How a lubricated screw compressor works

internal parts of a lubricated screw compressor
air compressor
In this article, we will analyze the operation of a lubricated screw compressor, its construction and the different types that can be found in the market.


The compressors of the screw are teams of positive displacement. The operating principle of these compressors is based on the reduction of the volume of air in the compression chamber where it is confined, producing the increase of the internal pressure until reaching the expected design value, at which time the air is released to the compressor.system.


The screw technology is more advanced than its predecessor, the reciprocating or piston compressor. Both systems are positive displacement; The main difference between these technologies is in the way of compressing, which in the case of the screw compressor is continuous along the rotor and in the piston, it does so in two phases (aspiration and compression). This way of compressing the air in the piston generates alterations in the flow, while that of the screw produces a continuous flow of air.



Compression process


you can see in the diagram, the compressor consists of two rotors that rotate in parallel and in the opposite direction, inside a housing. These two rotors have a different geometry, one of them being the so-called male, which enters the cavity of the other called female, to create a chamber where the aspirated air accumulates.



The two rotors rotate inside the housing, displacing the air from the suction side to the discharge side. This effect is produced by its special design in the form of a helical screw.


The air circulates longitudinally through both screws directly to the area opposite to the aspiration, where the pressure increase occurs due to a reduction of the space. The continuous displacement of the screws accumulates air in the compression zone until reaching the pressure required by the design of the equipment, at which time the air is released in the discharge pipe, leaving the compressor in continuous operation pressurizing the system to which is connected


The helicoidal design is based on the Archimedes screw, which consists basically of a spiral mounted on a cylinder. In the case of compressors, this design is more complex because not only their profiles are developed in a special way to achieve maximum efficiency, but their work is based on the operation of the two screws (male and female) rotating in parallel.


Depending on the application, the working pressure or the design of each manufacturer, the assembly formed by the two screws and their casing can have different components. In this article we will analyze the basic components.



Components of a screw compressor

A screw compressor is not only formed by the rotors that compress the air, but also requires a series of additional equipment that allows to operate and control the operation of said rotors. In general, the elements that make up said compressor are:


  • Set of rotors or screws, also known among compressors as AirEnd.
  • Main drive motor. Usually it is electrical (three-phase or single-phase) or internal combustion, but depending on its application can be another type of drive such as those made by steam turbines, pneumatic, hydraulic motors, etc.
  • System of regulation and control of the aspiration. An important element in the operation of the compressor that allows regulation of the air intake in the equipment.
  • General lubrication system of the complete equipment. The compressors have installed a closed circuit that includes the system of filtering and cooling the oil.
  • Lubricant cooling system. Maintains the lubricant at the optimum working temperature.
  • Cooling system of compressed air. To reduce the temperature of the compressed air to the ideal one for its use.
  • Filter system. Indispensable to eliminate particles and polluting remains of compressed air.
  • Start-up and control equipment Each compressor, depending on its design or application, has installed a control system that regulates the operation of the equipment, its start-up and shutdown.
  • Common bench and protection canopy / soundproofing. Although there are executions that do not require a soundproofing cabin, most screw compressors have high sound levels and must be isolated to prevent hearing damage to operators or people who circulate near them. Nowadays, these equipments are quite silent thanks to these protections.



Internal operation of the lubricated screw compressor

The equipment described above is assembled and interconnected to form the lubricated screw compressor, as explained in the following diagram typical of this type of compressors:


INTERNAL PARTS OF A lubricated screw compressor



The assembly formed by the rotors and the drive motor (elements 3 and 4 of the diagram) form the base of the compressor, independently of whether said drive is with an electric, combustion, hydraulic motor, etc.


The air is sucked in by the compressor through the valve (2) and the filter (1) , entering the screw (3) through the suction zone. Once inside, the air circulates through the two screws and is compressed on the air / oil separation vessel (5) . Following the scheme, it can be seen that inside this receiver vessel (5) , the compressed air is forced to make a sharp turn, with the idea of getting rid of as much oil as possible. To improve the efficiency of this separation, the compressed air comes out through a separating filter with coalescing properties (6), which removes the remaining oil in the air stream to a very small residual (less than 3 ppm, depending on the manufacturer).


One of the characteristics of the compression process is the generation of heat. Obviously, the air can not be delivered to the system as it leaves the screw due to its high temperature, which can oscillate around 100 ° C. For this reason, the compressors are equipped with heat exchangers (7) with which they lower the temperature of the compressed air to the right one for its safe use. These exchangers can be Air / Air or Air / Water.


Before reaching the compressed air exchanger, passes through the valve of retention and low pressure (11) . This valve has a double mission. On the one hand, it maintains the internal pressure of the air circuit at the minimum values specified by the manufacturer and, on the other, it prevents the air from returning from the network.


The lubrication of these compressors is done with an oil specially formulated for this work. Once the oil is separated in the container (5) , it is guided by a closed circuit that includes a filtering system (8) to eliminate the impurities that it has been able to collect from the air, and a refrigerator (9) to reduce its temperature. Once clean and at the right temperature, the oil is injected back into the screw.


But the oil in these compressors should not be cold or excessively hot. For this reason, within the lubrication circuit, a thermostatic valve (10) determines whether the oil flows into the refrigerator or returns directly to the screw, depending on the temperature.


The lubrication oil of this type of compressors is a vital element for the operation and performance of the same. It is advisable to use a suitable oil, because in the compressor it is used for different functions:


1-  Lubricate . The oil is used to lubricate the bolts and bearings.


2-  Seal . It is very important that the minimum tolerance between the rotors be sealed with the lubrication oil itself and avoid the loss of efficiency of the assembly.


3-  Cool . The injected oil is the refrigerant fluid with which the compression heat is evacuated.



Compressor regulation

The regulation of a screw compressor is an important part of the operation of the equipment. Usually all compressors respond to a regulation based on the pressure of the system, this means that the compressor will be compressing air until the pressure of the system reaches the desired value.


For this reason, screw compressors work between maximum and minimum pressure to regulate their operation. There are several ways to do this work:


• In start-up and stoppage


The compressor starts up by starting its main engine when the system pressure has reached its minimum value, stopping the engine when the pressure reaches its maximum value. This system is unusual in lubricated screw compressors, being used only in some special applications and with low power equipment.


• Loading / unloading


This is the most usual system. As can be seen in the operating diagram of a screw compressor, upstream of the filter, the suction valve (2) is located . This valve regulates the air intake in the screw. When the system reaches the minimum pressure value, the suction valve opens and the screw sucks the outside air. With this maneuver it is said that the compressor has entered load.


When the maximum pressure is reached in the system, the suction valve (2) closes and the compressor stops supplying air. At this time it is said that the compressor is in discharge.


This maneuver is repeated as many times as necessary to maintain the system pressure between the selected values. Throughout this process the main engine remains in operation.


• Load / unload with proportional regulation


This system is being used little by little. The operation of the suction valve is similar to the previous system, but upon reaching the maximum pressure value, the valve does not close completely but begins to close partially to maintain a constant pressure value, reducing or increasing the air intake in the screw proportionally. If the flow reaches an excessively small point, the suction valve closes completely leaving the compressor in discharge.


• Variable speed


This is the most advanced system in the control of the flow of the compressor and has replaced the previous one for its efficiency in the regulation of the flow and the saving of energy.


It is based on the property of these compressors to be of positive displacement, that is to say that for each revolution they displace a determined volume of air. In this way, increasing the speed of the compressor increases the flow and reducing it reduces it.


This execution is widely used in installations where the user requires a very variable air flow due to the requirement of the process. With this regulation, it is possible to maintain a constant pressure in the system and an energy consumption proportional to the required flow, with the consequent energy savings.



Different executions of lubricated screw compressors

The usual thing about lubricated screw compressors is that all the elements described above are mounted on a frame, forming a single assembly. It is also common that the aforementioned set is protected inside a cabin that makes the protection and soundproofing functions, together with the control and start system to handle the complete set automatically.


There are executions with compressors of powers lower than 22 kW that are supplied without soundproofing because, due to their design and low turns, the sound level of the equipment in operation is well below the required safety levels.


Another way to mount the lubricated screw compressors of small powers is on the accumulator tanks, forming a compact assembly.


In the following image, screw compressors of some of the main manufacturers are shown. We can see an example of lubricated screw equipment (A) , a portable screw compressor (B) widely used in public works and a screw compressor driven by a steam turbine (C) .



All of them are examples of the different executions and developments that manufacturers can perform to adapt their screw compressors to the different needs of their customers.


The screw compressors have a manufacturing range with powers between 3 kW and 500 kW. They are possibly the most common compressed air equipment in the industry , with working pressures between 3 bar and 15 bar. Depending on the applications, you can find higher power equipment or pressures that can reach 27 bar.


In this article we have analyzed the operation of the lubricated screw compressors, but there are other designs within the screw equipment:


•   Exempt screw compressors


As its name suggests, no lubricant is injected into the screw of these compressors, although it is necessary to lubricate other parts of the screw itself such as bearings or synchronization gears.


•  Water injection screw compressors


It is a screw compressor that uses water as a lubricant. Its operation is similar to that of a lubricated screw compressor, but using water as a lubricating fluid, sealant and coolant.


Since water is not a good lubricant, these compressors have a somewhat special design, using ceramic-type rotors or a combination of a paddle rotor and a traditional screw.

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