Effect of design parameters on thermodynamic losses of the heat transfer process in LT-MEE desalination plant

Thermodynamic losses caused by the flow resistances and the boiling point elevation (BPE) in low-temperature multi-effect evaporation (LT-MEE) desalination plant were investigated. The purpose is to provide detailed considerations for the design and modeling of LT-MEE process. Design parameters were analyzed to evaluate their effects on thermodynamic losses. Results indicate that on the premise of constant distillate production, as the sequence number of evaporator/condenser increases, each flow resistance decreases first and then increases, while the variation of corresponding temperature depression always performs a rising tendency. With the increase of tube length, the proportion of saturated temperature depression caused by the in-tube condensation flow resistance increases, and that caused by the inter-tube flow resistance decreases. The tube length for the lowest temperature depression occurs when two temperature depressions are in similar proportions. The thermodynamic losses caused by the flow resistances and the BPE account for considerable percentages, especially when under smaller temperature difference. The operating characteristics of LT-MEE desalination plant are summarized based on the analysis. © 2015 Elsevier B.V. All rights reserved.

The paper calculated thermodynamic losses in the LT-MEE desalination plant based on a series of experimental correlations to study the effect of the tube length, the arrangement of tube bundle, the concentration ratio and the average apparent heat transfer temperature difference. Results suggest the following conclusions: (1) The operating characteristics of LT-MEE desalination plant are summarized as “small temperature difference, low flow resistance, saturated state and high sensitivity”. (2) The flow resistances first decrease and then increase along the evaporator/condenser sequence, while the variation of corresponding temperature depression displays a rising tendency all the time. (3) The proportion of temperature depression caused by the flow resistance of condensation process increases, and that of the intertube flow resistance decreases with the increase of tube length. The tube length for the lowest temperature depression occurs when the temperature depressions caused by two flow resistances are in similar proportions. (4) The regular triangle arrangement has lower temperature depression than the rotated square. The advantage will be narrowed with the increase of tube length. The temperature depression caused by the flow resistances decreases with the increase of concentration ratio and the decrease of average apparent heat transfer temperature difference. (5) BPE decreases with the increase of evaporator/condenser sequence and the decrease of concentration ratio. The effect of average apparent heat transfer temperature difference on BPE is more significant with the decrease of evaporator/condenser sequence. (6) The heat transfer temperature difference loss caused by flow resistances and the BPE account for considerable percentages and should be paid full attention in the plant design, especially at the condition of smaller temperature difference. Nomenclature a, b, c empirical constant BPE boiling point elevation of brine, °C D equivalent diameter of steam channel, m d diameter, m f coefficient of pressure drop G steam mass velocity, kg/(m2·s) L length, m m steam mass rate, kg/s N number of tube columns n R Re T v X x number of effects radius of bend, m Reynolds number temperature, °C velocity, m/s salinity, ‰ integral average dryness of steam Greek symbols α local loss coefficient β angle of bend, ° η kinematic viscosity, m2/s μ dynamic viscosity, Pa/s ρ density, kg/m3 Γ spray density, kg/(m·s) Subscripts v vapor w wire mesh bf brine flash df distillate flash dis evaporated from heat transfer tube Acknowledgments The authors are grateful for the support of the Key Project of the National Natural Science Foundation of China (No. 51336001).