Fluid Power /Pneumatics

Fluid Power / Pneumatics

Detailed Notes on Hydrostatic Transmissions Hydrostatics Index

Pneumatics

Compressed air systems are generally used to provide controlled motion using cylinder actuators for linear motion and rotary actuators rotary motion.. Typical uses of compressed air drives include..

Power tools
Valve Operators
Positioning cylinders
Hammer drills.
Paint Spray Guns
Air driven Hoists
Air Motors
Packaging systems
Pick and Place units
Air lift pads
Air Conveyor systems
Fluid agitation systems
Impact wrenches

The main disadvantage of pneumatic actuators compared with electrical and hydraulic actuators is that the motive fluid (air) is compressible and hence accurate speed control and position control is difficult and often requires ancillary systems..

The advantages of pneumatic systems are

The low cost of the components
The ease of design and implimentation
The huge range of available components
The use of air limits the force/torque that can be generated providing a safety feature

Compressed Air system Components

Operating Conditions.
Normal compressed air systems operate at a pressure of approximately 7 barg...
The compressor would need to be rated for some margin above the operating pressure e.g. 10-12 barg.
The equipment would be rated for use at pressures 4 barg to 6 barg.

A compressed air system generally includes the following components..

Air compressor..
This is either electric driven or driven by an internal combustion engine.. The compressor output has to be balanced with the demand and can be operated on a variable displacement basis or, more normally, it is operated intermittently under pressure control..

Air Receiver..
This is a vessel so sized that the pressure is maintained within set limits as the flow to the users varies.

Air Preparation
On a large system there may be a main air treatment system on the main header and a individual air treatment system for each group of users. An air treatment system will include an Filter, Lubricator, Isolation Valve, and Pressure Regulator (with gauge) as a minimum.

Air piping system
On normal systems the main air pipes will be suitably rated steel /galvanised iron /Wrought iron piping. The local piping to users will be copper and plastic piping can be used for small bore connections. The piping should include the necessary slopes down to separators for removing any moisture in the compressed air supply which forms in the piping over time. The velocity of air within the air supply header should be limited to about 15m/s.

Air Consumption Data
Note: The following information represents crude first estimate data. Actual information from suppliers literature should be used for detailed estimates..

Appliance	Size	Air Consumption
Appliance	Size	litres/s
Air motor	Up to 0.75kW	20/kW
	0.75-4.0kW	18 /kW
	over 4 kW	16 /kW
Drilling Machine	mm hole in steel	10
	9 mm hole in steel	12
	18 mm hole in steel	15
	25 mm hole in steel	20
	30 mm hole in steel	25
	38 mm hole in steel	30
	50 mm hole in steel	40
Spray Gun	Small	1 at 4 bar
	Medium	4 at 4 bar
	Large	10 at 4 bar
Grinder	18 mm wheel	5
	50 mm wheel	8
	100 mm wheel	20
	150 mm wheel	30

Flow of Compressed Air Through Pipes

Note: Table allows rough estimation of pressure drop along

Free air	Compressed	Pressure Drop of Air in bars /100m at 7bar gauge and 15degC along Sched 40 compressed air pipes
Nm³/m @ 1,013bara	m³/m @ 7barg	Pipe sizes (nb sched 40) and ID (mm)
Nm³/m @ 1,013bara	m³/m @ 7barg	1/8"nb	1/4"nb	3/8" nb	1/2"nb	3/4"nb	1"nb	1.25"nb	1.5"nb	2"nb
@ 15^o C		6,8mm	9,2mm	12,5mm	15,8mm	21,0mm	26,6mm	35,1mm	40,9 mm	52,5 mm
0,03	0,038	0,093	0,021	0,0045
0,06	0,0076	0,337	0,072	0,016	0,0051
0,09	0,0114	0,719	0,154	0,033	0,011
0,12	0,0152	1,278	0,267	0,058	0,018
0,15	0,019	1,942	0,405	0,087	0,027	0,0067
0,2	0,0253	3,357	0,698	0,146	0,047	0,011	0,0035
0,3	0,0379	7,554	1,57	0,319	0,099	0,24	0,0073
0,4	0,0506		2,71	0,458	0,170	0,041	0,012
0,5	0,0632		4,1	0,842	0,257	0,062	0,018
0,6	0,0759		5,9	1,19	0,37	0,88	0,26	0,066
0,7	0,0885		8,03	1,62	0,494	0,117	0,035	0,0086	0,0041
0,8	0,101			2,12	0,634	0,150	0,044	0,011	0,0053
0,9	0,114			2,64	0,803	0,187	0,055	0,014	0,0065
1,0	0,126			3,26	0,991	0,231	0,067	0,017	0,0079
1,25	0,158			4,99	1,55	0,353	0,102	0,026	0,012
1,5	0,19			7,2	2,19	0,499	0,147	0,036	0,017	0,0048
1,75	0,221			9,79	2,98	0,679	0,196	0,047	0,022	0,0064
2,0	0,253				3,82	0,871	0,257	0,062	0,029	0,0082
2,25	0,284				4,84	1,1	0,325	0,076	0,036	0,010
2,5	0,316				5,97	1,36	0,393	0,094	0,045	0,012
3,0	0,379				8,6	1,92	0,565	0,135	0,063	0,018
3,5	0,442					2,61	0,754	0,184	0,086	0,024
4,0	0,506					3,41	0,984	0,236	0,110	0,030
4,5	0,569					4,32	1,25	0,298	0,136	0,038
5,0	0,632					5,34	1,54	0,368	0,164	0,046
6,0	0,759					7,68	2,17	0,518	0,236	0,066
7,0	0,885						2,95	0,689	0,321	0,090
8,0	1,011						3,85	0,9	0,419	0,115
9,0	1,138						4,88	1,14	0,53	0,145
10,0	1,264						6,02	1,41	0,64	0,179
11,0	1,391						7,29	1,71	0,774	0,217
12,0	1,517						8,67	2,02	0,921	0,252
13,0	1,643							2,38	1,08	0,295
14	1,770							2,76	1,25	0,343
15	1,896							3,13	1,44	0,393
16	2.023							3,57	1,64	0,443
18	2,276							4,49	2,07	0,558
20	2,528							5,49	2,53	0,685
22	2,781							6,65	3,07	0,825
26	3,287								4,22	1,15
28	3,54								4,86	1,33
30	3,793								5,62	1,52
32	4,046								6,39	1,73
34	4,298								7,22	1,94
36	4,551								8,09	2,17
38	4,804									2,41
40	5,057									2,67
45	5,689									3,36

For other diameters

Δ P _other = Δ P. (ID/ID_other ) ⁵

For other pressures than 7 barg and other temperatures then 15 ^oC..

Δ P _other = Δ P . (7 +1,013) / ( P_other +1,013) . (273+ t _other) / 288

For other Lengths..Pressure drop is in proportion to length ..

Useful Links

Hydraulics/Pneumatice.. Relevant information and Links
Nogren.. A supplier of components for pneumatic systems
Parker .. Supplier of wide range of hydraulic and pneumatic components
Pneumatic systems.. A short downloadable paper providing basic information
Pneumatics Online.. An american based site- providing information and links related to pneumatic systems

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