thermal massflowmeters for compressed air

[cokeguy]

We aquired thermal mass flowmeters to measure compressor outlet flows (in the 70 psig, 8000-10,000 SCFM range), but we get very erratic readings from the instruments. I suspect it has something to do with moisture content (sometimes air is completely saturated), but the manufacturer mentions that air humidity shouldn´t affect the readings that much.

Anyone has succesfully used thermal mass flowmeters for compressed air in those ranges? We use them extensively in blowers and gas flow apps, no problems, but we are baffled by their reading in compressed air. operating conditions and ranges are within calibration specs, only moisture content isn't controlled.

thanks for your comments

 

[Mobius44]

While changes in humidity might not have much effect, condensed water droplets in the air stream could throw off the meters as they impact the thermal element.

Can you move the meter downstream of a drier or coalescing filter?

 

[cokeguy]

Actually the meter is downstream of 2 aftercoolers and a drier, however the air is almost always saturated when we measure relative humidity, since we have a less than optimal installed capacity of driers for our compressors.

The issue here is to find out if thermal type flowmeters are adequate for this applications, I don't want to lose more time recalibrating meters (as the supplier suggests, moving them upstream where air is less saturated but much hotter) if they are going to be very sensitive to moisture content. We just bought 3 of this meters for three similar compressors and none of them is working OK. since I need to buy more meters for the rest of our compressors, I want to be sure I buy the correct meter technology at the least cost. I understand vortex type meters are better for this app, but also much more costly.

another supplier recommends annubar type meters for this applications.

any comments? basically I want to know if should simply forget about the thermal meters or if they should work even in varying conditions.

thanks for your comments.

 

[Mobius44]

You'll have to put the thermal mass flowmeters somewhere without condensed water, because that throws off the flowmeters thermal sensing of the airflow.

Since you don't have adequate drying capacity, you could try the suppliers suggestion to pipe them upstream and hope it's hot enough to prevent the condensation.

If that doesn't work, then you can either improve drying capacity or try different meter type that won't have as many problems with condensed water. You also might research meters designed for wet steam, since it's probably a more common measurement of a two-phase mixture.

 

[djs]

I have a project where we are using about 25 thermal mass flow meters on compressed air. Pressure is about 14 psig, flows from 1000 scfm upto 5000 scfm. No dryers or any type of conditioning. These are operating in central Florida. No problems with erratic readings. Units are Endress+Hauser TMass 65I insertion meters.