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Design, Fabrication, Performance Testing, and Modeling of Diffusion Bonded Compact
Heat Exchangers in a High-Temperature Helium Test Facility
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
Sai K. Mylavarapu, M.S.
Graduate Program in Nuclear Engineering
The Ohio State University
2011
Dissertation Committee:
Prof. Xiaodong Sun, Advisor
Prof. Tunc Aldemir
Prof. Richard N. Christensen
Prof. Richard S. Denning
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Copyright by
Sai K. Mylavarapu
2011
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Abstract
Very High-Temperature Reactor (VHTR) is a leading candidate for the U.S. Department
of Energys Next Generation Nuclear Power Plant (NGNP) project. It is a helium gas-
cooled reactor with very high reactor core outlet temperatures (800-950oC) and offers
high-efficiency electricity generation and a broad range of process heat applications,
such as coal liquefaction, coal gasification, and oil recovery from shale. To efficiently
transfer the core thermal energy to a secondary fluid, high-temperature and high integrity
intermediate heat exchangers (IHXs) with high effectiveness are required. While there is
no proven IHX concept for NGNP applications yet, a concept called printed circuit heat
exchangers (PCHEs) appears most promising. The current research focuses on the
design, fabrication, thermal-hydraulic performance testing, and modeling of PCHEs
under high operating temperatures and pressures.
PCHEs are plate-type heat exchangers, fabricated by photochemical machining and
diffusion bonding. In the current research work, both these fabrication techniques have
been demonstrated on Alloy 617 plates, a high-temperature candidate material for VHTR
structural components. Two counter-current flow PCHEs have been designed and
fabricated using Alloy 617 plates and are installed in a small-scale high-temperature
helium test facility (HTHF). The HTHF has been designed and constructed at The Ohio
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State University as part of this research to facilitate experiments at temperatures and
pressures up to 800oC and 3 MPa, respectively.
Microstructural and mechanical characterizations studies performed on diffusion bonded
Alloy 617 specimens are discussed. This study provided confidence from a safety view
point insofar as the operation of the heat exchangers, under high temperature and pressure
conditions in the HTHF. Performance testing of the two counter-flow PCHEs in the test
facility has been completed at varied operating temperatures, helium pressures, and
helium flow rates. The PCHE inlet temperature and pressure were varied from 85-
390oC/1.0-2.7 MPa for the cold side and 208-790oC/1.0-2.7 MPa for the hot side,
respectively, while the mass flow rate of helium was varied from 15 to 49 kg/h. The
maximum helium temperature that has reached at the exit of the main heater is 823oC.
This range of mass flow rates corresponds to PCHE channel Reynolds number of 950-
4,100 for the cold side and 900-3,900 for the hot side (corresponding to laminar and
laminar-to-turbulent transition flow regimes). The experimental data have been analyzed
for the pressure drop and heat transfer characteristics of the heat transfer surface of the
PCHEs and compared with the available models and correlations in the literature. In
addition, numerical and theoretical treatment of hydrodynamically developing and
hydrodynamically fully developed laminar flow through a semicircular duct is presented.
In summary, the PCHE testing at high temperatures and pressures and the experience
accumulated during the design and construction of the HTHF and the PCHEs will be of
value to the high-temperature reactor research.
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Dedication
This document is dedicated to my parents.
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Acknowledgments
This work would not have been possible without the support of many people.
At the outset, I am greatly indebted to my advisor, Prof. Xiaodong Sun, for his expert
guidance and support throughout the duration of my study. I have learned immensely
from him and I am very thankful for his comments and suggestions during the research
meetings. I truly appreciate his confidence in me and allowing me to work on this
challenging project. He has been an excellent mentor and a guide.
I gratefully thank Prof. Christensen for his candid comments and expert discussion during
all the research meetings. His expertise on heat exchangers and experimental facilities
have been of great help.
I am very thankful to my committee members, Prof. Tunc Aldemir and Prof. Rich
Denning, for their comments and suggestions during my candidacy exam.
The support from the U.S. Department of Energy and Idaho National Laboratory for this
research work is gratefully acknowledged. My sincere appreciation and very many
thanks to the NGNP Project Manager, Mr. Michael Patterson for all his support.
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My heartfelt thanks to the NE faculty and NE/ME staff for their wonderful support during
my stay here at Ohio State. It has been a true pleasure and an honor knowing them. I
would like to thank all my friends and colleagues in the Nuclear Engineering Program.
My sincere thanks and appreciation to Noah Needler for helping me during the initial
stages of the project. I would like to personally thank David Arcilesi, Benjamin Doup,
Tae Kyu Ham, and Richard Glosup (order is not important) for making laboratory life
(and graduate life) interesting. All four have helped me a lot in one way or another
during the construction of the facility. Thanks to Richard Glosup and Benjamin Doup for
helping me with the experiments at various stages and willing to stay long nights during
the experiments. Thanks also to Ran Li for helping me with some of the CAD drawings.
I would be remiss if I did not acknowledge the support of Ralph Orr and Grace Hines.
Ralph Orr was instrumental in ensuring that all my purchase orders are expedited
(probably to escape my constant nagging).
Special thanks to my wife, Sai Prasanna Jayanthi for her continued support and help
throughout this period. She had to endure long hours of my absence during the
experiments and the dissertation. Finally, my warmest thanks to Dr. Satya Seetharaman
and Deepa for their wonderful company during our stay at the Buckeye Village.
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Vita
2003................................................................B. Tech. Mechanical Engineering,
Jawaharlal Nehru Technological University, Hyderabad, India
2004-2006 ......................................................Research Assistant, Indian Institute of
Technology, Bombay, India
2008................................................................M.S. Nuclear Engineering, The Ohio State
University
2007 to present ..............................................Graduate Research Associate, Nuclear
Engineering Program, The Ohio State University
Journal Publications
1. Mylavarapu, S.K., et al., 2009, "Investigation of High-Temperature Printed Circuit Heat Exchangers for VHTRs," Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, 131(6), pp. 062905-0107. 2. Mylavarapu, S.K., et al., 2011,"Fabrication and Design Aspects of High- Temperature Compact Diffusion Bonded Heat Exchangers," Nuclear Engineering and Design, doi:10.1016/j.nucengdes.2011.08.043
Fields of Study
Major Field: Nuclear Engineering
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Table of Contents
Abstract ............................................................................................................................... ii
Dedication .......................................................................................................................... iv
Acknowledgments............................................................................................................... v
Vita .................................................................................................................................... vii
List of Tables ................................................................................................................... xiii
List of Figures .................................................................................................................. xiv
Chapter 1 : Introduction ...................................................................................................... 1
1.1 Motivation ................................................................................................................ 1
1.2 Intermediate Heat Exchanger .................................................................................... 3
1.3 Research Objectives .................................................................................................. 5
1.4 Previous Work ........................................................................................................... 7
1.5 Dissertation Organization ........................................................................................ 11
References for Chapter 1 ............................................................................................... 13
Chapter 2 : High-Temperature Helium Test Facility and Printed Circuit Heat Exchangers
........................................................................................................................................... 15
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2.1 Overview ................................................................................................................. 15
2.2 High-Temperature Helium Test Facility ................................................................. 16
2.2.1 Introduction ...................................................................................................... 16
2.2.2 Description of the Test Facility ........................................................................ 16
2.2.3 Design Aspects of OSU HTHF ........................................................................ 21
2.2.4 Simplified Stress Analysis for Loop Piping Design Calculations .................... 28
2.2.5 High-Temperature Helium Test Facility Components ..................................... 29
2.2.6 Instrumentation ................................................................................................. 39
2.2.7 Quality Assurance ............................................................................................. 41
2.3 Leak Testing ............................................................................................................ 43
2.4. Printed Circuit Heat Exchangers: Fabrication and Design Aspects ................... 45
2.4.1 PCHE Fabrication Techniques ........................................................................ 45
2.4.2 Design Aspects of PCHEs ................................................................................ 55
2.5 Microstructural and Mechanical Property Characterization of Diffusion Bonded
Specimens...................................................................................................................... 59
2.5.1 Microstructural Examination ............................................................................ 60
2.5.2 Mechanical Property Testing ............................................................................ 64
References for Chapter 2 ............................................................................................... 67
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Chapter 3 : Developing and Fully-Developed Laminar Flow in a Semicircular Duct:
Theoretical and Computational Analysis .......................................................................... 71
3.1 Overview ................................................................................................................. 71
3.2 Dimensionless Groups and Basic Definitions ......................................................... 72
3.3 Scale Analysis for Laminar Flow in a Semicircular Duct ....................................... 76
3.3.1 Fully-Developed Laminar Flow ....................................................................... 77
3.3.2 Hydrodynamically Developing Laminar Flow ................................................. 79
3.4 Fully-Developed and Hydrodynamically Developing Laminar Flow in a
Semicircular Duct: Analytical and Numerical Treatment ............................................. 82
3.4.1 Analytical Solution for Laminar Fully Developed flow in a Semi-circular Duct
................................................................................................................................... 83
3.4.2 Computational Model: Results and Discussion ................................................ 92
References for Chapter 3 ............................................................................................. 103
Chapter 4 : Performance Testing of PCHEs in the High-Temperature Helium Test
Facility ............................................................................................................................ 105
4.1 Overview ............................................................................................................... 105
4.2 Heat Exchanger Performance Variables ............................................................... 105
4.3 Thermal-Hydraulic Models/Correlations .............................................................. 114
4.4 Procedure for Determining the Flow Friction Characteristics of the PCHE Heat
Transfer Surface .......................................................................................................... 118
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4.4.1 Contributions to Pressure Drop ...................................................................... 121
4.4.2 Discrepancy in Differential Pressure Across Hot Side of PCHE1 ................. 129
4.5 Proof of Linear Temperature Profiles ................................................................... 131
4.6 Magnitude of the Wall Conduction Resistance ..................................................... 133
4.7 Uncertainty Analysis ............................................................................................. 136
4.8 Flow Maldistribution ............................................................................................. 147
4.9 PCHE Performance Experiments in the HTHF: Results and Discussion ............. 149
4.9.1 Experimental Procedure ................................................................................. 150
4.9.2 Experimental Test Matrix ............................................................................... 151
4.9.3 Pressure Drop and Heat Transfer Characteristics ........................................... 155
Chapter 5 : Conclusions, Technical Challenges, and Future Work ................................ 181
5.1 Summary and Conclusions .................................................................................... 181
5.2 Technical Challenges ............................................................................................ 185
5.3 Areas for Future Research ..................................................................................... 188
Bibliography ................................................................................................................... 191
Appendix A : Room and Elevated Temperature Leak Testing Procedure ...................... 197
A.1 Introduction .......................................................................................................... 197
A.2 Leak Testing Methods .......................................................................................... 198
A.3 Procedure .............................................................................................................. 199
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Appendix B : Experimental Data .................................................................................... 214
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List of Tables
Table 2.1. Nominal Chemical Composition of High-Temperature Alloys ....................... 23
Table 2.2. Electrical characteristics of the heaters ........................................................... 31
Table 2.3. Heater design specifications ........................................................................... 32
Table 2.4. Cooler design specifications ........................................................................... 34
Table 2.5. Haskel 8AGD-2.8 gas booster specifications ................................................. 36
Table 2.6. Specifications of the pressure reducing regulator ........................................... 37
Table 2.7. Surface roughness of the interior surface of a representative flow channel ... 54
Table 2.8. Basic geometric and characteristic parameters for the PCHEs ....................... 57
Table 2.9. Tensile test results for diffusion bonded Alloy 617 specimen ......................... 65
Table 3.1. Flow parameters for hydrodynamically developing flow in a semicircular duct
........................................................................................................................................... 99
Table 4.1. c1 as a function of l/Do .................................................................................. 126
Table 4.2. Ranges and accuracies of instruments used in the HTHF ............................ 138
Table 4.3. Fluid properties and their uncertainties ........................................................ 139
Table 4.4. PCHE experimental test matrix of helium flow, temperature, and pressure . 152
Table B.1. Experimental Data of PCHE1 (HX1) and PCHE2 (HX2) in HTHF ............ 214
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List of Figures
Figure 1.1 Schematic of the very high temperature reactor ................................................ 3
Figure 2.1 Layout of the high-temperature helium test facility ........................................ 19
Figure 2.2 Photographs of the high-temperature helium test facility: (a) Low-temperature
side and (b) High-temperature side ................................................................................... 20
Figure 2.3. ASME allowable design stresses the HTHF candidate materials .................. 24
Figure 2.4. Pressure design thickness requirement for a nominal 1 inch, seamless Alloy
800H pipe at different pressures ....................................................................................... 26
Figure 2.5. Kanthal RAC Fibrothal tube heater with embedded heating element ........... 31
Figure 2.6. Cooler for cooling helium gas ....................................................................... 33
Figure 2.7. Haskel 8AGD-2.8 gas booster ....................................................................... 36
Figure 2.8. Photochemical machining process flow chart ............................................... 47
Figure 2.9. Photochemically etched Alloy 617 plates (a) straight pattern and z pattern,
(b) channel cross-section ................................................................................................... 48
Figure 2.10. (a) Diffusion bonded Alloy 617 heat exchanger block and (b) heat exchanger
with headers welded .......................................................................................................... 52
Figure 2.11. PCHE flow distribution header of Alloy 800H construction ...................... 52
Figure 2.12. Three-dimensional contour map of the interior surface of a channel .......... 54
Figure 2.13. Illustration of PCHE channel cross-section .................................................. 55
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Figure 2.14. Geometry of the straight channel (cold side) and z-channel (hot side) plates
used in the current PCHE design (all dimensions in inches) ............................................ 56
Figure 2.15. Representative SEM micrograph of the diffusion bond interface revealing
the presence of second phase particles that bound the Ni interlayer and separating the
Alloy 617 plates ................................................................................................................ 61
Figure 2.16. EDS spectrum indicating the chemical composition of the particles and
adjacent Alloy 617 matrix. ................................................................................................ 62
Figure 2.17. Inverse pole figure OIM micrograph depicting the grain structure across the
diffusion bond joint between plates of Alloy 617 ............................................................. 63
Figure 2.18. (a) SEM micrograph of a failed specimen crept at 900C/207 MPa and (b)
higher magnification view of the flat and brittle facture surface. ..................................... 66
Figure 2.19. Creep test result of Alloy 617 diffusion bonded specimen .......................... 67
Figure 3.1. Illustration of boundary layer development in a circular pipe ...................... 76
Figure 3.2. Circular sector duct......................................................................................... 83
Figure 3.3. Duct of semicircular cross-section ................................................................. 84
Figure 3.4. Plot of dimensionless axial velocity at various angular locations ................. 87
Figure 3.5. Mesh density on a cross-section ..................................................................... 94
Figure 3.6. Apparent Fanning friction factor as a function of dimensionless axial distance
........................................................................................................................................... 96
Figure 3.7. Incremental pressure drop number as a function of dimensionless axial
distance ............................................................................................................................. 97
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Figure 3.8. Ratio of the maximum axial velocity to mean axial velocity as a function of
dimensionless axial distance ............................................................................................. 98
Figure 3.9. Plot of blown-up view of the axial velocity as a function of duct length ...... 99
Figure 3.10. Contours of fluid axial velocity in the fully-developed region in a
semicircular duct ............................................................................................................. 100
Figure 3.11. Ratio of apparent Fanning friction factor to fully developed Fanning friction
factor in a semicircular duct ............................................................................................ 101
Figure 4.1. Nomenclature for fluid stream temperatures in PCHE ................................. 109
Figure 4.2. Thermal circuit for heat transfer in a heat exchanger ................................... 110
Figure 4.3. Local Nusselt number as a function of dimensionless axial distance for a
semicircular duct ............................................................................................................. 117
Figure 4.4. Differential pressure measurement location across the hot and cold sides of
the PCHE ........................................................................................................................ 120
Figure 4.5. Geometry of the header ............................................................................... 126
Figure 4.6. Plot of pressure drop across the hot and cold sides of PCHE1 and PCHE2
under isothermal test conditions ..................................................................................... 130
Figure 4.7. Energy balance for a counter flow heat exchanger ...................................... 131
Figure 4.8. Plot of numerically estimated wall temperatures on the hot and cold fluid
sides of the PCHE investigated ....................................................................................... 135
Figure 4.9. Experimental test matrix of temperatures, pressures, and flow rates till date
......................................................................................................................................... 153
Figure 4.10. Plot of steady-state mass flow rate recorded by a Venturi flow meter ...... 154
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Figure 4.11. Plot of quasi steady-state temperatures in PCHE2 ..................................... 155
Figure 4.12. Isothermal friction factor for the hot and cold fluid sides of PCHE1 ....... 157
Figure 4.13. Isothermal friction factors for the hot and cold fluid sides of PCHE2 ...... 158
Figure 4.14. Fanning friction factors as a function of Reynolds number for the cold side
......................................................................................................................................... 162
Figure 4.15. Fanning friction factor-Reynolds number product as a function of cold side
Reynolds number ............................................................................................................ 162
Figure 4.16. Plot of Nu as a function of Re for the cold side ......................................... 165
Figure 4.17. Hot (z-pattern) and cold (straight) side of the heat exchanger .................. 166
Figure 4.18. Plot of friction factor as a function of Re for the hot side .......................... 168
Figure 4.19. Plot of Nu as a function of Re for the hot side ........................................... 171
Figure 4.20. Comparison plot of Fanning friction factor for the hot and cold sides of
PCHE1 ............................................................................................................................ 172
Figure 4.21. Comparison plot of Fanning friction factor for the hot and cold sides of
PCHE2 ............................................................................................................................ 173
Figure 4.22. Plot of Fanning friction factor for PCHE1 and PCHE2 as a function of
Reynolds Number ........................................................................................................... 174
Figure 4.23. Comparison plot of Nusselt number as a function of Reynolds number for
hot and cold sides of PCHE1 .......................................................................................... 175
Figure 4.24. Comparison plot of Nusselt number as a function of Reynolds number for
hot and cold sides of PCHE2 .......................................................................................... 176
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Figure 4.25. Effectiveness of the heat exchangers as a function of the number of transfer
units for PCHE1 and PCHE2 .......................................................................................... 177
Figure 4.26. Plot of heat load as a function of Reynolds number ................................... 178
Figure A.1. Schematic of the process chilled water line..................................................204
Figure A.2. Compressed air line schematic ................................................................... 205
Figure A.3. Safety Switches ............................................................................................ 206
Figure A.4. Electrical Enclosure ..................................................................................... 206
Figure A.5. UDC 1200 Controller .................................................................................. 207
Figure A.6. UDC 2500 Controller .................................................................................. 209
Figure A.7. Acuvim II Power meter ............................................................................... 210
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Chapter 1 : Introduction
1.1 Motivation
The Very-High-Temperature Reactor (VHTR) is a potential candidate reactor concept for
the Next Generation Nuclear Plant (NGNP). The reactor design is a graphite-moderated,
helium-cooled, prismatic or pebble-bed core, thermal neutron spectrum reactor with a
once-through uranium fuel cycle and core outlet temperatures of 900-950oC [1.1]. The
VHTR concept, with a projected plant design service life of 60 years, is being researched
not only due to its near-term deployment potential but also because of its applicability
beyond the electrical grid by providing industry with carbon-free, high-temperature
process heat for a variety of applications, including hydrogen production, petroleum
refining, bio-fuels production, and production of chemical feed stocks for use in the
fertilizer and chemical industries. Fig. 1.1 shows a conceptual layout of the VHTR
illustrating the nuclear heat source and the associated power conversion unit and the
hydrogen generation plant [1.1].
The operating conditions of the VHTR represent a major departure from the existing
light-water cooled reactor technologies. The components of the heat transport system of
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VHTR will be subjected to elevated temperatures for long times where adequate and
reliable performance of materials is critical. Of all the high-temperature metallic
components, the one most likely to be heavily challenged in the NGNP will be the
intermediate heat exchanger (IHX) [1.2-1.4]. It is a major component of the Heat
Transport System (HTS) of the NGNP and directly affects the system overall efficiency.
It must be robust enough to effectively transfer the heat between the Primary Heat
Transport System (PHTS) and the Secondary Heat Transport System (SHTS). The
current Technology Readiness Level (TRL) status issued by NGNP to all components
associated with the IHX for a reduced ROT of 750-800oC is 3 on a scale of 1 to 10, with
1 being the least matured [1.3].
This warrants a substantial technical development effort before the IHX is ready for full
commercialization. The current research aims to address this by investigating a promising
potential IHX concept called printed circuit heat exchanger (PCHE) for its design,
fabrication, performance testing, and modeling under high operating temperatures and
pressures. To facilitate testing at high temperatures and pressures, a high-temperature
helium test facility (HTHF) has been designed and constructed. Two PCHEs have been
designed and fabricated using Alloy 617 plates and have been tested for their thermal-
hydraulic performance in the HTHF.
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Figure 1.1 Schematic of the very high temperature reactor [1.1]
1.2 Intermediate Heat Exchanger
The purpose of IHX is to efficiently and reliably transfer the heat generated in the core to
the power conversion system and for high-temperature process heat applications [1.2].
Several candidate materials and candidate configurations exist for the IHX; however,
their applicability for NGNP has not been analyzed or confirmed to date. Compact heat
exchangers such as PCHEs offer a promising alternative to conventional shell and tube
heat exchangers for NGNP applications. PCHEs are plate-type heat exchangers in which
fluid flow channels are photochemically etched on flat metal plates. The etched plates are
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then stacked together in a particular configuration and diffusion bonded together to form
a heat exchanger block. Flow distribution headers are then welded on to the PCHE block
to form the complete heat exchanger core. In this work, PCHEs fabricated using high-
temperature Alloy 617 plates and straight channels have been investigated.
PCHE as a Potential Design Option for IHX of VHTRs
VHTRs require high-temperature and high efficiency heat exchangers to effectively
transfer the heat from primary helium to the secondary fluid (helium, nitrogen/helium
mixture or a molten salt). Gas coolants typically have low heat transfer capability due to
their low volumetric thermal capacity and thermal conductivity. This necessitates the
requirement of a heat transfer surface with a very high surface area density (650 to 1300
m2/m3), i.e., a compact heat transfer surface. Compared to a non-compact heat
exchanger, compact heat exchangers, such as PCHEs, are characterized by a large surface
area density, resulting in reduced space, weight, support structure and footprint, energy
requirements and cost, as well as an improved process design [1.5]. Furthermore, due to
the nature of the fabrication techniques involved, PCHEs possess high-pressure
containment capability and with the right selection of structural materials can be designed
for high-temperature service applications as well. All these factors have a great influence
on the VHTR plant layout and design. Moreover, PCHEs have a sound technology base
in that they are being extensively used in demanding non-nuclear applications, albeit at
much lower temperatures, such as offshore oil platforms as gas coolers, compressor after
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coolers, etc. In light of the above, PCHEs have a tremendous potential to be an excellent
choice for IHX.
To the authors knowledge, only two PCHEs have been manufactured in the U.S. from
high-temperature materials, i.e., the two fabricated by the Ohio State University [1.6].
When this research started about 5 years ago, no heat exchangers in the U.S. were tested
under high-temperature helium conditions typical of VHTRs even though there was a
lack of experimental data in the open literature for PCHEs operating at temperatures and
pressures encountered in NGNP. Furthermore, if the IHX is fabricated by diffusion
bonding, the bond strength may become the controlling factor for life and is a key to the
performance capability of a high-temperature heat exchanger. In summary, it is
important to address key issues related to design and fabrication of compact heat
exchangers, such as PCHEs, and experimentally investigate their thermal-hydraulic
performance under high operating temperatures and pressures.
1.3 Research Objectives
Even though there has been some amount of prior experimental research performed with
the PCHEs at low temperature conditions, there was no database on their thermal-
hydraulic performance at operating conditions of VHTRs. The current research aims to
fill the void in the PCHE performance database under high-temperature and moderate
pressure conditions. In what follows, the objectives of the current research are outlined.
Design and construct a high-temperature helium facility that can enable thermal-
hydraulic performance testing of heat exchangers for temperatures and pressures
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up to 800oC and 3 MPa, respectively. (The test facility has been designed and
constructed and leak tested with helium at close to full design pressure and design
temperatures).
Design and fabricate countercurrent flow PCHEs with straight channels using
Alloy 617 plates. (Two PCHEs with straight flow channels have been designed
and fabricated by diffusion bonding of Alloy 617 plates).
Perform mechanical and material characterization studies on diffusion bonded
Alloy 617 specimens.
Develop a test matrix for the PCHE testing and perform thermal-hydraulic
performance testing of the PCHEs in the high-temperature helium facility at
temperatures and pressures up to 800oC and 3.0 MPa, respectively.
Develop an experimental database for PCHE thermal-hydraulic performance
under intermediate-to-high-temperature environments. Analyze and reduce the
experimental data for friction and heat transfer characteristics of the heat
exchangers. Benchmark the experimental data against available data and models
for straight flow channels of semi-circular cross-section. Straight channels are
employed in the current design as this geometry is relatively better understood.
Investigate hydrodynamically developing and hydrodynamically fully-developed
laminar flow through a semicircular duct and provide relations for determining
the hydrodynamic entrance length in a semicircular duct and the friction factor (or
pressure drop) in the in the hydrodynamic entrance region for laminar flow
through a semicircular duct.
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1.4 Previous Work
PCHE is a commercial product developed by a foreign vendor for off-shore applications
where compactness is very important [1.7]. PCHEs for these applications are of stainless
steel construction and are typically designed and fabricated to less restrictive ASME
Code requirements. On the contrary, any potential IHX design concept for NGNP
applications should (most likely) conform to more stringent ASME III Code
requirements. Southall et al. [1.8] discusses different compact heat exchanger
configurations, such as printed circuit, formed plate, and hybrid heat exchangers along
with their potential applications for the high-temperature reactors. According to him,
formed plate heat exchangers (FPHEs), fabricated from corrugated sheets, are more
economical for low pressure applications. PCHEs, however, are capable of sustaining
higher design pressure than FPHEs. A hybrid heat exchanger, on the other hand, has the
attributes of both the PCHEs and FPHEs and is more suited for applications where one
heat exchanger must satisfy design requirements for two very different fluids.
In the literature, some research work was performed with the PCHEs. Nikitin et al. [1.9]
and Ishizuka et al. [1.10] investigated, both numerically and experimentally, the heat
transfer and pressure drop characteristics of a 3-kW PCHE in a supercritical CO2 loop
with mass flow rates varying from 40-80 kg/h. The PCHE employed in their study was
manufactured by a commercial vendor and had plates stacked in a double banking
arrangement with a cold channel plate sandwiched between two hot channel plates and
consisted of herringbone type channel flow passages. In their study, the hot and cold side
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inlet temperatures were varied from 280-300oC and 90-108oC, respectively while the hot
and cold side inlet pressures were varied from 2.2-3.2 MPa and 6.5-10.5 MPa,
respectively. For the PCHE tested, they correlated local heat transfer coefficient and
pressure drop as a function of Reynolds number. A maximum heat exchanger
effectiveness of around 98% was reported in their study. However, their experimental
operating temperatures only reached 300oC and are far from high-temperature reactor
(HTR) requirements. Song et al. [1.11] performed experiments at low Reynolds numbers
with a commercial PCHE using air as the working fluid. For the PCHE tested, they
developed heat transfer and friction factor correlations and examined the adaptability of
Hesselgreaves correlation [1.12] to PCHE type heat exchanger.
Figley [1.13] developed a numerical model of a straight channel PCHE and performed
numerical analysis to investigate its thermal-hydraulic performance for varied operating
conditions and generate predictive data for the PCHEs fabricated at The Ohio State
University. The PCHE model dimensions are representative of the PCHEs fabricated and
installed in the high-temperature helium test facility. The operating conditions are
representative of the maximum design operating temperature and pressure for the high-
temperature helium test facility. The computational results of the convective heat
transfer coefficient and pressure drop from the PCHE model agree well with the available
models in the literature. Kim et al. [1.14] investigated, both numerically and
experimentally, the thermal-hydraulic performance of the PCHE using the KAIST
Helium Test Loop with mass flow rates varying from 40-100 kg/h. The PCHE employed
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in their experimental study is of Alloy 800H construction and fabricated by a commercial
vendor. In their study, the operating pressure range was 1.5-1.9 MPa and the hot and
cold side inlet temperatures were varied from 25-550oC and 25-100oC, respectively.
Based on the experiments, a global fanning factor and a global Nusselt number were
proposed for the PCHE tested. Furthermore, they validated their 3-D numerical model
against the KAIST helium experimental data.
Pra et al. [1.15] carried out steady and transient tests on a PCHE recuperator mock-up
and investigated their thermal-mechanical behavior in an air test loop for conditions
typical of High Temperature Reactor (510oC). The PCHE was supplied by a commercial
vendor and had convoluted flow passages (wavy or herringbone type). The steady-state
analysis of the PCHE was performed to evaluate its thermal-hydraulic performance and
examine its ability to reach HTR recuperator specifications. An effectiveness of 95%
was reported in their study. Furthermore, transient tests were performed to determine the
number of thermal load cycles required to generate a failure in the heat exchanger. For
the transient tests, the PCHE mock-up was subjected to severe temperature variations or
thermal shocks representative of a HTR recuperator. During the cold shock, the
temperature was varied from 510 to 108oC within 5 s. In a similar manner, a hot shock
was generated by varying the temperature from 180 to 510oC within 3 minutes. This
process was repeated 100 times and no fatigue failure of the mock-up was noticed.
Although these tests are less stringent compared to the IHX requirements for NGNP, they
however show that PCHE has a good potential for high temperature applications.
-
10
To reduce the high pressure drop associated with the herringbone type channel
configuration and improve the performance advantage, numerical studies have been
carried out on alternative channel geometries as well. Tsuzuki et al. [1.16] developed a
new PCHE with S-Shaped fins and performed a numerical analysis to evaluate the heat
transfer and pressure drop characteristics. In their analysis, they considered supercritical
CO2 as the hot-side fluid and H2O as the cold-side fluid. The inlet temperatures of the
hot and cold fluid are 118 and 7oC and their inlet pressures are 11.5 and 0.25 MPa,
respectively. Their analysis indicated a similar heat transfer performance and a reduced
pressure drop compared to zigzag PCHEs. Kim et al. [1.17] performed a three
dimensional numerical analysis to investigate heat transfer and pressure drop
characteristics of supercritical CO2 flow in a PCHE model that incorporates airfoil shaped
fins instead of zigzag or herringbone type channels. Numerical comparison between
airfoil fin and zigzag PCHE channel configurations indicated an appreciable reduction in
pressure drop in the case of the airfoil fin PCHE. However, the heat transfer rate per unit
volume is comparable in both the configurations. The reduction in pressure drop was
attributed to the streamlined shape of airfoil fins that help suppress generation of
separated flow.
In light of the above discussion, it can be concluded that most of the work on PCHEs is
computational and there are very limited experimental data on their thermal-hydraulic
performance under very high operating temperatures and pressures. Notwithstanding the
aforementioned works on PCHEs, to the authors knowledge, however, a detailed
-
11
thermal-hydraulic performance of the PCHEs with helium as the working fluid has not
been experimentally investigated for the temperatures and pressures typical of VHTRs.
Besides, there is essentially no experimental data available for the performance of a
PCHE when the operating conditions deviate from the nominal design condition.
Furthermore, the PCHEs employed in the aforementioned experimental studies are
developed by a commercial foreign vendor and the U.S. fabrication capability of PCHEs
has not been fully explored and confirmed. The current research addresses these issues
and provides an extensive database of the thermal-hydraulic characteristics of PCHEs
under high operating temperatures and pressures.
1.5 Dissertation Organization
This dissertation is comprised of five chapters and two appendices.
Chapter 1 introduces the problem statement and provides motivation for the current
research, provides some background information on the heat exchangers, lays down the
objectives of the current research work, and provides a compilation of the previous work
performed on PCHEs.
Chapter 2 provides the design and fabrication aspects of the high-temperature helium test
facility and the printed circuit heat exchangers. The simplified stress analysis performed
on the test facility piping and the heat exchangers are discussed. The processes related to
the design and fabrication the PCHEs, such as the photochemical machining and
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12
diffusion bonding techniques are discussed. Microstructural characterization and
mechanical testing of representative diffusion bonded specimens are discussed.
Chapter 3 addresses the hydrodynamically developing and fully-developed laminar flow
in a semi-circular duct. The method of scale analysis [1.18] has been employed for
providing order of magnitude estimates for the friction characteristics in
hydrodynamically fully-developed and hydrodynamically developing laminar flow in a
semicircular duct. Following this, an analytical and numerical treatment of these flows in
a semicircular duct is presented and the results discussed.
Chapter 4 discusses the performance testing of the PCHEs carried out in the HTHF.
Various heat transfer and pressure drop correlations applicable for circular and semi-
circular ducts in the laminar, turbulent, and laminar-to-turbulent transition flow regimes
are discussed. The data reduction procedure for the friction and heat transfer
characteristics are described in detail. The experimental data has been benchmarked
against available models and correlations in the literature and the friction and heat
transfer characteristics of the heat transfer surface of the heat exchangers have been
determined.
Chapter 5 summarizes the important results and findings of this dissertation research, the
technical challenges and the lessons learned in this research, and provides
recommendations for future work.
-
13
Appendices detailing the leak test procedure at room and elevated temperatures for the
high-temperature helium test facility (Appendix A) and PCHE experimental data
(Appendix B) follow.
References for Chapter 1
[1.1] Idaho National Laboratory Homepage, Available online at http://www.inl.gov/research/very-high-temperature-reactor/, accessed October 12, 2011. [1.2] Natesan, K., Moisseytsev, A., and Majumdar, S., 2009, "Preliminary Issues
Associated with the Next Generation Nuclear Plant Intermediate Heat Exchanger Design," Journal of Nuclear Materials, 392, pp. 307-315.
[1.3] INL, 2009, "Next Generation Nuclear Plant Project Technology Development
Roadmaps: The Technical Path Forward for 750-800oC Reactor Outlet Temperature," INL/EXT-09-16598, Idaho Falls, ID.
[1.4] INL, 2004, "Design Features and Technology Uncertainties for the Next
Generation Nuclear Plant," INEEL/EXT-04-01816, Idaho Falls, ID. [1.5] Shah, R.K. and Sekulic, D.P., 2003,"Fundamentals of Heat Exchanger Design," John Wiley & Sons, Hoboken, New Jersey: NJ. [1.6] Mylavarapu, S., Sun, X., Figley, J., Needler, N.J., and Christensen, R.N., 2009, "Investigation of High-Temperature Printed Circuit Heat Exchangers for VHTRs," Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, 131(6), pp. 062905-0107. [1.7] Southall, D., Le Pierres, R., and Dewson, S.J., 2009, "Design Considerations for Compact Heat Exchangers," Proceedings of ICAPP '09, paper no. 8009. [1.8] Southall, D. C. and Dewson, S. J., 2010, "Innovative Compact Heat Exchangers," Proceedings of ICAPP'10, paper no. 10300. [1.9] Nikitin, K., Kato, Y., and Ngo, L., 2006, "Printed Circuit Heat Exchanger
Thermal-Hydraulic Performance in Supercritical CO2 Experimental Loop," International Journal of Refrigeration, 29(5), pp. 807-814.
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14
[1.10] Ishizuka, T., Kato, Y., Muto, Y., Nikitin, K., and Ngo, L., 2005, "Thermal-Hydraulic Characteristics of a Printed Circuit Heat Exchanger in a Supercritical CO2 Loop," NURETH-11, pp. 218-232.
[1.11] Song, S.C., 2005, "Thermal-Hydraulic Performance of a Printed Circuit Heat Exchanger in an Air Test Loop," M.S.Thesis, KAIST, Daejeon, Korea. [1.12] Hesselgreaves, J.E., 2001, "Compact Heat Exchangers: Selection, Design, and Operation," Pergamon Press, New York: NY. [1.13] Figley, J.T., 2009, "Numerical Modeling and Performance Analysis of Printed Circuit Heat Exchanger for Very High Temperature Reactors," M.S. Thesis, The Ohio State University, Columbus, OH. [1.14] Kim, I.H., No, H.C., Lee, J.I., and Jeon, B.G., 2009, "Thermal-Hydraulic Performance Analysis of the Printed Circuit Heat Exchanger using a Helium Test Facility and CFD Simulations," Nuclear Engineering and Design, 239, pp. 2399- 2408. [1.15] Pra, F., Tochon, P., Mauget, C., Fokkens, J., and Willemsen, S., 2008, "Promising Designs of Compact Heat Exchangers for Modular HTRs using the Brayton Cycle," Nuclear Engineering and Design, 238(11), pp. 3160-3173. [1.16] Tsuzuki, N., Kato, Y., and Ishizuka, T., 2007, "High Performance Printed Circuit Heat Exchanger," Applied Thermal Engineering, 27(10), pp. 1702-1707. [1.17] Kim, D.E., Kim, M.H., Cha, J.E., and Kim, S.O., 2008, "Numerical Investigation
of Thermal-Hydraulic Performance of New Printed Circuit Heat Exchanger Model," Nuclear Engineering and Design, 238(12), pp. 3269-3276.
[1.18] Bejan, A., 2004, "Convection Heat Transfer." John Wiley & Sons, Inc., Hoboken, New Jersey: NJ.
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15
Chapter 2 : High-Temperature Helium Test Facility and Printed Circuit Heat Exchangers
2.1 Overview
The design and construction of the high-temperature helium test facility is discussed in
this chapter. The HTHF is primarily intended for performance testing of heat exchangers
with different configurations, however, it is designed with sufficient flexibility to allow
testing of other high-temperature components of VHTR such as valves, gaskets, etc. The
test facility components and its working will be explained in detail. A stress analysis of
the test facility piping shows that the test facility is conservative and can be safely
operated at the design temperatures and pressures. In addition, the design details and
fabrication aspects of two high-temperature Alloy 617 printed circuit heat exchangers are
presented in detail. Both the PCHEs have been installed in the HTHF for performance
testing. Furthermore, some microstructural and mechanical characterization studies
performed on Alloy 617 diffusion bonded specimens are discussed.
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16
2.2 High-Temperature Helium Test Facility
2.2.1 Introduction
The high-temperature helium test facility has been designed and constructed to facilitate
performance testing of heat exchangers at temperatures and pressures up to 800oC and 3
MPa, respectively [2.1]. The original design goal of the test facility maximum
temperature was 900oC [2.2], which has been reduced due to the reduced reactor outlet
temperatures (750-800oC) in the new DOE VHTR design specifications [2.3]. The
facility has been designed with sufficient flexibility to accommodate testing of heat
exchangers with different configurations and other critical components of VHTR, such as
valves, instruments, gaskets, and piping under high-temperature conditions. In designing
and constructing the facility, the requirements of ASME B31.3 Process Piping Code [2.4]
and ASME VIII and IX of the Boiler and Pressure Vessel Code [2.5] were followed.
Welding and post-weld examination (radiograph and penetrant tests) on the weld joints
was performed as per ASME Section IX. The facility, however, is not designed and
constructed to more restrictive ASME III code. Two counter-flow PCHEs are installed in
series in the facility for thermal-hydraulic performance testing under a wide range of
operation conditions.
2.2.2 Description of the Test Facility
Figure 2.1 shows a schematic of the high-temperature helium test facility and Fig. 2.2
shows photographs of the HTHF, fully insulated and with a protective steel enclosure
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17
around it. The test facility is initially vacuumed to the desired vacuum pressure of -14
psig using the vacuum pump. Following this, the facility is pressurized to the desired
pressure with helium gas obtained from the helium gas cylinder. After the facility is
charged with helium, the gas booster is turned on to circulate the helium gas in the test
facility piping and its components. Originally, a gas compressor was chosen in the
design, but was not adopted due to its very high cost. A gas chromatography was
considered in the design stage to provide a means of monitoring the helium environment
during testing. However, due to economic considerations, it is not used in the current
design. A 5-gallon surge volume tank and an inline pressure reducing regulator/valve
(PRV) located downstream of the booster help mitigate the pressure fluctuations due to
the reciprocating action of the booster and ensure a stable helium flow in the test facility.
After exiting the PRV, helium gas is heated by a pre-heater with a maximum capacity of
about 23 kW. The pre-heater is a combination of three radiant type heaters, each having
a maximum capacity of 7.6 kW, and arranged in a 3-phase delta configuration. Helium
gas leaving the pre-heater with a desired temperature is then forwarded to the cold side of
the first PCHE (labeled PCHE1) where it exchanges thermal energy with the respective
hot side. The helium gas exiting the cold side of PCHE1 is then forwarded to the cold
side of the second PCHE (labeled PCHE2). The facility is designed such that the fluid
exiting the cold side of the second PCHE enters the hot side of PCHE2 through a main
heater that heats the helium to around 900oC when operated at its maximum heating
capacity of 23 kW. The main heater is also a combination of three radiant type heaters
and is electrically configured similar to the pre-heater. Following this, the fluid enters the
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18
hot side of PCHE1 where it exchanges energy with the respective cold side. After
transferring energy to the cold side, the helium gas enters a cooler, where it transfers heat
to the process chilled water and gets cooled down to the inlet temperature of the gas
boosters, i.e., around 35oC. There are two bypass lines in the facility; one bypasses the
cold side of PCHE1 and the other bypasses the hot side of PCHE1. These bypasses help
realize different flow rates on the hot and the cold sides of PCHE1.
-
19
Figure 2.1 Layout of the high-temperature helium test facility
-
20
(a)
(b)
Figure 2.2 Photographs of the high-temperature helium test facility: (a) Low-temperature side and (b) High-temperature side
The facility is well instrumented with various sensors. Pressure transducers are installed
for measuring the gage pressure: a) at the inlet of the hot and cold fluid sides of both the
heat exchangers, b) at the upstream location of the three venturi flow meters, c) at the
inlet and exit of the PRV, and d) on the return line of the process chilled water line. In a
similar manner, differential pressure transducers are installed in the facility to measure
the differential pressure: a) across the hot and cold fluid sides of the PCHEs and b) across
the upstream and throat location of the three venturi flow meters. K-type Alloy 800H-
sheathed thermowells are used for measuring the temperature: a) at the inlet and exit of
the hot and cold fluid sides of the heat exchangers, b) at the upstream location of the
Venturi flow meters, c) at the exit of the heaters, and d) at the exit of the cooler. Three
Venturi flow meters measure the volumetric flow rates of helium gas flowing through the
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21
loop. Additionally, two high-temperature flow sensors designed by Delta M Corporation
are installed in the facility for prototype design testing and cross benchmark of the flow
measurements. A turbine flow meter installed on the process chilled water side of the
cooler allows monitoring the flow rate of the process chilled water. The inlet and exit
temperatures of the process chilled water are measured by two RTD temperature sensors.
Along with the information of the flow rate and inlet and exit temperatures of the chilled
water, the rate of the energy being removed by the chilled water can be calculated.
2.2.3 Design Aspects of OSU HTHF
In what follows, the design aspects and features of OSU HTHF are discussed. This
includes material selection for the piping, piping pressure design thickness estimation,
and a simplified piping stress analysis for the design operating conditions.
Candidate Materials for the OSU HTHF
The high-temperature helium test facility must withstand high-temperatures for a
considerable period of time without significant mechanical property degradation and
resist corrosion/oxidation and erosion from the helium coolant. Research grade helium
(99.999% pure) is used in the facility. The leading candidate materials that can withstand
such high temperatures are nickel-based superalloys. Various high-temperature materials
were reviewed for their high-temperature mechanical properties (tensile, creep and creep-
fatigue properties), physical properties (thermal conductivity and thermal expansion),
environmental resistance, fabrication and joining technology, availability and economics
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22
[2.2, 2.6]. This assessment is carried out to identify an appropriate candidate material for
the helium test facility piping and the heat exchangers for operating conditions typical of
VHTRs. Finally, four primary candidate alloys, listed below, are identified and assessed
for use at design temperatures of 800-900oC.
Alloy 617
Alloy 230
Hastelloy X
Alloy 800H
Among these alloys, Alloy 617 is a prime candidate for VHTR structural components,
such as piping, reactor internals, and intermediate heat exchanger (IHX). Table 2.1 lists
the nominal chemical composition of these alloys along with their Unified Numbering
Scheme (UNS) numbers and ASME specifications for seamless pipe and plate [2.4-2.5,
2.7-2.8]. All these alloys are Ni-base superalloys with the exception of 800H, which is
an iron-base superalloy [2.7].
ASME Allowable Design Stresses for Candidate Materials
The material selection for the test facility and heat exchangers is primarily based on
allowable stresses from the ASME Section II, Part D approved for ASME Section VIII,
Division I construction (non-nuclear construction) [2.5]. The design and construction of
OSU HTHF does not conform to ASME Section III, Subsection NH and is, therefore, not
intended for nuclear service. Figure 2.3 compares the ASME allowable design stresses at
different temperatures for the materials listed in Table 2.1 and is applicable for both
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23
seamless pipe and plate product forms. The ASME allowable stresses are based on the
average stress to cause rupture in 100,000-hour operation (about 11.4 years) in air [2.5,
2.9]. The code uses 0.67 times this average stress at each temperature to define the
allowable stress [2.5, 2.9]. The HTHF would be operated for a time much less than 105
hours and it is safe to infer that the test facility design based on allowable stresses for 105
hours of operation is very conservative.
Table 2.1. Nominal Chemical Composition of High-Temperature Alloys [2.4-2.5, 2.7-2.8]
Alloy UNS Number
Product Form Spec No.
Nominal Chemical Composition (wt.%)
617 N06617 Seamless pipe & tube SB-167 52Ni-22Cr-13Co-9Mo Plate, sheet, strip SB-168
230 N06230 Seamless pipe & tube SB-622 57Ni-22Cr-14W-2Mo-La Hastelloy X N06002 Seamless pipe & tube SB-622 47Ni-22Cr-9M0-18Fe
800H N08810 Seamless pipe & tube SB-407 33Ni-42Fe-21Cr
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24
Figure 2.3. ASME allowable design stresses the HTHF candidate materials [2.4, 2.5]
It should be mentioned that among these alloys, only Alloy 800H is ASME Code Section
III (nuclear service) certified for use in applications with temperatures up to 760oC and
that neither Alloy 617 nor Alloy 230 is currently approved for ASME Section III
applications. However, all these three alloys are approved for Section VIII, Division I
construction. As for Alloy HX, only a limited database exists for ASME III applications.
It is however certified for ASME VIII. In light of the above, all the design and
construction pertaining to this facility is based on ASME Code applicable for non-nuclear
service. From Fig. 2.3, it is evident that only alloys 617, 230, and 800H are approved for
temperatures up to 982oC while Alloy HX is approved for temperatures up to 900oC.
Furthermore, on comparing the allowable design stresses (or rupture strengths) for 105
300 400 500 600 700 800 900 10000
20
40
60
80
100
120
140
160
Temperature (oC)
Allo
wab
le S
tress
(MP
a)
Alloy 617Alloy 230Alloy 800HAlloy HX
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25
hours of operation at temperatures greater than 800oC, it is clear that Alloys 617 and 230
are the most suitable materials for high-temperature applications. At these temperatures,
the materials are in creep-rupture failure regime, and as such have a finite life. At the
time when this study on materials was carried out, Alloys 617 and 230 were available
only in plate configuration (and not in tube or pipe configuration), which precluded their
usage for the test facility piping. Therefore, from the considerations of availability and
economics, Alloy 800H was selected for the test facility piping. The selection dictated
the maximum allowable working pressure and temperature of the test facility.
Figure 2.4 provides the rationale for designing the test facility for a maximum allowable
working pressure and temperature of 3 MPa and 800oC, respectively. The test facility
piping size is 1 inch NPS and is based on an economic design velocity of 25 m/s. Noting
the fact that the maximum wall thickness for a commercially available 1 inch NPS Alloy
800H pipe is 6.35 mm (corresponds to a pipe schedule of 160), it can be noted from Fig.
2.4 that the required pressure design thickness (minus the sum of mechanical allowances
and erosion and corrosion allowances) corresponding to 3 MPa and 870oC is close to 6.35
mm. Accounting for erosion plus corrosion allowance of 1 mm, it can be inferred from
Fig. 2.4 the test facility can be safely operated for temperatures and pressures up to 850oC
and 3 MPa, respectively.
However, due to the high-temperature nature of the proposed experiments, the test
facility design temperature was scaled down to 800oC from a safety stand point. The
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26
pressure design thickness required for operating pressures greater than 3 MPa and at
800oC exceeds the commercially available wall thicknesses. In summary, the test facility
is designed and constructed to facilitate experiments at temperatures and pressures up to
800oC and 3 MPa, respectively. The test facility can enable testing at temperatures lower
than 800oC and pressures greater than 3 MPa and the corresponding temperatures and
pressures can be estimated by following a similar approach.
Figure 2.4. Pressure design thickness requirement for a nominal 1 inch, seamless Alloy 800H pipe at different pressures
550 600 650 700 750 800 850 900 950 10000
2
4
6
8
10
12
14
16
18
Temperature (oC)
Pre
ssur
e D
esig
n Th
ickn
ess
(mm
)
P = 1 MPaP = 2 MPaP = 3 MPaP = 3.5 MPaP = 4 MPaP = 5 MPaSch.160 Pipe
6.35 mm
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27
Pressure Design Thickness
The pressure design thickness in Fig. 2.4 was estimated based on the Boardman
expression from ASME B31.3 [2.4]. The minimum required thickness of the pipe mt
including allowances is given by the following expression
mt t c (2.1)
where t is the pressure design thickness in mm; c is the sum of mechanical, corrosion
and erosion allowances and is typically taken as 1 mm. The pressure thickness required
for a pipe is determined by the Boardman expression [2.4] as
2i
i
PDt
SE PY
(2.2)
where iP is the internal design gage pressure in MPa; D is the pipe outer diameter in
mm; S is the pipe material allowable design stress in MPa; E is the quality factor (for a
seamless pipe 1E ), and Y is the stress-temperature compensating factor. The E factor
is an allowable pressure stress penalty based on the method of manufacture of the pipe
[2.4, 2.9]. It reflects the quality of the longitudinal weld in seam-welded pipe and has a
value ranging from 0.6 for furnace butt welded (FBW) to 1.0 for seamless pipe (SMLS).
The Y factor is included to account for the non-linear reduction in allowable stress at
design temperatures above 482oC [2.9]. Other expressions (Lame and Barlow's
Equation) available in ASME B31.3 [2.4] for estimating the pressure design thickness
were used for the purposes of comparison.
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28
2.2.4 Simplified Stress Analysis for Loop Piping Design Calculations
A simplified stress analysis is performed to verify the piping pressure design thickness
calculations performed in the previous section. For thick walled cylinders, the hoop or
tangential stress, h , is calculated as [2.10-2.12]:
2 2 2
2
1
1h
o o oi o
i
o
i
r r rr
rr
P Pr rSE
(2.3)
where ir and or denote the inner and outer radii of the pipe, and iP and oP denote the
uniform internal and external gage pressures, respectively. For the current design, since
the external pressure on the piping is atmospheric, it is reasonable to assume zero
external gage pressure, i.e., 0oP . The maximum tangential stress always occurs on
the inner surface. The required radius ratio so that the maximum tangential stress is less
than or equal to the allowable stress, S , can be calculated from Eq. (2.3) by replacing r
with ir . When the internal pressure exceeds the external pressure, the limiting ratio is
given by
.2
i
i
o
oi
S PS P P
r
r
(2.4)
The thickness-to-diameter ratio can be expressed as,
2
o i
i
r rtd r
(2.5)
For the current design with a nominal line size of 1 inch and a pipe schedule of 160 and
using the allowable design stress value of 9.68 MPa [2.4, 2.5] for Alloy 800H at the
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29
design temperature of 850oC, we have the following values for the required and available
thickness-to-diameter ratios.
required
1.39 0.22
i
o i
tS PS P P d
(2.6)
and, available
0.31.6oi
r
r
td
(2.7)
From Eq. (2.6) and Eq. (2.7), we have the following:
oravailable required
2i
o i
o
i
r
r
S PS P P
t td d
(2.8)
It can be concluded from the above analysis that it is an acceptable mechanical design
and the test facility can be safely operated at pressures and temperatures of 3 MPa and
850oC, respectively. However, as mentioned earlier, the test facility design temperature
was scaled down to 800oC to make the design more conservative. Furthermore, the
allowable stresses (creep-rupture strength) are based on 105 hours of operation at the
respective temperature, which is far greater than the time this test facility would be
operated. Therefore, it is a very conservative and safe design.
2.2.5 High-Temperature Helium Test Facility Components
The design of the test facility components was primarily dictated by the high operating
temperatures, availability, and economics. A case in point would be the high-temperature
valves. A high-temperature valve rated to at least 800oC and 3 MPa at the exit of the
main heater or before the hot inlet of the second heat exchanger would provide more
flexibility in terms of the test facility operation but was not included in the design due to
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30
economics. Operating within the available budget and at the same time ensuring the
integrity of the facility components for use at high temperatures and pressures is a
challenge. The design process became iterative in that it involved repeatedly modifying
the design to fit the component/material availability and budget. In the following
subsections, the principal components of the OSU HTHF are discussed.
Electric Heaters and Power Controllers
Kanthals FIBROTHAL standard RAC tube modules [2.13] were employed as heating
source for heating the helium flowing through the test facility piping and its components.
Figure 2.5 shows a Kanthal FIBROTHAL RAC tube heater with the embedded heating
element and the ceramic fiber insulation. Table 2.2 lists the electrical specifications of
the FIBROTHAL RAC tube heater [2.13]. Each FIBROTHAL heater consists of
vacuum-formed ceramic fiber components with radiating heating element embedded into
it. The embedded heating element is a Kanthal Grade A-1 heating element with a
nominal chemical composition (% by wt.) of 22% Cr, 5.8% Al, and balance Fe. The
heating elements are designed for a maximum element temperature of 1300oC.
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31
Figure 2.5. Kanthal RAC Fibrothal tube heater with embedded heating element [2.13]
Table 2.2. Electrical characteristics of the heaters [2.13]
Heater
Voltage (V) Power (W)
at 60 A
Voltage (V) Power (W)
at 72 A
Voltage (V) Power (W)
at 85 A
Resistance R20 (Ohm)
RAC 70/500
63.1 3786
75.8 5454
89.5 7608
1.008
As mentioned earlier, the heating system in the HTHF comprises of a pre-heater and a
main-heater. The pre-heater and the main-heater are each a combination of three heaters
wired in a 3-phase delta configuration with each individual heater having a maximum
heating capacity of 7.6 kW. Therefore, a total of six heaters with a total maximum
heating capacity of about 46 kW act as the heating source to deliver helium to the
PCHEs.
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32
Two phase angle type SCR (Silicon Controlled Rectifier) power control units, one for the
pre-heater and the other for the main-heater, control the amount of power input to the
heaters. Two Acuvim II multifunction digital power meters continuously meter and
monitor the current and voltage data fed to the heaters with an accuracy of 0.2% of the
reading. In addition, two Honeywell UDC 2500 temperature controllers with a
thermocouple input and a 4-20 mA output, monitor and control the fluid temperature at
the exit of the heaters by providing continuous feedback to the SCR controllers and help
in realizing the desired heater outlet temperatures. Furthermore, six microprocessor-
based UDC 1200 limit controllers with a thermocouple input and a relay output that is
continuously fed to the power controller provide necessary safety by preventing
overheating of the heating elements. Table 2.3 lists the general design specifications of
the pre-heater and the main heater.
Table 2.3. Heater design specifications
Specification Pre-Heater Main Heater Nominal inlet
Temperature range (oC) 70-100 400-650
Maximum outlet temperature (oC) 350 850 Maximum mass flow rate (kg/h) 45 45
Working Fluid (Gas) He He Power (kW) 23 23
Cooler
The cooler is a tube-in-tube heat exchanger and uses process chilled water (PCW) as a
cooling medium to cool the hot helium gas (working fluid) exiting from the hot side of
PCHE1. The hot helium flows through the inner tube while the water flows in the
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33
annular region. The working fluid is cooled to the inlet temperature of the gas booster,
rated for a maximum inlet temperature of 100oC. Figure 2.6 shows the schematic of the
cooler installed in the HTHF. The specifications of the cooler are tabulated in Table 2.4.
A turbine flow meter is installed on the PCW line to measure the volumetric flow rate of
the cooling water. Two RTD sensors, at the inlet and exit of the PCW line, facilitate the
measurement of the inlet and exit temperatures of the process chilled water.
Figure 2.6. Cooler for cooling helium gas
The inner tube and the outer annulus of the cooler is constructed using SS316. The inner
tube through which hot helium flows is a 1/2 in. (12.7 mm) diameter tube with a wall
thickness of 0.065 in. (1.65 mm). The wall thickness of the inner tube through which hot
helium flows should be sufficient to be able to withstand temperatures up to 450oC
without compromising its integrity. For the purposes of the design, the maximum
operating pressure and temperature are taken as 510 psig and 450oC, respectively. The
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maximum allowable stress for SS316 at this temperature is 12.7 kpsi [2.4]. The pressure
design thickness required for the inner tube of the cooler is estimated from Eq. (2.2) as
0.41 mm. The required pressure design thickness is less than the tube wall thickness of
1.65 mm and hence the cooler can be safely operated without compromising its integrity.
Table 2.4. Cooler design specifications
Parameter Inner Tube Outer Tube Outside Diameter (inch) 1
Fluid Type Helium Water (PCW) Inlet Temperature (oC) 450 20
Outlet Temperature (oC) 100 27 Mass Flow (kg/s) 0.022 1.382
Volumetric Flow (Lpm) 389.9 83.28 Pressure Drop (kPa) 82.26 93.32 Heat Transfer (kW) 40.4
Effectiveness 0.81
Gas Booster
The purpose of the gas booster is to provide the driving head for circulating helium gas in
the test facility piping and its components. Initially, a compressor was used in the design
but was not adopted due to its higher cost. Currently, one gas booster has been installed
in the HTHF. A gas booster differs from the compressor in that it is air-driven and
requires no electrical motive force. Haskels 8AGD-2.8 model gas booster [2.13], shown
in Fig. 2.7, was selected for the HTHF. This is a single stage, double acting, high flow,
air driven, and reciprocating piston type non-lube-oil free gas booster. Table 2.5 lists the
specifications of 8AGD-2.8 gas booster [2.14].
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A gas booster essentially consists of an air drive section (drive cylinder) and a gas barrel
section (boost cylinder) isolated from each other by appropriate seals. The piston in the
drive cylinder is attached to the piston in the boost cylinder. As the drive piston
reciprocates, it compresses the gas in the boost cylinder. The boost cylinder is double-
acting, i.e., it pulls gas on one side while pumping it out on the other side. The maximum
pressure boost is equal to the drive piston area divided by the boost piston area multiplied
by the pressure feeding the drive cylinder. In other words,
do a sb
AP P P
A (2.9)
where Ad, Ab, Po, Pa, and Ps are the drive piston area, boost piston area, gas outlet
pressure, drive pressure, and gas supply pressure, respectively. For the gas booster
8AGD-2.8, the approximate area ratio of the air drive piston area to gas piston area is 2.8.
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Figure 2.7. Haskel 8AGD-2.8 gas booster [2.14]
Table 2.5. Haskel 8AGD-2.8 gas booster specifications [2.14]
Maximum Rated Gas Supply (psig) 800 Maximum Rated Gas Outlet (psig) 800
Static Outlet (Stall) Pressure Formula 2.8a sP P
Piston Displacement (in3/cycle) 125 Minimum Inlet Gas Pressure (psig) 100
Maximum Outlet Gas Pressure (psig) 800 Maximum Air Drive Pressure (psig) 130
Booster Cycling Rate (cycles/min) for continuous operation 60 Maximum Compression Ratio 25:1
The flow rate and discharge pressure of helium exiting the booster can be controlled by
throttling the drive air flow rate and/or regulating the drive air pressure. The leakage rate
from the gas booster is 0.1 SCFH (Standard Cubic Feet per Hour).
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Pressure Reducing Regulator/Valve
A pressure reducing regulator/valve was installed at the exit of the 5-gallon tank located
downstream of the gas booster. It is an air-loaded regulator and its function is to
maintain and control the outlet pressure within limits as other conditions vary and ensure
a stable flow of helium in the test facility. The specifications of the PRV are listed in
Table 2.6.
Table 2.6. Specifications of the pressure reducing regulator
Loading Mechanism Air Actuated, Non-Venting Approx. Air Load to Output
Ratio 6.25:1
Material of Construction SS316 Maximum Inlet Pressure
(psig) 600
Outlet Pressure (psig) 0-500 Temperature Rating (oC) -26-150
During initial experiments prior to the installation of the PRV, helium flow oscillations
with more than 30% variation were noticed during operation and were very
unpredictable. It was noticed that the helium flow oscillations were caused by the gas
booster operation in that it is designed to cycle at variable rates based on the desired
outlet pressure at the exit of the booster. The PRV helped smoothen the pressure spikes
and the oscillations in the helium flow by providing a nearly constant helium pressure
downstream of the regulator and therefore ensured a stable helium flow in the test facility
piping. The flow is now very stable with flow variations less than 1%.
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Vacuum Pump
A DuraVaneHV RVR002H high vacuum pump has been installed on the low temperature
side of the test facility near the helium charging line. Before every experimental run, the
facility is vacuumed to -14 psig to minimize the amount of air in the helium working
fluid. After sufficient vacuum is obtained, the facility is charged with the working fluid
to the desired pressure.
Working Fluid
Helium gas is employed as the working fluid for performance testing of the PCHEs in the
HTHF. High purity research grade helium (99.999% pure) has been used for the
experiments in the HTHF.
High-temperature Valves
Six high-temperature SS316 needle valves provided by Swagelok, Inc., have been
installed in the HTHF. These valves are rated for temperatures up to 650oC and utilize a
high-temperature Grafoil packing as a seal. The valves are installed at locations in the
facility that do not experience temperatures greater than 650oC, the maximum
temperature rating of the valves. The valves have however been a primary source of leak
at high temperatures. The leak is primarily attributed to the drying to nickel anti-seize
used during the valve assembly. This resulted in the failure of the packing material
leading to loss of helium through leakage. As a temporary solution, the valve packing
material was replaced whenever the valve failed.
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2.2.6 Instrumentation
Controls for the Pre-Heater and Main Heater
As mentioned earlier, the heating system employed in the HTHF comprises of a pre-
heater and a main heater. Two phase angle type Silicon Controlled Rectifier (SCR)
power control units, one for the pre-heater and other for the main heater, control the
amount of power input to the heaters. Two Acuvim II multifunction digital power meters
continuously meter and monitor the current and voltage data fed to the heaters with an
accuracy of 0.2% of the reading. In addition, six microprocessor-based UDC 1200 limit
controllers with a thermocouple input ( from the thermocouples located near the heating
elements) and a relay output that is continuously fed to the power controller provide
necessary safety by preventing overheating of the heater elements. Furthermore, two
Honeywell UDC 2500 temperature controllers with a thermocouple input and a 4-20 mA
output monitor and control the fluid temperature at the exit of the heaters by providing
continuous feedback to the SCR controllers and help realize the desired outlet
temperatures.
Temperature Sensors
The temperature sensors provided by Weed Instruments are socket-weld standard-duty
type thermowells of Alloy 800H construction and house ASME special tolerance K-type
thermocouples. Eleven such thermowells are used for the measurement of helium
temperature at various locations in the facility. The thermocouples used in the facility
have special tolerance with an accuracy of 1.1oC or 0.4% (whichever is greater) in the
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measurement range of 0 to 900oC. The thermocouples were calibrated by comparison
technique with a standard platinum resistance thermometer (SPRT) having traceability to
ITS-90. The calibration was performed with a thermocouple furnace in accordance with
ASTM E220-86 [2.15]. The maximum uncertainty of the SPRT used for calibration is
0.03oC. In addition, two ultra precise 4-wire RTD sensors with 1/10 DIN accuracy
(0.012oC) are installed in the facility to measure the inlet and exit temperatures in the
process chilled water line.
Pressure and Differential Pressure Sensors
Honeywell ST3000 smart pressure transducers with a 4-20 mA DC output are used for
measuring the helium pressures and differential pressures at/across different locations in
the facility. To measure the pressure or pressure differential in the loop at temperatures
above the operating range of transducers, the pressure transducer is isolated from the
pressure source by a long length of coiled tubing so that the helium temperature in the
sensing line (tubing) at the transducer is sufficiently reduced. All the pressure and
differential pressure transducers have been calibrated using standards whose accuracies
are traceable to NIST (National Institute of Standards and Technology). The accuracy of
the pressure transmitters are 0.375 psig. The accuracy of the differential pressure
transducers are 0.075% of calibrated span or upper range value (URV), whichever is
greater.
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Flow Meters
Three Venturi type flow meters measure the volumetric flow rates of helium gas flowing
through the facility. The flow meters have calibration information traceable to NIST
standards. Additionally, two high-temperature flow sensors designed by Delta M
Corporation are installed in the test loop for prototype design testing and cross
benchmark of the flow measurements against the Venturi flow meters. Furthermore, a
turbine flow meter installed on the process chilled water-side of the cooler allows
monitoring the flow rate of the process chilled water with an accuracy of 0.11% and has
calibration information traceable to NIST standard as well.
Data Acquisition System
All data acquisition (DAQ) and process control tasks are managed by a PC executing
LabView 8.5 under Windows XP. The DAQ system consists of NI compact DAQ
chassis and five NI 9211 modules, one NI 9205 module, and one NI 9217 module. The
NI 9211 module is a 24-bit, 4-channel thermocouple input module; NI 9205 is a 10 V,
16-bit, 32-channel single-ended or a 16-channel differential analog input module; and NI
9217 is a 4-channel, 24-bit, 100 RTD analog input module. A DC excitation power
supply is used to power all gage pressure and differential pressure transducers.
2.2.7 Quality Assurance
The Thermal Hydraulics Laboratory (THL) at OSU has a QA procedure in place that is
consistent with the QA guidelines provided by the U.S. Department of Energy. Personnel
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performing research on OSU HTHF were trained in the areas of testing and data
collection as per the QA program requirements to ensure that the produced data are
acceptable. Furthermore, the operation pr