Energy savings are hiding in your compressed air system

July 26, 2006
A practical guide to saving energy in typical compressed air systems.

The DOE's Compressed Air Challenge launched in January 1998 is dedicated to improving the efficiency of compressed air systems by 10% by the year 2010. The DOE plans to train and educate compressed air users in the best practices of saving energy in compressed air systems. But what is the state of compressed air systems in the USA? What are the basic techniques that help users save energy in compressed air systems? The predominant air compressor found in industrial settings is the lubricated rotary screw compressor. Screw compressors are economical, compact, efficient, quiet, and continuous in operation. In 1998 approximately 25,000 more units will enter the domestic market.

Of the current machine population, 85% operate in an inefficient control mode. Consider this. The vast majority of rotary screws at work in industry operate in modulation control. Modulation control matches the supply to the demand by throttling the intake air via an intake valve. Throttling the air this way leads to a vacuum below in the inlet valve. Modulation is an inefficient way to control a rotary screw compressor because it affects the inlet pressure of the air to be compressed. If a compressor is at sea level it takes in air at 14.7 psia. Inlet throttling can cause the air under the inlet valve to become 10.0 psia.

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Why is this important?

You remember that brake horsepower (bhp) is equal to the mass flow multiplied by the compression ratio. As compression ratios increase, so does brake horsepower. It takes a compression ratio of 7.8 to compress air from 14.7 psia to 100 psig (114.7 psia) while you need a compression ratio of 11.5 to compress the throttled air at 10 psia to the same 100 psig.

Since more work and higher compression ratios are necessary to compress air from lower inlet pressures. The inefficiency in this control scheme is obvious. At 50% load, the bhp required is still at 87% of the full load value. In other words, the compressor is delivering half its capacity but still drawing 87% of the horsepower. This is obviously not an efficient way to control an air compressor.

As can be seen on the modulation curve, the more loaded a rotary screw compressor runs, the more efficient it is. The goal to "load up" your rotary screw compressors as much as possible. Rotary screw compressors are meant to be base load machines with the capability of running 24 hours per day, 7 days a week, with little problem. As an extra, motor efficiencies also improve at full load which serves to reduce kW and power costs.

Often, the end user does not know at what load the compressor is operating. Some manufacturers provide percent of capacity gauges and read outs on their compressors, but most do not. To find out if your compressor is modulating and throttling the inlet, you can run a simple test. Measure the current to the main motor. If the amperage is less than the full load rating, you know that the inlet is throttled. Creating an artificial demand, like opening a drain valve to the point that the pressure drops to the full load pressure of the machine yields a full load amperage measurement.

Closing a service valve very slowly causes the system pressure to rise and the inlet to close to its extreme. This diagnostic test allows measuring the unloaded amperage on a machine controlled by pure modulation. Interpolate between these two test points to determine the approximate load. Certainly, installing a cfm flow meter more accurately determines the load on the machine.

Since your goal is to increase the load on a modulating machine, keep as few of the compressors on-line as possible.The last thing you need is to run multiple compressors in modulation control at various loads. That figure shows a network of 3-100 horsepower rotary screw air compressors moving 500 acfm each. The network capacity is 1,500 cfm. When the network experiences a demand of 750 cfm, the network pressure stabilizes at 107 psig with Unit 1 at 70% load, Unit 2 at 50% load, and Unit 3 at 30% load. This makes no sense if energy conservation is the goal.

If we stopped Unit 1, then Unit 2 would load up to 85% of capacity and Unit 3 would load up to 65% of capacity. The system pressure would then stabilize at 103.5 psig versus the previous 107 psig. This represents a 1.75% decrease in bhp per machine because of pressure reduction. Brake horsepower decreases 1% for every 2 psig pressure reduction.

The system would cost $139,700 per year to operate versus $103,263 for the system shown in Figure 4, assuming 7.5 cents per Kwh, 8,000 hours operation per year, and 90% motor efficiency. Therefore, by loading up fewer compressors that use modulation control saves $36,516 in power.

What other steps can be taken to reduce power and achieve savings?

There are at least five measures that ou can take to cut your costs.  Adjust your controls to operate in upper range modulation control. Some modulation control machines have blowdown capability using pressure switch control along with the ability to modulate in the upper range of capacity, thus the phrase "upper range modulation." When the machine modulates down to 70% capacity, the compressor shuts the inlet valve and relieves itself of internal pressure. This blowdown state draws only 25% of full load horsepower. Rotary screw compressors with oil pumps can blowdown to 15% or less of full load horsepower because they can blow down internal pressure to zero yet maintain oil pressure via their oil pump. Units without oil pumps must maintain 20 to 30 psig air pressure in the sump to circulate the oil. This requires more horsepower.

Putting a machine into upper range modulation may require extra system storage if rapid cycling from modulation to no load occurs.

You can also adjust controls to operate in load/no load control. If the compressor has blowdown capability and pressure switch control, it is likely that the compressor can be adjusted to run in load/no load control. Usually, the pilot control valve can be adjusted so that the pressure switch controls the load pressure and the unload pressure with no modulation. This control operates much like constant speed control on reciprocating compressors. Either the rotary screw is pumping 100% of its capacity or it is unloaded in blowdown condition.

To attain the power curve shown in Figure 6, it is necessary to add 10 gallons of air storage per cfm of supply. The 1,000 cfm system shown in Figure 4 requires 10,000 gallons worth of storage to transform it into the power curve shown in Figure 5. Since blowdown time (usually 60 seconds or more) is a factor on a compressor that cycles from load to no load, minimum cycles are necessary to attain economical power curves. The 10,000 gallons of storage is necessary to minimize cycling.

Load a rotary screw compressor on modulation control to 100% load, then trim with a reciprocating or a rotary screw compressor with variable displacement control. A variable displacement airend on a rotary screw compressor recirculates air from the airend back to the inlet to unload. This method opens clearance pockets in the ariend before the air is to be compressed. The more clearance pockets that open, the more unloaded the machine runs. This type of control is efficient because it unloads the compressor without changing the inlet pressure. A constant inlet pressure means a constant compression ratio. This reduces part load horsepower requirements and achieves performance.

Load a rotary screw compressor to 100% and use a smaller rotary screw compressor to trim at near 100 percent load. For example, if system demand is 740 cfm and there are two 100 hp, 500 cfm compressors supplying air, one could use a 50 hp, 240 cfm rotary screw as trim. Running a 50 hp unit at 100 percent load consume 55 bhp. Operating a 100 hp, 500 cfm compressor at 48 percent load on modulation control requires 93 bhp. The power savings would be $18,899 per year assuming 7.5¢ per Kwh, 8,000 hours operation per year, and 90 percent motor efficiency. The payback period for buying the 50 hp unit to serve as 100 percent trim would be approximately 0.75 year.

Then, of course, you can fix leaks. A plant would meet the goal of the DOE's Compressed Air Challenge if it fixed its air system leaks. As much as 20% of a compressor's output is dedicated to leaks. An ultrasonic leak detector and some labor could easily save $21,882 in energy in a 1,000 cfm system.

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