DIFFERENT TYPE OF SUPER-HEATERS IN POWER PLANT
Hello ! all power plant engineers today we discussed about the type of superheaters are used in the boiler for the purpose of raise the steam temperature upto the super heated level .
A superheater surface is that surface which has steam on one side and hot gases on the other. The tubes are therefore dry with the steam that circulates through them. Overheating of the tubes is prevented by designing the unit to accommodate the heat transfer required for a given steam velocity through the tubes, based on the desired steam temperature. To avoiding this problems , it is necessary to have the steam distributed uniformly to all the superheater tubes and at a velocity sufficient to provide a scrubbing action to avoid overheating of the metal. Carry-over from the steam drum must be at a minimum.
Classification of superheaters :-
Superheater are generally classified according to the shape of the tube banks and the position of the header and also according to whether they receive heat by radiation or convection although in some causes it may be a combination of both types radiation or convection.
Types of superheater
It’s totally depending on the which firing method is adopted , fuel fired etc. The superheater are placed in the boiler flue gas passes, horizontally, vertically or combined.
Pendant type superheater :-
The superheater may be of pendent type , hanging from and supported by their header.
Horizontal type superheater :-
In horizontally type superheater arranged across the boiler. This type of superheater is self draining which ia an advantage during lighting up and for this reason they are now favoured by designer for the primary section superheaters . The platen is a plane surface receiving heat from both side . The ratio of longitudinal pitching to transfer pitching is very low for platen superheater
Radiant superheater is located near the furnace, where it receives the majority of heat by radiation from the flue gases after burning the fuel.radiant superheater absorbed heat by direct radiation from the furnace and are generally located at the top of the furnace. In some older design the superheater tubes part of the furnace wall and received practically all the heat of radiation , radiant superheater has falling characteristics the steam temperature drops as the steam flow rises.
The conventional convection-type superheater uses two headers into which seamless tubes are rolled or welded. The headers are baffled so that the steam is made to pass back and forth through the connecting tubes, which carry their proportioned amount of steam, the steam
Convection superheater absorbed heat mainly by the impingement of flow of hot gas around the tubes . A purely convection superheater has a raising steam temperature characteristics. The mass flow and temperature of gas entering the superheater zone as well as the steam flow from the boiler increase with an increase the firing rate . These changes in temperature producing a great average temperature difference between the gas and steam , and this together with the higher gas mass flow , causes an increased rate of heat absorption , resulting in an increased steam temperature.
A combination of the falling steam temperature characteristics of the radiant superheater, together with the rising characteristics of the convection superheater is used in most of the installation for the purpose of maintaining constant steam temperature. It has the advantage of providing a constant steam temperature over a very wide range in load .
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