Single-Stage Oilless Screw Compressor System

This invention relates to a single-stage oilless screw compressor system which is suitable for use as a source of air supply in fields in which a clean gas (air) is required, such as a food industry, a testing and measuring instrument manufacturing industry, a painting and a press for semiconductor manufacturing apparatus.

An oilless screw compressor system is known as a system for producing a clean air or clean gas having no oil incorporated therein. In this known system, for example, disclosed in U.S. Pat. No. 3,367,562, a gas has its pressure raised by two-stage compression, that is, the gas is compressed by means of a low pressure stage compressor to about 3 kgf/cm.sup.2 (gauge pressure) and is cooled by an intermediate cooler and then the gas is compressed again by means of a high pressure stage compressor to a predetermined level between 7 and 8 kgf/cm.sup.2 (gauge pressure) and is cooled again by an aftercooler.

The system of the prior art for supplying a clean gas is of a two-stage compression type not of a single-stage compression. Even if a single-stage oilless screw compressor is combined with the two-stage compressor system, it would be impossible to eliminate obnoxious effects of high temperature because the temperature of the discharged gas of the single-stage compression oilless screw compressor would exceed 300.degree. C. In a known system, no means has been provided for eliminating defects which would be experienced due to the discharged has having a high temperature.

An object of this invention is to provide a single-stage oilless screw compressor system capable of supplying, by a single-stage compression, a clean gas of a pressure of over 4 kgf/cm.sup.2 (gauge pressure) which has no oil incorporated therein and has a pressure ratio (discharge pressure/suction pressure) of over four.

Another object is to provide a single-stage oilless screw compressor system capable of protecting a valve means for avoiding a backflow of compressed gas to the compressor from high temperature, to thereby stabilize the operation of the valve means and prolong a service life thereof.

Still another object is to provide a single-stage oilless screw compressor system capable of causing the temperature of the compressed gas of high temperature discharged from the compressor to drop to a temperature level suitable for the use of the compressed gas, with excellent effects.

To accomplish the aforesaid objects, the invention provides the outstanding characteristic that the system comprises a single-stage oilless screw compressor, a precooler mounted in a gas passage on the discharge side of the compressor, a check valve mounted on the outlet side of the precooler and a cooler mounted on the outlet side of the check valve.

In this specification, the term "single-stage oilless screw compressor" refers to a type of compressor which performs compression in a single stage with a pressure ratio (discharge pressure/suction pressure) of over four or preferably in the range between seven and eight and which performs operation without requiring oil fed to the operation chamber of the compressor.

FIG. 1 is a systematic view of the single stage oilless screw compressor system of one embodiment of the invention;

FIG. 2 is a sectional view of the precooler, showing its construction in detail; and

FIG. 3 is a sectional view of the compressor, showing its construction in detail.

As shown in FIG. 1, a single-stage oilless screw compressor system according to the invention comprises a drive unit generally designated by the reference numeral 1, such as a motor or an engine, transmission unit generally designated by the reference numeral 2, a single-stage oilless screw compressor generally designated by the reference numeral 3, a precooler generally designated by the reference numeral 4, a check valve generally designated by the reference numeral 5, a cooler generally designated by the reference numeral 6 and release cooler generally designated by the reference numeral 7.

The transmission unit 2 comprises a drive pulley 21, a follower pulley 22, V-belts 23, a gear box 24, a gear 25 and a pinion 26. The drive pulley 21 is secured to an output shaft of the drive unit 1, and the follower pulley 23 is secured to a shaft of the gear 25 meshing with the pinion 26 which is secured to a rotor shaft of the single-stage oilless screw compressor 3. The V-belt 23 is trained over the two pulleys 21 and 22. The drive pulley 21 has a diameter which is about twice as large as that of the follower pulley 22, so as to increase the rotational speed by about twofold. The number of teeth of the gear 25 is about five to six times as great as that of the pinion 26, so as to further increase the rotational speed by about fivefold or sixfold. Thus, as the rotation is transmitted from the pulley 21 to the pinion 26, the rotational speed of the drive unit 1 is increased to ten to twelve times as high as its original value at the pinion 26.

An oil-feeding pump 27 is coupled to the shaft of the gear 25 through gears 28 and 29, with the oil-feeding pump 27 functioning to draw a lubricant collecting in a bottom portion of the gear box 24 and feed the same to meshing surfaces of the gears 25 and 26 and the bearing of the compressor 3 et al.

As shown in FIG. 3, the compressor 3 comprises a casing 31 formed with a suction port 31A and a discharge port 31B, a pair of screw rotors 32M and 32F rotatably supported in the casing 31, bearings 33A, 33B and 33C supporting the pair of screw rotors 32M and 32F, suction side seal means 34A and discharge side seal means 34B interposed between the casing 31 and shafts of the screw rotors 32M and, respectively, synchronizing gears 35M and 35F causing the pair of screw rotors 32M and 32F to rotate synchronously, a suction port adjusting valve 36 mounted adjacent the suction port 31A of the casing 31, a cylinder 37 and a piston 38 for actuating the suction port adjusting valve 36, and an air release valve 39. The compressor 3 has a pressure ratio (discharge pressure/suction pressure) of seven and is capable of compressing air drawn by suction into the pair of screw rotors 32M and 32F to raise its pressure sevenfold.

As further shown in FIG. 1, the precooler 4 is connected to the discharge port 31B of the compressor 3 through a line 43 and comprises a shell 41 formed with an inlet 41A and an outlet 41B for cooling water, and a heat transfer tube 42 located inside the shell 41. While only one heat transfer tube 42 is illustrated in the drawings, it is possible to, for example, provide two heat transfer tubes. The heat transfer tube 42 is formed of steel (carbon steel) so as to be able to withstand a high temperature in the range between 300.degree. and 350.degree. C. or above. The check valve 5 is mounted on the outlet side of the precooler 4.

The cooler 6 comprises a shell 60 formed at opposite ends with flanges 60A and 60B, a tube nest 61 located inside the shell 60, a water chamber case 62 and a cover 63. The tube nest 61 comprises a tube 65 formed of copper which is in the form of an inverted letter U and connected at ends thereof to a tube plate 64, fins 66 fitted over the tube 65, and a plurality of baffle plates 69 supported in staggered relation by through bolts 68 connected at ends to the tube plate 64 in such a manner that the baffle plates 69 are spaced from one another at a predetermined distance by spacers 67. The cover 63 is connected to the flange 60A of the shell 60 at its top, and the check valve 5 is located inside the cover 63. The tube plate 64 is held between the flange 60B and the water chamber case 62, and the tube plate 64, flange 60B and water chamber case 62 are connected by bolts 8 into a unitary structure. The tube plate 64 cooperates with the water chamber case 62 to define an inlet water chamber 62A and an outlet water chamber 62B. An inlet port 60C and an outlet port 60D for a compressed gas are respectively formed in the cover 63 and a large diameter portion of the shell 60.

As shown most clearly in FIG. 2, air release cooler 7 comprises a shell and a heat transfer tube 71, with the air release cooler 7 being accommodated in the shell 41 with the precooler 4. The heat transfer tube 71 is located in the shell 41 and has an inlet 71A connected to the outlet side of the heat transfer tube 42 of the precooler 4 and an outlet 71B connected to the gas release valve 39 through a line 72.

Operation of the embodiment of the invention will now be described.

Actuation of the drive unit 1 provides a drive force which is transmitted to the compressor 3 through the transmission unit 2 which increases the rotational speed, to drive the pair of screw rotors 32M and 32F of the compressor 3 for rotation at high speed (about 50-100 m/sec in peripheral speed). As a result, air under a pressure of 1 kgf/cm.sup.2 is drawn by suction from the atmosphere through the suction port 31A into a working chamber defined by the pair of screw rotors 32M and 32F where the air is compressed into compressed air which is discharged through the discharge port 31B. At this time, the compressed air is under pressure of 7 kgf/cm.sup.2 and has a temperature in the range between 300.degree. and 350.degree. C. The compressed air of high temperature is cooled as it exchanges heat with cooling water in the shell 41 while flowing through the heat transfer tube 42 of the precooler 4, until its temperature reaches the range between 250.degree. and 100.degree. C. at the outlet section of the heat transfer tube 42. The compressed air thus precooled flows through the check valve 5 into the shell 60 of the cooler 6 where it flows downwardly in serpentine flow as it is guided by the baffle plates 69, to be forwarded through the outlet 60D to a station where it is put to use. Heat exchange takes place between the compressed air flowing downwardly in and through the shell 60 in serpentine flow with the cooling water flowing in and through the tube 65, so that the compressed air has a temperature of about 45.degree. C. at the outlet 60D.

Meanwhile, as the volume of the air in use decreases, the pressure rises on the discharge side of the compressor 3 to a level above a predetermined level. The rise in pressure is sensed by a manometer, not shown which generates a signal to feed air to a chamber 37A of the cylinder 37 for actuating the piston 38 to move the suction port adjusting valve 36 from a solid line position to a phantom line position, to thereby throttle the volume of air fed inti the compressor 3. The air release valve 39 is opened by the movement of the piston 38, and the compressed air compressed by the compressor 3 is cooled by the precooler 4 and then it is cooled by the cooling water in the shell 41. Some air flowing through the heat transfer tube 71 of the air release cooler 7 is released through the opened air release valve 39 into the atmosphere or the suction side of the compressor 3.

The invention has been shown and described as being incorporated in the compressor having a suction pressure of 1 kgf/cm.sup.2 and a discharge pressure of 7 kgf/cm.sup.2. However, this is not restrictive and the invention has application in a system having a discharge pressure of over 4 kgf/cm.sup.2.