Unlike manual dishwashing, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot water, typically between 55 and 75 °C (130 and 170 °F) at the dishes, with lower temperatures used for delicate items. A mix of water and detergent is circulated by a pump. Water 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, a heating element in the bottom of the tub heats the air to dry the dishes. Sometimes a rinse aid is used to eliminate water spots for streak-free dishes.
- 1 History
- 2 Characterization
- 3 Detergent
- 4 Rinse aid
- 5 Dishwasher salt
- 6 Hazing of glassware
- 7 Items unsuitable for the dishwasher
- 8 Drying
- 9 Level of sanitizing
- 10 Commercial dishwashers
- 11 Environmental impact
- 12 Alternative use as a cooking device
- 13 See also
- 14 References
- 15 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 good 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.[unreliable source?]
The first reliable (hand-powered) dishwasher was invented in 1887 by Josephine Cochrane and unveiled at the 1893 Chicago World's Fair. Cochrane was quite wealthy and never washed dishes herself; she reportedly invented the dishwasher because her servants were chipping her fine china.
In England, William Howard Livens invented a small dishwasher suitable for domestic use in 1924. It was the first modern dishwasher, and incorporated most of the design elements that feature 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.
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.
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.
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. 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 each other.
The inside of a dishwasher in the North American market are either stainless steel or plastic. Stainless steel tubs resist hard water, provide better sound damping, 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 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.
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.
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).
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.
A dishwasher should never be emptied before a complete process has been signified to be finished by the control system, as this will often leave the contents unwashed or still in a saturated state. It is a common misconception that to empty a dishwasher before the end of a cycle will save energy, as many of the contents may need to be re-run, hence almost doubling running costs.
Depending on the model, some dishwashers can be plumbed into either the hot or cold water supply, taking into account the maximum incoming water temperature recommended by the manufacturer where hot water is used. Plumbing a dishwasher into the hot or warm water supply can improve cleaning performance and reduce food debris in the interior of the dishwasher. A few dishwashers may spend much less time on the wash phase if the incoming water is hot, which can compromise cleaning, so results will vary. Dishwashers may cost less to operate on a hot water supply if there are other forms of energy available that allow water to be heated more cost efficiently than an electrically powered heating element in a dishwasher.
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. Undamped, low-end dishwashers generally output noise levels of anywhere from 65–70 decibels. Most manufacturers generally use their own nomenclature with trademark for sound damping.
|This section needs additional citations for verification. (April 2012)|
Different kinds of dishwashing detergent contain different combinations of the items in the list below. Not all of the ingredients below are used in some detergent.
- Oxygen-based bleaching agents (older-style powders and liquids contain chlorine-based bleaching agents)
- Break up and bleach organic deposits .
- Non-ionic surfactants
- 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.
Prior to the invention of the dishwasher in 1886, hand-washing primarily with simple detergents was common. The invention of the machine prompted the use of stronger detergents and rinse agents, thus saving time. 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 (sometimes called rinse agent) contains surfactants that 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.
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, but 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.
Pouring detergent 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.
Some newer dishwashers allow the use of "all in one" tablets/detergents (which include an amount of water softener along with detergent and a rinse agent) and are marketed as an alternative to using separate salt and rinse aid, but dishwasher salt must still be added to the salt compartment in very hard water areas. The use of such "all in one" detergents does not mean that separate salt and rinse aid is not required, as omitting separate salt and rinse aid will impair the cleaning and drying results and may cause limescale damage, also, incorrect use of "all in one" tablets/detergents may not be covered under the dishwasher's warranty..
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.
Hazing of glassware
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:
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. 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.
Glassware placed such that it is physically touching can abrade and produce a milky surface.
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
Items unsuitable for the dishwasher
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. 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 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, however 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 (most 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.
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.
Level of sanitizing
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 °F-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 similar 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, 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. 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.
The dishwasher has made cleaning and drying dishes much easier and more efficient. 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.
Comparison with washing by hand
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 and 300 liters of water and between 0.1 and 8 kWh of energy, while the numbers for energy-efficient automatic dishwashers were 15 to 22 liters 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 liters of water.
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). 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.
Alternative use as a cooking device
Many recipe websites have noted that a dishwasher can be used to cook certain foods, in particular salmon. It is very important that all foods be completely sealed to avoid contamination with soaps or rinse aids, and that the maximum temperature available in the washer is taken into account (as certain foods should not be prepared below a certain temperature, for food safety reasons).
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