ACFM TO SCFM

The flow rate of compressed air is commonly used to determine the size and type of equipment of any compressed air system. Cubic feet per minute (cfm) is frequently used as the unit of measure and is a general term that warrants more definition. Actual cubic feet per minute (acfm) is the volumetric flow rate of air not the mass flow of air. The typical compressor supplier specifies a rated acfm when operating at determined discharge pressure. This may imply that the compressor produces the specified cfm at the discharge of the compressor. However, that is not the case. The rating is the volumetric air flow through the inlet air filter when operating at the discharge pressure. Sometimes, the term inlet cubic feet per minute (icfm) is used to make this clear and is a more precise definition to further define acfm.

However, site conditions vary with elevation above sea level and annual weather. These conditions affect the density of air which means the volumetric flow to deliver the same amount of mass flow into the plant will vary. Standard cubic feet per minute (scfm) is used to specify the mass flow rate to the inlet of the air compressor referenced at a specified pressure, temperature, and relative humidity. This is the best way to define the plant’s compressed air needs as it completely defines the usage of air regardless of the weather for the day and location.

Unfortunately, there is no agreed-upon standard for reference conditions used among air compressor suppliers. The most commonly used are:

• United States standard (USA) at 14.7 psia, 60° F, 0% RH
• The American Society of Mechanical Engineers (ASME) at 14.7 psia, 68° F, 36% RH
• Compressed Air and Gas Institute (CAGI) at 14.5 psia, 68° F, 0% RH

In a competitive bidding process, air compressor suppliers use the CAGI standard which allows the buyer to compare compressors but may not provide a clear picture of the expected mass flow or the energy consumption measured at the power meter. Notice that CAGI defines a set of tolerances of ± 4% on volumetric flow rate and ± 5% on specific energy consumption. These tolerances along with the common use of acfm can artificially make an air compressor unit seem more efficient while the actual energy for a given mass flow may be 10-20% higher.

UCA uses scfm defined at the United States standard with ± 0% tolerance on flow and power when investigating and discussing the customer’s air requirements and specifying the equipment in order to make clear of the system’s capabilities. 

An additional 3 psi of pressure drop increases the typical compressed air system’s energy consumption by 1-2%. Since energy consumption costs of compressed air systems are about 60% of total operating costs, improper sizing of piping can have a long-term financial impact. Undersized piping networks can cause excessive pressure drops causing energy consumption to escalate to meet the pressure requirements of air. Oversizing piping networks can provide a very-low pressure drop but can cause excessive installation costs.

UCA optimally sizes all compressed air piping according to the customer’s air requirements and analyzes the cost-benefit of selecting various types and sizes of pipe.

It is common to see a UCA project be justified by the power savings alone. Custom compressed air systems that are built with the mindset of lowest upfront costs will have rotary air compressors and either refrigerated or heatless dryers (depending on the dewpoint requirements). This may look financially sound in the short-term but in the long-term can consume excessive power and additional air that significantly impacts the power cost. Also, it can be a huge factor since common compressed air system's energy consumption costs are 60% of total operating costs. Some plants even add on additional units thinking they are expanding production, but in reality, the capacity of the system has degraded causing plant managers to buy and operate more units. Some air compressor systems use only amp meters to estimate the power and the flow which eliminates the opportunity to observe the effect of degradation. UCA has a long-term mindset when building new compressed air systems and evaluates the entire life-cycle cost of new plants when making equipment decisions. UCA monitors the performance using high-accuracy power meters and flow meters. 

Air properties and density will vary with site location.  The less-dense the air (e.g., higher elevations), the more power required by the air compressor to produce the same amount of airflow. Since higher temperatures and elevations lower the density of air, that means an air compressor operating in Pennsylvania closer to sea level during the winter will produce more air than the same compressor (at the same power input) operating during the summer in New Mexico. DIY compressed air systems can be under and over-specified without a solid understanding of ambient design conditions including historical weather trends and site elevation.

UCA investigates each potential site’s annual ambient conditions and sizes the new compressed air system optimally based on their customer needs and typical-worst case weather conditions.

Compressed air consumes 10% of the nation's energy produced. The energy costs due to compressed air are typically unrealized within a plant’s total energy bill for the entire site. 

UCA uses high-accuracy power meters to monitor the true energy costs for the compressed air system.