Ever wonder why ceiling fans only go so fast? Ever think that if manufacturers just made ceiling fans spin faster, they would move more air? Well, maybe you don't think of these kinds of things, but as the webmaster for the Largest Ceiling Fan Website online, I can't help but have such thoughts.
The fact is: There are UL standards that place limitations on ceiling fan manufacturers that prevent them making fans that spin too fast. These limitations for safety purposes and are meant to minimize the risk of injury should a person raise their hands (or any other object) into the path of the ceiling fan blades while the fan is in operation. Imagine if some ceiling fan blades moved as fast as an airplane propeller and had blades like a machete! Well, UL standards prevent manufacturers from making such products...but they also present challenges to engineers who attempt design ceiling fans for maximum airflow and efficiency.
The first basic limitation is that no residential ceiling fans are allowed to be installed such that the blades are less than 7 feet from the floor. The second basic limitation has to do with the RPMs at which a fan can spin relative to the thickness of the blades. The concept is that thinner blades can cause more damage of laceration, so they are not allowed to spin as fast as those with thicker blades.
The RPMs allowed for any residential ceiling fan that can be installed on a ceiling less than 10 feet high are restricted by UL based on the blade span and thickness of the blades (see chart below). Any blade less than 3/16" thick cannot exceed the RPMs for 1/8" thick blades. Any blades 3/16" thick or greater cannot exceed the RPMs for 3/16" thick blades. In no case can the fan blades be less than 1/8" thick. No ceiling fan is allowed to be installed with the blades closer than 7 feet from the floor. These limitations are in place to minimize the risk of injury should a person raise their hands (or any other object) into the path of the fan blades while the fan is in operation.
The amount of air that a ceiling fan produces is certainly affected by how fast is spins. Yet RPMs alone do not produce airflow. The aerodynamics of the blades is every bit as important as the RPMs. If a fan has blades that are straight and flat, it doesn't matter how fast it spins, it's not going to generate much airflow. Take the same blades and give them a 15 degree pitch and the fan will move air, but the RPMs will be slower since the angled blades are now having to perform some work, which puts drag on the motor. You can equate this to the difficulty of rowing a boat when you place the oars flat in the water compared to turning them at an angle. Flat is easy to row, but will not get you anywhere. Angling the blades just a few degrees makes it more difficult to row the boat, but you actually start moving. So imagine the difference in airflow between 2 fans that operate at the same RPM yet one has flat blades and the other has angled blades. The fan with angled blades must have a more powerful motor in order to maintain the same RPM as the fan with flat blades. So judging the airflow performance of a fan strictly by comparing RPMs is of little value. What you really need to know is how much airflow the fan will produce when operating at a particular RPM.
The data below is based on ceiling fans that have a reverse function. Non-reversing ceiling fans are allowed to have slightly higher RPMs. UL actually states the limitations based on the maximum allowed speed of the tip of the fan blade at 3200 feet/minute for 3/16" thick blades and 2400 feet/minute for 1/8" thick blades. We have provided the following data by creating a formula that does this calculation based on the overall bladespan of any particular ceiling fan ranging from 24" to 84".
Based on UL 507 Standards (link to PDF removed by request of UL - 5/30/2013)