cannot prevent this water vapor from being taken into
the compressor. Instead, the water vapor is usually
squeezed out of the air during compression and
transformed into steam by the heat of compression.
The steam condenses to form moisture droplets
downstream from the compressor, as the compressed
air charge is cooled. Since moisture can damage some
of the machines that use compressed air (pneumatic
tools, for example), the moisture must be removed
from the air before it is sent to the storage tank. So, a
FILTER and a MOISTURE SEPARATOR
ASSEMBLY are placed between the compressor and
the storage tank. The assembly removes most of the
moisture, or any other impurities, entrained in the air
before it is sent on to storage. The assembly is fitted
with a valve or drain cock, so accumulations of water
and dirt can be drained now and then by the operator.
The tank that stores the compressed air is called
the AIR RECEIVER, as shown in figure 6-42. In this
way, demands for compressed air are made upon the
receiver, rather than directly on the compressor itself.
And there is little chance of the demand for air
exceeding the supply. To this end, the air receiver has
with it some type of control system or device to
monitor the supply of compressed air in the receiver.
The control device may be a PRESSURE SWITCH
that senses predetermined thresholds or levels of
When the compressor has sufficiently
charged the receiver with compressed air, the pressure
switch automatically opens and shuts down the
compressor. If and when the demand for compressed
air begins to drain the receiver to a preset pressure
threshold, the pressure switch closes and automatically
starts the compressor.
In systems where the demand for air is more or less
constant and prolonged, a type of CONSTANT-
SPEED CONTROL can be used. The compressor is
permitted to run continuously to keep the receiver
charged with air, while the constant-speed control
functions somewhat like a pressure relief valve. If the
pressure of the compressed air in the receiver rises,
because of a momentary drop in demand within the
system, the control simply vents the excess
compressed air to the atmosphere, rather than shut
down the compressor.
Figure 6-42.LP air compressor piping arrangements.