Dispersed gases
Many dedusting and purification systems starts for the designer of the installation in a defined duct or even in the outlet flange of the main equipment (outlet of a boiler, kiln, etc.). But not always is in this way, sometimes the dirty gases are “dispersed”.
For example, when dumping bulk from a truck to a hopper, or from a silo to an open rail hopper, or polishing and finishing work on parts that give off small clouds of dust while being worked on.
In these open systems of “dispersed” gases, the first thing to do is to “confine” them; that is, design a type of enclosure, shed or hood so the gas and dust can be evacuated through a pipe to a corresponding filtration system.
Hood design
Hoods can be designed for numerous applications, and there is an abundance of reference information. It is impossible to generalise, but the hood should as well sealed as possible to the dust produced; and the further away from the hood, the larger it needs to be, and the greater the suction flow.
Below are some farther concepts:
Velocity gradient
The diagram below shows the much lower capture speed the farther away from the mouth of the hood.
Different speeds in the area of the hoods and/or catchment cabins
– Capture speeds: speed at a point away from the hood, sufficient for a dust particle to move towards the hood.
– Open velocity phase: velocity at the entrance to the hood.
– Slot velocity: speed through the slit.
– Plenum velocity: vertical speed in the plenum.
– Duct velocity: velocity in the duct.
Importance of air currents and temperature
Air currents in the catchment area affect the selection of the evacuation speed, which should be increased outdoors or in drafty areas, for example.
Similarly, different temperatures affect the movement of the gases, promoting or hindering suction. Colder gases are heavier and hot gases lighter, so hot gases tend to go upwards, which helps in their evacuation via ceiling hoods.
Dirty air duct speeds
The following must be established for the dirty air ducts or for the supply of gas or air with dust towards a bag filter:
– Sections with slopes as high as possible to reduce dust deposit
– Selection of speeds for a given flow of gas. Below are some considerations:
Low speeds cause less pressure drop, less abrasion, but with a higher risk of clogging. This is more acceptable for more vertical paths.
High speeds cause more pressure drops, more abrasion and have less risk of clogging; and are more appropriate for paths with a slight incline.
Fluidised duct
A solution designed by GORCO in extreme cases, when it is impossible not to have an almost horizontal path, is the design of quadrangular pipes with a fluidised textile base, behaving like an airslide.
GORCO Experience and supply
In general, GORCO can study the design, subsequent supply and assembly of all the ducts from the suction points to the bag filter; including flow rates, hoods, pipe sections and shapes and flow regulation valves to control suction in pipe networks from different points (or “grabs”).
There are hundreds of difficult cases that have come our way throughout our 45 years of existence.
