Are cooling fans really cool?

OK, I received several nice questions from various people asking why the fan was put on the bottom of the enclosure blowing inwards on a blog post picture. The funny thing is that the location of cooling fans is like fitness, religion, and politics…everyone has a very passionate and specific belief in what is correct.

IMHO, the answer is application specific. Take this application in the picture. The location was intentional to the application.


Bottom fed Cooling Fan
Cooling Fan Fed from the Bottom
Top mounted cooling fan
Cooling fan pulling from the top


Testing in the Past

Years ago, we had a testing chamber that allowed us to change the environment to test VFDs under various conditions (temperature, voltage, humidity, and pressure.) This was back in 2001 and as an afterthought, we decided to test various configurations of whether the fans blowing in or out in different configurations mattered. The results have stuck with me over the years and have made their way into my designs. The optimal location for cooling at a slightly negative pressure was placing the fan on the upper and opposite location from the filter drawing air outwards. If the fan was located on the bottom, then we saw the air (with added smoke for visibility) circulate more than the other method creating a slightly positive pressure. The temperature difference in the two methods was about 25% better with the output fan versus the input fan. Put another way, if your were to model this, the vector comets would show less turbulence drawing outward.

Drive Companies like the Exhaust

So, this was re-enforced by drive companies like ABB, Lenze, and a few others with conversations over the last several years. In the application picture with the bottom fan, the 25% loss of cooling was acceptable (70* ambient temperature in the installation location) and the positive pressure was highly desirable because of potential conductive dust present around the location of installation. Of course, AC units and heat exchangers were options, but out of the budget range.

Fan Companies like the Inlet

Pentair, like many other cooling fan companies, states the following:

“Fans can be used at the exhaust to draw air through an enclosure, or at the inlet to blow air into the enclosure. Generally, a blowing fan at the air inlet is recommended for the following reasons:1. A fan at the inlet will raise the internal air pressure within the enclosure, which will help to keep dust and dirt out of an enclosure that is unsealed or opened frequently.2. A blowing fan at the inlet will produce slightly more turbulence, which improves the heat transfer characteristics within the enclosure.3. Fan life is prolonged since it is located in the path of the entering cooler air.”

Last Words

All I can do is rely on what I tested. This is the main reason that we at B&D have our own labs. We like to actually test things for ourselves instead of just relying other folks. I welcome comments and questions.

C. Tolbert

VP Engineering