Civil Engineering Reference
In-Depth Information
that ejector types use a high-velocity steam or gas to transport the inflowing gas;
If the discharge pressure is kept constant, the intake pressure decreases
according to the compression rate (vacuum compressor).
All major types of compressor (reciprocating, vane, screw, dynamic) are used
for air compression. If the compression rate, that is the ratio between the
absolute discharge pressure and the absolute intake pressure, is too high, it
may become necessary to combine groups of elements in series to form a
multistage system with two or more steps of compression. The gas is often
cooled between stages to reduce the temperature and thus the volume entering
the following stage. Theoretical minimum power with perfect intercooling and
no pressure losses between stages is obtained if the ratio of compression is the
same in all stages.
In what follows, the basic principles of compression processes are reported and
applications to compressed-air system are pointed out.
11.2
Basic Principles of Compressed-Air Systems
The two basic theoretical compression processes, the isothermal and the adiabatic,
concern both positive-displacement and continuous-flow compressors, although in
practice neither of the two processes can be implemented.
11.2.1 Isothermal Compression
The temperature is kept constant as the pressure increases. This requires heat to be
removed continuously and expelled from the compressor.
Compression from state 1 to state 2 follows the formula:
p
1
v
1
¼
p
2
v
2
¼
constant
11.2.2 Adiabatic Compression
No heat is added to or removed from the compressor during the compression
isentropic process (or reversible).
