New vac, it's a question of suckage. . . . . . . . . .

[fluffflinger]

Help needed (in plain english please).

Just been given a new shop vac. Has usual gauge hose outlet, internal is 32mm or thereabouts. When I detached the hose from the body of the vac the outgoing flange is 55mm internal. Therefore the area of the aperture is approx 3 times that of the hose outlet.

Unsurprisingly the amount of suck without hose is amazingly increased. I tested this by the ultra scientific method of holding my hand over it, impressed aren't you?

Is this the increased diameter producing this effect or the lack of resistance previously created by the length of hose? Or, as I presume, a combination of both?

If indeed there is a potential for me to suck more efficiently my final question is this:

Would I be much better off gluing a piece of 63mm into the body of the vac (easily achieved with a file short length of rigid pipe and some Pink Grip) and using a length of 63mm flexi hose for my router table and under blade collector on the bandsaw and just using a reducer when connecting up to sanders etc or continue as I have previously making connections from the smaller diameter hose to larger outlets (which always seemed wrong)?

The only change is butchering the shop vac (very secondhand and free by the way) as I have 63mm flexi hose and pipe in a box somewhere.

All help gratefully rec'd.

Thanks in anticipation of some guidance by those who understand physics better than moi.

 

[CHJ]

Had the same with my cheap shop vac. (Lidl) made new coupler and fitted hose to match inlet size and just about doubled the rate of air moved. (think new hose cost as much as the vac though)

 

[fluffflinger]

Thanks Chas.

That's what I thought, luckily I already have the pipe so no extra expense would be incurred.

It begs the question of why the manufacturers build in this significant loss of performance, apart from the extra expense of he larger hose and tools of course? But honestly who wouldn't pay a bit more to suck twice as much .

 

[drillbit]

I don't understand... So the original vac in both cases had a larger diameter intake than the hose which was attached to that intake? So in effect, the hose was reducining the diameter of the vac intake, or the other way around?

And it sounds like switching to a larger diameter hose increases the airflow? I was suprised at this... I would have thought reducing the hose size would increase the airflow. The way I think of it is like if I were sucking through a straw, I could pick up a tabletennis ball, but sucking through a drainpipe, no way I could create enough suction.

So could you guys explain? Why does the airflow increase with the larger diameter pipe? Because it matches the intake diameter? Or because it is larger? Eg. if you had a larger pipe on a smaller intake, would it also suck more?

 

[Jensmith]

So if I have a 4" outlet on my Camvac but I use a 2.5" pipe to connect to my machines I would improve suckage by reducing the outlet to 2.5"?

 

[Jonzjob]

I think that there is a bit of confusion here between speed of air flow and volume of it.

A small hose will give you increased air speed and you will hear the fan run a bit faster than when you remove the hose and fit a larger diameter hose and even faster if you block the hose with your hand. That's basause if you block the hose the fan is doing less work because there's no air to suck. More work with the small hose and even more with the large hose as there is more air going through it.

So although the air flow through a large hose is slower there is more volume of air going through.

Not really well explained, but I hope you get the drift?

 

[CHJ]

These are the hoses (black) originally supplied with the 'Vac'.

Their two main functions being:
1. To reduce the size of pipe bore to fit the 'standard household' type fittings and connect to smaller hand tools.
2. Restrict the airflow to such an extent that the 'Vac' Fan spent most of its time screaming at higher revs because it was being starved of air and collapsed the two hoses to minimum length even without the tools fitted.

Increasing the bore size of the hose to that of the inlet connector increased the airflow enough to make it a viable collector for a benchtop thicknesser on most of the pieces I plane, the Chop Saw enclosure, notorious for spreading dust, and 12" Disc sander.

 

[CHJ]

So if I have a 4" outlet on my Camvac but I use a 2.5" pipe to connect to my machines I would improve suckage by reducing the outlet to 2.5"?

You may increase the actual vacuum level obtained at the end of the hose/pipeline by taking the fan further up its 'suck curve' but at the expense of air volume movement, if you restrict too far the fan efficiency will drop drastically because of air starvation.

But because you are restricting the airflow from the equipments designed inlet size you are more than likely to reduce the performance considerably.

 

[siggy_7]

And it sounds like switching to a larger diameter hose increases the airflow? I was suprised at this... I would have thought reducing the hose size would increase the airflow. The way I think of it is like if I were sucking through a straw, I could pick up a tabletennis ball, but sucking through a drainpipe, no way I could create enough suction.

For a given power of suction, you will move a larger volume of air for a larger hose. This is because the mean flow speed within the hose will go down as you increase the hose area, as frictional losses are a function of velocity squared, and cross-sectional area is a function of diameter squared, if you double the diameter you will increase your area in the hose by a factor of 4, reduce the mean velocity by a factor of 4, and so reduce your frictional losses in the hose by a factor of 16 - for a given volume of flow.

What this means for a constant motor "suck" is that considering the hose alone and for a constant motor power with no change in height, the velocity should be invariant with hose diameter (see Darcy-Weisbach equation, friction factors and pumping power formulae). Therefore, doubling your hose diameter should leave the velocity in the hose unchanged and increase your volumetric flow by a factor of 4. In a real system you need to consider other losses in the system which become significant (e.g. inlet/exit losses) as you reduce the losses in the hose. Therefore, increasing the diameter will cause a reduction in velocity through the hose as these other losses increase, whilst reducing the diameter will case an increase in velocity through the hose as these other losses reduce.

The reason you can't pick up the table tennis ball with a drain pipe but you can with a straw is the volume of air contained within each. Think of a compressed air cylinder - if you double its volume, you double the energy stored within it for the same pressure. So a drain pipe is going to require a lot of energy (and lung volume!) for you to generate the same interal pressure, compared with the straw. This is a static pressure problem not really relevant to the issues in a dynamic flow/pressure problem.

 

[CHJ]

..... I would have thought reducing the hose size would increase the airflow.

No it reduces the airflow, it may increase the level of reduced pressure/vacuum available at the working end but only if the airflow is sufficient not to starve the fan but this does not mean it will collect any more dust..

...... The way I think of it is like if I were sucking through a straw, I could pick up a tabletennis ball, .

You are increasing vacuum level at he expense of volume, there is no air movement at all on you table tennis ball.

....but sucking through a drainpipe, no way I could create enough suction

that's because of your inability to remove the air volume in the pipe due to lung volume limit.

.. Why does the airflow increase with the larger diameter pipe? Because it matches the intake diameter?

Yes, in simple terms the inlet hole size is now the major limiting factor.

Iif you had a larger pipe on a smaller intake, would it also suck more?

No, the inlet hole size is the restricting factor and airspeed in the larger hose would be slower and therefore unable to transport the dust particles

 

[Jensmith]

So with my 4" outlet I should use all 4" ducting / flexible pipe?

But in that case, how do I connect it to my tiny machine outlets without reducing tge airflow?

I do have the motor screaming issue depending on what I'm using it with.

 

[mack22]

What a fascinating topic, having just used Hetty to vacuum the stairs I read this item, so I thought let put it to the test re loss of suction via the hose, reducing the hole in Hetty by use of the centre of one of my circular saw blades, ( being the exact size of the hose input at the working end) I used a champaign cork attached to an elastic band and measured the difference, it took a pull of about 13'' from the hose end and about 14'' from the saw blade hole, so not much difference with a hose or not.
I appreciate it was a totally un-scientific approach, the cork was wind tight, and to allow for elastic variations I had several go's, perhaps others might come up with a more practical test..
Re the discussion about bore size I assume that atmospheric pressure cant be discounted, as the larger the area the greater the effect..

 

[CHJ]

So with my 4" outlet I should use all 4" ducting / flexible pipe?

But in that case, how do I connect it to my tiny machine outlets without reducing tge airflow?
...

You have come up against one of the proverbial problems.

In actual fact if your tool is the source of the airflow restriction then you may not have a working system if you just up the size of the hose.

This is because the air volume moving in the large hose will slow down as soon as it leaves the tool restriction, this may cause the sawdust to just collect in the hose until such time its bore is restricted enough to speed the airflow up enough to support the dust. (Think of it as a long drop box that you may have seen others mention)

If however you can connect more than one tool or a localised dust entrapment cowl via splitters at the same time, and have some form of switching them open or shut (Blast gates or similar) you may be able to operate your 'vac' more efficiently and still move the dust from your smaller tools.

One thing that a small outlet on a tool does is speed the airflow up at that point, this in some circumstances can improve the ability to start heavy debris moving if it is near the tool outlet and a small hose helps keep it moving. This is of course at the expense of the 'vac' having to work harder.

In some instances if the 'vac' is over specified it's efficiency can fall off because the fan can not grab enough air and stalls just resulting in the motor running ever faster to no effect.

 

[Jonzjob]

I have one of these http://www.axminster.co.uk/axminster-ax ... rod794272/ and would recomend them to anyone looking to get a very good dust extractor. I also got the tool hose to go with it and as you say Chas the air speed in the tool hose is far greater than the large one. The most noticable dofference is when I plug my Bosch Delta sander on it. It really speeds up the fan because of the restriction at the tool end. I don't have any power tools that the tool hose doesn't fit and if I do find one then I have a stepped hose reducer that will fit.

 

[siggy_7]

So with my 4" outlet I should use all 4" ducting / flexible pipe?

But in that case, how do I connect it to my tiny machine outlets without reducing tge airflow?

If your extractor generates sufficient suction to move air through a 4" hose at a fast enough speed to avoid dust and chips building up within the hose, then yes I would imagine that 4" ducting is what you would want to install for best extraction on larger machines. The problem you have with smaller inlets at the machine end is that the entry losses through the small inlet become where most of your suction power is consumed. On the plus side this generates high momentum airflow at the inlet which helps drag debris in, on the downside it starves the system of airflow so in the larger ducting the flow speed reduces, potentially allowing the debris to be deposited within it. To get round this you would periodically need to "flush" the system by opening up a bigger inlet, drawing more airflow through and clearing the debris, or providing a permanent second flow inlet partially open that allows the flow volume through the ducting to be larger. You sacrifice some suction at the extraction end with the second approach, but your extractor will run much happier and cooler - I think you'd find the loss of suction quite small.

 

[Jensmith]

So with my 4" outlet I should use all 4" ducting / flexible pipe?

But in that case, how do I connect it to my tiny machine outlets without reducing tge airflow?

If your extractor generates sufficient suction to move air through a 4" hose at a fast enough speed to avoid dust and chips building up within the hose, then yes I would imagine that 4" ducting is what you would want to install for best extraction on larger machines. The problem you have with smaller inlets at the machine end is that the entry losses through the small inlet become where most of your suction power is consumed. On the plus side this generates high momentum airflow at the inlet which helps drag debris in, on the downside it starves the system of airflow so in the larger ducting the flow speed reduces, potentially allowing the debris to be deposited within it. To get round this you would periodically need to "flush" the system by opening up a bigger inlet, drawing more airflow through and clearing the debris, or providing a permanent second flow inlet partially open that allows the flow volume through the ducting to be larger. You sacrifice some suction at the extraction end with the second approach, but your extractor will run much happier and cooler - I think you'd find the loss of suction quite small.

Ah! Well, like you and CHJ said, I have actually noticed that I'm getting dust collecting inside the tubes and not being pulled through to the extractor. I didn't realise that's why it was happening.

I got the extractor 2nd hand and it did come with a 'Y' connector so I could split the hose and have another 4" inlet partially open to do as you suggest.

I'll give that a try as my extractor does seem to struggle at times but there's no way I can connect a 4" hose to my machines. Either no room or they're too small in the first place! Some of my tool outlets are about 1.5" or less which is tiny really.

One other thing I've noticed - I have a 'cyclone' bin - the Camvac old style which does collect all the bigger material and what goes through to the extractor is the very fine stuff. However, because it's so fine it seems to be clogging up my filters quite quickly ie 1 month of light usage. It's the Camvac 3 stage filtration system. The outer bag gets coated in dust and the paper bags also get covered quite quickly.

Is this normal? The nature of what I do means I do produce more fine dust that might be typical in a normal workshop but given the size of the extractor 90L it would be good if it didn't clog quite so frequently.

 

[fluffflinger]

Well I'm definately going to butcher the shop vac based on the advice rec'd and the real world experience of Chas.

Just wondering about trying to find another similar vac and connecting them together via a manifold of some sort. Would I have created the shop vac equivilent to a twin motored Camvac. Any ideas what size of outlet would that support 100mm maybe? Actually I never really understood how Camvac offer there machines in both 2.5" and 4" versions powered by seemingly the same motors?

Thanks to all who have contributed thus far, my favorite has to be mack22 innocently hoovering his stairs and only a brief while later he is dangling a cork down his vacuum hose attached to a rubber band through the middle of a saw blade, priceless. Nothing like a Heath Robinson lash up to make me smile :lol: :lol: :lol: Surely it was thinking like this that put the Great in Great Britain.

Keep up the good work guys this place is priceless.

 

[CHJ]

.....Just wondering about trying to find another similar vac and connecting them together via a manifold of some sort. Would I have created the shop vac equivilent to a twin motored Camvac. ....

Most unlikely, every aspect of the two machines and their connections would have to be matched, and as already been mentioned a small difference in pipe layout etc. can make a big difference.
Connected in parallel the more powerful or efficient setup will tend to starve the weaker setup.
Connected in series they may provide more vacuum (more suck), but that depends on the connections and the matching of the characteristics, might even result in one or the other in the chain wrecking its motor because it was spinning too fast because of lack of load on its impellor. Strange things can happen with universal motors (those with brushes) when run unloaded such as when a fan stalls, just depends on the initial design.


Another aspect that has not been mentioned, it is worth checking how the universal motor in your 'vac' is cooled.
Most household 'vacs' rely on the airflow through the machine and debis filters to cool the motor.
The better designed of the shop 'vacs' have a separate cooling fan external to the working fan, so do not rely on the airflow through the system and any blocked filters.

 

[Digit]

And it sounds like switching to a larger diameter hose increases the airflow?

All other things being equal the 'airflow' is determined by the available power. Lengthening a hose will increase resistance and thus decrease the speed of that flow.
Again, all other things being equal, the volume of air moved will remain the same what ever the dia of the vac inlet, thus a smaller dia hose, lenghts being similar and not so long that the hose resistance to flow comes into effect, should result in the same airflow and an increased speed in the smaller dia hose compared to the larger one.
Thus for a longer hose the larger dia hose will win.

Roy.

 

[fluffflinger]

Chas

This is just theorectical but go with me I am learning alot here and thanks for your help thus far.

Let's say you have two identical shop vac's, same make, model, filters etc and you housed the power units in a common chamber rather than existing individual vac bodies.

So basically you have made a new vac but with two motors, rather than using some kind of external manifold. I can imagine that no two machines would be absolutely identical but if their spec's were, on paper at least the same, and as they are both sharing a common resistance from a single inlet surely they would peacefully co-exist but produce increased suction? Or am I just barking mad?