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First off, remember that FPM should always be approximately 4000 in the branches and 3500 in the main duct. There are several steps involved:
Some typical tool requirements are:
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| Tablesaw | 350 - 600* |
| Bandsaw | 400 |
| Jointer | 350 |
| Planer | 450 |
| Mitre Saw | 300 |
| Router Table | 350 |
Static Pressure values for 1' of straight spiral pipe. Values for 3500FPM are about 20% less than at 4000FPM, and have been calculated from the 4000 FPM figures:
| Duct Diameter | SP @ 3500 FPM | SP @ 4000 FPM |
| 3" | 0.08 | 0.10 |
| 4" | 0.056 | 0.07 |
| 5" | 0.044 | 0.055 |
| 6" | 0.036 | 0.045 |
| 7" | 0.030 | 0.038 |
| 8" | 0.024 | 0.030 |
Other duct elements are ratios of the values in the above table. For example, a 4" diameter 45º Elbow would use the figure for the 4" duct above (0.056 @ 3500 FPM, or 0.07 @ 4000 FPM), and then multipy it by a factor of 4 to 7 to get the total SP drop for the Elbow.
| Duct Element | Multiplier |
| 45º Elbow | 4x - 7x |
| 90º Elbow | 8x - 15x |
| Wye ("Y") | Need info |
| Flex Duct | 2.5x - 3x |
| Other Correction Factors |
| 30 micron Filters add 1" SP* |
| Cyclone adds 1-3" SP |
| Dust Hood adds 1" SP |
* Subtract 30% from effective CFM if using 5 micron instead of 30 micron filters and 50% if using 1 micron instead of 30 micron.
Example: a 90 degree elbow is the equivalent of 8 feet of pipe. So 5' of 4" Flex Pipe, then a 90, then 20' of 4" spiral pipe = 10+8+20 = 38' x .07 = 2.66" SP
For a simple system, such as when you plan to drag a flex hose and/or collector to each tool individually, you can start with the tool with the highest requirement such as a Planer that requires 450 CFM. Add 1" SP for the dust hood, 0.18" per foot of flex ducting (say 12'), and 1" for filter bags. This will give you a static pressure for the duct system of 4.16" SP. Thus you need a system that can deliver a minimum of 450 CFM @ 4.16" SP. The 1200 CFM unit above will meet this requirement (a 650 CFM unit may fall short). But, if you decide to go with 5 micron bags (highly recommended) instead of 30 micron, you will reduce the available CFM by approximately 30%. So on the 1200 CFM unit at 4.01" SP (close to what we need) you will get 560 CFM, which should still be close enough (since 4.01" SP is less than the 4.16" we need then we will need a slightly higher CFM to compensate, but since we have 110 extra CFM delivered we should be OK).
If you get tired of emptying the bags and cleaning chips from the impeller (blower) you will probably add some type of chip separator. This will increase your SP by anywhere from 1" to 3" (depending on design) and may put you in a position of moving up to a 1500 or 1900 CFM unit.
If, on the other hand you decide to install permanent ducting, you will
need to calculate your SP for the highest resistance run in your system.
My worst run is from my TS:
| Description | Dia" | Feet | Factor per foot* | Total SP" |
| Hood | - | - | - | 1 |
| FlexDuct | 5 | 4 | 0.165 | 0.66 |
| Pipe | 5 | 6 | 0.055 | 0.33 |
| Flex | 5 | 4 | 0.165 | 0.66 |
| Pipe | 5 | 3 | 0.055 | 0.165 |
| 90º | 5 | 9 | 0.055 | 0.495 |
| Vertical Rise | 5 | 10 | 0.055 | 0.55 |
| 90º | 5 | 9 | 0.055 | 0.495 |
| Main | 6 | 28 | 0.035 | 0.98 |
| Flex | 6 | 1 | 0.165 | 0.165 |
| Cyclone | - | - | - | 1 |
| 90º | 7 | 13 | 0.026 | 0.338 |
| 90º | 7 | 13 | 0.026 | 0.338 |
| Vert | 7 | 8 | 0.026 | 0.208 |
| 90º | 7 | 13 | 0.026 | 0.338 |
| Filter Loss | - | - | - | 1 |
| Total = 8.722" SP |
* NOTE: in this calculation I calculated 5" Ell's at an equivalent of 9' and 7" Ell's at an equivalent of 13' which are the exact values for the Oneida pipe I am using. Flex duct uses the exact SP value as provided by manufacturer.
Thus, I need 500 CFM @ 6.5" SP at the cyclone entrance or 500 CFM @ 8.8" SP at the dust collector. A 2hp/1500 unit would be close, but still fall under a bit, so I'd have to look at a 3hp/2000CFM unit if I go with this approach.
Oneida and Air Handling Systems both provide some good information on designing systems and calculating SP". Make sure you order the Air Handling Systems printed catalog. Besides some good design information not included on their website, they have some very useful duct elements.
After all of my research, I determined that the Oneida 1.5hp integral system was the best option for me. No other combination of units can provide the same filtration from a health standpoint at anywhere near the $600 cost of the Oneida. The Oneida takes up a 2' x 2' floor space that is significantly less than the 3' x 5' area a big-bag + cyclone option will waste and will not require that everything be housed in a separate closet.
It is possible to meet the second requirement of just chip collection (eg, we don't care about our health) for less money, especially if we don't mind building some stuff ourselves, emptying bags instead of cans, and/or dragging flex duct to each tool each time we use them. If you only use power tools occasionally, you can probably get by with a 1200 CFM DC, 5 micron bags, and maybe a garbage can cyclone (from Veritas) or homemade chip collector for less than $350. Some people have also purchased 650CFM systems and been happy with the chip collection. It will really come down to what you are trying to accomplish.
Better can mean two things: A) Filters smaller particles and B) More Surface Area.
Most systems come with 30 micron bags. If you place the system outside or in it's own closet with better filters to filter the air returning to the shop then this might be OK, but most likely you will want a better filter. Most mfr's offer 5 micron and 1 micron bags and a few have 0.3 micron bags. Some of these bags will significantly increase your SP, but some of the mfr's have stated that they will not. Comments from people who have used them range from no impact at all to "no suction left". Before you purchase ask the mfr to clearly outline to you what impact the new bags will have on SP compared with the stock bags.
As we discovered above in our discussion on Face Velocity, you can almost always improve a system by increasing the surface area (larger bags).
The integrated Oneida cyclone/filter is a special case that is unlike
any other system currently avaiable, and deserves special mention. The
pleated design of the filter cartridge on the Oneida provides for a large
surface area in a small volume. Keep in mind that with the 'big bag' type
systems most of the bag volume is unused, especially if you have a chip
collector. Since the 1st stage chip collector gets everything but the really
fine dust, all you really need is filter surface area and enough air space
to carry the air to the filter surface and distribute it evenly. For instance,
on the big bag systems, you could flatten the bags down to about 2" - 4"
inside airspace and everything would work as well (OK, almost as well,
you need something to distribute the air evenly across the surface area).
Also, the cyclone on the Oneida is efficient enough to remove a large portion
of the dust prior to the filter, so what does make it to the filter already
has a fairly low dust content, thus extending the time between cleanings.
Replacing the lower bag with a chip collection can or plastic bag
The short and easy answer to this is absolutely do not do it. Doing this will severely reduce the available filter area and thus increase the face velocity through the remaining filter area. The net result will be lower cfm for your tools and a greater amount of harmful dust exhausted into the air that you are breathing. The more difficult answer is maybe. As long as you provide adequate filter area in the top bag for the air volume you are moving (see face velocity above), making this change will make emptying the chips much easier. The problem will be getting an appropriate amount of filter topside. With even the smallest system you will need at least 40 sq ft of 16oz felt fabric which is a 5' high bag with a diameter of 2.5' (this is a very large bag).
Duct sizing depends on your system, but generally for a single person shop you will have a 5" or 6" main duct with 4" or 5" branches. It is important that you maintain approximately 4000 FPM in the branches and 3500 FPM in the mains. The main duct is usually a straight horizontal run and thus a lower velocity will effectively convey the wood chips while a branch circuit usually has bends and vertical sections which need a higher velocity to insure that chips and other debris stay airborn. Higher velocities also mean higher resistance and will have a negative impact on CFM thus your desire to use the lowest velocity possible that will insure all debris are carried to the cyclone or collector.
Oneida
and Air Handling
Systems both provide good information
on their sites for duct sizing and will design a ducting system for you
if you order from them.
The use of plastic ducting has been a subject of much debate. Personally I don't believe the cost of metal is enough over plastic to make it an issue for me (depending on where you buy what). All commercial shop systems utilize spiral metal pipe. Many people prefer plastic since it is less expensive, easier to put together, and generally more available at local stores. However, plastic does require a ground wire either inside or out to eliminate static electricity which lessens the 'ease of install' element. If you do choose plastic, make sure that you get a large enough inside diameter and that the wall thickness is appropriate for the system you are installing. For those of you worried about the issue of static discharge causing a dust explosion in plastic duct, look here for a very thorough discussion of the topic, inlcuding way too many physics lessons.
HVAC ducting can work, but there are 2 critical elements to be aware
of though. 1) You need to make sure that you use wide elbows. The elbow
or WYE should have a radius of at least 1.5 times the diameter of the pipe
(EG, a 5" elbow should have a radius of at least 7.5") Wider is better.
2) Use a minimum of 26 gauge metal and preferably 24 gauge. Both are available
from HVAC suppliers. The lighter 28 and 30 gauge stuff sold at home stores
can deform or crush when the system is running (kinda fun to watch though
if it's someone else's system).
Is that nice looking clear 2" dust collection pipe OK?
My local store sells a nice looking line of tools including a dust collection system. The dust system includes some cool looking 2" clear plastic pipe and has connectors for each of their tools. Is this a good choice? No. This type of system is completely inadequate for stationary woodworking tools. Recall the statements just above on typical duct sizes.
Locating a dust collection system
This also depends on several factors. Preferably you want to place the system in an enclosed dust room (actually a closet) with a filtered air return to the shop. This will lower dust in the shop air as well as reduce the noise level. You should never place a dust collector near any type of open flame or heat source such as a furnace, stove or water heater, nor is it a good idea to place the system near an entrance to your house. If you like science experiments, put a big bag type system with 1 micron filters in your finishing room.
Also, if you buy a cyclone that you want to mount to a wall make sure it's not a wall with a room on the other side since air vibration will cause a 'buzz'. Options are to build a stand for the cyclone, use rubber isolation mounts between the cyclone and the brackets, or mount isolation hangers in the ceiling with threaded rod used to hang the cyclone.
Exhausting directly to the outside
If you are in a temperate climate this might be a good option. There are several things to keep in mind. Exhausting the air outside will cause negative pressure in your shop. If this is within your house, such as a basement, this could create a downdraft in your furnace or water heater flue creating a very serious - and potnetially life-threatening - CO (Carbon Monoxide) problem. If you're exhausting air you need to make sure that the air can be easily replaced through some type of vent. If you're not in a temperate climate you may need to consider some type of air-to-air exchanger so that you don't just exhaust heated or air-conditioned air, but instead transfer the heat/cold to the fresh air coming in. Also be aware that the exhausted air will contain a fair amount of particulate matter that could damage or ruin the looks of your house or garden plants near the exhaust area.
Two critical things to keep in mind when building your own are to
make sure that you use a strong enough metal (24-20 gauge galvanized steel)
to avoid collapse from the negative air pressure and it is extremely important
that the dimensions are correct. DrVermin posted the following a while
back concerning some basic dimensions:
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| 350-550 | 1 | 16 | 6 | 4 | 6 | 5 | 16 | 18.5 |
| 550-800 | 2 | 18 | 7 | 6 | 7 | 5 | 18 | 20.5 |
| 900-1200 | 3 | 21 | 8 | 7 | 8 | 5 | 21 | 24.5 |
A: The diameter of the top of the cyclone; i.e. the part that is not
tapered.
B: The diameter of the pipe coming out of the top of the cyclone; i.e.
the outlet.
C: The diameter of the pipe coming in to the cyclone: i.e. the inlet.
D: The diameter of the opening at the bottom of the cyclone where the
chips and solids fall into a bin.
E: The vertical distance from the top of the outlet pipe to the top
of the untapered part of the cyclone.
F: The vertical height of the top (untapered part) of the cyclone.
G: The vertical height of the tapered part of the cyclone.
* Remember that the Actual CFM is the operating CFM of the system after
all Static Pressure is calculated, not the CFM on the box of the dust collector.
Fans for building your own DC, Cyclone, or Air Filter
This is really two questions. DC's and cyclones need a high power (1 hp minimum) centrifugal fan, while ambient air filters use much smaller fans - typically fractional HP squirrell-cage blowers (a type of a centrifugal fan). There are numerous sources for these fans including such industrial supply houses as Grainger, MSC, and McMaster-Carr. Grainger only likes to deal with other businesses, but McMaster and MSC are usually friendlier to individuals. Some woodworking catalogs also have some products here (blowers alone or kits). Penn State Industries is one example.
Good-quality polyester filter felt can be ordered from Penn State or Oneida
How can I turn my DC on automatically when I open a blast gate?
First, I would recommend using the magnetic door/window sensors used in an alarm system instead of microswitches. These will not have the dust/clog problems of typical micro-switches. Attach the magnetic (no wires) portion to the blast gate itself and the wired portion so that it is in contact (or close) with the magnet when the gate is either open or closed (your choice). These switches are available as normally open (NO) and normally closed (NC) depending on how you want to wire your system. (NO) are the most common for alarm systems.
There are two other good alternatives: 1) Place eye hooks in the ceiling above critical tools and run some string through these to a pull-pull on/off switch controlling the dust collector. Then you can pull the string to turn the system on or off. Remember to put a weight (ring of walnut works good) on the end of the string to keep it taut. 2) Use a handheld remote. These systems are widely available from home automation supply houses including http://www.homeauto.com and http://www.x10.comand woodworking catalogs carry some versions of these systems made specifically for DC's.
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