How to choose a cooling tower

How to choose a cooling tower (practical experience in cooling tower selection) A selection of energy-saving experience. Cooling towers are generally used as auxiliary equipment in central air conditioning systems, and their selection is often not given enough attention. However, if the cooling tower is not selected properly, there may be a mismatch in cooling capacity (without considering the severe decrease in cooling efficiency of the cooling tower during hot and humid summer weather). In mild cases, the energy consumption of refrigeration equipment increases, while in severe cases, the refrigeration host may not work properly, leading to malfunctions and machine trips; Or improper selection, unable to cooperate with frequency conversion energy-saving control; Or the service life of moving parts is short, requiring frequent maintenance and repair, which is not suitable for situations where reliability requirements are very high and shutdown for maintenance is not allowed. How to choose the model of cooling tower? How to avoid common problems in the selection of cooling towers? How to ensure that the cooling tower can still operate normally in summer to meet the required cooling capacity? The author summarizes some experiences for sharing. Based on years of practical experience, the selection of cooling towers generally requires attention to two aspects, namely the structural selection of cooling towers and the capacity selection of cooling towers. 1、 The commonly used types of cooling towers for structural selection include counter flow round towers or cross flow square towers, while closed cooling towers are rare and have special applications, which will be skipped here. If there is sufficient funds, the author suggests choosing a cross flow square tower cooling tower.
Cross flow towers have many advantages, such as low water loss rate, large ventilation area, and good cooling effect. One major advantage is that its water distributor is fixed, unlike the rotating water distributor of a circular tower that requires water flow to push, making it very suitable for cooling water pumps using frequency conversion energy-saving systems. After many cooling pumps adopt variable frequency energy-saving, the water distributor of the round tower cooling tower cannot maintain normal rotation speed or even stop rotating due to insufficient water flow after the cooling water flow rate decreases. The water flow cannot be evenly distributed but converges into water flow, which accelerates the flow rate, greatly reduces the contact surface and contact time with the air, and severely weakens the cooling effect. As the temperature of the cooling water increases, the refrigeration efficiency of refrigeration equipment such as refrigerators will decrease (within the normal working range), and they will not achieve the expected energy-saving effect. However, due to the fixed water distributor, the water flow distribution of the cross flow cooling tower is basically uniform when the water flow rate decreases, and the water flow velocity will be relatively slower, which will help to achieve better cooling effect. That is, the temperature of the cooling water can be lowered lower after better cooling, and the refrigeration equipment can achieve better refrigeration efficiency and reduce operating energy consumption. In addition, some large capacity cross flow towers are divided into several modules, each with an independent fan, water distributor, packing layer, etc., which can work independently, making their work flexible and diverse. If the fan can be stopped separately for maintenance, it can better meet the energy-saving control of the fan during quarterly changes, with simple control of start and stop without the need for complex variable frequency control. 2、 In general design, the capacity selection of cooling towers is considered to have some margin, but in actual operation, it is found that it is far from enough. As an auxiliary equipment, cooling towers are generally designed and selected based on the cooling capacity required by the refrigeration equipment. As the selection of refrigeration equipment takes into account margin, when matching cooling towers, it is generally just right, and a margin coefficient is rarely considered for the cooling tower. In actual operation, in hot and humid weather, the air conditioning load is high, and the refrigeration equipment basically reaches the design maximum value for operation. However, at this time, due to the working environment conditions of the cooling tower being far worse than the design conditions, the high relative humidity of the ambient air cannot effectively evaporate and take away the heat load from the cooling water, resulting in a serious decrease in the efficiency of the cooling tower and inability to achieve the designed cooling capacity. The accumulation of cooling water circulation in refrigeration equipment leads to an increasing temperature of the cooling water, which directly affects the operation energy consumption of the refrigeration equipment, the water loss rate of the cooling tower, and even leads to the protection tripping of the refrigeration equipment, or a large amount of low-temperature tap water needs to be directly added to the cooling water to maintain the temperature requirements. Therefore, when selecting a cooling tower, it is necessary to fully consider the factors that seriously reduce the cooling efficiency of the cooling tower due to environmental conditions not meeting the design temperature and humidity conditions in hot and humid weather. In addition, it is necessary to consider the factors that reduce the working efficiency of the packing layer, fan, and water distributor of the cooling tower after several years of operation. When selecting the capacity of the cooling tower, there must be sufficient margin. Based on my personal experience, choosing a cooling tower with twice the cooling capacity is not an exaggeration. If the standard cooling water flow rate is half of the standard flow rate of the selected cooling tower, the water flow will be dispersed. Whether under normal conditions or in abnormal weather conditions with severe stuffiness, better cooling effects can be achieved, and the water temperature will be lower after cooling; Although the hot and humid weather affects the cooling efficiency of the cooling tower, due to the large capacity of the cooling tower, the actual cooling capacity can basically meet the needs of the refrigeration equipment, and the temperature of the cooling water can be guaranteed not to be too high. The investment proportion of cooling towers in the overall system is generally small. Even if a cooling tower that is twice the size is chosen, there will not be much more investment. However, during operation, the temperature of the cooling water can be controlled at a lower state, and the refrigeration efficiency of the refrigeration equipment is high. The energy consumption and water loss saved in operation for one year are basically enough to buy a matching cooling tower. Both theory and practice have shown that within a reasonable range, for every one degree decrease in cooling water temperature, the average refrigeration efficiency increases by 3%. If the cooling tower is set too large, the cooling water temperature can be reduced by about 2-4 degrees. Therefore, increasing the selection of cooling towers is absolutely cost-effective and not wasteful. When selecting a cooling tower with a capacity greater than twice the actual demand, it is also important to note that when selecting a circular tower, the rotating water distributor needs to be driven by water flow. When the water flow is less than its design value, the overall efficiency of the cooling tower cannot be guaranteed. So when using a circular tower, at least two cooling towers with the same flow rate should be selected. Normally, only one cooling tower should be opened; When the weather is hot and humid, after turning on the cooling pump, both cooling towers can be put into operation. The horizontal flow square tower does not have the above concerns due to the fixed water distributor. Due to the significant seasonal variation of air conditioning load, its cooling capacity also varies significantly. After the total cooling amount is determined, the configuration scheme of most small capacity cooling towers is superior to the configuration scheme of large capacity and small quantity cooling towers in terms of energy-saving operation and flexibility in shutdown maintenance. Summary: If the cost is sufficient, it is best to choose multiple independent cross flow cooling towers with equivalent capacity.