First we will consider the mechanics of renewable
media dust collectors. The media has openings which are a small percentage of
the area of a media. This type of media, which we will consider, collects dust
by a sieve process. It will collect dust by the size of the holes in the media.
Tortuous path filters often applied in HVAC will not be considered. A measure of the relative areas is
permeability. Permeability is a measure of the flow through a square foot of
media at a 0.5 inches water column pressure drop (0.5 “ w.c.). At this pressure
drop the flow has a direct relationship to the area of the openings in the
filter media. How much dust the filter media can hold is a direct function of
the permeability. If a media has a permeability of 20, it will hold five times
as much dust holding as has a media with a permeability of 5. Also the
effective open area of the media is five times as great and is proportional to
the permeability
Dust Collection Process
A self cleaning collector of any type operates by
collecting dust in the holes of the media until the holes get filled with dust.
As they get more filled with dust the pressure drop goes up and the
permeability goes down. In a disposable filter the media is replaced. In a self
cleaning filter, the media is regenerated by media cleaning.
Function of Media Cleaning
The cleaning systems then remove dust from the holes,
and the permeability goes up and the pressure drop goes down. The permeability
does not return to its original (virgin) state. Some of the openings get
smaller which increases the collection efficiency. The difference in
permeability is due to formation of a filter cake on the media.
Filter Cake
In the formation of a filter cake the efficiency of
collection for a test dust improves dramatically. On a standard laboratory test,
typical results are:
Original
collection efficiency 80-85%
Final
efficiency with filter cake 99-99.9%
Original
permeability 22-24
Final
overall permeability 8-12
Media Cleaning Systems
There are various methods to clean the media.
a)
Washing with liquids. (For tortuous path media)
b)
Mechanical cleaning (shaker systems)
c)
Reverse flow through the filter media
We will only consider mechanical and reverse air
cleaning designs.
Mechanical Cleaning
Until the 1960’s, mechanical cleaning (Shaker) of the
filter media were the only self cleaning fabric filters available. These were
and are applied at filtering velocities of up to 8 feet per minute. Normally
they were selected on applications where cleaning was required about once every
three or four hours or where the process was intermittent so the collector flow
could be stopped to clean. If the collector was cleaned during process flow the
dust would not fall down into the collection hopper. To make it suitable for
continuous processes, three or more collectors were installed in parallel. The flow
was stopped with dampers to allow cleaning of the media in one module while the
full flow was handled by the remaining parallel collector. The flow through the
collector had to be stopped so that the mechanical action could be effective.
If the cleaning was delayed until the pressure drop rose too high the dust
might become imbedded in the media with enough force so that all or part dust
would not be ejected during the cleaning (shaking) procedure. This raises the
pressure drop and results in reduced flow at much higher pressure drop. They
have an advantage on some dusts. In a pulse jet compressed air cleaning
collector the dust must agglomerate into larger particles before it will fall
into the collection hopper. With a mechanical collector this is not necessary.
There is a system similar to mechanical cleaning,
“Plenum Pulses”, in which compressed air cannons agitate the bags in multiple
compartment without reverse air. Those will be covered in a future addendum to
this presentation.
Reverse Air Cleaning
collectors
There are several different types of reverse air
cleaning systems
1) Compartmented Reverse Air with continuous
reverse air flow fans
2) Continuous Cleaning reverse air collectors
with rotating or traveling manifolds with common dirty air plenums.
3) Intermittent fan pulse units with common dirty
air housing with traveling compartmented manifolds and common clean air
plenums.
4) High Pressure Pneumatic powered pulse jet
collectors (70-100 psig)
5) Low Pressure Pneumatic powered pulse jet collectors (7 to 22 psig)
Compartmented Reverse Air Cleaning.
These
are commonly referred to Reverse Air Collectors. The collector is divided into
individual modules. In one of the
modules a damper closes to the flow in the filtering direction. Another fan blows air backwards through the
module back through the duct inlet manifold, at approximately the same volume
as the filter flow. This backwards flow, then flows back through the inlet
manifold where it vents to the other compartments. This gradually pressurizes
the clean side of the bags, dislodging the dust so it falls in the hopper. In a
six compartment unit, the exhaust fan must be sized to handle 6/5 (1.2) times
the process exhaust flow. This process continues until all the compartments are
cleaned. It’s a very gentle cleaning system which was originally applied to
collectors at high temperatures which required woven fiberglass bags. The
mechanical shakers were too violent for the fiberglass bags. The filtering
velocity was lowered enough to run at low pressure drops usually lower than 2
inches w.c. They were generally bigger and more expensive than shaker collectors
and the later developed pulsed cleaning collectors. They can handle difficult
to agglomerate dusts as well as standard dusts. They are able to operate on
applications where the process gas stream operates at or near the dew point
temperatures. This is because the reverse cleaning fans experience temperature
regain in passing through the fan. Reverse air temperature is higher than the
process gas temperatures. When operated at the correct filter velocities, the
filter element bag life is often indefinite measured in years rather than
months. For a particular volume on a continuous process, they are many times
the size of shaker and pulse cleaning collectors. This type of reverse cleaning
applies reverse pressure very slowly and because of the inertia of the dampers.
It is a gentle cleaning action which increases with time. It also allows
cleaning of woven fiberglass media which often fails as the threads bend and
fracture as in mechanical cleaning and pulsed cleaning systems. .
Continuous cleaning
Collectors with traveling or rotating manifolds (Fan powered reverse air)
These were a variation of the compartmented reverse
air collectors. In these units, the rotating or traveling manifolds
continuously blew pressurized cleaning air backwards through either single or
multiple bag sections of the collector.(Note: They were not suitable for woven
fiberglass media.) These were able to operate at low pressure drop at filtering
velocities in the 6-8 fpm range. The manifolds divided the compartments into 8
or more sections and the reverse air fans provided two or three times the heat
regain to cleaning air. This lowered the requirements on the main exhaust fans.
These were successfully applied to the grain, and food industries where operating
near the dew point causes premature filter element failures for other types of
collectors. The pressure drops across the filter cakes were very low. These
collectors were applied to many other dusts to give excellent service. Examples
of these collectors were supplied by Sly Mfg. (Dynacone), Carter Day Corp (Type
RJ old style) and Pneumafil Corp. (Reverse Fan). These collectors had followers
on the traveling manifolds to prevent the ejected dust from short circuiting to
the bags that were previously cleaned.
Intermittent Cleaning Fan
Pulse Units
These collectors
were an outgrowth of the continuous cleaning collectors to operate at high
filter ratios and remedy the limitations of the upward velocity component for continuous
cleaning applications. These collectors also discovered the most important
principle of reverse air cleaning collectors. They discovered that the capacity
of a filter element depended on two factors:
A)
The open area of the openings in the bags. If a bag permeability were doubled the dust
holding capacity doubled. The capacity of a bag could be doubled by either
doubling permeability or doubling the bag area. This allowed operation at high
filtering velocities. It also allowed making the collectors much smaller. Thousands of these collectors
were applied at filter velocities of 10 to 20 feet per minute.
B)
The quantity air volume capacity per filter element in the filtering mode is
directly related to the reverse air volume. Increasing the length of the bag or
increasing permeability without increasing reverse air flow does not increase
volume which the filter element can handle. There must be sufficient open
area and sufficient reverse air volume to clean the open area. Doubling the
filter element length or area does not increase filter element volume rating.
As the bags were made longer, the size of the reverse
air fan was increased.
When introduced in the late 1960’s, they quickly
replaced the mechanical shakers and pulse collectors in the grain and food
industries. They were able
to handle heavy and continuous loads where dew point normally ran close to the
dry bulb temperatures. The reverse air fan had temperature regain so that the
reverse air cleaning gas stream was 3-6 degrees F higher than the process gas
streams.
Intermittent Fan Pulse units
with common dirty air housing with traveling or sequencing cleaning
compartments
These designs were able to operate at high filtering
velocities and pressure drops in the 1.5
-3 in.w.c. They were largely applied on trash recovery operations, on food
operations and cement handling applications, where operations near the dew
point were common.
High Pressure Pneumatically
Pulse Jet Continuous cleaning collectors (70-100 psig)
Because the diaphragm valves that powered the reverse
air valves operated in less than20 milliseconds, these collectors routinely
handled loads in the 150-300 grains per cu. ft. and were applied to Pulverizers
manufactured by Pulverizing Machinery Corp. (Later Renamed MikroPul Corporation) The Pulverizes added heat to the
process air and hundreds of installations were installed. These ran at
filtering velocities of 6-9 fpm. In the late 60’s they were applied to other
applications. Hundreds of applications were vented at filtering velocities of
12 to16 fpm. The precise selection of filter velocities was complex and was
related to load, temperature, dust density and to a lesser extent to the dust
load. Typical pressure drops ran between
2.5 and 3 “ w.c. Filter element life was 6 to 10 years.
Redesign of Pulse Collector
cleaning System in 1969
Under competitive pressures the bag lengths were
increased from 6 ft (6 sq. ft.) to 10 ft (10 sq ft.). This was accomplished by
increasing cleaning jet velocity from 12,000 feet per minute to 20,000 ft per
minute and the reverse air volume 240 cfm to 400 cfm. This seemed to be a
consistent design. But the result was that during cleaning the dust was ejected
towards adjoining rows in the filter mode at 20,000 fpm, reducing the bag
Permeability by 60 to 80%. The pressure drops at the same filtering velocities
went up to 6.5 to 9.5 inches w.c. To compensate for these higher pressure drops
the industry lowered specifications to filtering velocities to between 5 and 6
fpm. These collectors at the reduced filtering velocities ran at pressure drops
of 4.5 to 6.5 in. w.c. Filter element life was 20 to 30 months.
Low pressure compressed air
cleaning systems were introduced in 1970, but the jet velocities were the same
as the high pressure cleaning systems introduced in the late 60’s. They had the
same issues in performance.
Advanced Technology Design
1978 Continuous Cleaning Pulse Jet with 70-100 psig
These were introduced using the principles outlined
above.
AA) The reverse air volume was increased by 4-5
times
BB) Reverse Air Jet velocity was decreased back
to under 12,000 fpm.
CC)
The inlets we redesigned to allow the finer dust components to fall into the
hopper unimpeded by the upward velocities of the dirty air entering the filter
compartment. These were widely applied on many different applications from
cement to paper dust over the next 27 years.
The low cleaning jet velocities allowed the filter
media to maintain permeability so that they ran at filtering velocities in the15 to19 fpm range. Pressure
drops were lowered to 3.5 inches. And filter element lives were over 48 months.
In the late 1980’s the same technology was introduced to the low pneumatic
pressure powered units
Consideration from Refrigeration Cycle in Compressed
Gases for pneumatically powered pulse
jet collectors
In Thermodynamics, it can be noted that as a
compressed air expands it cools. This principle is used in designing air
conditioning and refrigeration designs. The net effect is, that on all
pneumatically powered collectors, the jet temperature of the jet/air mixture, when
compressed air is mixed with air from the clean air plenum, is 4-8 degrees F
cooler than the air in the clean air and dirty air plenums. One approach, to
counteract this when process gas streams are near the dew point, is to heat the
compressed air in the pulse pipes high enough so the jet temperature is higher
than the process temperature.
For more on the evolution of baghouses
Read more about ... Advanced Technology Baghouses
For more on the evolution of baghouses
Read more about ... Advanced Technology Baghouses
