Thermal performance analysis of heat exchanger for closed wet cooling tower using heat and mass transfer analogy

Abstract

In closed wet cooling towers, the heat transfer between the air and external tube surfaces can be composed of the sensible heat transfer and the latent heat transfer. The heat transfer coefficient can be obtained from the equation for external heat transfer of tube banks. According to experimental data, the mass transfer coefficient was affected by the air velocity and spray water flow rate. This study provides the correlation equation for mass transfer coefficient based on the analogy of the heat and mass transfer and the experimental data. The results from this correlation equation showed fairly good agreement with experimental data. The cooling capacity and thermal efficiency of the closed wet cooling tower were calculated from the correlation equation to analyze the performance of heat exchanger for the tower.

Performance Analyses of Counter-Flow Closed Wet Cooling Towers Based on a Simplified Calculation Method

“…A decrease of ambient wet-bulb temperature is beneficial to improve the cooling capacity of a CWCT because the temperature difference between the air and the cooling water is increasing. This indicates that the cooling water outlet temperature has a strong positive correlation with the ambient wet-bulb temperature, which is consistent with the findings of Zheng [4], Yoo [22], and Sarker [32]. The cooling tower effectiveness was not considerably affected by the ambient wet-bulb temperature in this study, which was consistent with the results reported by Facão [11].…”

“…This phenomenon is probably the consequence of joint contributions of the spray water, the spray, and the splash. When the external surface of the heat exchanger is fully wetted by the spray water, an increase of spray water flow rate does not significantly improve the thermal performance, which is consistent with the results reported by Facão [11], Zheng [21], and Yoo [22]. The spray water temperature is a dependent variable, which is mainly determined by the status of air and process water [34,35].…”

“…Apparently, the rise of air flow rate promotes the heat and mass transfer between the air and the spray water [7], which contributes to improving the thermal performance of CWCTs. Similar results were reported in [2,11,22,32,35]. However, the bigger the air flow rate, the larger the fan energy consumption.…”

“…As shown in Figure 1a, a force-draught CWCT with parallel counter-flow construction is designated as PCFCWCT in this study. As the best one-dimensional flow pattern of CWCT [20], the majority of flow patterns [7,9,11,18,[21][22][23][24] were that cooling water and spray water parallel flowed from the top to the bottom, while air flowed in the opposite direction. As shown in Figure 1b, the other one with cross counter-flow construction is designated as CCFCWCT, in which spray water flows from the top to the bottom, while process water and counter air flow in the horizontal direction.…”

“…According to the Merkel approach, the cooling capacity of a CWCT is expressed as follows [22,32]: The correlation between the dynamic viscosity and the temperature of water can be calculated by the following empirical formula [31]:…”