Multi-stage compressed gas filter

A filter for removing contaminants from compressed gas is disclosed. The filter features an inverse flow regime wherein gas entering the filter first flows downwardly and then reverses direction before encountering any filter media. The filter has a connection head with a gas inlet, a gas outlet and oppositely disposed openings separated by a central partition. The openings are sealingly enclosed by elongated tubular housings which releasably attach to the head. The gas inlet communicates with one side of the partition and the outlet communicates with the opposite side. A plurality of individual filter elements are arranged on either side of the partition and filter out aerosol, vapor and particulate contaminants from the compressed gas. The filter elements are releasably mounted to the central partition and can be removed and replaced independently of one another. The tubular housings are vertically oriented and a valve is provided in the bottom of the lower housing to drain accumulated liquid from the filter.

FIELD OF THE INVENTION 
This invention relates to filters for filtering contaminants such as solid 
particles, aerosols, water vapor and oil vapor from compressed gas. 
BACKGROUND OF THE INVENTION 
Compressed gases such as compressed air, nitrogen, oxygen, helium and so 
forth are used extensively throughout a wide spectrum of industries such 
as automotive, chemical, electronic, aerospace, oil refining and health 
care industries to name but a few. There is often a need in these 
industries for compressed gas which is free of contaminants such as 
particulate matter, aerosols (liquid droplets), water vapor, oil vapor or 
other undesired agents. For example, if used to spray paint, it is 
important that oil, water and particulate matter be removed from the 
compressed air to prevent the paint from becoming contaminated, thereby 
ruining the paint finish. Similarly, if compressed air provides a source 
of breathing air, for example, by SCUBA divers, firemen, air crew members 
or hospital patients, it is critical that particulate and oil contaminants 
be removed from the air to prevent ingestion of the contaminants into the 
lungs of the person. 
Gas compressors themselves are often the source of contaminants in 
compressed gas. Air drawn into a compressor, for example, is often not 
properly filtered at the compressor intake, thus allowing particulates 
commonly in the air, such as pollen, smoke and dust, to be drawn in and 
introduced into the compressed air supply. The moving parts of compressors 
such as pistons or impellers are lubricated with oils which can vaporize, 
atomize, oxidize or otherwise breakdown due to agitation by the moving 
parts and the increase in temperature associated with compressing a gas. 
The lubricating oil or its breakdown by-products become entrained in the 
compressed gas either as an aerosol (oil droplets suspended in air) or a 
vapor (oil in the gaseous state) as the gas leaves the compressor. 
Finally, water is a natural component of air, and unless deliberately 
removed, water will comprise a natural part of compressed air as well in 
both aerosol (water droplet) and vapor (gaseous) form. 
SUMMARY AND OBJECTS OF THE INVENTION 
The invention provides a multi-stage filter to remove contaminants such as 
particles, water vapor and aerosols, oil vapor and aerosols and the like 
from compressed gases. The multi-stage filter comprises a connection head 
having upper and lower oppositely facing receptacles separated by a 
partition. The partition has first and second sides within each 
receptacle. An aperture through the partition provides communication 
between the two receptacles. Within the connection head is an inlet for 
the compressed gas, the inlet being connected to a gas inlet passageway 
which extends through the head to the first side of the partition. A gas 
outlet passageway extends from the second side of the partition through 
the head to a gas outlet. 
Two elongated tubular housings are releasably attached to the connection 
head, a respective housing enclosing each of the upper and lower openings. 
Within each elongated housing is at least one filter element, the elements 
being disposed on the opposite sides of the partition and releasably 
attached to a respective side. The filter elements sealingly surround the 
aperture through the partition such that gas passing through one filter 
element will then pass through the aperture and into the other filter 
element. A first of the filter elements has a gas impermeable sidewall 
leading to an open end arranged opposite to the partition where gas can 
enter the first filter element. The impermeable side wall is disposed in a 
spaced apart relationship with the surrounding tubular housing, thus 
forming a duct from the partition to first filter element open end. 
The gas impermeable sidewall construction of the first filter element 
provides an inverse flow regime to the filter, whereby compressed gas 
entering the gas inlet is directed to the inlet passageway which channels 
the gas to the first side of the partition, whereupon the gas travels 
lengthwise of the gas impermeable sidewall (through the duct formed 
between the sidewall and the elongated housing) and, abruptly changing 
direction, the gas then enters the first filter element at the end of the 
sidewall opposite from the partition. Passing in a counterflow direction 
through the first filter element, the gas passes through the partition 
aperture and into a second of the filter elements on the second side of 
the partition and in coaxial alignment with the first. The gas passes 
through the second filter element and into the outlet passage which 
channels the gas to the compressed gas outlet where the gas exits the 
filter. 
The inverse flow regime greatly aids in the separation of water aerosol 
entrained in the gas, much of the water separating from the gas when the 
gas reverses direction to enter the first filter element. It is therefore 
preferable to orient the filter vertically with the first filter element 
positioned beneath the second because a vertical orientation allows water 
separated from the gas to accumulate at the bottom of the lower elongated 
housing where it can be drained by a valve positioned at the housing 
bottom. It is likewise preferable to have the first filter element be a 
coalescing type which separates water aerosol from the gas as well. Water 
separated from the gas by the coalescing filter element will drain to the 
bottom of the lower elongated housing along with the water separated by 
the gas flow reversal; the water can then be removed via the valve as 
described above. 
To dry the gas further by removing water vapor it is preferable to use a 
desiccant type filter element as the second filter element. Desiccant 
filters adsorb and retain a large fraction of the water vapor in the gas 
and must be replaced when they become saturated and are no longer 
effectively removing the vapor. It is preferable to use desiccant material 
which has a colorimetric indicator, i.e., the desiccant material changes 
color as it becomes saturated with water, a complete color change of the 
filter media indicating when the filter element should be replaced. When a 
colorimetric desiccant filter is used as the second filter element it is 
convenient to have a means to view the filter element for periodic 
inspection and servicing. To this end the elongated housing enclosing the 
upper opening of the connection head is made from a transparent material 
which allows a full view of the second filter element. Since the elongated 
housings are releasably attached to the head and the filter elements are 
releasably mounted on the partition changing filter elements can be 
accomplished quickly and easily by simple routine maintenance procedures. 
Oil can also be removed from the gas via either a coalescing filter as the 
second filter element. Coalescing filters remove oil aerosol entrained in 
the gas. Oil retained in a coalescing filter element effects a visible 
discoloration of the filter media indicating oil saturation and the need 
to change the filter element. A transparent elongated housing greatly 
facilitates visual inspection of oil adsorbing colorimetric filter 
elements and oil coalescing filter elements as described above for water 
vapor filters. 
A third filter element is preferably mounted within the gas impermeable 
sidewall of the first filter element in coaxial alignment with the first 
filter element. It is advantageous to use the third filter element to trap 
and remove particulate contaminants from the gas such as pollen, dust, 
smoke particles and the like. It is preferable to locate the particulate 
filter element downstream of the water coalescing filter so that the 
particulate filter remains relatively dry during filter operation. 
It is an object of the invention to provide a single filter for compressed 
gas having multiple filter elements which can filter a plurality of 
different types of contaminants from the gas. 
It is an object of the invention to provide a filter having an inverse flow 
regime. 
It is another object of the invention to provide a filter which allows for 
visual inspection of the individual filter elements. 
It is a further object of the invention to provide a filter wherein the 
individual filter elements are readily and easily replaced independently 
of one another.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, a filter 10 according to the invention comprises a connection 
head 12 formed by a peripheral wall 13 having a lower opening 14 
oppositely disposed from an upper opening 16 separated by a partition 18 
best seen in FIG. 2. Openings 14 and 16, in conjunction with partition 18, 
form lower receptacle 15 and upper receptacle 17 for receiving filter 
elements described below. Connection head 12 further has a compressed gas 
inlet 20 which communicates with a gas inlet passageway 22 extending 
through head 12 to a first side 18a of partition 18. Connection head 12 
further has a gas outlet passageway 24 extending from a second side 18b of 
partition 18, the passageway communicating with a gas outlet 26. 
Preferably, gas inlet 20 and gas outlet 26 are coaxially aligned on 
opposite sides of connection head 12, thus allowing filter 10 to be 
readily inserted into a straight run of a compressed gas line "G" without 
modifying the gas line by jogging the line to account for inlet and outlet 
misalignment on the filter connection head 12. Partition 18 has an array 
of spaced apertures 28, best illustrated in FIGS. 2 and 3, apertures 28 
providing communication between lower receptacle 15 and upper receptacle 
17. Connection head 12 is preferably formed from a strong and durable 
material such as steel or an aluminum alloy and may be cast or machined. 
Lower opening 14 is enclosed by a first elongated tubular housing 30 which 
is releasably attached to head 12, preferably by integral screw threads 32 
at one end of housing 30 engaging mating screw threads 34 disposed around 
lower opening 14 as seen in FIGS. 2 and 3. First housing 30 sealingly 
engages head 12 by means of an o-ring seal 36. When housing 30 engages 
lower opening 14 o-ring 36 is compressed between partition first side 18a 
and a lower housing sealing surface 38 disposed on lower housing 30 
adjacent to integral screw threads 32. Similarly, upper opening 16 is 
enclosed by a second elongated tubular housing 40. Second housing 40 is 
releasably attached to head 12 via integral screw threads 42 on housing 40 
engaging mating screw threads 44 disposed on head 12 around opening 16 as 
illustrated in FIGS. 2 and 3. Second housing 40 sealingly engages head 12 
via an o-ring 46 retained within an annular cavity 48 circumferentially 
disposed adjacent to integral screw threads 42. O-ring 46 is compressed 
between cavity 48 and a raised sealing surface 50 disposed around upper 
opening 16 to effect the seal as best seen in FIG. 2. 
Preferably, second housing 40 is formed from a transparent material, such 
as polycarbonate, allowing individual filter elements, described below, to 
be viewed for inspection and maintenance. First housing 30 is formed from 
the same material as connection head 12. 
As shown in FIG. 2, at least two filter elements are disposed within filter 
10 on opposite sides of partition 18. A first filter element 52 is 
positioned below partition first side 18a within first housing 30. Filter 
element 52 is disposed within the bore of a tube 54 seen in detail in FIG. 
3. Tube 54 has a gas impermeable sidewall 56 attached to a cruciform 
mounting structure 58 located at one end of the tube as best shown in 
FIGS. 3 and 5. Mounting structure 58 has a threaded hole 60 at its center 
which engages a mounting stud 62 which projects substantially 
perpendicularly from the center of partition 18. Engagement of the threads 
of mounting stud 62 with the threads of hole 60 allows tube 54 to be 
quickly and easily releasably mounted on partition 18. A raised lip 64 
extends from the mounting end of tube 54 and sealingly surrounds aperture 
array 28 in partition 18 by engaging first partition side 18a when tube 54 
threadedly engages stud 62. The opposite end of tube 54 is open to allow 
gas to enter and pass through the filter elements as described below. 
A second filter element 66 or 66a (illustrated in FIGS. 3 and 3a) is 
disposed within second elongated tubular housing 40 as seen in FIG. 2. 
Both types of second filter elements 66 and 66a have a bottom 68 
preferably composed of a pliant material such as rubber or plastic. The 
bottom, shown in detail in FIGS. 3 and 3a, comprises a filter sealing 
surface 70, gas apertures 72 to permit gas to enter filters 66 and 66a and 
a threaded mounting hole 74. Preferably, threaded stud 62 passes through 
the partition 18 and projects upwardly so that it can be threadedly 
received within the hole 74. Filter sealing surface 70 sealingly engages 
second partition side 18b and surrounds aperture array 28 in partition 18. 
A third filter element 76 is preferably placed within tube 54 interposed 
between first filter element 52 and mounting structure 58. With multiple 
filter elements 52, 66 and 76 within the same filter 10 it is possible for 
one filter 10 to remove a plurality of different types of contaminants 
from a compressed gas as described in detail below. 
In the preferred embodiment first filter element 52 is a coalescing filter 
comprising a bed of stainless steel mesh for removing water aerosol (water 
droplets) entrained in the compressed gas. Third filter element 76 
comprises a particulate filter, such as a bed of cotton polyester yarn and 
stainless steel wire for trapping particulates such as pollen, dust, smoke 
and the like. The second filter element desirably comprises an oil 
coalescing filter, such as a borosilicate glass microfibers cylinder 67 
for removing oil aerosols (illustrated at 66 in FIG. 3). Alternately, the 
filter element could comprise a water desiccant filter, as illustrated at 
66a in FIG. 3a. Filter element 66a contains desiccant material 69, 
preferably aluminum oxide fired at high temperatures as marketed, for 
example, by ALCOA Industrial Chemicals Division as Product No. F200. 
Alternative materials include molecular sieves, silica gel and various 
disposables, as are known in the art. However, silica gel is a less 
desirable material in that it tends to fractionate and disposables are 
expensive to dispose of due to environmental concerns and regulations. The 
operation of these filter elements is described below. It is preferable 
that the second filter element have calorimetric properties, i.e., that 
the filter element undergo a visual color change as it becomes saturated, 
indicating when the filter element should be changed. 
Filter Operation 
As seen in FIG. 2, contaminated compressed gas flows through gas line G and 
enters compressed gas inlet 20 in connection head 12 and passes further 
through gas inlet passageway 22 where the gas stream exits passage 22 and 
enters an annular space or duct 78 between tube sidewall 56 and first 
tubular housing 30. Because tube 54 is sealingly attached to partition 18 
and sidewall 56 is impermeable the gas must flow lengthwise of tube 54 
into first housing 30 and then reverse direction to continue into the open 
end of tube 54. The ducting action of sidewall 56 and first housing 30 
creates an inverse flow regime within the filter 10 whereby the 
contaminated gas stream must abruptly reverse direction to pass through 
the filter elements. The reversal of the gas stream serves to separate 
water droplets entrained in the gas. The gas also undergoes a change in 
velocity, slowing down as it transitions from the relatively small 
cross-sectional area of duct 78 to the relatively large area of tube 54. 
The velocity reduction aids in the condensing of water vapor from the gas 
and reduces the likelihood of condensed droplets becoming entrained in the 
gas. The water thus separated accumulates at the bottom of housing 30 
which is equipped with a drain valve 80. Drain valve 80 has a threaded 
nipple 82 which passes outwardly through an aperture 84 in the bottom of 
housing 30. Aperture 84 is sealed by a gasket 86 compressed between drain 
valve 80 and the bottom of housing 30 by the action of retaining nut 88 
which pulls valve 80 against gasket 86 when retaining nut 88 is tightened. 
Water which accumulates at the bottom of housing 30 is drained by opening 
drain valve 80, which is preferably a float type valve which opens 
automatically when the water reaches a predetermined level. Drain valve 80 
is otherwise normally closed. 
Because considerable water is separated from the gas due to the inverse 
flow regime it is highly desirable to orient filter 10 vertically as shown 
in FIGS. 1 and 2 by positioning first housing 30 vertically below 
connection head 12 and have the gas first pass downwardly into housing 30 
and then reverse direction and flow upwardly into the open end of tube 54 
where the gas encounters the first filter element 52. Water droplets still 
entrained in the gas after the flow reversal are separated by coalescing 
action as the gas stream flows upwardly through the stainless steel mesh 
bed of filter element 52. Separated water drains from filter element 52 
into the bottom of housing 30 where it is in turn removed from housing 30 
through drain valve 80 as described above. 
The gas stream next encounters third filter element 76 where particulate 
matter is entrapped in a bed of cotton polyester mesh and stainless steel 
wire. Leaving filter element 76, the gas, free of condensed water and 
particulate contaminants, passes by mounting structure 58 and through 
aperture array 28 in partition 18, as seen in FIGS. 2 and 5. Because 
filter sealing surface 70 sealingly surrounds aperture array 28 on the 
second partition side 18b the gas must pass upwardly through gas apertures 
72 and into second filter element 66 as seen in FIGS. 2 and 4. 
The water vapor still entrained in the compressed gas as the gas leaves 
filter element 76 may be removed through the use of a water desiccant 
media as second filter element 66a. Second filter element 66a comprises a 
transparent gas impermeable container 71 attached to filter bottom 68. The 
container holds a bed of desiccant material 69, preferably comprised of 
aluminum oxide spheres. Gas containing water vapor enters filter element 
66a through gas apertures 72 and is forced upwardly through the entire 
desiccant bed, exiting container 71 through exit vents 73 at the top of 
the container. As the gas passes through the desiccant, the water vapor is 
adsorbed and retained in the filter; the compressed gas, now free of water 
aerosols, particulates and water vapor, flows within second elongated 
housing 40 into gas outlet passage 24 and exits filter 10 through gas 
outlet 26. 
When the second filter element 66a is a desiccant filter to remove water 
vapor, it will periodically become saturated with moisture and will no 
longer effectively remove water vapor from the gas. Preferably, a 
colorimetric material, such as silica gel beads coated with cobalt 
chloride, is used to indicate saturation. The beads change color from blue 
to pink to white upon saturation giving a visual indication that the 
filter element 66a requires changing. Second housing 40 is made of a 
transparent material to allow maintenance personnel to observe the color 
of the beads within filter element 66a without removing housing 40. Filter 
10 is furthermore designed to enable the filter elements to be changed 
independently of one another by simply unscrewing the respective housing 
(30 or 40) from connection head 12, unscrewing the spent filter element 
(66a or tube 54) from mounting stud 62, replacing the spent filter element 
with a new filter element and reattaching the respective housing. 
It is frequently desirable to remove oil from the gas, and for this 
purpose, the second filter element comprises an oil coalescing filter, as 
illustrated by filter element 66. Oil aerosols are effectively removed by 
means of a borosilicate glass microfibers cylinder 67 comprising filter 
element 66 as shown in FIGS. 2 and 3. Oil coalescing on the borosilicate 
fibers causes the normally white fibers to discolor, thus visually 
signaling when the filter element needs replacing. 
Filter 10 is preferably provided with a mounting bracket 90 which is 
attached to connection head 12 and allows filter 10 to be supported on an 
external structure 92 as illustrated in FIG. 1. The bracket permits 
vertical orientation of the filter 10 and prevents the weight of the 
filter from being borne on gas line G. 
Filter 10 can be installed in a branch of a compressed air system to supply 
contaminant free compressed air for specific applications. The capability 
of filter 10 to remove a plurality of contaminants from the compressed air 
allows filter 10 to replace multiple individual filtering units designed 
to filter out only one specific type of contaminant. Preferably, filter 10 
is disposed in close proximity to the station at which the compressed air 
will be used, for example, at a spray painting booth. Filter 10 is further 
advantageously positioned to permit maintenance personnel easy access to 
inspect the condition of the various filter elements within the filter and 
readily replace the elements as needed.