Use both 4 in ports at the same time on a 2hp dust collector

[Larry Frank]

I find some of the assertions in this thread interesting.

If you have one 4" port open at a machine, you will get a certain flow at the machine. If you open a second 4" port, the overall flow through the dust collector will increase but the flow at your machine will decrease. What is important is the flow at the machine that you are using. Of course if your machine is not very powerful and you are running 6" ducts, you may need to open a second port to increase speed in the 6" pipe to have sufficient air flow to keep dust and chips moving.

The other point is that it is only necessary to increase the port size on your machine if you do have sufficient air flow because of your dust collector. With my dust collector, I get about 900 cfm through one open 4" blast gate on my system. This is more than enough for good dust collection. If I open a second 4" blast gate, I get about 1300 cfm but the flow from the first blast gate goes down slightly. If I open a single 6" blast gate, I get about 1500 cfm. Note: I am running a 5 hp Oneida Super Dust Gorilla. With this machine, the 4" ports on my machines work fine. However, with a smaller machine such as the HF, this may not be the case. It important to take all of the advice and try to analyze it as it applies to your machine, duct work and your requirements.

I did not use a fan anemometer to get the above numbers but measured the static pressure and used the performance curve that I had previously developed using a hot wire anemometer and a digital pressure gauge.

[Mike Chalmers]

That would be a measure of speed, not volume, wouldn't it? Any time you narrow an opening it will increase velocity, but the amount of air moving through it will not increase. From what I've been reading, it's not the speed as much as it's the combination of speed and the amount (volume) of air that keeps things suspended and moving. If there's not enough to keep it suspended in the airstream, it can drop out and create eddies and be far less efficient at keeping the fine stuff moving along. Anything not moving will create drag and slow things further, though it may not be evident at the port until it's severe.

While what you say is true, it is also not relevant to the point I am making. The premise was that two 4" ports are open, but only one is actually in use. In that situation, it is obvious that more FPM is going to yield more CFM.

[Mike Chalmers]

I find some of the assertions in this thread interesting.

If you have one 4" port open at a machine, you will get a certain flow at the machine. If you open a second 4" port, the overall flow through the dust collector will increase but the flow at your machine will decrease. What is important is the flow at the machine that you are using. Of course if your machine is not very powerful and you are running 6" ducts, you may need to open a second port to increase speed in the 6" pipe to have sufficient air flow to keep dust and chips moving.

The other point is that it is only necessary to increase the port size on your machine if you do have sufficient air flow because of your dust collector. With my dust collector, I get about 900 cfm through one open 4" blast gate on my system. This is more than enough for good dust collection. If I open a second 4" blast gate, I get about 1300 cfm but the flow from the first blast gate goes down slightly. If I open a single 6" blast gate, I get about 1500 cfm. Note: I am running a 5 hp Oneida Super Dust Gorilla. With this machine, the 4" ports on my machines work fine. However, with a smaller machine such as the HF, this may not be the case. It important to take all of the advice and try to analyze it as it applies to your machine, duct work and your requirements.

I did not use a fan anemometer to get the above numbers but measured the static pressure and used the performance curve that I had previously developed using a hot wire anemometer and a digital pressure gauge.

What he said.

[Mike Chalmers]

You will have more air flow at the opening of the 4", but less at the exit of the 6".

Yet no dust or chips in the mains.