Dust extraction - sizing

[andrew mcgregor]

Hi everyone.
I have an existing dust extraction system installed in my shed that isn’t performing as well as I’d like so I’m about to upgrade it. I’ll do all the usual things like replacing elbows with sweeps and other things to minimise suction losses.

I’m also considering upgrading the ducting from the existing 100mm to 150mm but I first want to make sure this isn’t a false step. My extractor is a 1.5hp 2 stage cyclone unit with a 150mm inlet and produces a nominal 1100 cfm of airflow (please excuse the imperial units).

The maximum length of the main ducting is about 6 metres with 3 inlet points, each supporting a few machines. The biggest loads would from the table saw at 400cfm and the thicknesser at 600cfm (estimated). It’s a small hobby workshop so I would never run more than one machine at a time.

My question stated simply is, will an upgrade of the main duct from 100mm to 150mm increase the suction or decrease it?

Thanks in advance for any input.

Cheers

Andrew

[JaapBasson]

Yes it will help a lot if you increase the inlet to 150mm. I would recommend that you increase the discharge ducting by 1 and a third. I assume that you are using a BCC blower?

[tonyfoale]

Many people go for oversize ducting. It needs to be large enough to handle the flow without too much pressure drop but not any larger. The larger you go the lower will be the duct velocity. It might seem counter intuitive but blockages are more likely with overlarge ducts because of the low velocity which allows the dust particle to drop out of the flow.
To answer your direct question, firstly with a question. Just what do you mean by "suction"? The suction at the work place depends on the suction at blower minus the pressure loss in the ducting. So as long as the duct is large ENOUGH then the pressure loss will be low in comparison to the suck at the blower. However, air velocity is what you need to consider as far as the ability to pull in the dust is concerned.

[hemmjo]

YES indeed, velocity is the most critical part of a dust collecting system. Velocity is a function of flow volume and duct size. I taught in a school shop that had a very large, very expensive, poorly designed, system. It had mostly 3 or 4" drops, with a 6" on the 24" planer. Each had a gate to block off flow to unused machines. As I recall there was about 8 drops. Those entered into a large duct over head. It started at about 24" at the collector and dropped in size after passing a few drops. Ending about about 10".

The 24" main duct was way too large, even if all of the drops were open. They could not provide enough air flow to keep velocity high enough in the main duct it was always plugging up. The was especially true with the dust collecting filters just a little bit dirty.

The link here is in regard to an HVAC blower system. But the principles are the same. Lots of factors come into play in your design. Bigger is not always better in this case.

There are tables that show required velocities to capture various particles.

Good Luck.

[mccwho]

Read through the post by hemmjo,above !

You could also find several papers on the internet about Fireman's hydraulics. That is an excellent source of technical information as they have to know about water flow, hose size, pressure and number of connections. It does not matter that they focus on water. Water is a fluid as is air, air is just a different density.

There are more PDF's here than you can shake a stick at.

https://www.google.com/search?q=fire...client=gws-wiz

[andrew mcgregor]

Thanks to all for your advice. I’ll get started on the project with a greater degree of confidence.

[nova_robotics]

Tonyfoale and hemmjo are absolutely correct. Prioritize duct velocity over pressure drop.

I too shall unapologetically mix units. 100 mm duct has a cross sectional area of 0.00785 m^2. 150 mm duct has a cross sectional area of 0.0177 m^2. At 1100 CFM (31.1 m^3/min or 0.518 m^3/sec), that gives us average duct velocities of 65.9 m/s for the 100mm duct and 29.3 m/s for the 150mm duct. Using hemmjo's very excellent table (https://www.engineeringtoolbox.com/p...rts-d_134.html) it looks like you're teetering on the edge of running into problems with that 150mm duct because your air velocity is too low.

Stick with the 100mm duct.

[andrew mcgregor]

Nova robotics - thanks for the excellent calculations. I’d done some of the same myself but I have more confidence in your numbers.

Based on your calculations with 150mm ducting and a duct velocity of 29.3 m/s, using Hemjo’s table it looks to me like that velocity will accommodate all or most activities in my wood workshop - in many cases with reasonable head room. Or am I missing something in that simple interpretation eg losses due to various factors?

Regards

Andrew

[nova_robotics]

Nova robotics - thanks for the excellent calculations. I’d done some of the same myself but I have more confidence in your numbers.

Based on your calculations with 150mm ducting and a duct velocity of 29.3 m/s, using Hemjo’s table it looks to me like that velocity will accommodate all or most activities in my wood workshop - in many cases with reasonable head room. Or am I missing something in that simple interpretation eg losses due to various factors?

Regards

Andrew

That's average velocity within the duct. Boundary layer effects mean the air velocity near the walls of the duct are much slower, and around elbows and other features you'll find all kinds of dead air. If you're getting near those velocities you're going to accumulate little deposits of crud all throughout your ductwork. Imagine how bad the hose on your vacuum cleaner would get if you turned the speed of the motor all the way down to the point where it would just barely lift dust. That's the situation you're trying to avoid.

[tonyfoale]

That's average velocity within the duct. Boundary layer effects mean the air velocity near the walls of the duct are much slower, and around elbows and other features you'll find all kinds of dead air. If you're getting near those velocities you're going to accumulate little deposits of crud all throughout your ductwork. Imagine how bad the hose on your vacuum cleaner would get if you turned the speed of the motor all the way down to the point where it would just barely lift dust. That's the situation you're trying to avoid.

Take notice and use 100 mm duct.