U.S. patent application number 10/985528 was filed with the patent office on 2005-05-26 for method and apparatus for reducing noise in a roots-type blower.
This patent application is currently assigned to Pulmonetic Systems, Inc.. Invention is credited to DeVries, Douglas F., Williams, Malcolm.
Application Number | 20050112013 10/985528 |
Document ID | / |
Family ID | 46123835 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050112013 |
Kind Code |
A1 |
DeVries, Douglas F. ; et
al. |
May 26, 2005 |
Method and apparatus for reducing noise in a roots-type blower
Abstract
A Roots-type blower comprises a housing defining a rotor chamber
and an inlet and outlet to the rotor chamber. First and second
rotors are mounted in the rotor chamber, each rotor defining a
plurality of lobes, adjacent lobes and the housing cooperating to
define gas transport chambers. The blower is configured so that a
net flow rate of gas into a gas transport chamber is generally or
approximately constant, whereby a change in gas pressure in the gas
transport chamber is generally or approximately linear, as the gas
transport chamber approaches the outlet. In one embodiment, this is
accomplished by providing flow channels extending from the outlet
towards the inlet, and from the inlet towards the outlet,
corresponding to each rotor. The flow channels permit gas to flow
from the high pressure outlet to a gas transport chamber and from
the gas transport chamber to the low pressure inlet. The resulting
amelioration of pressure spikes associated with flow back
substantially reduces the operational noise level of the
blower.
Inventors: |
DeVries, Douglas F.;
(Kenmore, WA) ; Williams, Malcolm; (San Clemente,
CA) |
Correspondence
Address: |
THE HECKER LAW GROUP
1925 CENTURY PARK EAST
SUITE 2300
LOS ANGELES
CA
90067
US
|
Assignee: |
Pulmonetic Systems, Inc.
|
Family ID: |
46123835 |
Appl. No.: |
10/985528 |
Filed: |
November 10, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10985528 |
Nov 10, 2004 |
|
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10912747 |
Aug 4, 2004 |
|
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60492421 |
Aug 4, 2003 |
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Current U.S.
Class: |
418/206.1 |
Current CPC
Class: |
A61M 16/0069 20140204;
A61M 2205/583 20130101; A61M 2230/205 20130101; A61M 2205/3584
20130101; A61M 2016/1025 20130101; A61M 2205/70 20130101; F04C
18/126 20130101; A61M 16/12 20130101; A61M 11/00 20130101; A61M
2205/8262 20130101; A61M 16/0063 20140204; A61M 2016/0021 20130101;
A61M 2016/0036 20130101; A61M 2205/8206 20130101; A61M 2205/16
20130101; A61M 2205/42 20130101; A61M 2205/3553 20130101; A61M
2209/086 20130101; A61M 2205/3569 20130101; A61M 2205/52 20130101;
A61M 16/205 20140204; A61M 16/206 20140204; A61M 16/0057 20130101;
A61M 16/0066 20130101; A61M 2230/432 20130101; A61M 16/021
20170801; A61M 2205/8237 20130101; A61M 2205/3368 20130101; F04C
29/0035 20130101; A61M 2202/0208 20130101; A61M 2205/3317 20130101;
A61M 2205/505 20130101; A61M 2205/581 20130101; A61M 2205/3365
20130101 |
Class at
Publication: |
418/206.1 |
International
Class: |
F01C 001/18 |
Claims
What is claimed is:
1. A noise reducing configuration for a Roots-type blower
comprising: a housing defining a rotor chamber, said rotor chamber
comprising having an inlet and an outlet; a first and a second
rotor rotatably mounted in said chamber, each rotor defining a
plurality of lobes, adjacent lobes of each rotor cooperating with
said housing to define at one or more times gas transport chambers,
said rotors configured to move gas from said inlet via said gas
transport chamber to said outlet; and at least one outlet gas flow
channel extending from said outlet along an inner surface of said
housing at said rotor chamber in a direction opposite a direction
of rotation of said rotor, said at least one outlet gas flow
channel configured to permit gas to flow from said outlet into a
gas transport chamber as said lobes of said rotor rotate towards
said outlet, said at least one outlet gas flow channel configured
so that a pressure of said gas in said chamber as said chamber
moves towards said outlet changes at an approximately linear
rate.
2. The blower in accordance with claim 1 including at least one
outlet gas flow channel for each of said rotors, said outlet gas
flow channel having a first end and a second end, said second end
located at said outlet and said first end spaced therefrom in the
direction opposite said direction of rotation of said rotor.
3. The blower in accordance with claim 1 wherein each outlet gas
flow channel has a cross-sectional area which increases moving in
the direction of the first end to the second end thereof.
4. The blower in accordance with claim 1 wherein said outlet gas
flow channel has a cross-sectional area which increases
non-linearly moving in the direction of the first end of the second
end thereof.
5. The blower in accordance with claim 3 wherein said increase in
area is associated with at least an increase in a depth of said
channel.
6. The blower in accordance with claim 1 including at least one
inlet flow channel corresponding to at least one of said rotors,
said at least one inlet flow channel extending from said inlet
along an inner surface of said rotor chamber in an opposite
direction as the direction of rotation of said rotor, said inlet
flow channel configured to permit gas to flow from a chamber to
said inlet.
7. The blower in accordance with claim 1 wherein said Roots-type
blower comprises part of a mechanical ventilator.
8. The blower in accordance with claim 1 wherein said rate of
change of pressure of said gas varies from linearity by no more
than about 10%.
9. The blower in accordance with claim 1 wherein said rate of
change of pressure of said gas varies from linearity no more than
about 5%.
10. A noise reducing configuration for a Roots-type blower
comprising: a housing defining a rotor chamber, said rotor chamber
having an inlet and an outlet; a first and a second rotor rotatably
mounted in said chamber, each rotor defining a plurality of lobes,
adjacent lobes of each rotor cooperating with said housing to
define at one or more times gas transport chambers, said rotors
configured to move gas from said inlet via said gas transport
chamber to said outlet; and at least one outlet gas flow channel
extending from said outlet along an inner surface of said housing
in a direction opposite a direction of rotation of said rotor, said
at least one outlet gas flow channel configured to permit gas to
flow from said outlet into a gas transport chamber as said lobes of
said rotor rotate towards said outlet, said at least one outlet gas
flow channel configured so that a gas flow rate from said outlet
into said gas transport chamber is approximately constant.
11. The blower in accordance with claim 10 including at least one
outlet gas flow channel for each of said rotors, said outlet gas
flow channel having a first end and a second end, said second end
located at said outlet and said first end spaced therefrom in the
direction opposite said direction of rotation of said rotor.
12. The blower in accordance with claim 10 wherein each outlet gas
flow channel has a cross-sectional area which increases moving in
the direction of the first end to the second end thereof.
13. The blower in accordance with claim 10 wherein said outlet gas
flow channel has a cross-sectional area which increases
non-linearly moving in the direction of the first end of the second
end thereof.
14. The blower in accordance with claim 12 wherein said increase in
area is associated with at least an increase in a depth of said
channel.
15. The blower in accordance with claim 10 including at least one
inlet flow channel corresponding to at least one of said rotors,
said at least one inlet flow channel extending from said inlet
along an inner surface of said rotor chamber in an opposite
direction as the direction of rotation of said rotor, said inlet
flow channel configured to permit gas to flow from a chamber to
said inlet.
16. The blower in accordance with claim 10 wherein said Roots-type
blower comprises part of a mechanical ventilator.
17. The blower in accordance with claim 10 wherein said gas flow
rate changes by no more than about 10%.
18. The blower in accordance with claim 10 wherein said gas flow
rate changes by no more than about 5%.
19. A noise reducing configuration for a Roots-type blower
comprising: a housing defining a rotor chamber, said rotor chamber
having an inlet and an outlet; a first and a second rotor rotatably
mounted in said chamber, each rotor defining a plurality of lobes,
adjacent lobes of each rotor cooperating with said housing to
define at one or more times gas transport chambers, said rotors
configured to move gas from said inlet via said gas transport
chambers to said outlet; and at least one outlet gas flow channel
extending from said outlet along an inner surface of said housing
in a direction opposite to a direction of rotation of said rotor,
said at least one outlet gas flow channel configured to permit gas
to flow from said outlet into a gas transport chamber as said lobes
of said rotor rotate towards said outlet, said at least one outlet
gas flow channel defining a flow area which increases generally
non-linearly towards the direction of said outlet.
20. The blower in accordance with claim 19 wherein a width of said
at least one outlet gas flow channel is generally constant and a
depth of said at least one channel increases non-linearly towards
the direction of said outlet.
21. The blower in accordance with claim 19 including at least one
outlet gas flow channel for each of said rotors, said outlet gas
flow channel having a first end and a second end, said second end
located at said outlet and said first end spaced therefrom in the
direction opposite said direction of rotation of said rotor.
22. The blower in accordance with claim 19 wherein said increase in
area is associated with at least an increase in a depth of said
channel.
23. The blower in accordance with claim 19 including at least one
inlet flow channel corresponding to at least one of said rotors,
said at least one inlet flow channel extending from said inlet
along an inner surface of said rotor chamber in an opposite
direction as the direction of rotation of said rotor, said inlet
flow channel configured to permit gas to flow from a chamber to
said inlet.
24. The blower in accordance with claim 19 wherein said Roots-type
blower comprises part of a mechanical ventilator.
25. A noise reducing configuration for a Roots-type blower
comprising: a housing defining a rotor chamber, said rotor chamber
having an inlet and an outlet; a first and a second rotor rotatably
mounted in said chamber, each rotor defining a plurality of lobes,
adjacent lobes of each rotor cooperating with said housing to
define at one or more times gas transport chambers, said rotors
configured to move gas from said inlet via said gas transport
chambers to said outlet; at least one outlet gas flow channel
corresponding to said first rotor, said at least one outlet gas
flow channel extending from said outlet along an inner surface of
said housing in a direction opposite a direction of rotation of
said first rotor, said at least one outlet gas flow channel
configured to permit gas to flow from said outlet into a gas
transport chamber between two lobes of said first rotor as said
lobes of said first rotor rotate towards said outlet; at least one
outlet gas flow channel corresponding to said second rotor, said at
least one outlet gas flow channel extending from said outlet along
an inner surface of said housing in a direction opposite a
direction of rotation of second first rotor, said at least one
outlet gas flow channel configured to permit gas to flow from said
outlet into a gas transport chamber between two lobes of said
second rotor as said lobes of said second rotor rotate towards said
outlet; at least one inlet gas flow channel corresponding to said
first rotor, said at least one inlet gas flow channel extending
from said inlet along an inner surface of said housing at said
rotor chamber in a direction of rotation of said first rotor, said
at least one inlet gas flow channel configured to permit gas to
flow from said gas transport chamber between two lobes of said
first rotor back to said inlet as said lobes of said first rotor
rotate towards said outlet; and at least one inlet gas flow channel
corresponding to said second rotor, said at least one inlet gas
flow channel extending from said inlet along an inner surface of
said housing at said rotor chamber in a direction of rotation of
said second rotor, said at least one inlet gas flow channel
configured to permit gas to flow from said gas transport chamber
between two lobes of said second rotor back to said inlet as said
lobes of said second rotor rotate towards said outlet.
26. The blower in accordance with claim 25 wherein said inlet and
outlet gas flow channels corresponding to said first and second
rotors are configured such that a net rate of gas flow into said
gas transport chambers is approximately constant.
27. The blower in accordance with claim 25 wherein said inlet and
outlet gas flow channels corresponding to said first and second
rotors are configured to cause an approximately linear rate of
pressure change within said gas transport chambers.
28. The blower in accordance with claim 25 wherein said outlet gas
flow channels corresponding to said first and second rotors have a
cross-sectional area which increases generally non-linearly moving
in the direction of said outlet.
29. The blower in accordance with claim 28 wherein said outlet gas
flow channels corresponding to said first and second rotors have a
cross-sectional area which increases continuously moving in the
direction of said outlet.
30. The blower in accordance with claim 25 wherein said Roots-type
blower comprises part of a mechanical ventilator.
31. The blower in accordance with claim 26 wherein said gas flow
rate changes by no more than about 10%.
32. The blower in accordance with claim 26 wherein said gas flow
rate changes by no more than about 5%.
33. The blower in accordance with claim 27 wherein said rate of
change of pressure of said gas varies from linearity by no more
than about 10%.
34. The blower in accordance with claim 26 wherein said rate of
change of pressure of said gas varies from linearity changes by no
more than about 5%.
35. A method for configuring a gas flow path for providing a flow
of gas between a port of a Roots-type blower and a gas transport
chamber formed between lobes of at least one rotor of said blower,
comprising the steps of: selecting a length for said flow path;
selecting a desired gas transport chamber function that defines
desired values of a characteristic of gas in said gas transport
chamber as a function of rotor position; selecting an area function
that defines a cross-sectional area of said flow path along said
length of said flow path; calculating estimated values of said
characteristic of said gas in said gas transport chamber
corresponding to said area function; comparing said estimated
values to said desired values; repeating said steps of selecting an
area function, calculating estimated values, and comparing said
estimated values to said desired values until said estimated values
are approximately equal to said desired values.
36. The method of claim 35 wherein said length of said flow path
comprises a taper angle.
37. The method of claim 36 wherein said rotor position is
represented by a taper time.
38. The method of claim 35 wherein said characteristic of said gas
in said gas transport chamber comprises a pressure of said gas.
39. The method of claim 38 wherein said desired gas transport
chamber function comprises an approximately linear rate of change
in pressure of gas in said gas transport chamber.
40. The method of claim 35 wherein said characteristic of said gas
in said gas transport chamber comprises a flow rate of gas into
said gas transport chamber.
41. The method of claim 40 wherein said desired gas transport
chamber function comprises an approximately constant rate of gas
flow to said gas transport chamber.
42. The method of claim 35 wherein said area function comprises a
constant component and a variable component.
43. The method of claim 42 wherein said constant component
comprises a leakage area.
44. The method of claim 42 wherein said variable component
comprises a polynomial.
45. The method of claim 44 where said polynomial comprises a
polynomial of the form Et.sup.4+Ft.sup.7+Gt.sup.12 where "E," "F,"
and "G" are constants and wherein "t" is a normalized taper
time.
46. The method of claim 45 wherein E equals approximately 0.007
in..sup.2, F equals approximately 0.02 in..sup.2, and G equals
approximately 0.007 in..sup.2.
47. The method of claim 45 wherein E equals approximately 0.001
in..sup.2, F equals zero, and G equals approximately 0.001 in.
48. The method of claim 35 wherein said port comprises an outlet
port of said blower.
49. The method of claim 35 wherein said port comprises an inlet
port of said blower.
50. The method of claim 35 further comprising the step of
configuring a gas flow channel that corresponds to said area
function.
51. The method of claim 50 wherein said gas flow channel comprises
a generally constant width.
52. The method of claim 51 wherein a depth of said gas flow channel
increases along its length in a generally non-linear manner.
53. The method of claim 50 wherein said gas flow channel comprises
an outlet flow channel.
54. The method of claim 50 wherein said gas flow channel comprises
an inlet flow channel.
55. The method of claim 35 wherein said Roots-type blower comprises
part of a mechanical ventilator.
56. A method for configuring a gas flow path for providing a flow
of gas between a port of a Roots-type blower and a gas transport
chamber formed between lobes of at least one rotor of said blower,
comprising the steps of: selecting a length for said flow path;
selecting a desired gas transport chamber function that defines
desired values of a characteristic of gas in said gas transport
chamber as a function of rotor position; selecting an initial
incremental rotor position; calculating an initial desired
cross-sectional flow area corresponding to said gas transport
chamber function at said initial incremental rotor position;
selecting a succeeding incremental rotor position; calculating a
succeeding desired cross-sectional flow area corresponding to said
gas transport chamber function at said succeeding incremental rotor
position; repeating said steps of selecting a succeeding
incremental rotor position and calculating a succeeding desired
cross-sectional flow area for rotor positions traversing said
length of said flow path.
57. The method of claim 56 wherein said length of said flow path
comprises a taper angle.
58. The method of claim 57 wherein said rotor position is
represented by a taper time.
59. The method of claim 56 wherein said characteristic of said gas
in said gas transport chamber comprises a pressure of said gas.
60. The method of claim 59 wherein said desired gas transport
chamber function comprises an approximately linear rate of change
in pressure of gas in said gas transport chamber.
61. The method of claim 56 wherein said characteristic of said gas
in said gas transport chamber comprises a flow rate of gas into
said gas transport chamber.
62. The method of claim 61 wherein said desired gas transport
chamber function comprises an approximately constant rate of gas
flow to said gas transport chamber.
63. The method of claim 56 wherein said port comprises an outlet
port of said blower.
64. The method of claim 56 wherein said port comprises an inlet
port of said blower.
65. The method of claim 56 further comprising the step of
configuring a gas flow channel that corresponds to said desired
cross sectional flow areas.
66. The method of claim 65 wherein said gas flow channel comprises
a generally constant width.
67. The method of claim 66 wherein a depth of said gas flow channel
increases along its length in a generally non-linear manner.
68. The method of claim 65 wherein said gas flow channel comprises
an outlet flow channel.
69. The method of claim 65 wherein said gas flow channel comprises
an inlet flow channel.
70. The method of claim 56 wherein said Roots-type blower comprises
part of a mechanical ventilator.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This patent application claims the benefit of the filing
date of pending U.S. patent application Ser. No. 10/912,747, filed
Aug. 4, 2004, which claims the benefit of U.S. Provisional Patent
Application Ser. No. 60/492,421, filed Aug. 3, 2003.
FIELD OF THE INVENTION
[0002] The present invention relates to Roots-type blowers and,
more particularly, to a method and apparatus for reducing the noise
generated by such a blower.
BACKGROUND OF THE INVENTION
[0003] Roots-type blowers have potential application in a wide
variety of environments. They are relatively efficient, and can
produce a wide range of delivery pressures and volumes. However,
they produce a high level of noise. The high noise level produced
by Roots blowers has limited their use in environments where such
high noise levels are unacceptable. One such environment is
providing breathing assistance to patients by means of a mechanical
ventilator.
[0004] For a variety of reasons, there are instances when
individuals (patients) with acute and chronic respiratory distress
cannot ventilate themselves (i.e. breathe). In those circumstances,
such patients require breathing assistance to stay alive. One
solution is to provide those patients with a medical device called
a mechanical ventilator, which assists with their breathing.
[0005] A purpose of a mechanical ventilator is to reproduce the
body's normal breathing mechanism. Most mechanical ventilators
create positive intrapulmonary pressure to assist breathing.
Positive intrapulmonary pressure is created by delivering gas into
the patient's lungs so that positive pressure is created within the
alveoli (i.e. the final branches of the respiratory tree that act
as the primary gas exchange units of the lung). Thus, a mechanical
ventilator is essentially a device that generates a controlled flow
of gas (e.g., air or oxygen) into a patient's airways during an
inhalation phase, and allows gas to flow out of the lungs during an
exhalation phase.
[0006] Mechanical ventilators use various methods to facilitate
precise delivery of gas to the patient. Some ventilators use an
external source of pressurized gas. Other ventilators use gas
compressors to generate an internal source of pressurized gas.
[0007] Most ventilator systems that have an internal gas source use
either constant speed or variable speed compressors. Constant speed
compressors are usually continuously operating, rotary-based
machines that generate a fairly constant rate of gas flow for
ultimate delivery to the patient. These constant speed systems
generally use a downstream flow valve to control flow of the gas to
the patient, with a bypass or relief mechanism to divert excess
flow that is at any time not needed by the patient (e.g. during
exhalation).
[0008] Variable speed compressors operate by rapidly accelerating
from a rest state to the rotational speed needed to produce the
flow rate necessary during the beginning of the inhalation phase,
and then decelerating to a rest or nearly rest state at the end of
the inhalation phase to allow the patient to exhale.
[0009] Two types of variable speed compressor systems are typically
employed in the mechanical ventilator art: piston-based systems and
rotary-based systems. An example of a prior art variable speed
compressor system for use in a mechanical ventilator is described
in U.S. Pat. No. 5,868,133 to DeVries et al. This system uses drag
compressors to provide the desired inspiratory gas flow to the
patient.
[0010] Rotary compressor systems deliver the required gas flow
during inhalation by accelerating the compressor rotor(s) to the
desired speed at the beginning of each inspiratory phase and
decelerating the compressor rotor(s) to a rest or nearly rest speed
at the end of each inspiratory phase. Thus, the rotary compressor
is stopped, or rotated at a nominal base rotational speed, prior to
commencement of each inspiratory ventilation phase. Upon
commencement of an inspiratory phase, the rotary compressor is
accelerated to a greater rotational speed for delivering the
desired inspiratory gas flow to the patient. At the end of the
inspiratory phase, the rotational speed of the compressor is
decelerated to the base speed, or is stopped, until commencement of
the next inspiratory ventilation phase. Prior art systems typically
use a programmable controller to control the timing and rotational
speed of the compressor.
[0011] Great strides have been realized in reducing the size of
mechanical ventilators. Ventilators are now available that are
portable, and allow users a limited degree of autonomous mobility.
Further reducing the size and power requirements of mechanical
ventilators hold the potential of giving patients even greater
freedom of movement, enhancing their quality of life.
[0012] Because of its relative efficiency, a Roots blower can
potentially contribute to the reduction in size and power
consumption of mechanical ventilators. However, heretofore it has
not been possible to reduce the noise created by a Roots blower to
the level that is acceptable for a mechanical ventilator.
[0013] Roots blowers use a pair of interacting rotors. Each rotor
has two or more lobes. The rotors are rotated inside a housing
having an inlet and an outlet. The rotors rotate with the lobes of
one rotor moving into and out of the spaces between the lobes of
the other. Gas is moved through the blower in chambers formed by
adjacent lobes of a rotor and the adjacent rotor housing wall.
These chambers will be referred to herein as "gas transport
chambers."
[0014] Noise is generated by roots blowers in a number of ways. One
type of noise is caused by pulsing flow. As the rotors rotate, the
gas transport chambers between the lobes of each rotor are
sequentially exposed to the outlet. As each chamber is exposed to
the outlet, a lobe of the mating rotor rotates into the chamber,
displacing the gas in chamber to the outlet, causing a
flow/pressure pulse. In the case of a pair of rotors each having
two lobes, during each cycle of the blower, there are four pulses
generated by the displacement of gas by the gas transport chambers.
These pulses generate a substantial amount of noise.
[0015] A second type of noise is generated by a phenomenon known as
"flow back." As each rotor rotates, it inducts gas at low pressure
at the inlet. This gas is generally trapped in the gas transport
chambers as the rotor moves towards the outlet. When this pocket of
gas initially reaches the outlet, it is exposed to higher pressure
gas at the outlet. At that time, the higher pressure gas at the
outlet rushes backwardly into the gas transport chamber that
contains the lower pressure gas that is being delivered from the
inlet.
[0016] This reverse gas flow is very sudden. Its duration and
magnitude depends on a number of factors, including the rotational
speed of the rotors and the difference between the pressure of the
gas in the gas transport chamber (which is typically close to the
inlet pressure) and the pressure at the outlet. As a result of this
sudden reverse gas flow, a pressure spike of substantial amplitude
is generated. This pressure spike is generated multiple times per
cycle of the blower, each time a gas transport chamber is exposed
to the outlet. The resulting series of pressure spikes creates
continuous noise at a level that is unacceptable for many
applications, including mechanical ventilators.
[0017] FIGS. 1 and 2 are diagrams that illustrate the change in gas
flow back rate and associated change in gas pressure, respectively,
immediately after a gas transport chamber of a Roots blower in
accordance with the prior art is exposed to the outlet area. As
illustrated, gas flow rate changes very abruptly with time, as does
gas pressure.
[0018] Some attempts have been made to reduce the noise level of
Roots blowers. To reduce the "pulsing" type of noise, the lobes of
the rotors have been reconfigured so that they have a helical,
rather than straight, shape. When the lobes of the rotors are
straight, the gas flow into and out of the gas transport chamber is
very abrupt. When the lobes are helical in shape, each lobe
displaces gas over a larger angle of rotation. This spreads the
displacement of gas over an angle of rotation, lessening the
magnitude of the pressure pulse caused by the gas displacement, and
reducing the noise created by the blower. However, this lobe design
does not address the problem of flow back, since the relative
pressure between the gas at the outlet and gas being delivered from
the inlet is still the same.
[0019] Attempts have also been made to reduce flow back noise.
Various kinds of channels or passages have been provided that allow
some gas to flow from the outlet to the gas transport chamber prior
to the time the chamber reaches the outlet, thereby increasing the
gas pressure in the chamber and reducing the pressure spike that
occurs when the gas in the chamber is exposed to the higher outlet
pressure. Heretofore, however, it has not been possible to reduce
the noise of a Roots blower to the extent required for use in a
noise sensitive application such as a mechanical ventilator.
[0020] A Roots-type blower which is configured to generate less
noise is desired.
SUMMARY OF THE INVENTION
[0021] The invention is a method and apparatus for reducing the
noise generated by a Roots-type blower. The invention has
particular use in mechanical ventilators, though the advantages
thereof may be realized in many different noise-sensitive blower
applications.
[0022] The blower of the present invention comprises a housing
defining a rotor chamber and an inlet and outlet to the rotor
chamber. First and second rotors are mounted in the rotor chamber,
each rotor defining a plurality of spaced lobes. Adjacent lobes of
a rotor cooperate with the housing to define a series of generally
closed chambers that move from the inlet to the outlet as the
rotors are rotated. These chambers are referred to herein as "gas
transport chambers." In one or more embodiments, the blower is
configured with helical rotors as known in the art to reduce noise
caused by pulsing flow.
[0023] In addition, the blower is specially configured so that the
pressure within a gas transport chamber increases from the inlet
pressure to the outlet pressure in a generally or approximately
linear manner as the chamber approaches the outlet.
[0024] In one embodiment, the net flow rate of gas from the outlet
into the gas transport chamber is regulated to control pressure
change within the chamber. In one embodiment, a flow path from the
outlet to the gas transport chamber and/or from the gas transport
chamber to the inlet is provided. The flow path is configured such
that a net flow rate of gas from the outlet into the gas transport
chamber is generally or approximately constant during the time the
gas transport chamber approaches the outlet, such that the
resulting pressure change in the chamber is generally or
approximately linear.
[0025] In one or more embodiments, the flow path comprises at least
one outlet flow channel formed in the interior surface of the
housing. The outlet flow channel extends from a point before the
outlet (when considering the rotational direction of the rotor) to
the outlet. The flow channel is configured to permit gas to flow
from the outlet into a gas transport chamber while the gas
transport chamber is proceeding towards the outlet. In one
embodiment, the cross-sectional area of the outlet flow channel
increases non-linearly and continuously moving from its first end
towards the outlet to maintain an approximately constant flow rate
of gas into the gas transport chamber and/or an approximately
linear rate of change in pressure in the chamber. In one
embodiment, an outlet flow channel is provided corresponding to
each rotor.
[0026] In one embodiment, the flow path comprises at least one
inlet flow channel formed in the interior surface of the housing
extending from a point after the inlet (when considering the
rotational direction of the rotor) to the inlet. The inlet flow
channel is configured to permit gas to flow from a gas transport
chamber to the inlet as the gas pressure in the gas transport
chamber rises as a result of gas entering the gas transport chamber
from the outlet via the outlet flow channel. In one embodiment, an
inlet flow channel is provided corresponding to each rotor.
[0027] In one embodiment, both outlet and inlet flow channels are
provided. The flow channels at the inlet and outlet work
cooperatively to control the net flow of gas into the gas transport
chamber and thereby the rate of change of pressure in the gas
transport chamber.
[0028] One or more embodiments of the invention comprise a method
for determining the configuration of the flow path so as to achieve
a desired rate of pressure change in the gas transport chamber. In
one embodiment, an initial flow channel configuration is chosen,
and then a modeling process is used to determine the pressure
change over time within the gas transport chamber at desired
operating parameters (e.g. rotational speed, temperature, inlet and
outlet pressures) using known equations that govern compressible
gas flow. If the resulting pressure change over time does not match
the desired result, the flow channel configuration is adjusted and
the pressure change within the gas transport chamber is again
determined for the modified flow channel configuration. A number of
iterations may be performed until a satisfactory result is
achieved. In one embodiment, a desired result is an approximately
or generally linear change in the gas transport chamber pressure
over time.
[0029] Alternatively, in another embodiment, instead of starting
with an assumed flow channel configuration and adjusting it
iteratively until a satisfactory pressure profile is achieved, a
desired pressure profile may be used to directly analytically
and/or numerically calculate the flow channel profile that will
achieve that desired pressure profile.
[0030] Further objects, features, and advantages of the present
invention over the prior art will become apparent from the detailed
description of the invention which follows, when considered with
the attached figures.
DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a diagram illustrating a change in gas flow back
rate over time for a gas transport chamber of a Roots-type blower
in accordance with the prior art;
[0032] FIG. 2 is a diagram illustrating change in pressure over
time during flow back for the prior art blower illustrated in FIG.
1;
[0033] FIG. 3 is a perspective exploded view of a Roots-type blower
in accordance with an embodiment of the present invention;
[0034] FIG. 4 is a perspective end view of a housing of the blower
illustrated in FIG. 3;
[0035] FIG. 5 is an enlarged view of an outlet of the blower
housing illustrated in FIG. 4, as viewed from an interior of the
housing;
[0036] FIG. 6 is a first side view of the housing illustrated in
FIG. 3, illustrating an outlet of the blower housing;
[0037] FIG. 7 is a cross-sectional view of the housing illustrated
in FIG. 6 taken along line 7-7 therein;
[0038] FIG. 8 is an enlarged view of a flow channel an a portion of
the outlet of the blower housing illustrated in FIG. 7;
[0039] FIG. 9 is an enlarged view of a portion of the housing
illustrated in FIG. 7 taken in the direction of line 9-9
therein;
[0040] FIG. 10 is a cross-sectional view of a housing of a blower
in accordance with an embodiment of the invention in which inlet
and outlet flow channels are provided;
[0041] FIG. 11 is an enlarged view of an inlet flow channel of the
blower illustrated in FIG. 10;
[0042] FIG. 12 is a flow diagram illustrating steps of a first
method for determining the configuration of a flow path in
accordance with an embodiment of the invention;
[0043] FIG. 13 is a flow diagram illustrating steps of a second
method for determining the configuration of a flow path in
accordance with an embodiment of the invention;
[0044] FIG. 14 is a diagram illustrating changes in gas flow rate
over time in accordance with a blower configured in accordance with
the present invention; and
[0045] FIG. 15 is a diagram illustrating change gas pressure over
time in accordance with a blower configured in accordance with the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0046] The invention is a method and apparatus for reducing
Roots-type blower noise. In the following description, numerous
specific details are set forth in order to provide a more thorough
description of the present invention. It will be apparent, however,
to one skilled in the art, that the present invention may be
practiced without these specific details. In other instances,
well-known features have not been described in detail so as not to
obscure the invention.
[0047] In general, the invention comprises a Roots-type blower. The
blower includes two inter-meshing rotors which rotate within a
housing. The rotors draw gas from an inlet and deliver it through
the housing to the outlet. The rotors each have two or more lobes.
Adjacent lobes on each rotor, in combination with the housing,
define gas transport chambers that transport the gas from the inlet
to the outlet.
[0048] In one embodiment, the blower includes one or more flow
channels that define flow paths permitting gas to flow from the
outlet to the gas transport chambers. In one embodiment, one or
more of such outlet flow channels are provided corresponding to
each of the rotors.
[0049] In one embodiment, the blower additionally includes one or
more flow channels that define flow paths permitting gas to flow
from the gas transport chambers to the inlet. In one embodiment,
one or more of such inlet flow channels are provided corresponding
to each of the rotors. In one embodiment, flow channels are
provided at both the inlet and outlet.
[0050] In one or more embodiments, one or more flow channels are
configured to regulate the net rate of gas "flow back" into a gas
transport chamber such that the gas flow rate is generally or
approximately constant, and/or such that changes in gas pressure
within the chamber are generally or approximately linear. The
configuration of the invention thus reduces pressure spikes
associated with gas flow back, thus substantially reducing the
noise generated by the blower.
[0051] FIG. 3 shows a Roots-type blower 20 in accordance with an
embodiment of the invention. As illustrated, the blower 20
comprises a housing 22, a first rotor 24, and a second rotor
26.
[0052] As described in more detail below, the housing 22 may have a
variety of configurations. As illustrated, the housing 22 comprises
a casing defining a rotor chamber 28. As illustrated, the chamber
28 has the configuration of two intersecting cylinders.
[0053] In one embodiment, the casing is a walled structure. The
external shape of the casing may vary. In one embodiment, it is
generally cube-shaped. In that configuration, the housing 22 has a
first side 30 and a generally opposing second side 32. The housing
22 has a first end 34 and a generally opposing second end 36, and a
top 38 and a bottom 40.
[0054] In one embodiment, the chamber 28 has a longitudinal axis
which extends through the first and second ends 34,36 of the
housing 22. In one embodiment, the first end 34, of the housing is
open, while the second end 36 of the housing 22 is closed. This
permits the rotors 24,26 to be inserted into and removed from the
housing 22 via the first end 34.
[0055] In this embodiment, a first end plate 42 is used to close
the first end 34 of the housing 22. A cover plate 44 is located at
the second end 36 of the housing 22. As illustrated, the first end
plate 42 has recessed portions 51 and 53 for accepting bearings 55
and 57, respectively. The cover plate 44 has an inset or recessed
portion 46 for accepting gears 64 and 66 that are mounted on second
ends 60 and 62 of shafts 50 and 52, respectively, that protrude
through bores in second end 36 of housing 22 when rotors 24 and 26
are mounted on shafts 50 and 52 and inserted into chamber 28. In
the embodiment of FIG. 3, gears 64,66 are supported in a driving
relationship by bearings 63 and 65 mounted in appropriate insets or
reset portions in the second end 36 of the housing 22 (not shown)
that are similar to insets or reset portions 51 and 53 in first end
plate 42. In operation, gears 64 and 66 are protected and covered
by the cover plate 44, being located in the recess or inset 46
thereof. Gears 64 and 66 intermesh with one another and insure
that, in operation, rotors 24 and 26 rotate in proper relationship
to one another so that their respective lobes 70 mesh but do not
actually touch.
[0056] The first end plate 42 and the cover plate 44 are preferably
removably connectable to the housing 22. The first end plate 42 and
the cover plate 44 may be connected to the housing 22 using one or
more fasteners. In one embodiment, one or more pins 48 extend from
the first end 34 of the housing 22 for insertion into mating
apertures in the first end plate 42. These pins 48 aid in
maintaining the first end plate 42 in aligned connection with the
housing 22. One or more threaded fasteners 50 are extended through
the first end plate 42 into engagement with the housing 22, thereby
connecting or fastening the first end plate 42 to the housing 22.
The cover plate 44 is preferably similarly connected to the housing
22.
[0057] In the embodiment illustrated, the first rotor 24 is mounted
on a first shaft 52 and the second rotor 26 is mounted on a second
shaft 54. Alternatively, the rotors 24 and 26 may be integrally
formed with shafts 52 and 54, respectively. In the embodiment
illustrated, a first end 56 of the first shaft 52 extends through
the first end plate 42. Means are provided for driving the first
shaft 52. This means may comprise, for example, a brushless DC
electric motor. One embodiment of such an electric motor is
described in U.S. patent application Ser. No. 10/847,693 filed May
18, 2004, owned by the same assignee hereof. Of course, the means
for driving the first shaft 52 may comprise a variety of elements.
Further, the means by which the first shaft 52 is driven by that
means may vary, such as by direct drive or indirect drive.
[0058] In one embodiment, a first end 58 of the second shaft 54 is
supported for rotation by the first end plate 42. This may be
accomplished by bearing 57 or similar means.
[0059] It will be appreciated that the rotors 24,26 may be driven
by means of a variety of other drive configurations than as
specifically illustrated and described above. For example, each of
rotors 24,26 may be independently driven by separate but
synchronized electric motors, or the rotors could be arranged in a
driving relationship with one another in other fashions.
[0060] In one or more embodiments, except for an inlet and an
outlet, the housing 22 is generally sealed so that gas enters and
exits only through the inlet and outlet. In the embodiment
illustrated, the first and second ends 34,36 of the housing 22 are
thus sealed. Thus, in the embodiment illustrated, the first end
plate 42 is sealed to the housing 22. Any of a variety of seals,
bushings and the like, as known in the art, may be used to seal the
connection of the first end plate 42 to the housing 22 and various
of the other component connections, such as at the interface of
shafts and the housing 22 and the first end plate 42.
[0061] Of course, the housing 22 may have a variety of other
configurations, and other approaches and/or components for sealing
the housing can be used. For example, the second end 36 of the
housing 22 might also be open and then closed or covered with and
end plate, or the first end 34 of the housing 22 might be closed
and the second end 36 might be open.
[0062] The first and second rotors 24,26 preferably have two or
more lobes 70. In a preferred embodiment, each rotor 24,26 has
three lobes 70. The rotors 24,26 could have, however, as few as two
and as many as four or more lobes. The lobes 70 preferably follow a
helical path around the axis of shaft 52 or 54, respectively. In
one embodiment, first and second ends of each lobe 70 are offset
from one another by about sixty degrees (60.degree.) radially about
the rotor/shaft.
[0063] The lobes 70 extend outwardly from a center of each rotor
24,26. A space is defined between adjacent lobes 70. As is known,
the lobes 70 of one rotor 24,26 are configured to mesh or engaged
with the other rotor by entering the space defined between adjacent
lobes of the other rotor. When the rotors 24,26 are mounted in the
housing 22, the adjacent lobes 70 of each rotor 24,26, in
cooperation with the interior of the housing 22, as shown in FIG.
10, define gas transport chambers 103 configured to transport gas
from the inlet to the outlet.
[0064] As illustrated in FIG. 3, the first and second rotors 24,26
are mounted in the rotor chamber 28. The rotors 24,26 are mounted
so that their shafts 52,54 extend parallel to one another and
perpendicular to the first and second ends 34,36 of the housing
22.
[0065] The blower 20 has an inlet through which gas is drawn and an
outlet through which gas is expelled. As illustrated in FIG. 4, an
inlet 80 is located in the first side 30 of the housing 22. Gas is
delivered to the inlet 80 of the housing 22. In one embodiment, the
inlet 80 may open directly to the atmosphere. In another
embodiment, a gas delivery path, such as a gas delivery tube, may
define a gas flow path to the inlet 80.
[0066] An outlet 82 is formed in the second side 32 of the housing
22. As described in more detail below, gas is delivered by the
rotors 24,26 from the inlet 80 to the outlet 82. The gas which is
delivered to the outlet 82 by the gas transport chambers is
expelled from the housing 22 through the outlet 82.
[0067] In one or more embodiments, the inlet 80 and outlet 82 are
generally triangular in shape. This configuration is particularly
applicable when the rotors 24,26 have helical lobes. In particular,
when the lobes of the rotors 24,26 are helical, the respective tops
of the inlet 80 and outlet 82 preferably slope downwardly at a
similar angle as the lobes of the top rotor 24, and the respective
bottoms of the inlet 80 and outlet 82 preferably slope upwardly at
a similar angle as the lobes of the bottom rotor 26.
[0068] Of course, the configuration of the inlet 80 and outlet 82
may vary, particularly when the configuration of the rotors 24,26
varies. For example, if the rotors 24,26 have straight lobes, then
the inlet 80 and outlet 82 might be rectangular in shape.
[0069] As described to this point, the rotors 24,26 are rotated in
the housing 22 by a drive element such as a brushless DC motor. As
the rotors 24,26 rotate, they deliver gas from the inlet 80 to the
outlet 82. Gas is delivered in the gas transport chambers 103
situated between a "front" lobe and a "rear" lobe as shown in FIG.
10. As the "front" lobe of a chamber passes the inlet, the gas
transport chamber is exposed to the inlet and filled with gas at
the inlet pressure. As the rotor continues to rotate, the "rear"
lobe passes the inlet. At this time, the gas transport chamber is
generally enclosed by the front and rear lobes and the interior
surface of the housing, and is not directly exposed to either the
inlet or outlet.
[0070] As the rotor continues to rotate, the leading or front lobe
reaches the outlet. As the gas transport chamber is exposed to the
outlet, gas initially rushes from the (higher pressure) outlet into
the gas transport chamber. This "flow back" creates an undesirable
pressure spike. As the rotor continues to rotate, the corresponding
lobe of the second rotor rotates into the gas transport chamber,
displacing the gas therein to the outlet, thereby delivering the
gas from the inlet to the outlet. The rotor then rotates farther,
with the gas transport chamber eventually rotating back to the
inlet.
[0071] In accordance with the invention, the blower 20 is
configured so that the rate of the gas flow back into the gas
transport chamber is generally or approximately constant, and/or so
that the rate of change of gas pressure in the gas transport
chamber is generally linear as the gas transport chamber approaches
the outlet. In one or more embodiments this is accomplished with
one or more gas flow channels incorporated in the rotor housing. In
one embodiment of the invention, flow back is controlled using one
or more gas flow channels that extend from the outlet to the
interior of the housing and, in a preferred embodiment, with one or
more gas flow channels leading from the interior of the housing to
the inlet.
[0072] As best illustrated in FIGS. 4 and 5, in one embodiment, a
gas flow channel is provided corresponding to each rotor 24,26, the
flow channel extending from the outlet 82 into the rotor chamber 28
of the housing 22 (such a flow channel is referred to herein as an
"outlet" flow channel). In one embodiment, a first outlet flow
channel 90a extends from the outlet 82 in a first circumferential
direction, that channel cooperating with the first rotor 24, and a
second outlet flow channel 90b extends from the outlet 82 in a
second circumferential direction, that channel cooperating with the
second rotor 26. One of the outlet flow channels 90a will now be
described in greater detail, it being understood that in a
preferred embodiment, the channel 90b corresponding to the other
rotor is similar.
[0073] As described in detail above, the rotors 24,26 are
configured to rotate within the rotor chamber 28 defined by the
housing 22. In the embodiment illustrated in FIGS. 4 and 5, one
rotor 24 is mounted above the other rotor 26. In this embodiment,
the top rotor 24 rotates in a clock-wise direction in the view of
the housing 22 illustrated in FIGS. 4 and 5. In other words, when
the rotor 24 is mounted in the housing 22, each lobe 70 thereof is
moving downwardly as it sweeps towards the outlet 82.
[0074] Referring still to FIGS. 4 and 5, the outlet flow channel
90a begins at a point along the interior of the housing 22 before
the outlet 82, when considering the rotational direction of the
lobes 70 of the rotor 24. As illustrated in FIG. 5, the outlet flow
channel 90a has a first end 92, a second end 94, and a pair of
sides 96,98. The second end 94 of the outlet flow channel 90a
preferably terminates at the outlet 82. The first end 92 is, as
described, located along the housing 22 at a point before the
outlet 82 when considering the rotational direction of the lobes
70.
[0075] In one or more embodiments of the invention, the rotors
24,26 of the blower 20 have a maximum exterior dimension. The
outer-most dimension is defined by the tips, or "lands," of the
lobes 70. The rotor chamber 28 is generally configured so that the
rotors 24,26 fit within the chamber 28 in close tolerance. In one
embodiment, this tolerance results in a clearance spacing between
the interior surface of the housing 22 and the lobes 70 of the
rotors 24,26 of about 0.003 inches. The actual spacing may vary. As
will become more apparent below, the size and configuration of the
flow channels may vary depending upon this clearance spacing.
[0076] In one or more embodiments, the depth of the outlet flow
channel 90a increases moving in a circumferential direction from
the first end 92 towards the second end 94. The sides or side walls
96,98 of the channel 90a extend along the length of the outlet flow
channel 90a from its first end 92 to its second end 94. The width
of the outlet flow channel 90a is defined by the distance between
the side walls 96,98. In one or more embodiments, the depth of the
outlet flow channel 90a increases in a generally continuous,
non-linear manner, while the width stays the same.
[0077] The depth of the outlet flow channel 90a corresponds to the
height of the side walls 96,98 at a particular location along their
length. As illustrated, each side wall 96,98 extends upwardly from
a bottom surface 100. Each side wall 96,98 terminates at the
interior surface of the housing 22. In the embodiment of FIG. 5,
because of the roughly triangular shape of outlet 82, side wall 98
is shorter in length than side wall 96.
[0078] As described above, outlet flow channel 90a is configured to
permit gas to flow from the outlet 82 into a gas transport chamber
as that gas transport chamber approaches the outlet 82. The
configuration of the outlet flow channel 90a, including its size
and shape, is preferably selected so that the rate of gas flow into
the gas transport chamber as the gas transport chamber approaches
the outlet 82 produces an approximately linear increase in pressure
in the gas transport chamber over time. The rate of change in gas
pressure within the gas transport chamber is generally related to
the rate of gas flow into the gas transport chamber. The rate of
gas flow into the gas transport chamber is generally related to the
pressure difference between the outlet and the gas transport
chamber, and the cross-sectional area of the outlet flow channel
90a at the point at which "front" lobe 70 of that gas transport
chamber is located at any point in time. That cross-sectional area
is a generally quadrilateral-shaped area formed on one side by the
radially outward edge of lobe 70 and on the other three sides by
the three sides of outlet flow channel 90a. In one embodiment, it
has been found that increasing the area of the outlet flow channel
90a continuously and non-linearly achieves a generally or
approximately constant gas flow rate into the gas transport
chamber, and thus an associated generally or approximately linear
rate of change of pressure, within the gas transport chamber. In
particular, the cross-sectional area of the outlet flow channel 90a
preferably increases continuously and non-linearly moving from the
first end 92 of the channel 90a towards the outlet 82.
[0079] In operation, as a gas transport chamber of the rotor 24
passes the inlet 80, it is filled with gas at the ambient pressure
at inlet 80. The ambient pressure at inlet 80 is generally lower
than the outlet pressure at outlet 82. As the rotor 24 rotates and
the gas transport chamber reaches the first end 92 of the channel
90a, higher pressure gas from the outlet 82 begins to flow into the
gas transport chamber. At this time, the pressure difference
between the gas at the outlet and the gas in the gas transport
chamber is at its maximum value. Because the gas flow rate into the
gas transport chamber is dependent on this pressure difference, to
achieve a generally linear increase in pressure over time, the size
of the channel 90a at end 92 is at a minimum.
[0080] As the rotor 24 continues to rotate towards the outlet 82,
the pressure of the gas in the gas transport chamber begins to rise
due to the flow of gas though channel 90a into the gas transport
chamber. Because the pressure difference between the gas transport
chamber and outlet 82 drops, the size of the channel 90a is
increased to provide a larger cross-sectional flow area to maintain
an approximately constant gas flow rate into the gas transport
chamber, and thus achieve an approximately linear increase in
pressure in the gas transport chamber.
[0081] Eventually, the front lobe reaches the outlet 82 and the gas
transport chamber is directly exposed to the outlet 82. Because the
pressure of the gas in the gas transport chamber and at the outlet
have already substantially equalized, there is no abrupt pressure
change, and noise is substantially reduced.
[0082] As the rotor 24 continues to rotate, a mating lobe 70 of the
other rotor 26 begins to fill the gas transport chamber, displacing
the gas therein out to the outlet 82.
[0083] Operation of the blower 20 with respect to the other rotor
26 is similar, with gas permitted to flow back from the outlet 82
into a gas transport chamber between lobes 70 of the rotor 26 via
the outlet flow channel 90b.
[0084] In one or more embodiments of the invention, and as
illustrated in FIGS. 10-11, the net flow rate of gas into the gas
transport chambers and the resulting pressure changes are
preferably further controlled by providing one or more inlet flow
channels 102a,b. These inlet flow channels 102a,b define a flow
path permitting gas within the gas transport chambers of the rotors
24,26 to flow back towards the inlet 80. As described above,
appropriately configured outlet flow channels 90a,b are effective
in creating a generally or approximately constant gas flow rate
into a gas transport chamber as the gas transport chamber
approaches the outlet 82, thus producing a generally or
approximately linear change in gas pressure in the gas transport
chamber. It has been determined, however, that when the flow path
from the gas transport chamber to the inlet is also provided, the
ability to control the net flow rate into the gas transport
chambers, and thus the change in gas pressure in the gas transport
chambers, may be further enhanced.
[0085] As such, in a preferred embodiment of the invention, flow or
relief channels or passages 102a,b similar to the outlet flow
channels 90a,b described above, are located at the inlet 80 (and
are thus referred to herein as "inlet" flow channels). Preferably,
an inlet flow channel 102a,b is provided in the housing 22
corresponding to each rotor 24,26. Inlet flow channels 102a,b are
used to control the rate at which gas flows back or "leaks" from a
gas transport chamber to the inlet.
[0086] To permit the rotors 24,26 to rotate within the housing 22,
there must be some clearance between the "land," or outermost
portion, of each lobe, and the adjacent housing wall. This small
clearance results in leakage from the outlet into the gas transport
chamber (via the clearance area between the "front" lobe of the gas
transport chamber and the housing wall) and from the gas transport
chamber back to the inlet (via the clearance area between the
"back" lobe of the gas transport chamber and the housing wall). It
will thus be appreciated that selection of a particular lobe
clearance has an effect on the net flow of gas into the gas
transport chamber.
[0087] In one embodiment, the configuration of the inlet flow
channels 102a,b is selected, in conjunction with the outlet flow
channels 90a,b at the outlet 80 and the inherent leakage resulting
from the lobe clearance and, so that the net gas flow rate into the
gas transport chamber is generally or approximately constant and/or
the change in gas pressure in the gas transport chamber is
generally or approximately linear as the gas transport chamber
approaches outlet 82. The inlet flow channels 102a,b may have a
variety of configurations. In one embodiment, the inlet flow
channels 102a,b have a similar configuration to the outlet flow
channels 90a,b. Specific methods for determining the configuration
of the inlet and outlet flow channels are described in greater
detail below.
[0088] A variety of variations of the invention are contemplated.
One or more outlet and/or inlet flow channels or passages are
preferably provided for both rotors. It is possible, however, to
provide flow channels or passages for only one rotor.
[0089] As described, the flow channels or passages are preferably
configured to result in a generally or approximately constant gas
flow rate into a gas transport chamber, and thus to create a
generally or approximately linear change in pressure in the gas
transport chambers as the gas transport chambers approach outlet
82. The terms "generally" or "approximately" contemplate some
deviation from an exact achievement of the desired goal. In one
embodiment, the results achieved deviate by no more than about 30%,
preferably no more than about 20%, and most preferably no more than
about 5%-10% from the desired results.
[0090] FIGS. 14 and 15 illustrate the net flow rate of gas into a
gas transport chamber and the resulting change in pressure, over
time, of one embodiment of a blower in accordance with the present
invention. As illustrated in FIG. 14, the flow rate is generally
and approximately constant over a period of time t. As illustrated
in FIG. 15, the resulting change in pressure is generally or
approximately linear.
[0091] One or more embodiments of the invention comprise methods of
determining the configuration of the flow channels to generally or
approximately achieve the desired flow/pressure characteristics.
One embodiment of the invention is a method for determining the
change in pressure in a gas transport chamber versus time based
upon a number of variables, including an assumed flow channel
profile. The method may be performed iteratively. For example, the
assumed flow channel profile may be varied until a satisfactory
pressure change profile is achieved.
[0092] In one embodiment, an iterative method of determining the
configuration of outlet and inlet flow channels of the blower is
performed by modeling the blower on a computing device. In a
preferred embodiment, modeling is performed using Vis Sim software
available from Visual Solutions, Incorporated of Westford, Mass.,
USA. The method could, however, be done manually or using other
appropriate software. The method could also be accomplished
physically by building models and measuring data from use of the
models.
[0093] As described in more detail below, in one method of the
invention a variety of assumed or selected parameters or variables
relating to or associated with the configuration of a blower (such
as size/shape of the flow channels and operating parameters such
inlet/outlet pressures, temperatures, delivery rates, and rotor
speed) are used to calculate changes in pressure in the gas
transport chambers over time, or to calculate other values of
characteristics of the gas in the gas transport chamber, such as,
for example, a flow rate of gas into the chamber.
[0094] In accordance with one method of the invention, the selected
parameters and/or variables are utilized to calculate or determine
changes in pressure over time in the gas transport chambers of the
blower as the rotors of the blower rotate based on compressible
flow equations, as are known in the art. Steps in accordance with
one embodiment of a method of invention are illustrated in FIG. 12
and are described in more detail below.
[0095] In a first step S1, the length of time that it takes for a
lobe of a rotor to traverse the angle ("taper angle") over which
the desired pressure compensation of the gas transport chamber is
to be accomplished is determined. This time is referred to herein
as the "taper time." The taper time depends on the taper angle and
the rotational speed of the rotor. In embodiments in which both
outlet and inlet flow channels are used, there may be separate
taper angles, and therefore taper times, for the outlet and inlet
flow channels, respectively. For example, in the embodiment of FIG.
10, taper angle 180 is applicable to the outlet flow channel 90b,
while taper angle 190 is applicable to inlet flow channel 102b.
Alternatively, a single taper angle may be applicable to both the
outlet and inlet flow channels.
[0096] Thus, in one embodiment, the "taper time" is determined from
an assumed operating rotational speed (measured in rpm) of the
rotors and the applicable taper angle. In one embodiment, the taper
time is calculated from the taper angle and the rotational speed of
the rotor as follows:
Taper time=(1/(rpm/60))*(taper angle/360)
[0097] If the blower is intended to be used over a range of
rotational speeds, there will be a different taper time applicable
to each rotational speed. The method may be performed at a variety
of operating speeds within the operating range to select a flow
channel profile that provides the most satisfactory pressure change
profiles over the operating range.
[0098] After the taper time is determined, at step S2, the inlet
and outlet cross-sectional flow areas (also referred to herein as
"inlet orifice area" and "outlet orifice area," respectively) as a
function of rotor position are determined using an assumed flow
channel profile. The orifice areas may alternatively be represented
as functions of the normalized taper time instead of as functions
of rotor position. That is, they may be represented as functions of
"t", where "t" equals the period of time from the time at which a
lobe of a gas transport chamber begins to traverse the taper angle,
divided by the taper time. For example, "t" will equal zero (0)
when the lobe is at the beginning the taper angle, and "t" equals
one (1) when the lobe reaches the end of the taper angle.
[0099] As indicated above, the total orifice area through which gas
flows from the outlet to the gas transport chamber (in the case of
an outlet flow channel) or from the gas transport chamber to the
inlet (in the case of an inlet flow channel) at any point in time
as the rotor rotates is the sum of the cross-sectional area of the
flow channel at the point at which the tip (or "land") of the
applicable rotor lobe is located at that time plus the
cross-sectional area of the clearance gap between the lobe and the
housing (the "leakage area").
[0100] In one embodiment, the depth of each flow channel varies
along its length, getting deeper closer to the outlet (in the case
of the outlet flow channel) or to the inlet (in the case of the
inlet flow channel), while its width remains fixed. As part of one
embodiment of the iterative method of the invention, an initial
area profile of each flow channel is assumed, and then the
resulting rate of change of pressure is calculated. Adjustments to
the assumed profile are made, and then the rate of change of
pressure is again calculated. This iterative process is followed
until a satisfactory rate of change of pressure over the desired
operating range of the blower is achieved.
[0101] In one or more embodiments, profiles for the areas of the
outlet and inlet flow channels are assumed to be in the form of
higher order polynomials. In one or more embodiments, the total
outlet flow orifice area (including the leakage area) is assumed to
have the form:
A.sub.O(t)=Et.sup.4+Ft.sup.7+ Gt.sup.12+L
[0102] In the above equation, "A.sub.O(t)" is the cross-sectional
area of the total outlet orifice area (the sum of the outlet flow
channel cross-sectional area and the leakage area) as a function of
the normalized taper time "t" (that varies from 0 to 1). E, F and G
are constants, and L is the leakage area. In one embodiment, values
of 0.007 in., 0.02 in..sup.2, and in..sup.2 are selected as values
of constants E, F and G respectively.
[0103] In one or more embodiments, the width of the outlet flow
channel is assumed to be fixed, and the depth of the outlet flow
channel at any location along the outlet flow channel will be equal
to the outlet flow channel cross-sectional area divided by that
width.
[0104] In one or more embodiments, the outlet flow channel
cross-sectional area is equal to the total outlet orifice area
minus the leakage area:
A.sub.OC(t)=A.sub.O(t)-L=Et.sup.4+Ft.sup.7+Gt.sup.12
[0105] Thus, the depth of the outlet flow channel as a function of
the normalized taper time has the form:
D.sub.O(t)=A.sub.OC(t)/W.sub.O-=(Et.sup.4+Ft.sup.7+Gt.sup.12)/W.sub.O
[0106] Where D.sub.O(t) is the outlet flow channel depth as a
function of "t," "A.sub.OC(t)" is the outlet flow channel area as a
function of "t", "W.sub.O" is the outlet flow channel width, and E,
F and G are constants. In one or more embodiments, "W.sub.O" is the
width of the outlet flow channel as measured across the land of a
lobe as it traverses the outlet flow channel.
[0107] In one or more embodiments, the total inlet flow orifice
area (inlet flow channel cross-sectional area plus leakage area) is
assumed to have the form:
A.sub.I(t)=H(1-t).sup.4+I(1-t).sup.7+J(1-t).sup.12+L
[0108] In the above equation, "A.sub.I(t)" is the total inlet flow
orifice area as a function of the normalized taper time "t" (that
varies from 0 to 1), and H, I, and J are constants. In one
embodiment, values of 0.001 in..sup.2, 0, and 0.001 in..sup.2 are
selected as values of constants H, I, and J, respectively.
[0109] In one or more embodiments, the width of the inlet flow
channel is assumed to be fixed, and the depth of the inlet flow
channel at any location along the inlet flow channel will be equal
to the inlet flow channel cross-sectional area divided by that
width.
[0110] In one or more embodiments, the inlet flow channel
cross-sectional area is equal to the total inlet orifice area minus
the leakage area:
A.sub.IC(t)=A.sub.I(t)-L=Ht.sup.4+It.sup.7+Jt.sup.12
[0111] Thus, the depth of the inlet flow channel as a function of
the normalized taper time has the form:
D.sub.I(t)=A.sub.IC(t)/W.sub.I-=(Ht.sup.4+It.sup.7+Jt.sup.12)/W.sub.I
[0112] Where D.sub.I(t) is the inlet flow channel depth as a
function of "t," "A.sub.IC(t)" is the inlet flow channel area as a
function of "t", "W.sub.I" is the inlet flow channel width, and H,
I and J are constants. In one or more embodiments, "W.sub.I" is the
width of the inlte flow channel as measured across the land of a
lobe as it traverses the inlet flow channel.
[0113] In a third step S3, the flow rate of gas through the inlet
and outlet orifice areas is determined as a function of time "t"
based on the size of the orifice areas and pressure differences
between the gas transport chamber and the inlet and outlet. In
particular, the flow rate of gas from the outlet through the outlet
orifice area back into the gas transport chamber ("Q.sub.In"), and
the flow rate of gas out of the gas transport chamber through the
inlet orifice area back towards the inlet ("Q.sub.Out") are
determined. In one or more embodiments, Q.sub.In and Q.sub.Out are
determined using compressible gas flow equations as are known in
the art. For example, the equations set forth in J. D. Anderson,
The Analysis & Design of Pneumatic Systems, published by
Krieger Publishing Co. may be used. The total net flow rate into
the gas transport chamber is the difference between these two flow
rates.
Q=Q.sub.In-Q.sub.Out
[0114] In a step S4, the gas transport chamber pressure as a
function of the normalized taper time is determined from the net
flow of gas into the chamber and dead space compliance of the
chamber. In this manner, it can be determined whether the pressure
varies undesirably. The pressure may be analytically or numerically
determined using well known principals and equations governing gas
flow though orifices and into chambers. In one or more embodiments,
a graph may be generated, the graph indicating pressure with
respect to time.
[0115] In one or more embodiments, the pressure P in the gas
transport chamber is calculated using the following equation: 1 P (
t ) = 0 t [ Q / 60 ] t .times. 1 / C
[0116] where P is the gas transport chamber pressure, Q is the net
flow rate into the gas transport chamber, and C is the dead space
compliance of the gas transport chamber. In one embodiment, P is in
cm H.sub.2O, Q is in liters per minute and C=0.00000167 liter/cm
H.sub.2O.
[0117] As indicated, in one application of the method of the
invention, it is desired that the change in pressure of gas in a
gas transport chamber is generally or approximately linear as the
gas transport chamber approaches the outlet. At step S5, a
determination is made as to whether the rate of change in pressure
determined at step S4 is sufficiently linear for the purposes for
which the blower is to be used. If it is determined at step S5 that
the rate of pressure change is not as linear as desired, then the
outlet and/or inlet area function(s) may be modified at step S6
(e.g., by changing the flow channel depth profile by changing
coefficients, taper angle or form of the function) to attempt to
formulate an area function that will result in a more linear rate
of pressure change. Steps S2 through S5 may then be repeated to
determine whether the modified area function achieves a more linear
result. Once it is determined at step S5 that the result is
satisfactory, the area function and/or flow channel profile that
produces that result is utilized in fabrication of the blower at
step S7.
[0118] In one or more embodiments, a characteristic of the gas in
the gas transport chamber other than the pressure may be of
interest. For example, instead of having a desired relationship
between pressure and taper time, a desired relationship may be
specified between the rate of flow of gas into the gas transport
chamber and the taper time. This desired relationship between the
values of a characteristic of the gas in the gas transport chamber
(e.g. pressure or flow rate) may be referred to as a "desired gas
transport chamber function." Whatever the desired gas transport
chamber function is, the iterative method of the invention may be
performed until the difference between the estimated or projected
values for the characteristic in question (calculated using the
assumed area function) and the desired gas chamber function are
satisfactory.
[0119] In accordance with the embodiment of the method as
described, in one or more embodiments, both inlet and outlet flow
channels are used to achieve the desired relationship (e.g.
generally or approximately linear pressure change in a gas
transport chamber or generally or approximately constant flow rate
of gas into the gas transport chamber). Alternatively, the method
can be performed using only an inlet flow channel or outlet flow
channel.
[0120] In one application of the method of FIG. 12, for a roots
blower having rotor dimensions of 1.0 in. length and 0.88 in.
diameter and operating at an inlet pressure of zero gauge pressure
and an outlet pressure of 40 cm H.sub.2O and over a range of
rotational speeds of 1000 to 12,000 RPM, with a leakage area of
0.0017 in..sup.2 and for an outlet taper angle of 60 degrees and an
inlet taper angle of 60 degrees, the following flow channel area
functions (in in..sup.2) were determined to provide a satisfactory
approximately linear pressure rise in a gas transport chamber:
A.sub.OC(t)=0.007t.sup.4+0.02t.sup.7+0.007t.sup.12
A.sub.IC(t)=0.001t.sup.4+0.001t.sup.12
[0121] Selecting widths of the outlet and inlet flow channels of
0.375 in. and 0.10 in., respectively, the resulting depth profiles
for the outlet and inlet flow channels in inches are:
D.sub.O(t)=0.0187t.sup.4+0.0533t.sup.7+0.0187t.sup.12
D.sub.I(t)=0.01t.sup.4+0.01t.sup.12
[0122] Another embodiment of the invention is an
analytical/numerical method of determining the desired
configuration of the blower. This embodiment will be described with
reference to FIG. 13. In this embodiment, instead of assuming an
area function for the outlet and/or inlet flow channels, the
desired pressure function is used to analytically and/or
numerically calculate the area function that will achieve that
desired pressure function. The flow channel dimensions are then
selected to achieve the required area function. In one embodiment,
only an outlet flow channel is utilized. In one embodiment, the
required orifice area is calculated at discrete intervals during
the taper time, and the area function is derived from the resulting
discrete orifice area values.
[0123] In a first step S1, the "taper time" is calculated. As
indicated above, the "taper time" is the time that it takes for a
rotor lobe to rotate through the selected taper angle. This time
may be calculated in similar manner to that described above with
respect to the previously described method. A desired iteration
time interval and a desired pressure change profile are also
selected. For example, a desired iteration time interval may be
expressed as a fraction of the taper time, for example, the
iteration time interval may be selected to be {fraction
(1/2000)}.sup.th of the taper time.
[0124] In a step S2, the desired flow rates at the outlet orifice
areas at a particular normalized time "t" is calculated from the
desired rate of change of pressure at that time as specified by the
desired pressure change profile. During the first iteration, "t"
will be equal to the selected iteration time interval divided by
the taper time. During each successive iteration, "t" is
incremented by the iteration time interval divided by the taper
time. The iterations will continue until "t" equals one (1).
[0125] In a step S3, the outlet orifice area required to achieve
the desired flow at the current time "t" is calculated using
compressible gas flow equations as are known in the art.
[0126] At step S4, the value of t is incremented by the iteration
time interval, and the process returns to step S2. Steps S2 to S4
are repeated until the value of t reaches 1. The outlet orifice
areas calculated for each time t may then be used to construct a
plot of desired orifice area versus time, which, turn, can be used
to select a outlet flow channel profile (after taking account the
clearance leakage area's contribution to the total outlet orifice
area).
[0127] If a blower according to the present invention is intended
to operate over a range of rotational speeds and/or outlet and
inlet pressures, the methods of FIG. 12 or 13 may be performed at a
number of points within the intended operating range, and an
average area function/depth profile may be selected from the area
functions/depth profiles determined at each of the operating
points, or the area function/depth profile that produces the most
satisfactory results over the range of operating points may be
selected.
[0128] The blower configuration of the invention is advantageous in
that it generates substantially less noise than a blower not having
the configuration.
[0129] It will be understood that the above described arrangements
of apparatus and the method therefrom are merely illustrative of
applications of the principles of this invention and many other
embodiments and modifications may be made without departing from
the spirit and scope of the invention as defined in the claims.
* * * * *