electrical units, so there are fewer constraints on roof-mounting.

An alternative to heating the water to generate steam is to spray water through nozzles, so that the water is atomised and absorbed into the air (adiabatic humidification). The water may be sprayed into ductwork or directly into the space being humidified - though there are a number of constraints that need to considered with the latter. Such systems also need o incorporate anti-bacterial measures such as ultra violet disinfection combined with demineralised or reverse osmosis water.

All forms of adiabatic humidification use less energy than self-generating steam humidifiers, though there is quite a lot of variation within the adiabatic category. For instance, low pressure nozzles use pressurised air to atomise the water so energy is consumed by the air compressor. High pressure nozzles, on the other hand, take the energy for atomisation from high pressure water,
so here the high pressure pump is the main energy consumer. Ultrasonic humidifiers have low energy consumption but also very low outputs.

Evaporation is another option and evaporative humidifiers have the lowest energy consumption with the ability to use any cold water supply.

As a company that supplies all types of humidity control we don't have a particular axe to grind. The important thing is to recognise is that effective humidification isn't as simple as it may at first seem, so it makes sense to team up with companies that have both the experience and the product range to offer the most appropriate solution.

// The author is sales director of Humidity Solutions //
10 September 2013

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