Air flow into a piston compressor

[Elevated]

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?

 

[BigInch]

Put an accumulator bottle between your meter and the intake. That should stabilize the flow into the bottle enough so you can measure it there.

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.

 

[vpl]

Use the pitot tube method: Create a grid that envelopes the intake area. Take pitot tube measurements in the center of each of the grid points. Average the measurements.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[zdas04]

Measure the outlet (use an AGA-3 device and calculation that gives you the volume flow rate at standard conditions, I like V-Cone's for this kind of application). Then you can convert the SCF numbers to suction conditions, interstage conditions, or interstage cooler outlet conditions with just a bit of math.

David

 

[dcasto]

Big Inch has the simplest method. Area Piston * stroke * rpm to get CFM. You'll have to adjust for loses in valves and clearance, but I'd guess using a VE (volumetric efficiency) of about 70% to be a good number. Just multiply the CFM by 70% to get the volume.

 

[chicopee]

I am with Big Inch. Volumetric efficiency of 70% per dcasto is acceptable with my ME handbboks.

 

[Elevated]

What I am trying to do is use a physical actual flow measurement to calculate the efficiency. The compressor is for educational purposes and for the student to basically use what biginch stated in comparison to the actual flow to get volumetric efficiency. So I need some physical way to measure the inlet flow. vpl can you elaborate on your method somewhat it looks like we could so something like that. also the compressor is so big that we put a 10 gallon surge tank in between the inlet and flow and is still very unstable.

 

[hydtools]

Build a 4-ft cube out of plywood, sealed except for intake with meter and outlet connected to compressor intake. Nearly 500 gal volume. Could add an interior baffle so the ports cannnot 'see' each other.

Ted

 

[tr1ntx]

Try a larger surge "tank", maybe build a plywood box and seal it with caulk or rubber seals.

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.

 

[vpl]

Elevated,

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

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[MikeHalloran]

Air to water heat exchangers typically use fans to move the air. Pitot tube scanning is the most sensible way to integrate the velocities over the flow area. Except for blade interactions that would be too fast for a liquid manometer to see, the velocity at any given station should be constant, whereas there may be different flows at different stations because of the nature of fans.

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