Compressed Air Systems

Sizing matters: How to avoid oversizing or undersizing your compressed air system

Build out your plant's complete compressed air demand profile to avoid oversizing or undersizing your system

By Steve Bruno, Atlas Copco Compressors

When considering an air compressor purchase, the primary concern should be receiving the right amount of compressed air for the lowest overall cost. Far too often, only the purchase price of the new machines is considered, and this is a small fraction of the total lifecycle cost. Energy costs are the real driver of a compressor’s total cost of ownership, and in many cases energy costs in the first year of use will total more than the purchase price of a new air compressor.

As a user, it is critically important to understand the performance of the air compressors that any engineering group or company will be providing and to choose the most energy-efficient solution to save on long-term energy costs. That’s why, when researching and comparing air compressors, it’s important to examine all aspects of the costs associated with operating and maintaining the unit while making sure it’s correctly sized for the job.

Properly designing your air system

When designing an air system and choosing the compressors, tanks, and air treatment products, knowing how and when the compressed air will be used is as important as knowing how much total air is required. It is critical to include an accurate air use profile in the purchasing process to maximize performance and reduce costs.

Users have many different methods for determining their plant’s compressed air demand profile – from extensive and comprehensive air audits performed using data loggers to ballpark estimates developed with the use of a stopwatch and based on the total compressed air cubic feet per minute (CFM) demand of each piece of equipment that is running simultaneously.

Unfortunately, the use of inaccurate or incomplete compressed air demand profiles is likely to lead plants to oversize or undersize compressors. The following two scenarios illustrate the need for a complete air demand profile rather than the use simply of estimations of pressure and flow.

Scenario 1

An engineering group responsible for the design of an entire plant sent requests for quotes (RFQs) for an air compressor that could produce at least 100 cfm at 125 pounds per square inch (psi). The compressor suppliers were not given a chance to perform an audit and had to specify the compressor based on this information alone. Relying on the available data, the user selected a single 25 hp fixed-speed compressor.

Users have various reasons for sending RFQs and preventing sales engineers from visiting the plant or talking to the organization’s engineers. However, this can lead to improperly sized air compressor systems and unscheduled repairs. If an audit had been performed, it would have shown that the facility was using 100 cfm 25% of the time and 50 cfm 75% of the time.

The choice of a 25 hp VSD compressor in this case would have saved the user approximately $4,000 per year versus the fixed-speed machine – money that could be added to the organization’s bottom line.

Scenario 2

An engineering group sent RFQs for a compressor that could produce an average of 200 cfm at 125 psi. The engineering group required the use of VSD compressors because it knew the system would experience some variance in demand and because it wanted to claim an energy rebate from the state. Based only on this information, a single 50 hp VSD compressor was selected by the user.
A few weeks after installation, the user’s system began shutting down because of low-pressure alarms. What went wrong?

This scenario illustrates how incomplete information can affect the production capabilities of a plant. If an audit had been performed, it would have shown that the facility requires 175 cfm 30% of the time and 200 cfm 60% of the time. However, during one important sandblasting process, the facility’s compressed air demand rises to 250 cfm; when this happens, the 50 hp VSD compressor is insufficient.

A better solution would have been a variable-speed compressor that accommodates the entire 175 to 250 cfm range or a separate smaller fixed-speed compressor paired with the VSD machine to provide the additional 50 cfm only during the sandblasting process. Ultimately, this manufacturer will feel the burden of paying more every month in electricity costs and maintenance for the air system long after the installation.

The role of CFM in performance and efficiency

Accurately determining CFM demands is a critical first step in selecting an air compressor. Improperly sizing a compressor because this step was skipped can cost thousands of dollars in wasted electricity if an oversized compressor is selected. On the other hand, if a facility requires more demand than originally thought, a new compressor may be insufficient.

Critical topics to understand include the differences among CFM, ACFM, and SCFM and how to compare the specifications of different air compressors:

  • CFM (cubic feet per minute) describes the volume flow rate of compressed air. Note that CFM must be further defined to include pressure, temperature, and relative humidity, as changes in these measurements will affect the necessary CFM that is delivered to the process. 
  • ACFM (actual CFM) is the air flow at local conditions. For example, if a compressor is installed in Denver, then the ACFM is lower than if it were installed at sea level. 
  • SCFM (standard CFM) is the amount of air flow at sea level, 68°F, and 0% relative humidity. ACFM must be used to determine exactly how much air is needed in a facility. ACFM can then be easily converted into SCFM and used as a reference for an apples-to-apples comparison of different air compressors.

How can a user identify which compressors are the most efficient? Compressors vary in CFM ratings, horsepower ratings, and total package energy use, which is a lot to research and compare. Also, for compressors with the same advertised horsepower, the CFM output can vary by as much as 25% among companies and models. The goal is to match the right amount of CFM at the necessary pressure with the lowest horsepower possible. This will allow the facility to manufacture at the lowest cost and generate the highest profit.

The Compressed Air and Gas Institute (CAGI) is an association comprising compressed air manufacturers, each of whom agreed to have its compressors independently tested and verified. Once verified, certified data sheets are published publicly by CAGI on each company website with key information that customers need to make the buying decision easier.

These CAGI data sheets are the air compressor version of a car’s Monroney or window sticker. Each CAGI sheet lists the amount of CFM the compressor model produces at a given pressure, the maximum psi that can be produced, and the efficiency of the compressor in kW/100 cfm, which is similar to an automobile’s miles per gallon (mpg) rating.

Figure 1 shows a CAGI data sheet from a 100 hp compressor rated for 100 psi, as indicated in blue boxes on lines 4 and 6. Important measurements to consider are indicated in green and include:

  • Line 3, which lists the capacity of the compressor in CFM at the full load operating pressure (100 psi in this case). Notice that this compressor produces 525 cfm.
  • Line 12, which shows the amount of electricity (in kW) required to create 100 cfm at the listed pressure (100 psi). This measurement is referred to as the specific power of the compressor and is the best apples-to-apples comparison of compressor efficiency among different compressor models. This value can be used to calculate the actual cost to run the compressor; in this case, the compressor uses 16.7 kw to generate 100 cfm.

Also, when replacing an existing compressor, I recommend checking the CFM rating and then choosing a new compressor with the same CFM rating. It might be unwise to swap out a compressor for another one with the same horsepower rating, because the newer model is likely able to produce more CFM while using less horsepower. A compressor with a smaller horsepower rating but an equal rate of airflow can lower your energy costs without any reduction in productivity.

Follow a disciplined process to savings

To make an educated choice, it is critical to understand a system’s compressed air demand profile and then determine how to deliver the right amount of compressed air at the lowest overall cost. All compressors are not designed equally, so this can be challenging; however, conducting an audit in advance and then matching the CFM requirements with the smallest possible horsepower compressor can greatly simplify the process.

Remember, a higher-horsepower air compressor uses more electricity than a smaller one, so the important question to ask is: “Am I getting any more CFM for my increased cost?” Following this process will result in the lowest overall cost of ownership and will help prevent buyer’s remorse.