A dehumidifier is typically a household appliance that reduces the level of humidity in the air, usually for health reasons. Humid air can cause mold and mildew to grow inside homes, which pose various health risks. Very humid climates or air make some people extremely uncomfortable, causing excessive sweating that can't evaporate in the already-moisture-saturated air. It can also cause condensation that can disrupt sleeping, or prevent laundry from drying thoroughly enough to prevent mustiness. Higher humidity is also preferred by most pests, including clothes moths, fleas, cockroaches, and dust mites. Relative humidity in dwellings is preferably 30 to 50 percent.

By their operation, dehumidifiers produce an excess of water which has been removed from the conditioned air. This water, usually called condensate in its liquid form, must be collected and disposed of. Some dehumidifier designs dispose of excess water in a vapor, rather than liquid form. Energy efficiency of dehumidification processes can vary widely.

Dehumidifiers are also used in industrial climatic chambers, to control relative humidity within certain rooms to stay at levels conducive to processing of products.

Processes

Mechanical/refrigerative

Mechanical/refrigerative dehumidifiers, the most common type, usually work by drawing moist air over a refrigerated coil with a small fan. Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses on the evaporator coils, and drips into a collecting bucket. The air is then reheated by the warmer side of the refrigeration coil. This process works most effectively with higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective. They are most effective at over 45 percent relative humidity, higher if the air is cold .

Air conditioners

Air conditioners inherently act as dehumidifiers when they chill the air, and thus there is a need to handle the accumulated water as well. Newer high-efficiency window units direct the condensed water to increase cooling of the condensing coils (warm side) which evaporates the water into the outdoor air, while older units simply allowed the water to drip outside. Central air conditioning units typically need to be connected to a drain.

An air conditioner is very similar to a dehumidifier. Air in a dehumidifier passes over a series of cooling coils (the evaporator) and then over a set of heating coils (the condenser). It then goes back into the room as drier air with its temperature elevated.

However in an air conditioner, air passes over the cooling coils (the evaporator) and then directly into the room. The warmed refrigerant then goes through a tube outside the volume being cooled where the heating coils (the condenser) are located, and outside air passes over it and then stays outside. The water which condenses on the evaporator in a dehumidifier is caught in the drain pan or drain hose. The water that condenses on the evaporator in an air conditioner is usually routed thorough a duct to the outside of the window, to remove excess humidity from the conditioned space.

Absorption/dessicant

This basic dehumidification process uses a special humidity-absorbing material called a desiccant, which is exposed to the air to be conditioned. The humidity-saturated material is then moved to a different location, where it is "recharged" to drive off the humidity, typically by heating it. The desiccant is usually mounted on a belt or other means of transporting it during a cycle of operation.

Electronic

Electronic dehumidifiers use a Peltier heat pump to generate a cool surface for condensing the water vapor from the air. The design is simpler as there are no moving parts, and has the benefit of being very quiet compared to a dehumidifier with a mechanical compressor. However, because of its relatively poor Coefficient of Performance (energy efficiency), this design is mainly used for small dehumidifiers.

Ionic membrane dehumidifier

Ionic membranes are used in many industrial areas such as fuel cell technology, chemical engineering, and for water improvement. A specialized type of membrane can be used as an "ionic pump" to move humidity into or out of a sealed enclosure, operating at a molecular level without involving visible liquid water.

The solid polymer electrolyte (SPE) membrane is a low power, steady state dehumidifier for enclosed areas where maintenance is difficult. The electrolytic process delivers dehumidifying capacities up to 0.2 grams/day from a 0.2m³ (7 cu ft) space to 58 grams/day from an 8m³ (280 cu ft). SPE systems generally do not have high dehumidification capacities, but because the water vapor is removed through electrolysis, the process is maintenance free. The process also requires very little electrical energy to operate, using no moving parts, making the ionic membranes silent in operation and very reliable over long periods of time. SPE dehumidifiers are typically used to protect sensitive electrical components, medical equipment, museum specimens, or scientific apparatus from humid environments.

The SPE consists of a proton-conductive solid polymer electrolyte and porous electrodes with a catalytic layer composed of noble metal particles. When a voltage is applied to the porous electrode attached to the membrane, the moisture on the anode side (dehumidifying side) dissociates into hydrogen ions (H+) and oxygen: the hydrogen ions migrate through membrane to be discharged on the cathode (moisture discharging) side where they react with oxygen in the air, resulting in water molecules (gas), being discharged.

Makeshift dehumidifiers

Because they operate in the same basic way as dehumidifiers, window air conditioner units are sometimes used as makeshift dehumidifiers by sending their heat exhaust back into the room instead of outside. This can produce the same net result as using a dehumidifier, a room atmosphere that is much less humid but slightly warmer. This improvised arrangement may not be as energy efficient as a machine designed for the purpose, since most window air conditioners are designed to dispose of condensate water by re-evaporating it into the exhaust air stream, even if the air conditioner is modified to allow some of the condensed water to be drained away instead.

In addition, most air conditioners are controlled by a thermostat which senses temperature, rather than the humidistat typically used to control a dehumidifier. While temperature and humidity in a closed space are related, it is difficult to control humidity by sensing only the temperature.

Water

Collection

Most portable dehumidifiers are equipped with a condensate collection receptacle, typically with a float sensor to detect when the collection vessel is full, to shut off the dehumidifier and prevent an overflow of collected water. These buckets will generally fill with water in 8–12 hours and will need to be manually emptied and replaced several times per day, to ensure continuing dehumidification.

Many portable dehumidifiers can also be adapted to connect the condensate drip output directly to a drain via an ordinary garden hose. Some dehumidifier models can tie into plumbing drains or use a built-in water pump to empty themselves as they collect moisture. Alternately, a separate "condensate pump" may be used to move collected water to a disposal location when gravity drainage is not possible.

Potability

Generally, dehumidifier water is considered a rather clean kind of greywater: not suitable for drinking, but acceptable for watering plants, though not garden vegetables The health concerns are:

the water may contain trace metals from solder and other metallic parts, most significantly lead (which is quite dangerous), but also copper, aluminum, and zinc. The trace metals pose a danger if used on edible plants, as they can bioaccumulate; however, the water is usable for irrigation of non-edible plants.

various pathogens, including fungal spores, may accumulate in the water particularly due to its stagnancy; unlike in distilled water production, the water is not boiled, which would kill pathogens (including bacteria);

as with distilled water, minerals are largely absent, hence it is somewhat flat-tasting.

Food-grade dehumidifiers, also called called atmospheric water generators, are designed to to avoid toxic metal contamination and to keep all water contact surfaces scrupulously clean. The devices are primarily intended to produce pure water, and their dehumidification effect is viewed as secondary to their operation.

Ice buildup

Under certain conditions of temperature and humidity, ice can form on the dehumidifier cooling coils. The ice buildup can impede airflow and eventually form a solid block of ice encasing the cooling coils. This buildup prevents the dehumidifier from operating effectively, and can cause water damage if condensed water drips off the accumulated ice and not into the collection tray. In extreme cases, the ice can deform or distort mechanical elements, causing permanent damage.

Many better quality dehumidifiers have a so-called frost or ice sensor. which simply turns off the machine, allowing the icy coils to warm and defrost, before automatically restarting. Most ice sensors are simple thermal switches, and do not directly sense the presence or absence of ice buildup. An alternate design senses the impeded airflow, and shuts off the cooling coils in a similar manner.

See also

Atmospheric water generator, a machine that extracts pure drinking water from air

Food dehydrator, device for decreasing humidity of food, to retard spoilage

Humidifier, an appliance that increases the humidity of air

Thermoelectric cooling, Peltier dehumidifiers

References

Further reading

Energy Star Qualified Dehumidifiers table listing removal rates and efficiencies. Long Island Power Authority, January 2011.

AHAM Dehumidifier Product Certification Program. Association of Home Appliance Manufacturers.

Dehumidifier Pint Capacity selection guide based on room size and humidity. SimplyHumidifiers.com

External links

Dehumidifier recalls. U.S. Consumer Product Safety Commission.

DIY Dehumidifier.

Category:Home appliances Category:Cooling technology

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