Understanding energy losses in a compressed air system

[tenichols94]

TL;DR

When measuring pressure drop across a compressed air system the delta_P is different across the a component depending on the downstream flow path. I would like to know why, and how to quantify the energy loss across the component.

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. Let's 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.

 

[jrmichler]

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

No. The pressure loss in compressible flow is a function of both density and velocity.

A full answer to your questions requires some study on your part. Find the textbook that the nearest engineering university uses for their undergrad fluid dynamics class, and get a copy. There will be a chapter on compressible flow in pipes. Study that. Since you have a master's degree, this should not be too difficult. It will take some time, but you need broader knowledge than you can get from an internet forum.

Not knowing anything about the component(s) under test, be advised that it makes a difference whether the flow through the component under test is laminar, turbulent, or part of each.

 

[Dullard]

Also note:
'CFM' is a volumetric flow, and is rarely measured directly. Depending on the device that you're using to measure it, you may need to correct for pressure and gas composition - for the exact same reasons that jrmilcher cites in his first sentence.

 

[ShellScot]

No. The pressure loss in compressible flow is a function of both density and velocity.

A full answer to your questions requires some study on your part. Find the textbook that the nearest engineering university uses for their undergrad fluid dynamics class, and get a copy. There will be a chapter on compressible flow in pipes. Study that. Since you have a master's degree, this should not be too difficult. It will take some time, but you need broader knowledge than you can get from an internet forum.

Not knowing anything about the component(s) under test, be advised that it makes a difference whether the flow through the component under test is laminar, turbulent, or part of each.

That could not have been said better! Devices are key to understanding Flow Controlling dynamics. There are many physics laws (known and unknown) that will astonish you!