Heating, Cooling and HVAC

A dehumidifier is a household appliance that reduces the level of humidity in air.

Mechanical/refrigerative dehumidifiers
Mechanical/refrigerative dehumidifiers 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.

Most dehumidifiers can be adapted to connect the drip output directly to a drain. Manufacturers may price dehumidifiers differently solely on the size of the collection bucket. There are usually sensors to detect when the collection bucket is full.

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.

Desiccant dehumidifiers
A desiccant dehumidifier is a device that employs a desiccant material to produce a dehumidification effect. Desiccant materials have a high affinity for water vapor. 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 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.

In general a desiccant dehumidifier comprises four major components. The component that holds the desiccant of which there are several types, a fan to move the air to be dehumidified (process air) through the desiccant holder, a fan to move the low humidity air for drying the desiccant through the desiccant holder and a heater to heat the air that will be used to dry the desiccant (regeneration air).

Uses:
Dehumidifiers are used to improve the indoor environment of a dwelling or building. They are used in homes, hospitals, and in other settings.

Other environments such as a rooms dedicated to housing sensitive electronic equipment (servers for example) may use dehumidifiers as a low-cost method to prevent long-term effects of moisture.

A dehumidifier may also be used to help dry out a damp basement after a rainstorm.

Dehumidifiers may be used in libraries recovering from water damage. Desiccant dehumidification is an effective method of quickly reducing the relative humidity in an enclosed chamber, resulting in drying not only the air in the chamber but also materials, furniture and books. Books may be dried in place on shelves using this method, however it is not recommended for use in situations involving saturated books

Relative effectiveness
Refrigerant dehumidifiers are more efficient when the air is quite humid (over about 45% relative humidity, higher if the air is cold). Sometimes, they are used in tandem, such as when the air is humid, and needs to be made very dry.

Difference between Dehumidifiers and Room Colers
The window air conditioner will cool the room or basement and shut off when it is cool enough to satisfy the thermostat. The RH may well be in the 90% range at that time. So you will have a nice cool but damp room or basement. The dehumidifier has a sensor inside which responds to humidity. It will run until the air becomes dry to the setpoint of the dehumidifier - if you want the benefits of both then you will probably need both. Remember, cold air can not contain the same amount of water that the same volume of hot air can so if you cool the room or basement down to 50 degrees the relative humidity could be 100% but the total quantity of moisture in the air will be less than it was when the air was 80 degrees. So you might consider running an air conditioner and a small heater to warm the air back to 70 degrees or so to decrease the relative humidity. But . . . . . . . that is exactly what a dehumidifier does!! The heat from the motor and the heat of condensation are kept in the room to rewarm the air which was cooled by passing over the evaporator coils in the unit.

Drinking the water
Water collected from any dehumidifier is technically distilled water in that it does not contain minerals and other particulates that are removed in a true distillation process. However, a true distillation process condenses the steam of boiled water, and the boiling process kills any microbes and fungi that may be present in the pre-distilled water. Since the surface coils in a dehumidifier are not kept to food-grade cleanliness, microbes and fungi can (and probably do) exist (needs citation) in the collected water and it is therefore not considered safe to drink. In fact, as the water may sit for a while in the collection bucket, the water may be quite stale.

http://en.wikipedia.org