Notions regarding the design of suction systems for industrial ventilation

Air flow in ducts is one of the basic processes of ventilation and air conditioning. Knowledge of this phenomenon and the laws according to it is indispensable for drawing up the sizing calculations of the duct networks, for determining the energy needed to move the air as well as for determining the aerodynamic parameters related to the ventilation ducts. Industrial buildings include large spaces with various sources of releases of harmful substances. Type of these sources and their location depends on the technological process in each section or room.The main role of industrial ventilation systems is to provide a continuous source of fresh air supply from the outside, to keep the temperature and humidity at comfortable levels, to maintain an adequate supply of oxygen in the work area, to control concentrations of hazardous explosive and / or toxic gases in the air and at work, to remove unwanted odors from a particular area and to remove and dilute airborne contaminants. The paper presents the design of exhaust systems, the calculation of suction ventilation systems and the choice of ducts with higher aerodynamic parameters in the choice of fans, using nomograms.


Introduction
The ventilation and air conditioning installations in the rooms aim to achieve a comfortable and hygienic environment. In the case of industrial rooms, these installations create in addition to a favorable environment for the good development of the technological process, and comfort conditions that lead to a significant increase in labor productivity.
The design and construction of an individual ventilation system must harmonize the comfort requirements with those of the management of the ventilation system by the user, without inadvertently affecting the other functions of the system.
When harmful emissions produce a defect in the room that endangers human health or affects their comfort, the use of special ventilation systems is required [1].

Design of exhaust systems
Designing an efficient suction system is a very complex process that involves knowing a large number of factors. Each suction port must be designed in such a way that the dust in question does not allow toxic and / or explosive gases to be recirculated, but at the same time, it becomes necessary to capture coarse particles that can clog the ventilation ducts [2].
In Table 1, shows the influence of air velocities at the end of the tubes column, as a percentage of the tube diameter, as a function of the air velocity in the center of the tube, as a percentage of the average velocity. The values in Table 1 are specific to the circular columns. If the air velocity at the end of a column with a diameter of 100 mm is 20 m / s, the suction at a distance of 100 mm from it (ie 100% of the diameter) will only be 7% of 20 = 1.4 m/s. For this reason, the suction mouths must be designed so that the source of dust or explosive and / or toxic atmospheres is isolated as far away as possible. If the toxic / explosive atmosphere is moving rapidly, the suction mouth must be placed in the direction of its movement.
Contaminated air trapped by a suction port must be transported along the duct at such a speed as to avoid increasing the concentration of gases in the duct or depositing dust on its walls.
Considering the case where 10 m 3 /s are transported on a column with a length of 100 m at a speed of 10 m/s and a cross section of 1m 2 is required. If a coefficient of friction of K = 0.003 is adopted, the results presented in Table 2 are obtained: There are tables, with the help of which the pressure losses in circular columns can be found, for different values of the air flow. These tables can also be used for rectangular columns if the equivalent diameter of the column is calculated in advance from the equation: In the tables, this value is used in association with the air speed and not with the air flow, in order to determine the pressure losses. The reason is that a rectangular column has a much larger area than a circular one for the same equivalent diameter, consequently being able to carry a higher air flow at the same pressure loss, if the pressure losses correspond, then the air velocities are identical, after as can be seen from Table 3. In addition to the internal resistance to air flow due to viscosity, the frictional resistance due to the friction of the air against the pipe walls, the air flow is also slowed by the presence of fillings, curves and variations in the cross section of the pipe.
Duct fillings slow down the flow of air in two ways, namely: -First, fillings produce a reduction in the free section of the pipe which results in an increase in air velocity which, as shown by the Atkinson equation, leads to an increase in frictional resistance; -Secondly, however, the air flow lines are interrupted by the presence of fillings producing eddy currents that lead to energy losses.

Calculation of suction systems
The calculation of any ventilation system starts from the farthest point from the fan. If a very simple suction system is taken into account, as the one represented in Figure 1 and starting from the assumption that it is necessary to aspirate a flow of 1 m 3 /s from point A and 0.5 m 3 /s each from points B and C, and the minimum air transport speed is 14 m / s, the falls The column inlet pressure is 78 Pa in dynamic pressure in A, 29 Pa in B and 49 Pa in C. [2]. The approximate pipe dimensions for the specified speeds and volume shall have diameters of 300 mm from point A to point D, 200 mm for the BD and CE branches, 360 mm for the DE pipe and 400 mm for the EF pipe.
For a flow of 1 m 3 /s, on a pipe with a diameter of 300 mm between the points AD the air speed will be 14.15 m/s, which corresponds to a dynamic pressure (at sea density) of 118 Pa. The pressure drop at the inlet to point A will be: H a = 7 8 / 1 0 0 x 1 1 8 = 9 2 P a ( 2 ) The negative static pressure in the pipe near point A will thus be 118 +92 = 210 Pa. The pressure loss in a 300 mm pipe carrying 1 m 3 / s of air is 78 Pa per 100 meters and therefore on 25 meters we will have: 25 m x 78/100 = 20 Pa (3) The negative static pressure at D will thus be 210 + 20 = 230 Pa. The BD branch can be calculated in exactly the same way to require a negative static pressure of 153.5 Pa, for a flow rate of 0.5 m 3 /s. But the pressure at D is 230 Pa. Therefore, the volume of air that will actually flow through the 200 mm pipe corresponding to the BD branch will not be 0.5 m 3 /s: This is too much and will increase the fan parameters and costs. The recalculation will be made for a pipe of 180 mm on the BD branch instead of 200 mm, resulting in an air volume of 0.49 m 3 /s, and an air speed of 19.3 m/s. Although this speed is very high and will lead to increased abrasion in this branch, this alternative is preferable to using a 200 mm pipe.
On the DE branch the flow will be 1.49 m 3 /s. The pressure loss along 10 m of pipe with a diameter of 360 mm will be 77 Pa, and the suction at E will be 230 + 77 = 307 Pa.
Having the data for the BD branch, the 180 mm pipe will now be considered for the CE branch as well. The calculation shows that a depression of 307 Pa will aspire to a flow rate of 0.51 m 3 /s, thereby with a speed of 19.8 m/s The pressure drop on the 400 mm column related to the EF branch for a flow rate of 2.0 m 3 /s, will be 75 Pa with an air speed of 15.9 m/s. Therefore, the fan must be able to supply a negative static pressure on the suction side of 307 ± 75 = 382 Pa when circulating 2,00 m 3 /s. The pressure on the discharge side will depend on the design of the pipe and the filter on that side.
In this example a real problem arose, solving this case leads to considerations regarding both the appropriateness of mounting the fan near point D instead of point F because it is centrally located and reduces the need for air and column leakage losses (point A).
An elbow at 90 in a column of tubes has a much greater strength than the equivalent length of the straight column. Its actual strength depends mainly on the angle of the elbow, which is usually expressed as the ratio of the rays. The radius ratio is defined as the ratio between the average radius of the elbow and the diameter of the column, where the average radius is the inner radius plus half the diameter of the pipe.
In Figure 2, R1 is the outer radius, R2 is the middle radius and R3 is the inner radius. The ratio of the rays is therefore R 2 /D. The ratio of the rays is therefore. A radius ratio of two or more indicates a smooth elbow while a smaller value indicates an elbow with a small connection angle, as shown in Table 4.  The pressure drop at an elbow of 45 ° represents 50% of that of an elbow of 90 °. If, in fig. no.2, column AD would have two elbows at 90º and one at 60º with a radius ratio of 2.0 for the purpose of calculations, its length should be 25+ (11 + 11 + 7) x 0.3 = 33.7. In order to minimize both pressure losses and friction, it is desirable to use as few bends as possible when designing the suction systems ( Figure 3). In Figure 3 a) two bends at 90º are used in order to join two columns located at different levels. In Figure 3 b) the same result can be obtained with the help of two elbows at 45º situation in which the resistance is reduced by half. In the case of an elbow in a rectangular column there is an additional factor that affects the pressure loss, namely the shape coefficient. This factor is defined as the ratio between its width and height. A square column has a shape coefficient = 1. A wide elbow with a wide width has a high shape coefficient (2 or more) which leads to low pressure losses. A narrow elbow has a low coefficient of shape (0.5 or less) and is characterized by high pressure losses.
The branch should enter the conical portion at an angle of 45 ° or even smaller, preferably 30 °. The air velocities in the branch and in the main column, upstream and downstream of the connection, must be as identical as possible.

Ventilation ducts with superior aerodynamic parameters
When air flows through a ventilation column, the pressure required to push or suck air through it depends not only on the internal friction force but also on the dimensions, length and shape of the pipe, the roughness of its walls, the nature of the obstructions inside, air speed and density.
Air leaks in both the main and secondary columns are another source of air leaks. Factors that amplify losses are:  Location of fans;  Diameter and length of the column;  Pressure drop and column condition. In design, the largest possible diameter must be chosen from a practical point of view.

Column types with higher parameters
The air ducts must be made of materials with the lowest possible roughness and which make it possible to achieve tight constructions. The materials used for the air ducts must be non-combustible, especially in cases where the ducts pass through several rooms and through which toxic and / or explosive gases are conveyed. The pipes with the highest transport capacity (m 3 /h air) are round or square, and for those with a rectangular section, aspect ratios greater than 8: 1 must be avoided. It is recommended that the value of these ratios be as small as possible, and in current applications, at most 4:1. [1].
The amount of air flowing through a duct depends on the cross-sectional area (duct opening area) and air velocity. Air that moves too slowly will allow contaminants, such as dust, to deposit and accumulate, and these particles will eventually clog the pipe. Air that carries heavy dust particles too quickly can cause noise problems and cause excessive duct abrasion. [3][4][5]

Types of pipes used in industrial ventilation
There are several types of pipes made of different materials and having the following constructional characteristics [5]:

Calculation of losses in ventilation columns
There are two types of air leaks through a ventilation duct, namely: dynamic pressure losses and friction losses [1][2][3].
The pressure drop at each inlet, outlet and outlet must be calculated individually, this being based on the dynamic air pressure at that point in the system.
The dynamic air pressure is given by the relation: The pressure drop in a pipe is given by Atkinson's equation: Since this relation is valid only for the standard air density (ρst = 1.2 kg/m 3 ), for the generalization of this relation and for other values of the air density, this relation becomes: For all relationships, the definitions are:

Evaluation of fan curves and pressure loss curves
To determine the friction losses, proceed as follows (according to the diagram in Figure 4) [4,5]:  the average air flow from the column is determined (air at the work front +1/2 losses due to leaks);  follow the line corresponding to the diameter of the column we want to analyze, and read on the axis the static pressure drop / 30 m;  multiply this figure obtained by the length of the column to obtain the friction losses.
For example, 800 m 3 / min are required, the length of the flexible discharge column is 600 m and the diameter is 900 mm and the leakage losses are 115 m 3 /min. The average air flow in the column will be 850 m 3 /min, resulting in a pressure loss of 20 daPa/30 m or 400 daPa in total.
For the choice of the fan ( Figure 5) it starts from the total pressure, and at the intersection of its afferent line with the vertical raised near the value of the air flow, it is evaluated if the intersection point is above or below the fan curve.