DEVELOPMENT OF COMPACT HEAT EXCHANGER WITH DIFFUSION WELDING

Transcription

1 DEVELOPMENT OF COMPACT HEAT EXCHANGER WITH DIFFUSION WELDING K. KUNITOMI, T. TAKEDA Q Department of HTTR Project, Japan Atomic Research Institute, Ibaraki T. HORIE, K. IWATA Heat Exchangers Division, Sumitomo Precision Products Co., Ltd., Hyogo Japan Abstract A plate fin type compact heat exchanger (PFCHX) normally uses brazing for connecting plates and fins. However, the reliability of brazing is insufficient when PFCHXs are used for a long duration as primary or secondary components in nuclear plants. Particularly, PFCHXs used as a recuperator of gas-turbine plant with the High Temperature Gas-cooled Reactor (HTGR) or Intermediate Heat Exchanger (IHX) in future generation HTGRs need high reliability in a high temperature region. We have been developing the PFCHX with diffusion welding between plates and fins. The tensile and creep strength in the diffusion welds are superior to those in the brazing especially in high temperature condition. The developing PFCHX consisting of Ni based HastelloyXR plates is expected to be used over 900 C. Prior to Vie development of a full scale PFCHX, the small PFCHXs with the diffusion welding were designed, manufactured and installed in a test loop to investigate the welding strength and reliability. The eariy tests showed that reliability of the diffusion welding is very high, and the PFCHX with the diffusion has a possibility for the IHX or recuperator. Thermal performance tests were also carried out to obtain effective thermal conductance. This paper describes the design and test results of the small compact heat exchanger with the diffusion welding. 1. Introduction The High Temperature engineering Test Reactor (HTTR) [1] being constructed in Japan Atomic Energy Research Institute (JAERI) is a graphite-moderated and heliumcooled reactor with an outlet gas temperature of 950 C. The HTTR will be used to establish and upgrade the technological basis for advanced HTGRs and to conduct various irradiation tests for innovative high temperature basic researches. It can be also used for demonstration of various nuclear process heat applications that are strongly required from various standpoints such as CO 2 emission reduction, efficient use of energy and so on. The Intermediate Heat Exchanger (IHX) [2] which can be used over 900 C condition is an essential component for process heat applications. The Helically coiled IHX (HCIHX) of 10MW has been developed and installed in a primary cooling system (PCS) in the HTTR. Various R&D tests and design works were carried out to develop materials and component structures in the HCIHX. The HCIHX will be tested in high temperature 165

2 condition during the HTTR operations in order to investigate thermal-hydraulic characteristics and structural integrity. The technology basis for the HCIHX will be established in these tests in the HTTR. The problems of the HCIHXs are their size and cost. In the HCIHX used in a MW thermal HTGR plant, its manifold to support heat transfer tubes cannot be easily manufactured because a welding structure is not proper to keep creep strain and creep fatigue damage lower than an allowable level. Even if a big forging manifold is manufactured, the plant loses economical competitiveness. On the other hand, a compact heat exchanger (CHX) has times larger heat transfer area per volume than the HCIHX. The CHX is expected to be the best heat exchanger in the future generation HTGRs with process heat application. It can be also used as a recuperator in a Gas-Turbine with the Modular High Temperature Reactor (MHTGR). In our study, a new CHX can be used over 900 C condition is designed and manufactured. The CHX consists of only plates welded with diffusion. The tensile and creep strengths in the diffusion welding are superior to those in the brazing especially in high temperature condition. Major features of the HCIHX in the HTTR and present CHX are described in Chapter 2 and 3, respectively. The design characteristics of the new CHX with diffusion welding and its test and results are described in Chapter Intermediate Heat Exchanger for the HTTR Figure 1 shows a reactor cooling system of the HTTR. The reactor cooling system of the HTTR consists the PCS, a secondary helium cooling system, pressurized water cooling system and vessel cooling system. The PCS consists of a main cooling system, an auxiliary cooling system. The main cooling system has two heat exchangers, the 10MW HCIHX and a 30MW pressurized water cooler. The HCIHX for the HTTR is a vertical helically-coiled counter flow type heat exchanger in which the primary helium gas flows on the shell side and the secondary in the tube side as shown in Fig.2. The major specification is shown in Table 1. The primary helium gas of the maximum 950 C enters the HCIHX through the inner tube in the primary concentric hot gas duct. It is deflected under a hot header and discharged around the heat transfer tubes to transfer primary heat to the secondary cooling system. It flows to the primary circulator via an upper outlet nozzle and returns between the inner and outer shell in order to cool the outer shell. On the other hand, the secondary helium gas flows downwards inside the heat transfer tubes and is heated up to 905 C. It flows upwards inside the central hot gas duct. The inner insulation is installed inside the inner shell to maintain the temperature of the inner shell under the allowable one. The thermal insulator outside and inside the central hot gas duct prevents the heat transfer between the primary and secondary coolant except the heat transfer area on the heat transfer tubes so that high heat transfer efficiency can be obtained, and the temperature of the central hot gas duct is maintained under the allowable one. The pressure of the secondary helium gas is controlled higher than that of primary helium gas for prevention of fission product release even if the heat transfer tube should be broken. A tube support assemblies hold the heat tubes. Both central hot gas duct and heat tube support assemblies are hanged with a vessel top so that thermal expansion cannot be constrained. Material of the heat transfer tubes and the hot header is Hastelloy XR, and inner and outer shells is made of 2 1/4Cr-1Mo. The Ni-base Cr-Mo-Fe superalloy Hastelloy XR was so developed as to have a superior corrosion resistance under the exposure to 166

3 CONTAINMENT VESSEL VCS AIR COOLER WATER PUMP AIR COOLER ACS MCS MCS : Main cooling system IHX : Intermediate heat exchanger PPWC : Primary pressurized water cooler PGC : Primary gas circulator SPWC : Secondary pressurized water cooler SGC : Secondary gas circulator ACS : Auxiliary cooling system AHX : Auxiliary heat exchanger AGC : Auxiliary gas circulator VCS : Vessel cooling system CS : Concrete shield Figure 1 Reactor Cooling System of the HTTR 167

4 Secondory Helium (to Secondory PWC) Secondory Helium Urom Secondory PWC) Cold Header Primory Helium do Primory Cos Circulator ) Tube Support Bsam Tube Support Assembly Primory Helium (from Primary Gos CirculoiDt) Inner Shell Ouier Shell Cenlrol Hot Gcs Duel (Center Pipe) Thermol Insuioior Helically Coiled Heol Transfer Tube Hot Header Primory Helium Hrom Reoctojl Primory Helium (loreocior) Figure 2 Bird's eye view of the IHX in HTTR the HTTR primary coolant. The 2 1/4Cr-1Mo is also superior in anti-corrosion and strength in high temperature condition. 3. Compact Heat exchanger The CHXs widely used in conventional plants such as fuel cells power generation plant, co-generation plant and gas-turbine plant have several advantages described as follows; (1) A large heat transfer area per heat exchanger volume can be offered. (2) Various kind of fin types and flow patterns can be selected. (3) Compact size and high thermal performance can be obtained. 168

5 Tablei Major specification of IHX Type Design pressure Outer shell Heat transfer tube etc. Design Temperature Outer shell Heat transfer tube Operating condition Flow rate of primary He Inlet temp, of primary He Outlet temp, of primary He Flow rate of secondary He Inlet temp, of secondary He Outlet temp, of secondary He gas Heat capacity Dimension Diameter of heat transfer tube Thickness of heat transfer tube Outer diameter of outer shell Total height Material Outer and inner shell Heat transfer tube Helically-coiled counter flow Rated operation 15 t/h 850 C 390 C 14 t/h 300 C 775 C 4.7 MPa 0.29 MPa 430 C 955 C 10 MW 31.8 mmod 3.5 mm 2.0 m 10.0 m High temp, operation 2 1/4Cr-1 Mo Hastelloy XR 12 t/h 950 C 390 C 12 t/h 300 C 905 C Figure 3 shows a typical CHX used in conventional plants [3]. It consists of a plates and fins core, header and nozzles. The plates, fins, and blazing metals, are bonded by brazing in a vacuum furnace. Brazing technology for the CHX was well developed, and strength and anti-corrosion characteristics of the brazing material were confirmed by several tests. Their composing materials are stainless, aluminum and so on. These CHXs have never used as primary heat exchangers in unclear power plants. However, they have a possibility to be used less than 600 C. 4. Development of Compact Heat Exchanger with diffusion welding 4.1 Development items and schedule A final goal of this study is to develop the CHX that can be used as a primary intermediate heat exchanger in a future generation HTGR plant or recuperator in the MHTGR with the direct gas-turbine (GT-MHTGR). 169

6 SEPARATE PUTS CORRUGATED FIN "V ^SPACER 3AR BRAZING FILLER METAL Figure 3 Typical CHX used in conventional plants Requirements to the IHX are to keep primary coolant during normal operations and off-normal conditions and transfer heat efficiently. The boundary between primary and secondary coolant must be assured when a significant system failure occurs in a secondary system. The recuperator in the GT-MHTGR is not used in higher temperature condition than the IHX. However, it is not easy to meet their requirements because the recuperator is a key component to improve efficiency for the GT-MHTGR[4]. Requirements to the recuperator in the gas-turbine system are as follows, (1) Thermal effectiveness is over 90%. (2) Pressure drop is less than 2% of total pressure drop. (3) Pressure difference between low and high temperature helium gas is the maximum 5MPa. (4) Life time is 40 years. (5) Structural integrity is assured in off-normal transients, start-up and shut-down. (6) In Service Inspection is necessary when it is used in a direct cycle plant. Effectiveness, pressure drop and differential pressure between two sides for the recuperator used in an existing gas-turbine plant are approximately 88%, 6% of GT plant total pressure drop and 1.4MPa, respectively. In addition, components with the brazing welding have never been used in primary components. The CHX with diffusion welding is one of the best candidates to solve above problems. In the first stage, the feasibility study of the CHX with the diffusion welding will be carried out by designing and manufacturing a small CHX. In addition, small specimens of the diffusion welding will be manufactured to measure their strength and observe their welding surfaces. In the second stage, after comparison between the CHX with the conventional brazing and new diffusion welding is conducted, the best CHXs for the GT- MHTGR and IHX are selected. In the final stage, the full scale model test will be conducted if necessary. 170

7 4.2 Design characteristics of the CHX with diffusion welding In order to establish technological base for the CHX with the diffusion welding, the small CHXs with the diffusion welding were designed, manufactured and installed in an air loop to investigate the welding reliability and confirm the structural integrity. Table 2 shows major design characteristics of the small CHXs with diffusion welding. They only consist of Hastelloy XR plates with flow paths. The flow paths are machined by a numerical control machining system. Those plates are welded with the diffusion. They do not use the separate fins shown Fig. 3. These are unique characteristics of the new CHX and it is easy to be manufactured comparing with the CHX with brazing diffusion. Moreover, considering that it is used over 900 C condition, Hastelloy XR is selected for the plates material. Hastelloy XR is the same material as that used in the HCIHX in the HTTR and superior in creep and tensile strength in a high temperature region. Figure 4 shows the cross sectional view of the CHX. 4.3 Diffusion welding method and results First, the best condition for the diffusion welding is determined by the tests using two small thin plates. Those plates are set in a chamber and pressurized each other by a hydrostatic generator outside the chamber. Figure 5 shows a schematic diagram of the chamber and its affiliated system. In our study, a contact pressure, ambient temperature and holding time, which significantly affect the diffusion welding, are selected as a parameter. Besides these parameters, the ambient pressure is determined as low as possible so that the impurity composites are not included between two plates. The ambient pressure is set to 6x10' 3 Pa by vacuum pumps in this test. After the diffusion process, those plates are removed from the chamber and their surfaces are observed by an optical electron microscope and a scanning electron microscope(sem). Figure 6 shows the enlarged cross section between two plates by the optical electron microscope. Voids between two plates decrease as the contact pressure increases. The voids are not found when the contact pressure is more than 49MPa. Effects of ambient temperature to the welding are significant. As the ambient temperature changes from 1100 C to 1150 C, voids and inclusions between two plates disappear. However, when the ambient temperature is more than 1150 C, the Table 2 Major specification of the new CHX Type Size Material Design pressure Differential pressure between low and high temperature fluid Number of stages Inlet temperature Plate heat exchangers with diffusion welding 170mmx 90mm* 49mm Hastelloy XR 8.0MPa 5.0MPa 10 Maximum 950 C 171

8 J U8.7) S7_ 10 o lo Mechanical - booster pump Hydrostatic generator Upper ram] Test M piece Lower ram Oil rotary pump Cryo pump R0.1 -R0.5 m in :~WrLn P777] Carbon heater Chamber 5.5 Figure 4 Cross sectional view of the new CHX Figure 5 Schematic diagram of the chamber and its affiliated system

9 X400 X400 X400 Figure 6 Cross section of two thin plates by the optical electron microscope deformation occurs at the edge of the plates. As for the holding time, if the holding time is over 30 minutes, the deformation at the edge is not negligible. The deformation deteriorates not only the thermal performance but also structural integrity for long duration of operations. This results prove that the ambient temperature of 1150 C, contact pressure of 49MPa and holding time of 30 minutes are selected as the best condition for the diffusion welding. In the next tests, test specimens of the diffusion welding will be manufactured and tensile and creep strength will be obtained. The Electron Probe Micro Analyzer analysis will be also conducted to investigate inclusions in the diffusion welding. 173

10 4.4 Thermal performance of the CHX The CHX is installed in an air loop to obtain its thermal conductance. Figure 7 shows a flow sheet of the air test loop. A compressor transfers low temperature air into the CHX. After that, it is heated up to the maximum 120 C in an electric heater and flows back to the CHX. The heat is transferred from high to low temperature air in the CHX. An exhausted air from the CHX is released to the atmosphere. Four K-type thermocouples are installed at each inlet and outlet nozzle of the CHX, and the flow rate of air is measured by a vortex flowmeter. As a result, the thermal conductance is calculated by an inlet and outlet temperatures, flow rate and specific heat of air. Table 3 shows the results of the thermal performance test. The thermal conductance in the CHX obtained by this experiment is approximately 10% higher than that of the analytical result. The pressure drop of the experiment is 7.3% higher than that of the analysis. Internal air flow in the CHX is more turbulent than expected because the length of the flow paths in the CHX is so short that air flow cannot be stable. Optimization of heat transfer characteristics has not been carried out because the focal point of this study is to develop the diffusion welding. The thermal conductance of this CHX does not meet the requirement from the GT-MHTGR. However, the thermal conductance is sufficiently improved by modification of the plate shape or installation of turbulent promoters in the flow paths. For example, a modified CHX where a twisted tape[5] is installed in the flow paths is assumed and its thermal conductance is evaluated. Figure 8 shows relationship between the effectiveness and the CHX height. The modified CHX with diffusion welding can achieve effectiveness of 95% and its size meet the requirement (1.5m/width x i.5m/length * 5.5m/height/unit * 6 units). The CHX with diffusion welding has the possibility to be used for the recuperator in the GT- MHTGR. The CHX with the diffusion welding would also become feasible for the IHX because the strength of the diffusion welding in high temperature condition is apparently higher than that of brazing, and the thermal performance and cost are superior to the HCIHX. Electric heater Vortex flowmeter Compressor Atmosphere Difference pressure transmitter it) transmitter Atmosphere (T) Thermocouple Figure 7 Flow sheet of an air test loop 174

11 Table 3 Comparison between experimental and analytical thermal conductance and pressure drop of the new CHX Amount of Heat transfer: Inlet and outlet temp, of lower fluid: Inlet and outlet temp, of higher fluid: 533W 23.9 C, 66.0 C o C,73.7 C Items Experiment Analysis Error Thermal 66.8 W/m 2 K 74.2 W/m 2 K 9.8% conductance Pressure drop 5.9 kpa 5.5 kpa 7.3% Modified CHX Present CHX 0.8 I Height of the CHX for the recuperator (m) Size : the maximum width 1.5mx1.5m Heat Capacity: 78.4MWx6units Reactor Power: 450MW Figure 8 Relationship between the effectiveness and height of the CHX 175

12 5. Concluding Remarks It is confirmed that the CHX with diffusion welding is one of the best candidates for the IHX in the future generation HTGR and the recuperator in the GT-MHTGR. The early tests prove that the ambient temperature of 1150 C, contact pressure of 49MPa and holding time of 30 minutes are selected as the best condition for the diffusion welding. The welding surfaces between plates are sound in this case. The tensile strength and creep strength of the welding will be tested this year. As for the thermal performance, the thermal conductance would meet the requirement from the GT-MHTGR by modification of the plate shape or installation of turbulent promoters in the flow paths. REFERENCES [1] T. Tanaka et al.,"construction of the HTTR and its Testing Program", presented at TCM and workshop on "Design and Development of Gas-Cooled Reactors with Closed-Cycle Gas Turbines", INET, Tsinghua University, Beijing, China, 30 Oct. -2 Nov., (1995). [2] K. Kunitomi et al.,"stress and Strain Evaluation of Heat Transfer Tubes in Intermediate Heat Exchanger for HTTR", ASME/JSME Nuclear Engineering Conference, , San Francisco, March (1993). [3] S. Sato, Private communication, (1995). [4] GCRA, "Evaluation of the Gas Turbine Modular Helium Reactor", DOE-HTGR , Dec. (1993). [5] M. Mori and L. M. Lidsky, private communication, (1990). 176