Drop Legs for Compressed Air Lines

The importance of assuring that compressed air is as clean and dry as possible before using it in any type of application is universally appreciated. The process of compressing air not only concentrates impurities in the atmosphere, but adds many more, rendering the final product totally unacceptable for many of the very wide range of applications involving the use of compressed air.

There are many divergent methods available to remove contaminates from compressed air and most of them consist of passing the air through a barrier. The barrier may be chemical or mechanical, but it will always have a limited period of effectiveness, the period being governed by the time it takes to become saturated with contaminates. When this happens the barrier must be cleaned or replaced, involving maintenance time and costs, or the contaminates will be forced into the air line. Anything which will prolong the effectiveness of the barrier is not only a cost saver but also an assurance of a more consistant supply of properly filtered air.

The most basic and effective method of removing the bulk of contaminates carried in an air flow is the use of a drop leg. This strictly mechanical device never clogs up, never wears out and, apart from having to have the accumulation of contaminates drained off, requires no maintenance. It uses a natural phenomena of air flow to separate gas and contaminate molecules and the greater the flow, the more efficient it becomes.

Drop legs have been in use since the first compressed air lines were installed. They originated in a similar capacity for steam lines and were converted to use with compressed air without any major changes. Their effectiveness is qualified by the fact that most suppliers of pneumatic equipment, including filters and dryers, recommend that a drop leg be installed prior to their equipment.

A conventional drop leg consists of a single tube carried past the point of take off of an air drop, an air drop merely being the pipe which carries the air down to working level from the overhead main pipe, or "leader". The diameter of the down pipe is usually expanded below the take-off point, capped and fitted with a drain cork. These legs function due to the fact that air molecules travel in a straight line and, having been directed into a flow path by the down pipe, continue in that path until they impinge on the cap at the base of the tube. The gas molecules rebound and continue to move with undiminished energy, but anything which has no energy of its own, usually such as water, dirt and oil, is left at the point of impingement.

This is, of course, a "catch as catch can" arrangement and rapidly loses its original efficiency when any accumulation of contaminates occurs. The turbulence then actually re-introduces the lighter contaminate matter into the air flow, leaving only the heavier matter at the base of the tube. This is evident when draining a drop leg and a heavy sludge is drawn off. Nevertheless, this is the very stuff which is most dangerous to filter screens and tools so the leg serves a very worthwhile purpose.

It has been proposed to provide drop legs with a separate air outlet. As far as we are aware, such air outlets have required the air, after striking the impingement surface to pass downwardly below that surface in a turbulent manner such as to cause reentry into the outlet air stream of at least the lighter contaminants which may have been removed. See U.S. Pat. Nos. 2,467,408 and 3,345,807.

The general objective of the present invention is to provide drop legs for compressed air lines of greatly increased efficiency in removing contaminants from the air.

In accordance with one aspect of the invention, this objective is attained with drop legs each of which has an outlet above the impingement surface to receive directly rebounding air and continue its upward course and at the same time draining separated contaminants into a zone isolated from turbulence. Each such drop leg has a straight inlet tube disposed to discharge air downwardly against the impingement surface and of sufficient length to ensure that the air molecules and entrained matter are all travelling in a straight line, the tube length approximately fifteen times the internal diameter of the tube. Best results are attained with the impingement surface inclined inwardly and downwardly in a manner such that rebounding air is directed generally towards the outlet. In practise the outlet is a straight length of tubing which may include a filter. The upper ends of the inlet and outlet tubes are held by a yoke.

While such drop legs effectively collect contaminate matter, matter in vapor form presents a problem and another aspect of the invention is the provision of drop legs which also enable such vapors to be removed. To that end, the drop leg has an impingement plate which is a free fit within the housing and a bulkhead below the impingement plate which is sealed to the housing and establishes an upper chamber vented to the atmosphere and a lower chamber. The drain opens into the lower chamber. The periphery of the impingement plate and the housing establish an orifice such that air can flow into the upper chamber but with an attendant pressure drop thus effecting the cooling of the impingement plate, condensing water vapor and coalescing oils. Where required, the dimensions of the orifices of such drop legs may be varied.

Another objective of the invention is to provide a housing well suited for drop legs of either of the above types. In accordance with this aspect of the invention, the housing consists of a length of tubing and upper or top and lower or base plates dimensioned to fit in the ends of the tube to the extent permitted by shoulders with which the end plates are formed. A tie rod extends axially through the tubing and one end is threaded into and sealed to the top plate and its other end extends freely through and is sealed to the base plate against leakage. A nut threaded on the exposed end of the bolt holds the assembly together.

Where only an impingement plate is needed, the plate has an axial bore to receive the tie bolt and where a bulkhead is also required an axial bore for the tie bolt extends through both the impingement plate and the bulkhead.

Another objective of the invention is to combine the impingement plate and the bulkhead into a module including a support to seat on the base plate with the drain an axial passageway through the module freely receiving the tie bolt.

Another important objective of the invention is to provide for the adjustment of the dimensions of the orifice. An objective attained by providing the tube with a step overlying the periphery of the impingement plate and defining therewith the orifice and by providing means to effect the movement of the impingement plate towards or away from the step to effect wanted changes in the dimensions of the orifice. A particular objective is to utilize the tubular housing and a module, both described above with a compression spring between the impingement plate and the top plate of the housing to hold the module seated against the bottom plate. The bottom plate and the bulkhead are slidably sealed to the housing. Accurate adjustments of the orifice are effected by utilizing the turning of the tie bolt nut in one direction to permit the slight movement of the base plate away from the top plate required to enable the spring to effect the slight repositioning of the impingement plate and in the other direction to advance the base plate towards the top plate against the resistance of the spring to effect the wanted repositioning of the impingement plate.

Other objectives of the invention and the manner in which they are attained will be apparent from the description of presently preferred embodiments and the claims.

The accompanying drawings illustrate presently preferred embodiments of the invention of which

FIG. 1 is a vertical section, on an increase in scale, of a drop leg filter in accordance with one such embodiment of the invention with the inlet and outlet tubes broken away to foreshorten the figure;

FIG. 2 is a side elevation of a drop leg filter in accordance with another such embodiment; and

FIG. 3 is a section taken substantially along the indicated line 3--3 of FIG. 2.

The embodiment of the invention illustrated by FIG. 1 has a housing, generally indicated at 5, shown as consisting of a tube 6, an upper or top end plate 7 and a bottom end or base plate 8. The end plates are dimensioned to fit within the ends of the tube 6 and are provided with seals 9 and formed with shoulders 10 against which the ends of the tube 6 may be seated. The housing 5 is completed by a tie bolt 11 which extends axially through the tube 6 with one end threaded in and sealed to the upper end plate 7 and the other end extending freely through the bottom end plate 8 and provided with a seal 12. A nut 13, such as an elastic or self locking nut, is threaded on the exposed end of the bolt 11 into contact with the bottom end plate 8 until the shoulders 10 butt against the ends of the tube 6. It is important to note that the housing will remain assembled if the nut 13 is backed off to a limited extent as the seals 9 will then frictionally hold the tube 6.

The top plate 7 has ports 14 and 15 which are located on opposite sides of the center line of the housing 5. A straight rigid tube 16 is sealed in the port 14 with its upper end extending through a yoke 17 and provided with a fitting 18 for use in connecting to it the upstream section, not shown, of a compressed air line. The length of the tube 16 is such that the molecules of and entrained matter in the air stream discharged into the housing 5 will be travelling in straight lines. A set screw 19 serves to lock the yoke 17 to the tube 16.

A like but somewhat shorter tube 20 is secured in the port 15 with its upper end sealed in a bore 21 in the yoke 17. The yoke 17 has a laterally opening port 22 which intersects the bore 21 and is dimensioned to enable the downstream section, not shown, of the compressed air line to be coupled thereto.

A module, generally indicated at 23, is confined within the housing 5 and includes an impingement plate 24 having a downwardly and inwardly inclined surface 25 directly exposed to the ports 14 and 15 and spaced from the top plate 7 to establish a separating chamber, generally indicated at 26. The impingement plate 24 is a free fit within the housing to provide an orifice 27. The module 23 also includes a bulkhead 28 provided with a seal 29 and is a sliding fit in the tube 6. The module has an axial hub 30 which extends below the bulkhead 28 and rests on the bottom plate 8. The bulkhead 28 thus establishes with the impingement plate 24 an upper chamber, generally indicated at 31 and with the bottom plate 8 a lower chamber, generally indicated at 32. The tube 6 has vents 33 opening from the upper chamber 31 to atmosphere. The hub 30 has an axial bore 34 which freely receives the bolt 11 and establishes a drain leading from the center of the surface 25 and opening through ports 35 into the lower chamber 32 which serves as a settling chamber for separated contaminants.

The tube 6 is provided with an internal step 36 in the separating chamber 26 which overlies the upper periphery of the impingement plate 24 to define therewith an annular orifice 37 dimensioned to permit a limited air flow from the separating chamber 26 into the subjacent chamber 32 such that it is attended by a pressure drop and the resultant cooling of the impingement plate 24 and also the bulkhead 28.

The downwardly and inwardly inclinded surfaces 25 are directly exposed to the inlet air stream with the inclination of the now cooled surfaces such that the air stream rebounding from them is directed generally towards the outlet port 15 with separated particles and condensates entering the bore 34, the effective dimensions of which are the annular space surrounding the tie bolt 11, and discharged into the settling chamber 32 which is either continuously or periodically drained through a port 37 provided in the bottom plate 8.

As the disclosed embodiments provide for the adjustment of the orifice 37 as required by operating conditions, the dimensions of the orifice 27 may be relatively large. Adjustments of the orifice 37 are effected by small axial adjustments of the position of the module 23 and accordingly of the impingement plate 24 relative to the step 36 to decrease or widen the orifice 27. For that purpose, a compression spring 39 in the chamber 26 surrounds the bolt 11 and is seated on the surfaces 25 of the impingement plate 24 close to the drain and is backed by the top plate 7 between the ports 14 and 15.

By turning the tie bolt nut 13 in the loosening direction, the spring 39 is operable to force the module 23 and the end plate 8 away from the top plate 7 and by turning the nut 13 in a tightening direction, the base plate 8 and the module 24 are moved against the resistance of the spring 39 towards the top plate 7 thus enabling fine adjustments of the orifice to be accurately made.

As the embodiment of the invention illustrated by FIGS. 2 and 3 is generally similar to that illustrated by FIG. 1, only different or added features will be detailed but corresponding parts are identified by the same reference numerals distinguished, however, by the suffix addition "A".

In FIGS. 2 and 3, the outlet tube 20A is of reduced length and is connected to the base plate 40 of a filter, generally indicated at 41.

The yoke 17A has a vertical bore dimensioned to receive a holder 42 having an end extending below the yoke 17A and provided with a socket 43 intersected by the laterally opening outlet port 44 on opposite sides of which there are seals 45 carried by the holder 42.

The filter 41 consists of a tube 46 with the base 40 and an annular head 47 fitted within and sealed thereto. The head 47 fits on the exposed end of the holder 42 and into an annular seat 48 surrounding the lower end of the bore 41.

A filter element 49 surrounds and is spaced from a supporting tube 50 having lengthwise slots 51 with one tube end within the socket 43 and its other end supported by a headed connector 52 seated in and closing an axial bore 53 in the base plate 40. It will be noted that the outlet tube 20A opens into the filter 41 at one side of the filter element.

From the foregoing, it will be apparent that drop legs in accordance with the invention, with or without filters are not only extremely efficient in use but also are well adapted to meet manufacturing requirements.