Condenser air leakage and condensate subcooling

[Fantasma]

I have always accepted that increasing air leakage into a condenser leads to subcooling of the condensate in accordance with Dalton’s Law of Partial Pressures. A recent turbine-generator test series included an output test at a simulated high cooling water temperature. The method chosen for the simulation was to bleed air into the unit to increase the condenser pressure. The apparent subcooling in the condenser decreased from about 2F at 2.25”Hg for normal operation to about 1F at 3.75”Hg at the test conditions. Dalton’s Law is obviously not applicable to this arrangement; does anybody know why and what is happening?
The condenser is a transverse, underslung, twin shell, tubular surface type. The instrumentation was unchanged for the tests at the two different operating conditions. Pressure measurement was by 0.1% pressure transducers, with one connected at each side of each LP turbine exhaust duct. A platinum resistance thermometer, directly immersed in the condensate, measured the temperature in the bottom of each condenser. Data from tests on other similar units showed a similar effect. Some of the units had air admitted directly into the air pump suction through the vacuum-breaking valve. The air was admitted into the other unit through spare connections next to the turbine to condenser joint. This suggests that the location of the air bleed may not have a significant effect on what is happening.

 

[jsteverm]

What you are seeing is a reduction in effective heat transfer surface as air is blanketing the air removal zone of the condenser. Subcooling occurs as condensate drips over tubes colder than the saturation temperature as it falls to the hotwell. With the higher back pressure caused by air blanketing the difference between Tsat and Circ water is less, thus less subcooling.

 

[poetix99]

"Fantasma", you are making a false conclusion that is faulty on several levels.

The key point the "jsteverm" is correct about is that the effectiveness of the heat transfer (a condenser is, of course, a heat exchanger) is greatly diminished when air is introduced into the condenser. This is essentially the entire point. This is why you can raise the condenser pressure by bleeding air into the condenser. This isn't a function of Dalton's Law in any useful way; it is an energy balance!

There would be no air removal equipment if this was not critical to the proper function of a surface condenser.

Actually, the temperature difference between Tsat and Twater is GREATER when the condenser pressure (and hence saturation temperature) increases. The condensate subcooling decreases (if you wish to believe measurements of 1 and 2 Fahrenheit degrees temperature difference) because, as "jsteverm" said, the average residence time of the condensate on the cold tubes has probably decreased.

Get a good reference book on Steam Condensers - try a keyword search of this website; there have been some books mentioned in other threads.