Thread: Fibreglass Cyclone Project

Originally Posted by John Samuel

Bob,

That's my take on it too. No matter how I crunch the dynamic numbers, they don't make a lot of sense in terms of flow rate, except to say the lower the pressure the higher the flow (I tried crunching using the inverse (or delta between 297mm and the dynamic measure), but that does not add up either).

So, if I want good dynamic numbers I will need to put my hand deep into my pocket.

Yep - and it's not just a matter of getting a pitot tube.

A reliable pitot tube with a calibration will cost about $60. A simple U-tube will give you reliable relative measurements but if you want some sort of accuracy then it will need to be calibrated.
A cheap manometer with claimed better than 1% accuracy can be obtained for about $40 (Digital Manometer Differential AIR Pressure Meter Gauge | eBay) this is the unit I have
Then you will need to compute the air speed from the pressure using the calibrated algorithm supplied with the pitot tube.

Air speed can be measured directly using an anemometer which come in several types, the main ones being "vane or propellor" and the "hot wire"
I do not recommend the vane types as they interfere too much with the flow. This type is not designed to operate inside a tube or duct and will significantly over estimate the air speed. I have numerous email correspondence with a member of a men,s shed who used a vane type to measure the performance of their DCs and his results were about 20 - 30% too high. I had a tricky time trying to convince him that the vane type are not suited to this task.

Quality hot wire anemometers with a calibration certificate are around $500.
I have been waiting for some time for chinese versions to become available and I just checked ebay and there is one available for ~$200 + $19 postage.
NEW TES 1340 HOT Wire Thermo Anemometer Digital Anemometer AIR Wind Flow Meter | eBay
It claims to be +/- 3% accuracy which is quite good. In most cases the limit is not the device but (just like the pitot tube) orientation into the flow and turbulence.
I am tempted to get one of these and compare it to my calibrated unit.

The typical range of speeds the hot wire units can measure is 0 - 20 m/s. Some units can measure 25 m/s and some even 30 m/s but even the 30 m/s will not be enough to measure the flow directly inside a typical DC ducting setup. Even if the air speed inside a duct is low enough to measure, the measurements are only reliable if there is straight ducting at least 5 (and preferably 10) ducting diameters either side of the test point. In most cases this means using a test pipe attached to the ducting. Seeing as a test pipe is needed then attaching a larger diam test pipe than the one being tested is possible and as the larger diameter slows the air speed down, a hot wire unit with a 0-20 m/s range is normally sufficient to measure the highest flows.
I have 3 test pipes, 4" diam to measure air speeds in 2" and smaller ducting, 6" to measure between 2 and 4" ducting, and 9" to measure 6" ducting.
The 4" test pipe should be at least 4" x (5 +5 ) or 40" long, the 6" should 60" long and the 9" should be 90" long.
The 9" ducting test pipe I have is only about 60" long but I got it for free and that stuff is fairly expensive.

Wait there is more.

There are hot wire units that claim to measure CFM directly but this is bollocks because the air speed varies across ducting.

So, then the air speed needs to be measured systematically across the air stream and a integrated sum of representative cylindrical volume flows determined. Its a right PITA but spreadsheets help.
Whatever method is used to measured the air speed sometimes you will find the pressures (or air speeds) will fluctuate significantly and you will need to manage this by repeated measurements and averaging of some kind.

See - easy huh!

Yes,

Saw that Chinese unit this morning. It measures to 30M/sec, or 6,000 FPM, which should do the job when I have a machine hooked up. If it won't, could I measure CFM in my 8 inch exhaust duct? Seems that air out ought to equal air in, and an 8 inch duct is nearly twice the cross section of a six inch duct. I could measure about four feet from the nearest bend, within a couple of feet of the outlet.

Thoughts?

Originally Posted by John Samuel

Yes,

Saw that Chinese unit this morning. It measures to 30M/sec, or 6,000 FPM, which should do the job when I have a machine hooked up. If it won't, could I measure CFM in my 8 inch exhaust duct? Seems that air out ought to equal air in, and an 8 inch duct is nearly twice the cross section of a six inch duct. I could measure about four feet from the nearest bend, within a couple of feet of the outlet.

Thoughts?

Borderline, but should be OK especially as it is an outlet.

Thought I’d add an update as the forum seems to be a bit slow lately. Have just built a 6” outlet for my jointer using my slowly developing fibreglassing skills.

Although not strictly required, I went a bit OTT and took the opportunity to build this using a proper split mold which involved more work but has resulted in a much better finish.

On my jointer the dust chute runs from under the cutter head to the original 4” outlet at the base of the cabinet. This made connection of the new outlet simply a matter of cutting a larger opening in the cabinet and bolting the new outlet over this.

Photos show a couple of views of the new outlet installed on my jointer with another showing the MDF plug, the red fiberglass split mold and the finished article (after trimming off the rough edges).
WP_000068.jpgWP_000065.jpgWP_000064.jpgWP_000063.jpg

Very professional looking.

Do you still have the molds? you could probably sell a couple although shipping would be a bit of a killer.

Have you done anything to allow the jointer to breathe more easily - most a well and truly choked.

Originally Posted by BobL

Very professional looking.

Do you still have the molds? you could probably sell a couple although shipping would be a bit of a killer.

Have you done anything to allow the jointer to breathe more easily - most a well and truly choked.

Bob,

The jointer cabinet has a couple of pressed louvre panels and the whole machine is lifted off the ground by the mobile base so there’s plenty of air drawn into the machine. Checking with my cobbled together manometer, the whole setup adds a bit under 3/4" of static pressure to my system which I think is not too bad a result. I think my previous 4" connection increased static pressure by a couple of inches.

One of my reasons for making a decent mold was the possibility of being able to make some more of these if there is any interest. The finished weight is around 1kg so the postage wouldn’t be too bad.

Hi Muchacho,

I wish I had your fibreglassing skills.

Cheers

Doug

Are you at all able to reproduce the actual cyclone unit from a mold lets say?

I'm surprised I hadn't asked this question before.

The finished product looks so superior to some others.

A real credit to you.

Originally Posted by Timless Timber

Are you at all able to reproduce the actual cyclone unit from a mold lets say?

I'm surprised I hadn't asked this question before.

The finished product looks so superior to some others.

A real credit to you.

Thanks for for your kind feedback. The cyclone is made using a couple of basic timber forms to produce the main shell and the round to rectangular inlet transition. The top was formed as a flat ring and then twisted to the required spiral shape. All the pieces were then bonded together to get the final shape. It is a rather cumbersome way of forming the cyclone but it saved a heap of work producing a proper multi piece mould for what was possibly a single use.

Time for an update on my ongoing cyclone project. I had always planned some airflow testing, but wanted to first square away a minor modification to try and squeeze the last few cfm’s out of my marginally powered system.

My 12 inch fan had previously been modified to have an 8 inch intake to suit my cyclone but I’d always considered the 5 inch outlet to be the archilles hell so I built a new 6 inch outlet for the fan housing.

As per the attached photos, I shaped up a sacrificial male former right on top of the existing casing and formed a 2 piece shell over this. Next, I took to the casing with an angle grinder and cut off the offending casing and former and then bonded the new shell back over the remaining section of casing.

The result was not as dramatic as thought it could have been, but managed to drop the static pressure through my machines to around 3 inches which I was happy with.




Now the interesting part. I hired a hot wire anemometer and knocked up a test pipe with a mount for the probe and a tapping for a simple water manometer and set to work. The set up is shown on the photo below. It also shows multiple layers of shade cloth held over the inlet with a hose clip which I found to be the easiest way of changing the static pressure. I tried out a few other methods to restrict the flow but they all caused too much turbulence in the pipe with fluctuating airspeed measurements. I started with about 16 layers to give me the first data point and unfolded a couple of layers at a time to give nice spread of readings.

The final attachment shows the completed fan curve. As mentioned above I have about 3 inches of static pressure when extracting through my thicknesser and planer which corresponds to about 600 cfm.

So overall I’m pretty happy with that has been achieved. I was hoping to have another 100 cfm or so, but I’ve well and truly satisfied my main objectives which were to rid my workspace of the health damaging filter bag extractor, free up valuable floor space and keep within tight financial and power supply constraints.

Nice job with the fibreglass work.

Good idea with the shade cloth to control the flow - I'll have to use that next time

On re-reading the whole thread I'm now wondering if your 8" inlet may in fact be too big and not enable the impeller to generate the ~8.5" of WC pressure that these 12" impellers can usually generate?

One thing is not clear to me - is the 6.5" of WC SP that you see with or without the cyclone?

Bob, Thanks for your response.

All of my readings were taken with the fan drawing through the cyclone and exhausting outside through a 600 x 400 louvre in my shed wall. This setup causes about 1” of static pressure which is corresponds with the highest flow readings on my chart.

I’m not sure about whether my 8” inlet is responsible for my apparently low max pressure reading. I never pay much attention to this parameter because I have no need to run the fan with a stalled airflow. In my opinion the important part of the fan curve is the flow readings in the zone relating to static pressures of typical workshop setups.

I think the main reason for differing fan performance comes down to the particular impeller and casing details. I have a pressed metal casing with semi-circular cross section with a six bladed 12” x 2 7/8” impellor. I note in your “The Generic 2HP DC” thread you have a square edged casing with 7 bladed impellor. Theoretically the extra blade is doing more work on the airstream and is possibly the cause of your improved performance.

In your thread you report initial testing of about 600 cfm with the fan only running through a 4.5” inlet (and I believe the stock 5” outlet ?) and drawing 5 Amps. After opening up inlet and outlets, you report an improvement to around 1100 cfm.

The maximum airflow I measured was about 800 cfm with cyclone and louvre attached at 1” SP running though an 8” inlet and 6” outlet drawing around 4.2 Amps. If I had disconnected the cyclone I estimate the SP would reduce to around 0.5” and extrapolating my chart would give about 1000cfm.

One could conclude from this that after our respective modifications, the two fans are infact very similar in top end performance, and my 8” inlet modification is probably not detrimental.

Reading your informative thread further, there’s a post describing testing your modified dusty, now with its bag attached at 840 cfm. This compares very close to my highest readings.

The point I’m trying to make?? I think that this shows that a 2HP cyclone can be made to perform as well as a modified bagged dust extractor. I’d go as far as saying if the cyclone is exhausted outside, it will perform better than a dusty over time as there are no bags to choke.