dishwasher

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A two drawer DishDrawer dishwasher.

A dishwasher is a mechanical device for cleaning dishes and eating utensils. Dishwashers can be found in restaurants and private homes.

How dishwashers work

Unlike manual dishwashing, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot (55–65 degrees Celsius or 130–150 degrees Fahrenheit) water on the dishes. A mix of water and detergent is used for cleaning purposes, followed by clean water to remove the detergent residue. Some dishwashers have multiple wash and rinse periods within the complete cycle. In some dishwashers, a rinsing aid can be added to the rinse cycle.

Manual dishwashers

The word 'dishwasher' (or abbreviated as simply "dish") may also refer to a person who washes dishes in a restaurant, hotel or other private or commercial setting. Pots and pans are also washed by hand by scrubbing them in a detergent and water mix, immersing them in a rinse of plain water, and then immersing them in a water/sanitizer solution for a period. Silverware is washed by placing loose silverware in a tray, washing them several times like this, then sorting them into circular holders, and washing them again in the dishwasher. Colloquially, a dishwasher may be known as a pan-diver, from the french "plongeur", and made famous by George Orwell in Down and Out in Paris and London. Commonly used also is the term "KP" for Kitchen Porter, who would have a variety of other duties.

History

The examples and perspective in this section deal primarily with the United States and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (April 2009)

The first reports of a mechanical dishwashing device are of an 1850 patent by Joel Houghton of a hand-powered device.

Modern dishwashers are descended from the 1886 invention of Josephine Cochrane, also hand-powered, which she unveiled at the 1893 Chicago World's Fair. Cochrane was quite wealthy and was the granddaughter of John Fitch, the inventor of the steamboat. She never washed dishes herself and only invented the dishwasher because her servants were chipping her fine china.

Models installed with permanent plumbing arrived in the 1920s. In 1924, William Howard Livens invented a small dishwasher suitable for domestic use. It had many of the features of a modern dishwasher, including a front door for loading, a wire rack to hold crockery and a rotating sprayer.^[1] Livens' invention was not, however, a commercial success. Electric drying elements were added in 1940.

Adoption was greatest at first in commercial environments, but by the 1970s dishwashers had become commonplace in domestic residences in the US.

Characterization

Capacity

The international standard for the capacity of a dishwasher is expressed as standard place settings. Dishes or plates of irregular sizes may not fit properly in a dishwasher's cleaning compartment, so it is advisable to check for compatibility before buying a dishwasher.

Commercial dishwashers are rated as plates per hour. The rating is based on standard sized plates of the same size. The same can be said for commercial glass washers, as they are based on standard glasses, normally pint glasses.

Size

Dishwashers that are installed into standard kitchen cabinets have a standard width and depth of 60 cm (Europe) or 24 inches (US), and most dishwashers must be installed into a hole a minimum of 86 cm (Europe) or 34 inches (US) tall. Portable dishwashers exist in 45 and 60 cm (Europe) 18 and 24 inch (US) widths, with casters and attached countertops. Dishwashers may come in standard or tall tub designs; standard tub dishwashers have a service kickplate beneath the dishwasher door that allows for simpler maintenance and installation, but tall tub dishwashers have approximately 20% more capacity and better sound dampening from having a continuous front door.

Features

The inside of a dishwasher, called the tub, can be composed of plastic or stainless steel. Stainless steel tubs resist hard water, provide better sound dampening, and preserve heat to dry dishes faster. They also come at a premium price. Older models used a baked enamel on steel and are prone to chipping and erosion; chips in the baked enamel finish must be cleaned of all dirt and corrosion then patched with a special compound or even a good quality two-part epoxy. All European made dishwasher feature as standard a stainless steel interior, even on low end models. The same is true for a built-in water softeners.

Mid-to-higher end North American dishwashers often come with hard food disposal units, which behave like miniature garbage (waste) disposal units that eliminate large pieces of food waste from the wash water. One manufacturer that is known for omitting hard food disposals is Bosch, a German brand; however, Bosch does so in order to reduce noise. If the larger items of food waste are removed before placing in the dishwasher, pre-rinsing is not necessary even without integrated waste disposal units. Pre-rinsing under a running tap beforehand uses more water and is not required.^[2]

Many newer dishwashers feature microprocessor-controlled, sensor-assisted wash cycles that adjust the wash duration to the quantity of dirty dishes (sensed by changes in water temperature) or the amount of dirt in the rinse water (sensed chemically/optically). This can save water and energy if the user runs a partial load. In such dishwashers the electromechanical rotary switch often used to control the washing cycle is replaced by a microprocessor but most sensors and valves are still required to be present. However, pressure switches (some dishwashers use a pressure switch and flow meter) are not required in most microprocessor controlled dishwashers as they use the motor and sometimes a rotational position sensor to sense the resistance of water, when it senses there is no cavitation it knows it has the optimal amount of water.

Most dishwashers include a large cone or similar structure in the bottom dish rack to prevent placement of dishes in the center of the rack. The dishwasher directs water from the bottom of the dishwasher up through this structure to the upper wash arm to spray water on the top dish rack. Some dishwashers, including many models from Whirlpool and Kitchenaid, use a tube attached to the top rack that connects to a water source at the back of the dishwasher which allows full use of the bottom rack. Late-model Frigidaire dishwashers shoot a jet of water from the top of the washer down into the upper wash arm, again allowing full use of the bottom rack (but requiring that a small funnel on the top rack be kept clear).

Some dishwashers include a child-lockout feature to prevent accidental starting or stopping of the wash cycle by children. A child lock can sometimes be included to prevent young children opening the door during a wash cycle. This prevents accidents with hot water and strong detergents used during the wash cycle.

Sound damping

The Modern dishwashers are quieter than older models. Using blankets, panels, and sound-absorbing materials in various configurations, dishwashers can achieve sound damping levels down to 39 decibels or so. Undampened, low-end dishwashers generally output noise levels of anywhere from 65–70 decibels. Manufacturers generally use their own nomenclature with sound damping, e.g. QuietGuard (Kenmore), QuietPartner (Whirlpool), Whisper Package (Maytag), etc. How this nomenclature translates into decibel level is different for every manufacturer. Some brands of dishwasher known for being quiet include Fagor, Bosch, Kuppersbusch, Asko, Smeg and Miele.

Detergent

Detergent tablet

Dishwashing detergent contains:

• Phosphates (30 %)
  • Dissolves calcium and magnesium ions to prevent 'hard-water' type limescale deposits.
• Oxygen-based bleaching agents (older-style powders and liquids contain chlorine-based bleaching agents)(1-5 %)
  • Breaks up and bleaches organic deposits.
• Non-ionic surfactants (5%)
  • Lowers the surface tension of the water, emulsifies oil, lipid and fat food deposits, prevents droplet spotting on drying.
• Enzymes
  • Breaks up and dissolves protein-based food deposits, and possibly oil, lipid and fat deposits. Proteases do this by breaking down the proteins into smaller peptides that are more easily washed away.
• Anti-corrosion agents
  • Often sodium silicate, prevents corrosion of dishwasher components.

Dishwashing detergent may also contain:

• Anti-foaming agents
  • Used as foam decreases the effectiveness of the washing action.
• Additives to slow down the removal of glaze & patterns from glazed ceramics
• Perfumes
• Anti-caking agents (in granular detergent)
• Starches (in tablet based detergents)
• Gelling agents (in liquid/gel based detergents)
• Sand (inexpensive powdered detergents)

Dishwasher detergents are strongly alkaline (basic).

Inexpensive powders may contain sand, which can be verified by dissolving the powder in boiling water and then passing the solution through a coffee filter. Such detergents may harm the dishes and the dishwasher. Powdered detergents are more likely to cause fading on china patterns ^[3].

Biodegradable detergent

Besides chemical detergents for dishwashers, biodegradable detergents also exist for dishwashers. These detergents may be more environmentally friendly than conventional detergents.

Hand-washing detergent

Prior to the invention of the dishwasher in 1886, hand-washing primarily with simple detergents was common. The invention of the machine promted the use of stronger deterents and rinse agents, saving many men the stuggle trouble that came with the cleaners. Hand-washing dish detergent (washing up liquid) should not be used in a dishwasher, as it will create a large foam of bubbles which will leak from the dishwasher. If hand-washing detergent is accidentally used, the foam may be removed by spraying with salt, and the dishwasher should be forced into a drain cycle to remove the detergent and water.

Rinse aid

Rinse aid (sometimes called rinse agent) contains surfactants that uses Marangoni stress to prevent droplet formation, so that it drains from the surfaces in thin sheets, rather than forming droplets.

The benefits of using it are that it prevents "spotting" on glassware (caused by droplets of water drying and leaving behind dissolved limescale minerals), and can also improve drying performance as there is less water remaining to be dried.

Dishwasher salt

In some countries, especially those in Europe, dishwashers include a built-in water softener. Dishwasher salt is used to recharge the resin in the built-in ion-exchange system. Unlike the sodium chloride salt used for culinary purposes, the sodium chloride salt used for a dishwasher water softener does not have added impurities (such as iodide salts). Additionally, salt used for dishwasher water softening systems must have minimal iron and manganese salt content, as these mineral ions tend to form precipitates that clog the ion-exchange resin.

If a dishwasher has a built-in water softener there will be a special compartment inside the dishwasher where the salt is to be added when needed. This salt compartment is separate from the detergent compartment, and generally located at the bottom of the wash cabinet (this is below the bottom basket). On most dishwashers, an automatic sensing system will notify the user when more dishwasher salt is required.

Pouring table salt, detergent (or anything other than proper dishwasher salt) into the salt compartment will damage the water softening system, however this can be reversed if the user acts very quickly and the dishwasher is NOT used: with a suitable wet and dry vacuum cleaner, remove the foreign substance e.g. detergent, followed by adding water again and removing the water with the wet and dry vacuum and repeating the process several times.^[4] Only salt specified as "Dishwasher Salt" can be used. However, it is possible to use salt granules or tablets sold for whole house softening units if that salt is also very high purity processed sodium chloride salt. Very pure whole house water softener salt (in 18 kg bags) is about one fifteenth the cost per equivalent weight of salt specifically marketed for dishwashers.

Some dishwasher detergents are marketed for use in hard water areas for dishwashers which do not have a built-in water softener (and therefore do not use any dishwasher salt). These detergents use higher levels of phosphates to increase the solubility of hard water ions. In very hard water areas, the amount of phosphate may still be insufficient and the manual addition of dishwasher salt into the detergent compartment is recommended. Adding salt along with the detergent does not soften the water as does a dishwasher with an ion-exchange water softener, but the water will gain some additional ability to dissolve hard water ions. Note, however, that as water drops remaining on the dishware evaporate, deposits of the salt will likely remain. To combat this, the use of a rinsing agent which cause the water to "sheet" will help eliminate the spotting. Some newer dishwashers allow the use of "all in one" tablets (which include an amount of salt along with detergent and a rinse agent) instead of using separate salt and rinse aid, but dishwasher salt must still be added to the salt compartment in very hard water areas.^[5] However, best results are achieved by using separate salt, rinse aid and detergent rather than using the combined "all in one" detergents with these included. Incorrect use of "all in one" tablets/detergents may not be covered under the dishwasher's warranty; it is advisable to check the instruction book when using these types of tablets/detergents.^[6]

Hazing of glassware, prohibition on dishwashing lead crystal

Glassware washed by dishwashing machines can develop a white haze on the surface over time. This may be caused by any or all of the below processes, only one of which is reversible:

Limescale deposit

If the dishwasher has run out of the salt that recharges the ion exchange resin that softens the water, and the water supply is "hard", limescale deposits can appear on all items, but are especially visible on glassware. It can be removed by cleaning with vinegar or lemon juice, or a proprietary limescale removal agent. The dishwasher should either be recharged with salt, adjusted appropriately for the hardness of the supply water—or possibly this is a symptom of failure of the ion exchange resin in the water softener (which is one of the more expensive components). The resin may have stopped working because it has been poisoned by iron or manganese salts in the supply water.

Silicate filming/etching/accelerated crack corrosion

This film starts as an iridescence or "oil-film" effect on glassware, and progresses into a "milky" or "cloudy" appearance (which is not a deposit) that cannot be polished off or removed like limescale. It is formed because the detergent is strongly alkaline (basic) and glass dissolves slowly in alkaline aqueous solution. It becomes less soluble in the presence of silicates in the water (added as anti-metal-corrosion agents in the dishwasher detergent). Since the cloudy appearance is due to nonuniform glass dissolution, it is (somewhat paradoxically) less marked if dissolution is higher, i.e. if a silicate-free detergent is used; also, in certain cases, the etching will primarily be seen in areas that have microscopic surface cracks as a result of the items' manufacturing.^[7] Limitation of this undesirable reaction is possible by controlling water hardness, detergent load and temperature (see Maytag Web site, Troubleshooting on Filling the standard tub detergent dispenser). The type of glass is an important factor in determining if this effect is a problem. In hard-water areas more detergent is needed to help prevent etching, and some dishwashers can reduce this etching effect by automatically dispensing the correct amount of detergent throughout the wash cycle based on the level of water hardness programmed. GE Appliances website

Physical abrasion

Glassware placed such that it is physically touching can abrade and produce a milky surface.

Devitrification

Components found in dishwasher detergents can chemically scour the glass, causing tiny crystals, which can precipitate further crystal growth that can turn entire glasses cloudy^[8]

Lead crystal should not be cleaned in a dishwasher as the corrosive effect of dishwasher detergent is high on such types of glass—that is, it will quickly go 'cloudy'. In addition, the lead in the crystal glass can be converted into a soluble form, which could endanger the health of subsequent users.^[9]

Items that should not be put in a dishwasher

Some items can be damaged if washed in a dishwasher because of the effects of the chemicals and hot water. Lead crystal will be irreversibly damaged if put in a dishwasher, while aluminium items will discolour. Saucepan manufacturers often recommend handwashing due to the harsh effects of the chemicals on the pan coatings.^[10] Valuable items—such as antiques—should be washed by hand as they may be dulled or damaged, and detergents will gradually fade the glazing and print. Sterling silver and pewter will oxidize and discolour from the heat. Furthermore, pewter has a low melting point and may warp in some dishwashers.^[11] Cast iron is likely to rust in a dishwasher.

Items soiled by wax, cigarette ash or anything which might contaminate the rest of the wash load (such as poisons or mineral oils) should not be put in a dishwasher. Objects contaminated by solvents may explode in a dishwasher. Glued items, such as some cutlery handles or wooden cutting boards, may be melted or softened if dishwashed, especially on a hot wash cycle when temperatures can reach 75 °C; these high temperatures can also damage plastic items which are labelled as not being dishwasher safe, however some plastic items can be distorted or melted if placed in the bottom rack too close to an exposed heating element, hence many dishwasher-safe plastic items advise placing in the top rack only (many newer dishwashers have a concealed heating element away from the bottom rack entirely). Squeezing plastic items into small spaces may cause the plastic to distort in shape.

Dishwashers should only be used to wash normal household items, like plates, cutlery, cups, mugs, kitchenware etc. Items such as paintbrushes, tools, furnace filters etc. should not be put into a dishwasher as this will cause the subsequent washes to become contaminated and may cause damage to the appliance.

Drying

The heat inside the dishwasher dries the contents after the final hot rinse. Plastic and non-stick items may not dry properly^[12] compared to china and glass, which hold the heat better. Some dishwashers incorporate a fan to improve drying. Older dishwashers with a visible heating element (at the bottom of the wash cabinet, below the bottom basket) may use the heating element to improve drying, however this uses more energy.

Governmental agencies often recommend air-drying dishes by either disabling or stopping the drying cycle to save energy.^[13]

Level of sanitizing

Dishwashers do not sterilize the utensils, as proper sterilization requires autoclaving at 121 °C with pressurized wet steam for at least 15 minutes. Commercial dishwashers can use one of two types of sanitizing methods: hot water sanitizing (using final rinse water at a temperature of at least 83 °C (180 °F)), or chemical sanitizing (by injecting chlorine in the final rinse water). Not all dishwashers are capable of reaching the high temperature required for hot water sanitizing. Medical grade dishwashers and sanitizers are starting to use ultrasonic cleaners^{[citation needed]}, which use a liquid bath treated with sonics to remove particles and sterilize instruments.

Most consumer dishwashers use a 75°C thermostat in the sanitizing process. During the final rinse cycle, the heating element and wash pump are turned on, and the cycle timer (electronic or electromechanical) is stopped until the thermostat is tripped. At this point, the cycle timer resumes and will generally trigger a drain cycle within a few timer increments.

Most consumer dishwashers use 75°C rather than 83°C for reasons of burn risk, energy consumption, total cycle time, and possible damage to plastic items placed inside the dishwasher. With new advances in detergents, lower water temperatures (50–55°C) are needed to prevent premature decay of the enzymes used to eat the grease and other build-ups on the dishes. This also saves energy and can allow the washer to be hooked directly to the hot water supply for the house.

In the US, residential dishwashers can be certified to a NSF International testing protocol which verifies the cleaning and sanitation performance of the unit.^[14]

Commercial dishwashers

Large heavy-duty dishwashers are available for use in commercial establishments (e.g. hotels, restaurants) where a large number of dishes must be cleaned. Commercial dishwashers work similar to a commercial car wash with "teeth" pulling the rack through a small chamber. It can wash a rack of dishes, or a rack of 25 glasses in just approximately one minute. In the UK, the British Standards Institution set standards for dishwashers. In the US, the NSF International (an independent not-for-profit organization) sets the standards for wash and rinse time along with minimum water temperature for chemical or hot water sanitizing methods.^[15] There are many types of commercial dishwashers including under counter, single tank, conveyor, flight type, and carousel machines.

Environmental impact

In the European Union, the energy consumption of a dishwasher for a standard usage is shown on a European Union energy label.

Comparison with washing by hand

This section requires expansion.

Comparing the efficiency of automatic dishwashers and hand-washing of dishes is difficult because hand-washing techniques vary drastically by individual. A 2004 peer-reviewed study concluded that the best automatic dishwashers available at the time, when fully loaded use less electricity, water, and detergent than the average European hand-washer. The most efficient hand-washers in that study, however, were far more energy efficient than the dishwashers.^[16][17] In particular, dishwashers that are capable of heating water internally, like cold fill European models, do not lose heat during transport to the faucet as the quantity of water to be loaded is usually very little, on average of 4-5 litres for each section of the wash. The study does not address costs associated with the manufacture and disposal of dishwashers, the cost of possible accelerated wear of dishes from the chemical harshness of dishwasher detergent or the value of labour saved.

Detergents and rinse aids

Most dishwasher detergent contains complex phosphates, as they have several properties that aid in effective cleaning. However, the same chemicals have been removed from laundry detergents in many countries as a result of concerns raised about the increase in algal blooms in waterways caused by increasing phosphate levels (see eutrophication). The state of Maryland, USA, is considering a bill to limit phosphates in dish detergent to help clean up the Chesapeake Bay.^[18]

In addition, rinse aids have contained nonylphenol and nonylphenol ethoxylates. These have been banned in the European Union by EU Directive 76/769/EEC.

References

1. ^ William Howard Livens (1924-10-23). "Improvements in apparatus for washing household crockery and the like". FR579765. UK Intellectual Property Office. http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=FR579765&F=0. Retrieved 2008-03-04. 
2. ^ http://www.which.co.uk/advice/how-to-be-greener-with-your-dishwasher/eco-friendlier-ways-to-use-your-dishwasher/index.jsp
3. ^ Hotpoint Service - Appliance Repairs, Spare Parts and Accessories for Hotpoint, Creda, Ariston
4. ^ http://www.miele.co.uk/dishwashers/troubleshooting/
5. ^ Do I need to add salt? - Which? tests dishwasher detergent tablets
6. ^ http://www.which.co.uk/reviews/dishwasher-detergent/page/features-explained/
7. ^ http://www.scienceblog.com/community/older/2003/F/20033788.html Dishwashers and microscopic scratches}
8. ^ Ancient glass and vitrification
9. ^ "Lead Contamination In Our Environment"
10. ^ Cooking equipment and dishwashers
11. ^ Dishwashers and silver
12. ^ http://www.siemenscustomerservice.com/Siemens/Siemens_42121.html
13. ^ Top 10 Tips for Renters : ENERGY STAR
14. ^ NSF Consumer Information: Residential Appliances and Other Products for the Home
15. ^ NSF article on dishwashers
16. ^ Stamminger, Rainer (2003). "A European Comparison of Cleaning Dishes by Hand". Proceedings of EEDAL conference. http://www.landtechnik.uni-bonn.de/ifl_research/ht_1/EEDAL_03_ManualDishwashing.pdf. Retrieved 2008-02-17. 
17. ^ Is a machine more efficient than the hand? http://www.landtechnik.uni-bonn.de/ifl_research/ht_1/homeenergy_0504_dishwashing.pdf
18. ^ Md. Dish Soap Bill Might Help Clean Bay

See also

• Home appliances

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