There's water (vapor) in the air: benefits and considerations of humidification

While too much or too little humidity may cause discomfort for or even impose certain risks on some people, just the right amount can be beneficial. Relative humidity ranging from 30 to 60% is recommended for the prevention and treatment of disease as noted in the 2012 ASHRAE HANDBOOK-HVAC Systems and Equipment.

02/29/2016



While too much or too little humidity may cause discomfort for or even impose certain risks on some people, just the right amount can be beneficial. Relative humidity ranging from 30 to 60% is recommended for the prevention and treatment of disease as notWhile too much or too little humidity may cause discomfort for or even impose certain risks on some people, just the right amount can be beneficial. Relative humidity ranging from 30% to 60% is recommended for the prevention and treatment of disease as noted in the 2012 ASHRAE HANDBOOK - HVAC Systems and Equipment. For some specific applications in health care facilities, ASHRAE/ASHE Standard 170-2008 "Ventilation of Health Care Facilities" Addendum "d" has expanded the range to 20 to 60%. Relative humidity levels within this range are considered optimal, whereas humidity levels above and below these values increase the chance of infection. In addition to the increased possibility of infection, low humidity dries hair, skin, eyes, and mucous membranes in respiratory systems. It also causes drying, shrinking, and cracking in wood, fabric, and paintings, and static electricity discharges between people and electronic equipment, including computers. From a thermal comfort standpoint, ASHRAE Standard 55 "Thermal Environmental Conditions for Human Occupancy" does not specify a minimum humidity level, but its appendix acknowledges that other factors, such as some of those indicated above, may affect acceptability of minimum humidity levels.

To avoid low humidity, humidifiers are used to add water vapor to the air. Water vapor can be added to the supply air distribution system serving spaces or to the occupied spaces directly. Here we'll review some common types of humidifiers.

Atomizing humidifiers introduce fine water droplets, similar to fog, directly into the airstream. No unwanted heat is added during this adiabatic humidification process. Two types of atomizing humidifiers include:

  • Pressurized water atomizing humidifiers - force water at high pressure through special nozzles to generate very fine water droplets that evaporate into the air.
  • Compressed-air atomizing humidifiers - use compressed-air and special nozzles to break up the water into a fog that evaporates into the air.

You should consider the mineral content of the water to minimize mineral fallout dusting on humidified surfaces. Higher purity water, such as reverse osmosis or deionized water, is frequently used with these types of humidifiers.

Direct steam injection humidifiers inject steam into a supply airstream or directly into an occupied space. Unwanted heat can be added during this isothermal humidification process, but can be minimized with insulated distribution tubes or manifolds. Three types of direct steam injection humidifiers are:

  • Distributer tube humidifiers - available with a single steam distributer tube or multiple steam distributer tubes designed for dry steam injection.
  • Panel distribution manifold humidifiers - designed for short absorption distances and dry steam injection and consist of headers and multiple distribution tubes in various configurations.
  • Area-type direct steam injection humidifiers - discharge steam directly into the occupied space via propeller fans; a utilitarian approach for industrial types of applications.

Supply steam for direct steam injection must be evaluated for chemical treatment prior to consideration for use in direct injection humidification. ASHRAE Standard 62.1-2013 "Ventilation for Acceptable Indoor Air Quality" references 21 CFR 173.310 for steam humidification, which is the document that indicates the FDA-established permissible exposure limits (PELs). ASHRAE/ASHE Standard 170-2008 allows the use of chemically treated steam for direct injection humidification subject to FDA-established PELs. However, ASHRAE HVAC Design Manual for Hospitals and Clinics - Second Edition indicates that the ASHRAE Standard 62.1 committee is considering future updates to the standard that may forbid the use of chemical additives in steam humidification systems. "Clean steam," made from purer, non-chemically treated water such as reverse osmosis or deionized water, can be used for direct steam injection humidification when use of chemically treated steam is a concern.

Self-contained humidifiers create steam from a water source using a heating source. Unwanted heat can be added during this isothermal humidification process, but can be minimized with insulated distribution tubes or manifolds. The following are three types of self-contained humidifiers:

  • Electric resistance humidifiers - heat the water directly to generate steam. Full range of makeup water purity is acceptable.
  • Electrode humidifiers - pass a current through the water to heat the water to generate steam. Makeup water needs to be adequately conductive in order to be used. Purer water sources cannot be used for makeup water source.
  • Gas-fired humidifiers - use gas combustion burners in conjunction with heat exchangers to heat the water to generate steam. Full range of makeup water purity is acceptable.

Steam-to-steam heat exchanger humidifiers create steam from makeup water source using a separate higher pressure steam source to heat the water to generate steam. Full range of makeup water purity is acceptable. Unwanted heat can be added during this isothermal humidification process, but can be minimized with insulated distribution tubes or manifolds.

In self-contained and heat exchanger humidifiers, evaporating water leaves mineral deposits in the humidifier. These humidifiers are equipped with controls that periodically remove part of the contents of these vessels to minimize scaling. The steam may be piped into a distributer tube or panel distribution manifold mounted in the duct system, or it may be mixed with air and discharged directly into the occupied space via a small fan.

In addition to determining which type of humidifier you should specify for your project, also think about what might be the best location for it. When deciding on the location of humidifiers, you need to consider downstream absorption distance. Sufficient distance needs to be provided downstream so that all of the water vapor is fully absorbed into the airstream before it can impinge on any components within the ductwork or air handling unit. If the air impinges on components before fully absorbed, it could result in undesirable condensation. So, whichever humidifier type works best for your project, always be mindful of its location.


Paula Gillette is a licensed professional engineer in 10 states and an ASHRAE committee member on Standard Project Committee 189.3 "Standard for the Design, Construction and Operation of Sustainable High-Performance Health Care Facilities," as well as involved with Technical Committee 9.6 "Health Care Facility Air-Conditioning Applications." This article originally appeared on ARCOM. ARCOM is a CFE Media content partner.

Edited by Ksenia Avrakhova, production coordinator, CFE Media, ksia94@gmail.com.



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