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Posted by Pete on January 19, 2004 at 10:38:40:
In Reply to: But I need to add..... posted by Yodaddy on January 18, 2004 at 15:03:35:
: Operating a boiler at reduced pressure and high firing rates can cause problem.
: The boiler is designed to contain X number of tubes of a certain size to operate safely and efficiently at a certain pressure and up to a certain output level...That said,
: When you are at a lesser pressure, the steam bubbles in the tubes and drum are larger than they would be at the higher pressure. Those steam bubbles are generated against the metal tube wall and then flow upwards into the drum....So at high firing rates and lower pressure you are allowing the tube metal to be exposed to higher temps than it would be normally....That is because there is actually less water in the tube due to the larger bubbles. It is water that cools those tubes. Larger bubbles can disrupt the flow characteristics that complete section of the boiler can over heat.....If that difference is substancial, it may exceed the design rating of the boiler and shorten tube life.
: In the steam drum those larger bubbles can cause carry over when at high firing rates. Carry over can be destructive to your turbine.
: This question is best answered by the boiler and turbine manufacturer. They know the limits of their equipment.
Don't forget that as the pressure drops, the difference in the densities of the steam and water increases. This means you will have a higher circulation rate inside the boiler, which translates to higher fluid velocities inside the tubes.
I don't believe nucleate boiling will be compromised in the situation Bethw describes. I also doubt that carryover will be an issue.
I fully agree that the boiler manufacturer should be consulted..
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