Outdoor Fan: Blades

SIZE

The size of an outdoor fan is referred to by its diameter. The diameter is determined by measuring the fan straight across from blade tip to blade tip at the widest point. The size of the area you need to cool will help determine the size of fan blades to use. The longer and the wider the blades the more air they will move.

The following chart shows some typical ceiling fan sizes and their recommended areas for cooling. This is just to give you a general idea -- the numbers are extremely flexible depending on the configurations of any one fan (its CFM, efficiency and wattage) and of any one area.

FAN SIZE		AREA		DIMENSIONS		TYPE OF AREA
Inches	Centimeters	Square Feet	Square Meters	Feet	Meters	Description
32"	81cm	up to 64	up to 6	up to 8x8	up to 2.4x2.4	small porch, shed, baseball dugout
34-36"	86-91cm	up to 100	up to 9.3	10x10	3x3	gazebo, greenhouse, festival booth
38-42"	96-106cm	up to 144	up to 13.4	12x12	3.7x3.7	medium patio, carport
44-46"	112-117cm	up to 225	up to 20.9	15x15	4.6x4.6	large patio
48-54"	122-137cm	up to 400	up to 37.2	20x20	6x6	medium courtyard
56"	142cm	up to 485	up to 45	22x22	6.7x6.7	barn, pavilion
58-60"	147-152cm	up to 600	up to 55.7	25x25	7.6x7.6	event tent, livestock shelter

Larger areas may require more than one fan. Rectangular, odd-shaped areas, or partially divided areas are best cooled by multiple smaller fans.

This chart doesn't really apply to standing fans. Most standing fans blades are simply not that large. Plus standing fans of comparable sizes generally move more air. To size ceiling fans and floor or standing fans, go back to the standards of CFM, efficiency and wattage, and use the rule:

To estimate how much airflow to look for in a fan
multiply the area in square feet or square meters by 3 to 4 to estimate CFM or CMM

MATERIALS

Blades should be made of sturdy, warp-resistant materials. An outdoor fan, being exposed to much harsher elements than an indoor fan, needs to have blades that resist corrosion and warping. The most common materials are aluminum and various kinds of plastics, such as ABS. Heavy duty fans may also incorporate fiberglass into their plastic / polypropylene blades. Materials like rattan and most woods won't hold up to long periods of outdoor use. But aluminum and plastics can be treated to look very much like them and still be durable enough for outdoor use.

NUMBER

Three is generally the most efficient number of blades for outdoor cooling. Four to six blades are usually used because the extra 1 to 3 blades makes for a quieter fan. More blades than six is almost always less efficient, but it can be more decorative. Fewer than three blades doesn't take advantage of a motor's power and moves less air. Choose a three-blade outdoor fan when noise level is not as important but cost and efficiency are crucial.

Any more than six blades can put more strain on the motor than resulting air movement would justify. The exceptions are specialty ceiling fans that may use up to ten blades and extend 24 feet across. These use a combination of specialized blades and motors.

PITCH AND TILT

The angle at which the fan's blades are angled up from the flat plane of their rotation is referred to as the "blade pitch". The greater the pitch, the greater the airflow. But as pitch increases so does the drag. So as the pitch increases, so must the power of the motor. Blade pitch usually varies between 9 and 15 degrees. Very strong motors can drive blades with a pitch in the 20s which makes for exceptional air flow.

Some outdoor fans modify not only the pitch of the blades but also their tilt. Tilting the blades back (or up, in the case of ceiling fans) distributes the blowing air more evenly around an area. So there's less air flow directly in front of the fan, but more in the outer reaches of the fan's air flow. The air flow is "evened-out."

SHAPE

The wider the blade, the more air it will move … up to a point. The blades on some very decorative fans can be so wide they would nearly overlap each other. This may not allow enough air in-between the blades for good air movement. If beauty is more important than maximum air flow, these can be a fine option.

Some fans have just the tips of their blades tilted. This doesn't "even out" the air flow the way tilting the entire blade does, but it can affect a wider area of air flow than fans with straight blades. In short, it enlarges the area a fan can affect. This is often seen in standing fans.

Still other fans have specialized shapes. Some are shaped like airplane wings with more surface area on the top than on the bottom. Some curve forward. You may need to contact the manufacturer to learn how these special shapes affect the air flow.

SPEED

Faster blade rotation is basic to greater airflow, which means greater cooling. Blade rotation is sometimes called "fan speed" and is measured in RPM (revolutions Per Minute). It is simply the number of times any one blade makes a full rotation in one minute. An outdoor fan with two speeds will have a lower RPM with less air flow on LOW and a higher RPM with greater air flow on HIGH. Look at the CFM / CMM rating for each speed.

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