Edited by Kenneth J. Korane
• Properly sized components are the first step to efficient systems.
• Supplying actuators with more pressure than needed wastes energy.
• Decentralized valve manifolds significantly reduce line losses.
Bosch Rexroth, www.boschrexroth-us.com
Industrial equipment that generates or uses compressed air presents many opportunities to cut your energy bill. After all, energy costs make up more than half the total cost of ownership of pneumatics today. So a constructive evaluation of pneumatic applications can quickly pay off. Here are three key factors to consider.
Properly sized components are a prerequisite for low-energy consumption. Yet it’s still common practice to oversize components to build in a margin of “safety.” Companies could afford overdesigned systems when energy costs were of little concern. But that’s not the case today. Oversizing unnecessarily increases air consumption, and larger systems cost more to purchase and operate than properly sized ones. They are also heavier and bigger, issues when weight is critical or mounting space is at a premium.
Reducing a cylinder’s diameter saves on the volume of air consumed each cycle, provided it still generates the necessary force. Our experience shows that in typical installations, at least 15% of the air volume can be saved by using compact and economical designs with correctly sized cylinders and valves, and optimized nominal flows. That equates to an energy savings of 15%. Fortunately, leading manufacturers offer online configuration tools and energy-saving calculators that help designers size components to precisely meet application requirements.
For example, at a lumber processor we increased a sawmill machine’s output 13% by simply reducing the size of the pneumatics on the sorter gates. The cylinder bore dropped from 3.5 to 2 in., air valves from 0.5 to 0.25 in., and pressure from 100 to 60 psi. These seemingly minor alterations effectively increased production speed and cut energy use by 75%.
Pneumatic systems frequently waste energy by supplying higher pressure than an actuator needs. For instance, in many applications cylinders either push or pull a load, but not both. Yet most often machines use the same pressure for both extend and retract strokes, which is extremely inefficient.
Supplying the right pressure for each task, using pressure regulators, can lower energy consumption by more than 25%. For instance, “smart” regulators combine digital control electronics with proportional valves. They constantly compare preset pressure limits with actual values to ensure exact metering.
For example, take the case of a pneumatic cylinder used in a press-fit assembly operation. Pressing the part required 100-psi cylinder pressure, and 100 psi was also used to retract the unloaded cylinder — even though retraction required less than 10 psi. Adding a pressure regulator between the valve and cylinder port reduced retract pressure to 10 psi. The machine then used 38% less energy/cycle, and the savings paid for the new components in approximately 2,100 cycles.
25%. Lower energy consumption from using pressure regulators to supply just the right pressure for each task an actuator performs.
35%. Energy saving from decentralized valve/actuator units that eliminate pressure losses through long lines from the control cabinet to the pneumatic drive.
Every application is different, so the overall machine design and types of components influence potential savings. However, these comparisons are based on modern components and systems, and values are relatively conservative. Savings could be much greater in applications with outdated components. The older the system, the greater the potential savings.
Applications that use rodless or lifting cylinders, such as automotive-assembly elevators, can typically benefit from nonload-stroke pressure regulation. In general, the larger the cylinder, the larger the efficiency gain. The key: look for actions with significant differences in the forces required for each stroke direction.
Energy savings is just one benefit of pneumatic pressure regulation. Because cylinder speed is governed by the rate at which air exhausts on the nonload side of the piston, reducing nonload pressure permits higher piston speeds on the working stroke. This decreases cycle time. It also reduces shock, vibration, and noise and extends life for components and the machine itself. And reducing air pressure inherently reduces the amount and cost of any leaks in the system.
When OEMs build standard machines for a variety of end users, the equipment must be designed and sized to accommodate users with the lowest available pressure. This usually means specifying larger, more-expensive components that increase air consumption. This issue can be partially resolved by regulating the machine’s supply pressure down to the design pressure.
Finally, operators commonly increase supply pressure on regulators in hopes of improving performance, but this wastes significant amounts of money in air and operating costs for no actual benefit — if components are sized correctly. It is important to monitor and ensure machine pressure remains within designated limits to avoid wasting energy.
Decentralized air supply
Centralized valve manifolds are typically cumbersome, require long air lines, and consume a lot of energy. Manufacturers now offer small, decentralized valves and manifolds that concentrate pneumatic functions at the point of use. Valves can mount directly to cylinders without hose connections. This direct connection eliminates pressure losses through long lines from the control cabinet to the pneumatic drive. Valve/actuator units can reduce tubing connections by 50% and cut energy use by 35%. Decentralized systems can also yield faster response times and higher cycle frequencies.
Manufacturers now offer decentralized valve units made of engineered polymers that are small, light, chemically resistant, and able to withstand harsh operating conditions. Some valves and manifolds are rated IP69K and have sanitary design and materials suitable for food processing. This eliminates the need to house pneumatic valves in remote stainless-steel enclosures with long tubes running to the actuators.
Every pneumatic system can save energy by avoiding leaks. Valves and deteriorated seals are two common sources. Some 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 comparable valves with soft seals can significantly reduce leakage.
Another source of leaks is industrial seals attacked by chemicals in the air system. If standard Buna seals degrade, switch to HNBR, Viton, PTFE, or polyurethane.
Modern air-preparation units are available with an integrated air-volume sensor. The sensor emits an electrical pulse each time a specific volume of compressed air has passed through the air-preparation package. The electrical pulse signals can be totaled by the controller and therefore actual air consumption (and energy costs) can be calculated for the machine over a period of time. This also lets users detect increases in air consumption of a machine that indicate developing leaks or nonscheduled changes to the operating pressures for the motions of the machine. The real life cost of leakage and overpressurization can be counted as well as the cost savings from correcting these problems.
Also consider energy-saving controls for on/off valves. Most installations do not automatically shut off air to idle machines, and having maintenance personnel turn off air to specific machines is generally too cumbersome. And some subsystems may require pressure even after the machine is turned off, such as air bearings, air gauging, and cleaning. Frequently, however, a lower supply pressure or high pressure for short durations can satisfy these functions.
For machines that do not run around the clock, an automatic air-reduction control reduces supply pressure and air consumption when the plant is not operating. Typically, the system can be overridden if higher pressure is required during off hours.