Brooks Instrument Blog

Written by:
Brandon Hansen

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, you’re welcome to enter some application and contact information into this page to be contacted by your local Brooks product expert.

Feel free to give my colleagues and I a call if we can help as well. We can be reached at 215-362-3500, ext 3000.

Written by:
Brandon Hansen

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.

Written by:
Brandon Hansen

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.

Written by:
Jim Hollis

Next week is the Pittcon trade show. Boy, that came up quickly! If you are planning to head to Orlando, FL for Pittcon this year, be sure to stop by and see us at booth 961. In fact, click here if you want to schedule an appointment with one of our sales engineers at the show! If you do stop by, you’ll be able to check out our award winning products like the brand new GF40/80 Series mass flow controller / mass flow meter, the XacTorr Series capacitance manometer, SolidSense II pressure transmitter, and so much more!

Just so you know what to look for on the show floor, below is a picture of our booth. So be sure to hunt us down at Pittcon. We’d love to hear from you!

Written by:
Jim Hollis

Winning Control Global’s Readers Choice Awards is probably one of the best awards that we can win because the winners are chosen by the users of instrumentation. You can check out the full press release about this here, but below is the gist.

Brooks was honored in three categories, variable area flowmeter, positive displacement flowmeter and thermal mass flowmeter. For variable area flow meter, this was the 15th consecutive year we have place 1st in this category. And for positive displacement flow meter, this is the 18th consecutive year we have placed 1st. Quite an achievement if I do say so myself! For thermal mass flow meter, we placed 4th this year.

Thanks to Control Global and a special thank you to all the readers of Control Magazine that voted for Brooks!

Written by:
Jim Hollis

Designed for semiconductor, MOCVD, and other gas flow control applications that require a high purity all-metal flow path, the Brooks GF100 Series mass flow controllers deliver outstanding performance, reliability, and flexibility. Highlights of the GF100 series industry leading features include: ultra fast 300 millisecond settling time, MultiFlo™ gas and range programmability, optional pressure transient insensitivity (PTI), local display, extremely low wetted surface area, and corrosion resistant Hastelloy® sensor tube and valve seat.

GF120XSD is an extension to Brooks GF100 Series thermal mass flow controller family (MFC). Designed for extreme low pressure operation, the GF120XSD has been optimized for the precise delivery of high value, low pressure specialty gases. Read more…

Written by:
Jim Dillon

Yesterday I covered flow rate and reference conditions and talked about how important (or not) these are to specifying a variable area (VA) flow meter. Today, I’ll finish up my tips by reviewing fluid density and viscosity as well as accuracy.

Fluid: Density and viscosity
We always have questions about the fluid such as gas or liquid.  What are the density and viscosity?  Is it corrosive or opaque?  If it a know fluid such as air, nitrogen, water, etc.  The questions get much easier because the world has defined how these known fluids behave so we can easily determine density and viscosity for common fluids such as air, water, nitrogen, etc.  Which leads to the questions as to why do we need to know fluid density and viscosity?  Fluid density and viscosity are important because these two values allow us to select the right flow meter (meter size).  We call this sizing.  What is behind sizing?  Briefly, performance data has been collected on all of the different meters we offer.  We query the performance data and look for flow meters that fit the supplied process conditions (density and viscosity).  Usually there are many flow meters that fit your conditions.  From there it becomes a matter of preference, available options, price or accuracy.  This leads me to my last topic on VA meters, which is accuracy. Read more…

Written by:
Jim Dillon

In yesterday’s blog post, I took you through why we need to know about your normal and maximum operating temperatures and pressures in your application to specify the right variable area (VA) flow meter for your process. Today, we’ll dive into flow rates and reference conditions.

3. Flow rate – minimum, maximum, normal
Of course flow rate is an obvious requirement but it is more complicated than it appears.  The goal is to specify a VA meter where the normal operating flow is in the 60% to 80% of the meter’s range.  Why you ask because a variable area meter is more accurate in the upper part of its range.  I will talk more about accuracy in my next blog post.  Of course a flow meter needs to be chosen that handles the minimum and maximum flows too.  The other component of flow rate is the units.  Read more…

Written by:
Jim Dillon

Variable area (VA) flow meters will respond like the canary in the mine when the air quality changes.  If process conditions change there usually is an impact on flow rate.  For example if back pressure changes on gas flows the float/flow will change just as changes in liquid viscosity will have a similar impact to the float/flow.

VA meters reacting to changes in process conditions can be a good or bad thing based on a user’s viewpoint, which brings us back to the real question.  So why do we need so much information to specify the proper VA meter?  I will go through the questions and explain why it is necessary.  The information needed is: Read more…

Written by:
Jim Hollis

I was just informed that as of yesterday, January 3, 2012, our long-time Brooks sales rep, Webco Controls, Inc, merged with Miller Energy Inc. This is pretty exciting news for our customers in the Northeast United States who deal with Webco. This merger will really enhance the technical inside sales support for customers as well as providing them a very broad range of instrumentation and valve manufacturers. Read more…