Clothes dryer

A modern front load tumble clothes dryer for the home. Click image to enlarge

A clothes dryer or tumble dryer is a household appliance that is used to remove 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.

Using these machines may cause clothes to shrink and or become bigger if u r fat become less soft (due to loss of short soft fibers/ lint) and fade. For these reasons, as well as environmental concerns, many people use open air methods such as a clothes line and clotheshorse.

Fuel types

Coin-operated, gas-powered dryer found in a laundromat

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 Canada, where NEC and NEMA standards are in use, 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 centre-tapped single-phase circuit. Small "portable" clothes dryers, popular with urban dwellers, normally use conventional 110 volt connections. In the rest of the world, most electric dryers in homes are somewhat smaller than those used in North America, typically with a load capacity of 5 kg (11 lb). These dryers are also easily connected to standard single-phase 240V/50Hz 10Amp powerpoints (outlets).

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 metal heat exchanger; a fan pulls in ambient air past the heat exchanger to raise its temperature, while combustion byproducts stay separate from the heated air and are vented to the outside. The air temperature can be altered by adjusting the size of the gas flame or, more simply, by extinguishing it and relighting it under thermostat control. Gas dryers still require electricity to spin the clothes, but the amount of electrical power needed for this is sufficiently low that a dedicated 240-volt connection is not needed and the machine can run on standard 120-volt electricity, although it will now require a dedicated gas connection instead.

Some types of clothes dryers installed in steam-heated buildings use a steam-based system to warm the incoming air, in lieu of a electric heating element or gas-fired heater. In this case, hot steam is piped through the dryer's steam coil (a steam-capable heat exchanger) that transfers thermal energy to the incoming air. A solenoid valve at the steam inlet, controlled by the dryer, can set the temperature of the heated air simply by opening and closing it at regular intervals. Like a gas dryer, an electrical connection is still required to spin the drum, operate the steam solenoid, and power the controls.

Traditional dryers

Traditional 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.

The traditional design makes no effort to recycle the heat put into the load, and so is 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, may increase humidity within a dwelling. Humidity from these devices causes a likelihood of mold, mildew, and bacterial growth inside a home. Indoor venting may also be against local regulations. Gas dryers must especially be vented outdoors in all cases, as the products of combustion are vented along with the moist air. It is recommended that dryers, whether electric or gas, be vented outdoors with a 4-inch diameter piping.

An Indoor Lint Trap Kit poses a similar and serious concern of increased humidity within the dwelling. Numerous articles have been written about the hazards of not actually venting a dryer to the exterior. Terminations into the attic, soffit or crawlspace are similarly hazardous and should be avoided at all costs.

Ventless dryers

Spin dryers

Spin dryer type TS66

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.

In some cases, dryers may use spin alone. Larger gyms and natatoria ("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 after.

Condenser dryers

Just as in a normal dryer, condenser 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, condenser 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.

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. 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, using a heat pump to dehumidify the process 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. 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.

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. 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.

Dehumidifier Dryers

By keeping a low humidity, dehumidifier dryers encourage fast evaporation without high heat. This type of dryer is suitable for clothes that can withstand tumbling but not high heat.

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.

Solar clothes dryer

The solar dryer is a box shaped static construction which encloses a second compartment where the clothes are held. It uses the sun's heat without direct sunlight reaching the clothes.

Static

Clothes dryers can cause static cling, through the triboelectric effect.

History

A hand-cranked clothes dryer was created in 1800 by M. Pochon from France. Electric tumble dryers appeared in the early 20th century. 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 severly kinked and blocked dryer transition hose used to vent a tumble dryer. In this case the dryer was located or pushed back to far against the wall. Lower image shows initial lint build-up in the flex transition hose. Click image to enlarge

As stated above a tumble dryer works by forcing hot dry air through a turning drum. The combination of the rapid replacement of hot dry air and the slow tumbling dries the clothes. Moisture and lint are bi-products of this 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 of flex transition hose found immediately behind the dryer, the 4 " rigid galvanized pipe and elbow fittings found within the wall framing, and the vent duct hood found outside the house.

Obviously a clean, unobstructed dryer vent improves the safety and efficiency of your dryer. As the dryer duct pipe becomes obstructed and filled with lint, drying time increases and causes the dryer itself to overheat. Likewise, this prevents the ample removal of air and moisture from the dryer, and a typical 45 minute cycle turns into an hour and 40 minute cycle, resulting in a waste of energy, additional clothes fibers lost and the frightening possibility of a fire. Clothes dryers are one of the most costly appliances in your home to operate.

Several factors can contribute to or accelerate rapid lint build-up. These include excessively 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, i.e. crawl space or attic.

Safety

Dryers expose flammable materials to heat. Underwriters Laboratories suggests:

• Clean the lint filter after every cycle for safety and energy efficiency.
• Provide adequate ventilation around the dryer.
• Keep glass fiber, rubber, foam and plastic items out of the dryer -- or anything that has had a flammable substance spilled on it.

Other Suggestions:

• Either DIY or have your dryer vent duct cleaned professionally at a minimum of every two years. Preferred: every 18 months. Generally involves a rotating brush on a flexible stem and a blower or vacuum.

Environmental Impact

The average home clothes dryer has a carbon footprint of approximately 2 kilograms (4.4 pounds) of CO₂ per load of laundry dried.

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).

Sensor dryers can automatically sense that clothes are dry and switches off the dryer. This means over drying is not as frequent. Most of the european market sells sensor dryers now and are normally available in condensor & vented dryers.

See also

• Drying cabinet
• Sheila Maid
• Combo washer dryer
• WikiHow - How to Vent a Dryer

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.
• Comparison of foil and aluminum flex duct In this link there is a good visual comparison of the two most popular transition flex hoses used to vent a tumble dryer. It is interesting how the material collapses into the interior of the pipe closing down the actual diameter.