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A dehumidifier is generally a household appliance which reduces the level of humidity in the air, usually for health or comfort reasons, or to eliminate musty odour. Large dehumidifiers are also used in commercial buildings such as indoor ice rinks to control the humidity level.
- 1 Overview
- 2 Applications
- 3 Processes
- 4 Condensate
- 5 Ice buildup
- 6 See also
- 7 References
- 8 Further reading
- 9 External links
By their operation, dehumidifiers extract water from the conditioned air. This collected water (usually called condensate) cannot be used for drinking, so it must be discarded. Some designs, such as the ionic membrane dehumidifier, dispose of excess water in a vapor rather than liquid form. The energy efficiency of dehumidifiers can vary widely.
Relative humidity in dwellings is preferably 30 to 50 percent.
Humid climate, or humid air within buildings, makes some people extremely uncomfortable, causing
- excessive body perspiration that cannot evaporate in the moisture-saturated air
- body moisture precipitation
- condensation to drip from cold-water pipes
- laundry to retain moisture and develop mustiness
Excessively humid air in buildings can cause or increase the growth of various pathogenic pests, including
- fungi, such as mold and mildew
- insects, such as clothes moths, fleas, cockroaches, woodlice and dust mites.
Excessively dry air in buildings can also cause health problems, including
- skin cracking
- respiratory irritation
Dehumidifiers are also used in industrial climatic chambers, to reduce relative humidity to levels conducive to processing of certain humidity-sensitive products.
These methods rely on drawing air across a cold surface. Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses on the surface, separating the water from the air.
Mechanical/refrigeration dehumidifiers, the most common type, usually work by drawing moist air over a refrigerated coil with a fan. The cold evaporator coil of the refrigeration device condenses the water, which is removed, and then the air is reheated by the condenser coil. The now dehumidified, warm air is released into the room. This process works most effectively at higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective. It is most effective at over 45 percent relative humidity; higher if the air is cold. 
Conventional air conditioners
A conventional air conditioner is very similar to a mechanical/refrigeration dehumidifier and inherently acts as a dehumidifier when chilling the air. In an air conditioner, however, the air passes over the cold evaporator coils and then directly into the room. It is not re-heated as in a refrigeration dehumidifier by passing over the condenser. Instead, the refrigerant is pumped by the compressor to the condenser which is located outside the room to be conditioned. The heat is then released to the outside air.
The water that condenses on the evaporator in an air conditioner is usually routed through a drain channel to the outside, thus removing extracted water from the conditioned space. Newer high-efficiency window units use the condensed water to help cool the condenser coil by evaporating 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, because the quantity of condensate water generated by such systems can be quite large over time. If the condensate water is directed into the sewer system, it should be suitably trapped. Otherwise, back pressure can allow smells or sewer gases to enter the house. The water should not be directed into a septic system of a house, because large central air conditioning systems will increase the water burden put on septic systems. If the height of the air handler (containing the evaporator) is above the ground level or in the attic of a house, condensate lines can also often be routed into rain gutters. Air handlers located in the basement of a house require condensate pumps to pump the water up to ground level.
Electronic dehumidifiers use a Peltier heat pump to generate a cool surface for condensing the water vapor from the air. The design is simpler and has the benefit of being more quiet compared to a dehumidifier with a mechanical compressor. However, because of its relatively poor Coefficient of Performance, this design is mainly used for small dehumidifiers.
Because window air conditioner units have condensers and expansion units, some of them can be used as makeshift dehumidifiers by sending their heat exhaust back into the same room as the cooled air, instead of the outside environment. If the condensate from the cooling coils is drained away from the room as it drips off the cooling coils, the result will be room air that is drier but slightly warmer.
However, many window air conditioners are designed to dispose of condensate water by re-evaporating it into the exhaust air stream, which cancels out the air humidity decrease caused by the condensation of moisture on the cooling coils. To be effective as a dehumidifier, an air conditioner must be designed or modified so that most or all of the water that condenses is drained away in liquid form, rather than re-evaporated. Even if condensate is drained, an air conditioner that drains is still less efficient than a single-purpose dehumidifier with a design optimized for dehumidification. Dehumidifiers are designed to pass air directly over the cooling coils and then the heating coils in a single efficient pass through the device.
In addition, most air conditioners are controlled by a thermostat which senses temperature, rather than a humidistat that controls humidity and is typically used to control a dehumidifier. A thermostat is not designed for the control of humidity, and controls it poorly if at all.
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This 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 can be mounted on a belt or other means of transporting it during a cycle of operation.
Dehumidifiers which work according to the adsorption principle are especially suited for high humidity levels at low temperatures. They are often used in various sectors in industry because humidity levels below 35% can be achieved.
Ionic membrane dehumidifier
An ionic membrane can be used 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 dehydration 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 (vapor), being discharged.
Most portable dehumidifiers are equipped with a condensate collection receptacle, typically with a float sensor that detects when the collection vessel is full, to shut off the dehumidifier and prevent an overflow of collected water. In humid environments, these buckets will generally fill with water in 8–12 hours, and may need to be manually emptied and replaced several times per day to ensure continued operation.
Many portable dehumidifiers can also be adapted to connect the condensate drip output directly to a drain via a hose. Some dehumidifier models can tie into plumbing drains or use a built-in water pump to empty themselves as they collect moisture. Alternatively, a separate condensate pump may be used to move collected water to a disposal location when gravity drainage is not possible.
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 may 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.
Food-grade dehumidifiers, also called atmospheric water generators, are designed to avoid toxic metal contamination and to keep all water contact surfaces clean. The devices are primarily intended to produce pure water, and the dehumidifying effect is viewed as secondary to their operation.
Under certain conditions of temperature and humidity, ice can form on the dehumidifier's evaporator coils. The ice buildup can impede airflow and eventually form a solid block encasing the 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.
Better quality dehumidifiers may have a frost or ice sensor. These will turn off the machine and allow the ice covered coils to warm and defrost. Once defrosted, the machine will automatically restart. Most ice sensors are simple thermal switches and do not directly sense the presence or absence of ice buildup. An alternative design senses the impeded airflow and shuts off the cooling coils in a similar manner.
- Air ionizer, a different device for conditioning air
- Atmospheric water generator, a machine that extracts pure drinking water from air
- Food dehydrator, device for decreasing moisture in food, to prevent spoilage
- Humidifier, an appliance that increases the humidity of air
- Thermoelectric cooling
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