Methods of Superheat Temperature Control

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Methods of Superheat Temperature Control

For proper and efficient operation of power plant it is essential to control the temperature of steam according to load on the power plant.
The reduction of about 20°C temperature of superheated steam causes the increase in 1% heat rate (H.R.) [H.R.-Work output x 3600 Efficiency (kJ / kWh) my]
However, the increased temperature of superheated steam above designed value may result into overheating of superheaters, reheaters and turbine blades.
It may lead to failure of system. Thus, it is of utmost importance to control the temperature of steam of superheaters and reheaters.
Various methods of temperature control are :
  1. Control of gas circulation by dampers.
  2. Combined radiant-convective superheaters in series.
  3. Desuperheating and attemperation.
  4. Separately fired superheater.
  5. Gas recirculation method (furnace temperature control)
  6. Excess air.
  7. Tilting burners.
  8. Turning the burners.

These are being discussed below in details :


1. Control of gas circulation by damper :
  1. In case the mass flow rate of gas circulation is reduced to convective superheater having constant flow rate of steam, it would result into reduction of temperature of superheated steam.
  2. This can be achieved by placing dampers in the gas path before the convective superheater to bypass some of the gases around the superheater.
2. Combined convective-radiant superheaters in series :
The radiant and convective superheaters in series give almost constant temperatures of steam over wide range of loads.

3. Desuperheating and attemperation : 
  1. Attemperation means reduction in temperature of superheated steam. It is also called as desuperheating. Attemperation can be achieved either by injecting water spray before the superheater or by injecting water in between the sections of a superheater. (Refer Figure A).
  2. Figure A (a) shows the control of temperature by injecting water through nozzles into the stream of steam before convective superheater (CSH).
  3. The water from boiler or economizer is sprayed by nozzles at the throat of a venturi constructed into steam pipe.
  4. The sprayed water mixes with incoming high velocity steam. 
  5. The water receives heat energy from incoming steam and it vapourizes. As a result, the temperature of steam reduces. 
  6. The vapourized water and steam both flow down and finally supplied to CSH. 
  7. A thermal sleeve made of high chrome steel is provided. 
  8. It helps in protecting the main stream in pipe from thermal shocks caused by water droplets which may remain unvapourized. 
  9. These water particles might cause an impact on steam pipe leading to its failure if sleeve is not provided. 
  10. In both the methods described above of attmperation, it is necessary to use highly pure water to avoid scale formation and loading on turbine blades.
Temperature Control in Superheaters by Desuperheating
Figure A
4. Separately fired superheater : 
  1. Instead of using the flue gases of a boiler for superheating of steam, we may use separately fired furnaces for the same purpose.
  2. This method leads to a very high degree of control of temperatures of superheated steam. 
  3. In spite of its precise control on temperatures, this method is not employed in modern steam power plants because it is not economical. For this reason, this method is only used in industries.
5. Gas recirculation method : 
  1. This method is used when the load on the power plant is low. In this method, part of gases leaving the economizer (sometimes even from exit of air preheater) are re-circulated back into the furnace as shown in Figure B by schematic diagram. 
  2. The mixing of lower temperature gases from economizer with the furnace gases causes the reduction of flame temperatures in the furnace with increased mass of gases. The increased mass of gases at lower temperature is circulated to CSH at increased total energy. 
  3. This increases the heat absorption. Thus, the reduction in flame temperature in radiant zone, causes the increase in temperature attained by superheated steam in radiant superheater.
  4. Figure C shows the effect of amount of gas recirculation on economizer, superheaters and reheaters and the furnace. Use of this method increases the fan power requirements.
Gas recirculation
Figure B

Effect of Gas Recirculation on Furnace Temperature and Heat Absorption
Figure C
6. Excess air : 
The admission of excess air into combustion chamber will have the effect of lowering the furnace temperature. The radiant heat transfer is proportional to the difference in the fourth power of the temperatures of furnace and the water wall (i.e T4g – T4w). The lower radiant heat absorption by water walls will reduce the water wall temperature but will increase the steam temperature. However, the increased mass flow rates of gases due to excess air will cause high heat rejection into chimney. Therefore, it will lower the boiler efficiency.

7. Tilting burners :
  1. By tilting burners downward in a furnace , much of the heat will absorbed by the water walls.
  2. Thus, the gas entering the superheater will be relatively at a lower temperature.
  3. This method is shown in Figure D which is quite an economical method of temperature control.
Tilting Burners
Figure D
8. Turning the burners :
  1. Furnaces of modern steam power plants have several burners as shown in Figure E.
  2. Depending upon the load on power plant and temperature of steam needed , the burners are put on and off e.g. when high temperature of steam is required, upper burners are lighted up while at partial loads the lower burners are put on.
  3. The use of upper and lower burners corresponds to the tilting of burners upwards and downwards respectively as discussed above.
Turning the burners
Figure E

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