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A dishwasher is a mechanical device for cleaning dishware and cutlery. Unlike manual dishwashing, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot water, typically between 45 and 75 °C (110 and 170 °F), at the dishes, with lower temperatures used for delicate items.
A mix of water and detergent is pumped to one or more rotating spray arms, which blast the dishes with the cleaning mixture. Once the wash is finished, the water is drained, more hot water is pumped in and a rinse cycle begins. After the rinse cycle finishes and the water is drained, the dishes are dried using one of several drying methods. Typically a rinse aid is used to eliminate water spots for streak-free dishes and glassware resulting from hard water or other reasons.
In addition to domestic units, industrial dishwashers are available for use in commercial establishments such as hotels and restaurants, where a large number of dishes must be cleaned. Washing is conducted with temperatures of 65–71 °C (149–160 °F) and sanitation is achieved by either the use of a booster heater that will provide a 82 °C (180 °F) "final rinse" temperature or through the use of a chemical sanitizer.
- 1 History
- 2 Design
- 3 Process
- 4 Cleaning agents
- 5 Effects on crockery
- 6 Efficiency
- 7 Adoption
- 8 Environmental impact
- 9 Alternative uses
- 10 See also
- 11 References
- 12 External links
The first reports of a mechanical dishwashing device are of an 1850 patent in the United States by Joel Houghton for a hand-powered wood device. This device was made of wood and was cranked by hand while water sprayed onto the dishes. This device was both slow and unreliable. Another patent was granted to L.A. Alexander in 1865 that was similar to the first but featured a hand-cranked rack system. Neither device was practical or widely accepted.
The first reliable hand-powered dishwasher was invented in 1887 by Josephine Cochrane with the help of George Butters and was unveiled at the 1893 World's Fair in Chicago, Illinois. Cochrane's inspiration was her frustration at the damage to her good china that occurred when her servants handled it during cleaning.
In the United Kingdom, William Howard Livens invented a small, non-electric dishwasher suitable for domestic use in 1924. It was the first dishwasher that incorporated most of the design elements that are featured in the models of today; it included a front door for loading, a wire rack to hold the dirty crockery and a rotating sprayer. Drying elements were even added to his design in 1940. It was the first machine suitable for domestic use, and it came at a time when permanent plumbing and running water in the house was becoming increasingly common.
Despite this, Liven's design did not become a commercial success, and dishwashers were only successfully sold as domestic utilities in the postwar boom of the 1950s, albeit only to the wealthy. Initially dishwashers were sold as standalone or portable devices, but with the development of the wall-to-wall countertop and standardized height cabinets, dishwashers began to be marketed with standardized sizes and shapes, integrated underneath the kitchen countertop as a modular unit with other kitchen appliances.
By the 1970s dishwashers had become commonplace in domestic residences in North America and Western Europe. By 2012, over 75 percent of homes in the US and Germany had dishwashers.
Size and capacity
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) or 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.
The international standard for the capacity of a dishwasher is expressed as standard place settings. 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.
Present-day machines feature a drop-down front panel door, allowing access to the interior, which usually contains two or sometimes three pull-out racks; racks can also be referred to as "baskets". In older U.S. models from the 1950s, the entire tub rolled out when the machine latch was opened, and loading/removing washable items was from the top, with the user reaching deep into the compartment for some items. Youngstown Kitchens, which manufactured entire kitchen cabinets and sinks, offered a tub-style dishwasher, which was coupled to a conventional kitchen sink as one unit.
Today, "dish drawer" models mimic this style, while the half-depth design eliminates the inconvenience of the long reach that was necessary with older full-depth models. "Cutlery baskets" are also common. A drawer dishwasher, first introduced by Fisher & Paykel in 1997, is a variant of the dishwasher in which the baskets slide out with the door in the same manner as a drawer filing cabinet, with each drawer in a double-drawer model being able to operate independently of the other.
The inside of a dishwasher in the North American market is either stainless steel or plastic. Stainless steel tubs resist hard water, provide better sound damping, and preserve heat to dry dishes more quickly. They also come at a premium price. Older models used 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 dishwashers feature a stainless steel interior as standard, even on low end models. The same is true for a built-in water softener.
The flutes (or valve meters) of the dishwasher are prevalent in American models (with some appearing in European and Asian models influenced by US design) due to the higher pressure of the American water system (which averages at 90 torrs/min, as opposed to the 65 torrs/min pressure in other countries). The flutes help drain the excess water, preventing entropy within the system due to higher pressures at a lower volume. This is a removable fixture, as some areas require a higher or lower discharge based on their water system.
European dishwashers almost universally use two or three spray arms which are fed from the bottom and back wall of the dishwasher leaving both racks unimpeded and also such models tend to use inline water heaters, removing the need for exposed elements in the base of the machine that can melt plastic items near to them. Many North American dishwashers tend to use more basic and old fashioned water distribution and exposed elements in the base of the dishwasher. Some North American machines use 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).
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.
Many new 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. A bimetal switch or wax motor opens the detergent door during the wash cycle.
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.
The heat inside the dishwasher dries the contents after the final hot rinse; the final rinse adds a small amount of rinse aid to the hot water, as this improves drying significantly. Plastic and non-stick items may not dry properly 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.
North American dishwashers tend to use heat-assisted drying via an exposed element. European machines and some high end North American machines use passive methods for drying – a stainless steel interior helps this process and some models use heat exchange technology between the inner and outer skin of the machine to cool the walls of the interior and speed up drying. Most dishwashers feature a drying sensor and as such, a dish-washing cycle is always considered complete when a drying indicator, usually in the form of an illuminated "end" light, or in more modern models on a digital display or audible sound, exhibits to the operator that the washing and drying cycle is now over.
Governmental agencies often recommend air-drying dishes by either disabling or stopping the drying cycle to save energy.
Different kinds of dishwashing detergent contain different combinations of ingredients. Common ingredients include:
- Bind calcium and magnesium ions to prevent 'hard-water' type limescale deposits. They can cause ecological damage, so their use is starting to be phased out. Phosphate-free detergents are sold as eco-friendly detergents.
- Oxygen-based bleaching agents (older-style powders and liquids contain chlorine-based bleaching agents)
- Break up and bleach organic deposits.
- Non-ionic surfactants
- Lower the surface tension of the water, emulsifies oil, lipid and fat food deposits, prevents droplet spotting on drying.
- Alkaline salts
- Anti-corrosion agent(s)
Dishwashing detergent may also contain :
- Anti-foaming agents
- Foam interferes with the washing action.
- Additives to slow down the removal of glaze & patterns from glazed ceramics
- Anti-caking agents (in granular detergent)
- Starches (in tablet based detergents)
- Gelling agents (in liquid/gel based detergents)
- Sand (inexpensive powdered detergents)
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.
Besides older style detergents for dishwashers, biodegradable detergents also exist for dishwashers. These detergents may be more environmentally friendly than conventional detergents.
Hand-washing dish detergent (washing up liquid) creates a large foam of bubbles which will leak from the dishwasher.
Rinse aid (sometimes called rinse agent) contains surfactants and uses Marangoni stress to prevent droplet formation, so that water 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. A thinner sheet of water also has a much larger surface-area than a droplet of the same volume, which increases the likelihood of water molecules evaporating.
In some countries, especially those in Europe, dishwashers include a built-in water softener that removes calcium and magnesium ions from the water. Dishwasher salt, which is coarse-grained sodium chloride (table salt), is used to recharge the resin in the built-in ion-exchange system. The coarse grains prevent it from clogging the softener unit; unlike certain types of salt used for culinary purposes, it does not contain added insoluble anticaking agents or magnesium salts. The presence of magnesium salts will defeat the purpose of removing magnesium from the water softener. Anticaking agents may lead to clogging or may contain magnesium. Table salt may contain added iodine in the form of sodium iodide or potassium iodide. These compounds will not affect the ion-exchange system, but adding table salt to the dishwasher's water softening unit can damage it.
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.
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.
Effects on crockery
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:
- Silicate filming, etching, and 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. Limitation of this undesirable reaction is possible by controlling water hardness, detergent load and temperature. The type of glass is an important factor in determining if this effect is a problem. 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.
- 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
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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. Some items can be damaged if washed in a dishwasher because of the effects of the chemicals and hot water. Aluminium items will discolor. Saucepan manufacturers often recommend handwashing due to the harsh effects of the chemicals on the pan coatings. Valuable items, such as antiques or hand-painted items, should only be washed manually 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.
Items soiled by wax, cigarette ash or anything which might contaminate the rest of the wash load (such as poisons or mineral oils) should be washed by hand. 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 put in a dishwasher, especially on a hot wash cycle when temperatures can reach 75 °C (167 °F); these high temperatures can also damage plastic items which are designated as only being washed by hand. Some plastic items can be distorted or melted if placed in the bottom rack too close to an exposed heating element; therefore, most dishwasher-safe plastic items recommend 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, such as 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.
Knives and other cooking tools that are made of carbon steel, semi-stainless steels like D2 or specialized, highly hardened cutlery steels like ZDP189 should also not be placed into a dishwasher, as these steels are not corrosion resistant or far less corrosion resistant than the austenitic stainless steels used for cookware. Also, very sharp edges can become dulled or damaged from colliding with other items and/or thermal stress from the washing cycles, and can pose an injury hazard if another person unloading the dishwasher does not expect such items in the dishwasher. Ceramic edges are very brittle and can take damage from collision with dishwasher parts or other contents.
Cast iron cookware is normally seasoned with oil or grease and heat, which causes the oil or grease to be absorbed into the pores of the cookware, thereby giving a smooth relatively non-stick cooking surface. Such cookware should not be washed in a dishwasher as the combination of alkali based detergent and hot water will strip off this cooking surface, requiring reseasoning before the item may once again be used.
In the European Union, the energy consumption of a dishwasher for a standard usage is shown on a European Union energy label. In the United States, the energy consumption of a dishwasher is defined using the energy factor.
Most consumer dishwashers use a 75 °C (167 °F) 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 (167 °F) rather than 83 °C (181 °F) for reasons of burn risk, energy and water 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 / 122–131 °F) are needed to prevent premature decay of the enzymes used to eat the grease and other build-ups on the dishes.
Large heavy-duty dishwashers are available for use in commercial establishments (e.g. hotels, restaurants) where a large number of dishes must be cleaned.
Unlike a residential dishwasher, a commercial dishwasher does not utilize a drying cycle (commercial drying is achieved by heated ware meeting open air once the wash/rinse/sanitation cycles have been completed) and thus are significantly faster than their residential counterparts. Washing is conducted with 65–71 °C / 150–160 °F temperatures and sanitation is achieved by either the use of a booster heater that will provide the machine 82 °C / 180 °F "final rinse" temperature or through the use of a chemical sanitizer. This distinction labels the machines as either "high-temp" or "low-temp".
Some commercial dishwashers work similarly to a commercial car wash, with a pulley system that pulls the rack through a small chamber (known widely as a "rack conveyor" systems). Single-rack washers require an operator to push the rack into the washer, close the doors, start the cycle, and then open the doors to pull out the cleaned rack, possibly through a second opening into an unloading area.
In the UK, the British Standards Institution set standards for dishwashers. In the US, 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. There are many types of commercial dishwashers including under counter, single tank, conveyor, flight type, and carousel machines.
Commercial dishwashers often have significantly different plumbing and operations than a home unit, in that there are often separate spray arms for washing and rinsing/sanitizing. The wash water is heated with an in-tank electric heat element and mixed with a cleaning solution, and is used repeatedly from one load to the next. The wash tank usually has a large strainer basket to collect food debris, and the strainer may not be emptied until the end of the day's kitchen operations.
Water used for rinsing and sanitizing is generally delivered directly through building water supply, and is not reusable. The used rinse water falls into the wash tank reservoir, which dilutes some of the used wash water and causes a small amount to drain out through an overflow tube. The system may first rinse with pure water only, and then sanitize with an additive solution that is left on the dishes as they leave the washer to dry.
Additional soap is periodically added to the main wash water tank, from either large soap concentrate tanks or dissolved from a large solid soap block, to maintain wash water cleaning effectiveness.
Comparing the efficiency of automatic dishwashers and hand-washing of dishes is difficult because hand-washing techniques vary drastically by individual. According to a peer-reviewed study in 2003, hand washing and drying of an amount of dishes equivalent to a fully loaded automatic dishwasher (no cookware or bakeware) could use between 20–300 litres (5.3–79.3 US gal) of water and between 0.1 and 8 kWh of energy, while the numbers for energy-efficient automatic dishwashers were 15–22 litres (4.0–5.8 US gal) and 1 to 2 kWh, respectively. The study concluded that fully loaded dishwashers use less energy, water, and detergent than the average European hand-washer. For the automatic dishwasher results, the dishes were not rinsed before being loaded. 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, the comparison for cleaning cookware, or the value of labour saved; hand washers needed between 65 and 106 minutes. Several points of criticism on this study have been raised. For example, kilowatt hours of electricity were compared against energy used for heating hot water without taking into account possible inefficiencies. Also, inefficient human washers were compared against optimal usage of a fully loaded dishwasher without manual pre-rinsing that can take up to 100 litres (26 US gal) of water.
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). 17 US states have partial or full bans on the use of phosphates in dish detergent, and 2 US states Maryland and New York ban phosphates in commercial dishwashing. Detergent companies claimed it is not cost effective to make separate batches of detergent for the states with phosphate bans (although detergents are typically formulated for local markets), and so most have voluntarily removed phosphates from all dishwasher detergents.
In addition, rinse aids have contained nonylphenol and nonylphenol ethoxylates. These have been banned in the European Union by EU Directive 76/769/EEC.
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