Patent Application
20170314547
This invention relates to reciprocating compressors with open crankcases, specifically, to a method and device to reduce or eliminate contamination of ambient air into the compressed gas.
Maintaining the purity of gases during compression in reciprocating compressors is an important problem in compression technology. The compressed gas can become contaminated with the lubricants used in the compressor, or by ambient gasses, commonly air, entering the compressor during the suction stroke of the compressor. These contaminations of the compressed gas can have a detrimental effect in certain applications.
To avoid the lubrication contamination problem, many manufacturers are offering piston compressors that do not require lubrication in the cylinders. These compressors are commonly referred to as ‘oil-less’, ‘oil-free’ or ‘dry-running’. Where strict separation between the gas being compressed and ambient air is critical, complex and intricate compressor designs may necessary. As no piston seal (or ring) can perfectly seal against the cylinder wall, some leakage into and out of the cylinder, is inevitable. To mitigate this vulnerability, additional barriers to gas escape and intrusion can be designed into the compressor. For example, labyrinth compressors may have complex piston rod designs and multiple sealed chambers around the moving shafts. These designs can be very effective at maintaining compressed gas purity, but their intricate and precise designs come with an accordingly higher capital cost.
As many compressor applications are not sensitive gas purity, compressor designs in which the piston seal is the only barrier between the compressed gas and ambient air, are common. They may have only a simple polymer seal between the piston and cylinder wall. They have open crankcases to allow ambient air to circulate through the compressor crankcase, to provide cooling to the compressor piston, cylinder, and the drive motor. They operate at fairly mild pressures, up to a few hundred psi, and are generally less than 15 hp. These compressors are often used for compressed air applications such as is common in an automotive repair shops. These compressors are very economical and reliable for applications that are not sensitive to the purity of the compressed gas.
As open-crankcase, compressors age, the piston seals begin to leak, often within a few hundred hours of run time. Because of some specific design features of the piston seal, this leakage may not seriously affect the compressor performance with regards to outlet pressure and flow rate, however, it often leads to contamination of the compressed gas with ambient air. The compressor will begin to pull ambient air into the cylinder, from the crankcase, around the worn piston seals, during the suction stroke of the compression cycle. This ambient air then becomes part of the compressed gas.
This piston seal leakage vulnerability has made these low-cost open-crankcase, oil-free, compressors, unsuitable for applications in which purity of the compressed gas is critical.
A common application requiring high compressed gas purity, is on-demand industrial oxygen systems. Users of these systems include glassblowers and welders. These systems employ pressure swing adsorption oxygen generators, which extract and purify oxygen from ambient air. The oxygen is delivered from the generator at fairly low pressure, usually 5-40 psi. This pressure is too low for many fuel gas/oxygen torches or for effective transport in a long manifold. Therefore, the oxygen is often compressed to a more usable pressure in the general range of 80 to 300 psi.
Many attempts to employ an open-crankcase piston compressor in this application have had limited success. The oxygen produced by the generator is usually 90% to 95% oxygen. When the oxygen is compressed in an open crankcase piston compressor, it can become diluted with ambient air. Commercial systems using this type of compressor often suffer significant oxygen purity loss (10-20%) across the compression stage.
The oxygen produced by the generator is usually 90% to 95% oxygen, and has a fixed flow rate. The flow rate capacity of the open-crankcase piston compressor is sensitive to the back pressure of downstream processes. For example, a compressor capable of compressing 90 liters per minute to 15 psi, may only be able to compress 30 liters per minute to 100 psi. This variable compressor capacity makes it impossible to properly match the oxygen generator and compressor flow rates.
When the compressor capacity is less than the generator flow rate, the compressor intake pressure will rise, and may eventually overload the compressor. This can cause the compressor drive motor to overheat and stall. Pressure relief devices can be installed on the compressor inlet to avoid overloading the compressor. This type of protection wastes the excess oxygen.
When the compressor capacity is greater than the generator flow rate, the compressor intake pressure will drop. Thermodynamic principles dictate that lowering the inlet pressure will cause a dramatic rise in the compressor outlet gas temperature. The elevated temperature in the compressor increases the piston seal wear and will accelerate the onset of leakage around the piston. Once the piston seal efficiency has been compromised, the higher pressure of the ambient air relative to the pressure in the cylinder during the suction stroke, will force ambient air past the worn seals, into the cylinder.
An oxygen system using an open-crankcase compressor as just described, can appear to be functioning properly in regards to pressure and flow rate, yet exhibit a loss of oxygen purity of 15%-20% across the compression stage of the system. The performance of welding and glassblowing torches is seriously degraded by the use of low purity oxygen. Flame temperatures drop, and flame chemistry becomes more reducing, as the oxygen purity drops. At oxygen purity levels of 80% or less, the flame is nearly un-useable for many applications.
Because of these shortcomings these economical oil-less open-crankcase compressors are not well suited to applications which are sensitive to the compressed gas purity.
There are many other applications that are sensitive to compressed gas purity including medical oxygen systems and inert gas systems.
Accordingly, this invention solves many of the problems that are encountered by using an oil-less open-crankcase piston compressor in a high purity gas system.
This invention enables economical oil-less open-crankcase piston compressors to perform in purity sensitive applications that until now, required the use of more complex, more expensive compressor designs.
This invention reduces or eliminates, a vacuum condition from developing at the intake of the oil-less open-crankcase piston compressor.
This invention greatly reduces the gas temperature rise through the oil-less open-crankcase piston compressor, resulting in increased service life.
This invention prevents ambient air from entering the oil-less open-crankcase piston compressor during the suction stroke, allowing the compressor to function properly even as the piston seals age and wear.
Compressor 10 is an open-crankcase compressor.
Low Pressure Pulsation Dampener 20 is a volume bottle.
Pressure Regulator 30 is a pressure regulator.
Heat Exchanger 50 is heat exchanger.
High Pressure Pulsation Dampener 40 is a volume bottle.
Pressure Relief Valve 60 is a pressure relief valve.
Compressor 10 is an open-crankcase oil-less reciprocating (piston) compressor. In operation, it receives gas at its inlet, and compresses it to a higher pressure at its outlet. Compressor 10, by its very nature, will generate pressure fluctuations at both its inlet and outlet. These pressure fluctuations correspond to the stroke of each piston.
High Pressure Pulsation Dampener 40 and Low Pressure Pulsation Dampener 20 are volume bottles used to dampen the pressure fluctuations created by Compressor 10. A volume bottle, is a pressure vessel usually three to ten times the swept volume of a compressor cylinder. The compressed gas in the volume bottle absorbs and dampens the pressure fluctuations generated by Compressor 10. Dampening the pressure fluctuations is necessary to prevent damage to, and allow proper function of, Pressure Regulator 30.
Pressure Regulator 30, is a common mechanical pressure regulator. Its function is to control pressure on the low pressure side of Compressor 10. When the Pressure Regulator 30 senses a pressure on the low pressure side of Compressor 10, that is below Pressure Regulator 30's setpoint, Pressure Regulator 30 opens its internal control valve and allows gas to flow from the high pressure side to the low pressure side of Compressor 10. When the pressure on the low side Compressor 10 rises back to Pressure Regulator 30's setpoint, the control valve inside Pressure Regulator 30 will begin to close. In this manner, Pressure Regulator 30 controls the pressure on the low pressure side of Compressor 10.
Heat Exchanger 50 is a finned tube heat exchanger. The temperature of the gas rises as it is compressed in the Compressor 1. Heat Exchanger 50 will transfer the excess heat from the compressed gas to the ambient air through its fins. This is necessary to prevent excessive heat buildup in the system.
Pressure Relief Valve 60 is a mechanical pressure relief valve. It monitors the gas pressure at the inlet to Compressor 10, and if the pressure exceeds the Pressure Relief Valve 60 setpoint, Pressure Relief Valve 60 will open and release the excess gas. Once the pressure at the inlet of Compressor 10 falls below the Pressure Relief Valve 60 setpoint, Pressure Relief Valve 60 will close.
The path from the outlet of Compressor 10, through High Pressure Pulsation Dampener 40, through Heat Exchanger 50, through Pressure Regulator 30, through Low Pressure Pulsation Dampener 20 and to the inlet of Compressor 10, forms a recycle loop. High pressure compressed gas from the outlet of the Compressor 10, is allowed to flow through this recycle loop, back to inlet of the Compressor 10. The gas flow in this recycle loop is controlled by Pressure Regulator 30. Pressure Regulator 30's pressure setpoint is set so as to prevent the pressure at the inlet of Compressor 10 from falling excessively below the pressure of the ambient air.
In summation, Compressor 10 compresses the gas. Heat Exchanger 50 removes excess heat from the compressed gas. Low Pressure Pulsation Dampener 20 and High Pressure Pulsation Dampener 40, protect Pressure Regulator 30 from pressure fluctuations generated by Compressor 10. Pressure Regulator 30 controls the gas flow through the recycle loop to prevent excessively low pressure at the inlet to Compressor 1. Pressure Relief Valve 6 protects the Compressor 1 from excessively high inlet pressure.
Another embodiment of this invention omits Pressure Relief Valve 60. If the source gas is of insufficient flow rate to exceed the capacity of Compressor 10, Pressure Relief Valve 60 may not be necessary. Additionally, some commercially available designs of Pressure Regulator 30, may include some pressure relief capability.
Another embodiment of this invention may omit either, High Pressure Pulsation Dampener 40, Low Pressure Pulsation Dampener 20, or both. In some cases, the function of a pulsation dampener may be accomplished by using a length of pipe or flexible tubing of sufficient volume to effectively dampen the pressure spikes created by Compressor 10. This could eliminate the need for a discreet pulsation dampener.
Another embodiment of this invention may use a pulsation dampener design other than a volume bottle, for High Pressure Pulsation Dampener 40 and Low Pressure Pulsation Dampener 20. There are many pulsation dampener designs that are commercially available.
Another embodiment of this invention may omit Heat Exchanger 50. Compressor 10, High Pressure Pulsation Dampener 40, Low Pressure Pulsation Dampener 20 and the piping or tubing associated with recycle loop, all have some heat exchange capacity which may be sufficient to mitigate excessive heat buildup.
Another embodiment of this invention might replace the finned tube Heat Exchanger 50, with another type heat exchanger. There are many heat exchanger designs that are commercially available.
Another embodiment of this invention may replace Pressure Regulator 30, with another device that accomplishes the same function. There are many commercially available options, a combination of a discreet pressure sensor, discreet control valve, and programmable logic controller (PLC) is one example.
Additional embodiments can be assembled using any number of combinations of these alternative embodiments.
The intrusion of ambient air into the Compressor 10 requires both of two conditions: first, a path for the ambient air to flow through and second, a pressure differential of sufficient magnitude to push the ambient air through this path into Compressor 10.
Compressor 10 piston seals inevitably wear and begin to leak, providing a path for the ambient air to flow, and thereby satisfying the first necessary condition for the intrusion of ambient air into Compressor 10.
The method of this invention greatly reduces the development of the second necessary condition, a pressure differential of sufficient magnitude (a vacuum condition) to push the ambient air past the worn seals, into the Compressor 10.
When Pressure Regulator 30 senses a vacuum condition at the inlet of Compressor 10, relative to the ambient air pressure, Pressure Regulator 30 allows a portion of the compressed gas from the outlet of Compressor 10 to flow back to the inlet of Compressor 10. This ‘recycled’ compressed gas reduces the vacuum relative to the ambient air pressure, at the inlet of Compressor 10, and hence prevents the intrusion of ambient air into Compressor 10.
Recycling compressed gas from the outlet of a compressor back to the inlet is often referred to as ‘recycle’ capacity control. Reciprocating compressors do not respond well to ‘choke’ flow control. Simply restricting flow to a reciprocating compressor can cause excessive pressures and temperatures. Recycle capacity control allows the compressor to operate in harmony with upstream and downstream process demands while avoiding these extremes. This invention is a new use for recycle control. The intent is not to alter capacity, but to prevent the intrusion of ambient air, by preventing the formation of a vacuum condition inside an open-crankcase compressor.
Thus the reader will see that the gas compression system of this invention provides a highly reliable and economical device that can deliver compressed gas free from contamination with ambient air.
While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one preferred embodiment. Many other variations are possible. Any compressed gas application, industrial or medical, that is sensitive to contamination by ambient air would benefit from this invention. Nitrogen, argon, fuel gases, and carbon dioxide are examples gasses that are commonly used applications that are sensitive to contamination with ambient air. Accordingly, the scope of the invention should be determined not by the embodiment(s) illustrated, but by the appended claims and their legal equivalents.
