Getting Humidification Right

By Barbara Horwitz-Bennett, Contributing Editor April 1, 2006

Good indoor air quality depends, in part, on good humidification. But consulting engineers often must win owners over to the benefits of quality humidification.

CSE: What obstacles must be overcome to convince an owner about the importance of installing a quality humidification system in his or her facility?

MUMMA: The main points that must be communicated are: need, benefit and cost-benefit ratio.

Essentially, the owner needs to realize that providing good IAQ reduces productivity loss and illness. For example, Lawrence Berkeley Lab estimates that poor IAQ costs U.S. businesses up to $208 billion per year, whereas winter or dry climate humidification can help reduce these losses.

COOPER: It’s true that the most significant obstacle is our ability to convey to the owner the tangible benefits of the humidification system. In addition to increased employee productivity, other benefits include the fact that building materials, furnishings and finishes will last longer and look better. Furthermore, computer network equipment and other instrumentation will function more reliably in a controlled, stable climate and be safe from the effects of static electricity.

There is a perception that humidification systems require greater maintenance than other building systems, but this is not accurate. Thus, an informed discussion with the owner is paramount.

LENTZ: To summarize, the two different kinds of obstacles that have to be overcome are the perception of need on the part of the decision makers and the technical issues of applying humidification systems, which can have tremendous life-cycle cost implications for the facility.

For the determination of need, the issues can be occupant health, productivity or process needs within the facility, such as maintaining control over static electricity or the physical degradation of a product or material. Outside of health-care facilities, probably the most difficult argument to sell is occupant health. There doesn’t seem to be a lot of hard data that can be readily understood by laypersons to support the issue, and humidity control can be a major cost issue if poorly executed.

The technical issues can also be overwhelming, especially when one is dealing with an existing facility. Again, there are multiple issues involved. One that is all too often not considered, usually to the detriment of the facility, is whether or not the facility can even support humidification. This means that there must be vapor retarders properly located within the wall and roof systems to effectively contain moisture within the occupied space and prevent condensation within wall and roof insulation systems. When humidification systems are added to facilities that are not equipped with appropriate vapor retarders, serious problems can occur, especially in the area of mold and mildew growth. So, if a decision is made to add humidity control, this may require significant modification to a structure in addition to the humidification equipment itself.

CSE: Are there climates or facility types that are most prone to humidification problems, and consequently, IAQ issues?

MUMMA: In arid climates, and those with a month or more where the outside air temperatures drop below about 30°F, good humidification is more critical. Otherwise, the introduction of significant quantities of dry outdoor air can lead to spaces with single-digit relative humidity, which can negatively affect all types of facilities.

LENTZ: It’s important to note that there is not necessarily a direct connection between providing humidification and IAQ issues. Actually, there are many facilities that have never been equipped with humidification systems that provide environments proven to be quite acceptable to the occupants. This is particularly true as one moves toward milder climates. The IAQ issue is more often associated with conditions of inadequate humidity control, such as excessive humidity levels that can produce moisture condensation inside wall or roof systems. In the southeastern U.S., we see these conditions more during hot and humid weather. In the north, IAQ problems result from both excessive and inadequate humidity levels during cold weather operation.

Problems associated with inadequate humidity conditions are more prevalent in the north because of the low moisture levels in the ambient air during the more prolonged cold weather. This is actually aggravated in facilities complying with the ventilation requirements of ANSI/ASHRAE Standard 62.1 due to the greater use of outdoor air. Facilities with the highest outdoor air ventilation requirements have the greatest difficulty maintaining humidity levels during cold weather, but the energy required to meet this need is highly dependent on what types of air-handling and humidification systems are employed.

COOPER: As for types of facilities, health care—including operating rooms, labor-delivery rooms, nurseries, emergency departments and patient rooms—requires controlled humidity to minimize potential bacterial growth, and this is directly related to the successful prevention and treatment of disease. Controlled humidity in laboratories promotes stable research environments, and in animal laboratories, a stable, safe and healthy environments for the animals.

Industrial facilities, including those designed for bulk storage, require controlled humidity to preserve sensitive materials such as paper, wood and textiles. Similarly, manufacturing facilities, such as semiconductor or pharmaceutical plants, also benefit from humidification, as variations in humidity can cause imperfections in the base properties of chemicals and materials used in the critical processes. Data and communications facilities require regulated humidity to maintain a proper environment for the instrumentation to function reliably and free from static electricity. Libraries and museums require controlled humidification to secure the integrity of books, artifacts and other valuable items. And in schools with proper humidity, students spend more time in class and less time home sick.

CSE: What are some common mistakes when specifying humidification systems?

COOPER: One example is that the impact of a 100% economizer cycle on humidification loads must be accounted for to avoid undersizing systems. Properly purified water must also be supplied to ensure operation through the system’s useful life. FDA-approved chemicals or a chemical-free water treatment system should be considered as well when using direct steam injection systems to protect building occupants from potentially harmful effects of boiler water treatment chemicals.

Yet another issue is the location of the humidifier within the system. Sufficient space downstream must be provided for evaporation so that moisture does not condense out onto air-handling components or in the ductwork. Upsizing central humidification systems to satisfy the requirements of a small number of critical spaces, in lieu of utilizing booster humidifiers for those spaces, should also be avoided. It is important to remember that certain types of humidifiers cool the airstream—those that involve phase change—while others heat the airstream. The associated impact on HVAC loads must be taken into consideration as well. Some types of humidifiers can also be quite noisy when operating, particularly the high-pressure steam and high-pressure atomizing types. Consequently, they must be located appropriately, with necessary noise control provisions. Adequate access to humidification equipment must also be provided in such a way that service is feasible without entering critical spaces. And appropriate controls must be included to address special circumstances such as part-load conditions in a variable-air-volume system, and also to precisely control temperature.

LENTZ: First of all, it’s important to understand that no one humidification system is always the right choice. Also, maintenance issues are more readily minimized with certain types of humidifiers than others. There is also a lot that can be done by the system designer to minimize these maintenance issues. Basically, how these systems are applied, or misapplied, has a lot to do with whether a design is perceived as successful, or not.

The next biggest mistake is usually the choice of humidification process. All too often, this is based a designer’s bias rather than the merits of the process with respect to the application. This bias is fed by disinformation and a lack of critical evaluation on the part of the design engineers.

Humidifier control is another area where we see mistakes being made. Every humidification process is controllable; the question is what is the best way for each process. This could be because of the relatively poor understanding of the subject of HVAC system control on the part of designers, but it is also influenced by their choice of process. All humidification processes fall into one of two major categories: evaporative or steam-injection. Each has multiple sub-categories, and each of these has its own unique characteristics. For instance, within the steam-injection category, there is the boiler-fed steam-jacketed humidifier, the boiler-fed clean steam-injection humidifier, the electric clean steam-injection humidifier and the electronic steam humidifier, to name the most common. Within the evaporative category, there is the rotating wetted media—also known as a “slinger” type, the high-pressure atomizing type, the high-pressure air-atomizing type, the “mister” type, ultrasonic type and rigid saturated media type.

The first major distinguishing characteristic between the two main categories is the source of energy. In the case of the steam-injection category, every unit of moisture introduced into the airstream requires the expenditure of virgin energy assets. This makes these types of humidifiers horrifically expensive to operate. This is often justified by the belief that this type of humidifier will not introduce living organisms into the airstream. While this is technically true, humidifiers are not the only process through which biological organisms can enter the airstream, and biological colonies can grow within duct systems regardless of humidifier type used. In the case of the evaporative category, the heat of vaporization is adiabatically drawn from its environment, creating a cooling effect. And the adiabatic nature of evaporative humidification has extremely powerful energy conservation advantages over any type of steam-injection process.

While certain types of evaporative processes have the potential to introduce microbes into the airstream, others do not. The mechanism has to do with the droplet size produced. The droplet must be large enough to support a viable organism and small enough to be buoyant in the airstream. Also, if the water source is functionally organism-free, the droplet size becomes irrelevant. Another concern is hard water deposits in the water. When a droplet is introduced into the air, the water will vaporize and the deposits will create a fine dust consisting of desiccated salts, which can attack the surfaces on which it lands. On the other hand, the rigid saturated and wetted media types will act as a wet scrubber, removing not only airborne particulates—dust—but also water soluble inorganic oxides—CO 2 , NO 2 , SO 2 , HCl, O 3 , cyanides and amines—as well as many water-soluble organic compounds like esters, alcohols and formaldehyde.

CSE: What are some of the newer, more advanced humidification products now available on the market?

LENTZ: Unfortunately, when it comes to humidification, “new and improved” usually just means the “same old stuff” by another name. I think the overall best humidity control system is the rigid saturated media humidifier used with an apparatus dew-point control strategy. From the standpoint of performance and benefits derived, it leaves everything else in the dust. It scrubs the air of both particulates and water-soluble gaseous contaminants, can provide humidity control with a precision of 0.5% RH, uses waste heat as its source of heat of vaporization and can be used to reduce annual cooling energy as a direct evaporative cooler. At the same time, it does need to be provided with an automatic desiccation cycle, daily sump purge to prevent the build-up of contaminants in the sump and a low-level ozonation process to prevent the formation of bio-slimes. If properly designed, when the water flow over the media is chilled, it can eliminate the need for a cooling coil. However, it is not right for every application.

COOPER: Newer technologies that are now commercially available include high-pressure fog systems, which use atomization to create very small water particles for rapid evaporation and shorter absorption distances; and ultrasonic systems, which use a piezoelectric transformer to oscillate water particles at high frequency, breaking the water into a fine mist that easily evaporates. High-pressure fog systems are often applied in larger air-handling systems, while ultrasonic systems are most often used in smaller, more specialized areas such as computer rooms. Chemical-free water treatment systems produce a pulsed, induced field that changes the way minerals in water precipitate. These systems can provide excellent performance, are environmentally friendly and provide savings to the owner by eliminating chemical costs, more efficient heat transfer, lower energy consumption and reduced equipment maintenance.

MUMMA: In my opinion, total energy recovery devices represent a wonderful way to recover moisture generated in the space, as well as that generated by the humidification equipment. It can significantly reduce both the operating cost of humidification and the associated first cost of the humidification equipment. When properly applied, the first cost of the total energy recovery equipment is completely offset by the reduction in the design chiller capacity. In fact, the total energy recovery device can actually be looked at as a free humidifier supplement. Importantly, in some arid climates, it can save substantial quantities of water, thus generating LEED rating points.


Michael F. Cooper , P.E. , Principal and Senior Mechanical Engineer, Harley Ellis Devereaux, Detroit

Mark Lentz , P.E. , President, Lentz Engineering, Sheboygan Falls, Wis.

Stanley A. Mumma , Ph.D., P.E. , Professor of Architectural Engineering, Penn State University, University Park, Pa.

The Key is Enthalpy

The design and contracting communities don’t always understand the sheer magnitude of the energy conservation potential of evaporative processes or the correct methods of application and control of the processes. However, as knowledge of the evaporative process grows within the design disciplines, we will begin to see new HVAC system strategies that are capable of taking advantage of adiabatic processes, not just for humidification or for cooling, but also for the use of enthalpy as a control parameter over the entire range of operating conditions.

Enthalpy is the sum of latent and sensible heat. HVAC systems have been traditionally controlled from temperature, which is a direct measure of sensible heat energy. The problem is with the control and management of latent heat energy. Most HVAC system strategies are open-loop with respect to this variable. HVAC systems that employ refrigeration for dehumidification and steam injection for humidification are open-loop on latent heat energy. This leaves half of the equation for enthalpy basically uncontrolled and its energy conservation potential lost. One of the keys to true high-performance HVAC system design is the ability to generate changes in psychrometric state points by taking advantage of the adiabatic conversion of sensible heat to latent heat, and latent heat to sensible heat.