Hmmm ... single stage! In my limited experience, the large compressors have been 2 stage. Now that I think about it, 2 stage gives you higher pressure, but single stage greater cfm (for same horsepower). I think.
It is a four cylinder, two stage. Each bank has a low pressure and a high pressure cylinder. Admittedly I haven ever seen that particular configuration but the documentations shows that. It is in fact a QT-15, not a QT-10 as it says on the tank.
The high pressure cylinder is just barely visible beyond the low pressure, look low, just above the crankcase and you can see the high pressure cylinder.
NICE compressor. Quincy's are really top of the line. You need to arrange the VFD to get it up to speed before the unloaders kick out. There may be some way to control the time delay, although this one doesn't look like it has electrical controls for the unloader.
I got a smaller, single-stage Quincy at a scrap yard that had a continuous-run motor and totally pneumatic pressure controls (using the unloaders). I built a microcontroller-based dual-mode controller for it that unloads for spin up, waits for oil pressure to build, then loads the compressor, and keeps the motor on for one minute after no more air is needed. The unloader is controlled with a solenoid air valve. The motor is controlled with a 50 A, 400 V solid state relay. See pix at
box above the motor is the controller. The white thing to the right of the tank is a refrigerated air dryer that I hauled out of the trash at work.
With 2 HP, the sensible thing was to go with a single-phase motor. You'd need to derate the VFD for the compressor application, but it ought to work. The slow start of the 10 Hp motor/flywheel should be much nicer to the neighborhood, too.
Surprisingly, in agreement with what you just said about QT-15, the Quincy service rep emailed me the manual for QT-15. I thought that he simply made a mistake. But he told me, once I said there is two cylinders on each side of the V, that "I know exactly what pump you have". And he sent me the QT-15 manual. Now I see this in a different lightl. It looks like a QT-15 with a 10 HP motor.
I think the unloaders are those brass pieces sticking out of the cylinder heads. (Those look different than mine.) What controls the unloader can vary. On compressors with motor controls, they often use a valve that is operated by oil pressure. When the oil pressure is developed, the valve shuts off air and the unloaders allow the compressor intake valves to work.
Yes, mine are operated by a diaphragm about 1.5" diameter. Maybe due to the higher pressure of the 2-stage, the unloaders are smaller. Yes, some copper lines apply air to the unloaders to lock the intake valves open.