The effect of plenum configuration on the performance of air-cooled heat exchangers

Salta, Carlos Antonio (1995-01)

Thesis (MEng)--Stellenbosch University, 1995.

Thesis

ENGLISH ABSTRACT: The main objective of this study is to determine the effect of plenum configuration on the performance of an air-cooled heat exchanger (particularly the fan system's operating point). Various plenum configurations, ranging from simple box-shaped plenums to plenums incorporating round-to-rectangular transition pieces, are tested for a normal flow heat exchanger, a delta heat exchanger and an experimental compact heat exchanger referred to as a double delta heat exchanger. The performance of the various heat exchangers relative to each other is also evaluated. The most important conclusions are as follows : The plenum losses (and hence the fan system performance) of a normal flow heat exchanger having a box-shaped plenum are not very sensitive to plenum length. However, a minimum plenum length of 0.4 fan diameters is advised. The plenum losses occurring in the abovementioned fan system result in approximately 28% of the kinetic energy introduced by the fan being dissipated within the plenum. It is also shown that in a well designed air-cooled heat exchanger the reduction in thermal efficiency due to a distortion of the air velocity distribution through it, is small. For the case of a delta heat exchanger system, the plenum losses result in approximately 70% of the kinetic energy introduced by the fan being dissipated within the plenum. Plenum losses occurring within a double delta heat exchanger system result in approximately 95% of the kinetic energy being lost. In both cases the incorporation of a round-to-round diffuser in the plenum configuration results in a decrease in plenum losses and hence a noticeable increase in the fan system performance. The incorporation of a round-to-rectangular diffuser in the plenum configuration also leads to a decrease in the plenum losses. However, due to the poor static pressure recovery characteristics of the round-to-rectangular diffuser, the increase in the fan system performance is not as large as that achieved with a round-to-round diffuser.

AFRIKAANSE OPSOMMING: Die hoofdoel van hierdie ondersoek is om die invloed van die plenum konfigurasie op die vermoë van 'n lugverkoelde warmteuitruiler (veral op die waaierstelsel se bedryfspunt) te bepaal. Verskillende plenum konfigurasies, van eenvoudige reghoekige plenums tot die wat rond-tot-reghoekige oorgangstukke insluit, word getoets vir 'n normaal-vloei warmteuitruiler, 'n delta warmteuitruiler en 'n dubbel-delta warmteuitruiler. Die werkverrigting van die verskillende warmteuitruilers word ook met mekaar vergelyk. Die belangrikste gevolgtrekkings is die volgende : Die plenumverliese (en dus die waaierstelsel se werkverrigting) van 'n normaal-vloei wartmeuitruiler met 'n reghoekige plenum is nie baie sensitief vir plenumlengte nie. 'n Minimum plenumlengte van 0.4 waaier deursnitte word egter aanbeveel. Die plenumverliese wat voorkom in bogenoemde waaierstelsel veroorsaak dat ongeveer 28% van die kinetiese energie wat deur die waaier opgewek word, verkwis word binne in die plenum. Dit word ook getoon dat vir 'n goed-ontwerpte lugverkoelde warmteuitruiler die afname in termiese doeltreffendheid as gevolg van 'n distorsie in die lug snelheidsverdeling deur die warmteruitruiler, klein is. Met 'n delta warmteuitruiler word ongeveer 70% van die kinetiese energie wat deur die waaier opgewek word, weens plenumverliese verkwis. Vir die geval van 'n dubbel-delta konfigurasie veroorsaak plenumverliese dat bykans 95% van die kinetiese energie verkwis word. In beide gevalle veroorsaak die insluiting van 'n rond-tot-rond diffusor in die plenumopstelling 'n afname in plenumverliese en gevolglik 'n merkbare toename in waaierstelselwerkverrigting. Die insluiting van 'n rond-tot-reghoekige diffusor in die plenum lei ook na 'n afname in plenumverliese. Weens die swak statiese drukherwinnings eienskappe van die rond-tot-reghoekige diffusor is die toename in waaiersteselwerkverigting egter nie so groot as die van die rond-tot-rond diffusor nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/58727
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