Duct design for dust or refuse exhaust

Long experience has decided the most suitable diameters of the connections to exhaust hoods for all the usual machines to which dust or refuse collection is applied. These standards are available from machine manufacturers or system designers.

The velocity necessary to provide adequate margin for the sus­pension of the particles in the airstream is also known for most types of dust or refuse. Table 3.9 shows some examples.


Duct velocity m/s

Grinding wheel dust


Buffing wheel dust


Sawdust, dry


Wood chips, normal machines


Wood chips, high speed machines


Wood sand papering machines


Table 3.9 Duct velocities for types of dust or refuse

The range of air velocity used by engineers is from about 12 to 25 m/sec, but 18 to 23 m/s covers the usual requirements. For unit collectors or individuals grinding or buffing machines, lower velocities are common in the short connecting pipes e. g. 18.5 m/s for grinders and 17 m/s for buffing machines.

Many plants are at work successfully which were designed for constant air velocity in all mains and branches. Some designers vary the velocity in a system in different branches according to the types of machines connected. For example, in a wood re­fuse plant the branches to sawdust-producing machines may be designed for 18 m/s; with those to chip-producing machines at 20 to 23 m/s, and with all mains at a nominal 20 m/s. This may vary slightly in mains due to approximations for duct diameters to the nearest 5 mm.

General notes

In an extensive woodworking plant, a separate system may be installed to deal with the saws, as sawdust can be sold. Another separate system deals with planers and moulders etc., the chips collected being discharged to a boiler or a refuse destructor.

Wood sandpapering machines should be handled by a sepa­rate plant, or as individual units, as this dust is extremely fine and it requires a textile filter to collect.

Grinding machines and buffing machines should no be con­nected to the same exhaust plant. Sparks from grinding might ignite lint from the buffs with risk of fire.

When a woodworking machine has multiple connections, e. g. a four-cutter or six-cutter moulder, it is important to keep in mind the effect of it being out of service with blast-gales (dampers) on connections closed. This might result in too low a velocity in the main to carry the refuse from other machines still in service on this section. Actually, when the material is in the main, the mini­mum carrying velocity is considerably less than those men­tioned, say 75% of normal, and this allows some latitude. Expe­rience is the only guide in difficult cases.

Design scheme

On an outline plan of the factory, mark the positions of ma­chines with their exhaust points and sizes according to the schedule. Lay out a suitable run for ducting, noting that branches in an exhaust system enter the main at 30°, or through patent junctions with almost parallel entry.

From the diameter of connection and selected velocity calcu­late the flow or obtain this from a manufacturer’s data. The di­ameter of the main is then calculated in its graduated sizes as branches enter, from selected velocity and total flow at any given point. Work to the nearest 10 or 5 mm in main sizes. This alters the selected velocity slightly and the final figure is used for friction calculation.

Calculation of resistance

1. Estimate entry loss at the hood most remote from the fan.

2. Calculate the approximate equivalent length in metres of this most remote branch from hood to main. That is, the length of straight piping plus equivalent length in metres for bends.

3. Branch loss at entry to main from B to A for exhaust sys­tems is less than in blowing. (Figure 3.83.)

Now total up the equivalent length of branch, estimate its friction loss in mm w. g. Add entry loss from item 1.

Duct design for dust or refuse exhaust

Figure 3.84 Entry of air from a branch

4. Duct design for dust or refuse exhaust

Figure 3 83 Loss in equivalent diameters of B

figure 3 83 loss in equivalent diameters of b









подпись: 45* 30* 15* nih
9 4 2 1

Balancing of dust extract systems

Balancing of the system is the adjustment of resistance so that in the example in Figure 3.85 the resistance from remote hood at A to the fan inlet at B is approximately equal to the resistance of the branch near the fan from C to B. If not balanced, C would exhaust too much air and A too little, as compared with that to meet designers requirements.

подпись: balancing of dust extract systems
balancing of the system is the adjustment of resistance so that in the example in figure 3.85 the resistance from remote hood at a to the fan inlet at b is approximately equal to the resistance of the branch near the fan from c to b. if not balanced, c would exhaust too much air and a too little, as compared with that to meet designers requirements.
Measure each length of main between the entry of branches and allow for any addition from item 5. Neglect tapers and include as straight duct.

5. The entry of air from the branch, if at an appreciable angle to main, causes a loss in the main from C to Adue to turbu­lence, and is shown in Figure 3.84. A summary is given in Table 3.10. (Note this is in diameters of Aand not B.) Esti­mate this loss at each branch of entry and add to the fric­tion of the section of main following any given point of entry.

Diam A Diam B




0° Parallel junction

Loss in diameters of A


























Table 3.10 Loss in branch in diameters of A (from step 5.)


Add values of steps 1, 2, 3, 4 and 5 and mark the total on the diagram at each point of entry. It may be conveniently shown in a square thus:


The complete frictional and turbulent resistance of the suction main is entered at the fan inlet as suction side re­sistance depression. If velocity pressure is added, it is then static suction, but most performance tables for fans are based upon fan static pressure and so this is the figure required when dealing with the fan speed etc.

Note: The resistance depression to be set up by the fan must include the separating apparatus. In wood refuse sys­tems a cyclone separator is used and is always on the discharge side of the fan. Hence, to the resistance de­pression on the suction side from step 7, must be added to the frictional resistance of the discharge duct with its bends, and the resistance of the cyclone sepa­rator. The latter will normally have a resistance of 35 to 50 mm.

In dust systems either a cyclone or a textile bag filter may be used as decided by experience of the particular application. These may be installed on either suction or discharge side of the fan. If on the suction side, the resistance depression must be added to step 7, plus the resistance of the discharge duct on the fan with its weather cap.

If on the discharge side, then the resistance of the piping, to­gether with that of the cyclone or bag filter added to step 7, will represent the fan static pressure.


Duct design for dust or refuse exhaust

Figure 3.85 Example of dust extract system balancing

Any artificial resistance put into the circuit must be of such na­ture that dust, sawdust or woodchips cannot build up on it to cause a blockage. An orifice in a plate inserted between a pair of flanges in branch C could be used to impose artificial resis­tance for balancing, but it would probably build up and cause a choke.

Experience has shown that when the air is carrying material, the best restriction is in the form of a conical piece, see Figure 3.86, inserted into the end of a branch where it joins the main.

Duct design for dust or refuse exhaust

Figure 3.86 Internal conical piece for balancing

Material passes easily through this and the desired added re­sistance is attained by a suitable diameter of the small end of the cone.

The cone is inserted in the inlet of its patent junction with the main, and has an included angle of 30° to 40°. If a relatively small reduction is required, say 5 mm or less than branch diam­eter, then the end of the branch itself is closed to the required di­mension and inserted into its junction with the main.

If the velocity were exactly equal throughout the entire system this balancing would involve only the question of so much added resistance. As mentioned, there may be some differ­ences in velocity in branches and in the main, due to the ideas of the designer.

So balancing is worked on static suction depression and when these are equal in the branch and in the main at any given point of entry, the system is balanced. All branches are, of course, treated as required. See the formula illustrated in Figure 3.87.


Static ‘





| Final


Due to




Difference _

Recovery ‘

— suction

« in


By (

^ “ depression

““ main at



I Of


In branch

In velocity



And in cone

After cone,





Suction | depression »



——- v——-

Nett cone effect

Duct design for dust or refuse exhaust


Duct diameter

Figure 3.88 Velocity pressure in cones

If a cone is inserted in a long length of piping there is consider­able recovery, as measured by tests. When inserted in thejunc — tion, the air leaving is in a turbulent state, and any recovery is balanced by a loss. Experience of results on the method of cal­culation described has indicated that any recovery may be ne­glected.

The cone inserted in a branch must have the same net effect as the difference in static depressions. As no recovery is assumed after the cone, this difference is equal to the increase in velocity pressure from the branch to the mouth of the cone. If the initial velocity pressure in the branch is known then the final velocity pressure at the mouth of the cone has to be vpi + difference in

Depressions. The required diameter of the mouth of the cone to produce this velocity pressure is given in Figure 3.88.

From the required additional pressure, read across to branch

■ .. i… , , . , cone diameter

Velocity m/s and then down to value of———————- .

Duct diameter

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