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Filters















3


Bag filters (Suite 3)














3.10


Elements of sizing (suite):






3.10.7

Case of the pulse jet filter:









During
the online cleaning process of the bags of the pulsed air filter, only a
small fraction of the dust coating the




filter bag falls into the recovery hopper.









The
rest of the dislodged dust is redeposited on the bags by following the gas
flow in the filter.





It
is clear that the redeposited dust has different characteristics of pressure
drop than the dust previously deposited.





A
modelization of the online cleaning was developed by Dennis and Klemm and
this has resulted in the following




equation:























With:








S is the resistance of the filter








Se is the resistance of the just cleaned filter







(K2)c is
the specific resistance of the recycled dust







Wc is the
areal density of the recycled dust








K2 is the specific resistance of the freshly deposited dust







Wo is the areal density of the dust deposited later








This
model has the advantage that it can easily report the three filtration
regimes of a pulse jet filter, ie:




clean filter, new dust
deposited and redeposited dust.








Considering
the operating conditions, the values of Se, (K2)c and Wc can be assumed to be constant,




so that they can be
grouped into the following relationship:






















With:








ΔP is the total pressure drop








V is the filtration velocity
























This
equation describes the behavior of the pressure drop for a single filter bag
but also determines the pressure




drop of the baghouse due
to the parallel arrangement of the bags.








The
disadvantage of the model represented by the last two equations is that the
constants, Se, (K2)c, andt Wc,
are




are difficult to know.









The solution is to
develop other empirical equations for special cases.







For example, for the
combination of Dacron felt with fly ash, the proposed formula is:




















With:








V in ft/min








Pj is the pressure of the pulsed air, normally between 60 and 100
psig







Of course, the above
equation can not be applied to other cases.








The formula calculating
the total pressure drop becomes:




















With:








ΔP in mmH2O








V in ft/min








Pj in psig
(psi)








K2 in inH2O/(ft/min).(lb/ft2) and generally between 1,2 and 40







C in g/ft3








tf in min






3.10.8

Example calculation for pulsed air filter:









Data:








* Quantity of Gas: 110 000 m3/h








* V equals 7 ft/min








* Pj equals
100 psi








* K2 equals 10








* C equals 1,4 g/ft3








* tf equals 40
min









Solution:








By applying the above formula, we have:

























There is also an
empirical formula to calculate K2.








The
formula, developed in the 70s by Dennis and David, shows that K2 depends on the average particle
diameter




and the speed of filtration.









By adding the particle
density and viscosity of gas, we have:
































With:








D is the mean diameter by
mass of the particles in m







μT is the gas viscosity at the
operating temperature in kg/m.s







μ20°C is the viscosity of the gas
at the temperature of 20°C (1,81.10ˉ⁵ kg/m.s)






ρ is the material density in kg/m3








V is filtration velocity in m/s









K2 is in mmH2O/(m/s).(kg/m2) or in inH2O/(ft/min).(lb/ft2)





3.10.8

Filtration efficiency:









The efficiency is given
by the following formula:





























With:








η is the efficiency








C is the concentration of
dust at the inlet in kg/m3 or g/m3







Co is the concentration of dust at the output in kg/m3 or g/m3








The efficiency is usually above 99%.






3.11


Calculators:









Two calculators are available for:








* Filter cleaning by reverse air








* Filter cleaning by pulsed air








Note: These calculators
are based on empirical formulas.







We must also be careful
with units which are AngloSaxon or in the international system (SI).






For the link to the calculator, see below.













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All rights reserved © 20122015 The Cement Grinding Office
