23-09-2014, 02:48 PM
A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such as jackhammers. The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors. Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws. The effecti
DETAILS OF PROJECT -
INTRODUCTION OF COMPRESSOR
Anair compressoris a device that converts power (usually from an electric motor, a diesel engine or a gasoline engine) into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts. There are numerous methods of air compression, divided into either positive-displacement or negative-displacement types.
TYPES OF COMPRESSOR
1 According to the design and principle of operation:
1.Reciprocating compressor
2.Rotary screw compressor
3.Turbo compressor
2 According to the number of stages
1. Single stage compressor
2. Two stage compressor
3 .Multi stage compressor
Three stage compressor
3 According to the pressure limits
1. Low pressure compressors
2. Medium pressure compressors
3. High pressure compressors
4. Super high pressure compressors
4 According to the capacity
1. Low capacity compressors
2. Medium capacity compressors
3. High capacity compressors
5 According to the method of cooling
1 .Air cooled compressor
2. Water cooled compressor
Positive displacement
Positive-displacement air compressors work by forcing air into a chamber whose volume is reduced to compress the air. Piston-type air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons. They use one-way valves to guide air into a chamber, where the air is compressed. Rotary screw compressors also use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, whose volume is reduced as the screws turn. Vane compressors use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume.
Negative displacement
Negative-displacement air compressors include centrifugal compressors. These use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air, which pressurizes it.
Applications
· To supply high-pressure clean air to fillgas cylinders
· To supply moderate-pressure clean air to a submergedsurface supplied diver
· To supply moderate-pressure clean air for driving some office and school building pneumaticHVAC control systemvalves
· To supply a large amount of moderate-pressure air to powerpneumatic tools, such as jackhammers
· For fillingtires
· To produce large volumes of moderate-pressure air for large-scale industrial processes (such as oxidation for petroleum coking or cement plant bag house purge systems).Most air compressors either are reciprocating piston type, rotary vane orrotary screw. Centrifugal compressors are common in very large applications. There are two main types of air compressor\'s pumps: oil-lubed and oil-less. The oil-less system has more technical development, but is more expensive, louder and lasts for less time than oil-lubed pumps. The oil-less system also delivers air of better quality.
COMPRESSOR USE IN INDUSTRY
SCREW TYPE COMPRESSOR
Arotary screw compressoris a type ofgas compressorwhich uses a rotary type positive displacement mechanism. They are commonly used to replacepiston compressorswhere large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such asjackhammers.
The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors.
Operation
Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.
The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors, and between the rotors and the chamber for sealing of the compression cavities.
Size
Rotary screw compressors tend to be compact and smooth running with limited vibration and thus do not require spring suspension. Many rotary screw compressors are, however, mounted using elastomer vibration isolating mounts to absorb high-frequency vibrations, especially in rotary screw compressors that operate at high rotational speeds. Rotary screw compressors are produced in sizes that range from 10cubic feet per minuteto several thousand CFM. Rotary screw compressors are typically used in applications requiring more airflow than is produced by small reciprocating compressors but less than is produced by centrifugal compressors.
Applications
Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.
Additionally, they are becoming increasingly popular in municipal wastewater treatment facilities, for their increased efficiency and thus, lower power consumption.
Oil-free
In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet (56.634 cubic meters) per minute (measured at 60 °C and atmospheric pressure).
Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point-of-use. Subsequently, air treatment identical to that used for an oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.
Oil-flooded
n an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed gas stream downstream of the compressor. In many applications, this is rectified bycoalescer/filter vessels.In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be remControl schemes
Among rotary screw compressors, there are multiple control schemes, each with differing advantages and disadvantages.
Start/stop
In a start/stop control scheme, compressor controls actuate relays to apply and remove power to the airend motor according to compressed air needs.
Load/unload
In a load/unload control scheme, the compressor airend remains continuously powered. However, when the demand for compressed air is satisfied, instead of disconnecting power to the compressor airend, the inlet valve is closed,unloadingthe compressor. This reduces the number of start/stop cycles for electric motors over a start/stop control scheme in electrically-driven compressors, improving equipment service life with a minimal change in operating cost. This scheme is utilised by nearly all industrial air compressor manufacturers. When a load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuously unloaded operation, it is known as a dual-control or auto-dual scheme.
Modulation
Instead of starting and stopping the compressor or actuating the inlet valve between two distinct positions, a modulation control scheme proportionally adjusts the inlet valve open and closed, altering the compressor discharge according to demand. While this yields a consistent discharge pressure over a wide range of demand, power consumption is significantly higher than with a load/unload scheme, resulting in approximately 70% of full-load power consumption when the compressor is at a zero-load condition. This control scheme was popularized in industrial air compressor applications byKOBELCO,KaeserandGardner Denver.
Due to the limited adjustment in compressor power consumption relative to compressed air output capacity, modulation is a generally inefficient method of control when compared to variable speed drives. However, for applications where it is not readily possible to frequently cease and resume operation of the compressor (such as when a compressor is driven by an internal combustion engine and operated without the presence of a compressed air receiver), modulation is suitable.
Variable displacement
Utilized by compressor companies Quincy Compressor,Kobelco,Gardner Denver, andSullair, variable displacement alters the percentage of the screw compressor rotors working to compress air by allowing air flow to bypass portions of the screws. While this does reduce power consumption when compared to a modulation control scheme, a load/no load system can be more effective when large amounts of storage (10 gallons per CFM). If a large amount of storage is not practical, a variable displacement system can be very effective, especially at greater than 70% of full load.
One way that variable displacement may be accomplished is via the use of multiple lifting valves on the suction side of the compressor airend, each plumbed to a corresponding location on the discharge. In automotive superchargers, this is analogous to the operation of a bypass valve.
Variable speed
While an air compressor powered by a variable speed drive can offer the lowest operating energy cost without any appreciable reduction in service life over a properly maintained load/unload compressor, the variable frequency power inverter of a variable speed drive typically adds significant cost to the design of such a compressor, negating its economic benefits if there are limited variations in demand. However, a variable speed drive provides for a linear relationship between compressor power consumption and free air delivery. In harsh environments (hot, humid or dusty), variable speed drives may not be suitable due to the sensitivity of the equipment.
Superchargers
The twin-screw type supercharger is apositive displacementtype device that operates by pushing air through a pair of meshing close-tolerance screws similar to a set of worm gears. Twin-screw superchargers are also known as Lysholm superchargers (orcompressors) after their inventor, Alf Lysholm.Each rotor is radially symmetrical, but laterally asymmetric. By comparison, conventional "Roots" type blowers have either identical rotors (with straight rotors) or mirror-image rotors (with helixed rotors). The Whipple-manufactured male rotor has three lobes, the female five lobes. The Kenne-Bell male rotor has four lobes, the female six lobes. Females in some earlier designs had four. By comparison, Roots blowers always have the same number of lobes on both rotors, typically 2, 3 or 4. The working area is the inter-lobe volume between the male and female rotors. It’s larger at the intake end, and decreases along the length of the rotors until the exhaust port. This change in volume is the compression. The intake charge is drawn in at the end of the rotors in the large clearance between the male and female lobes. At the intake end the male lobe is much smaller than its female counterpart, but the relative sizes reverse proportions along the lengths of both rotors (the male becomes larger and the female smaller) until (tangential to the discharge port) the clearance space between each pair of lobes is much smaller. This reduction in volume causes compression of the charge before being presented to the output manifold.
Comparative advantages
The rotary screw compressor has low leakage levels and low parasitic losses vs. Roots type. The supercharger is typically driven directly from the engine\'s crankshaft via a belt or gear drive. Unlike theRoots type supercharger, the twin-screw exhibits internal compression which is the ability of the device to compress air within the housing as it is moved through the device instead of relying upon resistance to flow downstream of the discharge to establish an increase of pressure.
The requirement of high-precisioncomputer-controlledmanufacturing techniques makes the screw type supercharger a more expensive alternative to other forms of available forced induction. With later technology, manufacturing cost has been lowered while performance increased.
All supercharger types benefit from the use of anintercoolerto reduce heat produced during pumping and compression.
A clear example of the technology applied by the twin-screw in companies likeFord,Mazda,MercedesandMercury Marinecan also demonstrate the effectiveness of the twin screw. While some centrifugal superchargers are consistent and reliable, they typically do not produce full boost until near peak engine rpm, while positive displacement superchargers such asRoots type superchargersand twin-screw types offer more immediate boost.
BREAKDOWN IN SCREW COMPRESSOR
When the compressor is run outside of design limits, the first thing that happens is that the discharge temperature/superheat increases. This increase in temperature is first felt inside the cylinders and at the discharge valves. As the temperature increases, the lubrication to the cylinder and piston is literally cooked away causing the now dry rubbing surfaces to score and fine iron particles are scrapped from the cylinder wall and end up in the compressor sump oil and around the compressor motion gear. This overheated oil as it passes through the discharge valves causes the oil to coke or carbonise on the valve plate causing the valve to leak bye, elevating the temperature even higher. This gas leaking past the valves and pistons will elevate the compressor suction pressure and cause loss of capacity in the machine. As the oil in the compressor starts to break down due to the heat, acids and carbon are produced. These acids will, to a certain extent, be absorbed by the drier desiccant, but will eventually start attacking the motor winding insulation and cause either a spot burn or full motor burn out. Here you can see the fi ne iron particles caused by the loss of lubrication due to the oil being ‘cooked away’ . Also notice the discolouration due to the carbonising of the oil. Both the valve and valve plate are heavily carbonised by the degrading oil.
Overheating
The 2 grooves seen in the bore of the cylinder are a direct result of gudgeon pin plastic caps melting allowing the pin to grind into the cylinder wall. The binding on the motor has been burst due to the motor running hot in an overheated state. This will eventually lead to a motor burnout which you can see is beginning to happen with the discoloured winding. The scores on the pistons indicate that the compressor has been running in a dry overheated condition which will lead to metallic debris being distributed throughout the machine and would most likely lead to a spot burnout of the motor. The weak acids that are produced will slowly migrate throughout the entire system and may cause other problems later on. As the oil continues to degrade, its ability to lubricate and support the bearing loads
Typical causes of Overheating
1) High Compression Ratios
2) Too low Suction pressure
3) Too high a discharge pressure
4) Too high Suction Superheat
5) Lack of Liquid Sub-cooling at Expansion Valve inlet
6) Air or other Non Condensable in condenser
7) Choked drier
8) Choked Condenser coil
9) Condenser fan faulty or wrong rotation
10) Under sized condenser
11) Too high Suction pipe pressure drop
12) Oil logged evaporator
13) Poorly insulated Suction pipe work
14) Incorrectly set Crankcase limit valve
Preventative Measures required
1) Check design operating conditions and adjust accordingly
2) Check condenser condition and rectify
3) Check suction superheat and adjust as required
4) Check or Log all system pressures and temperatures
Typical Damage to Compressor parts
1) Scored cylinders and pistons
2) Excess wear to moving parts due to poor lubrication
3) Carbonised valves and valve plates
4) Broken valves
5) Iron particles around compressor and in the sump oil
6) Burst motor winding bindings
DETAILS OF PROJECT -
INTRODUCTION OF COMPRESSOR
Anair compressoris a device that converts power (usually from an electric motor, a diesel engine or a gasoline engine) into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts. There are numerous methods of air compression, divided into either positive-displacement or negative-displacement types.
TYPES OF COMPRESSOR
1 According to the design and principle of operation:
1.Reciprocating compressor
2.Rotary screw compressor
3.Turbo compressor
2 According to the number of stages
1. Single stage compressor
2. Two stage compressor
3 .Multi stage compressor
Three stage compressor
3 According to the pressure limits
1. Low pressure compressors
2. Medium pressure compressors
3. High pressure compressors
4. Super high pressure compressors
4 According to the capacity
1. Low capacity compressors
2. Medium capacity compressors
3. High capacity compressors
5 According to the method of cooling
1 .Air cooled compressor
2. Water cooled compressor
Positive displacement
Positive-displacement air compressors work by forcing air into a chamber whose volume is reduced to compress the air. Piston-type air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons. They use one-way valves to guide air into a chamber, where the air is compressed. Rotary screw compressors also use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, whose volume is reduced as the screws turn. Vane compressors use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume.
Negative displacement
Negative-displacement air compressors include centrifugal compressors. These use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air, which pressurizes it.
Applications
· To supply high-pressure clean air to fillgas cylinders
· To supply moderate-pressure clean air to a submergedsurface supplied diver
· To supply moderate-pressure clean air for driving some office and school building pneumaticHVAC control systemvalves
· To supply a large amount of moderate-pressure air to powerpneumatic tools, such as jackhammers
· For fillingtires
· To produce large volumes of moderate-pressure air for large-scale industrial processes (such as oxidation for petroleum coking or cement plant bag house purge systems).Most air compressors either are reciprocating piston type, rotary vane orrotary screw. Centrifugal compressors are common in very large applications. There are two main types of air compressor\'s pumps: oil-lubed and oil-less. The oil-less system has more technical development, but is more expensive, louder and lasts for less time than oil-lubed pumps. The oil-less system also delivers air of better quality.
COMPRESSOR USE IN INDUSTRY
SCREW TYPE COMPRESSOR
Arotary screw compressoris a type ofgas compressorwhich uses a rotary type positive displacement mechanism. They are commonly used to replacepiston compressorswhere large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such asjackhammers.
The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors.
Operation
Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.
The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors, and between the rotors and the chamber for sealing of the compression cavities.
Size
Rotary screw compressors tend to be compact and smooth running with limited vibration and thus do not require spring suspension. Many rotary screw compressors are, however, mounted using elastomer vibration isolating mounts to absorb high-frequency vibrations, especially in rotary screw compressors that operate at high rotational speeds. Rotary screw compressors are produced in sizes that range from 10cubic feet per minuteto several thousand CFM. Rotary screw compressors are typically used in applications requiring more airflow than is produced by small reciprocating compressors but less than is produced by centrifugal compressors.
Applications
Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.
Additionally, they are becoming increasingly popular in municipal wastewater treatment facilities, for their increased efficiency and thus, lower power consumption.
Oil-free
In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet (56.634 cubic meters) per minute (measured at 60 °C and atmospheric pressure).
Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point-of-use. Subsequently, air treatment identical to that used for an oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.
Oil-flooded
n an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed gas stream downstream of the compressor. In many applications, this is rectified bycoalescer/filter vessels.In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be remControl schemes
Among rotary screw compressors, there are multiple control schemes, each with differing advantages and disadvantages.
Start/stop
In a start/stop control scheme, compressor controls actuate relays to apply and remove power to the airend motor according to compressed air needs.
Load/unload
In a load/unload control scheme, the compressor airend remains continuously powered. However, when the demand for compressed air is satisfied, instead of disconnecting power to the compressor airend, the inlet valve is closed,unloadingthe compressor. This reduces the number of start/stop cycles for electric motors over a start/stop control scheme in electrically-driven compressors, improving equipment service life with a minimal change in operating cost. This scheme is utilised by nearly all industrial air compressor manufacturers. When a load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuously unloaded operation, it is known as a dual-control or auto-dual scheme.
Modulation
Instead of starting and stopping the compressor or actuating the inlet valve between two distinct positions, a modulation control scheme proportionally adjusts the inlet valve open and closed, altering the compressor discharge according to demand. While this yields a consistent discharge pressure over a wide range of demand, power consumption is significantly higher than with a load/unload scheme, resulting in approximately 70% of full-load power consumption when the compressor is at a zero-load condition. This control scheme was popularized in industrial air compressor applications byKOBELCO,KaeserandGardner Denver.
Due to the limited adjustment in compressor power consumption relative to compressed air output capacity, modulation is a generally inefficient method of control when compared to variable speed drives. However, for applications where it is not readily possible to frequently cease and resume operation of the compressor (such as when a compressor is driven by an internal combustion engine and operated without the presence of a compressed air receiver), modulation is suitable.
Variable displacement
Utilized by compressor companies Quincy Compressor,Kobelco,Gardner Denver, andSullair, variable displacement alters the percentage of the screw compressor rotors working to compress air by allowing air flow to bypass portions of the screws. While this does reduce power consumption when compared to a modulation control scheme, a load/no load system can be more effective when large amounts of storage (10 gallons per CFM). If a large amount of storage is not practical, a variable displacement system can be very effective, especially at greater than 70% of full load.
One way that variable displacement may be accomplished is via the use of multiple lifting valves on the suction side of the compressor airend, each plumbed to a corresponding location on the discharge. In automotive superchargers, this is analogous to the operation of a bypass valve.
Variable speed
While an air compressor powered by a variable speed drive can offer the lowest operating energy cost without any appreciable reduction in service life over a properly maintained load/unload compressor, the variable frequency power inverter of a variable speed drive typically adds significant cost to the design of such a compressor, negating its economic benefits if there are limited variations in demand. However, a variable speed drive provides for a linear relationship between compressor power consumption and free air delivery. In harsh environments (hot, humid or dusty), variable speed drives may not be suitable due to the sensitivity of the equipment.
Superchargers
The twin-screw type supercharger is apositive displacementtype device that operates by pushing air through a pair of meshing close-tolerance screws similar to a set of worm gears. Twin-screw superchargers are also known as Lysholm superchargers (orcompressors) after their inventor, Alf Lysholm.Each rotor is radially symmetrical, but laterally asymmetric. By comparison, conventional "Roots" type blowers have either identical rotors (with straight rotors) or mirror-image rotors (with helixed rotors). The Whipple-manufactured male rotor has three lobes, the female five lobes. The Kenne-Bell male rotor has four lobes, the female six lobes. Females in some earlier designs had four. By comparison, Roots blowers always have the same number of lobes on both rotors, typically 2, 3 or 4. The working area is the inter-lobe volume between the male and female rotors. It’s larger at the intake end, and decreases along the length of the rotors until the exhaust port. This change in volume is the compression. The intake charge is drawn in at the end of the rotors in the large clearance between the male and female lobes. At the intake end the male lobe is much smaller than its female counterpart, but the relative sizes reverse proportions along the lengths of both rotors (the male becomes larger and the female smaller) until (tangential to the discharge port) the clearance space between each pair of lobes is much smaller. This reduction in volume causes compression of the charge before being presented to the output manifold.
Comparative advantages
The rotary screw compressor has low leakage levels and low parasitic losses vs. Roots type. The supercharger is typically driven directly from the engine\'s crankshaft via a belt or gear drive. Unlike theRoots type supercharger, the twin-screw exhibits internal compression which is the ability of the device to compress air within the housing as it is moved through the device instead of relying upon resistance to flow downstream of the discharge to establish an increase of pressure.
The requirement of high-precisioncomputer-controlledmanufacturing techniques makes the screw type supercharger a more expensive alternative to other forms of available forced induction. With later technology, manufacturing cost has been lowered while performance increased.
All supercharger types benefit from the use of anintercoolerto reduce heat produced during pumping and compression.
A clear example of the technology applied by the twin-screw in companies likeFord,Mazda,MercedesandMercury Marinecan also demonstrate the effectiveness of the twin screw. While some centrifugal superchargers are consistent and reliable, they typically do not produce full boost until near peak engine rpm, while positive displacement superchargers such asRoots type superchargersand twin-screw types offer more immediate boost.
BREAKDOWN IN SCREW COMPRESSOR
When the compressor is run outside of design limits, the first thing that happens is that the discharge temperature/superheat increases. This increase in temperature is first felt inside the cylinders and at the discharge valves. As the temperature increases, the lubrication to the cylinder and piston is literally cooked away causing the now dry rubbing surfaces to score and fine iron particles are scrapped from the cylinder wall and end up in the compressor sump oil and around the compressor motion gear. This overheated oil as it passes through the discharge valves causes the oil to coke or carbonise on the valve plate causing the valve to leak bye, elevating the temperature even higher. This gas leaking past the valves and pistons will elevate the compressor suction pressure and cause loss of capacity in the machine. As the oil in the compressor starts to break down due to the heat, acids and carbon are produced. These acids will, to a certain extent, be absorbed by the drier desiccant, but will eventually start attacking the motor winding insulation and cause either a spot burn or full motor burn out. Here you can see the fi ne iron particles caused by the loss of lubrication due to the oil being ‘cooked away’ . Also notice the discolouration due to the carbonising of the oil. Both the valve and valve plate are heavily carbonised by the degrading oil.
Overheating
The 2 grooves seen in the bore of the cylinder are a direct result of gudgeon pin plastic caps melting allowing the pin to grind into the cylinder wall. The binding on the motor has been burst due to the motor running hot in an overheated state. This will eventually lead to a motor burnout which you can see is beginning to happen with the discoloured winding. The scores on the pistons indicate that the compressor has been running in a dry overheated condition which will lead to metallic debris being distributed throughout the machine and would most likely lead to a spot burnout of the motor. The weak acids that are produced will slowly migrate throughout the entire system and may cause other problems later on. As the oil continues to degrade, its ability to lubricate and support the bearing loads
Typical causes of Overheating
1) High Compression Ratios
2) Too low Suction pressure
3) Too high a discharge pressure
4) Too high Suction Superheat
5) Lack of Liquid Sub-cooling at Expansion Valve inlet
6) Air or other Non Condensable in condenser
7) Choked drier
8) Choked Condenser coil
9) Condenser fan faulty or wrong rotation
10) Under sized condenser
11) Too high Suction pipe pressure drop
12) Oil logged evaporator
13) Poorly insulated Suction pipe work
14) Incorrectly set Crankcase limit valve
Preventative Measures required
1) Check design operating conditions and adjust accordingly
2) Check condenser condition and rectify
3) Check suction superheat and adjust as required
4) Check or Log all system pressures and temperatures
Typical Damage to Compressor parts
1) Scored cylinders and pistons
2) Excess wear to moving parts due to poor lubrication
3) Carbonised valves and valve plates
4) Broken valves
5) Iron particles around compressor and in the sump oil
6) Burst motor winding bindings