By Eamonn Ryan

With a wide range of humidifiers on the market and an increased requirement by end users to reduce energy costs and CO2 emissions, RACA Journal looks at the pros and cons of the different types of humidification systems available today.

Without food, a person can live for five to 10 days. Without water they can carry on for two to three days. Without air they die in three minutes. Until recently, we never had to fear for our precious air, of which we consume copious amounts — between 300ℓ and 600ℓ every hour.

Today, the quality of the air we consume is of senior importance, and something which controls that quality and comfort is its relative humidity (RH) level. Humidity is essentially the percentage of water as a vapour in any given space. A common analogy is the concept of a cubed sponge that would be proportionately saturated based on the ideal gas law factors. The ‘ideal gas’ law describes the relationship between pressure, volume and temperature in air.

At a specific temperature of 25° Celsius in summer, that sponge could be 60% full, while at 25°C in winter it could be 30% full. As contraction and expansion of air occurs, the volume either gets smaller or increases. 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.

Duncan Buxton, marketing manager, Condair, offers the following suggestions as to the diverse types of humidification systems.

Humidity is critical to the precise functioning of laboratories. Image credit: National Cancer Institute

Humidity is critical to the precise functioning of laboratories. Image credit: National Cancer Institute

Steam

The traditional type of humidifier installed by AC contractors is the electrode boiler humidifier. The unit creates steam by passing current through water inside a replaceable or cleanable plastic cylinder. They can be used with steam lances to humidify ducts or fan units to directly humidify a room’s atmosphere. An electrode boiler humidifier has a low capital cost and is simple to install making it a favourite among contractors. However, it does have the highest running cost of any humidifier, making it less popular with end-users.

As steam is produced in an electrode boiler, any minerals in the water are deposited within the plastic cylinder as limescale. Periodically these cylinders require replacement, which is a straightforward operation. However, for high duty applications the spares and service costs associated with maintenance in a year can be more than the initial purchase price.

In recent years there has been a trend to move away from the electrode boiler to resistive steam humidifiers as they do not require replaceable boiling cylinders and therefore have lower running costs. The capital cost is higher, but this is quickly paid back in reduced spares and servicing expense.

Some resistive systems, such as the Condair RS, have innovative scale management features which allow limescale to be removed quickly and easily without needing to open the main body of the humidifier.

Like the electrode boiler, the resistive humidifier is simple to install and can be used to humidify directly in a room’s atmosphere or in a duct. Steam offers intrinsically hygienic humidification so is ideal for use in sensitive applications such as healthcare environments.

However, both electrode and resistive steam humidifiers are electrically powered so a major disadvantage to the end-user is their high energy consumption. This also has implications for those end-users looking to minimise their CO2 emissions.

Christiaan Schalekamp, Technical Sales Director at Humidair. Image credit: © RACA Journal | Eamonn Ryan

Christiaan Schalekamp, Technical Sales Director at Humidair. Image credit: © RACA Journal | Eamonn Ryan

Cold water

Cold water humidifiers present a low energy and low maintenance alternative to steam humidification in certain applications. An evaporative humidifier can deliver high-capacity humidification to an air handling unit (AHU) while running on just 0.5kW of energy.

The energy used to evaporate the cold water is taken from the air rather than electricity or gas. This process provides adiabatic cooling to the air as well as humidification, which can be an added advantage of cold water systems. Up to 12oC of cooling can be achieved.

For many comfort applications additional pre-heating is needed to raise the temperature of the air back to the desired level. However, using gas pre-heating or heat exchangers, cold water humidification is still 75% cheaper in energy costs than using electric steam humidifiers.

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The two main types of cold water humidifiers are spray and evaporative. A spray humidifier consists of a series of nozzles that produce fine droplets of water that rapidly evaporate in the air stream. They can be located either inside a duct or mounted directly in a room. Humidity control is good at ±4%RH, and outputs can be very large with a single system delivering over 1 000ℓ per hour, while running on only 3.2kW of electricity.

Evaporative humidifiers are located inside a duct or AHU and consist of a continually moistened evaporative matrix through which the air flows. As the air passes through the wet matrix it picks up the moisture and is cooled. As there is no aerosol with these systems they can be located inside a short section without any risk of the humidity wetting out on to a nearby bend.

Hygiene in cold water humidifiers is assured through the use of drain and flush cycles. With the humidifier design preventing water from remaining inside the system, bacterial growth is controlled. Silver ion dosing can also be used in cold water humidifiers on the incoming water supply. Silver has a powerful antibacterial action that has a residual effect throughout the humidifier, further restricting bacterial growth.

Installation of cold water humidifiers is straight forward, especially as there is no requirement for three-phase power or gas supply. Maintenance is much less onerous than steam humidifiers as the minerals in the water are either removed by water treatment plant or simply flushed to drain, in the case of the evaporative humidifier.

This double saving of reduced energy cost and low maintenance combined with the additional benefit of adiabatic cooling makes cold water humidifiers a very attractive alternative to traditional steam humidification.

SCE logo. Image credit: © RACA Journal | Eamonn Ryan

SCE logo. Image credit: © RACA Journal | Eamonn Ryan

The specification of humidification

Ensuring you specify the correct humidification system largely depends on being able to extract the right information from the end-user on how the system will be employed. However, it can be difficult to build-up enough experience to know the right questions to ask when projects involving humidifiers don’t occur that frequently in an HVAC consultant’s career.

Buxton offers the following answers to some frequently asked questions:

What level of humidity is required and what level of fluctuation is acceptable? Different applications will require various levels of humidity control. The most common application for an HVAC consultant will be the office environment’s requirement of between 40-60% RH, whereby people are comfortable and static build-up is reduced. Many manufacturing industries will require a more specific level of humidity control: 50-60% RH for printing; 65-75% RH for textile manufacture; or 45-55% RH for an ideal museum environment.

How long will the humidifier be running for and when can it be shut down? If a humidification system is going to be used 24/7 then the number and type of humidifiers will need to reflect this. A critical system that needs to be constantly delivering a certain level of humidity must include run and standby humidifiers as every humidifier in the world needs to be shut down occasionally for maintenance.

How important are running costs and the environmental impact of the system? Running costs vary widely with different types of humidifiers. Some steam systems can use 150 times more energy than an efficient evaporative humidifier and require six times more to be spent on them in servicing and spare parts. The initial purchase cost is a lot less for the steam system but an error in the initial product selection can cost the client (and the environment) dearly over the life of the unit.

What types and how much energy is available? This is a critical question as it’s not unheard of for contractors to arrive on site to install equipment only to find out that the amount of electricity required to run a humidification system is not available. For really large duties, the energy requirements of using an electrical system can become prohibitive and either evaporative, spray or gas humidifiers may be a more viable option for the end-user.

What quality water is being used and what level of maintenance is acceptable? Water quality and maintenance are intrinsically linked when dealing with humidifiers as a poor water quality inevitably leads to a higher servicing requirement. The minerals left behind in the humidifier when the water is either boiled or evaporated into an atmosphere need to be dealt with. If the water has a high mineral content but a high level of maintenance is unacceptable, water treatment should also be specified.

What evaporation distance is required? For humidifiers providing moisture to ducts or AHU systems, the humidifier must be able to evaporate the moisture into the air stream before it meets physical obstructions, like duct corners, otherwise this will cause condensation.

Where are the humidifiers to be situated? If access to the location is restrictive, certain humidifiers may be easier to install than others. Also, if a unit is located in an awkward position, then servicing may be difficult or sometimes impossible.

Factories making textiles require humidity control. Iamge credit: Remy Geiling/Unsplash

Factories making textiles require humidity control. Iamge credit: Remy Geiling/Unsplash

The correct level of humidity in an indoor environment

Humidity conditions can vary sharply according to location — not only inland or coastal factors but differing regional weather cycles — thereby challenging the engineer to implement the correct affordable solution in terms of capital cost and energy consumption. This could include extremes such as permanent high humidity levels because of continual rain, or low levels due to habitats where RH can be as low as a couple of percent such as in the Karoo.

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With this constant change in ambient condition it is critical to accurately control relative humidity in many different applications. Many humidifier users work with “Hygroscopic” materials. This means that a material can lose moisture to the air in dry conditions or absorb moisture in humid conditions. Each material has an “equilibrium relative humidity” at which the material is at balance with the environment. Controlling the relative humidity to this equilibrium level prevents loss or moisture absorption as temperature and ambient air moisture content changes. Failing to do so can be hugely problematic in terms of productivity, quality and profitability.

For instance, at printing works the RH is essential to their paper handling. Paper is supplied to printers with a specified water content. If this dries then the paper can change dimension and curl, giving huge problems as it is fed through the printing machines.

People are also hygroscopic! As well as controlling humidity in working environments, hospitals use humidifiers in operating theatres to prevent moisture being drawn from any body tissue exposed during operations. This can cause premature drying and promote the formation of a scab from coagulated blood. The humidity levels in maternity and obstetric departments should always be maintained as babies are particularly sensitive to a dry atmosphere. There is a need for high humidity in burn centres to prevent drying and assist healing.

Tea, textiles, tobacco and crops in long term cold storage will all lose moisture in dry environments giving problems with quality and weight loss and so directly affecting the bottom line profitability of the customer. Pharmaceutical products must be manufactured in carefully controlled environments to ensure quality, repeatability and conformance with the approved conditions for the particular product.

Museums and art galleries should be kept at stable temperature and humidity levels to prevent damage to priceless artifacts.

Data centres or server rooms should be suitably controlled conditions. Less than 40% RH, there would be the risk of short circuiting due to static charge; above 65% and it would increase the risk of corrosive conditions on the electronics.

The applications for humidifiers are almost endless!

Equipment function and differentiating methods

Humidity control either adds or removes moisture from the air which can be achieved in a variety of methods using sensors. Humidification and de-humidification are separate functions with separate equipment. A full design should be completed and is always recommended.

One type of humidifier 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.

Pieter Aldred, managing member of Humidair, says: “For humidification, it is preferred to use a ducted air distribution system to introduce the required humidification. This is not always possible though when the required design RH is higher than the evaporator dew point temperature of the air in the AHU 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.”

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, rear wall mount, or ceiling mounted.

“If you want to raise RH, designers can focus on units that use adiabatic or ultrasonic techniques,” says Aldred. “Simply, these use a process of either evaporative media, misting or fogging to push water into the air. Adiabatic techniques generally require little energy. 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.”

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He continues, “In a de-humidification process, techniques include; the condensation method – that is only highly effective in warm climates; a heater that dries the air out; a heat pump is essentially a reversed air-conditioner; and then desiccant dehumidification which is required mostly in specialised processes of 20% RH or less. 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 which creates condensation and needs to be removed. Other production types may require negative degree dew points too.”

Museums and art galleries should be kept at stable temperature and humidity levels to prevent damage to priceless artifacts. Image credit: Federico Scarionati/Unsplash

Museums and art galleries should be kept at stable temperature and humidity levels to prevent damage to priceless artifacts. Image credit: Federico Scarionati/Unsplash

Christiaan Schalekamp, technical sales director at Humidair, 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 as to which option is more ideal 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. We regularly find the saying ‘you get what you pay for’ to be true, and ‘the humidification problem is as big as the price you have to pay to control it.’”

Humidification equipment efficiency is often product-quality, or water-quality dependant. 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).

Structured media technology has seen great development in recent years

While basic principles remain constant, efficiencies have improved. For instance, reheating can be provided via heat recovery from a four-pipe chiller or a VRF unit, or a heat pipe, avoiding the high running cost of resistance heating techniques. Newer technologies are often more energy efficient and designed to reduce their carbon footprint through improved system optimisation.

Their value lies in the fact that the air will only absorb what it can absorb and cannot be over humidified — as occurs with steam methods. Adoption of better engineering practices such as the elimination of resource intensive methods are also seen as technology advances as some countries have banned the use of electrical re-heating in favour of other methodologies.

“Today there are tremendous benefits and innovations that older systems were not designed to offer. Modernisation of RH control solutions has 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.

Getting it right first time

The last thing any contractor needs is to have to return to site to correct an issue with a humidifier installation that has arisen at the commissioning stage. To help avoid this, here are some of the most frequent installation issues encountered, with advice from Comair on how to get it right first time.

Location, location, location! The most convenient place to put a steam humidifier is not always the best. Consider access for servicing and put the unit as close to the point of steam delivery as possible so that the steam pipes can be kept short. If steam piping is too long steam will condense prior to being released in the duct, reducing system efficiency. Ideally keep them under 5m lagged copper or 3m flexible steam hose. Take 1m off for every elbow. Also avoid sags or deadlegs where condensate can build-up without running to drain.

Consideration should be given to drain piping too. Steam humidifiers can release water between 60-100°C so low-grade plastic piping can melt and bend. An air gap is always required but don’t position a humidifier over an open drain. Steam rising from the drain can re-enter the unit and cause corrosion or electrical faults in the circuit boards.

Locate the steam lance centrally in the duct, with the holes pointing upwards and at a right angle to the airflow. Common problems occur when steam lances are positioned too close to bends causing steam to condense on the duct prior to absorption. To maintain efficiency, the lance must always be angled to direct condensate into the lance’s drain.

Some philosophies surrounding adding moisture

Factors to take into account when designing a system for a specific space, taking into account energy efficiency.

There are various different methods to remove moisture or to add moisture to a building for specific conditions. The conditions themselves — the moisture content of the air that you require — determines what method you need to add or remove moisture. In addition, there are always heat recovery methods to employ, says John Andersen, director of Specialised Climate Engineering.

“The choice of system really depends on the space and what you require in that space: if it’s people, there’s specific criteria; if it’s materials, there are different criteria, because most materials have moisture isotherms. Then depending on the type of material, whether it be for example a museum, archive, chemical store or goods store, one has to appreciate that one has to not only control humidity for people but also the materials stored there,” explains Andersen.

He notes that most material has an equilibrium (isotherm) on the basis of temperature and humidity. “Raise the temperature and humidity and the material will re-establish an equilibrium at a higher moisture content. Examples are that nylon contains an enormous amount of water statically, while for polyethylene bottles you have to remove as much of the moisture as possible, or there can be quality problems.” Decay of products is associated with high moisture while with too low moisture, materials can become brittle, while static electricity issues also prevail.

Andersen explains that regarding human occupation of buildings, there are both SANS standards and ASHRAE standards. “There is a climate ‘window’ associated with comfort: the right temperature and the right humidity. There can be low temperature with high humidity, or alternatively higher temperature with lower humidity. For humans it’s all about how humidity evaporates from our skin, called vapour pressure deficit or difference, which gives us our comfort. Having the right temperature is important from an energy perspective, but having the right humidity is even more so,” says Andersen.

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“However, controlling moisture is expensive, for instance requiring the adding of water in winter. The phase change energy when adding water to air using adiabatic methods is still the cheapest way of adding water to air, for example, in a large occupied building space. From a fresh air makeup perspective, the fresh air rate that gets pressured into an occupied building should also be extracted with heat recovery — we don’t do this in South Africa. What is ideal is to install what is called an air-to-air heat exchanger so that the air going in absorbs 80% of the energy value of the stale air going out of the building.

“Say, for example, our dry highveld climate is typically at four degrees and 90% humidity, for roughly about five grams of moisture content per kg of air. But occupants want about 20 degrees at about 55%, which is around nine and a half grams of moisture. There’s a four and a half gram difference of moisture content. If one simply brings that air in from outside to heat it up, it then dries people’s eyes or sinuses making the occupants prone to infections. Therefore, moisture needs to be added and this can be expensive if using steam as an example.”

Andersen notes that adiabatic methods (or free energy) are much cheaper in terms of energy consumption. Also useful is where there’s a refrigeration plant, heat pump or solar energy whereby excess energy like hot water can be brought in as pre heat, with the pre heat allowed into any evaporative method. It gets even better if you use an air to air heat exchanger, where your in-air and out-air is in the same AHU physically transferring the higher energy value to the lower energy value. In winter it would be a balance between the air leaving for exhaust versus the air coming in which is fresh. “That’s a wonderful clean building philosophy in which at the same time you’re recovering 80% of the energy used.”

Dehumidification: Some philosophies surrounding removing moisture

Dehumidification is not only a winter issue in wet climates like the Cape, but ironically a  summer dryness one too.

In winter, with wet conditions and cold climates as well as hot very humid conditions, the typical experience at home, or in the commercial business space is normally corrosion, mould growth and decay — which can be expensive from a health and maintenance perspective. The cost of dehumidification is significant from a Capex perspective, but the benefits are significant, making the introduction a worthwhile Opex saving.

In summer, in very humid environments, dehumidification internally using the air-conditioning method within the building space leaves the occupants with some extreme humidity relief. However, it is often necessary to supplement this dehumidification with a dedicated drying system that brings the moisture level into the comfort window, ensuring health and a corrosion or decay free environment. Fresh air make-up systems in these humid environments require dehumidification in order that fresh air is introduced at the same moisture level as the ideal building climate.

In summer, in hot dry environments like the highveld, the constant use of air-conditioning to keep the building space cool, has a negative drying effect, where adiabatic cooling with humidification with fresh air make-up would be energy friendly resolution of this issue.

Humidair at Frigair 2022. Image credit: © RACA Journal | Benjamin Brits

Humidair at Frigair 2022. Image credit: © RACA Journal | Benjamin Brits

Cooling for free on off-peak scenarios using adiabatic methods.

Andersen gives the example of a situation where a building is cooled using evaporative cooling methods, using a contact media associated method. “Say it was using fresh air to do free cooling, circulating fresh air with a blend of building air, and the refrigeration compressors would only switch on when the humidity as well as temperature bands got breached — then refrigeration needed to be used.”

“An innovation is ‘indirect cooling’, where you’re able to do the same amount of cooling but without adding moisture to the air still using the evaporative method, by utilising a heat exchanger with the adiabatically cooled wet-bulb air or water. Another alternative is to use cooling towers, which aren’t as efficient from a wet-bulb approach, so the direct contact evaporative media is still the most efficient option.”

“Large hyper scale data centres use massive amounts of energy on racks so their power utilisation efficiency is required to be under one. They use contact evaporative media for direct and indirect cooling, either two stage or single. High moisture content in the air means that you can’t have high humidity leaving the system going into the building space, so one needs to be able to cool that air without adding moisture. This is accomplished using the energy-friendly indirect method. Project owners might not like it because the opex cost can be a little higher than standard air conditioning because it requires additional maintenance — but that maintenance cost is much less than the energy cost saving; a worthwhile ROI.”

Andersen notes that the concept of ‘full lifecycle cost’ is extremely important in designing an HVAC system, with a five-to-seven-year lifecycle being the optimum for contact media. This model looks at issues such as predictive analysis to look at water quality and its potential to create scaling as water evaporates to leave salt. The elements therefore can be changed while the base system can continue because it’s made out of high-quality steel that’s good for 15–20 years.

Energy considerate decisions

High tech industrial park. Image credit: Glsun Mall/Unsplash

High tech industrial park. Image credit: Glsun Mall/Unsplash

“Dehumidification is generally expensive if not specified using the correct method for the associated condition. There are other highly efficient methods with a cooling coil using refrigerant, which is extremely efficient for dehumidification at high temperatures and high humidities. Beyond that point, the efficiency of the plant from a coefficient of performance perspective needs to be evaulated: The COP drops with temperature. The energy consumed by the plant gets higher compared to the kilowatts of output of the plant as the temperature drops,” says Andersen.

“It depends on what humidity you want to dry the air to — if it’s within the realms of human comfort then HVAC does an extremely good job. However, if you’re able to use waste heat from your cooling plant, then the sorption dehumidification method starts becoming exciting. You’re already cooling the building and have a large amount of energy available from your air conditioning plant in the form of hot gas with mild 55 degree water available that can be used to regenerate a desiccant wheel which does the rest of the dehumidification for free to save a lot of energy on the latent load.” Coils are designed to have a high sensible (real energy temperature removal portion versus a latent portion — moisture content removal). Cooling coils are typically designed to remove heat and if they have to remove a lot of moisture then the efficiency of the coil is compromised against the design standard.”

“Finally, it also depends on where you are — for example Durban having a high moisture content. Having a combination system to reduce the moisture content of the air to bring the air to dew point and then using desiccant humidification thereafter to do the rest of the work to achieve a lower humidity setpoint is actually more efficient. It’s not as energy efficient below 18 degrees air-on to use the refrigeration method as compared to desiccant. There is a balance point where using refrigerants is not much more economical from a kilogram per watt consumption perspective than compared to using desiccant. Combination desiccant and refrigeration cooling has a higher capital cost associated with it, but long term the machines last 15 to 20 years to give a good return on investment,” says Andersen.

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