Energy Efficiency: Compressed Air Systems

This is the last article of a five-part series on industrial energy efficiency. This month we will address how Compressed Air Systems are prime targets for energy efficiency measures.

Compressed air is used in many industrial processes, such as sandblasting, injection molding, spray painting, and equipment heating and cooling, to name just a few. Air compression motors have high electrical demands. In fact, up to 20% of total electrical use in certain industries can come from air compression systems.

Which makes these systems prime targets for energy efficiency measures.

High Maintenance

If you use compressed air equipment, you probably know that the cost of the equipment itself is often a fraction of the cost of operating and maintaining it.

In fact, the cost of operating a compressor for just one year usually equals or exceeds the initial cost of the unit. So a reduction in operation and maintenance expense will create substantial savings over the lifetime of the system.

Let’s take a look at some of the low-cost or no-cost measures that can help minimize the expense of operating compressed air systems.

Air Leak Surveys

An industrial plant that has not been well maintained will typically leak about 20% of total compressed air production capacity. But this can be reduced to less than 10% of compressor output by proactively detecting and repairing leaks.

The best way to detect leaks is to use an ultrasonic acoustic detector. This device can recognize the high frequency hissing sounds associated with air leaks.

The units are portable and consist of directional microphones, amplifiers, and audio filters. They typically use either visual indicators or earphones to detect leaks.

Ultrasonic detectors filter out background noises within the audible range. As a result, leaks can be heard in even the noisiest environments.

The benefits of ultrasonic leak detection include versatility, speed, ease of use, the ability to perform tests while equipment is running, and the ability to find a wide variety of leaks. In addition, any operator can become competent after about 15 minutes of training.

Fixing Air Leaks

Air leaks occur most often at joints and connections. Which means stopping leaks is often as simple as tightening a connection. But it can also be as complex as replacing faulty equipment (couplings, fittings, pipe sections, hoses, etc.).

In many cases, leaks are caused by bad or improperly applied thread sealant. This is why it’s so important to select high-quality components, and install them properly with the appropriate thread sealant.

Did you know that non-operating equipment can be an additional source of leaks? To remedy this problem, any equipment no longer in use should be isolated with a valve in the distribution system.

You can also reduce air leaks by lowering the demand air pressure of the system. The lower the pressure differential across a hole or leak, the lower the rate of flow. A lower rate of flow translates into reduced leakage rates.

 

Once leaks have been repaired, the compressor control system should be re-evaluated and adjusted (if necessary) to realize the total savings potential. A proactive leak prevention program will go a long way toward improving the performance of your plant’s compressed air systems.

Recovering Waste Heat

As much as 80%-90% of the electrical energy used by an industrial air compressor is converted into heat. In many cases, a heat recovery unit can recover 50%-90% of this available thermal energy and put it to use heating air or water.

Typical uses for recovered heat include supplemental space heating, industrial process heating, and water heating. (Recoverable heat from a compressed air system is usually not hot enough to produce steam directly.)

For example, packaged air-cooled, rotary screw compressors are very amenable to heat recovery for space heating or other hot-air uses. Packaged compressors are typically enclosed in cabinets and already include heat exchangers and fans. So the only system modifications needed would be additional ducting (and possibly another fan).

Similarly, by using a heat exchanger, you can produce hot water. This is done by extracting waste heat from the lubricant coolers found in packaged water-cooled, reciprocating or rotary screw compressors.

Compressed Air Storage

An effective control strategy for your compressed air system should include adequate storage.  Employ storage to cover peak air demands by strategically locating receivers. This reduces both the amount of pressure drop and the rate of pressure decay.

For systems with highly variable air demand, you can achieve tight control by combining storage with a pressure/flow controller.  Narrowing the pressure variation with better controls not only uses less energy; it also minimizes any potential negative effects on product quality.

A Final Note

The final low- to no-cost measure recommended for improved energy efficiency pertains to inappropriate uses of compressed air. These include any application that can be done more effectively or efficiently by another method. The following table illustrates:


Sources:

Sustainable Plant

Compressed Air Best Practices

US Dept. of Energy

Univ. of Minnesota Technical Assistance Program