A well-designed rain cap can reduce the pressure drop of a cyclone separator

The pressure loss of a cyclone separator is divided into three main contributions from: the cyclone inlet, cyclone body, and vortex finder (gas outlet). The gas swirl inside the cyclone body is very useful for particle separation, but as this swirl exits the cyclone through the gas outlet, the rotational energy which is stored within is actually lost. This is the main source of the pressure loss of a cyclone separator.

A lot of studies have been conducted trying to reduce the pressure loss caused by this rotational energy in the exiting gas flow through modifications on the gas outlet:

1. Modify the length and diameter,
2. Change its shape from cylinder-shape to cone-shape.

These modifications can indeed reduce the pressure loss, but cannot be done to existing cyclones inexpensively.

Some studies looked outside of a cyclone separator. Here are two examples.

1. In one study, in order to minimize the cyclone’s pressure loss, a pressure recovery type diffuser known as a radial diffuser is used on top of the cyclone body connecting with the gas outlet. This diffuser converts the rotational energy of the exiting gas back into static pressure. The study showed that with this diffuser up to 30% pressure recovery is possible (without impairing the separation efficiency). After the rotation attenuates, the gas flow leaves the diffuser chamber tangentially. This diffuser is much like a scroll outlet, but with a pressure plate within it placed parallel to the cyclone top cover.
2. Another study tested a specifically-designed rain cap on two cyclones with different inlet velocities. It is much simpler with the gap between the top and bottom parts adjustable. The test results showed that it can reduce the pressure loss by between 8.7% and 11.9%.

Demonstration of a rain cap on the top of a cyclone

sketch of a specifically-designed rain cap

When designing a rain cap  or a radial diffuser to fit on a cyclone, stresses caused by wind load and weight also require consideration.

The common practice is for the gas outlet to end just above the cyclone top, or to cover it with a shallow cone (traditional rain cap) to keep rain and snow out. Though less common, some installations have covers (called spin caps) that direct exhaust gas flow away from prevailing winds or an adjacent structure. A traditional style rain cap on a cyclone doesn’t help to recover pressure loss. Actually, it is used on any vertical gas outlet, like a stack.

To replace a traditional cap with an inexpensive, specifically-designed one will definitely save a lot of energy cost in the long run, especially for a high efficiency cyclone, which normally requires a high pressure drop.

Both radial diffusers and specifically-designed rain caps don’t influence the collection efficiency of cyclone separators as expected per the studies, since they are all installed after a cyclone.

Airvate strongly recommends,

1. if you are installing a new cyclone, ask the supplier to use a radial diffuser if applicable; and,
2. replace existing traditional style rain caps with specifically-designed ones.

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.

Keyword:

Rain cap, cyclone separator, pressure loss, pressure drop, static pressure, radial diffuser, vortex finder, gas outlet