Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH 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. 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. / AFRIKAANSE 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 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71673 |
| Date | 12 1900 |
| Creators | Ackers, Mogamat Sadley |
| Contributors | Reuter, H. C. R., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 123 p. : ill. |
| Rights | Stellenbosch University |
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