Volume flow rate is dependent on pressure and temperature...
Mass, is as stated, constant.
Yet....
A compressor on a flowbench at ____ shaft rpm and ____ PR will move ____lbs/min @ ___ degrees ambient and @ ____ psia ambient.
This test will coincidentally also equate to a specific CFM flow as well.
Here's a point to ponder, if mass is the constant here...
You take the exact same scenario of shaft speed and PR that you tested before. Now only change two things, the pressure and temperature of the room in which the test is performed. Double the ambient temperature and double the ambient pressure and re-record your mass flow reading.
Do you suppose that the mass flow will be the exact same when the exact same shaft speed and PR are tested? As you stated that mass was constant and volume depended on pressure and temp?
Of course not. The reality is that mass flow would be around half what you recorded the first time, whereas the volume flow would still match what you had noted on test one.
In
reality the mass flow is the one dependent on temp/pressure while CFM flow is
not. If you think doubling the ambient temperature and pressure is impractical, I would urge you to consider the conditions found at the second stage compressor inlet for a moment.
So help me out here...
The axes of the compressor map are PR for Y, and either mass or volume flow for X. A third reference comes in the way of the shaft speed arcs on the graph itself.
Here's my point, plain and simple:
For
any given PR/Shaft speed point that I plot on
any available compressor map, there will be a single point on the X axis that coincides with this plotted point. Many people who consider themselves "fluent" in the language of the mechanical engineer will assert that mass flow is the correct value to be placed on that X axis to denote compressor outflow for any given PR/shaft speed.
However, as I have stated more than once in this thread, if mass is the correct unit of measure for compressor wheel flow performance, then why does the map become completely useless as soon as you begin dealing with a compressor working under non STP conditions, such as is the case with any second stage compressor?
Or, another way of reasoning out this issue as I see it:
Having noted the max flow for each respective wheel as shown on their compressor maps, while my first stage compressor is capable of moving ~140lbs/min, my second stage is only capable of flowing ~72lbs/min max.
This
should raise concern, I would hope. Because either one of two subsequent issues must be addressed in that case.
1. Is my second stage being operated at a point around
two times the maximum value shown on it's compressor map at all times when I am running WOT in my truck? If so, what in God's name is the shaft speed at that time? Since ~72lbs/min occurs at a shaft speed of 110,000rpm, if I was fully utilizing my first stage compressor, and moving roughly twice that mass through the system, am I to assume that the second stage compressor is then approaching 200,000rpm? I think we can all rule that out right now.
2. If I pull in ~140lbs/min on the first stage, and the second stage is only capable of moving ~72lbs/min at max flow (so says the compressor map) then is it possible that close to 70lbs/min of atmosphere is consumed by the system somewhere between the first stage compressor inlet and the second stage compressor inlet? I don't think we need to delve into E = MC^2 to see the magnitude of nuclear reactor that would entail... So no, the mass is not disappearing between stages, so we still cannot explain how the second stage is moving roughly twice the max flow listed on it's map.
My rationale, as I've made clear in the thread, is that the wheel is being
incorrectly measured in terms of mass flow vs shaft rpm and PR, when in fact, the wheel is quite literally moving ____ CFM for any given operational point, and the ones creating these maps are incorrectly looking at mass flow, and labeling as such.
This is only made more clear in the fact that the entire mess above, that is unexplainable (at least by me) in terms of mass flow maximum for a second stage wheel, is simultaneously
Perfectly explained if the wheel is instead viewed in terms of
volume flow. All of a sudden compressor flows make perfect sense, and plotting the operational point for the second stage goes from a point 8 inches to the right of the paper/screen to a point coincidentally right where you expected one to be. And coincidentally, right to a point where outlet temps and shaft speeds are right where they should have been all along.
Why are compressor maps labeled in terms of mass flow when it appears an incorrect indicator of wheel performance? I have no idea. Maybe the engineers in the room can shed light on it.
Fwiw, I find it completely possible that mass flow is indeed correct. I merely have no way to show it, nor explain it, while to date, volume flow appears to work flawlessly in the absence of such an explanation.
