More on Dust Collection

I’ve been doing quite a bit of research and reading on the topic over the course of the day, and the amount of information out there is huge, somewhat conflicting, and the bottom line on it, is the problem is not particularly straightforward.  So much for finding a simple answer.

Some of the things I have been determining are:

a large pipe size for the main trunk line would be best.  Whether I am prepared to increase the size of the pipe run I currently have is a difficult question.  There is a definite cost element there, although the pipe I have could be redeployed to a water catchment system I have wanted to do for ages, so that may be an option.

the flexible pipe that is typically sold for dust extraction is really inappropriate for long runs, and at best should only be used (as I have) between the machine and the main trunk line.  It is not smooth inside, so that dramatically increases the friction in the pipe, significantly affecting performance.

the flexible section I was using at the end of the main trunk to connect to the DC is a bad idea, so moving the DC is probably the most sensible, and perhaps erecting some noise damping is the best compromise.

keeping the pleated filter as clean as possible will really affect collection performance.

you can overdo the amount the motor can pull – in other words, if you run the collector pulling too much air (too large a pipe, and/or venting without collecting, you can result in the motor drawing too many amps and burning out.  Still getting my head around that one – something is not quite clicking on the logic states.

10 Responses

  1. Stu, I went from using the flexible hose in my old basement shop to using 4″ solid PVC in my new detached shop. Using the same Jet 1100 CFM Dust Collector. My dust collection has dramatically improved at the table saw and chop saw. I would recommend switching to the larger pipe.

    A J

    • I went with a 4″ PVC system last year (as featured in Episode 31), but the question is now whether the system main trunk should be increased to 5″ or even 6″.

  2. You need to be careful with the upsize of the main line. As you increase the duct size, the velocity drops. You do not want to go lower than 3500-4000 fpm or the air will not effectively carry away the sawdust. However, this is where it gets tricky. If you keep the size too small, it will increase the pressure drop and the dust collector will not move enough air (this is when you start over -amping the fan motor).

    • It has (and still is) surprising just how many variables (and how much information, and misinformation) are involved in what on the face of it, should be a simple exercise. Your point about overamping due to a too-small a diameter pipe makes a lot more sense than what I read about too large a pipe causing it to occur!

      Perhaps instead of the basic flowmeter I have been using, a doppler flowmeter would be better to accurately assess particle speed in the pipes. Unfortunately, the last one of these I had access to was when I was working at a Geothermal power station, and was assessing fluid speeds in the main cooling pipes. Not something an average woodworker has in their shed!

      • Actually, a flowmeter is necessary. The easiest thing to do is just measure the static pressure in the pipe. This is essentially an HVAC problem with a different desired outcome.

  3. Hi Stu, have you made any further progress with your dust collection?

    Is there a viable way to measure fpm for the hobbiest?

    I am getting a new dust collector this Friday, i will be using a 125mm backbone i just want to make sure my runs don’t exceed the machines capability’s.

    thanks
    Joe

    • Gidday Joe – guess your dusty has arrived! There is a lot of articles out there about the whole question, including the size of the main trunk etc. Exceeding the machine’s capacity does seem to be a bad thing – overheating of the motor. My system is working ok, with 100mm runs everywhere and blast gates limiting flow to the one machine currently in use.

      Measuring flow rate is more difficult than I was hoping – there is a significant edge effect, so measuring peak flow rate at the centre of the pipe gives just that-peak flow. Dividing by the square root of 2 rings a distant memory for approximately adjusting the peak to the mean flow. Wish I still had easy access to doppler flow meters – that’d help a lot!

  4. I know this is an old post, but I have to chime in:

    Too little air restriction on a blower can indeed overload the motor. Think of it as the work actually being done – less air restriction means MORE air gets pumped – thus more work, thus more load on the motor.

    Another way to picture it: when you block the hose on a shop vacuum, what does the motor do? It speeds up – because there is less load on the motor. If adding restriction increased the motor load, blocking the hose would make the vacuum motor bog down.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: