One of the most effective ways to save energy when it comes to compressed air is to reduce system flow. This requires dealing with the end uses and abuses of compressed air in an effort to eliminate or reduce the flow of compressed air that results from wasteful practices.
One often-cited opportunity for improvement is leak reduction – a valuable endeavor, but more can be done. Additional significant savings can be achieved by finding and fixing “inappropriate uses,” which we can define as some sort of process that could be more cost-effectively supplied by another energy source. This is not a simple process: There are hundreds if not thousands of individual uses of compressed air in most plants. With that in mind, though, let’s explore some of the more common potentially inappropriate uses of compressed air and look at examples of how plants have addressed these energy-wasters.
How efficient is compressed air?
One of the problems with compressed air is the high cost of the power input that feeds the air compressors compared with the actual mechanical power that comes out at a compressed-air-powered device.
An example often used is the output of 1 horsepower using a vane-style air motor. A vane motor rated at 1 hp produces about 0.746 kW of rotational power from its shaft and translates this to a rotary motion that can drive a tool or a mixer. If typical specifications are consulted, we might find that this motor will consume 40 cfm at 100 psi to produce this level of power.
Now, if we look at a typical air-cooled lubricated rotary screw compressor running at 100 psi, we can find that it would consume a specific power of 18 kW for every 100 cfm of compressed air it produces. We can easily find this number from the Compressed Air & Gas Institute (CAGI) specifications published by most member manufacturers. Therefore, doing the math, we find that it takes about 18 × 40/100 = 7.2 kW of power input to the air compressor to produce about 0.746 output at the air motor shaft. Almost 10 times more power is required than is produced at the motor shaft. Most of the power is lost because of the heat of compression.
But it gets worse. The numbers presented assume a lossless compressed air system in which all of the air produced goes directly to the air motor without any leakage, pressure loss, or inefficiency resulting from partial loading of the compressor. In real life, these losses come into play to increase the actual cost of an air motor. Typically, about 30% of the compressed air produced at an air compressor is lost due to leakage before it gets to the motor. There’s also typically about 10 psi of system pressure loss that stems from air dryers, filters, and losses that occur from connectors and hoses, so the compressors must produce 110 psi to feed the motor 100 psi. This adds about 5% to the energy input.
Moreover, a lightly loaded compressor will often consume more than double its CAGI-specific power rating because of partial-load characteristics not mentioned in the CAGI specification (the power specification for a fixed-speed compressor is valid only at full load). This can make the compressor-specific power more like 36 kW per 100 cfm. Add all of these costs to the equation, and the result is that the air compressor might consume the equivalent of 21.6 kW to produce 1 hp of shaft power – about 30 times the input power, making the wire-to-work efficiency about 3%.
In quite a few cases, it may be possible to replace a compressed-air-powered motor with a direct-drive electric motor, especially if it is a permanently mounted device. Of course, electric motors are not 100% efficient, but let’s assume an efficiency of 80% and calculate the cost savings potential from there. An electric motor would consume about 0.9 kW while producing 1 hp of shaft output, thus consuming 7,884 kWh per year at 0.9 kW × 8,760 hours. This would cost about $788 per year if the motor were running full time, assuming a cost of 10 cents per kWh.
An air motor under the same duty would consume about $6,300 worth of power in a lossless system and as much as $18,600 if the system has typical losses applied. This simple mathematical exercise shows why compressed air is often called one of the most expensive utilities in an industrial plant. A useful exercise, then, is to find and classify the various uses of compressed air in a plant as appropriate or potentially inappropriate. An inappropriate use would be something that can be turned off or replaced with some source of energy that is more cost-effective. Applying energy conservation measures to compressed air end uses is often a quite effective solution in addressing the high cost of compressed air.
Potentially inappropriate uses
Not all uses of compressed air are inappropriate; this is the reason for stating that some end uses are “potentially” inappropriate. The appropriateness often depends on many factors.
For example, vane-type air motors are used for ergonomic reasons in hand tools, as electric-powered tools are often too heavy for extended use. And air motors are used in hazardous locations where the spark from an electric motor might cause an explosion and fire. In these cases, the higher cost of their operation is justified. But in cases where electric motors can be used and these factors do not come into play, then the use of compressed air may be inappropriate.
There are numerous ways in which compressed air can be used and abused in a plant. Table 1 shows some of the more-common ones often discovered during plant surveys. Once these are found, some investigation needs to take place to find out whether there is a less-energy-intensive way to perform the same task. When a possible alternative is found, then a business case needs to be made to determine whether the change would be economical.
If enough investigation is done, you will become skilled at finding ways to amend end uses to reduce compressed air costs. The following case studies are examples of actual projects done over the past few years; they illustrate what can be done with some careful thought and action to reduce inappropriate compressed air use.