Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses. Domestically, water is traditionally heated in vessels known as water heaters , kettles , cauldrons , pots , or coppers.
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Hot water storage tank
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.
Domestically, water is traditionally heated in vessels known as water heaters , kettles , cauldrons , pots , or coppers. These metal vessels that heat a batch of water do not produce a continual supply of heated water at a preset temperature. Rarely, hot water occurs naturally, usually from natural hot springs. The temperature varies with the consumption rate, becoming cooler as flow increases. Appliances that provide a continual supply of hot water are called water heaters , hot water heaters , hot water tanks , boilers , heat exchangers , geysers Southern Africa only , or calorifiers.
These names depend on region, and whether they heat potable or non-potable water, are in domestic or industrial use, and their energy source. In domestic installations, potable water heated for uses other than space heating is also called domestic hot water DHW. Fossil fuels natural gas , liquefied petroleum gas , oil , or solid fuels are commonly used for heating water.
These may be consumed directly or may produce electricity that, in turn, heats water. Electricity to heat water may also come from any other electrical source, such as nuclear power or renewable energy. Alternative energy such as solar energy , heat pumps , hot water heat recycling , and geothermal heating can also heat water, often in combination with backup systems powered by fossil fuels or electricity.
Densely populated urban areas of some countries provide district heating of hot water. This is especially the case in Scandinavia , Finland and Poland. District heating systems supply energy for water heating and space heating from combined heat and power CHP plants , waste heat from industries, incinerators , geothermal heating, and central solar heating. Actual heating of tap water is performed in heat exchangers at the consumers' premises.
Generally the consumer has no in-building backup system, due to the expected high availability of district heating systems. In the United States today, domestic hot water used in homes is most commonly heated with natural gas, electric resistance, or a heat pump. Electric heat pump hot water heaters are significantly more efficient than electric resistance hot water heaters, but also more expensive to purchase.
Some energy utilities offer their customers funding to help offset the higher first cost of energy efficient hot water heaters. Hot water used for space heating may be heated by fossil fuels in a boiler, while potable water may be heated in a separate appliance.
This is common practice in the US, especially when warm-air space heating is usually employed. In household and commercial usage, most North American and Southern Asian water heaters are the tank type, also called storage water heaters , these consist of a cylindrical vessel or container that keeps water continuously hot and ready to use.
Typical sizes for household use range from 75 to liters 20 to US gallons. These may use electricity , natural gas , propane , heating oil , solar , or other energy sources. Natural gas heaters are most popular in the US and most European countries, since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use.
This is a popular arrangement where higher flow rates are required for limited periods. Water is heated in a pressure vessel that can withstand a hydrostatic pressure close to that of the incoming mains supply. A pressure reducing valve is sometimes employed to limit the pressure to a safe level for the vessel.
In North America, these vessels are called hot water tanks , and may incorporate an electrical resistance heater, a heat pump , or a gas or oil burner that heats water directly. Where hot-water space heating boilers are installed, domestic hot water cylinders are usually heated indirectly by primary water from the boiler, or by an electric immersion heater often as backup to the boiler. In the UK these vessels are called indirect cylinders , or direct cylinders , respectively.
Additionally, if these cylinders form part of a sealed system, providing mains-pressure hot water, they are known as unvented cylinders. In the US, when connected to a boiler they are called indirect-fired water heaters.
Compared to tankless heaters, storage water heaters have the advantage of using energy gas or electricity at a relatively slow rate, storing the heat for later use. The disadvantage is that over time, heat escapes through the tank wall and the water cools down, activating the heating system to heat the water back up, so investing in a tank with better insulation improves this standby efficiency.
Larger tanks tend to provide hot water with less temperature fluctuation at moderate flow rates. Volume storage water heaters in the United States and New Zealand are typically vertical, cylindrical tanks, usually standing on the floor or on a platform raised a short distance above the floor. Volume storage water heaters in Spain are typically horizontal. In India, they are mainly vertical. In apartments they can be mounted in the ceiling space over laundry-utility rooms.
In Australia, gas and electric outdoor tank heaters have mainly been used with high temperatures to increase effective capacity , but solar roof tanks are becoming fashionable. Tiny point-of-use POU electric storage water heaters with capacities ranging from 8 to 32 liters 2 to 6 gallons are made for installation in kitchen and bath cabinets or on the wall above a sink. They may be used when retrofitting a building with hot water plumbing is too costly or impractical.
Since they maintain water temperature thermostatically, they can only supply a continuous flow of hot water at extremely low flow rates, unlike high-capacity tankless heaters. Smaller water heaters are sufficient, as ambient weather temperatures and incoming water temperature are moderate. A locational design decision may be made between point-of-use and centralized water heaters. Centralized water heaters are more traditional, and are still a good choice for small buildings.
For larger buildings with intermittent or occasional hot water use, multiple POU water heaters may be a better choice, since they can reduce long waits for hot water to arrive from a remote heater.
The decision where to locate the water heater s is only partially independent of the decision of a tanked vs. Tankless water heaters—also called instantaneous , continuous flow , inline , flash , on-demand , or instant-on water heaters—are gaining in popularity. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication.
Tankless heaters may be installed throughout a household at more than one point-of-use POU , far from a central water heater, or larger centralized models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are a plentiful continuous flow of hot water as compared to a limited flow of continuously heated hot water from conventional tank water heaters , and potential energy savings under some conditions.
The main disadvantage is their much higher initial costs, a US study in Minnesota reported a to year payback for the tankless water heaters. Stand-alone appliances for quickly heating water for domestic usage are known in North America as tankless or on demand water heaters. In some places, they are called multipoint heaters , geysers or ascots. In Australia and New Zealand they are called instantaneous hot water units. In Argentina they are called calefones. In that country calefones use gas instead of electricity.
A similar wood-fired appliance was known as the chip heater. A common arrangement where hot-water space heating is employed is for a boiler also to heat potable water , providing a continuous supply of hot water without extra equipment.
Appliances that can supply both space-heating and domestic hot water are called combination or combi boilers. Though on-demand heaters provide a continuous supply of domestic hot water, the rate at which they can produce it is limited by the thermodynamics of heating water from the available fuel supplies. An electric shower head has an electric heating element which heats water as it passes through.
These self-heating shower heads are specialized point-of-use POU tankless water heaters, and are widely used in some countries. Invented in Brazil in the s and used frequently since the s, the electric shower is a home appliance often seen in South American countries due to the higher costs of gas distribution.
Earlier models were made of chromed copper or brass, which were expensive, but since , units made of injected plastics are popular due to low prices similar to that of a hair dryer. Electric showers have a simple electric system, working like a coffee maker, but with a larger water flow.
A flow switch turns on the device when water flows through it. Once the water is stopped, the device turns off automatically. An ordinary electric shower often has three heat settings: high 5.
The power consumption of electric showers in the maximum heating setting is about 5. The lower costs with electric showers compared to the higher costs with boilers is due to the time of use: an electric shower uses energy only while the water flows, while a boiler works many times a day to keep a quantity of standing water hot for use throughout the day and night.
Electric showers may save energy compared to electric tank heaters, which lose some standby heat. There is a wide range of electric showers, with various types of heating controls. The heating element of an electric shower is immersed in the water stream, using a nichrome resistance element which is sheathed and electrically isolated, like the ones used in oil heaters, radiators or clothes irons, providing safety.
Due to electrical safety standards, modern electric showers are made of plastic instead of using metallic casings like in the past. As an electrical appliance that uses more electric current than a washer or a dryer, an electric shower installation requires careful planning, and generally is intended to be wired directly from the electrical distribution box with a dedicated circuit breaker and ground system.
A poorly installed system with old aluminum wires or bad connections may be dangerous, as the wires can overheat or electric current may leak via the water stream through the body of the user to earth. Increasingly, solar powered water heaters are being used. Their solar collectors are installed outside dwellings, typically on the roof or walls or nearby, and the potable hot water storage tank is typically a pre-existing or new conventional water heater, or a water heater specifically designed for solar thermal.
The most basic solar thermal models are the direct-gain type, in which the potable water is directly sent into the collector. Many such systems are said to use integrated collector storage ICS , as direct-gain systems typically have storage integrated within the collector. Heating water directly is inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection if any , can easily heat water to temperatures unsafe for domestic use, and ICS systems suffer from severe heat loss on cold nights and cold, cloudy days.
After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger , transferring its heat to the potable hot water. When the panels are cooler than the storage tank or when the storage tank has already reached its maximum temperature, the controller in closed-loop systems stops the circulation pumps.
In a drainback system, the water drains into a storage tank contained in conditioned or semi-conditioned space, protected from freezing temperatures. Flat panel collectors are typically used in closed-loop systems. Flat panels are regularly used in both pure water and antifreeze systems.
Placed in a rack, evacuated tube collectors form a row of glass tubes, each containing absorption fins attached to a central heat-conducting rod copper or condensation-driven. The evacuated description refers to the vacuum created in the glass tubes during the manufacturing process, which results in very low heat loss and lets evacuated tube systems achieve extreme temperatures, far in excess of water's boiling point.
In countries like Iceland and New Zealand , and other volcanic regions, water heating may be done using geothermal heating , rather than combustion.
Where a space-heating water boiler is employed, the traditional arrangement in the UK is to use boiler-heated primary water to heat potable secondary water contained in a cylindrical vessel usually made of copper —which is supplied from a cold water storage vessel or container, usually in the roof space of the building. This produces a fairly steady supply of DHW Domestic Hot Water at low static pressure head but usually with a good flow. In most other parts of the world, water heating appliances do not use a cold water storage vessel or container, but heat water at pressures close to that of the incoming mains water supply.
Other improvements to water heaters include check valve devices at their inlet and outlet, cycle timers, electronic ignition in the case of fuel-using models, sealed air intake systems in the case of fuel-using models, and pipe insulation.
Вы уничтожите этот алгоритм сразу же после того, как мы с ним познакомимся. - Конечно. Так, чтобы не осталось и следа. Сьюзан нахмурилась. Она понимала, что найти принадлежащую Хейлу копию ключа будет очень трудно. Найти ее на одном из жестких дисков - все равно что отыскать носок в спальне размером со штат Техас.
В зависимости от уровня допуска они попадали в те отсеки банка данных, которые соответствовали сфере их деятельности. - Поскольку мы связаны с Интернетом, - объяснял Джабба, - хакеры, иностранные правительства и акулы Фонда электронных границ кружат вокруг банка данных двадцать четыре часа в сутки, пытаясь проникнуть внутрь. - Да, - сказал Фонтейн, - и двадцать четыре часа в сутки наши фильтры безопасности их туда не пускают.
Так что вы хотите сказать.
Б нем не чувствовалось грубой силы Грега Хейла, скорее - жестокость отчаяния, внутренняя бездушная решительность. Сьюзан повернулась. Человек, попытавшийся ее удержать, выглядел растерянным и напуганным, такого лица у него она не видела. - Сьюзан, - умоляюще произнес Стратмор, не выпуская ее из рук.
Любовь и честь были забыты. Мечта, которой он жил все эти годы, умерла. Он никогда не получит Сьюзан Флетчер. Никогда. Внезапная пустота, разверзшаяся вокруг него, была невыносима. Сьюзан равнодушно смотрела на ТРАНСТЕКСТ. Она понимала, что огненный шар, заточенный в керамическую клетку, скоро вырвется наружу и поглотит .
Не можем ли мы подкупить Танкадо. Я знаю, он нас ненавидит, но что, если предложить ему несколько миллионов долларов. Убедить не выпускать этот шифр из рук. Стратмор рассмеялся: - Несколько миллионов.
Этот новый стандарт шифрования означал бы, что АНБ может прослушивать кого угодно, где угодно и когда угодно. - Ты прав - и так и должно быть! - сурово отрезала Сьюзан.
Когда она уезжает. Двухцветный словно будто только что очнулся. - Когда? - Он заржал. - Она давно уехала. Отправилась в аэропорт несколько часов. Самое место, где толкнуть колечко: богатые туристы и все такое прочее. Как только получит денежки, так и улетит.
Беккер почувствовал тошноту.
Escortes Belen, - ответил мужчина. И снова Беккер изложил свою проблему: - Si, si, senor. Меня зовут сеньор Ролдан. Буду рад вам помочь. У нас две рыжеволосые.
Хорошо, - сказала. - Я немного погорячилась. - Немного? - Глаза Бринкерхоффа сузились. - У Стратмора стол ломится от заказов. Вряд ли он позволил бы ТРАНСТЕКСТУ простаивать целый уик-энд.
- Хорошо, хорошо. - Мидж вздохнула.
Я отлично все понял! - Он уставил на Беккера костлявый указательный палец, и его голос загремел на всю палату. - Вы не первый.
Отключить все это без подготовки - значит парализовать разведдеятельность во всем мире. - Я отдаю себе отчет в последствиях, сэр, - сказал Джабба, - но у нас нет выбора.
Однажды вечером на университетском представлении Щелкунчика Сьюзан предложила Дэвиду вскрыть шифр, который можно было отнести к числу базовых.
Весь антракт он просидел с ручкой в руке, ломая голову над посланием из одиннадцати букв: HL FKZC VD LDS В конце концов, когда уже гасли огни перед началом второго акта, его осенило.
Шифруя послание, Сьюзан просто заменила в нем каждую букву на предшествующую ей алфавите. Для расшифровки Беккеру нужно было всего лишь подставить вместо имеющихся букв те, что следовали непосредственно за ними: А превращалось в В, В - в С и так далее.
Отпил глоток и чуть не поперхнулся. Ничего себе капелька. В голове у нее стучало. Повернувшись, она увидела, как за стеной, в шифровалке, Чатрукьян что-то говорит Хейлу.