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The benefits and things to pay attention to when applying cyclones in series

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Introduction

The design of using a single cyclone connected to each particulate matter source point is common in many industrial applications.

In spite of the fact that each cyclone has been designed to handle separation of particles, there are many situations where a single cyclone is inadequate for the particle collection task. In such situations, it is often feasible to use multiple units either in series, in parallel, or both.

Multiple-cyclone separators in series consist of a number of individual cyclones. They are usually used in two cases:

  1. inlet dust loading is high and,
  2. emission from single cyclone configuration would be too high. 

Cyclones in series

When cyclones in series are feasible, a second, or even a third separator can be added in series with the first stage separator to collect additional solids. It is customary to refer to the individual stages as the “primary,” “secondary,” and “tertiary” stages or simply as the “first,” “second,” and “third” stage.

Sketch of cyclones in series

Collection efficiency improved by two cyclones in series significantly 

A study of identical cyclones in series by Whitelock and Buser stated that the overall collection efficiency is 7% higher for two in series than that of a single cyclone configuration with all the other conditions kept the same. However, the use of three or four identical cyclones in series only slightly increased efficiency.

Collection efficiency is not sensitive to inlet velocity for cyclones in series

Another study by Gillum and Hughs showed that the overall collection efficiency of two cyclones in series, 2D2D as the primary and 2D2D or 1D3D secondary, did not vary when the inlet velocity ranged from 40 ft/s to 60 ft/s, but the total system pressure drop for the lower inlet velocity was about half of that of the higher inlet velocity.

Changes of air conditions after primary cyclone affects the performance calculations of secondary and/or following cyclone

The overflow from the first cyclone is charged to the second one and so on. Cyclones working in series can be modeled using the same model as for a single cyclone. To predict the efficiency of stage II or stage III, it is necessary to know the concentration (dust loading) and particle size distribution of the feed solids, which is the concentration and particle size distribution in the overflow from the cyclone stage immediately preceding it. In addition, any significant decrease in pressure or change in temperature across a prior stage requires a re-computation of the gas flow rate, viscosity and density reporting to the following stage.

The following table shows examples of airflow rate changes caused by the pressure loss in the first cyclone (see the table below, ambient pressure before first cyclone and no temperature change were assumed.) If temperature changes too, then the air flowrate, air density, and air viscosity all should be re-computed for the performance calculation of the following cyclones.

 

Pressure loss by the first cyclone

(inch H2O)

Airflow rate entering the second cyclone increase (approximately equal to increase of pressure loss, and density decrease)

4

1 %

6

1.5 %

8

2 %

10

2.5 %

15

3.8 %

20

5.2 %

25

6.5 %

 

Dust loading also affects the pressure loss of a cyclone: higher dust loading will make it lower.

The dust loading to the secondary cyclone will normally be much less than that to the primary cyclone, often less than 1%.

Suppose the dust loading entering the primary cyclone is 100 grains/ft3, and collection efficiency of primary cyclone is 95%, then there will be 5% or 5 grains/ft3 dust entering the secondary cyclone. Then the pressure loss of secondary cyclone will be about 20% higher than that of the primary cyclone.

In one of Gillum et al.’s studies, the pressure losses of two 2D2D cyclones in series were measured and the pressure loss of the secondary cyclone is higher than that of the primary.

When designing two cyclones in series, the total pressure loss of the series cannot be simply obtained by multiplying the pressure loss of the primary cyclone by 2, since the air conditions for the primary cyclone and the secondary cyclone are significantly different.

Sometimes we were told that “If a cyclone collects 90% of 5 micron particle, then if two of them were put in duty in series, then the overall collection efficiency of the two in series will have a 99% collection efficiency of 5 micron particles.” This statement is wrong, and the reason is the same as above: the two cyclones are working with different airflow conditions.

Conclusions

  1. Apply two cyclones in series in cases of high dust loading and strict emission requirement.
  2. Properly choose low inlet velocity for a two cyclones in series configuration to reduce pressure loss and erosion.
  3. Air conditions for the primary cyclone and the secondary cyclone are normally significantly different.

Cyclone services by Airvate that you 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.

  • Existing cyclones in series don’t work as expected.
  • Have to change dimensions of inlet, gas outlet, and/or dust outlet, and want to know how the changes affect cyclone performance.
  • Need a new cyclone that has to meet the specified requirement.
  • Need a new cyclone with the best available performance in a limited space.
  • Want to avoid or reduce the wear-out or blockage that always happens to your cyclone.
  • Want to reuse an existing cyclone and know its performance in a new process.
  • Want to optimize the performance of an existing low-efficiency cyclone.
  • The existing cyclone collection efficiency is too high for the process and you want to lower it.
  • The existing cyclone has either too high or too low of a pressure drop
  • Want to optimize the performance or solve any problem of the combo of an existing cyclone (as a pre-cleaner) and a baghouse.
  • 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.

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