Measures of Flow
Understanding SCFM, ACFM and ICFM measures of flow and what is needed to provide a solution
Volumetric flow measurements can be confusing, and some equipment manufacturers tend to be a little vague in order to reflect positively on the performance of their equipment. The first reason for the confusion is the simple fact that gases are compressible. As such, variations in pressure, temperature, and humidity significantly change the volume and density properties of the gas. Additionally, there are many variations in the “standard” definitions for the different flow rate measurements. It can all seem pretty confusing.
- Standard Cubic Feet per Minute (SCFM) is used as a common reference for flow rate performance. However, there are at least 13 recognized variations to the “standard” conditions, published by agencies like CAGI, NIST, ANSI, ISO, EPA, U.S. Army, etc. At Hycomp, we primarily use the standard conditions of Pressure (14.696 psia) and Temperature (60 F). SCFM is used mostly as a common reference point for comparing the performance of different pieces of equipment.
- Actual Cubic Feet per Minute (ACFM) is used to express the volumetric flow based on defined pressure and temperature conditions at the location in the system where the measurement is being taken. ACFM could reflect the flow at the inlet to a compressor. ACFM could reflect the flow at the discharge of an air booster. ACFM could reflect the flow at the point of use of plastic bottle blowing equipment. It all depends on what is defined. It is important not to confuse ACFM with atmospheric conditions at the site, although since this could be part of the “system”, ACFM and atmospheric conditions could be same.
- Inlet Cubic Feet per Minute (ICFM) is used to express the volumetric flow based on the actual pressure and temperature conditions at the inlet to the equipment. ICFM and ACFM at inlet conditions are, of course, identical. Sometimes ICFM is used interchangeably with the term Free Air Delivery (FAD). The generic term, Cubic Feet per Minute (CFM), is used at times by manufacturers, but it is inherently misleading since it is not tied to any frame of reference.
With all of these units of measure, the most critical step to understanding them well is to be sure that the definitions and the standard conditions being referenced by all equipment manufacturers and users are matching up with each other. Without that, you are literally comparing apples to bananas. There are plenty of resources on the internet that can help with definitions of standards and making conversion calculations so that the comparisons are valid. Proper understanding and use of common definitions will certainly help eliminate confusion.
A few general rules to apply are: (1) Spend some time with equipment providers and equipment users to be sure that everyone is using the same standard definitions. (2) In general, use SCFM to compare equipment capacities. (3) In general, use ACFM to make calculations regarding actual conditions and equipment loading. (4) Always consider the worst case site and/or system conditions as part of the calculations. (5) Call Hycomp and arrange an E2E discussion to sort it all out.
Mike Byrd of Air Services Company recently had a complex booster application for PET plastic bottle blowing that he was working on with his customer. The system involved multiple equipment providers. The customer was having trouble getting clear information regarding flow requirements and specifications. During an Engineering to Engineer (E2E) discussion with the end user, all the parties were able to clarify the flow measurement information by settling on the same standard definitions as a common baseline. A consensus was reached quickly, and the right solution was proposed. After that E2E discussion, Mike reported back to Hycomp: ”I just want to let you know how well the E2E conversation we had last week went. Shortly after our conversation ended, I received a phone call from my customer thanking me for taking the time to speak with them. Their high-level management personnel all had questions that they needed answered so they could make a more educated decision on their equipment purchase.” Mike concluded, “By the end of the conversation we all had a much better understanding of compressed air boosters, flow ratings, the customer’s process, and the Hycomp solution. I am confident that this conversation “boosted” our odds at getting an order and gave the customer more confidence in the Hycomp product. Thanks for your help.” Mike was right, 7 days later the customer issued a purchase order for a Hycomp Oil Free Air Booster.