Screw compressors have gradually replaced obsolete analogs with piston compression heads in general industrial applications. The main advantage of screw compressors is their ability to operate around the clock. Also the advantages of these units include low noise and vibration levels. All these advantages are due to the design features.
Industrial screw compressor design
The modern screw compressor refers to positive displacement compression machines. This group also includes reciprocating and scroll machines. Turbo compressors are dynamic compression machines. Piston units are not sufficiently reliable, while scroll and centrifugal units are difficult to manufacture and - as a consequence - more expensive. Screw compressors optimally combine high reliability, long motor life and acceptable cost.
The main part of the unit is the screw pair. This is the discharge unit. One screw has a convex spiral shape, the second - concave. Both rotors are in a sealed chamber. The distance between the screws as well as to the cavity walls is measured in microns. Due to this gap, mechanical wear of the parts is excluded.
During operation, this gap is filled with lubricating fluid. An oil wedge seals the cavities formed by the screw cavities. The screw pair is driven by an electric motor. In inexpensive models, the torque is transmitted via a V-belt drive. In modern units, the rotation from the motor is transmitted to the augers by a gearbox or a direct drive system with frequency control.
Principle of operation of the screw compressor
During rotation, each depression of the concave rotor is periodically closed by the opposing projection of the convex screw. As the rotary motion continues, the closed cavity gradually shifts from one edge of the screws to the opposite edge, decreasing in volume. The air is compressed.
When the pressure in the chamber reaches a predetermined threshold, the valve opens and the working medium flows into the pneumatic line. Each screw has five grooves or depressions. At a rotation speed of 2400 rpm, you get about 12000 injection cycles per minute. That's 200 times per second. Thanks to this discharge feature, screw compressors are deprived of one of the disadvantages of their piston competitors - pulsation of the medium pressure.
To ensure autonomous operation, developers equip screw compressors with various automatics:
1. Thermal protection sensor. This sensor is installed on the branch pipe coming out of the rotor unit. If the temperature of the compressed air at the outlet of the compression chamber exceeds 105°C, the controller stops the motor.
2. The driving electric motor is fitted with an automatic shutdown device that reacts to a sudden increase in energy consumption.
3. A safety valve on the oil separator. This device is triggered if the pressure in the controlled unit exceeds the set value.
4. Oil mixture thermostat. Directs the lubricating fluid from the separator to the discharge unit either directly or through the cooling radiator. Controls the temperature.
5. Idle valve. Shuts off the suction valve when the screw compressor is operating at idle speed.
6. Minimum and maximum pressure switches. These devices automatically maintain the degree of compression of the working medium. If the limit is exceeded - stop the engine, if it falls to the lower threshold - start.
7. In expensive modifications, instead of pressure switches, electronic controllers are used, which regulate the rotor speed. As a result, the performance of the equipment is smoothly regulated in the range from 10 to 100%.