Dehumidifier - AbsoluteAstronomy.com

A dehumidifier is typically a household appliance that reduces the level of humidity

Humidity

Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

in the air, usually for health reasons. Humid air can cause mold

Mold

Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...

and mildew

Mildew

Mildew refers to certain kinds of molds or fungi.In Old English, it meant honeydew , and later came to mean mildew in the modern sense of mold or fungus....

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

Condensation

Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

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

Tineidae

Tineidae is a family of moths in the order Lepidoptera. Collectively, they are known as fungus moths or tineid moths. The family contains considerably more than 3,000 species in over 300 genera. Most of the tineid moths are small or medium-sized, with wings held roofwise over the body at rest...

, fleas

Flea

Flea is the common name for insects of the order Siphonaptera which are wingless insects with mouthparts adapted for piercing skin and sucking blood...

, cockroaches

Cockroach

Cockroaches are insects of the order Blattaria or Blattodea, of which about 30 species out of 4,500 total are associated with human habitations...

, Woodlice 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

Energy efficiency

Energy efficiency may refer to:*Efficient energy use, sometimes simply called energy efficiency.*Energy conversion efficiency, the ratio between the output and input of an energy conversion machine....

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.

Mechanical/refrigerative

Mechanical/refrigerative dehumidifiers, the most common type, usually work by drawing moist air over a refrigerated

Refrigeration

Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

coil with a small fan. Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses

Condensation

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

Dew point

The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

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 also a need to handle the accumulated condensate. Newer high-efficiency window units use the condensed water to help cool the condensing coils (warm side) 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.

A conventional air conditioner is very similar to a mechanical/refrigerative 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. The water which condenses on the evaporator in a dehumidifier is disposed of in the drain pan or drain hose.

However in an air conditioner, air passes over the cooling coils (the evaporator) and then directly into the room. Spent refrigerant then is pumped by the compressor through a tube to outside the space being cooled, to where the heating coils (the condenser) are located. The waste heat is transferred to the outside air, which passes over the condenser coils and remains outside. The water that condenses on the evaporator in an air conditioner is usually routed thorough a drain channel to the outside of the window, thus removing extracted water from the conditioned space.

Adsorption/desiccant

This basic dehumidification process uses a special humidity-absorbing material called a desiccant

Desiccant

A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....

, 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.
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.

Electronic

Electronic dehumidifiers use a Peltier heat pump

Thermoelectric cooling

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other side against the...

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

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

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 mechanical/refrigerative dehumidifiers, window air conditioner units are sometimes used as makeshift dehumidifiers by sending their heat exhaust back into the room instead of outside the space. 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

Thermostat

A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...

which senses temperature, rather than the humidistat

Humidistat

An electronic device analogous to a thermostat but which responds to relative humidity not temperature. Humidistats are utilized in a number of devices including dehumidifiers, humidifiers, and microwave ovens...

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

Greywater

Greywater is wastewater generated from domestic activities such as laundry, dishwashing, and bathing, which can be recycled on-site for uses such as landscape irrigation and constructed wetlands...

: 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
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...

of non-edible plants.
various pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s, including fungal spores, may accumulate in the water particularly due to its stagnancy; unlike in distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...

production, the water is not boiled, which would kill pathogens (including bacteria);
as with distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...

, minerals are largely absent, hence it is somewhat flat-tasting.

Food-grade dehumidifiers, also called atmospheric water generator

Atmospheric water generator

An atmospheric water generator , is a device that extracts water from humid ambient air. Water vapor in the air is condensed by cooling the air below its dew point, exposing the air to desiccants, or pressurizing the air. Unlike a dehumidifier, an AWG is designed to render the water potable...

s, are designed 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.

External links

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

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