Do pressure regulators restrict the mass flow rate?

Engineering Asked by Izak the coder on August 7, 2020

Suppose we have a tank containing compressed air. If a hole is made in the tank the air will start flowing out. The mass flow rate of exiting air will be heavily dependent on the diameter of the orifice (ie the narrowest opening on the system) – the bigger the orifice the greater the initial mass flow and vice versa.

Now, let’s say we want a certain mass flow rate (x) coming out of the tank, whilst maintaining a constant pressure in the exiting gas. We can achieve the desired mass flow rate
by making an appropriately sized opening in the tank (I’ll call this opening O[i]) and adjusting the pressure using a pressure regulator. The issue here is that the pressure regulator could have a smaller opening (O[pr]) on its inside, ie O[i]>O[pr]. This would consequently reduce the mass flow rate coming from the tank.

It’s obvious that a pressure regulator can’t be used above a specific desired mass flow rate. What other devices could be used? A wider pressure regulator maybe?

I have included a diagram of a pressure regulator, which clearly shows the small opening inside.

I do that kind of thing regularly. My advice would be as follows:

1. Work out what kind of pressure you need in the tank for the maximum flow rate
2. Use a pressure regulator, as you mention in your question, rated to the pressure you worked out in #1.
3. Make sure the supply pressure to the tank is sufficiently above the maximum pressure you worked out in #1 and within range of the pressure regulator chosen in #2.
4. To control the mass flow rate, rather than using an orifice, use a mass flow controller on the outlet of the tank. This will ensure that you get the desired mass flow rate, while maintaining the correct pressure inside the tank. They can be expensive, but are well worth the money. I have had good experience with Bronkhorst ones.

Disclaimer: I do not work nor do I have any affiliation whatsoever with Bronkhorst, it's just from experience of using their products.

Answered by am304 on August 7, 2020

Your assessment of the situation is correct - the solution is of course to specify a pressure regulator with a large enough orifice that it does not cause you a problem.

Pressure regulators are specified with a Maximum Flow Rate, in the same way they are specified with Maximum Operating Pressure. You simply need to pick one that is suitable for your application.

Answered by Jonathan R Swift on August 7, 2020

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