General concepts of industrial ventilation to reduce pollutant emissions
Within large-scale enclosures, industrial ventilation is a relevant factor, since its different forms and utilities increase the comfort of those who inhabit them and leave behind possible health problems.
General ventilation is a broad term that refers to the supply or extraction of air from an area, premises or building; helps eliminate or reduce undesirable environmental conditions, such as excess heat, cold, humidity and concentration of pollutants (particles, gases, vapors and /or mists, or aerosols, among others) that exceed safety limits.
According to its objectives, it can be classified in different ways. In industrial plants, two types are handled:
Drive systems. Used to inject air.
Extraction systems. Responsible for eliminating contaminants generated by an operation in order to maintain a healthy work environment.
A complete ventilation system must include drive and extraction, and can have both positive and negative pressure as required.
General extraction systems
They are used to control the thermal environment or eliminate the contaminants that are generated in an area, by sweeping internal air (with the term called dilution system).
With respect to contaminated areas, such a system is only used when the contaminant is dispersed in the work environment. Its purpose is the renewal of the entire volume of polluted air in a certain time.
This is usually done by installing air extraction devices on one side of the building and having outside air inlets on the opposite wall (this would be the basic principle of an air sweep). To calculate ventilation, it is necessary to know the volume of the room and the number of times per hour that air change is required (number of renewals or changes per hour).
There are tables of renewals or changes by hour of premises according to the process that is applied. These systems are subdivided into two types.
Dilution ventilation. It consists of the dilution of polluted air into unpolluted air, with the aim of controlling health risks, fire and explosion; annoying odors and contaminants
This type of ventilation can also include the control of environmental pollutants (vapors, gases, mists and particles) generated inside closed buildings.
This methodology is less satisfactory than localized extraction to control health risks. Although in certain circumstances it may provide the same level of protection at a lower cost, with the disadvantage of polluting clean areas.
Figure 1. General extraction system, Volvo plant, welding area, 2010 Ventilation for thermal control. Its operation lies in the control of the environmental conditions associated with very hot industrial environments, such as those of foundries, laundries, bakeries, among others; in order to prevent damage or discomfort. (See Figure 1)
Principles of dilution ventilation
1.- Choose, from the available data, the amount of air sufficient to achieve a satisfactory dilution of the pollutant. The values indicated in tables imply a perfect dilution and distribution of air and solvent vapours. These data must be multiplied by the value of K chosen (for a given equation)
2.- Locate, if possible, the extraction points near the sources of the contaminant, in order to benefit from "punctual ventilation"
3.- Locate the points of introduction and extraction of the air, in such a way that the air passes through the contaminated area. The worker must be placed between the air intake and the polluting source
4.- Replace the extracted air by means of a replacement system (the air provided must be heated during cold seasons). Dilution ventilation systems usually handle large amounts of air using low-pressure fans. For the system to work satisfactorily, the extracted air is essential
5.- Prevent the extracted air from being reintroduced into the premises by unloading it at a sufficient height above the ceiling or by ensuring that no window, outside air intake or other opening is located near the point of discharge, including the direction in which the air blows
Dilution ventilation to protect health
The use of this system for such purposes is subject to four limitations:
(1) The amount of pollutant generated should not be too high, as in that case the necessary air flow would be excessive.
(2) Workers must be sufficiently far from the source of contaminant or dispersion, which must occur in concentrations low enough so that the exposure of workers does not exceed the corresponding TLV value (Threshold Limit Values), used as a guide for the control of health risks caused by toxic substances or materials present in the air. They are expressed as the moderate mean concentration over time, during which almost all workers can be exposed (eight hours a day) without adverse effects.
3) The toxicity of the contaminant must be low
4) The dispersion of the pollutant must be reasonable and uniform
Dilution ventilation finds its most frequent application in the control of organic vapors, whose TLV is equal to or greater than 100 ppm. To successfully apply the principles of dilution to this kind of problem, it is necessary to have real data on the rate of steam generation or on the rate of evaporation of the liquid. Normally, this data can be obtained in the plant itself if it has adequate records on the consumption of materials.
The general equation of dilution ventilation indicates the ventilation flow required to keep the concentration constant within a given value for the rate of contaminant generation. It is deduced from the pollutant balance in the premises, assuming that the air introduced is free of pollution.
Accumulation = Generation – Elimination
Through a series of deductions, we would have the following formula:
Figure 2. Smoke extraction system, Almexa, smelting furnaces, 2009 Q = G / C . K
Where:
G = Generation speed
Q = Actual fan flow m³/h
C = Concentration of gas or vapour
K = Factor that depends mainly on TLV values (see Figure 2)
Localized extraction system
These systems are designed to capture and eliminate contaminants, prior to their expulsion into the general environment of the workplace. It consists of catchment hoods, transport pipe, scrubber, fan and, finally, a chimney with sampling ports and platform (optional).
The hood is the entry point into the extraction system; in addition, the most important. Its essential function is to create an airflow that effectively captures the pollutant.
Properties of contaminants
Inertia: gases, vapors and liquids do not have significant inertia. The same applies to small dust particles with a diameter of 20 microns or less (including respirable particles).
These types of materials move if the air around them does. In this case, the hood must generate a sufficient capture speed to control the movement of air loaded with pollutant and, at the same time, overcome the effect of air currents (turbulence) produced in the premises by other causes, such as movement of people, vehicles, windows or doors.
Effects of density
Often, the location of bells is mistakenly decided based on the assumption that the pollutant is more or less heavy than air. In most applications related to health risks, this criterion is of little value.
Dust particles, fumes, vapors and gases that can pose a health risk behave as if they were "air", without moving appreciably up or down due to their own density, but exclusively following the air currents.
The usual movement of air ensures a uniform dispersion of pollutants, except in operations with great heat or cold detachment, or when a pollutant is generated in large quantities and it is possible to control it before it disperses in other areas.
Types of bells
Although the bells are built in a wide variety of configurations, and what we intend is to encapsulate as much as possible the emission point, it is possible to classify them into two large families: cabins and exterior hoods.
The type of hood to be used will depend on the physical characteristics of the equipment, machinery or installation; the mechanism for generating contaminants, and the relative position of the equipment and the worker. Simply put: it's a tailor-made suit.
Cabins. They are bells that partially or entirely enclose the process at the point of generation of the pollutant. A complete cabin would be, for example, that of a laboratory with mittens, where there are no openings or the open area is small. A partial booth refers to a laboratory hood or the classic paint booths. A stream of air entering the cabin through its opening will retain the contaminant inside, preventing it from reaching the work environment.
The cab is the type of hood to choose whenever the configuration and operation of the process allow it. If complete isolation is not possible, one that complies to the greatest extent possible should be employed.
Exterior bells. This is the name given to those that are located adjacent to the source of contaminant, but without enclosing it, placed at the top or side of the emission point, such as the grids along the mouth of a rectangular opening on a welding table.
When the contaminant is gas, steam or fine dust, and is not emitted with a significant speed, the orientation of the hood is not critical; however, if the pollutant includes large particles that are emitted with an appreciable speed, the hood must be placed in the direction of that emission. An example would be a grinding operation for an emery grinder.
If the process emits very hot polluted air, it will rise due to its lower density. The use of an external hood, located laterally to the updraft, may not produce adequate uptake, because the current induced by the hood is insufficient to counteract the flow of air of thermal origin. This will be especially true for very high temperature processes, such as melting furnaces. In such cases, it is advisable to employ a hood placed on top of the process.
An exterior hood variant is the drive-pull system. In this case, a jet of air is driven through the polluting source to an extraction hood. The contaminant is controlled, essentially, by the jet while the function of the hood is to receive and aspirate it. The essential advantage of the drive-extraction system is that the driven jet can move in a controlled way over a much greater distance than it is possible to control the suction flow of a hood.
This system works successfully for certain surface treatment operations, where open vats are used, but it is possible to use it in other processes. It can happen that the jet of impulsion increases the exposure of the workers if it is not designed, installed or used properly. Special care must be taken in its design, manufacture and placement, and use.
Bell design parameters
The collection and control of pollutants is carried out by the air flow produced by the hood. The movement of air into the opening must be intense enough to keep the pollutant under control until it reaches the hood. Air movements generated by other causes may distort the flow induced by the hood and require higher air flows, in order to overcome such distortions or turbulence.
The elimination of the possible causes of such movements is an important factor in achieving effective control of the contaminant, without having to resort to excessive suction flows and incur the associated high costs. Important sources of turbulence include:
1.- High temperature processes or operations that generate heat, as they give rise to air currents of thermal origin
2.- Movement of machinery, such as deburring wheels or conveyor belts
3.- Movement of materials, such as in the unloading of carts or the filling of containers
4.- Movement of the operator
5.- Drafts in the premises (which are usually considered 0.25 m / s, but can be much higher)
6.- Rapid movement of air produced by localized cooling or heating equipment
The shape of the hood, its size, location and air flow are the main design variables.
Capture speed. It is called the minimum air velocity produced by the hood, which is necessary to capture and direct the pollutant. The air velocity achieved is a function of the flow of air sucked in and the shape of the hood.
Hoods that suck in exceptionally high air flows (e.g. large side hoods used for demolding in foundries) may require lower flow rates than those deduced from the recommended capture rates for small hoods. This phenomenon can be attributed to:
The presence of large masses of air moving in the direction of the bell That the pollutant remains under the influence of the bell for a longer time, than in the case of small bells The fact that a high flow rate provides considerable dilution, as explained above There are tables made expressly on capture speeds (reference: Industrial ventilation).
Determination of the suction flow. Air moves into the suction mouth of a bell from all directions, except for the limitations set by the existence of deflector screens, walls and other physical impediments. For a cabin, the capture rate at its opening(s) is the ratio of the extraction flow rate between the area of the opening(s). The capture speed at any point outside the hood shall be that corresponding to the surface of equal speed passing through that point for the suction flow used.
Minimum duct speed
The dynamic pressure in the duct (PI), used to determine the pressure loss in the hood, is determined from the air velocity of the duct area, immediately after its connection with the hood. This speed is fixed by the type of material that is transported through the duct (air + type of contaminant).
For systems that handle particles, it is necessary to establish a minimum design or transport speed, in order to prevent their deposition and clogging of the duct. On the other hand, too high speeds imply a waste of energy and can quickly cause abrasion of the ducts. The recommended minimum design speeds are higher than the theoretical values (existing tables; reference: Industrial Ventilation) and experimental values in order to take into account contingencies, such as the following:
If one or more branches become clogged or are taken out of service, the total flow rate in the system will be reduced and therefore the speed in at least some of its parts will decrease The deterioration of the ducts, for example by dents, will increase the resistance and decrease the flow and speed in the damaged part of the system Leaks in the ducts will increase the flow rate and speed in the direction downstream of the leak, but will decrease the flow upstream and in some other parts of the working system Corrosion or erosion of the fan blades or the slippage of their traction bands will reduce the flow rate and speeds Speeds must be adequate to trap and drag back dust that may have been deposited due to improper use of the extraction system
Figure 3. Toluca air movement and control duct, welding area, 2010 Designers should bear in mind that, in certain conditions such as sticky products, situations in which condensation may occur in the presence of dusts, strong electrostatic effects, among others; speed alone may not be sufficient to prevent tamponade; so it may be necessary to take action.
Also, it is important to consider an increase in transport speed or design. The conduction of air and particles or contaminants trapped in it is important.
There are standards for the construction of the pipe that affects the pressure drop of the system (see image 3). In addition, the minimum speed in the duct is extremely important (transport speed or design).
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