Superheater, function, advantages, disadvantages, classification


Functions of superheater:

The function of superheater in the thermal power plant is to remove the last traces of moisture from the saturated steam coming out of boiler and to increase its temperature sufficiently above saturation temperature as per requirement at inlet of steam turbine. The steam produced in the boiler is nearly saturated. This steam as such as should not be used in the steam turbine because the dryness fraction of the steam leaving boiler will be low. This results in the presence of moisture which causes corrosion and erosion of turbine blades.

Advantages of superheated steam (superheater):

(1) The steam turbine efficiency increased by approximately 29% of each percent of moisture removed and superheating reduces the percentage cut off in steam turbine, therefore steam consumption per unit power output is reduced.
(2) The superheating increases overall cycle efficiency as well as avoids to much condensation in the last stages of turbine which reduces the blade erosion and corrosion.
3) Superheated steam has a higher heat content and hence capacity to work is increased. Hence an economy in steam consumption in steam turbine is achieved

Superheaters consists of groups of tubes made of steel (carbon steel for steam temperature up to 510˚C, carbon molybdenum steel for steam temperature up to 570˚ C, and Stainless steel for steam temperature up to 650˚ C with an outside diameter ranging from 1.5 Cm to 6.4 cm. The metal used for superheater must have high temperature strength, high creep Strength and high resistance to oxidation. The superheater tubes are heated by the heat of flue gases during their passage from the furnace to chimney. Heat from hot gases to the steam in the Superheater is transfer at high temperatures. Therefore, primary section of superheater is arranged in counterflow and secondary section in parallel flow to reduce the temperature stressing of the tube wall.

Slagging of the superheater:

In modern steam power plant, the combustion temperature is higher than the fusing temperature of ash. Therefore, there is tendency of the ash to collect in fluid form on the superheater tubes and this is known as slagging of superheater. The problem of slagging the superheater can be minimized or eliminated by following methods as…
(1) Place radiant superheater as possible as close to furnace.
(2) Provide bank of water filled tubes in front of convective superheater to restrict radiation heat transfer well as to limit slag accumulation on the convective superheater
(3) Utilize combined convection – radiation superheater

Classification of superheaters:

1. Based on mode of heat transfer
(a) Convective superheater
(b) Radiant superheater
(c) Combined convective and radiant superheater (Pendant superheater)

2. Based on their position in the furnace with respect to water tubes
(a) Overdeck superheater
(b) Innerdeck superheater
(c) Inner tube superheater
(d) Inner bank superheater

Convective superheaters are located in the convective zone of the furnace usually before economizer. The convective superheater absorbs heat from flue gases through convection. The convective superheater gives rising characteristics i.e. the temperature of superheat increases with the increase in steam output as shown in Fig. 1 because with the increase the steam output both gas flow Over superheater tubes and steam flow inside the tubes increase which causes increase in heat transfer rate and mean temperature difference.

effect of steam output load on superheater temperature
Fig.1 Effect of load on superheater temperature

Radiant superheater is located in the furnace between the furnace walls (radiant zone). It absorbs heat from burning fuel through radiation. The radiant superheater has two main disadvantages as it may get overheated due to high temperature of furnace and it has a dropping characteristics i.e. the temperature of superheat falls with the increase in steam output as shown in Fig. 1, because with the increase in steam output, the furnace temperature rises at a much less rate than the steam output. Since radiant heat transfer is function of furnace temperature the heat transfer increases slowly with steam flow or the steam temperature decreases

Combined type of radiant and convection (Pendant) superheater is shown in Fig. 2. Both radiant and convective superheaters are arranged in series in the path of flue gases. Radiant superheater receives heat from the flue gases by radiation and convective superheater receives heat by convection. About 3/4th of convective surface is made counter flow while the remaining 1/4th is parallel low. If the entire coils are counterflow then exit end of the superheater coil will be subjected maximum temperature difference (outside maximum temperature of gas and inside maximum temperature of steam). Hence, parallel flow exit end of superheater protect against overheating and metal failure.

convective and radiant combined pendant superheater simple diagram
Fig.2 Pendant type suerheater

You will be able to answer following questions after referring below topic
Q1. Why superheating of steam is required?
Q2. Why are superheaters used in Power plants?

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