Abstract:
A flow restrictor is provided to reduce pressure in the flow of a fluid, such as a hydrocarbon-based fuel. The flow restrictor takes the form of a capillary that is void of any abrupt flow disruptions. The flow restrictor may be used in place of an orifice and provides the advantage that it has a larger diameter than an orifice of similar function. Precipitation is less likely to form on the restrictor and any precipitation is less likely to have an adverse affect on performance.

Description:
This application claims the benefit of U.S. Provisional Application 60/909,405, filed Mar. 30, 2007, the entire disclosure of which is incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to a flow restrictor, and specifically relates to a flow restrictor that does not include abrupt flow disruptions and may be used with hydrocarbon-based gases. 
     BACKGROUND OF THE INVENTION 
     Flow restrictors, used for reducing pressure and regulating flow, are used in a vast number of different applications. For instance, CO 2  insect traps, which produce CO 2  from air and a combustible fuel, typically use two flow restrictors. A diaphragm regulator is first used to reduce the pressure of the combustible fuel from approximately 120 psi down to 11 in H 2 O (0.4 psi). However, such insect traps are very low pressure devices and require further pressure reduction. A second flow restrictor is used because the diaphragm regulator has poor reliability at pressures much below 11 in H 2 O. In order to further reduce the pressure, a fixed orifice may be used. When operating properly, the fixed orifice provides a very inexpensive means to accurately regulate pressure of the combustible fuel. 
     However, it is possible that precipitation will form on the orifice where there is a change in the pressure. The likelihood of precipitation is magnified by the abrupt disruption in the flow. Many orifices used for precise pressure drop applications are very small. They may be 10-thousandths of an inch or smaller. Due to the size of the orifice, even very small quantities of precipitate thereon can negatively impact the performance of the device. Further, if too much precipitate accumulates, the orifice can be blocked entirely, and the device may not operate. In the case of a CO 2  insect trap, even relatively small changes in the overall flow rate can negatively impact performance of the device, by impacting the stochiometry of the reaction. 
     Thus, there is a need for a flow restrictor in such applications that effectively reduces fluid pressure without abrupt disruptions in the flow and without relying on a very small diameter orifice. 
     SUMMARY OF THE INVENTION 
     The present invention provides a flow restrictor for reducing the pressure of a fluid as it flows through a device. The flow restrictor is suitable for use with hydrocarbon based fluids, such as propane or petroleum gas. In one embodiment, the flow restrictor is in the form of an elongated capillary tube. 
     The flow restrictor may be used in an inventive CO 2  insect trap utilizing the advantages of the restrictor. The CO 2  insect trap includes a suction device, such as a blower or fan, for pulling intake air into the trap. The intake air carries insects, for example mosquitoes, along with it into the trap. A catch included in the trap catches the insects preventing them from escaping. The catch can take various forms. For instance, it may be a rigid container with screen mesh, webbing or small holes, or it can be a net bag. 
     The trap can also include a device to kill the insects once they are caught. Alternatively, the insects may simply be detained indefinitely, or can be let go once they are no longer a nuisance. The trap also emits CO 2  in order to lure insects close enough that they are sucked in by the intake air. The CO 2  is produced from a combustible fluid in a catalytic reactor. The combustible fluid is mixed with at least a portion of the intake air and the mixture is oxidized on the catalyst to produce CO 2  and H 2 O. The reaction products are emitted from the trap in order to lure insects. The CO 2  can be mixed with some of the incoming air after it has been produced to help carry it away from the trap. The rest of the intake air is expelled as waste air. 
     The combustible fluid is held at high pressure, in liquefied form, in a cylinder adjacent the trap. The pressure of the combustible fluid is reduced using a two stage pressure reduction scheme. First, the pressure is dropped from high pressure to a more moderate pressure using a regulator, such as a diaphragm regulator. The pressure is then dropped again using the inventive flow restrictor. 
     In one embodiment, the flow restrictor is in the form of a long capillary that gradually reduces the pressure of the fluid. The head loss is caused by shear and frictional losses in the capillary. The length of the capillary provides that no abrupt disruptions in the flow are required to decrease the pressure. Instead, the pressure is gradually reduced over the length of the capillary. Thus, it is less likely that condensation is formed at a single point within the restrictor. Further, the length also provides a sufficient decrease in pressure without requiring a very narrow diameter opening. Thus, any condensation that forms is less likely to have adverse affects on the performance of the restrictor because it will obstruct a smaller fraction of the opening and is unlikely to block the opening. 
     The required length of the passage may be reduced by introducing a gradual and continuous change in direction to the fluid, increasing the pressure drop per unit length. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the invention will become more apparent by referring to the drawings, in which: 
         FIG. 1  is a cross sectional view of a CO 2  trap including an embodiment of a flow restrictor according to the present invention; 
         FIG. 2  is a perspective view of the CO 2  trap shown in  FIG. 1 ; 
         FIG. 3  is a cross sectional view of an embodiment of a flow restrictor according to the present invention; and 
         FIG. 4  is a perspective view of the flow restrictor shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show an insect trap  1  incorporating an embodiment of a flow restrictor according to the present invention. The trap  1  includes a housing  2  formed of sidewalls  4  and a top  6 . Within housing  2  is a suction device  8 . The suction device  8  can be a rotational unit that is driven by a motor or an engine. For example, the suction device  8  can be a blower or a fan. The suction device  8  is shown diagrammatically in  FIG. 1  as two rotor blades. The object of suction device  8  is to draw air through the CO 2  trap  1  and any known method for doing so can be used. 
     Intake air  10  is pulled into the trap  1  at the top of the housing by suction device  8 . Along with the intake air  10 , insects are sucked into the trap. Once within the housing  2 , the intake air  10  passes through a catch  12  wherein the insects contained in the air stream are captured. After passing through the suction device  8 , or blower, the air is separated for various uses. Much of the air passes directly through trap  1  as waste air  14 . A first portion  16  of the intake air  10  is used in the reactor  18  to create CO 2 . The first portion  16  of air is combined with a combustible fuel  20  in the reactor  18 . The combustible fuel  20 , which may be a hydrocarbon-based gas, is fed to the trap  1  from a fuel source, such as tank  24 . Within the tank  24 , the combustible fuel  20  can be, for example, liquid propane or liquefied petroleum gas. The outlet of tank  24  can be directly adjacent the trap, or the combustible fuel  20  may flow to the trap  1  through a conduit, as shown. A regulator is included to reduce the pressure of the combustible fuel from the pressure level maintained in the tank. The pressure of the combustible fuel  20  is then further reduced using the flow restrictor  26  of the invention. 
     In the reactor  18 , the first portion of air  16  is mixed with combustible fuel  20  and oxidized. To initiate the reaction, a spark generator  32  is included in the catalytic reactor  18 . The reactants  28  are then fed through a catalyst  30  that is part of reactor  18 . The catalyst  30  operates to convert the reactants into CO 2  and H 2 O. The mixture emerging from the catalyst is rich with CO 2 , and though it also contains nitrogen, water and possibly other products, it will be referred to as CO 2 . The CO 2    34  leaves the reactor  18  and is swept through exit passage  36  by a second portion of air  38 . The second portion of air  38  is propelled by suction device  8  and is able to carry the CO 2  out of insect trap  1 . 
     The flow restrictor  26  of the invention is shown in detail in  FIG. 3 . The restrictor includes an elongate conduit or capillary  40  with a narrow diameter. Although the capillary  40  has a narrow diameter, it can be more than an order of magnitude larger than the diameter of a conventional orifice used to form the same pressure drop. In one embodiment of the invention, the capillary  40  is free of any abrupt flow disruptions, such as sharp turns or immediate obstructions. Any turns in the capillary may have a large radius of curvature with respect to the capillary diameter. For example, the radius of curvature may be more than three times the diameter of the capillary. For very minute flow disruptions, the radius of curvature of any curves of the capillary can be more than ten times the diameter of the capillary  40 . 
     Although capillary  40  can be a straight conduit, curves in the capillary allow the flow restrictor  26  to be more compact. The curves also provide additional shear stress which aids in the pressure reduction provided by the restrictor  26 . In one embodiment, the entire length of the capillary  40  is curved. One example of a continuously curved capillary  40  is a capillary that consistently curves in the same direction for the entire length of the capillary  40 . 
     The flow restrictor  26  shown in  FIGS. 3 and 4  is a particularly low cost embodiment of the invention. The capillary  40  is formed between a plug  42  and a surrounding tube  44 , such as a pipe. The combustible fuel  20  is provided to the flow restrictor  26  at an inlet side  50  of the tube. The fuel  20  then flows through the restrictor  26  to an outlet side  52  of the tube. A groove  46  is cut in the outer edge or surface  48  of the plug  42  to thereby provide the capillary  40  in the form of the groove  46  between the plug  42  and the inner surface of the tube  44 . Alternatively, the capillary  40  may be provided as a physically separate tube received within the groove  46 . The outer edge or surface  48  of the plug  42  and the inner surface of the tube  44  form a seal therebetween. The inlet end  54  of the groove  46  is in communication with the inlet side  50  of the tube and the outlet end  56  of the groove  46  is in communication with the outlet side  52  of the tube. Thus, the only path available from the inlet side  50  to the outlet side  52  is the groove  46  itself. The groove  46  may take any form on the outside of the plug  42 . In the illustrated embodiment, the groove  46  is in the form of threading across the entirety of the plug  42 . Threading is an advantageous form of the invention, because it provides a continuous, consistent curve in the capillary  40 . Thus, there are no abrupt flow disruptions and the capillary  40  may be long in relation to the length of the plug  42 , since it is essentially “wound” several times around the plug  42 . Further, threading machinery is readily available, and thus, the restrictor can be manufactured at low cost. 
     In one embodiment, the plug and tube may both be made of metal. To increase the integrity of the seal between the plug  42  and the tube  44 , one of these pieces may be formed of a softer metal than the other. For example, the tube may be formed of steel, while the plug is made of brass. Variations of this embodiment may also be made. For instance, the groove  46  can be cut in the tube instead of in the plug  42 . Alternatively, portions of the groove  46  can be included in both the plug  42  and the tube  44 . 
     Although the preferred form of the invention has been shown and described, many features may be varied, as will readily be apparent to those skilled in this art.