Thread: Duct size.

Soredust,

Talk to Shedman or Chris Parks, but by my reckoning if you are going to run a 6 inch main you really only needed a CV 1800, like me.

A Max allows you to run 2 machines simultaneously, or perhaps better, to run one machine at a time and have permanently open in the middle of the shop another 6 inch duct to scrub the air in the shop.

A 6 inch main probably won't hurt you (I may stand corrected), but it seems a waste of capacity. If I had my time again I would buy a Max and have a 6 inch port permanently open.

Hi Soredust, this depends on a few things: If you are going to expand in future with more machinery added, 8 inch main is what you need, also if you have a fairly large workshop, you may need 8 inch main due to the static loss.

I guess in the big scheme of things, making the main duct 8" will future proof it. As for scrubbing the air, leaving such a big system running for hours at a time would be a costly measure...would it not?

8 inch duct...

I do believe somewhere i read that Bob mentioned (and hopefully he will correct me here if i am wrong) that for the 8 inch main - if cost of 8 inch pipe is too dear - you COULD make up 8 inch square box duct from say MDF or similar, and just have square to round transitions coming off it, at all your 6 inch PVC drops to each machine etc.

If you think about it... the bottom face, if screwed on, in manageable lengths - could be screwed off from underneath with a battery drill, if you ever get a major blockage of some sort in the main duct. Heck if you wanted too you could rebate clear plastic perspex strips say 4 inches wide into all the bottom plates - so you can look up into the main duct all along it's length (with a torch) to know exactly "where" you had to drop a panel to clean a blockage.
Just a suggestion... I'm giving it serious thought....at least.
Maybe there are good reasons not too?.

Originally Posted by Soredust

I guess in the big scheme of things, making the main duct 8" will future proof it. As for scrubbing the air, leaving such a big system running for hours at a time would be a costly measure...would it not?

Use a 225mm main duct as the inlet transition on the Max is bigger than the 1800 and is designed to take the bigger duct. As for leaving it going, the current draw is not as big as some think, maybe ten amps with one gate open but maybe less and the motor speed can be wound down with the VFD to lower that. The VFD keyboard can be remotely mounted using an ethernet cable so it can be placed near where you are working and just wind the speed up or down as necessary. The Max moves so much air that it won't need hours of running to exhaust the air in the workshop. You can take two approaches, centre the open gate over where you are working or put it into the furthest part of the workshop away from the main air entry to encourage a complete exchange of air. I have always supported the former scenario but the latter might be worth thinking about. The Max will easily handle two gates open at a time. Corrimal Men's Shed shed run their's continuously and just use the gates as controls and amps fluctuate between 6 to about 12 fully loaded to no load. BTW a good amp gauge can be had here...

amp gauge | eBay

Pick the cheapest 50 AMP digital instrument (note some are 9.99 amps) and give the man some about $10 to send it to you. They really are great value and dead simple to install. The VFD will read amps but I use one of these to free up the display to read speed.

Originally Posted by Soredust

I guess in the big scheme of things, making the main duct 8" will future proof it. As for scrubbing the air, leaving such a big system running for hours at a time would be a costly measure...would it not?

I don't run mine for hours if I want to scrub the air.

Future proofing sounds like a good idea to me. I only wish I'd bought a Max instead of a CV 1800.

Scrubbing the air need not take hours. If, for instance, I have been spraying and I want to clear the shed of fumes, I use the CV. Sometimes I do the same if I am making dust that is difficult to catch at source ... hand sanding, drilling and etc. Generally, depending of how much fine dust/fumes I have been making, I'll run the system for about 10-15 minutes to clean up the air.

The gurus tell us that we should run the system for 20 minutes after any dust making activity. I don't. Instead I built pretty good ports/shrouds to maximise capture at source, and only run the system for a couple of minutes after making dust.

Please don't beat me ... my shop is very clean ... one day I might even get an instrument just so air quality can be measured.

I agree with John that if efficient collection methods are used then the need to run the DC to vent the shed air after dust making activity is very much reduced but without extensive testing it's difficult to say if it is eliminated. Wood dust generation is unpredictable and even testing does not reproduce well so it depends on how safe you want to play it. Just using a different size or shaped piece of timber at a slightly different angle than usual can be enough to deflect a spray of invisible dust away from a duct collection point.

The amount of venting needed after the last dust making activity depends on a bunch of stuff such as, the efficiency of capture at source, type of dust making activity, the type and dimensions of material, how long the dust making activity has gone on for, location of the dust collection points and fresh air entry points, and the volume of the shed. The other thing to remember is no amount of venting will remove any settled dust which is fluffled back up into the air by any manner of other activity including just walking past a dusty surface.

Ideally the dust collection and fresh air entry points should be even spread and opposite each other but if a DC is vented outside the shed the fresh air entry points should be opposite to the wall where the DC is located.

A commonly used method to work out venting time is to divide the shed volume by the flow rate of the DC.
eg 100 m3 shed and 1500 cfm (42 cubic metres per minute) or 100/42 = 2.4 minutes

In practice this 2.4 minutes will at best only vent about half (50%) the shed air.
In the next 2.4 minutes 50% of the remaining 50% will be vented which means 75% of the air will be vented
In the next 2.4 minutes 50% of the remaining 25% or a total of 87.5% will be vented
So the following series of 7 numbers goes 50, 75, 87.5, 93.75, 96.87, 98.44, 99.22

This it takes 7 x 2.4 or 17 minutes to get to remove 99% of the dust. This is where the conservative ~20 minute vent figure comes from.
The 99% removal is just a figure that is used - it may be overkill for some cases and insufficient for others

If the DC is only 400 cfm then 9 minutes is needed to move 100 m³ so 7 x 9 or 63 minutes is needed to get the levels down to 99% of the starting value.

Of course the dust capture at source for a 400 cfm system is unlikely to be anywhere near as efficient as it should be for a 1500 cfm so the using a 400 cfm system will result in the shed containing more dust so effectively a 400 cfm system has to be running indefinitely and in some cases will not keep up with continual use of a shed during a working day.

So the benefits of a higher capacity system are that (provided hoods etc are modified) more dust is captured at source and then when any dust does escape that venting time is much reduced because it is a higher capacity system and because less dust has escaped in the first place.

So back to the original question. Since no one wants to have to undertake a complex calculation about how long to leave a DC running after the last dust making activity so I use what I call a short and long vent time. If it is say a single cut with a TS or drilling a couple of holes with the DP I leave the DC running for just a couple of minutes. If it is longer than this then I use the 20 minute rule.

How long you leave it running is of course up to you.

The running time after dust creation should not have any limitations on it and taking this view negates any doubts of how long etc. The first thing you should do as you walk in to begin work is turn it on as just walking in raised the dust or so we are told.

The DE, let's say a CV Max as a health investment is quite a considerable amount of money and after that the cost to have it running continuously is nearly negligible. If you accept this then the best way is to leave it running continuously and control it via waste gates or the VFD. How much is your health worth? if it cost three dollars a day to run it is that a burden? i doubt it. With proper sound proofing the noise is cut to a mild hum of about 72 db and conversation can be conducted at normal levels so running it continuously is not unpleasant. I do not turn mine off while I am in the shed doing woodwork and with mine it is a bit of a pain as being an 1800 I have to manually open the gates due to it not supporting two gates open at the same time when using a machine.

If you take the view that continuously running the DE is a cost burden you are short changing yourself and the return on investment will be a lot lower.

If we know....for the Cv1800 or CV max, what the engine HP etc and watts etc is when running (after start up lets say), and we know - what the cost of electricity is per gigawatt hour or however it is metered, is it hard to come up with a per hour running cost as a "general indication", a bit like Bobs back of envelope 20 minute rule to clear the shed.

No need for a scientific paper on it just a ball park figure (coz likely out power suppliers all levy slightly different rates around the country).

It's something I've wondered to myself about....

Back when I had the factory and electrically operated kiln and fans... the power bills were almost $4K for 3 months... and I'd near fall over backwards every time they came in the mail. I have to admit that the possibility of big power bills for the 3 phase dusty has been worrying me...

It would be nice to know that it cost approximately XYZ cents an hour to run..... roughly - then one can figure your likely hours a week / month year and know what your getting yourself into costs wise to run your dust system, for the way you work in your shed is all.

Is there an easy way to do this calc?... (like miles per gallon in your car?).

Anyone have a easy equation to plug in the cost per kilowatt hour and size / type of engine... and come out with a guesstimate per hour running cost?

Just curious if I can build a MEG to run the dusty for the cost of a years power off the grid is all.

TT,
At the very outside its kw rating of your motor x energy price/ kwhr
Yours is a 3.75kW motor at 25 cents a kWHr = 94c an hour to run or ~$8 for an 8 hour day.

In practice it will be less than this because it's the actual Current drawn by the motor x Voltage = running power in Watts.
On a 3.75 kW motor the running current will be more like 10A x 250 V =2.5 kW or 62.5c/hr or about $5/day

But I don't like working with any machinery noise that is unnecessary and even a 65dB hiss gives me the irrits so this is why I turn it off if it's not needed.
I think it's the hiss frequency as much as the volume.

That's probably why I don't like using vacuum cleaners or some power tools, angle grinders in particular give me a shiver, even using good muffs and ear plus.

I'm currently in the process of building a new blower for my forge because the current blower (an 1800W vacuum cleaner) totally spoils the ambiance of the process.

Maybe this is also why I like working alone mainly with hand tools or quieter machinery like lathes.

In terms of shed venting it's cheaper and much quieter to use dedicated ventillation fan squirrel cage fans that vent directly ie without going through any ducting.
An unrestricted 1/2HP squirrel cage fan can move ~1000 cfm at <55 dB - this is what I use when welding and to vent the forge gasses from the shed.

Thanks for that Bob,

$5 a days not too bad - like yourself I'd only switch it on when using it. But it's surprising how those $5 all add up.... over a couple / 3 months!.

I might have to make mine a coin op machine... You puts your $1 & $2 coins in... and press play, and when the $ run out, it shuts off...

You gots to pays to plays! At least the power bills wouldn't come as such a shock at the end of 3 months!

Your trigger is a hiss... and mine is the sound of breaking glass! Maybe I was a Glazier in a previous life, and died in a glass breakage accident or summut...coz whenever Glass breaks (can be a simple drinking glass in the kitchen), it sets me off... for no rational reason. I've been know'd to spit the pacifier more than once at simple glass breakage.

Even I have no idea why... now I know about it I can usually bite my tongue...unless it happens by surprise - i.e. I don;t "see it coming"

SWMTSWTP & the tin lids all know better than to be around me when glass breaks.

Funny how certain sounds or smells can trigger a disproportionate response eh!

Originally Posted by Timless Timber

Thanks for that Bob,

$5 a days not too bad - like yourself I'd only switch it on when using it. But it's surprising how those $5 all add up.... over a couple / 3 months!.
.
.

They sure do add up but OTOH how likely is it that you will really need to use it 20 full days a month?

The hiss or breaking glass coming from you DE might be a sign that the VFD is not tuned correctly. When we did mine, my son played with the frequency the VDF ran at (Not the frequency the motor runs at) to make the sound less irritating. can be a big diffreance.
Just a thought for you guys

R

Originally Posted by rusel

The hiss or breaking glass coming from you DE might be a sign that the VFD is not tuned correctly. When we did mine, my son played with the frequency the VDF ran at (Not the frequency the motor runs at) to make the sound less irritating. can be a big diffreance.
Just a thought for you guys

R

The hiss I am referring to is just the air being sucked thru the ducting - I don't have a VFD on my DC.