The document discusses different types of compressors used to compress gases. It describes four main methods of compression: 1) trapping gas in an enclosure and reducing the volume, 2) trapping gas and compressing it by backflow before releasing it, 3) using rapidly rotating impellers to impart velocity and pressure to flowing gas, and 4) entraining gas in a high velocity jet to convert velocity to pressure. Positive displacement compressors use methods 1 and 2 through intermittent compression, while dynamic compressors use method 3 for continuous compression. Common compressor types include reciprocating, sliding vane, liquid ring, rotary lobe, helical lobe, centrifugal, and axial compressors.
2. Introduction
To understand how gases and gas mixtures behave, it is necessary to recognize that
gases consist of individual molecules of the various gas components, widely separated
compared to their size. These molecules are always travelling at high speed; they strike
against the walls of the enclosing vessel and produce what we know as pressure.
Temperature affects average molecule speed. When heat is added to a fixed volume of
gas, the molecules travel faster, and hit the containing walls of the vessel more often and
with greater force. This then produces a greater pressure.
If the enclosed vessel is fitted with a piston so that the gas can be squeezed into a
smaller space, the molecule travel is now restricted. The molecules now hit the walls with
a greater frequency, increasing the pressure, consistent with Boyle's Law
3. Introduction
However, moving the piston also delivers energy to the molecules, causing them to move
with increasing velocity. As with heating, this results in a temperature increase.
Furthermore, all the molecules have been forced into a smaller space, which results in an
increased number of collisions on a unit area of the wall. This, together with the increased
velocity, results in increased pressure.
The compression of gases to higher pressures results in higher temperatures, creating
problems in compressor design. All basic compressor elements, regardless of type, have
certain design-limiting operating conditions. When any limitation is involved, it becomes
necessary to perform the work in more than one step of the compression process. This is
termed multistaging and uses one basic machine element designed to operate in series
with other elements of the machine.
7. Introduction
This limitation varies with the type of compressor, but the most important limitations
include:
Discharge pressure—all types.
Pressure rise or differential—dynamic units and most displacement types.
Compression ratio—dynamic units.
Effect of clearance—reciprocating units (this is related to the compression ratio).
Desirability of saving power.
8. Methods of Compression
Four methods are used to compress gas. Two are in the intermittent class, and two are
in the continuous flow class. (These are descriptive, not thermodynamic or duty
classification terms.)
1. Trap consecutive quantities of gas in some type of enclosure, reduce the volume
(thus increasing the pressure), then push the compressed gas out of the enclosure.
2. Trap consecutive quantities of gas in some type of enclosure, carry it without
volume change to the discharge opening, compress the gas by backflow from the
discharge system, then push the compressed gas out of the enclosure.
9. Methods of Compression
3. Compress the gas by the mechanical action of rapidly rotating impellers or bladed rotors
that impart velocity and pressure to the flowing gas, (Velocity is further converted into
pressure in stationary diffusers or blades.)
4. Entrain the gas in a high velocity jet of the same or another gas (usually, but not
necessarily, steam) and convert the high velocity of the mixture into pressure in a
diffuser.
Compressors using methods 1 and 2 are in the intermittent class and are known as
positive displacement compressors. Those using method 3 are known as dynamic
compressors. Compressors using method 4 are known as ejectors and normally operate
with an intake below atmospheric pressure.
10. Methods of Compression
Compressors change mechanical energy into gas energy. This is in accordance with
the First Law of Thermodynamics, which states that energy cannot be created or
destroyed during a process (such as compression of a gas), although the process
may change mechanical energy into gas energy. Some of the energy is also
converted into non-usable forms such as heat losses.
Mechanical energy can be converted into gas energy in one of two ways:
1. By positive displacement of the gas into a smaller volume. Flow is directly
proportional to speed of the compressor, but the pressure ratio is determined by
pressure in the system into which the compressor is pumping.
2. By dynamic action imparting velocity to the gas. This velocity is then converted into
pressure. Flow rate and pressure ratio both vary as a function of speed, but only
within a very limited range and then only with properly designed control systems.
12. Methods of Compression
Total energy in a flowing air stream is constant. Entering an enlarged section, flow
speed is reduced and some of the velocity energy turns into pressure energy. Thus
static pressure is higher in the enlarged section.
14. COMPRESSOR TYPES DESCRIPTION
Positive displacement machines work by
mechanically changing the volume of the working
fluid.
Dynamic machines work by mechanically changing
the velocity of the working fluid.
15. Reciprocating
Reciprocating compressors are
positive displacement machines in
which the compressing and
displacing element is a piston
having a reciprocating motion
within a cylinder. The overall cycle
is shown in Figure with the four
typical phases of intake,
compression, discharge, and
expansion. Inlet valves are open
from 4 to 1, and discharge valves
are open from 2 to 3.
16. Sliding Vane
Sliding vane compressors are rotary
positive displacement
compressors. A slotted cylinder is
fitted with non-metallic vanes and
placed eccentric inside a tube. As
the slotted cylinder is turned, the
vanes slide along the inner wall of
the tube forming regions of
changing volume.
17. Liquid Ring
Liquid ring compressors utilize a
squirrel cage fan type impeller
which is placed eccentric inside a
tube. A compatible liquid is
introduced into the chamber along
with the gas to be compressed.
Because of the centrifugal force
and the shape of the internal cavity,
the liquid forms an eccentric shape
producing regions of changing
volume. The liquid must be
separated from the compressed
gas after the compression process
and recirculated.
18. Rotary Lobe
Two straight mating lobed impellers
trap the gas and carry it from
intake to discharge. There is no
internal compression.
19. Helical or Spiral Lobe Compressors
Two intermeshing rotors compress
and displace the gas. The gas is
trapped in the rotor pockets at one
end; it is compressed between the
intermeshing rotors and discharged
at the opposite end. Some helical
screw compressors operate with
fluid and these are called flooded
screw compressors. The fluid
provides a liquid seal around the
rotors, absorbs the heat of
compression, allowing the machine
to produce a greater pressure rise.
The fluid must be removed from the
gas after the compression process.
20. Centrifugal Compressors
Centrifugal compressors are
dynamic machines in which the
rapidly rotating impeller accelerates
the gas. The process flow
propagates from axial to radial
(perpendicular to shaft centreline)
into a stationary diffuser converting
velocity to pressure.
21. Axial Compressors
Axial compressors are dynamic machines in which the gas flow is accelerated in an
axial and peripheral direction by the rotation of specially shaped blades. The process
flow is parallel to shaft centreline. Stator blades allow the recovery of velocity to
pressure.
