Screw compressors are categorized as intermittent motion compressors (non-continuous flow). They can also be categorized with medium flow and medium pressure compressors compared with other types of compressors on pressure /flow chart for compressor types.
Screw compressor is called so as it contains two screws, one of them is driver mounted with prime mover (motor, turbine) and the other one is driven. According to the theory of operation, the two screws are mounted together by means of gears to rotate opposite to each other according to direction of rotation of driver (counter clockwise) squeezing the compressed air with oil in the compression zone to raise its pressure.
The compressed air is compressed with oil (flooded type) inside the compressor, and then the compressed oil and air are introduced to an oil separation unit to separate oil and air. Then the air is removed to the discharge line and oil is returned to the oil filter for filtration, then to the suction of compressor again. This type of compressor is characterized by large axial displacement during operation, at vibration measurement; the axial displacement of it is higher than any other compressor of different type.
Oil and screws
The function of oil inside the compressor is for lubrication of screws, bearings and seals during operation, and secondly, for cooling screws. As air compression rises temperature (thermodynamic process) and overheating affects screws badly and can cause localized thermal stresses in mid length of screw (hotspots). Over expansion of screws can lead to screw rubbing either with casing or with other screws but screws are manufactured from material of low thermal expansion to avoid overheating or expansion of screws due to bad lubrication or cooling.
Another function of oil is creating a layer on screws, to protect its surface as any dust particulates escaping from air suction filter can cause erosion and scratches on the screw profile. Overheating of screws can lead to over expansion, increasing amount of run out of screw, to adjust on balancing machine. On a balancing machine, the position and angle of defection in the screw can be determined, and then by heating it, can be restored. But this procedure is used in low percentage run out as it may cause localized thermal stress in the screw for bigger percentage.
Note: This procedure can be used also for screws of oil screw pumps to maintain its screws.
Screws maintained using this procedure are marked by black spots at the place of heating.
Oil problems
Oil used in this type of compressors should be of the type that can sustain high temperature as outlet temperature can reach 99 ᵒC which is very high. Some types of oil don’t afford this high temperature so it starts to precipitate particulates in system fouling it. These particulates block oil filters and oil separators cartridges, which make the rate of cartridge change very rapid.
Filters and contaminates
Torn or blocked air suction filters can cause serious damage to the compressor because if it is saturated with dust or oil, the air flow will bypass it and enter the compressor with dust causing scratches on the screws. Any dust or sandy weather around compressor suction will affect filters badly as sand particulates are very soft and can pass through suction filter holes. But it mixes with oil and gets precipitated in oil filters contaminating it and increasing changing rate.
PCV internal leakage
Oil can return to contaminate the air suction filter if the pressure control valves (PCV) on discharge line after oil separator is having any defects either in springs or seals, or any internal cracks in the internal valve body. As if the discharge valve is opened to discharge system (if valve defected), the system pressure (discharge pressure) will make a backflow for air to return inside the compressor through the internal leakage of PCV to push the oil backward to the suction of compressor till it reaches the air filter. The symptoms are higher ampere of the running motor and lower discharge pressure.
High motor ampere (power)
There are other reasons to increase the motor ampere (power) which is higher temperature of suction air, malfunction of oil cooler, decreasing voltage or torque of motor (motor driven), blockage in system filters, any increase in screw run out or misalignment.
Any dust or particulates from suction air or from bad oil can reach the bearings causing erosion and causing damage to the internal parts of bearings. Any defect in the bearings causes more motor ampere (power) and even leads to motor overload. Because of friction (particulates in lubricating oil causes erosion and friction).
Increasing screw run out will rise vibration levels as it generates induced force that increases with time, this force requires power to increase it. This increase in power will appear as increase in ampere and power consumption.
Screw scratches
Any scratch or rubbing in screws will start is small rate then increase with continuous running , rubbing means friction between screw and static casing leading to metallic deposits in oil , so periodic oil samples are is required from compressor to inspect oil to check contaminates and ashes.
Higher ampere (power) means higher noise level but also any defect in screws will make higher noise level (in start of problem).
Couplings and vibrations
Couplings are very important item in compressor train as it is responsible for power transmission from prime mover to driven machine.
Types of couplings
- Coupling consisting of rubber saddle shaped pieces mounting between motor hub (mover hub) and compressor hub
- Jaws coupling mounting between 2 hubs
- Membrane coupling
Coupling failure
Rubber couplings get torn with successive startup and shut down (alternative loading), the tearing place is at the bolt hole region. Rubber couplings usually get torn from the bolt hole region because of high stiffness of material against loading. Placing lower stiffness material can accommodate more flexibility with operation, these material can be rubber but with same properties of that of belts.
Belts are flexible and have the ability to sustain variable loading (tensile stress). Some of the high load belts are reinforced with steel beams inside it to be more yielding in sustain tensile stress. This material can sustain variable loading of successive startup and shutdown, variable torques without any tearing problem for coupling. Jaws coupling get broken down from the jaws itself with fluctuating loading.
Coupling and misalignment
Membrane coupling get broken down if there is any small percentage of misalignment which leads to coupling to alternate loading on it in opposite directions. After certain time the membrane will start to be broken under fatigue and fail. As a symptom of failure start, the torque transmission from motor to coupling will start to decrease as a part of it is lost in figure of losses in coupling, (energy absorbed by coupling).
This percentage of misalignment can cause bearings to be defected (breaking cage and friction in bolts) with continuous loading as the induced force of vibration will be excited to reach higher values with continuous operation.
