A sawmill was having trouble finding a flow meter to measure the compressed air that was produced by compressors totaling 1,500 cfm utput. This flow was running through a 3-in. pipe. The plant personnel contacted a compressed air expert to see if he could help. They were surprised by his answer.
The most common and affordable flow meter for dry air is the thermal mass type. To measure flow with a thermal mass meter, the meter probes must be inserted into the flow of compressed air through holes or valve taps. The air flow cools the probes, with the cooling effect proportional to the mass of compressed air flowing past. If the pipe internal diameter is known, then the cubic feet per minute and feet per second velocity can be calculated.
But a thermal mass flow meter assumes laminar flow, which occurs in smooth pipes with low velocity flow. Pipes with high flow have turbulent air flowing inside them, which makes the flow of compressed air past the flow meter probes unpredictable. With high velocities, the accuracy of this meter type is poor.
The reason the plant personnel were having problems finding a flow meter is that their main header pipe was too small, with velocities over 60 fps would occurring at peak. The compressed air expert replied to the plant personnel that they were having trouble finding a flow meter because their main pipeline was too small.
Sometimes equipment sizing problems can be a hint that there are unexpected system design problems. It pays to find out why.