Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers F.S.K. Warnakulasuriya, W.M. Worek * Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA Received 24 August 2006 Available online 23 October 2007 Abstract Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solu- tion sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems. In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capac- ity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side. Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated. In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U overall varied from 970 W/m 2 °C (170 Btu/h ft 2 °F) to 2270 W/m 2 °C (400 Btu/ h ft 2 °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Absorption refrigeration; Plate heat exchangers; Highly viscous flows in heat exchangers; Power-law equation 1. Introduction In multi-effect absorption machines, the equipment size is controlled by the mass and heat transfer rates in the high stage absorber. Previous studies have shown that the limit- ing mechanism in the absorption process is mainly chemi- cal diffusion and heat transfer [1–4]. It is obvious that enhancement of the absorption process reduces the physi- cal size and weight of the overall absorber and lowers the overall power consumption of the system. The spray absorption process is an adiabatic process. Therefore using a spray absorber to enhance the mass transfer rates as compared to a conventional absorber requires the addition of an external heat exchanger (sub- cooler), as is illustrated in Fig. 1. To optimize the size of the unit, characterizing and optimizing the sub-cooler is important to successfully implement the spray absorption concept. In this work, a commercially available plate heat exchanger with an absorbent salt solution LZB TM , supplied by Trane Company, as the heat transfer fluid is used for investigation. The main objectives of this work are to specify the best suited heat exchanger configuration, define the optimum operating conditions, and to discuss the advantages and 0017-9310/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2007.04.054 * Corresponding author. E-mail address: wworek@uic.edu (W.M. Worek). www.elsevier.com/locate/ijhmt Available online at www.sciencedirect.com International Journal of Heat and Mass Transfer 51 (2008) 52–67