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The Truth About Airflow and Amperage

2010 January 29
by admin

When you’re in the cooling-fan business for 49 years, you hear a lot of ideas and theories on what makes one fan better than another. You also run across some pretty interesting ways that people use to determine what fan they need to best fit their application.

One of the assumptions we frequently hear is that the fans that draw the most amperage are better. We’d like to set the record straight with some facts and definitions, as well as some actual product-testing results.

There are a lot of factors that determine the efficiency of an electric fan; the amount of amperage the motor requires is only one of these factors. Amperage and airflow are related, but you must also consider the pitch of the fan blades, the diameter of the fan blades, the fan shroud and also what it is that you are trying to move the air through.  For example, is the fan trying to move air through an oil cooler that is only ¾-inch thick, or is it trying to pull the air through a 5-core radiator that has an air conditioning condenser and charge-air-cooler stacked in front of it?

It may be easiest to understand if you consider a simplified comparison to the relationship between fuel economy (rated in miles per gallon, or mpg) and horsepower. Horsepower is a measurement of power output, while fuel economy measures how much fuel was consumed. The two measurements are related, but it doesn’t necessarily require a 10-percent increase in fuel economy to create 10-percent more horsepower.

Airflow

All manufacturers rate their fans by cubic feet per minute (CFM) of airflow at 0 inches static pressure. This is the amount of air the fan moves without anything in front of it. A typical, 2-core radiator will create about 0.5-inches of static pressure in front of the fan.

The factors that determine the CFM rating of a fan are the motor, the fan speed (revolutions per minute or rpm), fan-blade design pitch, number of blades, diameter, area and design of the fan shroud it is built into and static pressure.

With a cooling fan, the greater the airflow, the better that fan’s ability to cool a radiator.

Amperage

Amperage used to measure how much load an electrical device draws when in operation. Automotive fans are rated at 13.5 volts, and the rating is after the motor is at operating RPM. The initial start up of the fan will have an amperage spike of up to 60 percent, which lasts only a millisecond.

Compare the factors that shape the CFM rating of a fan to these items that affect the amperage draw of an electric fan motor: efficiency of the motor, weight of the fan blades, resistance of the electrical components, mechanical resistance of fan rotation (bearings and bearing tolerances) and the amount of airflow (CFM) through the fan.

“Wait a tick,” you may be saying. If they’re not related, why is airflow a factor in amperage? We didn’t say they weren’t related; we said that amperage doesn’t necessarily mean better cooling. You must consider the amount of airflow produced for the amperage required to produce it.

Unlike airflow – where more is better – higher amperage draws mean a greater load on your vehicle’s charging system. That load costs you horsepower and fuel economy.

Testing

To validate our point, we tested three electric fan combos. We chose applications with shrouds that were as close to the same size as possible, and each combo housing two electric fans. Here are the stats on all three fan combos:

We tested all three in our wind tunnel at Flex-a-lite®. We created 0.5 inches of static pressure to simulate a 2-core radiator. The chart below shows the results, and shows the difference between rated airflow and tested airflow.

While the difference in rated CFM between the Flex-a-lite fan and the Fan “Italy” was 168 cfm with zero static pressure. That difference was further reduced at 0.5 inches static pressure. You can also see that Fan “Turkey” looked okay with its stated CFM rating at zero pressure. But with a less powerful electric motor, it moved significantly less air under load compared to the other fans.

Summary

You can see from this example, that choosing an electric fan on airflow (CFM) or amperage draw alone is not ideal. The best electric fan is one that provides the best airflow under load with a reasonable amount of electrical consumption. This will give you the best cooling performance with the smallest impact on your vehicle’s charging system. You can find more information and specifications for all our electric fans by clicking here to go to Flex-a-lite.com.

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