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Choosing the Right Mass Flow Controller for Your Application

Chapter 1

Flow Rate

MFCs are used to measure and control the flow of gasses in a process.

The overall flow rate is an important consideration

  • Measurements
  • Turndown Ratio
  • Body Size

Some processes require very small flows usually specified in standard cubic centimeters per minute (SCCM). Higher flows are typically given in standard liters per minute (SLPM) or standard cubic feet per minute (SCFM).

Many manufacturers make devices able to measure flows of 200 sccm up to 30 slpm. The same body size can be used for this wide range of flows; however, the internal components are changed to give a more precise flow range.

Typically, once you choose a maximum flow, you are able to measure and control down from there a certain amount. Having a 10 to 1 turndown is common. Being able to control to a 50:1 or even 100:1 is available from some of the leading manufactures.

Setting the maximum flow to something very small like 10 sccm or even 3 sccm and then being able to measure and control from that point down to 2% is remarkable, but is standard for some.

Once the desired flow is more than 30 slpm (or 1 scfm), usually the physical size of the MFC body changes a bit. This allows greater flow as there are larger passageways inside the device. Flows to 100 slpm or 200 slpm are typical in this arrangement.

Once the flow gets higher than that, the body size really jumps and this allows flows up to 2500 slpm to be controlled or 9000 slpm flows to be measured (without a control valve).

Elastomer Sealed Thermal MFC

These have the widest flow and pressure ranges.

Explore this MFC
Chapter 2

Gas Conditions

Are you flowing one of the simple, common gasses such as air, oxygen, nitrogen, helium, hydrogen, argon, CO or CO2? If so, a simple design of the MFC can be used.

If your gas is very dangerous, you may be required to treat it differently.

Contamination

If it is susceptible to contamination, such as reacting with moisture, the interior of the MFC may be polished and the connections changed from screwed or compressed fitting to higher purity type connections such as VCR or VCO which utilize metal seals and are considered when high and ultra high purity gases are being used.

Positive Pressure

If the process is being run at a positive pressure, the design of the MFC is such that it can contain that pressure. Some MFCs are limited to 100 psi or 500 psi maximum working pressures. Industrial MFCs are usually standard at 1500 psi (100 bar) and can be made to withstand 4500 psi (300 bar) pressure sometimes found in catalyst research, hydrogenation of food or drugs, or many petrochemical processes.

Under Vacuum

On the other hand, if your process operates under vacuum, the design of the MFC has to be adjusted accordingly. Gases behave differently at positive pressures than they do under high vacuum and calibrating an MFC for vacuum service is a precise process.

Temperature

Temperatures of the gases are to be considered. Usually the gases are measured and controlled at relatively mild temperatures and elevated after the MFC if high temperatures are required in the process. There are temperature limits inside an MFC due to the measuring technology which utilizes the thermal properties of the gas to determine the flow. So, typically, 140-160°F is the working limit for gasses in MFCs.

Metal Sealed Thermal MFC

These are ideal for processes sensitive to moisture or oxygen

Explore this MFC

Key Takeaways

  • Having a 10 to 1 turndown is common. Being able to control to a 50:1 or even 100:1 is available from some manufacturers.
  • Gases behave differently at positive pressures than they do under high vacuum.
  • Typically, 140-160ºF (60-70ºC) is the working limit for gases in thermal MFCs.
Continue to Chapter 3

Accuracy