Hosing down your losses

Proper selection and application of a flex hose and its quick disconnects is critical to achieving optimum compressor performance. Learn how to spot cost-effective air hoses and connectors.

By Hank van Ormer

PlantServices.com

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.

Performance data for air-operated tools

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
	Max hp	0.35	0.47	0.765	0.927	1.74	2.32
	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
	Max hp	0.41	0.58	0.95	1.15	2.16	2.88
	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
	Max hp	0.5	0.69	1.13	1.38	2.58	3.44
	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
	Max hp	0.6	0.8	1.4	1.5	3.0	4.0
	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
	Max hp	0.6	0.9	1.5	1.8	3.4	4.6
	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. hose

Never 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.