The major costs associated with humidification systems are: Capital Expenditure (CAPEX) – purchase and installation and Operating Expenditure (OPEX) – maintenance and energy.

Humidifiers all add moisture to the air, with a variety of different technologies available which achieve this. These include: 

• Steam (resistance/ electrode/ gas-fired)

• Evaporative pads/ media

• High pressure

• Twin fluid

• Ultrasonic

• Rotary atomiser

Isothermal systems

Isothermal systems provide external heat to water. This is then used for evaporation into the air and so these systems do not affect the temperature of the air to be humidified. Isothermal humidification systems are steam-based.

Electrode humidifiers pass a current through water, producing steam. These systems have relatively low capital costs (and are simple to install) but have high operational costs both in terms of energy usage and maintenance. This is largely due to the boiling cylinders, with these requiring regular replacement.

Resistive humidifiers use heating elements to produce steam. This results in lower operating costs than electrode boilers as they require less maintenance/ replacement parts, although they are more expensive to purchase. They also have high energy costs as they use the same amount of electricity as electrode boilers (unless RO systems are used, in which case wastage of heated water is reduced).

Gas-fired steam humidifiers offer significant savings on energy costs over electric steam systems since gas has a much lower cost per kWh. However, this is counter-balanced by their higher capital costs.

Adiabatic/ cold water systems

While steam has historically been the de-facto system used for humidification, adiabatic systems are increasingly common. These evaporate water using heat from the air and therefore have considerably lower energy costs than isothermal systems. 

As adiabatic systems extract heat from the air, they cause a temperature drop. In summer or in installations where there is waste heat (such as datacentres) this temperature drop is desirable, providing a much less energy intensive source of cooling than the conventional mechanical systems.

Where this temperature drop is non-desirable, pre-heating will be required to maintain desired temperatures. This increases energy costs, though savings over electric steam humidifiers can still be realised as this heating can often be provided by gas-fired systems.

Also, while steam is inherently hygienic, adiabatic systems necessitate installation of UV and RO systems, adding to capital costs. Both UV and RO systems use relatively low amounts of energy, but this still needs to be factored in when evaluating operating costs.

Evaporative pads trickle water over a fill material. Air passes through this, extracting moisture. These systems have low capital costs and are often the cheapest to run energy-wise. However, they have high water usage and, depending on the system, there can be relatively high maintenance requirements as the media require monitoring and cleaning/ replacement.

High pressure spray systems force water at high pressure through small orifices with some systems also incorporating impaction pins. This produces small droplets which then evaporate into the air. These systems are mid-range in terms of capital costs and have low operating costs. However, for duct installations these may require droplet separators to be installed downstream which increases the costs of installation.

Twin fluid systems use a mixture of compressed air and water for atomisation. The sprays produced are largely composed of very fine droplets, which evaporate quickly. This technology is better suited for operations requiring lower volumes as the costs of producing large amounts of compressed air (both capital and operational) usually make them non-competitive for larger scale applications.

Ultrasonic systems use oscillating piezoelectric transducers to cavitate water, producing extremely small droplet sizes. Operating costs are low, but capital costs are often prohibitive. Also, due to their limited capacity per unit they are not easily scalable for operations requiring higher volumes.

Rotary atomiser systems are usually mid-range for installation costs and present one of the cheapest options in terms of energy usage. Maintenance requirements for these systems are relatively low as no high pressures or compressors are required. However, like high pressure systems, in ducts these may require droplet separators to be installed downstream which increases the costs of installation

A useful metric for comparing energy costs is kWh/kg of water introduced for humidification. Although these vary from system to system, ballpark figures are:

• Steam                ~0.75-1 kWh/Kg

• Evaporative pads/ media        <0.001 kWh/Kg

• High pressure spray systems        ~0.004 kWh/kg 

• Twin fluid systems            ~0.021 kWh/kg

• Ultrasonic systems            ~0.060 kWh/kg 

• Rotary atomiser systems        <0.003 kWh/kg 

Systems requiring RO and UV can expect additional energy usage of approximately 0.003 kWh/kg, although this varies with capacity.

For all humidifier types, there is also the benefit that avoiding low relative humidity allows people to feel comfortable at lower temperatures- leading to potentially significant savings on heating system energy costs.

There are clearly a wide variety of humidification options, each having different advantages and disadvantages from a technological, CAPEX and OPEX standpoint. Each application therefore requires a careful balancing act between all these factors. However, there may be a variety of options which may be suitable and, choice is never a bad thing.