Types of screw compressors and their advantages

Screw compressor is the most effective tool for organizing pneumatic systems, because it has compact dimensions, low weight, low noise and vibration compared with other types of units designed to compress gases and air.

Differences and advantages of screw compressors over piston compressors

The first difference between screw and piston compressors is in their design. The main thing is what distinguishes screw compressors from reciprocating compressors - this is a compression mechanism.. In the screw units are used rotors with helical teeth, rotating towards each other. And in the piston - the piston, which makes a reciprocating movement inside the cylinder. Due to the design differences described above, the screw compressor has a low weight and compact dimensions.

Also different method of injection and air storage. Screw machines create a constant air flow. Piston compressors, on the other hand, deliver air in pulses that correspond in frequency to the movements of the piston. Therefore, to create a constant flow to the piston apparatus is connected to the receiver.

The advantages of screw compressors over piston compressors are obvious.

1. Power saving. It is saved by using the latest generation screw blocks and automatic control of the air supply. Due to this, the consumption of electricity is reduced by approximately 30%.
2. Low maintenance cost. On average, maintenance of piston assemblies is required every 500 hours of operation. Screw machines need inspection after 4000-8000 hours of operation.
3. Long service life. Compressors with a screw principle of action can operate without repair for several years in a row. This is due to the lack of a valve system and the presence of a simple lubrication and cooling system. On the screw pair of the unit manufacturer for 2 years. But, as practice shows, the devices can work without replacing a screw pair of 7-8 years. During this time, under the conditions of an enterprise, about 5 piston-type compressors having a similar capacity have to be changed.
4. Low installation and commissioning costs. As already mentioned, the screw units are small and almost do not produce noise and vibration. Therefore, money is saved on installation and installation of equipment, since it is not required to be installed on a foundation or in a separate room.
5. Excellent technical specifications. Screw units are highly competitive equipment, possessing the following technical characteristics: Efficiency up to 95% (for piston units, efficiency does not reach 60%); capacity over 40 m³/ min; output pressure up to 9 kgf / cm².

Device and principle of operation of screw compressors

The main component of the screw compressor is screw block (see figure below). It consists of a body (1) in which a screw pair is located (2 and 3).

The rotors in the middle part have thickenings on which the screw profile is cut. These screws are set so that between them there is a gap in the range from 0.1 to 0.4 mm. The rotor pair is mounted either on bushings or on bearings. The rotation of the screws is synchronized with the help of gears (4) fixed on the shafts of the rotors. To ensure the tightness of the body, it is assembled with glands and seals.

Important! The drive screw of the unit has a convex and wide tooth shape, while the follower screw is thin and concave.

In the case of the compressor, cavities are also provided for cooling (5), into which, if necessary, fluid is supplied. The drive of the compressor can be both direct, and belt.

The principle of operation of the rotary unit is as follows.

1. When the screws rotate, air begins to flow through the inlet to the suction cavity where the rotor pair is located. At this stage, the air fills the screw depressions along the entire length (Fig. 1).
   

2. When the rotors rotate towards each other, the suction volume is cut off from the inlet.At this stage, oil is injected to seal the gaps between the screws and lubricate them. Also, the incoming oil absorbs the heat generated by the compression of air, performing the function of cooling. With further turning of the screws, the volume of the working chamber decreases, and the pressure in it increases.
   
3. Further, at the moment when the screw bottoms are connected to the compressor outlet, the compression in the chamber stops and the output of the compressed air-oil mixture begins through the outlet window of the unit.
   

The device of the screw compressor is noticeably different from the device of the piston unit. Below is the screw compressor circuit, which includes the following elements.

1. Filter. Designed to clean the atmospheric air drawn into the unit.
2. Suction valve. Interferes with emission of oil and air at a stop of the compressor.
3. Screw block. It is the main working unit of the unit, consisting of a screw pair placed in the housing. A thermal protection sensor is installed next to the nozzle (18), which turns off the engine if the temperature above 105 ° C is at the outlet of the screw block.
4. Belt drive. Designed to transmit rotational motion from the engine to the screws. The drive consists of 2 pulleys. One pulley is mounted on the motor shaft, and the other on the drive shaft of the screw block.
5. Pulleys The speed of rotation of the rotor pair depends on their size. The pulleys are interconnected by means of a driving belt.
6. Engine. Sets the rotational movement of the belt drive, which, in turn, drives the screw block.
7. Oil filter. Designed to clean the oil returning to the rotary unit.
8. Primary oil separator. In this unit, the oil is separated from the air using centrifugal force.
9. Oil separator filter. Designed for secondary cleaning of air from oil residues, that is, better. At the exit from the filter in the air, residual oil vapors can be detected in the amount of 1.3 mg / m³. This indicator for piston units is unattainable.
10. Safety valve. Provides safety when operating the unit. If the pressure in the oil separator (8) is exceeded, the valve will turn on, dropping it to an acceptable level.
11. Thermostat. Thanks to him, the optimum temperature of the oil composition is maintained. The latter can freely pass by the cooling radiator until it reaches a temperature of 72 ° C.
12. Oil cooler This tank receives heated oil, separated from the air, to cool to the desired temperature.
13. Air cooler. Allows you to cool the air before being supplied to the points of consumption to a temperature of 15-20 ° C higher than the ambient temperature.
14. Fan. Designed to cool all components of the unit.
15. Valve idling. It is electropneumatic and is designed to control the suction valve (2).
16. Pressure switch. Thanks to him, the work of the device is provided in automatic mode. In the latest generation of compressors, an electronic control system is installed instead of a pressure switch.
17. Manometer. Shows the pressure level inside the unit.
18. Outlet Through it, compressed air enters the points of consumption.
19. Visual inspection device. Made in the form of a transparent thickening on the tube. With it, you can control the process of returning oil.
20. Minimum pressure valve. It is in the closed state until the pressure rises to 4 bar. Since this element separates the pnevmoline from the compressor, it performs the function of a non-return valve when the unit stops or switches to idle mode.

All listed parts and components of the screw compressor are placed in metal case covered with sound-absorbing compound. Depending on the manufacturer and model of the device, its device may differ slightly from the above.

If you consider in detail principle of operation of the screw compressorthen it looks like this (see figure below).

1. When the unit is turned on through the filter (1), air begins to be drawn in.
   

2. Next, the air enters the suction regulator (2), and then moves to the rotor unit (3).
3. In the rotary unit, air is mixed with oil and its subsequent compression. The oil enters the unit in precisely dosed portions.
4. The air-oil mixture enters the separator (8) and passes through the cartridge (9), where it is separated into oil and air.
5. Further, clean air passes through the cooling radiator (13) and exits the unit.
6. The oil, which was separated in the separator (8), again enters the rotor unit.The temperature of the returning oil depends on which circle it will move - large or small. If the oil is too hot, the thermostat valve (11) is activated and redirects it in a large circle through the oil cooler (12).
7. Before coming from the radiator to the screw block, the oil is cleaned in the filter (7).
8. The helical pair is driven by the engine (6) and V-belt transmission (4 and 5).

Modes of operation

Screw compressor units, even the simplest, have 5 modes of operation.

1. Start. This is the start-up mode of the unit, which eliminates the overload of the power grid. The voltage is applied to the engine gradually, so that it starts to work only after 10-15 seconds. after pressing the power button.
2. Idling. In this mode, the apparatus is being prepared for full-load operation. The rotors are driven by the engine and begin to inject air, but at low power.
3. Work mode. In this mode, there is a complete operation of the unit, the output of which is compressed air.
4. Standby mode. It is activated when a certain pressure is reached in the system.In standby mode, all processes in the compressor are stopped until the pressure in the system drops to the level at which the device is turned on.
   Tip! This mode is very convenient when the compressor is used periodically, during the working day, since there is no need to de-energize the unit. His work is only suspended for a certain period.
5. Stop. This mode leads to a smooth shutdown of the device. In the beginning, it goes to idle, after which it turns off completely. This mode reduces the likelihood of breakdowns and wear of parts due to a sharp pressure drop or voltage.

Some models of screw compressors have Stop-Alarm mode. This mode is activated when any equipment malfunctions occur, or when the pressure and temperature in the unit rises to critical levels. Stop-Alarm mode, as a rule, works automatically. But to turn it on manually there is a button located on the control panel of the device.

Varieties of screw compressors

Existing types of screw compressors determine their scope of use.For example, industrial oil-filled units are versatile and widely used in various fields. But the use of oil-free apparatuses is in demand only in those areas where a high degree of purification of compressed air is required, for example, in the food, chemical and pharmaceutical industries.

Oil free machines

An oil-free compressor does not use oil for air compression as lubrication and cooling of the rotary unit; therefore, the compressed air produced by the device does not contain lubricant particles. Oil-free units are divided into 2 subspecies: screw dry compression and water filled.

Screw compressors dry compression equipped with synchronous motors driving screws that are not in contact with each other. “Dry” devices have lower productivity (3.5 bar by 1 step) than oil filled devices. When connecting the second stage, you can increase this figure to 10 bar. But this measure will only increase the cost of equipment, which is already quite high due to the use of twin engines.

Water filled apparatus They are the most technological and combine all the advantages of both oil-free and oil-filled devices. Water filled devices are capable of compressing force up to 13 bar (1 step). Also model data are eco-friendly, because instead of oil for cooling they use plain water. Since water has a high heat capacity and thermal conductivity, then, regardless of the level of air compression, it heats up to a maximum of 12 ° C due to a metered injection. From this it follows that with a decrease in the heat load on the parts of the unit increases their service life, as well as increases the safety and reliability of the equipment as a whole.

Important! The air leaving the water-filled unit does not need to be cooled, since the water that circulates in the system will always have an ambient temperature.

Water filled compressors have virtually no waste during operation. Also, these devices are cheaper to manufacture, because in their design there are no oil filters and containers for used oil.

Oil fill devices

The oil unit, as mentioned above, has 2 rotorsone of which is leading.To prevent physical contact between the rotors, oil is injected into the unit. It should be supplied with a speed of 1 l / min per 1 kW of power of the device. Oil compressors have noise in the range of 60-80 dB.

According to engine power, compressors can be from 3 to 355 kW, and according to performance - from 0.4 to 54 m³/ min High-performance equipment, as a rule, is stationary and is installed in workshops. But still there are mobile screw compressors, both gasoline and diesel.

Common malfunctions of screw compressors and their elimination

Prolonged operation of any equipment leads to the fact that it requires either service or serious repair. There are no exceptions and compressors, the main node of which is a rotary unit.

Repair screw compressors with their own hands is quite possible in the following cases:

• the device hardly starts;
• the compressor does not restart;
• there is no compressed air in the outlet of the unit;
• low productivity;
• excessive oil consumption;
• involuntary actuation of the safety valve;
• shutdown apparatus thermostat;
• disconnecting the unit with a network chopper;
• breakdown of the rotor unit;
• high blood pressure.

The device does not start well

The reason that the unit starts up with difficulty may be low ambient temperature. The compressor will start up only after warming up the room in which it is installed.

Device does not restart

This breakage is caused poor suction valve closure. The problem is solved by cleaning the valve. If this procedure does not solve the problem, the suction valve should be replaced.

Lack of compressed air

If there is no compressed air in the outlet of the apparatus, then this is a sign regulator closure. To fix the problem, you need to check the operation of the pressure switch. It is this node that supplies power to the valve, which is electromagnetic, which, in turn, is connected to the regulator.

Poor performance

The decrease in equipment performance is also associated with the closure of the regulator. In this case, the failure is caused by clogging of the latter. In order for the performance of the device to return to normal, you need to remove the suction filter, open or dismantle the regulator,and clean it well.

Excessive oil consumption or leakage

High oil consumption can cause broken filterinstalled in the oil separator, or leakage of seals of the same filter. In both cases, the problem is solved by replacing these parts.

Important! An unclosed regulator or excessively high pressure in the system can cause leakage of oil. In the first case, you should check the health of the solenoid valve and the regulator. In the second - to check the gauge.

Opening safety valve

This breakdown may occur if oil separator filter clogged. It is required to check whether there is a pressure differential between the oil separator, that is, its tank and the pipeline in which the compressed air is located. The problem is solved by replacing the filter.

Thermostat triggering

There are several reasons for the unit to turn off the thermostat unit.

1. High ambient temperature. Provide a room with good ventilation equipment, then press the “reset” button and restart the unit.
2. Oil cooler clogging. It is required to clean the cooler using a solvent liquid.
3. Low oil level. It is necessary to add the necessary amount of the latter.
4. Thermostat malfunction. The item should be replaced with a working one.

Disconnecting the motor by the circuit breaker

Tripping the circuit breaker may cause low voltage. You should check the voltage and, with its normal values, restart the device by pressing the “Reset” button.

Also the circuit breaker may operate when engine overheating. First of all, you need to check the heat sink from the electric motor. If the heat removal mode is not violated, then restart the equipment. In the case when the restart does not occur, you should wait a few minutes and try again.

Breakdown of the rotary unit

If you pay attention to the description of the rotary unit, which was cited above, it will become clear that it can only be repaired in case of failure of the bearings.. In the event of rotor jamming, repair of screw blocks should be entrusted to service center specialists.

High blood pressure

If the pressure rises above the maximum permissible values, then first of all regulator checked. Perhaps there is no command to close it.Make sure the solenoid valve is in the closed state. If necessary, these parts should be replaced.