Performance of a divided flow exchanger

Abstract

An analysis is presented of a 1-2 divided flow exchanger where the temperature of shell and tube streams depend on tube arrangement. Effect of having unbalanced tube passes is assessed for two possible cases. Graphs are plotted that show the variation of thermal effectiveness and logarithmic mean temperature difference correction factor with ratio of heat capacity rates and number of transfer units. Combination of such exchangers is also studied.On a analysC un tchangeur Ccoulement divisC 1-2 pour lequel la temptiratwe des courants dans les tubes de la calandre est fonction de la disposition des tubes. L'effet de passe-tubes non tquilibrCs a CtC CvaluC pour deux cas possibles. On prCsente des diagrammes illustrant la variation de I'efficacitC thermique et du facteur de correction des differences des tempkratures moyennes logarithmiques avec le rapport des capacitks calorifiques et le nombre d'unitCs de transfert. La combinaison de tels Cchangeurs est Cgalement CtudiCe Keywords: divided flow, exchanger design, thermal effectiveness. divided flow exchanger performs equivalent to a A TEMA E exchanger and still leads to a pressure drop of about 118 times that of the latter on shell side. Hence when shellside pressure drop limitations arise, the divided flow exchanger is preferred.Several studies on divided flow or TEMA J exchangers and comparisons with PCF (parallel and counter flow) or TEMA E exchangers are reported in the literature (Shah and Mueller, 1985). Pioneering work was done by Kern and Carpenter (1951). From heat balance, a differential equation was set up for the fixed head end. The floating head end was taken to be a TEMA E exchanger. Heat loads and mean temperature difference for the two ends were first determined. Equating overall heat load divided by mean temperature difference with the sum of ratios gave the mean temperature difference of the exchanger. This was followed by the study of Iqbal and Stachiewicz (1962). Jaw (1964) determined expressions for thermal effectiveness for number of tube passes equal to 1, 2, 4 and infinity. In the analysis by Jaw, mean temperature difference for the exchanger is obtained only once and not by combining values of the two ends. This approach is followed in the present study because it does not involve trial and error.More recently, Murthy (1983) determined the performance of 1-1 and 1-2 TEMA J exchangers and showed that for 1-2 case it is beneficial to transfer slightly less heat in the tubesheet as compared to floating head end of the exchanger of Figure 1. This happens because increase in heat load to left half makes second pass of left half of the exchanger less effective. Zhuang (1987) analysed combinations of divided flow exchangers and presented the results in a graphical form. Xuan et al. (1991) pointed out that studies were restricted to the middle entrance of shellside flow and even distribution of mass flow rate in each half of the exchanger. The optimum entrance location of the shellside fluid depends on factors such as the number of tube passes, the...