Design Guidelines for Humidification Systems


Use available resources to calculate load correctly

When specifying a humidification system to meet design conditions, expect a maximum humidification load of two to three pounds per hour for every 100 cfm of outside air introduced. A humidified building constantly loses moisture, primarily due to air changes. As a general rule, humidification load is driven by the amount of outside or makeup air entering a building or space, and calculated based on the difference between entering conditions and desired conditions. Load calculation methods vary, depending on whether the building uses mechanical, natural, or economizer ventilation.

A quick and accurate way to calculate load correctly is to use DriSteem's DriCalc sizing and selection software.

Choose energy source to fit application

To convert one pound of water to vapor requires approximately 1,000 Btu. Choosing the correct energy source for this conversion not only depends on energy efficiency but on application specifics as well.

Boiler steam: Direct injection or steam-to-steam humidifiers
If there is an existing onsite pressurized steam boiler with available capacity, a cost-effective and energy efficient humidification method is to inject pressurized boiler steam directly into a space or ducted airstream.
Learn more: DriSteem pressurized steam products

Some owners object to direct boiler steam injection humidification because boiler water anticorrosion chemicals are emitted into the airstream with the humidification steam, creating a potential health hazard. To take advantage of on-site boiler steam while avoiding the disadvantage of boiler chemicals in the airstream, consider a closed loop steam humidification system, such as a steam-to-steam evaporative steam generator. A steam-to-steam system routes boiler steam through a heat exchanger located in a boiling chamber filled with clean water and creates clean humidification steam that doesn't contain boiler chemicals.
Learn more: STS humidifier

Gas steam-to-steam humidifiers
Evaporative steam generators can operate using electricity or gas as energy sources. Historically, natural or LP gas humidification systems have provided significant energy savings over electric humidification systems. However, gas systems require flue venting and some require a sealed combustion air supply. If these requirements can be met, a gas fired evaporative steam generator is an cost-effective choice.
Learn more: GTS humidifier

Electric steam-to-steam humidifiers
A significant advantage of electric resistive element or electrode evaporative steam generator systems is that they can be installed wherever power is available. Because of their relatively small size and ability to integrate into existing systems, electric humidification systems are used for many different application types.

Electric humidification systems can provide tight RH control - to within 1% of set point - achieved usually with an electric resistive element steam generator operating with solid state relay (SSR) control and using deionized (DI) or reverse osmosis (RO) treated water. This capability is critical for manufacturing processes requiring tight RH control. The many benefits of electric humidification, including lower upfront costs than gas humidification, frequently outweigh the typically higher energy costs.
Learn more: Electric steam humidifiers

Compare isothermal humidifier energy costs
Isothermal humidification systems use electricity or gas as an external heat source to change water to steam. An isothermal humidification system operating for one year in Minneapolis - with a 100 lb/hr humidification design load humidifying 13,000 cfm with 30% outside air to meet desired conditions of 69° and 40% RH - would produce approximately 290,000 lb of humidification steam in one year. The Energy Information Agency lists average industrial utility rates for Minnesota (as of July 2005, the most current rates available at this writing) to be $0.056/kWh for electricity and $7.15/1,000 cu ft for natural gas.

Based on these rates, and adjusted for typical evaporative steam generator efficiencies, the energy cost to humidify the space described above for one year is approximately $5,600 using an electric evaporative steam generator and approximately $2,750 using a gas-fired evaporative steam generator, a significant savings. Given the fluctuating nature of energy costs, the easiest and most accurate way to compare electric and gas humidification energy costs is to use a humidifier manufacturer's energy calculation program that creates a customized report based on your local energy rates.
Learn more: EnergyCalc software

Adiabatic (unheated water) humidifiers
An alternative to isothermal humidification is adiabatic humidification. Adiabatic systems such as high-pressure atomization and wetted media systems disperse water droplets or fog into air that has enough latent heat energy to cause dispersed water to change state to vapor. As the dispersed water or fog changes state, air temperature drops, and RH increases.

Adiabatic humidifiers utilize a variety of technologies to introduce water into air, either by dispersing water droplets or by allowing water to evaporate from a wetted media, causing relative humidity (RH) levels to increase and air temperature (dry bulb) to decrease.

Perhaps the most touted benefit of adiabatic humidification is air cooling. Because adiabatic humidifiers draw heat from air for evaporation, they can produce a 20°F or more temperature drop. In fact, every pound of adiabatic humidification added to air removes approximately 1000 Btu of heat. Twelve pounds of water added as humidification equals about one ton of cooling. This is a significant benefit in climates where air is consistently warm and dry, or in spaces where there is an additional heat load such as from equipment in a computer room. 
Learn more: High-pressure system

Choose makeup water type carefully

Humidification systems operate using many water types, including:

  • Potable water, directly from a city or well water source.
  • Softened water, either heated or unheated.
  • Reverse Osmosis (RO) water, which has gone through a filtering process to remove most minerals, as well as some contaminants.
  • Deionized (DI) water, which is generally considered to be the purest water.

Mineral concentration in makeup water supply will determine how well various types of humidifiers will work in a given area and directly relates to the amount of maintenance an evaporative steam generator requires. As mineral concentration increases, so does maintenance.

Before selecting a system, water should be tested to ensure proper humidifier type selection and understanding of the ongoing maintenance requirements.

Well water is typically the most contaminated and therefore the least desirable source of water for any humidification system. If well water is the only source available, pre-treatment before it enters the humidifier is essential. City water sources are significantly cleaner and more reliable than well water and are a good choice for supplying water to your system.

Many cities provide softened water making it even more desirable as a source. Pre-treatment of the water may still provide significant value by extending intervals between humidifier maintenance and extending the life of the equipment.

Environments that require a humidifier to be continuously online use demineralized (RO and/or DI) water. DI water must be used in applications where humidification steam must be free of minerals and contaminants, or where uninterrupted service is required.
Learn more: Water treatment systems

Equipment location is critical

An isothermal humidification system includes a pressurized steam boiler or an evaporative steam generator, control components, and a dispersion assembly. While an evaporative steam generator or atomizing pumping station can be placed in numerous locations, proper placement of the dispersion assembly and control components is crucial.

Know absorption distance to choose dispersion assembly
Available absorption distance is the key factor affecting dispersion assembly choice. In AHUs or ducts, dispersed steam or mist must be absorbed into the air before contacting duct elbows, fans, vanes, filters, or any solid object, or dripping will occur. For example, if there is 10 ft of available unobstructed duct or AHU length downstream of the dispersion assembly, several types of dispersion assemblies will achieve absorption within that distance. Applications with 18 in. or less of available unobstructed duct or AHU length downstream of the dispersion assembly have fewer dispersion assembly choices to achieve absorption. Know the available absorption distance before selecting equipment.
Learn more: Non-pressurized dispersion products

Be aware of operating sound characteristics when choosing system component locations. Steam generators, pressurized dispersion assemblies, and adiabatic pumping stations generate some noise. Dispersion tube assemblies and fans generate minimal noise. Isolate noise-generating equipment is areas where noise isn't an issue. Connect to fans in open spaces or dispersion assemblies in ducts and AHUs with piping or hose.

Provide maintenance access
Choose a location that allows access to equipment for convenient visual inspection and maintenance. Pressurized steam dispersion systems include devices (such as a steam control valve, a steam trap, and a strainer) that require periodic maintenance. While these devices can operate for long periods without attention, they must be accessible for inspection.

Depending on makeup water type, evaporative steam generators require periodic cleaning or cylinder replacement, so allow room for access to cleanout plates and cover removal.

Electrical components are housed within the humidifier cabinet or located remotely. Allow 36 in. clearance in front of electrical panels.

Control choices impact system choice and performance

A wide range of relative humidity control — from ±1% to ± 5% RH — is achievable with current humidification system control technology; however, note that humidity control rests not only with the control system, but with the complete building system. Individual building dynamics, such as temperature or air changes, can affect the accuracy and control capability of any humidification system. 

The factors to consider when specifying humidity control are:

Desired relative humidity percentage (set point)
Typical RH ranges are:
     •  Comfort, static control: 35% to 40% RH.
     •  Paper storage, printing: 40% to 50% RH.
     •  Clean room: 35% to 55% RH.
     •  Medical facilities: 35% to 50% RH for general areas; 60% to 80% for critical care areas.

Control accuracy required
How critical is control accuracy and what is an acceptable variance from the desired RH set point? Some manufacturing processes tolerate RH fluctuations of only ±1% from set point; humidification provided to improve human comfort can fluctuate up to ±5% from set point.

Space temperature
The amount of moisture air can hold correlates directly to air temperature — this is why it's called "relative humidity." If air temperatures fluctuate, RH levels fluctuate. Accurate RH control requires stable temperature control.
Component quality
The control system is only as good as the least accurate component in the system. Selecting humidity controls that match the application can eliminate many system difficulties.

Component location
Sensor and transmitter location has a significant impact on humidifier performance.
Learn more: Location, location, location: Sensor and transmitter locations are critical

Component types
Two control components that help ensure drip-free dispersion are a duct high limit humidistat and an airflow proving switch connected to the humidifier controller.

A duct high limit humidistat ensures that RH does not exceed a typical set point of 85% to 90% at the humidistat location, downstream of the dispersion assembly. To ensure proper operation, locate the duct high limit humidistat far enough downstream of the dispersion assembly to allow steam to become fully absorbed and equally distributed in the airstream. Otherwise, short cycling can occur, resulting in an unsatisfied humidification demand.

Systems with constant air volume can use on/off-type high limit humidistats. Systems with VAV require a higher level of control than a constant air volume system and must use modulating high limit humidistats to track airflow changes. Changing airflow quantities require the use of both space- and duct-mounted humidity controls used in conjunction with a programmable logic controller to modulate humidifier output.

An airflow proving switch, mounted in the duct downstream of the dispersion assembly, switches open a safety circuit if airflow in the duct stops or decreases below an acceptable level, disabling the humidifier. Constant volume systems can use pressure- or sail-type airflow proving switches. VAV systems must use sail-type airflow proving switches because pressure-type switches are not accurate at low airflows.

A common control component used in cold climates is a temperature offset transmitter mounted on an inside pane of an exterior window. This device transmits window temperature to the controller, which lowers the RH set point when the outdoor temperature drops, preventing window condensation.