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

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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.

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  • Well it makes sense for a 24/7 plant but not for a variable production plant. It is better to have a small make up compressor that runs a little off-shift to keep the full pressure up and allow for timing to see if there are leaks. This compressor needs a timer to prevent it running during high air usage and start again off shift. This rule applies to shore side as well as shipboard. Be sure the compressors are set to alternate as well as a selector switch automatic or manual and hour meter to keep track of running time. Be sure the compressor operation is part of your maintenance program.

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