Dust Collector Question

[James Hardee]

Let's assume that a system has a single 4" duct that is 10 ft. long with four blast gates. Let's also assume that the only open blast gate is the first one. Would the suction at the first blast gate be increased if another blast gate was placed in the main duct immediately after the first blast gate?

[Jesse Busenitz]

Are you saying that the second BG would be open? Someone correct me if I'm wrong, but I think your system works best when you have to allow enough air to be coming into your duct work to match what your collector is trying to pull through the blower,
if that is clear as mud.....

[Jason Beam]

The answer isn't that simple. It depends on the blower and the amount of static pressure both in the line and what the blower works optimally at.

In theory, if the blower functioned better at 11" SP than 19" SP and opening the first gate introduced 19" SP and opening the 2nd gate lowered the SP to 11", then yes, it's possible that gate 1 will see better airflow.

But say that isn't the case. Say the blower is better at 11" SP but opening the 4" gate only generates 9" SP. Opening the 2nd gate may have zero impact, or it may lower the overall flow at gate 1.


In the end, it's all about static pressure both of the line and the blower's rating.

[Jim Andrew]

Google Bill Pentz. He has a website and tons of info on dust systems for home shops.

[Ole Anderson]

Correct me if I am wrong (I am sure someone will) but virtually all blowers function on a curve that shows as the air flow increases, the static pressure drops. In other words, as you open more and more gates the total airflow in the main will increase and the available suction or static pressure will decrease. The more static pressure (suction) available, the more flow you will get into an individual blast gate. Open up a second gate, the static pressure in the main drops and then so will the flow (CFM) into the already open blast gate. Other than to maintain a minimum flow in a main duct, it makes no sense to open up more ducts to get better airflow in a working duct. Even then you just need to open up a second gate only occasionally to sweep the main of any settled chips.

[Mike Chalmers]

Google Bill Pentz. He has a website and tons of info on dust systems for home shops.

Just keep in mind that Mr Pentz is a perfectionist due to his own personal issues. Most people cannot afford the recommended system. In my opinion, it is not needed if common sense and good practices are used regularly.

[Wade Lippman]

FWIW Guy in CS at Grizzly told me that if I had a 6" line with 4" drops on my 3hp cyclone, having an extra blast gate would increase suction at the gate I was using because it would allow the impellor to run faster.

I never tested it, but have found that forgetting to close a few blast gates doesn't hurt anything. That is contrary to common sense, but seems to be true.

[Ole Anderson]

FWIW Guy in CS at Grizzly told me that if I had a 6" line with 4" drops on my 3hp cyclone, having an extra blast gate would increase suction at the gate I was using because it would allow the impellor to run faster.

I never tested it, but have found that forgetting to close a few blast gates doesn't hurt anything. That is contrary to common sense, but seems to be true.

That is not correct.. The impeller will run faster if there is less load on it and that happens when there is less air flow. Not more. Impeller will turn slightly faster with all gates closed and no air moving. Does your shop vac speed up or slow down when you put your hand over the end of the hose? Answer: It speeds up because there is less load as it isn't doing as much work as it is not moving air.

If you have a big enough blower, 3 hp or more, forgetting to close a 4" blast gate may not seem to make much difference, but if you were to measure the airflow of the one working gate, I guarantee you would see a measurable reduction in airflow as you open more gates. And long term you would see your Dylos particle count start to climb.

And don't get me started on putting holes in your router enclosure to get better dust collection. Cleaner enclosure, ok.

[Lornie McCullough]

James.... I interpret your question as asking if the volume of the closed system beyond your first opened blast gate affects the efficiency of the system. Hence you are asking if decreasing that volume with an additional blast gate immediately behind the first opened gate will increase efficiency.

If I am interpreting correctly, I would say: The closed system behind your opened blast gate is evacuated quickly enough that you would not notice any improvement in your system with an additional blast gate. This assumes, of course, that your closed blast gates are not leaking any air......

Lornie

[Thomas Canfield]

The cyclone efficiency is based on the velocity (air flow volume) and particle density/size. That is why opening multiple gates on a larger system can provide better operation in some cases. One really has to look at all the components in the system, but I usually have an extra gate or so open on my 2HP system to get higher air flow through cyclone for better dust removal and less dust to filter.

[David Kumm]

Separate questions here. You can increase or change cyclone efficiency by opening a second gate- although opening a second gate when the first is the size of a main is pretty incremental- due to increasing velocity into the cyclone to a more optimal number for its design. Opening a second gate will not increase cfm through the first. Generally it will decrease it but at best it will leave it the same if the impeller and main is sized for it. While the cyclone may get more efficient, if the cfm at the machine you are using is decreased, more crap will get into your lungs and never hit the filters. You have to be careful matching the flow to the impeller and cyclone. Dave