Compressor - Regulator - Plenum - Exit. Regulator not providing correct plenum pressure

[jlchard1]

Hi,

I have a workbench test set up which consists of a compressor, followed by a 6bar supply line, followed by a regulator (for bringing the pressure down to 2bar) followed by a plenum chamber that leads to a slot that vents to atmospheric. The slot is approximately the same area as the supply line pipe. There is a pressure tapping in the plenum chamber to measure stagnation conditions. I want a plenum stagnation pressure of 2bar. The flow rate at this pressure is 0.1294kg/s which converts to 3240 l/min given the density of the air at this point. The regulator I have has flow characteristic curves for an input pressure of 7bar but show an output of 2bar is easily achievable at this flow rate. I understand the input pressure is 6bar in my case however, given the high max flow rate shown by the characteristic curve I believe I should still be able to achieve 2bar output with a 6bar input. The problem: The maximum plenum pressure I am able to achieve is about 1.3bar. If I try and increase the pressure past this nothing happens, it is as if the flow is choked. Given that the regulator should be able to produce 2bar output with a 6bar input at the design flow rate, why would my regulator be unable to produce 2bar plenum conditions? Any help is much appreciated. Any more details required please ask.

Thankyou

James

 

[LittleInch]

A diagram would help, but how have you worked out "The flow rate at this pressure is 0.1294kg/s which converts to 3240 l/min given the density of the air at this point"

Maybe your flow is more and hence your regulator is maxed out? Can you measure flow? Can you reduce the slot size?

Are you sure all the figures etc are all in absolute or gauge pressure? - Sorry if that sounds a bit simple, but you would be surprised by how many people get it wrong...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[jlchard1]

Hi

I have uploaded a diagram. The flow rate was calculated using the compressible mass flow equation at the nozzle exit (it is a convergent nozzle). Assuming the temperature is 293K I calculated density using the ideal gas law and then converted the mass flow rate into a volumetric flow rate. The pressure comes from a large compressor which supplies the building I work in. My part of the building has been regulated down to 6bar from 16. One suggestion was the use of a reservoir after the regulator with a control valve downstream of it. I would then pressurize the reservoir to 2bar and then open the control valve. Does this sound like it could solve the problem? If so, how?

James

 

[LittleInch]

What is the precise model number of your regulator? There seems to be an option that limits the output to the 0.2 MPa setting (ends in a 1) and at your 3,500l/min that would equal about 0.15 MPa. see

http://content.smcetech.com/pdf/AR_EU.pdf

Otherwise I suggest you post your calculation about the mass flow rate for others to pick over, noting that your comment "The slot is approximately the same area as the supply line pipe" doesn't seem to be correct. A 10mm tube has an internal area about 75mm2, your slot is 240mm2?? You could still be getting a lot more flow than you think / calculate, but I can't tell from current data.

Are you sure you're still getting 6 bar inlet when flowing?

I don't follow where the reservoir and control valve fit in relative to the plenum chamber and nozzle or whether your flow is a short term thing or a constant flow or what?


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[jlchard1]

AR60-F10E-Y is the precise model which has an identical characteristic curve to the one shown in the diagram. I get an inlet area of 3.14 x 10^-4 m2 for a pipe RADIUS of 10mm and the slot area is 2.9 x 10^-4 m2. I don't currently have any means of measuring the inlet pressure when flowing. The max flow rate (provided 2bar can be achieved in the plenum) is based on this equation:

http://www.grc.nasa.gov/

http://WWW/k-12/VirtualAero/BottleRocket/airplane/mflchk.html

and the exit Mach number was calculated using the isentropic pressure ratio equation

http://www.grc.nasa.gov/

http://WWW/k-12/airplane/isentrop.html.

I rearranged the ideal gas equation to get density, under the assumption that temperature is approximately ambient, and then divided mass flow rate by density to get volumetric flow rate. What is the difference between the flow characteristic curve and the max flow rate obtained using the regulator flow coefficient?

 

[LittleInch]

Ok,

apologies for the error in the pipe size, I saw 10mm and assumed diameter as not many people quote radius.

I'm no expert on fluid flow like this, but if you are trying to equate a rectangular slot and using a circular nozzle formula I suspect that is where your errors might lie.

have understood that right?

what I was suggesting was that you actually post the calculations you have got, not just the formula.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way