Energy savings are in the air

Anywhere compressed air can be eliminated represents an opportunity to reduce system operating expenses. Often, this reduction means fewer compressors must operate to meet the plant's air requirements.

By Christopher Schmidt, Mark D'Antonio and Alan MacDougall

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Compressed air is a common power source in many industrial applications; it is clean, powerful, readily available, simple to route and easy to use. Compressed air is as essential as electricity, gas or oil and often accounts for a significant percentage of a facility's electricity bill. It is very expensive to generate compressed air , operating energy is the single greatest cost during the lifetime of compressed-air systems. Unfortunately, this energy cost often is overlooked when specifying equipment or considering operating costs.

Depending on the type of compressor control (unload, modulation, variable frequency drive, etc.), we have observed that every 5 vol% reduction in end use or leaks results in energy savings between 1.5% and 4%. Consequently, anywhere compressed air can be conserved or eliminated represents an opportunity to reduce operating expenses associated with the system. In many instances, this reduction may mean that fewer compressors must operate to meet the plant's air requirements.

Control air demand
Numerous applications rely on compressed air. Some of the more prevalent uses include air knives, blow-off nozzles, vortex tubes, material handling, diaphragm pumps, and static or deionizer bars. Compressed air also handles a number of control and actuation applications through the use of air-actuated controls and pneumatic cylinders.

Many compressed air applications, although appropriate, are not configured to efficiently use the air. Other applications, often referred to as inappropriate uses, employ compressed air although it may not be the most economical form of energy for the application.

Appropriate uses often provide opportunities for reducing compressed air consumption and optimizing the efficiency of an air-powered process. Many of the inappropriate uses can be substituted with an alternative, more efficient solution.

When evaluating compressed-air use, it is important to consider air-supply control, or as is often the case, lack of control. Many of the uses mentioned above are for processes that have an intermittent need for air, but air is supplied continuously. This unnecessarily consumes compressed air.  Automating compressed-air control with electronic or mechanical control devices generally is the most effective method for avoiding these situations. This reduces the load on the compressor and results in energy savings.

Automatic valves reduce air use
Electronic control can be achieved by using solenoid valves, pilot valves and air manifolds. These electronic, pneumatic control valves can be linked to a timer or sensor, or they can be interlocked with the machine operation to automatically shut off the air supply when it is not needed.

For example, a processing facility fills containers with a liquid on a packaging conveyor line. After the containers are filled, all moisture from the outside of the bottle needs to be removed. The facility uses compressed-air knives around the conveyor to dry off the containers. Compressed air continuously is discharged from the air knives at 60 psig even though containers pass by them only 60% of the time. By installing solenoid valves and sensors, the compressed-air consumption of the air knives was reduced by 40%, or by 350 scfm (Table). As a result, the need for the trim air compressor was eliminated.

Another method of controlling air supply is through the use of mechanically actuated valves. Unlike solenoid valves, which require electrical wiring, sensors and oftentimes logic controls, these mechanical valves are actuated by levers, pedals, cams, springs or other mechanical devices that are controlled by operators or the process itself.

Regulate to reduce air flow
Compressed-air flow in open end-use applications, such as blow-off, material handling, part clearing and moisture removal, often is uncontrolled or manually controlled by equipment operators via in-line valves. Open or crimped tubing may be used as a delivery nozzle so pressures are unregulated.

Regulating pressures and using flow-control devices for delivery can result in more efficient use of compressed air. Such flow-control devices include air-saver nozzles, air amplifiers and air knives. These devices use flow dynamics and geometric design to entrain ambient air into the compressed-air stream. By doing so, a greater volume of air is delivered at the point-of-use, thereby reducing the quantity of compressed air that must be supplied.

A typical example of excessive consumption was observed at a facility where various processes used compressed air either to blow off moisture or to remove excess material. The blow-off was achieved with streams of high-velocity compressed air delivered through copper tubing focused at the desired locations. The installation of more efficient air-saver nozzles met the process requirements and reduced the compressed-air consumption for the applications by 50%, or 80 scfm (Table).

Compressed air alternatives
Compressed air frequently is used for applications that may be more economically served by another power source. As a result, there are various ways compressed air is used that are considered "inappropriate" since other energy sources could serve equally well with more favorable operating costs. These more cost-effective forms of power should be considered before selecting compressed air.

For example, an air motor rated at 1 hp output requires 7-8 hp of electrical energy input to the compressor to develop the volume of air needed to drive the motor. Using an electric motor for the same task will require considerably less energy and therefore will be more efficient.

The main concern in the chemical industry is that the motor be explosion-proof because of the risk of fire or explosion. Totally enclosed fan-cooled (TEFC) explosion-proof motors are readily available, but may cost one-and-a-half to three times as much as an equivalent air motor. However, the operating costs during the life of the electric motor will be far less. In a recent facility audit, replacing air motors with electric motors would satisfy the process requirements and eliminate 180 scfm of compressed air consumption.

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