Pressure drop during reflux condensation of steam in an inclined elliptical tube

Abstract

ENGLISH ABSTRACT: Zapke and Kroger developed a model for the pressure drop across flattened tubes employed in reflux condensers. This model is based on adiabatic two-phase countercurrent experiments and incorporates the theory on fluid friction during condensation by Groenewald. Zapke also developed a correlation with which the flooding velocity during adiabatic countercurrent flow may be predicted. The applicability of both the pressure drop model and the flooding correlation to reflux condensation had prior to this investigation not been verified by experiment. In this investigation the header-to-header pressure drop, the occurrence of flooding and the heat transfer performance of an inclined air-cooled elliptical tube in which reflux condensation of steam occurs were studied. The qualitative effect of the presence of a backflow region on the performance of the tube was also investigated. The 7 m long tube has a height of 97 mm (major axis) and a width of 16 mm (minor axis) and a sharpedged (90°) inlet. The tube is inclined at 60° to the horizontal. Steam temperatures are in the range of 45 °C to 65 °C. The pressure drop measurements obtained in this investigation showed that the header-to-header pressure drop in the tube is predominantly dependent upon the superficial vapor Froude number at the tube entrance. The pressure drop is also accurately predicted by the Zapke-Kroger pressure drop model in the range 0.1 ≤ Fr Hsv ≤ 0.4. At a certain steam flow rate a sudden sharp increase in the pressure drop occurs. This phenomenon is known as flooding. It was found that flooding during reflux condensation is governed by the superficial densimetric vapor Froude number with the duct height taken as the characteristic dimension. The measured vapor velocities at flooding are also in close agreement with those predicted by the Zapke flooding correlation. Flooding was found to have a limiting effect on the heat transfer rate of the elliptical tube in which all the steam condenses in the reflux mode. Finally, the formation of a steam backflow region in the tube resulted in the accumulation of noncondensable gases and the formation of a dead or cold zone. This led to a decrease in the heat transfer rate of the tube.

AFRIKAANSE OPSOMMING: 'n Model is deur Zapke en Kröger ontwikkel om die drukval in plat buise wat in teenvloei kondensers gebruik word te bereken. Hierdie model is op adiabatiese tweefase teenvloei eksperimente gebaseer en sluit die teorie oor wrywing tydens kondensasie deur Groenewald in. Zapke het ook 'n korrelasie ontwikkel waarmee die vloedingsnelheid tydens adiabatiese teenvloei voorspel mag word. Die toepaslikheid van beide die drukval model en die vloedingskorrelasie op teenvloei kondensasie is egter voor hierdie ondersoek nog nie eksperimenteel bevestig nie. In hierdie ondersoek is 'n lugverkoelde elliptiese buis waarin teenvloei kondensasie van stoom plaasvind gebruik om die drukval en vloeding in die buis asook die warmteoordragsvermoë van die buis te bestudeer. Die aanwesigheid van 'n terugvloei gebied op die buis se verrigting is ook ondersoek. Die 7 m lange buis bet 'n hoogte van 97 mm (hoof-as) en 'n breedte van 16 mm en is teen 'n hoek van 60° met die horisontaal gemonteer. Dit bet 'n skerp (90°) inlaat. Die stoom temperature wissel tussen 45 oc en 65 °C. Die drukval metings wat tydens hierdie ondersoek geneem is, bet dit duidelik gemaak dat die drukval oor die buis hoofsaaklik afhanklik is van die damp Froude getal by die buis inlaat. Die drukval word ook akkuraat deur die Zapke-Kröger drukval model voorspel in die gebied 0.1 ≤ Fr Hsv ≤ 0.4. By 'n sekere stoom snelheid vind daar 'n skerp styging in die drukval in die buis plaas. Hierdie verskynsel staan bekend as vloeding. Daar is bevind dat die densimetriese damp Froude getal met die buishoogte as die karakteristieke afmeting vloeding domineer. The gemete vloedingsnelhede stem ook goed ooreen met die wat deur die Zapke vloedingskorrelasie voorspel word. Vloeding blyk ook 'n beperkende effek op die warmteoordragstempo van die elliptiese buis te hê wanneer al die stoom in 'n teenvloei wyse gekondenseer word. Ten slotte, as gevolg van die aanwesigheid van 'n stoom terugvloeigebied het nie-kondenseerbare gasse in die buis versamel wat tot die vorming van 'n dooie of koue sone gelei het. Dit bet tot 'n vermindering in die buis se warmteoordragsvermoe gelei.