Reduced noise screw machines

Abstract

A reduced noise screw expander is described, which comprises a main rotor and a gate rotor each having an N profile. The rotors are designed so that the torque on the gate rotor caused by pressure forces is in the same direction as the torque on the gate rotor caused by frictional drag forces. A method of designing a screw machine exhibiting reduced noise is also described. The screw machine has two or more rotors having an N profile, and the method involves determining a ratio r/r.sub.1, where r is the main rotor addendum and r.sub.1 is the radius of the rack round side, and ensuring that this ratio is greater than 1.1 where the screw machine is to be a screw compressor or less than or equal to 1.1 where the screw machine is to be a screw expander.

Claims

1. A screw expander comprising: a main rotor and a gate rotor, wherein, when viewed in cross section, profiles of at least those parts of lobes projecting outwardly of a pitch circle of the main rotor and profiles of at least depressions extending inwardly of a pitch circle of the gate rotor are generated by a same rack formation, said rack formation being curved in one direction about an axis of the main rotor and in an opposite direction about an axis of the gate rotor, a portion of the rack formation which generates higher pressure flanks of the rotors being generated by rotor conjugate action between the rotors, and wherein the rack formation has a ratio r/r.sub.1 less than or equal to 1.1, where r is a main rotor addendum and r.sub.1 is a radius of a rack round side so that a torque on the gate rotor caused by pressure forces from the rotor conjugate action between the rotors is in a same direction as a torque on the gate rotor caused by frictional drag forces.

2. The screw expander of claim 1 wherein the rotors are designed such that during operation of the screw expander, contact between the rotors is made at a rotor flat flank.

3. A method of manufacturing a screw machine exhibiting reduced noise properties and having two or more rotors, the method comprising: determining a ratio r/r.sub.1, wherein r is a main rotor addendum and r.sub.1 is a radius of a rack round side; and forming the screw machine such that that the ratio r/r.sub.1 is greater than 1.1 where the screw machine is to be a screw compressor or less than or equal to 1.1 where the screw machine is to be a screw expander, wherein when viewed in cross section, profiles of at least those parts of lobes projecting outwardly of a pitch circle of one or more main rotors and profiles of at least depressions extending inwardly of a pitch circle of one or more gate rotors are generated by the same rack formation; said rack formation being curved in one direction about an axis of the or each main rotor and in an opposite direction about an axis of the or each gate rotor; and, a portion of the rack formation which generates higher pressure flanks of the rotors being generated by rotor conjugate action between the rotors.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein:

(2) FIGS. 1(a)-1(d) illustrate prior art examples of rotor profiles;

(3) FIGS. 2(a)-2(d) illustrate prior art examples of rotor profiles;

(4) FIG. 3(a)-3(c) illustrate prior art examples of rotor profiles;

(5) FIG. 4(a)-4(c) illustrate screw compressor rotors designed in accordance with the present invention which make contact on the rotor round flank;

(6) FIG. 5(a)-5(c) illustrate screw compressor rotors designed in accordance with the prior art, which make contact on the rotor flat flank;

(7) FIG. 6 illustrates an example of a rack profile for generating rotor profiles according to the present invention;

(8) FIG. 7(a) illustrates the results of experimental tests performed on prior art screw compressor rotors;

(9) FIG. 7(b) illustrates the results of experimental tests performed on screw compressor rotors designed in accordance with the present invention;

(10) FIG. 8(a) illustrates the results of experimental tests performed on prior art screw expander rotors; and

(11) FIG. 7(b) illustrates the results of experimental tests performed on screw expander rotors designed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(12) The parameters r and r.sub.1 are indicated in FIG. 6, which shows an example of a rack profile. Referring to FIG. 6, the lobe of this profile is divided into several arcs similar to the profile in FIG. 2(c). In this example, the segment D-E is a straight line; the segment E-F is a trochoid; the segment F-A is a trochoid; the segment A-B is a circle; the segment B-C is a straight line; and the segment C-D is a circle.

(13) Referring to FIG. 6, r is the main rotor addendum, which is the radial distance from the pitch circle of the main rotor to the outermost point A of the lobe; r.sub.1 is the radius on the rack round side, i.e. the radius of the arc between points A and B in FIG. 6; .sub.1 is the transverse pressure angle on the rack round side; and r.sub.3 is the rack root fillet radius on the rack round side.

(14) According to the present invention, it has been calculated that if the ratio r/r.sub.1 is more than 1.1 then the gate rotor torque will be in a first direction, whilst if the ratio r/r.sub.1 is equal to or less than 1.1, the gate rotor torque will be in a second direction, i.e. opposite to the first direction. Extensive experimentation has proven that a ratio r/r.sub.1 of more than 1.1 results in reduced noise in the case of N rotor screw compressor rotors, whilst a ratio r/r.sub.1 equal to or less than 1.1 results in reduced noise for N rotor screw expanders. These relationships are summarised below in equations 15 and 16.

(15) $\begin{array}{cc}\mathrm{Compressor}\mathrm{rotors}\text{:}\frac{r}{r_1}>1.1& \left(15\right)\\ \mathrm{Expander}\mathrm{rotors}\text{:}\frac{r}{r_1}1.1& \left(16\right)\end{array}$

(16) Accordingly, the screw expander in accordance with the first aspect of the present invention comprises r and r.sub.1 parameters satisfying the condition of equation 16 above.

(17) In accordance with a second aspect of the present invention, there is provided a method of designing a screw machine exhibiting reduced noise properties, the screw machine comprising two or more rotors having an N profile as defined herein, which is generated from a rack formation, wherein the method involves determining a ratio r/r.sub.1, where r is the main rotor addendum and r.sub.1 is the radius of the rack round side, and ensuring that this ratio is greater than 1.1 where the screw machine is to be a screw compressor or less than or equal to 1.1 where the screw machine is to be a screw expander.

(18) In accordance with a third aspect of the present invention, there is provided a method of manufacturing a screw machine exhibiting reduced noise properties and having two or more rotors having an N profile as defined herein, which is generated from a rack formation, wherein the method comprises determining a ratio r/r.sub.1, where r is the main rotor addendum and r.sub.1 is the radius of the rack round side, and ensuring that this ratio is greater than 1.1 where the screw machine is to be a screw compressor or less than or equal to 1.1 where the screw machine is to be a screw expander.

(19) Within the present inventive concept there is provided a screw machine designed or manufactured in accordance with any of the above methods.

(20) According to a fourth aspect of the present invention there is provided a power generator comprising the screw expander of the first aspect of the present invention or a screw expander designed or manufactured in accordance with the second or third aspects of the present invention.

(21) Tests

(22) Two sets of rotors were designed to accommodate the above mentioned claims for reducing screw compressor and expander noise and increasing their operational reliability. The first set of rotors was for a screw compressor and the second set of rotors was for a screw expander.

(23) The process of designing and making the compressor rotors involved modifying a standard set of N profile compressor rotors. Measurements taken of the standard rotors showed that the ratio r/r.sub.1 was less than 1.1, and experimental tests showed that the torque caused by pressure forces acted in an opposite direction to the drag torque. Accordingly, contact between the rotors occurred on the rotor flat flank.

(24) The modification of the standard rotors involved increasing the transverse pressure angle .sub.1 on the rack round side. Referring again to FIG. 6, it will be appreciated that increasing the angle .sub.1 results in a decrease in the radius r.sub.1 on the rack round side, and hence an increase in the ratio r/r.sub.1. .sub.1 was increased sufficiently such that the ratio r/r.sub.1 was more than 1.1. This resulted in relatively thicker lobes on the gate rotor and relatively thinner lobes on the main rotor, when compared with the standard N profile compressor rotors.

(25) Experimental tests were performed on the standard and modified compressor rotors and the results are presented in FIGS. 7(a) and 7(b), which show two lines corresponding respectively to the main and gate rotor torques resulting from pressure forces. The main rotor torque is larger than the gate rotor torque and hence is shown above the gate rotor torque. The results for standard compressor rotors are shown in FIG. 7(a), whilst the results for the modified compressor rotors are shown in FIG. 7(b). Referring to the lower lines in both figures, it can be seen that modifying the compressor rotors caused a change in the torque sign on the gate rotor resulting from pressure forces: the torque sign on the gate rotor for standard rotors was negative, whilst the torque sign on the gate rotor for the modified rotors was positive. The tests also proved that the modified compressor rotors were significantly quieter than the standard rotors and did not suffer materially from rattle and chatter yet there was no significant loss in efficiency.

(26) The process of designing and making the expander rotors involved modifying a standard set of N profile expander rotors. Measurements taken of the standard rotors showed that the ratio r/r.sub.1 was greater than 1.1, and experimental tests showed that the torque caused by pressure forces acted in an opposite direction to the drag torque. Accordingly, contact between the rotors was made on the rotor round flank.

(27) The modification of the standard rotors involved decreasing the transverse pressure angle .sub.1 on the rack round side. Referring again to FIG. 6, it will be appreciated that decreasing the angle .sub.1 results in an increase in the radius r.sub.1 on the rack round side, and hence a decrease in the ratio r/r.sub.1. .sub.1 was reduced sufficiently such that the ratio r/r.sub.1 was less than 1.1. This resulted in relatively thinner lobes on the gate rotor and relatively thicker lobes on the main rotor, when compared with the standard N profile expander rotors.

(28) Experimental tests were performed on the standard and modified expander rotors and the results are presented in FIGS. 8(a) and 8(b), which show two lines corresponding respectively to the main and gate rotor torques resulting from pressure forces. The main rotor torque is larger than the gate rotor torque and hence is shown above the gate rotor torque. The results for standard expander rotors are shown in FIG. 8(a), whilst the results for the modified expander rotors are shown in FIG. 8(b). Referring to the lower lines in both figures, it can be seen that modifying the expander rotors caused a change in the torque sign on the gate rotor resulting from pressure forces: the torque sign on the gate rotor for standard rotors was positive, whilst the torque sign on the gate rotor for the modified rotors was negative. The tests also proved that the modified expander rotors were significantly quieter than the standard rotors and did not suffer materially from rattle and chatter and there was a slight increase in efficiency due to the contact between the modified rotors occurring on the flat flank as opposed to on the round flank in the case of the standard rotors.

(29) Various modifications may be made to the examples described above without departing from the scope of the invention as defined in the following claims.