Air flow into a piston compressor
Air flow into a piston compressor
(OP)
I have a two stage air compressor and need to know the inlet airflow. The problem is any standard air flow meter will oscillate too much to be able to get a valid reading since the airflow is not constant. Is there any easy way to get an inlet air flow reading?





RE: Air flow into a piston compressor
If you know the first stage cylinder inside diameter and the piston stroke length find that volume and multiply by the rpm. That flowrate will be at the first stage intake pressure, so convert that to SCFM.
RE: Air flow into a piston compressor
Patricia Lougheed
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RE: Air flow into a piston compressor
David
RE: Air flow into a piston compressor
RE: Air flow into a piston compressor
RE: Air flow into a piston compressor
RE: Air flow into a piston compressor
Ted
RE: Air flow into a piston compressor
10 gal = 1.34 cu. ft. ----- not much compared to the inflow to a large compressor
What pressure does it pump up to? How large is its tank? How long does it take to go from 0psi tank pressure to trip point?
If it will fill a 30 gal tank to 120 psi in 2 minutes, that would be 9.16 x 30 = 275 gal = 36.75 cu. ft. / 2 min = 18.4 cfm (very roughly). So your box would swap itself out every 4.4 seconds. (I just used some numbers to illustrate, 30gal @ 120psi in 2 min is a pretty substantial shop-type compressor.)
To get stable readings, I'm guesstimating you would want 20 seconds of cycle time on your dampening box.
RE: Air flow into a piston compressor
The method I described was used (at least when I still was inspecting about 5 years ago) at some nuclear power plants to measure the air intake flow on air-to-water heat exchangers. The utility basically marked a grid on the outside air intake screen (grid size was dependent on the size of the heat exchanger), used a pitot tube, averaged the measurements and then used the applicable formulas to convert to an actual flow rate. The results had to be within pre-established acceptance criteria for flow.
There's a lot of information about how to use pitot tubes readily available on the internet, in far greater detail then I could tell you given how long it's been since I've inspected this area.
Patricia Lougheed
******
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RE: Air flow into a piston compressor
In this case, because of the compressor piston, the measurement needs to be integrated over time, hence the suggested accumulators/volume boxes.
The pitot tube method might work if the compressor (size not in evidence) is in a closed room that's large relative to the piston displacement, and the sole inlet screen to that room is not so large as to make the flow velocity below the range of a pitot tube. But the flow is likely to be much lower than that produced by a fan of similar HP, so the velocity will also be low, and the pitot tube measurements will be tricky.
There's another problem with making pitot tube measurements of flow going _into_ a screen in a wall/face; the isobars on the inlet side of the screen will tend to be nearly hemispherical, so the streamlines except at the screen center will not be normal to the screen. So pitot tube orientation becomes critical/tricky.
Put the air flow meter you already have in a hole in the compressor room door, or some other single hole, and caulk or close all the other openings.
Mike Halloran
Pembroke Pines, FL, USA