CFM Meaning and Airflow Calculations

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In many scientific studies, CFM is an important calculation. It also has uses in many industrial applications, including air compressors used by general contractors, fans used in the HVAC industry and safety hoods used for chemical work.

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What is CFM?

CFM stands for Cubic Feet Per Minute and is an important calculation of the flow of something. It is commonly used to measure airflow and is used in discussing the velocity of gas.

It is not commonly used to measure liquids as volumetric measurements such as "Gallons" or "liters" is handier for liquid applications.

 The measurement is positively correlated with airflow so that the more of a substance that is moved, the higher the CFM number.

 

CFM Vs. PSI

PSI stands for Pounds per square feet and is commonly associated with a discussion of CFM.

Where CFM describes, the rate and volume of air moved, PSI describes the amount of pressure that is applied.

CFM and PSI are negatively correlated. If you are looking at an application with a higher CFM, the PSI will be lower. If the PSI is higher, the CFM will be lower.

This is due to Bernoulli's principle that state an increase of the speed (CFM) of a fluid occurs simultaneously with a decrease in the pressure (PSI). Accordingly, when working with CFM in industrial applications, both CFM and PSI are often considered to gain the correct volume and pressure of air for the application.

 

CFM and AirFlow Efficiency

Often, when consumers are shopping for a product such as a fan, an exhaust vent, or an air compressor, they do not examine airflow efficiency.

In simple terms, airflow efficiency refers to the amount of air that can be moved with each watt of power. The calculation is the CFM of the product divided by watts so that:

 

CFM/Watts=Airflow Efficiency

This is less of a concern on short-use items such as an air compressor that may cycle on and off and remain off for days at a time. A fan, HVAC unit or exhaust vent that may be on for large portions of the day can provide monetary savings by being more efficient.

 

Below is a discussion of some of the popular questions surrounding CFM and applications in the real world.

 

HVAC CFM Calculations

The choice of sizing in an HVAC duct depends greatly on the amount of air that needs to be moved to adequately change the temperature in a room.

In order to know the CFM of a duct, you must know the velocity or FPM of the airflow. The higher the airflow, the higher the CFM. However, duct size has a higher correlation with CFM than increased FPM. An 8-inch duct with 2000 FPM can move more air volume than a 5-inch duct with 6000 FPM.

The standard air duct velocity for a residential duct is about 600 to 700 FPM.

 

How Much CFM Can A 4 Duct Handle?

A 4 inch round duct with 600 FPM will produce about 210 CFM.

 

How Much CFM Can A 7 Duct Handle?

A 7 inch round duct would produce around 640 CFM when operating at 600 FPM.

 

How Much CFM Can A 8 Round Duct Handle?

An 8-inch duct can transport significantly more airflow with around 838 CFM at the standard measurement of 600 FPM. This size and larger are often used as transport ducts for connecting rooms.

 

How Much CFM Can A 10 Round Duct Handle?

A 10-inch duct is a real air mover that can manage around 1,310 CFM when being blown at 600 FPM.

 

How Many CFM Can A Human Blow?

This is one of those questions that the internet struggles to find an answer for. One answer on Quora uses Bernoulli's principle to calculate that you could blow at about 41 m/s. However, that calculates out to over 86,000 CFM, which seems quite ludicrous.

The lung capacity of a male is about 6 liters of air. Most of us can only blow about 4.6 liters of air. Accordingly, it makes more sense to measure human capacity in liters per minute, with the understanding that few of us can blow for longer than a few seconds.  So Liters per second or even cubic feet per second would be a better measurement.

 

How many PSI Can human Lungs Blow?

Humans live in a world of 1 atmosphere, which is about 14 PSI. So our lungs have to be able to move air in and out in this environment. When tested with digital gauges, most humans can only blow about 2 PSI.

 

Proper CFM for Kitchen Hood

Kitchen hoods come in different sizes designed to account for the amount of ventilation they need to provide.

A Kitchen hood should provide at least 100 CFM for every linear foot of stove width.

Most American stoves are about 30 inches wide, which means that a standard residential kitchen hood should provide 250 CFM of air flow.

This should be the same for both a gas range as well as electric.

 

CFM vs. MPH for Leaf Blowers

Various marketing companies use different measurements for advertising their leaf blowers. This can lead to significant confusion for the homeowner who wishes to compare two models.

Miles per hour describes the speed at which the air moves. However, as we see from Bernoulli's principle, the higher the speed, the less volume can be moved.

For a leaf blower, it is better to choose one with slightly lower speed but a greater volume or CFM. This enables you to pick up more leaves with the airflow and move them around.

That said, you do need some speed -- or MPH -- out of your blower. A high volume fan with no speed will create a pleasant breeze -- not enough to clear the leaves in your hard.

A better calculation -- where you can find it -- is to measure the leaf blowers in Newtons. Newton Meters combines CFM and MPH to create a calculation of the amount of work that can be performed.

 

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