DEHUMIDIFIERS


Commercially, dehumidifiers are used to condition air for comfort and to protect areas from damage due to high humidity.


Industrially, they serve four purposes:

  1. Prevention of clogging and sticking in equipment, also called moisture regain
  2. Prevention of condensation
  3. Prevention of corrosion, and rotting
  4. Drying of heat-sensitive materials

Dehumidification can be performed by desiccant-based or condensation-based processes.



(Copyright Bry-Air, Inc., Sunbury, OH)

DESICCANT DEHUMIDIFIERS


Desiccant dehumidifiers are most often used in industry. They are used to protect equipment and keep large areas dry.



(Copyright Bry-Air, Inc., Sunbury, OH)

GENERAL INFORMATION

Desiccant based dehumidification is most effective when a low moisture level or dew point is required. The desiccant absorbs water vapor from the air stream as it passes through the dehumidifier.


(Copyright Bry-Air, Inc., Sunbury, OH)
(Copyright Calorex Heat Pumps Ltd, UK)

EQUIPMENT DESIGN

The desiccant surface may be introduced into the air-stream in several ways. Holding the material in a wheel-shaped matrix, as shown in this animation, is one of the most common methods.



Humid air, exemplified by the blue arrows, is forced through the desiccant material. The desiccant adsorbs the water vapor in the humid air, and the dry air, shown by the gray arrows, is vented back into the room. The desiccant is dried and reactivated as it rotates by using an incoming stream of warm air to evaporate the adsorbed water, as shown by the pink arrows. The hot air is vented, as demonstrated by the purple arrows, and the process is repeated.


A desiccant cartridge, such as the one shown in the left picture below, contains desiccant material. The desiccant is often silica gel, which is pictured below on the right. The individual beads have diameters in the millimeter range.


(Copyright Ted Pella, Inc., Redding, CA)
(Copyright Delta Adsorbents, Roselle, IL)

Desiccant materials have a very low vapor pressure compared to the vapor pressure of the humid air entering the dehumidifier. This difference in vapor pressures drives the water vapor out of the air and into the desiccant, leaving drier air behind.


USAGE EXAMPLES

Desiccant based dehumidifiers are used most commonly in industry and for large areas, such as the hockey rink pictured below. Dehumidifiers keep the ice surface clear of condensation and fog by removing moisture from the air. Without a dehumidifer, condensation would freeze on the ice and create a poor skating surface.



(Copyright Joan Broadwell, West Bloomfield, MI)

ADVANTAGES

DISADVANTAGES

  • Removes water as a vapor rather than a liquid, leaving no chance of freezing equipment.
  • Maintains a level of humidity well.
  • More economical than condensation method at lower dew points.
  • Desiccant material requires periodic replacement.
  • Does not allow precise temperature control.
  • Produces only saturated air. Additional equipment is required if the relative humidity needs to be controlled.

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CONDENSATION DEHUMIDIFIERS


Condensation dehumidifiers cool humid air to its dew point temperature, which causes the water vapor to condense.



(Copyright Calorex Heat Pumps Ltd, UK)

GENERAL INFORMATION

Condensation dehumidifiers, such as the one shown on the left, are very common residentially. An industrial condensation dehumidifier is shown on the right.


(Copyright Sylvane, Inc.,
Roswell, GA)
(Copyright Calorex Heat
Pumps Ltd, UK)

Condensation-based dehumidifiers lower the temperature of the air. As a result, water vapor condenses and the air becomes drier.


EQUIPMENT DESIGN

This picture illustrates the way the cooling coils condense water vapor. The ice water in the glass cools the glass surface. Water vapor from the surrounding air condenses on this cooled surface.



(Copyright The Department of Chemical Engineering,
University of Michigan, Ann Arbor, MI)

Similarly, when humid air comes in contact with the cooling coils, its temperature is lowered, resulting in condensation.


The animation below shows condensation-based dehumidification in operation. A stream of humid air, exemplified by the blue arrows, passes over the cooling coils. The temperature at the cooling coils is at or below the air's dew point temperature, causing the moisture to condense out. The condensed water is collected or pumped out and the dry air, shown by the grey arrows, is blown out of the dehumidifier.



USAGE EXAMPLES

Dehumidifiers are commonly used to keep moist areas, such as basements, dry. Condensation-based dehumidifiers are used in environments in which humidity must be controlled, such as the fire station shown below.



(Copyright Calorex Heat Pumps Ltd, UK)

ADVANTAGES

DISADVANTAGES

  • More economical to operate at higher dew points than desiccant dehumidifiers.
  • Maintains a level of humidity well.
  • Amount of water removed may be controlled.
  • Cooling coils may freeze up.
  • Not as efficient when low dew points are needed.
  • Can only handle a relatively small amount of air.

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ACKNOWLEDGEMENTS


Bry-Air, Inc., Sunbury, OH
Calorex Heat Pumps Ltd, UK
Delta Adsorbents, Roselle, IL
MultiTherm LLC, Malvern, PA
Sylvane, Inc., Roswell, GA
Ted Pella, Inc., Redding, CA

REFERENCES


Speltz, Katharine. "Properly Select and Design a Process Dehumidifier." Chemical Engineering      Progress. Nov. 1998: 65-70.


DEVELOPERS


Daniel Viaches
Christy Charlton
Steve Wesorick
Steve Cotton


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