There are six characteristics that determine the capacity of a on line cleaning fabric filter element
The Quality Air Managment Institute
Chapter 12; Bags shapes, lengths, capacities Flat envelop bags etc.
There are six characteristics that determine the capacity of a on line cleaning
fabric filter element:
1) Reverse Air Jet Volume and pressure characteristics
2) Permeability of the bag
3) The cleanable surface area / Shape of the filter element
4) Exit Pressure drop of gas leaving the mouth of the bag.
5) Ability of the ejected dust to agglomerate and fall into the hopper.
6) Dust and Distribution within the filter compartment
In this discussion we will assume that the elements are sealed between the clean
air plenum and the dirty
air section of the filter housing.
1) Reverse air cleaning jet characteristics. If a filter element of any shape
has a filtering volume of 100
CFM, and we have a reverse jet air volume of 50 CFM, the jet will have no effect
on the cleaning of the
bag. It will reduce the flow in the bag from 100 to 50 CFM. If we increase the
flow in the jet to 100 CFM
the flow in the filter will stop but it will not accomplish any cleaning.
Finally if we increase the jet flow to 200 CFM, the flow in the filter will be
stopped and there will be a
cleaning flow of : 200 CFM - 100 CFM for a cleaning flow of 100 CFM
The net cleaning flow will clean a certain area of the filter element. Assuming
25 CFM will clean one
square foot of filter media, the filter element could be four square ft. Any more
filter media would be
superfluous. In fact, in some applications it would allow combustible dust to
accumulate on the surface
and constitute a fire hazard. If we had a filter bag with ten square feet of media,
we would need at least
350 CFM (350-100 = 250 CFM net flow) flow in the cleaning jet to clean the bag.
If we wanted to filter
200 cfm, we would need 450 CFM in the cleaning jet (450-200 = 250 CFM net flow)
to clean the filter
element. You will note that while the filter area of the element enters into the
process, the volume of the
cleaning jet is a greater factor.
The net velocity pressure of the jet as it enters the bag opening, whether with
or without a so-called
“venturi” or “venturi tube”, must be higher than the operating pressure drop.
If the net velocity pressure is
8 inches wg, it will continue to clean on- line up to that pressure drop across
the filter element. A good
design will be to design for an operating pressure drop between 6 and 8 inches.
Some shapes of filter
elements notably pleated element can be designed for higher net operating velocity
pressure.
2) Permeability is a measure of the resistance of flow through a media.The pressure
drop across the
media is one portion of the pressure drop. The other portion consists of the pressure
drop across the filter
cake and is affected by filter ratio. In the example being considered we will
consider two medias:
Standard 14 oz felt media with a permeability of 22 and a laminated membrane filter
with a permeability of
8.
The bags are filtering 100 CFM or 200 CFM through our 10 sq. ft. bag. The pressure
drop across
the media is directly proportional to flow (or filter ratio)
AT 100 CFM and a permeability of 22. Pressure drop is 10/22 x0.5 in. w.c. = 0.23
w.c . For a flow
of 200 CFM, pressure drop is 20/22 x 0.5 in w.c. = 0.45 inches.
At 100 CFM and a permeability of 8, pressure drop is 10/8 x 0.5 inches = 0.63
inches water
column. At 200 CFM and permeability of 8 pressure drop is 20/8 x 0.5 inches =1.25
inches w.c.
If we assume a cake permeability of 10, at 10 fpm and 20 fpm the pressure drop
across the cake
would be 10/10 x0.5 = 0.5 in. w.c. and 20/10 x 0.5 = 1.0 in. w.c.
In designing a collector it is necessary to design for an operating pressure drop
across the media of less
than 2 inches with a filter cake permeability of 5-10.
The filter ratio is limited by the permeability of the media being employed. In
the example, the
maximum filter ratio for the Laminated media would be approximately 5-10 FPM compared
to 20-25 FPM
for the felted media. The permeability of the filter cake may vary from 5 to 10.
Some sintered media have permeabilities as low as 2.5 and this makes their use
very expensive although
for higher temperature processes this may be applicable.
3) Cleanable surface area.
For envelop bags and cylindrical filter elements, the complete surface area is
cleanable. In
pleated bags some of the area may be ineffective because of pinching of the pleats.
As the pressure drop increases the pleats often squeeze together and block filtering
flow. The solution is
to either widen the pleat spacing or stiffen the media. ( heavier weight of thread
coatings are also
effective).
4) Exit pressure drop: For a good reverse jet cleaning design the bottom of the
filter element must be
equal to or bigger than the opening at the exit of the clean side of the bag.
This is true no matter what the
shape of the bag and includes envelope bags. The common cylindrical bag will be
considered but the
principles apply to all bag shapes.
Taking our example we will consider 4 inch diameter bags of various length with
or without venturies and
operating at a filter ratio of 12.
NO VENTURI, 10 and 29 ft length, 4’ dia bag 10 feet long nominal without a venturi
Volume 12 x12.5 sq.
ft/bag =150 cfm. The flow coefficient through an orifice is 2.5. The flow coefficient
through this insert (or
adapter where cage fits in) is approximately 2.25. Area of orifice is 0.086 sq.ft.
150/ 0.086 = 1744 fpm is
0.19 in. w.c. leaving pressure drop = 0.46 in. If we use 24 fpm filter ratio the
pressure drop leaving the
bag is 1.6 in w.g. A good design would keep ratio press drop under one. The same
would happen if we
put in a 20 foot long bag and tried to run it at 12:1 ratio.
WITH STANDARD VENTURI, the venturi selected has a minimum diameter of 1.625 inches
and evase
leaving cone and a bell mouth entering cone. The area of the throat is 0.014 and
the leaving pressure
drop is 2.67 in. w.c. If we increased the bag length to 20 feet at the same ratio
the leaving pressure drop
would be 10 inches w.c.
Note these leaving pressure drops must be added to the drops across the media
and the filter
cake. The fact is that only a fraction of the bag will be cleaned unless the cleaning
volume is
increased proportionally to the new bag area.
5) Ability of the ejected dust to fall into the collection Hopper; with a hopper
inlet to the dust collector
the key is the upward or can velocities. If we have a hopper inlet and longer
bags the can velocities
become too high for the dust to fall into the hopper. If a collector with ten
feet bags has a 300 fpm can
velocity, putting in longer bags requires making the spacing 100 % wider if they
are spread out in a single
direction. High inlets can be used but the entrance to the bag compartment must
be altered so that the
gas entering between the bags has lower velocities to prevent bag wear.
6) Dust and gas distribution
Flat and oval bags: Sly Manufacturing introduced the first reverse fan cleaning
collector with envelope
bags 30 years ago. Carter Day introduced the oval bags in 1971 on both their fan
pulse collectors and
their low pressure RF units. There were also a couple designs introduced in Europe
that have been sold
to different companies. They offer the presumed advantage of squeezing together
a lot of media in a
specific cube. In doing this the distribution suffers. Either the horizontal velocities
going into the bags is to
high with horizontal inlets or the can velocity is too low. Sly had problems with
their Dynaclone on fume
venting from melting furnaces. The reverse air fan is not pulsed and the bag cleans
too long and much of
the cleaned fume dust has a tendency to migrate to other bags. They have tried
to block off two bags and
clean the middle one but it has not resolved the problem.
Carter Day, on their fan pulse collectors, put in a damper on their rotating manifold
arm to block the dust
from the cleaning row to be drawn into the other collectors. There pulse was at
least one second long.
THE CARTER DAY RF was developed to handle fume dust. The RF with its shorter pulse
was quite
successful