Title of Invention	PLURAL-STAGE PLURAL SCREW POSITIVE DISPLACEMENT MACHINE
Abstract	A plural-stage positive displacement plural screw machine of the kind in which each stage comprises first and second helically shaped rotors meshing together within a chamber having an inlet port and an outlet port, the outlet port of one chamber b,eing connected to the inlet of the another, wherein the first rotors of each stage are formed by portions of a continuous first rotor and the second rotors of each stage are formed by portions of a continuous second rotor, each said continuous rotor being supported in bearings at opposite ends of a housing forming the said chambers, the said chambers being separated by partition means through which the continuous rotors extend in a sealed manner.

Title of Invention

PLURAL-STAGE PLURAL SCREW POSITIVE DISPLACEMENT MACHINE

Abstract

A plural-stage positive displacement plural screw machine of the kind in which each stage comprises first and second helically shaped rotors meshing together within a chamber having an inlet port and an outlet port, the outlet port of one chamber b,eing connected to the inlet of the another, wherein the first rotors of each stage are formed by portions of a continuous first rotor and the second rotors of each stage are formed by portions of a continuous second rotor, each said continuous rotor being supported in bearings at opposite ends of a housing forming the said chambers, the said chambers being separated by partition means through which the continuous rotors extend in a sealed manner.

Full Text	This invention relates to plural-stage plural screw positive displacement machines such as compressors and expanders. The invention is particularly, but not exclusively, concerned with twin screw compressors. These are positive displacement rotary machines which consist, essentially, of a pair of meshing helical lobed rotors, contained in a casing. Machines of this type are broadly classless into two types; namely: a) Oil injected compressors in which a relatively large mass, though a very small volume, of oil is admitted with the gas to be compressed. The oil acts as a lubricant between the contacting rotors, a sealant of any clearances between the rotors and between the rotors and the casing and also as a coolant of the gas during the compression process This cooling effect improves the compression efficiency and permits pressure ratios of up to approximately 15:1 in a single stage without an excessive temperature rise. b) Oil free compressors: in which only dry gas is admitted into the working chamber. These require external timing gear, in order to prevent rotor contact, and internal shaft seals located between the bearings at each end of the rotor shaft and the main body of the rotor. The shaft seals are needed to prevent oil, which is supplied to the bearings through an external lubrication system, from entering the working chamber and thereby contaminating the gas being compressed. Because there is no injected oil to cool the gas in this machine, pressure ratios are limited to approximately 3:1, depending on the type of gas being compressed. Above this value the temperature rise associated with compression creates problems associated with rotor and casing distortion. In both types of machines, high pressure differences create large internal leakage and this reduces their efficiency. In order to overcome the problems of leakage in both types of machine and temperature rise in oil free compressors, it is common practice for oil injected compressors to become multi-stage machines at pressure ratios of more than approximately 10:1 and for oil free compressors to be built with two or more stages at pressure ratios of greater than about 3:1. In the bulk of applications, two stages are sufficient. A two stage compressor requires two sets of rotors, each rotating in a separate chamber. The two chambers are, however, sometimes machined in one casing. The first, or low pressure, stage must be large enough to take all the flow rate of gas required at the induction conditions. However, the second, or high pressure, stage will be much smaller since the gas is already partially compressed. Usually it is the practice to cool the gas being compressed after it leaves the low pressure (LP) stage before it enters the high pressure (HP) stage. The HP stage may then be made even smaller by virtue of the reduction in volume of the gas and, more importantly, the total power input to the machine is thereby significantly reduced. Normally the HP stage is made with rotors of similar proportions to those in the LP stage but with smaller diameters and driven at a higher rotational speed in order to maintain approximately the same tip speed. In order to minimize cost, it is usual to have the two compressor stages driven by a single external motor and coupled by gears so that the two stages are parallel to each other. Alternatively, it is possible to have a separate drive for each stage and thereby eliminate the coupling gears but the cost of this is greater and it frequently involves additional step up gearing between the HP rotors and their drive motor. According to the invention, there is provided a plural-stage positive displacement plural screw machine of the kind in which each stage comprises first and second helically shaped rotors meshing together within a chamber having an inlet port and an outlet port, the outlet port of one chamber being connected to the inlet of the other, wherein the first rotors of each stage are formed by portions of a continuous first rotor and the second rotors of each stage are formed by portions of a continuous second rotor, each said continuous rotor being supported in bearings at opposite ends of a housing forming the said chambers, the said chambers being separated by partition means through which the continuous rotors extend in a sealed manner. The invention will now be further described by way of example with reference to the diagrammatic drawings, in which:- Fig. 1 is a longitudinal section, with some parts omitted for clarity, of a two-stage compressor taken through the axes of the two rotors. Fig. 2 is a longitudinal section through the axis of the main rotor in the direction of the arrows II of Fig. 1, and Fig. 3 shows the two meshing rotors on an enlarged scale. The two-stage compressor shown in the drawings has a casing (1) defining a chamber within which are mounted a main rotor (3) and a gate rotor (4). The main rotor (3) has five helical lobes which mesh with six helical grooves in the gate rotor (4). Each rotor is supported at each end by -bearings in the housing (1) in the normal manner. Each rotor (3, 4) has a low pressure portion, (3L, 4L) which together form a low pressure compressor section. This section has a relatively large inlet port (5) and a smaller outlet port (6) formed partly in a side wall of the housing (1) and partly at (7) in a partition (8) which forms an end wall for the low pressure compressor section. The partition (8) is formed in two halves, being divided along a plane through the axes of the two rotors and extends into grooves (9) and (10) in the rotors (3) and (4). The rotors are extended beyond the grooves (9) and (10) to form high pressure stage portions (3H) and (4H). The high pressure stage has an inlet (11) which is connected to the low pressure stage outlet (6) but lies on the opposite side of the plane through the axes of the rotors. The port (6) is connected to the high pressure inlet port (11) either directly by a passage within the casing or through a heat exchanger. Part of the high pressure inlet port can be formed at (12) in the high pressure side of the partition (8). At the opposite end of the high pressure stage, the casing (1) forms a high pressure outlet port (13). It will be noted that the high pressure stage portions of the rotors are shorter than the low pressure stage portions but that the profiles are identical so that the low and high measure portions can be formed by a continuous machining or grinding operation with the grooves (9) and (10) being formed either before or after such operation. The main advantage of this arrangement compared to the conventional arrangement of two separate sets of rotors in two different diameter bores is the huge saving in manufacturing cost in that: (a) the four rotors required for two stage compression can be made on only two shafts by a single profile forming operation followed by cutting a slot to separate them into two sections. (b) the HP and LP bores can be machined in a single operation, being subsequently subdivided into two chambers by means of a separation plate. (c) the need for either gearing to couple the drives : of both the LP and the HP stages, or for separate drive motors for each stage, is eliminated. We Claim : 1. A plural-stage positive displacement plural screw machine of the kind in which each stage comprises first and second helically shaped rotors meshing together within a chamber having an inlet port and an outlet port, the outlet port of one chamber being connected to the inlet of the another, wherein the first rotors of each stage are formed by portions of a continuous first rotor and the second rotors of each stage are formed by portions of a continuous second rotor, each said continuous rotor being supported in bearings at opposite ends of a housing forming the said chambers, the said chambers being separated by partition means through which the continuous rotors extend in a sealed manner. 2. A machine according to claim 1, wherein the continuous rotors are each of constant profile along their lengths within the chambers, and have circular profiles where they extend through the partition means. 3. A machine according to claim 2, wherein the circular profiles are formed by annular grooves in the rotors into which divided wall portions of the partition means extend. 4. A machine according to any of the preceding claims, wherein the interconnected outlet port of one chamber and inlet port of another are adjacent the partition means separating the chambers and are on opposite sides of a plane through the axes of the two rotors. 5. A machine according to any preceding claims, wherein at least one of the interconnected outlet and inlet ports are formed at least partly in the partition means. 6. A machine according to any preceding claims, wherein the lower pressure portions of the rotors are longer than the higher pressure portions. 7. A machine according to any preceding claims, having timing gears and interconnecting continuous rotors. 8. Plural-stage plural screw positive displacement machine, substantially as hereinabove described and illustrated with reference to the accompanying drawing.

Full Text

This invention relates to plural-stage plural screw positive displacement machines such as compressors and expanders.
The invention is particularly, but not exclusively, concerned with twin screw compressors. These are positive displacement rotary machines which consist, essentially, of a pair of meshing helical lobed rotors, contained in a casing.
Machines of this type are broadly classless into two types; namely:
a) Oil injected compressors in which a relatively large mass,
though a very small volume, of oil is admitted with the gas to be
compressed. The oil acts as a lubricant between the contacting rotors, a
sealant of any clearances between the rotors and between the rotors and
the casing and also as a coolant of the gas during the compression
process This cooling effect improves the compression efficiency and
permits pressure ratios of up to approximately 15:1 in a single stage
without an excessive temperature rise.
b) Oil free compressors: in which only dry gas is admitted into the
working chamber. These require external timing gear, in order to
prevent rotor contact, and internal shaft seals located between the
bearings at each end of the rotor shaft and the main body of the rotor.
The shaft seals are needed to prevent oil, which is supplied to the
bearings through an external lubrication system, from entering the
working chamber and thereby contaminating the gas being compressed.
Because there is no injected oil to cool the gas in this machine, pressure

ratios are limited to approximately 3:1, depending on the type of gas being compressed. Above this value the temperature rise associated with compression creates problems associated with rotor and casing distortion.
In both types of machines, high pressure differences create large internal leakage and this reduces their efficiency. In order to overcome the problems of leakage in both types of machine and temperature rise in oil free compressors, it is common practice for oil injected compressors to become multi-stage machines at pressure ratios of more than approximately 10:1 and for oil free compressors to be built with two or more stages at pressure ratios of greater than about 3:1. In the bulk of applications, two stages are sufficient.
A two stage compressor requires two sets of rotors, each rotating in a separate chamber. The two chambers are, however, sometimes machined in one casing. The first, or low pressure, stage must be large enough to take all the flow rate of gas required at the induction conditions. However, the second, or high pressure, stage will be much smaller since the gas is already partially compressed. Usually it is the practice to cool the gas being compressed after it leaves the low pressure (LP) stage before it enters the high pressure (HP) stage. The HP stage may then be made even smaller by virtue of the reduction in volume of the gas and, more importantly, the total power input to the machine is thereby significantly reduced. Normally the HP stage is made with rotors of similar proportions to those in the LP stage but with smaller diameters and driven at a higher rotational speed in order to maintain approximately the same tip speed. In order to minimize cost, it is usual

to have the two compressor stages driven by a single external motor and coupled by gears so that the two stages are parallel to each other. Alternatively, it is possible to have a separate drive for each stage and thereby eliminate the coupling gears but the cost of this is greater and it frequently involves additional step up gearing between the HP rotors and their drive motor.
According to the invention, there is provided a plural-stage positive displacement plural screw machine of the kind in which each stage comprises first and second helically shaped rotors meshing together within a chamber having an inlet port and an outlet port, the outlet port of one chamber being connected to the inlet of the other, wherein the first rotors of each stage are formed by portions of a continuous first rotor and the second rotors of each stage are formed by portions of a continuous second rotor, each said continuous rotor being supported in bearings at opposite ends of a housing forming the said chambers, the said chambers being separated by partition means through which the continuous rotors extend in a sealed manner.
The invention will now be further described by way of example with reference to the diagrammatic drawings, in which:-
Fig. 1 is a longitudinal section, with some parts omitted for clarity, of a two-stage compressor taken through the axes of the two rotors.
Fig. 2 is a longitudinal section through the axis of the main rotor in the direction of the arrows II of Fig. 1, and
Fig. 3 shows the two meshing rotors on an enlarged scale.

The two-stage compressor shown in the drawings has a casing (1) defining a chamber within which are mounted a main rotor (3) and a gate rotor (4). The main rotor (3) has five helical lobes which mesh with six helical grooves in the gate rotor (4). Each rotor is supported at each end by -bearings in the housing (1) in the normal manner.
Each rotor (3, 4) has a low pressure portion, (3L, 4L) which together form a low pressure compressor section. This section has a relatively large inlet port (5) and a smaller outlet port (6) formed partly in a side wall of the housing (1) and partly at (7) in a partition (8) which forms an end wall for the low pressure compressor section.
The partition (8) is formed in two halves, being divided along a plane through the axes of the two rotors and extends into grooves (9) and (10) in the rotors (3) and (4).
The rotors are extended beyond the grooves (9) and (10) to form high pressure stage portions (3H) and (4H). The high pressure stage has an inlet (11) which is connected to the low pressure stage outlet (6) but lies on the opposite side of the plane through the axes of the rotors. The port (6) is connected to the high pressure inlet port (11) either directly by a passage within the casing or through a heat exchanger. Part of the high pressure inlet port can be formed at (12) in the high pressure side of the partition (8). At the opposite end of the high pressure stage, the casing (1) forms a high pressure outlet port (13).
It will be noted that the high pressure stage portions of the rotors are shorter than the low pressure stage portions but that the profiles are

identical so that the low and high measure portions can be formed by a continuous machining or grinding operation with the grooves (9) and (10) being formed either before or after such operation.
The main advantage of this arrangement compared to the conventional arrangement of two separate sets of rotors in two different diameter bores is the huge saving in manufacturing cost in that:
(a) the four rotors required for two stage compression can be made on only two shafts by a single profile forming operation followed by cutting a slot to separate them into two sections.
(b) the HP and LP bores can be machined in a single operation, being subsequently subdivided into two chambers by means of a separation plate.
(c) the need for either gearing to couple the drives : of both the LP and the HP stages, or for separate drive motors for each stage, is eliminated.

We Claim :
1. A plural-stage positive displacement plural screw machine of the
kind in which each stage comprises first and second helically shaped
rotors meshing together within a chamber having an inlet port and an
outlet port, the outlet port of one chamber being connected to the inlet of
the another, wherein the first rotors of each stage are formed by portions
of a continuous first rotor and the second rotors of each stage are formed
by portions of a continuous second rotor, each said continuous rotor
being supported in bearings at opposite ends of a housing forming the
said chambers, the said chambers being separated by partition means
through which the continuous rotors extend in a sealed manner.
2. A machine according to claim 1, wherein the continuous rotors
are each of constant profile along their lengths within the chambers, and
have circular profiles where they extend through the partition means.
3. A machine according to claim 2, wherein the circular profiles are formed by annular grooves in the rotors into which divided wall portions of the partition means extend.
4. A machine according to any of the preceding claims, wherein the interconnected outlet port of one chamber and inlet port of another are adjacent the partition means separating the chambers and are on opposite sides of a plane through the axes of the two rotors.

5. A machine according to any preceding claims, wherein at least
one of the interconnected outlet and inlet ports are formed at least partly
in the partition means.
6. A machine according to any preceding claims, wherein the lower
pressure portions of the rotors are longer than the higher pressure
portions.
7. A machine according to any preceding claims, having timing gears
and interconnecting continuous rotors.
8. Plural-stage plural screw positive displacement machine,
substantially as hereinabove described and illustrated with reference to
the accompanying drawing.

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Patent Number

216680

Indian Patent Application Number

707/MAS/2002

PG Journal Number

17/2008

Publication Date

25-Apr-2008

Grant Date

18-Mar-2008

Date of Filing

20-Sep-2002

Name of Patentee

ELGI EQUIPMENTS LIMITED

Applicant Address

TRICHY ROAD, SINGANALLUR, COIMBATORE - 641 005,

Inventors:

#	Inventor's Name	Inventor's Address
1	JAYARAM VARADARAJ	ELGI EQUIPMENTS LIMITED, TRICHY ROAD, SINGANALLUR, COIMBATORE - 641 005,

PCT International Classification Number

F01B 21/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA