WO2014172277A1

WO2014172277A1 - Internal cooling of a working fluid that is compressed in a piston-cylinder assembly

Info

Publication number: WO2014172277A1
Application number: PCT/US2014/033999
Authority: WO
Inventors: Nicholas White; Christopher PARAFINCZUK; Blake CARL; Tim BECK; Robert ANDERSEN JR.; Thomas BIAGI
Original assignee: Parker-Hannifin Corporation
Priority date: 2013-04-15
Filing date: 2014-04-14
Publication date: 2014-10-23

Abstract

A piston-cylinder assembly for compression of a working fluid includes a cylinder having a cylindrical wall defining an interior chamber that is closed at one axial end by a cylinder end cap, and a piston disposed in the interior chamber for reciprocating movement. A working fluid inlet provides fluid flow into the interior chamber when the piston is moving away from the cylinder end cap, and a working fluid outlet provides fluid flow out of the interior chamber when the piston is moving towards the cylinder end cap. The cylinder end cap has a submerged cooling portion that protrudes into the interior chamber, and is spaced radially inwardly from the cylindrical wall for defining an annular space through which the working fluid flows for cooling of the working fluid. A secondary cooling mechanism may be provided for cooling the working fluid in combination with the submerged cooling portion.

Description

INTERNAL COOLING OF A WORKING FLUID THAT IS COMPRESSED IN A

PISTON-CYLINDER ASSEMBLY

Related Applications

This application claims the benefit of U.S. Provisional Application No.

61/812,063, filed April 15, 2013, which is incorporated herein by reference.

Field of the Invention

The present invention pertains to cooling systems for cylinder components including moveable piston elements, including fluid pump systems having piston- based cylinders for pumping a working fluid from an inlet to an outlet, and systems and methods for providing cooling associated with the cylinders in such systems.

Background of the invention

Fluid pumps and compressors are utilized in a variety of applications, including various motors and engines, medical applications, hydraulic systems, gas delivery systems including natural gas delivery, and others. In many such systems, a moveable piston actuated within a cylinder pumps a working fluid from an inlet to and through an outlet. Depending upon the application, the working fluid may be air or other gases, hydraulic fluid and other forms of oils, and numerous others.

Compressing a fluid via an isothermal process requires less energy than compressing a fluid it via an adiabatic process, but the compression inside of a typical cylinder is more adiabatic than isothermal. Fluid inside of a moving cylinder is constantly being heated through friction losses and compression, and the fluid must therefore be cooled to provide a more isothermal process. In conventional systems, the fluid may be cooled using an external tank cooler, and tank cooling may be sufficient under certain circumstances. A highly compressible fluid, such as air, however, undergoes a substantial change in temperature when compressed, requiring more input work to obtain the same compression. To obtain a more isothermal compression process, the heat must be extracted during compression rather than removing the heat after the compression cycle using an external tank cooler. Current systems for extracting heat during compression have proven deficient. Attempts to cool a working fluid have included injecting a coolant spray into the compressed gas, or circulating the compressed gas through a heat exchanger that is located outside the cylinder. Effective cooling of the working fluid, however, remains difficult to achieve, particularly when it has been deemed desirable to remove heat from the working fluid while the working fluid is being compressed within the cylinder. In a more general sense, it is desirable to provide effective cooling for systems including cylinders at various stages in fluid

transmission processes.

It is known that heat may be transferred between two fluids separated by a thermally conductive wall. Such heat transfer between two fluids can be idealized via three thermal resistances in series, where two of the resistances are the convective heat transfer, one for each fluid, and the third thermal resistance of the conductive heat transfer of the wall. The conductive heat transfer resistance of the wall is a property of the wall material and wall thickness. The convective heat transfers of the two fluids are affected at least in part both by the material properties of the fluid as well as the velocity of flow, and the heat transfer area between the fluids. Convective heat transfer coefficients exhibit a positive correlation to both the fluid flow velocity and the heat transfer area. Accordingly, increasing either the velocity of the fluid flow, the heat transfer area, or both will increase the convective heat transfer coefficient of the fluids, which enhances the overall heat transfer coefficient and the total heat transfer of the system.

Summary of the Invention

The present invention provides improved systems and methods for cooling a working fluid that is compressed in a piston-cylinder assembly. The cylinder of the piston-cylinder assembly is uniquely provided with a cylinder end cap having a submerged cooling portion that protrudes into the interior chamber of the cylinder and is spaced radially inwardly from the cylindrical wall of the cylinder, which defines an annular space through which the working fluid flows for cooling of the working fluid. The submerged cooling portion provides increased fluid velocity in the annular space and increased area for heat transfer, thereby improving the cooling of the working fluid.

An aspect of the invention is a piston-cylinder assembly for compression of a working fluid. In exemplary embodiments, the piston-cylinder assembly includes a cylinder having a cylindrical wall defining an interior chamber that is closed at one axial end by a cylinder end cap, and a piston disposed in the interior chamber of the cylinder for reciprocating movement. A working fluid inlet proximate the cylinder end cap provides fluid flow into the interior chamber when the piston is moving away from the cylinder end cap, and a working fluid outlet proximate the cylinder end cap provides fluid flow out of the interior chamber when the piston is moving towards the cylinder end cap. The cylinder end cap has a submerged cooling portion that protrudes into the interior chamber of the cylinder, and is spaced radially inwardly from the cylindrical wall of the cylinder for defining an annular space through which the working fluid flows for cooling of the working fluid. A secondary cooling mechanism may be provided for cooling the working fluid in combination with the submerged cooling portion of the end cap.

The submerged cooling portion may axially overlap the working fluid inlet and outlet such that fluid flowing into and out of the interior space is caused to flow through the annular space. The submerged cooling portion may include coolant passages through which a coolant can flow to extract heat from the submerged cooling portion and in turn from the working fluid. The submerged cooling portion may be conical such that the annular space progressively increases in cross- sectional area going in a direction toward the piston.

According to another aspect of the invention, a system for compressing a working fluid includes the described piston-cylinder assembly, a working fluid source connected to the working fluid inlet, a working fluid sink connected to the working fluid outlet, and a heat exchanger circuit connected between coolant inlet and outlet ports of the piston-cylinder assembly for the flow of coolant fluid. The system may include a first stage piston-cylinder assembly and a second stage piston-cylinder assembly, wherein the piston-cylinder assembly including the submerged cooling cap is at least the second stage piston-cylinder assembly. The system further may include one or more secondary cooling mechanisms for cooling the working fluid in combination with the submerged cooling portion. Secondary cooling mechanisms may be used in combination with each other, and take a variety of configurations and be located in various points in the system.

Secondary cooling mechanisms, for example, may be provided in any one of or combination of a cylinder end cap, including the end cap with the submerged cooling portion, the working fluid inlet and/or outlet, an interstage line between multiple stage cylinders (e.g., first and second stage cylinders), and/or piston rod-side cooling within a piston cylinder assembly.

These and further features of the present invention will be apparent with reference to the following description and attached drawings. In the description and drawings, particular embodiments of the invention have been disclosed in detail as being indicative of some of the ways in which the principles of the invention may be employed, but it is understood that the invention is not limited correspondingly in scope. Rather, the invention includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto. Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in

combination with or instead of the features of the other embodiments.

Brief Description of the Drawings

Fig. 1 is a schematic diagram depicting a perspective view of an exemplary natural gas storage and delivery system, into which the apparatuses of the present invention may be incorporated.

Fig. 2 is schematic diagram depicting an exemplary cylinder configuration that may be utilized as part of the system of Fig. 1 .

Fig. 3 is a schematic diagram depicting a cross-sectional view of an exemplary piston-cylinder assembly in accordance with embodiments of the present invention.

Fig. 4 is a schematic diagram depicting an exemplary cooling system employing the piston-cylinder system of Fig. 3. Figs. 5A-5D depict a series of cross-sectional views illustrating flow of a working fluid through the piston-cylinder assembly during reciprocal movement of the piston.

Fig. 6 is a schematic diagram depicting an exemplary secondary cooling mechanism for use in connection with a cooling end cap.

Fig. 7 is a schematic diagram depicting a cross-sectional view of fluid connecting lines or passages, showing an exemplary finned configuration.

Fig. 8 is a schematic diagram depicting an exemplary secondary cooling mechanism utilizing a Venturi fluid passage.

Fig. 9 is a schematic diagram depicting an exemplary secondary cooling mechanism utilizing a one piece manifold unit.

Fig. 10 is a schematic diagram depicting an alternative exemplary secondary cooling mechanism utilizing a one piece manifold unit, in a cross-sectional view.

Fig. 1 1 is a schematic diagram depicting a cross-sectional view of another exemplary piston-cylinder assembly in accordance with embodiments of the present invention, using piston rod side cooling.

Fig. 12 is a schematic diagram depicting a cross-sectional view of a three stage piston for use in a piston rod side cooling system comparable to that of Fig. 1 1 .

Fig. 13 is a schematic diagram depicting a cross-sectional view of a cooling piston for use in the piston-cylinder assembly of Fig. 1 1 .

Fig. 14 is a schematic diagram depicting a cross-sectional view of another exemplary piston-cylinder assembly in accordance with embodiments of the present invention, using an extended cooling end cap for localized cooling adjacent corners of the piston head.

Figs. 15A and !5B are schematic diagrams depicting another exemplary piston-cylinder assembly in accordance with embodiments of the present invention, using cooling pins.

Detailed Description

Embodiments of the present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. It will be understood that the figures are not necessarily to scale.

The present invention pertains to various features of cooling mechanisms and apparatuses for cooling a working fluid in piston-based pumps, compressors, and like devices. For purposes of illustration, the invention is described in part in connection with an exemplary usage within a natural gas storage and delivery system. Such systems may be used to store and supply natural gas for natural gas powered vehicles, natural gas heaters and other devices powered by natural gas, home heating, and any other suitable usages for natural gas. It will be appreciated, however, that usage of the present invention in a natural gas storage and delivery system is a non-limiting example, and the present invention may be employed in any suitable system in which it is desirable for cooling a working fluid, including working fluids in piston-based pumps, compressors, and like devices.

For illustrative purposes, Fig. 1 is a schematic diagram depicting a

perspective view of an exemplary natural gas storage and delivery system 210, into which the apparatuses of the present invention may be incorporated. The system 210 may include a natural gas inlet line 212 for receiving natural gas from an outside provider, and a natural gas outlet line 214 for delivering natural gas to a storage device or user device, as are known in the art. The natural gas inlet line provides a gas supply into a pump or compressor system including multiple piston-based cylinders. Typically, the natural gas is provided from an outside provider. In exemplary systems, gas may be supplied at a pressure of about 17psiA and may be compressed it to 3600 psiA for storage. The system 210 is operative to convert the inlet supply into an outlet supply at a suitable flow rate and pressure for storage and/or use. In exemplary systems, the natural gas supply enters into a first stage cylinder 216, which pumps the gas through an interstage line (not visible in Fig. 1 ) into a second stage cylinder 218. The second stage cylinder 218 then pumps the natural gas through the outlet line 216 to a suitable storage or use device.

Significant heat is generated by operation of the first stage and second stage cylinders, with the heat generation by the second stage cylinder being particularly substantial. Accordingly, a cooling end cap 220 typically is incorporated as part of the second stage cylinder to remove heat from the working fluid within second stage cylinder (e.g., natural gas), although additional cooling mechanisms may be provided elsewhere in the system, such as associated with the first stage cylinder and gas transmission lines. However, because heat generation is most substantial at the second stage cylinder, the heat removal capabilities of the cooling end cap 220 is particularly significant. As further detailed below, the present invention pertains to an enhanced configuration of the second stage cooling end cap, both by itself and in combination with additional cooling mechanisms that may be located at various points in the system.

The first and second stage cylinders 216 and 218 may be driven by an additional drive cylinder 222. In exemplary embodiments, the drive cylinder 222 is a hydraulic drive cylinder. The system may include a hydraulic fluid reservoir and pump system 224. The system 224 may include one or more pumps that pump the hydraulic fluid utilized to drive the drive cylinder. Examples of suitable pumps include multi-gear pumps, such as two-gear or three-pumps, or variable

displacement pumps. A motor 226, for example an electric motor, may be employed to drive the hydraulic pumps.

In exemplary embodiments, the hydraulic fluid also may be employed as a coolant fluid for cooling the working fluid. In such embodiments, under operation of the pumps, hydraulic lines 228 may transport hydraulic coolant fluid to the cooling end cap 220. In a cooling operation, the hydraulic coolant fluid in the cooling end cap removes heat from the working fluid. The heated hydraulic fluid is further pumped through a heat exchanger 230, which is driven by a heat exchanger motor 231 . The heat exchanger re-cools the hydraulic coolant fluid, which is returned to complete the coolant fluid circuit.

Fig. 2 is schematic diagram depicting an exemplary cylinder configuration 232 that may be utilized as part of the system of Fig. 1 . A drive cylinder 234 drives a first stage cylinder 236 and a second stage cylinder 238 via a common drive shaft 240. A natural gas supply is introduced into the first stage cylinder 236 from a gas supply 241 . As the drive cylinder operates to pull the drive shaft left, the gas is compressed by the first stage cylinder and moved out of the first stage into an interstage line 242. The interstage line 242 connects the first stage cylinder 236 to the second stage cylinder 238. As seen in Fig. 2, the system is configured such that the drive cylinder drives the first and second stage cylinders with opposite stroke. Accordingly, as the gas is forced into the interstage line 242 from the first stage cylinder, gas is pulled from the interstage line 242 into the second stage cylinder 238 as the working fluid space within the second stage cylinder expands. The

subsequent compression stroke of the second stage cylinder forces the gas into the outlet line 244 and into a storage tank (or alternatively a use device) 246. During such compression stroke of the second stage cylinder, gas is pulled from the gas supply 241 into the first stage cylinder 236 as the working fluid space within the first stage cylinder expands. Proper flow direction through the system is maintained by a plurality of valves 248.

As referenced above, the present invention pertains to an enhanced configuration of cooling mechanisms. Such cooling mechanisms may include an enhanced second stage cooling end cap, both by itself and in combination with additional secondary cooling mechanisms that may be located at various points in the system. Again, It will be appreciated that the present invention may be employed in a natural gas storage and delivery system as a non-limiting example, and the present invention may be employed in any suitable system in which it is desirable for cooling cylinder based systems of any type, such as cooling a working fluid in piston- based pumps, compressors, and like devices including cylinders.

Fig. 3 is a schematic diagram depicting a cross-sectional view of an

exemplary piston-cylinder assembly 10 in accordance with embodiments of the present invention. For enhanced and substantial cooling effect, the piston-cylinder assembly 10 may be employed as a second stage cylinder in a system comparable to that of Figs. 1 and 2. Alternatively or additionally, the piston-cylinder assembly 10 may be employed as a first stage cylinder, or as the single cylinder in a one-cylinder system.

The exemplary piston-cylinder assembly 10 is operative for compression and transmission of a working fluid from a working fluid inlet 12 to a working fluid outlet 14. The piston-cylinder assembly 10 includes a cylinder 16 having a cylindrical wall defining an interior chamber 18 for the working fluid, which is closed at one axial end by a cylinder end cap 20. A piston 22 is disposed in the interior chamber of the cylinder for reciprocating movement. The working fluid inlet 12 is proximate the cylinder end cap 20, through which fluid can flow into the interior chamber 18 when the piston is moving away from the cylinder end cap, and the working fluid outlet 14 is proximate the cylinder end cap 20 through which fluid can flow out of the interior chamber 18 when the piston is moving towards the cylinder end cap 20.

In exemplary embodiments, the cylinder end cap 20 has an axially outer end cap 21 that may be similar to a conventional end cap, and a submerged cooling portion 24. The submerged cooling portion 24 defines a coolant chamber 26 through which a coolant can flow to extract heat from the submerged cooling portion, and in turn from the working fluid. The cooling portion 24 protrudes axially inwardly from the outer end cap 21 into the interior chamber 18 of the cylinder 16, and is spaced radially inwardly from the cylindrical wall of the cylinder for defining an annular space 28 through which the working fluid flows for cooling of the working fluid. Hence, the cooling portion 24 is submerged in the working fluid in the interior chamber of the cylinder defined between the piston 22 and the outer end cap 21 . The outer end cap 21 includes a coolant inlet port 30 for introducing coolant into the submerged cooling portion at relatively low temperature as compared to the working fluid. The outer end cap 21 also includes a coolant outlet port 32 for withdrawal of the coolant after the coolant in the submerged cooling portion has absorbed heat from the working fluid.

As referenced above, convective heat transfer coefficients exhibit a positive correlation to both the fluid velocity and the heat transfer area. Accordingly, increasing either the velocity of the fluid, the heat transfer area, or both will increase the convective heat transfer coefficient of the fluids, which enhances the overall heat transfer coefficient and the total heat transfer of the system. The annular recess 28 provides a narrower chamber for flow of the working fluid adjacent the submerged cooling portion 24 relative to the interior chamber 18. Accordingly, the working fluid flows with increased velocity within the annular recess 28, thereby enhancing the convective heat transfer from the working fluid into the coolant within the cooling portion 24. This enhanced effect provides for increasing heat transfer as the piston compresses the working fluid as the fluid is pumped from the inlet to the outlet. The cooling portion 24, being submerged within the working fluid, further provides for a substantial heat transfer area. In this manner, the configuration of the present invention provides enhanced cooling effects as compared to conventional configurations by increasing both flow velocity and heat transfer area, and does so during the compression stroke of the piston. As shown in the exemplary embodiment of Fig. 3, the submerged cooling portion 24 may have a truncated conical shape such that the annular space between the cooling portion and wall of the cylinder progressively increases in cross-sectional area going in a direction toward the piston. With such configuration, the submerged cooling portion axially overlaps the working fluid inlet and outlet such that fluid flowing into and out of the interior space is caused to flow through the annular space. The working fluid inlet and outlet are located at diametrically opposite sides of the annular space laterally adjacent an upper half portion of the annular space.

Although the submerged cooling portion 24 of Fig. 3 has the shape of a cone, the cooling portion 24 may be any suitable shape or size that increases the heat transfer area between the cooling portion and the working fluid, while allowing for coolant to flow through the submerged cooling portion. The submerged cooling portion may have an external wall to which fins may be added at both an interior and exterior surface of the wall to increase the heat transfer area and therefore the heat transfer. There can also be a number of embodiments where instead of using an extruded cooling end cap as shown (a cooling cone), typical radiator arrangements may be employed, such as tube and fin, or plate and tube. Turbulators also may be used to perturb the fluid flow as it passes through a heat exchanger to increase the convective heat transfer coefficient.

The cylinder also has an end cap 34 at the rod end of the piston 22 opposite the end cap 20 with the submerged cooling portion 24. The rod end cap 34 has a bore through which the piston rod of the piston 22 extends. The piston is configured such that reciprocating back and forth movement of the piston drives the fluid flow, as is conventional. The piston may be driven by any suitable means.

Fig. 4 is a schematic diagram depicting an pump system 40 employing the piston-cylinder system 10 of Fig. 1 in combination with a coolant circuit for cooling. Accordingly, like components are identified with the same reference numerals in Fig. 4 as in Fig. 3. The pump system 40 further includes a working fluid source 42 connected to the working fluid inlet, and a working fluid sink 44 connected to the working fluid outlet. The working fluid source can be, for example, a reservoir for the working fluid. The working fluid sink can be, for example, a storage device such as a gas storage device referenced above. A heat exchanger circuit 46 is connected between the coolant inlet and outlet ports. The heat exchanger circuit may include a coolant pump 48 that provides a source of coolant. The heat exchanger circuit further may include a heat exchanger 50 remote from the piston-cylinder assembly 10, which removes the heat from the heated coolant. The heat exchanger 50 may be of any suitable type of heating changer for removing heat from the coolant that has passed through the submerged cooling portion 24. The coolant pump 48 provides coolant flow, and the coolant is heated up in the submerged cooling 24 while absorbing heat from the working fluid as described with respect to Fig. 3 The heat in the coolant subsequently is rejected to the atmosphere (or otherwise) via the heat exchanger 50.

The pump system 40 further may include appropriate flow control devices for preventing reverse flow of the working fluid during normal operation of the piston- cylinder assembly 10. In the exemplary embodiment of Fig. 4, an inlet check valve 52 is fluidly connected between the working fluid source and working fluid inlet, and an outlet check valve 54 is fluidly connected between the working fluid sink and the working fluid outlet. The working fluid is supplied to the cylinder by the working fluid source 42, which is separated by the inlet check valve 52 to control the direction of flow. The outlet check valve 54 controls the flow of the working fluid to the working fluid sink 44, which could represent another cylinder, a reservoir, or some

component that operates on or with the working fluid.

As described above, the end cap 20 at the working end of the cylinder is submerged at least partially in the working fluid. The flow of working fluid from the inlet 12 and out the outlet 14 will cool the working fluid flow as it enters and exits the cylinder body. The working fluid can be any suitable fluid, such as water, hydraulic oil or other oils, air or other gases such as natural gas, or others.

To further conserve energy consumption of the system, the coolant pump 48 may be turned on and off depending on position and velocity of the piston, as well as the temperature of the working fluid. When the piston is retracting, working fluid will begin to enter the cylinder through the check valve at ambient temperature, and therefore the coolant may actually increase the temperature of the working fluid. Furthermore, when the cylinder first begins to extend, very little heat will be generated, and therefore the difference in temperature between the working fluid and the coolant will be minimal such that only a small amount of heat can be extracted regardless of the magnitude of the heat transfer area and heat transfer coefficient. During such periods period where heat transfer may be limited, the coolant pump may be turned off or operated at a feed rate lower than the feed rate used when the piston is nearing the submerged cooling portion 24 of the end cap. In this manner, the coolant flow through the heat exchanger circuit 46 may be controlled to maintain efficient heat transfer while conserving the energy consumption of the system. A suitable electronic control device may be incorporated as part of the coolant pump 48.

Figs. 5A-5D depict a series of cross-sectional views illustrating flow of a working fluid through the piston-cylinder assembly 10 during reciprocal movement of the piston 22 within the cylinder body 16. As the piston moves toward the submerged end cap 24 as shown in Fig. 5A, the working fluid inside the cylinder body will be compressed, and therefore the temperature of the working fluid will increase. As the piston continues to move toward the submerged end cap 24, the outlet check valve referenced above will open, and some of the working fluid will forced at an elevated pressure out of the cylinder through the outlet port 14 and to the working fluid sink. At the same time, the working fluid is rejecting heat through the submerged cooling portion 24 of the end cap as shown in Fig. 5B.

The submerged cooling portion 24 reduces the effective flow area for the working fluid near the working fluid outlet, thereby increasing the working fluid velocity which increases the convective heat transfer coefficient, and therefore the overall heat transfer. Fig. 5C illustrates that eventually the piston 22 will be fully extended (and may or may not dwell at full extension) while still continuing to reject heat through the submerged cooling portion 24. The piston 22 will eventually begin to retract, and at this time the pressure inside of the cylinder will decrease, along with the temperature of any remaining fluid inside the cylinder. At some point while the cylinder is retracting as shown in Fig. 5D, the pressure inside the cylinder body 16 will be less than that of the working fluid source, and fluid will flow into the cylinder body through the inlet check valve referenced above and through the fluid inlet 12, until the piston is fully retracted. The piston may dwell at maximum retraction and then the cycle repeats.

The illustrated piston-cylinder assembly 10 and pump system 40 may have a variety of usages. For example, a particular application may be a home natural gas compression unit for the purpose of storing the natural gas to be used on a CNG converted vehicle. Such a system may correspond to that described with respect to Figs. 1 and 2. During the compression process, heat is generated and removing this heat both allows for a more efficient compression process as well as allowing for more natural gas to be stored in the same size tank. With a lower temperature, the same amount of natural gas can be stored at a lower pressure in the same size container according to the ideal gas law. Furthermore, the work required to obtain the same compression ratio in an isothermal (constant temperature) process is less than the work required in an adiabatic (no heat transfer) process according to pVⁿ = C, which is obtained by rearranging the ideal gas law. As another non-limiting example, the assemblies and systems according to the invention may be used for cooling the working fluid of a hydraulic system, and potentially eliminate the need for an external heat exchanger for the working fluid.

In comparison to conventional cooling processes, such as cooling via an external tank, water jacket, or injected fluid, the present invention has a lesser need for materials, and enables the use of a smaller coolant pump as less volume of fluid will be required as well as less power used for the coolant pump. Although the cooling end cap will generally have a much smaller heat transfer area as compared to a conventional water jacket, hence the less materials, the cooling cone will be localized to the point of maximum temperature differential within the cylinder.

Effective cooling, therefore, is achieved using less materials. A cooling jacket is often distributed over the exterior of the entire cylinder, but the cooling cone will be located in a single cylinder region where the temperature differential is the highest. The cooling end cap can also be used to increase the convective heat transfer coefficient by perturbing the flow as it is being compressed, in comparison to a conventional water jacket which generally requires smooth walls. Finally, more precise control of the dead volume at the end of stroke is achieved using the cooling cone embodiment as compared to a flat end cap.

Using a three dimensional cooling surface, such as the described cone or a more traditional fin and tube cooler, improves the heat transfer from the working fluid in comparison to a conventional flat plate cooling device. The heat transfer area using a three dimensional surface is much greater than if a flat plate is used. Furthermore, the shape of the three dimensional surface disturbs the fluid and increases its velocity, thereby increasing the convective heat transfer coefficient and total heat transfer. The benefit of achieving higher cooling will lead to a more isothermal compression, which means less input energy is required to compress a volume of fluid the same amount.

As compared to conventional configurations for cooling a working fluid, the present invention has a variety of advantages, including for example:

- Reduced materials

- Reduce flow required from coolant pump and thereby less input energy

- Cooler located at area of maximum temperature

- Increases turbulence/velocity of flow thereby increasing the convective heat transfer coefficient

- More precise control of dead volume.

Particularly as compared to a conventional natural gas cooling end cap, the present invention enables:

- Increased convective heat transfer coefficient and heat transfer area for the compressed natural gas

- Increased heat rejection from the compressed natural gas

- Lower temperature of the compressed natural gas thereby reducing the

energy for compression

- More precise control of dead volume.

In exemplary embodiments, a secondary cooling apparatus or mechanism may be provided, in which the submerged cooling portion described above may be combined with at least one such additional cooling apparatus or mechanism.

Secondary cooling mechanisms may be used in combination with each other, and take a variety of configurations and be located in various points in the system.

Secondary cooling mechanisms, for example, may be provided in any one of or combination of a cylinder end cap, including the end cap with the submerged cooling portion, the working fluid inlet and/or outlet, an interstage line between multiple stage cylinders (e.g., first and second cylinders), and/or piston rod-side cooling within a piston cylinder assembly. Referring again to Fig. 4, one type of secondary cooling mechanism is shown. In the specific example of Fig. 4, at least one evaporation coil may be provided in the fluid pathways of at least one of the working fluid inlet and working fluid outlet to further cool the fluid within such fluid pathways. For example, a first evaporation coil 60 may be provided on the working fluid inlet side adjacent the inlet check valve 52, and/or a second evaporation coil 62 may be provided on the working fluid outlet side adjacent the outlet check valve 54.

In such embodiments, the coolant used will be a fluid such that the coolant travels through the evaporation coiling coils that may be wrapped around the inlet, interstage, and outlet lines. Such a configuration may be regarded as having the properties and temperature associated with a two phase flow in which the

evaporation point occurs where some of the fluid is gas and some is liquid. As heat is extracted from the natural gas lines, the temperature of the cooling fluid will not change as it is experiencing a phase change as the amount of fluid in the gas state is increasing and the amount of liquid is decreasing. An advantage of such system is that the coolant does not change temperature as it absorbs heat, which keeps the difference in temperature between the coolant and the gas larger. To make a "closed system" the coolant after cooling the working is be sent back to a condenser element, as is known in the art, where the heat picked up during the cooling is extracted. After cooling the working fluid, the coolant will be mostly gas and a little liquid, and after leaving the condenser the coolant will mostly be in a liquid state.

As another variation on this embodiment including two-phase cooling, the naturalize gas alternatively may be heated at the inlet to provide an enhanced inlet flow, and then cooled later within the transmission system. In such embodiment, the evaporation coil at the inlet essentially would provide heating at the inlet as part of the two-phase cooling, and then localized cooling would be performed at other transmission lines within the system.

Fig. 6 is a schematic diagram depicting another exemplary embodiment of a secondary cooling mechanism for use in connection with a cooling end cap. In the example of Fig. 6, a cooling end cap 64 for a typical cylinder is shown in block form. The cooling end cap 64 may be a cooling end cap of a second stage cylinder that has the submerged cooling end cap configuration of Fig. 3. The cooling end cap 64 is in communication with one or more fluid passages 66 for communicating the working fluid into the piston-cylinder assembly (on the inlet side) or from the piston assembly (on the outlet side).

In exemplary embodiments of the secondary cooling mechanism of Fig. 6, the cooling end cap 64 may be provided with a plurality of cooling fins 68 to provide an increased surface area for cooling. The secondary cooling mechanism further may include an air flow blown over the cooling end cap 64 by a fan 65, the air blown by the fan being at a temperature less than the ambient temperature so as to cool the working fluid flowing through the cooling end cap and adjacent fluid passage 66. A Peltier device 67 may be provided in electrical communication with the fan 65. The Peltier device 67 converts waste heat withdrawn from the working fluid by the cooling end cap into electrical energy to power the cooling fan. Such a configuration adds to the energy efficiency of the system. The cooling fan further may be adjustable to control the amount of air flow, and/or to adjust the direction of the fan so as to provide a cooling air flow to different segments of the broader system as desired.

As another example of a secondary cooling mechanism, the various fluid passages in the system may be configured to increase the surface area for cooling. For example, Fig. 7 is a schematic diagram depicting a cross-sectional view one of the fluid connecting lines or passages 66, showing an exemplary finned

configuration. In the example of Fig. 7, at least one of internal fins 70 or external fins 72 may be provided so as to enhance the surface area for cooling as a secondary cooling mechanism. In a natural gas system such as that depicted in Figs. 1 and 2, a finned fluid line configuration may be provided in any line of the system, such as the inlet gas line, outlet gas line, or interstage line between first and second stage cylinders. A finned configuration is particularly suitable for the outlet gas line and the interstage line, where heat removal is most significant due to the heat generated by compression within the cylinders.

Fig. 8 is a schematic diagram depicting another example of a secondary cooling mechanism utilizing a Venturi fluid passage. The configuration of Fig. 8 relies on the Venturi effect to provide enhanced cooling, which is particularly suitable for use in the natural gas delivery system referenced above. In one embodiment, a fluid passage for the working fluid (e.g., natural gas) may be configured as a Venturi passage. For example, referring to Fig. 8, any suitable fluid passage 66 may include a larger diameter, low pressure inlet 80. A booster may be provided so as to increase the pressure and flow at the inlet. The working fluid (e.g., natural gas) then flows into a compressed passage 82, through which the working fluid has an increased flow velocity and pressure by virtue of the Venturi effect. As a result of the increased flow velocity and pressure, heat is generated and dissipated to the outside, as shown by the depiction of heat waves emanating from the compressed passage 82. The working fluid then flows through an outlet 84, which is configured so that the working fluid has a greater density, but the same pressure, as on the inlet side. To maintain the same inlet and outlet pressures, the outlet 84 is provided with a smaller diameter as compared to the inlet 80. The pressures are maintained with different diameters of the inlet and outlet due to the heat that was removed as the working fluid flowed through the compressed passage 82. In a preferred

embodiment, Venturi cooling may be provided at the natural gas inlet so that the inlet flow essentially is pre-cooled prior entering the first stage cylinder. Venturi cooling also may be suitable within the interstage line so that the flow essentially is pre- cooled prior entering the second stage cylinder.

In another embodiment based on Venturi cooling, a Venturi configuration may be employed as part of the embodiment of Fig. 6. In such embodiment, the Venturi passage of Fig. 8 may be placed between the fan 65 and the cooling end cap 64 such that the air from the fan is blown through the Venturi passage. In this

embodiment, the Venturi principles described above operate to cool the air from the fan 65, and the cool air the flows over the cooling end cap 64 as detailed above in connection with Fig. 6.

Fig. 9 is a schematic diagram depicting another example of a secondary cooling mechanism utilizing a one piece manifold unit 86. The one piece manifold unit integrates a working gas fluid flow 88 and a coolant fluid flow 90. As seen in Fig. 9, the directions of flow of the working fluid and coolant fluid may be opposite to each other (although this is optional), and heat is transferred from the working fluid to the coolant across the manifold portion 92. The manifold components may be secured with a bolt-on fastening elements and sealed with o-rings. The manifold

configuration provides improved cooling without the need for specialized check valves. This manifold configuration is particularly suitable for systems that rely on hydraulic cooling fluid, and the one piece manifold configuration has modular tube and fitting components that provide for easy assembly and disassembly. For example, referring to the exemplary natural gas system of Figs. 1 and 2, the manifold unit 86 of Fig. 9 may be provided as part of the interstage line that connects the output of the first stage cylinder to the input of the second stage cylinder. An advantage of using an integral manifold unit in the interstage line is that the gas flow and cooling liquid are transported in the same assembly. This configuration increases heat transfer and reduces the number of component parts of the system, for more efficient manufacture and assembly. Accordingly, the hydraulic line that transports the coolant fluid to the cooling end cap is combined with the working fluid interstage line, which creates a simpler package with increased heat transfer. The check valves that control fluid flow through the interstage line also may be incorporated into the manifold unit of Fig. 9, which further simplifies the

arrangement of the various system components.

Fig. 10 is a schematic diagram depicting another exemplary configuration of a one piece manifold unit 86, in a cross-sectional view, which may be employed in comparable manner as the one-piece manifold unit of Fig. 9. The configuration of Fig. 10 utilizes essentially concentric fluid passages for the working fluid and the coolant, which for example may be a hydraulic fluid. An inner tubing 94 defines an inner fluid passage 96 for the working fluid. In addition, an outer tubing 98

cooperates with the inner tubing 94 to define an outer fluid passage 100 for the coolant fluid flow. Similarly to the embodiment of Fig. 9, the directions of flow of the working fluid and coolant fluid may be opposite to each other (but this is optional), and heat is transferred from the working fluid to the coolant fluid across the inner tubing 94. The inner tubing may have an extruded profile of fins 95 to increase the area of heat transfer.

As referenced above, one or more of the various secondary cooling

mechanisms and apparatuses of Figs. 6-10 may be used in combination with the enhanced, submerged cooling end cap described with respect to Figs. 3-5. In this manner, full cooling performance may be achieved, including pre-cooling the fluid at the initial fluid inlet, cooling within an interstage line between sequential stage cylinders (e.g., first and second stage cylinders), cooling after compression at the fluid outlet, and cooling by providing an enhanced cooling end cap in one or more cylinders (e.g., first and/or second stage cylinders). As referenced above, the enhanced submerged cooling end cap typically would be provided at least in the second stage cylinder, where heat generation is most substantial.

In addition to cooling within a cylinder with an enhanced cooling end cap, embodiments of the present invention further provide for cooling on the piston rod side of the piston within the cylinder. Figs. 1 1 -13 depict exemplary embodiments of piston rod-side cooling configurations and features, which may be employed in combination with the enhanced submerged cooling end cap. As with the enhanced cooling end cap, piston rod-side cooling typically would be provided at least in the second stage cylinder, where heat generation is most substantial, although piston rod-side cooling also may be employed in a cylinder at any stage. Piston rod-side cooling further may be employed in combination with any of the secondary cooling apparatuses and mechanisms described above.

Fig. 1 1 is a schematic diagram depicting a cross-sectional view of another exemplary piston-cylinder assembly 1 10 in accordance with embodiments of the present invention, using piston rod side cooling. The piston-cylinder assembly 1 10 includes a cylinder 1 12 having a cylindrical wall defining an interior chamber 1 14 for the working fluid, which is closed at one axial end by a cylinder end cap 1 16. A piston 1 18 including a piston rod 120 connected to a piston head 122 is disposed in the interior chamber of the cylinder for reciprocating movement.

In the embodiment of Fig. 1 1 , an outer housing 124 encloses the cylinder 1 12, and the outer housing and cylinder define a sleeve 126. The sleeve 126 defines a fluid flow passage between external coolant ports 128 and internal cooling ports 130. The internal cooling ports 130 provide a passage for coolant into a coolant space132 in the area of the piston rod 120 and on an opposite side of the piston head 122 relative to the interior chamber 1 14 for the working fluid. The coolant fluid flows in a heat exchanger circuit 134 including a heat exchanger 136.

In this embodiment, as the piston moves to compress the working fluid in the interior chamber 1 14, coolant fluid is pulled from the heat exchanger circuit 134, through the external coolant ports 128 and into the sleeve 126. The coolant fluid further is pulled through the internal cooling ports 130 and into the coolant space 132 adjacent the piston rod 120. Heat from the compressed working fluid transfers into the coolant liquid in the sleeve 126 and coolant space 132. As the piston retracts, the heated coolant fluid is forced out of the coolant space 132 through the internal cooling ports 130, and back through the sleeve 126 and external coolant ports 128. The coolant then travels through the coolant circuit 134 to the heat exchanger 136, which removes the heat from the coolant. The cycle repeats as the piston

compresses and retracts.

Fig. 12 is a schematic diagram depicting a cross-sectional view of a three stage piston-cylinder assembly 140, including a three stage piston 144 moving within a cylinder 142 as are known in the art. The three stage piston-cylinder assembly may be employed in a piston rod-side cooling system comparable to that of Fig. 1 1 , with the addition of the outer housing and sleeve, and the heat exchanger circuit. The piston-cylinder assembly 140, including a three-stage piston, typically would be used as a singular assembly alternatively to a multi-stage system having first and second stage cylinders. The side chambers 143 for the working fluid would tend to be low pressure regions that would act comparably to a first stage cylinder, and the central chamber 145 for the working fluid would tend to be a high pressure region that would act comparably to a second stage cylinder.

Fig. 13 is a schematic diagram depicting a cross-sectional view of cooling piston 150 for use in the piston-cylinder assembly, such as for example that of Fig. 1 1 . It is known that during compression, the piston surface itself generates heat and preferably may be independently cooled. In the embodiment of Fig. 13, the piston rod includes a fluid flow path for transferring coolant fluid into the piston head for cooling the surface of the piston head. In particular, the piston 150 includes a piston rod 152 and a piston head 154. The piston rod has a coolant fluid inlet 156 that is in fluid communication with the coolant space 132 of Fig. 1 1 .

During compression, as coolant fluid is drawn into the coolant space, coolant fluid further is drawn through the inlet 156 and through an inlet passage 158 in the piston rod. From there, coolant fluid flows through a first connecting passage 160 into spaces and flow paths 162 within the piston head 154. The coolant fluid in the piston head draws heat from the surface of the piston, which has been heated during compression. In this manner, localized cooling of the piston head is achieved. The heated coolant fluid then flows back into the piston rod via a second connecting passage 164 and into an outlet passage 168. The heated coolant fluid continues to flow, next through an outlet 170 and back into the coolant space shown in Fig. 1 1 . A plurality of plugs 172 and stoppers or control valves 174 may aid in directing the flow of coolant fluid through the piston rod and piston head. As with other piston rod-side cooling, the additional features of Fig. 13 typically would be provided at least in the second stage cylinder, where heat generation is most substantial, although such features also may be employed in a cylinder at any stage. The configuration of Fig. 13 also may be employed in combination with any of the secondary cooling apparatuses and mechanisms described above.

Fig. 14 is a schematic diagram depicting a cross-sectional view of a portion of another exemplary piston-cylinder assembly 180 in accordance with embodiments of the present invention, using an extended cooling end cap for localized cooling adjacent corners of the piston head. Fig. 14 depicts a portion of a piston head 182 adjacent a portion of the space 184 in which the working fluid is compressed. The piston head moves adjacent cylinder wall. For convenience of a close-up view, a portion of the assembly 180 adjacent a corner 188 of the cylinder wall 186 is shown. It will be appreciated that opposite ends of the assembly 180 are comparably configured.

It is known that the last portion of the piston stroke, e.g., last ¾" in a typical assembly, generates the highest pressure gradient resulting in the highest portion of heat generation. To provide for enhanced cooling, in the assembly 180 the end cap 190 is an extended cooling end cap that extends around the corner portion 188 of the cylinder wall, and this portion of the cylinder wall is in contact with the piston head during the final portion of the piston stroke. For example, when the last ¾" of the piston stroke generates the highest pressure gradient, the extended cooling end cap extends around an amount of the cylinder body greater than the ¾", such as by one inch, to ensure full coverage of the piston head during final stroke portion.

In exemplary embodiments, the assembly 180 has a coolant fluid port 192 for receiving coolant fluid from a heat exchanger circuit as described above. The fluid port 192 may provide both an inlet for cool coolant fluid, and an outlet for heated coolant fluid, in reciprocating fashion during the piston stroke. Coolant fluid paths 194 may follow the circumference of the outer diameter of the cylinder body. A plurality of fluid seals 196 provide sealing relative to the fluid flow, and a plurality of working fluid seals 198 provide sealing relative to the working fluid flow. In exemplary embodiments, the cap 190 may be a unitary cast component, including casting of the fluid paths 194 and a plurality of dimples 200 may be formed on an inner surface of the cap to increase the surface area for heat transfer with minimal dead volume increase. The cast of the fluid flow paths eliminates the need for separate drilling during manufacture, and also result in fewer points for potential leakage. The extended cooling end cap190 of Fig. 14 may be utilized in combination with a submerged cooling end cap portion as described with respect to Figs. 3-5.

Figs. 15A and !5B are schematic diagrams depicting another exemplary piston-cylinder assembly 250 in accordance with embodiments of the present invention, using cooling pins as a submerged cooling portion. Fig. 15A shows a cross sectional view, and Fig. 15B shown an isometric view. Assembly 250 includes a cylinder 252 including an end cap 254 at the top of cylinder, A piston 256 operates by reciprocating movement within the cylinder to pump a working fluid.

The embodiment of Figs. 15A and 15B is a variation on the embodiment of Figs. 3-5, in which the submerged cooling portion of the end cap is configured as a plurality of first cooling pins 258 that extend from the end cap 254. The cooling pins are in fluid communication with a source of coolant fluid. In exemplary

embodiments, the piston 256 includes a plurality of second cooling pins 260 also in fluid communication with the source of coolant fluid. The coolant fluid flows in the direction of the arrows shown in Fig. 15A. As the piston compresses the working fluid, the working fluid is in contact with the various cooling pins to transfer heat out of the working fluid. The pin configuration of Figs. 15A and 15B provides for an increased surface area for heat transfer. As shown particularly in Fig. 15A, to further increase the surface area for heat transfer, the first and second sets of cooling pins are configured to overlap in an intermeshing fashion during the latter stage of the compressions stroke.

In accordance with the above description, an aspect of the invention is a piston-cylinder assembly. In exemplary embodiments, the assembly may be configured for compression of a working fluid and include a cylinder having a cylindrical wall defining an interior chamber that is closed at one axial end by a cylinder end cap, a piston disposed in the interior chamber of the cylinder for reciprocating movement, a working fluid inlet proximate the cylinder end cap through which fluid can flow into the interior chamber when the piston is moving away from the cylinder end cap, and a working fluid outlet proximate the cylinder end cap through which fluid can flow out of the interior chamber when the piston is moving towards the cylinder end cap. The cylinder end cap has a submerged cooling portion that protrudes into the interior chamber of the cylinder and is spaced radially inwardly from the cylindrical wall of the cylinder for defining an annular space through which the working fluid flows for cooling of the working fluid.

In an exemplary embodiment of the piston-cylinder assembly, the submerged cooling portion axially overlaps the working fluid inlet and outlet such that fluid flowing into and out of the interior space is caused to flow through the annular space.

In an exemplary embodiment of the piston-cylinder assembly, the working fluid inlet and outlet are located at diametrically opposite sides of the annular space.

In an exemplary embodiment of the piston-cylinder assembly, the working fluid inlet and outlet are laterally adjacent an upper half portion of the annular space.

In an exemplary embodiment of the piston-cylinder assembly, the submerged cooling portion includes coolant passages through which a coolant can flow to extract heat from the submerged cooling portion and in turn from the working fluid.

In an exemplary embodiment of the piston-cylinder assembly, the cylinder end cap includes coolant inlet and outlet ports.

In an exemplary embodiment of the piston-cylinder assembly, the submerged cooling portion is conical such that the annular space progressively increases in cross-sectional area going in a direction toward the piston.

In an exemplary embodiment of the piston-cylinder assembly, the end cap extends around a portion of the cylinder, and the assembly further includes coolant fluid paths that follow a circumference of an outer diameter of the cylinder.

In an exemplary embodiment of the piston-cylinder assembly, the submerged cooling portion is configured as a plurality of first cooling pins that extend from the end cap.

In an exemplary embodiment of the piston-cylinder assembly, a plurality of second cooling pins extend from the piston, and the first and second cooling pins are configured to overlap in an intermeshing fashion during a compression stroke of the piston. Another aspect of the invention is a cylinder based system. In exemplary embodiments, the system is configured for compressing a working fluid and includes a piston-cylinder assembly. The piston-cylinder assembly includes a cylinder having a cylindrical wall defining an interior chamber that is closed at one axial end by a cylinder end cap, a piston disposed in the interior chamber of the cylinder for reciprocating movement, a working fluid inlet proximate the cylinder end cap through which fluid can flow into the interior chamber when the piston is moving away from the cylinder end cap, a working fluid outlet proximate the cylinder end cap through which fluid can flow out of the interior chamber when the piston is moving towards the cylinder end cap, and coolant inlet and outlet ports in the cylinder end cap for coolant fluid. The cylinder end cap has a submerged cooling portion that protrudes into the interior chamber of the cylinder and is spaced radially inwardly from the cylindrical wall of the cylinder for defining an annular space through which the working fluid flows for cooling of the working fluid. The system further includes a working fluid source connected to the working fluid inlet, a working fluid sink connected to the working fluid outlet, and a heat exchanger circuit connected between the coolant inlet and outlet ports for the flow of coolant fluid into and out from the submerged cooling portion.

In an exemplary embodiment of the system, the system further includes a first stage piston-cylinder assembly and a second stage piston-cylinder assembly, and the described piston-cylinder assembly is the second stage piston-cylinder assembly.

In an exemplary embodiment of the system, the system further includes a secondary cooling mechanism for cooling the working fluid in combination with the submerged cooling portion.

In an exemplary embodiment of the system, the secondary cooling

mechanism comprises at least one evaporation coil located in at least one of the working fluid inlet or working fluid outlet.

In an exemplary embodiment of the system, the secondary cooling

mechanism comprises a plurality of fins extending from the end cap, and a fan that blows cooling air over the end cap. In an exemplary embodiment of the system, the system includes a first stage piston-cylinder assembly and a second stage piston-cylinder assembly, and the described piston-cylinder assembly is the second stage piston-cylinder assembly. An interstage line provides a fluid connection between the first stage piston-cylinder assembly and the second stage piston-cylinder assembly, and the secondary cooling mechanism comprises a finned configuration provided in at least one of the working fluid inlet, working fluid outlet, or interchange line.

In an exemplary embodiment of the system, the system includes a first stage piston-cylinder assembly and a second stage piston-cylinder assembly, and the described piston-cylinder assembly is the second stage piston-cylinder assembly. An interstage line provides a fluid connection between the first stage piston-cylinder assembly and the second stage piston-cylinder assembly, and the secondary cooling mechanism comprises a Venturi passage located in at least one of the working fluid inlet and interstage line.

In an exemplary embodiment of the system, the system include a first stage piston-cylinder assembly and a second stage piston-cylinder assembly, and the described piston-cylinder assembly is the second stage piston-cylinder assembly. An interstage line provides a fluid connection between the first stage piston-cylinder assembly and the second stage piston-cylinder assembly, and the secondary cooling mechanism comprises a one-piece manifold unit located in the interstage line, the one-piece manifold unit including fluid passages for both the working fluid and the coolant fluid.

In an exemplary embodiment of the system, the piston comprises a piston rod attached to a piston head, and the secondary cooling mechanism comprises a coolant space for receiving coolant fluid, the coolant space being located adjacent the piston rod on a side of the piston head opposite of the piston head relative to the interior chamber for the working fluid.

In an exemplary embodiment of the system, the piston rod comprises a fluid flow path for transferring coolant fluid into the piston head for cooling a surface of the piston head.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

Claims

1 . A piston-cylinder assembly for compression of a working fluid, comprising:

a cylinder having a cylindrical wall defining an interior chamber that is closed at one axial end by a cylinder end cap ,

a piston disposed in the interior chamber of the cylinder for reciprocating movement,

a working fluid inlet proximate the cylinder end cap through which fluid can flow into the interior chamber when the piston is moving away from the cylinder end cap, and

a working fluid outlet proximate the cylinder end cap through which fluid can flow out of the interior chamber when the piston is moving towards the cylinder end cap,

wherein the cylinder end cap has a submerged cooling portion that protrudes into the interior chamber of the cylinder and is spaced radially inwardly from the cylindrical wall of the cylinder for defining an annular space through which the working fluid flows for cooling of the working fluid.

2. The piston-cylinder assembly of claim 1 , wherein the submerged cooling portion axially overlaps the working fluid inlet and outlet such that fluid flowing into and out of the interior space is caused to flow through the annular space.

3. The piston-cylinder assembly of any of claims 1 -2, wherein the working fluid inlet and outlet are located at diametrically opposite sides of the annular space.

4. The piston-cylinder assembly of any of claims 1 -3, wherein the working fluid inlet and outlet are laterally adjacent an upper half portion of the annular space.

5. The piston-cylinder assembly of any of claims 1 -4, wherein the submerged cooling portion includes coolant passages through which a coolant can flow to extract heat from the submerged cooling portion and in turn from the working fluid.

6. The piston-cylinder assembly of claim any of claims 1 -5, wherein the cylinder end cap includes coolant inlet and outlet ports.

7. The piston-cylinder assembly of any of claims 1 -6, wherein the submerged cooling portion is conical such that the annular space progressively increases in cross-sectional area going in a direction toward the piston.

8. The piston-cylinder assembly of any of claims 1 -7, wherein the end cap extends around a portion of the cylinder, and the assembly further includes coolant fluid paths that follow a circumference of an outer diameter of the cylinder.

9. The piston-cylinder assembly of any of claims 1 -6 and 8, wherein the submerged cooling portion is configured as a plurality of first cooling pins that extend from the end cap.

10. The piston-cylinder assembly of claim 9, wherein a plurality of second cooling pins extend from the piston, and the first and second cooling pins are configured to overlap in an intermeshing fashion during a compression stroke of the piston.

1 1 . A system for compressing a working fluid, comprising

a piston-cylinder assembly comprising:

a cylinder having a cylindrical wall defining an interior chamber that is closed at one axial end by a cylinder end cap,

a piston disposed in the interior chamber of the cylinder for

reciprocating movement,

a working fluid inlet proximate the cylinder end cap through which fluid can flow into the interior chamber when the piston is moving away from the cylinder end cap, a working fluid outlet proximate the cylinder end cap through which fluid can flow out of the interior chamber when the piston is moving towards the cylinder end cap, and

coolant inlet and outlet ports in the cylinder end cap for coolant fluid, wherein the cylinder end cap has a submerged cooling portion that protrudes into the interior chamber of the cylinder and is spaced radially inwardly from the cylindrical wall of the cylinder for defining an annular space through which the working fluid flows for cooling of the working fluid;

the system further comprising:

a working fluid source connected to the working fluid inlet,

a working fluid sink connected to the working fluid outlet, and

a heat exchanger circuit connected between the coolant inlet and outlet ports for the flow of coolant fluid into and out from the submerged cooling portion.

12. The system of claim 1 1 , comprising a first stage piston-cylinder assembly and a second stage piston-cylinder assembly, wherein the piston-cylinder assembly recited in claim 1 1 is the second stage piston-cylinder assembly.

13. The system of any of claims 1 1 -12, further comprising a secondary cooling mechanism for cooling the working fluid in combination with the submerged cooling portion.

14. The system of claim 13, wherein the secondary cooling mechanism comprises at least one evaporation coil located in at least one of the working fluid inlet or working fluid outlet.

15. The system of any of claims 13-14, wherein the secondary cooling mechanism comprises a plurality of fins extending from the end cap, and a fan that blows cooling air over the end cap.

16. The system of any of claims 13-15, comprising a first stage piston- cylinder assembly and a second stage piston-cylinder assembly, wherein the piston- cylinder assembly recited in claim 1 1 is the second stage piston-cylinder assembly; and

an interstage line that provides a fluid connection between the first stage piston-cylinder assembly and the second stage piston-cylinder assembly;

wherein the secondary cooling mechanism comprises a finned configuration provided in at least one of the working fluid inlet, working fluid outlet, or interchange line.

17. The system of any of claims 13-16, comprising a first stage piston- cylinder assembly and a second stage piston-cylinder assembly, wherein the piston- cylinder assembly recited in claim 1 1 is the second stage piston-cylinder assembly; and

wherein the secondary cooling mechanism comprises a Venturi passage located in at least one of the working fluid inlet and interstage line.

18. The system of any of claims 13-17, comprising a first stage piston- cylinder assembly and a second stage piston-cylinder assembly, wherein the piston- cylinder assembly recited in claim 1 1 is the second stage piston-cylinder assembly; and

wherein the secondary cooling mechanism comprises a one-piece manifold unit located in the interstage line, the one-piece manifold unit including fluid passages for both the working fluid and the coolant fluid.

19. The piston assembly of any of claims 13-18, wherein the piston comprises a piston rod attached to a piston head, and the secondary cooling mechanism comprises a coolant space for receiving coolant fluid, the coolant space being located adjacent the piston rod on a side of the piston head opposite of the piston head relative to the interior chamber for the working fluid.

20. The piston assembly of claim 19, wherein the piston rod comprises a fluid flow path for transferring coolant fluid into the piston head for cooling a surface of the piston head.