Gases intermediate heat exchangers

Both gas turbine and process heat versions of the HTGR are based on the demonstrated high-temperature capability of the fuel and core structure. However, some development in the metallic components, such as the turbine, hot duels and intermediate heat exchanger is necessary, Present commercial alloys would have limited lifetime under service conditions at 1650°F (899°C) and above. However, currently envisioned advancements in ceramics and carbon-carbon composites indicate that high-temperature nonmctallic substitutes for metallic alloys will soon be available. These materials advances are the key to making future application of the IITGR a reality. 

Takeda T, Kunitomi K, Horie T, Iwata K. Feasibility of the applicability of a diffusion-welded compact intermediate heat exchanger to next-generation high-temperature gas-cooled reactors. Nucl Eng Design, 1997 168 11-21. 

Takeda, T., Kunitomi, K., Horie, T., Iwata, K., Feasibility study on the applicability of a diffusion-welded compact intermediate heat exchanger to next-generation high temperature gas-cooled reactor, Nucl. Eng. Des. 1997, 168,11-21. Bier W., Keller W., Linder G., Seidel, D., Schubert, K., Martin, H., Gas-to-gas heat transfer in micro heat exchangers, Chem. Eng. Process. 1993, 32, 33-43. Schubert, K., Brandner J., Fichtner M., Linder G., Schygulla, U., Wenka, A., Microstructure devices for applications in thermal and chemical process engineering, Microscale Therm. Eng. 2001, 5,17-39. www.fzk.de, Forschungszentrum Karlsruhe, 17 July 2004. 

Equation (6.14) is used for the temperature of gas in the tubes of the intermediate heat exchanger used in the multistage adiabatic reactor system. Heat is transferred from the hot gas to saturated water on the shell side generating steam at temperature Tst. The overall heat transfer coefficient Uux is constant and is equal to 0.227 kJ s-1 m-2 K-1. The heat transfer area per volume Ahx/Vhx is 157 m2/m3 ... 

H. Sato (JAEA) presented a paper discussing detection methods and system behaviour assessments for a tube rupture of the intermediate heat exchanger (IHX) for a sulphur-iodine based nuclear hydrogen plant. A rupture could be detected by monitoring the secondary helium gas supply using a control system that monitors the differential pressure between the primary and secondary helium gas supply. Isolation valves would be used to reduce the helium flow between the primary and secondary cooling systems. The study showed that the maximum temperature of the reactor core does not exceed its initial value and that system behaviour did not exceed acceptance criteria. 

Figure 3 briefly shows the structure, major specification and the history of the HTTR R D by JAEA. The reactor core is composed of graphite prismatic blocks. Fuels are inserted in the blocks as a shape of cylindrical graphite compacts in which tri-isotropic (TRISO)-coated fuel particles with U02 kernel are dispersed. The coolant helium gas of the HTTR is circulated at 4 MPa to an intermediate heat exchanger where high temperature heat is transferred to hydrogen production process. 

If a gradual addition of reactant has kinetic advantages compared to the total addition to the feed (here a suitably designed intermediate heat exchanger ensures a uniform distribution and mixing within the reaction gas stream). 

Fig. 25.11. Sankey energy flow diagram for a 1000ton/day sulfur-burning double absorption sulfuric acid plant (feed gas 10% S02). A Blower B Sulphur furnace C Waste heat boiler D Catalyst bed 1 E Steam superheater F Catalyst bed 2 G Boiler H Catalyst bed 3 J Intermediate heat exchangers K Intermediate absorber L Converter bed 4 M Economizer N Final absorber O Air dryer P Acid coolers. (Courtsey Lurgi GmbH, Frankfurt, Germany.)...

Helium gas of the Hl IR coolant is circulated under a pressure of 4 MPa, and through an intermediate heat exchanger, high temperature heat is transferred to the hydrogen production process as shown in Figure 5. The first criticality of the HTTR was achieved in 1998, and the Full power operation of 30 MW was attained in 2001. Then, the reactor outlet temperature was 850°C. Safety demonstration tests have been conducted since 2002. In April 2004, we conducted first high-temperature operation of 950°C (3). 

Tire reactor core, composed of graphite blocks, is so designed as to keep all specific safety features. In the cooling system, the intermediate heat exchanger (HEX) is equipped to supply high-temperature helium gas to some process heat application system being coupled to the HTTR in the future. 

Fig. 2-13 Principle gas flow in a helium-to-helium intermediate heat exchanger, from [22]... 

A probabilistic safety analysis has been conducted at the Research Center Jiilich for the process heat variant of the German modular HTGR with the purpose to identify differences compared with the electricity generating HTR-MODUL [55]. The process heat HTR-MODUL consists of a pebble bed core with 360,(KX) spherical fuel elements to produce a thermal power of 170 MW. Helium coolant gas inlet / outlet temperatures are 300 and 950 °C, respectively. The system pressure is 4 MPa. The connection to the secondary circuit is given by a He / He intermediate heat exchanger. 

Two concepts of a He - He intermediate heat exchanger for a heat rating of 125 - 170 MW have been selected. For both, a 10 MW test plant has been operated in the KVK loop verifying the operation of reformers with convective helium. A 10 MW decay heat removal system cooler, hot gas ducts including insulation and liner, hot gas valves, and a steam generator were other components of the KVK loop. Furthermore, a helium purification system was operated in a bypass of the main system. Starting in 1982, the KVK facility was operated for 18,400 h with approx. 7000 h above 900 C [28]. Hot gas duct with internal insulation was operated at temperatures up to 950 °C. The KVK experimental loop has demonstrated reliability and availability even of newly developed components. 

An intermediate heat exchanger (IHX) circuit separates the nuclear from the chemical system. It thus minimizes contamination of the process heat exchanger and allows it to be placed outside the reactor containment, prevents water and process gas ingress into the nuclear core in case of a tube rupture, and reduces hydrogen permeation from the process gas into the primary helium as well as tritium permeation in the reverse direction. Drawbacks of an IHX are its technical problems due to the higher temperature compared with the reformer, its additional electricity demand of 50 MW(e) in a 3000 MW(th) process... 

These factors create prerequisites for impurities accumulation in the zones with limited mass transfer to the main sodium flow, namely surface of cover gas plenum of the reactor and possible stagnation zones. At the same time, corrosion products would deposit on the non-isothermal surfaces of the intermediate heat exchanger until the certain moment determined by the critical thickness of deposits layer. 

Within the primary circuit radioactive coolant is protected against air by the steel barriers and cover argon gas. Radioactive sodium of the primary circuit is separated from non-radioactive sodium of the secondary circuit by the steel tubes of the intermediate heat exchangers. 

The construction of the HTTR started in March 1991, with first criticality in 1997 to be followed after commissioning testing. At present the HTITI reactor building and its containment vessel have been constructed and its main components, such as a reactor pressure vessel, an intermediate heat exchanger, hot gas pipings and core support structures, have been installed in the containment vessel. Fuel fabrication has been started as well. 

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