understanding energy loss in compressed air system

Aug 2019
6
0
When measuring pressure drop across a compressed air system shown in the included figure, I get different results depending on the system downstream of the actual component I am measuring pressure drop across. Btw this is a real experiment that has been ran. The numbers below are different but a similar scenario is happening.

Case 1: The flow rate is 10 [cfm] and the pressure at P1= 63 and at P2=32 which gives us Delta_P1=31. So that would mean delta_P2 > 0.


Case 2: The flow rate is 10 [cfm] and the pressure at P1= 70 and at P2=4 which gives us Delta_P1=66. Lets assume for this case the delta_P2 = ~0. So we can assume that the outlet is exposed to the atmosphere.


To my knowledge for incompressible (fluid) systems the delta_P in at a given flow rate will be the same no matter the location in the system, which is why head loss charts can be so helpful in designing fluid systems. But as seen in this compressed air system the down stream head loss can have an affect on the pressure loss at the component that I want the measurement across.

Should the pressure loss be constant and my pressure gauges be read wrong?

Also, how can I quantify the energy loss in the system? At the inlet and outlet I have pressure, volumetric flow rate, and density at these points when making certain assumptions.

I want to use this experimental apparatus to help us understand which component flow better.
 

Attachments

Jun 2016
1,143
514
England
What units are your pressures in?

what is the speed of the air at the various pinch points?
Are you sure your incompressible assumption is valid?
 
Aug 2019
6
0
What units are your pressures in?

what is the speed of the air at the various pinch points?
Are you sure your incompressible assumption is valid?
The meter is vented to atm and is calibrated to display scfm (14.7 psi and 68F).
But to answer your question I don't experimentally find it but Using the mass flow rate, which I know, and making some assumptions I can approximate the cfm at the pressure points.

Well for incompressible fluids the head loss it a function of the system design (e.g. pipe length), fluid, and flow rate...
 
Jun 2016
1,143
514
England
What I was pointing toward was that if the speed of flow is too fast,
or the rate of acceleration of the flow at any point is too high,
then the simplifying incompressible assumptions cannot be made.

I don't know enough about your experimental setup to say that is the case,
I am just flagging it up as a possibility.