Civil Engineering Reference
In-Depth Information
to throttle the excess air to intake pressure and put it through the compressor
again.
Intake throttling reduces discharge pressure for a given intake volume or
reduces volume for a given discharge pressure. This method is widely used
with constant-speed machines.
For large units, adjustable inlet guide vanes are installed ahead of the first
impeller. The power saving is less than with variable-speed drives, but it is still
significant;
4. Axial compressors. Adjustable vanes are typically used on the several early
stages of axial compressors, since they are very sensitive to the gas angle attack
and have a very short operating range.
The variable speed drive compressor is widely used to regulate the air
flow according to the end user need. It is generally installed in com-
pressed air plant together with others compressors in order to smooth
the flow diagram and reduce the no-load operating hours of fixed
speed compressors. The percentage ratio between no-load and total
operating hours should not exceed 10 %: in this case a value of
0.11 kWh per 1 Sm
3
(0.0031 kWh per 1 Sft
3
) of inlet air in standard
atmospheric conditions to compress at 0.8 MPa (116 psi) can be
achieved.
11.6
Compressed-Air Systems: Distribution Lines
and End Users
11.6.1 The Distribution Lines
The aim of the distribution system is to carry compressed air from the compressor
to the end users at the required pressure, generally in the range 0.5-0.8 MPa
(73-116 psi). The speed generally ranges from 5 to 10 m/s (11.4-32.8 ft/s); the
shorter the line, the higher the speed.
Most systems consist of a main line from which secondary lines go to different
production areas where individual feeder lines carry the compressed air to the end
users.
For heavy intermittent demands, air receivers installed as near as possible the
demand point can reduce pressure losses.
Pressure losses must be considered in selecting the pipe sizes by using tables and
charts from specialized literature.
