Cooling Tower Fan Efficiency Requires Careful Analysis
Hello, and welcome back to the Industrial Cooling Solutions, Inc. blog! If you are just now joining us, we are in the middle of a multi-part blog series that is dedicated to detailing the factors that influence the efficiency levels of industrial cooling tower fans. If you have not read our previous post, we highly suggest that you go do so now. While you may find the information in this second post useful, it is build upon important ideas that are presented in the first post and, as such, will make more sense if you read the posts in sequential order. Now that we have that small disclaimer out of the way, we are going to use today’s post to continue where we left off last time and discuss some of the factors that influence the overall efficiency of a cooling tower fan system. While each factor may seem small when isolated, together they can greatly decrease the efficiency of a cooling tower, effectively rendering the tower useless for its intended application. Keep reading below to learn more.
Potential Losses in System Efficiency Can Occur in Several Separate Areas
When it comes to overall system efficiency in cooling tower fan assemblies, there are three main areas that need to be considered by researchers and engineers alike.
- Losses caused by the system design of the fan rather than by variable physical properties.
- Losses caused by variable environmental properties.
- Performance losses caused by the recirculation of hot air.
Of these three main areas of potential loss, only the second category is easily fixable. Below, we have listed out the details surrounding the first category that affects the efficiency of a cooling tower fan system.
Losses Caused by System Designs
While the variables that might potentially decrease the efficiency of a cooling tower fan system are sometimes easy to identify, most of the time they are not. One factor that can greatly affect the overall efficiency of the system is the design of the fan blade. For the most part, modern axial fans found in industrial cooling towers are made from either FRP materials or aluminum. Aluminum fan blades, by their very nature, are always of a uniform design, whereas FRP fan blades can be molded into pretty much any shape that an engineer may want. No matter which type of fan blade material is used, the main purpose of the fan assembly is to produce uniform airflow over the entire plane of the fan. Uniform airflow ensures that the optimal amount of force is produced to adequately dissipate the heat that is introduced into the cooling tower. To determine that a fan blade design is able to produce the amount of airflow that is needed, the work done at any radius along the fan blade is a function of blade width (angle of attack and tangential velocity squared).
The Shape of Fan Blades Plays an Important Role in Efficiency
With the above information in mind, it can be concluded that as a point on the fan blade decreases from the tip toward the hub of the fan assembly, the tangential velocity decreases sharply. To compensate for this and produce uniform airflow, the twist of the blade along with its width must also increase. This becomes an issue when dealing with aluminum fan blades because if the blade width cannot be increased, the twist of the blade must be increased to compensate. Due to the elasticity limits of aluminum, this twist can only be taken to a certain level before the fan blades break. Luckily, FRP fan blades have no such limitation because they are molded into a single piece, allowing the most ideal shape to be more closely achieved.
Another factor that is affected by the design, shape, and twist of the fan blade is the fan operating point, or the point where the system resistance line meets the fan performance line. In laymen’s terms. The operating point is the blade pitch angle that produces the necessary air flow against the required system resistance of the cooling tower. Depending upon the fan speed, only a single pitch angle will be able to satisfy the system design operating condition. To put all of this together, within certain limits, the speed of a cooling tower fan can be adjusted so that the most optimal pitch angle can be selected that will satisfy the required system resistance.
Faulty Design Leads to Faulty Performance
When considering the above two points, it is easy to see how poor fan blade design, as well as a poor selection in the operating point of the fan system, can contribute to a loss in efficiency of a particular cooling tower fan system. Once in place, these two factors are not always the easiest things to fix, meaning that it is always in the best interest of the engineer and designer to make sure that their initial system design is as efficient as possible. If these considerations are not taken into account, valuable time, energy, and money will have to be spent rectifying the issue.
Thank you for reading part two of our blog series on the factors that should be taken into consideration in relation to cooling tower fan efficiency. If you would like to learn more about our selection of FRP fan blade designs and the applications in which they can be used, please contact us today at Industrial Cooling Solutions, Inc. We have completed dozens of cooling tower projects around the world and we are certain that we can handle any and all needs you may have when it comes to optimizing your fan system.