Compressors
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Purpose:
This training
Manual is limited to mechanical description of compressors. The control
philosophies are mentioned sometimes to give a better understanding of the
machines.
1 Introduction
2 Compression Methods
3 Positive displacement compressors
3.1 Reciprocating compressors
3.1.1 Mechanical Piston
Reciprocating Compressor
3.1.1.1 Components and
Constructions
3.1.1.2 Frame lubrication
3.1.1.3 Cooling
3.1.2 Diaphragm Compressors
3.1.2.1 Construction and
Principle of Operation
3.1.2.2 Head Integrity
Detection System
3.2 Rotary compressors
3.2.1 Sliding vane
compressor
3.2.2 Helical lobe
compressor (screw compressor)
3.2.3 Straight lobe
compressor (blower)
4 namic Compressors
4.1 Ejector
4.2 Centrifugal Compressor
4.2.1 Arrangement
4.2.2 Mechanical components
1 Introduction
Compression of gas has one basic goal to deliver gas at a
pressure higher than that originally existing.
The inlet pressure level can be any value from a deep vacuum to a high
positive pressure. The discharge
pressure can range from sub atmospheric levels to high values in the tens of
thousands of pounds per square inch. The
fluid can be any compressible fluid, either gas or vapor, and can have a wide
molecular weight range. Applications of
compressed gas vary from consumer products such as the home refrigerator, to
large complex petrochemical plant installations.
2 Compression Methods
Compressors have numerous forms, the exact configuration
being based on the application. The are
two basic compression modes: positive displacement and dynamic. The positive displacement mode of compression
is cyclic in nature, in that a specific quantity of gas is ingested by the
compressor, acted upon, and discharged, before the cycle is repeated. The dynamic compression mode is one in which
the gas is moved into the compressor, and discharged without interruption of
the flow at any point in the process. Figure 1 diagram shows the relationship
of the various compressors by type. Figure 2 shows the typical application
range of each compressor.
Figure 1
Figure 2
3 Positive displacement compressors
3.1 Reciprocating compressors
Reciprocating compressors are
manufactured into different types and styles.
The most recognizable types are:
·
Mechanical
piston Reciprocating compressor
·
Diaphragm
compressor
3.1.1 Mechanical Piston Reciprocating Compressor
The reciprocating compressor is
probably the best known and the most widely used of all compressors. It consists of a mechanical arrangement in
which reciprocating motion is transmitted to a piston which is free to move in
a cylinder. The displacing action of the
piston, together with the inlet valve, causes a quantity of gas to enter the
cylinder where it is in turn compressed and discharged. Action of the discharge valve prevents the
backflow of gas into the compressor from the discharge line during the next
intake cycle. When the compression takes
place on one side of the piston only, the compressor is said to be single
acting. The compressor is double acting
when the compression takes place on each side of the piston. When a single cylinder is used or when
multiple cylinders on a common frame are connected in parallel, the arrangement
is referred to as a single stage compressor.
When multiple cylinders on a common frame are connected in series,
usually through a cooler, the arrangement is referred to as a multistage
compressor. Figure-3,4,5 give some examples of reciprocating
compressors.
Figure-3
Figure-4
Figure-5
3.1.1.1 Components and Constructions
Cylinders
Cylinders for compressors used in
the process industries are separable from the frame. The are attached to the frame by way of an
intermediate part known as the distance piece.
All cylinders are equipped for cooling, usually by means of water jacket
or air fins. Larger cylinders normally
have enough space for clearance pockets.
A clearance pocket is used for capacity control in some compressor. Figure
6 is an illustration of a cylinder with an unloading pocket in the
head.
Figure-6
Pistons and Rods
The piston must translate the energy from the crankshaft
to the gas in the cylinder. The piston
is equipped with a set of sliding seals referred to as piston rings. Rings are made of a material that must be reasonably
compliant for sealing, yet must slide along the cylinder wall with minimum
wear. Figure 7 shows a piston with piston rings. In some processes, it is preferred to use a
labyrinth piston (See Figure 8). The piston rod is threaded to the piston and
transmits the reciprocating motion from the crosshead to the piston.
Figure 7
Figure
8
Valves
The compressor cylinder valves
are of the spring-loaded, gas-actuated. Two of the many basic valve
configurations are depicted in Figure 10
& 11. Damaged valves can cause
noticeable decreases in compression efficiency.
The valves can normally be removed and serviced from outside the
cylinder without dismantling any other portion.
The inlet and discharge valves should not be physically
interchangeable.
Figure10
Figure
11
Distance piece
The distance piece is a separable housing that connects
the cylinder to the frame. The distance
may be opened or closed and may have multiple compartments. The purpose of the distance piece is to
isolate that part of the rod entering the crankcase and receiving lubrication
from the part entering the cylinder and contacting the gas. This prevents lubricant from entering the
cylinder and contaminating the gas. Some typical distance piece arrangements
are shown in Figure 12.
Figure 12
Rod Packing
A packing is required whenever
piston rod protrude through compressor cylinder and distance piece. The packing may consist of a number of rings
of packing materials and may include a lantern ring (see Figure 13). If cooling of
packing is required, the packing box may be jacketed for liquid coolant.
Figure 13
Crankshaft and
bearing
The crankshaft is drilled with passages to allow for
pressure lubrication of the bearings and crosshead. Figure
14 shows a drilled crankshaft. The
main and connecting rod bearings are mostly split-sleeve, insert type. Figure
15 shows a split sleeve bearing caps.
Figure 14
Figure 15
3.1.1.2 Frame lubrication
The pressurized lubrication
system is a more elaborated lubrication method (see Figure 16) the system has a main oil pump, either crankshaft or
separately driven, a pump suction strainer, a cooler when needed, a full-flow
oil filter and safety instrumentation.
Figure 16
3.1.1.3 Cooling
For large process gas
compressors, forced cooling through the cylinder barrel and heads is most
common. If water is used, it is very
important that clean treated water be used.
The purpose of cylinder cooling is to equalize cylinder temperatures and
prevent heat buildup. This cooling only
removes the frictional heat, the heat of compression is removed by the inter-
or aftercoolers.
3.1.1.4 Capacity
control
Capacity control is so important. The reciprocating compressor cannot
self-regulate its capacity against a given discharge pressure; it will simply
keep displacing gas. The four famous
capacity control methods are bypass, suction throttling, suction valve
unloading and clearance pockets.
One of the simplest methods of controlling is to bypass,
or recycle the compressed gas back to the compressor suction. This is accomplished by piping from the
compressor discharge line through some type of control valve and going back to
the compressor suction line. In addition
to being simple, this system also has the advantage of being infinitely
controllable.
Probably the most common method of controlling compressor
capacity is via suction valve unloading.
The technique here is to physically keep the cylinder from compressing
gas by maintaining an open flow path between the cylinder bore and the cylinder
suction chamber. The cylinder will take
in gas normally; however, instead of completing the normal cycle of compression
and discharge, the cylinder will simply pump the gas still at suction pressure
back into the suction chamber via this opening pathway.
Although not very widely used, suction throttling is
another method of controlling the capacity of a reciprocating compressor. The technique is to reduce the suction
pressure to the compressor by limiting or throttling the flow into the
cylinder. Suction throttling has its
limitations. It takes a fairly dramatic reduction in suction pressure to give
any sizeable reduction in capacity.
Additionally, as the suction pressure is reduced and the discharge
pressure held constant, the compression ratio is increased. This causes higher discharge temperatures and
also higher rod loads.
Clearance pocket is essentially an empty volume, typically
in the outer head of the cylinder, with a valved passage to the cylinder bore
(see Figure 17). During normal operation, the valve is closed
and the cylinder operates at full capacity.
For reduced capacity operation, the valve is opened, and the cylinder
capacity is reduced by the effect of added clearance.
Figure 17
3.1.2 Diaphragm Compressors
The diaphragm compressor (Figure 18) is designed to compress gases without the use of a
dynamic seal. This allows the unit to handle gases that cannot be processed
with an ordinary compressor. It can be used for gases that demand the ultimate
in cleanliness or for hazardous gases, this with no gas pollution.
The diaphragm compressor has a conventional crankshaft,
connecting rod and piston. The piston, however, does not compress gas. It
forces hydraulic fluid against a flexible metal diaphragm. The diaphragm
compresses the gas by deforming against a smooth, domed contour, eliminating
the need for a dynamic seal.
Figure
18
3.1.2.1 Construction and Principle of Operation
The diaphragm compressor has a
conventional crankshaft, connecting rod and piston. The piston, however, does
not compress gas. It forces hydraulic fluid against a flexible metal diaphragm.
The diaphragm compresses the gas by deforming against a smooth, domed contour,
eliminating the need for a dynamic seal.
Upon each complete ascending
and descending stroke of the piston, a compensating pump actuated by an
eccentric on the shaft sends a quantity of fluid greater than the quantity that
escapes between the piston and the cylinder and ensures the application of the
diaphragm to the gas plate, thereby reducing the dead space to the minimum.
The excess oil expelled by the
compressor is evacuated by a calibrated valve called pressure limiter and
returns to the casing. The direction of the oil circulation,
casing-compensator, compensator-cylinder, and cylinder casing ensured by the
non-return valves and the pressure limiter.
The piston moves in the cylinder and pushes the hydraulic
fluid in the head producing an oscillating movement of the diaphragm group (Figure 19).
The diaphragm group consists of three diaphragms clamped
and seated at the periphery between the gas plate and oil plate (Figure 20).
The oil plate has the role of distributing the hydraulic
fluid uniformly under the diaphragms and the gas plate. The gas plate contains
the suction and the discharge valves. The discharge valve is located at the
center of the gas plate for optimum capacity. The two plates are specially
contoured on their internal faces and their assembly forms the compression
chamber. Their profile is carefully designed so as to minimize the stress in
the diaphragms.
Figure
19
Figure
20
3.1.2.2 Head Integrity Detection System
Running a diaphragm compressor
with a cracked or broken diaphragm will damage the machined surfaces of the
upper or lower plates, pollute handled gas and create a problem of gas leakage
to atmosphere. It is important, should a diaphragm break, the compressor must
be stopped immediately.
For these reasons the compressor
is fitted with a diaphragm crack detection system. The system uses 3 diaphragms
sandwiched together. Should a diaphragm either on the oil or the gas side
cracks, then the pressure between the intermediate diaphragm and the cracked
diaphragm will rise. An instrument tapping from this intermediate diaphragm is
fed to a pressure switch set to sense rising pressure. This switch is in turn
connected to the compressor control gear which shuts down the drive motor.
Intermediate diaphragm has a
pressure-tapping slot or grooves located across sealing area at diaphragm
periphery.
When the 3 diaphragms are
located together, the tapping slot or groove in the intermediate diaphragm is
positioned into circumpherencial grooves machined between gas and oil plate to
guide the any leakage to the detection system.
Figure 21 & 22 show a diaphragm crack detection system.
Figure
21
Figure
22
3.2 Rotary compressors
3.2.1 Sliding vane compressor
This type of compressor has a
rotor eccentrically mounted inside a cylinder, which is mostly water jacketed
for cooling purpose. The rotor is fitted
with blades that are free to move radially in and out of longitudinal
slots. These blades are forced out
against the cylinder wall by centrifugal force. Figure 23 shows the sliding vane compressor and the operation
principle.
Figure23
3.2.2 Helical lobe compressor (screw compressor)
Helical
lobe compressors (Figure 25) are
rotary positive displacement machines in which two intermeshing rotors, each
with helical form, compress and displace the gas. The rotor with the lobe is called a male
rotor and the rotor with the interlobe is called a female rotor. Figure
26 shows a typical screw compressor rotor combinations. In some types, oil or liquid is injected
inside the compressor area to cool down the compressed gas. This is because some gases can polymerizes
when it is compressed due to increase in temperature. The screws are kept without touching due to
the timing gears (see Figure 27). Because screw compressor is a positive
displacement machine, the most advantageous method of achieving capacity or
volume flow control is obtained by variable speed motor or installing a bypass.
Figure
25
Figure 26
Figure 27
3.2.3 Straight lobe compressor
Straight lobe compressors are
rotary positive displacement machines in which two straight mating lobed
impellers trap gas and carry it from intake to discharge. This type of compressor is mostly used in
pneumatic conveying systems. In
petrochemical industry, it can be used to convey powder and pellets. Figures
28 to 29 shows a typical blower
and the principle of operation.
Figure 28
Figure 29
4 Namic Compressors
4.1 Ejector
Ejectors
are principally used to compress from pressure
below atmospheric to a discharge close to atmospheric. The ejector has no moving parts and it is
simple and it has no wearing parts (see Figure 30). The ejector is operated directly by a motive
gas or vapor source. Air and steam are
probably the most common of the motive gases.
The ejector uses a nozzle to accelerate the motive gas into the suction
chamber where the gas to be compressed is admitted at right angles to the
motive gas direction. In the suction
chamber, the suction gas is entrained by the motive fluid. The mixture moves into a diffuser where the
high velocity gas is gradually decelerated and increased in pressure.
Figure-30
4.2 Centrifugal Compressor
A centrifugal compressor is a
continuous flow unit in which the mechanical action of rotating vanes or
impellers imparts velocity and pressure to the flowing medium. The velocity energy is then converted to
additional pressure. See Figure 31 & 32 for typical centrifugal compressors.
Figure-31
Figure-32 4.2.1 Arrangement
One
type of compressor is an overhung type centrifugal compressor (see Figure 33). It is basically an overhung style machine
mounted on a gearbox and uses the gear pinion shaft extension to mount an
impeller.
Another
type is a multi-stage centrifugal compressor.
In this type, many impellers are attached to the rotor. This type of
compressors is one of the most important compressors in the industry today (see
Figure 34).
Figure 35
shows a schematic layout of integrally geared compressor. It consists of three impellers, the first
located on one pinion, which would have a lower speed than the other pinion
that has mounted the remaining two impellers.
Figure 33
Figure
34
Figure-35
4.2.2 Mechanical components
Casings
The casing for centrifugal
compressor can be horizontal or vertical split.
The horizontal split casing is generally used for low-pressure operation
relative to vertical split casing. Figure
36 and 37 shows a vertical and
horizontal split compressor. Normally,
all the connections such as the suction and discharge nozzles are arranged on
the bottom section of the casing so that the upper section can be easily
removed for maintenance work.
Figure 36
Figure 37
Internals / diaphragms
The internal flow-conducting
components comprise an inlet ring, the intermediate diaphragms and the
discharge volute (see Figure 38). The diaphragms form diffuser for each
impeller and the return duct leading to the intake of the next impeller. The discharge volute conducts the gas to the
discharge nozzle of the compressor.
Figure-38
Shaft and impellers
The rotor consists of the shaft,
impellers, shaft sleeves, a balancing piston and the thrust collar for the
axial bearing. The number and size of
the impeller depends on the process requirements. Figure 39 shows a shaft
and impellers together.
Figure-39
Bearings
Radial
bearings or journal bearings are usually pressure lubricated. Most compressor use two bearings on opposite
ends of the rotor assembly or on the overhung design located adjacent to each
other between the drive coupling and the impeller. It is highly desirable for
ease maintenance to have the bearing horizontal split (see Figure 40). Double-acting
thrust bearings are also there to absorb the axial forces. There are tilting pad type and should be
suitable for both directions of rotation (see Figure 41). Magnetic
bearings maybe employed instead of oil lubricated bearings for certain
applications (see Figure 42). The
advantages of magnetic bearing are the reduction of mechanical losses, no
supply of oil necessary and the adjustability of the radial and axial positions
of the rotors.
Figure
41
Figure
42
Shaft end seals
Many of the gases to be
compressed are combustible, explosive, toxic or harmful to the environment, and
under no circumstances can these be allowed to enter the atmosphere. Depending on the service conditions, any of
the following seals can be applied:
Labyrinths seals
Oil lubricated mechanical
contact seals
Oil lubricated floating-ring seals
Dry-running gas seals
To Be Continued .......
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