Thermostatically controlled intercooler system for a multiple stage compressor and method

Abstract: A thermostatically controlled intercooler system for use with a multiple stage compressor to prevent the condensation of water from the compressed gas, such as air, within the system, whereby the temperature of a compressed gas entering a second compressor stage following the first compressor stage is controlled to a target temperature, ideally up to about 250°F., as will not allow the partial pressure of the water therein to exceed the saturation limit of the water content after the compressed gas is further compressed in the following compressor stage.

Description: FIELD OF THE INVENTION The present invention relates, in general, to multiple stage compressors and, more particularly, this invention relates to a method of operating a thermostatically controlled intercooler system for use with multiple stage compressors that functions to control the inlet temperature of the compressed gas at a second and/or subsequent compression stage to effectively prevent the condensation of water within the compressor. By permitting controlled, preselected amounts of uncooled compressed gas from one compressor stage to by-pass the in-line intercooler, the combined temperature of the compressed air emerging jointly from the intercooler and by-pass can be controlled to a predetermined value, selected to assure that upon further compression and heating the temperature of the compressed gas will not allow the partial pressure of the water therein to exceed the saturation limit of the water content at that temperature. BACKGROUND OF THE INVENTION Mechanical, single stage, air compressors are well known in the art which comprise one of several different types, such as piston and cylinder type, centrifugal type, axial-flow type, turbine type and even other types. The simplest and most common type in use is the piston and cylinder type. In this type compressor air, or any gas, is admitted via a valve into the cylinder where a reciprocating piston therein compresses the air or gas within the cylinder and displaces the compressed air to a conduit or reservoir from which it can be taken for use as may be required. Multiple stage air compressors are also well known in the prior art, the most relevant example being shown in JP 08049674 A. Such multiple stage compressors are utilized to compress air and/or other gas to pressures which are higher than can normally be achieved with a single stage compressor. These multiple stage compressors normally comprise a plurality of mechanical single stage compressors of any one type or the other, interconnected in series, wherein the compressed gas is passed from one stage to the next with the pressure thereof being increased at each succeeding stage. In the typical multiple stage compressor of the piston and cylinder type, air or gas, at ambient pressure and temperature, is admitted into the cylinder of the first compressor stage where a first reciprocating piston serves to compress the air therein and displace it to the second stage and so on through all the stages in the system, with each stage further compressing the previously compressed gas until the final desired pressure is achieved. It is also well known that most multistage compressors normally include a cooling step of the compressed air between at least some of the various compressor stages so that the overall compression may be more isothermal than adiabatic. That is to say, because of the ideal gas law, (PV = nRT), each compression stage of the air will, of course, cause an increase in pressure, P, as intended, and will also cause a directly proportional increase in the air temperature, T. While this in not normally a problem in a typical single stage compressor, where a defined volume of air is compressed but once, the relatively high air pressures obtained in most multistage compressors can result in the compressed air having excessive and problematical temperatures, but for the intermediate cooling of the compressed air between the various compression stages. For example, compressed air temperatures in excess of 500°F is not only a hazard to persons therearound, but can cause operating difficulties of various different forms, such as malfunctioning valves and other compressor components. As a result, practically all commercially available multiple stage compressors include an intercooler system of some sort between at least some of the compression stages for the purpose of preventing excessive heating of the compressed gases compressed to such high pressure levels. It is also well known that water exists as vapor in practically any ambient air to be compressed in a conventional compressor, which is quantified as the relative humidity of the air. The relative humidity of the air, expressed as a percent value, is the ratio of (a) the water vapor actually present in the air, in comparison to (b) the saturation vapor pressure at the temperature in question. The saturation vapor pressure is a function of the air temperature, so that as the temperature increases for any given sample of air, the saturation vapor pressure increases, and accordingly, the relative humidity decreases. In a compressor, the above natural conditions can create a problem. Obviously, when the air is compressed, with little or no externally caused change in temperature, the temperature of the compressed air is increased in proportion to the increase in pressure, as noted above. Because the saturation vapor pressure of water is dependent on the temperature of the air, it follows that when the temperature is increased the saturation vapor pressure is also increased. Thereafter, if the compressed air is cooled by any means, such as an intercooler for example, and thereafter again compressed, it is not uncommon for the water vapor pressure in the twice-compressed air to actually exceed the saturation vapor pressure for the twice-compressed air, even at the new further elevated temperature, This is, particularly, the case if the twice-compressed air is allowed to further cool thereafter. Therefore, it is not uncommon for this phenomenon to cause significant amounts of water to be condensed as liquid within the system. Free water within the compressor, however, is known to cause a variety of problems, such as oxidation (rusting) of compressor components, and more importantly, cause condensed water to be admixed into the lubricating oil within the compressor sump. Such dilution of the lubricating oil in the compressor with water can seriously impair the normal operation of the compressor as well as reduce its overall useful life. Therefore, it is highly desirable to eliminate, or to at least substantially minimize, the condensation of such water within any compressor, particularly any such water that may find its way into the lubricating oil. SUMMARY OF THE INVENTION This invention is predicated on our joint conception and development of a thermostatically controlled intercooler system for use with a multiple stage compressor, which can virtually prevent, or at least significantly minimize, the condensation of water from the compressed air within the system. In the thermostatically controlled intercooler system of this invention, the inlet temperatures of the compressed air at least at some of the compression stages following the first stage, is controlled to a value that will prevent the partial pressure of the water vapor in the compressed air from exceeding the saturation vapor pressure at the pressure and temperature achieved in the next following compression stage, to virtually prevent, or at least greatly minimize, condensation of water within the compressor. Such a temperature control is effected by permitting controlled, preselected amounts of uncooled compressed air from the previous compressor stages to by-pass the next, in-line intercooler, and be blended with the cooled, compressed air emerging from the intercooler. Such blending of cooled and uncooled compressed air will permit control of the temperature of the compressed air entering the next following compression stage, and with attention to such temperature control it is possible to select and control the temperature of that compressed air to a value that will not be increased upon further compression to a level that will allow the partial pressure of the water therein to exceed the saturation limit of the water content at that temperature. OBJECTS OF THE INVENTION Accordingly, it is one of the primary objects of this invention to provide a new and improved multiple stage gas compressor having a significantly reduced tendency for water condensation therein. Another object of the present invention is to provide a new and improved thermostatically controlled intercooler system for use with a multiple stage compressor that significantly reduces the tendency for water condensation within the multiple stage compressor. A further object of the present invention is to provide a new and improved thermostatically controlled intercooler system for use with a multiple stage compressor that controls the inlet temperature of the compressed air at a second and/or subsequent compression stage to thereby substantially eliminate the condensation of water within the compressor. Still another object of the present invention is to provide a new and improved thermostatically controlled intercooler system for use with a multiple stage compressor that controls the inlet temperature of the compressed air at a second and/or subsequent compression stage by permitting controlled amounts of uncooled, compressed air to by-pass the intercooler, thereby raising the temperature of the inlet compressed gas entering the next compressor stage so that its temperature can be controlled to a level that will prevent the partial pressure of water therein from being increased to a level above the saturation level, to thereby prevent or minimize water condensation within the compressor. These and other objects and advantages of the present invention will become more readily apparent to those persons skilled in the compressor art after a full reading of the following detailed description, particularly, when such description is taken in conjunction with the attached drawings as described below. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified, schematic view of a two-stage gas compressor incorporating a thermostatically controlled intercooler system according to a presently preferred embodiment of this invention; and Figure 2 is a simplified, schematic view of a three-stage gas compressor incorporating a thermostatically controlled intercooler system according to another presently preferred embodiment of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Prior to proceeding with a detailed description of the subject invention, it is noted that for the sake of clarity, identical components which have identical functions have been identified with identical reference numerals throughout the several views of the attached drawings. Reference to Figures 1 and 2 will illustrate in schematic form two presently preferred embodiments of this invention, namely, Figure 1 being a multiple stage compressor having two compressor stages with a single intercooler therebetween, and Figure 2 being a multiple stage compressor showing the first three compressor stages in a system having a plurality of compressor stages, with an intercooler between at least the second and third compressor stages and between the third and any subsequent compressor stages. In each of the figures, the multiple stage compressors, generally designated 10, include a first compressor stage 10a, which is schematically illustrated as a piston and cylinder type of compressor having a piston 12 mounted for reciprocal movement within a cylinder 14. While such piston and cylinder type of compressors are perhaps the most common, it should be appreciated that this invention can be incorporated into multiple stage compressors based on other forms of mechanical compressors, such as a centrifugal type, an axial-flow type, a turbine type and other types, particularly any such multiple stage compressor wherein an intercooler is provided between any two compressor stages for the purpose of preventing the compressed gas from becoming excessively heated. As is well known, piston and cylinder type mechanical compressors are provided with a suitable valve arrangement (not shown) which are closed during a compression stroke to permit the gas therein to be suitably compressed, with an outlet valve (not shown) opening thereafter, permitting the compressed gas to be directed out of the cylinder 14 and into a discharge line 18. Thereafter, the outlet valve is closed as an inlet valve (not shown) is opened so that the reciprocating piston 12 will draw in fresh gas at ambient pressure to repeat the compression process. Such valving arrangement is well know to those familiar with the art and need not be further described here. As in some two-stage compressors, an intercooler 16 is provided which is adapted to cool the gas compressed after a compressor stage, such as a first compressor stage 10a, before it is further compressed and heated in the next following compressor stage, i.e., compressor stage 10b. Accordingly, such discharge line 18 is provided which is adapted to convey the gas compressed in compressor stage 10A into the intercooler 16 so that the gas heated in compression stage 10a by virtue of its compression can be cooled, at least to some degree, before it is further compressed in compressor stage 10b. As with other elements of the compressor described above, the intercooler 16 is also well known to those familiar with the compressor art. Such intercooler 16, normally, comprises a radiator-type of cooler wherein the heated gas is passed through a plurality of thin radiator tubes 20 spaced by cooling fins (not shown). Therefore, such intercoolers 16 need not be further described here in detail. As shown in Figure 2, the multiple stage compressor 10 may comprise more than two compression stages, such as three compression stages 10a, 10b and 10c, as shown in Figure 2, or even more, with an intercooler 16 operably disposed between any pair of adjacent compression stages 10. However, since excessive heating of the compressed gas is not normally a problem until a third or later compression stage is involved, some commercially available multiple stage compressors may only utilize an intercooler before the third and any subsequent compression stages. In a like manner, as shown in the Figure 1 embodiment, an intercooler 16 is provided, in the embodiment of Figure 2, which is adapted to cool the gas compressed in compressor stage 10a before it is further compressed and heated in the second compressor stage 10b. Again, a discharge line 18 is provided which is adapted to convey the gas compressed in the first compressor stage 10a into the intercooler 16, so that the gas heated in compression stage 10a by virtue of its compression, can be cooled, at least to some degree, before it is further compressed in compressor stage 10b. In a similar manner to that described above, another intercooler 16b is provided which is adapted to cool the gas compressed in the second compressor stage 10b before it is further compressed and heated in compressor stage 10c. Again, a discharge line 18b is provided which is adapted to convey the gas compressed in compressor stage 10b into the intercooler 16b so that the gas heated in compression stage 10b by virtue of its compression can be cooled, at least to some degree, before it is further compressed in compressor stage 10c. In a like manner, additional intercoolers 16 are commonly operably disposed between any pair of subsequent compressor stages 10 for the purpose of cooling the previously compressed gas before it is further compressed, and accordingly further heated in such following compression stage. The operation of the systems described above are conventional in the prior art and need not be further described here. The crux of this invention resides in a selective by-pass system, incorporated with the intercoolers 16, which permits control of the compressed gas temperature admitted into any selected compressor stage after the first compressor stage so that such inlet temperature can be deliberately controlled and maintained at a predetermined level which will assure that upon further compression the gas temperature will not be increased, upon further compression, to a level that will allow the partial pressure of the water therein to exceed the saturation limit of the water content at that temperature. Accordingly, with reference to Figure 1, the inventive elements include a three-way valve 30 adapted to receive compressed gas cooled by the associated intercooler 16 and direct the compressed gas to the subsequent compressor stage 10b for further compression. In addition, a by-pass line 34 is also provided which interconnects the associated compressor stage 10a directly with the three-way valve 30. Accordingly, the by-pass line 34 is adapted to pass compressed gas directly from the associated compressor stage 10a to the three-way valve 30 without such compressed gas passing through the intercooler 16. Therefore, the three-way valve 30 is adapted to selectively convey either cooled or uncooled compressed gas, or a controlled mixture thereof, onto the next succeeding compressor stage 10b. In addition to the three-way valve 30 and by-pass line 34, the inventive elements of this invention further includes a control means 36 for controlling the operation of such three-way valve 30. Obviously, control means 36 should be a control adapted to selectively pass either cooled or uncooled compressed gas, or mixtures thereof, as necessary to maintain a preselected target temperature of the compressed gas entering into the compression stage 10b. The preselected target temperature, will of course vary from one system to the next, but as noted above, it should be a temperature which is determined to be one that upon further heating as a result of further compression in a second compression stage 10b, will not be increased to a level that will allow the partial pressure of the water therein to exceed the saturation limit of the water content at that temperature. While the above description is addressed primarily to the thermostatically controlled intercooler system between the first and second compression stages of the multiple stage compressor shown in Figure 1, it should be readily apparent that any thermostatically controlled intercooler system should be substantially the same regardless of its location with respect various compressor stages. The only differences in essence will be the target temperatures sought. While a number of differing control means could be provided, and are within the scope of the present invention, we have preferred to use an "off-the-shelf" three-way type valve having a built-in temperature controlled valve operation. Specifically, we have successfully used both a 38mm (1.5-inch) and a 61mm (2-inch) three-way valve manufactured by FLUID POWER ENERGY having the built-in, out-put temperature controller, whereby the desired out-put temperature can be selected and set on the valve, with the valve then automatically out-putting a mixture of the two in-put gasses as necessary to provide an out-put blend thereof matching the preselected temperature. Since such valves are commercially available, it is believed that any further description and discussion thereof is not necessary here. It may be apparent to those skilled in the art that exacting controls are not in fact necessary. For a conventional two-stage compressor utilizing two compression cylinders in the first stage and one compression cylinder for the second stage, with an intercooler between the first stage and the second, we have determined that water condensation can be significantly reduced, or eliminated, if the ultimate temperature of the compressed gas, after compression in the second stage, remains below about 260°C (500°F). In the above noted system, we have learned that such a goal can be achieved by controlling the compressed gas temperature entering the second stage at a level of generally no more than about 121°C (250°F). Therefore, if the temperature of the compressed air exiting the first stage is at or below about 121 °C (250°F)., then all of that compressed air can be passed directly to the second stage without any of it having to be diverted through the intercooler. Only when the temperature of the compressed air emerging from the any compressor stage is in excess of about 121°C (250°F)., will it be necessary to divert a portion thereof through the following intercooler. By utilizing the temperature controlled three-way valve described above, the valve will itself adjust the temperature of the out-put. Further testing of the above described apparatus having a thermostatically controlled intercooler, as described herein, in contrast to a prior art compressor being identical in all other respect except for the absence of the inventive control, has shown that the prior art compressor has routinely caused water build-up in the lubricating oil in amounts exceeding 1.0 percent, and even 2.0 percent water after operating for a defined test period of time. An identical compressor having the thermostatic control of this invention, has managed to keep such water build-up in the lubricating oil at amounts consistently below about 0.1 percent, when operating the controls as necessary only to keep the temperature of the compressed gas entering the second compressor stage, preferably, at or below about 93 °C (200°F). Having described in detail a presently preferred embodiment of this invention, it should be apparent that various other embodiments could be utilized and modifications incorporated therein by those persons who are skilled in the compressor art without departing from the scope of the appended claims.

Claims: A method of operating a multiple stage gas compressor (10) to minimize condensation of water therein, having the steps of: (a) compressing a gas in a first compressor stage (10a); (b) connecting the first compressor stage (10a) with an intercooler (16) via a discharge line (18); (c) passing compressed gas compressed in the first compressor stage (10a) through the discharge line (18) into the intercooler (16); (d) cooling the compressed gas in the intercooler (16); (e) passing cooled compressed gas cooled by the intercooler (16) to a three-way valve (30); (f) directing the cooled compressed gas from the three-way valve (30) to a second compressor stage (10b); (g) interconnecting the first compressor stage (10a) directly with the three-way valve (30) via a by-pass line (34) adapted to pass uncooled compressed gas directly from the first compressor stage (10a) to the three-way valve (30) without the uncooled compressed gas passing through the intercooler (16); and (h) controlling the three-way valve (30) such that compressed gas emerging therefrom and directed to the second compressor stage (10b) comprises one of the cooled compressed gas from the intercooler (16), the uncooled compressed gas from the by-pass line (34), and a mixture of the cooled and uncooled compressed gases, as necessary to effect a preselected target temperature of the compressed gas directed by the three-way valve (30) to the second compressor stage (10b), characterised in that the built-in temperature control of the three-way valve assures that upon further compression, the gas temperature will not be increased to a level that will allow the partial pressure of the water therein to exceed the saturation limit of the water content at that temperature. The method according to claim 1, further characterized by the steps of: (i) compressing the compressed gas further in the second compressor stage (10b); (j) connecting the second compressor stage (10b) with a second intercooler (16b) via a second discharge line (18b); (k) passing compressed gas compressed in the second compressor stage (10b) through the second discharge line (18b) into the second intercooler (16b); (l) cooling the compressed gas in the second intercooler (16b); (m) passing cooled compressed gas cooled by the second intercooler (16b) to a second three-way valve (30); (n) directing the cooled compressed gas from the second three-way valve (30) to a third compressor stage (10c); (o) interconnecting the second compressor stage (10b) directly with the second three-way valve (30) via a second by-pass line (34) adapted to pass uncooled compressed gas directly from the second compressor stage (10b) to the second three-way valve (30) without the uncooled compressed gas passing through the second intercooler (16b); and (p) controlling the second three-way valve (30) such that compressed gas emerging therefrom and directed to the third compressor stage (10c) comprises one of the cooled compressed gas from the second intercooler (16b), the uncooled compressed gas from the second by-pass line (34), and a mixture of the cooled and uncooled compressed gases, as necessary to effect a preselected target temperature of the compressed gas directed by the second three-way valve (30) to the third compressor stage (10c), the built-in temperature control of the three-way valve assuring that upon further compression, the gas temperature will not be increased to a level that will allow the partial pressure of the water therein to exceed the saturation limit of the water content at that temperature. The method according to claim 1, further characterized by the step of regulating the preselected target temperature of the compressed gas directed by the three-way valve (30) to the second compressor stage (10b) to be between 93°C (200°F) and 121°C (250°F). The method according to claim 1, further characterized by the step of presetting an output gas temperature of the three-way valve (30) to effect the preselected target temperature. The method according to claim 1, further characterized by the step of controlling the three-way valve (30) to prevent a final temperature of the compressed gas compressed in the second compressor stage (10b) from exceeding 260°C (500°F). The method according to claim 2, further characterized by the step of regulating the preselected target temperature of the compressed gas directed by the second three-way valve (30) to the third compressor stage (10c) to be between 93°C (200°F) and 121°C (250°F). The method according to claim 2, further characterized by the step of presetting an output gas temperature of the second three-way valve (30) to effect the preselected target temperature. The method according to claim 2, further characterized by the step of controlling the second three-way valve (30) to prevent a final temperature of the compressed gas compressed in the third compressor stage (10c) from exceeding 260°C (500°F).

Inventors: CUNKELMAN BRIAN L, DRUMMOND ROGER, GOETTEL WALTER E, WAGNER DANIEL G, CUNKELMAN, BRIAN L., DRUMMOND, ROGER, GOETTEL, WALTER E., WAGNER, DANIEL G.
Applicants: WESTINGHOUSE AIR BRAKE CO, WESTINGHOUSE AIR BRAKE COMPANY
Application year: 1997
Main classification: F04B25
IPC codes: F04B25, F04B39, F04B23