Dip tubes have a significant influence on the separation efficiency of a cyclone. In modern suspension preheaters, it is standard to use cyclones with a dip tube for improved separation.
Advantages of having immersion tube -
· Better segregation between raw meal and gases avoiding unnecessary material recirculation
· Better heat profile across the preheater tower
· Lower exhaust gas temperature
· Reduced fuel and production increase.
It is always possible to run without the dip-tube at the bottom stage cyclone of preheater — normally happens due to premature failure, halfway between next kiln linings replacement.
Causes for failure -(1) Abrasion and wear, (2) Thermal stresses, and (3) Wrong selection of metallurgy.
Now here it’s important to assess the shortcomings of running the bottom-most cyclone without an immersion tube.
With decreased settling efficiency of bottom stage cyclone would be unfavorable in terms of heat consumption.
In multi-stage cyclone preheaters, the low separation efficiency of gas and solid phase reduces the thermal efficiency of heat exchange units at all stages, thus greatly reducing the thermal efficiency of the whole system.
From the above chart, we can see that the thermal efficiency of the whole system changes obviously in the range of the separation efficiency of the bottom stage cyclone, especially when the separation efficiency is less than 50%. When the separation efficiency is more than 50%, the influence of each cyclone on the thermal efficiency of the system is basically the same; when the separation efficiency is less than 50%, the influence of the separator efficiency from the 4th stage cyclone to the last cyclone (stage1) on the thermal efficiency of the system is gradually weakened.
Also, there is a possibility of an increase in exit gas temp at PH top because increasing hot dust circulation will raise the gas temp of the remaining stages.
We may think that there will be no additional dust losses from the system because the next four stages (for 5 stage ILC kiln) would get collected. Similarly, there will be a minor benefit in overall ∆P at PH fan inlet.
Finally, it may be said here — removing the dip-tube will increase dust recirculation within the system.
Intermediate cyclones and especially bottom cyclones have typical lower efficiency (75–85%) — depending on cyclone design and condition of the dip tube. The exact measurement of settling efficiency of intermediate cyclones is complex and often impossible.
Nevertheless, the impact can be high — efficiency drop of the bottom stage from 75% to 50% (for example, dip tube damage) increases the preheater exit gas by 10 to 15°C.
Here the main objective is to determine the amount of additional dust loss against actual condition (with dip-tube).
Normal dust loading factor to bottom stage cyclone (5th cyclone) in the range of 1.25–1.30 Kg/Kg clinker. As previously mentioned, the settling efficiency of the cyclone is expected to be 70–80%, therefore, escaped dust quantity may come around 0.33 Kg/Kg clinker @ 880 0C during normal operation.
And same escaping dust quantity may further add up to 0.65 Kg/Kg clinker due to poor collection factor — caused by no immersion tube in it. While in recapturing process in later stages, it may lose temp 250 to 300 0C.
Therefore, an estimated heat loss is calculated in the range of 20–25 K.Cal/Kg clinker when bottom cyclone collection efficiency gets reduced by 50%. Also, there will be an impact on optimum feed rate and more chances of material build-up due to unusual dust load.
Running bottom stage cyclone with lesser collection efficiency would result in production loss and an associated increase in heat consumption.
The utilization of the thermal energy in the flue gasses is essential for obtaining an overall acceptable fuel economy of the cement plant.