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How to Make Simple Calculations for Fume and Dust Extraction Systems?

 

 

The selection of components to be used to absorb and filter welding fumes, metal dust and similar industrial pollutants requires a series of calculations. These calculations are usually quite complex. In this article, we will try to explain it in its simplest form.

 

First of all, it is necessary to determine the transport rates according to the type of pollutants.

The following formula can be used to calculate the minimum transport speed in its simplest form. (Minimum transport velocities of mineral and metallic dust in exhaust systems. R.E. Pocovi, G.Villaflor, J.E. Flores.)

 

V_d= 10,4 x p^0,37 x d^0,26

V_d= minimum design air velocity, m/sec

P= solid particle density, gr/cm³

d= average diameter of solid particles, mm

Sample calculation:

Cement specific gravity: 3,2 g/cm³. 3,2 ^0,37 = 1,54

Cement particle size: 0,03 mm. 0,03 ^0,26 = 0,40

V_d= 10,4 x 1,54 x 0,40 = 6,40 m/s

 

The result here gives the minimum design air velocity to be considered in duct design. In general applications, air velocities higher than these values are selected. Actual horizontal and vertical conveying air velocities can also be calculated with more complex equations.

The preferred sample transport velocities for some particles are as follows. (Quoted from the Nordfab company)

Nature of Contaminant	Contaminant Examples	Design Velocity  (m/s)
Smoke, vapors, gases	All smoke, vapors and gases	Any desired velocity  (economic optimum velocity usually 5-10 m/s)
Fumes	Welding	10-13
Very fine light dust	Cotton lint, wood flour, litho powder, toner powder, paint pigments	13-15
Dry powder & dusts	Cotton dust, shavings (light), leather shavings, fine rubber dust, Bakelite molding powder dust, jute lint, soap dust, plastics dust	15-20
General industrial dust	General material handling, grinding dust, coffee beans, buffing lint (dry), wool just dust (shaker waste), shoe dust, granite dust, silica flour, brick cutting, clay dust, cement dust, brick dust, gypsum dust, foundry (general), limestone dust, packaging and weighing asbestos dust in textile industry, animal feed products	18-20
Heavy dust	Sawdust (heavy and wet), wood blocks, metal turnings, sand blast dust, foundry tumbling barrels and shake-out, hog waste, brass turnings, cast iron boring dust, lead dust	20-23
Heavy or moist	Lead dusts with small chips, moist cement dust, asbestos chunks from transite pipe cutting machines, buffing lint (sticky), quick-lime dust, wood waste (transport systems)	23 and up

 

 

Once the transport velocity has been determined, the air velocity at which these particles can be safely transported will be determined. It is necessary to pay utmost attention not to go below this transport speed in the system design.

Dusts in terms of particle size;

-Very fine powders (0.1-50 µm)

-Fine powders (50-100 µm)

-Coarse powders (100-1000 µm) can be classified as.

 

Very fine powders are categorised under 3 groups in the industry:

-Ultra fine powders (around 0.1-1µm)

-Superfine powders (around 1-10 µm)

-Granular fine powders (10-100 µm)

 

After determining the air velocity, we can calculate our flow rate according to the pipe / duct section we will use.

Air flow rate calculation formula:  Q = V x A

Q = Flow rate, V = Air velocity (m/s), A = Cross-sectional area (m²)

EXAMPLE:

20 m/s air velocity and 200 mm diameter circular duct will be used.

Area of 200 mm diameter pipe: 0.031416 m²

Q= (20 x 0,031416) x 3600 = 2262 m³/hour

This is how we can determine the minimum air flow rate we will need to safely transport this pollutant in a 200 mm diameter circular duct.

But what should be the pressure value of our fan that will produce this flow rate?

At this point, the calculations become a little more complicated.

Regardless of whether it is a central or mobile system, all components to be used in the design will have pressure effects.

In the design of a mobile smoke/dust extraction machine, we first need to know the total pressure loss caused by the design of the machine. Because the fan we choose must overcome this total pressure. During the design phase of such devices, it is necessary to make a design according to the lowest possible pressure losses, taking into account many factors such as dirty air inlet ducts, sections where the air is directed, pressure losses of the filters we will use (filter manufacturers publish these values in their technical documents) and so on. Of course, it is also necessary to calculate the pressure losses of the equipment such as suction arm and hose to be used with this machine. For example, pressure losses will be much higher in machines and systems whose suction arm is designed with a completely flexible hose.

In centralized systems, in addition to the design pressure losses of the filter unit, pressure losses should be calculated properly by calculating many factors such as pipe / duct sections, lengths, structures and numbers of fittings. A fan must be selected according to these calculations.

The following formula can be used to calculate pressure losses in piped systems;

Δpb= λ*(l/D)*(ρ/2)*v² *10^-5

λ Pipe friction coefficient, l Pipe length, D Pipe inside diameter, ρ Density, v Air velocity     

 

Different formulas are used for elbows and duct connection equipment.

In addition, the friction factors in the pipes we will use must be calculated precisely. It should not be forgotten that the air flow for the pipe is turbulent in such systems. The number to be obtained with the formula roughness value of the pipe we will use (ε) / pipe diameter (d) will give us the f (friction factor) value in the Moody diagram, depending on the Reynolds number we will calculate.

Re = (p.V.D) / μ

Re: Reynolds number, p: Density (kg/m³), V: Velocity (m/s), D: Pipe diameter (meters), μ: Dynamic viscosity (N-s/m²))

Figure 1 Moody diagram

 

All pressure losses that may occur from the suction nozzle to the fan should be calculated and a fan that can overcome this pressure should be selected according to the curve published by the fan manufacturer.

 

Figure 2 Example of a fan curve

As can be seen in the example shown in Figure 2, the efficient operating point of this fan is approximately 500 Pascal pressure (2600 rpm) at a flow rate of approximately 3500 m³/h. In other words, this fan can provide a healthy 3500 m³/h air flow rate in a system with a total pressure loss of 500 Pascal.

In the same example table, you can see that at 3000 rpm rotation speed, approximately 2900 m³/h air flow rate can be provided at 1000 pascal pressure. As you will notice, at the same flow rate, the pressure dropped to 700 pascal when the engine speed was reduced to 2600 rpm. In this case, the rotation speed of the motor, i.e. the fan, is also an important factor in our designs

Pipe diameters and lengths must be precisely determined in central extraction system designs.

For example, if a line with a length of 15 meters is to be designed and 5 extraction arms with a diameter of 150 mm are to be placed on this line, the pipe diameter to which the arm at the end will be connected will not be the same as the diameter of the nearest pipe to be connected to the filter system.

Figure 3 Example of pipe diameters in centralised system installation

Pipe diameters are designed in such a way that they can handle the air flow rate added to the system and do not affect the air velocity.

To explain through the example in Figure 3; let's calculate the transport speed as approximately 16 meters/second according to the particles to be sucked. Let's assume that we will suck 1000 m³/hour of air through all 150 mm diameter suction arms. We have naturally connected this arm to a pipe of the same diameter, i.e. 150 mm diameter. According to the air flow rate of 1000 m³/h and the circular duct section of 150 mm diameter, the air velocity will be approximately 16 m/second.

From the second branch, we provided 1000 m³/h air suction, so that the flow rate was 2000 m³/h. We will not reduce the air speed below 16 m/s. If we continue with the same diameter, the air speed will increase excessively and the dynamic pressure will increase. For this reason, we increased the diameter of our connecting pipe with the second branch to 200 mm. Our air velocity became about 17 m/s. By not increasing the air velocity too much, we prevented the increase in dynamic pressure, and by not decreasing it, we remained faithful to our transport speed so that the particles would not accumulate in the pipe. We can complete the system design by making similar calculations for all subsequent arm connections.

To repeat; we will stick to the transport speed, avoid high speeds and prevent possible dynamic pressure increases (let's not forget Bernoulli; static pressure + dynamic pressure = total pressure).

We have to work to ensure the lowest possible flow rate and pressure loss in our mobile and centralized smoke/dust extraction designs. Because as the air flow rate and pressure loss increase, the fan that can provide these values and the motor values that will turn this fan will also increase. In this case, energy consumption increases unnecessarily.

Fan and motor selection with high efficiency design is also important for energy saving. These types of fans and motors, which increase investment costs, provide high savings in operating costs. Fans with EC motors, which are becoming more widespread today, offer very high efficiency and much lower energy consumption.

For more energy savings in central suction systems, it is also important to use electronic designs that can detect the arms that are not currently in use and close them with a flapper (or inform the system when the flapper is manually closed) and automatically adjust the motor speed accordingly.

HiVent Technology, 02/12/2024, Ankara