Thursday, August 14, 2008

Saving Energy with the Fan Law

The fan law is a set of formulas describing the behavior of air flow and horsepower requirements at different fan speeds. This law has one very interesting component, the relationship between fan speed (RPM) and fan horsepower requirements. As the fan speed changes, the horsepower change is the cube of the change in speed. In other words, if the fan speed is decreased by 10% (or a new speed of .9 of the original speed) the fan horsepower is reduced 27%. This is calculated by taking the speed ratio and cubing the number or multiplying it by itself 3 times. Therefore a .9 change in speed is .9*.9*.9 or .729 of the original. What this means is a relatively small change in a fan speed results in huge horsepower and dollar savings.

Take for example, a 50 hp fan motor running at full capacity. A 10% reduction in speed reduces the horsepower requirements to 37 horsepower. Over a year, this speed reduction can easily account for thousands of dollars.

There are many instances were a 10% speed reduction or more are possible all the time or even some of the time.

The two ways to reduce a fan speed are mechanical (pulleys) or electrical (variable speed drive). If you know that a fixed reduction is possible at all times, a simple pulley change will give you predictable savings. If fan speed reductions aren’t precisely known or if the speed change varies (less air in the winter, more in the summer) an variable speed drive or inverter is the best solution.

An inverter is a relatively inexpensive item when compared to the annual savings. They are easy to install, easy to use and very reliable. There are other benefits to an inverter such as reduced start speed minimizing mechanical stress and electrical demand spikes.

Numerous inverter manufacturers are available. When choosing an inverter, make sure it is fan rated. There are some special software settings on an fan rated or HVAC rated inverter that make it especially suited for this purpose.

Some of the most economical choices for fan rated inverters are AutomationDirect at or AC Tech at

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