Some insights into energy savings in water-driven fan cooling towers

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This post was last edited by WSRYLONG on 2019-12-11 at 08:48. Some insights into the energy savings of water-driven fan cooling towers: Using a turbine instead of an electric motor to drive the cooling tower fan, namely what is known as a water-driven fan cooling tower, has its earliest documented record in China in a Chinese invention patent application disclosure titled “Energy-Free Cooling Tower”, dated October 29, 1998; the applicant was Zhang Fei Kuang from Shangyu City, Zhejiang Province. Around 2000 years ago, Zhang Fei Kuang was the first to develop a double-click type turbine for use in cooling towers, replacing electric motors to drive the fans in these towers; this technology was then widely adopted, and it can be considered the first generation of turbines for cooling towers. Since then, the domestic market for hydrodynamic fan cooling towers has gradually developed. At present, there are dozens of companies engaged in the modification of cooling tower turbines and in the production of such turbines, forming an industry of considerable scale. The turbines used vary widely, but the quality of these turbines varies greatly, with both high-quality and low-quality options available. From the perspective of actual performance, many cases of retrofitting fans with water-driven systems have encountered various problems during use. Severe vibration and fan speeds that do not meet the required levels are the main shortcomings, which result in inadequate cooling effects. Early cooling tower turbines were essentially based on the design principles of power generation turbines, with only minor structural changes. Later, it was realized that the operating conditions of cooling tower turbines are very different from those of power generation turbines, and it is not appropriate to rely blindly on the principles applicable to power generation turbines; as a result, some theoretical research was conducted to improve the suitability of cooling tower turbines. On this basis, new products such as ultra-low specific speed mixed-flow turbines designed specifically for cooling towers, and shaft-mounted radial-flow turbines for industrial cooling towers have been developed. Although the new type of cooling tower turbine represents a significant improvement over earlier models, cases of insufficient fan speed still occur frequently. When preparing to carry out energy-saving upgrades on the water turbine in a cooling tower, users should not act hastily based solely on the promotional materials provided by the manufacturers. Be sure to check whether the manufacturer has case studies with similar parameters, and conduct on-site investigations to understand the actual usage situation ； The manufacturer is required to give a clear commitment regarding whether it can ensure that the fan’s speed remains unchanged, thus maintaining the cooling effect, under the condition that the water pump’s flow rate and current stay constant. Our company is a specialized firm focused on energy-saving upgrades for circulating water systems. We primarily carry out energy-saving modifications to water pumps, with the main approach being to replace the impellers; in special cases, we replace the entire pump. We also consider the cooling tower turbine as an option, so we have been keeping an eye on and researching related technologies. We are not averse to cooling tower turbines, nor do we intend to disparage this technology; however, in a spirit of seeking truth from facts and out of a sense of responsibility toward our customers, it must be pointed out that in many cases, or even in most cases, cooling tower turbine-based energy-saving technologies are not suitable for use. In other words, in many situations, employing cooling tower turbines for energy-saving upgrades is not a good choice. A brief analysis of this view is as follows: regardless of the type of cooling tower turbine, it is the excess water head from the recycled circulating water that is used to drive the fan. Well, since there is an excess head, we can simply carry out energy-saving modifications to the water pump to eliminate this excess head at its source; there’s no need to take a roundabout route and go through multiple steps. There are several differences between the energy-saving renovation of water pumps and that of cooling tower turbines: 1. The renovation of water pumps addresses the issue at its root, eliminating excess head from the source, thus achieving twice the result with half the effort ； The cooling tower turbine first creates excess water before attempting to recycle it, which is redundant and results in less efficient performance. 2. The energy-saving effect of pump renovation is better. Cooling tower turbines reuse the excess head, involving an additional energy conversion process compared to pump modifications, resulting in a **reduced overall efficiency**. Assuming that the existing excess energy in the circulating water system is Pf, with efficiencies of the pump, motor, and turbine being ηb, ηd, and ηl respectively, the actual power consumption associated with Pf is Pf/(ηb*ηd). The energy that can be recovered by the turbine is Pf*ηl. The unnecessary power consumption of the system after modifying it with a turbine is Ph = Pf/(ηb*ηd) – Pf*ηl. With ηb=0.8, ηd=0.95, and ηl=0.8, Ph equals 0.516 Pf. This means that the system still wastes 0.516 times the amount of excess energy Pf. In contrast, modifying the pump eliminates this excess energy at its source, resulting in zero excess energy and a reduction in energy consumption of 0.516 Pf. 3. The renovation work for water pumps involves minimal workload, has a short construction period, allows for restoration, and the restoration process is very convenient. When carrying out energy-saving upgrades to water pumps using the impeller replacement method, once the energy-saving impeller is ready, the pump is disassembled, the rotor assembly is taken out, the original impeller is removed, and the new impeller is installed; after that, the pump is reassembled and put back into operation. The pump body, foundation, and pipelines of the original water pump require no changes, making the construction modifications very convenient and simple. Moreover, the renovation of cooling tower turbines involves a large amount of work and a long construction period; it is not possible to restore them, or if restoration is attempted, the amount of work required and the losses incurred will be significant. 4. The cost of modifying water pumps is low, and the fees are low as well. Take my company as an example: I provide the technology and the funds, carry out the transformation first and then charge fees; no charges are made if it is not successful. The total amount charged for energy-saving services generally does not exceed the savings in electricity costs over 8–10 months, and it is charged only once. In other words, the customer saves more in electricity costs within a year than the total amount paid for the services, resulting in a surplus after the service fees are settled. Compared to the electricity costs incurred before the renovation, the cost of the energy-saving renovation service does not impose any additional burden on the customer; on the contrary, the customer can save this amount of money in the same year, and even earn a profit as well. The cost of upgrading cooling tower turbines is high, and the fees are high as well. Taking a 1000 m3/h circulating water system as an example, the cost of upgrading our pumps is around one hundred thousand yuan, while the cost of upgrading the turbines in the cooling towers is around several hundred thousand yuan, generally no less than twice our pump upgrade cost. Energy-saving retrofits using cooling tower turbines are suitable only in the following two cases: 1. There must be a surplus head in the circulating water system, and this surplus head cannot be eliminated, for example due to pressure requirements in the process ； 2. The excess head in the circulating water system is an inevitable phenomenon that cannot be eliminated. For example, if there is a large height difference in the water usage devices, of over 30 meters, a high head pressure is required to deliver water to the highest points. This results in a significant pressure difference between the circulating water at higher levels and the return water from the cooling systems. When the resistance in these pipelines is low, it is inevitable for an excess head to occur. In both cases, retrofitting with a turbine is a good energy-saving solution that will yield significant energy savings. In fact, there aren’t many cases that meet these two conditions; even those in which turbine modifications have been carried out often do not satisfy them. In such cases, using a pump for energy-saving modifications is better than using a turbine. Finally, two points need to be mentioned: first, some manufacturers of cooling tower turbines use the rated power of the original fan motor when conducting energy-saving assessments to calculate the power savings rate. As we all know, the actual power consumption of fan motors is lower than their rated power; moreover, the machines are not turned on in winter. Therefore, using the rated power to calculate the energy savings rate will definitely result in a higher figure than the actual energy savings rate. Secondly, after the turbine is modified, and with the original circulation water pipeline and valve settings remaining unchanged, the current drawn by the water pump may decrease slightly. Under no circumstances should this be considered an energy-saving effect resulting from the turbine modification; rather, it is due to an increase in system resistance that leads to a reduction in flow rate. These are just my personal opinions; mistakes and omissions are inevitable. I welcome everyone’s discussions and suggestions.