Pressure control with adjustable flow, art or science?

Setup with mass flow controller and pressure controller

Mass flow controller with pressure controller in-line

As with any inter-dependent parameters there are consequences for changing one parameter if the controller of the other is responsive. For example; if you installed a mass flow controller and a digital pressure controller in-line, each with their own inputs and outputs, changing the set-point on the digital pressure controller would adjust the valve and therefore the flow. This reaction would be detected by the mass flow controller as either a decrease or increase in flow and the mass flow controller valve would adjust accordingly to achieve its given set-point. This second reaction would then be seen as a change in pressure by the digital pressure controller and you can see how quickly this would become a feedback loop of increasing instability.

One of the most frequent challenges that we come across, and one that people are always surprised we can solve, is the ability to control and meter flow and pressure in-line. Many of the applications we are involved in require control and measurement of the flow and pressure of the fluid. However, as the two parameters are different but inter-dependent it can make it difficult to determine the optimal process conditions in which both parameters are fully supporting each other. Our experience teaches us that each case has its own unique solution. We learned by experience that it is very beneficial to draw together the process owners of the application under discussion. I/we use this ‘drawing’ skill to help identify which areas are critical and where value could be added through the use of digital instrumentation. This can make it easier to determine what the best course of action is.

Applications for use:

The need to control/meter both pressure and flow can be important in:

Consumption reactions
Biochemistry
Fuel cell development
Burner/Ignition lance applications
Air permeability testing

Application examples

If you need to measure the Air permeability of medical packaging; you need to set a standard pressure while also metering and controlling the Air required to maintain the standard pressure. This measure gives you the flow required to achieve stability and therefore the Air permeability.

In consumption reactions you may want to hold your reaction vessel at a set pressure and control/meter the feed gas to maintain the set pressure. This set-up, with electronic instruments and PC/PLC communication software will give you total consumption, consumption rate and allow you to profile the life cycle of the reaction you are conducting. Adding digital control can allow you to control maximum flow as you build up to desired pressure levels which can be important in certain applications.

As with all application solution requirements, talking the application aim through with your supplier can be crucial to getting the best result.

Have a look at the related measurement technologies: