HEAT EXCHANGER DESIGN

Transcription

1 ABOVE GROUND GEOTHERMAL ALLIED TECHNOLOGIES HEAT EXCHANGER DESIGN FOR MATERIALS RESEARCH Holger Heinzel HERA NZWC

2 Technologically Advanced Low Enthalpy Conversion Systems Knowledge Base Project context Expert design tool Material Knowledge Base Research Understanding and Modelling Scaling Mechanism Heat Transfer Performance Data Expander Technology Research Material Knowledge Base Research Research team - HERA Welding Centre - University of Canterbury Timeframe 01/10/ /10/2016 Control Technology Research Standardised System Concepts Heat Exchanger Technology Development Turbo-Machinery Technology Development Control Systems Development

3 Research Aim What material performs best for any given application in the AGGAT environment? Objectives Identification and characterisation of standard and novel materials and surface modifications for components within an ORC plant Built up industry capability to manufacture and deliver equipment and sample materials required for the research and consulting services. Performance parameters Corrosion performance scaling heat transfer thermal and corrosion fatigue ability to fabricate economic sustainability

4 Material selection Goal: best performance at minimal life cycle cost Required: performance criteria for components in AGGAT environment Pathway: Identify material solutions through research and testing In geothermal binary plant: Heat exchanger main challenge: geothermal brine organic medium

5 Common problems Fouling and Scaling is the accumulation of unwanted material on solid surfaces to the detriment of function Fouling caused by coarse matter Scaling crystallization of solid salts, oxides and hydroxides Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment

6 Influencing factors / Effects Factors influencing corrosion and scaling ph Temperature Velocity of flow Pressure Microbial growth Suspended Solid Material and Deposits Effects on corrosion/scaling on Heat exchanger Reduced (thermal) efficiency Reduced flow Induced under-deposit corrosion Increased use of cooling water may induce vibrations Turbines Reduced efficiency Increased probability of failure Minimize fouling and corrosion Selection of low corrosive material Specification of surface condition Selection of coating

7 Primary fluids Chemical composition of geothermal brines (worldwide incl. NZ) Country Name Type degc ph Li Na K Rb Cs Mg Ca B HCO 3 SiO 2 SO 4 Cl - Seawater < Colombia Ruiz acid spring Colombia Ruiz neutral spring Guatemala Zunil well Mexico Araro spring NZ Maui well NZ Morere spring < NZ Ngawha spring NZ Ngawha well NZ Wairakei spring NZ Wairakei well NZ Waitangi Soda spring NZ White Island spring < Solomon Is. Paraso spring Vanuatu Yasur spring min avg max Each location poses a challenge in its own rights Highly variable

8 Material solutions Material selection Plan carbon / low alloy steels Stainless steel Ti and Ti alloys Nickel based alloys Copper alloys Tantalum & Zirconium Al and Al alloys Fibre reinforced materials Coatings Epoxy coatings Ceramic filled Polymer coatings Phenolic resin Inorganic and composite coatings Metal coatings Manufacturing option Pipe welded from narrow strip material

9 Material test facility Test material performance under conditions similar to ORC plant Chemical composition of brine Physical conditions (Temp, pressure) Flow conditions

10 Design objectives Test material performance under conditions similar to ORC plant Chemical composition of brine // Physical conditions //Flow conditions Replicate standard HX design Standard material dimensions Standard material shapes Cooling of brine to less than 80 C - arsenic or antimony sulphide scaling Allow different materials to be tested simultaneously

11 Test rig: 1 st test site Wairakei Geothermal Field Geothermal brine Temperature C 135 Pressure bar 4-5 Chemistry ph 18 ºC Barium mg/l Boron mg/l 25 Bromide mg/l 4.7 Calcium mg/l 16.8 Chloride mg/l 1850 Potassium mg/l 184 Silica (as SiO2) mg/l 559 Sodium mg/l 1130 Sulphate mg/l 39 Antimony (Screen level) mg/l 0.11 Arsenic (Screen level) mg/l 4.3 Cooling water Type Grey water Temperature C enviro

12 Geothermal test rigs Salton Sea, USA Gross Schoenebeck, Germany Soultz-sous-Forets, France Mammoth, USA

13 HX types Type of heat exchangers Shell and Tube / Plate / U-tubes Tube arrangements Straight / U-tubes Flow arrangements Counter flow / parallel flow / cross flow Straight tubes Plate heat exchanger

14 HX calculations I Calculation steps Fluid temperatures, fluid properties, geometry Reynolds numbers Nusselt numbers Heat transfer coefficients Temperature drop Simplifications Single pipe Heat transfer coefficient constant over tube length Fluid properties of brine similar to normal water No axial heat transfer over tube length Mathcad Express Excel sheet Iterative process

15 HX calculations II Shell and Tube HX with baffles Cross- and Counter-flow zones Half HX-model (symmetric) Sectioning into finite volumes Separate wall-temperature calculation in each baffle area GNU-Octave 4 temperature matrices: Geothermal brine Cooling fluid Wall temperature tube-side Wall temperature shell-side

16 Outlet temperature of brine [degc] HX Calculation results Example: Results for max brine flow rates Length of tube: Length of tube [m] Temperature of geothermal brine Temperature of cooling fluid

17 Material test rig Shell and Tube Heat exchanger (small scale) Single pass of hot brine Vertical arrangement Brine in tubes Cooling water in shell

18 Test rig: Instrumentation Adjustment of flow(s) through HX Monitoring and Recording of Process Data Pressure In and Out Temperature Hot/cold side In /Out Flow Hot/cold side In /Out

19 Summary Customized field test rig designed to investigate materials performance in the AGGAT environment Comparative analysis of 19 tubes of different materials Design optimized for increased likelihood of scaling Results will benefit design of AGGAT components