Background Art Air compressors for industrial use are characterized as reciprocating, screw and turbo compressors. The reciprocat- ing, screw and turbo compressors are used for less than 50 horsepower (hp), about 50 to 200 hp., and over 600 hp., respectively. The turbo compressor is of excellent durabil- ity and provides clean air excluding oil as compared with reciprocating and screw ones, and is superior to them.

However, since there is a limit to the development of gear techniques, a turbo compressor of less than 600 hp. has not been manufactured, and Japanese IHI that developed the 110,000-RPM increasing gear mechanism first proposed a 100- hp turbo compressor. The turbo compressor which depends on the quality of the increasing gear cannot be manufactured to a small size of less than 100 hp. In the presently-

available turbo compressor since impellers are driven by a motor and an increasing gear, the outlet width of an impel- ler of the final stage compressor becomes too small, and there is a limit to being in a high compression ratio with a small amount of gas. For example, when trying to produce gas of 10 bar by a 200-hp turbo compressor, the outlet width of the impeller of the final stage compressor is about 2mm, so an axial clearance cannot be secured and the efficiency of the final stage compressor is too low to be of practical use. Even in case of producing a pressure of 20 bar by a 500-hp turbo compressor, it cannot operate because of the same reason as the above.

Therefore, in spite of various advantages of the turbo compressor, it cannot be used for less than 200 hp.

In order to realize a turbo compressor with a small amount of air, the present invention employs compressors driven by an external power and a turbine driven by a part of the compressed air, and connected to the final stage of the compressor to produce about a pressure of 11 bar with a small amount of gas. For low-stage compressor such as a first-stage or second-stage compressor, their impellers are driven by a conventional method such as increasing gear, and a high-stage compressor (third-, fourth-, and final stage compressor) employs a turbine driving mechanism using

compressed air produced from the low-stage compressor in order to provide the satisfactory rotational speed not to decrease the outlet width of the impeller of the high-stage compressor in case of a small amount of gas (generally, less than l. OKg/sec). According to a conventional gear driving, the rotational speed can hardly exceed 70,000 RPM because of the limit of gear mechanism, and in order to produce a high compression ratio with the small rotational speed, the outlet width of the impeller becomes small, which cannot be of practical use.

Summary of the Invention It is an object of the present invention to provide a small turbo compressor which can obviate disadvantages of conventional compressor techniques, and assures an increase in energy efficiency, supply of clean compressed air, and satisfactory operation in a high compression region with a small amount of gas.

In order to achieve the above object, the present invention provides a multi-stage turbo compressor including a turbine driven by a high-pressure gas from a low- stage compressor's outlet; and a high-stage compressor driven by a power transmitted through an axis directly connected to the turbine. The gas that passed the turbine

is returned to a first-stage compressor's inlet. First and second compressors are driven by a high-speed motor directly connected thereto. Or, the first and second compressors are driven by a motor whose rotational speed is increased by an increasing gear.

Brief Description of Drawincrs The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: FIG. 1 schematically shows the basic concept of the present invention having two centrifugal compressors and one centrifugal turbine; FIG. 2 depicts impellers for a 30-hp compressor; and FIGS. 3 and 4 each depict modified examples of the present invention.

Best Mode for carrvincr Out the Invention FIG. 1 depicts the basic concept of a turbo compressor of the present invention. As shown in FIG. 1, first-stage and second-stage compressors are driven by an engine or

motor, and an inter-cooler is used between each stage to reduce the consumption of power. The turbine is driven by air of about a pressure of 4 bar from an outlet of the second-stage compressor, and the third-stage compressor connected to the turbine is operated more than 100,000 RPM, thus producing a pressure of 10 bar. with a small amount of gas. It is hard to apply the conventional turbo compressor to the amount of gas of less than lKg/sec, and the present invention provides a method of overcoming the conventional restrictions by driving the high-stage compressor with the turbine. The first-and second-stage compressors inhale gas more than the air actually produced, and it is possible to operate in the region where the impeller's efficiency is high. Recently, Japanese IHI disclosed a two-stage small turbo compressor of 100 hp with a newly developed gear of 110,000 RPM, but this compressor does not provide a pres- sure of 8 bar and over because of its final stage. This pressure does not reach a pressure of 10 bar that a screw compressor provides, and since the number of the axial rotation of the increasing gear must exceed 170,000 RPM for use of third-stage compressor, the pressure cannot be more raised and its application is impossible.

The following is a result obtained from a 100-hp com- pressor producing a pressure of 12 bar by the use of a gear

of 70,000 RPM significantly lower than IHI's one of 110,000 RPM.

Pressure Temperature Amount of gas Notes (bar) (K) (Kg/sec) 0.1.00 300.00 0.2219 under the atmospheric condition 1.1.00 288.80 0.3221 condition of mixture of air from turbine's outlet and inhaled air 2.2.80 409.30 0.3221 First-stage compressor: Compression ratio 2.8 & Efficiency 0.82 3.2.80 310. 00 0.3221 Heat exchanger's pressure loss is disregarded for simple calculation 4.6.72 418.80 0.3221 Second-stage compressor: Compression ratio 2.8 & Efficiency 0.81 5.6.72 310.00 0.2219 Heat exchanger's pressure loss is disregarded for simple calculation 6.12.0 379.90 0.2219 Third-stage compressor: Compression ratio 1.786, Efficiency 0.82,

Rotational speed 170,000 RPM. Compression ratio is determined from second- stage compressor and turbine's power balance.

7.1.0 264.1 0.1002 Air from the turbine outlet The respective performances of the aboves are as follows:

Item Value Evaluation Amount of 4.02 CFM conventional screw compressor-4; gas for IHI's is not high in IHI pressure so cannot be compared Energy 0.932 IHI's-about inter-cooling efficiency is performed twice in the present invention while inter-cooling is once performed for IHI Amount of 402 CFM 400 for conventional screw com- inhaled pressor; 465 for IHI gas Output 12 Kg/cm2 it is similar to conventional pressure screw compressor's; 8 for IHI; 8 for oilless screw compressor In conclusion, the compressor of the present invention is superior to the conventional screw compressor and small

compressor in performance. The inventive compressor has high supply pressure and high energy efficiency, and if a 110,000-RPM gear is employed, a compressor of less than 50 hp. can be manufactured. In the meantime, the inventive compressor's efficiency is lowered compared to a large turbo compressor's but it is excellent as a small turbo compressor.

The present invention is compared with the IHI's com- pressor by calculating the performance when the output pressure is 8 bar like the IHI's 100-hp compressor that is known as the smallest one. Present IHI Notes invention Amount of gas 4.795 4.665 4 for conven- for 1 hp. tional com- (CFM/HP) pressor Energy 0.874 0.849 efficiency If the pressure is lowered, the increase in the effi- ciency becomes small. When considering the mechanical loss, the efficiency of the present invention is similar to IHI's. Up to now, there is no turbo compressor of less than 100 hp. that is of practical use, and such a manufacture of a compressor of 100 hp. by using the present invention is

of great significance.

FIG. 2 shows an example of an impeller of each centrif- ugal compressor (first-stage, second-stage and third-stage compressors from the left). 110,000-RPM, 110,000-RPM, and 220,000-RPM gears are respectively used for first-, second- and third-stage compressors. Outlet widths of the impellers are 4.94m, 4.02mm, and 2.16mm, respectively, and the effi- ciency of each stage is 80%, 82.9% and 82.3%, which shows that the compressors are manufactured in the optimum rota- tional speed (about 100). If the third-stage compressor uses a 110,000-RPM gear, its outlet width of 2.16mm becomes less than l. mm, which increases a loss due to leakage, and cannot be of practical use.

FIG. 3 depicts an example of driving first-and second- stage compressors by using a gear, and FIG. 4 shows an example of driving first-and second-stage compressors directly connected to a high-speed motor. Referring to FIG.

1, the power consumed by the first-stage compressor can be saved by returning the air, passed the turbine, to the inlet, and when discharging the air to the outside, if necessary, it can serve as an air conditioner (the outlet temperature of the turbine is about 6 C during summer). In the case where the discharged air is returned to the inlet, the inhaled air does not leak to the outside, and differ-

ent kinds of gases other than the air may be used.

As described above, the small turbo compressor of the present invention provides the following advantages: first, the present invention is capable of providing a high pressure with a small amount of gas that the conventional turbo compressor cannot provide; second, according to the present invention, a turbo compressor can be manufactured without using any precise gear; third, the present inven- tion can supply clean air without oil that the conventional screw compressor cannot provide; and fourth, the trouble-free inventive compressor assures a long-time use while the conventional screw compressor is of low durability and needs frequent repairs.

The inventive small turbo compressor has the above features, and can replace the conventional screw compressors as 50-hp to 200-hp air compressors.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as recited in the accompanying claims.