Dishwasher

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A Dishwasher

A two drawer DishDrawer dishwasher.

This article is about the appliance. For manual dishwashing, see dishwashing.

A dishwasher is a mechanical device for cleaning food dishes and eating utensils. They can be found in restaurants and also in the kitchens of many private residences.

[edit] 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. First detergent-added water is used for cleaning purposes, then 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. As there is no human contact during the process, strong detergents may be used which would be too alkaline for habitual exposure to the skin. Many dishwashers also contain a heating element to achieve fast drying of the dishes which, in some models, can also be used to heat the hot water to the desired wash temperature.^[1]

[edit] History

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 as her servants were chipping her fine china.

Models installed with permanent plumbing arrived in the 1920s, and 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 developed world.

[edit] Characterisation

[edit] 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.

[edit] 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.

[edit] 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 price premium. 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.

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 simply wastes water.

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.

Some dishwashers include a child-lockout feature to prevent accidental starting or stopping of the wash cycle by children.

[edit] Sound Damping

Using blankets, panels, and sound-absorbing materials in various configurations, dishwashers can achieve sound damping levels down to 44 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, i.e. QuietGuard (Kenmore), QuietPartner (Whirlpool), Whisper Package (Maytag), etc. How this nomenclature translates into decibel level is different for every manufacturer.

[edit] Detergent

Dishwashing detergent contains:

Phosphates
- Solublises 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)
- Breaks up and bleaches organic deposits.
Non-ionic surfactants
- Lowers the surface tension of the water, emulsifies oil, lipid and fat food deposits, prevents droplet spotting on drying.
Enzymes
- Breaks up and solublises protein-based food deposits, and possibly oil, lipid and fat deposits. They do this by emulsifying them.
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).

Intuition suggests that a powder will provide a better scrubbing action due to a soft media sandblasting effect, though liquid detergents have marketed themselves as premium products. Inexpensive powders sometimes actually contain sand, which can be verified by dissolving the powder in boiling water and then passing the solution through a coffee filter; these detergents should be avoided to prevent wear of the dishes and the dishwasher.

[edit] Hand-washing detergent

Hand-washing dish detergent is not to 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 soap and water.

[edit] Rinse aid

Rinse aid contains surfactants that prevent droplet formation by reducing the surface tension of the water, so that it drains from the surfaces in thin sheets, rather than forming droplets.

The benefits of using it are that it prevent '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.

[edit] Dishwasher salt

Dishwasher salt is used to recharge the built-in ion-exchange water softener in dishwashers. Unlike salt used for culinary purposes, it does not have added iodide salts. However, it should have minimal iron and manganese salt content, as these mineral ions tend to form precipitates that clog the ion-exchange resin.

Some dishwasher detergents are marketed as not requiring the use of dishwasher salt. These instead use increased levels of phosphates to increase the solubility of hard water ions. In very hard water areas, the amount of phosphate may be insufficient, requiring the additional use of salt in any case.^[2] Some newer dishwashers have a setting for "all in one" tablets.

[edit] Hazing of glassware, prohibition on dishwashing lead crystal

Glassware that is washed by dishwashing machines often develops 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 more soluble in the presence of silicates in the water (added as anti-metal-corrosion agents in the dishwasher detergent). In certain cases, the etching will primarily be seen in areas that have microscopic surface cracks as a result of the items' manufacturing.^[3] Limitation of this undesirable reaction is possible by controlling water hardness, detergent load and temperature (see Maytag Web site). The type of glass is an important factor in determining if this effect is a problem.
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^[4]

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 is not good for the health of subsequent users.^[5]

[edit] Items that should not to 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.^[6] 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 discolor from the heat. Furthermore, pewter has a low melting point and may warp in some dishwashers.^[7] Items soiled by wax, cigarette ash or anything which might contaminate the rest of the wash load (such as poisons or WD-40) should not be put in a dishwasher. Objects contaminated by solvents may explode in a dishwasher. Glued items, such as some cutlery handles, may be melted or softened if dishwashed, especially on a hot wash cycle where 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).

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.

[edit] Level of sterilisation

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

Most consumer dishwashers use a 75°C thermostat in the sanitising 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 buildups on the dishes. This also saves energy and can allow the washer to be hooked directly to the hot water supply for the house.

[edit] 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 machines can wash a rack of dishes in just a few minutes. In the UK, the British Standards Institute 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.^[8] There are many types of commercial dishwashers including under counter, single tank, conveyor, flight type, and carousel machines.

[edit] Dishwashers and the environment

[edit] Dishwashers versus washing dishes by hand

Comparing the efficiency of automatic dishwashers and hand-washing of dishes is difficult because hand-washing techniques vary drastically by individual. At least one privately funded, non-peer-reviewed study concludes that automatic dishwashers when fully loaded use less water than even the most efficient hand-washers, while relative energy use depends on hand-washing technique and number of place settings washed per load (a smaller load still favours hand washing)^[9] The study does not address costs associated with the manufacture and disposal of dishwashers or the cost of possible accelerated wear of dishes from the chemical harshness of dishwasher detergent.

Many people are sceptical of these claims, and one private individual has shown that dishwashers are comparable to efficient hand-washing in water usage;^[10] however this small-scale study did not include any measurements of other environmental impact.

[edit] Waterless dishwasher concept

A team of students from the University of New South Wales in Sydney won top honours in the 2004 Electrolux Design Laboratory competition for their Rockpool waterless dishwasher design. Its primary feature was the use of supercritical carbon dioxide in place of water in closed-loop operation. Pressurized carbon dioxide behaves like a nonpolar solvent, which can effectively remove grease. It also demonstrates the physical properties of a liquid, hence the solvent effectiveness, and a gas, which fills its container and thus does not require as much mechanical agitation. The grease is filtered from the carbon dioxide and removed from the system. Supercritical carbon dioxide has previously been used in dry cleaning. The primary benefit of this concept is the fact that it uses no water directly.^[11]

[edit] Dishwasher 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 is considering a bill to limit phosphates in dish detergent to help clean up the Chesapeake Bay. ^[12]

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

[edit] References

[edit] See also

[edit] External links

Dishwasher Care UK site containing dishwasher advice
Dishwasher FAQ
How to Choose a Dishwasher
Vintage Appliance Commercials, including commercials for 1950s dishwashers.

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Categories: Domotics | Home appliances