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Boiler feedwater

Boiler feedwater is an essential part of boiler operations. The feed water is put in to the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the feed water is used it is then dumped to the main condenser. From the condenser it is then pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feed water is never open to the atmosphere. This cycle is known as a closed system or Rankine cycle.

Characteristics of boiler feedwater
Water absorbs more heat than any other substance at a given temperature change. This quality makes it an ideal raw material for boiler operations. Boilers are part of a closed system as compared to open systems in a gas turbine. The closed system that is used is the Rankine cycle. This means that the water is recirculated throughout the system and is never in contact with the atmosphere. The water is reused and needs to be treated to continue efficient operations. Boiler water must be treated in order to be proficient in producing steam. Boiler water is treated to prevent scaling, corrosion, foaming, and priming. Chemicals are put in to boiler water through the chemical feed tank to keep the water within chemical range. These chemicals are mostly oxygen scavengers and phosphates. The boiler water also has frequent blowdowns in order to keep the chloride content down. The boiler operations also include bottom blows in order to get rid of solids. Scale is precipitated impurities out of the water and then forms on heat transfer surfaces. This is a problem because scale does not transfer heat very well and causes the tubes to fail by getting to hot. Corrosion is caused by oxygen in the water. The oxygen causes the metal to oxidize which lowers the melting point of the metal. Foaming and priming is caused when the boiler water does not have the correct amount of chemicals and there are suspended solids in the water which carry over in the dry pipe. The dry pipe is where the steam and water mixture are separated.

Boiler feedwater treatment
Boiler water treatment is used to control alkalinity, prevent scaling, correct pH, and to control conductivity. The boiler water needs to be alkaline and not acidic, so that it does not ruin the tubes. There can be too much conductivity in the feed water when there are too many suspended solids. These correct treatments can be controlled by efficient operator and use of treatment chemicals. The main objectives to treat and condition boiler water is to exchange heat without scaling, protect against scaling, and produce high quality steam. The treatment of boiler water can be put in to two parts. These are internal treatment and external treatment. (Sendelbach, p. 131) The internal treatment is for boiler feed water and external treatment is for make-up feed water and the condensate part of the system. Internal treatment protects against feed water hardness by preventing precipitating of scale on the boiler tubes. This treatment also protects against concentrations of dissolved and suspended solids in the feed water without priming or foaming. These treatment chemicals also help with the alkalinity of the feed water making it more of a base to help protect against boiler corrosion. The correct alkalinity is protected by adding phosphates. These phosphates precipitate the solids to the bottom of the boiler drum. At the bottom of the boiler drum there is a bottom blow to remove these solids. These chemicals also include anti-scaling agents, oxygen scavengers, and anti-foaming agents. Sludge can also be treated by two approaches. These are by coagulation and dispersion. When there is a high amount of sludge content it is better to coagulate the sludge to form large particles in order to just use the bottom blow to remove them from the feed water. When there is a low amount of sludge content it is better to use dispersants because it disperses the sludge throughout the feed water so sludge does not form.

Deaeration of feed water
Oxygen and Carbon Dioxide are removed from the feed water by deaeration. Deaeration can be accomplished by using deaerators heaters, vacuum deaerators, mechanical pumps, and steam-jet ejectors. In deaerating heaters steam sprays incoming feed water and carries away the dissolved gases. The deaerators also store hot feed water which is ready to be used in the boiler. '''This means of mechanical deaeration is also used with chemical oxygen scavenging agents to increase efficiency. (Sendelbach, p. 129)''' Deaerating heaters can be classified in to two groups. The two deaerating heaters are spray types and tray types. With tray type deaerating heaters the incoming water is sprayed into steam atmosphere in order to reach saturation temperature. When the saturation temperature is reached most of the oxygen and non-condensable gases are released. There are seals that prevent the recontamination of the water in the spray section. The water then falls to the storage tank below. The non-condensables and oxygen are then vented to the atmosphere. The components of the tray type deaerating heater are a shell, spray nozzles, direct contact vent condenser, tray stacks, and protective interchamber walls. The spray type deaerater is very similar to the tray type deaerater. The water is sprayed into a steam atmosphere and most of the oxygen and non-condensables are released to the steam. The water then falls to the steam scrubber where the slight pressure loss causes the water to flash a little bit which also helpswith the removal of oxygen and non-condensables. The water then overflows to the storage tank. The gases are then vented to the atmosphere. With vacuum deaeration a vacuum is applied to the system and water is then brought to its saturation temperature. The water is sprayed in to the tank just like the spray and tray deaeraters. The oxygen and non-condensables are vented to the atmosphere. (Sendelbach, p. 130)