Innovative compressed air solutions

As the cost of energy continues to rise, so does the cost of operating a compressed air system. Floor space in plants has become increasingly expensive as well. Cost pressures like these force plant managers to identify and employ innovative methods to cut compressed air costs.Fortunately, air compressor technology continues to improve.

By Elie Lutfy, Atlas Copco Compressors, Inc., Holyoke, MA March 11, 2002
Key concepts
Using better controls and small, local compressors can reduce the cost of compressing air.
Variable speed drives closely match compressor output to demand.
Workplace compressors reduce pressure drops and leaks.
Variable speed drives
Compressor installations
Workplace concept
VSD compressor advantages
Advantages of the all-in-one concept include:
Small changes that can have an impact on energy savings.
Advantages of the workplace concept include:

As the cost of energy continues to rise, so does the cost of operating a compressed air system. Floor space in plants has become increasingly expensive as well. Cost pressures like these force plant managers to identify and employ innovative methods to cut compressed air costs.

Fortunately, air compressor technology continues to improve. Control systems have evolved from modulation to full load/no load, as one way to reduce energy consumption. Enclosures have been designed to lower compressor noise levels.

About 15% of the power consumed by some heatless desiccant dryers is for regeneration. Desiccant dryers are available that provide the required low pressure dew point through heat of compression not regeneration. If only a small amount of the air needs a low dew point, dry only that amount using desiccant dryers and the rest with refrigeration dryers or point-of-use dryers.

Other cost-saving concepts that save energy include variable speed drives, all-in-one compressor packages, and small workplace compressors that can be located near the point of use (Fig. 1).

Variable speed drives

Research has shown that most compressors run below their maximum capacity. For more than 70% of plants, air demand varies between 40% and 80% of system capacity over the course of the workday.

Since all conventional compressors are most efficient at full load, such unused capacity wastes energy. The use of high efficiency motors and compressors with energy efficient controls has helped improve the situation.

Variable speed drive (VSD) compressors use the exact amount of energy needed to produce a specific air demand with no wasted energy. VSD compressors adjust to a plant’s air demand automatically, acting as base unit or trim compressor to provide the greatest energy savings at any given time.

The concept of VSD is not new. However, its application to air compressors is relatively new. Some companies tried to adapt VSD technology to existing compressed air systems by adding a frequency controller on the input power line, which varies the speed of the motor and achieves some energy savings. While these systems work, the energy savings are limited because standard production motors cannot run below a certain speed without heating problems.

Other manufacturers have designed compressors specifically to be variable speed. These packages combine special purpose motors with standard frequency and voltage converters programmed with unique software. These efficient VSD compressors can have a payback period of less than two years.

Compressor installations

A typical compressor installation has the compressor room and workplace separated, requiring long piping to get compressed air to the workplace.

A compressor room can include compressors with built-in after coolers and water separators, and refrigerant air dryers that eliminate most of the remaining water from the compressed air down to about 39 F. Filters help eliminate oil from compressed air down to 1 ppm or lower.

Other equipment includes an air receiver and oil-water separators that remove oil from water in the condensate and provide clean discharge water to help meet local and federal environmental laws. Separators are not required when oil-free compressors are used.

All these items must be connected with piping And installed in a dedicated area. Assuming a 10-hp compressor, the floor space required for such an installation is about 80 sq ft. A 100-hp installation usually requires 200 sq ft.

To reduce the amount of floor space required, some manufacturers have created systems that place some of these items inside the compressor package. For example, a refrigerated dryer, air filter, or automatic drain can be built in. Other manufacturers have created all-in-one designs that place these and other features, such as heat recovery systems, inside the compressor while providing sufficient room to allow access to all components for maintenance and service.

Workplace concept

The workplace concept addresses two basic customer concerns, piping from the compressor room to the workplace, which results in pressure drops, and air system leaks, which increase installation and operating costs.

When a company grows and more compressed air is required, adding a compressor can require upgrading pipe sizes, otherwise the pressure drop becomes too high.

With this in mind, system designers asked, “Why not put the compressor where people work?” The primary challenges to overcome were space requirements and noise levels.

The problem is solved by combining the all-in-one concept with new technology based on finite element modeling that allows better vibration and acoustic compressor package designs to achieve very low noise levels (Fig. 2).

Edited by Joseph L. Foszcz, senior editor, 630-320-7135,

VSD compressor advantages

Outstanding energy savings (up to 35%)

Constant pressure

Wide range of discharge pressures (40-180 psig)

Low starting currents and high power factor, eliminating peak current penalties

Advantages of the all-in-one concept include:

Floor space. A compressor installation that typically occupies 80 sq ft requires only 10 sq ft of floor space with an all-in-one system; installations needing 200 sq ft can be reduced to just 20 sq ft.

Single-point monitoring of all built-in ancillaries from the compressor panel with available remote indication

Single-point connections

Less piping eliminates excessive pressure drop

Lower installation cost.

Small changes that can have an impact on energy savings.

Use pressure drop (DP) gauges on air filters and change the filter elements regularly.

Set compressors to provide minimum acceptable discharge pressure.

Find and fix air leaks. At 125 psig, a 1/8-in. hole allows 32 cfm to escape. This is equivalent to blowing off 8 hp or $2640/yr.

Consider using heat recovery systems. This could be a simple redirection of hot discharge cooling air to heat adjacent space. With either air or water-cooled compressors, heat could also be recovered from the oil system and used to heat water.

Buy compressors with energy efficient controls.

Advantages of the workplace concept include:

All-in-one design

Quiet operation

No piping from compressor room to workplace

Fewer air leaks in piping

Low pressure drop in piping

Receiver suspended from the ceiling saves even more space