Kesseli Recup Igti Paper

MST TOC Proceedings of ASME Turbo Expo 2003 Power for Land, Sea, and Air June 16–19, 2003, Atlanta, Georgia, USA

GT2003-38938 MICRO, INDUSTRIAL, AND ADVANCED GAS TURBINES EMPLOYING RECUPERATORS James Kesseli Ingersoll-Rand Energy Systems Portsmouth, N.H.

Thomas Wolf Ingersoll-Rand Energy Systems Portsmouth, N.H.

James Nash Ingersoll-Rand Energy Systems Portsmouth, N.H.

Steven Freedman Consultant Deerfield, Ill.

DESCRIPTION OF THE INGERSOLL-RAND RECUPERATOR The patented recuperator manufactured by Ingersoll-Rand Energy Systems (U.S. patent 5,983,992) represents a significant advancement in recuperator technology. It offers superior life and a design that can be manufactured at reasonable cost. The Ingersoll-Rand recuperator is a counterflow plate-fin heat exchanger with crossflow headers. This is a compact arrangement, offering a large gas-entry frontal area and a large amount of heat exchanger area per unit volume.

ABSTRACT Recuperators increase system efficiencies in gas turbine engines by recovering exhaust heat to the compressor discharge stream. In this study, the performance and economics of recuperation are evaluated and presented for a practical range of effectiveness with typical pressure-loss-fractions. The strong correlation between recuperator cost and engine specific-power is shown, using a recuperator designed and manufactured at a highly automated facility by Ingersoll-Rand. This commercially available recuperator is also described, with specific emphasis on features contributing to its exceptional durability. INTRODUCTION In 1994, Ingersoll-Rand began development of a new recuperator in response to the needs of the emerging microturbine and advanced gas turbine industries. These gas turbine engines operate at high gas-side temperatures (600700°C) and need higher durability than those of the previous generation. Of equal importance has been recuperator cost, which has been a significant barrier to their wide scale application in cycles optimized for recuperation.

The fundamental building block of the Ingersoll-Rand recuperator is a unit cell (Fig. 1), the repeating element of a recuperator core. The unit cell includes all the features for flow distribution, heat transfer and pressure containment. Gas fin segment Stamped parting plate Air fin segment

This paper describes some of the unique design features contributing to the exceptional durability of the Ingersoll-Rand recuperator. Secondarily, this paper provides a quantitative cost basis for the recuperator. This information can be used by prospective developers of recuperated gas turbines to optimize various cycle and engine design parameters on a life-cycle-cost basis.

Air distribution headers

Gas fin segment

Stamped parting plate

Figure 1. Recuperator Unit Cell (Exploded)

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The cell is a brazed assembly composed of symmetrical gas fin segments on the outside of two parting plates. Between the parting plates are an air-fin matrix and two air distribution headers. The perimeter of the unit cell parting plates, forming the pressure boundary between the air and exhaust gas streams, is totally welded and brazed.

DESIGN FOR MANUFACTURING Unit-cell construction gives the core a scalable geometry, facilitating adaptation to a wide range of engine sizes. The simple construction technique is very efficient in its use of materials and is well suited to automated assembly methods. Ingersoll-Rand Energy Systems currently produces five different recuperator products optimized for specific engines. Although each product requires unique assembly and tooling combinations, all are manufactured on a common production line and share the sizable investment in capital equipment.

The air distribution headers between the parting plates channel airflow through the matrix fin. When brazed, the airmatrix fin and the header fins carry the pressure load created by the compressor discharge pressure. The brazing also forms an ideal joint between the fin and plate, assuring excellent thermal performance.

The manufacturing process uses high-volume progressive stamping dies, capable of producing several hundred finished parts per hour from rolled sheet stock. A continuous-flow robotic braze application system, produces assembly-ready parts with exceptionally high precision and quality. Simple selffixturing features in the seven unit cell elements facilitate brazing of the cell in a high-volume process. Due to the lightweight cell structure, the brazed materials require relatively short exposures at the braze melt temperature, preventing the grain growth and other undesirable material transformations that plagued earlier furnace-brazed multi-layer core heat exchangers. The high level of automation and refinement of the manufacturing process yields very low attrition rates.

Construction of a modular recuperator core (Fig. 2) is accomplished by stacking and joining individual cells at the stamped circular flanges. These flanges are designed to facilitate welding and accommodate substantial thermal strain. When stacked to the full core height, the welded circular flanges form the distribution manifolds for the air entering and exiting the core. These manifolds form a flexible welded bellows, extending the entire length of the recuperator core. The assembled Ingersoll-Rand recuperator is shown schematically in Fig. 3. It can be characterized as a hybrid design, exploiting the best aspects of both primary and extended-surface plate heat exchangers while overcoming their inherent weaknesses. The core is fabricated entirely from lightgauge sheet, eliminating the stiffness discontinuities present in recuperators employing edge bars. Moreover the core has an accessible and entirely welded pressure boundary. The result is a compact core with independent air-fin and gas-fin optimization. Its critical advantage is tolerance of the severe thermal gradients that often occur during transient operation. The design also eliminates the need for the complex preload mechanism ordinarily required for primary-surface recuperators.

The brazed unit-cell is a complete recuperator, containing enhanced air and gas-side heat transfer surfaces. Each cell is pressure tested to its proof-test limit to assure mechanical integrity. For most applications, a nominal 300-percent proof pressure test of may be imposed. The assembled recuperator core may contain from 1 to 200 cells, depending on customer specifications. The unit-cells are joined at the circular manifold hoops employing a highly automated robotic orbital welder. In situ and final core welding tests are subsequently performed to verify leak-tight requirements and guarantee the integrity of the finished cores. These basic recuperator manufacturing advantages contribute to low cost, even at modest sales volumes.

Unit-cell construction makes the recuperator exceptionally durable compared with conventional configurations, historically plagued by fatigue and creep problems. Without rigid connection between individual cells, the Ingersoll-Rand core has minimal internal mechanical restraint and avoids thermal strain associated with more rigid designs. Fatigue-life is extended well beyond that of the conventional monolithic construction. Unit cell

For large packages, multiple cores can be joined with Vband clamps. Due to the inherent flexibility of the welded cores external bellows expansion joints are not required in core-to-core attachments. Hot air out

Circular flange

Cold air in Exhaust gas out

Manifolds created by welded circular flanges

Exhaust gas in

Figure 2. Core Fabrication

Figure 3. Recuperator Flow Paths

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RECUPERATOR COST MODELING AND BRAYTON CYCLE ANALYSIS The recuperator core represents the complete heat exchanger, containing integral air-side manifolds, counterflow heat exchange surface, and all necessary internal headers. The core alone constitutes the majority of the recuperator system costs, but in all applications additional packaging, ducting, and mounting accessories are required for a practical functional system. Since these accessories are dependent upon the engine size and configuration, it was not possible in this study to add their contribution to the cost model. To avoid underestimating these relevant costs, budgetary estimates for the balance of the recuperator system typically range from 25 to 50 percent of the core price. In military applications, which have shock load and other non-commercial requirements, the cost of the balance of the recuperator package may range from 50 to 100 percent of the core price.

RECUPERATOR PERFORMANCE ANALYSIS Recuperators consisting of unit-cell building blocks can be fabricated to meet a wide range of gas turbine cycle specifications. Typical recuperator performance is specified as air-side pressure drop, gas-side pressure drop, and thermal effectiveness. The task of the recuperator designer is typically to meet these specifications, at minimum cost, for the following inputs: Air-side: Mass flow rate into recuperator • Compressor discharge temperature • Compressor discharge pressure •

Gas-side: Mass flow rate into recuperator • Turbine exit temperature • Turbine exit pressure (typically equal to the stack loss plus the recuperator pressure loss) •

As indicated by the recuperator sizing inputs listed in the previous section, the engine designer’s choice of cycle parameters and performance specifications dictates the size of the recuperator. The task of the recuperator designer is to manipulate the geometry parameters into the lowest cost package while meeting the engine designer’s performance specifications.

To optimize the Ingersoll-Rand unit-cell recuperator, the designer manipulates the following design parameters. • • • • • • • • • • •

Air side counter-flow fin density Air-side fin geometry Air-side fin thickness * Air-side fin height Gas-side fin density Gas-side fin geometry Gas side fin thickness Gas side fin height * Cell count Plate thickness * Material selection * dictated by frame model

The following parametric studies provide a quantitative indication of the size and cost of the compliant recuperator. Due to the sizable number of free parameters, the following analysis attempts to categorize engines according to three practical groupings (Table 1): Microturbines Low pressure ratio, uncooled turbine sections, with low-cost hot sections limited to operation from 800 - 1000° C. Component efficiencies are typical of the under-250 kWe equipment class. Small and mid-sized gas turbines Moderate pressure ratios, no bleed air fraction, and higher component efficiencies. For this class of engine, typically in the 1 to 5 MW power level, turbine inlet temperatures of 9001100°C are evaluated.

Currently, Ingersoll-Rand manufactures unit cells with three counterflow parting plate configurations (“footprints”), and two parting plate draw-depths (fin heights). Each footprint requires a special stamping die, generally an expensive tool. The three dies in operation at Ingersoll-Rand accommodate plate thickness from 0.008 to 0.016 inches (0.2 to 0.4 mm). Typical fin gauges range form 0.003 to 0.008 inches (0.07 to 0.2 mm).

Large industrial gas turbines Operating with cooled turbine sections, and high component efficiencies. Table 1. Engine Categories Microturbines

To predict the thermal effectiveness and gas and air-side pressure losses, Ingersoll-Rand uses computational models validated by instrumented core rig tests. In commercial and U.S. Navy recuperator designs, these models have been used to predict pressure drops with an accuracy of ±0.5 percent. With uniform flow distributions, the thermal effectiveness predictions match the measurements of controlled experiments with an accuracy of ±0.3 percent. Under conditions with highly skewed gas-side flow distribution, variances of 1 percentage point have been observed.

Turbine inlet temp range (°C)

800-1000

900-1100

Turbine cooling bleed (percent)

0

0

82

84.5

87

83.5

85.5 (gas generator) 84 (power turbine)

88 (gas generator) 86 (power turbine)

Compressor polytropic efficiency (percent) Turbine polytropic efficiency (percent)

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Small and MidSize Gas Large Industrial Gas Turbines (SGT) Turbines

1100-1300 3 at 1100°C 5 at 1200°C 7 at 1300°C

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In the following analysis, recuperators have been sized for an array of Brayton cycle statepoints. Recuperator cost has been normalized for engine shaft output power. To limit the extensive number of variables in the study, all recuperators are sized using the Ingersoll-Rand’s commercially available Frame 4 parting plate configuration (“footprint”) and only the air- and gas-side fin densities and cell count are varied.

• • • •

Figures 4 and 5 display cycle efficiency for lines of constant turbine inlet temperature, assuming the engine configurations defined in Table 1. The recuperator configurations in Figure 4 have been sized for an effectiveness of 90 % with a combined pressure drop percentage (∆P/P air + ∆P/P gas) of 4 %. This pressure loss has been apportioned between the two sides of the recuperator in the manner that yields the least expensive recuperator. The air-side pressure drop include losses in the bellows-like inlet and outlet manifolds formed by the assembled recuperator cells. The gasside pressure losses includes friction losses within the core plus entrance and exit effects, based on a uniform velocity profile. In a typical recuperator installation, a turbine exit diffuser is integrated within the recuperator plenum, though these losses are not debited to the recuperator. In Figure 5, a recuperator of 85% thermal effectiveness with 6% total pressure drop has been sized for the three engine groups defined in Table 1.

•

Recuperator cost can also be shown to correlate with engine specific power, defined as shaft power/compressor mass flow rate. Figures 6 and 7 illustrate this correlation, with recuperator core cost reduced dramatically for increasing specific power. The second distinct effect is the step function change in the pricing caused by the switch from the austenitic alloy to the superalloy, defined to occur at the turbine exit temperature of 700°C. (The heavy dotted lines in Figs. 6 and 7 shows the demarcation between recuperator alloys.) This is believed to be a representative limit for the IR recuperator, employing our specialized alloy 347 metallurgy. At sustained conditions near this limit, more detailed evaluation is recommended as oxidation rates are know to be accelerated. The IN625 recuperator cost is shown the be higher due to three effects; 1) the high price of the alloy, 2) slower processing times, and 3) its lower thermal conductivity.

The annotated values on each TIT curve in Figures 4 and 5 represent the specific cost of the recuperator core. The specific core cost is defined as the normalized recuperator core cost divided by output shaft power in US$/kW. The recuperator cost model assumes an abrupt transition between austenitic stainless steel and the high nickel class of superalloys when the turbine exhaust temperature reaches 700° C. This point of demarcation is indicated by the symbol [Җ]. Specific power figures to the left of this symbol represent the higher material pricing and slower processing rates of alloy-625.

CONCLUSIONS The preceding analysis has been presented to serve as a preliminary design guide for gas turbine engine designers contemplating conversion to a recuperated cycle. The results show that a given temperature ratio has a characteristic optimum pressure ratio for maximizing efficiency. This optimum pressure ratio naturally occurs at a relatively low specific power, generally well below that of a practical simple cycle gas turbine. Furthermore, the economics of the recuperator significantly improve with increasing specific power. The competing relationship between cycle efficiency and capital cost suggest the need for a detailed life cycle cost analysis and a well conceived business plan. The comparative analysis shows that the 90% effective with 4% pressure drop costs about 50% more than the recuperator with 85% effectiveness and 4% pressure drop.

Cost has been estimated using material and labor rates from 2002 at the Ingersoll-Rand recuperator manufacturing facility in Mocksville, N.C. The specific cost figures presented are solely for the purpose of providing a estimates for cycle optimization analyses, and should not be construed as indicative of a quotation. Also, the figures are representative of volume production orders and a mature product configuration. Additionally the following commercial conditions are assumed: • • •

Austenitic stainless steel (typical AISI 347) purchased at 3.18 $/lbm. High nickel superalloy (typical alloy-625) purchased at $11.03 /lbm. Forming and processing rates for alloy-625 are slower than those of alloy-347. Recuperator plate geometry of IR Frame 4 model, the more compact Frame 5 and Frame 3 geometries will result in higher costs. Pressure test included in build cost.

Prices are FOB Mocksville, NC, excluding shipping and special packaging. Price excludes insulation, case, piping, seals, and mounting structure. Price excludes core-to-core flanges and special attachments.

ACKNOWLEDGMENTS The authors wish to recognize the roughly 9 years of intensive efforts by both Gary Manter, for persistently working out the complexities of the manufacturing process, and Alex Haplau-Colan for his analytical and innovative contributions to the thermal-structural design.

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Figure 4. Gas turbine cycle analysis for the three engine scenarios and recuperator of 90% effectiveness and 4% total pressure drop. Specific cost values for the recuperator core are annotated on each curve of constant TIT.

20.1 20.8

Figure 5. Gas turbine cycle analysis for the three engine scenarios and recuperator of 85% effectiveness and 6% total pressure drop. Specific cost values for the recuperator core are annotated on each curve of constant TIT.

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Figure 6. Gas turbine cycle analysis for the three engine scenarios and recuperator of 90% effectiveness and 4% total pressure drop. The heavy dotted line shows the boundary between the choice of a alloy-347 and alloy 625.

Figure 7. Gas turbine cycle analysis for the three engine scenarios and recuperator of 85% effectiveness and 6% total pressure drop. The heavy dotted line shows the boundary between the choice of a alloy-347 and alloy 625.

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