Humidity: Essential to be just right

Humidity: Essential to be just right

By Benjamin Brits

From personal comfort and health to medical facilities, electronics and various production processes, the level of moisture in any environment is most often critical to achieving optimal conditions.

When considering modern day life, we are often so consumed with digital devices and technology that we don’t think about the specific details that make life possible, easier and in many instances functional before comfort even comes into range.

Humidity conditions vary quite broadly depending mostly on location and can be an engineering challenge to implement the correct solutions while not breaking the bank in terms of energy consumption, although, some humidification solutions are amongst the cheapest and simplest forms of environment conditioning.

“Most commonly both humidification and de-humidification are applied simultaneously  where close control conditions are required to meet operational set points. An example of the need for simultaneous control is within indoor growing [organic] facilities where the transpiration rate varies during the growth stages, but relative humidity must be maintained to optimise plant growth. Alternatively, either humidification or de-humidification would be required if conditions outside of a desired range would have a negative impact or could cause damage over time,” says Christiaan Schalekamp, technical sales manager at Humidair.

So, from cold rooms and surgeries, to preserving history, artefacts, collector’s items, or your favourite wines, and even in the production of foods, the control of moisture within a particular desired range is required in so many instances.

Older technologies are most often way less economical to run, consuming more energy than what is needed by today’s standards.

Variations in space temperature and humidity may also modify the dimensions and mechanical properties of many materials, systems or processing plants causing challenges and possibly irreversible outcomes through damage to equipment, health compromise or batch rejection. Here for example, one could consider the storage of timber, or the effects low humidity levels could have on electronic equipment static.

“Humidification levels can add greatly to ideal conditioning; however, it is expensive in terms of maintenance and running costs and therefore is normally not commonly implemented in an office or retail environment. It is critical thought in facilities such as operating theatres, certain laboratories, and factories. For instance, at a printing works the relative humidity is essential to their paper handling. De-humidification is part of the cooling process in conventional air conditioning systems since the water condenses on the cold coils, but this is not specifically ‘controlled’ if not a critical requirement,” says Willie Kotze, mechanical building services associate at Zutari.

The need for humidity control

Around the world and even within South Africa, environments can be drastically different from one another. This not only includes inland or coastal factors but differing weather cycles depending on regions. This could be the extremes of any of the earth’s elements – permanent high humidity levels because of continual rain, or low levels due to habitats where relative humidity (RH) can be as low as a couple of percent such as in the South Australian desert.

The ideal gas law describes the relationship between pressure, volume, and temperature in air. The reason why humidity control is required comes down to the nature of the expansion and contraction that occurs due to the changing conditions throughout the day, season, or climate conditions. Expansion and contraction thus create differences in RH and the difference in RH creates various impacts on spaces or materials that could be extremely sensitive to said changes.

“Generally, a 50% RH indoors is ideal for humans to feel comfortable and is also the lowest point in terms of ideal conditions for viruses and bacteria – too humid, and bacteria and mould growth may occur. If air is too dry, dust and static electricity becomes a risk which is not ideal in several settings,” says Kotze.

RH maintains the equilibrium conditions around many hygroscopic materials (product or substances that absorb moisture from the air). This equilibrium prevents loss or moisture absorption as temperature and ambient air moisture content changes. Without a stable RH, products could crack, warp, and deteriorate over time. Stability would be highly relevant in such facilities that produce chemicals that may react adversely outside of set parameters.

Variations in space temperature and humidity may modify the dimensions and mechanical properties of many materials.

A common analogy in describing humidity, which is essentially the percentage of water as a vapour in any given space, is the concept of a cubed sponge that would be proportionately saturated based on the ideal gas law factors.

“At a specific temperature (25 degrees in summer), that sponge could be 60% full, while at 25 degrees in winter it could be 30% full. As contraction and expansion of air occurs, the volume either gets smaller or increases. You can then imagine the level of moisture in that sponge changing. As RH increases it will eventually reach saturation known as the dew point – the air has cooled down to the point where the water inside the air is at 100%. It has got nowhere else to go and so results in condensation,” explains John Andersen, director at Specialised Climate Engineering.

In a data centre as another example or a server room in a bank, engineers are looking at a ‘dynamic window’ or ideal range where if a shift outside of the window occurs, conditions will not be favourable. Less than then 40%, there would be the risk of short circuiting due to static charge. Humidity above 65% you increase the risk of corrosive conditions on the electronics. Each application would have specific ranges to maintain. Other examples in the medical arena are the need for high humidity in burn centres to assist healing, or low humidity in pharmaceutical manufacturing.

The world is also moving quickly towards accommodating healthy environments which includes all the places we live and work. Here humidity is not only about comfort anymore.

“If you do not have enough moisture in a room, the dryer the air is the RH will decrease because of the expansion of the air. This means that breathing that air in, it is going to have a physical impact on occupants through drying out their sinus, it is going to dry the alveoli in their lungs and possibly progress further to de-hydration. On the converse, RH reaching 100% is sufficient to germinate a spore, bacteria, or mould. Once this has happened, all you need is above 75% RH and growth will continue. These scenarios need to be managed correctly in terms of achieving the best people-health, which is vital,” says Andersen.

Other than healthy environments, electronics and processing sites, maintaining the correct humidity band is also important for other materials such as fabrics and items that rely on moisture levels such as wooden goods that can be damaged through continual environment changes. Other applications where humidity control is essential are places such as archives, libraries and museums that have a variety of materials under high levels of protection due to the nature and importance of what is often stored.

In fact, the inclusion of the optimal band of humidity can be applied to almost any item, procedure, or process to produce any goods we have access to today from the paper used to print this publication, the screen you use to consume digital media or the pen on your desk or workstation. Any process involving the use of hygroscopic materials would also be critically reliant on the correct humidity. This could further include food stuffs such as sugar or salt mills, and grain silos, which are susceptible to clumping or coagulation.

Equipment function and differentiating methods

As a brief overview of an extensive topic, humidity control in simple terms is either adding or removing moisture from the air and this can be achieved in several ways using different methods and sensors. We have to identify humidification and de-humidification as separate functions because the equipment for each outcome is different from the other and prevailing climate temperature conditions often determine what method is best suited for specification.

“One type of humidifier typically boils water to create dry steam which could be supplied in air handling units. De-humidification can be affected by a cooling coil followed by a reheating coil or resistance heater. Alternatively, desiccant dryers can be used if low relative humidity is necessary for the particular application. Custom solutions are possible through incorporation of these functions into bulk air supply or by direct in-space inclusion. However, a full design should be completed and is always recommended,” Kotze says.

In some humidification techniques one must make sure that the water source is relatively pure to avoid trace elements being left behind through the evaporative function that forms salt dust which is not good for a system in terms of corrosion or scaling. This may require various pre-treatments such as chlorination or reverse osmosis. However, some electrode-type units, depend on minerals in the supply water to function.

“For humidification, it is preferred to use a ducted air distribution system to introduce the required humidification as Willie has described. This is not always possible though when the required design RH is higher than the evaporator dew point temperature of the air in the air handling unit which limits results. It is always better to introduce the humidity in the return air duct or AHU. Supply air is often close to, or at dew point temperature, adding to risk of oversaturation. We suggest to only use supply air with over humidity protection in the form of a humidistat or high limit sensor down the line. Absorption lengths must also be calculated carefully. The alternative is to humidify within the space and both steam and most atomising humidifiers are suitable for indoor or industrial humidification requirements,” notes Schalekamp.

Dehumidification equipment is generally installed serving the space directly. Process air is re-circulated in multiple passes. The unit can be free standing, wall mounted within, ducted outside, or ceiling mounted.

The inclusion of the optimal band of humidity can be applied to almost any item, procedure or process to produce any goods we have access to today

“If you want to raise RH, designers can focus on units that use adiabatic or ultrasonic techniques. Simply, these use a process of either evaporative media, misting or fogging to push water into the air. Adiabatic techniques generally require very little energy too. These techniques reduce the temperature of the air and have added cooling benefit in summer months when temperatures are high, and humidity is low. Further, from a fresh air makeup perspective according to occupational health and safety standards, these techniques speak to using fresh air that is cooled and humidified for free or vice versa. Other techniques involve thermal functions as Wille alluded to. Heating water to the point of phase change is more expensive. Techniques here would include steam. Steam is appropriate where it is generally cold because you are adding heat as well. You can therefore also reduce wasting energy to increase your conditioning temperatures. This is not a cost-effective solution in hot environments though unless it’s a specific requirement,” notes Andersen.

He continues, “In a de-humidification process, techniques include the condensation method – that is only very effective in warm climates, a heater that dries the air out, a heat pump that most people only think about in terms of heating a geyser or pool but is essentially a reversed air-conditioner, and then desiccant dehumidification which is required mostly in specialised processes of 20% RH or less at lower temperatures or for dew-points less than zero degrees C. Desiccant techniques are also ideal for cold and wet climates, as well as certain process cooling such as cooling injection moulds where a combination of systems may be in operation that creates condensation and needs to be removed. Other production types may require negative degree dew points too.”

Schalekamp adds, “The various principles of operation for both humidification and dehumidification would be the same, although refinement and quality could differ dramatically between brands. The decision of which option is ideally suited depends mostly on the output capacity, control accuracy and energy source availability. In general, when humidification is required, gas or electrical heated steam humidifiers are preferred. For humidification by means of evaporation or atomising (adiabatic) the energy consumption remains similar because preheating of air is required to counter adiabatic cooling and to allow efficient absorption. In our opinion, direct evaporative humidification is not suitable as a humidifier but rather for evaporative cooling and the benefit is free. Refinement such as wastewater management, water absorption efficiency, control accuracy, as well as economical maintenance requirements and product quality is evident in the different brands available. We regularly find the saying ‘you get what you pay for’ to be so true.”

Humidification equipment efficiency is often product quality, or water quality dependent in the form of wastewater. Wasting hot water as a result of unsuitable water quality or bad equipment design affects both electricity and water efficiency use. Evaporative humidifiers can be big water wasters depending on the design and required flushing and draining requirements to prevent bacterial growth such as legionnaires disease (condensing type).

“Desiccant de-humidifiers require greater applied energy than condensing types, However, if the requirement is for low temperature or low RH requirements, there is no choice but to use a desiccant solution”, says Schalekamp.

Technology advances

Kotze says, “Basic principles have remained the same, however, efficiencies have improved. Reheating for instance can be provided via heat recovery from a four-pipe chiller or a VRF unit, or a heat pipe for instance, avoiding the high running cost of resistance heating techniques.”

Older technologies are most often way less economical to run, consuming more energy than what is needed by today’s standards. When there are newer, far more energy efficient alternatives out there that are also designed to reduce a collective carbon footprint through improved system optimisation, it just makes sense to take the upgrade step.

“Structured media technology has seen great development in recent years. The use of this method of humidification has become very useful because the air will only absorb what it can absorb and cannot be over humidified as with steam methods. Recovery systems in terms of entropy is another area in humidity control which has gained a lot of value as technology moves forward and does save significant energy. Sustainability is another perspective that has seen advancement as suppliers aim to achieve net zero outcomes. The same applies for water utilisation efficiency – how much water is used, is water being wasted and how to incorporate the use of grey water in certain systems that use evaporative media. Adoption of better engineering practices such as the elimination of resource intensive methods are also seen as technology advances as some countries for example have banned the use of electrical re-heating totally because other methods are available for this need,” says Andersen.

Today’s modern humidifiers use more premium parts and materials, and they are designed to perform at their peak for far longer. This improved longevity lends to quite a solid value proposition – you just cannot get the same level of performance and dependability out of a decades-old system. Computer chips are faster, filter media has been meticulously overhauled, and the user experience is literally a night-and-day improvement from the devices of old.

“Today there are so many benefits and innovations that older systems were not designed to offer. Modernisation of RH control solution has also been driven by electronic devices such as integrated controllers built into units. The controller allows for easy parameter adjustment, onscreen trouble shooting and extensive error reporting. The addition of communication with building management systems via BACnet, IP, MSTP, Modbus or Lon Works enables greater facility control and reporting”, Schalekamp concludes.

Note: Look out for further technical articles to appear in future issues of the RACA Journal on this topic.

Sources:

  • Carel Group White Paper
  • Humidair
  • Specialised Climate Engineering
  • Zutari

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