Compressors

Compressors
Purpose:
To give a basic knowledge about the different types of compressors.
 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 40                         


 
                   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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