Parametric Study of Wet Air Return Ratio and Energy Saving Control Strategy for Cooling Tower

• Main Authors: Chia-Wei Liu, 劉家瑋
• Other Authors: 蔡尤溪
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/5c552v

博士 === 國立臺北科技大學 === 機電科技研究所 === 99 === Parameters of wet air return and energy saving control strategies for cooling towers are analyzed by theoretical modeling and partly field measurements. There are four major topics in this thesis, namely, the energy saving control of condensing water temperature, the condensing water flow rate control, the capacity coupling of cooling towers, and the parameters that affect the wet air return. A system performance factor (SPF) is used in the study of condensing water temperature control. A computation scheme is developed to seek the optimum condensing water temperature (OCT) so to maintain the maximum SPF for the chillers and the cooling towers as a system. A mathematical regression for the control of OCT is presented that include multi-control parameters such as outdoor wet bulb, chiller load ratio, cooling tower factors, etc. It was found that for a typical building case with a baseline condensing water temperature control, the OCT control strategy would give the energy saving rate of 4.35～5.74％ for Taipei, and 3.46～4.2％ for kaohsiung. This control strategy is applicable to regions with wide variation of annual wet bulb temperature. For the control of condensing water flow rate, an optimal flow rate (OFR) scheme is presented that to achieve the maximum SPF for the condensing water system. A regression function is presented for the OFR control strategy. However as water pump control is added in the control, additional energy saving of 1.13～9.71％ is possible for Taipei, and 0.59～4.03％ for Kaohsiung. For OFR control, the effects to the chiller efficiency have to be taken into consideration. A subject often neglected in the research literature is the capacity coupling of cooling towers, relative to the chiller capacity. For the system as a whole, the increase in the cooling tower capacity would further increase the system performance either for OCT and OFR controls. For Taipei and the OCT control as compared to the base case, 1.1～1.6 times of cooling tower capacity would further allow energy saving of 0.59～2.67％. For OFR the further energy saving is 5.3～7.08％. The payback return is calculated and for OCT is between 8～10.7 years. However for OFR the payback is between 0.9～4 years. There is a limit to the capacity coupling due to cooling tower efficiency and the minimum flow rate and temperature requirement for the condensing water. Otherwise the payback years would be longer. Lastly the influencing parameters for the wet air return to the cooling towers are studied. It is a subject of practical importance but not found in the literature. Parellel arrangement of two banks of cooling towers is used in the studied. This study considers the annual tower load, weather variation, the length of tower bank, the spacing of the two banks of towers, and the flow characteristics of towers. The interactions of these factors are also considered. Two ambient wind directions, parallel and normal to the tower banks, are also considered in the analysis of total wet air return ratio. This study also considers the effects of wet air return on the system performance of the cooling towers and the chillers. A regression function is presented to predict the wet air return ratio. The research results of this study can be used in the design and installation of cooling towers, in order to increase the operating energy efficiency.