PPeerrffoorrmmaannccee aanndd TThheerrmmoo-MMeecchhaanniiccaall CCoosstt EEvvaalluuaattiioonn ooff AAPPII 666611 AAiirr-CCoooolleedd HHeeaatt Exchangers by Mogamat Sadley Ackers Thesis ppprrreeessseeennnttteeeddd iiinnn fffuuulllfffiiilllmmmeeennnttt ooofff ttthhheee rrreeeqqquuuiiirrreeemmmeeennntttsss fffooorrr ttthhheee dddeeegggrrreeeeee of MMaasstteerr ooff EEnnggiinneeeerriinngg ((MMeecchhaanniiccaall)) in the Faculty of EEnnggiinneeeerriinngg at Stellenbosch University Supervisor: Prof. Hanno Carl Rudolf Reuter December 2012 Stellenbosch University http://scholar.sun.ac.za Declaration By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellenbosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification. Signature: …………………………. Date: .............................................. Copyright © 2012 Stellenbosch University All rights reserved i Stellenbosch University http://scholar.sun.ac.za Abstract The optimal design of a heat exchanger for a specified heat transfer, pressure drop and set of ambient conditions entails minimising space, weight, material usage and overall cost. However, the variables which influence the performance as well as the overall cost of a heat exchanger are not related in a simple way and it is not obvious which variables play the most important roles (Perry & Green, 1997:11-44). Air cooled heat exchangers (ACHEs) are normally designed in three stages, by different experts in the field, and with the aid of specially designed software. This project combines these thermal, mechanical and cost estimation processes into a MS Excel model which makes it easier to see the influence that design parameters have on the overall cost of the heat exchanger. A thermal model was created to design an API 661 (2006) ACHE. The results from this model compared well with those of HTRI Xchanger Suite 6.0 software, with HTRI being more conservative in its design mode. A mechanical design model was then developed, which uses as inputs the outputs of the thermal design. The output from this mechanical design model is the minimum material thicknesses based on the stress criteria of Appendix 13 of ASME VIII div 1 (2007) Boiler and Pressure Vessel Code. An experiment on a finned tube bundle was performed in a wind tunnel facility to determine performance characteristics and compare these to existing correlations in literature. The results showed that both the heat transfer coefficient (h) and loss coefficient (Eu) correlations proposed by Ganguli et al. (1985) closely predict the measured data, and were consequently used in the thermal design model. During this experiment it was also shown that the tube bundle reached 8 % - 9 % of its allowable internal fouling factor, due to rust build up inside the tubes, and in a testing period of only nine days. The thermal and mechanical models were then combined with a cost estimation process to perform both a thermal and mechanical parametric study. The thermal study showed that to obtain an optimal solution, the design must attempt to maximise the length, increase the width rather than the number of bays, make use of two bundles per bay with fewer but larger fans and employ a large number of tube rows with the least number of tube passes. These guidelines were used to create an initial design; Excel Solver was then applied to locate the optimum combination of bundle length and width that result in the minimum heat exchanger cost. ii Stellenbosch University http://scholar.sun.ac.za Two mechanical considerations were investigated, both requiring additional welding and thus increased welding cost. Firstly the use of stay plates result in reduced required plate thicknesses according to the stress criteria since it provides additional stiffness in the header box design. Secondly the use of more (but smaller) nozzles as opposed to less (but larger) nozzles was also considered. The mechanical parametric study showed no specific trends, but both considerations should still be checked as it can be cost beneficial in a specific design. iii Stellenbosch University http://scholar.sun.ac.za Opsomming Optimale ontwerp van ’n warmteoordraer vir ’n gespesifiseerde warmteoordrag, drukval en stel van omgewingstoestande behels die minimalisering van ruimte, gewig, materiaalverbruik en algehele koste. Die veranderlikes wat egter die werkverrigting, sowel as die algehele koste, van ’n warmteoordraer beïnvloed, hou nie in ’n eenvoudige sin met mekaar verband nie, en dit is nie vanselfsprekend watter veranderlikes die belangrikste rolle speel nie (Perry & Green, 1997:11-44). Lugverkoelde warmteoordraers (air-cooled heat exchangers of ACHEs) word normaalweg in drie fases ontwerp deur verskillende kundiges in die veld en met behulp van spesiaal ontwerpte programmatuur. Hierdie studie kombineer dié termiese, meganiese en kosteberamingsprosesse in ’n MS Excel-model, wat dit makliker maak om van te stel wat die invloed wat ontwerpparameters op die algehele koste van die hitteruiler is. ’n Termiese model is geskep om ’n “API 661 (2006) ACHE” te ontwerp. Die resultate van hierdie model het goed vergelyk met dié van die HTRI Xchanger Suite 6.0-program, met HTRI meer konserwatiew in die ontwerp af. Na die termiese model geskep is, is ’n meganieseontwerp-model ontwikkel, wat as insette die uitsette van die termiese ontwerp gebruik het. Die uitset van hierdie meganieseontwerp-model is die minimum materiaaldikte gebaseer op die spanningskriteria van Bylae 13 van “ASME VIII div 1 (2007) Boiler and Pressure Vessel Code.” Daar is ’n eksperiment op ’n vinbuisbundel in ’n windtonnelfasiliteit uitgevoer om werkverrigtingskarakteristieke te bepaal en dit met bestaande korrelasies in die literatuur te vergelyk. Die resultate het getoon dat sowel die warmteoordragskoëffisiënt (h) en die verlieskoeffisient (Eu) korrelasies, voorgestel deur Ganguli et al. (1985), die data wat gemeet is akkuraat voorspel, en gevolglik is die korrelasies in die termieseontwerp-model gebruik. Gedurende die eksperiment is ook getoon dat die buisbundel 8 % - 9 % van sy toelaatbare interne-aanpakkingfaktor bereik het vanweë roesopbou binne-in die buise, en dit in ’n toetsingtydperk van slegs nege dae. Die termiese en meganiese modelle was toe gekombineer met ’n kosteberamingsproses om ’n termiese sowel as ’n meganiese parametriese studie uit te voer. Die termiese studie het getoon dat, om ’n optimale oplossing te verkry, die ontwerp moet poog om die lengte te maksimeer; die wydte eerder as die aantal strate (bays) te vermeerder; van twee bundels per straat gebruik te maak met minder, maar groter waaiers; en ’n groot aantal buisrye met die kleinste hoeveelheid iv Stellenbosch University http://scholar.sun.ac.za buisdeurvloeiweë in te span. Hierdie riglyne is gebruik in ’n aanvanklike ontwerp, waarna die Excel Solver gebruik is om die optimale kombinasie van bundellengte en –wydte vas te stel met die oog op die laagste moontlike warmteoordraerkoste. Twee meganiese oorwegings is ondersoek wat albei addisionele sweiswerk sou vereis en dus tot verhoogde sweiskoste sou lei. Eerstens lei die gebruik van ankerplate (stay plates) tot ’n vermindering in die vereiste plaatdiktes volgens die spanningskriteria, aangesien dit addisionele stewigheid in die spruitstukhouerontwerp bied. Tweedens is die gebruik van meer (maar kleiner) spuitstukke teenoor minder (maar groter) spuitstukke ook oorweeg. Die meganiese parametriese studie het geen spesifieke voorkeurneigings getoon nie, maar altwee oorgewings moet nog getoets word want dit kan koste voordelig word in 'n spesifieke ontwerp. v Stellenbosch University http://scholar.sun.ac.za Acknowledgements I would like to express my sincerest gratitude to the following people/organisations for their contribution towards making this study possible: • Prof H.C.R Reuter for his support, knowledge and guidance. • My family and friends for their support, patience and encouragement. • GEA Aircooled systems (Pty) Ltd for the funding and information applicable to this project. • To Mr C Zietsmann and Julian Stanfliet for their assistance with the finned tube performance testing in the wind tunnel laboratory. vi Stellenbosch University http://scholar.sun.ac.za Table of contents Declaration ..................................................................................................................... i Abstract ......................................................................................................................... ii Opsomming .................................................................................................................. iv Acknowledgements ...................................................................................................... vi Table of contents ......................................................................................................... vii List of figures ................................................................................................................ x List of tables ................................................................................................................ xii Nomenclature ............................................................................................................. xiv 1. Introduction ........................................................................................................... 1 1.1. Background .................................................................................................... 1 1.2. Motivation ...................................................................................................... 6 1.3. Objectives ....................................................................................................... 6 1.4. Thesis outline ................................................................................................. 7 2. Thermal design according to API 661 (2006) ...................................................... 7 2.1. Introduction .................................................................................................... 7 2.2. System description ......................................................................................... 8 2.3. Thermal model ............................................................................................... 9 2.3.1. Geometry .............................................................................................. 10 2.3.2. Draft equation ....................................................................................... 14 2.3.3. Fan characteristics ................................................................................. 17 2.3.4. Water side pressure drop ....................................................................... 18 2.4. Thermal model ............................................................................................. 20 2.4.1. Thermal model algorithm explained ..................................................... 20 2.4.2. Thermal rating mode ............................................................................. 23 2.4.3. Thermal model design mode ................................................................. 24 3. Mechanical design of an air-cooled heat exchanger header box according to ASME VIII Div 1 (2007) Addenda 2009b ........................................................ 26 vii Stellenbosch University http://scholar.sun.ac.za 3.1. Introduction .................................................................................................. 26 3.2. Mechanical design procedure ....................................................................... 26 3.2.1. Design Inputs and outputs ..................................................................... 27 3.2.2. Vessel configuration ............................................................................. 29 3.2.3. Overall dimensions ............................................................................... 30 3.2.4. Corrosion allowance (Ca) ..................................................................... 30 3.2.5. Corroded condition ............................................................................... 35 3.2.6. Header design ....................................................................................... 36 3.2.7. Nozzle design ........................................................................................ 42 3.2.8. Tube-to-tubesheet welds ....................................................................... 44 4. Finned tube bundle performance characteristics ................................................ 48 4.1. Introduction .................................................................................................. 48 4.2. Literature review .......................................................................................... 48 4.3. Description of test facility ............................................................................ 49 4.4. Measurement devices and techniques .......................................................... 52 4.4.1. Temperatures ........................................................................................ 52 4.4.2. Pressures ............................................................................................... 53 4.4.3. User interface ........................................................................................ 53 4.4.4. Mass flow rates ..................................................................................... 54 4.5. Test procedure .............................................................................................. 55 4.6. Data Processing ............................................................................................ 56 4.7. Results .......................................................................................................... 57 4.7.1. Isothermal test ....................................................................................... 57 4.7.2. Energy balance ...................................................................................... 57 4.7.3. Row effect ............................................................................................. 58 4.7.4. Heat transfer coefficient ........................................................................ 59 4.7.5. Pressure drop ......................................................................................... 60 4.7.6. Rust effect ............................................................................................. 61 viii Stellenbosch University http://scholar.sun.ac.za 4.8. Summary of results and conclusion ............................................................. 64 5. Parametric study ................................................................................................. 65 5.1. Cost estimation ............................................................................................. 65 5.2. Parametric study procedure .......................................................................... 66 5.3. Results of thermal parametric study ............................................................. 67 5.3.1. Length versus width .............................................................................. 67 5.3.2. Length versus number of bays .............................................................. 67 5.3.3. Width versus number of bays ............................................................... 68 5.3.4. Fans per bay versus fan diameter .......................................................... 68 5.3.5. Number of bundles per bay versus number of bays .............................. 68 5.3.6. Passes versus width ............................................................................... 69 5.4. Cumulative thermal parametric study .......................................................... 69 5.4.1. Case 1 .................................................................................................... 69 5.4.2. Case 2 .................................................................................................... 70 5.5. Excel solver .................................................................................................. 70 5.6. Results of mechanical parametric study ....................................................... 72 5.6.1. Nozzles .................................................................................................. 72 5.6.2. Stay plates ............................................................................................. 73 5.7. Cost breakdown ............................................................................................ 73 6. Conclusions and recommendations .................................................................... 75 6.1. Conclusions .................................................................................................. 75 6.2. Recommendations ........................................................................................ 77 7. References ........................................................................................................... 78 Appendix A – Properties of fluids .............................................................................. 80 Appendix B - Thermal design sample calculation ...................................................... 83 Appendix C - Mechanical design sample calculation ................................................. 98 Appendix D – Measured data and results of wind tunnel experiment ...................... 108 Appendix E - Wind tunnel experiment sample calculation ...................................... 116 ix