Can I Verify the Flow Rate of an Exhaust Fan Based on Specifications?

In summary, the conversation discusses the specifications and electrical requirements for an exhaust fan. The speaker is unsure if they can accurately measure the given flow rate and may need to make assumptions. They also mention the importance of considering the fan curve and the system curve when determining the actual volume of the fan. The fan's single published CFM number may not be useful in design, and the system itself will determine the pressure the fan experiences.
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JoshDeM
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I've been given some specifications by a company for an exhaust fan that I am looking at purchasing for my company. They tell me it is 10,900 cfm. Then given some specs on the electrical requirements. I am wondering with the information given could I check their given flow rate?

I imagine that I will need to make a few assumptions along the way. I want to make sure they aren't out to lunch.
Below are the specs

243675
 
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  • #2
No.
The actual volume for the fan will depend on inlet and outlet conditions. The manufacturer will have (or you may not want to do business with them) a fan curve showing the relationship between pressure and flow. unless you're using it with no ducts and no restrictions, you'll need to figure out where your system curve intersects their fan curve. Figuring out your system curve can be a real PITA.
 
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  • #3
Dullard said:
you'll need to figure out where your system curve intersects their fan curve.
Yes! The single published CFM number isn't very useful in design, it's really for sorting fans to consider more carefully. Start by finding the "fan curve" (CFM vs. pressure) in the data sheet. Notice that for almost every fan you will actually get much lower CFM than that single number at most pressures. Systems are usually designed to operate in the middle, flatter part of the fan curve, where the flow is less sensitive to back pressure. It is your system (enclosure, ducting, etc.) that will determine the pressure that the fan will experience. As Dullard said, your system has its own flow vs. pressure curve.
 
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1. How do I calculate the flow rate of an exhaust fan based on its specifications?

The flow rate of an exhaust fan can be calculated by dividing the volume of air it can move in a given time (usually measured in cubic feet per minute or CFM) by the time it takes to move that volume of air (usually measured in minutes). This information can usually be found in the fan's specifications sheet.

2. Can I trust the flow rate listed in the exhaust fan's specifications?

The flow rate listed in the specifications of an exhaust fan is typically accurate, as it is measured and verified by the manufacturer. However, it is always a good idea to double check the information and make sure it meets your specific needs before purchasing the fan.

3. How does the size of the exhaust fan affect its flow rate?

The size of an exhaust fan can greatly affect its flow rate. Generally, larger fans have a higher flow rate as they are able to move more air. However, the flow rate can also be affected by other factors such as the fan's design and motor power.

4. Is the flow rate of an exhaust fan affected by the length of the ductwork?

Yes, the length of the ductwork can affect the flow rate of an exhaust fan. The longer the ductwork, the more resistance the fan will face in moving the air, which can decrease the flow rate. It is important to consider the length and layout of the ductwork when selecting an exhaust fan.

5. Can I increase the flow rate of an exhaust fan by adding more blades?

No, the number of blades on an exhaust fan does not directly affect its flow rate. The flow rate is primarily determined by the size and design of the fan, as well as the power of its motor. Adding more blades may actually decrease the efficiency of the fan and result in a lower flow rate.

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