Fan manufacturers provide volumetric air flow ratings for their equipment based on standardized conditions. At Eldridge, when we design ventilation systems, we typically utilize these standard air flow ratings. However, certain applications involve non-standard air conditions, requiring us to calculate the air volume under these specific conditions to ensure accurate fan sizing. In this blog, I’ll walk you through the conversion formula and provide examples to clarify when to use SCFM or ACFM.
Conversion Formula
Let’s begin by establishing what constitutes standard air conditions. Fan ratings are based on a standard air density of 0.075 lbs./cu.ft., which corresponds to sea-level conditions with a pressure of 14.7 psi and a temperature of 70 degrees F. When manufacturers specify a fan’s volumetric air flow, they’re referring to these standard conditions, expressed as standard cubic feet per minute or SCFM.
When measuring volumetric air flow under non-standard conditions, we refer to it as actual cubic feet per minute or ACFM. The conversion between SCFM and ACFM requires the following formula:
ACFM = SCFM x (P Standard/P Actual) x (T Actual/T Standard)
This formula demonstrates that both pressure reduction and temperature increase affect air volume similarly. Let’s examine some examples to illustrate this concept.
Temperature Change
Consider a scenario where we’re designing a fume extraction system with a ducted outlet. The furnace produces air at 700 degrees F, and we’ve calculated that 15,000 SCFM is required for effective fume capture. The system will be installed in Houston, Texas, where we can assume standard pressure conditions.
To properly size the fan for this application, we need to calculate the ACFM for air at 650 degrees F. Here’s how the calculation works:
ACFM = 15,000 x (14.7/14.7) x (700+460)/(70+460)
The formula uses the Rankine temperature scale, which requires adding 460 to Fahrenheit values for conversion. After performing the calculations, we find that the ACFM equals 32,830. This demonstrates that at 700 F, the required ACFM for effective fume capture is more than double the SCFM value.
Pressure Change
The most prevalent factor causing pressure variation is altitude change. Let’s consider a scenario where we’re designing a dust collection system for a client’s manufacturing facility in Denver. Given Denver’s elevation of approximately 5,280 ft, the atmospheric pressure measures 12.1 psi. Our dust collection system design calculations indicate that capturing dust effectively at various collection points requires 20,000 SCFM. Since the facility maintains indoor heating during winter, we can assume a consistent year-round temperature of 70 degrees F. For this case, we’ll apply the following calculation:
ACFM = 20,000 x (14.7/12.1) x (70+460)/(70+460)
The resulting ACFM calculation yields 24,298. This demonstrates that due to Denver’s reduced atmospheric pressure, our client needs a fan capable of moving roughly 20% more air compared to an identical dust collection system operating at sea level.
Conclusion
The examples we’ve analyzed clearly demonstrate the critical importance of accurately converting SCFM to ACFM when air conditions deviate from standard parameters. Failing to perform these conversions precisely often results in selecting an undersized fan for the specific application, potentially compromising system performance and effectiveness.