A screw compressor is a type of rotary compressor which compresses air due to screw action. The main advantage of using this compressor is that it can supply compresses air continuously with minimum fluctuation in delivery pressure. It is usually applied for low-pressure applications up to 8 bars.
Construction of screw Compressor :
- It is a positive displacement compressor in which compression is accomplished by the enmeshing of two mating helically grooved rotors suitably housed in a cylinder equipped with inlet and discharge ports as shown in Fig.
- The male rotor (driving rotor) and consists of a series of lobes (usually four) along the length of the rotor that mesh with similarly formed corresponding helical flutes (usually six) on the female (driven rotor) rotor and a very small clearance between them.
- The entrapped gas is progressively compressed as it moves through the narrowing passage ways formed by the lobes.
- The inlet and outlet flow from the compressor are neither radial as in the case of roots blower nor axial but oblique.
- The casing is made of high grade cast iron and is ribbed for extra strength.
- Rotors are forged from normal carbon steel and must dynamically balance after machining.
- Sleeve bearings are used as shaft seals within the compressor.
In a screw compressor one of the shafts is driving shaft and the other is driven shaft. The driving shaft is connected to the driven shaft via timing gears which help to match speeds of both the shafts. The driving shaft is powered by an electric motor generally. The two shafts are enclosed in an airtight casing.
The working cycle of the screw compressor has three distinct phases as following:
(i) Suction process
(ii) Compression process
(iii) Discharge process
(i) Suction process:
As the rotors rotate, air is drawn through the inlet opening to fill the space between the male lobe and the female flute.
As the rotor continues to rotate, the air is moved past the suction port and sealed in the interlobe space.
(ii) Compression process:
As the main rotor turns, the air trapped in the ineterlobe space is moved both axially and radially.
The air is compressed by direct volume reduction as the enmeshing of the lobes progressively reduced the flute volume and compression occurs.
(iii) Discharge process:
At a fixed point where the leading edge of the flute and the edge of the discharge port co-inside, compression ceases and the air is discharged into the delivery line, until the flute volume has been reduced to zero.
The reason behind this is that the efficiency of any compression process will be highest if it is carried out isothermally in comparison to adiabatically or polytropically. Thus to maintain the temperature of air during compression, the heat developed is continuously exchanged with flowing water. A graph can help you understand this logic more clearly.
As you can see the shared area will be minimum for isothermal compression thus requiring minimum work for the compression process which will lend you higher efficiency.
This is the working principle of a screw principle
Types Of Screw Compressor:
The principle for a rotating displacement compressor in twin screw form was developed during the 1930s, when a rotating compressor with a high flow rate and stable flow under varying pressure conditions was required.
The twin-screw element’s main parts are the male and female rotors, which rotate in opposite directions while the volume between them and the housing decreases. Each screw element has a fixed, build-in pressure ratio that is dependent on its length, the pitch of the screw, and the form of the discharge port. To attain maximum efficiency, the build-in pressure ratio must be adapted to the required working pressure.
The screw compressor is generally not equipped with valves and has no mechanical forces that cause unbalance. This means it can work at a high shaft speed and can combine a large flow rate with small exterior dimensions. An axial acting force, dependent on the pressure difference between the inlet and outlet, must be overcome by the bearings.
Oil-free screw compressors
The first twin screw compressors had asymmetric rotor profile and did not use any cooling liquid inside the compression chamber. These were called oil-free or dry screw compressors. Modern, high-speed, oil-free screw compressors have asymmetric screw profiles, resulting in significantly improved energy efficiency, due to reduced internal leakage.
External gears are most often used to synchronize the position of the counter-rotating rotors. As the rotors neither come into contact with each other nor with the compressor housing, no lubrication is required inside the compression chamber. Consequently, the compressed air is completely oil-free. The rotors and housing are manufactured with ultimate precision to minimize leakage from the pressure side to the inlet. The build-in pressure ratio is limited by the limiting temperature difference between the inlet and the discharge. This is why oil-free screw compressors are frequently built with several stages and inter-stage cooling to
reach higher pressures.
Liquid-injected screw compressors
In liquid-injected screw compressors, a liquid is injected into the compression chamber and often into the compressor bearings. Its function is to cool and lubricate the compressor element’s moving parts, to cool the air being compressed internally, and to reduce the return leakage to the inlet.
Today oil is the most commonly injected liquid due to its good lubricating and sealing properties, however, other liquids are also used, for example, water or polymers. Liquid-injected screw compressor elements can be manufactured for high-pressure ratios, with one compression stage usually being sufficient for pressure up to 14 and even 17 bar, albeit at the expense of reduced energy efficiency.
Advantages Of Screw Compressor :
Screw compressors advantages are as follows,
- The screw compressor combines many advantageous features of both centrifugal and compressors, along with some of its own. As it is a positive displacement machine, high-pressure refrigerants, such as R-22 and ammonia are used in it. As it is a high-speed rotary machine, a large volume can be handled by it. It is, therefore, found extremely suitable for large capacity low-temperature applications such as in food refrigeration.
- Like reciprocating compressors, it has no surging problems. It has small pipe dimensions and positive pressures due to the use of high-pressure refrigerants. Like centrifugal compressors, it has high compression efficiency, continuous capacity control, unloaded starting, and no balancing problems. Also, the compressor is suitable for large capacity installations.
- Low maintenance cost
- Not usually noisy.
- Can run fully loaded for extended periods of time
- Good for geat recovery for space heating
- Good efficiency for oil-flooded models
Disadvantages of screw Compressor:
Screw Compressor has some disadvantages are as follows
- Can use 40 % – 90% of full load power when unloaded
- Not good for very small or very large loads.
Applications of Screw Compressor :
Applications of screw compressors are as follows ;
1) Delivery capacity: from 200 to 20000 m3/hr
2) Pressure ratio: less than 3 bar in one stage and up to 12 to 14 bar in two stages.
3) It is used in industrial refrigeration.
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