Intro

In this post we’ll explain a quick, simple, and inexpensive way to check on the performance of commonly used cooling fans using a handheld anemometer. This technique will assist you in comparing your cooling fan’s current performance against its nameplate specifications, giving you a tool to track their condition. Having information about fan condition will help you schedule maintenance intervals or determine if the fan needs to be replaced.

Cooling fans are critical components used for regulating the temperature of engines, motors, circuitry, inhabited spaces, and experiment samples. It isn’t difficult to imagine scenarios in which expensive and sensitive equipment could be damaged due to the failure of a cooling fan. I feel confident assuming that we’d all like to avoid the potentially disastrous costs and consequences that fill the epilogues of equipment failure events. A non-zero number of these events could have been avoided by monitoring, maintaining, and—when necessary—replacing the problematic fans (sometimes for as little as a few dollars).

Basic maintenance for the most common fans we are likely to interact with usually consists of nothing more than blasting off the dust and grime that accumulates on the blades with a little compressed “air”. Fan replacement is rarely necessary. Even the least expensive fans can perform for years! Fans are cheap, easy to acquire, and require very little skill or knowledge to implement and maintain. These qualities make these ubiquitous little workhorses well-suited for most temperature regulating duties.

So, what if we wanted a way to check up on our aging fans? You know the ones—faithfully purring away in the shadows, cooling critical equipment longer than anyone can remember. If only there was some convenient metric we could use to compare the old fan’s performance against what it was back when the fan-storks first delivered Baby Fan to our doorstep all those years ago. We could gain some insight into the present “health” of the fan on its stoic journey towards inevitable doom.

Fortunately for us, manufacturers include their fan’s volumetric flow rate (VFR) as one of the listed performance specifications! We can approximate the VFR for our fans by taking a few simple measurements and making a few simple calculations. Comparing the fan’s current VFR against its rated VFR can help alert us to small problems before they become bigger ones. (Note: We don’t like “expensive noises”, “unwelcomed smells”, or error codes any more than you do).

Volumetric flow rate is just a fancy way of saying, “This is how many volume-units passed by here per some unit of time,” and is calculated below with its dimensional units given in the brackets:

Equation 1:

where Q is the volumetric flow rate, V is the fluid velocity, and A is the cross-sectional area of the fluid stream (area normal to the flow direction). Commonly used units are m³/s, CFM (aka: ft³/min), and ml/hr. The handheld anemometer will give us a velocity, and we will calculate the area via simple measurements taken from the fan.

(Note: Why “Q” for “volumetric flow rate?” Well, way back in 1840, Jean Léonard Marie Poiseuille was busy experimentally deriving a really great formula to get the pressure drop of blood within blood vessels. He realized that volumetric flow rate was an important “…quantity…” for his formula.)

We are using an AC-powered ducted fan (Orion Fans, model OA80AP-11-1 TB, 115 VAC, 50/60 Hz, 14/12 W), with a nameplate volumetric flow rating of 30 CFM.

The cooling fan consists of a fan blade and motor suspended in a ducted cast metal or molded plastic mounting box by support arms that house and route the electrical wiring from the central motor to the connection points on the outside of the box.

Thus, the cross-sectional area can be approximated as the difference between the cross-sectional area of the duct and the cross-sectional area of the central hub of the fan that houses the motor and associated electronics.

Equation 2:

where A is the cross-sectional area, D­²_duct and D²_hub are the diameters of the duct and hub, respectively. (Note: We can safely use the full diameter of the duct in our area calculation—as opposed to the smaller diameter of just the fan’s blade-span—because ducted-fans force more air to receive more work from the blades. We’re essentially taking an average velocity of all the air that the fan is moving. The air between the ends of the blade tips and the inner wall of the duct is moving at a speed close enough to that average velocity that we will want to make sure we include it. Additionally, the area estimate can be further refined by subtracting the cross-sectional area of the support arms from the cross-sectional area of the duct along with that of the hub, but we chose to neglect those measurements for our test.)

Measurements and Calculations

Although we operate in the United States—a country that insists on clinging to the imperial system (despite legally defining nearly all of our imperial units as their SI counterparts with the Metric Conversion Act of 1975)—we will perform our core calculations using SI units before converting to cubic feet per minute [CFM] at the end.

Our cooling fan duct-diameter measured 75 mm (0.075 m), and the hub measured 53 mm (0.053 m). Plugging those values into eq. 2, we calculate the cross-sectional flow area to be approximately 2212 mm², or 0.002212 m².

Our fan-type anemometer measured the air speed at 6.78 m/s. This was the maximum reading we observed, achieved by holding the back of the meter flat against the down-stream side of the fan’s mounting box, and making small adjustments until no higher velocity could be measured.

(Note: We were careful not to contact the moving parts of either the cooling fan or meter, and that the air flow was clear of obstructions)

The position of the meter when we took this velocity reading was such that the meter’s fan-type center axis was positioned just inside of the cooling fan hub’s outer edge and was parallel with the rotational axis of the cooling fan vanes. Both were positioned such that the support arms of the meter and cooling fan were oriented so as to maximize the overlapping ducted regions of the fan and meter (achieved by conducting a “highly precise” visual inspection in strict compliance with accepted industry standards).

Plugging our measured and calculated values into eq. 1:

For CFM, simply multiply Q by the unit conversion ratio:

Analyzing the Results

Since our fan is factory rated to deliver 30 CFM, we can be reasonably certain that our calculated value of 32.8 CFM indicates that it is functioning at full performance (or near enough to full performance that we don’t need to be concerned). We might start to worry if our calculated value for the fan’s volumetric flow was significantly below 30 CFM, i.e. 10% or more below rated, or $\leq$ 27 CFM. Holding it to tighter tolerances could necessary if the equipment relying on the fan was especially critical, sensitive, or costly. This is a rough technique used to quickly and conveniently check the performance of cooling fans. We make sure to use our best judgement when it comes to making important preventative maintenance decisions, and we support you doing the same!

Thank you!