Thread: Fletty's new shed WIP ....or is it a retrospective??

What is confusing about 766 & 774? I have to ask otherwise we will never know.

Lappa - VFDs are a huge topic.
To get all the relevant info about VFDs into one place would be like trying to do the same with dust extraction.
Some blokes from a mens shed asked me to just write it all down for them.
I told them a) I was not qualified and b) I dd not have a couple of months to spare.
It's probably better for you to start a new thread in the electronics forum and ask specific question that you need answered.

Originally Posted by fletty

Ive just got the grandurchins to bed after reading many many stories. I've read nursery rhymes, fireman Sam, Peter Rabbit, Noah's Ark..

I highly recommend "the Wind in the Willows" (Graeme Greene) as a suitable soporific read to encourage the grand urchins to sleep

With all the discussion about VFD's going on here I thought I'd ask what would the maximum speed a 16' impeller can be SAFELY driven in a CVMax? I'm ordering a Max and was wondering how much extra would be safe for big jobs like sucking dust from sanding a big bowl or a downdraft table. I vaguely recall reading somewhere on these forums of 70 cycle/hertz being used.

Thanks
Pete

P.S. CV said they don't know of anyone doing it, so couldn't advise.

Originally Posted by QC Inspector

With all the discussion about VFD's going on here I thought I'd ask what would the maximum speed a 16' impeller can be SAFELY driven in a CVMax? I'm ordering a Max and was wondering how much extra would be safe for big jobs like sucking dust from sanding a big bowl or a downdraft table. I vaguely recall reading somewhere on these forums of 70 cycle/hertz being used. .

Safety would depend on how well the impeller was made / balanced.

But even if the impeller was safe the motor might not be.

70Hz represents a 1.4x increase in speed over 50Hz and in theory a 40% increase in flow, but the increased power required would be (~1.4)^2=1.96, or 96% more power than usual.
So if the current is around 9A at 50Hz then at 70Hz the current would be nearly double that or 18A.
This would cook a 3 or 4HP and most 5HP motors.

The movement of 40% more air assumes the rest of the DC system will actually cope with or allow the extra air to be moved though it. In practice what happens is the system rapidly builds up back pressure because the ducting and air passages though the impeller are not large enough limiting the amount of air moved and this in turn may limits the extent of current increase. The system thus cavities way more than it should wasting power for marginal increases in flow.

So what about strapping on a 7.5HP motor to the impeller?? Well it turns out that it is more efficient to use the 7.5HP to drive a bigger impeller back at 50/60Hz than to use a small impeller and drive it faster. It's all about machine the impeller size to the motor.

The next thing is the noise - even a few Hz increase can really ramp the noise up.

A safe way to get more flow in a system is to be more attentive about flow losses. This is where ducting size and layoutdesign, machine port sizes and cabinet flow losses and things like Bell mouth hoods really make a difference. On a down draft table the overall design, especial the hole sizes and shapes will make a much bigger difference to the ultimate dust removal than what you can do at the DC end of things.

I have some experience with down draft tables as we used them everywhere in open clean labs at unis where squeeze bottles of reagents like acids needed to be left out in the lab for ongoing use rather than putting them away every time. What happens with changes in atmospheric air pressure is that squeeze bottles pump out small amounts of acid vapour out and this needed to be captured. We experimented with a range of hole sizes in the high density Polyethylene sheet bench tops and found that making the holes as big as possible really helped with the flow. This of course impacts on the usable surface area left to work on but I reckon smaller bell mouth shaped holes on both sides would allow smaller hole to be used more efficiently. Acid vapour is denser than air so as it left the bottle it would fall onto the table.

The problem with wood dust is its generation increases the temperature of the dust and a substantial amount of fine dust can easily escape upwards away from a down draft table. If you want to capture fine dust generated by sanding larger objects, then a fume hood or spray booth style setup with a downdraft table to catch the fines and a canopy to grab the rising superfines might be the way to go. In that case I wouldn't bother to use a DC to vent the canopy, I would just use a high flow squirrel cage fan (lots of those available in NA) and vent the stuff directly outside. Might not work so well in your winter.

Originally Posted by QC Inspector

With all the discussion about VFD's going on here I thought I'd ask what would the maximum speed a 16' impeller can be SAFELY driven in a CVMax? I'm ordering a Max and was wondering how much extra would be safe for big jobs like sucking dust from sanding a big bowl or a downdraft table. I vaguely recall reading somewhere on these forums of 70 cycle/hertz being used.

Thanks
Pete

P.S. CV said they don't know of anyone doing it, so couldn't advise.

That statement was written by me after I had telephone discussions with Bill Pentz and I raised it with him. His view was above 70hz a material collision with the impeller could not be contained with any certainty within the fan housing. Since then I have seen a material collision at Corrimal Men's Shed and the results and a full report can be seen on their website. It is the only incident I am aware of but others could have occurred and not been reported of course.

Thanks Bob and Chris.

I was only thinking of "boosting" the rpm to at most 3800 rpm to 4000 rpm from the 3460 we get on our 60 cycle power and only if it was safe to do so. If the impeller isn't safe to do that with I'll limit the VFD to 60 cycle and be very content. That's why I asked.

Bob experience tells me that dust drawn up past my face to be a bad thing in general. If the dust produced can't be drawn down then having it pulled away to a backdraft pickup across the sanding table is the next best. Unfortunately having -20C for 3 or more months with periods down to -30C and -40C rule out any venting outside. When the weather is warm enough then sanding can take place outside in the almost constant prairie wind. At least until the mosquitoes carry me away.

Pete

Sanding is a real problem as is the dust escape from wood turning, here is a real life experience I was involved in. I had a customer ring me prior to starting a business making chairs on a commercial basis and he was concerned about the amount of sanding that would be done and the dust problem. His idea was to use a CV Max and build a dedicated sanding room that exhausted at the top and make up air was introduced at the bottom to feed the cyclone. He was lucky as his father was a HVAC engineer, understood the problems and designed the whole thing from scratch.

I have never seen how a DD table can be anything like effective to be honest as there are simply too many problems surrounding them and common sense tells me that covering the holes in the table stops it working and that is only the first thing that is wrong with the concept. I reckon a simple sanding room could be made for under a $100 using some electrical conduit for a frame and plastic sheeting or what ever was cheap as the walls. If a lathe was inside that the operator would be better off as well. You need the air to travel past the operator over the lathe to be exhausted behind and above him or her and be able to feel the draught of the air movement.

Originally Posted by QC Inspector

Bob experience tells me that dust drawn up past my face to be a bad thing in general. If the dust produced can't be drawn down then having it pulled away to a backdraft pickup across the sanding table is the next best. Unfortunately having -20C for 3 or more months with periods down to -30C and -40C rule out any venting outside. When the weather is warm enough then sanding can take place outside in the almost constant prairie wind. At least until the mosquitoes carry me away.

Sure I can appreciate these problems.
We have a similar but opposite temperature problem in summer.

I wish I had kept the results from the experiments we did on downdraft tables in the 1980's. I seem to recall flow rates were in the few hundred CFM range for a 600 x 450 mm table and we had around 4-5 of these in each lab. This was more than enough for the small amount of heavier than air acid vapour being generated. Like CP what I fear is that this low flow rate and resulting air speeds might be too slow so that warm dust may escape even a back draft collection before capture.

If the collection though a back draft table is say 200 CFM (5.7 cubic metres), across a 1 m cubed volume this represents an air speed of 5.7 m/min or about 10 cm/s. Warm fine dust will have a vertical velocity of about the same ball park so if the back draft collection is used about half the dust generated will escape. A hood over and walls around the table to prevent the fine dust escaping and collection slots at the top of the back would capture all the fines. Along with collection slots to collect the heavier than air fumes, this is typically of what fume hoods use. And this is effectively the same as what CP is suggesting, i.e. a small vented l room to do the sanding in.

My whole shed is a bit like this - there are 2 squirrel cage fans that can draw up to 2800 CFM then the 1000 CFM from the DC makes 3800 CFM of total forced ventilation.
My shed has a volume of ~3800 cuFt, so that is one shed air change per minute.
I cross checked this against the (0.75 m/s) predicted air speed though the door of the shed and the measured speed was 0.72 m/s)

For spindle work on a lathe a Bell Mouth hood and a good airflow will easily grab the fine dust.

on sanding ...

at Tech we had an arrangement similar to a spray booth.
a powerful fan -- something like a 5' diameter aircraft propeller was mounted behind a filter blanket and maintained an air flow of around 5 m/s across the sanding area. The fan was part of an industrial dc that exhausted through a set of filters the size of 3 semi-trailers.

Originally Posted by ian

on sanding ...

at Tech we had an arrangement similar to a spray booth.
a powerful fan -- something like a 5' diameter aircraft propeller was mounted behind a filter blanket and maintained an air flow of around 5 m/s across the sanding area. The fan was part of an industrial dc that exhausted through a set of filters the size of 3 semi-trailers.

That should work given it's around 19000 CFM - must have been a bit noisy?

only a little noisy -- it was definitely a PPE zone as was the sanding room which contained the disc / spindle sander (which was a monstrous lump of cast iron), the big linisher and a 3 m x 1.2 m belt sander.

Originally Posted by Chris Parks

What is confusing about 766 & 774? I have to ask otherwise we will never know.

Post 766 "Short answer, a VFD will not control a SINGLE phase motor only three phase motors."

Post 774 " Originally Posted by LappaSo a VDF CAN control a single phase motor?



Yes but I see no reason to do this - in fact I can see lots of reasons not to do, it ESPECIALLY on a DC."

Cheers

At the last workplace they controlled the dust with 4 downdraft tables from Donaldson Toritt and 2 large booths plus a Hydrotron double downdraft tables. The down draft tables were about 4' x 8' (1200mm x 2400mm) 5hp, 3000 cfm drawing the air through 4 cartridge filters of 250 square feet each. The company added large a HEPA filter after the cartridges. Reasonably effective for the metal polishing and edge breaking as long as the operators stayed over the table and didn't work at the front edge and ground to direct the dust down or to the back, there were short walls on the sides and back. Otherwise it went everywhere.

The Hydrotron was a back to back with the tables the same size as the others, the difference was that they had a water tank below the table and the air was pulled through it and up between the two tables, through water impinging filters, before the impellers, with the air directed through a roof and then blowing down to the front of the tables, creating an air screen. I believe it had a pair of 5hp motors too. It was very effective and with the water below the table very safe because sparks were extinguished.

The booths (Donaldson Toritt again) consisted of 6 units side by side of 5hp impellers, 4 cartridge filters of 190 square feet each and exhausting through the HEPA filter at the top. They about 10' deep and 30' or 40' wide and about 10' high. Stips of plastic curtains hung down in front short of the floor a couple feet. They worked best when the operator stood with his right shoulder to the intake so the dust coming off the disc was directed towards the intake.

I got my stash of 28 used cartridges form the tables and booths and had them cleaned. They are what I will be using for my cyclone. I never got a chance to look at the innards of the water filtered tables to see how the managed the airflow through the water. I wish I had because they worked so well and think a home version would be the cats backside.

Hydrotron Wet Dust Collectors

Pete