Clayton W. Fryer
IMI Norgren Inc.
Despite escalating energy prices, many industrial users of compressed air remain blissfully unaware of its true costs. In fact, the notion that compressed air is free is a common misconception even though the largest single electrical appliance in a manufacturing plant is often the compressor motor.
Here's a look at some commonsense ways to determine the operating costs of typical pneumatic circuits. By knowing the facts, designers can make better decisions that lead to more-efficient systems. Because compressed-air costs are typically hidden in the operating overhead of most companies, any cost reductions immediately fall to the bottom line as added profit.
The first step to improving the bottom line is exposing three popular myths.
Compressed air is free. In the past, most people would not take the time to find out the cost of the compressed air an actuator requires. Yet experts indicate that producing compressed air typically costs from $0.15 to $0.40 per 1,000 standard cubic feet (scf).
Hydraulic-system designers must calculate the size of a power unit to efficiently operate hydraulic cylinders. However, little effort has been made to recognize the similarities between hydraulic and pneumatic systems, including the need for safety, conservation, component sizing, and cost justification.
One benefit of rising energy costs is a growing interest in the costs of pneumatic systems. For example, users are starting to specify 60 psi as the maximum operating pressure, rather than 90 psi or higher. However, many manufacturers are reluctant to discuss costs for fear of encouraging customers to find a substitute for pneumatics. Such fears are overblown. Compressed air is readily available, affordable, clean, and is less of a force hazard than hydraulics, and will continue to be the best choice for many industrial applications.
Pipe size is the right size. Another popular myth involves sizing circuit components. For instance, if a cylinder has a 0.5-in. port, most installers would automatically use 0.5-in. flow controls, pipe and fittings, valves and, quite possibly, a 0.5-in. FRL. This approach leads to oversized, high-priced components and higher operating costs over the life of the equipment. As a result, entire systems are frequently oversized, the focus of the third popular myth.
If a little is good, a lot is better. When sizing cylinders, many designers add an extra safety factor to cover stiction and cylinder breakaway forces. But oversizing results in poor actuator performance, wasted compressed air, and high up-front component costs. For example, oversizing an actuator by one bore size can increase compressed-air costs by 50%. And sizing a cylinder to move more than twice the required load at design pressure adversely affects speed, and also increases compressed-air costs. If care is taken early in the design process, every component upstream of the actuator valves, conductors, fittings, filters, regulators, and lubricators stand a better chance of being sized correctly, resulting in an efficient system.
The first step in properly sizing a system is to calculate cylinder flow. It takes into consideration the forces required to move a load at the specified pressure, extend and retract stroke volumes, cycles per minute, air pressure, and converting it all to standard cubic feet per minute (scfm). Most pneumatics manufacturers use scfm to size system components, and it is also linked to sizing valves using coefficient of flow, Cv.
After calculating cylinder flow, designers can readily determine the correct tubing, fittings, valves, and FRLs for an application.
Consider the following example: A horizontal cylinder must move a 500-lb load in the extend direction only. The retract stroke has no effective load. The load moves 12 in. at 30 cycles/min, 8 hr/day, 5 days/week, and 50 weeks/yr. Compressed-air pressure is 80 psig and cylinder velocity should be as fast as possible.
Accurately calculating cylinder flow rate requires five steps.
1. Size the cylinder for maximum performance.
Based on common practice and cylinder orientation, the force multiplier (performance factor) ranges between 1.25 and 2.0 times the load being moved, at the specified pressure. This is more than enough to compensate for friction and other factors.
To size a cylinder for maximum performance (fastest stroke) apply the 2 rule. Multiply the load by two to get the correct cylinder at the specified pressure. In this case, 500 lb 2 = 1,000 lb. With F = PA, solving for the cross-sectional area determines the cylinder bore. Here, d = 3.99 in., very close to a standard NFPA 4-in. bore. It is important to note that a larger or smaller bore size moves slower at 80 psig. Use a 4-in. bore.
2. Calculate total volume/cycle.
Recognize that the extendstroke volume exceeds the retract-stroke volume on typical double-acting, single-rod cylinders, due to the volume the rod displaces. Using a standard 1.0-in. rod (after checking for rod buckling), extend volume Ve = Al, where l = stroke length.
12.57 in. 2 12 in. = 150.84 in. 3
Retract volume Vr, considering the rod, is:
(12.57 - 0.79 in.2) 12 in. = 141.36 in. 3
Total volume/cycle = 150.84 + 141.36 = 292.20 in. 3
For a 5-in. bore the total volume/ cycle is 461.88 in. 3, or a 58% increase over a 4-in. bore cylinder.
3. Calculate total volume/ min.
Multiply total volume/cycle by cycles/min, 292.20 30 = 8,766 in.3/min.
4. Convert in.3/min to cu ft/min (cfm).
(8,766 in.3/min)/1,728 in.3/ft3 = 5.07 cfm.
5. Convert cfm to scfm.
This conversion requires the compression ratio of compressedair. This converts compressedair to standard conditions (14.7 psia, 36% relative humidity, and 68°F) and gives the working pressure in absolute terms. In most industrial applications, ambient temperature and humidity can be ignored because they have little impact on the calculations.
Compression ratio is (80 psi + 14.7 psi)/14.7 psi = 6.44.
Multiply compression ratio by cfm to get scfm, 5.07 6.44 = 32.65 scfm. Use this information and manufacturers' catalog data to properly size valves (Cv) and FRLs for the system.
Cylinders running 5 days/week, 50 weeks/yr will operate 120,000 min/yr/8-hr shift. In the above application, that equates to 32.65 scfm 120,000 min = 3,918,000 scf of compressed air consumed by this single cylinder annually. If the average cost of compressed air is $0.25/1,000 scf, operating the 4-in. bore cylinder costs $979.50/yr. For the 5-in. bore cylinder, that figure jumps to $1,549.22/yr.
Fortunately, there are a number of ways to reduce operating costs. For example, because the 500-lb load only moves in the extend direction, lowering the return air pressure can produce substantial savings. A regulator that reduces return pressure from 80 to 20 psig cuts compressedair-consumption from 15.8 to 5.8 scfm, and reduces compressedair costs for the retract stroke from $474.08 to $174.03, a 63% reduction. Even if a 0.5-in regulator costs $50, payback only takes about two months.
Additional savings lie in properly sizing valves, fittings, and tubing. Using flow coefficients (Cvs) to size a system provides reasonably accurate results and a margin of safety for most design calculations.
Cvs help evaluate typical circuits for potential bottlenecks. Every component has a flow coefficient and, with some effort, it is possible to determine the Cvs of cylinder ports, flow controls (both in the free-flow and the wide-open controlled flow directions), pipe or tubing, fittings, and directional-control valves.
System Cv is always less than that of the component with the smallest Cv. A strong recommendation is to make the most-restrictive component the most expensive component usually the directional-control valve. This minimizes the cost of system components. It is fair to say that 0.5-in. directional valves cost more than 0.25-in. valves. On the other hand, there is only a marginal cost difference between 0.25 and 0.5-in. pipe or tubing.
There are other approaches to conserving compressed air. If cylinder speed is not important, using a force multiplier between 1.25 and 2.0 times the load yields smaller cylinders and lower air consumption. And using singleacting cylinders could significantly reduce long-term costs.
Perhaps the most-impressive savings in pneumatic circuits are realized by eliminating leaks. In hydraulic systems, leaks are quickly repaired due to obvious hazards and the associated cost of hydraulic oil. With pneumatics, leaks are often ignored until they become uncomfortably loud and annoying, or cause such a significant pressure drop that repair is a must.
Numerous studies suggest air losses due to leaks range from 20 to 45%. Leaks, left unattended, grow due to the abrasive effect of airline contamination and particulates attacking the leak orifice. Eliminating wasted air reduces operating costs and justifies the expense of a maintenance patrol that searches out and quickly repairs air leaks.
Another, less-obvious source of waste is excessive pressure drop across air filters. In hydraulic systems, excessive pressure drop across a filter can have serious consequences for the entire hydraulic power unit even resulting in catastrophic failures. Poor maintenance on a pneumatic filter rarely results in sudden system failure. However, excessive pressure drop across the filter is an ongoing, albeit hidden, cost.
Using pressure-differential indicators on filters and changing filter elements regularly avoids the escalating cost of pressure drop across the filter element. Electronic and mechanical indicators are readily available and provide reminders to service the elements. Pressure switches to monitor pressure drop also help ensure efficient use of compressed air and optimum system performance.
The High Cost Of Leaks
|Orifice size, in.||
Leak rate scfm @ 100 psig
|Values are based on leakage 24 hr/day, 7 days/week, over 50 weeks. Costs are based on compressed air at $0.25/1,000 scf.|
A = Cylinder bore, in. 2
d = Cylinder diameter, in.
F = Load, lb
l = Stroke length, in.
P = Air pressure, psi
Ve = Cylinder extend volume, in. 3
Vr = Cylinder retract volume, in. 3
IMI Norgren, (303) 794-2611, norgren.com