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Cylinder free air consumption
Free air or air at atmospheric pressure is readily available at zero cost. For air to acquire energy in the form of
pressure it must be compressed and stored.
The compression process only occurs through the expenditure of another type of energy, often electrical,whichhas
a production cost, contributing to the cost of the pneumatic energy.
The cost of the pneumatic energy is not related to the volume of
compressed air
but to the volume of
free air
.
The amount of free air that the compressor must draw in and compress to the desired pressure is defined as
Nm
3
normal cubicmeter. Assuming that a compressor with a power of 11
KW
supplies a quantity of air of 1400
Nl/min
.
that is 84
Nm
3
/h
(considering a pressure of 10
bar
), that 1
KW
of electrical power costs 0,10 € and that the
compressor stays active for one hour, the cost for the production of compressed air is:
Cost = 11 * 0,10= 1,1€/h
Assuming that the pneumatic equipment consume 600
Nl/min
.
→
36
Nm
3
/h
it means that the compressor keeps
running for:
36 / 84= 0,43 * 100=43%
The daily cost of electrical power to produce this quantity of air is:
Cost = 1,1 * 0,43=0,473€
for 220working days
220 * 8= 1760 hours
1760 * 0,473=
832,48€
In order to increase the pressure, the compressor must draw in and compress a larger volume of “free air”; the
higher the pressure value necessary, the longer the operating time, and consequently the higher the cost.
Figure 38
Consumption Q
: is the amount of free air expressed in
Nl
that the compressor must supply to the cylinder to
perform a given job.
Q
s
: with the piston against the front end-caps the volume to be filled is that corresponding to the positive stroke;
its value is determined by the product of the surface of the pistonmultiplied by the stroke itself. Once the volume
is known,we can calculate howmany
dm
3
of compressed air are contained in a cylinder chamber.With a pressure
of 6
bar
for example, the relative pressure of 6
bar
corresponds to an absolute pressure (6+1) = 7
bar
.
It is the absolute pressure value that is usedwhen calculating free air consumption.
Q
t
: similar to
Q
s
except that due to the presence of the piston rod, the volume of the negative chamber is lower
than that of the positive chamber.
n
: number of cycles per minute. This data is necessary to determine the amount of free air that the compressor
must provide to the cylinder so that it can perform the operations provided in the unit time. The sum of
Qs
and
Qt
,
(i.e. the air flow required for the positive and negative strokes) is to bemultiplied by the number of cycles that the
cylinder completes perminute. In the case ofmultiple cylinders operated simultaneously, the respective
Qs
and
Qt
must be summed to calculate the amount of free air necessary.
For an accurate calculation of
Q
consumption, it is necessary to consider any differences in the pressure and the
number of strokes (positive and negative) per minute.
Example:
cylinder with diameter
D
=32
mm
rod
d
= 12
mm
stroke
C
= 200
mm
number of cycles
n
= 10
per minute
positive chamber supply pressure
p
s
=6
bar
negative chamber supply pressure
p
t
=4
bar
.
Area of positive chamber
S
s
=
π
* D
2
3,14 * 32
2
=
803
mm
2
4
4
3
82
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