Abstract:
A storage vessel is filled with compressed gas by filling a first tank with gas from a low pressure gas source. Hydraulic fluid is drawn from a reservoir and pumped into the first tank in contact with the gas. This causes the gas in the first tank to flow into the storage vessel as it fills with hydraulic fluid. At the same time, gas is supplied from the gas source to a second tank. Hydraulic fluid previously introduced into the second tank flows out to the reservoir as the second tank fills with gas. When the first tank is full of hydraulic fluid, a valve switches the cycle so that the hydraulic pump begins pumping hydraulic fluid back into the second tank while the first tank drains. The cycle is repeated until the storage vessel is filled with gas to a desired pressure.

Description:
This application claims the provisional filing date of application filed Aug. 23, 2001, Ser. No. 60/314,506 entitled “Wet Compressor System”. 
    
    
     TECHNICAL FILED 
     This invention relates in general to equipment for compressing gas, and in particular to a system for compressing gas from a low pressure source into a storage vessel at a higher pressure. 
     BACKGROUND OF THE INVENTION 
     Compressed natural gas is used for supplying fuel for vehicles as well as for heating and other purposes. The gas is stored by the user in a tank at initial pressure of about 3,000 to 5,000 psi., typically 3600 psi. When the compressed natural gas is substantially depleted, the user proceeds to a dispensing station where compressed natural gas is stored in large dispensing tanks at pressures from 3,000 to 5,000 psi. The dispensing station refills the user&#39;s tank from its dispensing tank. 
     If the station is located near a gas pipeline, when the station&#39;s storage vessels become depleted, they can be refilled from the natural gas pipeline. For safety purposes, the pipeline would be at a much lower pressure, such as about 5 to 100 psi. This requires a compressor to fill the dispensing tank by compressing the gas from the gas source into the dispensing tank. Compressors are typically rotary piston types. They require several stages to compress gas from the low to the high pressure used for natural gas vehicle applications. These compressors generate significant amounts of heat which must be dissipated in inner cooling systems between the compression stages. These compressors may be expensive to maintain. 
     Also, in certain parts of the world, natural gas pipelines are not readily available. The dispensing stations in areas far from a pipeline or gas field rely on trucks to transport replacement dispensing tanks that have been filled by a compressor system at a pipeline. The same compressors are used at the pipeline to fill the dispensing tanks. 
     Hydraulic fluid pumps are used in some instances to deliver hydraulic fluid under pressure to a tank that contains gas under pressure. A floating piston separates the hydraulic fluid from the gas. The hydraulic fluid maintains the pressure of the gas to avoid a large pressure drop as the gas is being dispensed. 
     SUMMARY OF THE INVENTION 
     In this invention, gas is compressed from a gas source into a storage tank by an apparatus other than a conventional compressor. In this method, a first tank assembly is filled with gas from the gas source. Hydraulic fluid is drawn from a reservoir and pumped into the first tank assembly into physical contact with the gas contained therein. This causes the gas in the first tank assembly to flow into the storage reservoir as the first tank assembly fills with hydraulic fluid. The second tank assembly, which was previously filled with hydraulic fluid, simultaneously causes the hydraulic fluid within it to flow into a reservoir. The hydraulic fluid is in direct contact with the gas as there are no pistons that seal between the hydraulic fluid and the gas. 
     When the first tank assembly is substantially filled with hydraulic fluid and the second tank assembly substantially emptied of hydraulic fluid, a valve switches the sequence. The hydraulic fluid flows out of the first tank assembly while gas is being drawn in, and hydraulic fluid is pumped into the second tank assembly, pushing gas out into the storage vessel. This cycle is repeated until the storage vessel reaches a desired pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a system constructed in accordance with this invention. 
     FIG. 2 is a schematic of an alternate embodiment of the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, first and second tanks  11 ,  13  are shown mounted side-by-side. Each tank is a cylindrical member with rounded upper and lower ends. Fins  15  optionally may be located on the exteriors of tanks  11 ,  13  for dissipating heat generated while their contents are being compressed. Tanks  11 ,  13  have gas ports  17 ,  19 , respectively, on one end for the entry and exit of gas  20 , such as compressed natural gas. Hydraulic fluid ports  21 ,  23  are located on the opposite ends of tanks  11 ,  13  in the preferred embodiment for the entry and exit of hydraulic fluid  24 . 
     Hydraulic fluid  24  may be of various incompressible liquids, but is preferably a low vapor pressure oil such as is used in vacuum pumps. Preferably tanks  11 ,  13  are mounted vertically to reduce the footprint and also to facilitate draining of hydraulic fluid  24  out of hydraulic ports  21 ,  23 . However vertical orientation is not essential, although it is preferred that tanks  11 ,  13  at least be inclined so that their gas ports  17 ,  19  are at a higher elevation than their hydraulic fluid ports. 
     Fluid level sensors  25 ,  27  are located adjacent gas ports  17 ,  19 . Sensors  25 ,  27  sense when hydraulic fluid  24  reaches a maximum level and provide a signal corresponding thereto. Very little gas will be left in tank  11  or  13  when the hydraulic fluid  24  reaches the maximum level. Minimum fluid level sensors  29 ,  31  are located near hydraulic fluid ports  21 ,  23 . Sensors  29 ,  31  sense when the hydraulic fluid  24  has drained down to a minimum level and provide a signal corresponding thereto. Fluid level sensors  25 ,  27 ,  29  and  31  may be of a variety of conventional types such as float, ultrasonic, or magnetic types. 
     A solenoid actuated position valve  33  is connected to hydraulic fluid ports  21 ,  23 . Position valve  33  is shown in a neutral position, blocking any hydraulic fluid flow to or from hydraulic fluid ports  21 ,  23 . When moved to the positions  33   a  or  33   b , fluid flow through hydraulic fluid ports  21  or  23  is allowed. Position valve  33  is also connected to a fluid supply line  35  and a drain line  37 . Fluid supply line  35  is connected to a hydraulic fluid pump  39  that is driven by motor  41 . A check valve  43  prevents re-entry of hydraulic fluid  24  into pump  39  from supply line  35 . A conventional pressure relief valve  45  is connected between supply line  35  and drain line  37  to relieve any excess pressure from pump  39 , if such occurs. In this embodiment, pump  39  is a conventional variable displacement type. As the pressure increases, its displacement automatically decreases. 
     A reservoir  47  is connected to drain line  37  for receiving hydraulic fluid  24  drained from tanks  11 ,  13 . Reservoir  47  is open to atmospheric pressure and has a line  49  that leads to the intake of pump  39 . A splash or deflector plate  48  is located within reservoir  47  for receiving the flow of hydraulic fluid  24  discharged into reservoir  47 . The hydraulic fluid  24  impinges on splash plate  48  as it is discharged. This tends to free up entrained gas bubbles, which then dissipate to atmosphere above reservoir  47 . 
     When position valve  33  is in position  33   a , pump  39  will pump hydraulic fluid  24  through hydraulic fluid port  21  into first tank  11 . Simultaneously, hydraulic fluid  24  contained in second pump  13  is allowed to flow out hydraulic fluid port  23  and into reservoir  47 . A control system  51  receives signals from sensors  25 ,  27 ,  29  and  31  and shifts valve  33  between the positions  33   a  and  33   b  in response to those signals. 
     A gas supply line  53  extends from a gas source  54  to gas port  17  of first tank  11 . Gas source  54  is normally a gas pipeline or gas field that supplies a fairly low pressure of gas, such as between about 5 and 100 psi. A gas line  55  leads from gas supply line  53  to gas port  19  of second tank  13 , connecting gas ports  17 ,  19  in parallel with gas source  54 . Gas ports  17 ,  19  are continuously in communication with gas source  54  because valves  59  located between gas source  54  and gas port  17 ,  19  are normally in open positions. 
     A storage vessel line  61  extends from each of the gas ports  17 ,  19  to a storage vessel  63 . Check valves  57  in lines  53  and  55  prevent any flow from tank  11  or  13  back into gas source  54 . Check valves  64  mounted between storage vessel line  61  and gas ports  17 ,  19  prevent any flow from storage vessel  63  back into tanks  11 ,  13 . Also, check valves  64  will not allow any flow from gas ports  17 ,  19  unless the pressure in gas ports  17 ,  19  is greater than the pressure in storage vessel line  61 . Storage vessel  63  is capable of holding pressure at a higher level than the pressure of gas in gas source  54 , such as 3,000 to 5,000 psi. Storage vessel  63  may be stationary, or it may be mounted on a trailer so that it may be moved to a remote dispensing site. Storage vessel  63  is typically a dispensing tank for dispensing compressed gas  20  into a user&#39;s tank. 
     In operation, one of the tanks  11 ,  13  will be discharging gas  20  into storage vessel  63  while the other is receiving gas  20  from gas source  54 . Assuming that first tank  11  is discharging gas  20  into storage vessel  63 , valve  33  would be in position  33   a . Pump  39  will be supplying hydraulic fluid  24  through supply line  35  and hydraulic fluid port  21  into tank  11 . Gas  20  would previously have been received in first tank  11  from gas source  54  during the preceding cycle. Hydraulic fluid  24  physically contacts gas  20  as there is no piston or movable barrier separating them. In order for gas  20  to flow to storage vessel  63 , the hydraulic fluid pressure must be increased to a level so that the gas pressure in tank  11  is greater than the gas pressure in storage vessel  63 . Gas  20  then flows through check valve  64  and line  61  into storage vessel  63 . 
     Simultaneously, hydraulic fluid port  23  is opened to allow hydraulic fluid  24  to flow through drain line  37  into reservoir  47 . The draining is preferably assisted by gravity, either by orienting tanks  11 ,  13  vertically or inclined. Also, the pressure of any gas  20  within second tank  13  assists in causing hydraulic fluid  24  to flow out hydraulic fluid port  23 . When the pressure within tank  13  drops below the pressure of gas source  54 , gas from gas source  54  will flow past check valve  57  into tank  13 . 
     Pump  39  continues pumping hydraulic fluid  24  until maximum fluid level sensor  25  senses and signals controller  51  that hydraulic fluid  24  in tank  11  has reached the maximum level. The maximum level is substantially at gas port  17 , although a small residual amount of gas  20  may remain. At approximately the same time, minimum level sensor  31  will sense that hydraulic fluid  24  in tank  13  has reached its minimum. Once both signals are received by control system  51 , it then switches valve  33  to position  33   b.    
     The cycle is repeated, with pump  39  continuously operating, and now pumping through fluid port  23  into second tank  13 . Once the pressure of gas  20  exceeds the pressure of gas in storage vessel  63 , check valve  64  allows gas  20  to flow into storage vessel  63 . At the same time, hydraulic fluid  24  drains out fluid line  21  from first tank  11  into reservoir  47 . These cycles are continuously repeated until the pressure in storage vessel  63  reaches the desired amount. 
     Ideally, the signals from one of the maximum level sensors  25  or  27  and one of the minimum level sensors  29  or  31  will be received simultaneously by controller  51 , although it is not required. Both signals must be received, however, before controller  51  will switch valve  33 . If a maximum level sensor  25  or  27  provides a signal before a minimum level sensor  27  or  29 , this indicates that there is excess hydraulic fluid  24  in the system and some should be drained. If one of the minimum level sensors  29  or  31  provides a signal and the maximum level sensor  25 , or  27  does not, this indicates that there is a leak in the system or that some of the fluid was carried out by gas flow. Hydraulic fluid should be added once the leak or malfunction is repaired. 
     A small amount of gas  20  will dissolve in hydraulic fluid  24  at high pressures. Once absorbed, the gas does not release quickly. It may take two or three days for gas absorbed in the hydraulic fluid to dissipate, especially at low temperatures when the hydraulic fluid viscosity increases. Even a small amount of gas in the hydraulic fluid  24  makes pump  39  cavitate and the hydraulic system to perform sluggishly. 
     If excess gas absorption is a problem at particular location, the release of absorbed gas  20  from the hydraulic fluid  24  can be sped up by reducing the molecular tension within the fluid. This may occur by heating the hydraulic fluid in reservoir  47  in cold weather. Also, the hydraulic fluid could be vibrated in reservoir  47  with an internal pneumatic or electrical vibrator. Splash plate  48  could be vibrated. A section of drain pipe  37  could be vibrated. Heat could be applied in addition to the vibration. Furthermore, ultrasound vibration from an external source could be utilized to increase the release of gas  20  from the hydraulic fluid  24 . Of course, two reservoirs  47  in series would also allow more time for the gas  20  within the returned hydraulic fluid  24  to release. 
     FIG. 2 shows an alternate embodiment with two features that differ from that of the embodiment of FIG.  1 . The remaining components are the same and are not numbered or mentioned. In this embodiment, rather than a variable displacement pump  39 , two fixed displacement pumps  67 ,  69  are utilized. Pumps  67 ,  69  are both driven by motor  65 , and pump  67  has a larger displacement than pump  69 . Pumps  67 ,  69  are conventionally connected so that large displacement pump  67  will cease to operate once the pressure increases to a selected amount. Small displacement pump  69  continuously operates. Controller  71  operates in the same manner as controller  51  of FIG.  1 . The two pump arrangement of FIG. 2 is particularly useful for large displacement systems. 
     The second difference in FIG. 2 is that rather than a single tank  11  or  13  as shown in FIG. 1, a plurality of first tanks  73  are connected together, and a plurality of second tanks  75  are connected together. The term “first tank assembly” used herein refers to one (as in FIG. 1) or more first tanks  11  or  73 , and the term “second tank assembly” refers to one (as in FIG. 1) or more second tanks  75 . 
     First tank assembly  73  comprises a plurality of individual tanks connected in parallel. Also, each of the tanks of second tank assembly  75  are connected in parallel. Each tank assembly  73 ,  75  has a gas port header  74  that connects all of the gas ports together. Each tank assembly  73 ,  75  has a hydraulic fluid head  76  that joins all of the lower ports. Consequently, each of the tanks within first tank assembly  73  or within second tank assembly  75  will fill and drain simultaneously. A single minimum fluid level sensor  77  is used for the first tank assembly  73 , and a single minimum level sensor  77  is used for the second tank assembly  75 . Only a single maximum level sensor  79  is needed for each of the tank assemblies, as well. 
     The embodiment of FIG. 2 operates in the same manner as the embodiment of FIG. 1 except that multiple tanks are filling and emptying of hydraulic fluid at the same time. Tank assemblies  73 ,  75  could be used with a variable displacement pump such as pump  39  in FIG.  1 . Similarly, the two-pump system of FIG. 2 could be used with the single tank system of FIG.  1 . 
     The invention has significant advantages. It allows compression of gas from a low pressure to a high pressure with a single stage. Less heat should be generated and less expenses are required. 
     While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention.