How to work out losses in reducing HVLP extractors?

[cusimar9]

In an effort to improve the dust collection in my workshop I've just invested in a Record Power CX2600 HVLP extractor, it's rated at 1.15 m3 /hr or 320 ltr/sec

Unfortunately I didn't fully grasp the fact that HVLP work differently to HPLV extractors and am now trying to work out what that will mean for my shop.

Neither my table saw nor chop saw have 100mm extraction ports so I'm resigning to the fact that I will lose some of the airflow when I reduce down to the approx 40mm extraction ports for these.

I've seen a few setups where others have done this but I would like to understand exactly what the effect of this reduction will do. Will it literally reduce the airflow by 60%? How do you work it out?

I do still have my 53 ltr / sec HPLV extractor but, even if the Record Power is reduced by 60% it would still be pulling in 128 ltr / sec which is more than double my HPLV extractor output.

Is that right or am I way off?

 

[cusimar9]

At the risk of answering my own question, I came across this thread

http://www.thewoodhaven2.co.uk/viewtopi ... 43&p=16000

(this is based on a discussion of 150mm extractor inlet and 100mm exhaust ports)

Restriction of flow is relative to a combination of diameter and length. A short run of pipe will not benefit much from being a larger diameter, but a long run will. There is no absolute rule, but I would suggest any runs of say 3m or more it would be worth using 150mm. Short runs, such as the machine inlets you mentioned can be in 100mm with no significant degradation.

 

[jimmy_s]

In your case you will probably have an abrupt contraction from the innards of your thicknesser to 40mm and then an abrupt expansion to your larger duct size. These will cause a significant pressure loss. The actual loss generally relates to the velocity pressure in the duct when considering fittings etc. The velocity pressure is proportional to the square of the velocity, in narrow diameter fittings the velocity will be higher giving a larger pressure drop.

Normally for vent systems the duct velocities are kept low as this keeps the pressure losses low, reducing fan power and normally noise. However for dust extraction systems low velocities are hopeless as dust etc is not transported along the ducts and it builds up inside, clogging up.

To work out your flowrate you would need a fan curve for the fan - I dont know if Record can supply this or not. With this and a rough idea of the ductwork configuration you can plot the airflow/ pressure loss in the ductwork onto the curve. Where the 2 intersect is your operating point. Its easy enough to work out.

 

[cusimar9]

Thanks Jimmy, hmm perhaps not so easy to work out for the layman! In the absence of anything more scientific I may have to just "stick my hand over the end" and see!

 

[wcndave]

As I understand it, HVLP systems provide a constant speed of air movement, and therefore volume is related to the square of the diameter (A = pi r^2) so going from 10cm to 4cm is going from 78 sq cm to 12 sq cm, so not 40% more like 15%!

Whereas Vac systems offer constant pressure, hence a smaller opening has higher suction (as you know when you put that very thin attachment on when doing your house / car)

Therefore a vac can't do massive volumes of air necessary for say planers and good TS extraction, as it's lt/m is fixed, and a chip extractor can't do good suction on small ducting as it's speed is fixed.

At least that's how I understand it works at a basic kind of level.

I have 2.5" ducting all over one wall, and it's no good for my TS / PT, so I am getting a chip extractor. I expect it will be no good for attaching to my 2.5" ducting despite having a 15 fold increase in m^3 per minute over my vac.

What I will do is take various readings using my anemometer and post some results.

 

[siggy_7]

To properly work this out, you need to know the performance curve of your blower. This gives the inlet pressure vs flow rate characteristic. It will typically look something like this, which will work out something approximate for your blower:

http://www.denysschen.com/curveplot/CPcurveVIV.asp

Manufacturers will almost universally quote the flow rate when nothing is connected - i.e. zero pressure drop from a short distance upstream of the inlet. As soon as you connect something to the inlet, there is some resistance to flow along the circuit. In the limit of zero flow (e.g. if you stuck your hand across the inlet), you would get to the stall pressure of the blower - this is the maximum static pressure that it can generate.

For any working system you will have something in between. The flow rate from the extraction point on your machine to the inlet of the extractor is determined by the pressure drop across it, which will match the pressure drop across the blower. So you have a problem where you know the relationship between pressure and flow (from the blower curve), and you put together a formula for the pressure drop across your system as a function of flow. Work out the curve for your extraction line as a function of flow and pressure, and then look up where this curve crosses the blower curve, and voila you have your real world flow figure.

The question remains as to how to work out the pressure drop across your extraction line as a function of flow rate. If you have a system with a very obvious pinch point (a very small area in comparison to the rest of the line), you could assume that all the pressure loss is across that pinch point. Small area means faster moving air, and losses are proportional to the square of the flow speed - so half the area, twice the flow speed and four times the losses. It's easy to see why small areas will really starve your system of flow.

If you have a well designed system with no major constrictions, then the total pressure drop will be a function of entry/exit losses associated with changes in area, and frictional losses along lengths of ducting. You then put together an equation that sums all the resistances together for a given flow rate. The maths is pretty basic and putting a simple spreadsheet doesn't take that long.

 

[wcndave]

I did 2 a-levels in maths, and one in physics, and I'm scratching my head at your post!

I'll try to read it again after a beer...

 

[siggy_7]

Oh dear, sorry! It wasn't meant to sound complicated. I have an unfortunate ability to make things sound more complicated than they are. Basically what Jimmy said in his first post covers most of it.

Sent from my SM-N9005 using Tapatalk

 

[PAC1]

Short answer is it will not work. In theory you will have a sixth of your 320 litres per second or 50 litres per second but I doubt it will achieve 30 as you will be starving the fan of air. Could you use the vacuum for the mitre saw and the other unit with 100 flexi on larger machines?

 

[RogerS]

Interesting thread. I wonder if there is a sort of reverse analogy with power shower pumps? I installed one and because it was easier fed it with a couple of short legs of 15mm reduced down from the recommended 22mm - a short run of about 500mm. The cavitation in the pump was chronic and before long the impellers were knackered.

 

[jimmy_s]

Roger

I suppose it does - in as much as the total pressure is made up of 2 components - static and velocity pressure. If you fit a small diameter pipe to a water based system with a low total pressure acting on it depending on the fluid you can end up with cavitation issues. With water - particularly hot water you increase the velocity pressure in the narrow bore tube and as such the static pressure must drop. If it drops far enough - below the saturated vapour pressure of the water - bubbles form quickly knackering the impeller as they implode.

 

[RogerS]

Many thanks, Jimmy, for the excellent explanation. I always wondered why/how cavitation occurred...extending the 22mm pipe is on my TUIT list.

 

[wcndave]

question about measuring with anemometer, if I put it on a 6cm pipe, the fan size of the anemometer covers if perfectly, so I can measure velocity, and flow.

If I put it into the airflow of a 10cm pipe I get one reading, and if I cover the gap with my had of course it goes faster, however that's at the end of a 10cm run, so which would be considered the "true" reading at that point?

I am measuring along various points in my system to see what causes drop off, and vac vs chip extractor on various dimensions of ducting. I guess it's the former number, as the later is effectively for a smaller diameter, as my hand covers the rest.

Just would like you fluid dynamics experts out there to confirm ;-)

 

[jimmy_s]

wcnsdave,

Normally commissioning engineers balancing ventilation systems using a anemometer or pitot tube certainly on a larger duct would measure the air speed at a number of points and take an average and then work out the flowrate by multiplying the average velocity by the area of the duct. The other way its done is that the engineer makes a "hood" you could make one from a bit of cardboard or something with a gradual taper from the duct or grille to suit the diameter of the anemometer. The engineer would out the flowrate based on the area of the instrument, the losses in a gradual taper are low and will not make a significant difference to your readings. You have made a sort of hood by cupping your hands around the duct you - might not get as accurate a reading as your hands will create an uneven flow pattern also you will create a higher pressure loss due to the more abrupt transition but due to the characteristics of the fans you are testing (both being able to generate a high pressure) I wouldn't think it would make a lot of difference to your readings as the pressure loss caused by your hands cupping the anemometer would be low compared with the pressure the fan develops. It would be different if you were working with a fan that developed a high flowrate at a low pressure such as a propeller fan, but in your case both fans will be developing a significant pressure otherwise they would not be able to be ducted.

I hope that makes sense!

Jimmy

 

[siggy_7]

By covering the gap with your hands you are reducing the cross sectional area and thus your measurements will be negatively affected. I would take the reading with the meter centred in the duct if possible. If the meter is calculating a flow rate rather than a velocity then you need to correct for the larger pipe area as not all the flow is going through the meter.

Sent from my SM-N9005 using Tapatalk

 

[cusimar9]

Well, I can confirm that, by reducing a 100mm pipe down to 50mm for 3 metres, the resulting airflow is useless, far less than my cheap HPLV extractor.

I may have just wasted £160

 

[wcndave]

Thanks Siggy, that's what I guessed, reading the speed "in line" without sealing hte gap, and of course multiplying out by the area I am taking the measurement from. I do have a flow setting, however I find it easier to just measure speed. Then I can write in the speed and diameter in cm, in to a spreadsheet, and it just tells me the cfm / m3h / m3s / m3m etc.

Have some illuminating results, that improved my understanding of my system above the "gluing washers to a ping pong ball and seeing if I can suck it up" excercise ;-)

 

[wcndave]

Sorry if I am hijacking the post, however seemed a good a place as any!

I tested all parts of my shop vac system and got the following results:

The Makita 440A vac is rated as 3.5 m3/m (123 cfm)

• Vac with no filter / bag runs at 17m/s with a 6cm diameter outlet, so 181 m3/h or 107 cfm (I will stick to m3/h from now on)
• adding the paper bag reduces this to 171 (94%)
• adding the "net" which I used for larger collection, eg planer, reduced to 135 (75%)

Taking out the net which I don't use now, as I have a "drop box"

• Out of the drop box with no bag (how I was using it typically for planer) 160 (88%)
• Out of drop box with a HEPA bag in the can, (I now use this before vac for power tools to avoid dust in filter) 157 (87%)
• With 2m flexi pipe for planer 170 (93%) for some reason an increase on the reading directly out of the bin
• Festool 38mm 5m Hose 64 (35%)
• Black normal 28mm 4m hose 20 (11%)

So for some reason I got higher flow rate next to planer, than further upstream, despite it being 60mm all the way...

Also with the hose, I may have odd results as the diameter of the hose is less than the anemometer. Certainly my hoses offer good vac suction.

And, when measuring the 10cm outlet from the drop box, with no shielding, I had 14.6 m/s which is 372 m3/h and therefore 205% of the vacs capability! Must be something to do with those pressure drops over different sizes that I don't quite grasp the science of. Yet.

I then attached to the 60mm (2.5") ducting system

At the mess of gates (four of them all in one place), I was getting 14 m/s or 142 m3/h or 84 cfm or 79% of what came out of the vac direct with no filters / bags.

Now 142 seems very low, I read that about 400+ is decent for TS / Planers, however for my lathe, router table, disc sander, bobbin sander and woodrat, it seems perfectly adequate.

Now, on to my christmas present, chip extractor!

 

[wcndave]

I've been meaning to get one for ages, as my TS cabinet is FULL of fine chipboard dust despite weekly cleans. The planer clogs up the pipes all the time, and generally my vac is just not up to it.

I have a soil pipe which I secured to the rough floor before they poured the concrete for the final floor, and the vac had no chance, it's full of wood waste now.

So the Jet is rated at 1600 m3/h at 15cm and 1150 at 10cm.
It comes with 2x10cm outlets, so how quite one gets to 15cm I don't know. Anyway...

Putting together was easy, it's the one with the cone "vortex" in it, which may not be as good as a Thein, the dust in bag does swirl around, however seems pretty good at protecting the filter. I got the one with the fine filter cartridge and can I just say well done to Axi for getting it to me in time for xmas. They were out of stock only last week.

When I put it together I was thinking "this is massive", however it's growing on me now. Or rather shrinking on me.

I tested the flow at source, using one 10cm outlet, and the other capped off. I got 904 m3/h or 532 cfm. Pretty close to the rated value.

Opening the second gate to let the fan "breathe" as some has suggested, reduced the air speed in the other. However static pressure drops etc could be something else...

Connecting it to a 60mm reducer after 1m flexible 10cm pipe, I was getting same airspeed, however with the reduced opening that's 295 m3/h, however still much more than the 140 I was getting from the vac.

However after running it through the ducting, I was down to 140. The same as the vac. Opening the second port on the DE made no difference.

So my first question is: if I get the same value as vac, and it's more convenient, any reason not to use this, and trundle my vac around for power tools (It's a pain to re-attach to ducting every time), or does running it like this cause damage to the DE due to low air flow through the motor?

I then ran it through the soil pipe to the table saw, with 2 blast gates at exit of floor, so I can attach to planer too.

I got 763 m3/h so about 84% of the source flow, which ought to be plenty.

I then used one of these:

To run a 60mm hose to the top of the table saw. However the suction is very very weak indeed.

Question 2: Am I better of with a Y splitter, and then reducing one branch to 60mm and running it like that?

Whilst I was leaning on my table saw checking the flow, I noticed a pain in my thigh, occurring at regular intervals. Turned out it was regular electric shocks. Also my blast gates produced a 1" spark when I reached out to close one. (I have the aluminium ones).

So looks like some grounding is in order. I already get about 30-50 shocks a day, every door handle, every drawer handle, every tap, sometimes plain walls, concrete and even bits of wood. I wish it meant I had some super power, but "staticman" already describes me accurately for 10 hours a day.

I plan to try wrapping copper round the hoses / pipes, and at one end just screw to the DE base (I assume that's grounded, Q3, how can I tell?)

At the TS end, given the under concrete run, I guess I could just attach to the TS cabinet. Neither devices have marked earth screw locations, so not sure...

Also I might try stripping some of the hose wire, and tightening the jubilee clip over it, and then connect that with a short length at the end.

Any ideas on static would be great, it seems to cause as many sparks as sharpening or hand planes debates, but most of the sites I read were 'murrican...

 

[siggy_7]

At the mess of gates (four of them all in one place), I was getting 14 m/s or 142 m3/h or 84 cfm or 79% of what came out of the vac direct with no filters / bags.

Now 142 seems very low, I read that about 400+ is decent for TS / Planers, however for my lathe, router table, disc sander, bobbin sander and woodrat, it seems perfectly adequate.

Dust collection relies on the velocity of air to entrain particles into the air stream. Table Saws and planers represent a bit of a challenge for dust collection because the cutting surfaces are large, so to get a sufficient air velocity around the cutter to entrain all the dust you need a lot of air flow (because you are trying to catch dust over a wide area). A lower flow rate will still do a job (and for tools where the cutter is nicely enclosed or where all the dust is naturally thrown by the cutter towards the dust collection port, a very good one) but the higher flow rates for a tablesaw or planer will improve collection efficiency. I would generally treat manufacturer's recommendations with a pinch of salt though as I don't think they are too scientific in how they derive their flow rate recommendations, just go with what works.

Opening the second gate to let the fan "breathe" as some has suggested, reduced the air speed in the other. However static pressure drops etc could be something else...

Hmm. Allowing a secondary air flow into the system can offer the following benefits:

• Increasing the amount of air in the extraction pipe increases the flow speed in the pipe - if you have a small area inlet around the tool and larger diameter pipes, then the extra air flow introduced near to the tool reduces the settling of dust in the pipes which may otherwise occur if the flow speed in the pipes is too low
• If you are using an inertial separator (i.e. a cyclone), the extra air flow will improve the efficiency of the separator if the flow rate from the tool alone is too low
• If you have a shop vac type extrator, too little air flow will overheat the motor because of the cooling arrangements. Adding in extra air flow prevents this

From the description of your system, it seems none of these apply so I would leave the second gate shut.

Connecting it to a 60mm reducer after 1m flexible 10cm pipe, I was getting same airspeed, however with the reduced opening that's 295 m3/h, however still much more than the 140 I was getting from the vac.

However after running it through the ducting, I was down to 140. The same as the vac. Opening the second port on the DE made no difference.

So my first question is: if I get the same value as vac, and it's more convenient, any reason not to use this, and trundle my vac around for power tools (It's a pain to re-attach to ducting every time), or does running it like this cause damage to the DE due to low air flow through the motor?

Seems like your ducting is really throttling the performance of the system. I see no reason why you can't use the new high flow extractor. The fan will be powered by an induction motor outside of the fan casing, and the motor will have its own cooling fan built in (unlike vac type extractors where it's all the same air). So the motor won't overheat if the fan is operated closer to stall.

Question 2: Am I better of with a Y splitter, and then reducing one branch to 60mm and running it like that?

Most definitely. You want to give the air as smooth a passage as possible. Changes of direction are accelerations on the air speed, and the sharper that change in direction the more energy is sapped from the flow to deliver that harder acceleration. Y pieces are much kinder in this regard. Use large radius bends and gradual transitions in pipe cross sectional area.

I don't know too much about static, but anything metallic on the extractor should be grounded to the earth wire in the power lead. You can usually spot the grounding connections, follow wires from the mains lead and see where they go to. You can check grounding with a multimeter measuring resistance.