BACKGROUND OF THE INVENTION The present invention relates to gas turbines. More specifically, the present invention relates to a system for capturing heat rejected from the portion of the compressor discharge air used to cool the turbine section of the gas turbine by transferring the heat to a fluid to be injected into the combustion section, such as gaseous fuel, without the use of an intermediate heat transfer fluid. A gas turbine is comprised of three main components: a compressor section in which air is compressed, a combustion section in which the compressed air is heated by burning fuel and a turbine section in which the hot compressed gas from the combustion section is expanded. To achieve maximum power output of the gas turbine, it is desirable to heat the gas flowing through the combustion section to as high a temperature as feasible. Consequently, the components in the turbine section exposed to the hot gas must be adequately cooled so that their temperature is maintained within allowable limits.

Traditionally, this cooling is achieved by flowing relatively cool air over or within the turbine components. Since such cooling air must be pressurized to be effective, it is common practice to bleed a portion of the air discharged from the compressor section and divert it to the turbine components for cooling purposes.

Although the cooling air eventually mixes with the hot gas expanding in the turbine, since it bypasses the combustion process much of the work expended in compressing the cooling air is not recovered in the expansion process. Consequently, to maximize the power output and efficiency of the gas turbine, it is desirable to minimize the quantity of cooling air used.

Unfortunately, as a result of the temperature rise which accompanies the rise in pressure in the compressor, the air bled from the compressor is relatively hot -- i.e., 315-485°C (600-900°F) depending on the compression ratio. Consequently, the air bled from the compressor must often be cooled to ensure that its temperature is low enough to adequately cool the turbine components. Moreover, as is well known in the art, the quantity of air bled from the compressor for cooling purposes can be reduced by cooling the air prior to directing it to the turbine components, thereby increasing its capacity to absorb heat. In the past, an air-to-air cooler was often used to cool the cooling air. In this arrangement, the air bled from the compressor flows through finned tubes over which ambient air is forced by motor driven fans, thereby transferring heat from the compressed air to the atmosphere. Although this method achieves adequate cooling, it detracts from the efficiency of the gas turbine since the heat energy associated with the work expended to compress the cooling air is lost to atmosphere.

Another approach involves the use of dual shell and tube heat exchangers and an intermediate heat transfer fluid. In this approach, air bled from the compressor is cooled by transferring heat from the compressed air to an intermediate fluid, such as water, in a first heat exchanger. The heat absorbed by the heat transfer fluid is then transferred to, for example, gaseous fuel in a second heat exchanger. Such an approach is disclosed in U.S.

patent 5,255,505 (Cloyd et al. ) , hereby incorporated by reference in its entirety.

Although this approach is more efficient than the air-to-air cooler, it suffers from several drawbacks. First, the requirement for two heat exchangers increases the size, cost and complexity of the system. Second, and perhaps more importantly, while small leaks in the tubes will not drastically compromise the system's integrity, a tube failure would result in a large leak and, potentially, contamination of the compressed air or fuel by the intermediate heat transfer fluid.

It is therefore desirable to provide a system for cooling the air bled from the compressor for cooling purposes in which the heat removed from the cooling air is returned to the cycle without the use of an intermediate heat transfer fluid.

SUMMARY OF THE INVENTION Accordingly, it is the general object of the present invention to provide a method and system for cooling air bled from the compressor of a gas turbine in which the heat removed from the cooling air is returned to the cycle, specifically, via a fluid, such as gas fuel or water, to be injected into the combustion section of the gas turbine without the use of an intermediate heat transfer fluid.

Briefly, this object, as well as other objects of the present invention, is accomplished in a gas turbine system having (i) a compressor section for producing compressed air, (ii) a combustion section for producing heated compressed gas by the combustion of a fuel in a first portion of the compressed air, (iii) a turbine section for expanding the heated compressed gas, and (iv) a heat exchanger having a core having means for cooling a second portion of the compressed air by (i) transferring heat from the compressed air to the core by convection,

(ii) then transferring the heat from a first portion of the core to a second portion of the core by conduction, and

(iii) then transferring the heat from the second portion of the core to the fuel by convection prior to the combustion thereof.

In a preferred embodiment of the invention, the heat exchanger is a printed circuit type heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a longitudinal cross-section, partially schematic, through a gas turbine system utilizing the compressed air cooling scheme of the current invention. Figure 2 is an isometric view, partially cut away, of the heat exchanger shown in Figure 1.

Figure 3 is a cross-section through a portion of the heat exchanger core shown in Figure 2.

Figures 4-6 are plan views, partially schematic, of the plates shown in Figure 3 taken along lines IV-IV, V- V, and VI-VI, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, there is shown in Figure 1 a longitudinal cross-section of a gas turbine system 1. The gas turbine is comprised of three main components: a compressor section 2, a combustion section 3, and a turbine section 4. A rotor 5 is centrally disposed in the gas turbine and extends through the three sections. The compressor section 2 is comprised of a cylinder 6 that encloses altemating rows of stationary vanes 7 and rotating blades 8. The stationary vanes 7 are affixed to the cylinder 6 and the rotating blades 8 are affixed to the rotor 5.

The combustion section 3 is comprised of a cylinder 9 which forms a chamber in which are disposed a plurality of combustors 10 and ducts 11 that connect the combustors to the turbine section 4. A fuel supply pipe 23 is connected to a fuel manifold 24 that distributes fuel to a nozzle 25 in each combustor 10. A portion of the rotor 5 extends through the combustion section 3 and is enclosed therein by a housing 12. Cooling air return pipes 13 and 14, discussed further below, penetrate the cylinder 9,

extend through the chamber and terminate at a manifold 15 that surrounds a portion of the housing 12.

The turbine section 4 is comprised of an outer cylinder 16 that encloses an inner cylinder 17. The inner cylinder 17 encloses alternating rows of stationary vanes 18 and rotating blades 19. The stationary vanes 18 are affixed to the inner cylinder 17 and the rotating blades 19 are affixed to a plurality of rotating disks 20 that form the turbine section of the rotor 5. In operation, the compressor inducts ambient air

21 into its inlet and discharges compressed air 22 into the chamber formed by the cylinder 9. The vast majority of the air 21 in the chamber enters the combustors 10 through holes therein (not shown) . In the combustors 10, fuel 26, heated as discussed below, is injected into and mixed with the compressed air 22 and burned, thereby forming a hot, compressed gas 27. The hot, compressed gas 27 flows through the ducts 11 and thence through the alternating rows of stationary vanes 18 and rotating blades 19 in the turbine section 4, wherein the gas expands and generates power that drives a load (not shown) connected to the rotor 5. The expanded gas 28 then exits the turbine, whereupon it may be exhausted to atmosphere or directed to a heat recovery steam generator. The rotating blades 19 and disks 20 in the turbine section are exposed to the hot gas 27 from the combustors 10, which may be in excess of 1300°C (2370°F) , and are subjected to high stresses as a result of the centrifugal force imposed on them by their rotation. Since the ability of the materials that form the blades and disks to withstand stress decreases with increasing temperature, it is vital to provide adequate cooling to maintain the temperature of these components within allowable levels. In the preferred embodiment, this cooling is accomplished by diverting a portion 29 of the compressed air 22 from the chamber formed by the cylinder 9 to the turbine section of the rotor 5. This diversion is

accomplished by bleeding air through an external bleed pipe 30 emanating from the cylinder 9. After being cooled, as explained below, the cooled cooling air 31 re-enters the gas turbine through return pipes 13 and 14. The return pipes direct the air to the manifold 15 after which the cooling air penetrates the housing 12 through holes 50 and enters an annular gap 52 formed between the housing 12 and the rotor 5. The cooling air 31 then enter the rotor 5 through holes 51 whereupon it flows through a plurality of intricate cooling passages (not shown) in the rotating disks and blades to achieve the desired cooling.

It is important to note that the cooling air 29 bypasses the combustors 10. Even though this air ' eventually mixes with the hot gas expanding in the turbine section 4, the work recovered from the expansion of the compressed cooling air is much less than that recovered from the expansion of the compressed air heated in the combustors. In fact, as a result of losses due to pressure drop and mechanical efficiency, the work recovered from the cooling air is less than that required to compress the air in the compressor. Hence, the greater the quantity of cooling air used the less the net power output of the gas turbine.

In accordance with the present invention, the quantity of cooling air 29 bled from the compressor discharge 22 is reduced by cooling the air, thereby increasing its capacity to absorb heat from and cool the turbine components, without losing the rejected heat from the cycle. This is accomplished by directing the hot cooling air 29 to a heat exchanger 53, which according to the current invention is preferably of the printed circuit heat exchanger type, as discussed more fully below.

The fuel 56 from a fuel source (not shown) is also directed to the heat exchanger 53 via fuel supply piping 58. The heated fuel 26 is returned to the gas turbine via piping 23. Thus, the cooling air 29 is cooled by rejecting heat to the fuel 56, thereby heating the fuel.

Since the heated fuel 26 is injected into and burned in the combustors 10, the heat it has absorbed from the cooling air 29 is returned to the cycle and reduces the quantity of fuel that must be burned to obtain the desired temperature of the gas 27 entering the turbine. Consequently, unlike traditional approaches to cooling the cooling air, the current invention does not result in significantly degrading the thermal efficiency of the gas turbine.

As previously discussed, the heat exchanger 53 according to the current invention is of the printed circuit heat exchanger type ("PCHE") . Such heat exchangers are available from Heatric Ltd., Dorset, England and from Gencorp Aerojet, Rancho Cordova, CA. As shown in Figure 2, the heat exchanger 53 is comprised of a housing 55 that encloses a core 54.

The heat exchanger housing 55 forms manifolds 61- 65. Manifold 61 is connected to the hot compressed air piping 30. It receives the hot compressed air 29 supplied to the heat exchanger 53 and directs that air to the core 54. Manifold 63 is connected to the cooled compressed air piping 40 and receives the cooled compressed air 31 from the core 54 that is to be discharged from the heat exchanger 53. Manifold 64 is connected to the fuel supply piping 58. It receives the fuel 56 supplied to the heat exchanger 53 and directs the fuel to the core 54. Manifold 62 is connected to the heated fuel piping 23 and receives the heated fuel 26 from the core 54 that is to be discharged from the heat exchanger 53.

The core 54 is formed from a solid mass, such as stainless steel, in which a large number of passages are formed. As shown in Figure 3, the core 54 is preferably formed by a series of metal plates 44-46 that are interleaved and then diffusion bonded together along their planar surfaces. For purposes of illustration, the surfaces along which the plates were joined is indicated in Figure 3 by the dotted lines marked by reference numeral 37. However, it should be understood that owing to the

fact that they are diffusion bonded together, the plates 44-46 form a contiguous mass without intermediate boundaries. As is well known in the PCHE art, each of the plates 44-46 contains a number of channels that were chemically milled into its surface prior to diffusion bonding using techniques similar to those used to form electrical printed circuits (hence the name "printed circuit heat exchanger").

According to the preferred embodiment of the current invention, there are three types of plates. The channels milled into plates 44 form passages 41 for the fuel 56, while the channels milled into plates 46 form passages 42 for the compressed air 29. The plates 45 are sentinel plates, discussed further below. The plates 44 alternate with plates 46 and, in the preferred embodiment, a sentinel plate 45 is disposed between each pair of plates 44 and 46.

As shown in Figure 4, the fuel passages 41 formed in plate 44 lie in a plane and extend along the entire length of the plate. The inlets of the passages 41 are formed in one half of one end edge 68 of the plate 44 and the outlets are formed in the opposite half of the opposite end edge 69 of the plate. The initial and final extent of the passages 41 are angled so that although the inlets and outlets of the passages are segregated into one half of the end edges 68 and 69, the major portions of the passages 41 are distributed over a major portion of the planar surface of the plate 44. The manifold 64 creates a chamber 35 that serves to distribute the incoming fuel gas 56 to the passages 41. The manifold 62 creates a chamber 32 that serves to collect the heated fuel gas 29 from the passages

41 for discharge from the heat exchanger 53.

As shown in Figure 6, the compressed air passages

42 formed in plate 46 also lie in a plane and extend along the entire length of the plate. The inlets of the passages

42 are formed in one half of one end edge 72 of the plate 46 and the outlets are formed in the opposite half of the

opposite end edge 73 of the plate. The initial and final extent of the passages 42 are angled so that although the inlets and outlets of the passages are segregated into one half of the end edges 72 and 73, the major portions of the passages 42 are distributed over a major portion of the planar surface of the plate 46. The manifold 61 creates a chamber 33 that serves to distribute the incoming hot compressed air 29 to the passages 42. The manifold 63 creates a chamber 34 that serves to collect the cooled compressed air 31 from the passages 42 for discharge from the heat exchanger 53.

Thus, as shown in Figures 2 and 3, the core 54 forms alternating parallel rows of fuel and compressed air passages that extend along its entire length, with each row of fuel passages 41 being sandwiched between two rows of compressed air passages 42. In the preferred embodiment, the rows of passages 41 and 42 are arranged so that the fuel 56 and compressed air 29 flow in a counter flow arrangement. However, by reversing the fuel piping 23 and 58 connections to the manifolds 62 and 63, a parallel flow arrangement could also be employed. In addition, by rotating one set of plates 90° prior to diffusion bonding, and adjusting the length and width of the plates, a cross flow arrangement, in which the passages 41 and 42 were oriented perpendicular to each other, could also be employed.

Regardless of which flow arrangement is selected, as the compressed air 29 flows through the passages 42 it transfers heat, primarily by convection -- that is, by heat transfer from a fluid to a solid --, to the adjacent portions of the core 54 that surround each of the passages 42. This heat is then transferred by conduction -- that is, by heat transfer through a solid -- to the adjacent portions of the core 54 that surround each of the passages 42. The heat is then transferred, primarily by convection, to the fuel 56 flowing through the passages 42. In this manner, heat is transferred from the compressed air 29 to

the fuel 56 directly -- that is, without the use of an intermediate heat transfer fluid.

As can be seen, the use of a PCHE type heat exchanger eliminates the concern that an intermediate heat transfer fluid will enter the compressed air or fuel. Moreover, PCHE type heat exchangers are compact and have low pressure drops. In addition, due to the multitude of small passages in the core 54 through which the fuel and compressed air flow, the likelihood of a catastrophic leak, such as occurs when a tube fails in a shell and tube type heat exchanger, is minimized.

Nevertheless, the danger exists that fuel will leak into the compressed air if the flow containment capability of the passages 41 and 42 are compromised -- for example, because of the formation of fatigue cracks that extend from a fuel passage 41 to a compressed air passage

42 or because corrosion wastes away the portion of the core 54 between adjacent fuel and compressed air passages.

As is known in the art, this situation can be addressed by the use of sentinel plates 45 disposed between each pair of plates 44 and 46, as shown in Figure 3. Channels milled into each sentinel plate 45 form a row of passages 43 disposed between the alternating rows of fuel passages 41 and compressed air passages 42. As shown in Figure 5, like the passages 41 and 42, the passages 43 formed in sentinel plate 45 lie in a plane and extend along the entire length of the plate. One end of each passage 42 is formed in a portion of the side edge 74 of the plate 45 that is adjacent to the end edge 70. Each passage 42 terminates in a dead end adjacent the other end edge 71 of the plate 45. The initial and final extent of the passages

43 are angled so that the major portions of the passages are distributed over a major portion of the planar surface of the plate 45. As can be seen in Figure 3, the distance between a sentinel passage 43 and either a fuel passage 41 or compressed air passage 42 is shorter than the distance between a fuel passage and a compressed air passage.

In the embodiment of the invention shown in Figure 1, the sentinel passages 43 are pressurized with an inert gas to a pressure higher than that of the fuel 56 or the compressed air 29. Since the distance between a fuel passage 41 and a sentinel passage 43 is shorter than the distance between a fuel passage and a compressed air passage 42, in the event of cracking or corrosion that compromises the flow containment capability of a fuel passage it is likely that such deterioration will place the compromised passage into the flow communication with a sentinel passage, rather than a compressed air passage. Consequently, the pressurized inert gas in the sentinel passage 43 will enter the compromised fuel passage "41, resulting in a drop in pressure in the manifold 65. Similarly, deterioration of a compressed air passage 42 will also cause a drop in pressure in the manifold 65. In either event, a pressure sensor 57' installed in the manifold 65 will alert operating personnel of the situation. The use of sentinel passages containing pressurized gas is known in the art.

However, according to the current invention and contrary to the previously known approach discussed above, the sentinel passages 43 may contain air at ambient pressure. This embodiment is shown in Figure 5. If fuel 56 enters a sentinel passage 43 it will flow into a chamber 36 formed by the manifold 65 and then into vent piping 49. A rise in the pressure in the vent piping 49, as sensed by a pressure sensor 57, will alert operating personnel to the presence of the leak so that appropriate corrective action can be undertaken before a dangerous situation arises.

According to the current invention, a rupture disc 48 is installed in the vent piping 49 so that the fuel 56 will be vented to atmosphere when the pressure reaches a pre¬ determined level. Alternatively, a hydrocarbon monitor could be installed in the compressed air piping 40 to allow detection of fuel leaks.

In the embodiment shown in Figure 1, heat captured from the compressed air 29 is transferred to the fuel 56 that is injected into the combustors 11. However, in many applications it is desirable or necessary to inject another fluid, such as water or steam, into the combustors

10 -- for example, to reduce the formation of oxides of nitrogen (NOx) , which are considered atmospheric pollutants, in the hot gas 27 or to augment the power output of the turbine. In such situations, the current invention may be utilized by directing water or steam, rather than fuel 56, through the heat exchanger 53. When the heated water or steam is injected into the combustors

11 to reduce NOx, the heat that the water/steam absorbed from the compressed air 29 is returned to the cycle. A traditional gas turbine cannot be operated without burning fuel. Therefore, if fuel is used in the heat exchanger 53, cooling of the compressed air 29 can always be assured as long as the heat exchanger 53 is in service. However, in a system in which water or steam is used in the heat exchanger, as suggested above, it may sometimes be undesirable or unnecessary to inject the water or steam. In such situations there would be no fluid flowing through the heat exchanger 53 to which the heat from the cooling air 29 can be rejected. However, since the cooling air 29 must be cooled nonetheless, an alternate medium must be found to which the heat from the cooling air can be transferred. According to the current invention, this problem is solved by the use of an auxiliary heat exchanger 66, shown in Figure 1, which may be a conventional air-to-air cooler of the fin-fan type over which ambient air 59 flows, that is connected in parallel with the heat exchanger 53. Even in a system utilizing fuel in the heat exchanger 53, the use of the auxiliary heat exchanger 66 will facilitate maintenance of the primary heat exchanger 53 without an extended outage of the gas turbine.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.