Fan manufacturers rate the volumetric air flow of their products based on air at standard conditions. When Eldridge designs most ventilation systems, we do so using these standard air flows for the fans. However, there are applications where the air is not at standard conditions and we need to determine what is the volume of air at those non-standard conditions so that we can accurately size the fan. In this article, I will explain the formula used to do the conversion and provide some examples so that you understand SCFM vs. ACFM.
Conversion Formula
Let’s first start by defining standard air conditions. Fans are rated based on standard air density of 0.075 lbs./cu.ft. which is based on air at sea level where the pressure is 14.7 psi and the temperature is 70 degrees F. When a fan manufacture rates the volumetric air flow of a fan, they are doing so at these standard conditions and we refer to that volumetric air flow as the standard cubic feet per minute (SCFM).
If the volumetric air flow that we are measuring is not at standard air conditions, that air flow 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 a decrease in pressure and an increase in temperature have the same effect on a volume of air. Let’s work through a couple of examples to see what I mean.
Temperature Change
For this first example, 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. The customer is located in Houston, Texas 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)
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 air that we need in order to capture the fumes has more than doubled from the SCFM.
Pressure Change
The most common reason for a pressure change is due to a change in altitude. For this example, we will assume that we are designing a dust collection system for a customer’s factory located in Denver. At an elevation of approximately 5,280 ft, the atmospheric pressure in Denver is 12.1 psi. Based on our design of the dust collection system, we have calculated that to capture the dust at several pick-up points we will need 10,000 SCFM. The factory is heated in the winter so we will assume that 70 degrees F is the average indoor temperature throughout the year. For this example, the calculation will look like this:
ACFM = 10,000 x (14.7/12.1) x (70+460)/(70+460)
The calculated ACFM is 12,148. Because of the lower atmospheric pressure, our Denver based customer will need a fan that moves approximately 20% more air than an equivalent dust collection system located at sea level.
Conclusion
The examples that we worked through show that when air conditions deviate from the standard, it is important to know how to convert SCFM to ACFM. The impact of not doing the conversion accurately often leads to a significant under sizing of the fan for the application.