Table of Contents
What exactly is CFM?
The easiest way to calculate airflow is in cubic feet per minute, or CFM. The size of an area is measured in square units (like square feet, square inch etc). Volumes (such as a room full of air) are measured in cubic feet per minute and CFM specifies how many cubic feet of air can be exchanged, transported or transferred per minute.
A 10,000 CFM system will be needed to replace all of the air in a 10,000 ft3 room every minute.
PSI vs. CFM vs. HP
The three key ratings that demonstrate what a compressor can do are PSI, CFM, and HP.
- The amount of pressure applied to a square inch of space is measured in PSI (pounds per square inch). PSI stands for pounds per square inch of force that an air compressor can produce.
- CFM stands for cubic feet per minute and it refers to the flow rate of a compressor or the amount of air it may generate at a given pressure level. Compressors with higher CFM ratings will typically provide more air, making them suitable for larger applications.
- HP stands for horsepower or the amount of work a motor can do. When deciding whether your compressor will work for you, HP is less critical than pressure and flow, as newer and more powerful compressors can do more with less HP. Doing more with less HP would save you resources and provide you with higher returns over the compressor’s lifetime.
How to Measure the Volume of a Specific Space?
Multiply length × width × height to measure the volume of a room. A room that is 50 feet long, 40 feet wide, and 20 feet tall is 40,000 feet³ (50× 40 × 20 = 40,000).
Some areas have irregular walls, bent ceilings or other characteristics that render measurement more difficult. Complicated shapes can be broken down into basic shapes to be measured and the results can then be added up to get the sum.
Simply multiply the total area by the average height to get the volume of any shape.
CFM (Cubic Feet per Minute) is an airflow measurement unit used in HVAC calculations.
Most of the time, we need to measure CFM for a room in order to instal fans, air purifiers, air conditioners, and other similar devices.
Example:
We have a standard bedroom of 600 square foot. How much CFM should the fan of that room have if we need to change all air twice per hour (every 30 minutes)?
CFM Calculation:
The airflow must be powerful enough to exchange the air of the entire volume of a 600 square foot space with an 8 square foot ceiling height twice per hour.
Volume of a room = 600 sq ft x 8 ft = 4,800 ft3. To change it 2 times per hour (ACH = 2), we need to deliver 9,600 ft3 per hour. CFM is a ft3 per minute unit. That’s why we need to divide the total volume by 60; hence 9,600/60 = 160 CFM.
Answer For CFM Conversion:
We need a 160 CFM airflow for a 600 sq ft standard room and 2 ACH).
Here’s a neat CFM calculator that calculates CFM based on length of room , width of room and ceiling height, and the number of air changes per hour (ACH).
CFM Airflow Calculation Chart
How Frequently The Air of a Room Should Be Exchanged?
Now that you know how much air is in a certain room, you have to figure out how quickly it must be exchanged. Specific airflow conditions vary depending on the setup of the room, but here are a few general exchange rates to consider:
Places that need air exchange rate of 1-4 minutes
Places where heat or gas is produced (ex. Engine room, boiler room). Since there are potentially hazardous exhaust gases in these spaces, all air should be cycled every 1-4 minutes. If your engine room is 1000 cubic feet, you’ll need a machine that can shift 250-1000 CFM.
Places that need air exchange rate of 2-5 minutes
Places where a mild amount of heat or gas is produced(ex. Kitchen, Lab). Moderate-to-high air circulation is needed in laboratories and areas where food is prepared or served (roughly every 2-5 minutes). Look for a device that can accommodate approximately 200-500 CFM for a 1,000 ft3 food-related area or laboratory.
Places that need air exchange rate of 3-7 minutes
Many industrial environments, while not as demanding as engine rooms or food preparation areas, also require consistent airflow to eliminate work-related fumes and keep the air clean. A device capable of pushing 140-335 CFM will be needed for a 1,000 ft3 industrial area, for example.
Places that need air exchange rate of 4-10 minutes
Conference rooms, retail stores, and offices, as well as homes and public spaces, are slightly less challenging when it comes to air exchange than the other styles of rooms listed above. Air exchange is normally not as essential in these spaces, and a 1,000 ft3 space will require a device capable of moving 100-250 CFM.
FAQ’s about CFM Calculation
How to calculate CFM airflow?
Determine the Flow Velocity in feet per minute, then multiply this number by the Duct Cross-Sectional Area to get Air Flow in Cubic Feet per Minute (CFM).
What is 1 CFM?
A particle counter's flow rate, measured in cubic feet per minute(CFM), is the rate at which a pump pulls air through the sample chamber. 1 CFM and 0.1 CFM are two typical flow rates for particle counters. Each flow rate can be useful depending on the application.
What is the difference between PSI and CFM?
The difference between PSI(Pounds per Square Inch) and CFM(Cubic Feet per Minute) are what they measure. Pressure is measured in PSI, while volume is measured in CFM. Air compressors' output is often measured in PSI and CFM. They represent the maximum air volume and pressure that an air compressor can generate to power air tools.
Is a higher CFM device better?
The volume of air delivered by a ceiling fan is the airflow and airflow is measured in CFM, which stands for cubic feet per minute. The higher the CFM, the more powerful the fan is, and the more air it pushes. So, a higher CFM is better if you need more air exchanged per minute.
Is too much CFM bad?
Only if you're warming or cooling with an a/c or heater and it's not keeping up because the air is being dumped too quickly, that’s the one scenario where high CFM would be a problem. Otherwise, the more venting the better.
How To Measure How Much CFM Is Needed?
You can measure the CFM required for your device once you've calculated the volume of the room and how quickly you'll need to exchange the air in a room. The total CFM you need for your device is calculated by multiplying the total volume of air (in cubic feet) by the exchange rate (how quickly you want to replace the air). Keep in mind that many applications (especially those involving large areas) employ multiple fans/blowers. To manage the total CFM, it's always more cost-effective to use a number of smaller units rather than a single large unit.
Is high CFM better in bathroom fan?
The right CFM rate for your bathroom depends on how larger the bathroom is. A fan with a high CFM rate should be able to refill the air in a standard size bathroom eight times per hour.
How much CFM is equal to 1 HP?
3 to 4 CFM is equal to 1 HP. The rule of thumb is each 1 HP motor produces 3-4 CFM of compressed air flow at 90 PSI.
How much CFM is a kW?
For handheld, rotary screw air compressor applications, a more accurate approximation of kW to CFM (and vice versa) is 3-4 CFM per 1 kW.
What is good CFM airflow?
A CFM between 4,000 and 5,000 is good, between 5,000 to 6,000 is better and over 6,000 is best.
Can you convert psi to CFM?
CFM and PSI have a simple linear relationship: if you get 10 CFM at 140 PSI, you'll get 5 CFM at 70 PSI.
Why is the CFM lower at higher PSI?
Higher PSI indicates that the compressor will hold more air in the tank, enabling you to use air tools for longer periods of time. The amount of air delivered by a compressor is measured in cubic feet per minute