Low-cost compact primary surface recuperator concept for microturbines

Applied Thermal Engineering 20 (2000) 471±497

www.elsevier.com/locate/apthermeng

Low-cost compact primary surface recuperator concept for microturbines Colin F. McDonald* McDonald Thermal Engineering, 1730 Castellana Road, La Jolla, CA 92037, USA Received 2 February 1999; accepted 16 March 1999

Abstract By the year 2000, microturbines in the 25±75 kW power range are projected to ®nd acceptance in large quantities in the distributed power generation ®eld, their major attributes include low emissions, multifuel capability, compact size, high reliability and low maintenance. For this type of small turbogenerator, an exhaust heat recovery recuperator is mandatory in order to realize a thermal eciency of 30% or higher. The paramount requirements for the recuperator are low cost and high e€ectiveness. These characteristics must be accomplished with a heat exchanger that has good reliability, high performance potential, compact size, light weight, proven structural integrity, and adaptability to automated high volume production methods. In this paper, a recuperator concept is discussed that meets the demanding requirements for microturbines. The proposed stamped and folded metal foil primary surface recuperator concept has as its genesis, a prototype heat exchanger module that was fabricated as part of an energy research program in Germany over two decades ago. This novel heat exchanger approach was clearly ahead of its time, and lacking an application in the late 1970s was, alas, not pursued and commercialized. Based on this earlier work, a further evolution of the basic concept is proposed, with emphasis placed on the following: (1) minimization of the number of parts, (2) use of a continuous fabrication process, (3) matrix overall shape and envelope ¯exibility (annular or platular geometry), (4) ease of turbogenerator/recuperator integration, and (5) a later embodiment of a bimetallic approach, towards the goal of establishing a compact and cost-e€ective recuperator for the new class of very small gas turbines that are close to entering service. For a representative microturbine, an annular recuperator would have only ®ve basic parts. The matrix cartridge would be essentially a plugin component, analogous to an automobile oil ®lter element. In this paper, the important role that the recuperator has on turbogenerator performance is discussed, together with a summary of the early prototype heat exchanger development. The major requirements, features and cost goals for a compact

* Tel.: +1-619-459-9389; fax: +1-619-459-6626. 1359-4311/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 9 9 ) 0 0 0 3 3 - 2

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Nomenclature a b c d f FZJ j kW MW P R SFC SP V W b DP DP=P d e l

heat exchanger length (counter¯ow plus end sections) matrix depth matrix height (over folded section) passage hydraulic diameter fanning friction factor Research Centre Juelich, Germany Colburn heat transfer factor kilowatt megawatt pressure compressor pressure ratio speci®c fuel consumption speci®c power recuperator matrix volume engine air ¯ow rate angle of wave pattern, and total surface compactness pressure loss heat exchanger pressure loss, percent metal thickness heat exchanger e€ectiveness wave length of corrugation

primary surface recuperator for microturbine service, are also covered. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Microturbine; Turbogenerator; Recuperator; Primary surface heat exchanger; Performance; Cost goals; Compact matrix cartridge; High-volume automated fabrication

1. Introduction For over half a century, the simple cycle type of gas turbine has been dominant in the ®elds of power generation, mechanical drives and aircraft propulsion. Performance advancements have been signi®cant, the major contributors being continuous gains in compressor and turbine eciencies, and increased turbine inlet temperature. While these will continue to increase, future gains in engine thermal eciency will likely be only incremental. Compressor and turbine eciencies are near plateauing, and further increases in turbine inlet temperature are governed by materials and blade cooling technologies. To improve engine thermal eciency by several percentage points requires the use of more complex thermodynamic cycles. Heat

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exchangers, in one form or another, are important in the quest for higher eciency gas turbines. This topic has been covered in previous papers by the author [1±3]. The use of exhaust heat recovery exchangers has always been an option for improving eciency, and they are applicable in principle to a wide range of gas turbine applications. Nevertheless, recuperators have found only limited acceptance because of their earlier bulky size, poor reliability and high cost. To date the use of a recuperator has been essentially a user option, but there is now an emerging engine application, namely microturbines, where its use is mandatory to achieve engine eciencies of 30% and higher [4]. The use of a recuperated cycle is mandatory for it to penetrate the power generation market, that in the 25±75 kW power range, is currently dominated by low SFC Diesel generator sets. Microturbines are facing a moving target since the performance of small Diesel engines continues to improve [5]. Today, a direct injection Diesel automotive engine with a variable geometry turbocharger and intercooling [6] is in service, with an eciency of 42.9% (SFC of 196 g/kW h). Advantage will surely be taken of this automotive Diesel engine technology for future industrial generator sets. It is projected that microturbines will enter the market place around the year 2000, and with a variety of applications, they have the potential to be manufactured in large quantities. Such a market opportunity presents a considerable and exciting challenge to heat exchanger designers. There are several recuperators available today that have overcome the early impediments of bulky size and poor structural integrity. However, these heat exchangers were never designed for high volume production, and having been manufactured in only small quantities have a high unit cost. For the microturbine market, e€orts need to be focused on establishing a low-cost recuperator, since this component represents a signi®cant percentage of the overall turbogenerator cost. It also represents the one gas turbine component that has received the least development attention in recent years. Clearly, intensive activities in the next two years or so are necessary to remedy this situation. Minimizing the number of parts in the recuperator assembly, together with assuring that one hundred percent of the surface geometry is e€ective in the heat transfer process and utilizing an automated manufacturing process for high volume production, are the key factors towards establishing a low-cost recuperator. Over two decades ago such a recuperator concept was patented and pioneered in Germany [7], and a prototype module was successfully fabricated and tested. Such a concept was ahead of its time. Lacking an application at that time, it was not deployed or commercialized. The author considers that the concept identi®ed in the late 1970s has features and merits that meet the needs of todays' small turbogenerators. An evolution of this proven type of heat exchanger construction is proposed to meet the demanding requirements of recuperators for the ®rst generation of microturbines for electrical power generation. To give some background to the discussion of the concept, this paper also covers the early recuperator development, since sadly it does not seem to be well known, particularly in the United States and the United Kingdom. This paper, of course, would not be complete without discussing recuperator cost. This topic is addressed essentially as a challenge to the heat exchanger industry, to fabricate a microturbine recuperator matrix cartridge for a value of about 1.5 times the material cost, when manufactured in large production quantities. It is postulated that the proposed primary surface recuperator concept has the potential for meeting this challenging goal.

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2. Microturbine deployment 2.1. Near-term deployment Microturbines can meet the total energy needs for a variety of complexes including hospitals, supermarkets, schools, factories, oce buildings and apartment houses [8,9]. The advantages of the gas turbine prime-mover over existing Diesel engine generator sets include the following: smaller size and weight, multifuel capability, lower emissions, lower noise, vibration-free operation and reduced maintenance. There are at least a dozen engineering organizations with microturbine units under development towards the goal of penetrating the commercial market starting around the year 2000. 2.2. Representative microturbine features While several di€erent microturbine concepts are being developed, the con®guration shown in Fig. 1 and discussed previously in [10] is convenient for highlighting the major engine features. To meet demanding cost goals the engine must be kept as simple as possible, and could include the following: (1) single stage radial compressor, (2) single stage radial in¯ow

Fig. 1. Recuperated microturbine concept (courtesy C. Rodgers, ITC).

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turbine, (3) high speed rotor supported on air bearings, (4) direct-drive high speed air-cooled generator, (5) multifuel combustor (conventional or catalytic), (6) a simple control system, and (7) a compact high e€ectiveness recuperator. To minimize the size of the overall turbogenerator package, the concept shown in Fig. 1 embodies a wrap-around annular recuperator. The actual gas ¯ow paths shown in the recuperator are unique to the proposed low-cost heat exchanger concept, and this will be discussed in a later section. The utilization of an annular recuperator contributes signi®cantly to minimizing the overall size of the turbogenerator package. The advantages of this approach also include the following: (1) good aerodynamic gas ¯ow paths resulting in low pressure losses, (2) lower acoustic signature, (3) built in rotor burst protection shield, (4) elimination of external ducts and thermal expansion devices, (5) minimizes the amount of external insulation needed, and (6) minimizes the overall heat exchanger cost. It should be, however, pointed out that there is considerable ¯exibility in the packaging of a microturbine, and engines with the recuperator installed behind the rotating machinery [11] can also meet users' needs. This approach may be attractive for applications where there is a desire

Fig. 2. Projected performance potential of recuperated microturbines.

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to bypass the recuperator, or where the heat recovery system includes a steam generator. Also, it is very amenable to the coupling of a microturbine with a high temperature fuel cell to give very high levels of overall eciency. 2.3. Microturbine state-of-the-art performance For simple cycle microturbines, it is dicult to achieve a thermal eciency much in excess of 20% [12] based on today's technology, thus a recuperator is mandatory to achieve eciencies of 30% and higher. The majority of microturbines currently being developed by various engineering organizations embody compact ®xed boundary recuperators, but the use of rotary regenerators in some others still continues. It is the view of many engineers, including the author, that the recuperator o€ers a simpler and more cost-e€ective approach. A further advantage is that with no moving parts, it does not require maintenance for the full life of the turbogenerator. A convenient and simple way of portraying the e€ect that two of the major cycle parameters, namely turbine inlet temperature and recuperator e€ectiveness, have on the thermal eciency of low pressure ratio (say around four to one) microturbines is shown in Fig. 2. Regime number 1 on this ®gure is representative of ®rst generation state-of-the-art recuperated microturbines, and shows an eciency of around 30%. An excellent example of an operating microturbine embodying proven technology is the 30 kW Capstone turbogenerator [8], and this unit is shown in Fig. 3. This very compact microturbine has a stainless steel wraparound annular primary surface recuperator. It is for engines characterized by operation in Regime number 1 that the proposed low-cost recuperator concept, discussed in later sections, is being initially proposed. 2.4. Microturbine performance evolution Following their introduction in the distributed power generation ®eld, development e€orts

Fig. 3. Compact Capstone 30 kW recuperated microturbine (courtesy Capstone Turbine Corporation).

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Fig. 4. Projected performance evolution of recuperated microturbines.

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will continue aimed at advancing microturbine performance and reducing emissions. With technology advancements in all of the components, but dominantly in the area of higher turbine inlet temperature, an orderly evolution over a period of several years is likely towards realizing the full performance potential of say 45±50% eciency for this type of heat engine. Overall eciency levels in excess of 60% may be possible with the future coupling of microturbines with high temperature fuel cells. While clearly speculative at this stage, a possible performance evolution trend curve is shown in Fig. 4. Of all the components, there will likely be advancements in recuperator technology that will have the strongest near-term impact on microturbine performance improvement. To realize a thermal eciency approaching 50%, there will certainly be a quest for higher e€ectiveness and lower pressure loss, and the impact that these have on recuperator size (and hence cost) is addressed in a later section. An important aspect of the aforementioned engine performance growth is that the recuperator must be capable of accommodating higher temperatures. As it will be discussed later, the fabrication of the proposed recuperator concept is readily adaptable to a bi-metallic approach, to yield a cost-e€ective heat exchanger as operating temperatures increase. Eventually, of course, a ceramic recuperator will be needed, but since this technology has been discussed previously by the author [13], the topic is viewed as being beyond the scope of this paper, which is really aimed at the near-term deployment of a low-cost metallic recuperator based on proven technology, for the ®rst generation of microturbines. 2.5. Microturbines for hybrid electric vehicles While the focus of this paper is on the utilization of a low-cost metallic recuperator for power generation microturbines, it is germane to brie¯y mention another engine application that is being pursued. For hybrid electric vehicles, the very small gas turbine has the attributes of multifuel capability, low emissions, and a compact and light weight package. However, in the past two years or so, it seems to have lost ground as a premium prime-mover in the emerging hybrid vehicle market place. In the near-term, small high eciency direct injected turbocharged and intercooled Diesel and gasoline engines are viewed as the most attractive engine option. For the longer term, fuel cells are currently receiving intense focus and, in particular, e€orts to reduce their cost. For the microturbine to be competitive, with eciency values of over 40%, it would have to utilize ceramic components in the complete hot end section of the engine. This would include the combustor, turbine, scrolls and heat exchanger [14,15]. It is recognized that ceramic technology is still in its infancy and today could not support an advanced 45±50% eciency microturbine with the level of reliability needed for automotive service. 3. Metallic recuperator state-of-the-art technology It is not the purpose of this paper to discuss in detail recuperator technology, since the author has addressed this topic in two previous publications [16,17]. There are basically three

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types of recuperators being used today, namely, (1) welded primary surface units, (2) furnace brazed plate-®n exchangers, and (3) tubular geometries. Todays' recuperators have overcome many of the problems encountered earlier, and have demonstrated the performance and structural integrity needed for microturbine service. However, there is one impediment that remains to be overcome, and that is their high unit cost. The recuperators of the aforementioned type so far produced in only small quantities, are characterized by having many subcomponents that require manual assembly. For example, a representative recuperator, currently being used in an operating microturbine, has over 2000 individual parts that require sophisticated metal forming, intricate ®tting, welding and subelement testing, before the ®nal matrix is assembled. It is encouraging that todays' recuperators have demonstrated structural integrity, namely that of remaining leak tight in the high temperature cyclic gas turbine environment. Utilizing high performance surface geometries, they also have envelopes that are compatible with the compact nature of the turbomachinery to give attractive overall packages. The major technology barrier is that of establishing a surface geometry and a form of construction that is compatible with high volume automated production methods, so giving a low-cost recuperator.

4. Recuperator requirements for microturbines Since establishing a recuperator of low cost is paramount for microturbine service, the requirements must be strongly focused on this aspect. In establishing a new recuperator concept, there are two formidable technology bases to draw from, namely, (1) existing recuperator technology know-how which has been generated over several decades and (2) manufacturing expertise from the heat exchanger industry, such as in the manufacture of automobile radiators, where several thousands of units are fabricated each day using a high degree of automation. Ideally, what is foreseen is a manufacturing facility in which a spool of thin metal foil enters one end, and a completed recuperator matrix leaves the other end, without the need for manual labor. This compact matrix could be regarded as a cartridge which could be readily installed into a simple casing, which may well be an integral part of the microturbine backbone or structure. To achieve low cost there are certain basic requirements that need to be adhered too, and indeed these have a strong impact on the heat exchanger form. One basic consideration is that in the metal forming process there should be no material wastage, that is to say absolutely zero scrap metal. Also, every square millimetre of metallic foil should be one hundred percent heat transfer e€ective, this implying that a primary surface geometry be selected. While very compact plate®n heat exchangers can be fabricated, they su€er from ineciency in the secondary surface ®ns. Because of the relatively low thermal conductivity of candidate high temperature materials (e.g., stainless steels), the secondary surface ®n eciency may only be on the order of 75±85% [18]. Simply stated, when seeking a minimum cost recuperator it does not seem to be prudent to include an element of parasitic expensive thin-foil metal that merely adds to the unit weight and cost without contributing fully to heat exchanger and engine performance.

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The furnace brazing of high temperature heat exchangers (in either a vacuum or an inert atmosphere) requires high capital cost equipment, demanding process control, expensive braze alloys, and is time consuming. Furnace brazing, therefore, militates against the aim of producing a minimum cost heat exchanger, and de®nitely should be avoided. In the quest for Table 1 Microturbine recuperator requirements Major design criteria Performance Surface geometry Fabrication

Type of construction Cost

Integrity Installation

Maintenance

Performance growth capability

Near-term goal Long-term goal

Low heat exchanger cost Meet demanding microturbine performance and economic goals High recuperator reliability High recuperator e€ectiveness …> 90%) Low pressure loss …< 5%) Good part load performance Prime surface geometry (no secondary surface ineciency) High surface compactness Superior thermal-hydraulic characteristics Minimum number of matrix parts Continuous/automated fabrication process Welded sealing (eliminate need for furnace brazing) Adaptable to high volume production methods Utilize heat exchanger industry experience (e.g., automobile radiators) Compact and light weight matrix Integral manifolds/headers Matrix envelope ¯exibility (annular or platular) No basic material wastage (zero scrap) Minimum (or zero) labor e€ort Standardization Materials selection for particular duty Unit cost goal not to exceed 1.5 times material cost Resistant to thermal cycling Remain leak tight for engine life Life goal of 50,000 h for microturbine generator sets Gas ¯ow path compatibility with turbomachinery Compact and light weight overall assembly Eliminate inter-connecting ducts Eliminate need for thermal expansion devices Ease of recuperator removal/replacement Plug-in matrix cartridge (analogous to oil ®lter element) Ease of leak detection testing Ease of weld repair Adaptable to future higher temperature microturbine variants Materials selection ¯exibility Adaptable to bi-metallic construction Retro®t capability with advanced heat exchanger concepts High volume production of cost-e€ective metallic recuperator with demonstrated performance and structural integrity for emerging family of microturbines Development of a ceramic recuperator to facilitate the full performance potential of microturbines to be realized (i.e.. 45±50% eciency)

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minimum cost, the necessary heat exchanger sealing operations should be done by an automated welding process. Furthermore, this is consistent with the above recommendation to select a primary surface type of construction. To minimize the number of parts in the heat exchanger means the elimination of the multielement stacked type of construction inherent in most existing types of primary surface and plate-®n recuperators. What is needed is a geometry and construction type that is compatible with an automated labor-free, fabrication process. The above would seem to suggest a continuous manufacturing process involving stamping, folding or spirally forming of the thin foil basic stock, followed by an automated weld sealing operation, and this will become more apparent in the following sections. There are, of course, many requirements for a microturbine recuperator, and the salient ones are summarized in Table 1. 5. Novel low-cost compact primary surface recuperator 5.1. Background Now, nearing the end of the 20th century, the proposed microturbine recuperator has as its genesis a novel heat exchanger concept pioneered by Dr. Manfred Kleeman and Dr. Siegfried Foerster as part of an energy related research project undertaken in Germany in the late 1970s [7,19,20]. The primary aim of their development work was the construction of a compact metallic recuperator module, which had the potential for fabrication using a continuous automated process. Over the years, many novel heat exchanger concepts have been invented and patented, but only a few have actually materialized as viable products. To their credit, the aforementioned inventors went beyond the analytical and design stage and fabricated a prototype heat exchanger module, and demonstrated its leak tight integrity. Further, rig testing

Fig. 5. Principle of stamped and folded matrix from thin-foil spool for prototype module (courtesy M. Kleeman and S. Foerster).

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was undertaken to establish thermal-hydraulic characteristics. Details of the program to fabricate this prototype rectangular recuperator module are given below. 5.2. Prototype primary surface recuperator module The primary surface heat exchanger concept is intrinsically simple, and fabrication basically starts with only a single part, namely a spool of thin foil metal. The major fabrication processes involve stamping and folding of this thin foil stock. The matrix is formed by the to and fro folding of a herringbone corrugated metal sheet, and is made leak tight by external welding. Starting with the spool of thin metallic foil, the ®rst operation involves the stepwise stamping of the heat transfer geometry. The continuous metal stamping and folding process for the prototype module is shown simply in Fig. 5. In production, the stamping and folding process would be automated. A photograph of the heat transfer surface geometry for the prototype matrix is shown in Fig. 6. During the folding process, two end or side strips (at each end of the unit) are introduced as shown in Fig. 7. The thickness of these strips is such that when the folded assembly is compacted, the ends of the matrix become a solid wall of metal. A simplistic view of the formed matrix is shown in Fig. 8. A unique ¯ow path results from the folded type of construction, since the ends of the matrix are sealed by welding. As shown in Fig. 8, the air and gas streams enter and leave from opposite sides of the core, giving a counter¯ow arrangement in the main part of the heat exchanger. These simple gas ¯ow paths

Fig. 6. Stamped and folded section of prototype matrix (courtesy M. Kleeman and S. Foerster).

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Fig. 7. View of matrix end closure with folded in end strips (courtesy M. Kleeman and S. Foerster).

can be readily accommodated in a straight forward manner in a microturbine as shown simply in Fig. 1. Returning to the fabrication of the matrix, the next step involves welding the closed solid formed metallic wall at each end of the unit to give a gas tight closure. In this operation, there is no danger of burning through the thin foil material since a solid metallic face has been formed by utilizing the aforementioned end strips. Unlike some other primary surface

Fig. 8. Flow con®guration in primary surface recuperator module (courtesy M. Kleeman and S. Foerster).

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recuperators, there are no additional formed elements inserted into the end headering areas. As can be seen in Figs. 5±7, the corrugated form extends for the full length of the unit, except for the two narrow uniformed regions at the ends of the matrix, these being necessary to accommodate the end strips. The crossing corrugations of the two adjacent foils over the full length of the matrix result in many contact points which serve as support structure and obviate surface nesting during engine operation, where there is an internal pressure di€erential of several atmospheres between the air and gas passages in the heat exchanger. A possible alternative end section con®guration will be addressed in a later section. The end closures of the prototype matrix were electron beam welded, as shown in Fig. 9. The completed basic matrix assembly (without the cover plates) is shown in Fig. 10, with a rectangular form being made by welding the end plates to the last fold of the convoluted assembly. To give the required counter¯ow con®guration, two external cover plates are required (as shown in Fig. 8). These are not mechanically attached to the matrix, and could be a part of the heat exchanger housing or engine structure, and this will be discussed in a later section. The formed herringbone corrugation in the counter¯ow section has a sine curve form, and a unique ¯ow geometry as shown in Fig. 11. Rig testing of the prototype module revealed turbulent ¯ow down to very low Reynolds numbers, and good thermal-hydraulic characteristics were obtained. While there are clearly many variables to be considered in the selection of the surface geometry, a representative set are identi®ed for the prototype module, and the major ones are identi®ed in Table 2. 5.3. Status of primary surface recuperator concept The aforementioned research program undertaken at FZJ in Germany in the late 1970s was successful in that it achieved the following: (1) identi®cation of a novel primary surface recuperator concept with low-cost potential, (2) identi®cation of a representative surface geometry, (3) demonstrated a fabrication process that could be automated, (4) fabrication of a compact prototype matrix, (5) demonstrated that welding of the matrix end faces gave a leak tight core, and (6) showed ¯exibility of the approach for several gas turbine applications.

Fig. 9. Welded end face of prototype module (courtesy M. Kleeman and S. Foerster).

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Fig. 10. Prototype rectangular matrix assembly before cover plate attachment (courtesy M. Kleeman and S. Foerster).

Alas, in the late 1970s no near-term applications materialized, and this novel primary surface heat exchanger concept has remained dormant and essentially unknown for two decades. During this period, the two inventors have become involved in other endeavours. For many years, heat exchanger specialists have been studying and analyzing di€erent recuperator concepts that could be manufactured for a lower cost than contemporary units, and these e€orts are now being focused on the microturbine market. Recuperator know-how generated from this application will also be valuable in the quest for lower cost heat exchangers that will be needed for the new class of high eciency recuperated industrial gas turbines in the 3±15 MW power range. In evaluating di€erent high temperature heat exchanger concepts, the author came to the conclusion that the aforementioned primary surface type of construction came closest to meet the demanding recuperator requirements (given in Table 1) for the emerging class of microturbines. However, it seems fair to project that innovativeness on the part of manufacturing engineers will lead to variants of the proposed concept, that meet these

Fig. 11. Herringbone corrugation details for compact primary surface gas turbine recuperator (courtesy M. Kleeman and S. Foerster).

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Table 2 Salient features of low-cost recuperator concept Feature

Demonstrated prototype heat exchanger element (Ref. [7])

Microturbine recuperator application

Surface geometry Matrix construction Exchanger assembly Material Flow con®guration Matrix envelope Material thickness (mm) Corrugation wave height h (mm) Hydraulic diameter d (mm) Matrix length a (mm) Matrix height c (mm) Matrix depth b (mm) Angle of wave pattern (deg) Total surface compactness (m2/m3) Matrix thermal density (MW/m3) Speci®c weight (kg/kg/s) Speci®c volume (kg/s/m3) Headering end section geometry Matrix end section closure Closure integrity Number of matrix parts Preferred geometry for microturbine Assembly/casing Ease of inspection/repair Development required

Primary surface recuperator Stamped and folded metal foil Manual Chrome Nickel Steel Counter¯ow Rectangular element 0.20 1.0 1.24 210 400 18 120 2400 6.3 60 24 Integrally formed Electron beam weld Leak tight demonstrated Five ± Two ¯at plates Very accessible Fabrication demonstrated

Heat exchanger technology status Adaptable to bi-metallic fabrication

Demonstrated ±

Primary surface recuperator Stamped and folded metal foil Continuous automated process St. St. or Nickel base alloy Counter¯ow (see Fig. 1) Annular or rectangulara 0.10±0.50a 0.6±1.50a 0.75±1.85a Up to 1500a Unlimiteda Up to 200a 120 3600±2400 Up to 10 50±60a 25±30a Integrally formed Laser or electron beam weld Zero leakage required 5 (foil spool + 4 end strips) Annular matrix Inner and outer cylindrical shells Welded faces accessible Tooling design and procurement for annular matrix State-of-the-art Yes

a Selection of optimum parameters and matrix geometry/envelope dependent on microturbine requirements/application.

requirements, and will yield low recuperator costs when fabricated using continuous and automated production facilities. 6. Concept applicability for microturbine recuperator 6.1. Major features for low-cost microturbine recuperator While there are many factors that must be considered in the design of a new recuperator concept, the following are viewed as being the most important: (1) primary surface geometry,

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(2) minimum number of parts, (3) no basic material wastage, (4) welded construction, (5) adaptable to a continuous automated fabrication process, (6) compact and light weight assembly, (7) matrix fabricable in an annular form, and (8) matrix to be fabricated as a simple cartridge for ease of installation, and removal and replacement. Perhaps of more importance than these, is that the selected concept should have a solid technology base and to have had its viability demonstrated in a prototype form. What is paramount, and overrides all other considerations, is that the recuperator must have a low unit cost. A proposed microturbine recuperator, that has as its genesis, the major features of the aforementioned proven prototype matrix fabricated and tested in Germany two decades ago, is discussed below. 6.2. Recuperator matrix con®guration While a rectangular recuperator module could be readily fabricated for installation behind the rotating machinery, focus here is on producing an annular matrix, which in a wrap-around form as shown in Fig. 1, results in an attractive and compact overall microturbine package. For such a con®guration, the di€erences in the matrix construction from the fabricated prototype module are highlighted in Table 2, and the salient ones are discussed below. The sequence of operations to fabricate the compact annular core is shown in a simpli®ed form in Fig. 12. A continuous automated matrix forming process is foreseen, starting with a spool of thin foil metal. The initial operation (number 1) involves the introduction of the two narrow end strips from spools at each end of the matrix. This is followed by the continuous seam welding of these strips to the thin foil stock (operation 2). The stamping operation (number 3) involves generating the corrugated form in the thin foil material. Unlike the rectangular prototype module, where a ¯at surface was formed (Figs. 5±7), the stamping operation for an annular core as shown in Fig. 12 involves generating an involute geometry. In the prototype module, the corrugated form extended for the full length of the matrix, with the crossing nature of the adjacent foils providing integrally formed headering areas. This provides a core of uniform sti€ness when compressed prior to the welding of the end faces. It is proposed that this concept can also be adopted for the annular matrix. An alternate approach, perhaps yielding a lower pressure loss, could involve forming the corrugations in the counter¯ow section only, with stand-o€ dimples formed in the end sections. The stamping operation, however, would be more complex, and additional metal-forming development would be necessary to determine feasibility. The proven and recommended approach, together with a possible alternative to the headering areas are highlighted as shown in the View A sketch, in Fig. 12. Instead of folding the stamped foil in a ¯at manner as in the rectangular prototype module, an involute form would be generated for the annular core, this being the fourth operation shown in Fig. 12. When the appropriate number of folds have been generated, the involute geometry would be compressed to form an annular matrix (operation 5). As in the case of the rectangular prototype module, the end faces of the formed cylinder would be a solid wall of metal. To complete the basic matrix, which only consists of ®ve parts (i.e., the metal foil and the four end strips), the two end faces must be welded to give a leak tight core (operation 6). Either electron beam or laser welding could be considered. Referring to Fig. 12, fabrication

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Fig. 12. Sequence of continuous and automated fabrication operations for compact primary surface annular recuperator matrix cartridge.

operations 1 to 6 would be done in a continuous process using automated machinery. With the use of appropriate ®ttings, the end sections of the completed matrix would be pressure tested to verify core leak tightness. The fabrication of an annular matrix would be done with a minimum of manual labor, and a high production rate should be realizable. 6.3. Recuperator/engine integration/packaging As mentioned earlier, the folded matrix has a unique gas ¯ow path as shown in Fig. 8, but in an annular wrap-around form, this can be readily accommodated in a state-of-the-art microturbine as shown in Fig. 1. The recuperator matrix can be considered to be an insertable and removable cartridge, and this is shown in the simpli®ed cross-section view of the recuperator given in Fig. 13. The idea of an installed cartridge is attractive, but because of the pressure di€erential between the two ¯uid streams, one cannot simply insert the matrix in the casings without some connections. A sealing means is necessary, and while this could likely be accomplished with circumferential

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Fig. 13. Simpli®ed section through recuperator showing annular matrix cartidge installed in a representative microturbine engine casing.

seal rings, this approach does not seem to be prudent in the hot gas environment. There are also other factors that must be considered in the integration of the recuperator with the rotating machinery, namely: (1) accommodation of the temperature gradients, and resultant thermal growth of the matrix, and (2) avoidance of bypass ¯ow between the matrix cartridge and the inner and outer casings. To assure absolute leak tightness, a welding operation is required. While shown in only a simplistic form in Fig. 13, welds X and Y located between the ¯exible membranes and the casings would have to be broken before the cartridge could be removed and replaced. Design details in these areas remain to be determined together with the identi®cation of the best

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solution for each particular engine con®guration. These would have to be veri®ed during the development phase. The cartridge approach gives the engine manufacture ¯exibility, particularly regarding performance upgrade over the lifetime of the microturbine. If for example, advancements are made in the compressor, turbine and ®ring temperature then an upgraded recuperator cartridge with higher e€ectiveness could be readily installed, it being optimized within the same envelope, to accommodate the microturbine's increased power output and higher eciency from the same frame size. Also, if unforeseen problems arise, the recuperator cartridge could be readily removed, inspected, repaired if necessary, or replaced with a new unit. 6.4. Recuperator speci®c size and cost The major theme of this paper has been to propose a simple and practical recuperator concept that has the potential for low-cost fabrication. The actual sizing of a recuperator is beyond the scope of this paper, and would require the detailed speci®cations of a particular microturbine, however, a generalized discussion on some of the major features impacting size is germane. The size of a recuperator is impacted by several parameters, and based on an extension of earlier work [21] the recuperator matrix volume can be considered approximately proportional to the following group of parameters, that include an engine power related function, heat exchanger parameters, and surface geometry characteristics: 0s 1 ! p e 1 f1 p  @ 3 A VA W= R  1 ÿ e j b DP=P …Power parameter† …Recuperator parameter†

…Surface geometry†

At this point, it is germane to brie¯y discuss the impact that these parameters, particularly the thermal e€ectiveness, have on heat exchanger size, and hence cost. From Fig. 2, it can be seen that to achieve an engine thermal eciency approaching 50%, a recuperator e€ectiveness of 0.95 is required. Such a high value for a ®xed boundary recuperator in an open cycle gas turbine is essentially unheard of, and is more associated with a rotary regenerator. If the heat exchanger mass ¯ow rates were kept constant, the impact of increasing the e€ectiveness from say 0.87±0.95 would result in an increase in recuperator size by a factor of about 2.8. However, in moving up along the technology trend line (in Fig. 2) from Regime 1 to 5, this is not the case. In advancing along the technology trend line from Regime 1 to 5, the speci®c power increases signi®cantly as shown in Fig. 4. Simply stated, if for advanced variants of the microturbine the power output was kept constant, the air¯ow passing through the engine would be reduced, this resulting in a smaller turbogenerator package. While the recuperator e€ectiveness is increased (to yield higher engine thermal eciency), the size of the recuperator is also a€ected by the reduction in air¯ow (the term W in the above equation), and this is shown in the upper part of Fig. 4). Considering Regime 1 as the base case, the relative size of the recuperator actually has a minimum value as the speci®c power increases. Such a curve

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shape was not envisaged by the author at the onset of the analysis, and is, of course, uniquely related to the pro®le of the technology trend line shown in Fig. 2. The relative recuperator size curve is clearly in¯uenced by heat exchanger evolutionary changes (e.g., the pressure drop, compactness and surface geometry characteristics in the above simple equation), and this is illustrated by curves A and B. From the above, one can conclude that a high value of recuperator e€ectiveness is not out of order when considering microturbine performance evolution. The selection of the actual surface geometry would be identi®ed from detailed thermal analyses and sensitivity studies to establish an optimum recuperator design for a particular engine speci®cation. To get an appreciation for the speci®c size of a `candidate' recuperator one could consider a state-of-the-art microturbine rated at say 50 kW, with a thermal eciency of 30%, and a speci®c power of 100 kW/kg/s. Utilizing a geometry with a representative surface compactness, a heat exchanger thermal density of about 8 MW/m3 (i.e., 125 kW/cm3) could be realized for an 85% e€ectiveness recuperator. The corresponding speci®c size would be about 25 kg/s/m3, which is reasonably consistent with published data for primary surface recuperators [22]. The recuperator matrix speci®c weight would be of the order of 60 kg/kg/s, this giving a core weight of about 30 kg. With a thin foil stainless steel cost of say $12/kg, the basic material cost in the core would be $360. If the aforementioned goal of a recuperator cartridge unit cost of about 1.5 times the material cost (when produced in very large quantities) is to be achieved, then the basic recuperator matrix core cartridge cost (excluding casings, ¯anges, manifolds and instrumentation) would be about $500. The basic recuperator matrix cartridge cost estimate of $500, for a 50 kW microturbine produced in very large quantities, is clearly tentative, and may well seem to some in the industry to be an overly ambitious value, and indeed it may be. However, it is recognized of course, that for recuperators produced in small quantities, the basic matrix core cost could be perhaps ®ve to ten times the aforementioned value of $10/kW (or $1000/kg/s in this case), much depending on the heat exchanger type, the manufacturer, and the amount of manual labor content required in their fabrication. In light of the paucity of recuperator cost data from industry, and available to researchers in the open literature, the above value could perhaps be viewed as an aggressive cost target or goal for heat exchanger specialists and manufacturing engineers to strive for in the next two years or so, as several microturbine variants enter service. 6.5. Recuperator development required 6.5.1. Near-term activities A detailed thermal-hydraulic analysis is required to identify the optimum geometry (e.g., material thickness, passage height, corrugation pitch, wave angle etc.) for a particular application. In the case of the stamping operation, an advancement over the prototype module will be required, particularly the forming of a core of increased depth (noting that dimension b is the core radial depth in the case of an annular matrix). The stamping of the foil and end strips in an involute form, and the subsequent folding operation, must also be demonstrated. The ®nal task will involve the simple procedure of compacting the multi-folded material to give the core the required annular geometry, and then performing the welding closure operation on

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both ends of the unit. This would be followed by a pressure/leak detection test to con®rm the seal integrity of the two end faces. Rig testing of the ®rst annular recuperator would include performance veri®cation of the e€ectiveness and pressure loss, and extensive thermal cycling to con®rm structural integrity. This would be followed by engine testing over the full operating spectrum. Bearing in mind that a prototype module has already been fabricated and tested, the next evolutionary step of producing a compact recuperator in an annular form is not viewed as an overly expensive and time-consuming endeavour, but rather just a matter of resolve. A challenging task for manufacturing engineers will be the design of the tooling and machinery for the automated fabrication of production units, this include the stamping, folding, compacting and welding operations. To support the manufacture of say 20,000 microturbines a year, it would be necessary to produce about 100 recuperators per day (say 12±15 per hour). To put this into perspective, the only factory that has produced recuperators in signi®cant quantities (i.e., over 15,000), was the former Lycoming facility in the United States, that had the capability to fabricate up to 300 primary surface recuperators per month, in support of the AGT 1500 tank engine. Whether each microturbine manufacturer has the goal of fabricating their own recuperator remains to be determined. If a centralized and specialist heat exchanger manufacturing corporation was to provide the gas turbine industry with low-cost primary surface recuperators, this would present a greater challenge to manufacturing engineers, and would involve several production lines fabricating di€erent customized recuperator design concepts.

6.5.2. Long-term activities With a production line in operation, it is expected that manufacturing engineers would continue to perfect the fabrication process, with the obvious goal of striving to further reduce the recuperator unit cost. With continuing modest advancements being made in compressor, turbine, generator, combustor and emission reduction areas, there will be incentives to increase the engine power output and eciency from a given engine frame size. A particular advancement that will impact the recuperator is an increase in turbine inlet (and hence outlet) temperature. For the ®rst generation of low pressure ratio microturbines, the recuperator will likely be fabricated from a stainless steel. However, with an increase in turbine inlet temperature, a point will be reached where the recuperator hot gas inlet temperature reaches about 7008C, and at values much above this, it is unlikely that the required heat exchanger lifetime (of say 50,000 h) could be realized with a stainless steel unit. It is true that a longer life recuperator could be readily fabricated from a superalloy to take the higher temperatures as shown in Fig. 14 (from Ref. [13]). However, the material cost alone would be several times more expensive than the stainless steel unit it was replacing. At this point, turbine outlet temperature (i.e., recuperator hot gas inlet temperature) is well worth mentioning, since it is an important parameter in the control system of single shaft microturbines. An attractive method of microturbine power regulation with frequency control, to yield high thermal eciency at part load, is to vary the rotational speed and keep the

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Fig. 14. Array illustrating the impact of major cycle parameters on microturbine performance.

turbine exit temperature constant. This method of control assures that the recuperator will not over temperature at engine part load conditions. Returning to the above materials consideration issue, it is true to say that for commercial service, utilizing superalloys with their higher temperature capability, in the recuperator would not be economically viable, but fortunately there is another approach which is more appealing, and this is discussed below. Since in a counter¯ow heat exchanger, the temperature gradient is in the axial direction, e€orts should be expended to establish a bi-metallic core, where the necessary high strength (and high cost) alloy is utilized only at the hot end of the recuperator. Naturally, the two selected materials must have compatible coecients of thermal expansion to avoid inducing thermal stresses in the core. The proposed bi-metallic approach could be readily implemented in the proposed primary surface recuperator. Instead of one spool of thin foil stock, as shown in Fig. 5, two spools of di€ering alloys could be used, with a continuous butt welding operation performed prior to the stamping process. In performing this operation, it might be wise to arrange that the butt weld have an irregular `jiggle' form to avoid a line of potential continuous weakness in the matrix. This approach could be considered analogous to the fabrication of hacksaw blades, where for economic reasons, only a thin strip of the necessary high strength metal for the teeth

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section is used. The bulk of the blade being made from a lower cost material, with the necessary bonding of the two metallic strips being done by a continuous welding operation. This paper has focused on the need for a low-cost metallic recuperator for ®rst generation state-of-the-art microturbines. It is, however, recognized that the full performance potential of microturbines can only be realized by substantially increasing the turbine inlet temperature, and this would necessitate the eventual use of ceramics in all of the hot-end components, including the recuperator [13]. Development e€orts are underway in this area, but with compact ceramic heat exchanger technology still in its infancy, this is viewed as a long-term endeavor, but one obviously well worth pursuing.

7. Conclusions In the current pre-production manufacturing phase of microturbines for power generation use, existing types of primary and plate-®n recuperators are meeting the engine manufacturers' needs in terms of performance and structural integrity, but unit heat exchanger cost is high. This is surely a re¯ection of two factors: (1) production in only small quantities, and (2) adherence to existing types of heat exchanger construction, which were not conceived for automated high volume manufacturing processes in the ®rst place. If the projections are correct for the introduction of microturbines in very large quantities starting around the year 2000, then there will be a strong demand for a cost-e€ective recuperator. The compact primary surface recuperator concept proposed in this paper is responsive to this need. It is of all-welded construction, thus avoiding the costly and time consuming high temperature furnace brazing operation, with its need for close process control, that is used in the manufacture of many heat exchangers for elevated temperature service. The annular recuperator proposed has only ®ve basic parts, and the matrix is formed by continuous stamping, folding and welding operations. The concept is very adaptable to an automated, high volume manufacturing process. Unlike many `paper concepts', perhaps the strongest point in the favor of the proposed primary surface recuperator is that it is based on proven technology. It has as its genesis a prototype matrix module fabricated and tested in Germany two decades ago, but never commercialized because of lack of a market. The market for this product could very well be the microturbine, and only a minor evolution of the fabricated prototype module is necessary to meet the needs of today's very small gas turbines. An advantage of the proposed wrap-around annular recuperator approach is that the heat exchanger matrix can be essentially regarded as a cartridge, and this facilitates ease of installation, removal, and later replacement with an upgraded performance unit if appropriate. In the very few technical papers published by recuperator manufacturers, the topic of heat exchanger cost is never mentioned, clearly because of the proprietary nature of their business. This paucity of cost information, is understandable of course, but it leaves an obvious void to those trying to get a full appreciation of heat exchanger technology. After all, we must remember that cost is the bottom line when it comes to recuperators for the microturbine market. This topic has been addressed in this paper, but more in the form of a challenge to

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industry, rather than giving de®nitive cost data, which can realistically come only from the manufacturing sector. Primary surface recuperators have fascinated designers since the gas turbine was ®rst introduced for power generation. Perhaps the best known one being the Nicholson concept [23], pioneered in the United Kingdom over four decades ago. It is of interest to note that variants of this basic concept are still being produced in small quantities today. The proposed primary surface recuperator, particularly in an annular form, is viewed as being amenable to automated high volume manufacturing processes, with minimal manual labor necessary. If produced in very large quantities, a recuperator unit cost goal of about 1.5 times the material cost has been put forward. As discussed in the text, this means that for a ®rst generation state-of-the-art microturbine in the 50 kW class, the basic recuperator matrix cartridge cost should not exceed about $500. It will be up to the heat exchanger manufacturing industry to see if this challenging goal of about $10/kW (or $1000/kg/s) for a new recuperator matrix can be realized in the next two years or so. The performance of microturbines will continue to improve, and a point will be reached when a stainless steel recuperator will no longer meet the life requirements in the resulting higher operating temperature environment. A cost-e€ective way to counter this will be to use a bi-metallic approach. The primary surface recuperator concept proposed in this paper is ideally suited to such a technology advancement. The microturbine is the ®rst class of very small gas turbines where a recuperator is mandatory to achieve an engine thermal eciency of 30% or higher, this being necessary for it to compete with contemporary Diesel generator sets. The type of compact primary surface recuperator outlined in this paper was pioneered over two decades ago, and since then signi®cant technology advancements have been made in many germane areas, including metal forming, welding and automated fabrication. The next step towards the deployment of a low-cost recuperator, is to a large extent, dependent on the innovativeness of manufacturing engineers. The annular concept discussed in this paper, could perhaps be viewed as the starting point of a needed e€ort leading to the identi®cation of a heat exchanger that will overcome the remaining recuperator impediment, namely that of high cost. With specialist attention, variants of the concept discussed in this paper, involving a continuous and automated stamping, folding and welding fabrication process, will surely emerge in the next two years or so. In addition, it should be mentioned that spirally wrapped, primary surface concepts also have the potential for low cost [24]. The successful operating experience of recuperated microturbines, particularly the demonstration of their projected high reliability, will give potential users of future larger industrial gas turbines additional con®dence that recuperated variants will meet demanding customer requirements, particularly trouble free, high availability operation. It would be remiss, however, not to mention in this paper that many users remain to be convinced about the projected widespread use of recuperated gas turbines, because of adverse experiences had in the past. The author, after four decades of involvement in the development of high temperature heat exchangers, is now optimistic about their widespread deployment, and early in the next century, recuperated engines could ®nally well be in the mainstream of gas turbine applications [25].

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Acknowledgements The proposed low cost, compact primary surface recuperator for microturbine use is an evolution of a heat exchanger approach invented, pioneered, and following the fabrication of a prototype module, experimentally veri®ed in Germany in the late 1970s by Dr. Manfred Kleeman and Dr. Siegfried Foerster. The author has known these two engineers for almost 25 years and is indebted to their innovativeness, which was perhaps two decades ahead of its time in terms of product commercialization. The author would also like to thank Colin Rodgers (ITC), a friend of many years, for several discussions, and his guidance on the performance of very small gas turbines.

References [1] C.F. McDonald, The increasing role of heat exchangers in gas turbine plants, ASME Paper 89-GT-103, 1989. [2] C.F. McDonald, Heat exchanger ubiquity in advanced gas turbine cycles, in: Proceedings of ASME COGENTURBO Congress, IGTI Vol. 9, 1994, pp. 681±703. [3] C.F. McDonald, D.G. Wilson, The utilisation of recuperated and regenerated engine cycles for high eciency gas turbines in the 21st century, Journal of Applied Thermal Energy 16 (8/9) (1996) 635±653. [4] C.F. McDonald, Heat recovery exchanger technology for very small gas turbines, Int. Journal of Turbo and Jet Engines 13 (4) (1996) 239±261. [5] F.F. Fischinger, The Diesel engine for carsÐ is there a future? ASME Journal of Gas Turbines and Power 120 (3) (1998) 641±647. [6] S. Birch, V5 engine for slippery passat, Automotive Engineering (1996) 70. [7] M. Kleeman, Neuartiger kompakter rekuperator in plattenbauweise, Brennst.-Warme-Kraft 31 (6) (1979) 256± 261. [8] P. Craig, The Capstone turbogenerator as an alternative power source, SAE Paper 970202, 1997. [9] J. Carne, A. Cavini, L. Linaaki, Micro gas turbine for combined heat and power, ASME Paper 98-GT-309, 1998. [10] C. Rodgers, Small turbogenerator (10±200 kW) design considerations, ASME IGTI 8 (1993) 525±542. [11] P. O'Brien, Development of a 50 kW low emissions turbogenerator for hybrid electric vehicles, ASME Paper 98-GT-400, 1998. [12] C. Rodgers, Turbochargers to small gas turbines, ASME Paper 97- GT-200, 1997. [13] C.F. McDonald, Ceramic heat exchangersÐ the key to high eciency in very small gas turbines, ASME Paper 97-GT-463, 1997. [14] C. Rodgers, C.F. McDonald, Automotive turbogenerator design considerations and technology evolution, SAE Transactions, Journal of Engines, Section 3 106 (1997) 2054±2070. [15] C. Rodgers, C.F. McDonald, Automotive turbogenerator design options, in: Paper C529/024/98, I. Mech. E., Proceedings on Combustion Engines and Hybrid Vehicles, UK, April 28±30, 1998, pp. 301±319. [16] C.F. McDonald, Gas turbine recuperator renaissance, Journal of Heat Recovery 19 (1) (1990) 1±30. [17] C.F. McDonald, Gas turbine recuperator technology advancements, in: Proceedings of the Inst. of Metals Conference on Materials Issues in Heat Exchangers and Boilers, UK, October 17±18, 1995, pp. 239±261. [18] C.F. McDonald, Recuperator development trends for future high temperature gas turbines, ASME Paper 75GT-50, 1975. [19] M. Kleeman, Auslegung eines neuartigen kompakten rekuperators, Dr. Ing. Dissertation, T.H. Aachen, Germany, 1978. [20] S. Foerster, M. Kleeman, Compact metallic and ceramic recuperators for gas turbines, ASME Paper 78-GT-62, 1978. [21] C.F. McDonald, Gas turbine recuperator technology advancements, ASME Paper 72-GT-32, 1972.

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[22] P.P. Walsh, P. Fletcher, in: Gas Turbine Performance, Blackwell Science Publications, Oxford, 1998, p. 288. [23] W. Hryniszak, Heat Exchangers-Applications to Gas Turbines, Butterworths, London, 1958. [24] C.F. McDonald, Spirally wrapped, low cost, primary surface recuperator cartridge concept for microturbines, ASME Paper 2000, to be published. [25] C.F. McDonald, Emergence of recuperated gas turbines for power generation, ASME Paper, 1999, to be published.