Abstract:
An air drying system for a vehicle including a membrane air dryer is provided. The air dryer is supplied with a stream of compressed air that has a low and constant concentration of oil vapor. A coalescing filter element may be used to maintain the oil vapor concentration. The filtered material may be discharged from the coalescing filter by a pressure swing drain valve, which discharges the filtered material upon unloading of the compressor.

Description:
FIELD OF THE INVENTION 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 10/274,215 filed on Oct. 18, 2002 for MEMBRANE AIR DRYER AND METHOD OF MOUNTING A MEMBRANE DRYER TO A VEHICLE, the entire disclosure of which is hereby incorporated by reference. 
     The invention relates to a vehicle air dryer and more specifically to a membrane air dryer for a vehicle. 
     BACKGROUND OF THE INVENTION 
     In air brake systems, such as those implemented in commercial trucks, compressed air from the vehicle compressor is dried prior to delivery to the vehicle brakes and other compressed air operated components. The compressed air is dried to make the vehicle brakes operate effectively, and also to prevent damage to the brakes and other downstream components. Typically, an air dryer with a desiccant bed is placed between the compressor and the brakes. The desiccant bed air dryer removes moisture from the compressed air thereby permitting only dried compressed air to continue downstream to the brakes and other components. 
     Several problems exist with desiccant bed air dryers. First, the desiccant degrades as it becomes contaminated with organic components, such as oil vapor or aerosols, for example. As such, the desiccant bed must be periodically changed in order to maintain an effective air dryer. Second, desiccant beads have a saturation point, beyond which no additional moisture can be absorbed. In order to maintain an effective desiccant air dryer, a periodic, purge of the desiccant is required in order to keep the desiccant from becoming saturated. In typical desiccant bed air dryers, a purge cycle is established when the compressor unloads, thereby allowing the desiccant to regenerate. In some situations, the compressor is required to maintain a lengthy cycle before it can be unloaded. These situations arc somewhat frequent in commercial vehicles, such as during rapid braking and use of ancillary air powered equipment. During these long compressor cycles, the compressor charging duration can exceed the desiccant water absorption capacity and subsequent wet air can pass downstream to the brake system, or other components, where it can cause damage or other problems. 
     While it is known that membrane air dryers can produce a steady stream of dried air without requiring periodic change or periodic purging, membrane air dryers have generally only been implemented in clean air systems. Contaminants, such as oil aerosols, hinder the performance of membrane air dryers since the hydrophilic membrane tubes are resistant to organic components. The organic contaminants enter the membrane and foul up the surface of the membrane. This fouling of the membrane by organic contaminants prohibits air from passing through the membrane, thereby reducing the effectiveness of the membrane air dryer. As such, membrane air dryers have not been used in connection with vehicles. 
    
    
     SUMMARY OF THE INVENTION 
     An air drying system for a compressed air system of a vehicle is provided. The system includes a membrane air dryer and a means for maintaining a fairly constant and fairly low oil vapor concentration in the compressed air system. In one embodiment, a coalescing oil filter is used to maintain the oil vapor concentration below a threshold level. Another embodiment includes a pressure swing drain valve that actuates upon unloading of the vehicle compressor to discharge the filtered material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The membrane air dryer of the present invention may be more readily understood by reference to the following drawings. While certain embodiments are shown as illustrative examples of the membrane air dryer, the scope of this application should not be construed as limited to these illustrative examples. 
         FIG. 1  is a cross-sectional side view of a membrane air dryer system of the present invention; 
         FIG. 2  is a cross-sectional side view of a membrane air dryer; 
         FIGS. 3A and 3B  are close-up cross-sectional views of the discharge and feed ends of a membrane air dryer; and 
         FIG. 4  is a cross-sectional view of the membrane air dryer shown in  FIGS. 3A and 3B , wherein the cross-section is taken through A—A. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The vehicle air brake system membrane drying system  10  generally consists of a coalescing filter element  20 , a membrane air dryer  30 , and a mounting arrangement or means for attaching the membrane air dryer to a vehicle  35 . While the details of the air brake system membrane drying system  10  are disclosed herein for a preferred embodiment of the invention, one skilled in the art should appreciate that the element of the membrane drying system  10 , namely the coalescing filter element  20 , a membrane air dryer  30 , and a means for attaching the membrane air dryer to a vehicle  35 , may take on a number of different embodiments. It should be understood that these additional embodiments are intended to be a part of this invention to the extent such embodiments fall within the scope of the claims. 
     The coalescing filter element  20  may be a single stage coalescing filter, such as, for example, a Bendix PURAGUARD™. The filter element  20  may include a filter housing  40 , a pressure swing drain valve  42 , such as, for example, Bendix DV-2, and a restriction orifice  44  within the filter element housing  40 . As shown in  FIG. 1 , air from a compressor enters through orifice  46  and then is forced through the filter element  20 . Air exists the filter element  20 , greatly reduced in oil vapor and aerosol concentration into filter space  48  prior to exiting the filter through exit port  50 . 
     A pressure swing drain valve  42  may be attached to the lower portion of the filter element housing  40 , thereby taking advantage of gravitational forces to assist in the collection of condensed fluids or heavier gases. One skilled in the art should appreciate that the pressure swing drain valve  42  may be located in other locations along or about the filter housing. Furthermore, the pressure swing drain valve may be integral with the filter housing, or attached as a separate unit. Additionally, the drain valve does not need to be pressure swing driven, and thus may be actuated by any other means, such as, for example, a manual drain valve or time-release drain valve. However, the use of a pressure swing drain valve is preferred since each time the compressor unloads, or stops compressing air, pressure from the discharge line falls thereby activating the pressure swing drain valve  42  which expels liquid water and/or oil from the brake system. At the connection between the pressure swing drain valve  42  and the filter space  48 , a restriction orifice  44 , or some other means for restricting air flow, may be placed in order to prevent pressure pulses from the compressor from activating the pressure swing drain valve  42  with each piston stroke. An orifice of about 0.046 inches is preferred. 
     The use of a coalescing filter  20  has been determined to facilitate in the drying of the compressed air by the membrane air dryer  30 . A coalescing filter, such as the Bendix PURAGUARD™ has the capability to maintaining the oil vapor and oil aerosol concentration in the compressed air stream at a fairly consistent level. Unlike conventional desiccant dryers which continue to degrade with each exposure to contaminants, such as oil vapor, a membrane air dryer will continue to produce approximately the same results as long as the oil vapor concentration is maintained at a fairly constant level below a threshold level, which is defined by the desired air drying results. Since the membrane air dryer is typically composed of hydrophilic elements, organic compounds, such as oil, build up on the surface of the membrane. At a given concentration, the air passing over the oil condensed on the surface of the membrane will push the oil through the membrane, thereby freeing the membrane for drying. As such, there is an equilibrium concentration of oil wherein the air drying capability of the membrane is fairly constant. The coalescing filter element  20  is used to maintain the concentration of oil vapor at an equilibrium level that will produce air of a predetermined dryness. Additionally, the coalescing filter  20  may reduce the concentration of water vapor in the compressed air stream, thereby reducing the load on the membrane air dryer  30 . 
     Filtered air leaves the filter element  20  through exit port  50  and passes to the membrane air dryer  30  through the supply port  60  located in the supply end cap  62 . The compressed air, after traveling through the plurality of membrane air dryer hollow fibers  64 , as shown in  FIG. 4 , is now dry and collected in the delivery volume  66  located in the delivery end cap  68 . The dried compressed air in the delivery volume  66  is either fed through the membrane air dryer core  70  as backflow, or through a delivery check valve  72  to the air supply tanks, brakes, or other downstream components. The backflow travels along the outside of the membrane air dryer fibers  64 , collects the water vapor and vents to the atmosphere through vent holes  74 . Preferably, the portion of air that is used as backflow is between about 15 and 20 percent of the total dried air flow. In the embodiment shown in  FIG. 3A , a metering orifice  76  is used to control the backflow rate. 
     Materials suitable to achieve the desired membrane morphology include those polymeric materials typically known for use in making permeable membranes exhibiting the specified morphology. Examples of the polymeric materials from which the hollow fibers are formed include, but are not limited to, polysulfones, polyethersulfones, polyarylsulfones, polycarbonates, nylons, polyimides, polyvinylidenedifluorides, polyvinylidenedichlorides, cellulosics, polyacrylonitriles, and others known to make permeable membranes, used alone or in combination. Inorganic materials capable of forming an appropriate membrane structure may also be suitable. Examples of such inorganic materials include, but are not limited to, ANOPORE® membranes, which are anodic aluminum oxide flat sheet membranes. ANOPORE® membranes are commercially available from Whatman, Inc. in Clifton, N.J. Other suitable flat sheet membranes may also be used. The desired morphology may be achieved with the suggested materials by a number of processing and formation techniques, among them phase inversion, thermal induced phase separation (TIPS), track-etching, or other known or envisioned means of fabricating permeable membranes with appropriate selective permeability. Further suitable membranes are disclosed in U.S. Pat. Nos. 5,525,143 and 6,083,297, each of which is incorporated herein by reference in its entirety. 
     The means for attaching the membrane air dryer  30  to a vehicle can take on a number of embodiments, such as, for example mounting bolts  78  through the vehicle frame  80  as shown in FIG.  1 . Further mounting arrangements are disclosed in commonly-owned co-pending U.S. application Ser. No. 10/274,215. Such mounting arrangements may also be dictated by the structure of the coalescing filter  20  and membrane air dryer  30 , such as an embodiment wherein the coalescing filter  20  and membrane air dryer  30  are integral. 
     It will be appreciated that the membrane air dryer assembly can take the form of various configurations and mounting arrangements. It should be further understood that the membrane air dryer and corresponding end caps can be used either with new equipment, or retrofit to attach to existing components. Such existing components may cause minor alternations to the design of the membrane air dryer; however one skilled in the art should appreciate that these minor modifications fall within the scope of this application. This invention is intended to include such modifications and alterations in so far as they fall within the scope of the appended claims or the equivalents thereof.