Humidifiers

Humidifiers are used to increase the relative humidity of air. The most common humidification methods use either steam or fog.

(Copyright Armstrong International, Inc., Three Rivers, MI)

Direct Steam

Direct steam humidification, one of the most common humidification methods, requires a central source of steam.

(Copyright Armstrong International, Inc., Three Rivers, MI)

General Information

Direct steam humidifiers use a central boiler as a source of steam, which is fed to the air through a dispersion grid system. A steam separator is usually needed to condition the boiler steam.

(Copyright Armstrong International, Inc., Three Rivers, MI)

Equipment Design

In direct steam humidifiers steam enters the system and flows into a strainer, which removes solid impurities. The steam continues around the steam jacketed dispersion tube, where its heat is used to reduce condensation of the exiting steam.

Next, the steam enters a separator, and condensate drains out. The steam flows into a drying chamber that removes any remaining condensate and reduces the noise of the escaping steam. The steam flowrate is regulated by a valve.

Finally, the steam is discharged through the dispersion tube, usually into air ducts.

(Copyright Armstrong International, Inc., Three Rivers, MI)

(Animation based on picture copyright Armstrong International, Inc., Three Rivers, MI)

Usage Examples

Steam humidification has numerous applications in industry. Because of its high temperature, steam is essentially sterile. This makes steam humidification useful in applications such as healthcare and cleanrooms. Humidification is also used in electrical circuit production facilities to reduce the risk of harmful static electricity effects. Shown below to the left, is an example of an operating room, where maintaining humidity levels is essential. To the right, is a printed circuit board which must be produced in the correct humidity to prevent damage from static electricity.

(Copyright Armstrong International, Inc., Three Rivers, MI)

(Copyright JS Humidifiers Plc, West Sussex, UK)

The paper and leather industries also use humidification to prevent their products from becoming dry and brittle, as demonstrated by the pictures below: The sheet on the left has the proper amount of moisture, while the sheet on the right lacks moisture and breaks easily.

(Copyright Armstrong International, Inc., Three Rivers, MI)

Advantages

Disadvantages

  • Excellent response to changes in settings
  • Good humidity control
  • Low operating cost
  • No effect on air temperature
  • Must have central steam source

Electronic

Electronic humidifiers use electricity to generate steam for humidification. They are often used when a central steam source is not available.

(Copyright JS Humidifiers Plc, West Sussex, UK)

General Information

In an electronic humidifier, electricity is used to generate steam. The steam is then fed into air ducts through a series of dispersion tubes, shown below. Most units also include automatic control systems to ensure constant humidity.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Equipment Design

An electronic electrode humidifier uses submerged electrodes to pass a current through water, causing it to boil and produce steam. The electrodes are housed in the steam generator, shown below. The steam is then fed to dispersion tubes. The water used in electrode humidifiers must be ionized enough to conduct electricity, or the water won't boil.

(Copyright Armstrong International, Inc.,

Three Rivers, MI)

The diagram below shows an example of submerged electrodes. Initially, only the lower section of the electrodes are exposed to the water. Over time scale builds up on the electrodes and reduces the conductivity. The humidifier recognizes the decrease in conductivity and automatically raises the water level to expose fresh parts of the electrodes to the water in order to maintain consistent humidity. This humidifier is equipped with a pump and drainage system in order to control the fresh water exposed to the electrodes.

(Copyright JS Humidifiers Plc, West Sussex, UK)

The picture below shows ionic bed humidifiers, which use immersed heating coils instead of electrodes to boil water. Because electricity is not passing directly through the water, water conductivity is not important.

(Copyright Armstrong International, Inc., Three Rivers, MI)

Ionic bed cartridges contain fibrous media that keep the water free of impurities. The ionic bed attracts solids from the water, reducing the buildup of solids on the tank walls. Maintenance therefore consists only of replacing the cartridges periodically.

Usage Examples

Electronic humidification is used in the same situations as direct steam humidification when a central source of steam is not available. It is used in a variety of applications, from textile factories (left) and museums (right) to maintaining constant humidity in test chambers. Electronic humidification is usually most efficient in smaller load applications.

(Copyright Walter Meier (Climate USA) Inc., Manheim, PA)

Advantages

Disadvantages

  • No central steam source needed
  • Low maintenance
  • No effect on air temperature
  • More expensive than other humidification options
  • Average response to changes in settings

Water Spray

Unlike other humidifiers, water spray systems use room temperature water instead of steam. The picture below shows a two head water spray humidification system.

(Copyright JS Humidifiers Plc, West Sussex, UK)

General Information

Spray humidifiers have specially designed nozzles that atomize water into a very fine fog, which is quickly absorbed into the surrounding atmosphere. As the liquid leaves the nozzle, the stream quickly expands, forming small particles. For more information, see the Nozzles section of the encyclopedia.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Equipment Design

The key to effective water spray humidification is in the nozzle design. Impingement nozzles are usually used for humidification purposes because they produce a very fine spray.

Air atomizing nozzles, such as the one shown, are also used for humidification. A high velocity stream of air enters from the top, while a stream of water enters from the bottom. The air stream serves to break up the water into small particles, which are driven through the orifice, producing the atomized water particles.

(Copyright Armstrong International, Inc., Three Rivers, MI)

Below is another design for an air atomizing nozzle. After exiting the orifice, the stream hits a resonator tip that further reduces the particle size. This produces a fog.

(Copyright JS Humidifiers Plc, West Sussex, UK)

This impingement nozzle is similar to the air atomizing nozzle shown above. The water feed is forced through the small orifice, then hits the resonator, creating very small particles.

The picture to the right shows an impingement nozzle in use. The fog flowrate can be varied by changing the pressure or flowrate of the water feed.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Usage Examples

Water spray humidification is useful in applications in which evaporative cooling is needed: The temperature of the atmosphere drops slightly as the air loses the heat necessary to vaporize the water particles.

For example, water spray humidifiers are used in timber processing, as shown below. Water sprays are also used in textile and paper mills to reduce fire hazards and in greenhouses to maintain humidity.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Advantages

Disadvantages

  • Low operating cost
  • Steam not needed
  • No moving parts to wear out
  • Slow response to changes in settings
  • Subject to corrosion and bacteria problems
  • Can decrease air temperature

Steam To Steam

General Information

For applications where chemical-free steam is needed, boiler steam may not be pure enough to use for humidification. In these cases, a steam to steam humidifier may be necessary.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Equipment Design

In steam to steam humidifiers, boiler steam is used only as a source of heat. Boiler steam enters a heat exchanger, where its heat is used to generate chemical-free steam from pure water. The boiler steam does not contact the regenerated steam, keeping it chemical-free. The regenerated steam is fed through a distribution tube and directly into an air duct. The regenerated steam is usually at atmospheric pressure, which makes equipment location very important.

(Copyright JS Humidifiers Plc, West Sussex, UK)

The pictures below show the inside of a steam-to-steam humidifier. On the right, the heat exchanger and tube used to transport condensed steam back to the boiler can be seen.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Usage Examples

Steam-to-steam humidifiers produce pure, sterile steam that can be distributed to air ducts or air handling units. These humidifiers are used for buildings and large institutions that have a central boiler system, such as hospitals, cleanrooms and data centers. Shown below to the left is an example of a cleanroom environment where maintaining humidity levels can increase production yields. On the right is an example of a data center. Maintaining humidty levels is essential in data centers because if humidity levels drop, static electricity can build up, leading to damage to servers and equipment.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Advantages

Disadvantages

  • More efficient than electronic humidifiers
  • Low operating cost
  • Slow response to changes in settings
  • Frequent maintenance required
  • Central steam source required

Evaporative

General Information/Equipment Design

One example of evaporative humidifiers are evaporative pan humidifiers. They are most often used in small areas that do not require a large humidification load. Steam, hot water, or electricity is used to boil water in a large flat open pan. The steam is absorbed into the atmosphere, increasing the humidity.

Usually, a fan or blower is used to disperse the steam, which humidifies the area. The water supply is regulated using a float valve, ensuring that the pan does not run dry.

Evaporative humidifiers can also be used in large, industrial scale situations to provide low energy humidification. A matrix, shown below, is connected to the air handling unit or duct. Water is supplied to the top of the matrix and flows down the surface. Warm air is then passed through the moist matrix, evaporating the water. The air then leaves the matrix to humidify the building or space.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Usage Examples

Evaporative pan humidification systems are only used occasionally in industry. They are efficient only for small load applications, and where a central steam source is not available.

The evaporative humidifier shown below is used for humidification and evaporative cooling in an industrial setting.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Evaporative humidifiers can also be used in a smaller scale, such as in the home or office. The humidifier shown below pulls air into the unit through a moistened pad and releases the humidified air back into the room.

(Copyright JS Humidifiers Plc, West Sussex, UK)

Advantages

Disadvantages

  • Low operating cost
  • Good vapor quality
  • No steam source required
  • Prone to bacteria growth
  • Slow response to changes in settings
  • Small capacity per unit size
  • Pan subject to corrosion
  • Causes slight air temperature rise

Acknowledgements

Armstrong International, Inc. , Three Rivers, MI

JS Humidifiers, PLc , West Sussex, UK

Walter Meier (Climate USA) Inc. , Manheim, PA

References

Clifford, George. Modern Heating Ventilating and Air Conditioning . Englewood Cliffs:      Prentice-Hall, 1990. Print.

DeBat, Richard J. "Humidity, the Great Equilizer." HPAC Heating, Piping, Air Conditioning .      v68, Oct 10 1996. Print.

McCabe, Warren L., Julian C. Smith, and Peter Harriott. Unit Operations of Chemical Engineering . 5th ed. New York: McGraw-Hill, 1993, 751-760. Print.

Olson, Arnie J. "Improve Indoor Air Quality with Chemical Free Humidification." Heating Piping and Air Conditioning . v67, July 7, 1995. Print.

Watt, John R. Evaporative Air Conditioning Handbook . 2nd ed. New York: Chapman and Hall,      1986. Print.

Developers

Michael Fein

Steve Wesorick

Joseph Palazzolo

Kelsey Kaplan

Keith Minbiole