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.
Compressed air often is used for a number of applications that easily could be served by a low-pressure (less than 15 psig) electric-blower system. For example, a facility has six extrusion lines that consume large volumes of compressed air for flock and coolant blow-off operations. Installation of centrifugal blowers to serve the same function eliminates the compressed-air consumption for this application and allows facility staff to shut off a 300-hp compressor.
Look for these common inappropriate uses of compressed air and consider the suggested alternatives:
Open Blowing: Compressed air is applied with an open, unregulated tube, hose or pipe. Typical applications for open blow-off are part and debris clearing, machine tool cooling, drying and cleanup. Alternative: Replace with blower systems or flow distributing devices, such as nozzles, air amplifiers or air knives.
Sparging/Agitation: The process of aerating, agitating, oxygenating or percolating a liquid with compressed air. Alternative: High-volume, low-pressure blower systems driven with electric motors can be used for many sparging or agitating applications.
Aspirating: Compressed air is used to induce the flow of another gas. Alternative: As with sparging above, blowers can be used to induce the flow of another gas.
Personnel Cooling: Operators use compressed air to cool themselves. Alternative: Space and occupant cooling should be adequately supplied by HVAC systems.
Air-Powered Diaphragm Pumps: Air-powered pumps are a convenient way to transfer chemicals or treated water since their plastic components are corrosion resistant. Alternative: Specially designed electrically driven pumps with plastic bodies are available for pumping chemicals and treated water.
Induced Vacuum Systems: Compressed air often is used to create a vacuum for process systems. Alternative: Independent vacuum systems should be used, such as electric vacuum-pumping systems, which are far more efficient.
Cabinet Cooling/Pressurization: Compressed air commonly is used to provide cooling or positive pressurization of electronic cabinets located in warm or dirty environments. Cooling typically is provided by air-powered vortex tubes that provide a cold air stream output from a compressed air input. Alternative: Cabinet cooling should be provided by blower or heat-pipe systems, depending on the environment. For dirty environments, install heat-pipe units that maintain the internal cabinet temperatures without interchange of external air.
Distribution system inefficiencies
Oftentimes the most substantial inefficiencies lie in the distribution system. These include the most common offender, air leaks, as well as excessive pressure settings, high pressure drop filters and poor piping design.
Lose the leaks
Air leaks are an inevitable part of any compressed air system and can account for 10% to 30% of a facility's compressed-air demand. Leak-abatement programs are highly effective in minimizing air loss in a system and should be included as part of a regular preventive maintenance program that ideally is performed quarterly.
Leaks usually range from less than 1 cfm to greater than 60 cfm. For example, a 1/8-in. round orifice (nozzle, hole, leak, etc.) discharges about 26 scfm of compressed air at 100 psig. Many leaks occur at threaded connections and are the result of decayed or absent pipe thread sealant at fitting junctions. Other typical leaks are found in rubber hoses, filter/lubricator/regulator gaskets or drains, cracked filter bowls and at quick-couplers. In a shampoo-production facility, 133 compressed-air leaks were found and tagged. The total leak load of these 133 leaks was estimated to be 337 scfm.
Reduce supply pressure
It is prudent to operate compressed-air systems at the lowest pressure that meets production requirements. It is common, however, to observe air supply pressures in excess of 100 psig at an industrial facility. In certain cases, this pressure setting is warranted. But in most cases it is due to the absence of or improper adjustment of a regulator since most industrial operations require 85 psig or less.
When the supply pressure is greater than required, larger volumes of air are expelled for any given end-use, including leaks, which equates to wasted energy. For example, reducing the compressor pressure settings by 2 psig typically will reduce energy consumption by 1% .
Each use should be regulated separately and be supplied with compressed-air pressures specifically set to meet the needs of that process. Select regulators that have low pressure drop, minimize pressure swings and provide consistent supply pressure. If only one application in the plant requires a significantly higher pressure, it likely is more efficient and cost-effective to use a booster or a separate compressor dedicated to that application.
The bottom line
As energy costs escalate, it increasingly is important to improve efficiency of demand-side uses of compressed-air systems. Controlling the volume of flow and supplying flow only when needed can all reduce consumption while still meeting production requirements. This can be done by using efficient nozzles and simple solenoids or mechanical valves. Also, numerous inappropriate compressed-air uses can be eliminated by adopting alternatives such as high-volume blowers, vacuum systems and electric-motor-driven systems.
Compressed-air systems widely are used in the industrial sector and act as a primary energy source for innumerable industrial applications. Paying careful attention to the demand side of the system can reveal significant opportunities for reducing compressed-air use and increasing energy savings.
Christopher Schmidt is project engineer for Energy & Resource Solutions, Haverhill, Mass. E-mail him at CSchmidt@ers-inc.com. Mark D'Antonio is vice president of engineering operations for ERS. E-mail him at MDantonio@ers-inc.com. Alan MacDougall is energy analyst for ERS. E-mail him at firstname.lastname@example.org.