Thesis (MScIng)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: An investigation was made into the use of pre-cooling of air with evaporative cooling
as a means of improving the performance of air-cooled heat exchangers (finned tube)
under conditions of ambient temperatures above maximum design values and during
times of increased load. A review of previous research on this subject indicated that
the concept is theoretically sound, but that practical application thereof is still limited.
It was found that one of the major areas of concern is the wetting of the heat
exchanger finned surface and subsequent corrosion.
Mathematical models were derived for the behavior of liquid droplets in free air
stream conditions and droplets that have penetrated a laminar hydrodynamic boundary
layer formed on a flat plate. These two models were combined to determine the
behavior of a liquid droplet for its entire lifetime. It was found that evaporation of
droplets in a boundary layer resulted in major improvements in heat transfer. In an
attempt to prevent droplets from impacting and wetting the finned tube heat
exchanger, the use of electrostatically charged water spray was investigated.
Experiments were performed to determine the charging performance of a capacitive
electrostatic nozzle. It was found that this type of nozzle successfully charged droplets
in a spray. Experiments were then performed whereby electrostatic spray was sprayed
on to a heat exchanger with a similar electric charge as the droplets. It was found that
droplet deposition decreased significantly as the charge on the droplets was increased.
However, total prevention of deposition could not be achieved, since the equipment
used could not produce high enough voltages. This concept shows some promise, and
it is recommended that further research be performed on it. At this stage, no reliable
method of evaporative precooling of air has yet been found. The only viable option
for cooling capacity shortages at present is the construction of large air-cooled heat
exchangers or the addition of wet cooling towers. / AFRIKAANSE OPSOMMING: Voorverkoeling van lug deur middel van verdampingsverkoeling vir die gebruik met
droë lug-verkoelde vinbuis warmteruilers is ondersoek as ‘n manier om die
verkoelingseffektiwiteit te verhoog gedurende tye van hoë omgewingstemperature en
verhoogde las. ‘n Ondersoek van navorsing op die gebied het getoon dat die konsep in
teorie moontlik is, maar dat daar nog geen praktiese implementering plaasgevind het
nie. Dit blyk dat die benatting en korrosie van die vinbuise een van die hoof probleme
is.
Wiskundige modelle is afgelei vir die gedrag van ‘n water druppel in ‘n vrye lug
stroom en vir ‘n druppel in ‘n laminêre hidrodinamiese grenslaag op ‘n plat plaat. Die
twee modelle is gekombineer om die gedrag van ‘n druppel gedurende sy totale
leeftyd te bepaal. Die model het getoon dat verdamping van ‘n druppel in die
grenslaag ‘n drastiese verhoging in die hitteoordrag koëffisiënt veroorsaak. Die
gebruik van elektrostaties gelaaide sproei om te verseker dat die vinbuis warmteruiler
droog bly is ondersoek. Eksperimente is uitgevoer om die elektriese laaivermoëns van
‘n kapasitiewe elektrostatiese mondstuk te bepaal. Daar is gevind dat die tipe
mondstuk suksesvol is in die laai van druppels. Toetse is toe uitgevoer waartydens
gelaaide druppels gespuit is op ‘n warmteruiler met dieselfde lading as die druppels.
Daar is gevind dat die duppel neerslag op die warmteruiler merkwaardig afneem
namate die lading op die druppels verhoog is. Die warmteruiler kon egter nie totaal
droog gehou word nie, aangesien die toerusting gebruik vir die toetse nie ‘n hoë
genoeg spanning kon gee nie. Hierdie konsep is belowend, en dit word aanbeveel dat
verdere navorsing daarop gedoen word. Op hierdie stadium is daar nog geen
betroubare metode gevind om die verkoelings effektiwiteit van lugverkoelde
warmteruilers met verdampings verkoeling te bewerkstellig nie. Die enigste sinvolle
opsie tans is die kostruksie van groter lugverkoelde warmteruilers of die konstruksie
van nat koeltorings.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/16321 |
| Date | 12 1900 |
| Creators | Esterhuyse, Bernard de Waal |
| Contributors | Kroger, D. G., University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | xiv, 141 leaves : ill. |
| Rights | University of Stellenbosch |
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