Control of dust is important in meeting the environmental, health and safety requirements. The systems of controlling dust will not just help reduce emissions at site but will also help protect employees. The exposure to dust can trigger various respiratory health conditions and complications. Remember that as the employer you will be held liable for the health of the employees especially if they develop the condition while working for you. Among the leading contributors of air pollution is the dust produced in construction, mining and in unpaved roads.
Other than being explosion hazard when dust is dispersed to the air, dust from coal site can cause black lung diseases. The dust produced from stone, gravel and sand processing may lead to the development of a condition known as silicosis. The purpose of dust control spray is to suppress and also collect the fumes and dust which is produced during the industrial process of production of steel, mining and in the course of manufacturing cement, among others.
Types of dust control:
There are two major options of dust control and they include the following –
Dry collection of dust: in this option, bag house which have fabric filters are used for collecting dust. The purpose of the fabric filters is to vacuum dust from its source and direct it to hopper from where it is collected and disposed properly.
Wet suppression of dust: This is the most common method of dust management and involves the use of water guns, water sprays and mists to arrest the dust on processing operations, roads and piles. Water spray systems are among the most popular option in the wet dust suppression technology.
For improved performance, most of the wet systems of dust control use surfactants and foams, especially for the materials which need specialized applications, for example coal and petroleum coke, which do not get wet easily. In this method, various factors, including surface tension, impact velocity, droplet size and water contact angles all play important role in determining how effective the dust control is.
The drop size:
Drop size refers to individual drops in the spray pattern produced by the nozzle. The spray pattern will produce liquid drops of various sizes. Different factors affect the size of drop and they include spray angle, spray pressure, capacity of the nozzle and liquid properties.
The air atomizing nozzles are known to produce the smallest sizes of drops. They are followed by the hydraulic fine spray nozzles then the hollow cone, the flat fan, and finally the full cone nozzles. For effective suppression of dust, the drop size should range from 20 to 200 µm as that is also the average size of the dust particles which get airborne. Higher atomization is needed in order to produce small size of droplet.
The atomization of dust control spray is usually achieved when water is pumped through the nozzles at higher pressure. It can also be achieved through the use of compressed air combined with water which is pumped at low pressure so as to produce fog or very small droplets of water. The basic line here is that if the diameter of the drop is larger than the diameter of the dust particle, then the dust particle follows the air steam surrounding the drop. However, if the diameter of the drop is of almost or equal size to that of the dust particle, the water droplet follows airstream and thus collides with that dust particle.
The spray angle ranges between 0o and 175o. Different angles are needed depending on placement of nozzle, the spray pattern and also the number of the nozzles used.
The operating pressure and rate of flow depends on the level of moisture need to be applied so as to suppress the dust. Important to note is that decrease in pressure reduces the size of drop. For enclosed areas, high pressure sprays should be used. When nozzles are operating at high pressure, it is important to place them close to the source of dust in order to reduce the amount or volume of air which sets in motion along that spray path.
The pattern of spray is important. The pattern should generally cloud over the dust in order to capture as many particles as possible.