Air filtration units and methods employing dust as filtration media and air flow rate as a directly controlled parameter

Air filtration systems and methods which intentionally use collected dust to enhance air filtration performance, use air flow rate control to optimally maintain electro-mechanical efficiency and media permeability, use high velocity discharge to "throw"clean air to preferable workplace areas, and use a cylindrical drum design for pleasing appearance and practical features. The systems and methods are particularly applicable as modular filtration units to textile processing environments, but are suitable for more general applications as well.

BACKGROUND OF THE INVENTION 
The present invention relates generally to workplace air filtration systems 
and, more particularly, to a modular air filtration unit which employs 
local dilution ventilation. While the present invention was developed 
particularly in the context of a textile processing environment wherein 
objectionable respirable dust includes relatively small amounts of lint 
fragments and very small (e.g. 15 micron) dust particles, the invention is 
also applicable to dust control applications in general. 
Increasing concern for the medical effects of airborne particulate, 
gaseous, and radioactive substances has caused industry to improve the 
quality of workplace air. An important general approach directed to this 
end is to employ what are commonly termed engineering controls. Examples 
of engineering controls are: (1) design and operation of production 
machinery so that unwanted emissions are minimized; (2) capturing the dust 
near the source(s) of emissions; and (3) utilizing general capture and 
dilution ventilation. Examples of other approaches are establishing work 
practices which minimize emissions or respiratory exposure, and requiring 
personal respiratory protection. 
A commonly employed engineering control technique is the use of air 
filtration devices for the collection and removal of toxic substances. 
This is especially true for dust or other particulate matter. In general, 
filtration is required either to meet OSHA standards or industrial hygiene 
guidelines in the workplace or EPA standards for emissions into the 
atmosphere. Moreover, increasing energy costs make it increasingly 
necessary to recirculate workplace air. The alternative of continually 
bringing outside air into the workplace can be very costly where heating 
or cooling must be employed. The need for recirculation systems places 
increased demands on the filtration equipment since such equipment must be 
both more efficient and more reliable. 
Filtration methods for collection and removal of particulate matter are 
well known in the art and generally fall into two categories: (1) central 
filtration and (2) modular filtration. 
In a central filtration system, dust-laden air is captured or picked up by 
drawing it into suction duct work. The air is transported to a central 
filtration unit, such as a bag house or a drum filter, is cleaned, and is 
then recirculated back to the workplace, discharged into the atmosphere 
or, usually, a mixture of the two. Thus, to achieve air recirculation in a 
central filtration system, there must in general exist both collection 
duct work and return duct work. In some cases, the return duct work may be 
part of an air-conditioning system. In any case, the air flow rates are 
very high and energy losses within the duct work are a major fraction of 
the total energy required. 
In a modular filtration system, to which the present invention is primarily 
directed, a major difference compared to central filtration systems is the 
elimination of the duct work such that workplace or machinery emissions 
are captured, filtered, and returned to a local area. This may be termed 
"local dilution ventilation". Thus, in one particularly useful embodiment 
of the broad concept of modular filtration, modular filtration units 
operate simply by generally capturing dusty room air and preferentially 
redistributing the clean, filtered air over the workers and/or machinery. 
Modular filtration systems are known in the art and typically comprise a 
blower pulling dusty air into one or more stages of filter media and 
discharging the air back to the workplace. The media choice is influenced 
by the size, concentration, and type of dust. The media must be cleaned or 
discarded after loading up and thus leads to a high operating cost which 
increases with increasing dust capture rate. 
Other dust collection systems are known, such as cyclones, scrubbers, and 
electrostatic precipitators. However, cyclones are ineffective and 
improper technology for respirable dust. Scrubbers are inapplicable to 
efficient modular filtration. Electrostatic precipitators are applicable 
to a restricted class of dusts, and in some explosion or fire-prone 
environments may not be employed at all. 
As will be apparent from the description hereinafter, by the present 
invention relatively lower-cost, effective and efficient modular 
filtration units are provided. Considering cost for example, in 1983 the 
typical installed cost for high-quality central filtration, having 
discharge air quality similar to that provided by the present invention, 
is in the order of $4.00 per cubic foot per minute (CFM). For purposes of 
comparison, the installed cost for systems employing modular air 
filtration units of the present invention is in the order of $1.36 per 
CFM. 
Moreover, operating costs for both types of system are dominated by the 
cost of electrical power to operate the blower motors, and the annual 
power cost per CFM of filtered air when employing modular air filtration 
units in accordance with the present invention is typically one-fourth 
that for central filtration. This is due not only to the fact that modular 
filtration inherently eliminates expensive losses associated with 
collection and return duct work, but also as a result of more efficient 
blower operation in accordance with control system aspects of the present 
invention. 
More particularly, for purposes of illustration but not limitation, it is 
pertinent to consider two typical application areas in the textile 
industry. 
First, it has heretofore been stated by many practitioners of engineering 
controls for cotton dust that there are no proven methods for controlling 
workplace respirable dust levels in certain processes, such as spinning, 
winding, or warping. The reason behind this statement is the extreme 
difficulty, indeed practical impossibility, of implementing source capture 
for such machines. For example, a spinning machine has perhaps 100 
spindles turning at 12,000 RPM and liberating dust and fiber and 
respirable fiber fragments in an obviously general manner that defies 
source capture. In a warper, which comprises a large frame or creel 
holding several hundred rolls of yarn which are then pulled onto a long 
beam for subsequent weaving, the emissions problem is similarly general 
and inadmissable to source capture. General capture using the modular 
filtration units described herein has proven to be extremely effective, 
especially when an induced air flow pattern resulting from a high-velocity 
clean air discharge of the modular air filtration unit is used. 
A second application area for modular filtration is to marginal areas which 
may already have engineering controls. A good example is carding, where 
dust is effectively removed from the aggressive action on individual 
fibers. These emissions must therefore be effectively contained either 
within the equipment or by source capture devices; otherwise, the 
workplace dust levels would be extremely high. Currently, almost all 
carding rooms have some type of dust capture system moving typically 500 
to 2000 CFM per carding machine. In many applications having 500-1000 CFM, 
the workplace dust levels resulting from this dust control equipment were 
well under the former OSHA Standard of 1000 .mu.g/m.sup.3 and many of them 
had respirable dust levels in the range of 300 to 500 .mu.g/m.sup.3. (It 
may be noted that the workplace dust levels depend heavily upon the type 
of stock and the speed at which it is processed, as well as upon the 
performance of the dust capture system.) 
The new Cotton Dust Standard [Dept. of Labor, OSHA. Occupational exposure 
to cotton dust. Federal Register, pp. 27350-27436, June 23, 1978] calls 
for a permissable exposure limit (PEL) in this process of 200 
.mu.g/m.sup.3. The employer has already invested in dust capture and air 
filtration equipment and in air-conditioning equipment whose size is 
related to the filtration equipment. One option for the employer is to rip 
out a well-designed and properly-operating dust capture and 
air-conditioning system and install a much more expensive one operating at 
a much higher air flow. In many cases, this also necessitates upgrading or 
rebuilding the air-conditioning system. In accordance with the present 
invention, marginal processes such as summarized above can be brought into 
control simply by the addition of dilution ventilation via modular 
filtration. 
Moreover, as illustrated next below, in some cases modular filtration 
designs can follow different design paths to accomplish respirable dust 
level reduction as the main objective. Alternatively stated, central 
filtration technology may be inapplicable or ineffective for respirable 
dust control as opposed to dust associated with waste-handling, the waste 
involved deriving from the fiber being processed. 
In particular, respirable dust concentrations in the workplace generally 
obey, at equilibrium, 
##EQU1## 
where, M.sub.r is the respirable dust emission rate in gm/min, and Q is 
the circulating, filtered air flow in m.sup.3 /min. .chi..sub.eq can be 
reduced only by reducing the respirable emissions M.sub.r or by increasing 
the ventilating or diluting air flow, Q. 
In some processes, for example carding, source capture can effectively 
reduce the M.sub.r component from machinery emissions. In other processes, 
especially from spinning through warping, source capture is not practical, 
as has been recognized in the Cotton Dust Standard and as briefly 
mentioned above. In these processes the only engineering control measure 
is increased dilution ventilation, which generally captures the airborne 
dust and recirculates filtered air to the workplace. 
It is most important to appreciate the magnitude of M.sub.r by a simple but 
realistic example. In a process area with Q=35,310 CFM=1,000 m.sup.3 /min 
and .chi..sub.eq =350 .mu.g/m.sup.3, 
##EQU2## 
In other words, a quantity of respirable dust small enough to be contained 
within a salt shaker (21 grams) of respirable dust emitted each hour into 
a textile workplace having 35,310 CFM of circulating air is responsible 
for the 350 .mu.g/m.sup.3 respirable dust concentration. If half this 
small amount of respirable dust could be captured, then the workplace dust 
concentrations would drop by half, from 350 to 175 .mu.g/m.sup.3, and the 
workplace would be placed in compliance with the 200 .mu.g/m.sup.3 OSHA 
PEL. 
For contrast, consider a dust collection and filtration system serving 
carding machinery. If the 35,310 CFM is supplied by an air washer to the 
workplace and all this air is returned to the air washer by being drawn 
into a dust collection system serving 35 cards processing 60 pounds per 
hour and removing approximately 2% of dust, trash, and other wastes, then 
the 35,310 CFM must transport away a waste component mass 
##EQU3## 
or 19,068/21=908 times as much waste mass per hour as is responsible for 
the 350 .mu.g/m.sup.3 workplace respirable dust concentration. 
It is an underlying recognition of the present invention that the 
technologies for respirable dust control and for process dust control 
should be vastly different because (1) the mass emission rates and (2) the 
particle sizes are vastly different, by orders of magnitude. It is a 
misapplication of technology to expect that central filtration, which can 
well handle large quantities of large particles, can effectively and 
generally apply to the control of micron-sized respirable dust, in grams 
per hour quantities. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an efficient and 
cost-effective modular filtration system. 
It is another object of the invention to provide a modular filtration unit 
which properly employs dust itself as its own filtration medium to achieve 
highly efficient and low-cost operation, in combination with either 
low-cost disposable filter substrate media or with cleanable-filter 
substrate media. 
It is another object of the invention to provide a modular filtration unit 
which operates in a cotton processing environment and which properly 
employs lint fragments and dust particles to form a highly efficient 
medium for respirable dust. 
It is another object of the invention to optimally maintain 
electromechanical efficiency of a motor-blower combination employed to 
draw air through the filter medium. 
It is yet another object of the invention to provide a modular air 
filtration unit which effectively avoids "short-circuiting" wherein clean 
filtered air is wastefully drawn right back into the filtration unit. 
It is another object of the invention to provide a modular air filtration 
unit which facilitates the inducement of a preferred air flow pattern 
aiding in the general collection of dust emitted from processing machinery 
or work activity. 
It is yet another object of the invention to provide esthetically pleasing 
and easily mountable modular air filtration units. 
Briefly, and in accordance with an overall concept of the invention, a 
modular filtration system is provided for optimally and cost-effectively 
using collected dust to enhance the air filtration unit of the unit. 
Measurements in connection with the present invention, employing the 
continuous aerosol monitor (CAM) electro-optical method described in 
commonly-assigned Shofner et al U.S. Pat. No. 4,249,244, have established 
that fiber fragments, which are ever-present in textile operations, 
constitute one of man's oldest and best filters. By properly building a 
lint mat from these ever-present fiber fragments in textile operations, 
and the word "properly" must be emphasized, an extremely efficient filter 
medium for respirable dust can be developed. Inasmuch as the collection 
efficiency of the filter substrate media on which the fiber fragments and 
respirable dust are captured is virtually insignificant, an important 
aspect of the invention is the utilization of relatively thin, inexpensive 
filter substrate media to hold the dust, which then becomes its own 
filter. In previous filter systems, it has been common to improperly form 
lint mats, with the result that either the respirable dust penetration is 
highly unsatisfactory, or the air flow is severely limited. 
An important aspect of the invention is the use of air flow rate as the 
controlled parameter, rather than static pressure developed across a 
filter element as is common in the prior art. Filter element area remains 
essentially constant, while the rate of filter substrate media cleaning or 
filter substrate movement is controlled as required to maintain a 
predetermined air flow rate. For reasons developed in detail below, this 
control approach has a number of significant advantages. In particular, 
the electromechanical efficiency of the motor-blower combination is 
optimally maintained. The air flow rate can be set to whatever level is 
required for the particular application depending upon the dust level to 
be maintained. Within the range of operating conditions of the invention, 
the rate of filter substrate media consumption depends directly on the 
amount of lint and dust removed from the air. 
Another overall aspect of the invention is the use of a relatively 
high-velocity discharge, in the order of 3000 feet per minute. This 
high-velocity discharge may be employed either with or without an air 
diffuser. The high-velocity discharge permits "throwing" the 
clean-filtered air to preferable workplace areas, and prevents the usual 
wasteful short-circuiting or recirculation of clean air. This high 
velocity discharge moreover enables inducement of a preferred air flow 
pattern which aids the general collection of dust emitted from processing 
machinery or work activity. 
Other general aspects of the invention lie in the mechanical aspects which, 
in general, involve cylindrical drums formed at least in part of rigid 
open mesh material such as expanded metal which remains stationary while 
filter substrate media is controllably drawn thereacross. The filter 
substrate media may either be a disposable media, in which case the filter 
substrate media is provided in elongated web form as applied from a feed 
roller. Alternatively, a cleanable filter media may be employed. 
Another aspect of the invention is an improved cleaning system for removing 
accumulated mat material from a cleanable filter substrate. 
Briefly, and in accordance with a more specific aspect of the invention, 
there is provided a modular air filtration unit comprising a unit housing 
formed at least in part of a rigid open mesh material, such as ordinary 
expanded metal, defining an air flow inlet. The expanded metal also serves 
as a media support element. In one form, the expanded metal portion is a 
semi-cylindrical shell, with the remaining portion of the cylinder 
completed by solid, air-impermeable metal. In another specific embodiment, 
the expanded metal shell is substantially completely cylindrical, with the 
exception of a single axially-extending solid strip of limited 
circumferential extent which comprises a portion of filter substrate media 
cleaning system, described hereinbelow. 
The modular air filtration unit additionally includes a motor-driven blower 
for drawing air into the housing through the expanded metal material and 
for forcibly discharging the air from the housing through a blower outlet. 
In the one embodiment, the air is forcibly discharged in a radial 
direction. In the other embodiment wherein the expanded metal media 
support extends substantially all the way around the cylindrical housing, 
the blower discharge is in an axial direction. 
A sheet of filter substrate media is positioned on the outside of the 
expanded metal mesh, and is supported thereby such that unfiltered air is 
drawn through the filter substrate media to form a mat thereon, and such 
that the mat thus formed serves as a filtration medium. In the one 
specific embodiment, a disposable filter substrate media is employed, and 
extends between a media supply roll and a media take-up roll, both 
oriented generally parallel to the axis of the housing. The web extends 
from the supply roll to the take-up roll across the outside of the media 
support portion and such that air can enter the housing only through the 
filter substrate media. In the other embodiment, a cylindrical sleeve of 
cleanable filter substrate media, such as the material known in the art as 
"fake fur", is positioned on the outside of the cylindrical shell and 
supported thereby. 
The filtration unit additionally includes a controllable 
media-replenishment device for drawing exposed filter substrate media 
across and off the expanded metal mesh at one portion thereof and 
simultaneously supplying clean filter substrate media at another portion 
of the rigid open mesh. In the case of the one embodiment wherein a 
disposable filter substrate media is employed, the media-replenishment 
device comprises quite simply the aforementioned rolls, together with a 
controllable drive motor for rotating the take-up roll. Accordingly, when 
the controllable drive motor is rotated, exposed filter substrate media is 
drawn across and from the expanded metal portion, while at the same time 
clean filter substrate media is drawn from the supply roll. In the other 
specific embodiment, the controllable media replenishment device comprises 
a filter substrate cleaning system including a controllable drive 
mechanism for rotating the cylindrical sleeve relative to the cylindrical 
shell. The media cleaning system further comprises the above-mentioned 
axially-extending solid strip of limited circumferential extent, and this 
axially-extending solid strip serves to locally block air flow. An 
axially-extending slot is formed in the solid strip, and a conduit is 
provided for directing compressed air radially outwardly through the slot 
to aid in removing accumulated mat. Finally, an axially-extending suction 
nozzle is positioned over the cylindrical sleeve immediately over the slot 
for aiding in removing and carrying away accumulated mat. 
The final overall element of the modular air filtration unit is a control 
system responsive to the rate of air flow through the unit and operable to 
substantially maintain a predetermined rate of air flow by activating the 
media replenishment device when sensed air flow decreases. 
As stated above, and as further explained in detail hereinbelow, a 
significant and advantageous aspect of the invention is that the rate of 
air flow is user-selectable in accordance with the requirements of the 
particular environment within which the modular air filtration unit is 
operated. Accordingly, the control system is user-adjustable. As a result, 
the mat permeability, motor-power consumption, and substrate media usage 
rate are each indirectly selected as a direct function of air flow rate. 
In addition, dust mat filter density and dust mat filter efficiency are 
each indirectly selected as an inverse function of air flow rate. 
As stated hereinabove, an important aspect of the invention is to properly 
form a dust mat or combination lint and dust mat filter which then serves 
as its own filter medium, with emphasis on the "properly". In accordance 
with the invention, this involves controlling the air flow rate such that 
the permeability of the mat filter is no greater than in the order of 200 
cubic feet per minute (CFM) per square foot of open area at a static 
pressure drop of 2 inches water column (WC). Lower permeability 
corresponds to better filtration and lower media consumption, but 
increases the physical size of the apparatus for a given flow. Clearly, an 
optimum must be determined and, in accordance with the invention, a value 
of in the order of 3450 CFM through about 18 square feet of expanded metal 
is good. Thus, the air flow control approach, which then indirectly 
controls the permeability of the dust mat is intimately tied to the 
formation of a proper dust mat. At this point it should be noted that the 
resistance of the filter substrate media is very low, relative to the dust 
mat, and that its capture efficiency for microdust is also very low. 
In accordance with the invention, it has been quantitatively discovered, 
using the CAM monitor system of the above-identified Shofner et al U.S. 
Pat. No. 4,249,244, that a lint mat in a textile processing application, 
and dust mats in general, can in fact be good filters, coupled with a 
combination lint mat and flow control approach optimally applicable to 
modular filtration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Installation in General 
With reference first to FIGS. 1 and 2, shown in overall perspective view is 
a single modular air filtration unit 20 in accordance with the invention 
intended particularly for use in textile processing applications. The 
particular unit 20 illustrated is intended for mounting by suspension from 
a ceiling by means of suitable suspension rods 22. From a comparison of 
FIGS. 1 and 2, it can be seen that unit 20 can be oriented in a variety of 
ways by varying the mounting. In particular, in FIG. 1 the unit 20 is 
oriented for vertically downward clean air discharge, and in FIG. 2 the 
unit 20 is oriented for horizontal clean air discharge. By way of example, 
the particular unit 20 has overall dimensions of 45 inches in length, 40 
inches in height, and 40 inches in width. Typical applications of the 
modular air filtration unit 20 involve multiple units, with each unit 20 
covering in the order of 500 to 2000 square feet of floor area. The air 
discharge rate is in the order of 3450 CFM, at a velocity of in the order 
of 3000 feet per minute. A 1.5 or 2 HP 3-phase blower motor is employed. 
In overall configuration, the modular air filtration unit 20 comprises an 
esthetically pleasing cylindrical portion, generally designated 24 and 
partly or fully covered with filter media 26, extending between end plates 
28 and 30, which also serve to terminate the mounting rods 22. Doors 32 
and 34 are provided for respectively providing access to a media supply 
roll 36 and a media take-up roll 38. Also visible in FIG. 2 is a media 
drive arrangement comprising a high torque gear motor 39 which rotates the 
media take-up roll 38 through a chain drive 40 when energized. 
In the overall operation, dusty air is drawn in through the cylindrical 
portion as depicted in FIG. 1, and is forcibly discharged through a blower 
outlet 41, shown in FIG. 2. 
FIGS. 3, 4 and 5 respectively depict typical and various applications for 
the modular air filtration unit 20. 
FIG. 3 illustrates the use of the unit 20 in a general dilution mode with a 
localized air flow pattern, as indicated by arrows 42, around the unit 20, 
and which indicate that minimal "short-circuiting" occurs. The flow 
pattern depicted in FIG. 3 is achieved by a combination of relatively high 
discharge velocity and an air diffuser, schematically depicted in 
cross-section as vanes 44. A "source-sink" characteristic is evident. 
FIG. 4 depicts a modular air filtration unit without a diffuser operating 
in an induced flow pattern in connection with a warping machine 48. The 
"source-sink" characteristics of the unit 20 occasioned by the 
high-velocity air flow are also apparent from FIG. 4. In general, a 
high-velocity, e.g. 3000 feet per minute, flow of air as generally 
represented at 50 is directed over the warping machine 48, and is then 
drawn back through the warper 48 as depicted at 52. In some applications, 
a wall 54 aids in redirecting the air flow back through the machine 48. It 
will be appreciated that a warping machine 48 has an inherent air flow 
pattern caused by yarn entrainment of air, and in such cases the modular 
air filtration unit of the invention aids this inherent air flow pattern. 
As a result, larger floor areas can be served by each modular air 
filtration unit 20 than might otherwise be the case. 
More particularly, roughly, floor space areas of 500 to 2000 square feet 
can be covered by each unit 20. However, this is highly 
application-dependent. To achieve a better estimate requires that various 
factors be taken into account, such as existing dust concentration, target 
dust concentration, existing air change rates, machinery dust emission 
rates, quality of air supply by other sources (e.g., air-conditioning, 
filter houses, etc.), and inherent air flow patterns, especially 
cross-flows from adjacent processes. As a very rough rule of thumb, 30% 
reductions are typically effected by modular air filtration units in 
accordance with the invention each serving about 1000 square feet of floor 
area. 
Referring next to FIG. 5, the modular air filtration unit 20 is employed in 
a partial source capture mode, particularly exploiting the sink feature of 
the unit 20. As is known, it is always better to capture dust before it 
spreads. Some machinery, such as draw frames, spinning end collection 
boxes, winders, and some spoolers, can have their discharges ducted 
generally toward, but not hard connected into, the modular air filtration 
unit 20. In FIG. 5, this principle is exemplified by a lint screen 
discharge duct 50. 
MECHANICAL FEATURES 
FIGS. 6 and 7 depict the internal construction of the unit 20, FIG. 6 being 
substantially complete, and FIG. 7 having a number of elements removed for 
clarity of illustration. 
The air filtration unit 20 comprises a unit housing, generally designated 
60, comprising the end plates 28 and 30 and an at least semi-cylindrical 
shell 62 including a semi-cylindrical inlet and media support portion 64 
formed of rigid open mesh material, such as expanded metal, with a pair of 
axially-extending boundaries 66 and 68. In the FIG. 6 view, the expanded 
metal portion 64 extends clockwise from the axially-extending boundary 66 
at approximately the eight o'clock position clockwise around to the 
boundary 68 at approximately the four o'clock position. The remainder of 
the shell 62 generally comprises solid material 72, a portion of which is 
also visible in FIG. 2. 
Within the housing 60 is a squirrel cage blower 74 driven by an AC 
induction motor 76, for example, 1.5 or 2.0 HP. For compact size, the 
blower motor 76 is preferably a three-phase motor. A suitable mounting 
arrangement 78 supports the blower motor 76. As may be best seen in FIG. 
7, the motor 76 has an extended shaft such that air axially enters the 
blower 74 from two sides. 
The blower 74 serves to draw air into the housing 60 through the expanded 
metal mesh material 64 and to forcibly discharge the air through the 
blower outlet 41. The blower outlet 41 is rectangular in cross-section, 
having cross-sectional dimensions in the order of 12.times.13 inches. 
In FIG. 6 it may be seen that the web 26 of filter substrate media extends 
from the supply roll 36 generally across the outside of the expanded metal 
media support portion 64 to be tightly wound up on the motor-driven 
take-up roll 38. A gear-driven high-torque motor 39 (FIG. 2) drives the 
take-up roll 38 as required by the flow-control system. 
FIG. 6 also depicts a lint mat 82 which is formed on the filter substrate 
media 26 from the lint and particle matter itself. The microdust 
filtration properties of this filter mat 82 are exceptionally good 
because, at least for textile processing application, the ever-present 
fiber fragments are small and numerous and effectively capture and retain 
microdust particles. As approximate examples of the sizes involved, the 
fiber fragments of interest are roughly 0.5 mm to 5 mm in length, and 3 
.mu.m to 30 .mu.m in width. It may be noted that cotton fibers from which 
the fragments are formed are about 25 mm in length, and 20 .mu.m to 30 
.mu.m in average diameter. The microdust particles are roughly spherical 
and have diameters of 15 .mu.m and smaller, for OSHA-defined, respirable 
cotton dust. The permeability of the filter mat 82, when properly formed, 
is in the order of 200 CFM per ft.sup.2 of open area at a static pressure 
drop of 2 inches WC. 
The filter substrate media 26 comprises a non-woven rayon fabric material 
having a high machine direction strength such that it can withstand the 
significant forces required to pull it around the metal mesh cylinder 64 
against the friction forces caused by static pressure drop across the 
media 26 plus lint mat 82. This media 26 preferably comprises a non-woven 
rayon fiber material having a pore size smaller than about 1 mm and a 
permeability in the order of 200 CFM per square foot of open area at a 
static pressure drop of about 0.1 inches WC. The weight is only about 1.5 
to 2.0 ounces per square yard. As herebefore stated, the filtration 
properties of the substrate media are immaterial; the media 26 serves only 
to capture the lint which constitutes the microdust filter. 
Suitable non-woven materials are well-known for other purposes and are 
employed, for example, for apparel purposes and men's collar linings, and 
as other apparel linings. Exemplary materials are available from J.P. 
Stevens and Co., Inc., High Point, N.C.; for example, J.P. Stevens Style 
Nos. MF-210 (1.5 oz. per square yard) and MF-710 (1.9 oz. per square 
yard). The material is employed in 45 inch wide rolls, with a length in 
the order of 220 yards. These materials, while intended for use for other 
purposes, i.e. apparel, are adequate for use in the subject invention. It 
is anticipated that improvements to the material (e.g. higher machine 
direction strength and lower weight) will be made in the future in view of 
the specific requirements of the subject invention. 
In connection with the comparative permeability of the media substrate 
material 26 (200 CFM per ft.sup.2 of open area at a static pressure drop 
of 0.1 inch WC) and of the filter mat 82 (200 CFM per ft.sup.2 of open 
area at a static pressure drop of 2 inch WC), for start-up purposes a 
manually-operated damper 83 is included to restrict air flow through the 
unit 20 until the lint mat 82 is formed. (See FIGS. 6 and 9.) Otherwise, 
the airflow exceeds the rated capacity of the blower 74, causing the motor 
76 to draw in excess of its rated current. 
An idler bar 84 and a ball-bearing idler roll 86 direct the substrate media 
26 such that the expanded metal portion 64 is always completely covered, 
to avoid leakage of air around the filter medium comprising the mat 82 
supported on the substrate media 26. 
As stated above, the exposed media 26 is tightly wound up onto the take-up 
roll 38 driven by the high-torque gear motor 39 and chain drive 40. The 
tight winding permits typical "doff" intervals of about three months. A 
new roll of media typically lasts about twelve months, but this is 
highly-application dependent, as will be seen from the discussion of 
operating parameters below. 
The final major element of the unit 20 is a control system, generally 
designated 89, and including a portion enclosed within a control box 90, 
and an airflow sensor 92 (FIG. 2). The control system 89 comprises 
essentially an adjustable pressure switch, the electrical function of 
which is depicted in FIG. 9, described hereinafter. More particularly, the 
pressure sensor 92 comprises a closed-end tube having a plurality of 
upstream-directed apertures (not shown) facing into the discharge air 
stream from the blower outlet 41 such that pressure within the tube is a 
direct function of flow rate. Since airflow is not constant at different 
points within the cross-section of the blower outlet, the particular form 
of pressure sensor 92 employed in effect averages or integrates air flow 
along a line, to provide a more accurate measurement. Thus, the sensor may 
be termed an "integrating bar", although other shapes may be employed, 
such as integrating loops. Static pressure for reference purposes is 
sensed by an open-end tube 94. 
FIG. 8 depicts a modification for pre-exposure dust mat formation. In 
particular, the FIG. 8 embodiment includes an additional idler roller 100 
arranged such that the filter substrate media 26 is constrained to follow 
an S-bend. As a result, dust is collected and forms a pre-exposure cake or 
mat 102 so that better microdust filtration is realized at 104 and at 82. 
FIG. 9 depicts the overall electrical schematic wiring diagram of the unit 
20 including, in particular, an adjustable pressure switch 106 included 
within the FIG. 2 control box 90 for periodically energizing the media 
drive gear motor 39 when air flow falls below a predetermined rate. In 
typical operation, the media drive gear motor 39 is energized 
approximately once per hour, and advances about three lineal inches of 
fresh media substrate 26 before air flow increases sufficiently to re-open 
the pressure switch 106. 
In operation, since the static pressure developed across the lint mat 82 is 
essentially constant at about 2 inches WC, it follows that flow through 
the unit 20 is determined by the permeability of the lint mat. A major 
advantage of the control approach is that flow, once set, is a known and 
dependable quantity and that the consumption of media is in proportion to 
that amount of lint and dust which needs to be removed. 
Alternatively stated, one of the most powerful features of the modular air 
filtration unit 20 is this ability to set just the flow required to 
achieve certain dust levels in the workplace. If the stock is cleaner, or 
production is lower, or the production or air-handling machinery emit less 
dust, the modular air filtration unit 20 airflow can be simply decreased 
with the result that electrical power, media consumption and dust 
concentration out of the modular air filtration unit 20 are all decreased. 
Since the units 20 of the invention tend to be associated with specific 
processing machines, the ability to adjust the dilution ventilation rate 
influencing dust levels around those machines is a very flexible feature. 
OPERATING AMETER INTERRELATIONSHIPS 
The dependences of media consumption, electrical power consumption, and 
discharge air quality upon the controlled parameter, filter airflow Q, are 
now described. 
Some of these relationships may be better understood in view of the plots 
of FIG. 10. In FIG. 10, the line 120 depicts a blower static pressure 
characteristic curve. What is significant to note is that blower static 
pressure is approximately constant over the useful CFM range of the unit. 
The two lines 122 and 124 depict pressure drop as a function of airflow 
for two different representative lint mat densities, the curve 122 
representative of a relatively lower permeability lint mat, and the line 
124 representative of a relatively higher permeability lint mat. 
1. Media Consumption. In general, static pressure drop .DELTA.p across 
flat, low face velocity filter media obeys, for a given type of dust and 
media, 
##EQU4## 
where H is the average surface density of dust held onto the media in 
grams/m.sup.2 and V is air velocity in m/sec. (Evidently, H at a point on 
the media varies from zero to some maximum value as the media is drawn 
around the drum. We can express the major operational parameter 
dependances in terms of the average holding capacity H.) But Q=VA, where 
A=modular air filtration unit open area 64, and .DELTA.p=constant 
.apprxeq.2" WC for a typical unit. It follows that 
##EQU5## 
or the higher the flow, the thinner the lint mat, as shown by the trend 
arrow 128 in FIG. 10. 
Now the mass of lint and dust in the lint mat M=HA; since H=constant 
(because .DELTA.p and Q are constant), 
##EQU6## 
Using Equation (5) we finally have 
EQU A.varies.MQ.sup.2 (8) 
which shows that media consumption is directly proportional to the amount 
of lint and dust captured, M. The dependence of A on Q.sup.2 is 
significant. Relative to 3000 CFM, media consumption on a given dust is 
32% higher at maximum flow of 3450 and 56% lower at 2000 CFM, or A covers 
a 3:1 range over the nominal MF operating range. 
2. Electrical Power Consumption. Since the static pressure .DELTA.p and 
electro-mechanical conversion efficiency for the modular air filtration 
unit 20 are approximately constant over the 2000-3450 CFM range, it 
follows that 
EQU Electrical Power Costs.varies.Q (9) 
The total operating costs for any filtration system must include media, 
repair parts, maintenance labor, and electrical power. Electrical power 
dominates. In the modular air filtration unit 20 2 HP delivers 3450 CFM of 
25 .mu.g/m.sup.3 air. Such high quality air cannot be produced with the 
typical central filtration system designed primarily for waste handling; 
the technology is inapplicable and 50-100 .mu.g/m.sup.3 is considered very 
good. When high efficiency central filtration systems are designed to 
deliver such good air quality, it is typical to find a 75 HP motor driving 
a 35,310 CFM blower. Thus the electrical power operating cost ratio is 
##EQU7## 
or about 4:1 favor of the modular air filtration unit. A more detailed 
operating cost analysis preserves this 4:1 advantage. 
3. Discharge Air Quality. The residual dust concentration in the modular 
unit discharge X.sub.MF should depend inversely on H, the lint mat 
density. This is expected to be the primary factor but we also expect 
slight dependences on A. At present there are not sufficient data to 
conclude the exact form of these dependences because all of the units 
installed to date have operated at constant flows. However, it can be 
stated that the range and typical values observed are 
##EQU8## 
4. Dust Concentration Transient Analysis. Next is presented a simplified 
analysis to support the discussion of a practical modular filter 
performance evaluation protocol using PCAM dust test equipment. Shofner et 
al [F. M. Shofner, A. C. Miller, Jr., G. Kreikebaum, "Measurement and 
Control of Non-Cotton Dust Contributions in the Cotton Processing 
Workplace: I-.chi..sub.at " presented at the ASME Symposium on Cotton 
Dust, Oct. 7-8, 1980, Atlanta] show that equilibrium respirable workplace 
dust concentrations obey 
##EQU9## 
where F is the penetration efficiency of dust in a test cubic meter of air 
upon one complete recirculation path, and M.sub.r and Q are as above for 
Equation (1). (In order to focus on the major parameters, the (1-F) term 
was omitted from Equation (1)). F therefore includes all losses. 
Typically, for good filtration, F.about.0.1 to 0.2. We assume the same 
penetration for all circulating flows in order to more simply show the 
major effects. 
FIG. 11 shows the transient behavior of workplace respirable dust 
concentrations when modular filters are turned ON and OFF over a period of 
the order of one hour. This period is short enough that machinery and 
processing conditions should remain constant, and that intense 
observations by test personnel can assure it, and long enough that 
equilibrium conditions are reasonably reached. The 15 minute PCAM 
averaging periods are ideal for this type evaluation. (For a PCAM test 
protocol for modular filter evaluation see J. H. Hanley, F. M. Shofner, 
"Application of Modular Filtration to Cost-Effective Cotton Dust Control 
in Textile Processes; Especially in Spinning through Warping", presented 
at and published in the Proceedings of the Seventh Cotton Dust Research 
Conference, 1983 Beltwide Cotton Production Research Conferences, San 
Antonio, Tex., Jan. 3-4, 1983.) 
As a matter of analytical interest, noting the assumption above and several 
further simplifying assumptions, including perfect and immediate mixing of 
machinery dust emissions, then .chi.(t) may be expressed, for the 
OFF.fwdarw.ON transient as an exponential decay from equilibrium 
concentration .chi..sub.eq, with modular filters OFF, to a new equilibrium 
.chi..sub.eq ', with modular filters ON, according to 
##EQU10## 
wherein M.sub.r =total respirable emissions into workplace, gm/min 
Q=existing circular air flow, m.sup.3 /min 
Q.sub.MF =Modular unit circulating air flow, in m.sup.3 /min 
F=net penetration efficiency for all circulation and filtration sources 
t=time after modular unit is turned ON, in minutes 
.tau.=time constant, minutes. 
The decay time constant .tau. is related to the traditional room air 
exchange time 
##EQU11## 
60/T is the customary air changes per hour. 
Thus if a room has 15 changes per hour, T=4 minutes, and in about 15 
minutes, the major part of the transient is complete. 
Note that the fractional reduction in dust levels as a consequence of 
turning the modular unit ON is 
##EQU12## 
For example, for a 33% reduction in dust levels, the added modular unit 
flow Q.sub.MF must be half the existing, effective circulating flow, Q. 
A similar expression holds for the OFF.fwdarw.ON transition. Note that 
.tau..sub.ON and .tau..sub.OFF are different. 
Performance Examples 
The following TABLE is a Case Study Summary summarizing the performance of 
experimental prototype units in accordance with the invention in 
representative textile processes. These particular units delivered about 
2500 CFM. Induced flow patterns were employed in warping and carding; 
general dilution was employed elsewhere. 
TABLE 
__________________________________________________________________________ 
CASE STUDY SUMMARY 
Estimated 
Observed MF Media 
.sup..chi. eq.sup.1, .mu.g/m.sup.3 
.sup..chi. eq.sup.'1, .mu.g/m.sup.3 
Reduction 
Predicted 
.sup..chi. in.sup.2 
.sup..chi. out.sup.3 
Coverage 
Consumption 
Process 
Without MF's 
With MF's 
R Reduction R 
.mu.g/m.sup.3 
.mu.g/m.sup.3 
Ft.sup.2 /MF 
Ft.sup.2 /Day 
Comments 
__________________________________________________________________________ 
Warping 
396 175 56% -- 690 25 1063 14.8 Large atomizer 
component. 
Nominal air 
changes 
Carding 
266 183 31% 45% 509 16 625 8.5 Large variation 
in .chi., both 
with- 
out and with MF's 
.sup..chi. AC 
.about. 80 
.mu.g/m.sup.3 
Drawing 
510 335 34% 40% 758 30 500 9.7 Definite cross- 
(max) flow. Unusual 
diffusers. Large 
emission from 
frames. 
Waste .sup..chi. AC 
.about. 
.mu.g/m.sup.3 
Baling 
669 431 36% -- 1466 
-- ? 15.7 Unusual layout 
Back 2 units in cor- 
Winding 
109 73 35% -- -- -- &gt;1050 -- ner of large 
room. Cross 
flows. 
__________________________________________________________________________ 
.sup.1 .chi..sub.eq = respirable workplace dust at equilibrium 
.sup.2 .chi..sub.in = total dust concentration onto modular filter unit 
.sup.3 .chi..sub.out = respirable dust concentration out, as 
isokinetically sampled with PCAM 
.sup.4 At .DELTA.p.sub.1 = 1.2" H.sub.0. Nominal .DELTA. p.sub.1 now 
1.9" H.sub.2 O for MF3450. 
Alternative Embodiment 
The foregoing descriptions with reference to FIGS. 1-11, which generally 
are illustrative of the principles of the invention, are specifically for 
textile processing applications, wherein the lint and dust mat, when 
properly formed, constitutes an excellent dust filter. However, as noted 
in the background, the amount of respirable dust that must be removed for 
engineering controls of the textile workplace is small, of the order of 
tens of grams per hour. In typical installations, even this low collection 
rate is distributed over several modular filters. 
There are numerous other applications where the permissible exposure limits 
are higher. Nuisance dust (as opposed to toxic dust) PEL's are of the 
order of 5 to 15 mg/m.sup.3. It follows that the dust collection rate is 
much higher for such applications and that the concept of disposable media 
can, in some cases, become economically unattractive. 
All of the major features of the apparatus described above are retained in 
a modular air filtration unit 200 designed for high dust concentration 
applications as shown in FIGS. 12 and 13. The unit 200 includes a 
cylindrical shell 202 formed of rigid open mesh material, such as expanded 
material as in the previous embodiments. A motor 204 and centraxial blower 
206 combination is provided for drawing air into the housing 202 through 
the cylindrical shell and for forcibly discharging the air from the 
housing through an axially-directed blower outlet 208. 
Rather than the disposable filter substrate media as employed in the 
embodiment described above, the filtration unit 200 of FIGS. 12 and 13 
employs a cleanable filter substrate media 210, comprising, for example, a 
material known in the filtration art as "fake fur". The filter substrate 
media 210 is configured into a cylindrical sleeve configuration and is 
positioned on the outside of the cylindrical shell 202 and supported 
thereby such that unfiltered air is drawn through the filter substrate 
media 210 to form a dust mat thereon, and such that the dust mat thus 
formed serves as a filtration medium. The substrate media 210 and the dust 
mat formed together comprise a primary filter F1. 
The unit 200 includes a filter substrate media cleaning system. More 
particularly, the cleaning system comprises a controllable drive mechanism 
for rotating the cylindrical sleeve 210 relative to the cylindrical shell 
202. 
In connection with the drive mechanism, it may be noted that the filter 
substrate media 210 actually comprises three individual sleeve segments 
212', 212", and 212'" attached at their circumferential edges to band-like 
nylon racks 214 rotated by a gear and media drive arrangement. 
The filter substrate media 210 is cleaned by a combination of compressed 
air and suction. To accomplish this, a portion of the cylindrical sleeve 
comprises an axially-extending solid strip 216 (FIG. 13) of limited 
circumferential extent for locally blocking radially inward airflow. An 
axially-extending slot 218 is provided in the axially-extending strip, and 
a conduit 220 directs compressed air radially outwardly through the slot 
to aid in removing accumulated mat from the filter substrate media 210. In 
addition, an axially-suction nozzle 222 is positioned over the cylndrical 
sleeve 202 immediately over the slot 220 for aiding in removing and 
carrying away accumulated mat. 
Airflow is sensed by an integrating ring 224 positioned at the blower 
outlet 208. The integrating ring 224 is similar in operation to the 
integrating bar of the previous embodiment, and comprises a tube with a 
plurality of apertures (not shown) pointing upstream such that static 
pressure builds in the tube dependent upon airflow velocity. The 
integrating ring 224 is connected to a control box 226 by means of a 
conduit (not shown). The control box 226 is comparable to the control box 
90 of FIG. 2, and comprises an adjustable pressure switch 106 as in FIG. 
9. 
The same control approach is applied to the modular air filtration unit 200 
of FIGS. 12-14 as in the previous embodiment. The rate of airflow through 
the unit 200 is sensed, and the media 210 is advanced and thereby cleaned 
in response to the airflow velocity. As before, user-selection of a 
particular velocity indirectly determines a particular permeability 
because the blower static pressure is approximately constant over the 
useful CFM range from the unit. 
The economics of increased capital cost for the unit 200 intended for high 
dust concentration applications are off-set by its lower operating costs 
relative to media consumption. The selection of either the disposable 
filter substrate media embodiment 20 or the cleanable filter substrate 
media embodiment 200 is determined through a trade-off between capital 
costs and operating costs. 
As in the previous embodiments, the axial flow from the centraxial blower 
206 is a directed high-velocity flow, and retains the high kinetic energy 
along through distance of the previous embodiment. As may be seen from the 
drawings, it utilizes a direct-drive motor, thus eliminating mechanical 
energy losses inherent in belt-drive designs. 
In a 200 .mu.g/m.sup.3 PEL process, it is important that the discharge 
having concentration .ltorsim.25 .mu.g/m.sup.3. In a 15 mg/m.sup.3 =15,000 
.mu.g/m.sup.3 process, the requirements on discharge air quality are 
obviously much less stringent. However, in those processes where the 
discharge is important, a secondary set of cleanable or disposable filters 
230 or F2, may be inserted as shown in FIG. 12. 
These F2 filters may be z-folded, axi-folded, and in some critical 
applications, HEPA media. Clearly, the choice of secondary filter media is 
more application dependent than the choice of the primary filter F1 
because the primary filter serves to capture dust which, in sufficient 
quantity and when properly formed, forms its own filter media. 
While specific embodiments of the invention have been illustrated and 
described herein, it is realized that numerous modifications and changes 
will occur to those skilled in the art. It is therefore to be understood 
that the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.