Civil Engineering Reference
In-Depth Information
Pressure losses range between 40 Pa/m and 150 Pa/m (between 1,768 and
6,631 psi/ft) with an air-flow speed from 5 to 10 m/s (16.4-32.8 ft/s).
Higher speeds would provoke higher losses. Leaks along the pipeline
may cause significant losses. Notice that a hole with a diameter of 5 mm
(0.02 ft) loses at least 5 kW of electric input power (with a leak of roughly
1Sm
3
/min, 35.3 Sft
3
/min).
Table
11.4
lists values of losses in typical operating conditions.
A useful test is to shut down all the equipment needing compressed air and to
measure the compressor's power requirement: the resulting value represents the
system leaks because no useful work is delivered to the end users.
The efficiency of the distribution line between two sections A and B (see
Fig.
11.3
) can be calculated as follows:
η
¼
ð
ln
ð
p
B
=
p
1
Þ=
ln
ð
p
A
=
p
1
Þ
Þ
ð
100
leak%
Þ
where
p
1
¼
compressor inlet pressure (usually the atmospheric pressure),
p
A
¼
line upstream pressure or discharge pressure from the compressor
(section A),
p
B
¼
p
B
<p
A
),
line downstream pressure (section B,
p
A
p
B
¼
¼
pressure drop between sections A and B, leak%
percentage
of flow leaks between sections A and B.
Typical values of the flow leaks range from 10 % to 30 % depending on the
operating conditions and on the maintenance of the pipeline; the total efficiency of
the distribution line ranges from 80 % to 60 %.
Figure
11.3
shows the energy balance of a complete system: electric input
energy, output energy as heat (losses and recovered heat), compressed air, pipe
distribution losses (leaks and pressure losses), and end users.
11.6.2 The End Users
The end users of compressed air for power purposes can be classified as positive-
displacement machines, where a piston is given a reciprocating motion within a
cylinder by alternate feeding of the air supply from one side of the piston to the
other, and rotating machines where pistons or turbines or blades transform the
energy accumulated in the compressed air into a rotating motion.
Table
11.5
lists typical values of air consumption for different tools and other
machines. A coefficient with a coefficient which takes into account the discontinu-
ous operating mode should be used. In practice, the capacity of the air compressors
is equal to the sum of the single end user air consumptions with a load factor of at
least 20 %.
