The screw compressor is one of the most commonly used types of compressors. It uses a series of screws to compress the fluid. This article explains the screw compressor working, types, and applications.
What is a Screw Compressor?
A screw compressor is a type of positive displacement compressor that uses two spiral screws to compress the gas. It uses a positive displacement rotary mechanism for the compression of gas.
The screw compressor has two below given meshing spiral screws (called rotors) for the compression process:
- Male rotor
- Female rotor
The rotors of these compressors have very tight tolerances. The rotor contains many blades that rotate according to the rotor movement. These blades are in close contact, typically only a few thousandths of an inch apart.
The effectiveness of this mechanism depends on the precise clearance between the rotor and the chamber to seal the compression cavity. But some leakage is unavoidable, and high rotary speed should be utilized to minimize the ratio between leakage and effective flow.
Compared to the root blower, the two rotors of the latest rotary screw air compressor have different designs. The male rotor has a convex blade that meshes with the concave cavity of the female rotor. The male rotor has a smaller number of blades compared to the female rotor blades. Due to the smaller number of blades of the male rotor rotates faster than the female rotor.
Rotary screw compressors are most widely used all over the world in different industries. These compressors replace conventional reciprocating compressors that require larger amounts of compressed gas. These types of compressors have no volumetric or mechanical losses and can work at the high speed of the shaft.
Screw Compressor Working Principle
A screw compressor works according to the positive displacement mechanism. It uses two spiral rotors (male and female rotors) for the compression of air or gas. These rotors mesh in such a way that the concave cavity of the female rotor meshes with the convex blade of the male rotor.
The female rotor has a more significant number of blades than the male rotor blades. Due to this reason, the female rotor rotates slower than the male rotor. The male rotor is connected to an electric motor via a shaft.
During the screw compressor working, the male rotor gets power from the electric motor via a shaft. As the male rotor receives power from the electric motor, it starts rotating and also rotates the female rotor.
When these two meshed rotors start rotating, a cavity produces at the suction end of the compressor. Due to this cavity, a pressure difference produces between the external and internal sides of the compressor. Due to this pressure difference, the compressor sucks air from the atmosphere into the compressor. This air is trapped between the female and male rotor blades because the compressor has no way for air to move directly from the inlet to the outlet.
As male and female rotors continue their rotation, the trapped air also continues its movement from the inlet to the outlet side. During this rotation, the rotors continuously compress the trapped air. As the air compresses according to the requirements, it discharges from the discharge valve and stores into the storage tank.
For a better understanding, watch the following video:
Read Also: Working of Centrifugal Compressor
Components of Rotary Screw Compressor
The rotary screw air compressor has the following parts:
- Compression cylinder
- Air filters
- Pinion gear
- Inlet valve
- Outlet valve
The screws of the compressors are also known as rotors. A screw compressor has two rotors (male and female rotors) for the compression of the gas. The male rotor is connected with a shaft. As the shaft rotates, it also rotates the male rotor.
The male and female rotors mesh with each other. The movement of these rotors plays an important role during the suction and compression of the gas. As the rotors start rotating, they suck the gas from the outside into the compressor. After suction, they further compress and discharge the gas.
2) Compression Cylinder
The compression cylinder of the screw compressor plays a vital part in the compression process. As the rotors start rotating, a vacuum produces inside the cylinder; due to that, air or gas enters the compression cylinder. This cylinder has a decreasing volume from the inlet to the outlet.
The male and female rotors rotate inside the cylinder. The number of cylinders of a compressor represents to compression stages for a compressor. If a compressor has one cylinder for the compression process, it is known as a single-stage compressor. If a compressor has more than one cylinder, it is known as a multistage compressor.
Read Also: Different types of Reciprocating Compressors
3) Air Filters
The air filter installs inside the inlet valve of the compressor. It stops air particles, dust, moisture, and other contaminations from entering the compressor. If this filter doesn’t install with the inlet valve, the contaminations may damage the internal parts of the compressor.
4) Inlet Valve
As the vacuum inside the rotary screw air compressor generates, the compressor sucks the gas through the inlet valve.
5) Outlet Valve
After increasing the desired pressure of the gas, the compressor uses an outlet valve to discharge the compressed gas.
Read Also: Different Types of Valves
An electric motor uses to supply power to the male rotor of the screw compressor. This motor is connected with the male rotor via a crankshaft. As the motor turns on, it turns the shaft which further rotates the male rotor.
The main objective of the housing is to prevent the internal components from any damage due to the fall of any external load. It acts as a safety guard for the inner parts of the compressor.
The bearings are installed on both ends of the rotor. They help in the rotation of the rotor and increase the life cycle of the rotor.
The main objective of the bearing is to balance the rotor blade and the rotation of the rotor. The screw compressors have anti-corrosive and anti-friction bearings. These are available for both oil-free and oil-lubricating compressors.
Types of Rotary Screw Compressors
A rotary screw compressor has the following major types:
- Oil-lubricated screw compressor
- Oil-free screw compressor
1) Oil lubricated Screw Compressor
These compressors have high reliability, durability, and efficiency. Due to these characteristics, these compressors are best appropriate for a wide variety of manufacturing processes. Therefore, these compressors use all over the world for different industries.
The oil-lubricated rotary compressor is best for medium-air pressure applications. Therefore, only those people use these compressors who require large amounts of medium-pressure air.
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In these compressors, oil is injected into the compression chamber of the compression. This injected oil lubricates and decreases the temperature of the compressor components. This oil also assists the compressor in minimizing the compression chamber leakage and eliminating heat during compression.
The lubricants increase the service life of the compressor parts and also help to form a tight seal. The oil-lubricated rotary screw compressor has low noise compared to other types.
The oil-injected screw compressors may work the whole day without any negative effects. Due to this, this rotary compressor is beneficial for manufacturing applications where a stopped compressor can affect production.
The oil-injected screw compressors are also helpful for applications where air demands are tough to guess, or you don’t want to control demand.
The compression chamber controls the injected oil temperature from 60 to 700°C. The outlet temperature should be kept above the pressure dew point to avoid condensation of water mixed with oil.
The thermostatic valve controls the amount of oil that circulates in the oil cooler or in the bypass line for maintaining the required temperature within a wide range of ambient temperatures.
2) Oil-free lubricated Screw Compressors
It is known as an oil-free screw compressor because it doesn’t use any oil during the compression of gas or air. These compressors use lubricants for reducing the motor temperature and for eliminating heat by the motor. Some types of these compressors utilize water as an alternative lubricant.
An oil-free screw compressor doesn’t use oil in the compression chamber like an oil-injected compressor. This compressor efficiently eliminates the risk of contamination from the compression chamber without the use of any oil.
The oil-free rotary compressor is the best choice when air purity is important for your products, and even the most minor impurities can spoil the process and damage the end product. Therefore, many manufacturing industries use these types of screw compressors to make the air free from contamination.
Oil-free compressors are often used in sensitive industrial functions. Because these industries meet strict standards for pharmaceuticals, beverages, and food. These are also best for the stringent needs of the oil & gas industry.
Read Also: Different Types of Dynamic Compressors
Advantages and disadvantages of Screw Compressors
The screw compressor has the following significant advantages and disadvantages:
Advantages of Screw Compressors
- These compressors have continuous operation. You don’t need to turn on or off to a screw compressor.
- They have easy maintenance.
- They have no duty cycle.
- These compressors have a quiet operation than piston compressors.
- A screw compressor can work in the most challenging conditions because it has a high airflow rate and can work at severe temperatures.
- These are energy-efficient compressors.
- They require low cost for maintenance.
- There is no capacity loss with time.
- These are highly efficient for oil-flooded models.
Disadvantages of Screw Compressors
- These compressors have a high installation cost
- With time, the maintenance or repair cost of the rotary screw increases
- High service cost
- It requires a more experienced operator for its maintenance
Applications of Rotary Screw Compressors
- Screw compressors are most commonly used to supply compressed air for different industrial applications.
- These compressors have continuous operation. Therefore, these are best for applications such as automated manufacturing and food packing system.
- In automotive industries, a rotary screw compressor uses for aeration, product finishing, plasma welding, plasma cutting, and air tools.
- These are used for petrochemical plants.
- Screw compressors are also employed for mining and metallurgy
- The rotary compressors use for printing and papermaking applications
- These also use in electrical and mechanical machinery
- They use for textile and clothing applications
- They also use for transportation applications
- These compressors also use for construction and building material applications.
- They use for household appliances.
Screw Compressor VS Reciprocating Compressor
The main difference between the screw compressor and reciprocating compressor is given below:
|Reciprocating compressor uses a piston to compress the fluid.
|Screw compressor uses two intermeshing rotors to compress the fluid.
|It has a low cost.
|It has a high cost compared to a reciprocating compressor.
|Reciprocating compressors have low efficiency.
|The screw compressor has approximately 30% more efficient than a reciprocating compressor.
|They require high space for installation.
|These require less installation space.
|These have high noise.
|Screw compressors produce low vibration and noise.
|It doesn’t have a continuous operation.
|It has a continuous operation.
|These can’t produce high compressed air than screw compressors.
|These compressors can produce high compressed air than reciprocating compressors.
|It has dead space between the valve and piston.
|There is no dead space between the valve plate and the piston.
|It is more frequent and more complex.
|It is less frequent and less complex.
|It has a low service cost.
|It has a high service cost.
What is a Screw Compressor?
A compressor that uses two or more screws for fluid compression is known as a screw compressor.
What are the types of Screw Compressors?
- Oil-free lubricated screw compressor
- Oil lubricated screw compressor
What are the Parts of Screw Compressor?
- Compression Chamber
- Air Filters
- Inlet and outlet Valve
Why use a screw compressor?
Screw compressors are most commonly used because of their different advantages over other types of compressors, mainly for applications demanding continuous and high air demand. These benefits include very low maintenance requirements, easy design, very low vibrations, lower noise level, and higher efficiency at full load. The rotary compression mechanism in a screw compressor permits it to work continuously and smoothly.
Which oil used in screw compressor?
The oil used in a screw compressor usually varies according to the screw compressor type and its application. Synthetic or mineral lubricants are most commonly used to ensure the smooth working, minimize friction between the rotors, and offer efficient heat dissipation.