Optimum thermal design of humidification dehumidification desalination systems Mostafa H. Sharqawy ⁎, Mohamed A. Antar, Syed M. Zubair, Abubaker M. Elbashir Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia HIGHLIGHTS • Humidification dehumidification (HDH) cycles are investigated. • Theoretical analysis of water heated and air-heated HDH cycles are presented. • Practical design aspects of HDH cycles are given. • Humidifier and dehumidifier design models are provided. abstract article info Article history: Received 24 November 2013 Received in revised form 10 June 2014 Accepted 11 June 2014 Available online 5 July 2014 Keywords: Humidification Dehumidification Desalination HDH Optimization Design Humidification dehumidification (HDH) process is used for producing fresh water from saline water at sub- boiling temperature. This process uses a low-temperature source such as solar energy or waste heat source. Although these heat sources are available with minimal operating cost, an optimum thermal design is required to maximize the water production rate for a given heat input. In this paper, the main design and performance pa- rameters are investigated for two HDH cycles namely, water-heated and air-heated cycles. First-law based ther- mal analyses are provided and performance charts are presented by considering assumptions. The design details of both the humidifier and dehumidifier are presented to determine their sizes under different design conditions. It has been demonstrated that optimum mass flow rate ratios exist for each cycle such that the gained-output ratio (GOR) is maximized. In addition, it is demonstrated that higher GOR can be obtained by using large-size hu- midifiers and dehumidifiers due to increasing their effectiveness. Moreover, increasing the temperature of water entering the humidifier reduces GOR for the water-heated cycle whereas it increases for the air-heated cycle. A comparison is also carried out between the two cycles to provide guidelines for designers in terms of, power re- quirements and components size. © 2014 Elsevier B.V. All rights reserved. Nomenclature A Surface area m 2 a Effective surface area per unit volume m 2 m -3 C Minimum to maximum heat capacity ratio C min Minimum heat capacity WK -1 c p Specific heat at constant pressure J kg -1 K -1 h Specific enthalpy J kg -1 h fg Enthalpy of vaporization J kg -1 h c Convective heat transfer coefficient Wm -2 K -1 h d Mass transfer coefficient kg m -2 s -1 h g Specific enthalpy of saturated water vapor J kg -1 H Height of humidifier/dehumidifier m k Thermal conductivity Wm -1 K -1 L Length m Le f Lewis factor Me Merkel number ˙ m Mass flow rate kg s -1 P Pressure kPa R Thermal resistance KW -1 U Overall heat transfer coefficient Wm -2 K -1 V Volume m 3 w Salinity – grams of solutes per kg of solution g kg -1 W Width of humidifier/dehumidifier m Z Dimensionless height of packing Greek symbols ε Effectiveness η Fin or overall surface efficiency ρ Density kg m -3 ω Specific humidity kg vapor kg -1 dry air Subscripts a Air br Brine DH Dehumidifier fw Fresh water H Humidifier Desalination 349 (2014) 10–21 ⁎ Corresponding author. Tel.: +966 013 8607161. E-mail address: mhamed@kfupm.edu.sa (M.H. Sharqawy). http://dx.doi.org/10.1016/j.desal.2014.06.016 0011-9164/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal