With over ten thousand air system audits under our belt, we’ve seen it all and learned a few things. One of the most common problems we see is that most systems have far more capacity than needed. On average, users operate at 44% of peak capacity. It’s so common, we’d say it is an epidemic, and even our own customers are not immune despite our efforts to inoculate with education.
How does this happen? In many cases, users select compressors based on what they already have, adjusted with some prognostication about whether they expect to grow, add or eliminate production lines, etc. Generally, very little measurement and analysis goes into it. Plant operators are usually comfortable up-sizing a compressor for the safety factor. They don’t want to hear complaints of equipment with low pressure alarms, nor do they want to re-revisit compressed air system design every few years as they grow. So they purchase as big as their budget allows at the outset. When involved, consulting engineers may add to the problem by making conservative assumptions that all pneumatic equipment will operate fully loaded, all the time. Then they take this bad estimate and add a safety factor. In nearly all cases, there’s fudge factor on top of fudge factor. All believe they are acting in the interest of reliability, without understanding the significant negative impact on energy consumption.
Compressed air efficiency is best measured in terms specific power, which is kW/100cfm, and the Compressed Air and Gas Institute (CAGI) has an excellent program that encourages compressor makers to publish the specific power for each compressor. This is a great point for comparing two compressors side by side, but it cannot be used to predict what the user’s actual system performance will be. As the car sellers say: “your mileage may vary.” So much depends on how the compressors are run. The CAGI datasheets for fixed speed machines assume 100% load, which rarely happens in practice. From our many system studies we know that systems are grossly over sized. Whether a single machine or multi-compressor system, under-utilized compressors do not operate at their datasheet spec.
Let’s look at some actual examples of over-sized systems and the costs that resulted.
The chart above shows how the performance of compressed air systems declines dramatically as demand decreases (shown for the most common types of screw compressors in the field). This is measured in specific power (kW/100cfm), which increases as compressors operate further away from their full output capacity. We’ve added data points showing where a few actual customers operate on this curve to show that this graph is actually showing ideal (e.g. laboratory) conditions. As you can see, some are off-the-charts inefficient, but achieving efficient operation is certainly possible.
This is a greenfield plant (i.e., new construction) where the company specified dual 125 hp compressors, (2) 230 cfm refrigerated dryers, 1000 gallons of storage, an air main charging valve, and a master system controller. They spent $1.10/1000 cubic feet! Their system could be replaced with a pair of 15 hp units.
The current facility operates with a 50 hp screw compressor, a 285 cfm refrigerated dryer, and a 400 gallon receiver tank. Typical operation showed the facility running ~11 hours a day Monday through Thursday, with no operation Friday through Sunday. According to the data on the screw compressor’s controller the average system pressure was approximately 115 psig. The peak demand measured was 65 cfm and the average flow was 22 cfm. This unit is over-sized for the current demand. The calculated system specific power was 65.54 kW/100 cfm. The company would be much better off with a pair of 10 hp compressors. They spend $1.09/1000 cubic feet for their air!
Retail equipment manufacturer
The facility currently operates with (3) water-cooled 200 hp compressors, (3)100 cfm refrigerated dryers, and 3,800 gallons of dry storage. The data was provided from the master system controller. This system is highly variably in demand (891 to 2417 cfm) but was designed with 3 units to supply this full range efficiently. While this is an outstanding example of a well-designed system, they could get even better specific performance if they drop their pressure below the average of 115 that they currently maintain. They spend $0.35/1000 cubic feet including their cost of cooling water.
The cost per unit of compressed air goes up as the % load goes down, which means that your yield on this costly input goes down as well. Don’t be yet another statistic with an over-sized and inefficient compressed air system. Educate yourself on the life cycle cost benefits of multiple smaller units that will provide low costs, high efficiency and reliability. Below are a couple of useful resources: