Two basic types of superheaters are available depending upon the mode of heat transfer from the flue gas. One type is the convention superheater ,for gas temperatures where the portion of heat transfer by radiation from the flue gas is small. Convective heat transfer is highly dependent on the characteristics of gas flow (i.e., quantity and velocity) At maximum load, or 100% steam output, the gas weight is highest; therefore, heat transfer is also high. The higher heat absorption results in higher steam temperatures leaving the superheater and a higher boiler output,or higher total absorption in the superheater per pound of steam.
A radiant superheater receives energy primarily by thermal radiation from the furnace with little energy from convective heat transfer. Radiant heat transfer is highly dependent on the high flue gas temperatures produced by the combustion of fuel. The steam temperature leaving the radiant superheater is lowest at higher loads for two reasons. Radiant absorption does not increase as rapidly as boiler output, and at higher loads there is more steam to cool the effects of the radiant absorption. The radiant superheater usually takes the form of widely spaced steam – cooled wingwalls or pendant superheat platens located in the furnace. It is sometimes incorporated into the furnace enclosure as curtain walls.
The design of radiant and convective superheaters requires extra care to avoid steam and flue gas distribution differences that could lead to tube overheating. Superheaters generally have steam mass fluxes of 1,000,000 Lb/h ft2 or higher. These are set to provide adequate tube cooling while meeting allowable pressure drop limits. The mass flux selected depends upon the steam pressure and temperature as well as superheater thermal duty. In addition, the higher pressure loss associated with higher velocities improves the steam - side flow distribution.
Translate the following sentences into Russian paying attention to the words in bold. Compose Wh questions to these sentences.
1. Designing an efficient superheater raises the boiler efficiency.
2. A superheater is known to raise the temperature of a steam coming from the steam drum.
3. It should be noted that a superheater is an important part of a boiler.
4. It is the design of a superheater that is considered in this article.
5. Radiant superheaters are those which absorb heat by radiation.
Put the words in the right order and form sentences.
1. .Superheated, is produced, steam, by, saturated steam, causing, to flow through, from a boiler, a heated tube or superheater.
2. There will be, in steam pressure, a decrease, fluid friction, due to, in the superheater tubing.
3. The first law of thermodynamics, all the work, states that, done by the turbine, from the energy, comes, in the steam, turbine, flowing through.
4. The products of combustion, sufficiently, must be cooled, the superheater, before they enter.
Write questions to which the following sentences are the answers.
1. Maximum work is obtained when fluid expands without friction and without heat transfer to the surroundings.
2. The increase of temperature of superheated steam has considerable advantages to the use of superheated steam in turbines.
3. Superheaters may be classified as convection or radiant superheaters.
4. Radiant superheaters. are located in furnace walls where they “see” the flame and absorb heat by radiation with a minimum of contact with hot gases.
5. The products of combustion must be cooled sufficiently before they enter the superheaters tubes.
Write an abstract of the text “Steam temperature adjustment and control”.
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