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CFM 101: What is CFM And How To Calculate CFM

Imagine getting your very own apartment after years of saving up. But with new things, comes new responsibilities. You have to ensure enough light and air passage in your apartment in order to make it a comfortable abode.

What is CFM

Again, let’s say the office you work at has an amazing space designated as a smoking zone. But due to bad ventilation, the smokes tend to persist in the room for a longer period, causing discomfort to all the smokers while smoking.

What’s the solution to this problem? An easy fix for such problems is to have a proper air ventilation system. But how do you know which fans are suitable according to the size of your room? It is simple – calculating the CFM of the fan!

What Is CFM?

CFM or its elaboration “cubic feet per minute” is referred to as the amount of air that can be moved per cubic feet in a minute. Merely, it can also be termed as “airflow”. When a fan is set to its maximum speed, the CFM is calculated by utilizing both the volume of air and the rate at which the fan blades move.

The measurement of CFM is of immense significance when choosing the right fans for your room to guarantee the perfect balance of airflow, helping you stay healthy and cool at all times.

Even the manufacturers regard the CFM as a key issue in product manufacturing to compete against different brands and come up with the most air efficient fans.

What Is the Correlation between CFM and Air Efficiency?

As we already mentioned, top-rated fan manufacturers focus on producing the most air efficient fans by focusing on the CFM. But why is that? Does a higher CFM always mean that the fan is more air efficient?

For determining the airflow efficiency, the measured CFM is divided by the amount of energy (in Watts) that is used up when the fan is being operated at its maximum speed level. In such cases, the standard or minimum CFM to label a fan as an efficient one is considered to be 75.

A drawback to high CFM fans to be considered as an inefficient one is when it consumes high watt energy. So, air-efficient fans need to have both high CFM along with relatively lower wattage consumption. That’s why you always need to opt for an electric fan, taking both of these aspects in mind.

Calculating CFM: How to Calculate CFM (Cubic Feet Per Minute)

Now that we already know the significance of measuring the CFM, let’s move on to the next step – “how do we measure the measure the CFM?” Quantifying the CFM can be a bit tricky if you’re bad at math, but we have you covered! You just have to follow our instructions, and you’re good to go.

Here’s the detailed process on how to figure out the CFM easily:

Method 1: Using the Air Changes Calculation to Determine Room CFM

The air change rate is basically the measure of how long it takes for an enclosed area to replace the inside air by the outside airflow through ventilation.

And the unit used to express this air change rate is the ratio of cubic meters per hour and the volume of air within the room. Put simply, it is the number of times the air is exchanged between the inside and outside of a space.

For instance, if the volume of air in a room is replaced twice in an hour, then space is said to have two air changes per hour or 2 ACPH.

Air Change Formula to Calculate CFM

Although a number of complex computer analysis can be done to calculate the air change rate, a much simpler mathematical formula can also be used for the same purpose. This formula is incredibly straightforward and does not require any expertise to determine the ACPH (air change per hour).

You can easily use the following equation to do so –                 

To calculate the air change in a room, the airflow is measured (CFM) and multiplied by 60 minutes per hour. It is then divided by the volume of the room you wish to measure (in cubic feet):

Air Changes / hr = (CFM x 60 minutes) / Volume of the room (cubic feet)

In simpler words, we change the CFM or Cubic Feet per Minute into CFH or Cubic Feet per Hour and divide it with the volume of the room. The volume of the room is determined by multiplying the room’s height, length, and width (Volume = Height x Length x Width).

An example to clear out your confusions-

Suppose the CFM of your room is 400, and the room measurements are as follows:

Height = 10 feet

Length = 20 feet

Width = 15 feet

Then the ACPH (air change per hour) would be –

Air change / hr = (400 CFM x 60 minutes) / (10’ x 20’ x 15’)

Air change / hr = 24000 / 3000

Air change / hr = 8

Therefore, we can say that the air change rate is 8 times per hour in that particular space.

How Frequently Do I Need to Exchange Air?

Frequency in the exchange of air differs due to differences in room sizes and types of the enclosed area. While a smoking zone requires considerably more air change rate, the ACPH for a basement is quite low.

Here is a list of the air change frequencies of some common spaces-

Residential

Usual Air Change

Basement

3 – 4

Bedroom

5 – 6

Bathroom

6 – 7

Family Living Room

6 – 8

Kitchen

7 – 8

Laundry

8 – 9

Office

 

Business Office Room

6 – 8

Lunch Break Room

7 – 8

Conference Room

8 – 12

Copy and Print Room

10 – 12

Computer Room

10 – 14

Smoking Zone

13 – 15

Restaurant

 

Dining Area

8 – 10

Kitchen

14 – 18

Bar

15 – 20

Public Area/Building

 

Hallway

6 – 8

Stores

6 – 10

Church

8 – 12

Restroom

10 – 12

Auditorium

12 – 1

Smoking Room

15 – 20

Room CFM Formula

At times, the airflow or CFM of a particular space may be unknown to us. On certain occasions where you are confused about what CFM fans might be required for your room size or type, you can easily use the standard air change table above to calculate the airflow.

To do so, you simply change the equation or formula for the air change measurement. Hence, you can just calculate the volume of the room in cubic feet, multiply it by the desired air change per hour (our table can really come in handy!), and divide the product by 60 minutes.

Or in a more straightforward way, this is how to do it –

CFM = (Volume of the Room x Air Changes / hr) / 60 minutes

A sample to make it easier for you –

Suppose you want to help determine the CFM of your friend’s newly bought store. It necessitates an air change value of around 8 times per hour and has a volume of around 6000 cubic feet (10’ x 30’ x 20’). The required CFM would be –

Required CFM = (10’ x 30’ x 20’ x 8 ACPH) / 60 minutes

Required CFM = 48000 Cubic feet / 60 minutes

Required CFM = 800

This indicates that for proper airflow in the store, you would need to install a fan with around 800 CFM. This may vary in certain cases if you decide your air change rate to be a bit higher or lower.

Reference: https://www.contractingbusiness.com/service/article/20868246/use-the-air-changes-calculation-to-determine-room-cfm

Method 2: Using CFM Calculator

With the advancement of technology, you don’t really have to go all the way to use your math skills (lucky save if you’re bad at it!) and waste your time to determine the CFM that is needed for your space. Your savior is really just a CFM calculator!

What Is a CFM Calculator?

Calculators or, more precisely, CFM calculators help you to measure the airflow in a much more hassle-free way. Interesting, isn’t it? You can easily find such calculators online, which are, in fact, free!

How to Use CFM Calculator to Measure CFM

CFM Calculator

Using a CFM calculator is pretty easy and does not require any expertise at all. All you have to do is measure the height, length, and width of your enclosed space and input it into the virtual calculator along with the frequency of your desired air change rate. And voila! The calculator shows you the required CFM!

Some CFM calculators also come in with options to input the wattage consumption as well as the number of light sources in the room, which helps in a much more precise calculation.

Pretty amazing how technology can help you to always be at ease by just sitting back and relaxing, right?

Conclusion

Even though determining the CFM and air change rate may seem like an extra burden to most people, you now know how essential it really is to ensure appropriate air ventilation for the residential or commercial spaces. Then why wait? Go and spread your knowledge to your friends and family now!

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