dehumidify

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A typical portable dehumidifier.

A dehumidifier is mostly a household appliance that reduces the level of humidity in the air, usually for health reasons, as humid air can cause mold and mildew to grow inside homes, which has various health risks. Relative humidity is preferably 30 to 50%.^[1] Very high humidity levels are also unpleasant for human beings, can cause condensation and can make it hard to dry laundry or sleep. Higher humidity is also preferred by most insects, including clothes moths, fleas and cockroaches. Dehumidifiers are used in industrial climatic chambers for keeping certain level of humidity.

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, 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.^[2] They are most effective at over 45% relative humidity, higher if the air is cold. ^[3]

Desiccant Type

Desiccant materials have a high affinity for adsorbing water vapour. Today, most desiccant dehumidifier manufacturers use the desiccant material in a 'rotor' form. Rotors are manufactured from alternate layers of flat and corrugated sheets impregnated with the active component (desiccant). This forms a vast number of axial air channels running parallel through the rotor structure. As air passes through these channels, moisture is transferred between the air and the desiccant.

A desiccant dehumidifier employs a silica gel desiccant rotor or other desiccant material to remove moisture from air in its vapour form. As they do not rely on cooling the air to produce condensation, they perform exceptionally well when used in cooler climates or when lower dew points are required. As there is no water produced during the drying process, these units work effectively at sub-zero temperatures. When dehumidifying humid air for industrial processes, it is quite common to employ both pre-cooling (moisture removed as condensation) and a desiccant rotor (moisture removed by adsorption) in the same air system.

Typically their moisture content is a function of the relative humidity of the surrounding air. Exposed to low relative humidities desiccant materials come to equilibrium at low moisture contents and exposure to high relative humidities results in equilibrium at high moisture contents. The process involves exposing the desiccant material to a high relative humidity air stream, allowing it to attract and retain some of the water vapor and then exposing the same desiccants to a lower relative humidity air stream (heated) which has the effect of drawing the retained moisture from the desiccant. The first air stream is the air that is being dehumidified while the second air stream is used only to regenerate the desiccant material so that it is ready to begin another cycle. Note that the first air stream's water vapor content is reduced while the second air stream's water vapor content is increased. Typically the low relative humidity air stream is air taken from any available source and heated to reduce its relative humidity. Hence desiccant dehumidifiers consume heat energy to produce a dehumidifying effect.

Generally a desiccant dehumidifier comprises five major components:

• the component that contains the desiccant material (desiccant rotor), of which there are several types.
• a fan to move the air to be dehumidified (process air) through the desiccant rotor or material.
• a fan to move hot air (reactivation air) through the desiccant rotor or material.
• a heater to heat the air that is used to regenerate the desiccant material.
• a mechanical device to slowly rotate the desiccant rotor or material bed.

Electronic

Electronic dehumidifiers use a peltier heat pump to generate a cool surface for condensing the water vapour from the air. This type of dehumidifier has the benefit of being very quiet when in use as there is no mechanical compressor. This design is mainly used for very small dehumidifiers due to the simple design and low cost of parts.

Air conditioners

Air conditioners automatically act as dehumidifiers when they chill the air and thus need to handle the accumulated water as well. Newer window units use the condensing coil and fan to evaporate the accumulated water into the outdoor air, while older units simply allow the water to drip outside. Central air conditioning units 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 dryer 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 heated freon then goes through a tube outside the house and into the heating coils (the condenser) 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 runs thorough a duct to the outside of the window.

Makeshift Dehumidifiers

Because air conditioners operate in the same basic way as dehumidifiers, window units are often used as makeshift dehumidifiers by sending their exhaust back into the room instead of outside. This produces the same result as using a dehumidifier, a room atmosphere that is much less humid but slightly warmer.

Water

A dehumidifier partially disassembled.

Collection

Most dehumidifiers can be adapted to connect the drip output directly to a drain via a garden hose, though they usually also come with a collection receptacle. There are usually sensors to detect when the collection device is full, and shut off the dehumidifier. These buckets will generally fill with water in 8-12 hours and will need to be emptied and replaced. Some dehumidifiers can tie into plumbing or use a water pump to drain themselves as they collect moisture.

Potability

General dehumidifier water is considered a rather clean kind of greywater: not suitable for drinking, but acceptable for watering plants, though not garden vegetables.^[2][4] The concerns are:^[4]

• the water may contain trace metal from the solder, most significantly lead (which is quite damaging), but also copper, aluminium, and zinc;
• various pathogens accumulate in the water, particularly due to its stagnancy, including fungal spores; unlike in distilled water, 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.

The trace metal poses a danger if used on edible plants, as they can accumulate; however, the water is otherwise usable for irrigation.

One can make food-grade dehumidifiers (avoiding toxic metal and keeping the collection tank clean), which are called atmospheric water generators.

See also

• Atmospheric water generator, a machine that makes pure drinking water from air
• Food dehydrator, device for decreasing humidity of food
• Humidifier, an appliance that increases the humidity of air
• Thermoelectric cooling, Peltier dehumidifiers

References

Further reading

Wikimedia Commons has media related to: Dehumidifiers

• Tables with many dehumidifier models. With columns for liters of water removed per day, and liters per kilowatt-hour: Energy Star Qualified Dehumidifiers [2].
• AHAM Dehumidifier Product Certification Program [3]. Association of Home Appliance Manufacturers.