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Posted by Bob D. on December 24, 2003 at 05:23:03:
In Reply to: boiling point calculations under pressure posted by Dave Sutherland on December 23, 2003 at 21:54:37:
Simply use the steam tables. Remember that the drum is at saturation (this is theory guys). Any superheating occurs later, outside the drum. In your case, go down the column that has pressure, add 14.7 to get your drum pressure in PSI absolute (110 PSI gage + 14.7 = 124.7 PSI absolute, use 125, it's close enough). At 125 PSIA, the saturation temperature is read off directly as 344.3 deg. F.) That is, by definition, the boiling point of water at that pressure. The phase change, from liquid to saturated vapor at those conditions takes 875.35 BTU for each pound of water. That's called the latent heat of vaporization. If the drum pressure were to instantaneously drop to 80 PSIG, a certain percentage of the liquid volume in the drum would "flash" to steam. You can calculate the percentage pretty easy. For illustration purposes, you have to assume that it happens instantaneously, so you can ignore mass flow into and out of the drum.