Spare the details, we're chasing a mysterious instability in control of a 7 MPa industrial boiler. Still to be confirmed, but there are reports that the air flow measurement is done by measuring the DP across the firebox, rather than a conventional flow element in the FD fan duct. I thought I'd seen it all, that scheme seems a dodgy practice to me, and quite potentially the source of instability. Has anyone come across it before?
DP = differential pressure? (and what does FD mean?).
P-51 (yes, the WW-II fighter) radiator ducting was arranged such that at the design altitude, speed, and engine output, the engine cooling assembly generated thrust, which it could not do at a stand-still. So the pressure drop vs. flow rate curve had to have at least one section with a negative slope.
Other airplanes have managed to have cooling arrangements that generate thrust, too.
Given that, I would be very hesitant to trust any flowmeter that works by measuring the pressure drop across a heating element.
This is the standard method used on every marine propulsion boiler control system I have ever seen or heard of. These were typically 1200 PSI and 100,000 lb/hr or a bit larger. These small (by stationary plant standards) boilers had each of 2 or 3 blowers separately ducted directly to the boiler wind box with a very short duct, and no reasonable place available for flow elements. The air flow transmitter high pressure tap was at a low flow section of the wind box (air casing), far from burners or blower ducts, and to the best of my hazy recollection the LP tap to the firebox was actually between the generator bank and the economizer, since the combustion chamber is an unfriendly place prone to slagging and there is not much additional pressure drop across the generating bank compared to the burner air throat, Tap location varied from system to system, but all 3 generations of control systems I worked on back in the
70's used this method of air flow measurement without any problems.
Thanks Glen, that's very useful.
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