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Leaf Filter Screen Cake Trap
The problem of an improperly constructed
filter cake caused my company to lose $250,000 profit over a two month period. It is a
factor of leaf filtration poorly understood by plant operators and maintainers.
Successful leaf filtration requires a properly formed and structured filter cake. It is
the cake that does the filtering. The screen simply supports the cake. The cake is
intentionally built-up on the screen to have certain properties. If those properties are
not present, you will not get a good flow, or a clean product, from the filter.
The filter cake must trap the material to be removed, yet still pass the fluid being
filtered. Hence the passage ways through the filter cake must be large enough to allow a
reasonable flow rate, yet be narrow and twisted to stop foreign material getting through.
Over a period time the removed material build's-up and grows thick on the screen. Once the
passage ways through the filter cake get blocked the flow rate drops and the pressure
difference across the cake rises. At that point the flow rate is too slow the cake must be
discharged and a new filter cake applied on the clean screen.
In creating the cake it may be necessary to first apply a precoat on the screen.
Especially when removing very fine particles. The precoat is usually a medium of fine
fibres that layer in criss-cross fashion over the screen. Diatomaceous Earth (skeleton of
diatoms - tiny planktonic organisms) is a common precoat. The precoat bridges over the
screen openings and stops the screened material, forcing it to build-up and thicken on the
"The sand particle size range had altered!
The passages that used to naturally occur when the sand cake developed had disappeared!
The answer to the problem was to find out why the particle range had altered."
When using precoat you do not want to
blind the screens by filling up the holes. Select a screen hole size that is suitable to
the particle size range to be removed, while still allowing any precoat to sit over the
holes without blocking them. It may be necessary to conduct some trials and to get advice
from both the filter manufacturer and the precoat material supplier when sizing the screen
and precoat medium.
There are also situations when a body feed is continuously added into the steam to be
filtered. The intention being to continuously create fine passage ways through the growing
filter cake. By making the cake porous, it is possible to get longer filter runs before
the flow drops too low, and the cake has to be removed.
The problem that occurred at my company was a classic misunderstanding of how to form good
filter cakes. We filter sand out of a viscous liquid. The sand is used to make the filter
cake. No precoat is required. If we take what we now know about creating a filter cake, it
is clear that there must be sufficient numbers of small passages through the cake to pass
Typically the sand cake was 5 mm thick at the start of a filter run and grew to 15 mm
thick before cleaning the screens. The sand needed to pack together, yet leave minuscule
passages around the sand particles. To do this it was necessary to have sizes and shapes
of sand that left gaps in-between when they pack together.
The symptom we suffered was very low flow rates no matter what pump pressure was applied.
We just could not get liquid through those screens!
From what you now know about making good filter cakes, can you guess what had happened?
The sand particle size range had altered! The passages that used to naturally occur when
the sand cake developed had disappeared! The answer to the problem was to find out why the
particle range had altered.
We found the problem to be a tank agitator that had worn down its blades from stirring
action against the sand. With the blades reduced in diameter there was not enough
agitation to suspend the bigger, heavier grains of sand. They remained at the bottom of
the mixing tank and only the smaller, lighter grains were transferred into the process.
When the smaller sand particles were used to make the filter cake they packed together
closer than ever before and produced very small passages between the grains. So small in
size that little liquid could pass between them.
Be careful what happens when your filter cake is constructed. Because the cavities in the
cake must be sufficient in quantity and size to get the flow through the cake, yet still
filter-out your contaminant.
ASTM Viscosity Calculator
By utlizing the ASTM D341 equation, this
calculator will extrapolate a visocity curve based on the recommended method. The
user enters two viscosity points at two known temperatures and a curve is generated.
Grab this free spreadsheet here.
MS Excel Spreadsheet Calculation Contest
We're looking for the best process engineering calcuation
spreadsheets out there. Enter our contest and a chance to win a $100 prize!
The spreadsheets will be judged on the following criteria:
us your spreadsheet today. The winner will receive a $100 cash prize!
Who are the Best and
As a process engineer, you've most likely
utilized a supplier as a technical resource to help you solve problems in your plant.
A supplier with the technical expertise that you need can be an extremely valuable
resource and often the advice is free!
But, have you had a supplier go above and
beyond your expectations? Maybe a supplier spent days at your plant site to help you
solve a nagging problem. Or, perhaps a supplier recognized a process change or
modification that saved your company thousands of dollars.
Tell us about your experiences and we'll share
them in our upcoming newsletters.
Software Titles Available
We've released a brand new software titles in
our online store.
EZZE SIM is an Excel Monte Carlo
Simulator Add-in which contains two working Dryer (fluid bed & spray)
simulations. The simulations can be used for your drying application or as a template to
develop your own process simulation using the step by step instructions contained in the
online users guide. If your process can be described mathematically (mass - energy
balances and process correlations) you can simulate the process!
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