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Superheater temperatures

Q: The superheater cools gas from 620ºC to 440ºC and, in the process, superheats steam from 225ºC to 400ºC. Eight tubes in the superheater developed leaks and steam mixes with the gas. To stop this, tubes were plugged and make it difficult to control temperatures. After plugging, temperatures are: gas 620ºC to 525ºC; steam 225ºC to 370ºC. How we control temperatures within the design limits?

A: The plugging of tubes in the heat exchanger has reduced the available surface area for heat transfer and the problem can only be corrected by replacing the tubes or providing an additional exchanger in series with the existing unit.