StaticCalc Duct Sizing Calculator

8/4/2019 StaticCalc Duct Sizing Calculator

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Introduction: We built and shared this calculator because most small shop woodworkers don't realize how mu

impacts dust collection. Most wrongly think of their dust collector as a huge shop vacuum and that causes all k

shop vacuum that can lift a column of water 35" or more, the blowers used in dust collection generate only a ti

Air at dust collector blower pressures will barely compress air at all, so almost any small pipe, bend , wye fittin

other obstruction will act just like a partially opened water valve and kill our airflow. This leaves us with two cho

horsepower and larger blower until we overcome all that resistance or design a system with minimal resistance

smallest, most cost efficient blower.

Click here to see the Exhaust Requirements for Woodworking Document

Click here to see the Exhaust Hoods samples

Tool Modifications: To meet government air quality mandates that went into effect in the late eighties, the ma

collection equipment had to take a fresh look at dust collection. Until then dust collection meant keeping shop f

chips that would otherwise be swept up with a broom. These firms found that to also ensure collecting the fine

collect the dust right at the source, meaning each tool as the dust was made. If they let the fine airborne dust

hours for a good exhaust fan or air cleaner to bring the dust levels down to low enough to meet government st

the air streams from our tool blades, bits, cutters, belts, motor fans, etc. from spraying this fine dust all over, th

stationary tool. Almost all required a new hood, larger ports, internal ducting, and sometimes new pannels. Sm

similar changes to our tools if we want good fine dust collection.

Disclaimer!!

USE THIS AT YOUR OWN RISK! HIRE A PROFESSIONAL ENGINEER to design, specify, test, and certi

collection system if you have a commercial or an industrial application, allergies, other medical probleyou, a large shop, work with hazardous materials, or are subject to regulatory oversight. Don Beale, Bil

and all other references and links cannot be held liable for this calculation's applicability to your specific situati

Warning this is only an approximation!

This information provides small shop woodworkers with a better sense of what they need to collect the wood d

calculations are approximations based upon values from industrial fittings, pipes and dust hoods which may b

what you may use. Moreover, these calculations also require you to add in other known large losses such as t

of separators (trashcan or cyclone) and filters.

Dust Collection Air Flow Requirements: The major dust collection suppliers also did the testing to determin

fine dust collection. We need enough airspeed to move the dust and enough air volume to carry the dust. Air e

that it takes an air speed of roughly 3700 feet per minute (FPM) to move the chips and heavier sawdust up ver

airborne dust only required moving about 50 FPM, so no additional air speed was needed for good fine dust c

air volume would be small as well, so existing systems would work just fine. The testing showed a totally differ

from the ground up with good fine dust collection engineered in to protect and control the fine dust until it can

vacuum that only moves 50 cubic feet per minute (CFM) provides good fine dust collection. Unfortunately, in o

shop owners buy older and hobbyist tools that have little to no fine dust collection built in. Our larger tools are i

commercial tools that air engineers found almost all need modified and then supplied with about 800 CFM air

At first this does not make sense because it takes almost double the volume of air to collect fine dust that take

airspeed to move. We already know this is true if we think about using our shop vacuums. They only pick up ri

because sucked air comes from all directions at once causing airspeed to decrease at roughly pi times the cub

inlet. A monster 800 CFM dust collector blower that moves 4000 FPM right next to the inlet, moves less than t

overcome the room air currents and collect the fine dust just nine inches away! To keep commercial shops froindoor air quality air engineers had to roughly double the total airflow plus redo most machine hoods, ports an

shop owners must do the same if we want good fine dust collection. Moreover, many woodworking unions and

standards are too low because workers keep getting ill, most hobbyists should build their dust collection syste

STATIC PRESSURE CALCULATION SHEET
http://cnets.net/~eclectic/woodworking/cyclone/AAFRequirements.gifhttp://cnets.net/~eclectic/woodworking/cyclone/AAFHoods.gifhttp://cnets.net/~eclectic/woodworking/cyclone/AAFHoods.gifhttp://cnets.net/~eclectic/woodworking/cyclone/AAFRequirements.gif


2/14

Return Duct: If there will be return duct moving the cleaned air back inside your shop from an outside cyclone

2000 FPM speed. To determine that return duct size simply select the size to run at 2000 FPM or less with you

it in each branch calc. The same applies when sizing the return for a cyclone or muffler.

Blast Gates: Normally we put a blast gate as close as we can to the main for each machine to leave the least

piles when their runs are not open. Putting the blast gate down low lets lots of debris build up in the pipe. Whe

material can slam around potentially ruining blowers and especially filters.

Basic System Design:

There are two real ways to design your dc system in order to get optimum results.

Tool Ports: It takes one 6" port to support the 800 CFM required for most larger hobbyist machines. For machhobbyists should generally use a wye that splits their 6" ducting into two runs. The smaller should use 3.5" dia

larger should use 5" ducting connected to a 5" port. This combination maintains the same area as the 6" main

restrictions, plugging, dust piles, and poor dust collection. Generally, we have to modify our hobbyist tools to a

Dust collectors do not have enough pressure to provide good fine dust collection for tools that cannot be modif

diameter ports. Tools with smaller ports require use of a shop vacuum that generates at least 60" of pressure t

needed plus often a movable hood and downdraft table.

2) Small Shop: Most small shop owners cannot build dust collection systems similar to commercial systems b

service, wiring, and don't want to pay the cost to buy or run a huge motor. As a result, our systems are design

more efficient blowers that can only collect from one machine at a time . All other ducting runs are closed off

us use a blower just big enough to meet the airflow needed for fine dust collection at their largest machine and

run. We should size our ducting and dust collector for our max cfm requirements at our largest machine and si

longest possible run. This will allow only one machine at a time but there will be no build up and all dust and le

around without needing to buy a bigger blower and motor.

Down Drops: Commercial dust collection systems size each down drop to carry just the air needed for good c

tool. They also use many different sized ducting runs for the mains to keep the airspeed ample to avoid pluggi

being so complex that most ducting engineers use commercial programs to compute all the ducting sizes, part

firms now offer similar services to hobbyists using these same commercial programs. The results are a nightm

use a down drop smaller than our main in a one ducting run open at a time hobbyist system, the small down dr

airflow. A 4" down drop connected to a 6" main looks pretty and appears to work well, but drops the main airsp

immediately building up dangerous dust piles that can grow huge. These piles pose a serious fire hazard and

restored, these piles break loose and slam into things potentially ruining motor bearings, impellers, and your fil

Main Duct: If you want the most possible CFM you need the biggest duct you can get that keeps the air velocimoving instead of clogging your ducting. Most hobbyist vendors sell 4" dust collection duct and flex hose as th

to collect chips, but strangles the airflow needed for good fine dust collection. A typical 2 hp hobbyist dust coll

impeller moves a maximum of about 1200 CFM, but a 4" duct airflow drops that airflow to only about 450 CFM.

turning a 16.5" diameter impeller to force a 4" duct to carry 800 CFM. Most prefer a less expensive solution. In

airflow, our blowers must keep the duct airspeed high enough so we don't get plugging or piles of wood dust.

hazard and ruin both blowers and filters when they break loose and go slamming around. Air engineers found

of 4000 FPM keeps the ducting clear. Because FPM=CFM/duct size, a little math shows we need 6" diameter

4000 FPM. Most hobbyists need all 6" hoods, 6" main ducting lines, and 6" down drops.

1) Commercial: Size your ducting/dc for the total requirements of the entire shop with NO BLAST GATES all.

commercial dust collection systems with all ducting runs are open at once to collect the dust from all machines

All ductwork drops are sized to provide the minimum cfm requirements for each machine. The main becomes

sized pipes sized to meet the CFM of all downstream ductwork. The Air Laws show us that to double the airfl

power. Just a two person shop running a couple of runs at the same time requires a 10 hp motor turning a hug

impeller. Motors this big normally only work with three-phase power, require commercial instead of residential i

much heavier wiring than found in most residences.


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Assumptions, comments, and notes:

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Instructions: READ THE EXAMPLE PROBLEM! THIS IS REQUIRED!!!

CFM Required: 800 Target Air Velocity 4000

Optimum Duct Diameter 6.06Ducting:

3.5 4 5 6 7 81.75 2 2.5 3 3.5 4

9.621 12.566 19.635 28.274 38.485 50.265

0.0668 0.0873 0.1364 0.1963 0.2673 0.3491267 349 545 785 1069 1396267 349 545 785 800 800

11974 9167 5867 4074 2993 22924000 4000 4000 4000 2993 2292

Hoods:

Enter the number of wye connections where the airflow splits equally, e.g. 6" to 6" and 6".

Duct Radius

Duct Area Sq. Inches

Duct Area Sq. FeetCalculated CFM

Input in the total number of feet of straight duct measured on a centerline through the fittings.

Return can be added in to each run and it will be included in the total drop. See the sample problem.

Duct Diameter Inches

8", 10", and 12" loss factors are for furnace pipe, to be used on RETURN ONLY.

Pipe loss factor based on 4000 FPM.

One VP acceleration factor is included in the hood entry.

From a CFM Requirements table Input the CFM required for our largest tool (normally 800 or 1000).

This sheet assumes that the total area of the duct below the blast gate roughly equals the area of the mai

less area can increase the overall resistance ten fold or more

Hood entry loss is for a flanged pipe entry. OTHERS VARY - SEE FAQ

90 loss factor is for smooth stamped steel, R/D = 2.0

Wye loss factor is for a 30-degtree branch entry, on a 15-degree taper (where it is reduced).Flex pipe losses vary - SEE FAQ

The system is made of 3.5", 4", 5", 6", 7" or 8" metal dust collection ducting and nothing smaller than 3.

anywhere because we know that would kill the airflow from our blower needed for fine dust collection.Loss factors are based on metal with a roughness factor .0005 feet

These loss factors are based on the following assumptions:

Resistance Calculation: Small shop owners only use one ducting run open at a time, so we size our systems

power our largest need. Knowing that most will move their machines around over time, it is best to figure what i

machine overhead and our longest ducting run. The longer the hose and the more bends and fittings we use i

machine collection, the higher the pressure drop and bigger blower it will take. Hobbyists need to calculate the

with the most hoods and ducting overhead, then add to that the resistance of all the pipe and fittings in their la

add the other overhead resistance including cyclone, muffler, and filters. That total resistance corrected for air

pressure drop in our system. We use that total worst case resistance with a standard engineering fan table toimpeller diameter. This simple resistance calculator helps you work through what you need for your own shop.

Transition losses are not included.

Any wye connection that does not result in the outlets being of equal or larger area than the incoming ma

should not be used with this static pressure calculator.

THIS SHEET WILL TELL YOU WHAT THE LOSSES ARE; IT WILL NOT TELL YOU WHAT IS REASON

input 10000 CFM, the 4" velocity goes to 114592 FPM, which is 1302 MPH. And the hood entry alone in

water, which is 44 PSI.

Input the target air velocity in FPM for your ducting (4000 for sawdust and 2000 FPM for clean air)

This immediately calculates the most efficient optimum duct diameter.

Run the calculation for only your longest, most complex branch from the hood to the collector.

Calulated. Velocity, FPM

Delivered. Velocity, FPM

Delivered CFM

Input 1 for the number of hoods under the correct duct diameter column.

Enter the total number of bends counting 0.5 for each 45 degree and 1.0 for each full 90.


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# of 90's: 1.5

# of Wyes: 1

23

1.5Other:

Trashcan Separator S.P. Enter 4.5" if used

Cyclone S.P. 3.5

Filters S.P. 0.5

Muffler S.P. 0.15

Other S.P. 2.32

TOTAL SP LOSS: 8.16 inches WC

SUBTOTAL LOSSES: 3.5" diameter 4" diameter 5" diameter 6" diameter 7" Diameter 8" diameter

Hood: 0.00 0.00 0.00 0.00 0.00 0.0090's: 0.00 0.00 0.00 0.32 0.00 0.00

Wye: 0.00 0.00 0.00 0.18 0.00 0.00

Pipe: 0.00 0.00 0.00 0.99 0.00 0.00

Flex: 0.00 0.00 0.00 0.19 0.00 0.00

Subtotal losses: 0.00 0.00 0.00 1.69 0.00 0.00

By placing our cyclone right at the end of this duct run we only need a single 45 degree elbow and 1.5' of 6" flex hose to

main. The flex hose helps control noise by not allowing our ducting to serve as a sounding board, plus it makes our conn

In this example shop neither the planer run nor the radial arm saw run is ample to size our system. The ductin

longest possible for this shop and our radial arm saw has the most overhead. The planer overhead is small be

6" port. The radial arm saw needs both a sweep up hood plus flex hose blade guard pickup hood. Figuring o

shop requires finding your largest resistance tool (normally the one with the most hoods and the most

the resistance for the longest run, then adding the resistance for the tool with the highest ducting resi

of your separator, cyclone, and filters.

Example

For starters, it is useful to have a sketched layout of your shop, with the tools in place, and the centerlines of all the duct

the shortest runs, with the least number of bends and with the right sized ducting. Practically, because equipment will ch

need to create a system that is flexible and will permit changes. The best compromise is centering the dust collector as

either a center main or a pair of mains on opposite walls. In this example, we have a 20 x 20 shop. Power concerns forc

centered on the east wall and run an 18' long duct down the center of the shop.

Note that when the spreadsheet is first loaded, it has the values filled in for this example. The following steps show how

Note: if SP over 12" you need to us

Std. Bag =2.5",


5/14

Duct Size

Inches

Duct Area

Square Inches

CFM Starting

with 6" pipe

at 800 CFM

1 0.79 22.2

2 3.14 88.9

2.5 4.91 138.9

3 7.07 200.0

3.5 9.62 272.2

4 12.57 355.6

5 19.63 555.6

6 28.27 800.0

7 38.48 800.0

8 50.27 800.0

Highest Resistance Tool: Based upon the above discussion we now need to enter in 3.5" column 1 hood an

enter 1 hood, 1 each 90 elbow, and 3' of pipe. In the 6" size column add 1 more 90 degree elbow (2 each 45s)

pipe. This gives us 4.62" of total resistance, so the average resistance is half that or about 2.32" w.c. When w

average instead of add the resistance values.

Even with the right sizing, dual port machines may still need further adjustment to keep a larger duct f

from a smaller! Any size variation, constriction in the tool, and even resistance of the different hoses will signi

goes to each collection point. Proper balancing can require use of a wye with a baffle to direct the airflow and

airflow to both legs.

From the above RAS ducting picture taken from the ducting hoods section of these web pages, you see a 6" m

splits off the 5" for the rear sweep up hood and a 3.5" for the blade guard. The RAS hood turns up on the wall

the wall, and enters the wye. The 3.5" flex hose is 5' long and enters the other leg of the wye. We then go from

straight pipe to the main.

For our RAS and many other tools such as our table saw, band saw, etc., we end up splitting the down drop in

picked the RAS for this example because it has one of the highest ducting overheads of any hobbyist tool. For

hood that sits behind the blade and a smaller flex hose that attaches to the blade guard. To keep from killing a

of ducting, the area of these two ducts must be very close to the same area as our 6" diameter down drop duct

inches. The following table shows ducting areas, so you can pick a combination of offshoots that come close t

main. We ideally would split our 6" main into a 5" and a 3.5" because adding the areas of the 5" and 3.5

equals the area of the 6" duct. Practically, going with 5" and 4" makes more sense because 3.5" flex h

For this duct to work with our machines we also often have to change out the origingal too small machine chip

RAS this requires making a 3.5" port on the blade guard and a 5" back connection on the rear pickup hood.

Radial Arm Saw Ducting Size: The CFM Requirements Table shows our RAS only needs 500 CFM. If we inp

required airflow, then our calculator wrongly tells us we should use a 5" down drop to this machine. If we were

ducting solution where all ducts are left open with a blower big enough to power all at once, that would be the

drop. In a hobbyist system where we only power a single run at a time, we instead must use the same sized pi

enter 500 CFM in our calculator for the required CFM. It immediately computes a need for 5" ducting and sho

main drops to only 2546 FPM. We need close to 4000 FPM to keep uphill runs from plugging and to avoid buil

mains hobbyist need 6" down drops to each machine.


6/14

Longest Ducting Run: Our scaled drawing shows the longest run starts at the cyclone with 1.5' of flex hose, 1

degree elbow, 5' of straight pipe, another 45 degree elbow, blast gate, and 5' of straight pipe going down to a f

the upper and lower collection hose. Since we already calculated the overhead after the wye, we will just calcu

wye. This leaves us entering in the 6" column no hoods, 1.5 for 90 degree elbows, 23' of straight pipe duct, 1

a total ducting resistance of 1.69" w.c.

Overhead: Now to this we enter 3.5" for an average cyclone with neutral vane, 0.5" for under 225 square feet

muffler that sits between the blower and filters. To this we must add our 2.32" oof overhead for our largest toolresistance for the longest run of 8.16" w.c. total.

Hobbyist Solution: Standard engineering fmaterial movement fan tables let us use our 800 CFM and 8.16" pr

the minimum sized blower we need. The below table shows our 8" of resistance requires about 2.18 hp with a

about 890 CFM. This is why I say almost all average small shops will burn up 2 hp motors unless the airf

our needed 800 CFM. Because impellers are made in even inch increments for larger than 12" in diameter th

sources for 13" impellers. Bumping up to a 14" impeller with an open short run on a minimal resistance tool onl

generating a need for over 4.4 hp which will quickly burn up even a 3 hp motor. Most hobbyist cyclone vendors

powered by 2 and 3 hp motors. These motors burn up quickly unless they kill our airflow to well under 800 CF

down drops. Although a nice advertising ploy that shows very high CFM maximum numbers, a far smarter solu

provides better separation is to use a real 5 hp motor with a custom made 15" impeller, exactly as recommend


7/14

Commercial Solution: If we were building a commercial system we would add up the CFM requirements for e

would tell us how many CFM our blower would need to produce. We would likewise total the resistance of ever

ducting to tell us how much resistance our blower would have to overcome. For this simple two machine shop,

1300 CFM at 18" of resistance. This requirement goes right off our hobbyist fan table scale and puts us into a

the Cincinnati Fan, Inc. table we need 5.9 hp that requires a 7.5 hp motor turning a 16.5" diameter impeller to

collection on these two tools at once.


8/14

D. Beale/B.Pentz

Version 9.3 -- 1/03/06

ch their duct size and layout

inds of problems. Unlike a

y fraction of that suction.

, small port, restriction, or

ices. We can addto permit us to use the

jor suppliers of dust

loors clear of the dust and

airborne dust they had to

scape into the air, it took

ndards. They found to keep

ey had to redo almost every

all shop owners must make

y performance of any dust

s, people working forll Pentz, American Air Filter,

n.

st in their shops. These

significantly different than

ose that occur with the use

what was needed for good

ngineers long ago learned

tical ducting. The fine

llection. Most suspected the

nt story. If a tool was built

e collected, a good shop

r real world most small

dentical to smaller

olume to provide good fine

s a fraction as much

ht next to the inlet. This is

e of the distance from the

e 50 FPM we need to

being closed due to poorinternal ducting. Small

doctors now say the current

s to provide 1000 CFM.

April 13, 2012


9/14

, it only needs to provide

r max CFM machine. Leave

possible pipe to build up

n the gate gets opened this

ines with two pickups,eter ducting and port. The

to prevent airflow

d the 3.5", 5", and 6" ports.

ied to have at least 3.5"

o force the air collection

cause we lack the power

ed to use the smaller, much

with blast gates. This lets

highest resistance ducting

e the ductwork for the

s you move machines

ollection for each specific

g and piles. This ends up

, etc. Sadly, a number of

re because any time we

op strangles our needed

eed to under 2500 FPM

hen the airflow gets

ers.

y ample to keep the dustir standard. This works well

ctor with a 12" diameter

It takes a 5 hp motor

addition to maintaining that

ust piles in ducts pose a fire

esigning for a duct speed

ucting to move 800 CFM at

Air engineers design

running at the same time.

collection of ever bigger

w we need eight times the

e 18" diameter blower

ncoming power, and require


10/14

10 125 6

78.540 113.097

0.5454 0.78542182 3142800 800

1467 10191467 1019

in. Use of a smaller duct with

5" duct will be used

based upon what it takes to

it takes to power our largest

our ducting runs and

the resistance of our tool

gest ducting run, and then

flow loss gives the worst

ize our blower motor and

in will kill airflow and they

BLE!!! For example if you

a 4" branch goes to 1220"


11/14

10" diameter 12" diameter

0.00 0.000.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

connect the cyclone to the

ection easier.

run to our planer is the

ause it uses a simple single

t the maximum for any

small hoses), calculating

tance plus the overhead

run. Ideally we want to create

ange and move around, we

uch as possible then running

d us to put the cyclone

e got these numbers.

bigger diameter pipe!

50 sq ft filter=0.25", 600 sq ft

, and 2.25" for Pentz cyclone


12/14

5' of flex. In the 5" column

's and add 5' of straight

split a down drop we

om stealing all the flow

icantly alter how much air

ual gauges to ensure ample

ain going into a wye that

with a 90 degree, goes 3' up

that 6" wye with 5' of

o two collection pipes. We

the RAS we have a pick up

irflow by restricting the area

, roughly 28 1/4 square

the same area as the

" runs almost exactly

se is so difficult to find.

collection ports. For our

ut that 500 CFM for our

building a commercial

orrect sizing for this down

e as our mains. To see why

s the airspeed in the 6"

ing up dust piles. With 6"


13/14

8' of straight duct pipe, a 45

inal wye before splitting into

late up to and including that

ye, and 1.5' of flex hose for

of filter, and 0.15" for the

to give a total ducting

essure requirement to pick

13" diameter impeller to get

low is restricted to below

re are almost no affordable

y pulls about 5" resistance

offer 7" mains and cyclones

. Most do so by using small

ion that moves more air and

d for my cyclone design.


14/14

very machine and that total

y ducting run and machine

the total comes out to about

ommercial table. Checking

rovide concurrent fine dust

Documents

StaticCalc Duct Sizing Calculator