Clothes dryer

A modern front-load tumble clothes dryer for home application

A clothes dryer, tumble dryer, or drying machine is a powered household appliance that is used to remove moisture from a load of clothing and other textiles, usually shortly after they are washed in a washing machine. Clothes may also be dried by natural evaporation and, if available, sunlight on an outdoor or indoor clothes line or clothes horse.

Many dryers consist of a rotating drum called a "tumbler" through which heated air is circulated to evaporate the moisture, while the tumbler is rotated to maintain air space between the articles. Using these machines may cause clothes to shrink or become less soft (due to loss of short soft fibers/lint). A simpler non-rotating machine called a "drying cabinet" may be used for delicate fabrics and other items not suitable for a tumble dryer.

Tumbler dryers

Tumbler dryers continuously draw in the cool, dry, ambient air around them and heat it before passing it through the tumbler. The resulting hot, humid air is usually vented outside to make room for more dry air to continue the drying process. This design makes no effort to recycle the heat put into the load, and thus is considered environmentally wasteful. Nevertheless, it is simple and reliable, and therefore has been widely used.

Improvised methods of salvaging this heat for in-home heating, by use of inline vent boxes equipped with a flapper valve to redirect moist heated air to indoor areas, will also increase humidity within a dwelling. Although this may be beneficial in dry winter conditions, excess humidity from these devices increases likelihood of mold, mildew, and bacterial growth inside a home. Indoor venting may also be against local regulations. Gas dryers must always be vented outdoors, as the products of combustion are mixed with the moist air. Building codes and manufacturers' instructions usually recommended that dryers vent outdoors. An indoor lint trap kit poses a similar concern of increased humidity within the dwelling.

"Long run" dryers have an additional fan inside to boost the exiting moist air through longer sections of vent pipe, as in apartments or dwellings where the vent cannot make a short direct connection from the dryer to the outside.

Beyond issues with venting exhaust, other improvised efficiency efforts with conventional dryers attempt to harvest an input source of pre-heated air rather than using the conditioned air of the living space. One notable source of heat to pre-heat dryer air is to install ductwork allowing the device to suck hot air from a dwelling's attic.

Ventless dryers

Spin dryers

Spin dryer type TS66

These centrifuge machines simply spin their drums much 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.

In some cases, dryers may use spin alone. Larger gyms and swimming pools may have small spin dryers for the convenience of exiting patrons. Extracting much of the water from a swimsuit means fewer problems in gym bags and later, laundry hampers. Full drying is not necessary, as the patron typically launders the swimsuit soon afterwards.

Condenser dryers

Just as in a normal dryer, condenser or 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. The dryer 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. In some designs, cold water is used in the heat exchanger, eliminating this heating, but requiring increased water usage.

In terms of energy use, condenser dryers typically require slightly more power than conventional dryers, typically on the order of ~15%.

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 typical 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. Condenser dryers are a particularly attractive option where long, intricate ducting would be required to vent a traditional dryer.

Heat pump dryers

A closed-cycle heat pump clothes dryer uses a heat pump to dehumidify the processing air. Such dryers typically use less than half the energy per load of a condenser dryer. 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 afterwards for re-use. 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 up to 50% less energy required by either condensation or traditional dryers. Domestic heat pump dryers are designed to work in typical ambient temperatures from 5 to 30 °C. Below 5 °C, drying times significantly increase.

As with condensation dryers, the heat exchanger will not dry the internal air to as low a level of humidity as the typical 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.

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 dryers, 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. 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.

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, but relatively energy-efficient. It is only marginally faster than line-drying.

Solar clothes dryer

The solar dryer is a box-shaped stationary construction which encloses a second compartment where the clothes are held. It uses the sun's heat without direct sunlight reaching the clothes. Alternatively, a solar heating box may be used to heat air that is driven through a conventional tumbler dryer.

Microwave dryers

Japanese manufacturers have developed highly efficient clothes dryers that use microwave radiation to dry the clothes (though a vast majority of Japanese air dry their laundry). Most of the drying is done using microwaves to evaporate the water, but the final drying is done by convection heating, to avoid problems of arcing with metal pieces in the laundry. There are a number of advantages: shorter drying times (25% less), energy savings (17–25% less), and lower drying temperatures. Some analysts think that the arcing and fabric damage is a factor preventing microwave dryers from being developed for the US market.

Static electricity

Clothes dryers can cause static cling, through the triboelectric effect. This can be a minor nuisance, and is often a symptom of over-drying textiles to an extremely low humidity level. Fabric conditioners and dryer sheets are marketed to correct this condition, but there have been increasing health concerns about the effects of prolonged skin contact with the chemicals deposited by these products.

History

A hand-cranked clothes dryer was created in 1800 by M. Pochon from France. J. Ross Moore, an American inventor from North Dakota, developed designs for automatic clothes dryers during the early 20th century. His design for an electrically-operated dryer was developed and released to the public in 1938. Industrial designer Brooks Stevens developed the first electric dryer with a glass window in the 1940s.

Lint build-up (tumble dryers)

Upper image shows a severely kinked and blocked dryer transition hose used to vent a tumble dryer. In this case the dryer was located or pushed back too far against the wall. Lower image shows initial lint build-up in the flex transition hose.

Moisture and lint are byproducts of the tumble drying process, and are pulled from the drum by a fan motor and then pushed through the remaining exhaust conduit to the exterior termination fitting. Typical exhaust conduit comprises flex transition hose found immediately behind the dryer, the 4-inch (100 mm) rigid galvanized pipe and elbow fittings found within the wall framing, and the vent duct hood found outside the house.

A clean, unobstructed dryer vent improves the safety and efficiency of the dryer. As the dryer duct pipe becomes partially obstructed and filled with lint, drying time increases and causes the dryer to overheat and waste energy. In extreme cases, a blocked vent may result in a fire. Clothes dryers are one of the more costly home appliances to operate.

Several factors can contribute to or accelerate rapid lint build-up. These include long or restrictive ducts, bird or rodent nests in the termination, crushed or kinked flex transition hose, terminations with screen-like features, and condensation within the duct due to un-insulated ducts traveling through cold spaces, such as a crawl space or attic.

Safety

Dryers expose flammable materials to heat. Underwriters Laboratories recommends cleaning the lint filter after every cycle for safety and energy efficiency, provision of adequate ventilation, and cleaning of the duct at regular intervals. UL also recommends that dryers not be used for glass fiber, rubber, foam or plastic items, or any item that has had a flammable substance spilled on it.

A clothes dryer that has been damaged by fire.

In the United States, the U.S. Fire Administration in a 2007 report estimated that clothes dryer fires account for about 15,600 structure fires, 15 deaths, and 400 injuries annually, with 80% (12,700) of the fires in residential buildings. The Fire Administration attributes “Failure to clean” as the leading factor contributing to clothes dryer fires in residential buildings, and observed that new home construction trends place clothes dryers and washing machines in more hazardous locations away from outside walls, such as in bedrooms, second-floor hallways, bathrooms, and kitchens.

To address the problem of clothes dryer fires, a fire suppression system can be used with sensors to detect the change in temperature when a blaze starts in a dryer drum. These sensors then activate a water vapor mechanism to put out the fire.

Environmental impact

The environmental impact of clothes dryers is especially severe in the US and Canada, where over 80% of all homes have a clothes dryer. According to the US Environmental Protection Agency, if all residential clothes dryers sold in the U.S. were energy efficient, "the utility cost savings would grow to more than $1.5 billion each year and more than 22 billion pounds of annual greenhouse gas emissions would be prevented”.

Clothes dryers are second only to refrigerators and freezers as the largest energy consumers in America.

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). Sensor dryers can automatically sense that clothes are dry and switch off. This means over-drying is not as frequent. Most of the European market sells sensor dryers now, and they are normally available in condenser and vented dryers.

See also

• Drying cabinet
• Sheila Maid
• Combo washer dryer
• List of home appliances

External links

• "What You Should Know About Clothes Dryers." Popular Mechanics, December 1954, pp. 170–175, basic principles of dryers even today.