SCREW ROTOR COMPRESSOR

Abstract: A screw compressor drivingly connected to a prime mover with variable speed provided with an inlet port having a section acting as a by-pass at low rotor speeds only.

Description: SCREW ROTOR COMPRESSOR The present invention relates to a screw rotor compres¬ sor drivingly connected to a prime mover with variable speed, especially a frequency controlled electric motor, and having a port arrangement for variation of the volu et- ric capacity over a wider range than that corresponding to the speed ratio. A screw rotor compressor is disclosed ' in US patent No. 4 435 139 and comprises a casing composed of a barrel portion and two end plate portions enclosing a working space generally in the shape of two intersecting bores and provided with low pressure and high pressure ports. A pair of intermeshing male and female rotors provided with exter¬ nal helical lands and grooves are rotatably mounted within the working space for sealing cooperation with each other and with the walls of the working space. During the rota¬ tion of the rotors a pair of rotor grooves are brought into communication with the low pressure port and continuously filled with low pressure gas. Two such grooves brought out of communication with the low pressure port are then brought into communication with each other at the low pressure end wall, as a land of the female rotor starts to enter the male rotor groove, whereby the free volume of the male rotor groove decreases to a certain value before a male rotor land starts to enter the female rotor groove. The total volume of the two grooves combined in a chevron- shaped chamber will thus be reduced by about 20 % before the full intermesh of the rotors completely takes over the sealing at the axial low pressure end of the chamber. Owing to the centrifugal forces acting upon the gas en- closed in a groove when the rotors revolve, the pressure at the outer periphery will be higher than that at the bottom of the groove. This pressure differential increases with the speed of the rotor which means that the pressure rise at the bottom of the groove owing to the initial volume re¬ duction may be negligible at high speed whereas it may be significant at low speed. In order to reduce the volumetric capacity of a compres¬ sor running at low speed beyond the reduction corresponding to the speed reduction the actual invention comprises an extension of the low pressure port into the compression, phase area by cutting away a wedge-shaped portion of the end wall facing the bottom portion of a male rotor groove. At low rotor speed there will be a considerable pressure difference over this cut away portion between the groove and the low pressure channel, simultaneously as the time before the closing thereof by the male rotor land is also considerable. This means that a certain amount of the gas in the groove may be passed back to the low pressure chan¬ nel without the consumption of any significant power for the compression thereof. At high rotor speed on the other hand the pressure difference is negligible simultaneously as the time for closing the wedge-shaped portion is reduced which means that practically no leakage back to the low pressure channel will take place under such conditions. The invention will now be described more in detail in connection with the embodiment of a compressor shown in the annexed drawings, where Fig. 1 is a vertical section through a screw compressor, Fig. 2A is a partial transverse section taken along line 2-2 in Fig. 1 and showing a normal low pressure port, Fig. 2B is a partial transverse section taken along line 2-2 in Fig. 1 and showing a low pressure port according to the invention, Fig. 3 shows the volumetric capacity as a function of the rotor speed, Fig. 4 shows the torque required as a function of the rotor speed, and Fig. 5 shows the efficiency of the compressor as a func¬ tion of the capacity. The screw compressor shown in Figs. 1 and 2 comprises a casing 10 forming a working space 12 substantially in the form of two intersecting cylindrical bores having parallel axes. The casing 10 is further provided with a low pressure channel 14 and a high pressure channel 16 for the working fluid which channels communicate with the working space 12 through a low pressure port 18 and a high pressure port 20, respectively. In the compressor shown the low pressure port 18 is lo- cated in its entirety in the low pressure end wall 22 of the working space 12 and extends mainly on one side of the plane containing the axes of the bores. The high pressure port 20 of the compressor shown is located partly in the high pressure end wall 24 of the working space 12 and part- ly in its barrel wall 26 and it is in its entirety located on the side of the plane through the axes of the bores opposite to the low pressure port 18. In the working space 12 are provided two cooperating rotors, viz. a male rotor 28 and a female rotor 30, located with their axes coinciding with the axes of the bores. These rotors are journaled in the casing 10 in cylindrical roller bearings 32 in the low pressure end wall and in pairs of ball bearings 34 with shoulders in the high pres¬ sure end wall 24. One rotor is further provided with a stub shaft 36 projecting outside the casing 10. The male rotor 28 has five helical lands 38 and inter¬ vening grooves 40 having a wrap angle of about 300". The female rotor 30 has six helical lands 42 and intervening grooves 44 having a wrap angle of about 250 * . In the barrel wall 26 of the working space 12 are pro¬ vided a plurality of oil injection channels 54 opening at the intersection line 56 between the two bores forming the working space 12. These channels 54 form communications between an oil supply chamber 58 and the working space 12. Oil is supplied to this chamber from a pressure oil source not shown through a supply opening 60 under a pressure higher than the pressure prevailing in the working space 12 at the openings of the channels 54. In Fig. 2A a standard inlet port 18 is shown where the closing edge 64 thereof on the male rotor side is shaped correspondingly to the leading flank 62 of a male rotor land 38 which means that the groove 40 preceeding the land 38 is cut off simultaneously along its complete radial ex¬ tent. In Fig. 2B an inlet port 18 according to the invention is shown. The closing edge on the male rotor side is com¬ posed of two sections, an outer one 66 extending over about 2/3 of the radial extent of the port 18 and having a shape corresponding to that of the edge 64 in Fig. 2A and inner portion 68 providing a wedge-shaped extension 70 of the port 18 along the inner periphery thereof. As already stated above in the introduction of the spe- cification the extension 70 acts as a by-pass port at low rotor speeds whereas at high rotor speeds the extension 70 will not influence upon the function of the compressor. In Fig. 3 the variation of the capacity as a function of the rotor speed is shown by a continuous line compared with the corresponding variation when using a standard port shown by a dashed line. As the losses in the by-pass port are very low compared with the losses for decreasing the speed of a frequency controlled motor the torque for driving the rotors with a port according to the invention will be considerably less than that with a standard port. In Fig. 4 the variation of the torque as a function of the rotor speed is shown by a continuous line compared with the corresponding variation when using a standard port shown by a dashed line. The efficiency of the compressor at low speed will con¬ sequently be considerably higher with the new port compared to a standard one, which is shown in Fig. 5 shown by a con¬ tinuous line and a dashed line, respectively.

Claims: CLAIM Screw compressor drivingly connected to a prime mover with variable speed and comprising a casing with a working space having a barrel and a low pressure and a high pres¬ sure end wall and provided with spaced apart low pressure and high pressure ports at least partially disposed in said end walls, and a pair of intermeshing male and female ro¬ tors having external helical lands and grooves disposed in said working space, characterized in that the closing edge of the axial low pressure port on the male rotor side at the inner periphery of the port is provided with a wedge- shaped notch extending into the area of the end wall bound¬ ing the compression phase of the compressor.

Inventors: EDSTROEM SOEREN, SJOEHOLM LARS, EDSTROEM, SOEREN, SJOEHOLM, LARS
Applicants: SVENSKA ROTOR MASKINER AB
Application year: 1987
Main classification: F04C18
IPC codes: F04C18, F04C28, F04C29