Emerson Exchange 365

The post The Good, Bad and Ugly of Thermal Mass Flowmeters first appeared on the ISA Interchange blog site.

Thermal mass flowmeters can be a relatively inexpensive flowmeter that can handle extremely small flows (e.g., inline 1/16 inch meters) and large flows (e.g., multipoint insertion type in 60 inch ducts). Thermal flowmeters introduce heat into the flow stream and measure how much heat is absorbed using one or more temperature sensors. However, the meter requires a fixed specific heat capacity, no heat loss or gain from ambient conditions, a fixed composition, predictable heat distribution, no change in phase and no change in heat transfer coefficient unrelated to velocity (e.g., surfaces must be clean and dry). Thermal mass flowmeters are most frequently used for gas flow since heat absorption in liquids and solids can be problematic. The greatest success is seen with properly installed inline meters measuring single component gas flows in a very controlled environment.

The flow measurement uses two temperature sensors, the upstream one being heated by an electrical current. Flow is inferred either from the temperature rise for a constant current or from the amount of current needed to maintain a constant temperature differential.

Total shipments of thermal mass flowmeters have been increasing by about $5 million per year to become about 2 percent of the total worldwide market for all types of flowmeters that was about $5 billion in 2009.

Questions from ISA Mentor Program Participant Adrian Taylor

1. We have some installed on nitrogen lines to reactor inserts, these give poor agreement and we have a current project to change these out for Coriolis meters (although I suspect we are suffering the effects insufficient upstream piping diameters on these particular meters).
2. We also have some thermal mass flow switches on suction lines from some sample cabinets up to our hygiene vacuum system. The duty is air most of the time from a vent to the sample cabinet but following a sample there are also HCl vapors. The switch is used in an interlock to prevent the cabinet door being opened with insufficient vacuum for fumes from the sample. At the time of installation the switches worked perfectly but we’ve found over time there is a shift in the flow/voltage curve which has to be adjusted for. The elements are Hastelloy, which I think should be fine for the occasional ambient temperature HCl vapors. All I can think is the shift is due to either some kind of coating forming, or due to droplets forming on the element from the occasional vapors.
3. Finally as part of a turnkey burner replacement project we have just installed an array of thermal mass elements in a square combustion air duct on our site. This is intended to work in a similar way to a pitot array but for average mass flow rather than volume. The system has only recently been commissioned so I can’t comment on performance yet.

Answers from ISA Mentor Program Participant Hunter Vegas

Rule #1 (and it is a huge one) – If you are trying to use a thermal flow meter on air/gas/vapors never install it in a service where it can ever see a gas/vapor approaching dew point.  Even a tiny bit of liquid will create erratic flow readings.

Rule #2 – Most thermal flow meters do not promise high accuracy particularly if they are the insertion type. If you need highly accurate flow readings you should probably investigate a different meter type. However they do work well as a flow switch or a go/no go type of flow application like purges and the like.

1. I’m a little surprised you had trouble on nitrogen. Normally dry nitrogen is a good application, but you do need to have a decent meter run to get accurate results.
2. Can’t really speak to the sample cabinet meters, but if you are getting vapors that condense the reading will be off.  However it should recover if you are just flowing air unless you are getting some kind of a coating as you say. In that case it would definitely impact the meter. Realize too that different gasses will require different calibrations so if you have a mix of gases your accuracy will be impacted.
3. The array meter will only work if you are downstream of the economizers and have hot air coming through that is well above the dew point.  If the meters are in the suction duct where they’ll see outside air at ambient temperature they’ll work great until it is foggy or rainy and then the readings will likely turn unstable.

Greg McMillan’s Comments

I have seen thermal mass flow meters extensively use in laboratories and pilot plants to measure air, oxygen and carbon dioxide flows for bioreactors. As you can imagine, the gas and ambient conditions are exceptionally controlled, which may explain their success.

For more on the physical principles and practical considerations as to selection and installation of thermal mass flowmeters and all other types of measurements see my 2010 ISA book Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance.