Home > Coriolis and Thermal Mass Flow > Flow control problems when backpressure changes? Choke the flow! (Part 1)


April 12th, 2012

In gas flow control applications, inlet and outlet pressures are critical factors when configuring a flow controller to ensure that the desired flow rates can be maintained. Increase the downstream pressure following an orifice, and the amount of flow is typically reduced. In this series we’ll talk about a method you can use to specify a mass flow controller that ignores downstream pressure changes to provide reliable mass flow control into a range of pressures.

Repetitive increases and decreases to a gas flow controller’s downstream pressure are common in many of our customer’s applications. How common? Here are a few examples:

Biotechnology: A mass flow controller controls gas flow in a bioreactor to promote a desired biochemical reaction. There are a wide range of reactions or events in bioreactors such as: promoting tissue growth, assisting organisms to produce desired chemicals or medicines, developing enzymes to break down hazardous compounds, and many others. Tight gas flow control of oxygen is needed to help organisms that consume oxygen prosper inside a bioreactor. Many of these processes create other gases, (oxygen converted to carbon dioxide, for example) and different batch sizes or recipes require different gas flows. These factors change vessel pressure without removing the need for precise gas flow control.

Food Aeration: A mass flow controller injects gas into a food item. (Nitrogen is commonly used) As foods like butter, bread dough, chocolate bars, ice cream, and even Oreo cookie stuffing are processed, it’s quite common for a gas to be injected into the food to maintain a target consistency or texture. Different foods and batch sizes change the pressure needed to inject gas into the food. Inaccurate gas flow increases the amount of food rejected for poor quality.

Selective Catalytic Reduction: A mass flow controller injects gas flow into an exhaust stream to break down targeted hazardous gases or compounds for air quality purposes. For example, ammonia vapor is commonly used to breakdown nitrous oxides. The exhaust stream pressure changes as the equipment load changes, and the mass flow controller needs to provide tight mass flow control to break down enough of the compounds. Inaccurate injection gas control reduces air quality.

Vessel Fuel Research: A mass flow controller controls gas that fills a vessel to initiate and control a reaction. Hydrogen is often used for fuel research. A catalyst is placed or gradually fed into a reaction vessel along with the gas(es). The mass flow controller needs to maintain precise mass flow control into the vessel to maintain the desired reaction rate at the same time that the downstream pressure is increasing as the vessel pressure rises. Inaccurate gas control prevents the desired reaction(s) from occurring.

There are definitely other flow controller applications with a variable back pressure that were not included in this list to keep it a manageable size. Please post any you’d like to share in the comments – we’d love to hear more about your applications.

Many of our customers who need a gas flow controller for an application with downstream pressure changes take advantage of a flow effect called choked flow that allows the flow controller to ignore backpressure changes. We’ll talk more about this gas flow effect in the next post.

If you’d like to read a bit more about instrumentation and process control, feel free to check out more of my contributions summarized on my Google Plus profile.

About the author

Brandon is an Applications Engineer at Brooks Instrument. Prior to joining Brooks, Brandon worked for FTI Flow Technology and Key Instruments. He has extensive experience working with a range of process control instrumentation. Brandon holds a BS in Technical Management focused on Control Technology from Devry University. When he's out of the office, Brandon is typically trying to strike the balance between an automated home and a happy wife. Feel free to connect with Brandon on Twitter(@BrandonDHansen) or LinkedIn. (linkedin.com/in/brandondhansen)

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