Thread: Single Bag High CFM DE

DE manufacturers have a number of methods for specifying flow rates, as you might notice comparing units of similar nominal power.

The big differences in flow rate are caused by two things, input flow restrict and exit flow restriction. Input flow restriction is caused by ducting, bends, corregated flex tube etc, and is generally outside the manufacturers control beyond the machines input fittings.

Output flow restriction is created by the flow path to the bags and the exit resistance of the bags themselves. The typical fabric upper filter bag(s) have to trap the fine particles, but still maintain high flows. The bigger the bag surface area, the more flow you can achieve for a given filter performance. One of the better performance upgrades is to go to pleated paper filters, similar in concept to car air filters or shopvac filters. These offer a significant increase in airflow for similar particle capture ability because they increase the filter surface area significantly by pleating it into a lot of folds so it can be packaged into a similar envelope to the original equipment bag.

Looking at the Carbatech offerings, you are looking toward a 3HP twin bag unit, or a 2HP twin with pleated paper filters to get the nominal airflow you seek. Restricting exit surface area by blanking off one side of a twin bag unit would not work as it occupies the same floor space as it would with both bags, but generates significant back pressure limiting the flow.

I agree with Malb, if you remove one bag you can expect to halve the flow. In that case you might as well get a "2HP - one bag" unit. If you add a Pleated filter you might as well just add it to the 2HP unit.

BTW don't be fooled by the manufacturers specs. It is physically impossible for a 3HP DC to move 2200 CFM unless the bags are new and there is no ducting installed. Once the bags are slightly used and there is a bit of ducting and a few connections involved expect something like half that value.

Thanks malb and BobL, all makes perfect sense.

Do we know if the ratings for DEs are typically reported with or without the filter bag/s attached?

Everything being equal if we were to take two 2hp motors and impeller setups and have one attached to a bag and filter and the other to nothing (direct vent) the one venting directly into the air would be more efficient. But which setup are the manufactures giving us specs for?

Originally Posted by GarethR

Thanks malb and BobL, all makes perfect sense.

Do we know if the ratings for DEs are typically reported with or without the filter bag/s attached?

With a clean fliter bag and no ducting.

Everything being equal if we were to take two 2hp motors and impeller setups and have one attached to a bag and filter and the other to nothing (direct vent) the one venting directly into the air would be more efficient. But which setup are the manufactures giving us specs for?

Of course the less resistance on either in the inlet or outlet side of a DC the more efficient it will be be but running a DC without bags or ducting may also cause the motor to draw too much current and cause it to burn out.

Originally Posted by BobL

running a DC without bags or ducting may also cause the motor to draw too much current and cause it to burn out.

Has anyone every experienced this, because I suspect that it is not true.

Dusties that I have come across always use induction motors. Induction motors run at synchronous speeds (i.e. mains related speeds) under no load and draws their minimum current (35 to 40 % of rated load current) under this condition. As a load is plied to the motor, it slows slightly or "slips", and the current draw increases as load is applied. At its rated power, it has slowed typically about 5 to 8% and the current draw increases to it's rated current.

I currently work with a 10HP dusty on a CNC router and have previously worked with 2, 3, and 5 HP dusties on other gear, and all have slowed down as restriction increases, so restriction would cause an increase in current and a potential to burn out the motor if the rated current is exceeded for a prolonged time. The only dusty I have come across that burnt out was the 5HP mentioned above which lost a phase while running without phase loss protection. This causes the remaining phases to draw excessive current burning their windings.

I would anticipate a dusty running totally unloaded (no resistance at inlet and outlet) to draw about half it's rated current, i.e. more than the unloaded current but less than the rated current. This is based on the notion that the motor is doing some work moving air around the impeller and through the housing and intrisic ducting, but not doing as much work as it would fighting against resistance in the input ducting and semi clogged filters.

Originally Posted by malb

As a load is plied to the motor, it slows slightly or "slips", and the current draw increases as load is applied. At its rated power, it has slowed typically about 5 to 8% and the current draw increases to it's rated current.

The biggest load on a DC impeller and motor (and therefor the greatest current being drawn) is when it is moving the most air it can or when there is the least restriction on it. As restriction is applied, less air is moved and, somewhat counter intuitively, the impeller simply cannot grab as much air and so the motor does less work and so draws less current.

Here's the current V Area thru which air can move (air resistance) data for my nominally 3HP DC with the bags are attached and are moderately clean.

8" impeller entry wide open (ie the 4 x 4: ports removed) 0.35 sqft I= 9.6 A
4 x 4" open ports also 0.35 sqft I=9.4 A
3 x 4" open ports (ie one of the 4" ports is blocked) 0.26 sqft I - 9.1 A
2 x 4" open ports or 0.174 sq ft I = 8.7 A
1 x 4" open ports or 0.087 sqft I = 8.1 A
All 4 x 4" ports closed drops the current to 7.3A

It's pretty clear from this that as air resistance increases the current draw goes down.

With the bags removed and the 8" entry port open the motor draws 13.7A (or the equivalent of 4.4HP) It is not advisable to run the motor like this for too long as it will definitely over heat. Fortunately it has an overload sensor and will trip a current breaker. Motors that have no sensor are the ones that are in danger.

Note also the 0.2A difference in draw between the 8" post and the 4 x 4 which are exactly the same areas but the 4 x 4" ports are more restrictive which is why it is worth throwing these away and using the 8" impeller entry direct.

The DC would be run with ducting to the machines and would vent the finer dust externally via a cyclone.

....from what's been mentioned previously, would it be fair to ignore the manufactures spec & assume that the greater the filter area the greater the cfm of the dusty?

Originally Posted by snapman007

....from what's been mentioned previously, would it be fair to ignore the manufactures spec & assume that the greater the filter area the greater the cfm of the dusty?

Nope, a certain amount of HP is still needed to physically move a certain amount of air. A 1HP with a huge filter is just a waste of a huge fliter.

There are three things to consider when a DC system is designed.
1) The restriction imposed by air bags on the output side
2) The ability of the impeller and motor to move air
3) The restriction of ducting and machine it is connected to etc on the input side.
All 3 have to be optimized for a DC to work at maximum efficiency, just increasing bag size alone might not do anything

..if I'm sending this thread off on a tangent let me know.

So if we take the ducting out of the equation & look solely at the extractor/cyclone, in the Carbatec Catalogue for example, they have 2 different 3hp 2 stage dust cyclones. 1 = $2595 rated @ 2200cfm with filtration to 1 micron and the other 1 = $2995 rated @ 1650cfm with 2 micron filtration. So would Carbatec be measuring the performance of each machine differently because on paper the first 1 that is cheaper with better filtration performs better than the more expensive 1?

Originally Posted by snapman007

the Carbatec Catalogue for example, they have 2 different 3hp 2 stage dust cyclones. 1 = $2595 rated @ 2200cfm with filtration to 1 micron and the other 1 = $2995 rated @ 1650cfm with 2 micron filtration. So would Carbatec be measuring the performance of each machine differently because on paper the first 1 that is cheaper with better filtration performs better than the more expensive 1?

Online the specs are different again from what's in the 2011 catalogue so I suspect there have been input errors along the way, not ideal.

I would not have expected two similarly speced units to deliver such a difference in CFM ratings, ~500CFM+.

Originally Posted by snapman007

. . . . So would Carbatec be measuring the performance of each machine differently because on paper the first 1 that is cheaper with better filtration performs better than the more expensive 1?

I seriously doubt Carbatech measures anything and just accept whatever the manufacturer tells them. The same happens with electric motor power on most cheap WW machinery.

OTOH the unit with the pleated filter should have the greater flow and it does. The other unit has a spun polyester canister which is not that much better than a spun polyester bag. 3HP DCs that use bags need at least two bags to work effectively.

This then answers the original posters question.
If you compare the

CT703 - 1650 cfm 2 micron Polyester canister cyclone
with the
CT-003VB - 3100 cfm 5 micron polyester twin bags DC

You can see the CT-003VB has twice the rating of the "effectively single bag" CT703


BTW Single filter figure ratings are also somewhat meaningless. An OSH grade filter rating is a graph showing the % of particles filtered over a range of particle sizes. A minimum industry grade rating is the % of >0.3 micron particles filtered. The Carbatech cyclone with the 2 micron rated filter could be 90% efficient at 2 microns. The cyclone with the 1 micron rated filter might only be 70% efficient at 1 micron.

Originally Posted by BobL

OTOH the unit with the pleated filter should have the greater flow and it does. The other unit has a spun polyester canister which is not that much better than a spun polyester bag. 3HP DCs that use bags need at least two bags to work effectively.

I believe the two units that snapman007 is referring to have pleated filters, UB-3100ECK and CT-703.

Originally Posted by GarethR

I believe the two units that snapman007 is referring to have pleated filters, UB-3100ECK and CT-703.

Yep - I agree.
Carbatech website says the 703 has a 2 micron spun-bond polyester cartridge. The UB-3100ECK specs just say 1 micron cartridge, maybe this one has a greater surface area. Both cartridges look quite small to be on a 3HP system and I suspect represent the air flow limitations on both machines. The UB-3100ECK also has the dreadful 3 x 4" inlets whereas the 703 has 1 x 8" inlet.