Improving system efficiency and achieving quieter performance is vital to any application, and there are a number of key points to examine in order to select the right fan for your cooling application. Here are the principal variables to consider.
Airflow. Whether the application is a cooling tower or an air-cooled condenser, airflow is required to remove heat from the process. Airflow is determined by the air velocity (measured in feet per minute) required through the condenser coils or cooling tower fill media in order to produce the desired latent heat transfer.
Static Pressure. Static pressure is a measure of the amount of resistance the fan must overcome to deliver “X” amount of air velocity across the coils or through a fill media. Static pressure (PS) is measured in inches of water. Manufacturers test to determine the amount of static pressure for a coil or fill media.
Fan Diameter. The fan diameter is a variable, but is normally determined by design constraints. Coil dimensions, availability of Venturi orifices and package-size limits all affect the size of the fan. However, you should always try to maximize the fan diameter in order to provide maximum coil coverage and to reduce system static pressure.
Speed and Power. Operating speed and available power are also determined by design constraints or influenced by industry tradition. You must know the system’s desired operating speed and available horsepower in order to meet expectations.
Temperature and Elevation. Ambient operating temperature and elevation are two parameters that the fan engineer will need to know in order to make the best recommendation for the cooling application. Typically, coil and fan test data are converted to what is called standard air conditions (70°F at sea level). This gives a common starting point for calculating performance variance.
Actual Operating Environment. The operating environment has significant effect on the life of the fan. For example, coastal cooler applications are exposed to salt spray, and chemical process coolers are exposed to chemical corrosives, which affect the choice of fan materials.
Optimum Inlet Geometry and Fan Tip Clearance. The fan inlet geometry and tip clearance also affect fan performance. Performance is achieved using aerodynamically superior bell mouth inlets and tight fan tip clearances. The bell mouth opening reduces turbulence, increasing fan efficiency and reducing noise. In all applications, the fan will perform at its highest efficiency when the tip clearance is 1 percent of the fan diameter. In cases where this may not be practical, a range of 1 to 2.5 percent tip clearance is recommended. Fan performance significantly decreases as tip clearance exceeds 2.5 percent.
- Blade Profiles. Each blade profile -- paddle, airfoil, increasing arc and sickle -- offers a different primary benefit: low power consumption, high performance and low noise. Fan designers continue to improve blade profile designs offering both high performance and noise reduction.
Beating Poor Fan Performance: Did You Know?
Multiple factors influence fan performance including inlet geometry, tip clearance, obstructions in airflow and unknown system losses (e.g. poor ducting or location obstruction). Identifying these factors is the vital first step in developing a solution that satisfies your particular air-cooling requirements.
Using computer software, the fan application engineer reviews these performance variables in determining all possible combinations, and then builds the optimum selection for each application.
When working with an application engineer, make sure you utilize a selection method based on providing an efficient working performance range that builds in safety factors for both airflow and static pressure when conditions are less than ideal. Using this selection technique, the solution compensates for known and unknown system inefficiencies, reducing the potential for an under-performing recommendation.
This article was contributed by Multi-Wing America, Burton, Ohio. To learn more about industrial fans for process cooling applications, call (800) 311-8465 or visit www.multi-wing.net.