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PowerPedia:boiler

Lasted edited by Andrew Munsey, updated on June 14, 2016 at 10:01 pm.

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A boiler is a closed There was an error working with the wiki: Code[1].

Overview

Construction of boilers is mainly limited to There was an error working with the wiki: Code[2], or There was an error working with the wiki: Code[3] or There was an error working with the wiki: Code[4] type heating elements. Nuclear fission is also used as a heat source for generating There was an error working with the wiki: Code[9]. Heat recovery steam generators, or There was an error working with the wiki: Code[10]s use the heat rejected from other processes such as Gas turbines.

Boilers can also be classified into:-

There was an error working with the wiki: Code[11]s. Here the heat source is inside the tubes and the water to be heated is outside.

There was an error working with the wiki: Code[12]s. Here the heat source is outside the tubes and the water to be heated is inside.

The simple inefficient old type where there are no tubes and the fire heats one side of the water container.

The goal is to make the heat flow as quickly as possible from the heat source to the water. For example, steam There was an error working with the wiki: Code[13]s have There was an error working with the wiki: Code[14]s, where the fire is inside the tube and the water on the outside. These usually take the form of a set of straight tubes passing through the boiler through which hot combustion gases flows. In There was an error working with the wiki: Code[15]s the water flows through a large number of narrow tubes around the fire. The tubes frequently have a large number of bends and sometimes fins to maximize the surface area. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can contain the pressure with a thinner wall.

In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are mechanically assembled on site to create the finished boiler. There are other types of boilers, largely of historical interest. For example, the There was an error working with the wiki: Code[16] developed around There was an error working with the wiki: Code[17] by There was an error working with the wiki: Code[18] for generating steam for Steam engines. This was both stronger and more efficient than the simple boilers which preceded it. It was a cylindrical water tank around 27 feet long and 7 feet in diameter, and had a coal furnace placed in a single cylindrical tube about three feet wide which passed centrally along the long axis of the tank. The fire was tended from one end and the hot gases from it travelled along the tube and out of the other end, to be circulated back along flues running along the outside of the boiler before being expelled via the There was an error working with the wiki: Code[19]. This was later improved upon in the There was an error working with the wiki: Code[20] which had a pair of furnaces in separate tubes side-by-side. This was an important improvement since each furnace could be stoked at different times, allowing one to be cleaned while the other was operating. These designs are really primitive fire tube boilers, and led on to the There was an error working with the wiki: Code[21] which remains a popular fire tube design.

Superheated Steam Boilers

Most boilers heat water until it boils, and then the steam is used at There was an error working with the wiki: Code[22] (i.e., saturated steam). Superheated steam boilers boil the water and then further heat the steam in a superheater. This provides steam at much higher pressure, and increases the overall thermal efficiency of the steam plant due to the fact that all of the added heat can be extracted to perform useful work.

Superheated steam presents unique safety concerns, however, since the steam is at such high pressure (sometimes 1200 psi or higher). If there is a leak in the steam piping, water at such high pressure/temperature can cause serious, instantaneous harm to anyone entering its flow. Since the escaping steam will initially be completely superheated vapor, it is not easy to see the leak, although the intense heat and sound from such a leak clearly indicates its presence.

Supercritical boilers

There was an error working with the wiki: Code[5] boilers are used for the generation of electric power. They operate at "supercritical pressure". In contrast to a "subcritical boiler", a supercritical boiler has no water - steam separation. There is no generation of steam because the pressure is regulated above the "critical pressure" at which steam bubbles can form. Thus, the fluid generated is called "supercritical fluid". It passes below the critical point as it does work in the high pressure turbine and enters the generator's condensor. This is more efficient resulting in slightly less fuel use and therefore less There was an error working with the wiki: Code[23] production.

Hydronic boilers

Hydronic boilers are used in generating heat typically for residential uses. They are the typical power plant for There was an error working with the wiki: Code[24] systems fitted to houses in northern There was an error working with the wiki: Code[25], as opposed to the There was an error working with the wiki: Code[26] furnaces or wood burning stoves more common in There was an error working with the wiki: Code[27]. "Boiler" is clearly a misnomer for this kind of device, which is really nothing but a large water heater in which the water is never intended to boil but the name is universal and unlikely ever to change. The hydronic boiler operates by way of heating water/fluid to a preset temperature and circulating that fluid throughout the home typically by way of There was an error working with the wiki: Code[28]s, baseboard heaters or through the floors. The fluid can be heated by any means....gas, wood, fuel oil, etc, but in built-up areas where piped gas is available, Natural gas is currently the most economical and therefore the usual choice. The fluid is in an enclosed system and circulated throughout by means of a motorized pump. Most new systems are fitted with There was an error working with the wiki: Code[29]s for greater efficiency.

Hydronic systems are being used more and more in new construction in North America for several reasons. Among the reasons are:

They are more efficient and more economical than There was an error working with the wiki: Code[30] systems (although initial installation can be more expensive, because of the cost of the copper and aluminum).

The baseboard copper pipes and aluminum fins take up less room and use less metal than the bulky steel ductwork required for forced-air systems.

They provide more even, less fluctuating temperatures than forced-air systems. The copper baseboard pipes hold and release heat over a longer period of time than air does, so the furnace does not have to switch off and on as much. (Copper heats mostly through conduction and radiation, whereas forced-air heats mostly through There was an error working with the wiki: Code[6]. Air has much lower There was an error working with the wiki: Code[7].)

They do not dry out the interior air as much.

They do not introduce any dust, allergens, mold, or (in the case of a faulty heat exchanger) combustion byproducts into the living space.

Forced-air heating does have some advantages, however. See There was an error working with the wiki: Code[8].

Accessories

Boiler fittings

Safety valve

Water column: to help the operator tell if there is a satisfactory level of fluid in the boiler, a There was an error working with the wiki: Code[31] or water column is provided.

Bottom blowdown valves

Surface blowdown line

Circulating pump

Steam accessories

Main steam stop valve

Steam traps

Main steam stop/Check valve used on multiple boiler instaltions.

Combustion accessories

Fuel oil system

Gas system

Coal system

Automatic combustion systems

Controlling draft

Most boilers now depend on mechanical draft equipment rather than natural draft. This is because natural draft is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as the chimney height. All these factors make proper draft hard to attain and therefore make mechanical draft equipment much more economical.

There are three types of mechanical draft:

1) Induced draft: This is obtained one of two ways, the first being through use of a steam jet. The steam jet forces steam into the stack and allows for a greater flue gas velocity increasing the overall draft in the furnace. This method was more common on older steam driven locomotives. The second method is by simply using an induced draft fan (ID fan) which sucks flue gases out of the furnace and up the stack. All induced draft furnaces have a negative pressure.

2) Forced draft: With this method draft is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is usually passed through an air heater which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draft furnaces have a positive pressure.

3) Balanced draft: Balanced draft is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric.

Related articles

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There was an error working with the wiki: Code[33] (used by railway locomotives)

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References and external links

A short history of steamboat boilers

Water Heater Resource Info, articles, and more on boilers and heaters

Frederick M Steingress, Low pressure boilers. American Technical Publishers, 1986, ISBN 0-8269-4407-8

Different Types Of Boilers - The 3 Main Types Of Boiler.

Encyclopedia of Earth: Boilers

There was an error working with the wiki: Code[1], Wikipedia: The Free Encyclopedia. Wikimedia Foundation.

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