Brooks Instrument Blog

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.

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.