Unique Systems, Common Problems
Air compressors operate based on simple principles but control of entire compressed air systems can become quite complex. The primary goal of operators is to provide enough airflow to maintain production. System efficiency is likely an afterthought. Each compressed air system is unique, from the system that uses less than 100 cubic feet per minute (CFM) erratically to the system that produces hundreds of thousands of cubic feet per hour of every day of the year. While each system is unique, common trends do exist. For example, systems with more than one compressor tend to operate more efficiently when controlled by a sequencer.
Sequences of Operation
Some compressed air operators may be thinking, “Who needs a sequencer? I can manage the system well without one. I can set the operating parameters at just the right points to prevent issues.” While to some degree this is true, in a system with multiple large compressors, efficient operation can be difficult to obtain without a sequencer.
One of the primary limitations of not using an automatic sequencer is that it is particularly difficult to maintain efficient operation when your compressors are limited to one set of setpoints. For example, in a system with both a large and small compressor, which would be the best way to order the operation of these compressors? One option would be to have the large compressor operate all of the time to satisfy the loads and only turn on the smaller compressor once the larger compressor is maxed out. However, this may not be the most efficient sequence of operation, as the smaller compressor could be used at times when the loads can be met by that compressor alone, allowing the larger compressor to be shut off. This could be like driving a semi-truck to the grocery store to buy a gallon of milk. It can do the job but there are more efficient alternatives to the truck. Using a sequencer to control the staging of compressors will resolve this problem when compressed air loads, compressor capacities, and part load characteristics are properly accounted for.
In addition, sequencers are capable of modulating more than one compressor at the necessary points to achieve the best system efficiency. Referring to the chart below, it’s best to avoid operating compressor 1 below 30% of its load. Keep in mind that the curve is percent power versus capacity. Efficiency in brake horsepower per cfm is a different issue. This is like a large diesel engine in the truck versus a small gasoline engine in the car. The gasoline engine is assuredly less efficient, but not for fetching the gallon of milk.
