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Fluid Power / Pneumatics

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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
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)
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/IDother ) 5

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

Δ P other = Δ P . (7 +1,013) / ( Pother +1,013) . (273+ t other) / 288



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

Useful Links

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

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