BRAYTON CYCLE INDUSTRIAL AIR COMPRESSOR

Abstract: A Brayton cycle air compressor system having two turbine-compressors and a recuperated combustion chamber interconnected. A portion of the hot compressed high pressure air is used as combustion air for the combustion chamber. In addition, a portion of the combustion air may be used as the heat source for regenerating the desiccant in the optional air dryers.

Description: BRAYTON CYCLE INDUSTRIAL AIR COMPRESSOR BACKGROUND OF THE INVENTION This invention relates generally to air compressors and more particularly to gas turbine powered oil-free air compressors. A typical industrial air compression system is shown in FIG. 1. A prime mover 10, which can be an electric motor or an internal combustion piston engine, is connected by a gear or coupling 12 to a compressor 20. The compressor 20, typically a multi-stage intercooled compressor, can be a piston compressor, screw compressor or centrifugal compressor. Inlet air 22 is provided to the compressor 20 though an inlet air filter. After being compressed, the air is normally cooled in an aftercooler 25. The cooled compressed air is then delivered to an air receiver or storage tank 50 from which the compressed air flows into an air distribution system 52. When additional drying of the compressed air is required, the compressed air is processed through a dryer system 30. FIG. 1 shows a typical two tank desiccant dryer system. The dryer system 30 consists of two tanks 34a, 34b and associated valving, one system designated "a" and the other system designated "b" (Open valves are shown as "O" and closed valves are shown as "C"). As shown in FIG. 1, the "a" system is being regenerated and the "b" system is in service. Supply air inlet valves 36a, 36b direct the compressed air to the in-service tank 34a, 34b where a moisture absorbing desiccant absorbs moisture from the cooled compressed supply air. Supply air outlet valves 38a, 38b, direct the discharge of the in-service tank to the air receiver 50. Regeneration air inlet valves 37a, 37b direct either heated air from an auxiliary burner 32 or uncooled compressed air diverted from the compressor output prior to the aftercooler 25. In either case, the air is unsaturated and dryer than the cooled supply air. As a result of exposure to the unsaturated air, the previously absorbed moisture is released from the desiccant. The regeneration air including the removed moisture is directed by regeneration air outlet valves 39a, 39b to a regeneration dump air line 40. This regeneration air is either released to a sump or to a vent. The dumping of the regeneration air leads to a loss of the energy of compression in the compressed air and to the loss of the energy added by the auxiliary burner 32 (if used). The dumping of the regeneration air can cause a large pressure drop across the desiccant bed 34a, 34b being regenerated. Frequently, the pressure drop from the inlet to the outlet of the desiccant bed 34a, 34b is large enough to damage the desiccant bed. The foregoing illustrates limitations known to exist in present industrial air compressor systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. SUMMARY OF THE INVENTION In one aspect of the present invention, this is accomplished by providing a Bray ton cycle air compressor comprising: a combustion chamber; at least one turbine, the at least one turbine being driven by exhaust gases from the combustion chamber; and a compressor for supplying combustion air to the combustion chamber, the compressor being driven by one of the at least one turbines, the compressor having a capacity in excess of the capacity necessary to supply combustion air to the combustion chamber, the work by-product of the Brayton cycle air compressor being extracted as excess compressed air. The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a schematic diagram of a prior art industrial air compression system; FIG. 2 is a schematic diagram of one embodiment of a Brayton cycle industrial air compression system; and FIG. 3 is a schematic diagram of an alternate embodiment of a Brayton cycle industrial air compression system. DETAILED DESCRIPTION FIG. 2 shows one embodiment of a Brayton cycle industrial air compressor system having a compressor section (70 and 72) (with multiple stages) that supplies air to both the gas turbine engine (60, 64 and 66) and the air receiver 50. Each compressor stage 70, 72 is driven by a dedicated turbine 64, 66 respectively. Depending on the desired discharge pressure, one, two, three or more stages of compression may be used. A fraction of the air compressed in the multi-stage compressor (70 and 72) is delivered to an air receiver 50 and air distribution system 52, usually through a dryer system 30, while the remainder of the air proceeds through the Brayton cycle components, combustion chamber 60, optional recuperator 68 and turbines 64, 66. This dual use of the compressors (for both supply air and combustion air) eliminates the usual mechanical connections between the engine drive and the need for a separate conventional air compressor package. In thermodynamic terms, the work by-product of the Brayton cycle is extracted as excess air compression, rather than shaft power. One major use for this Brayton cycle air compression system is to produce oil-free compressed air by using turbine driven oil-free centrifugal air compressors. The use of a portion of the engine's air supply to dry (or regenerate) the desiccant serving the supply air (indicated as M„) is an additional efficiency and cost improvement over the conventional approach that expends a portion of compressor output for this duty. In the present invention, this drying air circuit does not result in an air loss (dump) and is a significant simplification over regenerative dryer systems that incorporate an auxiliary burner. In the present invention, the drying air and the moisture it receives are returned to the engine cycle for expansion through the driving turbines. The excess moisture is beneficial to the cycle. This return of the drying air and the acquired moisture also eliminates the common environmental problem of condensate disposal. Typically, an industrial air compression system contains numerous condensate traps to remove condensation from the piping of the system. The condensate from these traps can also be added to the combustion air M, where it is returned to the engine cycle. This eliminates a further environmental problem of condensate disposal. The embodiment shown in FIG. 2 also eliminates the two common means of regulating part-load operation. These are controllable inlet guide vanes and variable speed motor controllers. The embodiment shown in FIG. 2 can be controlled to provide constant pressure delivery over a wide flow range. Additionally, the machine can be easily adjusted to change the constant pressure set point to a different pressure with much greater range than conventional air compressors. Also, unlike a conventional constant speed, guide vane controlled compressor, the embodiment shown in FIG. 2 will operate more efficiently with lower pressure set points. Fuel is supplied to an air heating system such as an external combustion chamber with a heat exchanger (not shown) or an internal combustion chamber 60 through a fuel throttle valve 62. In the preferred embodiment, the fuel is mixed with preheated compressed combustion air in the combustion chamber 60. The preferred combustion chamber is described in U.S. Patent No. 5,450,724. The exhaust gases from the combustion chamber 60 are directed to a first stage turbine 64. A temperature sensor 76 measures the gas inlet temperature to turbine 64. The fuel supply is throttled as needed to maintain the inlet temperature between predetermined minimum and maximum limits. The exhaust from the first stage turbine 64 then enters a second stage turbine 66. As an option, a recuperator 68 can be added to the Brayton Cycle engine to improve the fuel efficiency. In this case, the exhaust from the second stage turbine 66 passes through the recuperator 68 where the exhaust gases preheat the combustion air M, prior to being exhausted from the system through vent 90. Inlet air 22 is supplied to a first stage compressor 72, through an inlet air filter (not shown). The first stage compressor 72 is driven by the second stage turbine 66. The compressed air from the first stage compressor 72 is cooled in an optional intercooler 74 prior to entering a second stage compressor 70, which is driven by the first stage turbine 64. Although the system shown in FIG. 2 is a two-stage system with two turbines and two compressors, the system can have a single stage or a three or more stages. In a single stage system, one compressor is driven by one turbine. In a three- stage system, a first stage compressor will be driven by a third stage turbine; a second stage compressor will be driven by a second stage turbine; and a third stage compressor will be driven by a first stage turbine. The discharge from the first stage turbine is directed to the second stage turbine and the discharge of the second turbine is directed to the third stage turbine. The discharge of the first stage compressor enters the second stage compressor and the discharge of the second stage compressor enters the third stage compressor. Air compression systems having more than three stages will be similarly configured. The discharge of the final (second stage in FIGS. 2 and 3) compressor 70 is divided into three portions, a supply air portion M„ a combustion air portion M„ and a regeneration air portion M r . The supply air M s after passing through air throttle valve 84 is cooled in an aftercooler 25 and then enters a receiver 50, typically through a drying system 30. The smaller fraction M r , regeneration air, flows through the drying system 30 to regenerate the desiccant in the drying system 30. The combustion air M, enters the recuperator 68 where it is preheated prior to entering the combustion chamber 60. The regeneration air M r is returned to the Brayton Cycle engine at the combustion chamber 60. At typical operating conditions, the ratios of M.rM.iM, are 64:34:2. However, as air demand varies, these ratios can vary. A pressure switch 88 is set to the desired high and low pressure set points at or near receiver 50. The fuel supply to the combustion chamber 60 is regulated by the fuel actuator valve 62 in response to the pressure switch 88. The temperature sensor 76 can override control from the pressure sensor 88 to maintain the turbine 64 inlet temperature within acceptable limits. FIG. 2 shows an optional dryer system 30. This system operates the same as the prior art dryer system described above with one major difference. The compressed air from compressor 70 is split into three portions, regeneration air M r , combustion air M, and supply air M,. The heated (heat of compression) regeneration air M r passes through the desiccant picking up moisture from the desiccant. The regeneration air M r , including the additional moisture, then mixes with the combustion air M, after the combustion air M„ passes through the recuperator 68. The additional moisture is beneficial to the Brayton cycle combustion chamber- turbine combination. Among the various benefits are reduced emissions, such as NO x . If necessary, for optimum performance, the regeneration air M r can enter the combustion chamber 60 separately, for example, into a pre-chamber. In some instances, throttle valve 85 is provided so that the pressure drop across throttle valve 85 and the desiccant bed 34a, 34b matches the pressure drop across the recuperator 68. This allows the regeneration air M r to mix with the balance of the combustion air M t . This prevents the addition of pressure losses to the Brayton Cycle which would adversely impact the fuel efficiency. For a typical system, the regeneration flow M r is 15 % of the supply air M,. One embodiment of the present invention utilizes conventional fixed geometry turbine and compressors (1, 2 or 3-stage). To regulate flow at the prescribed constant pressure set-point, a fuel valve 62 and air throttle valve 84, are actuated. The air throttle valve is a variable area valve 84, like a butter-fly valve, that regulates air flow to the receiver 50. Closing this valve 84 reproportions the flow through the turbine section of the engine. The fuel throttle 62 would respond to changes in the high spool compressor discharge pressure (64 in FIG. 2). This simple method allows system flow turn-down ratio of between 2 and 4 before engine on/off cycling would be required. This represents a much larger range than would be possible with a conventional compressor system as shown in FIG. 1. A second embodiment of the present invention described above can provide improved fuel efficiency at the part-load flow conditions. To maximize part load efficiency, for the 2:1 turn-down range, a variable nozzle feature can be added to the turbine sections. This added degree of control freedom will permit high turbine inlet temperatures to be maintained over a wider flow range and thereby improve fuel efficiency. For an even wider range of high efficiency, up to a 4: 1 turn-down range, a variable diffuser section can be added to the compressor sections. This additional feature would work in conjunction with the variable nozzle turbines and fuel throttle. An alternate embodiment of a Brayton cycle air compressor system is shown in FIG. 3. An electrical generator 100, 101, such as a generator or an alternator can be connected to one or both turbines 70, 72. With one or more generators 100, 101 connected to the turbines 70, 72, system efficiency will remain high throughout wide variations in the supply air delivered. When the air receiver 50 and the air distribution system 52 pressure increases to the set point of the pressure control valve 84, the valve will begin to throttle closed. As the back pressure on the compressors 70, 72 increases, the electrical generators 100, 101 will begin to pick up load. The system shown in FIG. 3 automatically shares load, i.e. , the work by-product of the Brayton cycle system, between the compressors 70, 72 and the electrical generators 100, 101. The temperature sensor 76 remains in the system to maintain the turbine inlet temperature within acceptable limits.

Claims: Having described the invention, what is claimed is: 1. A Brayton cycle air compressor comprising: a combustion chamber; at least one turbine, the at least one turbine being driven by exhaust gases from the combustion chamber; and a compressor for supplying combustion air to the combustion chamber, the compressor being driven by one of the at least one turbines, the compressor having a capacity in excess of the capacity necessary to supply combustion air to the combustion chamber, the work by-product of the Brayton cycle air compressor being extracted as excess compressed air. 2. The Brayton cycle air compressor according to claim 1, wherein the number of turbines is one. 3. The Brayton cycle air compressor according to claim 1, further comprising: an additional compressor, and wherein the number of turbines is two, each turbine driving one of the compressors. 4. The Brayton cycle air compressor according to claim 1, further comprising: two additional compressors, and wherein the number of turbines is three, each turbine driving one of the compressors. 5. The Brayton cycle air compressor according to claim 1, further comprising: a recuperator, the exhaust from one of the at least one turbines being provided to the recuperator, the combustion air passing through the recuperator prior to being provided to the combustion chamber, the exhaust gases preheating the combustion air. 6. An apparatus comprising: a combustion chamber; a supply of fuel being provided to the combustion chamber; a first turbine which is driven by exhaust gases leaving the combustion chamber; a second turbine which is driven by exhaust gases leaving the first turbine; a first compressor driven by the second turbine for compressing inlet air; and a second compressor driven by the first turbine for compressing the compressed air from the first compressor, the compressed air from the second compressor being divided into two portions, the first portion being provided to the combustion chamber, this first portion defining a combustion air portion, the second portion being extracted to form a supply of compressed air, this second portion defining a supply air portion. 7. The apparatus according to claim 6, further comprising: a recuperator, the exhaust gases leaving the second turbine being provided to the recuperator, the combustion air portion passing through the recuperator prior to being provided to the combustion chamber, the exhaust gases preheating the combustion air portion. 8. The apparatus according to claim 7, wherein the supply air portion of compressed air is provided to a distribution system; and having a pressure control means responsive to pressure in the distribution system for controlling the amount of fuel provided to the combustion chamber. 9. The apparatus according to claim 8, further comprising a temperature control means responsive to the temperature of the combustion chamber exhaust gases entering the first turbine for controlling the amount of fuel provided to the combustion chamber. 10. The apparatus according to claim 9, wherein the temperature control means maintains the temperature of the combustion chamber exhaust gases entering the first turbine between a predetermined minimum temperature and a predetermined maximum temperature. 11. The apparatus according to claim 8, wherein the distribution system includes a storage tank, the pressure control means being responsive to the pressure in the storage tank. 12. The apparatus according to claim 6, further comprising: a means for cooling the supply air portion; a dryer system for removing moisture from the supply air portion, the dryer system including a moisture absorption substance capable of being regenerated by the application of air having a temperature higher than the temperature of the cooled supply air portion, a further portion of the combustion air portion being applied to the moisture absorption substance for regenerating the moisture absorption substance, this portion of the combustion air portion defining a regeneration air portion. 13. The apparatus according to claim 12, wherein the regeneration air portion, including any moisture removed from the moisture absorption substance, is returned to the combustion air portion. 14. The apparatus according to claim 6, further comprising: an intercooler for cooling the compressed air from the first compressor prior to entering the second compressor. 15. The apparatus according to claim 6, further comprising: a first means for generating electricity driven by one of the first turbine or the second turbine. 16. The apparatus according to claim 15, further comprising: a second means for generating electricity driven by the other of the first turbine or the second turbine. 17. The apparatus according to claim 15, further comprising: a temperature control means responsive to the temperature of the exhaust gases entering the first turbine for controlling the amount of fuel provided to the combustion chamber. 18. A method of compressing air comprising: providing a supply of fuel; providing a supply of combustion air; heating the combustion air in a recuperator; mixing the combustion air and the fuel; burning the combustion air and fuel mixture; expanding the burned combustion air and fuel mixture in a first turbine; further expanding the burned combustion air and fuel mixture in a second turbine; compressing a supply of inlet air with a first compressor, the first compressor being driven by the second turbine; compressing the compressed air from the first compressor with a second compressor, the second compressor being driven by the first turbine; dividing the compressed air from the second compressor into two portions, providing the first portion to the recuperator, providing the second portion to an air distribution system; and providing the burned air and fuel mixture from the second turbine to the recuperator, the burned air and fuel mixture heating the portion of compressed air supplied to the recuperator. 19. The method according to claim 18, further comprising: measuring the pressure in the air distribution system; controlling the quantity of fuel mixed with the combustion air in response to changes in the pressure in the air distribution system. 20. The method according to claim 18, further comprising: measuring the temperature of the burned air and fuel mixture entering the first turbine; and controlling the quantity of fuel mixed with the combustion air in response to changes in the temperature of the burned air and fuel mixture entering the first turbine to maintain the temperature of the burned air and fuel mixture below a predetermined maximum temperature.

Inventors: BALDWIN STEPHEN PETER, KESSELI JAMES BARNETT, BALDWIN, STEPHEN, PETER, KESSELI, JAMES, BARNETT
Applicants: BALDWIN STEPHEN PETER, KESSELI JAMES BARNETT, NORTHERN RES & ENG, BALDWIN, STEPHEN, PETER, KESSELI, JAMES, BARNETT, NORTHERN RESEARCH & ENGINEERING CORPORATION
Application year: 1995
Main classification: F02C6
IPC codes: F02C6