Abstract:
A cam driven compressor includes a cam coupled to a plurality of cylinder and piston assemblies. Each cylinder and piston assembly comprises a piston located and movable within a respective cylinder. Each cylinder has a cylinder head. The compressor comprises a housing defining a cavity configured to receive a portion of a source gas from one or more of the cylinders in order to maintain a positive gas pressure within the cavity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to compressors and, in a first aspect thereof, more particularly relates to a compressor having a cavity at a positive internal pressure relative to the surrounding environment to reduce induction of contaminants into the product gas produced by the compressor. In another aspect, the invention relates to a compressor having a pressure relief valve in communication with the compressor cavity to maintain the compressor cavity at a desired positive pressure. 
         [0002]    Electrically driven compressors must convert rotary motion from a motor into linear motion to actuate a piston or a series of pistons to generate compressed gas. Most gas compressors accomplish this task by means of a crankshaft and connecting rod assembly similar to that found in internal combustion engines. By design, compressors known in the art will create a slight leak around the inter-stage seal located between the piston and cylinder. Some advantages to this design are the proven reliability and the high operating efficiency. One major disadvantage is that these compressors are referenced/vented to atmosphere so as to eliminate a positive pressure cavity. Not only does this waste any leaked gas, but any moisture in the atmosphere may be reintroduced into the product gas via back-diffusion. This back-diffusion of atmosphere decreases the product gas concentration and increases contaminants within the product gas. 
         [0003]    By way of example, Aviation Breathing Oxygen (ABO) requires product gas purities exceeding about 99.9%. Thus, current compressors may compromise the product gas purity through cavity leakages. Even at low compression pressures, moisture may be introduced into the product gas thereby leading to a failure in meeting the requirements set forth by MIL-PRF-27210J (Performance Specification—Oxygen, Aviator&#39;s Breathing, Liquid and Gas), particularly the requirement that the moisture content of the gas be less than 7 ppm. 
         [0004]    Thus, what is needed is a high pressure compression system that would allow use of high purity gases without impacting gas purity due to compressor leakages. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention addresses the above needs by providing a cam driven compressor including a cam coupled to a plurality of cylinder and piston assemblies. Each cylinder and piston assembly comprises a piston located and movable within a respective cylinder. Each cylinder has a cylinder head. The compressor comprises a housing defining a cavity configured to receive a portion of a source gas from one or more of the cylinders. A first cylinder may be a low pressure cylinder, wherein a portion of the source gas within the first cylinder is directed into the cavity. The housing may further include a pressure relief valve to prevent over-pressurization of the housing. The pressure relief valve may have a maximum pressure limit of 1 psig. The cavity may be filled with the portion of the source gas at a flow rate of about 200 scc/m and the housing may be selected to have a positive cavity pressure of about 1 psig. 
         [0006]    In a further aspect of the present invention, a method of producing a high purity high pressure gas comprises: a) providing a cam driven compressor, wherein a cam is coupled to a plurality of cylinder and piston assemblies wherein each cylinder and piston assembly comprises a piston located and movable within a respective cylinder, each cylinder having a cylinder head, the compressor comprising a hermetically sealed housing defining a cavity configured to receive a portion of a source gas within the cylinders; b) allowing a source gas to be supplied to the compressor; and c) allowing a portion of the source gas to pressurize the cavity of the housing. 
         [0007]    The first cylinder may be a low pressure cylinder and the portion of the source gas may be provided by the first cylinder. The method may further include the step of d) preventing over-pressurization of the housing through a pressure relief valve. The pressure relief valve may have a maximum pressure limit of 1 psig. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention will further be described, by way of example, with reference to the accompanying drawings: 
           [0009]      FIG. 1  is a front perspective view of an embodiment of a cam driven compressor in accordance with the present invention; 
           [0010]      FIG. 2  is a side perspective view of the embodiment shown in  FIGS. 1 ; and 
           [0011]      FIG. 3  is a cross section view of the cam driven compressor taken generally along line  3 - 3  in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring to the drawings, there is seen in  FIGS. 1 through 3  one embodiment of the inventive compressor assembly designated generally by the reference numeral  10 . Compressor assembly  10  includes a housing  12  and is configured to connect to a motor and speed reducer (not shown) via keyed bore  14  in shaft sleeve  16 . Compressor assembly  10  generally includes a housing  12  comprising a block comprised of corresponding block halves  12   a  and  12   b.  First, second and third stage cylinder  18 ,  20  and  22 , respectively, are spaced 120° apart and radially extend along respective axes X 1 -X 3 . 
         [0013]    For the sake of clarity, the moving components of the compressor have been removed. An example of such suitable components may be found within U.S. Pat. No. 8,684,704 (the &#39;704 patent) assigned to Carleton Like Support Systems, Inc., the entirety of which is incorporated by reference as if fully set forth herein. As recounted within the &#39;704 patent, a. compressor may include a cam positioned on shaft sleeve  16  with respective cam follower assemblies operably connected to the cam. Each cam follower assembly may include a respective roller element rotatably connected between respective roller brackets and associated end plates. Each cam follower assembly may further include a respective connecting rod connected to a respective roller element via a respective roller bracket at a first end thereof; and to a respective piston at a second end thereof. Each connecting rod telescopes within a respective linear bearing and each piston is reciprocally located in a respective cylinder  18 ,  20  and  22 . A compressor head  18   a,    20   a  and  22   a  mounts to the end of a respective cylinder opposite the end from which the respective connecting rod extends. 
         [0014]    In this manner, low pressure gas enters via an air tube  24  into first stage cylinder  18  and its included piston assembly via inlet port  26  thereof and enters cylinder  18 . When the highest lobe point of the cam reaches the piston assembly, its roller rides along the lobe point of the cam resulting in a piston upstroke (toward head  18   a ) and a first stage compression of the gas within cylinder  18 . The compressed gas exits head  18   a  at outlet port  26   a  and is directed through air tube  28  until it reaches head  20   a  wherein the first stage compressed gas enters through inlet port  30  into cylinder  20 . At this time, the piston within cylinder  20  begins a downstroke position as the gas enters its respective compression chamber. As the cam continues to rotate, its medium point approaches the cam follower assernhly associated with cylinder  20  which then begins its upstroke. High lobe point next approaches this assembly which completes the second stage compression of the gas within cylinder  20 . The compressed gas exits at outlet port  30   a  and is directed through air tube  32  until it reaches head  22   a  wherein the second stage compressed air enters through inlet port  34  into cylinder  22 . Again, as the cam continues to rotate, its medium lobe point approaches roller assembly associated with cylinder  22  which begins its upstroke. This roller then rides along the lobe high point of the cam resulting in a full piston upstroke and a third stage compression of the gas within cylinder  22 . The compressed then gas exits as high pressure gas (e.g., up to or exceeding 1000 psi), via outlet port  34   a  through air tube  36  which may be connected to an appropriate high pressure gas collection (e.g., air cylinder, not shown). As rotation of the cam continues, this cycle is repeated providing a continuous stream of high pressure gas at outlet port  34   a.    
         [0015]    Returning now to  FIGS. 1-3 , in accordance with an aspect of the present invention, block halves  12   a  and  12   b  are adapted to be joined so as to produce a sealed compressor body such as through a seal or gasket  38 . Similarly, cylinders  18 ,  20  and  22  may be hermitically sealed to block halves  12   a,    12   b.  Shaft sleeve  16  may also be sealed to block halves  12   a,    12   b  as is known in the art. In this manner, housing  12  may be fully sealed such that the cavity  40  defined by the joined block halves  12   a,    12   b  may be pressurized so as to be slightly above ambient pressure. To pressurize cavity  40 , a purge gas may enter volume  40  through leaks around the inter-stage seals between each piston and cylinder. 
         [0016]    Housing  12  may further include a pressure relief valve  42  in communication with cavity  40  to prevent over-pressurization of the cavity in the event that a seal fails during any of the compression stages. Pressure relief valve  42  may also help meter the cavity pressure. In accordance with an aspect of the invention, operational efficiencies were optimized to include a pressure relief valve  42  selected to have an upper operating limit of 1 psig with an upper purge flow rate of about 200 sec/m. Thus, cavity pressure may be metered at 1 psig due to the upper operating limit of pressure relief valve  42 . In an exemplary sample, oxygen supply gas having less than 1 ppm water at 25 psig was supplied to compressor  10 . The resultant high pressure outlet gas was found to contain less than 7 ppm water at 500 psig in about 95% relative humidity environment. In accordance with an aspect of the invention, the first stage cylinder  18  may be configured to receive a pure oxygen supply having greater than 99.9% oxygen, wherein cylinder 18 operates with maximum flows of about 4.0 slpm at maximum 40 psig at ambient temperatures. Third stage cylinder  22  may output greater than 99.9% oxygen gas with maximum flows of about 4.0 slpm at maximum 3,000 psig at ambient temperatures. 
         [0017]    While the above example recited a preferred relief valve/cavity pressure and purge flow rate, it should be understood by those skilled in the art that other values may be used depending upon system tolerances and required gas outputs. For instance, increased cavity pressures and/or purge flows may result in less contaminated outlet gases. It should be noted that cavity pressure and purge flow should be controlled, and preferably minimalized, to conserve the high purity gas and improve delivery efficiencies. 
         [0018]    Although the invention has been described with reference to preferred embodiments thereof, it is understood that various modifications may be made thereto without departing from the full spirit and scope of the invention as defined by the claims which follow.