Spot cooling fans serve as an effective solution for providing supplementary comfort cooling. These fans create airflow across individuals, enhancing their sweat evaporation rate. The effectiveness of spot cooling fans heavily depends on their sizing and placement to ensure optimal air movement. This requires a thorough understanding of two crucial factors: Spread and Throw. In this blog, I’ll discuss how Spread and Throw formulas can be used for sizing spot cooling fans.
Spot Cooling Application
We are not going to use Spread and Throw formulas to find the best location for an existing spot cooling fan. For this application, we are going to use the Spread and Throw formulas to size the spot cooling fan.
Let’s assume that we have a 6-foot-tall person working in front of a machine. We will want the spot cooling fan to provide air flow from head to toe. Also, the fan will be column mounted 8 feet from the floor and the column is 10 feet from the person as shown below:
Using Spread Formula to Find the Fan Diameter
Spread can be defined as the width of the divergence of the air stream measured in either a horizontal or vertical plane at a specific distance from the fan outlet. The Spread formula is S = D + .36X, where D = fan diameter and X = distance measured from the fan outlet.
To find the right spot cooling fan diameter for our application, we first need to find distance X. Using the Pythagorean Theorem (a2 + b2 = c2), X equals 12.8 feet. Using the Spread Formula to solve for D (D = S – .36X), where S = 6 feet and X = 12.8 feet, we can determine that the required fan diameter is 16.7 inches. For fan selection, the closest common size up is 18 inches so we will use an 18 inch diameter spot cooling fan for our application.
Determine Airflow Using the Throw Formula
Throw is characterized as the linear distance between the fan outlet and a specific average terminal velocity. Unlike Spread, Throw varies based on the fan’s air flow volume. Additionally, Throw calculations incorporate the Spread formula, resulting in a more complex equation:
In our application, we know that T, distance from fan to person, is 12.8 feet. We also know that D, fan diameter, is 18 inches or 1.5 feet. For TV, terminal velocity, we will use 200 feet per minute. At that rate, air flow across a person will provide sufficient sweat evaporation rate. The person will feel 8 to 10 degrees F cooler than the ambient air temperature.
Solving the Throw formula for CFM we get CFM = πTV x ((T + 2.7778D)/5.556)2. Using the inputs of T = 12.8, TV =200 and D =1.5, the required air flow is 5860 CFM.
Fan Selection
Using the Spread and Throw formulas we now know that to get the desired cooling effect for our application, we need to select an 18 inch spot cooling fan that provides 5860 CFM. Unfortunately, moving that volume of air through an 18 inch fan will create a high noise level of over 80 dBA. We don’t want to solve the cooling problem and create a noise problem. Increasing the fan diameter to either 24 or 36 inches will reduce the noise problem. However, we would then need to recalculate the required CFM. Changing D in the CFM formula to 2 and 3 feet, we find that the required air flow increases to 6860 and 9092, respectively.
Conclusion
Using Spread and Throw formulas can help you make a smart decision when purchasing spot cooling fans. Selecting a larger fan than is needed will cost more in capital and operating expenses. If you need help making the right choice in selecting spot cooling fans that will increase the comfort and productivity of your employees, contact a ventilation expert at Eldridge. We will find the right size spot cooling fans to create a successful environment in your facility.