Maximum Air Flow

[dusty]

We have been told that the Maximum CFM of the DC-3300 is 330CFM but that the individual hoses restrict the air flow to 200 CFM in any one hose (due to hose characteristics. It was further stated that with two ports open, each hose would provide 150 CFM. I don't know why it would not be 165 CFM (330/2) but that is for a different discussion.

Is it the 3 port manifold that imposes the limitation and if it is could the manifold be modified so as to allow the maximum 330CFM at one larger port?

[RFGuy]

Dusty,

All good questions. I don't have concrete answers, but I do remember that the 3 port manifold was quite restrictive. I have the 42" filter hood and it never fully inflates on my DC-3300, i.e. the top is rather limp and caves in a bit. As soon as I removed the 3 port manifold to do some experiments and measurements when I was considering connecting 4" PVC ductwork to it, the 42" filter hood filled completely. So the manifold is definitely restricting airflow, but is it 10%, 20% or ??? It has been awhile so I don't know if I still have the CFM measurements written down anymore. Unfortunately my DC-3300 is in my attic and with the current temps here is difficult to access. If I can get up there early one morning, I will try to do a few experiments and measure with my anemometer for you...

[dusty]

Dusty,

All good questions. I don't have concrete answers, but I do remember that the 3 port manifold was quite restrictive. I have the 42" filter hood and it never fully inflates on my DC-3300, i.e. the top is rather limp and caves in a bit. As soon as I removed the 3 port manifold to do some experiments and measurements when I was considering connecting 4" PVC ductwork to it, the 42" filter hood filled completely. So the manifold is definitely restricting airflow, but is it 10%, 20% or ??? It has been awhile so I don't know if I still have the CFM measurements written down anymore. Unfortunately my DC-3300 is in my attic and with the current temps here is difficult to access. If I can get up there early one morning, I will try to do a few experiments and measure with my anemometer for you...

I appreciate that.

When you say "Remove the 3-Port Manifold" are you saying remove the little box on the side of the DC? If I remove that should I just expect to find a larger portal.

[RFGuy]

When you say "Remove the 3-Port Manifold" are you saying remove the little box on the side of the DC? If I remove that should I just expect to find a larger portal.

Yeah, when you remove the 4 screws/bolts that attach the 3 port manifold to the DC-3300 you will be left with an inlet that is slightly larger than 4". Going off memory here since I can't see my DC-3300 right now, but it is very easy to remove this manifold and put it back on if you want to take a look.

[dusty]

When you say "Remove the 3-Port Manifold" are you saying remove the little box on the side of the DC? If I remove that should I just expect to find a larger portal.

Yeah, when you remove the 4 screws/bolts that attach the 3 port manifold to the DC-3300 you will be left with an inlet that is slightly larger than 4". Going off memory here since I can't see my DC-3300 right now, but it is very easy to remove this manifold and put it back on if you want to take a look.

I have been hesitant because I don't want to destroy a gasket unnecessarily

[RFGuy]

Dusty,

I have some numbers for you. Keep in mind, I have a 42" filter hood on and this DC-3300 was purchased new but is now 20+ years old (still in great shape though). Filter is NOT freshly clean, but isn't that dirty either. I measured everything using my anemometer and I chose to measure airspeed (ft/min) and to calculate airflow (CFM). The anemometer calculates airflow, but you have to specify the area of the inlet, so I just calculated airflow in an Excel sheet instead. No hoses were used, so measurements were taken right at the manifold. The first set of numbers below are the measured airspeeds and are shown in blue. The second set of numbers are the calculated airflow and are shown in red. So, there was a noticeable difference between the left or right ports and the downward facing one in the center. Given this, I only measured the left and right ports for the 2 ports open at a time measurement. As you can see from the numbers in red, my actual airflow numbers are quite a bit worse than the "design" numbers that Jim quoted. The highest airflow achieved was with only one port open and gave 142.7CFM. Even with all three ports open and adding each one together, it is only 218CFM total which is much less than the theoretical 330CFM spec'ed for the DC-3300. All of this just confirms what I have been saying for awhile which is that the DC-3300 is a very anemic dust collector. In fact, I got better performance with my ShopVac™ and switched to it instead.

Removing the 3 port manifold is very easy. Just 4 medium size screws to take off the Shopsmith nameplate, then 4 large screws to remove it from the DC-3300 housing. There are not seals here to my knowledge...the backside of the manifold fits snugly into a 4" inlet inside the DC-3300 housing. So, I removed the manifold and measured at least 5583ft/min, or 477CFM. I say "at least" because I don't have a 4" PVC setup to fit to my anemometer so I don't have a perfect seal around it like the other measurements. In reality, it could be even higher than this for the 4" measurement. So, as you can see the blower inside the DC-3300 is capable of more, but the 3 port manifold is very restrictive. I am comfortable in saying this because the area of the 3 ports (2.3" diameter) is nearly identical to the area of the single 4" inlet. With equal area, the airflow should be the same, but it is not, e.g. 218CFM on 3 open ports versus 477CFM on 1 4" port.

Presumably the DC-3300 upgrade (or DC-6000) will have significantly more airflow than the numbers below, but if they have the same 3 port manifold then I suspect they will be severely limited in performance as well.

AIRSPEED - <measured>
1 port open at a time: (units=ft/min)
Left: 4944 Center: 4494 Right: 4894

2 ports open at a time: (units=ft/min)
Left: 3411 Right: 3388

3 ports open at a time: (units=ft/min)
Left: 2584 Center: 2497 Right: 2479

AIRFLOW - <calculated>
1 port open at a time: (units=CFM)
Left: 142.7 Center: 129.7 Right: 141.2

2 ports open at a time: (units=CFM)
Left: 98.4 Right: 97.8

3 ports open at a time: (units=CFM)
Left: 74.6 Center: 72.0 Right: 71.5

[dusty]

Your measurements are very interesting and encouraging. Why? Because they look very similar to mine. I just finished taking the readings documented here. These are the averages of three independent sets of measurements. I do not have a means to measure CFM. My numbers are all RPM of the test fan.

Your numbers make me confident that I can use my numbers as the base line for my tests with the DC-6000.

Your numbers are more meaningful because they are air flow and air speed while my readings are all the RPM of a fan inserted into the flow stream and therefore reflect some inherent error. I tried hard to set set consistently so as to reduce that error.

The data shown in this chart is suspect. Addition measurements were taken today (same configurations) that resulted in different numbers

Capture.PNG (11.44 KiB) Viewed 2546 times

[Hobbyman2]

Food for thought , Years ago we had a gauge on our vacuum line next to the collection filter , when the gage was in the yellow we changed / cleaned the filter and emptied the saw dust ? pretty sure they are still out there , maybe even digital , any guessing on the numbers can be eliminated and may even help in trouble shooting a leaky system ? might not be worth the cost for a non commercial unit ? jmo

[dusty]

Food for thought , Years ago we had a gauge on our vacuum line next to the collection filter , when the gage was in the yellow we changed / cleaned the filter and emptied the saw dust ? pretty sure they are still out there , maybe even digital , any guessing on the numbers can be eliminated and may even help in trouble shooting a leaky system ? might not be worth the cost for a non commercial unit ? jmo

Definitely not in my plans. Remember, this is not a dust collection system. It is a simple dust collector with a hose. Oh yeah, it has a collector bag that I seem to punch a hole in frequently so leaks are inevitable.

[RFGuy]

Food for thought , Years ago we had a gauge on our vacuum line next to the collection filter , when the gage was in the yellow we changed / cleaned the filter and emptied the saw dust ? pretty sure they are still out there , maybe even digital , any guessing on the numbers can be eliminated and may even help in trouble shooting a leaky system ? might not be worth the cost for a non commercial unit ? jmo

Might be a good idea, though it is a different kind of measurement. So, I measured airspeed and calculated airflow using an anemometer. What you are describing is a manometer. Typically these are suction tubes with a loop and water inside to show how many inches of water column they measure, so it is measuring pressure, not airflow. Both are important, but for different reasons. Typically vacuums, such as a ShopVac excel at suction (static pressure), whereas true dust collectors are optimized more for airflow. Dust collectors have to have a minimum amount of suction or it will be difficult for them to get dust pulled into the hose inlet, but once it is there, airflow is more important to keeping it moving down the hose/duct to its final destination. Building a manometer to compare before/after of the DC-3300 upgrade would be cheap and easy. However I don't know how much information this would really give us, e.g. it may measure less suction than the original. This may be the case if the new blower was designed to optimize airflow more over static pressure. There are lots of videos online for how to make a manometer and I included one below for reference if anyone wants to try it.

CFM vs. Static Pressure, defined
Air volume is measured in cubic feet per minute (CFM) or liter per second (l/s)--the volume of air that the device can move from its intake to its exhaust in a given amount of time.

Static pressure is measured in inches of water (often referred to as inches water column, or inches WC), as in, how many inches the suction device can lift water up a pipe, or in millibar (mbar).

In simple terms, static pressure determines how much resistance (“static pressure loss”) your dust collector can overcome in order to move a given volume of air through the hoses and pipes. When it comes to designing a dust collection system, the raw CFM figure is initially more important than the raw static pressure figure, though static pressure does come into play once you start designing your ductwork (more on this later).
https://woodworking.stackexchange.com/q ... a-shop-vac