MRO Magazine

Air Force: New technology puts the squeeze on compressed air costs


February 14, 2001
By PEM Magazine

Air compressor technology is evolving at a rapid pace. The cost of energy and consequently the cost of operating an air system are on the rise. Floor space is also getting to be more expensive. Plant managers are looking into methods to cut down on these costs. Actions taken by the various local governments and power companies across Canada and the U.S. over the past decade, along with the various energy programs, have helped create awareness among many companies and the people responsible for operating them.

The leading manufacturers of air compressors listened to the various needs of their customers and over the last few years made many improvements to their compressors to fulfill some of those needs. Control systems were changed, for example, from modulation to full load-no load to reduce energy cost and most compressors had a canopy added to lower the compressor’s noise level to satisfy local environmental conditions.

Magazines and industry specialists made efforts to educate the public about many little things that can have big effects on energy savings. Some of these suggestions include:

  • Use pressure drop (DP) gauges on your air filters and change the filter elements regularly, as just 1 psi DP costs approximately $250 per year. (One HP running 8,000 hours per year with 90 percent motor efficiency, costs approximately $330 per year).
  • Set your compressor to provide the minimum acceptable discharge pressure
  • Check for, and fix, air leaks. A 1/8" hole allows 32 cfm to go through it at 125 psig. This is equivalent to blowing off 8 HP or $2,640 per year.
  • Be aware when using heatless desiccant dryers that they use 15 percent of the power used for regeneration. If only a small amount of the air needs low dew point, dry only that amount of compressed air via desiccant dryers and the rest via refrigeration dryers. Also, there are desiccant dryers on the market that provide the required low pressure dew point (PDP) and do not waste any energy for regeneration as they use the heat of compression. They provide a good option that is worth looking into.
  • Consider using heat recovery systems in your installation. This could be a simple redirection of hot discharge cooling air to heat adjacent space or a more advanced option that allows the plant to use the recovered heat anywhere needed via hot water. In this case, the compressor itself could be either air or water cooled since the heat is taken off the oil system.
  • Buy compressors with more energy efficient controls.

With new technologies available, the leading manufacturers of air compressors have introduced new concepts to the compressor industry. Some of the major innovations include:

  • Variable speed compressors
  • All-in-one compressor package concept
  • Workplace compressor concept

Research has shown that most compressor installations run below their maximum capacity. In fact, more than 70 percent of companies have varied air demand over the course of a full working day between 40 and 80 percent of the system capacity.

Since all conventional compressors are most efficient at full load, running them below capacity can waste expensive energy. Many plants have introduced programs to help improve the situation, including the use of high efficiency motors and compressors with more energy efficient controls.

The latest innovation in "better control" is the variable speed drive (VSD) concept. Simply put, a VSD compressor will use the exact amount of energy needed to produce a specific air demand with no wasted energy.

The concept of a variable speed drive is not new. The pump industry, and a few others, have used the idea for many years. Its application to air compressors, however, is relatively new. Many companies have tried to adopt VSD technology to existing compressed air systems by adding a frequency controller on the input power line thus varying the speed of the motor and achieving some energy savings.

These systems work, but they are limited in the amount of energy they could save as heating problems can occur if a standard production motor used on the compressor falls below a certain speed.

Some manufacturers have designed compressors from the ground up to be variable speed compressors. These packages have specially designed motors as well as standard designs, well-known frequency and voltage converters (properly programmed with the manufacturer’s software) and many other standard production compressor components.

VSD compressors add many advantages to the industry, including:

  • Outstanding energy savings (up to 35 percent);
  • Constant pressure;
  • The ability to accommodate a wide range of discharge pressure (40-180 psig);
  • Low starting currents and a high power factor eliminating peak current penalties;
  • Built-in converter for space saving and eliminating any need for special converter ventilation requirements;
  • The option of built-in dryers, filters, oil-water separators and many other ancillaries;
  • A choice of many sizes of both lubricated and oil-free screw compressors.

These highly efficient VSD compressors have a quick payback period through the substantial energy savings they can provide.

The VSD compressors adjust to plant’s air demand and act as base unit or trim compressor automatically, thus providing the plant with the most energy savings possible at any given time.

A typical installation commonly used in the industry has the compressor room and working area separated, where longer piping is required to get compressed air to the working stations. In the compressor room we find:

  • Air compressors with a built-in after cooler and water separator;
  • Refrigerant air dryers that eliminate most of the water from compressed air, generally down to a four-degree Celsius Pressure Dew Point (lower dew points require different types of dryers);
  • Air filters that help eliminate oil from compressed air down to 1 ppm or lower
  • An air receiver;
  • Oil/water separators that serve to separate oil from water in the condensate and allow cleaner water to go through the different sewage systems and subsequently meet local and federal environmental laws. These separators are not required when using oil-free compressors;
  • Various piping to connect all of the above together.

All these items need to be installed in a dedicated compressed air system area, usually a compressor room. The space required for such an installation, for a 10 HP compressor is usually around five metres long by 1.5 m wide or a floor space of 7.5 m2. A 100 HP installation usually requires around 6 x 3 m or 18 m2, which tends to be very expensive.

Some manufacturers have improved their designs and begun to install some of these items, such as a built-in refrigerated dryer or a built-in air filter or an automatic drain inside the compressor package. Other manufacturers have managed to install all of the above — and more — inside the compressor, while at the same time keeping the inside of each package "roomy" enough to allow easy access to all components for maintenance functions. This is known as the all-in-one concept.

The main advantages to the all-in-one concept are:

  • Minimal floor space: A compressor installation that used to occupy 7.5 m2 will now require only 1 m2 (10 sq. ft.) and those previously needing 18 m2 will now require 2 m2 (20 sq. ft.) of floor space;
  • Single point monitoring of all built-in ancillaries, generally from the compressor panel with possible remote indication;
  • Single point connections;
  • Less piping to eliminate excessive pressure drop.

Compressor ranges from 5 to 400 HP are now available on the market with this all-in-one concept.

Some manufacturers have taken the all-in-one concept even further and introduced a new workplace concept. The idea sprang from two basic customer concerns:

  • Piping from compressor room to workplace necessitates expensive pressure drops and even more expensive air system leaks;
  • When a company grows and more compressed air is required, adding a compressor will most likely necessitate upgrading of piping size or the pressure drop can become too high (i.e. 100 HP supplying 450 cfm and going through 2 1/2" pipe results in 6 psi DP per 1,000 feet of piping. When business doubles and another 100 HP compressor is added to the system, the piping will have to handle 900 cfm and consequently the DP becomes 24 psi (four times higher) which is excessive and expensive. Consequently piping would need to be upgraded to 3" or 3 1/2" pipe).

So the question for the designers became "Why not put the compressor where people work?" Obviously, space and noise levels were the main deterrents.

The space problem was solved with the introduction of the all-in-one concept. A new technology introduced with finite element modeling (FEM) allowed better vibration and acoustic compressor package designs and subsequently low noise levels that solved the noise problem.

The result is the workplace concept whose main advantages include:

  • All the same advantages of the all-in-one concept
  • No piping from compressor room to workplace
  • No air leaks in piping
  • No pressure drop in piping
  • A receiver that can be hung off the ceiling, called a slick receiver, to save even more space

The compressed air industry is changing rapidly with all the new technologies available. Before you buy a new compressor, take a little bit more time and look around to see what is out there on the market. You might find a compressor that fits both your present and future compressed air needs, while saving your plant a lot of money in the process.

Elie Lufty is, P.Eng., is the product manager, industrial air division, at Atlas Copco Compressors Canada. He can be reached at