Brooks Instrument Blog

In my last post, we discussed several applications for mass flow controllers where precise flow control is needed despite backpressure changes. I introduced a flow effect called choked flow, which many of our customers use in these applications to ignore downstream pressure changes. This is also referred to as sonic flow or critical flow.

To my flow-savvy readers: You’ll notice that I’m discussing choked flow in conceptual terms rather than demonstrating complex formulas and calculations. Don’t be alarmed! Feel free to post any additional thoughts you have on this topic in the comments below.

The sketch to the right shows gas flow through a typical orifice. The green shaded areas are high pressure, low velocity flow areas, and the blue area is a low pressure, high velocity flow area. Inlet gas flow speeds up as it compresses to pass through the orifice, then re-expands and slows back down on the outlet side. The flow rate through the orifice is primarily set by the inlet and outlet pressures, as well as the diameter of the orifice opening. Gas temperature also plays a part.

A gas expands in a space as its molecules collide with whatever else is present. (pipe walls, other gas molecules, etc.) Every gas expands at its own rate, and pressure increases in a gas are a result of squeezing more gas molecules into the same amount of space. Applying these factors to the orifice flow pictured, gas expansion causes some of the molecules that are expanding in the green area on the outlet side to collide with and deflect the “fast” molecules in the blue area. If the pressure rises in the green area on the outlet side, it’s because there are more gas molecules present in the same amount of space.

More molecules in the green outlet area mean that more molecules deflect the “fast” molecules in the blue area. This reduces flow velocity in the blue area, which is what reduces the flow rate passing through an orifice at higher backpressures. If the pressure drops in the green outlet area, it means that fewer molecules are present in that space, which results in fewer deflections of “fast” blue molecules. This causes a higher velocity in the blue area, and thus a higher flow rate when the backpressure drops.

Choked flow occurs when the flow velocity in the blue area reaches the speed of sound. At this velocity, the molecules in the blue area are essentially traveling faster than the molecules in the green outlet area are expanding. So deflection between molecules at the blue/green border doesn’t reduce velocity in the blue area. With a fixed inlet pressure, the outlet pressure can change over a wide range without changing the mass flow rate as long as the conditions to maintain choked flow remain in place.

So how can we reach the conditions needed to maintain choked flow? We’ll cover that in our final post in this series.

Where did the names choked flow, sonic flow, and/or critical flow come from? Please post where you think one of these names came from in the comments. The first poster that correctly lists the reason for each of the names will win a 4 GB jump drive in the shape of a mass flow controller.

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.

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.