FIOOUNG DURING REFLUX CoNDENSATIoN oF STEAM IN AN INCLINED ELLIPTICAL TUBE P.D. Schoenfeld* D.G. Krogert Received September 1999; Final version December 1999 In this erperimental inuestigation the pressure drop is Subscripts measured between the headers located at the ends of an a alr in,clined air-cooled elliptical tube in which refl,ur conden- c cross-section sation of steam occurs. The cross-section of the 7 m long d diameter tube has a height of g7 mm (major aris) and a width fl, flooding of 16 mm (minor ads). Steam temperatures are in the f inlet range of 45o C to 65o C. The pressure drop can be pre- / liquid dicted accurately using the Z apke- I(rri g er pressure drop .s steam, superficial model applicable to refl,ur condensers. At a certain steam t tube fl,ou, rate a sudden sharp increase in the pressure drop u vapour occurs. This phenoTne?t,on is lcnowrt, &s fl,ooding. The Dimensionless groups rneasured aapour uelocities at fl,oodin,g agree well with FrH,o = pur?, I [Q, - p,) gU] Superfi cial densimetric the ualues predicted by the Zapke-Krciger fl,ooding corre- vapour Froude number lation. The erperimental results also show that flooding Frn,t = ptr?,/ lQ, - p,) gdnl Superficial densimetric has a detrimental effect on the thermal effectiueness of liquid Froude number Ihe elliptical tube. Rero = puurodHlp, Superficial vapour Reynolds number Nomenclature ZK (p,dro) I pt = Oh-r Dimensionless liquid property A Are" (-2) parameter cpo Specific heat of air (J/kgK) Introduction Hydraulic diameter (m) dp Forced draft air-cooled steam condensers are increasingly e Heat exchanger effectiveness used in power generating plants, especially in arid re- Gravitational acceleration G (-/r') gions. The condensers consist of bundles of finned tubes H Height of duct (rn) arranged in an A-frame configuration above the fans as iJ n Latent heat of vap orrzation (J/kS) shown in Figure 1(a). Dimensionless eonstant, I\ eqn (3) The inclination angle of the finned tubes, which gener- Mass flow m rate (kg/r) ally have a round, elliptical or flat-profile cross-sectional n, Dimensionless constant, €etr (4) geometry, is approximately 60o to the horizontal. In Heat transfer rate A (W) the finned tubes the steam and condensate flow concur- T Temperature ('C) rently downward into the drainage header. To prevent Velocity (m/s) u subcooling of the condensate and the formation of dead Dynamic viscosity (kg/ms) tt zones due to the accumulation of non-condensable gases p Densitv (kg/m3) in the condenser, a secondary reflux condenser or de- o Surface tension (N/-) phlegmator is connected in series with the main con- Inclination angle (degrees) 0 denser. The dephlegmator ensures that there is a net outflow of steam from the bottom of the main condenser. This steam condenses in a reflux mode in the dephleg- mator, the steam flow being upwards countercurrent to * Department of Mechanical Engineering, University of Stellen- bosch, Private B.g X1, Matieland, T602 South Africa the condensate flow. Non-condensable gases that may t Department of Mechanical Engineering, University of have leaked into the system are removed at the top of Stellenbosch the dephlegmator by means of an ejector R & D lournal, 2000, l6( I ) Since the dephleghmator operates in the reflux con- 60o. An air outlet manifold (9) is mounted on top of the densation mode, flooding of the dephlegmator can oc- wooden casing and is connected to a centrifugal fan (10) cur. At a so-called flooding vapour inlet velocity, the which draws cooling air across the finned tube. condensate does not drain freely into the bottom header During an experimental run the air inlet and out- of the dephlegmator, but starts to accumulate in the let temperatures as well as the steam temperatures in tubes. This results in a large pressure drop across the the inlet and outlet headers are measured using cop- dephlegmator headers as well as a decrease in the ther- per constantan thermocouples. Calibrated propeller- mal effectiveness of the dephlegmator. type anemometers (11), situated in each lateral of the Numerous studies have been conducted to investigate air outlet manifold, measure the air volume flow rate the pressure drop and flooding in reflux condensers. over the finned tube while a pressure transducer mea- Banerjee et al.,L Girard k Chang2 and Obinela et al.3 sures the differential pressure between the inlet and top studied reflux condensation of steam in a vertical water- headers of the tube. The steam condensation rate is de- cooled glass tube with the top and bottom header pres- termined using a measuring cylinder (3) that is attached sures held constant duritg an experimental run. Russella to the steam generator. made use of a long air-cooled finned tube inclined at 57o to the horizontal in which steam at atmospheric pressure Experimental results was condensed. The various flow modes were observed through a sight-glass. Reuter & Kroger5 conducted ex- The pressure drop and flooding in the elliptical finned periments in a vertical and inclined water-cooled glass tube were investigated using steam ranging in tempera- tube in which low pressure steam was condensed in the ture from 45oC to 650C. It was found that the header- reflux condensation mode. Bellstedt6 studied reflux con- to-header pressure drop can be predicted accurately us- densation of low pressure steam in an air-cooled finned irg a two-phase pressure drop model applicable to reflux tube inclined at 60o to the horizontal. Groenewald7 con- condensers as was proposed by Zapke k Kriiger.8'e Ac- ducted similar experiments in a flattened finned tube. cording to Zapke k Kriigere the pressure drop is both a In this study the pressure drop and flooding during function of the vapour Reynolds number and the densi- reflux condensation of low pressure steam is investigated metric vapour Froude number. At low vapour flow rates in an air-cooled elliptical finned tube inclined at 60o to the pressure drop is Reynolds number related. How- the horizontal. The experiments were conducted in a ever, &t high vapour flow rates greater liquid-vapour in- steam temperature range from 45o C to 650C. teraction takes place, resulting in wave formation on the surface of the liquid. The vapour Froude number there- Experirnental apparatus fore becomes the governing dimensionless group and the duct height the characteristic dimension. The pressure The experimental apparatus is schematically depicted drop in the elliptical tube as predicted by the Zapke- in Figure l(b). Hot water (2), electrically heated, is Kriiger pressure drop model is compared to experimen- pumped through a shell-and-tube heat exchanger (1) in tal header-to-header pressure drop data as a function of which low pressure steam is generated. The steam is the vapour superficial velocity at the tube entrance as ducted to the steam inlet header (a) which has radial in- shown in Figure 2. Note that it is more meaningful to let and guide vanes that ensure that the steam is vortex plot the pressure drop data in terms of the vapour ve- free on entering the test tube. The test tube (5) is an locity since the pressure drop is a function of both the elliptical finned tube ,7 m long and with an inside height vapour Reynolds number and Froude number. (major axis) and width (minor axis) of g7 mm and 16 As was mentioned above, at high vapour velocities the mm, respectively. The inlet of the tube is square-edged densimetric vapour Froude number becomes the govern- (90').ThecroSS-Sectionalareaofthetube,A"t ing dimensionless group. At a certain vapour velocity, mm2 and the hydraulic diameter, dn = 25.922 mm. A namely the flooding velocity, a sharp increase in the top header (6) is connected to the upper end of the tube. header-to-header pressure drop occurs. The fact that A water-driven vacuum pump (7), connected to the sys- flooding is governed by the vapour Froude number as tem at the outlet header, ir used to obtain subatmo- opposed to the vapour Reynolds number is clear when spheric pressures in the system prior to an experimental considering Figures 3 and 4. They are plots of the pres- run and to remove non-condensable gases that may col- sure drop in the tube versus the vapour Reynolds nurnber lect in the tube during a run. The entire finned tube is and densimetric vapour Froude number, respectively. In mounted in a wooden casing (8), which also forms part Figure 3, the flooding vapour Reynolds number varies of a support frame. The frame is hinged at its lower end, between approximately 13 000 and 20 000 depending on thus making it possible to adjust the inclination angle of the steam temperature. In terms of the Froude number, the tube. In this investigation the tube is at an angle of flooding occurs at approximately 0.47 irrespective of the R & D lournal, 2000, 16( I ) fo ejector o0da Fons Dephlegmotor Condensote Figure 1 (a) Condenser with dephlegmator Elliptical nrbe Cross-section A-A Figure 1 (b) Schematic representation of the experimental apparatus R & D lournal, 2000, 16( I ) 4000 3500 3000 25W d /tl L< \/ 2000 aE 1500 r000 500 0 40 50 vsv (*ts) Figure 2 Header-to-header pressure drop in an elliptical reflux condenser tube with a square-edged inlet 4000 3 500 3000 2500 63 t-tn \/ 2000 lr -a I 500 I 000 500 0 10000 I 5000 20000 Re'sv Figure 3 Total pressure drop in the elliptical reflux condenser tube versus the superficial vapour Reynolds number R & D lournal, 2000, 16(I) steam temperature, &s shown in Figure 4.