Selecting the correct type and radius of elbows is very important for balanced pressure conditions in the air extraction and dedusting system. Generally speaking, when choosing the correct elbow for dust extraction we decide on two requirements:
- Requirement of the lowest possible ducting pressure drop
- Requirement of the elbow durability
Low pressure drop
Plays an important factor in the economics of the extraction system operation. The right design of elbows is as important as the economic operation of the bag filter system described in the chapter “Optimal baghouse pressure drop“.
The general rule is: The longer the ducting is, the higher the radius of the elbows is to be used. For simplicity, the situation can be represented by the fact that air always prefers the path of least resistance. This is manifested in the fact that the extraction points closer to the baghouse are dedusted better than the more remote ones. This is due to the pressure drop behind each branch and the fact that the extraction points furthest away are connected by the longest ducting route and therefore have the greatest pressure drop.
We often use a combination of elbow radii so that with increasing distance from the baghouse we increase the radius of the elbows to force the system to extract even more air at the furthest places. Near the baghouse, elbows with a radius of 1D are used, and 3D knees are used along the entire length of the main branch up to the outermost points. The balancing of the entire extraction system is then controlled by means of regulation slider or butterfly valves.
Here you can see a visualisation of simulated flow inside several elbows. The flow velocities are marked with different colors. The warmer the color, the faster the flow velocity is. A large increase in flow velocity can be seen in the sharp 1D radius elbow, this results in an increase of pressure drop inside the elbow. We can see a gradual change of flow velocity in the 3D radius elbow – resulting in much smaller pressure drop than the 1D elbow.
The lifetime of elbows in the dust extraction system is not limitless. The elbows and branches are suffer to abrasion from the extracted dust. This type of wear is caused by particles sliding on the outer arc wall where they hit every segment of the elbow. The segment joints are the most worn parts of the elbow. This wear will eventually manifest by scraping the elbow through the outer wall. In some exceptionally heavy plants, treating the elbows as spare parts or consumables is neccesary. Simulation of particle dust behaviors in different elbow radii is shown below.
Anti-abrasion damage protection can be applied in a variety of ways – such as internal rubber kneading or double-skinned design. However, we know from practice that it is a costly adjustment that only prolongs life, but does not remove the wear.
General rule is: The greater the abrasiveness of the dust extracted, the greater the radius of the knees should be, and the greater the thickness of the casing to be used. The knee will last longer.