Thread: BobL's Dusty setup

Originally Posted by Superbunny

BobL
You could have said here are the changes I made over night just to suit you, hope you like the way I covered up the ducting.

SB

Very good!

Great work Bob

The shed is definitely starting to look "lived in" but still plenty of room ... compared to the old shed

I didn't know there were so many fittings for 150mm ... pipeworks ... got that one!

I think I'll have to talk soon as just yesterday I picked up a couple of lengths of 150mm stormwater pvc to do exactly this ... oh and a 2HP dusty that I scored from an auction last week that's about to be converted into a (external) cyclone. You might be able to help me on that one too.

Talk soon
Ramps

Originally Posted by Ramps

Great work Bob

Cheers Ramps

The shed is definitely starting to look "lived in" but still plenty of room ... compared to the old shed

I didn't know there were so many fittings for 150mm ... pipeworks ... got that one!

I think I'll have to talk soon as just yesterday I picked up a couple of lengths of 150mm stormwater pvc to do exactly this ... oh and a 2HP dusty that I scored from an auction last week that's about to be converted into a (external) cyclone. You might be able to help me on that one too.

Talk soon
Ramps

Fire away!

I might chime in with a few qestions myself , others may be wondering the same thing/s...

A bit of preamble first tho....
If when we are measuring air flow in a duct using a pitot tube we are measuring two pressures.... velocity pressure (Pv) and static pressure (Ps)
Pv is as a result of the velocity of the air travelling in the pipe and is measured with the inlet to the pitot tube pointing direct into the air stream
Ps is from the restriction to air movement created by the fan...small pipe, lots of bends, long run, blocked pipe etc. etc. and is measured at 90° to air flow

The question/s are...
Is there a difference of Pv values if we measure on the suction side of the fan when compared to the exhaust side?
Is the Pv higher by the amount of Ps if we measured on the exhaust side?
and
Is Pv lower by Ps on the suction side?
My gut feeling is yes on both accounts but I could be wrong

Would I get acceptable Pv results if I simply pointed a piece of 1/4' or such sized steel/brass tube into the airstream?
or
Is the entrance into the Pv port something special fit for the purpose? Sized for the proposed expected air speed maybe?

The one I have used a long time ago did have a small hole at the inlet, maybe 1 to 1.5mm, this possibly dampens the fluctuations to give a steady reading in the manometer but Is there more to this? It was also bullet nose shaped at the inlet.

I read on Bill's site that the distances from the bend, Ps port and the Pv inlet in relation to each other (for a combined tube) gives the pipe a C value that is included in the calcs but if we are measuring with separate tubes as in BobL's pic above do we need to consider the C value?
I wonder if a few pics of the end of your pitot tube might be possible Bob?

Intergrating...mmmm it's been far too long since I have done any of that...
What was the reasoning behind needing to do that? Bob/Graeme has said that from an accuracy standpoint that a closer to a real value is obtained but in my head it's only mud mmm thinking thinking maybe something to do with straightening the curve to give a linear result aarrghhh to long ago I can't remember
That's probably enough for the mo.....


Pete

Originally Posted by pjt

The question/s are...
Is there a difference of Pv values if we measure on the suction side of the fan when compared to the exhaust side?
Is the Pv higher by the amount of Ps if we measured on the exhaust side?
and
Is Pv lower by Ps on the suction side?
My gut feeling is yes on both accounts but I could be wrong

Provided there are no leaks the answer should be yes.
The Pitot tube measures a pressure differential between "front on" and "side on" points in an airflow - it does not measure these relative to atmosphere so it does not matter if it measure it in the up or downstream of the fan section of the flow.

Would I get acceptable Pv results if I simply pointed a piece of 1/4' or such sized steel/brass tube into the airstream?
or
Is the entrance into the Pv port something special fit for the purpose? Sized for the proposed expected air speed maybe?

My understanding is that Pitot tubes ports are sized and streamlined to suit a range of air speeds. A 1/4" port is way too big, all the ones I have seen are generally around a mm in diameter

I read on Bill's site that the distances from the bend, Ps port and the Pv inlet in relation to each other (for a combined tube) gives the pipe a C value that is included in the calcs but if we are measuring with separate tubes as in BobL's pic above do we need to consider the C value?

I would say so.
My pitot tube was made for use in 200 or greater mm diam pipes and above so it is really too big for accurate measurement of 4" pipe and borderline for 6" pipe. However it agrees well with 2 hot wire anemometers (HWA) up to about 30 m/s which is all the HWAs go up to.

I wonder if a few pics of the end of your pitot tube might be possible Bob?

Happy to oblige.

Intergrating...mmmm it's been far too long since I have done any of that...
What was the reasoning behind needing to do that? Bob/Graeme has said that from an accuracy standpoint that a closer to a real value is obtained but in my head it's only mud mmm thinking thinking maybe something to do with straightening the curve to give a linear result aarrghhh to long ago I can't remember
That's probably enough for the mo.....

That is effectively what we are doing. We are effectively working out a cumulative average air speed across the tube to work out a final flowrate for the pipes.

Originally Posted by pjt

I wonder if a few pics of the end of your pitot tube might be possible Bob?

Here it is.



The front hole (Fp) has a diam of 2.0 mm (measured with set of fine drills) and there are 5 equidistant spaced holes on the side (Sp) with a diam of 1.0 mm.

If you can see it the side holes feed through to an outer tube surrounding an inner tube which connects through to the font hole. The pressure read is the difference between these two pressures.

I guess you know you can purchase quality calibrated pitot tubes on line for not too outrageous a price?

One way of getting cheap ducting and fittings is to make your own.

After consulting with Bill Pentz, who advised me that square ducting is more than OK, I made my own with material I had to hand, mostly out of found materials and some leftovers.

FWIW (not offering any advice on optimum sizes, ratios or layout), here are some samples of what I did. Minimum inlet diameter is 6".

Sample run...

Attachment 198965

Sample with internal baffles (to keep the airflow evenly distributed though bend...

Attachment 198961

Splitter...

Attachment 198962

Sample of internal structure of octagon profile...

Attachment 198963

Sample of transition/reducer piece...

Attachment 198964

Originally Posted by NeilS

One way of getting cheap ducting and fittings is to make your own.

Nice work Neil. They look like they are large enough to easily do the job.

I did think about making my own ducting and fittings but I figured making my own PVC blast gates was enough "in house ducting construction" at this stage since I need to do a significant shed fitout and renovation at the same time. One reason I personally wouldn't go with MDF for ducting or fittings is I don't like doing a lot of work with it if there is another way. I lost my sense of smell for 6 weeks about 10 years ago when working with MDF so I avoid it when I can. The idea of large areas of MDF hanging over my head and continually shedding particles and chemicals into the shed leaves me cold. Painting the exterior surfaces would reduce this problem but that means more work. I also wonder about how long MDF can retain a smooth surface inside a duct or fitting. I guess that depends on how humid the environment is where you live. Once again that could be counteracted by sealing and sanding and resealing - sounds like a lot more work still. When all this is taken into account PVC fittings don't look all that badly priced.

Originally Posted by BobL

.... I also wonder about how long MDF can retain a smooth surface inside a duct or fitting.
....
Once again that could be counteracted by sealing and sanding and resealing - sounds like a lot more work still. When all this is taken into account PVC fittings don't look all that badly priced.

Yeah, I used 6" PVC ducting on the DC system at the local men's shed, which made it real easy, but they were paying for it....

Mine are half MDF and half ply. You could do it all in ply, but exy if you had to buy.

I sealed the internal surfaces of the MDF/ply before installation. At this stage, no obvious deterioration after five years of constant use.

Just shared in case it's a solution for someone else.

Originally Posted by NeilS

I sealed the internal surfaces of the MDF/ply before installation. At this stage, no obvious deterioration after five years of constant use. . .

That's good to know. Probably constant use is better than intermittent use since the moving air probably keeps it dry.