Thread: Vented blower vacs.

Hi Bob,

I am not sure I quite get this.

You are assuming about 30 cfm of air circulating through the motor cooling loop. I can accept that as a fair estimation, but it is very unlike you not to have an exact measurement. You must have some basis in your mind for coming up with that figure, so that is fine. Just dont knock my rudimentary measurements in the absence of expensive equipment in my own experiments and setups .

Now you are saying that the dust contained in that air should not be a worry, because while the vac is running through the DC, there will be 100 cfm of air vented outside, drawing another similar amount of clean air into the shed. As you say, you can also open another 4" vent to remove another 400 cfm of air from the shed, replacing it with a similar amount of clean air from outside. All this seems to make sense.

Now here is the bit I am not sure of. The vac as you found it, you say is putting out 100 cfm through its blower and 30 cfm through the motor cooling loop, a total of 130 cfm. You say that the 30cfm of dirty air will be ok as it is scrubbed over time by the 100 cfm air turnover.

If that is the case, why choke the 4" dusty port by connecting it to the blower port of the vac?

The air coming out of the blower port has been filtered by the vac after collection from a tool so the dust coming out of the blower port should not be all that much worse than the dust coming out of the motor cooling loop, or is it? I did not think it would be but I could be wrong.

I understand that for high concentrations of dust in the air such as that coming straight off the tool, then catching it "at source" is ideal, but the dust is captured "at source" by the vac in this case with the majority of it captured in the vac's collector and filters. Now being a vac, it will not capture all the invisible dust at source, as you keep telling us, Bob, and quite rightly so.

This means that depending on what the dust source is, there may well be other airborne dust raised in the shed that may also need removing from the air. Possibly even using the vac to clean dust off the floor or bench would raise invisible dust back into the air.

What I am wondering is: You are talking about dealing with 130 cfm of air. Assume one 4" Dc port available at 400 cfm. You are choking the 400cfm back to 100cfm just by attaching it to the vac to collect air that has already been filtered, but as we all know still contains fine particles.

If 100 cfm can cope with the 30 cfm from the motor cooling loop, why not leave that 400 cfm vent running at full capacity instead of connecting it to the blower port on the vac? this way, there is a full 400 cfm to deal with the 130 cfm of slightly dusty air instead of choking it back to 100 cfm? This would surely also help with any invisible dust raised by the tool/cleanup activity being conducted.

But as you say, Bob, you can also open another 400 cfm to boost things along, then its 800 cfm to deal with 130 cfm of dust-laden air. Take it a step further - If you dont need to use one 4" hose to collect the 100 cfm from the vac, you could use a 6" hose and suck out 1200 cfm to be replaced by clean air.

Can you tell me what I am missing here?

Doug

Originally Posted by doug3030

Hi Bob,
I am not sure I quite get this.

I agree it is somewhat complicated and we need someone to ask the "please explain" especially if they see a problem.

You are assuming about 30 cfm of air circulating through the motor cooling loop. I can accept that as a fair estimation, but it is very unlike you not to have an exact measurement. You must have some basis in your mind for coming up with that figure, so that is fine. Just dont knock my rudimentary measurements in the absence of expensive equipment in my own experiments and setups .

I agree this is very slack of me and I should have provided more detail. The estimate is based on the very technical method of waving my hand in front of both orifices and feeling the breeze after doing many measurements of air flow you can get a fair estimate of the flow rates especially if you have an air stream with a known flow rate right next to the one you are trying to assess. The air flow out of the motor cooling loop was clearly less that the air coming out of the blower end of the VC. My estimate was that it was much less (maybe closer to 1/10th) that 1/3rd of the air coming out of the blower but instead I chose to be conservative and chose the 1/3rd value.

Now you are saying that the dust contained in that air should not be a worry, because while the vac is running through the DC, there will be 100 cfm of air vented outside, drawing another similar amount of clean air into the shed. As you say, you can also open another 4" vent to remove another 400 cfm of air from the shed, replacing it with a similar amount of clean air from outside. All this seems to make sense.
Now here is the bit I am not sure of. The vac as you found it, you say is putting out 100 cfm through its blower and 30 cfm through the motor cooling loop, a total of 130 cfm. You say that the 30cfm of dirty air will be ok as it is scrubbed over time by the 100 cfm air turnover.
If that is the case, why choke the 4" dusty port by connecting it to the blower port of the vac?
The air coming out of the blower port has been filtered by the vac after collection from a tool so the dust coming out of the blower port should not be all that much worse than the dust coming out of the motor cooling loop, or is it? I did not think it would be but I could be wrong.

The air coming out of the filter on a cheap VC is usually dirtier than from a motor cooling loop. More importantly there is a lot (at least 3 times and most likely 10+ times higher) more air coming out of the back end of a VC so the total amount of dust emission is higher.
On a quality VC unless there is an internal leak (more likely than one thinks) the air will probably not need to be vented outside the shed, but how does one ever know unless one tests before every use every time with a particle counter?
The dirtiness of the VC motor cooler loop depends on the age of the VC. On (even cheap) new VCs the loops output less than 0.1 mg/m^3 and on older dirtier VCs (irrespective of cost) the loops can emission can be as high as 4 mg/m^3.

Because If 100 cfm can cope with the 30 cfm from the motor cooling loop, why not leave that 400 cfm vent running at full capacity instead of connecting it to the blower port on the vac? this way, there is a full 400 cfm to deal with the 130 cfm of slightly dusty air instead of choking it back to 100 cfm? This would surely also help with any invisible dust raised by the tool/cleanup activity being conducted.

The reason the dirty air from the VC blower needs to be fed straight into the 100 mm ducting is because of the mismatch in the air speeds of the two air streams. The 100 cfm coming out of the DC is travelling much faster than the air stream going into the 100 mm duct. Remember the 400 cfm is being drawn from a 180º+ mushroom shaped volume into the 100 mm duct whereas the air coming out of teh blower is like water coming out of a hose. Unless the 100 cfm is fed directly into the 100 mm ducting the 100 cfm coming out of the blower will be slowed down as soon as it hits shed air and and scattered out into the shed. Feeding the 100 cfm directly into the 100 mm ducting forces the air to match speeds inside the 100 mm ducting ensuring no loss of dust in the transfer. A cleverer simple transfer device would be a short length of 50 mm ducting inserted inside and screwed to the side of a length of 100 mm ducting, leaving the rest of the 100 mm ducting open to suck air from the shed. I will make one and and show this tomorrow.

But as you say, Bob, you can also open another 400 cfm to boost things along, then its 800 cfm to deal with 130 cfm of dust-laden air. Take it a step further - If you dont need to use one 4" hose to collect the 100 cfm from the vac, you could use a 6" hose and suck out 1200 cfm to be replaced by clean air.

Can you tell me what I am missing here?

You got most of it right especially the last bit

Originally Posted by BobL

The estimate is based on the very technical method of waving my hand in front of both orifices and feeling the breeze.

Bob, I thought you had instruments to measure that. Have all the graphs etc in other threads been based on what you feel with your hand? could you not measure the air flow in both orifaces and made something out of it?

Originally Posted by BobL

The air coming out of the filter on a cheap VC is usually dirtier than from a motor cooling loop. More importantly there is a lot (at least 3 times and most likely 10+ times higher) more air coming out of the back end of a VC so the total amount of dust emission is higher.

So if I get what you are saying here, there may be more than 10 times the amount of dust-laden air being ejected by the vacuum that is not being picked up by the the choked dust extraction facility applied to the task? And this air is a lot dirtier than the air coming out of the motor cooling loop? how does collecting the one - tenth of this by using the DE help at all?

Doug

Originally Posted by BobL

On a quality VC unless there is an internal leak (more likely than one thinks) the air will probably not need to be vented outside the shed, but how does one ever know unless one tests before every use every time with a particle counter?

But we aren't talking about a quality VC here are we Bob? Its a score from dumpster diving, isn't it? Why was it in the dumpster in the first place? I just don't get it. Now we have 10 times more polluted air coming out of the back of the vac than what comes out of the blower port and you did not think about including this information in your first post in the thread? I do not understand. Sorry.

Doug

Originally Posted by doug3030

Bob, I thought you had instruments to measure that. Have all the graphs etc in other threads been based on what you feel with your hand? could you not measure the air flow in both orifaces and made something out of it?

I do have the instruments and if you like I can, like my hand, wave these in front of the the motor cooling loop orifices but the result will not be accurate because to make an accurate measurement the orifices needs to be a regular shape i.e. circular or square so that I can attach a tube to them and then insert the probe inside the tube. The curved sides of a vacuum cleaner with multiple outlet orifices angled all over the place make this very hard to do. I will do it when I can work out an easier way to do this.

So if I get what you are saying here, there may be more than 10 times the amount of dust-laden air being ejected by the vacuum that is not being picked up by the the choked dust extraction facility applied to the task?

I'm not sure what "air being ejected by the vacuum" you are referring to.
There are two main air/dust sources coming out of the VC.
- the air coming out of the motor cooling loop which is not collected directly by the DC
- the 100 CFM of air coming out of the blower port ALL of which is easily being collected by the DC connection. The VC puts out 100 CFM and the 100 mm DC ducting and DC can accept and will draw up to 400 cfm, provided the transfer takes place inside the DC duct.
The up to 10 times more polluted air I refer to is the same air as the air coming out of the blower.
On a new quality VC (I can't think of one that is a blower VC) both sources of air will be extremely clean, especially in the visible particle range, but for how long?

There may also be leaks but I have left these out of the calculations.
Leaks internal to the VC leaks will also be collected by the direct DC connection. However the leaks external to the VC which will not be collected by the direct DC connection but hopefully these will be picked up by the non-direct connection.

But we aren't talking about a quality VC here are we Bob? Its a score from dumpster diving, isn't it? Why was it in the dumpster in the first place?

Exactly, my point is you don't need a quality VC to do this. As I said that VC was on the side of the road because it had a filter holder fitting missing (which I was able to fix with a $3 piece of PVC) but in my VC to DC arrangement the VC filter can be completely removed as all that does is restrict the VC flow and the DC and 100 mm ducting can accept a lot more than 100cfm.

The idea of this thread was to provide woodworkers who already have a DC that vents outside, and a blower VC, a simple way to keep their sheds cleaner and avoid building a . As usual the devil is in the detail, in this case the dirty motor cooling loops. I would be really useful if VC manufacturers could funnel these directly into the blower port so that too could be easily scavenged and exhausted. If someone is about to buy a new VC then dirty motor cooling loop air will be less of a problem.

Originally Posted by doug3030

Bob, I thought you had instruments to measure that. Have all the graphs etc in other threads been based on what you feel with your hand? could you not measure the air flow in both orifaces and made something out of it?

This afternoon I used an airflow meter in an attempt to assess the airflow out of the Ryobi VC motor cooling loop. After several trials the maximum value I measured was 17 CFM which is roughly half way in between my initial hand waving estimate of 10 CFM, and the more conservative estimate of 30 CFM that I used to support my arguments. My original post still stands, but as the maximum cooling loop flow estimate has now almost halved this provides even more support that venting a blower VC with a 100 mm duct DC and leaving at least one other 100 mm DC duct free sucking in the shed is a cleaner way to use a blower VC and there should be no need to completely isolate the blower VC inside a box. This assumes the DC is venting outside your shed and of course if you don't have a blower VC then you will need a box to capture the VC air output.