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CFM, Horsepower, and
Cyclone Size

Some companies who make or sell dust-collectors make a big deal about the CFM ratings on their systems or the motor horsepower ratings. In far too many cases, their "specifications" appear to be virtually meaningless because they cannot exist in a normal installation. Unless you get a complete picture of your needs, then base it on "real numbers" rather than values measured in some undisclosed, arcane way that has nothing to do with actual use, it is difficult, and sometimes dangerous to trust their specifications. In reality, unless you know how their performance specifications were measured, you have no possible way of knowing whether their system will be appropriate for your needs or not.

Let us use a simple approach to solving the problem that will ensure a properly sized system that is capable of producing the needed results. This is done by following a few simple steps:

  1. Determine how many CFM are required by each individual machine or tool that will be used in the shop.

  2. Determine which machines or tools, if any, are likely to be used simultaneously.

  3. Calculate the minimum required CFM through the collector, and determine appropriate ducting sizes for each tool.

  4. Determine the proper cyclone size.

  5. Determine what kind of blower is needed.

  6. Determine blower motor size.

  7. Determine the corresponding electrical power requirments.

Individual Tool
CFM Requirments

Here is a list of machines found in a typical shop and the corresponding typical CFM required to properly remove dust from the machine to a dust collector during normal operating conditions:

Tool CFM Req'd Duct Size
Table Saw up to 16" Bottom: 450-500
Top: 350-400
Total Req'd CFM: 800-900
Bottom: 6"
Top: 4"
Jointer:    Up to 6"
6" to 12"
12" to 20"
20" to 24"
Planer 800 6"
Radial arm saw up to 20" 350-400 6"
Miter saws and similar 350-400 6"
Shaper or router table 350-400 6"
Band saw Under table: 350-400
Up side of bottom wheel: 350-400
6" x 4" x 4"
Disc sander up to 12" 350-400 6"
Belt sander up to 6"

Belt width 6" to 9"
Head end: 450-400
Tail end: 350-400
Head end: 450-400
Tail end: 350-400

Note that these CFM figures are minimums for good collection of fine airborne dust with well-designed collection hoods to direct the air effectively. There is no harm in running more CFM than is required for a given tool or group of tools being used simultaneously.

Duct sizes are specified as 6" because 4000 feet per minute minimum air velocity is required for ducts with air flowing vertically upward (only 3000 ft/min is needed in horizontal runs) to prevent plugging and keep dust dispersed in the flowing air. Do not use 4" or 5" ducting because the static pressure loss is too high, and will degrade cyclone performance and micro-fine dust separation. To maintain the required 4000 ft/min in vertical 6" ducts, minimum air flow through the duct must always be at least 800 CFM, regardless of what the tool itself actually requires. An 18" cyclone does not operate effectively below about 800 CFM.

The minimum CFM that must be flowing through the cyclone and the entire system is equal to the CFM needed for the tool in your shop that has the highest CFM requirement, unless you expect to use other tools at the same time.

Simultaneous Operation

If any of the tools in your shop are to be used simultaneously, such as when two people are working at the same time, list them in groups as appropriate, along with the CFM requirements for each. This must be done before the system CFM requirements can be ascertained.

For example, if you plan to run a shaper or router table at the same time as a 10" table saw, you would need a combined total of about 900 CFM for the saw plus an additional 350-400 CFM for the router or shaper, which adds up to 1200-1300 CFM. If the dust collector cannot deliver that much air flow, it would require that no other tools be used when the table saw is in use. However, an 8" jointer and a router table could be run simultaneously if the collector can produce at least 800-900 CFM (450-500 for the jointer and 350-400 for the router).

System CFM Required

The minimum CFM of air flow that must be available from the dust collector is the largest combined CFM of any two or more tools that will be used at the same time, or the largest CFM required by any individual tool that is never used when any other tool is running, whichever is greater.

Ductwork Sizing

Ductwork must be sized to match the CFM flowing through that part of the system. General guidelines are as follows:

4" Duct:

Used only for short-run connections from wye connections on bandsaws or closely-spaced router tables or to the overhead hood on a table saw or other similar arrangement. 4-inch ducting is not suitable for runs longer than about 2-4 feet, depending on available system static pressure.

5" Duct:

There is little reason, if any, to use 5" duct in a typical dust-collection system.

6" Duct:

The workhorse of effective dust collection systems. 6" ducting requires at least 800 CFM air flow through vertical runs in order to maintain 4000 ft/min minimum velocity, and is well-matched to 18" cyclones. When collecting dust from table saws, use 6" ducting underneath, 4" to the hood over the blade, and a 6" x 6" x 4" wye with a 4", 45-degree side leg. For bandsaws, use two 4" hoses from a symmetrical 6" x 4" x 4" wye.

8" Duct:

To maintain 4000 ft/min, 8" ducting requires at least 1400 CFM in vertical up-flows. For horizontal runs, 8" ducting must operate at not less than 1050 CFM. 8" ducting is rarely used outside of commercial shops running fairly large machines. However, an 8" duct works quite well when collecting simultaneously from two 6" vertical ducts from two adjacent machines requiring 800 CFM (or more) each, for example.

8" ducting with its higher CFM air-flow requirements to prevent clogging is suitable for use with 20" cyclones. It should not normally be used with 18" cyclone units due to higher static pressure loss and the potential for turbulence in 18" cyclones at higher air flows.

10" Duct:

Use this pipe only with very large cyclones running 3000 to 4000 CFM (24" diameter or larger). It is also suitable for connecting multiple 20" cyclones or 18" cyclones (with suitable adapter to fit the cyclone outlet) to a common shared high-volume blower unit feeding a shared filter bank.

Cyclone Sizing

Cyclones must be sized so that the incoming air velocity at the interior mouth of the inlet is at least 3000-3500 feet per minute. Cyclones also have an upper limit where turbulence sets in, causing disruption in separating fine dust.

18" Cyclone:

Inlet cross-section is 4.5" x 9.0". The minimum 3000 ft/min inlet air-velocity requirement is met at 850 CFM, and the unit should work well up to about 2000 CFM.

20" Cyclone:

Inlet cross-section is 5.0" x 10.0". The minimum 3000 ft/min inlet air-velocity requirement is met at 1050 CFM, and the unit should work well up to about 2800 CFM.

At higher air volumes through 18- and 20-inch cyclones, it may be necessary to use larger collection ducting in order to reduce static pressure losses through piping and fittings. To reduce air-flow resistance, use wide-radius elbows, bends, and fittings (centerline radius should be not less than 1.5 pipe diameters and larger where practicable).

Selecting a Blower

The blower is to a dust collection system what the engine is to a large truck. If the engine is too small, the truck cannot move at the required speed. If the blower is too small or turns too slowly to create the necessary air flow at the maximum normal system static pressure, it cannot properly capture and separate the dust being produced at the tools and machines in the shop.

Many producers of popular dust collectors do not publish any information about the air flow through their machines at various levels of static pressure so one can make an intelligent evaluation of their product. Consequently, it is usually wise to either tread very carefully before deciding to buy such products, or avoid them altogether, especially if the numbers are a bit suspect. One glaring indicator is that if the blower is not at least 14 inches in diameter, and turning at 3400 RPM or more, it will likely not be suitable for serious dust collection because it is physically impossible in most situations to develop the necessary static pressure without meeting those two minimum requirements.

For the cyclone to perform properly, the blower needs to develop the required air flow at not less than 8-10 inches of static pressure. This allows 2-3" of pressure loss through the cyclone, an additional 1-3" through the filters and associated ducting, while leaving at least 2-4" of static pressure for the ductwork and collection hoods. Our cyclones should operate in the area of 1.5-2" of static pressure at air flows near 3000-4000 ft/min at the inlet. However, if you operate at higher air flows, the static pressure loss increases proportionally; hence, the need for proper blower performance at least 8-10" static-pressure.

Motor Size and
Electrical Power Requirements

Once the total system CFM requirements are determined and the cyclone has been selected along with the appropriate blower, you are ready to address the electric motor for driving the blower, and arrange to provide appropriate electrical power. Note how this is backwards to what most people do when buying a consumer dust collector. Even many commercial shops use the same backwards approach then wonder why they don't get the results they expected.

In most cases, the fan or blower manufacturer will specify how much power (in horsepower) is required to turn the blower at the proper speed to produce the required air flow at the desired minimum static pressure. The motor should be at least that large. Thus, if a blower specifies 4.5 HP at the desired performance level, a 5-HP motor would be a good selection, but a 7 or 7.5 HP motor would also work well. However, a 7 HP motor will not produce more air flow at the specified speed than would a 5-HP motor. Air flow is determined entirely by the blower design, and as long as the motor produces at least the required horsepower and delivers it to the blower, increasing the motor's power rating will not move more air unless the speed of the impeller is increased. Increasing the motor size beyond the required level may result in a cooler-running motor and may welllead to longer life and lower maintenance, but that is not a guraranteed outcome in all cases.

Do not attempt to operate any blower at a speed higher than it was designed for. Doing so can cause blower failure and could result in serious bodily injury or even death.

Electrical Power:
An electrical circuit must be provided that is capable of delivering at least as much current as is specified on the motor nameplate. The circuit breaker or fuse must be rated at not less than 25% more than the motor nameplate current. Power panel breakers and fuses are not suitable for protecting large electric motors from overload or burnout. All dust collector motors must be equipped with current sensing devices that can detect an overload or overcurrent condition and immediately remove power from the motor. Failure to provide such protection can result in burned out motors or even a fire in the shop.

In general, our dust collection systems are suitable for use only on single-phase 240-volt power, or 208-volt 3-phase power if the blower is outfitted with a suitable 3-phase motor. All motor wiring should be performed by a qualified electrician. Improper wiring can cause serious injury, death, and severe equipment damage.

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Updated September 23, 2004

©Copyright 2004, Clarke F. Echols and Clean Shop Air, LLC. All rights reserved.
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