dust extraction system

[davyowen]

Sept 12, 2008 18:21:02 GMT engineerone said:

davy i am not trying to suggest i know these things in relation to leccy, however my mid 80's installed central heating has earth loops in every conceivable place, and even some not conceivable ;D

That's what happens when plumbers pretend to be electricians ;D

An earthing and bonding system installed by an idiot can actually make for a less safe environment by introducing potentials where there were originally none, so I just wanted to make it clear that radiators do NOT need to be bonded to the main earth terminal, nor do they require earthing. And please, we are in the UK, so don't call it grounding

[engineerone]

been looking at various things in relation to this, and at the risk of making myself look even more foolish than before

firstly, the conversation about the filtration was from tests i saw in the usa. there is an optimum level of clogging the filter, i think about 15% is the figure i saw, this is basically what is left when having used the filter to draw air through, you then knock it to reduce the amount of build up. so at that level, the filter is more effective for being slightly clogged.

now to air flow etc, i have checked out a number of machines through the axminster catalogue, and think i have found that in
the non pro workshop, then only a couple of types of machines have exhaust hoses larger than about 60mm. some p/t machines, and some sanders, but otherwise, most table and cabinet saws sold to "US" have extractor pipes no bigger than 60-63mm.

now although air is a fluid, when you instal an extractor system in the workshop, realistically, the air only "moves" when the machine is turned on. and since there is no tuning of the air flow, then only that air which was in the pipe, plus that dragged through the working machine needs to be moved. thus by drawing air from a 60mm pipe to a 100mm pipe, you are only moving that volume of air. in aerodynamic terms it will then flow somewhat more slowly.

so in principle, i am saying that unless your machines have extraction pipes larger than 60 mm you are wasting your time using 100mm pipe for the whole system.

paul

[mrgrimsdale]

air is a fluid

Not. It's all gas. Gasses are compressible and expand to fill the available space. Fluids are not and don't. They behave very differently in pumped/piped dynamic situations
PS no I'm wrong. 'Fluid' includes gasses. But they aint liquid!

so in principle, i am saying that unless your machines have extraction pipes larger than 60 mm you are wasting your time using 100mm pipe for the whole system.

Not so - larger diameter mean less resistance. Whether or not worth the extra expense will depend on many details. You will get lower losses however.

cheers
Jacob

[engineerone]

that's why aerodynamics are also called fluid dynamics ;D

whilst gas will expand to fill the space, you are ignoring the point.

properly sorted, a dust extraction system will have proper gates to stop the flow from any machine before and behind the machine you are expecting to use. so you would have a space of lets say 24feet by 4inches dia containing a lump of air. start the specific machine and that lump of air may well be replaced by 6 feet of air from a 2inch dia.pipe by expanding to fill the space, plainly said air will go more slowly into that gap.

of course with a saw you generally expect to only get dust, whilst with a planer/thicknesser, you may, depending on the thickness of cut, get lumps of wood down the pipe too, hence the value for 4inch pipe if using say a 12x8 machine where you take large cuts.

but how many home workshops do this all day everyday.?

paul

[jake]

You will get less resistance losses in the bigger pipe, even if it isn't full length.

Think of plumbing.

[engineerone]

yes jake but in hydraulics which include plumbing to some extent, you combine smaller and larger pipes to get movement in different places. and frankly with a disabled toilet, i regularly have to worry about the change in diameter between the s bend and the pipes involved

thing seems to be that the pipe diameter can change the price dramatically.

paul

[mrgrimsdale]

Mines one of these
It has 2x4" inlet spigots. I use one, with a cap on the other. Here's a question for the panel:
If I want to plumb it in semi permanently would it be a good idea to do it as a 4" ring circuit using both spigots and joining the pipe in a loop. More flexible design layout and cheaper than 6" pipe, but with increased effective cross sectional area?

cheers
Jacob

[engineerone]

i think you are right jacob, to be properly effective, you need to
have a stopped end. and along the length, various gates to reduce the amount of air the extractor has to move at any one time.

obviously if you are using two or more machines at the same time, then you need larger pipes, but frankly few amateurs or one man bands do. ;D

paul

[mrgrimsdale]

Wouldn't need stop ends - loops don't have ends. Nor gates - what for? Say you had 4 inlet Ts on the loop; you'd close the ones not in use. I'd only be using one at a time anyway. That's the only stops you'd need.

cheers
Jacob

[engineerone]

if you have a loop., over time you would get a build up against the gates that are closed, since they it would bypass the extractor.

and most people call the closing pieces gates

paul

[jake]

Sept 21, 2008 9:52:59 GMT engineerone said:

yes jake but in hydraulics which include plumbing to some extent, you combine smaller and larger pipes to get movement in different places. and frankly with a disabled toilet, i regularly have to worry about the change in diameter between the s bend and the pipes involved

thing seems to be that the pipe diameter can change the price dramatically.

paul

I meant water pipes, not soil pipes. There's always a benefit in terms of flow in keeping as much of a run in 22mm (or upwards) as you can, even if the last bit happens to be 15mm.

I doubt that the hole through most tap spouts is more than 8-10mm, which by your argument would make micro-bore a good enough choice for all the supply pipe.

That would save a lot of money and hassle, so there's a reason it isn't done - i.e. your argument is rubbish :-).

[engineerone]

you are misinterpreting me jake, as usual ;D

paul

[jake]

I'm not, you're missing the point. As usual. ;D;

[Keith]

Sept 21, 2008 11:31:20 GMT mrgrimsdale said:

. Here's a question for the panel:
If I want to plumb it in semi permanently would it be a good idea to do it as a 4" ring circuit using both spigots and joining the pipe in a loop. More flexible design layout and cheaper than 6" pipe, but with increased effective cross sectional area?

The problem I see is a decent system will take into account the direction of the air flow and angle the joints and fittings to give the lowest resistance. If the air can flow in either direction you could not do that and the increased turbulence would probably defeat the object and encourage dust to settle within the duct.

[engineerone]

cmon jake.

water is pressurised into the house, coming in from 4-6inch dia mains pipes into the 2inch i think branches.

my mains cold water is relatively high pressure and of course the pressure comes through the taps in that way. this is why sometimes european taps do not work in all situations in this country. since i live in a flat and on the ground floor, my hot tap flows more slowly than that of my neighbours higher up, whilst my cold is better.

but this is apples and oranges again, the water is pressurised from behind as it were, whilst a dust extraction system draws the air towards it.

paul

[modernist]

Sept 22, 2008 18:48:01 GMT engineerone said:

but this is apples and oranges again, the water is pressurised from behind as it were, whilst a dust extraction system draws the air towards it.

paul

No it's not, its pushed by the pressure behind ;D

When I use a metal tubed vacuum on my workshop floor with a plastic sweeper end and plastic drum I get a shock from the metal tube This is clearly static build up which cannot run to earth via the insulated ends. Might this not cause a spark or fire?

Brian

[jake]

Sept 22, 2008 18:48:01 GMT engineerone said:

but this is apples and oranges again, the water is pressurised from behind as it were, whilst a dust extraction system draws the air towards it.

How clever of the water to know it is being pushed, and the air to know it is being pulled.

What on earth difference do you think that makes?

[engineerone]

cmon jake be as logical as you would like me to be.


in dust extraction you are collecting dust produced, and using guess what an extractor to draw it into the filter.

whereas water is kept in place behind the washer until you open the tap.

paul

[mrgrimsdale]

In theory Jake is right in that water/air moves according to pressure differences, whatever the mechanics. Except it isn't that simple.
I use an old vacuum cleaner with a nozzle on the pipe, in reverse, as a dust blower like an electronic feather duster. It produces a strong focused blast of air which dislodges sawdust in far corners.
Used in normal sucking mode the air coming in to the nozzle is not at all focused and won't reach nearly so far.
So it's not the same.
How do you explain that? ;D

cheers
Jacob

[engineerone]

at last jacob we are getting to where barry and i started out, which was being dubious about some of the claims of various
people about diverse piped extraction systems, including cyclones.

as i have said the fact is that you are using a remote sucker to draw the air and dust from a specific point, in general we all tend only to use one machine at a time. although i accept that a properly designed system in a commercial workshop a number of machines may be used, the same principals apply.

you mention using the vacuum as a blower, it is interesting to me that until about 15 years ago, when b&d marketed the blower here, most people either used a broom, or if lucky, the compressor to direct air to a specific place, although we had had vacuums for over a 100 years by that time. wonder who noticed the difference and why it was not exploited sooner?

i have studied lots of machines, in pictures, and very few except as i have said before, some sanders, and some although fewer p/t's have hoses much bigger than 60-63 mm, so i really cannot see what advantage happens apart from the air slowing down as it enters the 4inch pipe. i frankly doubt whether many individuals take large cuts on their p/t's not least since jason is one person i know who is not too scared of bashing away at his machines, unless it is the spindle moulder ;D and i don't think he gets large chips flying off his p/t. so you are in an amateur workshop. or a one man place not shifting too many thick strips of wood, so what you are mainly moving is DUST. that you want to move quickly and economically.

so i believe not to many increases in pipe diameter is the way to go.

paul

[jake]

Sept 23, 2008 7:16:12 GMT mrgrimsdale said:

In theory Jake is right in that water/air moves according to pressure differences, whatever the mechanics. Except it isn't that simple.
I use an old vacuum cleaner with a nozzle on the pipe, in reverse, as a dust blower like an electronic feather duster. It produces a strong focused blast of air which dislodges sawdust in far corners.
Used in normal sucking mode the air coming in to the nozzle is not at all focused and won't reach nearly so far.
So it's not the same.
How do you explain that? ;D

I don't see the issue there - if you have a high pressure jet of air, flowing from the end of a small pipe, of course it is going to be more directional than sucking air from a large volume into a small hole. You'd be better off comparing how directional your 'sucked' air is once it has any 'experience' of the pipe.

[mrgrimsdale]

Sept 23, 2008 10:56:43 GMT jake said:

Sept 23, 2008 7:16:12 GMT mrgrimsdale said:

In theory Jake is right in that water/air moves according to pressure differences, whatever the mechanics. Except it isn't that simple.
I use an old vacuum cleaner with a nozzle on the pipe, in reverse, as a dust blower like an electronic feather duster. It produces a strong focused blast of air which dislodges sawdust in far corners.
Used in normal sucking mode the air coming in to the nozzle is not at all focused and won't reach nearly so far.
So it's not the same.
How do you explain that? ;D

I don't see the issue there - if you have a high pressure jet of air, flowing from the end of a small pipe, of course it is going to be more directional than sucking air from a large volume into a small hole. You'd be better off comparing how directional your 'sucked' air is once it has any 'experience' of the pipe.

Yes it makes no difference whether the air outside the pipe has falling pressure, or the pipe inside rising pressure. I was just trying to confuse the issue

[jake]

Sept 23, 2008 9:53:45 GMT engineerone said:

i have studied lots of machines, in pictures, and very few except as i have said before, some sanders, and some although fewer p/t's have hoses much bigger than 60-63 mm, so i really cannot see what advantage happens apart from the air slowing down as it enters the 4inch pipe.

Friction.

I take it you accept that it is easier to pull 1000l/m of air through 100mm pipe than 63mm pipe (ignoring the port diameter for now)?

That is largely because of the friction.

If you have 9.9m of 63mm pipe, and a 10cm 'port' of 63mm pipe, you have a full 10m of high friction to pull though.

If you have 9.9m of 100mm pipe behind the 10cm 'port' of 63mm pipe, the amount of friction in the 9.9m of 100mm pipe does not increase. So you have a system with a very small proportion of high friction, dominated by a very large proportion of low friction. The result is lower friction overall = greater flow*.

(*by an amount which will depend on how high a pressure the extractor can pull, i.e. the degree to which it can overcome the friction. For trad hvlp extractors, the effect on flow is much higher than for vacuums)

[mrgrimsdale]

Sept 23, 2008 9:53:45 GMT engineerone said:

snip
although we had had vacuums for over a 100 years by that time. wonder who noticed the difference and why it was not exploited sooner?

Actually quite a lot of old vacuum cleanrers had outlet nozzles too, sometimes under a grille, so it could easily be overlooked

snip
i really cannot see what advantage happens apart from the air slowing down as it enters the 4inch pipe.
snip

Large diameter = less resistance to air flow = faster net throughput = faster flow through the narrow bits = more suck at the sucking end and more blow at the blowing end.

[jake]

Sept 23, 2008 11:13:10 GMT mrgrimsdale said:

Yes it makes no difference whether the air outside the pipe has falling pressure, or the pipe inside rising pressure. I was just trying to confuse the issue

Fair enough, I think you succeeded.