In maintaining acceptable comfort conditions in workplaces, humidity control shouldn’t be an add-on that’s only included when budgets permit, it should be a standard requirement, says *John Barker.
Humidity control in the UK has something of a chequered history, with specifiers often choosing not to include it in their original designs or allowing it to be ‘value engineered’ out at a later stage. However, research consistently shows that humidity is an essential ingredient in the recipe for thermal comfort and wellbeing.
To understand this, it’s worth reviewing what we mean by ‘thermal comfort’. BS EN ISO 7730 (1) defines it as: ‘that condition of mind which expresses satisfaction with the thermal environment.’ Thus is describes each person’s relationship with their environment – often expressed as feeling too hot or too cold.
The fact that this perception varies from one individual to another means it’s quite difficult to quantify but in designing the environment correctly building services engineers can minimise the impact of personal factors. In this respect the environmental variables identified in HSE Guidance note 194 (4) – in relation to maintaining workplace health, safety and welfare – are air temperature, mean radiant temperature, ventilation, humidity, air velocity, climatic and seasonal variations in outdoor temperatures and solar intensity.
As noted above, humidity is one that has often been ignored, but BSRIA Applications Guide AG10/94.1; Efficient Humidification in Buildings (5), concluded that relative humidity (RH) is an important variable for thermal comfort and the well-being of humans.
Indeed, as buildings have become more airtight the need to control humidity in support of good indoor air quality (IAQ) has increased as it is no longer possible to rely on a building’s natural ‘leakiness’ to control humidity.
Good humidity control will keep the RH between 40% and 60%, as problems can arise if the RH is consistently either side of this range. Low humidity (<40%) can lead to skin and eye irritations and also increase the spread rate of pathogens such as the influenza virus. The survival rate of this virus is at its lowest at 40-60% RH at a temperature of 21°C
Low RH also makes occupants feel colder, so they turn the heating up to compensate, which increases energy consumption and costs. Raising the temperature also reduces the RH even further, exacerbating the problem. Conversely, RH at the upper end of the acceptable range will make the space feel
2-3°C warmer so that heating can be turned down.
If the RH is allowed to rise too high, though, it can lead to respiratory ailments, thermal discomfort, condensation and mould growth problems, resulting in potential damage to the building’s fittings and fabric.
Poor RH control has also been linked with Sick Building Syndrome and the intensity of chemical pollution caused by gases from materials used inside the buildings. This is because the RH affects the distribution rate of these gases and there may also be reactions between these chemicals and the water in the air. Allergic reactions are also influenced by RH.
Time to take control
In the UK, most humidity problems in commercial environments result from the RH being too low, so that a humidification system is required to maintain good IAQ. As well as designing humidification into new buildings there are also many existing buildings that would benefit from a retrofitted humidification system. But how best to go about it?
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.
The first consideration is how much moisture will be needed to achieve the required level of humidification – and then how best to introduce that moisture.
The most common humidification systems heat water to generate steam, which is then introduced to the ventilation system. When steam humidifiers are the most appropriate solution for a particular project the method of heating needs to be selected carefully to minimise energy consumption.
Increasingly, spraying cold water through nozzles at high pressure so the water is atomised and quickly absorbed into the air (adiabatic humidification), is being used as an energy-efficient alternative to steam.
High pressure adiabatic systems also produce evaporative cooling, which in some projects will obviate the need for additional mechanical cooling. For each 500 litres of water that is evaporated such a system provides around 345kW of cooling for a power input of just 4kW.
Whatever humidity control technology is selected, the important thing is the recognition of the key role that humidity plays in achieving and maintaining thermal comfort and good IAQ. The best solutions can then be specified, ideally with support from humidity control specialists.
The subject of thermal comfort is discussed in more detail in the white paper ‘Humidity and the Impact on Human Comfort’, published by the Humidity Group of the HEVAC Association
*John Barker is Sales Director with Humidity Solutions