Steam Superheater in Boiler: Definition, Parts, Types, Working


In this article, we will learn what is steam superheater in boiler, its parts, definition, working, types, etc. Electricity forms the lifeblood of modern civilization. It is obtained from many sources. One such prominent source is the thermal power plant. Here coal which is a non-renewable source is used. As coal is costly it becomes important to achieve high efficiency by using various methods. The cost of one electricity unit has to be low so that electricity becomes affordable. So only way to achieve this is by making the system more efficient and reducing losses. One such method is used of a superheater. It uses energy that is getting wasted and recovers heat from it which is in turn used to increase power output, reduce fuel and increase average plant efficiency. As it is very cost-effective, understanding it becomes very important.

What is Steam Superheater? Definition, Function

Definition of Steam Superheater

The superheater is a device which recovers heat from flue exhaust gases of boiler which are thrown out of the system

Function of Superheater

  • To superheat steam coming out of boiler.
  • To improve the efficiency steam power plant.

These are some basic functions of steam superheater, but we need to understand its significance by diving deep into this topic.

Firstly, the boiler is a place where steam is getting created. Fuel is burnt and heat is supplied to water which causes water to turn into steam. At any given time, whole space is neither whole steam nor whole water, but it is somewhere in between. It is a mixture of steam and water.

As steam is lighter it will be at the top while water will be at the downward portion. As this steam is supplied to the turbine through pipes. This steam naturally has some wetness into it, even though it is saturated steam. The superheater is used just to remove this wetness and to increase the temperature of steam above its saturation temperature. This is done by using exhaust gases.

These exhaust gases are coming after the burning of fuel. This is done in order to make steam dry and possibly to superheat it. This is done to avoid erosion of turbine blades and pipes as wet steam corrodes these instruments. To ensure their long life this is done.

Secondly as told earlier, as exhaust gases are released into the atmosphere, where it gets mixed up in air. The amount of heat carried by such gases gets wasted, as these same gases are used in the superheater, steam gets superheated, and its enthalpy increases which in turn causes a rise in the efficiency of the plant.

Types of Superheater

According to tube position

  • Non drainable superheater
  • Drainable superheater

According to mode of heat transfer

  • Convective superheater
  • Radiant superheater
  • Combined superheater

According to movement of flue gases

  • Counter flow superheater
  • Parallel flow superheater
  • Mixed flow superheater

According to tube design

  • Platen type superheater
  • Pendent type superheater

According to location of superheater

  • Internal superheater
  • External superheater

Parts of Superheater

To understand its construction, various other parts which work with superheater assembly must be understood.

Boiler

Consider that in this case Lancashire boiler is used. It is being used in the horizontal configuration. At the right side, fuel is burnt and the converging section shows exhaust leaving the boiler.

Header

It lies in between main pipes carrying steam from boilers and superheater.

Stop valve

They are used to operate steam flow and switch ON or OFF specific lines. This is done to shut of whole operation of superheating when it is not needed.

U shaped steel tube superheater

Superheater in as shown is set of tubes which is in U shape and 2 Header. Steam from boiler comes through pipes to superheater via header. These u tubes are located such way that they are in path of travel if exhaust/hot gases. It must be noted that the amount of hot gases passed over superheater tubes must be in proportion to steam passing through tubes and actual amount of degree of superheat required.

Damper

As suggested, the amount of superheat needs to be controlled. This can’t be done by reducing or increasing boiler fuel which will cause fluctuation in steam supply. This is done by use of damper. It is operated by hand wheel. These hot gases can be diverted and their amount is controlled to avoid the overheating. When this damper is in vertical position, at such instances these flue gases don’t pass over superheater, but they pass directly to bottom and from there released into the atmosphere. In such a situation, superheating doesn’t take place.

How Does Steam Superheater Work? Working Principle

As we can see there are 3 stop valves. This setup is operated in two.

When stop valve A is closed

In this scenario, stop valve A is closed but valve B and C are in open position. Wet steam coming from the boiler flows into pipes via valve C to the right side header. From there it enters the U tube and gets superheated by flue gases coming out of the boiler. After getting superheated it flows into the left-hand header and from there via valve B it gets withdrawn towards the turbine.

When stop valve C is closed

In this scenario, there is a requirement that steam should not be superheated. Hence valve C and valve B is closed. Wet steam is directly allowed to pass through valve A towards the turbine.

Advantages of Steam Superheater

  • Steam consumption is reduced. This is because wet steam carries less enthalpy as compared to superheated steam. So, when superheated steam is expanded in turbine it produces more work as compared to wet steam, this way for same amount of work less amount of steam is needed.
  • Amount of water needed to operate plant reduces as due to superheating less amount of water is needed. As superheated steam carries higher amount of energy.
  • Erosion of turbine blades is reduced. This also helps increase life of pipes. Also, wet steam gets condensed fast and gets collected in corners. This causes dripping and leakages in long term as pipe gets corroded.
  • Condensation losses are reduced. This happens when pipes carrying steam are not properly insulated. Due to which wet steam gets condensed within pipe and water gets collected. Steam passing over this water gets partly wet, and its enthalpy reduces.
  • Thermal efficiency of steam power plant is increases.
  • Efficiency of whole plant increases. This is due to fact that superheated steam gives saving in water consumption from 10% to 12% over that with saturated steam. Also, by superheating fuel consumption is reduced up to 10 to 15%. It is proven that every 5 degree Celsius rise in temperature of a steam will reduce steam cost by 1% in boiler.

Limitations of Steam Superheater

  • They are prone to accident when plant is shut off. Water gets accumulated in U tubes and if it is not drained, when suddenly it starts to heat, tubes may get burst and due to which steam may enter into boiler furnace and also affect crew members. Due to its complicated position, maintenance work is also very delicate and hard. But still its advantages outweigh its limitations by a very big margin that’s very sure.

Material of Steam Superheater

  • Chromium nickel or chromium molybdenum alloys are used to withstand temperature which are about 650 degrees Celsius to prevent metal cracks.
  • Low carbon steel is used when temperature of superheater is below 427 degrees Celsius.

Types of superheater

Non drainable superheater

Steam enters through the header. Coils may be vertical, horizontal, or inclined as per design. When superheater is not being used and steam gets condensed in its tubes which cannot be drained from it. Such types are called non-drainable superheater. These superheaters need more careful handling in its starting as compared to other superheaters. This is because condensate which is collected has to be removed so it must be done by slow rise in temperature with limit and below 550 degrees Celsius. This is done to avoid overheating of tube metal.

Drainable superheater

It is designed in such a way that a natural slope is provided to superheater tubes. When condensate gets accumulates in a tube, due to slope it gets removed and drained out.

Convective superheater

The convection mode of heat transfer dominates. It is placed in the convection zone. It is placed away from furnace. they are usually placed ahead of economizers and hence they are called as a primary superheaters. It is in area from where you cannot actually see flame. The main characteristic of such devices is that with increase in steam flow the temperature of steam also increases.

Radiant superheater

Radiation mode of heat transfer dominates. It is placed in the radiation zone of the combustion chamber near the water wall, so it absorbs heat by radiation. It is placed very close to the furnace. It is the area where you can actually see the flame. Main characteristics of such devices is steam flow is increased then the steam temperature will get reduced.

Combined superheater

It is placed not too close not too away from the furnace. It is just kept in a zone where heat transfer by radiation as well as by convection can take place.

Counter flow superheater

As the name suggest flow of steam and flow of flue gases will be in opposite direction. When steam is moving at entering coil, it will face flue gases which will be at a lower temperature as compared to flue gases it will face when it will be exiting superheater. This ensures complete superheating.

Parallel flow superheater

As the name suggests, the flow of steam and flow of flue gases will be in same direction. In such types of flue gases will be hottest at the entry stage where they will make contact with steam. As steam keeps moving ahead temperature of flue gases will keep decreasing. Heat transfer will go on decreasing as steam moves near the exit of the superheater.

Mixed flow superheater

This is a combination of both parallel and counter flow superheaters. In some portion steam is moving in the opposite direction to that flue gases while in another portion it moves parallel to the direction of hot gases. This way higher heat transfer is achieved.

Platen type superheater

It is related to tube design. Platen is a vertical single loop that has sets of closely packed tubes appearing like plates. They are placed in the upper part of the furnace so as to absorb the entire heat radiated by radiation. These platen coils are distanced from each other such that they are 600mm to 1000mm apart. This is done to avoid the formation of ash build-up and to remove it.

As flue gases often carry with themselves such small ash articles which have to be removed as they reduce heat transfer rate when deposited over pipes. It is always placed in a radiation zone; hence it is also a radiant type superheater.

Pendent type superheater

It is very easy to spot. Whenever multiple loops of tubes are used, such superheater is called pendent superheater. It is always placed in a convection zone.

Internal superheater

If a superheater is placed inside the boiler, then it is called an internal superheater.

External superheater

If superheater is placed outside boiler, then it is called an internal superheater. A separate firing burner is needed for such designs.

Temperature Characteristics

It is graph plotted between steam load and steam temperature. Steam load on X-axis and steam temperature in Y-axis.

Radiant type

It will gain heat by radiation. If boiler load is low, then less steam will be flowing through the superheater. Due to this outlet temperature will be high. When boiler load will be increased, then steam flow will be increased and output temperature rise will be less as the flow has been increased.

Convective type

It will gain heat by conduction. When boiler operating at low boiler load so less fuel burnt, that means fewer hot gases produced and hence less contact with superheater. This will give less temperature rise in a superheater. But if the load on superheater is increased more flue gases will be created which will, in turn, give more temperature rise in the superheater.

Combined type

They are placed in radiation and convection zone. Even if the load is increased or decreased, output temperature obtained is nearly constant. Hence these superheaters are mostly preferred.

Applications of Steam Super heater

  • Steam power plant
    • Chemical industries
    • Locomotives

Other similar devices which use flue gases

  • Economiser

Feed water is supplied to the boiler is heated by passing through flue gases so that less fuel is needed to increase its temperature.

  • Reheater

Even though they are similar to superheater, but their exit temperature is always less than superheater. Their pressure is also less so can be made by cheap material.

  • Air preheater

Air entering in the furnace is heated by using flue gases. This also increases boiler efficiency.

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