Civil Engineering Reference
In-Depth Information
Fig. 11.3
Energy flow from input section to mechanical energy end user section in a compressed-
air system
Table 11.5
Air requirements of compressed-air end user machines (absolute 0.7 MPa, 101.5 psi;
100 % load factor)
Air flow referred to
standard conditions
Air flow at 0.7 MPa
ΒΌ
101.5 psi
m
3
/min
m
3
/s
Sm
3
/min
Type of machine
Screw driver
0.15-0.3
0.0025-0.005
1-2
Concrete vibrator
1-2
0.016-0.032
7-14
Paint spray
0.2-0.5
0.003-0.0086
1.4-3.5
Steel drills
0.5-2
0.008-0.032
3.5-14
Chipping hammers
0.35-1
0.006-0.016
2-7
The power of an end user reciprocating motor depends mainly on the mean
pressure differential across the air inlet and outlet, the displacement of the piston
through the cylinder at maximum stroke and the number of strokes per unit of time.
The degree to which air can be expanded is determined by air moisture freezing
which may block the exhaust passages with ice, and not by the efficiency of the
system which would impose high expansion ratios. Freezing can happen with
expansion ratios as small as 2 to 1.
The work diagram of a piston motor includes two phases: a constant
pressure phase (air at the distribution-line pressure enters the cylinder
through valves and moves the piston, thus performing work) followed by
a limited expansion (inlet valves are closed and the piston cylinder
performs work by using the energy delivered during the expansion).
The first phase is generally the major one; air consumption can be
reduced for the same output work by cutting off the air entering as
early as possible (first phase), and then allowing expansion (second
phase). The opposite of the compression phase, this, is always a small
part of the total work in the end user equipment.
