Compressor aftercooler bypass with integral water separator

An aftercooler bypass system for selectively allowing a portion of the hot compressed gases exiting an air compressor to bypass an aftercooler and intermix with the cooled compressed gases exiting the aftercooler in order to ensure that the cooled compressed gases are above a threshold temperature when the ambient air temperature is at or below freezing. The system includes a valve for controlling the amount of air diverted around the aftercooler and a mixing chamber for allowing the valved air supply to intermix with the aftercooler outlet stream. Temperature sensor may be used to measure ambient air temperature and downstream air temperature to control the opening and closing of the valve and maintain the desired mixed air temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compressor aftercooler bypass systems and, more particularly, to an aftercooler bypass having integral water separator.

2. Description of the Related Art

Railway braking systems rely on, among other things, air compressors to generate the compressed air of the pneumatic braking system. As the compression of air results in heating of the air to temperatures that are too hot for braking systems, railway air compressors are generally provided with an aftercooler to cool the compressed air to 20° F. to 40° F. above ambient temperature. The cooled, compressed air is then supplied to the air supply system of a locomotive through a compressor discharge pipe that connects to the first main reservoir. This discharge pipe may be as long as 30 feet, and may necessarily include several ninety degree bends. In winter operation, when the ambient air temperature can be well below freezing (32° F.), water vapor and water aerosol in the compressed air stream can freeze in the compressor discharge pipe, thereby at least partially blocking the flow of air to the braking system and adversely interfering with the operation of the braking system.

As is well known to those skilled in the art, and described by a body of knowledge known as psychrometrics, the maximum total amount of water vapor in a volume of air is strongly dependent on the air temperature, as warm air is able to hold much more water vapor than cool air. This effect is characterized as the partial pressure saturation pressure. Further, as is also well known, the water vapor saturation partial pressure is the maximum water vapor in air at that temperature, regardless of air pressure. As air is compressed, the water vapor in the air will also be compressed, until the water vapor partial pressure equals the saturation pressure. The net result is that for a railway compressor with a 10.5:1 compression ratio, intake air as dry as 9.5 percent relative humidity will be at 100 percent relative humidity after compression. Lastly, due to the thermodynamics of air, the temperature of the air increases significantly as a result of compression. For a two-stage railway compressor, the second stage discharge temperature may be as high as 300° F. above ambient temperature.

Thus, based on the temperature dependent water vapor holding capacity of air and the effect of the compression on the water holding capacity of the air, the hot air discharged from the second stage of an air compressor may contain a significant amount of water vapor. As this hot air flows through a compressor aftercooler, the air temperature is reduced to 20° F. to 40° F. above ambient temperature. Air at this temperature can hold much less water vapor than air at the second stage discharge temperature, so the excess water vapor precipitates out as liquid water and/or water aerosol. When this liquid water is transported into the compressor discharge pipe, it may freeze if the discharge pipe and ambient air are cold enough. In addition, because the air exiting the compressor is 20° F. to 40° F. above ambient air temperature, it is subject to further cooling in the compressor discharge pipe. As the air temperature drops in the pipe, further water will precipitate out thereby compounding the problem.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises an air compressor for railway braking system that includes an integrated aftercooler bypass valve and integral water separator to prevent freezing of the compressor discharge pipe in winter operation. An integrated aftercooler bypass valve controllably connects the outlet of the second stage of the compressor to the outlet of the aftercooler. When the aftercooler bypass valve is open, then a fraction of the hot air from the compressor second stage outlet flows to the mixing chamber of the aftercooler bypass valve assembly, thereby bypassing the aftercooler. The remaining fraction of the hot air from the compressor second stage outlet flows through the aftercooler and is cooled to a temperature of 20° F. to 40° F. above ambient temperature as in conventional aftercooling systems. This cooled fraction of air from the aftercooler is directed to a second inlet port on the aftercooler bypass valve assembly to the mixing chamber, where it is mixed with the hot air from the first fraction of air. The combined air has a new temperature which is a mass-temperature average of the two air streams and the new outlet air temperature is the result of the relative mass flow of the two air streams, which is a consequence of the flow capacity of the open bypass valve. For example, the flow capacity of the open bypass valve could be selected to provide a new, mixed compressor outlet temperature of 140° F. above ambient temperature so that even if the ambient air temperature was −40° F., the outlet air temperature presented to the discharge pipe would be 100° F. The outlet air temperature can therefore be selected to have a high-enough temperature so that even after flowing through the cold discharge pipe the air has sufficient heat that it remains above 32° F., thus preventing freezing in the pipe.

When the bypass valve is closed, all of the hot air from the compressor second stage outlet flows through the aftercooler and is cooled to a temperature of 20° F. to 40° F. above ambient temperature. The aftercooler bypass valve is controlled to be opened or closed depending on optionally either ambient temperature and/or the compressor system outlet temperature. When the ambient temperature is below a threshold, such as 32° F., then the aftercooler bypass valve is opened. At temperatures above the control temperature, the aftercooler bypass valve is closed.

The aftercooler bypass valve assembly optionally includes an integral water separator to remove the liquid and aerosol water from the outlet air stream. By making the water separator part of the aftercooler bypass valve assembly, the water separator is operational when the aftercooler bypass valve is open and when it is closed. Furthermore, packaging the water separator with the aftercooler bypass valve assembly simplifies the design, reduces the cost, eliminates piping connections and makes for a more compact arrangement.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like reference numerals refer to like parts throughout, there is seen inFIG. 1an after cooler bypass system10. System10is interconnected to an air compressor12via a connector duct14that is fluidly interconnected to the second stage outlet16of compressor12so that at least a portion of the air exiting compressor12may be redirected to system10away from the aftercooler inlet pipe18of a conventional aftercooler20. Connector duct14diverts the compressed air exiting outlet16of air compressor12to a bypass valve assembly22having a mixing chamber24. Mixing chamber24is also is interconnected to the discharge flange34of aftercooler20, so that cooled air exiting aftercooler20may be intermixed with the hot air diverted by connector duct14. Valve assembly22further comprises a bypass valve26that may be selectively opened or closed, or at least partially opened, based on a threshold, such as the ambient air temperature. Valve assembly22preferably comprises a water separator28attached thereto and positioned proximately to mixing chamber24to assist with the removal of water from the intermixed air streams. The intermixed air in mixing chamber24may then be provided to the braking system via an outlet flange42that can connect to the conventional discharge piping used to conduct compressed air to the main reservoir of the braking system. When bypass valve26is closed, the cooled compressed air exiting aftercooler20will still pass through mixing chamber24so that water separator28can remove any undesired water and then exit to the braking system via flange42.

Bypass valve26is preferably dimensioned to provide a predetermined mixing ratio of bypassed air and thus result in a predetermined outlet temperature above ambient temperature when ambient air temperatures fall below as threshold, such as freezing. Alternatively, as explained below, valve26may be controlled to adaptively maintain mixed air temperature based on the ambient air temperature. Furthermore, as seen inFIG. 1, aftercooler bypass valve assembly22may be formed as a single, integral unit that may be installed or replaced as a single unit for easier installation or repair in the field.

Referring toFIG. 2, bypass valve26selectively allows compressed air leaving compressor12to bypass aftercooler20and then intermix with the cooled air leaving aftercooler20by discharge flange34. Thus, bypass valve assembly22provides a direct and short bypass of aftercooler20so that when bypass valve26is open, the flow resistance through bypass valve assembly22is less than the flow resistance through aftercooler20. As a result, a substantive fraction of hot air will preferentially flow through bypass valve26into mixing chamber24. This arrangement is significantly simpler and less costly than conventional approaches that necessitate the use of a three-way valve to simultaneously block the connection to an aftercooler while opening another connection to an aftercooler bypass line.

As seen inFIG. 1, water separator28preferably includes an automatic drain valve30to expel liquid and aerosol water from the outlet air stream. While drain valve30is shown schematically inFIG. 2as a solenoid valve on the bottom of the reservoir32of water separator28, drain valve30could additionally comprise a pneumatically piloted drain valve at the bottom of the reservoir, with the controlling solenoid integrated into the block of aftercooler bypass valve22. Reservoir32of water separator28may include an integral, pneumatic connection between the solenoid valve30in the valve block and the pneumatically piloted drain valve in the bottom of the reservoir, so that the water separator reservoir could be removed for maintenance without disturbing electrical wiring or piping.

While bypass valve26could be formed using a suitable two-way valve known in the art, bypass valve26may also be made in the same manner as the unloading valves64of the cylinder heads of air compressor12, as these valves are designed to operate reliably at the high temperature and pressure of the second stage cylinder outlet. For example, as seen inFIG. 3, bypass valve26may comprise a housing50having a control input52for controlling the position of a valve54positioned within housing50and biased by one or more springs56for movement between a closed position, where valve52engaged a seat58formed in housing50, and an open position, where valve52allow an inlet port60to be in communication with an outlet port62. Preferably valve54and seat58form a metal to metal contact for reliable operation at the high temperatures and pressures associated with system10. Inlet port60is interconnected to second stage outlet16of compressor12by connector duct14, and outlet port62is interconnected to mixing chamber24. Using the same manufacturing process for both bypass valve26and the unloading valves64of air compressor12reduces the variety of parts necessary for initial manufacture and for periodic remanufacture and maintenance.

While the forgoing description is discussed in the context of a two-state aftercooler bypass valve26, i.e., either open or closed, bypass valve26could optionally be a proportional valve that would allow the outlet temperature of aftercooler20to be controlled over a range of temperatures. For example, the outlet temperature could be controlled by an associated controller36having an ambient air thermometer38, or comparable sensor, as well as an inline temperature sensor40downstream of mixing chamber24. Thus, the outlet temperature could be set to 100° F. whenever the ambient temperature is at or below freezing by varying the opening of aftercooler bypass valve26to provide the needed high temperature air flow to mixing chamber24. For example, if the ambient temperature was above 32° F., then the aftercooler bypass controller36would close aftercooler bypass valve26and all the air volume would flow through the aftercooler so that the compressor outlet temperature is 20° F. to 40° F. above ambient temperature. Similarly, when temperatures were below 32° F., then the aftercooler bypass controller36would open bypass valve26enough to maintain an outlet temperature of about 100° F. or whatever temperature is desired. Thus, bypass valve26and controller36may be configured to provide closed-loop control of the outlet temperature, thereby providing a variable mixing ratio and a controllable outlet temperature independent of ambient temperature.