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Fans - Basic Properties, Terminology and Theory

What is a Tubeaxial Fan?

Tubeaxial Fan is a reliable efficient device that controls heat rise in electronic equipment. In most applications, the fan motor operates continuously when the system is on and in operation.

Tubeaxial fans are available in AC or brushless DC models. They are readily available and inexpensive with many sizes and performance standards to choose from. The efficiency of these fans is rated good to excellent with an axial air flow pattern (located on, around, or in the direction of an axis).
DC Tubeaxial fans is up to five times more efficient than its AC counterpart. The corresponding low heat dissipation of the DC Tubeaxial fan reduces the thermal load of the bearings, increasing life expectancy. Benefits in noise performance are also realized. Speed control is easier and it is also linear which makes it easier to apply. Voltage adjustments can be made to optimize the air flow to cooling requirements that can minimize noise. Generally, DC Tubeaxial fans will operate effectively with voltage reductions up to 50% and voltage increases up to 25% of their normal voltage. Functional monitoring, in many cases, is necessary to protect the devices being cooled. This is easy with DC Tubeaxial fans. An external circuit can evaluate speed/proportional commutation pulses from the internal fan electronics. The signal can be activated if the fan loses speed or stalls to shut down equipment or to activate or increase the speed of other fans.
Selecting a Fan for Your Application
Below is a list of design objectives to consider when making your choice:
  1. Determine total cooling requirements needed to operate the system efficiently and that provide the desired operating conditions to maximize the system components lifespan.
  2. The formula for determining the necessary air flow is:
    • Q = 1760 x KW divided by Delta Tk
    • Q = required air flow in CFM
    • KW = heat to be dissipated
    • Delta Tk = allowable temperature rise in Kelvins
    Other factors to consider is the cleanliness of the air passing through the system. Will the air need to be filtered? Will it need to be isolated from the air in the cabinet or within the system? Can it be exhausted into the local environment or does it need to be exhausted outside? Which is better - pressurized air, exhaust air or some kind of push/pull approach? How important are other factors such as noise, power consumption, or form-factor (size) in determining which air moving approach is best for this specific application?
  3. Define the total system impedance or System Characteristic curve of the enclosure/system. Once load parameters are defined in terms of heat dissipation or number of air changes per hour required, and the required air flow determined, the static pressure characteristics that the moving air will encounter as it passes through, over and around components located within the air flow path must be determined. Elements that impede the flow of air create a pressure rise within the system that restricts free flow and passage of air. The change in pressure, or Delta P, is the static pressure measured in inches of water.
  4. The System Characteristic Curve formula is:
    • Delta P = KQ to the power of " n"
    • K = system characteristic constant
    • Q = air flow, CFM
    • n = turbulence factor, 1 < n > 2
    • Laminar Flow, n = 1
    • Turbulent Flow, n = 2
  5. The final step overlays your system characteristic curve on the air performance curves of selected, alternative, air moving devices.
  6. Intersection points are "possible fits" as exemplified in the Operating Point graph. Static efficiency should be considered. It is the optimum relationship of air flow times static pressure divided by power. Rather than calculating this separately, it is most easily found by looking at the slope of the static pressure curve. When the delta slope is at its lowest point in the fourth quadrant (270 - 360 degrees), you can be assured that static efficiency is being maximized. The best air mover for your application will be at the point of intersection of the system characteristic curve and the air performance curve when the intersecting point is on a rising portion of the fan curve, and when the rate of change in the slope is minimized.
Basic Guidelines, Application & Use
It's important to combine all the design criteria and determine the best way to evaluate and prioritize all the factors that play a role in the final selection of the right air mover for your specific application.
  • Locate components with minimum heat rise closer to the air inlet and components with the highest heat rise closer to the exhaust end of the air flow path
  • A larger air moving device operating at a lower speed is the best way to reduce noise levels
  • If increasing the air flow causes a disproportionate increase in static pressure, the air passages or the exhaust outlets should be redesigned before any changes are made in the type or size of the air mover
  • 90% of the air flow passes through the blade or impeller tips; therefore, a filter must be placed far enough away to effectively use its entire surface area. Placing it too close will cause a ring to form close to the blade passage air.
  • When directed volumes of high velocity air, such as forced air cooling of high horsepower drive motors are required, centrifugal blowers should be selected. Preferable are ones with external rotor motors that mount directly to the impeller to eliminate the extended shaft, hub, set screws and any maintenance problems.
Fan/Blower Comparison Chart
These tips will help you select the best fan for your application.
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