Duct sizing opinions Trotec speedy 400

[Chris Bratton]

Getting ready to install my Speedy 400. I purchased the Penn State 1300 cfm blower. I was wondering if anyone has used 4in metal duct in a run of 36 ft. I was thinking about going to a 5in or maybe 6in but 6 in is huge to run along the ceiling of my shop.

[Mike Null]

My inclination would be to use the 6" duct. That's a long run for 4" duct.

[Gary Hair]

Getting ready to install my Speedy 400. I purchased the Penn State 1300 cfm blower. I was wondering if anyone has used 4in metal duct in a run of 36 ft. I was thinking about going to a 5in or maybe 6in but 6 in is huge to run along the ceiling of my shop.

Considering that I'm running about 20' with a blower with half the CFM of yours, I don't think it will matter what you use. 1300 CFM is a lot of air!

[Keith Winter]

I'd definitely go 6" you have a 4" and a 2" port on that laser. If you try to stuff it into a 4" pipe you will be redoing it in a year or so. Been there done that. Save yourself the trouble and run a 6" pipe to it.

6" is really not that big, check out the 10" or 12" that the HVAC folks use at the hardware store sometime, now that's big! ;)

[Mike Null]

The square inch area of the 4" duct is 12.56" The square inch area of the 6" duct is 28.26". The 6 inch duct more than doubles the capacity of the 4" duct.

[Keith Winter]

The square inch area of the 4" duct is 12.56" The square inch area of the 6" duct is 28.26". The 6 inch duct more than doubles the capacity of the 4" duct.

Perfect explanation Mike!

[Gary Hair]

The square inch area of the 4" duct is 12.56" The square inch area of the 6" duct is 28.26". The 6 inch duct more than doubles the capacity of the 4" duct.

One thing you are failing to consider is the length of the run and the volume of air in that 6" duct. Think of it this way - put a normal size straw in a glass of water and take a sip. You'll get a fair amount of water and not exert much effort. Try the same thing with the same length piece of 1" pvc pipe. You are not likely to draw the water up into the pipe, let alone get any in your mouth. Same thing with an exhaust fan and a pipe that is twice the volume of another. I'd go with a 4", straight-walled pipe and be done with it.

[Mike Null]

Gary
I'm using a 6" and you're using a 4". We're both happy; so much for that.

[Glen Monaghan]

You neglect the fact that the 6" pipe doesn't extend all the way into the machine, it's necked down to the (typically) 4" machine port. Consequently, the air flow through the machine will be at least as great and potentially greater with the 6" duct than with a 4" duct. For shorter runs, it will be essentially the same but, for longer runs and with bends, it will be greater due to the lower friction losses in the larger diameter ducts. Same thing as with garden hoses, they all have the same size connector on the end but you'll get more water through a 100 foot long, 1 1/4" inch diameter hose than a 100 foot long 3/4" inch diameter hose due to the lower friction within the wider hose.

[Gary Hair]

You neglect the fact that the 6" pipe doesn't extend all the way into the machine, it's necked down to the (typically) 4" machine port. Consequently, the air flow through the machine will be at least as great and potentially greater with the 6" duct than with a 4" duct. For shorter runs, it will be essentially the same but, for longer runs and with bends, it will be greater due to the lower friction losses in the larger diameter ducts. Same thing as with garden hoses, they all have the same size connector on the end but you'll get more water through a 100 foot long, 1 1/4" inch diameter hose than a 100 foot long 3/4" inch diameter hose due to the lower friction within the wider hose.

One last comment here then I'm done.

You can only move so much of anything through a given space, water, air, whatever. The 4" connections on the end will dictate the amount of air you can move, regardless of the size of the pipe in between. Try this - take a 1" pvc pipe and connect a 1/4" tube onto either end. Stick one end in a glass of water and see how difficult it is to drink through it. Not only do you have to fill up the larger diameter in between the 1/4" pieces before you get any water out of the tube, but you have a larger (heavier) volume of water to move. The 1/4" tubes on the ends restrict the flow of water and you'll actually get less through it than you would if you had 1/4" tubing the entire length. Same thing if you had 4" ends on a 6", 8", 10", or 12" pipe, it's actually going to make things worse, not better.

If you had 6" tubing all the way through then you will see a lot of difference, but not if you are restricting it on either end.

[Keith Winter]

One thing you are failing to consider is the length of the run and the volume of air in that 6" duct. Think of it this way - put a normal size straw in a glass of water and take a sip. You'll get a fair amount of water and not exert much effort. Try the same thing with the same length piece of 1" pvc pipe. You are not likely to draw the water up into the pipe, let alone get any in your mouth. Same thing with an exhaust fan and a pipe that is twice the volume of another. I'd go with a 4", straight-walled pipe and be done with it.

Having done both, I disagree. I replaced my 35 foot 4" main line with a 6" line, same blower, and the improvement of airflow was Dramatic to say the least.

[Keith Winter]

Kev, he has two ports the 4"/6" main run is splitting into so that's doesn't fit your pvc analogy.

[Matt McCoy]

Having done both, I disagree. I replaced my 35 foot 4" main line with a 6" line, same blower, and the improvement of airflow was Dramatic to say the least.

Just curious -- were you able to take measurements?

[Keith Winter]

Hey Matt,
It was a couple years ago but it was pretty dramatic. I've redone my duct system a number of times, I think I'm on version four or five now if you count the additions I've done over the years plus one or two complete redos. I've gone so far as to tape each and every joint and seam and add wider curve elbows to decrease friction. Each time it gets better and I learn something new.

Typically if you have two ports pulling down one fairly small duct, such as a 4", you need to upsize the main duct. In my case it was two speedy 300s in the OP's case he has a 4" port on the back and a 2" port below the cutting table on his speedy 400 since it's bigger. I want to say it was near a 100% increase in suction by increasing from a 4" to 6" with a harbor freight blower . If you're only going 5 or 10 feet that will be less dramatic but if you are going 30-40 feet like I did, and the OP is doing, combined with all the turns that happen typically in longer runs, you are going to see a big increase in through put due to the decreased friction. At least that was my experience.

[Keith Winter]

Btw what Kev is hinting toward is air velocity being too low and not suspending the particles due to too big of a pipe , hes not wrong that can be an issue. However it is unlikely considering the OPs situation and stated blower IMO. If we were talking dust collectors sucking up wood chips this would be a completely different conversion centered on static pressure. As is the thing I would worry about is too small of a pipe causing friction and decreasing static pressure and airflow on the longer runs and turns.

[Glen Monaghan]

The 4" connections on the end will dictate the amount of air you can move, regardless of the size of the pipe in between. Try this - take a 1" pvc pipe and connect a 1/4" tube onto either end. Stick one end in a glass of water and see how difficult it is to drink through it. Not only do you have to fill up the larger diameter in between the 1/4" pieces before you get any water out of the tube, but you have a larger (heavier) volume of water to move. The 1/4" tubes on the ends restrict the flow of water and you'll actually get less through it than you would if you had 1/4" tubing the entire length. Same thing if you had 4" ends on a 6", 8", 10", or 12" pipe, it's actually going to make things worse, not better.

If you had 6" tubing all the way through then you will see a lot of difference, but not if you are restricting it on either end.

It's not that simple. For short runs with a restricted end, you won't move noticeably more air using the 6" duct than 4" duct, but you won't move less, steady state. A smaller restriction at the end definitely affects the throughput, but (especially with air -as opposed to water-, and with a less drastic ratio between restriction size and pipe size than your example, which is 16:1 versus 9:4 for a 4" restriction into a 6" duct) the fluid (air, water, etc.) flows faster through the restriction than the larger pipe or duct. For short runs, the restriction effect dominates total flow but, as I said, for much longer runs the friction in the pipe/duct can be far more significant and using larger ducting to lower fluid speed can significantly reduce friction losses to increase total system flow. Sure, you have a larger volume of air in the 6" duct than in a 4" duct, but the air inside the duct is moving slower in the larger duct than in the smaller duct so it's not like you have to move the entire volume at the same linear rate as it's moving through the restriction.

The other thing you are focussing on is the transient effect or start up condition, where you go from no flow to full flow. Yes, if you look at instantaneous results when you first turn on the blower, the bigger duct will have a longer delay before you get full air flow at the laser because of the greater duct volume compared with a smaller duct, all other things being equal. However, after a couple of seconds and steady state is achieved, the bigger duct will allow as much or more air flow than a smaller duct. I suspect most people turn on their blowers and then start their laser, and their jobs run more than a couple of seconds, in which case those initial conditions are relatively insignificant.

And we haven't even considered the impact of your fan ratings (volume vs SP), but a 6" duct isn't going to reduce steady state total system flow compared with a 4" duct.

[Kev Williams]

There IS an easy way to figure out if you ARE losing air flow between 2 sizes of ducting...
343459
Open your breaker box, and clamp one of these around the black wire of the breaker feeding your blower.

Run the blower with no hose attached, take an amp readiing.
Now block the flow complete, take another amp reading.

Now you have your baselines. You'll find the blower used a lot less power with no air to push :)

Hook up your hose, then take a reading. If you're restricting air flow, the amperage will go down.

Change to smaller hose, again, air being restricted will result in lower amperage.

I have NO clue if the percentage of amperage gain would accurately correlate to the percentage of airflow loss between the baseline readings, but I would think it would pretty close--
for example (just guessing at numbers), lets say 6 amps full load, 3 amps no load, for a 3 amp spread-
lets say some 4" duct comes in at 5.8 amps, that's a .2 amp loss out of a 3 amp baseline spread-
.2 amps/3 amps is a 6.6% difference, so, roughly a 7% airflow loss?

Regardless of actual airflow loss, a decrease in power usage will indicate a decrease in airflow...

[Glen Monaghan]

I think there is a definite nonlinearity you are overlooking. With no hose/duct attached, the blower has to work at a certain level and so consumes X power. With a fully blocked inlet, (at least some) fans spin "freely" and so consume less power than with the wide open inlet. The nonlinearity part comes in as you add ducting, bends, filters, etc. because these "restrictions" or "loads" cause the blower to work harder up to some maximum, at which point the blower consumes the most power. If you further increase the load on the blower, it begins to starve for air and power consumption starts dropping off toward the "blocked" lower limit on power consumption. The actual power consumption vs air moved curve depends on the type of motor and type of blower/fan mechanism, so you can't do much with just those two readings.

Added example graphs below. The first one in particular shows a fan power curve that exhibits a hump shape. You can go to a SP (static pressure) level on the left scale (12 is shown) and go across to the SP curve and down to see how many CFM this fan will move at that level of restriction. Also, where that same SP-CFM vertical line intersects the BHP (brake horse power) curve, you go to the right side to read the BHP at that flow rate. This particular curve is for a centrifugal impeller, but I couldn't find the motor type used. The second graph shows a motor/impeller combination that actually uses more power at higher SPs and lower CFMs, definitely NOT the power reduction for a blocked inlet that you mention.

[Ivan Shuliak]

Chris, have already installed your blower? Or tried to turn it on? How loud is it?
I'm looking to redoing my current ductwork and it looks like this blower would ideal for what we need.
Also does anybody if it's going to work with Fan Speed Controller, Infinitely Variable, 230 V ac, 15A
If we don't need the full power and decide to reduce the speed/noise/power consumption?

[Bill George]

More air = more amperage or work done. Air has weight per cubic foot and also depends on the temperature and moisture content to some extent. Squirrel cage blowers blocking either the intake or outlet causes the amperage to drop. Other types of fan blowers, radial or backward inclined blades may not. Propeller fans restricting the air flow will cause amperage to increase.

[Chris Bratton]

Ivan, Yes I installed the blower. I've been using it for a few weeks now. The fan from pen state really moves alot of air. I changed my mounting location to shorten the length a little. It has 18 feet 5 in duct, 6 feet 4in duct and 1 ft 3 in duct into the back of the machine. The fan is surprisingly quite. The Trotec Atmos system is quit a bit louder. I have I have 3 other variable speed exhaust on my spray booths that are much louder.