Collection Efficiency

Extremely small particles (less than 1 μm in diameter) can be efficiently collected in a baghouse.
Emission regulations for various industries including municipal waste combustors and
hazardous waste incinerators require emission limits of 0.010 gr/dscf. Baghouse units
designed with overall collection efficiencies of 99.9% (varying particle sizes) are common.
Exhaust air from many baghouses can even be recirculated back into the plant for heating purposes,
as long as the gas stream is not toxic.

Baghouses are not normally designed with the use of fractional efficiency curves as are some
of the other particulate emission control devices. Vendors design and size the units strictly on
experience. The baghouse units are designed to meet particulate emission outlet loading and
opacity regulations. There is no one formula that can determine the collection efficiency of a
baghouse. Some theoretical formulas for determining collection efficiency have been suggested,
but these formulas contain numerous (3 to 4) experimentally determined coefficients in
the equations. Therefore, these efficiency equations give at best only an estimate of baghouse
performance.

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Bag Cleaning Comparisons

Air-to-cloth ratios describe how much dirty gas passes through a given surface area of filter in
a given time. A high air-to-cloth ratio means a large volume of air passes through the fabric
area. A low air-to-cloth ratio means a small volume of air passes through the fabric. When
using the A/C ratios for comparison purposes the units are (ft3/min)/ft2 or (cm3/sec)/cm2. Likewise,
when using filtration velocities the units are ft/min or cm/sec.
Reverse-air cleaning baghouses generally have very low air-to-cloth ratios. For reverse-air
baghouses, the filtering velocity (filtration velocity) range is usually between 1 and 4 ft/min
(0.51 and 2.04 cm/sec).
For shaker baghouses, the filtering velocity ranges between 2 and 6 ft/min (1.02 and 3.05
cm/sec). More cloth is generally needed for a given flow rate in a reverse-air baghouse than in
a shaker baghouse. Hence, reverse-air baghouses tend to be larger in size.
Occasionally, baghouse cleaning is accomplished by two methods in combination. Many baghouses
have been designed with both reverse-air and gentle shaking to remove the dust cake
from the bag. This cleaning is called shake and deflate.
Pulse-jet baghouses are designed with filtering velocities between 2 and 15 ft/min (1 to 7.5
cm/sec), with many velocities falling in the 2.0 to 2.5 ft/min range. Therefore, these units typically use felted fabrics as bag material. Felted material holds up very well under the high filtering
rate and vigorous pulse-jet cleaning. Due to their typically higher A/C ratios, pulse-jet
baghouses are generally smaller in size than reverse-air and shaker baghouses. Pulse-jet cleaning
methods have the advantage of having no moving parts within the compartments. In addition,
pulse-jet units can clean bags on a continuous basis without isolating a compartment
from service. The duration of the cleaning time is short (< 1.0 sec) when compared to the
length of time between cleaning intervals (approximately 20 min to several hours). The major
disadvantage of high pressure cleaning methods is that the bags are subjected to more mechanical
stress. Fabrics with higher dimensional stability and high tensile strength are required for
these units. Air-to-cloth ratios for the various cleaning methods are given in Table 1. Comparisons
of the cleaning methods are given in Table 2.

Table 1. Typical air-to-cloth ratio (filtration velocity) comparisons for three cleaning mechanisms

The A/C ratio (filtering velocity) is a very important factor used in the design and operation of
a baghouse. Improper ratios can contribute to inefficient operation of the baghouse. Operating
at an A/C ratio that is too high may lead to a number of problems. Very high ratios can cause
compaction of dust on the bag resulting in excessive pressure drops. In addition, breakdown of
the dust cake could also occur, which in turn results in reduced collection efficiency. The
major problem of a baghouse using a very low A/C ratio is that the baghouse will be larger in
size, and therefore have a higher capital cost.

Table 3-2. Comparison of bag cleaning parameters

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Filtration Velocity: Air-To-Cloth Ratio

The terms filtration velocity and air-to-cloth (A/C) ratio can be used interchangeably. The formula
used to express filtration velocity is:

The air-to-cloth ratio (also called the gas-to-cloth ratio) is defined as the ratio of gas filtered
in cubic feet per minute (cfm) to the area of filtering media in square feet. Typical units used to
express the A/C ratio are:

These A/C ratio units reduce to velocity units. The units for filtration velocity are ft/min or
cm/sec.
The term gross air-to-cloth ratio refers to the total amount of cloth area used to filter the
entire flue gas stream. The term net air-to-cloth ratio is used to describe the net amount of
cloth available for filtering when one baghouse compartment is taken off-line for maintenance
or bag cleaning. The term net, net air-to-cloth ratio describes the amount of cloth available
when 2 compartments are taken off-line.

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Pulse-Jet Baghouse

In a pulse-jet baghouse, felted filters are typically used as bag material (although woven fabrics can also be used). Since there are no openings in the fabric material, there is no initial cake buildup period. Effective filtration begins immediately as the dust is filtered by the bag. The performance curve of a pulse-jet bag (or row of bags) is given in Figure 1. The pressure drop across the bags is slightly higher than with woven filters. The baghouse is usually operated with pressure drops of 4 to 6 in. of H2O and occasionally as high as 10 in. of H2O. In a pulse-jet baghouse one row of bags is cleaned at a time. Some of the dust is knocked off the bags being cleaned while adjacent rows are still filtering. Bag cleaning cycles are initiated to keep the overall pressure drop across the baghouse within the designed range. If off-line cleaning is used, a compartment is taken out of service and bag cleaning is initiated in that compartment (module).

Figure 1. Performance curve of a pulse-jet bag or a row of bags

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Multicompartment Baghouse

In multicompartment baghouses where the various compartments are cleaned one at a time, the performance curve takes on a different shape. In this case the change in the curve is less pronounced than in Figure 1. The performance curve has a slight saw tooth shape for the net pressure drop across the entire baghouse (Figure 2). Each of the minimum points on the curve represents the cleaning of an entire compartment. The average pressure drop would be represented by the dotted line. For optimum filtration rate and collection efficiency, the baghouse should be designed to operate at a pressure drop that approaches a constant value. This involves careful selection of fabrics and cleaning mechanisms for the baghouse. The weave, and any pretreatment of the fabric can affect the cake repair time. Poor cleaning will increase the filter drag; therefore, the bags must be thoroughly cleaned to reduce the filter drag effect. If cake repair time can be minimized, the pressure drop will be lower. Consequently, the effective filtration rate will be longer for optimum filtering use.

Figure 1. Performance curve for a single woven bag

Figure 2. Overall pressure drop of a multi-compartment baghouse

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Single Bag

A filter performance curve of a single bag of a fabric is shown in Figure 1. The drag is

plotted versus the dust mass, or cake, deposited on the filter.

Figure 1. Performance curve for a single woven bag

The point cr on the graph is the residual drag of the clean filter medium. The filter drag increases exponentially up to a constant rate of increase. This is the period of cake repair and initial cake buildup. Effective filtration takes place while the filter drag increases at a constant rate. When the total pressure drop reaches a value set by the system design, bag cleaning is initiated. At this point, the pressure drop decreases (almost vertically on the performance curve) to the initial point. Cake repair begins when the cleaning cycle stops and the cycle repeats. Baghouses are designed to remove most of the dust cake during the cleaning process. However, shaking or reverse-air baghouses are designed so that during the cleaning cycle some dust will remain on the bags. Therefore, a dust layer will not have to be built up again on the openings in the weave of the fabric. If the fabric is cleaned too efficiently, the cake repair cycle would be as long as the initial cake buildup, lessening the overall effective filtration time of the baghouse.

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Filter Drag

Filter drag is the filter resistance across the fabric-dust layer. The equation for filter drag essentially gives the pressure drop occurring per unit velocity. It is a function of the quantity of dust accumulated on the fabric and is given as:

The true filtering surface of a woven filter is not the bag itself, but the dust layer. Dust bridges the pores or openings in the weave, plugging the openings with particles, increasing the drag rapidly.

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Pressure Drop

Pressure drop (Δp), a very important baghouse design variable, describes the resistance to air flow across the baghouse: the higher the pressure drop, the higher the resistance to air flow. Pressure drop is usually expressed in millimeters of mercury or inches of water. The pressure drop of a system (fabric filter) is determined by measuring the difference in total pressure at two points, usually the inlet and outlet. The total system pressure drop can be related to the size of the fan that would be necessary to either push or pull the exhaust gas through the baghouse. A baghouse with a high pressure drop would need more energy or possibly a larger fan to move the exhaust gas through the baghouse.

Many different relationships have been used to estimate the pressure drop across a fabric filter. In a baghouse, the total pressure drop is a function of the pressure drop across both the filter and the deposited dust cake. Some pressure losses due to friction also occur as the gas stream moves through the baghouse.

The simplest equation used to predict pressure drop across a filter is derived from Darcy's law

governing the flow of fluids through porous materials and given as:

The term k1 is the fabric resistance (also called drag) and is a function of exhaust gas viscosity and filter characteristics such as thickness and porosity. Porosity describes the amount of void volume in the filter.

The pressure drop across the deposited dust cake can be estimated by using Equation 2 (Billings and Wilder 1970). This formula is also derived from Darcy's law and the simplified form is given as:

The term k2 is the dust-fabric filter resistance coefficient and is determined experimentally. This coefficient depends on gas viscosity, particle density and dust porosity. The dust porosity is the amount of void volume in the dust cake. The porosity is related to the permeability. Permeability for the fabric only is defined in American Society of Testing and Materials (ASTM) standard D737-69 as the volume of air which can be passed through one square foot of filter medium with a pressure drop of no more than 0.5 inches of water. The term k2 is dependent on the size of the particles in the gas stream. If the particles are very small (< 2μm) k2 is high. If k2 is high, then the pressure drop will tend to increase and the bags will have to be cleaned more frequently.

Filtration velocity also has an effect on k2. In more recent tests, conducted in the late 1980's under controlled conditions, the relationships of k2, particle size, and velocity have been shown more clearly. Researchers including Dennis, Cass, and Cooper (1977) and Davis and Kurzyske (1979) showed that both particle size and velocity have an effect on k2.

The total pressure drop equals the pressure drop across the filter plus the pressure drop across the cake and is given as:

 

Use equations 3 and 4 only as an estimate of pressure drop across shaker and reverse-air cleaning baghouses. In the industrial filtration process, complicated particle-fabric interactions are occurring just after the filtration cycle begins. In addition, the filter resistance factor k1 can take on two values; one value for the filter before it is brought on-line and another after the filter has been cleaned.When the dust cake builds up to a significant thickness, the pressure drop will become exceedingly high (> 10 in. H2O or 25 cm H2O). At this time the filter must be cleaned. Some dust will remain on the cloth even after cleaning; therefore, the filter resistance level will be higher than during original conditions. A baghouse is normally operated with a pressure drop across the unit of 4 to 10 in. H2O. But many units operate at less than 6 in. of H2O. Bag cleaning is usually initiated when the pressure drop approaches this point.

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Fabric Filter Design Variables

Baghouses are designed by considering a number of variables: pressure drop, filter drag, air-to-cloth ratio, and collection efficiency. Although rarely done because it may not be possible or practical, it is a good idea to use a pilot-scale baghouse during the initial stages of the baghouse design. However, previous vendor experience with the same or similar process to be controlled will generally be adequate for design purposes. Careful design will reduce the number of operating problems and possible air pollution violations.

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