ltial to control the superheat temperature Question No.4: List the mhods you [8,

Question

ltial to control the superheat temperature Question No.4: List the mhods you [8, 10 Marks) n brief, explain why: i- too much steaming not desired in riser tubes carry over should pass to superh ii- No Vapor bubbles owing in econom b) A common turbine arrangement design is a combination of Curtis stage fol y large number of reaction stages. What are the advantages of such arrangement c) Compare the condition for maximum blade efficiency for impulse,too urts and 50% reaction stages. Compare the enthalpy drop per stage for the above 3 pes of blades for same blade speed and exit nozzle ange

Explanation / Answer

The circulation ratio in a natural circulation boiler varies from one riser tube to another. The riser tubes located opposite to the burners have more thermal loading and generate more steam. So, these will have less circulation ratio, while the risers are located in the corners of the furnace are realatively cooler, and so these will have more circulationarea.

Too much steaming is not desired in a riser tube and circulation ration needs to be maintained above 6. If steaming is more then hested surface will be there, there will be too much bubble formation with the result the bubbles may coalease and first form an unstable vapour film which continuosly collapse and reforms. In case it became stable, then it will offer large amount of thermal resistance.

2.

Steam purity is a fundamental specification for a boiler. While it is true it becomes harder to separate water and steam as the density difference between them becomes less with pressure, the drum design becomes more complicated to ensure no carryover occurs. Also it is not moisture that is the problem after all spray attemporators add plenty.

It is the solids in the drum water which must not be carried over. So as more carryover becomes unavoidable solids in water is reduced to compensate.

Abnormal or non designed rates of carryover occur due to faulty seals in drum internals or high level reducing separation room.

Priming is when a slug of water is sent to the superheater due to suddenly reducing pressure and increasing steam flow.
Foaming is when chemical concentration forms suds and bubbles.

Slight carryover is normally detected by condensate conductivity increase, severe carryover will reduce superheater temperature.

3.   

- Maintaining a normal water level in the steam drum so that periods of high demand do not cause priming.
- The design and efficiency of the moisture removing devices located in the steam drum. These can be any one or a combination of: dry pipe, cyclone separators and corrugated scrubbers. If these are not working properly, priming can occur.
- Foaming even minor foaming caused by high boiler water conductivity and/or contaminants in the water.

From above we can say that foaming can affect the water level and thermal conductivity.

Formation of bubbles will create the, a small stable layer which will cause to decrease the thermal conductivity which will further reduce the thermal efficiency of the boiler.

4.

Frequent sootblowing can damage boiler components and place a big load on plant efficiency but not enough it is a big problem.

This invention relates to improvements in and relating to soot blowing andtube cleaning ,ap: paratus for boilers, economizers or the like and more particularly for use in the removal of soot and other combustion products from the exterior of banks of tubes of tube boilers,

eco-nomizers or the like. Apparatus for this purpose have heretofore been proposed adapted to provide a plurality of jets in close proximity to the tubes whereby blasts of steam are impinged on the tubes,

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