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Filtration and Vacuum Specialists since 1976

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A Scientific Review of Dust Collection - Part 5

Mechanical Cleaning Collectors (Shaker Collectors)

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next several months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.


Basic Unit

Image of a Shaker collector

Some industrial processes emit large quantities of dust and the capture of these dusts is costly with non-cleanable types of filters; therefore, in most applications, cleanable filter arrangements are required. The simplest and oldest version of this type of filter (Figure 5-1) consist of a bag with the same diameter as a 55 gallon drum and an inlet sewn into the bottom of the bag. Typically, the fan's paddle wheel is the air mover and rec3eives the incoming dust. The filter bag is suspended from a rope that is convenient for agitation. These collectors have the fan on the dust laden side and were the first collectors to be adapted to manual paper trim operations.


The most common filter media used in these designs is a sateen weave cloth which is a very tight weave and very flexible. The dust collects on the inside of the bag with velocities ranging from 0.5 to 8.0 rpm The virgin media's initial permeability is between 30-50 cfm per square foot at 1/2 inch of water column which indicates an initial pressure drop of 0.1 to 0.2 inches water column across the media. As the dust collects on the inside of the bag, the pressure drop rises and, at some point between 2 and 3.5 inches of w.c., the filter media is cleaned. The bag is usually cleaned by a person actually pulling up and down on the rope. After this cleaning process, the pressure drop generally readjusts to a lower value in the range of 0.5 to 1.0 inch water column.



Figure 5-1

Filter Cake / Operating Characteristics
This difference between the initial pressure drop and the pressure drop after cleaning is due to what is referred to as a filter cake. Let us look at the operating parameters of this basic collector as shown in Table 5-1:

Air Volume Flow:
500 to 1000 CFM
Bag Size:
24 inch diameter
Bag Length:
10 to 14 feet
Bag Area:
88 sq. ft. @ 14 feet
Filtering Velocity:
5.7 fpm at 500 CFM
11.4 fpm at 1000 CFM
Initial Pressure Drop:
0.1 inches w.c.
Average dust holding capacity at 3" w.g.:
8-16 oz.
Dust holding capacity after cleaning at 1" w.g.:
2-4 oz.
Inlet load:
5 grains / cu. ft.
Maximum time between cleanings:
10 - 20 hours at 500 CFM
5-10 hours at 1000 CFM

The collector selection is affected by three basic parameters: Filtering Velocity, Dust Load, and Residual Pressure Drop

There is usually a time between cleanings that may be tolerated. If the requirement is for cleaning twice in an eight hour shift, the capacity selection would be 1000 CFM, but if the requirement is once per shift, the collector should be selected to operate at 500 CFM. If the inlet load is doubled, the time would be reduced to one-half between the cleaning cycle. The residual pressure drop is related to the dust holding capability of the filter media which, in turn, is related to the dust characteristics. Generally, the filter ratio is based on past experiences with the type of dust collection or the industrial process.

Envelope Filter Bags

In order to provide more filter media area in a given volume or floor space, other configurations are used. One of the earliest types is the unit dust collector (Figure 5-2) with envelope bags. The filter bags are sewn into envelopes with spacers to keep the media from collapsing on itself. The earliest spacers used wire inserts and some of the later designs use open foam. The collectors either shake manually with the cleaning operation being similar to the single bag collector or automaticity by motor power.

Since the typical media stretches after the cleaning cycle, the filter bags are held taut by springs that are attached to the closed end of the envelope. In fact, the stretch is enough that after a few weeks of operation, it is desireable to adjust the spring tension. These types of designs are effective on most mechanically generated dusts such as transfer points on belt conveyors, screening, clamping stations, grinding and abrasive blast cabinets. Conversely, they are less effective in collecting dust from processes such as dryers and furnaces. These types of dust particles seem to imbed themselves more deeply into the filter media. There is not enough energy in the cleaning mechanism to provide suitable cleaning to give reasonable filter element life and low residual pressure drops.

Internal View of an Envelope Filter Bag Unit


Figure5-2

Tubular Shakers

To handle the difficult embedded dust particles, the tubular shaker collectors were developed. These collectors (Figure 5-3) have the opening to the bag at the bottom of the collector, gathers the dust on the Insides of the bags, contains some form of tensioning device to keep the cloth tight on the bag, and uses a variation of some type of shaker mechanism. Typicality, each specific design can be evaluated by its mechanism designs. Other parameters include the relatively small bag opening diameters as compared to the overall length. Usually the bag diameter varies from three to twelve inches in diameter and has an overall length that corresponds to a length to diameter ratio of 20 to 35. This ratio gives the cleaning motion a better action and is able to remove the dust more successfully than other shaker types.

Many of these processes require continuous cleaning and cannot tolerate stopping the process for cleaning. To accommodate this requirement, continuous cleaning compartmental collector systems (Figure 5-4) were developed and consist of dividing the unit into multiple modules that have separate dampers.

By closing a damper none compartment and diverting the flow into the other compartments, the isolated module can be cleaned. These effective filter units have at least two compartments and are produced with as many as 20 compartments. First, the fan flow is stopped momentarily by a damper in one compartment and the isolated compartment unit is effectively now "off-line". Then, the cleaning action occurs with no fan flow moving through the compartment.

Tubular Shaker Image Internal Design

Figure 5-3

Tubular Shaker diagram
The main drawback of this type of collector is the off-line cleaning process as well as higher maintenance due to its internal moving components. Since it is required to operate as a low air to cloth ratio compared to other designs, this type of collector is usually larger and more costly than other models. Its main advantage is in low volume applications or in environments where compressed air is not available for filter cleaning requirements.


Figure 5-4






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