Almost every compressed air system uses flex hose to make the final connection to production machinery. Proper selection and application of this air hose and the quick disconnects is critical to achieving optimum performance.
Compressed air system audits often uncover significant opportunity for savings at such locations. Typically, total system pressure is unnecessarily high to offset pressure drops in small-diameter hose and incorrect quick disconnects.
The most important sizing data for any process is the air flow and minimum pressure required at the tool entry. If you don’t know these data, it’s easy for system analysts to measure them on-site. In areas where the pressure or flow are critical to productivity or quality, economical mass flowmeters and pressure gauges can be rigged for continuous machine monitoring.
Working by hand
Air-driven tools can illustrate the effect of hose and connector selection on productivity and quality. Most air tools are designed for a hose feed pressure of 90 psig. The tool designer really sizes for full flow at about 80 psig for optimum performance. Depending on the tool, pressure significantly higher than 90 psig may not increase performance, but lower pressure certainly will reduce it. In many cases, out-of-range air pressure can damage tools and reduce the time between rebuilds.
Standard impact tools, screwdrivers, grinders, chippers and banders prefer a constant 80 psig to 90 psig inlet pressure. The phrase “at rest pressure” has no meaning. Table 1, abstracted from selected air tool technical data sheets, clearly shows the general magnitude of performance loss at low pressure. At 70 psig, most tools will still operate, but below rating. At 60 psig, performance is seriously degraded and probably will be unacceptable. Operating below 60 psig isn’t really a viable option. However, unless specifically stated, no tool is designed for inlet pressure greater than 100 psig. Table 2 shows the approximate performance losses at various inlet pressures in 1-hp to 3-hp vane motor grinders and sanders. The power drops may preclude effective job performance. Along with the loss in power, which is most important, there’s also a loss in speed. Both factors affect productivity.
|Inlet air press (psig)||1/2 hp||3/4 hp||1 hp||1.5 hp||2 hp||3 hp|
|60||rpm at max load||8,500||5,809||3,810||5,550||3,730||3,900|
|scfm at max hp||20||20.1||27.5||30||51||67|
|Max torque ft/lb||0.36||0.88||1.67||1.67||3.7||5.0|
|70||rpm at max load||9,000||6,184||4,060||5,900||3,975||4,160|
|scfm at max hp||21||53||32||35||60||78|
|Max torque ft/lb||0.42||1.0||1.95||1.95||4.3||5.8|
|80||rpm at max load||9,500||6,429||4,250||6,190||4,160||4,350|
|scfm at max hp||22||27||36||40||68||89|
|Max torque ft/lb||0.5||1.2||2.2||2.2||4.9||6.7|
|90||rpm at max load||10,000||6,700||4,400||6,400||4,300||4,500|
|scfm at max hp||24||30||39||42||76||100|
|Max at torque ft/lb||0.55||1.3||2.5||2.5||5.5||7.5|
|100||rpm at max load||10,500||6,888||4,520||6,580||4,415||4,630|
|scfm at max hp||26||33||45||50||85||111|
|Max torque at ft/lb||0.6||1.4||2.8||2.8||6.1||8.3|
Beware of 3/8-inch. hoseNever select air hose unless you know the air flow and hose length the tool requires. The most common hose sizes for plant use range from 3/8 inch to 3/4 inch and handle 300 psig. Hose choice is often left to the operator, who usually wants 3/8-inch hose, regardless of application, because:
- 3/8-inch hose appears to be the lightest and easiest to handle.
- A 50-foot length of 3/8-inch or 1/2-inch hose weighs about 13 pounds, depending on grade but a 50-foot length of 3/4-inch hose weighs 22 punds.
- The operator might not be trained regarding the hose size required to run the tool.
A 3/8-inch hose isn’t a viable supply hose for industrial tools. The smallest size you should use is 1/2-inch. Table 3 refers to premium black industrial air hose. The data leads us to specific conclusions:
- 1/2-inch hose in 50 foot lengths is suitable only for 1 hp or smaller tools (approximately 30 cfm/hp).
- 3/4-inch hose is acceptable for 2 hp to 3 hp (60 scfm to 90 scfm), depending on the length of run.
- For runs greater than 50 foot, use larger hose or pipe, supported on the walls or ground as required, to eliminate pressure drop.
- For more comfort and easier operation, adding an 8 foot to 10 foot whip hose to the larger 3/4 inch or 1 inch main line will have minimal effect on performance, but still gives the operator the feel of a lighter hose.
- Don’t use any more hose than necessary. Coiling the extra just adds pressure drop. Cut the hose to the proper length and install fittings.
Don’t forget about OSHA safety requirements. Going from a 3/8-inch to 1/2-inch hose still allows personnel to handle smaller hose without the mandatory automatic air shutoff valve or safety velocity fuse. These fuses are an excellent safety device when applied correctly. Refer to U.S. Department of Labor, Occupational Safety and Health Administration – Power Operated Tools 1926.302, page 2, paragraph 1926.302(b)(7), which mandates a safety velocity fuse on all hoses larger than 1/2 inch inside diameter.