26th April 2008 - 01:20 AM
But for compressing air at the surface and feeding it through a pipe, here is a method I prefer to the centrifugal compressor-turbine: it's a rotary screw compressor
Unfortunately, there is no animation nor drawing at Wikihttp://en.wikipedia.org/wiki/Rotary_screw_compressor
so those who haven't seen one must try to imagine it, not really obvious:
It comprises two screws, with little metal and much air. These screws have opposite pitches, parallel axis, and are close enough that their threads fully interpenetrate, up to contact. Also, a precise casing closes the volume around both threads - so the casing has two overlapping holes, one for each screw.
Still there? Each thread step is closed, mostly by the casing, and by the interpenetration by the other screw.
No, the screws rotate in opposite directions, synchronously, so that the volumes enclosed in each thread step move parallel to the axis. This is a pump.
Small refinement for a gas compressor: the volumes decrease at the high-pressure end. There, the diameter is smaller, the thread shallower, and the pitch can be smaller. Not easy to manufacture, but we have CNC machines nowadays.
Such compressors run often at moderate speeds, but I would run them Allegro
so that leaks are less critical - though not at 240m/s.
One very nice feature is that we can spray cooling water in the compressor itself
, so that no exchanger is needed, and compression ratios can be high - maybe 2 steps for 300 bar. Erosion by water is reasonable, as we don't use turbine speeds.
I would really use immoderate amounts of cooling water, I mean, a volume not negligible compared to the air. Combined with the fast rotation, it helps keeping the compressor airtight. And let it spurt and spray the big way, as we must evacuate 240kW in a reduced volume.
I know there are hairsplitters among you (where is A.? I haven't read him for weeks, how is he doing?), who will suggest that a rotary screw compressor needs a gear to synchronize both screws and transmit half of the power from the motor to the opposite screw, and gears wear out, and all that stuff. But here comes the answer (in fact, for every rotary screw compressor):
Use TWO motors, one on each screw shaft, so that little torque must be transferred through the gear, which won't wear out then. Even better, put sensors on the gear, and regulate the power among both motors so that no torque at all is transferred. The gear works only as a synchronizer then, and as a passive safety.
One very nice feature is that this compressor works as an engine as well
. Reuse also the cooling circuit as warming and the electric motors as generators.
Clearly my preferred one