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Join Date: Feb 2019
Posts: 8

Flow Control

02/04/2019 7:20 PM

Hello all,

I am currently building a test bench that can input a high amount of flow, pressure and temperature into an Air Cycle machine. The inlet to the machine requirements are:

Flow = 71 lbs/min

P = 48 PSIa

T = 235 degree F

Diameter = 3 inches

In order to produce these conditions, I have a 75HP Compressor that produces approximately 335CPM, a 10,000 gallon storage tank and a 200KW Heater.

I am looking for a way to control the amount of flow and pressure but know that controlling 1 will affect the other in some way that will make it difficult to get a precise readout. Some questions I have are:

Should I control pressure or flow first or does it not matter?

What kind of control devices for flow/pressure should I look more into (valves, meters, etc)?

Can I control the flow or pressure after or before the heater?

Is the compressor supplying the storage tank at a different pressure than the inlet pressure going to affect the flow?

I am looking for any advice or companies that specialize on control valves or systems that can handle such conditions and greatly appreciate your input.

Blake

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Guru

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#1

Re: Flow Control

02/04/2019 7:51 PM

Not sure what you are trying to do. Are you testing these?

Air cycle machine

Maybe ask these guys: Air Cycle Machine MRO at Global Aerospace Corporation

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#2

Re: Flow Control

02/04/2019 8:22 PM

You can't control both pressure and flow unless you can also control the flow resistance. One possible way to do this is to ensure that the compressor/heater can produce a surplus of pressure/flow, and then bleed off any excess flow.

When you say CPM do you mean CFM? And then, do you mean SCFM entering the compressor, or CFM leaving the compressor?

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#5
In reply to #2

Re: Flow Control

02/05/2019 10:17 AM

Hi Tornado,

I gave you a "good answer" for the answers and the questions.

The 75 HP equates to 335 SCFM.

I'm a bit confused by the, Flow = 71 lbs/min!

Best regards,

John

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#10
In reply to #5

Re: Flow Control

02/06/2019 6:03 AM

Indeed. 335 scfm ≠ 71 lb/m, but maybe 335 output cfm does? (I haven't looked it up.)

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#11
In reply to #10

Re: Flow Control

02/06/2019 6:53 AM

Hi Tornado,

Just did a quick check and you're right. 1000 SCFM is 74 lbs/min. @ 14.7 psi so 335 SCFM I guess would be about a third of that given the pressure he stated.

Best regards,

John

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#12
In reply to #10

Re: Flow Control

02/06/2019 9:18 AM

taking the 335 x 3.3Bar you do get approx. 88 Lbs/min

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#13
In reply to #12

Re: Flow Control

02/06/2019 9:22 AM

Hi Don,

In the ball park!

Best regards,

John

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#14
In reply to #12

Re: Flow Control

02/06/2019 11:35 AM

335 scfm ÷ 4.3 bara ≈ 78 outlet cfm.

335 scfm x 0.076 lb/ft3 ≈ 25.5 lb/m. (The 0.076 depends on air temp.)

Some of the original numbers don't seem consistent.

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#3

Re: Flow Control

02/04/2019 9:04 PM

You need a computer that can control all the variables with predictable results....this requires testing and programming the results in for every combination, with parameters set for safety....This also means a step in heater with many stages...and a blower that can operate within the parameters...

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#4

Re: Flow Control

02/05/2019 10:07 AM

Lag the tank to retain as much of the heat generated by compression as possible.

Flow = 71 lbs/min?

Best of luck,

John

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#6

Re: Flow Control

02/05/2019 11:04 AM

Your data suggest that machine is designed in such a way that it will create a pressure of 48 PSI if you push in the air of 2350 at a flow rate of 71Lbs/min, and machine inlet to be connected with 3 inch pipe.

So, you just need to maintain flow rate, rest it will do itself.

This is based on the assumption that machine is per-designed and this data is taken from machine name plate.

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#7

Re: Flow Control

02/05/2019 12:05 PM

How about providing a sufficiently detailed diagram of exactly all of what you are trying to do, and how you are (tentatively?) trying to do it?

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#8

Re: Flow Control

02/06/2019 2:34 AM

First,I would control pressure with a pressure regulator,downstream of the tank.
Absolute Pressure regulators and controllers are available.
Flow is much easier to control with a steady pressure.
The compressor type is not specified,but variable speed drive would be preferable,but simple on-off of the compressor is acceptable,provided the cycle times are long enough to prevent overheating.
The storage tank should have sufficient pressure above setpoint to allow for variations in output from the regulator.
You should also incorporate a refrigerated air dryer after the compressor,before the tank,and a timed drain valve on the tank.
There will always be moisture in the compressed air.
Before you can control anything,you must measure it,so you will need a flow meter.
Many types are available,your choice,but consult with seller about the operating conditions..
I would control the temperature as near to the destination as possible with a PID controller,with the output modulating the power to the heater elements.

Here is a link to one of many companies that will help you design your complete system.

https://www.schneider-electric.com/en/brands/foxboro/foxboro-measurement-instrumentation-products.jsp

Here is a link to 50 industrial control companies:

https://www.controlglobal.com/articles/2016/positioned-for-recovery-top-50-automation-companies-of-2015/

Good Luck.

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#9

Re: Flow Control

02/06/2019 4:43 AM

What tolerances are you trying to achieve?

Taking the most simplistic view flow dictates the speed of the machine you will be testing while pressure dictates the power, so you need to define your equipment in terms of what you are attempting to test.

Is yours a reciprocating compressor or a screw compressor and is it fitted with variable speed drive? Reciprocating compressors deliver a pulsed air output that is usually equalized by feeding the air into a large tank and releasing it through a pressure regulator. Even with damping some pulsation will transmit through the tank. Screw compressors deliver a much smoother output. With a variable speed drive a screw can be used without the balancing tank by matching the load and output. That would be the most cost efficient in terms of running costs but the capital cost is higher. If you plan to run the test rig 24/7 running costs become a significant element of the equipment design. If you will be testing for a couple of hours per week then capital cost is usually dominant but try to convince the bean counters that overall efficiency is better for the environment and that has tangible PR value to your company which should be included in the costing equation.

You mentioned heating but not cooling. What is the maximum ambient (usually summer) temperature of the air intake? How much additional heat is generated by the compression? Is the resultant temperature above your specification? Any equipment that is out of spec in the summer months is poorly designed.

Do you heat early or later in the process? Your temperature sensor should be placed as close as possible to the test piece intake. That will compensate for temperature variations caused by changes in flow and pressure. A large differential in pressure over the regulator will cause the air to cool as it passes through. An increase in flow around bends in the delivery pipe will cause the air to heat. Heating/cooling air changes its volume so you need to do that before the flow and pressure regulators. It comes back the my initial question. What tolerances do you need?

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#15

Re: Flow Control

02/07/2019 9:31 AM

In controlling the flow I thought it would be interesting to see what the velocity is. Unfortunately, the pipe is just given as 3" so I have taken that as my ID. Also, I have taken air as the flowing medium. A very rough estimate is 129 ft/sec (39 m/s). However, it is ages since I did a calculation in Imperial so please bear with me. If the temperature is to be controlled, it will need a fairly robust thermowell [with sensor] for such a velocity.

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