U.S. patent application number 13/383401 was filed with the patent office on 2012-08-09 for dry screw driver.
This patent application is currently assigned to ROBUSCHI S.P.A.. Invention is credited to Paolo Cavatorta, Umberto Tomei.
Application Number | 20120201708 13/383401 |
Document ID | / |
Family ID | 41723031 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120201708 |
Kind Code |
A1 |
Cavatorta; Paolo ; et
al. |
August 9, 2012 |
DRY SCREW DRIVER
Abstract
An embodiment of a dry screw compressor has a male rotor at a
peripheral speed lower than approximately 80 m/s. The compressor
includes: a casing body having an inlet for a gaseous fluid to be
taken in and at least an outlet for the compressed fluid; at least
a male rotor and at least a female rotor meshed together, said
rotors being arranged inside said casing body whereby the ratio
between the length and the external diameter of the male rotor is
higher than or equal to approximately two, and the winding angle of
the male rotor is smaller than or equal to approximately
300.degree..
Inventors: |
Cavatorta; Paolo; (Parma,
IT) ; Tomei; Umberto; (Castenaso, IT) |
Assignee: |
ROBUSCHI S.P.A.
Milano
IT
|
Family ID: |
41723031 |
Appl. No.: |
13/383401 |
Filed: |
July 9, 2010 |
PCT Filed: |
July 9, 2010 |
PCT NO: |
PCT/IB10/01706 |
371 Date: |
April 26, 2012 |
Current U.S.
Class: |
418/75 |
Current CPC
Class: |
F04C 18/16 20130101;
F04C 18/084 20130101; F04C 2240/52 20130101 |
Class at
Publication: |
418/75 |
International
Class: |
F04C 15/00 20060101
F04C015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 10, 2009 |
IT |
PR2009A000054 |
Claims
1. A dry screw compressor having a male rotor whose peripheral
speed is lower than approximately 80 m/s, the compressor
comprising: a casing body having an inlet for a gaseous fluid to be
taken in and at least an outlet for the compressed fluid; at least
a male rotor and at least a female rotor meshed together, said
rotors being arranged inside said casing body; wherein the ratio
between the length and the external diameter of the male rotor is
higher than or equal to approximately two, and the winding angle of
the male rotor is smaller than or equal to approximately
300.degree..
2. The compressor according to claim 1, wherein the driving shaft
is a shaft on which said female rotor is keyed.
3. The compressor according to claim 2, further comprising an
electric motor operationally acting on the shaft of the female
rotor for starting its rotation around a first axis of
rotation.
4. The compressor according to claim 3, wherein in the rotor of
said electric motor is keyed on said shaft of the female rotor.
5. The compressor according to claim 3, wherein said electric motor
is a permanent magnet motor.
6. The compressor according to 1, wherein said outlet defines an
opening formed in the casing body, the actual size of said opening
being variable by means of shaping means in order to obtain a
predetermined compression ratio.
7. The compressor according to claim 1, wherein the compressor can
be used in applications under differential pressures approximately
between 1 bar and 3 bars.
8. The compressor according to claim 1, wherein the compressor can
be used in applications under vacuum up to a threshold absolute
pressure of approximately 150 mbars.
Description
PRIORITY CLAIM
[0001] The present application is a national phase application
filed pursuant to 35 USC .sctn.371 of International Patent
Application Serial No. PCT/IB2010/001706, filed
[0002] July 9, 2010; which further claims the benefit of Italian
Patent Application Serial No. PR2009A000054 filed Jul. 10, 2009;
all of the foregoing applications are incorporated herein by
reference in their entireties.
TECHNICAL FIELD
[0003] An embodiment relates to a dry screw compressor for a gas,
in particular air, for use in pressure applications (e.g. in the
conveyance of granulates or powders, or in water treatment, where
large amounts of air must be conveyed to start and help aerobic
reactions) and in vacuum applications (e.g. in gas, fume or steam
exhaust systems). In particular, an embodiment of the present dry
screw compressor is used in applications with low differential
pressures comprised between 1 bar and 3 bars and under vacuum to a
threshold absolute pressure of 150 mbars.
BACKGROUND
[0004] As already known, low differential pressure applications
(lower than 1 bar) use lobe compressors. These are compressors
wherein two lobe rotors (usually two or three lobes) with parallel
axes mesh together and synchronically rotate in opposite
directions.
[0005] However, these lobe compressors, although being structurally
simple, economical and able to guarantee a good flow, have a scarce
thermodynamic efficiency.
[0006] Therefore, it has been projected a screw compressor which
could work under low pressure, with a high flow and a thermodynamic
efficiency characteristic of an internal compression machine, but
whose structural characteristics were as similar as possible to a
lobe compressor.
[0007] As known, a normal screw compressor under high pressures
includes at least a male rotor and at least a female rotor meshing
together during the rotation around respective axes and housed
within a casing body. Each of the two rotors has screw-shaped ribs
that mesh with corresponding screw-shaped grooves of the other
rotor. Both the male and female rotor show, in cross section, a
predetermined number of teeth corresponding to their ribs and a
predetermined number of valleys corresponding to their grooves. The
casing body has an inlet for the gas to be taken in and an outlet
(also called "delivery outlet") for the compressed gas. The intake
gas is compressed between the two moving rotors and arrives to the
outlet under the requested pressure.
[0008] Furthermore, it is known that dry screw compressors,
generally indicated as "oil-free", as opposed to oil injection
compressors, are largely used in applications wherein the level of
contaminants must be kept below a determined percentage threshold
(usually very low).
[0009] In recent years, some manufacturers have proposed dry screw
compressors for differential pressures approximately between 3 and
10 bars, thus re-adjusting the technology of oil injection screw
compressors for applications under high pressures (higher than
approximately 10 bars).
[0010] However, the manufacture of such dry screw compressors is
quite sophisticated and expensive, since it typically must take
into account the remarkable mechanical and thermal stresses to
which the rotors are subjected. In particular, in order to avoid an
excessive bending under load, the ratio between the length and the
external diameter of the male rotor is usually approximately
between 1.5 and 1.8; this ratio may strongly limit the compressor
capacity and may require the insertion in the compressor structure
of a gear multiplier in order to start the rotors at very high
peripheral speeds, usually approximately >150 m/s.
[0011] By altering the delivery outlets, the aforesaid compressors
can be also used under differential pressures approximately between
1 bar and 3 bars. However, a drawback of these low pressure
compressors is represented by their having the same structural
complexity of high-pressure compressors.
SUMMARY
[0012] Therefore, an embodiment is a dry screw compressor which can
work under low pressure, with a high flow and a thermodynamic
efficiency typical of these kinds of machines.
[0013] In particular, an embodiment is a dry screw compressor under
low differential pressures (approximately between 1 and 3 bars) and
with a high flow, which is structurally simple, economical and easy
to maintain.
[0014] Moreover, a further embodiment is a dry screw compressor
which is also suitable for under vacuum applications, up to a
threshold of approximately 150 mbar of absolute pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Characteristics and advantages of one or more embodiments
will become more apparent from the following approximate, and hence
non-restrictive, description of one or more embodiments of a dry
screw compressor, as illustrated in the appended drawings,
wherein:
[0016] FIG. 1 illustrates a longitudinal cross section of a dry
screw compressor according to an embodiment;
[0017] FIG. 2 illustrates a three-dimensional view, in longitudinal
section, of some details belonging to an embodiment of the dry
screw compressor belonging in turn to the dry screw compressor of
FIG. 1;
[0018] FIG. 3 illustrates a cross section (not in scale) of an
embodiment of the rotors used in the compressor according to FIGS.
1, 2; and
[0019] FIG. 4 illustrates a three-dimensional schematic side view
(not in scale) of a male rotor used in the dry screw compressor
according to an embodiment.
DETAILED DESCRIPTION
[0020] With reference to the alleged FIGS., 1 indicates a dry screw
compressor for gas, in particular air, according to an
embodiment.
[0021] The compressor 1 can be used both under pressure and under
vacuum.
[0022] The compressor 1 includes at least a male rotor 2 and at
least a female rotor 3, meshed together (FIGS. 1, 2, 3).
[0023] The embodiment here described and illustrated provides for a
single male rotor 2 and a single female rotor 3 housed within a
single casing body 4.
[0024] In particular, this casing body 4 is obtained by coupling
two communicating cylinders (not shown) so that they define a
single cavity 5 housing the rotors 2, 3.
[0025] An alternative embodiment (not shown) provides for a
plurality of conjugated pairs of male rotors 2 and female rotors
3.
[0026] As shown in FIG. 1, the female rotor 3 is keyed on a shaft
17 (having an axis of rotation (01)), whereas the male rotor 2 is
keyed on a shaft 18 (having an axis of rotation (02)). In
particular, the first axis of rotation (01) is arranged at a
certain distance (I) (generally known as "center distance") from
the second axis of rotation (02). The first axis of rotation (01)
and the second axis of rotation (02) are parallel to each
other.
[0027] Each of said rotors 2, 3 has screw-shaped ribs meshing with
screw-shaped grooves formed between corresponding screw-shaped ribs
of the other rotor 2, 3. In this way, in cross section (FIG. 3),
the male rotor 2 shows lobes 6 (or teeth) and valleys 7 meshing
with corresponding valleys 8 and lobes 9 (or teeth) of the female
rotor 3.
[0028] Moreover, FIG. 3 shows some main dimensional parameters
characterizing the profiles of the rotors 2, 3. In particular, it
can be seen an external circumference (Cef) of the female rotor 3
and an external circumference (Cem) of the male rotor 2.
[0029] Moreover, as shown in FIG. 1, the length (Lm) of the male
rotor 2 corresponds to the length (Lf) of the female rotor 3.
[0030] Conjugated profiles identical to those shown in the present
FIG. 3 have been described and claimed in the international patent
application PCT/IB2010/051416, which is incorporated by reference,
and whose content is part of the present detailed description
since, in combination with the geometrical elements described
hereinafter, it allows to maximize the compressor capacity and to
minimize the gas leaks normally occurring in the coupling areas
between the rotors, and between the rotors and their casings.
[0031] In fact, an embodiment and with a particular reference to
FIG. 4, the "winding angle" (.phi.) is formed by the angle of a
generic helix 40 (described by the head of a generic tooth) between
a segment OA, coupling the axis (02) of the male rotor 2 to the
helix 40 on a first end plane (.pi.1) of the rotor 2, and a segment
O'B', also coupling the axis (02) to the helix 40 on a second end
plane (.pi.2) of the rotor 2 opposed to the first end plane
(.pi.1).
[0032] As also shown in FIG. 4, the rotor 2 includes three helixes
30, 40, 50, parallel to each other, described by the heads of the
relative teeth.
[0033] Furthermore, the term "length (Lm)" of the male rotor 2
defines the distance between the two end planes (.pi.1), (.pi.2),
the term "pitch (Pz)" between two helixes 30, 40 defines the
distance between point B and point B1, and the term "angle of the
helix" (.psi.) defines the angle between the tangent (r) to the
helix 40 in any point (P) and the axis (02) of the male rotor
2.
[0034] It has been found that the ratio between the length (Lm) and
the external diameter (Dm) of the male rotor 2 (see also FIG. 4),
in an embodiment, must be higher than or equal to approximately 2,
to maximize the compressor capacity and, therefore, together with
the conjugated profiles of the lobes of the rotors, to guarantee
high gas flows. Said (Lm)/(Dm) ratio may be approximately between 2
and 3. In this context, external diameter (Dm) means the diameter
of the external circumference (Gem) of the male rotor 2 (FIG.
3).
[0035] Moreover, it has been found that, in an embodiment, in order
to maximize the compressor capacity, if the other geometric
dimensions are equal, the maximum value of the winding angle
(.phi.) must be approximately 300.degree.; in fact, by increasing
the value of the winding angle (.phi.), and with an equal length
(Lm), an equal diameter (Dm) and an equal profile of the tooth of
the male rotor 2, the overlap between the teeth of the two rotors
2, 3 consequently increases, with a following reduction of the
total capacity of the compressor 1.
[0036] Moreover, the values (Lm), (Pz) and the angles (.phi.),
(.psi.) are geometrically related to each other.
[0037] Therefore, in an embodiment, it is possible to projectually
determine the optimal values of parameters (Lm), (Dm), (Pz),
(.psi.) in order to define an optimal value of the "winding angle"
(.phi.) giving the maximum gas flow at reduced peripheral speed of
the male rotor 2 and under reduced pressure.
[0038] The number of lobes 6 of the male rotor 2 may be different
from the number of lobes 9 of the female rotor 3. In particular,
the number of lobes 6 of the male rotor 2 may be lower than the
number of lobes 9 of the female rotor 3 by at least one unity. For
example, in the embodiment here described and illustrated, the
number of lobes 6 of the male rotor 2 corresponds to three, whereas
the number of lobes 9 of the female rotor 3 corresponds to five. In
another embodiment (not shown), the number of lobes 6 of the male
rotor 2 corresponds to four, whereas the number of lobes 9 of the
female rotor 3 corresponds to six.
[0039] The two rotors 2, 3 are kept in the reciprocal position by
means of the synchronization gear formed by two toothed wheels 20a
and 20b of the known kind (FIG. 1).
[0040] In order to allow a correct working of the compressor 1 in
an embodiment, the transmission ratio between the synchronization
gears 20a, 20b is equal to the ratio existing between the number of
teeth of the two rotors 2, 3.
[0041] The driving shaft is the shaft 17 on which the female rotor
3 is keyed because it is the one with more teeth, so that each
rotation of this shaft 17 corresponds to the filling of a larger
number of gaps and, in short, to a larger volume conveyed by the
compressor 1.
[0042] As shown in more detail in FIG. 2, the casing body 4 has an
inlet 10 for a gaseous fluid to be taken in, flowing according to
an arrow (F1), and at least an outlet 11 (or delivery outlet) for
the compressed fluid flowing according to an arrow (F2). Said
outlet 11 defines an opening 12 formed in the casing body 4.
[0043] The compressor 1 uses bearings of a known kind. In
particular, the radial loads are sustained by a first group 19a of
radial ball bearings arranged close to the inlet 10 and by a second
group 19b of cylindrical ball bearings arranged close to the outlet
11. The axial loads, on the other hand, are sustained by a third
group 19c of oblique contact ball bearings arranged beside the
bearings of the second group 19b.
[0044] In the particular embodiment shown in FIG. 1, the compressor
1 is provided with an electric motor 16 whose rotor is keyed on the
shaft 17 of the female rotor 3 for starting its rotation around the
first axis of rotation (01). The motor 16 may be a permanent magnet
motor. Such a permanent magnet motor 16 may be of the kind cooled
by water circulation. As an alternative, a permanent magnet motor
of the air-cooled kind may be used.
[0045] As previously stated, the motor 16 may be keyed on the shaft
17 of the female rotor 3, namely it may be aligned with said shaft
17.
[0046] When no speed variation of rotors 2, 3 is required, the
compressor 1 may be coupled to an electric motor (not shown) by
means of a "belt and pulley" drive (not shown).
[0047] The operation of the dry screw compressor according to an
embodiment is described hereinafter.
[0048] The gas (e.g. air) is taken in by the compressor 1 and,
through the inlet 10, enters into the casing body 4 (FIGS. 1, 2).
During the rotation, the screw-shaped ribs of the female rotor 3
mesh together with the screw-shaped grooves of the male rotor 2,
and vice versa. In embodiments with no contact between the rotors
2, 3, the correct transmission/multiplication ratio between the
rotors 2, 3 is actuated by means of the synchronization gears 20a,
20b.
[0049] By longitudinally crossing the casing body 4, the gas is
compressed between the "coils" of the two rotating rotors 2, 3,
thus reaching the outlet 11.
[0050] An embodiment wherein the opening 12 is arranged on the side
surface of the casing body 4, is used for "intermediate"
compression ratios R, e.g. approximately between 1 and 4; whereas,
in another embodiment, the opening 12 is arranged in correspondence
to an end of the casing body 4 (on the plane (.pi.1); see FIG. 1);
this latter embodiment solution is chosen for "high" compression
ratios (R), e.g. approximately between 4 and 10. Both embodiments
may be provided with shaping means (not shown) defining the actual
dimension of opening 12 corresponding to the desired compression
ratio (R).
[0051] The aforesaid description clearly shows the characteristics
of the dry screw compressor according to one or more embodiments,
as well as its advantages.
[0052] In particular, the ratio between the length and the external
diameter of the male rotor (higher than or equal to approximately
two) is made possible by low differential pressures (approximately
between 1 bar and 3 bars) or by the threshold absolute pressure of
approximately 150 mbars for under vacuum applications.
[0053] Moreover, the choice of the profile geometry and the
operation of the compressor by means of the shaft of the female
rotor allow to maximize the compressor capacity, with rotors of the
same length, thus allowing to reach the requested high flow at a
peripheral speed of the male rotor 2 lower than approximately 80
m/s.
[0054] Furthermore, the geometry of the profiles of the two coupled
rotors allows obtaining a shorter contact line between the rotors
with a better seal, thus reducing the blow by.
[0055] Moreover, thanks to the fact that the compressor works at
peripheral speeds of the male rotor lower than approximately 80
m/s, the peripheral speed of the female rotor is even lower, and,
therefore, the rotor of the electric motor can be directly keyed on
the shaft of the female rotor (namely with no interposition of
multiplying gears), thus obtaining a compressor which is
structurally simple, compact and having a higher energetic
efficiency. This makes use of the multiplying ratio of
synchronization gears of the rotors, corresponding to the ratio
between the number of lobes of the female rotor and the number of
lobes of the male rotor (in the described embodiment, it
corresponds to 5/3=1.66667). This avoids the utilization of
toothed-wheel multipliers integrated in the compressor, with a
resulting advantage in structural simplicity, encumbrance, cost and
noise.
[0056] Furthermore, the energetic efficiency of the compressor is
also provided by the use of a permanent magnet motor, characterized
by low consumptions over a large range of speeds. In particular,
this kind of permanent magnet motor has higher efficiencies than
the three-phase asynchronous electric motor used in the known art,
especially at reduced speeds. Among other things, the use of a
water-cooled permanent magnet motor allows a reduction in size and
weight of the motor, thus allowing its direct arrangement of the
shaft of the female rotor, exploiting the radial bearings of the
compressor.
[0057] Finally, the optimization of the energetic efficiency is
also obtained thanks to the use of a delivery outlet whose size
varies according to the desired compression ratio, thus producing
an extremely versatile and modular compressor.
[0058] From the foregoing it will be appreciated that, although
specific embodiments have been described herein for purposes of
illustration, various modifications may be made without deviating
from the spirit and scope of the disclosure. Furthermore, where an
alternative is disclosed for a particular embodiment, this
alternative may also apply to other embodiments even if not
specifically stated.
* * * * *