Clothes dryer
From Wikipedia, the free encyclopedia
 |
It has been suggested that Convectant drying be merged into this article or section. (Discuss) |
A clothes dryer or tumble dryer is a household appliance that is used to remove the moisture from a load of clothing and other textiles, generally shortly after they are cleaned in a washing machine.
Most dryers consist of a rotating drum called a tumbler through which heated air is circulated to evaporate the moisture from the load. The tumbler is rotated relatively slowly in order to maintain space between the articles in the load. In most cases, the tumbler is belt-driven by an induction motor.
Contents |
[edit] Fuel types
There are two general classes of rotating dryers: electric and gas. Both of these refer to the method used to raise the temperature of the air flowing through the tumbler, since the tumbling action is usually electrically powered.
The electric dryer generally uses a coiled wire that is heated with electric current. The amount of electric current is varied to adjust the air temperature. In the United States and other countries following USA wiring standards, electric dryers typically have a 4-wire NEMA 14-30 plug, rather than the 3-wire NEMA 5-15 plug used by most appliances, and need a 30-ampere, 240-volt centertapped single-phase circuit. Small "portable" clothes dryers, popular with urban dwellers, normally use conventional 110 volt connections.
The gas dryer employs a gas burner that burns natural gas, propane, or butane to form a jet of hot gases that are directed into a venturi chamber, which uses Bernoulli's principle to pull in ambient air and raise its temperature. The air temperature can be altered by adjusting the size of the gas flame or, more commonly, by merely extinguishing it and relighting it. Gas dryers require electricity to spin the clothes, but the amount of electricity is much smaller than in an electric dryer removing the need for a special connection. The motors normally run on standard 110 volt electricity.
[edit] Traditional dryers
Traditional dryers continuously draw in the cool, dry, ambient air around them and heat it before passing it through the tumbler. Afterwards, the resulting hot, humid air is simply vented outside to make room for more dry air to continue the drying process.
The traditional design makes no effort to recycle the heat put into the load, and so is quite inefficient. Nevertheless, the basic design is simple, reliable, and cheap.
It should be noted that some methods of salvaging this heat for in-home heating, by use of inline vent boxes equipped with a flapper valve to direct heat to indoor areas, poses the concern of increased humidity within a dwelling. Humidity from these devices causes a likelihood of mold, mildew, and bacterial growth inside a home. It is recommended that dryers, regardless of electric or gas, are vented outdoors with a 3 inch diameter piping.
[edit] Ventless dryers
[edit] Spin dryers
These machines simply spin their drums faster than a typical washer could in order to extract additional water from the load. They may remove more water in two minutes than a heated tumbler dryer can in twenty, thus saving significant amounts of time and energy. Although spinning alone will not completely dry clothing, this additional step saves a worthwhile amount of time and energy for large laundry operations such as those of hospitals.
[edit] Condensation dryers
Just as in a normal dryer, condensation dryers pass heated air through the load. However, instead of exhausting this air, the dryer uses a heat exchanger to cool the air and condense the water vapor into either a drain pipe or a collection tank. Afterwards, this air is run through the loop again. The heat exchanger typically uses ambient air as its coolant, therefore the heat produced by the dryer will go into the immediate surroundings instead of the outside, increasing the room temperature slightly. In some designs, cold water is used in the heat exchanger, eliminating this heating, but requiring increased water usage.
In terms of energy use, condensation dryers typically require less system-wide energy use than conventional dryers. The bulk of the savings results from the HVAC system from not having to heat or cool additional air to replace that exhausted by a traditional dryer. Typically this savings is sufficient to offset the slight increase in power draw, longer drying times, and ambient cooling requirements associated with condensation dryers.[1]
Because the heat exchange process simply cools the Internal air using ambient air (or cold water in some cases), it will not dry the air in the internal loop to as low a level of humidity as the fresh, ambient air. As a consequence of the increased humidity of the air used to dry the load, this type of dryer requires somewhat more time than a traditional dryer. Condensation dryers are a particularly attractive option where long, intricate ducting would be required to vent a traditional dryer.
[edit] Heat pump dryers
Whereas condensation dryers use a passive heat exchanger cooled by ambient air, these dryers use a heat pump. The hot, humid air from the tumbler is passed through a heat pump where the cold side condenses the water vapor into either a drain pipe or a collection tank and the hot side reheats the air. In this way not only does the dryer avoid the need for ducting, but it also conserves much of its heat within the dryer instead of exhausting it into the surroundings. Heat pump dryers can therefore use less than half the energy required by either condensation or traditional dryers.
As with condensation dryers, the heat exchanger will not dry the internal air to as low a level of humidity as the ambient air. With respect to ambient air, the higher humidity of the air used to dry the clothes has the effect of increasing drying times; however, because heat pump dryers conserve much of the heat of the air they use, the already-hot air can be cycled more quickly, possibly leading to shorter drying times than traditional dryers, depending on the model.
[edit] Mechanical steam compression dryers
A new type of dryer in development, these machines are a more advanced version of heat pump dryers. Instead of using hot air to dry the clothing, mechanical steam compression dryers use water recovered from the clothing in the form of steam. First, the tumbler and its contents are heated to 100 °C. The wet steam that results purges the system of air and is the only remaining atmosphere in the tumbler.
As wet steam exits the tumbler, it is mechanically compressed (hence the name) to extract water vapor and transfer the heat of vaporization to the remaining gaseous steam. This pressurized, gaseous steam is then allowed to expand and is superheated before being injected back into the tumbler where its heat causes more water to vaporize from the clothing, creating more wet steam and restarting the cycle.
Like heat pump dyers, mechanical steam compression dryers recycle much of the heat used to dry the clothes, and they operate in a very similar range of efficiency as heat pump dryers. Both types can be over twice as efficient as traditional dryers. However, the considerably higher temperatures used in mechanical steam compression dryers result in drying times on the order of half as long as those of heat pump dryers.
[edit] Dehumidifier Dryers
By keeping a low humidity, dehumidifiers encourage fast evaporation without high heat. This type of dryer is suitable for clothes that can withstand tumbling but not high heat.
[edit] Convectant Drying
Marketed by some manufacturers as a "static clothes drying technique", convectant dryers simply consist of a heating unit at the bottom, a vertical chamber, and a vent at top. The unit heats air at the bottom, reducing its relative humidity, and the natural tendency of hot air to rise brings this low-humidity air into contact with the clothes. This design is slow and relatively energy-efficient, but is only marginally faster than line-drying.
[edit] Static
Clothes dryers can cause static cling, through the triboelectric effect.
[edit] History
Contrary to Internet rumors, the first tumble dryer was not invented by American George T. Sampson.[citation needed] A hand-cranked version was created in 1799 by a Frenchman named Pochon. Sampson's United States patent (number 476,416), which he received on June 7, 1892, was for an improved rack for holding wet clothes near a heat source. Electric tumble dryers appeared in the 20th century.
[edit] Safety
Dryers expose flammable materials to heat. Underwriters Laboratories provided these pieces of advice:
- The lint filter should be cleaned after every cycle. This habit also improves energy efficiency.
- Be sure there is adequate ventilation around the dryer.
- Keep off the dryer glass fiber, rubber, foam and plastic items, and anything that has ever had any type of flammable substance spilled on it (like gasoline, white spirit, cooking oil or alcohol).
Source: Underwriters Laboratories
[edit] Environmental Impact
Energy Star, a United States government program concerned with energy efficiency in consumer products, does not rate clothes dryers.
In the European Union, the EU energy labeling system is applied to dryers; dryers are classified with a label from A (best) to G (worst) according to the amount of energy used per kilogram of clothes (kWh/kg).
[edit] See also
[edit] External links
- Analysis of the economy of gas versus electric dryers In the follow-up article, there is also a spreadsheet for you to calculate the monetary tradeoff.
- Energy Efficiency Breakdown of the costs, savings, and energy efficiency of Energy Star appliances.
