we have a ge 7fa 150megawatt gas turbine that is hydrogen cooled at start up they fill it and run with air. id like to know how many megawatts we will get with the air vs hydrogen? And why? How do you figure this is there a formula?

 

The best resource to answer this question is the manufacturer of the generator or packager of the unit. I'm not aware of a formula, but engineers can devise formulas for just about anything.

Since hydrogen is seven to ten times more effective at cooling a generator than air, the output when filled with air would be significantly less than when filled with hydrogen.

I recall seeing a set of curves once for a Frame 7E for operating with air instead of hydrogen. The unit was rated for approximately 82 MW when filled with 2 barg hydrogen. When filled with 1.5 barg air, the rating was only about 15 MW.

Simply spinning the generator rotor in air generates a fairly significant amount of heat; air is much more dense than hydrogen. And, that's without any heat generated from current flowing in the stator windings.

Hydrogen-cooled generators are usually filled with air and tested for leaks during initial commissioning (start-up), then purged with CO2, then charged with hydrogen for initial loaded operation. But I've only been present at one start-up where the unit was actually run to produce power while filled with air, and that was only because we couldn't obtain sufficient CO2 or hydrogen for purging and charging and a possible subsequent emergency purge, and the start-up manager insisted on closing the generator breaker for a short period to make a schedule milestone and collect a bonus payment. (It was his poor planning that caused the inability to get the CO2 and hydrogen to site in time for a proper purge and charge, but of course, we couldn't point that out to him.)

In the absence of a number or a formula, one could put air in the generator, but not at full rated pressure. (When the temperature in the generator rises, the pressure will also rise.) Then, it would be very important/critical to closely monitor the generator internal temperatures (stator temperatures; cooler gas inlet and outlet temperatures), as well as the pressure, while producing power ("loaded"). When the stator temperature and/or the cooler gas inlet/outlet temperatures approach rated, that would be the limit of operation.

Regardless of any formula or number.

And, operation on air might not be the best thing for the generator for any extended period of time. I would be very curious to know the reasoning for operation with air (as I'm always trying to learn something).

 

Tyler... answering the questions in your 05-Jan-11 (18:31) post:

Q1) Alternator Rating if Air Cooled
One Rule-of-Thumb (ROT) says that using air as the cooling medium will reduce an alternator's capacity about about 20%. However, three key factors are involved: a) H2 pressure; (b) the permissible temperature-rise of the medium; and (c) the efficacy of the rotor-mounted fans when operating in air. Caveat: heed CSA’s admonishment regarding contacting the machine’s manufacturer.

Q2) Formulas'
The only formula used relates to the required coolant flow-rate to remove a specific quantity of heat. An interesting fact is that whether air or H2 is used the their flow rates are almost equal. The difference, of course, is moving H2, rather than air, results in a much lower "windage loss factor," while absorbing heat at a higher rate.

If anyone is interested in the List of References I used to increase one’s knowledge on the subject, just contact me!

Phil Corso (cepsicon [at] aol [dot] com)