Elbows ahead of the inlet will influence the performance of a cyclone separator

 

Introduction

Cyclone separators have been applied in industrial fields for over a century to separate particles from gas streams. They have an inherent simplicity of design that usually encompasses no moving parts with the ability to collect fine particulates. Current designs of industrial high-efficiency cyclones can provide collection efficiency of 90 percent and higher for particles with diameters as small as 2 microns with specific gravity of 1.

The inlet flow features used in design are the mean velocity and mixed mean dust concentration at the inlet, and the type of inlet. Based on these assumptions, flow patterns inside of a cyclone and its performance (pressure loss and grade collection efficiency) are developed and calculated.

However, all design procedures appear to treat the cyclone as a device which operates independently from the flow configuration upstream. In particular, for a cyclone collecting dust from a gas flow, the orientation of elbows in the inlet duct is ignored.

An elbow can “push” the solids being conveyed in an air-conveying duct to its “outside” wall, so they become unevenly distributed in the gas flow, and are slowed down, especially in tightly curved elbows. Thus, the inertia of the solids becomes important in the distribution of the solids after an elbow. Also gravity may affect this distribution after a long horizontal travel. After entry into a cyclone, the inertia of the solids is expected to continue to influence its path within the cyclone, and even influence the path of the gas, if the solids content (or dust loading) is high.

Different inlet duct configurations influence the performance of a cyclone

Both lab tests on small scale cyclones and industrial experience on large cyclones have indicated that the elbow configuration upstream of a cyclone can influence its performance markedly. This could be seen in installations where there were two or more cyclones of identical geometry and size, treating similar dust flows, but with differing inlet duct configurations. These different configurations were installed inadvertently in the plant because the designer did not consider their differences important, and thought only of the convenience or cost, in laying out the connecting pipes.

7 different inlet duct configurations for a cyclone

One study showed that paper pulp collection was affected by elbows in a different way from powder collection. Paper pulp collection by a laboratory cyclone and an industrial cyclone 10 times the size indicates that for both, a down-flow elbow leading to a horizontal entry brings about a higher efficiency than an up-flow elbow. In contrast, for the powder, the vertical elbow from above had the higher emission.

At a plant scale, given a rising approach duct to the cyclone, coal dust has been found to collect better for an elbow with a horizontal component of the same rotation as the cyclone vortex than one against.

Common considerations for inlet duct design

Just like how we care about the inlet duct configuration for a fan/blower, we also have to pay attention to that for a cyclone.

Inlet ductwork should be designed so that it does not interfere with the flow of the cyclone. Indeed, the conditions upstream of all cyclones need to be considered and recorded with all experiments and designs. Often, no mention is made of these conditions.

Some common considerations for inlet duct design are:

1. Materials that will be collected should be taken into consideration.
2. Horizontal elbows that are within 4 duct diameters of the cyclone should be of the same rotation as the cyclone.
3. Some precautions that will prevent problems from upward vertical elbows include moving the elbow as far upstream of the cyclone as possible and using as long a radius elbow as is reasonably possible.
4. Avoid elbows close to the cyclone which will collect material on the “inside” wall of the inlet of the cyclone
5. Use a long horizontal inlet duct as much as possible.
6. If it is impossible to avoid an unfavorable entry, as in many existing installations, with solids flow likely concentrated in the upper part of the inlet, use an elbow with vanes or a deflector baffle to move it away, and also to slow it down.

Different applications of cyclones require inlet duct configurations that are suitable to them. The engineer who designs the duct network should always consult the cyclone designer to decide the best configuration available with the space limitation onsite.

Cyclone services by Airvate that you/ your customers can afford

Whether you are sizing a cyclone or using one now, if you need any services listed below, feel free to contact Airvate at info@airvate.com.

1. Have to change dimensions of inlet, gas outlet, and/or dust outlet, and want to know how the changes affect cyclone performance.
2. Need a new cyclone that has to meet the specified requirement.
3. Need a new cyclone with the best available performance in a limited space.
4.  Want to avoid or reduce the wear-out or blockage that always happens to your cyclone.
5. Want to reuse an existing cyclone and know its performance in a new process.
6. Want to optimize the performance of an existing low-efficiency cyclone.
7. The existing cyclone collection efficiency is too high for the process and you want to lower it.
8. The existing cyclone has either too high or too low of a pressure drop
9.  Want to optimize the performance or solve any problem of the combo of an existing cyclone (as a pre-cleaner) and a baghouse.
10.  Want to solve the uneven dust distribution problem in applications of dual-cyclone, quad-cyclone, or multi-barrel cyclone; with this issue, one or two cyclone barrels get more dust, and consequently, it causes wear-out in their inlet section and cone bottom more quickly than on others.
11. Existing cyclones in series don’t work as expected.