The volumetric airflow of industrial fans is measured at standard conditions which are referred to as standard cubic feet per minute or SCFM. Standard conditions are atmospheric pressure of 14.7 psi and air temperature of 70 degrees F. But what happens to airflow if the air temperature is not at standard conditions? In this article, I will explain the formula used to do the conversion and provide some examples of how air temperature impacts airflow.
Conversion Formula
If the volumetric airflow that we are measuring is not at standard air conditions, that airflow is referred to as actual cubic feet per minute (ACFM). To convert SCFM to ACFM we must use the following formula:
ACFM = SCFM x (PStandard/PActual) x (TActual/TStandard)
What the formula tells us is that when temperature increases, airflow increases and when temperature decreases airflow decreases.
Temperature Change
Let’s first look at an extreme example to show the point. Assume we are designing a fume extraction system that will have a ducted outlet. The temperature of the air coming out of the furnace is 650 degrees F. We have determined that the SCFM needed to capture the fumes is 10,000. Our fictitious customer is located in the Gulf Coast area so we will assume the air is at standard pressure.
To size the fan for this application, we need to determine the ACFM for air at 650 degrees F. The calculation will look like this:
ACFM = 10,000 x (14.7/14.7) x (650+460)/(70+460) = 20,943
The temperature scale in the formula is Rankine and to convert Fahrenheit to Rankine we need to add 460. Doing the math, we find that the ACFM is equal to 20,943. Because the air is at 650 F, the ACFM of airflow that we need in order to capture the fumes has more than doubled from the SCFM.
Here is another example for a frozen storage facility. We are sizing a desiccant dehumidifier to provide dry air and positive pressure for the space. The SCFM to provide the positive pressure is determined to be 2,000. The required temperature in the frozen storage is -10 degrees F. Here is the calculation of ACFM:
ACFM = 2,000 x (14.7/14.7) x (-10+460)/(70+460) = 1,698
What we have determined is that the ACFM is about 15% less than the SCFM. Because we determined that 2,000 SCFM is needed, we will have to increase the output of the fan by 15% to get the required airflow.
Conclusion
The examples that we worked through show that when air temperature deviates from the standard, it is important to know how to convert SCFM to ACFM. For general ventilation applications, the difference between SCFM and ACFM will not have a material impact on the performance of the ventilation system. However, for applications that deal with extreme temperatures or those that require specific airflows, the impact of not doing the conversion may lead to a significant over or under sizing of the fan for the application.
If your ventilation system or process air system is underperforming, contact a ventilation expert at Eldridge. We will provide a free system evaluation to determine how to make it right for your application.