Introduction to FAN SELECTION
Motor Horsepower |
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The motor horsepower for direct drive fans is | If this is the case, we will need a 1/2 hp motor |
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always sized by Greenheck and does not require | because our fan will have to run at almost |
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further consideration. For belt drive models, | 810 rpm (refer to performance box - 2052 cfm at |
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the catalog identifies which horsepower is | 0.625 in. Ps). Therefore, choosing a 1/2 hp motor |
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recommended. However, there are times when it | in this case is exercising good judgement. |
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is wise to bump the horsepower one size. For | The complete model designation for this |
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example, the hp recommended for the |
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application is |
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at 810 rpm (2375 cfm @ .5” Ps) is 1/3 hp. |
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Although a 1/3 hp motor is recommended, it is | Note: The |
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This means that if the static pressure is less than |
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not necessarily a good motor selection for this |
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estimated, say 0.25 in. Ps, the fan can be slowed down |
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application. Our static pressure of 0.5 in. was only |
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to accommodate this condition. |
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an estimate. It may actually turn out to be .625 in. |
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Belt Drive |
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One advantage of choosing a belt drive over a direct |
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drive is that it is capable of adjusting the fan rpm, |
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which enables the fan to move more air if necessary. |
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Motor pulleys are adjusted by loosening the set |
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| Belt |
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screw and turning the top half of the pulley (see |
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illustrations at right). This causes the pulley diameter | Opening | the pulley decreases | fan | rpm. |
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to change, which results in changing the fan rpm. | Closing the pulley increases | fan | rpm. |
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Applications | The information that follows will help walk you |
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Ventilating a building simply replaces stale or foul | through this type of problem and enable you to |
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select the right fan for the job. |
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air with clean, fresh air. Although the ventilation |
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process is required for many different applications, |
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the airflow fundamentals never change: | Fan Model |
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Undesired air out, fresh air in |
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Fans all perform the basic function of moving air |
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The key variables that do change depending on | from one space to another. But the great diversity of |
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fan applications creates the need for manufacturers |
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applications are the fan model and the air volume |
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to develop many different models. Each model has |
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flow rate (cfm). Other considerations include the |
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benefits for certain applications, providing the most |
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resistance to airflow (static pressure or Ps) and |
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economical means of performing the air movement |
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sound produced by the fan (Sones). |
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function. The trick for most users is sorting through |
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Occasionally, a customer will require a fan to perform |
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all of the models available to find one that is |
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a particular function, yet does not know which model | suitable for their needs. |
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to use or even what cfm is necessary. In this case, |
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some fan specification work must be done. |
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Fan specification is usually not a precise science and can be done confidently when the fan application is understood.
Based on the application, four parameters need to be determined. They are:
Fan Model
Cubic Feet per Minute (cfm)
Static Pressure (Ps)
Loudness limit (sones)
Propeller vs. Centrifugal Wheel
Propeller fans provide an economical method to move large air volumes (5,000+ cfm) at low static pressures (0.50 in. or less). Motors are typically mounted in the airstream which limits applications to relatively clean air at maximum temperatures of 110°F.
Centrifugal fans are more efficient at higher static pressures and are quieter than propeller fans. Many centrifugal fan models are designed with motors mounted out of the airstream to ventilate contaminated and high temperature air.
130F A N F U N D A M E N T A L S