Plant & Works Engineering
Why is humidity control so important?
Published:  18 May, 2016

Maintaining effective control of humidity is essential in industrial environments for both the health of the building and its occupants and, very often, supporting optimum performance of manufacturing processes. John Barker of Humidity Solutions explains.

It is well accepted that humidity control is an essential element in maintaining a healthy environment with good indoor air quality, and this is something that applies to any space where people are living or working. Relative humidity (RH) can also impact the building fabric, for example by encouraging condensation on cold surfaces that potentially leads to corrosion or mould growth (depending on the type of material) – or can create slip hazards.

In addition, RH can affect the integrity of stored materials and impair the reliability of machinery, resulting in disruption to the manufacturing process.

Health and wellbeing

When there are problems with humidity control in industrial and commercial environments it is usually because the RH is too low. This can make the eyes become dry and itchy, especially for contact lens wearers, and respiratory surfaces start to dry out, leading to dehydration. Low RH also increases 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.

Furthermore, poor RH control has 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 be reactions between these chemicals and the water in the air. Allergic reactions are influenced by RH too.

These issues are reflected in the environmental variables identified in HSE Guidance note 194 (4) - in relation to maintaining workplace health, safety and welfare. These are air temperature, mean radiant temperature, ventilation, humidity, air velocity, climatic and seasonal variations in outdoor temperatures and solar intensity.

Low RH will also cause various materials to dry out very quickly. Timber, for instance, is prone to shrinkage, defamation and cracks when stored in a low humidity environment.

Moreover, low RH 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.

Taking control

For all of these reasons, it makes commercial sense to include humidity control in any new build projects, whilst retrofitting to buildings that currently have humidity control should also be considered.

Good humidity control will keep the RH between 40% and 60% and, as noted above, the requirement will usually be to increase humidity using a humidification system.

In such cases, choosing the right type of humidification system is essential in terms of providing the right amount of moisture and maintaining the required RH. Account must also be taken of additional factors such as energy consumption, maintenance requirements and where to locate the humidification plant (particularly if plant room space is limited).

Traditionally, the solution has been to heat water to produce steam that is then introduced to the ventilation system - and this is still the most appropriate option for some projects. However, high pressure, low energy systems using cold water are becoming increasingly popular in industrial applications, as discussed in more detail below.

Steam humidification

There are several ways of generating steam for humidification, so the selection needs to be influenced by the specific requirements of project.

For example, electrode boilers will not supply more than 90kg/hr. of steam, while resistive generators can provide up to 120kg/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.

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%. 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 reduce the length of the ductwork required for high duties.

Clearly cost of ownership must be considered and there are several issues to take into account. In hard water areas the potential for limescale formation cannot be ignored and demineralised water is often recommended.

For example, we recently worked with a specialist packaging tester where the humidity was controlled by electrode steam boilers. These were experiencing regular scaling and were unable to maintain the required humidity without extensive maintenance. A more cost-effective alternative proved to be resistive steam humidifiers served with demineralised water.

Another consideration is where to locate the humidification plant – especially when plant room space is at a premium. Gas-fired units tend to have a relatively small footprint and can be supplied in skid-mounted configurations for installation outside the building. Electric units can also be weatherproofed, presenting fewer constraints if roof-mounting is the best solution.

Low energy, high pressure

A lower energy alternative to heating water is to spray water through nozzles, so that the water is atomised and absorbed into the air (adiabatic humidification). The water may be sprayed – at low or high pressure - into ductwork or directly into the space being humidified. Such systems also need to incorporate anti-bacterial measures such as ultra violet disinfection and can be combined with demineralised or reverse osmosis water.

All forms of adiabatic humidification use less energy than self-generating steam humidifiers, though there is some variation. Low pressure nozzles use pressurised air so energy is consumed by the air compressor. High pressure nozzles take the energy for atomisation from high pressure water, so here the high pressure pump is the main energy consumer, although very low.

Achieving effective humidity control while minimising lifecycle costs was a consideration for bespoke cardboard tube packaging manufacturer Visican, where there were problems with paper curl and static electricity resulting from low humidity in both the storeroom and the factory. The solution proved to be an adiabatic humidification system introducing water to the factory and store room through high pressure nozzles to maintain an RH of 55%.

A similar solution was used for a manufacturer of injection-moulded plastic drink bottles, again delivering moisture through fan-assisted nozzle heads, each configured to suit the area being served and to ensure effective humidification without risk of wetting or condensation on machines and products.

In both cases the use of adiabatic humidification delivered additional benefits through the cooling effects produced by evaporating water, which reduced the need for mechanical cooling in the spaces.

Given all of the factors discussed here, it is clear that achieving acceptable humidity control depends on addressing a number of variables within each project. For that reason, it makes sense to team up with companies that have a comprehensive range of different humidity solutions and the expertise to apply them to optimum effect.

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