Thread: DE Measurements

Originally Posted by Bauldy

Question 1: Can I use this method to give me a useful indication of any relative improvement, and if so, is there a particular methodology I should follow?

No they will not even give useful relative measurements .
Do not base any decisions on these instruments unless the test duct is some 10 times bigger in area
Read this post on air flow measurements DRAFT: FAQ - Dust Extraction (Practical Aspects)

I have read in these threads a bit about Static Pressure, which appears to be relatively simple to measure. I gather from comments that it is not an indication of air flow, but I would have thought it could give me some before and after conversion information. It also seems like an easy way to measure at different points along the ductwork, particularly if I have a mixture of 4" and 6" lines as comparison.

Relative measurements that make sense are possible and worth doing. Interpretation can be tricky but at least you will have an accurate gauge of if there is "more" or "less" flow.

Thanks Bob.

Bauldy

I have a florite you could you borrow...reads 0-3000 feet per min...need to do conversions to litres per sec or M3/hr etc etc...but thats easy if you can count from 1-10.

it was all we ever used back in the old days for air balancing... before $4K plus instruments came in.
wont be as accurate as my TSI Flowhood/Pitot and or my annie but it will be very close.

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Thanks Eskimo, Will make contact and discuss further.

Bauldy

Sorry you can use the fluorite to measure flow in 4 or 6" ducting as it restricts the flow.
It won't even give accurate relative measurements.

Thanks Bob. Suspected as much, but was going to research a bit more first.

Regards

Bauldy

Originally Posted by BobL

Sorry you can use the fluorite to measure flow in 4 or 6" ducting as it restricts the flow.
It won't even give accurate relative measurements.

It wouldnt be any different to using a cheap annemometer...to get more accurate figures a cone could be fabricated using cardboard and fitted to the end of the pipe with diffuser plate...but then is it correct. Without certified equipment one would never know.

I took it that Bauldy was only looking for indicative figures to see if there was any increase after mods were carried out?

What goes in, must must come out.

With the amount of air that they run with how accurate does one need/want to be.

Originally Posted by eskimo

It wouldnt be any different to using a cheap annemometer...to get more accurate figures a cone could be fabricated using cardboard and fitted to the end of the pipe with diffuser plate...but then is it correct. Without certified equipment one would never know.

I took it that Bauldy was only looking for indicative figures to see if there was any increase after mods were carried out?

What goes in, must must come out.

With the amount of air that they run with how accurate does one need/want to be.

I agree in the open air or large (>couple of square ft) ducting a cheap hot wire anemometer would be no better than those units

However, inside narrow duct work is another issue.
I tried making sense of those prop driven units in small duct work for about 12 months (that's all we had in the 1970's) and stopped using them once we got the hot wire anemometers.

Whether absolute or relative measurements are being made the flow must be measured inside the ducting otherwise the vagaries of entrance turbulence stuffs up the flow paths.
The problem is the props are so large they restrict and disturb the flow when placed into a small duct.

The ridiculous thing is that even though they restrict the flow they register as higher flow because the DC fan tries to pull as much air as it can at higher speed through the throttled space generated by the prop housing and vanes.

Here is the problem with relative measures.
Yes, if drastic changes are made (i.e. go from 100 to 150 mm ducting) then a relative change can easily be measured with prop based measuring gear.
However, these are a no brainer and don't need testing.
What most folks want to test are changes that will produce only small changes in flow (this ducting connection or that connection? this hood or this one? etc), but unfortunately they can produce major changes in the distribution of the flow inside the ducting, which is usually far from even.
It's possible for a change to produce a lower flow but now directed at the prop vanes so the prop will register as a high flow rate. Making changes or selections on this basis turns change selection into a lottery and can lead to a poorer performing system.

Hot wire anemometers at least allow a check for the extent of turbulence across a duct.

In practice it can be just as difficult to detect small changes to ducting and machine connections with a hot wire anemometer as it is with a prop based anemometer unless careful and repeated measurements are made. Folks are dismayed when I tell them it can take as much as an hour to, for example, accurately test the difference between two hoods. This is why I don't offer it as a free service.

The prop based units can be used accurately if they are used inside test ducts that are around 10 times the diameter of prop based units and the ducting needs to be long enough to enable the flow to stabilise - this is how we used them in the 1970's. Sounds good until one realises the prop was 75 mm in diameter so the test duct had to have a cross section of about 750 x 750 mm and at least a couple of m long and this had to be somehow shoe-horned into an existing ducting system.

Thanks Eskimo and Bob. Every piece of information is useful and helps with the understanding.

Regards

Bauldy

Since you are only interested if there is an improvement wouldn't a measure of the power consumption of the fan motor give you that info. The higher the flow the larger the load on the motor so if the motors current reading falls you have made an improvement. You would also need to check the supply voltage at the same time as this fluctuates and will affect the current reading.

Originally Posted by Bohdan

Since you are only interested if there is an improvement wouldn't a measure of the power consumption of the fan motor give you that info. The higher the flow the larger the load on the motor so if the motors current reading falls you have made an improvement. You would also need to check the supply voltage at the same time as this fluctuates and will affect the current reading.

no not neccessarily...static pressure can increase with little and in some cases no increase in airflow.
Conversely, a small amperage increase can result in a substantial increase in airflow
It all depends on where all criteria are/is on the fan curve.

Originally Posted by Bohdan

Since you are only interested if there is an improvement wouldn't a measure of the power consumption of the fan motor give you that info. The higher the flow the larger the load on the motor so if the motors current reading falls you have made an improvement. You would also need to check the supply voltage at the same time as this fluctuates and will affect the current reading.

The greater the flow, the more current is drawn by the motor, but the effect is non linear and the range of changes in current is small. Eg on my 3hp system the current draw on fully open is 9.4A and fully closed it's 7.7A. So the range is 1.7A with a resolution of 0.1A BUT STRICTLY SPEAKING the error of reading the initial current and final current have to be added so the measurement res is more like 0.2A so +/- 12%!

Many changes in ducting & junctions and dust collection ports are much smaller than this so result can easily be misinterpreted and changes made that make things worse.

Using an ammeter with more resolution will help but even though they display higher res doesn't always mean the last digit means very much.

Originally Posted by eskimo

no not neccessarily...static pressure can increase with little and in some cases no increase in airflow.
Conversely, a small amperage increase can result in a substantial increase in airflow
It all depends on where all criteria are/is on the fan curve.

For a given fan wouldn't an increase in static pressure result a decrease in airflow volume.

Originally Posted by Bohdan

For a given fan wouldn't an increase in static pressure result a decrease in airflow volume.

Flow is best determined using the difference between static and dynamic press, static press alone does not provide a complete picture.
The other import variable is fan efficiency. A fan may draw a greater/lesser current and/ or generate lower/higher static press but have poor efficiency at that point in its fan curve so it may not move as much air.