Pitot Tube Location for Filter Monitoring

[Kevin Jenness]

That's what I've done. I epoxied a thread X barb elbow between the 'funnel' and filter on the dust collector. Bought a 5' loop of clear plastic tube that fits over the barb end of the elbow. I tie-wrapped the other end of the clear tubing to form a loop or U. I put enough colored water in the clear tubing to come up about 10" on both legs. The one end remains open to the outside, I just very lightly packed some paper towel to keep any dust out of the colored water. If the filter is clean the colored water is about the same height in both legs of the clear tubing when the DC is running. I say about because there's always some back pressure. As the filter clogs the difference in height of the water columns will become greater. When to clean the filter? That's a judgment call.

That's just what I did. I simply want to monitor the dust loading in the filter tubes so I know when to give them a whack.

[Eric Arnsdorff]

This may be TMI but thought I’d comment about the uses of pitot tubes and pressure measurements in the context of the filter loading indicator.
There are “2” pressure measurements at each measurement point. One is the total dynamic pressure (used for determining flow velocities and rates) and the other is the static pressure (the measurement of interest for filter loading).

I put the number 2 in parenthesis because the total dynamic pressure is really a profile of many measurements across the flow stream geometry. This value can be averaged (hence the term averaging pitot tube) to obtain the total dynamic measurement. This value minus the static pressure gives you a reading you can determine the average flow stream velocity. This could be used for dust collector flow but likely could be problematic due to the clogging of ports and such. Additionally a true profile measurement requires a lot of effort (there are more details but likely not of interest).
The static measurement is a measurement perpendicular to the flow stream. This is the one you want to measure for filter back pressure. As the filter gets loaded the static pressure work rise. Please note it is perpendicular meaning don’t put a tube or port facing the flow stream. Just put it in the side wall of the pipe and not projected into the pipe. There are inaccuracies that can happen from how you put the port in. The smaller the better with minimal to no intrusions into the flow stream. However, likely any easily installed port will be adequate for relative pressure changes due to filter clogging. A pitot tube has holes for this measurement but a pitot tube is not required for this measurement. It is true that pitot will make multiple static pressure readings giving a more accurate reading but this level of accuracy isn’t needed and likely more of a maintenance issue than useful. I would not recommend a pitot tube for this measurement.

Hopefully, that helps to understand the pitot tube and lack of a need for such a device in this measurement.

In terms of filter loading you could use the single sided static pressure before the filter one side of your manometer in the pipe prior to the filter and the other leg open to atmosphere (or permeable filter material). This measurement would give a measurement related to the atmospheric pressure. As long as the exit side of the flow doesn’t change or get obstructed then this will likely be a usable measurement.
To improve the accuracy and account for any flow/pressure changes on the exit side downstream of the filter you can connect the other leg of the manometer to the exit of your filter. This is a differential pressure measurement. If your filter is at atmospheric pressure then no need to do this.
Therefore, a magnahelic type gage can be used and you can use differential measurements or gauge pressure (single sided) type measurements. It all depends on your setup and what you're trying to do. I suspect the typical dust collector filter measurement can be made with a simple water manomoter connected to the flow before the filter. Pressure gauges capable of small pressure measurements can also be used.

[Eric Arnsdorff]

One more comment concerning the location. Ideally you would have proper flow development leading to the measurement point and no disturbances after the measurement point. A rule of thumb is no direction or flow disturbance 10 pipe diameters (hydraulic diameter for non-round) prior to the measurement point t and 5 pipe diameters past the measurement point.
In the end you’re just looking to get a relative measurement and much less than these ideal conditions needed. Try to use these distances as a rough estimate but not at the expense/effort of changing things drastically.

Note: the number of pipe diameters vary by measurement type and to be technically accurate you’d need to follow the instruments requirements. I just gave a rough rule of thumb.