# Calculating the required fan size and cfm.

• Uridan
In summary, an electronic engineering student is researching control systems for a vertical wind tunnel and is struggling with calculating the necessary fans and airflow. They plan to use multiple fans to ensure the ball stays at the desired height and are seeking help with selecting appropriate fans for the project.
Uridan
hi,

I am an electronic engineer student and I am doing a study on control systems that can be used to control a ball's height in a vertical wind tunnel.

I have a problem in calculating the fans needed (as in size and number of blades) and the amount of cfm that the fans need to produce. The wind tunnel specs are:

Upper diameter: 26cm
mid diameter: 22 cm
lower diameter: 18cm
Tunnel height: 50cm

the ball that needs to be lifted requires 10m/s winds for terminal velocity.

I don't know that much about flow rates and fluid mechanics that is why I am asking :shy:.

My plan is to make a number of fans at the top of the wind tunnel so if one of the fans fail, the other fans will compensate to keep the ball at the pre set height.

Thanks, any help would be appreciated :)
Regards
Uridan

Welcome to PF.

If you have the dimensions (how big is the ball, though, and where will you be holding it?) and the required velocity, you can calculate the flow rate with simple geometry (cross sectional area times velocity). Obviously, you have some units to convert...

For fan selection, you have a fair amount of velocity there, which means a fair amount of pressure so I'm not sure an axial (bladed) fan will do. You may need a centrifugal fan. Once you have the airflow, though, start looking for fan curves of available fans and you'll start to get an idea of how real fans perform.

Hi Uridan,

Calculating the required fan size and CFM (cubic feet per minute) for your wind tunnel project will involve some basic fluid mechanics principles. First, you will need to determine the required air velocity to achieve the 10m/s winds for terminal velocity of the ball. This can be calculated using the formula V = √(2gh), where V is the velocity, g is the gravitational constant (9.8 m/s²), and h is the height of the tunnel (50cm in this case). This gives us a required air velocity of approximately 3.13 m/s.

Next, you will need to calculate the cross-sectional area of the tunnel. This can be done by using the formula A = πr², where A is the area, and r is the radius of the tunnel (13cm for the upper diameter, 11cm for the mid diameter, and 9cm for the lower diameter). This gives us a total cross-sectional area of approximately 850 cm².

To determine the required CFM, we can use the formula Q = AV, where Q is the flow rate (in cubic feet per minute), A is the area (in square feet), and V is the velocity (in feet per minute). Converting our values to feet, we get a required flow rate of approximately 146 CFM. This is the minimum flow rate needed to achieve the required air velocity in the wind tunnel.

As for fan size and number of blades, it will depend on the specific fan you choose to use. You will need to consider the fan's CFM rating, static pressure capabilities, and efficiency. It would be best to consult with a professional or do some research on different fan options to determine the best fit for your project.

In terms of having multiple fans in case of failure, it is a good idea to have a backup system in place. However, keep in mind that adding more fans may also increase the overall airflow and potentially affect the accuracy of the control system. It may be beneficial to have a backup fan with a lower CFM rating to maintain consistency in the airflow.

I hope this helps you in your project. Good luck!

Best regards,

## 1. How do I determine the required fan size for a specific area?

The required fan size can be calculated by considering the volume of the area in cubic feet and the desired air changes per hour. First, measure the length, width, and height of the space in feet. Then, multiply these values to get the volume in cubic feet. Next, determine the recommended air changes per hour for the type of space (e.g. 4-6 for offices, 8-10 for kitchens). Finally, divide the volume by the air changes per hour to get the required fan size in cubic feet per minute (cfm).

## 2. What is the relationship between fan size and cfm?

Fan size and cfm (cubic feet per minute) are directly related. The larger the fan size, the higher the cfm. This means that a larger fan will be able to move more air and provide better ventilation for a given space.

## 3. How do I calculate the cfm needed for a specific application?

The cfm needed for a specific application can be calculated by considering the ventilation requirements for the space. This includes factors such as the size of the space, the type of activity or equipment in the space, and the recommended air changes per hour for that type of space. Using these factors, the required cfm can be determined by multiplying the volume of the space by the recommended air changes per hour.

## 4. Can I use the same fan size for different types of spaces?

No, the fan size and cfm needed will vary depending on the type of space and its ventilation requirements. For example, a kitchen may require a higher cfm than an office due to the presence of cooking equipment and higher heat and humidity levels. It is important to calculate the required fan size and cfm specifically for each space.

## 5. Are there any other factors to consider when calculating fan size and cfm?

Yes, in addition to the size and type of space, other factors to consider include the location of the fan (e.g. ceiling or wall), the type of fan (e.g. axial or centrifugal), and any obstructions or obstacles that may affect airflow. It is important to consult with a professional or refer to fan sizing charts to ensure accurate calculations.

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