U.S. patent application number 14/358277 was filed with the patent office on 2014-10-23 for piston cylinder arrangement of an aerostatic linear compressor.
This patent application is currently assigned to WHIRLPOOL S.A.. The applicant listed for this patent is Dietmar Erich Bernhard Lilie, Henrique Bruggmann Muhle. Invention is credited to Dietmar Erich Bernhard Lilie, Henrique Bruggmann Muhle.
Application Number | 20140311337 14/358277 |
Document ID | / |
Family ID | 47552709 |
Filed Date | 2014-10-23 |
United States Patent
Application |
20140311337 |
Kind Code |
A1 |
Muhle; Henrique Bruggmann ;
et al. |
October 23, 2014 |
PISTON CYLINDER ARRANGEMENT OF AN AEROSTATIC LINEAR COMPRESSOR
Abstract
A piston (1) and cylinder (2) assembly that can reduce
efficiency losses due to the gas in a linear compressor with
aerostatic bearings. The space between the piston (1) and cylinder
(2) in the piston-cylinder assembly was designed to decrease radial
clearance (12) in the upper portion of the piston (1) when the
piston approaches the headpiece (3), i.e., when the density of the
gas not being used in the refrigeration process is the highest. The
piston (1) and cylinder (2) assembly must exhibit such a geometric
ratio that radial clearance (12) changes in inverse proportion to
the density of the gas in the radial clearance (12).
Inventors: |
Muhle; Henrique Bruggmann;
(Joinville, BR) ; Lilie; Dietmar Erich Bernhard;
(Joinville, BR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Muhle; Henrique Bruggmann
Lilie; Dietmar Erich Bernhard |
Joinville
Joinville |
|
BR
BR |
|
|
Assignee: |
WHIRLPOOL S.A.
Sao Paulo
SP
|
Family ID: |
47552709 |
Appl. No.: |
14/358277 |
Filed: |
November 14, 2012 |
PCT Filed: |
November 14, 2012 |
PCT NO: |
PCT/BR2012/000450 |
371 Date: |
May 15, 2014 |
Current U.S.
Class: |
92/169.1 |
Current CPC
Class: |
F04B 39/126 20130101;
F04B 53/162 20130101; F04B 39/122 20130101; F04B 39/0005
20130101 |
Class at
Publication: |
92/169.1 |
International
Class: |
F04B 39/00 20060101
F04B039/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 2011 |
BR |
PI1105479-4 |
Claims
1-23. (canceled)
24. A piston/cylinder assembly, comprising a piston (1) and a
cylinder (2), the piston (1) being displaceably positioned inside
the cylinder (2), the piston moving between a top dead center
(TDC/PMS) and a bottom dead center (BDC/PMI), between an inner wall
of the cylinder (2) and an outer wall of the piston (91) there
being a perimeter clearance (12) for aerostatic bearing arrangement
(1), wherein: there is a minimum perimeter clearance (12) at the
top portion of the piston (1) when the piston (1) is at its top
dead center (TDC/PMS), the clearance (12) is always smaller at the
top portion of the piston (1) than in any other region of the
piston/cylinder assembly.
25. The piston/cylinder assembly according to claim 24, wherein the
perimeter clearance (12) is variable from the bottom dead center
(BDC/PMI) to the top dead center (TDC/PMS).
26. The piston/cylinder assembly according to claim 25, wherein the
closer to the top portion of the piston (1) top the perimeter
clearance (12) is, the smaller it is.
27. The piston/cylinder assembly according to claim 26, wherein the
piston (1) comprises a variable cross-section. Page 4
28. The piston/cylinder assembly according to claim 27, wherein the
cylinder (1) comprises a variable cross-section.
29. The piston/cylinder assembly according to claim 28, wherein the
top portion of the piston (1) has a dimension larger than a
remaining portion of the piston (1).
30. The piston/cylinder assembly according to claim 29, wherein the
top portion of the cylinder (2) has a dimension smaller than the
remaining portion of the cylinder (2).
31. The piston/cylinder assembly according to claim 30, wherein the
piston (1) is conical.
32. The piston/cylinder assembly according to claim 31, wherein the
piston (1) comprises a circle-segment shape.
33. The piston/cylinder assembly according to claim 32, wherein the
cylinder (2) comprises frustum-type geometry.
34. The piston/cylinder assembly according to claim 33, wherein the
cylinder (2) comprises a circle-segment shape.
35. A linear compressor comprising a piston/cylinder assembly, said
piston/cylinder assembly of said linear compressor comprising: a
piston (1) and a cylinder (2), the piston (1) being displaceably
positioned inside the cylinder (2), the piston moving between a top
dead center (TDC/PMS) and a bottom dead center (BDC/PMI), between
an inner wall of the cylinder (2) and an outer wall of the piston
(91) there being a perimeter clearance (12) for aerostatic bearing
arrangement (1), wherein: there is a minimum perimeter clearance
(12) at the top portion of the piston (1) when the piston (1) is at
its top dead center (TDC/PMS), the clearance (12) is always smaller
at the top portion of the piston (1) than in any other region of
the piston/cylinder assembly.
36. A piston/cylinder assembly for a linear compressor, said
piston/cylinder assembly comprising a piston (1) and a cylinder
(2), the piston (1) being displaceably positioned within the
cylinder (2), the piston (1) moving between a high-pressure portion
(Pd) and a low-pressure portion (Ps), the high-pressure portion
(Pd) having higher gas density than the low-pressure portion (Ps),
a perimeter clearance (12) being defined between an inner wall of
the cylinder (2) and an outer wall of the piston (1) for aerostatic
bearing arrangement (1) with gas, wherein the dimension of the
perimeter clearance (12) varies in an inversely proportional manner
with respect to the gas density in the perimeter clearance
(12).
37. The piston/cylinder assembly according to claim 36, wherein the
piston (1) comprises a variable cross-section.
38. The piston/cylinder assembly according to claim 37, wherein the
cylinder (1) comprises a variable cross-section.
39. The piston/cylinder assembly according to claims 38, wherein
the top portion of the piston (1) comprises a larger dimension than
a remaining portion of the piston (1).
40. The piston/cylinder assembly according to claim 39, wherein the
top portion of the cylinder (2) has a smaller dimension than the
remaining portion of the cylinder (2).
41. The piston/cylinder assembly according to claim 40, wherein the
piston (1) is conical.
42. The piston/cylinder assembly according to claim 41, wherein the
piston (1) comprises a circle-segment shape.
43. The piston/cylinder assembly according to claim 42, wherein the
cylinder (2) is conical.
44. The piston/cylinder assembly according to claim 43, wherein the
cylinder (2) comprises a circle-segment shape.
45. A linear compressor comprising a piston/cylinder assembly, said
piston/cylinder assembly of said linear compressor comprising: a
piston (1) and a cylinder (2), the piston (1) being displaceably
positioned within the cylinder (2), the piston (1) moving between a
high-pressure portion (Pd) and a low-pressure portion (Ps), the
high-pressure portion (Pd) having higher gas density than the
low-pressure portion (Ps), a perimeter clearance (12) being defined
between an inner wall of the cylinder (2) and an outer wall of the
piston (1) for aerostatic bearing arrangement (1) with gas, wherein
the dimension of the perimeter clearance (12) varies in an
inversely proportional manner with respect to the gas density in
the perimeter clearance (12).
46. A piston/cylinder assembly, the piston (1) being displaceably
positioned within the cylinder (2), the piston (1) moving between a
top dead center (TDC/PMS) and a bottom dead center (BDC/PMI),
between an inner wall of the cylinder (2) and an outer wall of the
piston (1) there being a perimeter clearance (12) for aerostatic
bearing arrangement of the piston (1), the piston/cylinder assembly
comprising a varying cross-section defining a perimeter clearance
(12) which is minimum when the piston (1) is at its top dead center
(TDC/PMS), and the closer to a head (3) the piston (1) is, the
smaller the perimeter clearance.
Description
[0001] The present invention refers to a piston/cylinder assembly
of a linear compressor for cooling with aerostatic bearing
arrangement, more particularly to the dimension relationships of
the assembly so as to minimize losses.
DESCRIPTION OF THE PRIOR ART
[0002] In general, the basic structure of a cooling circuit
comprises four components, namely: the compressor, the condenser,
the expansion device and the evaporator. These elements
characterize a cooling circuit in which a fluid circulates so as to
enable the reduction of the temperature of an internal environment,
removing the heat from this medium and displacing it to an external
environment through said elements.
[0003] The fluid that circulates in the cooling circuit generally
follows this passage sequence: compressor, condenser, expansion
valve, evaporator and again the compressor, which characterizes a
closed circuit. During the circulation, the fluid undergoes
pressure and temperature variations that are responsible for
altering the state of the fluid, which may be either gaseous or in
the liquid state.
[0004] In a cooling circuit, the compressor acts like a heart of
the cooling system, creating the cooling fluid flow along the
components of the system. The compressor raises the temperature of
the cooling fluid through the rise in pressure inside it and forces
the circulation of this fluid in the circuit.
[0005] Thus, the importance of a compressor in a cooling circuit is
undeniable. There are various types of compressors applied to
cooling systems, and in the field of the present invention
attention will be focused only on the linear compressors.
[0006] Due to the relative movement between the piston and the
cylinder, it is necessary to provide the piston with bearing
arrangement. This bearing arrangement consists of the presence of a
fluid in the clearance between the outer diameter of the piston and
the inner diameter of the cylinder, preventing contact between them
and the consequent premature wear of the piston and/or cylinder.
The presence of the fluid between said two components serves also
to decrease the friction between them, thus causing the mechanical
loss of the compressor to be lower.
[0007] One of the ways of providing the piston with a bearing
arrangement is by means of aerostatic bearings, which, in essence,
consist in creating a gas bearing arrangement between the piston
and the cylinder so as to prevent wear between these two
components. One of the reasons for using this type of bearing
arrangement is justified by the fact that the has a much lower
viscous friction coefficient than any other oil, thus contributing
to cause the energy spent in the aerostatic bearing system to be
much lower than that of oil lubrication, thus achieving a better
output of the compressor. One advantage resulting from the use of
the cooling gas itself as a lubricating fluid is the absence of the
oil pumping system.
[0008] In FIGS. 1 and 2, it is possible to see that the gas
compression mechanism takes place through the axial and oscillating
movement of a piston inside a cylinder. At the cylinder top is the
head, which, in conjunction with the piston and the cylinder, forms
the compression chamber. At the head discharge and suction valves
are positioned, which regulate the entry and exit of gas in the
cylinder. In turn, the piston is actuated by an actuator that
remains connected to the linear motor of the compressor.
[0009] The compressor piston actuated by the linear motor has the
function of developing a linear alternating movement, causing the
piston movement inside the cylinder to exert a compression action
of the gas admitted by the suction valve, until it is in a position
to be discharged to the high-pressure side through the discharge
valve.
[0010] For the correct functioning of an aerostatic bearing
arrangement, it is necessary to use a flow restrictor between the
high-pressure region that involves the cylinder externally and the
clearance between the piston and the cylinder. This restriction
serves to control the pressure in the bearing arrangement region
and to restrict the gas flow.
[0011] Between the various possible solutions, it is usual to
employ the cooling-circuit gas itself for providing aerostatic
bearing arrangement of the piston. In this way, the whole gas used
in bearing arrangements represents a loss in efficiency of the
compressor, since the gas is diverted from its original function,
which is to generate cold in the evaporator of the cooling system.
Thus, it is desirable for the gas flow rate employed in bearing
arrangement to be as low as possible, so as not to impair the
compressor efficiency.
[0012] In order for the functioning of a cooling compressor to be
efficient, all the characteristic losses of this type of equipment
should be kept as low as possible, as for example, mechanical
losses (friction between components), electric losses (appearance
of parasite currents, resistance to motor current passage or
thermodynamic losses (leakages, flow of undesirable heat). With
regard to gas compression, in order for the efficiency of the
compressor to be high, it is necessary that all the work carried
out on the gas should be employed in the cooling system. For this
reason, any type of leakage or phenomenon that causes loss of gas
after the compression of the latter is undesirable.
[0013] Anyway, there will always be leakages, because, in order to
provide bearing arrangement, gas should be present between the
cylinder walls and the piston walls. However, the efficiency logic
requires the gas leakages to be kept as low as possible, in order
not to affect the compressor efficiency significantly.
[0014] The main sources of leakages in a compressor are discharge
valves and suction valves and the clearance between piston and
cylinder. The clearance between the piston and the cylinder will be
called perimeter clearance hereinafter.
[0015] For a better understanding of the phenomena that cause
decrease in the compressor efficiency, the region between the
piston top and the cylinder head is called compression chamber, and
there is where the high pressures on the gas take place. The region
that is between the piston bottom and the cylinder portion opposite
the head is called low-pressure region.
[0016] In linear compressors that make use of aerostatic bearing
arrangement, two phenomena related to loss of gas take place, which
will be the object of observation for understanding the present
technology.
Leakage
[0017] The phenomenon leakage is defined by the amount of gas that
circulates between the high-pressure region (above the piston top)
and the low-pressure region (below the piston bottom), through the
perimeter clearance. This leakage phenomenon always occurs when the
piston is in the compression phase, i.e., moving toward the head.
When this piston movement takes place, the gas is compressed up to
a discharge pressure (Pd) through the perimeter clearance,
throughout the clearance length (Cf), reaching the suction-pressure
region (Ps) located on the opposite side of the compression
chamber. It should be noted that this gas does not come out of the
compressor into the cooling system to play the main role, which is
to generate cold.
Irreversibility
[0018] To thermodynamics, irreversibility is a characteristic of
all the real processes and their sources are the dissipative
processes. Systems provided with aerostatic bearing arrangement
undergo the irreversibility phenomenon in the compression, caused
by the presence of a small portion of gas in the clearance between
the cylinder and the piston. Irreversibility can be understood as
being the loss of energy resulting from the flow of the small
portion of gas into and out of the perimeter clearance.
[0019] Considering the technology of linear compressors provided
with bearing arrangement, a loss of load is always associated to a
flow of gas, which inevitably consumes energy, the compressor being
negatively influenced by this irreversibility phenomenon.
The Problems
[0020] For a better understanding of the repercussions of the
leakage and irreversibility phenomena, FIG. 5 shows experimental
results that relate the power consumed by the said two effects as a
function of the clearance between piston and cylinder. It should be
noted that the losses due to irreversibility and leakage occur
simultaneously.
[0021] The graph in FIG. 5 does not leave any doubt about the
magnitude of the loss of efficiency, since the variation in
dimension between piston and cylinder on the order of 5 .mu.m
entails loss of power on the order of 2 W-10 W, that is, the
greater the clearance in the piston/cylinder assembly, the greater
the loss in power associated.
[0022] Therefore, there is no doubt that the technology of linear
compressors provided with aerostatic bearing arrangement needs to
have a solution that inhibits the enhanced loss of energetic
efficiency due to the perimeter clearance.
[0023] Thus, at present there are no linear compressors provided
with aerostatic bearing arrangement capable of effectively reducing
the loss of efficiency due to the use of cooling gas for providing
the piston with bearing arrangement. In other words, the present
invention manages to achieve a geometric and dimensional
relationship designed for inhibiting the loss of efficiency in
providing bearing arrangement by reducing the specific perimeter
clearance, as well as providing a solution of easy productive
implementation, guaranteeing benefits for the final user and, by
the result of better energetic efficiency, for the environment.
OBJECTIVES OF THE INVENTION
[0024] Therefore, it is an objective of the present invention to
minimize the losses of efficiency that occur on the gas of a linear
compressor provided with aerostatic bearing arrangement.
[0025] It is also an objective of the present invention to provide
spacing between the piston/cylinder assembly, so as to decrease the
clearance where there is higher gas density that is not employed in
the cooling process.
[0026] It is a further objective of the present invention to
provide a dimensional relationship and of the form in the
piston/cylinder assembly so as to guarantee maximum efficiency of a
linear compressor provided with aerostatic bearing arrangement.
BRIEF DESCRIPTION OF THE INVENTION
[0027] The objectives of the present invention are achieved by
means of a piston/cylinder assembly, the piston being displaceably
positioned within the cylinder, the piston moving between a top
dead center and a bottom dead center, wherein there is a perimeter
clearance between the inner wall of the cylinder and the outer wall
of the piston for providing the piston with aerostatic bearing
arrangement, wherein the minimum perimeter clearance occurs in at
the upper portion of the piston when the piston is at its top dead
center, and a linear compressor comprising the piston/cylinder
assembly described.
[0028] The objectives of the present invention are also achieved by
means of a piston/cylinder assembly for a linear compressor, the
piston being displaceably positioned within the cylinder, the
piston moving between a high-pressure portion and a low-pressure
portion, the high-pressure portion having higher gas density than
the low-pressure portion, a perimeter clearance being defined
between the inner wall of the cylinder and the outer wall of the
piston for providing the piston with aerostatic bearing arrangement
with gas, the dimension of the perimeter clearance varying in an
inversely proportional manner with respect to the gas density in
the perimeter clearance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The present invention will now be described in greater
detail with reference to examples of embodiment represented in the
drawings. The figures show:
[0030] FIG. 1 is a sectional view of a linear compressor provided
with aerostatic bearing arrangement of the prior art.
[0031] FIG. 2 is a sectional view of a linear compressor provided
with aerostatic bearing arrangement of the prior art showing the
gas pressures.
[0032] FIG. 3 is a sectional view of a linear compressor provided
with aerostatic bearing arrangement of the prior art showing the
gas pressures at instant i).
[0033] FIG. 4 is a sectional vie of a linear compressor provided
with aerostatic bearing arrangement of the prior art showing the
gas pressures at instant ii).
[0034] FIG. 5 is a graph of power loss due to the clearance between
cylinder and piston.
[0035] FIG. 6 is a graph of the pressure profile in the
piston/cylinder clearance as a function of the pressure, position
and time.
[0036] FIG. 7 is a graph of the gas-mass flows in the
piston/cylinder clearance in the top and bottom region of the
piston.
[0037] FIG. 8 is a graph of the gas-mass flows in the
piston/cylinder clearance in the top region of the piston.
[0038] FIG. 9 is a graph of the gas-mass flows in the
piston/cylinder clearance in the bottom region of the piston.
[0039] FIG. 10 is a sectional view of a piston/cylinder assembly
presenting an efficient solution.
[0040] FIG. 11 is a sectional view of a possible embodiment of the
piston/cylinder assembly of the present invention.
[0041] FIG. 12 is a sectional view of a possible embodiment
piston/cylinder assembly of the present invention.
[0042] FIG. 13 is a sectional view of a possible embodiment of the
piston/cylinder assembly of the present invention.
[0043] FIG. 14 is a sectional view of a possible embodiment of the
piston/cylinder assembly of the present invention.
DETAILED DESCRIPTION OF THE FIGURES
[0044] The present invention proposes a technological advance in
the piston/cylinder assembly of linear compressors with aerostatic
bearing arrangement, both in the energetic efficiency and in the
productive process.
[0045] According to the functioning principle of a cooling circuit
and as shown in FIG. 1, preferably, the gas compressing mechanism
occurs by the axial and oscillating movement of a piston 1 inside a
cylinder 2. At the head 3, one positions the discharge valve 5 and
suction valve 6, which regulate the entry and exit of gas into/out
of the cylinder 2. It should be further noted that the piston 1 is
actuated by means of an actuator 7 connected to the linear
compressor motor, and the latter is not the subject of further
explanations in this document.
[0046] The piston 1 of a compressor, when actuated by the linear
motor, has the function of developing a linear alternating
movement, providing a movement of the piston 1 inside the cylinder
2 that exerts a compression of the gas admitted by the suction
valve 6 to the extent in which the gas can be discharged to the
high-pressure side through the discharge valve 5.
[0047] The cylinder 2 is mounted within the block 8, and a cover 9
with the discharge passer 10 and the suction passer 11, which
connect the compressor to the rest of the system.
[0048] As said before, the relative movement between piston 1 and
cylinder 2 requires the bearing arrangement of the piston 1, which
consists of the presence of a fluid in the perimeter clearance 12
between the two walls, for the purpose of separating them during
the movement. An advantage of using the gas itself as a lubricating
fluid is the absence of an oil pumping system.
[0049] Preferably, the gas used for the bearing arrangement may be
the gas itself that is pumped by the compressor and sued in the
cooling system.
[0050] In this case, the gas is diverted, after compression, from
the discharge chamber 13, from the cover 9 through the channel 14,
to the pressurized region 15 around the cylinder 2, wherein the
pressurized region 15 is formed by the outer diameter of the
cylinder 2 and inner diameter of the block 8.
[0051] From the pressurized region 15 the gas passes through the
restrictors 16, 17, 18, 19 inserted into the cylinder wall 2 toward
the perimeter clearance 12 existing between the piston 1 and the
cylinder 2, forming a gas cushion that prevents contact between the
piston 1 and the cylinder 2.
[0052] With a view to restrict the gas flow between the pressurized
region 15 and the perimeter clearance 12, it is necessary to make
use of a restrictor 16, 17, 18, 19. This restriction serves to
control the pressure in the bearing-arrangement region and to
restrict the gas flow, since the whole gas used in the bearing
arrangement represents a loss of efficiency of the compressor,
since the main function of the gas is to be sent to the cooling
system and generate cold. Thus, it should be pointed out that the
gas diverted to bearing arrangement should be as little as
possible, so as not to impair the efficiency of the compressor.
[0053] In order to maintain the balance of the piston 1 within the
cylinder 2, at least three restrictors 16, 17, 18, 19 are
preferably necessary in a given section of the cylinder 2 and at
least two regions of restrictor 16, 17, 18, 19 are necessary on the
cylinder 2. The restrictors should be in such a position that, even
with oscillation movement of the piston 1, the restrictors 16, 17,
18, 19 will never be uncovered, that is, the piston 1 will not come
out of the actuation area of the restrictor 16, 17, 18, 19.
[0054] FIG. 2 presents information relating to the expressions
existing inside the cylinder/piston 1 assembly. The instant of FIG.
2 corresponds to a gas compression movement effected by the piston
1. At this instant there is a gas discharge pressure that is much
higher than the pressure existing in the opposite region of the
piston 1.
[0055] For a better understanding of the phenomena that entail the
decrease in efficiency of the compressor, the region between the
piston 1 top and the cylinder head 3 will be called high-pressure
region. The piston cylinder head 3 will be called low-pressure
region.
[0056] In turn, when the piston 1 top is at the point closest to
the cylinder head 3, this is called top dead center (TDE/PMS) and
when the piston 1 top is at the point farthest from the cylinder
head 3 this is called (LDE/PMI). Thus, the piston 1 travels a
linear movement between the top dead end (TDE/PMS) and the lower
dead end (LDE/PIM).
[0057] Of course the gas pressure at the moment of compression will
be higher in the high-pressure region. This gas flows to the
perimeter clearance 12, defined by the difference between the
piston diameter (Pd/Dp) and the cylinder diameter (Cd/Dc),
travelling the whole length of the clearance (Cf) which, in this
case, corresponds to the length of the piston 1. For a better
definition of the invention, for the purpose of the expressions
existing in the perimeter clearance 12, one should understand that
the top of the perimeter clearance 12 and the bottom of the
perimeter clearance 12 vary throughout the clearance (Cf).
[0058] As already demonstrated, the size of the clearances between
piston 1 and cylinder 2 entails a loss of efficiency of the
compressor in a considerably high relationship. In order to assess
the better solution, one should detect which of the factors leakage
and irreversibility has more influence on the loss of efficiency.
For this purpose, we use theoretical models.
[0059] Anyway, before the explanation on the result of the
simulation, it is necessary to comment a few characteristics on the
behavior of a gas. Thus, the heat exchange of a cooler is based on
the "General Equation of the Perfect Gases", which demonstrates
that in a gaseous mass the volumes and pressures are directly
proportional to their absolute temperatures and inversely
proportional to each other.
[0060] Additionally, it is necessary to synthesize a few
characteristics on the gas flow, which is established by the
perimeter clearance 12: [0061] as it is the case for any fluid, the
gas flow within the clearance exhibits a loss of load; [0062] the
gas is a compressible fluid, so that the loss of load causes the
gas pressure to vary throughout the clearance and, as a result, its
density varies; [0063] the pressure profile, consequently the gas
density, in the perimeter clearance 12 throughout the piston length
assumes different forms depending on the instant of the compression
cycle.
[0064] According to the characteristics described, two different
instants were considered for working out the theoretical model. The
instant 1 corresponds to FIG. 3 and occurs when the piston is at
its top dead end. In turn, the instant 2 corresponds to FIG. 4 and
occurs at the moment when the piston 1 is at the beginning of its
suction movement.
[0065] FIG. 6 shows the pressure profile in the perimeter clearance
as a function of the pressure, position and time of the piston 1
with respect to the cylinder 2. This graph shows that an
oscillation movement cycle of the piston 1 corresponds to the axis
X, and it is possible to identify, around 150 ms, the instants 1
and 2, the dotted line (see indications i1 and i2). The growing
variation at the axis Y corresponds to a position along the
clearance of the cylinder 2 with the piston 1. Finally, the rise in
pressure corresponds to the increase at the axis Z. This graph
enables one to consider that: [0066] i) at the instant 1 (i1), the
pressure profile throughout the piston 1 and the minimum in the
base region of the piston 1; in other words, the pressure at the
bottom is always minimum, regardless of the pressure at the tope of
the piston 1; [0067] ii) at the instant 2 (i2), the pressure
profile throughout the perimeter clearance 12 (dotted line) has its
maximum value in the central region of the perimeter clearance 12,
with the minimum pressure at the bottom and an intermediate
pressure at the top of the perimeter clearance 12.
[0068] The gas mass flow through the perimeter clearance 12 between
the piston 1 and the cylinder 2 behaves, at each moment, in
accordance with the pressure profile shown in FIG. 6 and the gas
density throughout the clearance 12. The diagram in FIG. 7 shows
the mass flows in the bottom and top regions of the piston 1
throughout the time equivalent to an oscillation of the piston 1,
indicating also the instants 1 and 2 (i1 and i2) already mentioned
in the graph of FIG. 6.
[0069] The graph of FIG. 7 shows that the flow that comes out of
the compression chamber 4 corresponds to the negative mass flow,
that is, in the top region (TP) or at the bottom (BP) of the piston
1. A positive flow represents the gas that returns to the
compression chamber 4.
[0070] One can notice that, during the larger part of the time, the
mass flow at the top of the piston 1 is different from the mass
flow at the bottom. One can further notice that, by the bottom
region of the perimeter clearance 12, there is a constant leakage
of gas (dotted line of negative values), further that the mass flow
thereof varies a little throughout the oscillation of the piston
1.
[0071] The continuous line that corresponds to the mass flow in the
perimeter clearance 12 in the top region of the piston 1 shows that
the gas comes out of the compression chamber 4 and goes into the
perimeter clearance 12 during a certain period of time (negative
mass flow--continuous line below the abscissa axis).
[0072] Additionally, at the beginning of the suction motion, the
gas that has remained in the perimeter clearance 12 is returned to
the compression chamber 4. Such a pressure, in the direction
opposite the suction pressure (Ps), which goes into the compression
chamber 4 through the suction valve 6, impairs the entry of the gas
into the compression chamber 4, thus interfering with the output of
the compressor.
[0073] Examining attentively FIGS. 3 and 4, which correspond to the
instants 1 (i1) and 2 (i2), respectively, in the light of the
graphs of FIGS. 6 and 7 one can see that the at the instant 1 (i1)
the piston is at the top dead center (PMS), where there is the
highest mass flow (2.8E-10 kg/s) coming out of the compression
chamber 4 and going into the perimeter clearance 12 in the top
region of the piston 1, the leakage through the bottom region of
the piston 1 being of 0.04E-10 k/s.
[0074] For the instant 2 the largest flow, of about 1.2E-10 kg/s,
takes place in the gas return in the top region of the perimeter
clearance 12 to the compression chamber 4. At the same instant, the
leakage through the bottom is on the order of 0.094E-10 kg/s.
[0075] In other words, for both instants 1 and 2, the gas mass flow
with high density (GAD) occurs in the top region of the perimeter
clearance 12, the gas flows with low density (GBD) occurring in the
bottom region of the perimeter clearance 12.
[0076] The diagrams of FIGS. 8 and 9 show separately the same
curves represented by the diagram of FIG. 7. By observing FIG. 8,
which represents the mass flow in the top region of the piston, one
concludes that the gas mass per compressor cycle that goes into the
perimeter clearance 12 is equivalent to the area between the
negative part of the mass flow curve and the abscissa axis (axis
xx). In turn, further for FIG. 8 the gas mass that returns to the
compression chamber 4 through the top of the diameter clearance 12
is equivalent to the portion of the graph represented above the
abscissa axis.
[0077] The difference between these two amounts of mass, or
graphically, the difference between the areas above and below the
abscissa axis of FIG. 8 corresponds to the gas mass equivalent to
the leakage of gas through the bottom of the piston 1, and the
latter, in turn, is represented by the filled area of the graph in
FIG. 9.
[0078] Therefore, one can conclude that of all the gas that goes
into the perimeter clearance 12 between the piston 1 and the
cylinder 2 little will escape through the bottom region in the form
of leakage. The largest part of the gas displaces between the
perimeter clearance 12 and the compression chamber 4.
[0079] Thus, the greatest part of the power lost because of the
perimeter clearance 12 existing between the piston 1 and the
cylinder 2 shown in FIG. 5 comes from the irreversibility effect,
not from the leakage effect.
[0080] The highest gas densities occur in the top region of the
piston 1 when the latter is closest to the head 3, due to the fact
that the high pressures in this region are capable of compressing
the gas into a smaller volume.
[0081] On the basis of the identification of the region of the
piston-1/cylinder-2 assembly responsible for the greatest loss of
efficiency of the compression, it is possible to achieve a solution
of high energetic efficiency, which is the focus of the present
invention.
[0082] The way to reduce the irreversibility effect caused by the
clearance between the piston 1 and the cylinder 2 is by keeping the
clearance as low as possible, so that here will be less volume
available for the accumulation of gas at high pressure in the
perimeter clearance 12 during the compression phase. In this way,
it is possible to establish a smaller gas flow between the
compression chamber 4 and the perimeter clearance 12.
[0083] However, the decrease of the perimeter clearance 12 between
the piston 1 and the cylinder 2 finds its limits in the pressure
limits of the manufacture process (machining processes) used for
making the piston 1 and the cylinder 2.
[0084] As a rule, the perimeter clearance between the piston 1 and
the cylinder 2 may be as follows: the lower the cylindricity errors
on the outer surface of the piston 1 and the inner surface of the
cylinder 2 the smaller the clearance. At present, this clearance in
cooling compressors is of about a few microns.
[0085] Additionally, it should be noted that the cylindricity error
obtained on parts like pistons 1 and cylinders 2 is dependent upon
the length of the cylindrical surfaces, that is, on the length of
piston 1 and cylinder 2. The relationship is established so that
the longer the part length, the greater the cylindricity which it
exhibits. Thus, an option of decreasing the cylindricity error to
enable one to reduce the perimeter clearance 12 might be simply to
reduce the length of the piston 1 and/or cylinder 2.
[0086] FIG. 10 shows a piston/cylinder assembly with a large
clearance in the top region of the piston 1 due to the high
cylindricity error of the cylinder.
[0087] The decrease in length of piston 1 and cylinder 2, however,
is not suitable for compressors that use aerostatic bearings
instead of lubricating oil, because they need longer piston 1 and
cylinder 2, so that the aerostatic bearings will provide the
necessary support for the piston 1, preventing contact between the
piston 1/cylinder assembly; otherwise, the assembly would undergo
premature wear and, as a result, loss of efficiency.
[0088] The problem to be solved by the present invention is,
therefore, one exclusive of compressors that use aerostatic
bearings. On the one hand, there are the difficulties mentioned in
the previous paragraph and, on the other hand, only compressors
with aerostatic bearings have a perimeter clearance 12 through
which the cooling gas flows.
[0089] Since it was not possible to reduce the length of the piston
1 and cylinder 2 to achieve a reduction of the cylindricity errors,
due to the questions of stability and bearing arrangement of the
piston 1 in the cylinder 2, a solution has been found which enables
one to achieve the effect of a shorter piston 1 or cylinder 2. Such
a solution results in a decrease in the perimeter clearance 12
between piston 1 and cylinder 2, without the need to reduce the
length of one of the parts of the piston/cylinder assembly.
[0090] According to what was demonstrated by the results of the
theoretical models, the smallest perimeter clearance 12 possible is
all the more necessary and beneficial the closer to the piston 1
top, that is, the closer to the region of the piston 1 the decrease
in the perimeter clearance 1 is carried out, the greater the effect
of reducing irreversibility, since it is in this region that the
largest gas-mass flows that go into and come out of the perimeter
clearance 12 take place.
[0091] It is not necessary to reduce the perimeter clearance 12
throughout the clearance length (Cf), nor during the whole cycle of
oscillatory movement of the piston 1, by rather at the moment when
pressures close to the discharge pressure occur in the compression
chamber 4, that is, when the piston 1 is close to the head 3.
[0092] In this regard, the problem of the perimeter clearance 12
can be solved by using a smaller clearance in the top region of the
piston 1 than in the bottom region of the piston 1.
[0093] Preferably, but not compulsorily, a solution of the present
invention for the irreversibility is by using components (piston
and/or cylinder) with a varying cross-section, so as to create a
specific portion in which the clearance will be effectively
reduced. These regions have lengths that are quite shorter than the
lengths of the components themselves and for this reason they will
exhibit lower cylindricity errors than those of internal
components.
[0094] Thus, exclusively in these regions the clearance between
piston 1 and cylinder 2 can be reduced.
[0095] FIGS. 11 to 14 show a few possible embodiments of the
piston/cylinder assembly that guarantee better compressor
efficiency. The piston 1, due to its smaller bottom diameter,
enables en increase in the clearance at the bottom of the
piston/cylinder assembly and the consequent decrease in the top
clearance of the piston 1.
[0096] It should be noted that whatever the solution the clearance
in the top portion of the piston 1 is always smaller than in any
other region of the piston/cylinder assembly. Additionally, the
closest to the head 3 the piston 1 is the smaller the perimeter
clearance.
[0097] FIGS. 11 to 14 show that one can find solutions in which the
bottom diameter of the piston 1 is reduced with respect to the rest
of its body (FIG. 11). The same result can be achieved through one
of more variable sections of the piston 1 and cylinder 2, while
achieving a perimeter clearance 12 that is reduced in the top
region of the piston/cylinder assembly.
[0098] FIGS. 12 and 13 show possible geometrical embodiments of the
piston 1/cylinder 2 assembly that make use of two different
sections on one of the elements piston 1 or cylinder 2 with the
objective of reducing the perimeter clearance 12 as the piston 1
gets close to the cylinder 2 top.
[0099] In FIG. 12, the piston 1 exhibits two different sections,
the section adjacent the top region of the piston 1 having larger
diameter than the region adjacent the lower portion of the piston
1, that is, the top portion of the piston has larger dimension than
the rest of the piston 1. Thus, as the piston 1 moves to the top of
a cylinder 2 that is slightly arched in its longitudinal direction,
the diameter clearance 12 reduces to a minimum when the piston 1 is
close to the cylinder 2 top. This slightly ached shape of the
cylinder 2 in its longitudinal direction may be defined as a
circle-segment top shape.
[0100] FIG. 13 shows a situation analogous to FIG. 12, but this
time it is the cylinder 2 that has two sections provided with
different diameters. Naturally, in order to guarantee a smaller
diameter clearance 12, the cylinder 2 undergoes a narrowing in the
section at the portion located closer to the cylinder top (the top
portion of the cylinder 2 has a smaller dimension than remaining
portion of the cylinder 2), which provides the minimum necessary
diameter clearance 12.
[0101] FIG. 14 shows another of these possible embodiments, which
can be achieved by means of a cylinder 2 that has a frustum-type
geometry, wherein the portion of smaller diameter would be in the
top region of the cylinder 2. Thus, as the top of the piston 1 gets
closer to the top of the cylinder 2, the perimeter clearance 12 is
reduced.
[0102] The solution of the present invention is, therefore,
achieved when one ensures a relationship in which the dimension of
the perimeter clearance 12 varies in an inversely proportional
manner with respect to the density of the gas present in the
perimeter clearance 12.
[0103] Preferred examples of embodiment having been described, one
should understand that the scope of the present invention embraces
other possible variations, being limited only by the contents of
the accompanying claims, which includes the possible
equivalents.
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