The Basics of Desiccant Dehumidification
March 8, 2009
Much confusion surrounds the use of desiccant dehumidifiers in the restoration industry. Some myths include:
- Desiccants are for crawlspace drying only
- Desiccants can only be used in cool conditions and won’t work in hot conditions
- Desiccants should only be used for hardwood floor drying
- Desiccants are the only dehumidifiers that can get the air very dry
- Desiccants are only useful when the air is dry
In reality, desiccant dehumidifiers have many uses and are an important addition to a restorer’s toolbox. Proper application of desiccant dehumidification yields great results.
How They WorkDesiccant dehumidifiers depend upon a chemical attraction called “adsorption” to remove water from the air. Because they do not use condensation to remove water like refrigerant dehumidifiers, they are not limited by low dew point temperatures; therefore, they can produce the driest air of any dehumidifier when used properly in very-low-humidity conditions.
A desiccant wheel consists of a grid of small air passages, similar to a large bundle of coffee straws, impregnated with silica gel. (This is the same silica that is used in the packets that keep shoes dry.) The desiccant wheel rotates slowly through different airflow zones of the desiccant system, adsorbing moisture in one zone and releasing it in another. The size of this rotor and the speed at which it rotates varies determines the unit’s dehumidification airflow and capacity.
A desiccant (See Illustration 1) uses one or more air blowers to first draw the moist air (A) through the desiccant wheel. In most models, 75 percent of this moist air stream is dried as it passes through the wheel, exiting the unit as warm, dry process air (B).
The remaining 25 percent of the air (D) is directed through a heater (E) and then back through the disc. This heated air removes the moisture from the disc, “reactivates” it, and exits the unit as warm, wet reactivation air (F). Heat removes moisture from the silica by adding enough energy to the water molecules to overcome the chemical attraction they have to the desiccant media. The heating function of a desiccant dehumidifier requires a high level of energy consumption, and can run on electricity or fossil fuels.
Configurations of DesiccantsDesiccant dehumidifiers range dramatically in size and configuration. Desiccant dehumidifiers used in restorative drying may be as small as a roll-on suitcase (88 CFM) or as large as a semi tractor-trailer (up to 25,000 CFM or more), with many sizes in between. Large desiccants are often equipped with their own onboard generators, running on propane or diesel fuel. When large catastrophic events, such as hurricanes, occur, large desiccants, like other trailer-mounted dehumidifiers, can provide a means of dehumidification when little or no power is available.
ApplicationsDesiccant dehumidifiers have many applications in restorative drying, but they are most commonly used on materials that are difficult to dry, such as such as hardwood floors, plaster and lath walls, or other dense materials. The extremely low-humidity air produced by desiccants can accelerate the drying of these materials when used in conjunction with air movers. This procedure is especially true with inter-air drying systems, which typically do not produce high airflow rates; feeding the inter-air system with super-dry, desiccant-processed air will maximize the drying efficiency of this equipment. Desiccants are also well known for their efficiency in large loss restoration (larger than 7,500 square feet) because one desiccant can be used in place of many smaller dehumidifiers.
The efficiency of a desiccant can be boosted by maximizing airflow and providing appropriate temperature and humidity. Here are some of the ways a desiccant’s performance can be enhanced:
- Keep the reactivation air (wet air) duct as short as possible. Long ducting will restrict airflow through the reactivation chamber and will inhibit water removal from the rotor. Long duct runs also can cause the reactivation air to cool below dew point and can fill the duct with water. This can cause damage to the dehumidifier and the structure being dried.
- Duct the process air inlet from the coolest, driest air available at the job site – whether it’s outside air, cold air from the air conditioner, or air from an unaffected area. Cold air enhances desiccant performance for two reasons. First, cold air is normally drier air, reducing the amount of energy required to remove moisture from the disc. Second, the cold air helps to cool the rotor after reactivation. The cooler this air, the faster the rotor will return to operable temperature and the better it can adsorb moisture once again.
- Desiccant dehumidifiers can be set up to create a positive, negative or neutral pressure. In cases where air is drier outside, a negative or positive pressure can be the best approach. In cases where air outside is wetter, consider using a neutral pressure setup. Here, recirculation will allow the dehumidifier to continually reduce the moisture inside the building without being hindered by infiltration of wetter air.
- In cases where the desiccant is too far from a window or exit, duct the warm, wet air into a refrigerant dehumidifier. The refrigerant can easily collect the moisture from this warm, wet air.
Many technicians are concerned that using a desiccant will dry the structure too much. It is true that a desiccant will, in the end, create drier air than a refrigerant or an LGR dehumidifier, but daily monitoring will prevent problems with over-drying. In the hands of a well-trained technician, desiccant dehumidifiers are an efficient tool to solve many restoration problems.