Dehumidifiers are devices that remove moisture vapor from the air. There are many applications where we want to reduce humidity and for each, there are dehumidifiers designed for the job.
Heat plays an important role in dehumidification and all commercial and industrial dehumidifiers use heaters.
- Refrigerant based units heat the exhaust air that had previously been cooled in the condensation stage.
- Desiccant units use heat to dry a moisture adsorbent material after it has been exposed to a humid airflow.
- Supplemental heat plays an assistive role in cold environments or extreme water removal situations like flood restoration drying.
Refrigerant based units heat air at the condenser side of the system. This is the business end of a dehumidifier. Air heaters may also be used for additional heating control. Desiccant systems require air heaters to dry (regenerate) moisture adsorbent material after it has removed moisture vapor from a moist air stream. Supplemental heaters use electric air heaters or a liquid fuel like propane.
Applications requiring air heaters are best served with a robust reliable solution engineered for their intended purposes. Dehumidification heating element design is critically important when it comes to reliability and performance. Tutco-Farnam has decades of experience designing and manufacturing electric air heaters for drying and dehumidification applications. Contact a Tutco-Farnam representative for more information.
Custom dehumidification heaters designed and manufactured by Tutco-Farnam.
Methods and Types
There are many different dehumidifiers on the market serving a multitude of applications. They are largely based on the two fundamental dehumidification methods mentioned above, Refrigerant and Desiccant.
Thermo-electric and Ionic membranes are two additional methods though they have limited use with moisture removal rates measured in ounces or grams per day. We won't be addressing them in this article.
Refrigerant based units (also referred to as compressor-based units or mechanical refrigeration) remove liquid moisture by way of condensation. Condensation starts at the dew point when the air has reached a relative humidity of 100%. The ambient air conditions are such that the air is saturated and can no longer hold the moisture it contains. Lowering process air temperature to this point of saturation results in air that is ready to give up its water. The condensation on the outside of a cold glass of water is an example of this condition. Air that comes into contact with the cold glass is unable to hold moisture. The moisture vapor goes through a state change becoming liquid water condensation on the cold glass surface.
We are taking a few liberties with the words ‘hold’ and 'give up'. In a more detailed study, we would do better to talk about vapor pressure seeking equilibrium, something I am conveniently ignoring.
In a refrigerant-based dehumidifier, dew collects on the surface of a cold metal coil called the evaporator. Here the moisture collection can be controlled, held, and removed from the area being dried. Refrigerant is decompressed to make the evaporator cold. It is similar to how an a/c unit works except that with refrigerant based dehumidifiers we don’t want the exhaust air to be cold. In an a/c unit, the warmth leaves the environment by way of the condenser which is external to the building being cooled. In a dehumidifier, the evaporator and the condenser are in the same unit releasing warmer air back into the room. The newly dried air is heated by the condenser before being exhausted from the unit. The air may take on additional heat energy from heaters situated at the exhaust end of the unit or by way of supplemental heater/blowers external to the unit. This dry heated air expands giving it the ability to ‘hold’ more moisture.
Some Refrigerant based units use Low Grain Refrigerants (LGRs) allowing their units to operate in lower temperature environments, achieve lower relative humidities and dry air faster than traditional refrigerant-based units. The addition of a built-in pre-cooling area allows the units to process air at lower temperatures than standard refrigerant-based units.
Desiccant is a material that has special moisture holding and releasing properties. H2O molecules are attracted to the desiccant. Moisture is then removed from the desiccant by applying energy to the ‘regeneration’ airflow in the form of heat. This heat energy helps to release the moisture-desiccant bond, resetting the desiccant to its dry condition. Desiccant air heaters are specifically engineered for this purpose.
Some desiccant materials are hygroscopic meaning they can absorb moisture while there are other desiccants that are non-hygroscopic and hold the moisture on the surface of the material only. Desiccant is used in many drying applications and can often be found in the form of beads. Packaged electronics commonly contain small desiccant packets filled with silica beads that keep the merchandise dry. Desiccant is a broad name that encompasses many different materials. The most popular are molecular sieve, clay, and silica gel.
Desiccant based dehumidification units remove moisture vapor with the help of an adsorbent form of silica desiccant. Moisture does not penetrate the surface making it easier and faster to remove the moisture when regenerating the desiccant. This non-hygroscopic moisture attracting material is at the heart of a desiccant dehumidifier. Water molecules in the process airflow come into contact with the desiccant and attach to the surface resulting in a dryer exhaust air. Dehumidifier desiccant is commonly found in the form of a honeycomb wheel (also called a rotor) that slowly rotates a dried (regenerated) portion of the wheel into the path of the process airflow. The honeycomb wheel consists of a corrugated material that has been coated with silica or some other form of desiccant. The rotor is housed within a frame called a cassette, which typically contains the motor that rotates the wheel along with a drive mechanism such as a belt or a chain.
Commercial desiccant units do not operate on the same principles of heat that are fundamental to a refrigerant-based system. The moisture gathers on the surface of the desiccant but does not become a liquid in this process. No phase change is required to capture the moisture and thus there is no condensation. This gives desiccant units much greater flexibility in the environments where they can be used. They can operate in cold environments where a refrigerant system would be ineffective. This also allows them to achieve a lower relative humidity.
Desiccant air heaters warm the moisture molecules on a desiccant rotor’s surface, releasing them from the desiccant and carrying them away in the exhaust airflow. Displayed to the right is a Tutco-Farnam desiccant heater used to regenerate a desiccant wheel. The moist exhaust air is evacuated from the area being dehumidified through an exhaust duct. In cases where ducting out of a room is not practical, moist air can be exhausted right into a refrigerant dehumidifier where the moisture is collected as liquid water. Some consumer-based desiccant dehumidifiers will contain an added condenser for capturing liquid water. That feature comes with operating temperature and relative humidity (RH) penalties inherent in the refrigerant-based unit.
Dehumidifier Comparison by Type
|TYPICAL OPERATING TEMPERATURE||LOWEST RELATIVE HUMIDITY WITH EFFICIENCY||NOTES|
|Refrigerant Units||65°F to 90°F||40%||Cheapest Option. Easy Setup.|
|LGRs||35°F to 90°F||30%||Outperforms Multiple Refrigerants|
|Desiccant||10°F to 90°F||5%||Operates in Cold Environments|
Where is Dehumidification Needed?
Dehumidifiers are used wherever humidity (or lack of humidity control) is a problem. This includes residential, commercial, industrial, and institutional applications. Common humidity issues include damage to property (ie. buildings, books, clothing), health issues, musty odors, mold, mildew, bacteria, mites, viruses, fungi, microorganisms, plant diseases, water damage, condensation damage, frost or ice buildup, high heating and cooling costs, workplace safety issues such as icy floors or fog, slow drying times and production environments where dehumidification control is required to produce a product or remove inconsistencies.
Commercial / Industrial Applications
Ice rinks, indoor pools, spas, pumping stations, water management, and wastewater treatment facilities are all applications that introduce large amounts of moisture to an environment. This results in an undesirable relative humidity that needs to be controlled.
Medical environments such as hospitals, surgical units, and emergency rooms are dehumidified to both keep people comfortable and to reduce hygiene issues. Greenhouses and large indoor growing rooms dehumidify to reduce plant diseases that otherwise flourish in warm moist environments.
Many valuable, irreplaceable, and historically important items will become damaged in the presence of humidity. They will cease to exist if ideal conditions for mold, mildew, and fungus are allowed to flourish. Archival storage facilities like libraries and museums are safe havens for important documents, books, and archaeological items and they require strict environmental control. Specialty storage for film and classic cars provides protective low humidity environments.
Even stricter climate control may be seen in environments like clean rooms, pharmaceutical manufacturing, drug packaging, sterilization processes, silicon wafer making, and precision manufacturing.
Supermarkets have complex environmental control needs. A typical US supermarket may have produce that gets misted with water, open display refrigerated foods, one or more large banks of freezers, condensation prevention heaters, hoods for prepared foods, and a big front door that opens and closes all day long. Without proper humidity and temperature control produce will rot or dry out, frozen food will frost, glass displays will fog, and customers will find the environment uncomfortable.
Big-box general merchandise stores that also sell groceries will segment their air control efforts into specific store regions. This helps to prevent your fruit and vegetables from tasting like the tires over in the auto supply section, for example.
All kinds of cold storage applications must contend with ice and frost condensation when moisture vapor is introduced. Everything from the transparency of refrigerated display cases to the fog and ice dangers of a food storage loading area presents a risk to a business's bottom line.
Food processing, packaging storage, and transportation environments are typically cold. When moist air enters these environments frost and ice collect on surrounding surfaces. This includes chiller walls and ceilings, loading docks, and the food being held or processed. Workplace safety, as well as product integrity, are at risk. These applications require dehumidifiers that can operate at low temperatures. Warmer environments like kitchens risk condensation damage as well as mold and mildew. Dehumidification is used to reduce condensation and make the environment uninhabitable to warm moisture seeking microorganisms.
Commercial grade portable units big and small are used in restoration, construction site drying, and water remediation. Dehumidification is used to drive the drying of an environment which may include water-soaked walls, floors, and furniture. Supplemental heat is frequently used to raise the temperature creating a larger difference in vapor pressure and increased condensation efficiency. The operator must take several factors into consideration in order to get the best performance without causing further damage.
Industrial process applications include optics, plastic resin, timer drying, paint booths, sand drying, surface preparation, product consistency, and industrial condensation prevention to name a few.
Applications by Category
The main reasons people use home dehumidifiers are for comfort, health, and property protection. The sweet-spot recommended by the EPA is an RH (relative humidity) between 30% and 50%. (https://www.epa.gov/indoor-air-quality-iaq/inside-story-guide-indoor-air-quality). In a residential environment, humidifiers are typically used when RH drops below 30% and dehumidifiers when RH is above 50%.
The National Research Council recommends maximum relative humidities for interior air based on a range of cold exterior temperatures.
|Outside Air Temperature||Max Recommended Relative Humidity|
Residential dehumidifiers come in three flavors.
- Simple, low-cost consumer models made for individual rooms. Refrigerant based units are most popular in the US. These are typically sold in big box stores and you can buy them online. Desiccant based units are growing in popularity in Europe. The regenerated air does not need to be pumped out of the room as the unit includes a condenser for capturing liquid water.
- Larger robust freestanding units come from specialty suppliers. They are commonly installed in the basement, attic or a crawlspace often with the capacity to handle multiple rooms.
- Whole house units are permanent fixtures tied right into the HVAC system of the house. These are professionally installed and have the ability to remove moisture at much higher rates and within a greater temperature and humidity range. Distributors typically sell the higher grade units through professional HVAC installers and do not recommend homeowners perform their own installations.