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
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
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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