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Flow control problems when backpressure changes? Choke the flow! (Part 3)

In this series, we've been discussing a gas flow control challenge that users face when backpressure changes. In the first post, we discussed several gas flow control applications where this is a concern. In the second post, I described a flow effect called choked flow, which occurs when gas flow through an orifice reaches the speed of sound. We already know that when gas is flowing through an orifice at the speed of sound, it's moving faster than the gas can expand on the outlet side. We can get the gas flowing through the orifice at this speed by adjusting the ratio of inlet to outlet pressure. The minimum ratio of these pressures that results in choked flow can be calculated from the isentropic expansion factor of the gas. This ratio is 1.8 to 2.2 for many common gases. This means that when gas flow control is needed into something with a changing pressure, we can disregard the downstream pressure changes with most gases by using an upstream pressure that is at least 2.2 times the highest downstream pressure. This ratio should always be calculated using absolute pressures. So if a desired mass flow rate needs to be maintained when downstream pressure ranges from 25 to 75 PSIA, the flow will stay steady if the inlet pressure to the orifice is set at 165 PSIA or higher. Now that we can use choked flow to maintain a mass flow rate into a changing backpressure, what happens if we need to increase the flow rate? Here are two options:
  • Increase Inlet Pressure: A higher inlet pressure increases the density of the gas, which increases the mass flow rate passing through an orifice. This can be achieved by adding a regulator upstream of an orifice, or with a pressure controller if automation or premium accuracy is desirable. This is not the preferred approach for many of our customers for three reasons: (1) the purchase of both an orifice and another instrument that can change the pressure is required; (2) there is no direct feedback of the flow rate to the user; and (3) choked flow can't occur with some orifice designs.
  • Increase the Orifice Size: This is the approach that users of mass flow controllers take. The control valve in a mass flow controller has numerous positions between fully open and fully closed. The valve position changes to achieve each desired flow rate, which essentially changes the size of the orifice in the valve. This is the preferred approach for many of our customers because it is a single instrument to install, it is automated, and it provides real-time feedback of the flow rate provided to the process.
But what if the maximum downstream pressure is higher than 75 PSIA? Our clients operating at higher pressures are having success with the market-leading SLA series mass flow controller.  The SLA can operate at pressures up to 4,500 PSIG. It is also capable of operating indoors, outdoors or in hazardous areas, and it provides numerous electrical communication options to meet the needs of a wide range of flow control applications. If you'd like to discuss an application like this in more detail, feel free to give my colleagues and I a call.

About The Author

Howard Kramer
Senior Applications Engineer

Howard is a senior applications engineer at Brooks Instrument.

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