Abstract:
An omnidirectional vent cap for venting holding tanks is provided. By providing a 360 degree venturi channel, the present invention can use a small movement of air to reduce the air pressure above the exhaust port to draw out vapors or gasses. Since the device is symmetrical about the central axis the movement of air can be from any direction relative to the device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/994,468 filed Nov. 23, 2004, which is pending. 
    
    
     TECHNICAL FIELD 
     The application relates to the field of ventilating caps for ventilating exhaust vents, stacks, chimneys or roof top vents for recreational vehicle holding tanks and the like; and to devices for preventing downdrafts or wind from blowing down a stack or chimney. 
     BACKGROUND 
     Ventilator caps are widely used to facilitate venting from gas exhaust outlets on building roofs and transportation vehicles. For example, recreational vehicles utilize a holding tank for storage and processing of sewage (black water) and/or grey water. Such holding tanks are generally vented to the exterior atmosphere by a vent pipe which typically exits the vehicle through the roof of the vehicle. A cap is provided on the vent pipe to prevent objects from entering the pipe. However, the unpleasant odors can accumulate in the holding tank and seep back into the living area of the vehicle. 
     To improve and facilitate the exhaust of gases from the holding tank, a rotating vent cap resembling a wind vane has been used to take advantage of wind movement. Such vent caps are pivotally connected to the outlet of the vent pipe and have a vane which causes the outlet opening to point downwind, thereby drawing vapors from the holding tank. A drawback of such vent caps however is that they require a certain minimum wind velocity to function effectively, in particular at least enough wind velocity to pivot the wind vane. In the absence of that minimum wind force, the device may not function. 
     Other venting apparatus are shown in the following U.S. Pat. No. 55,094 Hardy; U.S. Pat. No. 3,302,552 Walsh; U.S. Pat. Nos. 3,347,147 and 3,382,792 Howard; U.S. Pat. No. 3,509,811 Topp; U.S. Pat. No. 4,531,455 Palmer; and U.S. Pat. No. 4,603,619 Amphoux. 
     It is therefore desired to provide a vent cap which functions to improve the exhaust of gasses from holding tanks, attics, stacks, chimneys or the like omni-directionally with no moving parts and even in conditions of low velocity air movement. 
     SUMMARY OF INVENTION 
     An omnidirectional vent cap for venting holding tanks is provided. The invention provides a vent cap for venting an exhaust gas passage, comprising: i) a base having a central opening for communicating with the exhaust gas passage and an outer perimeter, and a ramp surface extending between the outer perimeter and the central opening, the ramp surface sloping upwardly from the outer perimeter to the central opening; ii) a top cover element supported above the base, having a diameter greater than the diameter of the central opening, having an outer edge and a convex lower surface extending at its lowest point below the outer edge to thereby in combination with the base cause a low pressure effect above the exhaust gas passage when air moves through the vent cap; and iii) means for supporting the top cover element above the base. 
     According to one aspect of the invention the ramp surface has an increasing slope as the distance from the central opening decreases. According to a further aspect the invention further comprises a plurality of spaced, radially extending vanes projecting upwardly from the upper ramp surface. According to a further aspect the top cover element can be removed and replaced onto the base. 
     By providing a 360 degree channel, the present invention can use a small movement of air to significantly reduce the air pressure above exhaust port to draw vapors or gasses from the exhaust pipe. Since the device is symmetrical about the central axis the movement of air can be from any direction relative to the device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In drawings which illustrate a preferred embodiment of the invention: 
         FIG. 1  is a perspective view of a first embodiment of the vent cap of the invention: 
         FIG. 2  is a front elevation view of a second embodiment of the vent cap having a variation in the shape of the support legs; 
         FIG. 3  is a detail view in front elevation of the base of the vent cap shown in  FIG. 2 ; 
         FIG. 4  is a top view of the base shown in  FIG. 3 ; 
         FIG. 5  is a cross-section of the cap shown in  FIG. 2  taken along lines  5 - 5  of  FIG. 6 ; 
         FIG. 6  is a top view of the vent cap shown in  FIG. 2 ; 
         FIG. 7  is a cross-section view showing a recreational vehicle holding tank; 
         FIG. 8  is a perspective view showing the installation of the vent cap shown in  FIG. 2 ; 
         FIG. 9  is a front elevation view of a third embodiment of the vent cap having a further variation in the shape of the support legs; 
         FIG. 10  is a perspective view from above of a fourth embodiment of the invention; 
         FIG. 11  is a perspective view from above of the embodiment of the invention shown in  FIG. 10  rotated slightly; and 
         FIG. 12  is a cross-section of the embodiment of the invention shown in  FIG. 11  taken along lines  12 - 12 . 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
       FIG. 7  illustrates a holding tank  10 , containing sewage  14 , and having an air inlet  16  and a vent pipe  18  extending through roof  20 . The vent cap  22  of the invention is shown in  FIG. 1 . It has a base  24 , top  26  and support legs  28  joining the base  24  and top  26 . It is preferably formed of molded plastic but could be constructed of other synthetic or natural materials including wood and metal or combinations of such materials. 
     Top  26 , as shown in  FIG. 6 , serves as a cap to prevent rain or debris from entering vent pipe  18 . It has a convex or domed upper surface  30  which provides an aerodynamic shape and permits water and debris to run off. Top  26  has a convex lower surface  32  which forms the upper surface of the channel formed between base  24  and top  26 . The central area or apex  33  of convex lower surface  32  (see  FIG. 5 ) extends below the outer edge  31  of upper surface  30  by a distance A. Slender legs  28 , that is, legs whose number and thickness are preferably minimized to reduce air turbulence and whose shape is similarly chosen to reduce air turbulence, support top  26  at the appropriate height above base  24 . It has been found that a useful minimum separation of base  24  from top  26  is about ½ the length of the ramp surface  40  as described below. 
     Base  24 , as shown in  FIG. 1-4 , is circular in top view and has a central cylindrical opening  34  for receiving the vent pipe  18 , as shown in dotted outline in  FIG. 3 . The bottom surface  36  of base  24  is flat. The circular outer circumference of base  24  forms a generally vertical rim or edge  38 . A radially extending ramp  40  is formed on the upper surface of base  24  between the outer edge  38  and central opening  34 . The surface of ramp  40  is curved, with a smoothly increasing tangential angle, being almost flat at edge  38  to about 80 degrees at opening  34 . The vertical height of the ramp rises about one-half its surface distance over the curvature of the ramp from outer to inner edge. The radial length of the ramp is preferably roughly the same as the radius of opening  34 . The shape of ramp  40  causes the moving air to be accelerated upwardly away from the exhaust port  34 . The shape of ramp  40 , in combination with the shape of surface  32 , causes a low pressure effect above exhaust port  34  causing the vapors or gasses to be drawn up the stack or chimney from the source to be vented. 
     To further improve the operation of the device, radial channeling vanes  48  which extend vertically from ramp  40  may be provided. Vanes  48  serve to focus and direct the air flow toward the center of the device, reducing the amount of air that slips around the side of the raised inner area of the device. Due to the radial direction of the vanes, the space between vanes decreases towards the center of the device, thereby funneling, concentrating and magnifying the air flow as it reaches the center of the device, and further increasing the air speed and lowering the air pressure in a zone above exhaust port  34 . The height of the vanes  48  above ramp  40  is preferably 5% to 10% of the length of the surface of ramp  40 . 
     In this way a small movement of air can be channeled by the vent cap  22  to significantly reduce the air pressure above exhaust port  34  to draw vapors from holding tank  10  even with light air movement from any direction. Since the device is symmetrical about the central axis the movement of air can be from any direction relative to the device. 
       FIG. 10-12  illustrate a further embodiment  62  of the vent cap of the invention. It has a base  64  with a diameter  65 , a top  66  and slender support legs  68  joining the base  64  and top  66 . In this case the legs  68  are formed integrally with the top  66  and have lower ends  69  which snap into apertures  70  in base  64  so that the top  66  can be removed or replaced on base  64 . A central opening of channel  72  of base  64  has a diameter  73  and is covered by screen  74 . Base  64  has a ramp  76  with radial channeling vanes  78  which extend vertically from ramp  76  as in the previously described embodiments. As shown in  FIG. 12 , top  66  has a diameter  77  that is at least 30% greater than the diameter  65  of the base  64 , and has a convex or domed central upper surface  80  with an annular, thin outer edge portion  81  extending around the convex central upper surface  80  and having a radially outer portion thereof in the form of an upturned outer edge portion  82 , giving it a saucer-like appearance. The legs  68  extend from an outer margin of the outer edge portion  81 , as seen in  FIG. 10 . Upturned outer edge portion  82  serves to re-direct down drafts away from the base, creating additional low pressure near the exhaust port and to further gather and concentrate horizontally moving air towards the center of the device. Convex central lower surface portion  84  forms the upper surface of the channel formed between base  64  and top  66  as in the previously-described embodiments, with the central area or apex  85  of convex central lower surface portion  84  extending below the lowest portion  86  of outer edge portion  82 . The convex central lower surface portion  84  has a diameter  87  that may be greater than the diameter  73  of the central opening of the channel  72 . As is also shown in  FIG. 12 , a lower surface area surrounding the convex central lower surface portion  84  is an annular shallow downwardly concave lower surface portion  88  of the top  66 , and is partly defined by the lower surface of a radially inner part of the annular thin outer edge portion  81 , and is circumscribed by the upturned outer edge portion  82 . 
     A particular advantage of the embodiment shown in  FIG. 10-12  is that the top  66  can be sold and purchased separately, for example in the event that the top or legs  68  are damaged or broken, without the need to buy or replace the base  64  also. 
     While the device has been described in the context of a vent cap for a holding tank, it will be apparent to the person skilled in the art that the present invention will have application in other venting applications also including chimneys, other types of roof vents, and other transportation vehicles. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.