Is compressed air perfection an attainable goal or a myth?

The journey toward compressed air perfection can result in significant energy savings.

By Ron Marshall, Manitoba Hydro

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In brief:

  • Perhaps true perfection does not exist in compressed air systems, but the journey toward perfection can result in significant energy savings.  
  • Almost all of the energy input to an air compressor is converted to heat.
  • There are a number of ways to produce compressed air more efficiently; most of these apply to equipment located in or near the compressed-air room.

The word “perfect” is not commonly used when describing the performance of a typical compressed air system. A good quality, fully loaded, air-cooled, lubricated rotary-screw compressor consumes about 16.5 kW of power to produce 100 cfm of compressed air at 100 psi. Refrigerated air drying of this air costs another 0.5 kW/100 cfm.

The word “perfect” is not commonly used when describing the performance of a typical compressed air system.

Once the compressed air gets to the final end user, it is converted back to mechanical energy. A rotary vane air motor, for example, something that might drive an industrial air-powered paint mixer, will consume about 60 cfm of compressed air to produce an output of 1 kW at the shaft of the motor. Putting it in the same terms as the air compressor, this represents about 1.7 kW of air motor output power per 100 cfm of compressed air used. Putting it simply, this means 10.3 kW of air compressor input power is consumed for every 1 kW of air motor mechanical output. This number relates to a “perfect” lossless compressed air system, no leaks, no waste and no pressure differential; and even without these issues the best power conversion efficiency achievable is slightly less than 10%.

Where does the energy go?

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This low-power conversion efficiency may surprise you, and you may wonder where all the remaining energy goes. If you visit your compressor room it will become obvious; the most significant output of an air compressor is heat. Almost all of the energy input to an air compressor is converted to heat. An air compressor produces about 2,545 Btu/hr for every brake horsepower (bhp) of compressor capacity. If you do the conversion, you’ll find of 2,545 Btus is equivalent to about 0.746 kW of electric heat (or about 1 hp). Because modern air compressors are typically loaded into the motor service factors the equivalent energy input to the air compressor is slightly more that the heat of compression that comes out. The difference is the energy imparted to the compressed air.

But let’s explore a real-world, typical “imperfect” system. Let’s choose a typical compressed-air system using three 150-hp, air-cooled lubricated screw compressors, using three separate non-cycling refrigerated air dryers, each with one particulate and coalescing filter installed (Figure 1). The system has 2 gal of storage per cfm or 1,400 gal, as recommended by the manufacturer, and the compressors will operate in load/unload mode with controls set up in a cascaded arrangement with 10 psi wide pressure band for each compressor with 5 psi between steps. The system loading is such that the system will operate with two normally running main compressors and one hot backup. Condensate drains for the compressor and dryer are timer-controlled. We can calculate what the conversion efficiency of this real-world system would be if applied to a typical shift-oriented industrial plant, operating continuously.

We can calculate what the conversion efficiency of this real-world system would be if applied to a typical shift-oriented industrial plant, operating continuously.
Figure 1. We can calculate what the conversion efficiency of this real-world system would be if applied to a typical shift-oriented industrial plant, operating continuously. (Source: Compressed Air Challenge)

The Compressed Air Challenge (www.compressedairchallenge.org) has discovered by sampling numerous industrial facilities that typically only 50% of all the compressed air produced by industrial air compressors is actually consumed by appropriate end uses (Figure 2). About one-quarter to one-third of the compressed air is wasted due to leaks before it gets to the end use. About one-eighth of the compressed air is extra flow consumed by end uses because the system pressure is higher than the user needs; this flow is called artificial demand. The remaining part is compressed air that shouldn’t be consumed at all because the uses are inappropriate — uses such as condensate drains that blast compressed air to clear separator bowls, blowing compressed air on a motor to cool it, or air-consuming equipment left on even when the machine is not producing any product. Our real-world-example system has these same characteristics.

About one-eighth of the compressed air is extra flow consumed by end uses because the system pressure is higher than the user needs; this flow is called artificial demand.
Figure 2: About one-eighth of the compressed air is extra flow consumed by end uses because the system pressure is higher than the user needs; this flow is called artificial demand. (Source: Compressed Air Challenge)

The real-world system has a typical 24/7 load profile, where compressed-air production is high for 16 normal hours and is lower for midnight shifts during weekdays, with minimal compressed-air load during weekend loads, mostly consisting of plant leaks. Studies by compressor manufacturers have shown that about 65% of all industrial systems operate with this type of profile.

This example production schedule might have the following characteristics.

Shift Annual Hours New Average cfm Average psi Required
Weekday normal production 4,100 1,000 115 2 compressors
Weekday midnight 2,000 625 120 1 compressor
Weekend/holiday 2,660 370 120 1 compressor
Total 8,760      

                
The calculated power consumption would be as follows

Shift Annual Hours Average cfm Average psi Compressor kW Dryer kW kWh
Weekday normal production 4,100 1,000 115 202.3 16.2 895,850
Weekday midnight 2,000 530 120 113.0 16.2 258,400
Weekend/holiday 2,660 370 120 95.8 16.2 297,920
Total 8,760          
Weighted Ave   701   165.8 16.2  
Specific Power       23.6    

                              
The operating power for this system is calculated using the Compressed Air Challenge’s power vs. flow curves, along with the Compressed Air and Gas Institute (CAGI) data sheet for the proposed compressor type. In this case, compressor data for a unit with 700 cfm output consuming 126 kW at 125 psi (18 kW/100 cfm) has been used. Data for the air dryers is for 700 cfm units consuming 5.4 kW each. Since all units are hot spares, the total consumption is 5.4 x 3 = 16.2 kW. The total compressed-air-system energy consumption is calculated by totaling the energy consumed for each shift type.

In the case of the main production shift, one compressor is fully loaded at 700 cfm output and the other compressor is partially loaded at 300 cfm. Since the compressors operate at 115 psi, about 10 psi below their rated pressure, the power used in the calculation is reduced by about 5% to account for lower power at lower-than-rated pressure (compressors consume about 1% less power for every 2 psi pressure reduction around 100 psi).

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