Page 303 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mechanical Separations 273

c. Chemical compos1t10n of vapor and of dust,
including any abrasive, hydroscopic or other char-
acteristics.

2. Define dust recovery, as percentage below a certain
particle size.
3. Indicate, if known, preferred bag material that
will withstand environment, e.g., fibers of glass,
polyester, TeOon®, Nomex®, polypropylene, poly-
ethylene, cotton, wool, nylon, Orlon®, Dacron®,
and Dyne I®. The type of weave of fiber should be
recommended by the manufacturer. The fabrics
may be felted or woven [47,48] in 'Weaves of
plain, satin, or twill, and should be resistant to
.. � any corrosive material in the solid particles or
Cleaning Air from the gas stream.
Atmosphere

Figure 4-66. Bag filtration with continvous reverse air cleaning. 4. Manufacturer should recommend
Courtesy of W. W. Sly Mfg. Co.
a. Bag size, ( diameter, length).
b. Bag holding hardware: anti-collapse spreader
rings, snap rings, etc.
Primary Manifold c. Number of baghouse compartments, number of
nozz!e
bags per compartment.
�.,__,-secondary d. Air/gas flow cycle to compartments.
nozzle e. Complete description with mechanical details of
Bag
bag cleaning system (shaking, air-jet, etc.)
f. Dust removal arrangement.

The cleaning system set up for a particular bag
house will determine whether the filtering system oper-
ates continuously or batch/intermittently. Some sys-
tems operate as a continuous batch, with sections of the
entering chambers being isolated by valving to auto-
FILTERING CLEANING
POSITION POSITION matically switch from one section of one bag house to
another. Thereby, one or more bag groups/sections fil-
Pulse-jet cleaning (above) uses a ter while another one or more are not operating, but
controlled blast of compressed air from
the primary into the secondary nozzle, are in the dust removal cycle. The permeability of the
which is magnified by induced air being fabric is generally stated as the clean airflow in (cu
drawn into the bag. The sudden release of
air causes the bag to expand fully, ft/min)/square foot of fabric at a pressure differential
throwir.g the dust from the outer surface. of 0.5 inches water as determined by the ASTM stan-
Dislodged dust falls into the collection
hopper. At right. types of duty cycles dard D-737 (Frazier Test) [47]. Whereas this test is use-
ful, several fabrics may have the same permeability yet
Continuous eutometic
have different fiber surfaces, and thereby do not per-

E
u ----- form the same for a specific application.
Operating cycle
i The felted fabrics are generally used for maximum
;;:: 0 - Intermittent recovery of product and are used at high face velocity for
airflow-Lo-cloth-area ratio. The felt promotes the greatest
Cleaning cycle
dust collection surface.
Time-----
Figure 4-67. Pulse-jet air cleaning of fabric bags. By permission, Monofilament fibers require special attention to
Power, November 1975, McGraw-Hill Co., Inc., New York, p. 41. ensure a uniform open space between the filaments.