Six steps to energy efficiency in pneumatic systems
Pneumatic systems are considered to be inherently inefficient, so little is done to improve them. However, this article will show how these systems can be optimized to not only improve energy efficiency, but also enhance productivity and lengthen the life of the machine.
The good news is while there are many sources of inefficiencies in pneumatic systems, implementing these strategies can reduce energy consumption by as much as 35%.
Options for improving efficiency:
- Find and fix leaks
- Correct compressor operation
- Optimize component sizes
- Use proper pressure
- Regulate the return stroke
- Hit the off switch.
Find and fix leaks
Just because leaks are very common in pneumatics systems, it doesn’t mean they can’t be remedied. The U.S. Dept. of Energy statistics indicate the average manufacturing plant experiences compressed air leakage in the range of 30% to 35%.
Fortunately, many of these leaks can be fixed or even prevented. Of the many points between the compressor and the load where leaks occur, valves and seals are two main areas for improvement. Deteriorated seals should be the first area to examine. It’s also important to understand the nature of leaks to select the best valve for the job.
Certain valve designs, such as lapped-spool valves with metal seals, have inherent internal leakage that is constant while air is supplied to the valve. Simply installing valves with soft seals can significantly lower leakage.
Nonetheless, it’s important to note that lapped spool and metal sleeve valve air consumption doesn’t vary during operation. On the other hand, a soft seal produces hundreds of times more leakage than the lapped spool-and-sleeve valve during an open crossover when the valve shifts. Therefore, total air leakage can be optimized by selecting the right type of valve for the application.
Environmental conditions, such as temperature, moisture content. and lubrication, all contribute to the leakage rate of a seal. Pneumatic systems in areas with high contamination risk can benefit significantly from an investment in resilient seals like Viton, Teflon, or polyurethane.
Correct compressor operation
After fixing leaks, compressors are the next biggest area for improvement. The U.S. Dept. of Energy reported in a 2012 study that manufacturers spend over $5 billion each year on energy for compressed air systems. This shouldn’t be surprising since they form the backbone of the pneumatic system.
Manufacturers that optimize their compressed air supply systems have been able to reduce their compressed air energy consumption in the range of 20% to 35%.
Detailing the methods for increasing compressor efficiency is beyond the scope of this article. However, the Dept. of Energy offers guidelines for determining the cost of compressed air in a plant, as well as tips on how to reduce compressor energy usage.
Optimize component sizes
It’s important to take the time upfront to correctly size the pneumatic system’s components because each component’s size affects other parts of the system. Buying smaller control valves may save money on the purchase price, but they will be more expensive over time. Smaller control valves will require the air compressor to work harder simply to get the proper pressure to the actuators, creating a long-term demand for more energy.
Another common problem comes from oversizing the cylinders more than necessary. Some oversizing is necessary to compensate for pressure fluctuations and air losses; however, components that are far too large account for one of the biggest energy losses in a pneumatics system.
For instance, a 3-in. cylinder requires more than double the volume of air of a 2-in. cylinder. However, this much extra capacity may not be needed. To avoid grossly oversizing, it’s important to remember most loads and speeds require only 25% additional capacity to ensure correct operation. By selecting the right amount of oversizing, cylinder efficiency can be improved by as much as 15%. When factoring in the number of cylinders that will operate thousands of times over their life span, the savings from right sizing becomes significant.
To assist with the many calculations and considerations that go into properly sizing components—such as if the load is rolled or lifted—there are software packages, online calculators, and even an iPhone app that can assist with component sizing. By spending a little more time understanding the system’s true requirements, the savings can be substantial (Figure 2).
Use proper pressure
It’s inevitable that small amounts of air pressure will be lost due to usage fluctuations, line and valve flow resistance, and other factors. However, many losses can be prevented by ensuring the distance between the air compressor or air supply point and the actuator isn’t longer than necessary.
A system that uses the shortest tubing possible will lower energy consumption. Tubing running between control valves and cylinders should be less than 10 ft whenever possible. When the distance is longer, the ability to position the load correctly may be compromised without more pressure.
Making sure the actuator uses only the pressure needed to perform the task is another way to improve energy efficiency. Too often the system is designed to deliver more pressure than needed to the actuator because energy efficiency isn’t considered, only the ability for the system to perform.
Another instance of actuators using more pressure than necessary occurs on the plant floor when operators increase supply pressure, believing it improves performance. Unfortunately, this operating practice often merely wastes energy and money.
Installing sensors that monitor pressure, and pressure regulators that maintain correct values, can keep pressure between the minimum and maximum parameters. Simply by adding pressure regulators to control the amount of pressure distributed, plants have achieved energy savings of up to 40%.
Regulate the return stroke
Supplying the correct pressure to the return stroke is often neglected when optimizing pneumatic systems for energy efficiency. The majority of applications typically move a load in one direction, but the machines use the same amount of pressure on the return stroke as they do on the working stroke.
There are a few ways to improve the efficiency of the return stroke. A spring return actuator on a single-acting cylinder usually works well with shorter strokes. The control valve in a spring return actuator ports the pressure (using the compressed air) for the working part of the stroke, and then exhausts the air. For the return stroke, the spring—or sometimes merely the weight of the mechanism—takes the cylinder back to the starting position.
An example of a spring return actuator is found in a material handling system with a conveyor pushing a box to a side conveyor. The cylinder is working only in one direction. While the working stroke of the process needs 100 psi to move the item, the return stroke requires only 10 psi. By installing a spring return actuator, the volume of air for the return stroke is eliminated, cutting the work required by the compressor in half. The job is performed correctly, and a great deal of money is saved over the thousands of cycles the action is performed.
A spring return actuator on a single-acting cylinder can also reduce energy demand for a pressing action. This type of application involves two items pushed together, such as a bearing into a housing or a plug into a hole. A significant amount of force is required for the pressing action, but only a small amount is needed for retraction. The spring return will provide the force needed for the retraction.
Regulating air pressure saves energy, and it will also minimize wear on the pneumatic and related components. By reducing pressure for the extract stroke to only what is needed, the machine is not subjected to unnecessary vibrations and shock.
Hit the ‘off’ switch
Air is often wasted on idle machines when there is no automatic way to stop airflow. In the past this was often performed manually, but staff reductions mean fewer maintenance workers are available to manually turn off air to individual machines.
Other workers don’t understand that some elements of the system, such as air bearings, which require pressure even after the machine is turned off, only need a fraction of the pressure required during operation. These operations can benefit from an automatic air reduction control package to lower the air pressure to necessary levels when the machine isn’t working. Typically, the cost of these devices is recovered within a few months.
Perhaps the ubiquity and relative simplicity of pneumatic systems leads companies to believe they are inherently energy inefficient and little can be done to improve the situation. However, many savvy plant managers are optimizing these systems to improve energy efficiency.
In the past, businesses were mainly concerned only with their pneumatic systems performing their job correctly. Little thought was put into correctly sizing components and making sure that only the pressure required was used.
However, today’s businesses can’t afford to waste energy in any area of the plant. The good news is that with some time dedicated to determining the actual requirements of the pneumatic system and selecting the right components, plants can expect improvements in both energy efficiency and productivity.
Pat Phillips is pneumatics product manager for AutomationDirect.
AutomationDirect is a CSIA member as of 2/26/2015