It is an ill wind that blows no good

[Bruce Seidner]

I have negotiated the vagaries of shop ceiling angle and depth with the aid of power and hand tools and inner tubes. I now have a straight shot to my blower and to the outside wall where I plan to discharge into my side yard/woods.

There is a 20 foot span between my last fitting, through a contrived wall that separates the "shop" from the crawl space, and to the foundation wall that separates my home from the great outdoors.

I can place my blower close to the shop wall, minimizing distance between it and the machines, or I can put it at the far end, next to the outside wall and have almost no distance to vent to the outside.

This is where I have read about the equivalence of low pressure, "liquid" like dynamics, and placement of the blower being of little difference. But I am unconvinced by the theory.

It seems that having the blower closer to the machines and a long discharge run to the outside wall makes more sense than a long run from the blower to the machines and a short run to the outside wall. On the shop side of the wall the blower has a main trunk of 8" which have a few wyes that drop to 6" and then to the machines. I have plenty of left over 8" pipe to put the blower any where I choose between the shop wall and the outside wall, an expanse of 20 feet. I am also playing with the idea of using inexpensive 12" galvanized pipe to exhaust the 20' span to the outside and sell the 8" PVC green sticks I am don't require. It seems that a 12" exhaust would preserve static pressure on the machine end and with the 5hp rebuilt motor there should be enough wind to get the wood outdoors out a straight but long exhaust pipe. But I am frequently wrong and seldom uncertain. I am very open to suggestion.

In a private forum the French have a funny but vulgar expression for how one holds one's liquor, in a public forum, I am asking where I should place my blower.

[Ty Williams]

Just as a matter of fluid dynamics, it's better to suck things a short distance and blow them a longer one. Since there's negative pressure (suck) between the machines and the air mover, you'd want that run to be shorter than the run to the outside, which us under positive pressure. The reason for this is that you can't draw a vacuum down below 0PSIa, which means you get a maximum pressure gradient of 14.7PISg on the inflow/vacuum side. However, you can "pump up" the pressure to whatever your hardware supports on the outflow side, ensuring you have enough pressure gradient to eject the chips all the way out.

This, btw, is the same reason that a well for water has to have the pump in the bottom of the well rather than on the surface once the well passes a certain depth. You can only suck water up so far but you can pump it as high as your pump is strong.

[Carl Beckett]

Just as a matter of fluid dynamics, it's better to suck things a short distance and blow them a longer one. Since there's negative pressure (suck) between the machines and the air mover, you'd want that run to be shorter than the run to the outside, which us under positive pressure. The reason for this is that you can't draw a vacuum down below 0PSIa, which means you get a maximum pressure gradient of 14.7PISg on the inflow/vacuum side. However, you can "pump up" the pressure to whatever your hardware supports on the outflow side, ensuring you have enough pressure gradient to eject the chips all the way out.

This, btw, is the same reason that a well for water has to have the pump in the bottom of the well rather than on the surface once the well passes a certain depth. You can only suck water up so far but you can pump it as high as your pump is strong.

+1 with Ty's comments. Put the longer run (and thus higher flow loss) on the output of the blower.

[Anthony Whitesell]

I had forgotten that part of the theory. I have still not pulled the trigger, due to time and money or the lack thereof, on changing from my single stage to setting up and using my cyclone. With this piece of information (or enlightment) I wonder if there is a better place for me to put the blower, away from the cyclone body, and use the push through design instead. Hmmm.

[Jim Andrew]

Anthony, if it were me, I'd wait till Bruce posts back with his results. The cyclone company told me that long runs were practically invisible to the cyclone, but I didn't ask about pushing dust out a pipe. Of course I filter my air back into the shop.

[ian maybury]

Without looking at the specifics of the installation Bruce the biggest factor regarding airflow/pressure drop is probably that (with a cyclone system) while you need to keep the ducting on the suction side of the cyclone and blower small enough to deliver the required transportation velocity (4,000fpm), you can make the ducting on the exhaust side much larger as there is only the requirement to transport whatever fine dust gets through the cyclone.

Whether this is likely to deliver a worthwhile reduction in pressure drop depends on how long the exhaust duct run is likely to be, and how many bends it may have. A 6in duct at 4,000fpm/790cfm drops about 5inWG/100ft (with a 90 deg bend equivalent to say 10-12 ft, and a 45 deg half that). Put that same 790cfm through an 8in duct and the drop reduces to about 1.25in/100ft. Which seems like a good saving in pressure drop, but if your exhaust run is only say 10ft it amounts to only about 0.5in WG of a saving.

It probably doesn't make a lot of difference after that. The density of the air on the suction side is slightly reduced, but a quick look suggests that the 10in WG suction we tend to run at is the equivalent of very roughly 900ft of altitude. Cincinatti Fan's altitude correction tables (if i'm reading them right) seem to suggest that this might de-rate the fan by the equivalent of its pulling an extra 2% of pressure - about 0.2in WG or so, which costs maybe 50cfm on a 3HP fan.

The information isn't readily available to judge the effect of a moderate pressure drop on a cyclone (the push/pull discussion we've had before), but it's probably not that big a deal as it's not like those with push systems seem to report any difference in performance. My guess is that the dynamics are such that there's probably lots of minor variations that have more effect on their performance.

Water pumps run into a different issue when trying to lift on the suction side - if it's to pull water through to prime they don't move air very well, and if it's just a simple issue of lifting water the impeller starts to cavitate so that as well a possibly doing damage the performance falls away.

Some fans will run into buffeting/stalling if restricted on the inlet side, but it doesn't much affect the radial centrifugal impellers we use which are not very efficient, but equally not very fussy about air speed at which they are operated.

ian

[David Kumm]

Bruce, if you are running a 5 hp motor with an appropriate sized impeller and 8" mains you have enough capacity to put your blower where you want it. With no filters on the backside even 8" discharge pipe won't reduce capacity in any measurable way. My system is a pull through with the blower up in the attic. My runs are 8" reduced to 7" and over 50' long on the input and 15' to the filters. You will have less resistance than I and will likely generate over 1600 cfm if you max the blower. Dave

[Bruce Seidner]

This is enormously helpful. And if I may beg a further indulgence, I have taken a snapshot of the fan curve for the New York G series 126 fan blower with the 5hp 3500rpm motor. The unit was a $200 Ebay salvage that did OK for it's apparent age and use, but I just had the motor rebuilt, new bearings and all, because I thought with the new shop rebuild, it should be the last shop rebuild. This time there are new benches and a sliding table saw extension and there will be no room whatsoever for a collector or cyclone. I have used the shoehorn to build what I have built and it may not work at all given the space but that is another story. I had been transfixed with all kinds of improbable schemes to make my current Pentz dimensioned/style cyclone fit, or make a Thien style collector fit, or just run the whole lot of it out of doors. I live with plenty of trees and space between myself and my neighbors and anyone who uses a fireplace for cozy evenings at home has created more of a carbon footprint or incrementally added to the pollution of our collective Eden. (So, please no well intentioned tsk, tsking from earnest Green Party types.)

I added the fan curve because I have some concern that because it is not merely the fines, but all the wood that is liberated by my machines that will be going out into the side yard garden, I want to be sure that the correct size duct is going the 20' from the blower to the external wall. The blower will be placed closer to the machines which leaves 20' of duct to the outside wall. The example of the well pump has been something I can organize my understanding around. It will go up with the Pentz example of moving a balloon about a room blowing on a straw as opposed to sucking on a straw as concrete examples of what one is dealing with in a duct system to remove dust.

In a system that is exhausting after a separator it is just essentially air that is moving. But here I am contemplating moving all of it, from jointer, to table saw, to errant rodents. So, that has me reconsidering the "advantages" of a larger 12" duct for exhaust and staying with the 8" pipe I have on the front end of the system. I have been a bit cavalier, both out of ignorance and because this blower is pretty strong for a guy who runs one machine at a time and has no intention of ever buying a lathe. The shop that carries wood in our area charges very little to thickness plane the lumber I buy and besides most of what I do is plywood. I got into this because I like to play with audio loud speakers and folded horn designs that create a labyrinth inside a box.

What size duct would someone with this data choose to exhaust all that one's machines throw out 20' to an outside wall?

[ian maybury]

The following are off the cuff thoughts Bruce, you'll have to run your own numbers to figure where you stand.

The curve for the 126 (on the graph) seems quite low for a 3,450 rpm/5HP fan. It seems to say it's gone by 10in WG, and in order to get say 1,200cfm/3HP it needs a total system pressure drop of no more than about 7in WG - more like the performance of a 12 or possibly 13in dia impeller, or that there's some other factor reducing the high pressure output.

Unless maybe the curve is very conservative or something?

I wonder if since you have a 5HP motor if a Clear Vue 15in or 16in impeller might be a decent fit in the casing? Don't know enough to predict how it would perform if the clearances were a bit tight vs. the Pentz design. Either way if your are blowing dust and chips through the fan is going to need to be able to handle blocks of wood etc. Meaning decent clearances, and a steel impeller.

Going without filters and a Pentz style cyclone should save 3 or 4in WG I think from memory. (has anybody got numbers?)

An 8in duct like that mentioned at 1250cfm/4,000fpm approx loses about 3in/100ft, and a 90 deg bend costs 15ft - you would need to figure how many bends and what length of straight duct run you are likely to have worst case to estimate a system pressure drop number. If it's coming out over 7in the CFM will be less than the above and subject to tailing off rapidly with pressure increases.

The downside of this sort of smallish impeller/big duct set up would be that you can get to quite high CFM when the resistance/pressure drop is low enough, but the airflow will drop very rapidly with even moderate increases in resistance. Meaning that even if the ducting flows freely enough it'd suffer a lot more badly than a unit with a larger impeller if hooked up to a machine with a restrictive hood. Which could quickly tip it into transportation problems caused by low airspeed too given the large duct.

An 8in duct is to my mind pushing into dodgy territory for these sorts of reasons on a fan like this - I'd be cautious about running it with looking very carefully at the sort of pressure drop my system was likely to create, and how free flowing the sort of equipment I was hooking it up to was.

No filters at least removes a major cause of increased pressure drop.

Don't underestimate the mess wood chips can make - a guy dropping a beech tree in pieces near my house covered a large part of the site in wood chips. Could the cyclone be fitted in outside? I think it might cost around 2 to 3in WG.

ian

[Michael W. Clark]

Bruce,
The straw example applies, but once you have the air in the ductwork, it will behave the same on either side of the fan as far as losses are concerned. If I understand right, you are not using a cyclone and dumping everthing outside. I think for this application you may want to keep the same size duct for the inlet and outlet. If possible, maybe duct it to an outside compost pile or bin to make clean-up easier. Maybe point it down and make clean-up easier. If you blow it horizontal, seems like it would make a huge mess. I used some pine chips from my planer one time for "mulch". The stuff stayed yellow/white for over a year and finally had to cover over it with real mulch. Just my thoughts.

If you had a cyclone, it does not care where the fan is located. It's performance is based on air volume, pressure drop, particulate size, etc. The advantage to putting the fan downstream of the cyclone is that the cyclone will get out all the big stuff and you can sometimes use a more efficienct fan. Once the air is clean, you can drop the duct velocity and save on static pressure. Of course, dropping the duct velocity means larger duct, which costs a little more. A typical economic point is to size the duct for clean air at about 2500 to 3000 FPM. This may vary depending on the cost of your materials. Some HVAC systems run even lower velocity because of cost and noise.

If you want to send or post a sketch of your system with rough duct lengths, elbows, etc., I can give you a static pressure estimate for your system.
If you have 8" duct, you would need about 1400 CFM to get 4000 FPM. This gives you a fan static pressure of approximately 6" according to your curve. 20' of 8" duct at 1400 CFM is about 0.6"wg of loss and 20' of 12" duct is 0.08" of loss at 1400 CFM. In the 12" duct, you are running about 1780 FPM, chips would likely deposit at some point.

Mike

[ray hampton]

If the end of the duct work are 2 or 3 feet lower than where the duct exit your blower will this help to keep the chips moving to the outside since they are moving downhill ?

[Michael W. Clark]

If the end of the duct work are 2 or 3 feet lower than where the duct exit your blower will this help to keep the chips moving to the outside since they are moving downhill ?

Ray,
Maybe, Could make a small section that is removeable in case of a plug.

(Good to see another Northern Kentuckian)

Mike

[ray hampton]

Michael, a clean -out plug is a good idea, were you born in KY ?

[Bruce Seidner]

Bruce,
If you want to send or post a sketch of your system with rough duct lengths, elbows, etc., I can give you a static pressure estimate for your system.
Mike

I don't have any drafting app that I can use so it is prose that will have to do. This is my thinking so far. All subject to change as I learn more about the actual rather than imagined facts of this layout. To keep fittings at a minimum I am mounting the blower behind a wall that separates the shop from the crawlspace at near joist height. This is what all the hand wringing was about and why I was wanting to use inner tubes to adjust for the differences in height that are small but a real pain to negotiate using 8" fittings.

There will be about 2 feet of straight duct to a 8x8x6 wye that comes straight out from the blower through the wall. The wye is mounted in the wall with the 6" inlet facing off to the right. This will be the 6" run to service the short "L" of a corner work bench. This bench section is 4'x8' and used for assembly and resembles a traditional bench with vices, etc., but is obviously twice the depth. There will be one 6x6x6 wye to drop in the middle of this duct to service sanding and power hand tools. The duct will continue to the end of the bench and likely service my mobile band saw, and mobile oscillating sander from the end.

So, worst case, this 6" duct will come off a 8x8x6 wye 2' from the blower and continue to the end of the bench and end up 6' across and 4' down with a large 2x radius 90 degree downturn to bring it just below bench height with another 2x radius 90degree fitting to bring it out to the front of the bench requiring about 2' of flexible duct to the machine, but 6" the whole way. Instead of the 2x 90degree fittings which i am making I could use 45degree fittings in combination to make 90's but looking at the tables this does not save much if the radius is 2x. This will likely service a 14" Jet band saw or a 1hp Grizzly oscillating sander. The Grizzly can be, well, grizzly so maybe this should be an 8x8x8 wye and a middle wye that is 8x8x6 so that it is 8" all the way to the machines?

The 8" wye coming straight from the blower will have a 2x radius 45degree fitting to a 2' straight to another 45degree to the long run of duct to the end of the long wall mounted bench that is 14' long and 3' wide. Just like the short L bench on the blower wall it will have a couple 8x8x6 wyes along the length of the bench to manage power hand tools, a miter saw that gets pulled down now and again and a bench sized Delta drill press. It is likely that these wyes would service a router table or the other mobile machines that are all the same height as this L shaped wall mounted bench. They are stored under a tall bench on the opposite wall that is 12' long and is not wood oriented. There is also the potential to make a run between the joists over to this bench and then down but I don't see a real need for this.

The worst case scenario is to the table saw or jointer that will be down at the end of the long bench. Here goes. From the blower there is 3' of straight duct to a 2x radius 45degree fitting to a 2' straight duct to another 2x radius 45degree fitting to 12' of straight duct to a 2x radius 90 degree fitting down 4' to another 2x radius 90 degree fitting out 4' to the TS which is not mobile due to the sliding attachment. I would be the first to submit this is not the best place to put this contraption but there is nowhere else. The table saw and jointer are collected top and bottom. I imagine this entire duct run to be 8" and then down below use some combination of wyes to service the dust collection at the top and bottom of these machines. Or perhaps it should be 6" ducting that comes down and makes two 90 degree bends to the machines because of lack of static pressure in an 8" duct. This Neanderthal does not know.

Thank you for noticing the lack of what one would anticipate a 5hp blower to produce in terms of static pressure. When I bought the blower it was caked with a sand like material and badly rusted internally. It is 14 gage steel and was clearly coated with something when it was new. I have re-habed the unit but the impeller is 12" and the inlet port is 8". With a 5hp motor might it have been spec'd for something heavier than wood dust? This is my layman guess based on my cycling experience and gearing on a bike. (I am not an engineer.)

In terms of the discharge, all points well taken and I will be mounting the blower at about 4' above the plastic covered dirt floor of the crawl space and it will drop about 3' over the 20' run to the outside wall. I had planned a type of awning that would cover the outlet and create an opposite wall that would block the discharge from spraying the yard. Instead it will be open to the planting bed behind some shrubs and I can blow it or shovel as needs be. I can't thank you all enough for the input. I have read a lot but don't have much actual understanding of the consequences and practice of what I have been reading. Your expertise is much appreciated.

[David Kumm]

Your system is well thought out but the 12" blower is inadequate. As Ian has said, check the housing to see if it would accept at least a 14" one. They are pretty cheap if you have the clearance and enough extra diameter. Dave