Pitot Tube Location for Filter Monitoring

[Kris Cook]

I am sure this has been asked and answered but couldn't find what I needed. I have a pitot tube, and magnehelic guage I plan to install in the airflow of my DC. I have a GO440 2 HP collector which I bought new 2 years ago when they were closing them out (that is how long it is taking me to get to this point on my new shop :rolleyes:).

Anyway, most of the posts I have read have to do with airflow measurements. I am not really interested in that, just want to establish a baseline with clean filters and monitor the pressure change to get a correlation on a dirty filter.

Wondering where the best location would be to install the pitot tube.

Thanks

[Larry Frank]

I do not use a pitot tube just a hose into the area just before the filter. I do not believe a pitot tube is needed.

[Rob Luter]

I agree with Larry. All you need is a pressure tap on either side of the filter. Pitot tubes are needed to measure velocity pressure. All you need to measure is static pressure. Dywer Instruments has static pressure tips (Link (https://www.dwyer-inst.com/Product/Miscellaneous/Accessories/StaticPressureSensors/StaticPressureTips-Accessories)) available. I've used them in the past in conjunction with dirty filter indicators (I used to be in the industrial HVAC business).

https://www.dwyer-inst.com/images/addtl/A_301_SS_1000x1000.gif

[George Yetka]

A magnahelic wont work for you as one side of the filter is the atmosphere. Rob has it right

[Curt Harms]

I do not use a pitot tube just a hose into the area just before the filter. I do not believe a pitot tube is needed.

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.

[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.

[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.

[Rob Luter]

A magnahelic wont work for you as one side of the filter is the atmosphere. Rob has it right

A Magnehelic will work. They'll measure differential pressure. That said, it's a pricy approach. I'd use a manometer or a Minihelic. I used to use pressure switches too, so I could turn on an indicator light at a specific pressure drop.

[Kris Cook]

OK. Thanks everyone. Eric your information was particularly helpful, although I thought my head was going to start hurting when I started reading it. Ended up with no pain and I will re-read before I put any holes in anything.

I already have the gauge so will install it. Based on Eric and others' input I probably won't install the pitot tube.

Thanks Again!

[Kris Cook]

I agree with Larry. All you need is a pressure tap on either side of the filter. Pitot tubes are needed to measure velocity pressure. All you need to measure is static pressure. Dywer Instruments has static pressure tips (Link (https://www.dwyer-inst.com/Product/Miscellaneous/Accessories/StaticPressureSensors/StaticPressureTips-Accessories)) available. I've used them in the past in conjunction with dirty filter indicators (I used to be in the industrial HVAC business).

The below pic is the tube I have. I was mistakenly calling it a pitot tube (apparently two holes vs. one hole). I may not use it as I mentioned.

463626

The rest of this stuff is what I ordered when I was planning my DC system - pressure differential, and dust bin sensor stuff. Can't have too many gizmos.

463627

[Bill Dufour]

For you location does not matter much. You do not really care about pressure levels and fan curves. just measure from the same location and watch the needle move as the filter gets dust caked. When it has dropped too much clean the filter. You get to decide how much is too much.
Bill D

[Larry Frank]

When you install a gauge to measure filter pressure, try doing it with various DC flows. As the flow goes up, the filter pressure will go up. I checked the pressure with a 2.5, 4, and6 inch blast gates open and wrote the numbers on a card near the filter for reference.

[Kris Cook]

Thanks to everyone for the input on this. I got the system up and running.

I tapped the plenum between the blower housing and the filter tube, and installed a 1/8" barbed fitting. The fitting protrudes into the flow about 1/2"

My duct runs are 6" spiral pipe. I have two trunk lines that are about the same length. My drops are 4" spiral pipe to the blast gates.

With one 4" gate open on either trunk line I am getting 0.3" WC.

Question - Is there any "rule of thumb" out there for what I should be looking for as regards the change in flow with a dirty filter? Full Scale Deflection is 5" WC. Gauge resolution isn't ideal (Ideally FSD would be 1" or less) but hoping this gauge will work.

[Thomas Wilson]

One thing to be aware of with measuring the pressure drop across the filter is that the resistance in the rest of the flow network affects flow and thus the measurement. You want to take a reading at some reference condition most likely with all blast gates open. This condition gives the greatest flow and greatest pressure drop across the filter. You take a reading with filter clean (new is good). This should give a reading in the 1-2 inches of H2O range. Keep that reading on file (ie, write it on the filter). You repeat the measurement at the same condition over time as the filter loads up. You are only concerned with how much it increases. If the reading increases by an inch of water or so, the filter loading is pretty high compared to the rest of the system pressure losses and you would gain some system flow by cleaning the filter. If this does not answer your question, let me know. I can talk endlessly on the topic of measurement.

I do not plan to have my pressure probe permanently installed in my new dust collector. If I decide I need to measure filter pressure drop, I will drill a hole in the pan at the bottom of the filter. I will seal around the tube with modeling clay for the measurement. I will seal the hole with tape from the inside when not measuring.

[David L Morse]

I tapped the plenum between the blower housing and the filter tube, and installed a 1/8" barbed fitting. The fitting protrudes into the flow about 1/2"
That 1/2" could be a problem. ASHRAE spec requires that it be flush with the wall. IME about an 1/8" protrusion caused around -0.2" wg error at decent flow rates. BTW, that's not a plenum; it's a transition.

Question - Is there any "rule of thumb" out there for what I should be looking for as regards the change in flow with a dirty filter?
Yes, but it's probably not very useful. By the time there's a noticeable change in flow the filter will be a huge pain to clean. Filter pressure is a much more sensitive indicator of filter loading.
fwiw, relative flow is proportional to the squareroot of one minus the ratio of the change in pressure to the total internal fan pressure. So let's say for a give system configuration your filter pressure changes from .3" to 3.3". IRRC, that DC has an internal pressure of about 15" so 1-3/15 = .8 and the squareroot of .8 is about .9, so about a 10% reduction for a 3" increase. You would not likely notice a 10% change in flow when it happens gradually over time.

EDIT: Just saw Thomas Wilson's post. He has a much better location for the pressure port. That, in fact, is the way I do it, so it must be right:D

[Kris Cook]

One thing to be aware of with measuring the pressure drop across the filter is that the resistance in the rest of the flow network affects flow and thus the measurement. You want to take a reading at some reference condition most likely with all blast gates open. This condition gives the greatest flow and greatest pressure drop across the filter. You take a reading with filter clean (new is good). This should give a reading in the 1-2 inches of H2O range. Keep that reading on file (ie, write it on the filter). You repeat the measurement at the same condition over time as the filter loads up. You are only concerned with how much it increases. If the reading increases by an inch of water or so, the filter loading is pretty high compared to the rest of the system pressure losses and you would gain some system flow by cleaning the filter. If this does not answer your question, let me know. I can talk endlessly on the topic of measurement.

I think this does answer my question. Thank you. I was going to use the single gate open as my reference condition but opening more gates makes sense. I will give that a try.

[Kris Cook]

That 1/2" could be a problem. ASHRAE spec requires that it be flush with the wall. IME about an 1/8" protrusion caused around -0.2" wg error at decent flow rates.

Wondering how that matters is we are talking about a relative change in pressure...

Edit - I backed the fitting out as far as I could and still maintain a seal. Turns out it does matter. The reading went up exactly 0.2". I will come up with a saddle or block to thread through and try to maintain the fitting flush with the wall.

Thank you.