Entropy generation analysis in an evaporative air-cooled heat exchanger

Abstract

In the present study, air-side entropy generation rate for an evaporative air-cooled heat exchanger has been investigated using thermodynamic second-law analysis. For this purpose, entropy generations due to heat transfer, friction loss and evaporation were taken into consideration. From the results of this study, it was observed that the total air-side entropy generation was increased by increasing the air-side Reynolds number. Actually, increasing of air mass flow rate, increases irreversibilities due to both evaporation and friction. IntroductionAir-cooled heat exchangers are utilized in various applications such as air-conditioning and refrigeration systems, chemical plants, and power plants. In an evaporative cooler, heat is usually transferred from the process fluid to the cooling air stream via spraying water over the tubes and extended surfaces or finned tubes. As water flows down the surface of tube, it is evaporated by air flowing over it, resulting in cooling of the process fluid.In a thermodynamic process, the loss of exergy is primarily due to the associated irreversibilities which generate entropy. There is a direct proportion between the wasted power and the entropy generation rate. If engineering systems are to operate such that their lost work is to be minimized, then the conceptual design of such systems and components must comply with the minimization of entropy generation (Bejan, 1994(Bejan, , 1996. The minimization of entropy generation in evaporative systems attributed to fluid flow, heat transfer and evaporation of water is of considerable practical interest with regard to heat transfer augmentation techniques. Our main objective is to increase the convective heat transfer coefficient as compared to that of an unaugmented surface. A parallel objective, however, is to achieve this improvement without an increase in pumping power as demanded by the forced convection arrangement. These two conflicting objectives lead to concept of optimal design of an engineering system. Optimization of these systems in general and heat exchangers in particular plays a significant role in the profitability of the overall systems. The current trend in the optimization of thermal systems considers both the first and second laws of thermodynamics as the criteria; i.e. the efficient usage of both energy and exergy shall be ensued (Bejan, 1996).Based on the entropy generation minimization principle, considerable optimal designs of thermal systems have been proposed. For example, Morosuk (2005) worked on entropy generation in conduits filled with porous medium. Received 2 August 2013Shoeib MAHJOUB*, Mohammad Reza SALIMPOUR**, Hossein SHOKOUHMAND* and Zabihollah BAHRAMI** E-mail: salimpour@cc.iut.ac.irKey words: Thermodynamic analysis, Entropy generation, Second law, Evaporative, Air cooled heat exchanger, Deluge waterStepping up of ambient temperature leads to increasing of irreversibility due to water evaporation. The results showed when the ambient temperature approaches to inlet process fluid temperature...

Local Entropy Production of the Parallel Flow and Counterflow Heat Exchanger

“…Specially elaborated is the case of so called balanced counterflow heat exchanger, where both streams M. Rauch, A. Galović, Local Entropy Production of the Parallel Flow and N. Ferdelji, S. Mudrinić Counterflow Heat Exchanger have equal heat capacities. Many papers researching entropy production in individual heat exchangers have been written, like in [3,4], where entropy production is calculated in a shell and tube heat exchanger, or in [5], where entropy analysis of a single air cooled heat exchanger is shown. An interesting entropy analysis of three different heat exchangers using nanofluid as a working fluid is given in [6].…”