Heat transfer temperature difference drop on falling film evaporation of horizontal tube bundle in rotated square-arranged

Abstract

Low-temperature multi-effect distillation that employs horizontal tube falling film evaporation technology is one of the most promising desalination methods with advantage of high heat transfer coefficient at low temperature, low water- spray density and small heat transfer temperature difference. Falling film evaporators have been widely used in absorb- type chillers, desalination devices and other areas. In a horizontal tube falling film evaporator, water and steam were flowed in different directions. The vertical falling film flow was driven by gravity, whereas the steam flow across the tube bundle was driven by pressure difference. Hence, the pressure drop and the consequent temperature drop of the steam are not only caused by the tube bundle but also by the falling water. In this paper, the effects of parameters such as spray density were determined. Experimental measurement of pressure drop data during the saturated steam flowing across aluminum brass tube bundles was carried out. The pressure drop coefficient was obtained from the experimental data. The rotated square-arranged tube bundle was chosen as a physical model to calculate the steam temperature difference drop on falling film evaporation. Based on the experimental data, the fitting pressure drop coefficient was employed in this calculation. The numerical codes were developed to simulate the steady-state performance of temperature difference drop in horizontal tube falling film evaporation. The iterative procedure was required based on the initial values with a close chain of equations and parameters. The solution was obtained through a step-by-step method. This quantitative temperature difference drops caused by the pressure drop can be calculated by the solution. The influences of spray density, saturated temperature, and tube column number on vapor temperature difference drop were analyzed respectively. The temperature difference drop caused by the pressure drop will decrease the temperature difference. The effect of temperature difference drop can be analyzed in two aspects. On the one hand, the temperature difference drop directly reduces the difference in heat transfer temperature, and thus, decreases heat transfer rate; on the other hand, the temperature difference drop generally increases the heat transfer coefficient under a certain heat transfer flux. When temperature difference drop is considered in a calculation, the heat transfer coefficient can be derived more accurately. Moreover, an accurate calculation of the temperature difference drop can provide a more detailed analysis on the influence factors of the heat transfer coefficient. At a certain tube arrangement and the same steam flow flux, the results showed that the temperature difference drop increases with the increase of spray density. It can be showed that the temperature difference drop at 50°C is 3.5 times of that at 70°C. However, the temperature difference drop decreases with the increase of saturated temperature. The temperature difference drop at spray density of 0.08 kg/(m s) is almost double of that at spray density of 0.02 kg/(m s). The calculations also show that the temperature difference drop is in parabolic rise with the increase of tube column number. When the temperature difference drop is set at lower than 0.3°C, the maximum tube column ( N max) can be obtained. The results showed that the N max increases with increasing saturated temperature, but decreases with increasing spray density. The N max can be employed to determine the heat transfer area for optimization design of the heat exchanger.