Both CFM and SCFM are units for measuring airflow. That can be an airflow generated by an air conditioner, furnace, ceiling fan, compressor, and other HVAC devices.
CFM stands for Cubic Feet per Minute. SCFM stands for Standard Cubic Feet per Minute. As we see, the only difference between SCFM and CFM is that inclusion of ‘Standard’. And it’s there for a very important reason.
Here’s the thing:
Not all air is the same. If you look at SCFM vs CFM, both are volumetric units of air. However, the same volume of air can have different temperatures, pressure, and humidity. That also means that the amount of air (mass of air, if you will) is not the same at all temperatures and air pressures.
Note: It’s a bit difficult to explain the differences between SCFM and CFM without going too much into physics and Ideal Gas Law. We try to explain it as simply as possible. Below you will also find an SCFM to CFM calculator that automatically converts SCFM to CFM. Here is the complete SCFM to CFM formula we derive further on:
CFM = SCFM × (14.7 psi ÷ Pactual ) × (Tactual + 459.67) ÷ (68°F + 459.67)
From experience, we all know that warmer air rises (due to lower density). We also know that moist air sinks (due to higher density). SCFM vs CFM is just a means to quantify exactly what amount of air is moving (not just what volume of air but the exact mass of air).
Example: A typical 1-ton air conditioner will generate 400 CFM of airflow. How many SCFM airflow we’re talking about here? It can be 350 SCFM, 390 SCFM, 450 SCFM airflow. In very specific cases SCFM can be equal to CFM; ie. 400 CFM.
This variable nature of air calls for standardization (based on air pressure and temperature). That’s why we have come up with SCFM – a standardized measure of airflow, expressed in CFM. All of this might sound a bit complex. Here’s the simplified version:
- CFM is airflow measured at any temperature and any pressure.
- SCFM is airflow measured at a specific temperature (68°F) and at a specific pressure (14.7 psi).
To truly understand what SCFM and CFM are and what are the differences between SCFM and CFM, let’s look at each of them individually. Knowing what each of them means will help us illustrate the main SCFM vs CFM differences:
Understanding SCFM (Standard Cubic Feet Per Minute)
If you check the Ideal Gas Law, you can see why not all air (gas) is the same. Here is the Ideal Gas Law equation:
P×V = n × R × T
You can see there are quite a few variables here: P is pressure, V is volume, n is the amount of gas, and T is the temperature (R is a constant). CFM will only tell us what the volumetric nature of gas is; all other variables are unspecified.
That can cause quite a lot of problems because you are unable to compare airflow in CFMs alone. That’s why SCFM was introduced. SCFM doesn’t only tell us what the volume of the air is (V), it also specifies what is the pressure (P), amount of gas (n), and temperature (T). SCFM differs from CFM mainly in that SCFM has these variables fixed.
The most common SCFM standardization specifications are set by the American Society of Mechanical Engineers (ASME). Pretty much all US manufacturers that produce devices with airflow will use the ASME airflow unit standardization.
Here are the specific metrics that define SCFM:
- Pressure (P) is 1 atmosphere or 14.7 psi.
- Temperature (T) is 68°F.
- Relative humidity is 36% (used for calculating ‘n’ or amount of air).
- Air density is 0.075 lbs/ft3 (used for calculating ‘n’ or amount of air).
Simply put, SCFM is a measure of airflow at exactly 1 atm pressure, at exactly 68°F pressure, and air density of 0.075 lbs/ft3 (36% relative humidity levels).
Now, in most cases, we don’t have exactly 1 atm pressure and 68°F pressure temperature, and so on. In all those cases, we don’t talk about pressures and temperatures; we only talk about the volumetric amount of air. And that volume of air is measured in CFMs.
Understanding CFM (Cubic Feet Per Minute)
CFM is the simplest way to describe the volume of air that is moving. 1 CFM airflow means that every minute, such a device will generate a total of 1 ft3 of airflow.
If you compare the CFM to SCFM, the CFM unit doesn’t tell us anything about temperature (T), pressure (P), humidity levels, and so on. It only tells us about 1 variable and that is the volume of air (V).
Of course, if we know at which temperature and pressure we are measuring this airflow in CFM, we can convert these CFMs into CFMs at standardized conditions (1 atm pressure, 68°F pressure). That’s the CFM to SCFM conversion.
Obviously, the reverse is also true. If we have SCFM, we can convert them to CFM, if we know at what pressure and temperature these CFMs will be.
How To Convert SCFM To CFM
Let’s say we have an air conditioner that generates 400 SCFM of airflow. We would like to know how many CFM does this air conditioner generates, and calculate it from SCFM.
For SCFM-part, we know that this AC will generate 400 SCFM of airflow. That is airflow at 1 atm (14.7 psi) pressure and 68°F temperature (or 293K).
We, however, are using this AC in a room with a bit higher pressure (say 15.3 psi) and a bit lower temperature (say 65°F or 290K).
Because of the difference in pressure and temperature, 400 SCFM will not equal 400 CFM. To adequately convert SCFM to CFM, we use the following SCFM to CFM formula:
CFM = SCFM × (14.7 psi ÷ Pactual ) × (Tactual + 459.67) ÷ (68°F+459.67)
This might look like a daunting equation, but it’s quite easy to use. You just plug in SCFM, P and T. Here’s what you get in our example:
CFM = 400 CFM ÷ ((15.3 psi ÷ 14.7 psi)×((68°F+459.67) ÷ (65°F+ 459.67))) = 382 CFM
As we can see, if we change the conditions at which we measure the airflow from SCFM conditions, the resulting CFMs can differ quite a bit.
Using this SCFM to CFM formula, we can devise a calculator that converts SCFM to CFM automatically. Here it is:
SCFM To CFM Calculator (Convert SCFM To CFM)
To calculate CFM from SCFM, you need to know what is the pressure (in psi) and temperature (in °F) you’re calculating the CFM at. Once you do, you can just plug them into this SCFM to CFM converter and get the resulting CFMs:
Here is an example of how this calculator works:
Let’s say we want to convert 500 SCFM to CFM at 60°F temperature and 16 psi pressure. You just use the slider, set the ‘500 SCFM’, ’60°F’ and ’16 psi’ and you get the result: 500 SCFM is equal to 452.41 CFM at those conditions.
Using this calculator, you can pretty much convert any SCFM to CFM at any condition. We have calculated an SCFM to CFM chart for pressures ranging from 10 to 20 psi and temperatures ranging from 0°F to 100°F to illustrate what’s the SCFM vs CFM difference at various conditions:
SCFM To CFM Chart (From 10-20 psi And 0-100°F)
Here are converted 100 SCFM airflow values to CFM at various temperatures and air pressures at 36% relative humidity:
|Temperature (°F):||CFM (10 psi):||CFM (12 psi):||CFM (14 psi):||CFM (16 psi):||CFM (18 psi):||CFM (20 psi):|
|0°F||128.06 CFM||106.72 CFM||91.47 CFM||80.04 CFM||71.14 CFM||64.03 CFM|
|10°F||130.84 CFM||109.04 CFM||93.46 CFM||81.78 CFM||72.69 CFM||65.42 CFM|
|20°F||133.63 CFM||111.36 CFM||95.45 CFM||83.52 CFM||74.24 CFM||66.82 CFM|
|30°F||136.42 CFM||113.68 CFM||97.44 CFM||85.26 CFM||75.79 CFM||68.21 CFM|
|40°F||139.20 CFM||116.00 CFM||99.43 CFM||87.00 CFM||77.33 CFM||69.60 CFM|
|50°F||141.99 CFM||118.32 CFM||101.42 CFM||88.74 CFM||78.88 CFM||70.99 CFM|
|60°F||144.77 CFM||120.64 CFM||103.41 CFM||90.48 CFM||80.43 CFM||72.39 CFM|
|70°F||147.56 CFM||122.96 CFM||105.40 CFM||92.22 CFM||81.98 CFM||73.78 CFM|
|80°F||150.34 CFM||125.29 CFM||107.39 CFM||93.96 CFM||83.52 CFM||75.17 CFM|
|90°F||153.13 CFM||127.61 CFM||109.38 CFM||95.70 CFM||85.07 CFM||76.56 CFM|
|100°F||155.91 CFM||129.93 CFM||111.37 CFM||97.45 CFM||86.62 CFM||77.96 CFM|
As we can see from the chart above, CFMs increase with increasing temperature and decreases with increasing pressure.
Hopefully, now the SCFM vs CFM relation is better understood. If you have any other questions, you can use the comments below and we’ll try to help you out as best we can.
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