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									         Designing Your Wood Shop Dust Collection System

First of all, let’s begin by answering a few concerns of the ordinary school wood
shop. Why should a dust system be my concern? What will be involved with it, &
what will it do for me? When does it need to be used? Where will it be located?

Dust systems are of various components, some simple while some can be
complicated. There are concerns of avoiding health risk because of the exposure to
dust and fumes. Various types of dust can remain air borne which can cause irritation
to humans for a long time as well as having an explosive nature which can be a
safety hazard as well. Any spark could set off a shop fire as well.

Producing a quality product is another advantage of a good dust collection system.
Accurate cuts, measurements, correct assemblies are the results of a clean air
environment. Other advantages are clean defect free finishes, as well as dent free
board from machines such as the planer.

Local exhaust systems operate on the principle of capturing dust at or near its source.
They are comprised of four (4) basic elements: the hoods, the duct system, the fan,
and the air cleaning device. The purpose of the hood is to collect dust generated in
the air. A duct system must the transport the air laden dust to the fan. The fan must
overcome all the losses due to friction, hood entry, and fittings in the system while
producing the intended flow rate. In the air cleaning device, dust is removed from the
air stream.


                                                       STACK
                   DUCT
   AIR

         HOOD



                                                               FAN




                                          -1-
The following basic definitions are terms used to describe air flow. The volumetric
flow rate is defined as the amount of air that passes a given location per unit of time.
It is related to the average velocity and the flow cross-sectional area.

One of the most important assumptions of industrial ventilation is derived from the
fact that matter cannot be created or destroyed. Once a mass of air is identified,
confined and moving, it cannot give up or create mass. Under steady flow conditions,
the same mass of air flows past any given cross section or point of the duct, but
changes the velocity or rate at which it flows pass that given point. If we assume
incompressible flow (an important assumption we make about we make about flow
in industrial dust collection systems), the same volume flow rate passed each point.
Air flow can be described mathematically by the equation shown below. The most
common units are shown below the equation.




                  CONTRACTIONS




                           Q = VA
where:
          Q = volumetric flow rate or CFM ,          Cubic Feet per Minute
          V = average velocity or     FPM ,          Feet Pre Minute
          A = cross-sectional area    SQFT ,         Square Feet

          SP = static pressure,         Inches of Water


Static Pressure (SP) is created by gravity and modified by the fan. SP is felt in all
directions within the duct, and is measured in such a way that the flow of air does not
effect the measurement. Negative static pressure tends to want to collapse the duct.
Positive static pressure want to blow it up like a balloon.

                                      -2-
Static pressure (SP) is measured with a manometer. If the end of the manometer
probe is inserted facing directly into the air stream, the manometer will not only
measure static pressure, but will also measure the influence of the air on the end of
the probe. (Imagine billions of air molecules crushing against the dead air space in
the probe. This creates pressure.) This combination of pressures is known as the
Total Pressure (TP). The pressure component solely attributed to the impacting
molecules is called the Velocity Pressure (VP). The Velocity Pressure can be
estimated by subtracting the static pressure from the total pressure. When measuring
VP and TP, we must find the average because they vary across the duct face.


                         TP = SP + VP

In order to make calculations using the pressure formula, the sign of each term must
be known and used. Use consistent units (e.g., inches w.g. or mm of w.g.) Any time
the static pressure within the duct is less than the atmospheric pressure, it has a
negative value. If the pressure is greater than atmospheric pressure, it has a positive
value.

The fan creates a pressure lower than atmospheric pressure on the upstream side of
the fan. Since pressure is reduced in the hood, air is pushed into the hood by
atmospheric pressure. From the hood, air is pushed into the duct, and so on. This low
pressure is called static pressure (SP). It is the potential energy of the system. It is
then converted to the kinetic energy of velocity, the heat energy of friction loses, and
so forth.




                   VP                       TP            SP




                                      -3-
         Think of SP as money in the bank. The fan puts SP in the bank. When we purchase
         something valuable, like air movement, we spend some of the SP money, i.e., we
         exchange SP for air movement in the form of velocity pressure, VP we convert
         static pressure to velocity pressure.

                       SP --- VP

         Unforturnately, the conversion of SP to useful purposes never occurs perfectly. Some
         of the SP is converted to useless forms of energy: heat, vibration, and noise. These
         are called “losses”. Think of the bank. If we exchange dollars for Japanese Yen,
         wemust pay an unwanted fee. If you buy a car, you usually have to pay an unwanted
         fees and taxes. When you spend some of your SP, you have to buy some losses, too.

                       SP --- Loses (heat, vibration, noise)

         When buying a car, you can shop around and get the best deal. With SP, you shop
         around and get the most efficient hoods, ducts, elbows, and other items and conserve
         your SP losses. Conserving SP losses conserves operating expense dollars. Thus the
         emphasis on good design.
                                                                    SP = 0
                  SP = -0.12”                                                         SP = +0.5”
                                                       SP = -2.6”
                                SP = -1.2”
                                                  SP = -1.6”
SP = 0
                                                                                      SP = +1.0”




                                             AIR CLEANER                                 FAN
                                                                         SP = -3.0”

         The objective of the hood is to control emissions. There are many hood designs
         which get the job done, but use car in choosing one that will enclose the emission,
         direct it into the duct system, while providing the least amount of static pressure loss.
         The greatest loss normally occurs at the entrance to the hood or duct, due to the vena
         contracta formed in the throat of the duct. The center of the vena contracta is usually
         found about one half of the duct diameter inside the duct. Ideal flow could be
         achieved if all the hood static pressure were converted to velocity pressure, i.e., there
         would be no hood entry loss. We know this is impossible since there is never a
         perfect conversion of static pressure to velocity pressure. Cardboard can make an
         excellent hood design, at least until the necessary parameters can be determined
         experimentally. Modifying an existing hood provides an excellent opportunity for
         building this type of model. Simply add or subtract cardboard pieces to the existing
         hood to determine the desired shape of the new hood design. When the ideal
         condition is achieved, have a permanent hood constructed.
                                                -4-
Ductwork has the job of carrying air and the dust away from the hood for treatment,
handling and/or exhaust. Ductwork must provide a conduit for the negative static
pressure to reach from the hood to the fan and for positive static pressure to reach out
of the stack or be returned into the shop. Ducts must be chosen and designed to last
throughout the expected service life of the system. How many systems have you seen
that are plugged with dust? Are bent or dented? Have holes worn in the elbows? Or
missing a way to clean out the duct? Perhaps you can help others avoid the problems.
Air does not flow smoothly in the ductwork. Because of its high velocity ( 4000-
6000 fpm, typically) it is either totally turbulent, or approaching it. This turbulence is
desired to assist in transporting dust particles through the ductwork. Actually, the air
doesn’t just keep dust mixed with air, the mixing action scours dust off the duct
surface when they settle.

Industrial duct usually consists of straight runs of duct, elbows, expansions, branch
entries clean outs, transitions, and stacks. Today, most systems use galvanized sheet
metal. But there are ducts made of PVC plastic, ABS plastic as well as fiberglass.
(Metal is the most recommended material because it will ground itself to relieve
static electricity, which builds up when air moves through the duct.) Sheet metal
ducts come in various lengths or sections, or may also be spiral-wound. Flexible
ducts are made of many different materials which may be required for the
application.
-5-
The purpose of the fan is to move air by overcoming the systems static pressure
losses. There are several types of fan designs which are used for dust collection
systems. Things to be considered when selecting a fan are: required flow rate, static
pressure requirements, what type of material may be going through it, safety, noise,
physical space limitations, and motor & drive layout. There is concern to match the
fan’s performance and the system requirements. A desired point of operation results
from the process of designing a duct system and selecting a fan. Each fan has a given
performance curve which varies based on : fan size, rotation rate (RPM), flow rate
(CFM), static pressure requirement, and horse power requirement (PWR). Fans can
be designed as a part of the cyclone or filter.

Air cleaning devices remove dust from the air stream as it passes through the device
and described as a dust collector. They are usually sized based on the volume of air
they can process at a given rate (Cubic Feet Per Min.) & a given cleaning efficiency
rate.
A safe recommendation in equipment selection is to select the collector that will
allow the least possible amount of dust to escape and is reasonable in the first cost
and maintenance while meeting all prevailing air pollution regulations. The cyclone
and fabric filter are the two (2) most commonly used for dust collection systems.
Cyclones separate the dust from the air stream by use or combination of centrifugal,
inertial, and gravitational forces. Collection efficiency is influenced by: dust particle
size, weight, shape, collector size, velocity, and dust concentration. Principle
advantages are: low cost, low maintenance, and relatively low pressure drops ( 0.75
to 3” water gauge). Fabric filters use a porous mass (fabric) through which the air
passes leaving the dust on the out side of the surface. They are rated by the amount
of air (CFM) which passed through a square foot of fabric each minute at a pressure
of 0.5” water gauge ( expressed as air to cloth ratio). Fabric filters are not 100%
efficient, but well deigned, adequately sized, and properly operated fabric collectors
can be expected to operate at efficiency of 99.9+% on a weight basis. They can be
cleaned using the shaker, pulse-jet, or reverse-air methods. There are a variety of
designs available based on: type of fabric, intermittent or continuous cleaning, air
flow pattern within the collector, concentration of dust, and housing configuration.

Now that we’ve gone over the basics of elements and operation of the dust system,
let’s get down to the job of designing of a wood shop dust collection system. The
first step of designing your system is to draw a floor plan of you shop area including
the following:

1. Location of the dust producing machines, indicating size & location of openings
   of each machine.
2. Desired location of the collector unit.
3. Determine the Floor to the centerline of the duct main trunk lines.
4. Locate any obstructions that would interfere with duct locations.

                                       -6-
As the first step, we must determine: the duct velocity (FPM), diameter of each drop
or machine to be connected to the system, diameter of the transitions and main duct
trunk line, and the system resistance (SP). From your shop layout, determine how
you will locate your drops for each machine, your main duct trunk lines, fan, and
collector. This will take into consideration avoiding any obstructions in the machine
drops as well as the main duct trunk line center line height from the start of each run
all the way to the collector.


A. The recommended duct velocity for wood dust is 4500 FPM in the drops & at
   4000 FPM in the main duct trunk lines.

B. Diameter of each drop or machine. Measure the opening provided for dust
   collection on each machine. Normally it is round while some may be square or
   rectangular. If so, convert the area to that of a round diameter.

Diameter of transitions or main duct trunk line. When sizing main lines, start
with the drop (machine) that is farthest away from the dust collector. Run this
diameter until the next drop (machine) enters the main duct trunk line. Increase the
main size at that junction to accommodate required C.F.M. of the two drops. Look at
the C.F.M. chart for the required C.F.M. for each opening, add the two values, look
up the closest value for the total of both the drops. Look under the diameters list the
determine the new diameter of this trunk line. Repeat this same process for each
addition of drops entering the main until it reaches the collector. The total required
C.F.M. of all the drops will be the required C.F.M. of your system. This is the
common practice for a dust collection system when all the machines are being used.
If you only use a few machines in your system, while using blast gates on machines
no in use, the designate your primary machines (ones you use most often) only
increase the main duct trunk line when adding a primary machine. Normally when
any machine is not being used, it is a good practice to close the blast gates of all
machines not in use. This will increase the suction for the other machines which are
in operation. (One word of caution, don’t close all the gates, because this will not
permit the fan to get air, which can cause the duct to collapse or fan motor to
overheat.)

Example: There are 3 machines in your layout: Table Saw - 4” dia. 395 CFM,
Shaper - 4” dia. 395 CFM, Planer - 5” dia. 615 CFM.
A 4” drop would run from the Table Saw until it joins the 4” drop from the shaper.
At this point the main starts and you would need to increase the diameter of the pipe
to handle the combined CFM (395 + 395 = 790 CFM). Using the CFM Chart, look
up the appropriate velocity (4000 CFM in the main for wood dust), Then look at the
corresponding diameter (6”). Therefore, run a 6” main pipe from this point until the
drop of the Planer joins the main.

                                     -7-
   At this point, you need to increase the main pipe to handle the total CFM (790 + 615
   = 1405 CFM). Using the chart again you will see that 1405 CFM is slightly more
   than the volume for 8” diameter. So, we drop back to 8: diameter, so as not to go
   below the transport velocity. Therefore, run a 8” main pipe from this point to your
   fan or collector.

   D    Calculate system Resistance (SP) The total static pressure is several factors
   added together. They are entry loss, dirty filter loss, SP of the worst drop, SP of the
   main trunk duct, and SP of return duct.

   1. There are more complicated ways to figure the entry loss of your system, but it
      usually equals a loss of 1” water gauge. (Use 1” as a constant).

   2. If your system has a filter, add 2” w.g. loss ( If you do not have a filter, add zero)

   3. The worst drop, is the drop with the greatest resistance, which is usually a smaller
      diameter with the most lineal footage of pipe and elbows, (farthest away from the
      fan). Static pressure (SP) of the worst drop an main duct can be calculated using
      the SP chart. The chart is based on 100 feet of pipe; therefore, you have to
      convert all elbows to an equivalent. When calculating the feet of pipe, count the
      lateral branches as 45 degree elbows. Flex hose has a lot of resistance depending
      on the corrugation. For this reason, try to keep hose to a minimum. Multiply you
      length of flex hose on your worst drop by 3 to calculate the equivalent length of
      straight pipe.

Example: Determine SP in Worst Drop             Description - 4”                   Equivalent
SP (inches of w.g.) 4” Table Saw                Straight pipe --------------------------20’

                                                2 - 90 Elbows------------------------12’

                                                1 - 45 Elbow--------------------------6’

                                                 5’ Flex hose (3X) -------------------15’

                                                 Total straight pipe equiv. -------------53’

                                                 395 CFM in 4” diameter = 8.5” SP /100’

                                                 395 CFM in 4” diameter = 4.51” SP /53’

                                                 6” diameter pipe ------------------------20’

                                                 790 CFM in 6” diameter = 4.5” SP / 100’

                                                 790 CFM in 6” diameter = 0.9” SP / 20’

                                          -8-
                                             Straight pipe 8” dia. --------------------25’

                                             2 -90 Elbows - 8” dia.-----------------30’

                                             Total straight pipe equiv.---------------55’

                                             1405 CFM in 8” diameter = 3.2” SP/ 100’

                                             1405 CFM in 8” diameter = 1.76”SP/ 55’

8” diameter pipe runs to self contained Dust Collector

Total Static Pressure = 1”+ 2” + 4.51 + 0.9” + 3.2” = 11.81” S.P Water Gauge


System Requirement - 1405 CFM at 11.81” SPWG

We have now covered all the aspects of designing the basic dust collection system for
a woodworking shop. There is and will always be many ways to layout, design and
calculate all the data required for and efficient system. The latest thing to come out is
an economical auto blast gate system by ECOGATE. This system is designed for
most all the drops to have blast gates which are controlled and monitor the fan speed
for the most efficient use of energy. With the price of energy (electricity) rising above
the $0.10 / KWH, this alternative can become quite attractive.

For the remainder of the session, we will spend our time in using the information to
do actual hands on design of three types of systems which are very common in our
world:

First, let’s design a system in which there are no limits. This will be a new wood
shop in which we can run any machine and have near perfect suction on all machines
at any time. Money will be of no concern. There will not be any blast gates, because
all the machines will be running all the time while the system is running.

Second, let’s design a system in which there are some limits. This will be a new
wood shop or it could be an existing shop. This system will require the use of blast
gates in order to shut off the machines not in use. There are only three people
working at any given time. There is a limited amount of money which can be used for
the project. How can we best utilize the design, layout and anything else we have to
get the best system for our money.


                                      -9-
Third, let’s look at the real world in most cases. We have the system in place in an
existing shop. There have been no changes made in the last ten years. We have
extremely limited funds left to do any thing, much less any to spend on the dust
collection system. What can we do, if anything, to improve what we have now? Can
we make any changes which would help the system to improve or have better
suction?

This is the situation in which most teachers find themselves. The reason for this
presentation is to assist you and help to make any improvements at all in your
shops, as well as provide information which can be used to teach students about the
need for dust collection. By adding blast gates made of a scrap piece of wood can cut
off any unused equipment. Since manufacturing is being done with the use of
common machines, say a group of 3 or 4, this could be used to design your cell dust
collection for these machines. There are many ways to reduce suction requirements in
the shops while still have good dust collection.

I hope you have learned something you can use for your situation. As manufacturing
techniques change, there will be other changes in the dust collection system. Only
you can determine what is best for you, along with how much it will cost.




References:

1. “Industrial Ventilation” by American Conference of Governmental Industrial
   Hygienists, Inc.

2. “Industrial Ventilation Workbook” by D. Jeff Burton, PE, CSP, CIH

3. “How to Design Your Air Handling System” by Air Handling Systems

4. “Duct Collection for the One Man Shop” by Anatole Burkin




                                -10-
      CFM requirements / dia. @ given velocity

Dia. 4000        4500 FPM 5000FPM
     FPM

3"         195         220         245
4"         350         395         440
5"         545         615         680
6"         785         885         980
7"        1070        1205        1335
8"        1395        1570        1745
9"        1765        1990        2210
10"       2180        2455        2725
12"       3140        3535        3935
14"       4275        4810        5345




Static Pressure based on     Elbow to Straight Pipe
100' Pipe                    Conversion

Dia   4000FP 4500FPM 90 Elb       45 Elb 1.5R
      M                 1.5R
3"          10       12         5       2.5
4"           7      8.5         6         3
5"         5.5      6.5         9       4.5
6"         4.5      5.5        12         6
7"         3.8      4.5        13       6.5
8"         3.2      3.8        15       7.5
9"         2.8      3.4      17.5      8.75
10"        2.4        3        20        10
12"          2      2.5        25      12.5
14"        1.6        2        30        15




                                         -11-

								
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