Fan Selection: The Key to Cooling Performance
Selecting the correct fan for your cooling operation can improve system efficiency and result in quieter performance.
Improved system efficiency and quiet performance can be obtained by considering design variables that help optimize the fan blade selection process. Airflow should be your first design consideration. Whether the application is a cooling tower or an air-cooled condenser, airflow is required to remove heat from the process. The amount of airflow required is determined by the air velocity rate required through the condenser coils or cooling tower fill media to produce the desired latent heat transfer. Air velocity is measured in feet per minute.
Coil or media manufacturers determine air velocity requirements through testing. Required velocity is multiplied by coil or media face area. This determines the amount of air volume the fan must produce to achieve the desired heat rejection.
Static Pressure. In the simplest terms, static pressure (PS) is a measure of the amount of resistance the fan must overcome to deliver a certain amount of air velocity across coils or through fill media. Static pressure is measured in inches of water. The manufacturer also determines a coil's or fill media's static pressure through testing.
Fan Diameter. Fan diameter is a variable, but normally it is determined by design constraints. Coil dimensions, package size limits and availability of venturi orifices affect fan sizing. The designer should always try to maximize an application's fan diameter as this will provide maximum coil coverage and reduce system static pressure.
Operating Speed and Available Power. Like fan diameter, speed and power are determined by design constraints or influenced by industry tradition. The fan application engineer will need to know the designer's desired operating speed and available horsepower to meet expectations.
Ambient Operating Temperature and Elevation. The fan engineer also will need to know the ambient temperature and elevation before making a recommendation for the cooling application. Typically, coil and fan test data are converted to standard air conditions (70°F [21°C] at sea level), which provide a common starting point for calculating performance variance.
Because a fan is a constant velocity machine, airflow is not affected by temperature or elevation. However, both a fan's ability to overcome static pressure and the amount of power required to operate it are affected by air density, and air density is affected by ambient operating temperature and elevation.
For example, assume a fan has 10,000 cfm airflow at 0.5" water gauge performance capabilities at standard conditions. Table 1 provides a comparison of fan operation in standard vs. actual conditions. Obviously, the fan's ability to generate pressure is reduced if the air has low density due to increased temperature and elevation. The table illustrates conditions that an air-cooled condenser application might be exposed to on a hot summer day in Denver.
With temperature and elevation knowledge, the fan application engineer can eliminate from the selection process those fans that would perform well at standard conditions but would become marginally effective under actual operating conditions.
Actual Operating Environment. While the operating environment will have little effect on airflow performance, it affects the fan's lifecycle. For example, coastal cooler applications are exposed to salt spray, and chemical process coolers are exposed to chemical corrosives. With an understanding of the fan's operating environment, the fan engineer can make material or coating recommendations that will provide the best solution for the operating environment. Table 2 shows available blade materials and their suitable operating environments.
Optimum Inlet Geometry and Fan Tip Clearance. Fan inlet geometry and tip clearance also influence fan performance. Most fan blade manufacturers test their products to provide maximum performance and efficiency. Maximum performance is achieved using aerodynamically designed bell-mouth inlets and tight fan tip clearances. The fan application engineer will need to know application inlet conditions so that safety margins can be added to the design parameters. This will compensate for differences between actual and test conditions (figure 1).
Minimizing Fan Noise. Noise always is a concern when designing applications in which air movement is required. The question is, how do you minimize it? The best way to minimize fan noise is to produce aerodynamically efficient operating conditions. This is achieved by selecting a fan blade that will operate within its maximum operating efficiency range. An aerodynamically efficient design will create less turbulence as energy is transferred to the air.
Fan blade diameter should be as large as the physical limits of the application will allow as this will reduce air velocity across the fan blade. To reduce tip-speed-generated noise, the fan should operate at the lowest speed at which application parameters can be met.
In addition to blade aerodynamic efficiency and reduced fan speeds, system aerodynamic efficiencies also must be improved to minimize noise. To improve system efficiency, the fan should operate in a high efficiency bell-mouth inlet with a bell radius equal to eight to 10% of fan diameter. Fan tip clearance should be as tight as practically possible.
Eliminating airstream obstructions that create wake turbulence -- particularly those obstructions on the air intake side of the fan blade -- also will reduce fan-generated noise. Obstructions that cannot be eliminated should be placed as far from the fan blade intake as possible to minimize the effect of wake turbulence.
Armed with information provided by the user, the fan application engineer can use computer software to review all possible combinations and provide a selection that produces the maximum performance range (figure 2).
When working with an application engineer, utilize a selection method that focuses both on providing an efficient working performance range and one that builds in safety factors for airflow and static pressure under less than ideal conditions. Using this selection technique, system inefficiencies that can affect fan performance -- both known and unknown -- are compensated for, reducing the potential of selecting a system that underperforms in your application.