Don't overlook the return stroke
Another way to conserve energy is by supplying the correct pressure for an actuator’s return stroke. Most applications only move a load in one direction. However, many machines use the same pressure for both the working and return strokes.
For example, a material-handling system that pushes boxes from one conveyor to another needs high cylinder force only in one direction. The working stroke may demand 100 psi to move a box, but the low-force return stroke only requires 10 psi. Using the same pressure in both directions wastes energy. Reducing the pressure on the return stroke saves 90% of the volume of compressed air. Because that conserves compressed air, a lot of energy is saved over the thousands of cycles that the action is performed.
Another important and often overlooked benefit of regulating air pressure to the minimum required level: It lessens wear and tear on the pneumatic and related components. Not overpressurizing the retract stroke reduces vibrations and shock to the machine. Moreover, adding a quick-exhaust valve can reduce cycle times because exhaust rate on the return stroke affects cylinder speed.
Processes with shorter strokes can use single-acting, spring-return cylinders. A control valve ports compressed air to the cylinder for the working part of the stroke, and then exhausts that air. During the return stroke the spring, or sometimes merely the weight of a mechanism, brings the cylinder back to the starting position.
A typical case where single-acting, spring-return cylinders can reduce energy demand involves presses. In this type of application, a cylinder pushes two items together such as a bearing into a housing, or a plug into a hole. The job demands a significant amount of force to press the parts together, but only a small amount to retract. This makes it a good candidate for energy savings by minimizing return-stroke air consumption.
Turn it off
Shutting down a machine when it’s not working seems like an obvious way to save energy. While some elements of a system, such as air bearings, can require pressure even when the machine is off, the required compressed airflow is usually much less than that needed during normal operations.
However, many installations have no automatic way to reduce or stop airflow to idle machines. Reduced staffing often means that manufacturers can no longer send maintenance workers to manually turn off air to specific machines. In these instances, automatic air-reduction controls will lower air pressure or, if appropriate, shut it off completely when the machine isn’t working, more than paying for itself in short order.
Leaks are common and expensive in pneumatics systems. Statistics from the U. S. Department of Energy show the average manufacturing plant loses 30 to 35% of its compressed air due to leakage. The good news is many leaks can be prevented or repaired.
There are many points between the compressor and the load where leaks can be fixed, with valves and seals two main areas for improvement. Deteriorated seals and certain valve designs, such as lapped-spool valves with metal seals, have inherent internal leakage that is constant as long as air is supplied to the valve. Switching to valves with soft seals can significantly lower this leakage.
However, it’s important to note that air consumption in lapped-spool and metal-sleeve valves doesn’t vary during operation. On the other hand, during an open crossover when the valve shifts, a soft seal produces hundreds of times more leakage than a lapped spool-and-sleeve valve. Therefore, selecting the right type of valve for an application can minimize air leakage.
It’s equally important to consider environmental conditions such as temperature and humidity, and type or lack of lubrication, as these all affect the leakage rate of a seal. In some instances, hardy and relatively expensive seals like Viton, Teflon, or polyurethane may be the best option.
Pneumatic systems aren’t quite as simple as they might first appear. The engineering concept of actuating valves and moving loads with air is quite straightforward, but optimizing pneumatic-system designs and maintenance involves many variables.
While operating conditions and component selection are large factors in the general inefficiency of these circuits, pneumatic systems can be greatly improved by implementing the concepts discussed here. OEMs play a big part because much of the energy inefficiency of pneumatic systems can be remedied at the design level. Machine users also have a crucial role to play as they are responsible for the overall operation and maintenance of a plant’s pneumatic system.
In today’s world, users are more aware of how energy consumption affects their bottom line. As such, OEMs must consider their customers’ TCO, not just upfront costs.
For more information, call AutomationDirect at (800) 633-0405 or visit www.automationdirect.com.