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

Selecting components for the extraction and filtration of industrial pollutants such as welding fumes, metal dust, and similar substances requires a series of calculations. These calculations are often quite complex. In this article, we will attempt to explain them in their simplest form.

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

The following formula can be used to calculate the minimum transport velocity in its simplest form. (Minimum transport velocities of mineral and metallic dust in exhaust

us systems. RE Pocovi, G.Villaflor, JE Flores.)

V_d= 10,4 p^0,37 . d^0,26

V_d= minimum design air speed, m/sec

P= solid particle density, g/cm³

d= mean 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 obtained here provides the minimum design air velocity to consider in duct design. In general, air velocities above these values are selected. Actual horizontal and vertical conveying air velocities can also be calculated using more complex equations.

Preferred sample transport speeds for some particles are as follows. (Quoted from Nordfab)

Polluting	Pollutant Examples	Design Speed (m/s)
Smoke, vapors, gases	All smoke, vapors and gases	Any desired speed (economic optimum speed is generally 5-10 m/s)
Smoke	Welding fumes	10-13
Very fine light powder	Lint, wood dust, toner dust, paint pigments	13-15
Dry powder and dusts	Cotton dust, sawdust (light), leather dust, fine rubber dust, Bakelite mold dust, jute lint, soap dust, plastic dust	15-20
General industrial dust	General material handling, grinding dust, coffee beans, polishing lint (dry), wool dust (waving waste), shoe dust, granite dust, silica flour, brick cutting, clay dust, cement dust, brick dust, gypsum dust, foundry (general), limestone dust, packaging and weighing of asbestos dust in the textile industry, animal feed products	18-20
Dense dust	Sawdust (heavy and wet), wood blocks, metal turnings, sandblast dust, foundry drum and shake dust, pig waste, brass shavings, cast iron drilling dust, lead dust	20-23
Heavy or damp	Small sawdust lead dust, moist cement dust, asbestos particles from transit pipe cutting machines, polishing lint (adhesive), lime dust, wood waste (transport systems)	23 and over

Once the transport velocity is determined, the air velocity required to safely transport these particles will be determined. In system design, utmost care must be taken to avoid falling below this transport velocity.

Dusts are classified in terms of particle size;

-Very fine powders (0.1-50 µm)

-Fine powders (50-100 µm)

-Coarse powders (100-1000 µm);

They can be classified as.

Very fine dusts are grouped into three groups in the industry:

-Ultra-fine powders (around 0.1-1µm)

-Super fine powders (around 1-10 µm)

-Granular fine powders (around 10-100 µm)

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

Air flow calculation formula: Q = V x A

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

EXAMPLE:

20 m/s air speed 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

In this way, we can determine the lowest air flow rate required to safely transport this pollutant in a 200 mm diameter circular duct.

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

At this point the calculations get a little more complicated.

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

When designing a mobile smoke/dust extraction machine, we first need to know the total pressure loss resulting from the machine's design. This is because the fan we select must overcome this total pressure. When designing such devices, it's essential to consider the dirty air intake ducts, the sections through which air is directed, the pressure losses of the filters we will use ( filter manufacturers publish these values in their technical documentation ), and many other factors, to achieve the lowest possible pressure losses. Of course, it's also necessary to calculate the individual pressure losses of equipment such as the extraction arm and hose that will be used with this machine. For example, pressure losses will be significantly higher in machines and systems designed with fully flexible hoses for the extraction arm.

In central systems, pressure losses must be calculated accurately by considering many factors, including the design pressure losses of the filter unit, pipe/duct cross-sections, lengths, and the structure and number of connecting elements. Fan selection must be based on 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 inner diameter

ρ Density

v Air speed

Different formulas are used for elbows and duct connection equipment.

Additionally, the friction factors in the pipes we will use must be precisely calculated. It should be remembered that air flow through the pipe in such systems is turbulent. The formula for the pipe's roughness value (ε) / pipe diameter (d) will yield the f (friction factor) value on the Moody chart, which is also dependent on the Reynolds number we will calculate.

Re = (pVD) / μ

Re: Reynolds number, p: Density (kg/m³), V: Speed (m/s), D: Pipe diameter (meter), μ: Dynamic viscosity (Ns/m²)

Moody diagram

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

Fan curve

According to the fan curve in the example, the efficient operating point of this fan is approximately 500 Pascal pressure (2600 rpm) at a flow rate of approximately 3500 m³/hour. In other words, this fan can effectively deliver 3500 m³/hour of air flow in a system with a total pressure loss of 500 Pascal.

In the same example table, you can see that an air flow of approximately 2900 m³/h can be achieved at a rotation speed of 3000 rpm and a pressure of 1000 pascals. As you'll notice, when the engine speed was reduced to 2600 rpm at the same flow rate, the pressure dropped to 700 pascals. Therefore, the engine's (or fan's) rotation speed also plays a significant role in our designs.

In central suction system designs, pipe diameters and lengths must be determined precisely.

For example, if a 15-meter-long line is to be designed and 5 150 mm diameter suction arms are to be placed on this line, the pipe diameter to which the farthest arm will be connected will not be the same as the pipe diameter closest to the filter system.

Example of pipe diameters in centralised system installation

Pipe diameters are designed to be increased in a cost-effective manner to handle the air flow added to the system and not affect the air velocity.

To illustrate, using the example in the figure, let's calculate the conveying speed as approximately 16 meters/second, based on the particles to be suctioned. Let's assume we'll be suctioning 1000 m³/hour of air through each 150 mm diameter suction arm. Naturally, we'll connect this arm to a 150 mm diameter pipe. Based on an air flow rate of 1000 m³/hour and a 150 mm diameter circular duct cross-section, the air speed will be approximately 16 m/second.

We also achieved 1000 m³/h of air intake from the second branch. This resulted in a flow rate of 2000 m³/h. We will not reduce the air velocity below 16 m/s. If we maintain the same diameter, the air velocity will increase excessively, and so will the dynamic pressure. Therefore, we increased the diameter of our connecting pipe to 200 mm, including the second branch. Our air velocity reached approximately 17 m/s. By keeping the air velocity constant, we prevented a rise in dynamic pressure, and by keeping the air velocity constant, we maintained our conveying velocity to prevent particles from accumulating in the pipe. We can complete the system design by performing similar calculations for all subsequent branch connections.

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

We must strive to achieve the lowest possible airflow and pressure loss in our mobile and centralized smoke/dust extraction designs. This is because as airflow and pressure loss increase, the fan and motor requirements required to achieve these values will also increase. This, in turn, unnecessarily increases energy consumption.

Selecting a fan and motor with a high-efficiency design is also important for energy savings. These types of fans and motors, while increasing investment costs, also provide significant savings in operating costs. EC-motor fans, which are becoming increasingly common today, offer very high efficiency while also consuming significantly less energy.

For greater energy savings in central suction systems, the use of electronic designs that can detect branches that are not currently in use, close them with a damper (or inform the system when the damper is manually closed), and automatically adjust the engine speed accordingly is also an important issue.