U.S. patent application number 14/234712 was filed with the patent office on 2014-06-05 for termperature control logic for refrigeration system.
This patent application is currently assigned to Carrier Corporation. The applicant listed for this patent is Carrier Corporation. Invention is credited to Minfei Gan, Gilbert B. Hofsdal, Hans-Joachim Huff, Wenhua Li, Aryn Shapiro, Jian Sun.
Application Number | 20140151015 14/234712 |
Document ID | / |
Family ID | 46750426 |
Filed Date | 2014-06-05 |
United States Patent
Application |
20140151015 |
Kind Code |
A1 |
Sun; Jian ; et al. |
June 5, 2014 |
Termperature Control Logic For Refrigeration System
Abstract
A refrigeration system includes a compressor having a first
stage (20a) and a second stage (20b); a motor (22) driving the
compressor; a heat rejecting heat exchanger having a fan (44)
drawing ambient fluid over the heat rejecting heat exchanger, the
heat rejecting heat exchanger including an intercooler (43) and a
gas cooler, the inter-cooler coupled to an outlet of the first
stage and the gas cooler (41) coupled to an outlet of the second
stage; a flash tank (70) coupled to an outlet of the gas cooler; a
primary expansion device (55) coupled to an outlet of the flash
tank; a heat absorbing heat exchanger (50) coupled to an outlet of
the primary expansion device, an outlet of the heat absorbing heat
exchanger coupled to the suction port of the first stage; and a
controller (100) for implementing a pulldown mode, a control mode
and a staging logic mode.
Inventors: |
Sun; Jian; (Fayetteville,
NY) ; Huff; Hans-Joachim; (Mainz, DE) ;
Shapiro; Aryn; (Syracuse, NY) ; Hofsdal; Gilbert
B.; (Chittenango, NY) ; Gan; Minfei; (Manlius,
NY) ; Li; Wenhua; (East Syracuse, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carrier Corporation |
Farmington |
CT |
US |
|
|
Assignee: |
Carrier Corporation
Farmington
CT
|
Family ID: |
46750426 |
Appl. No.: |
14/234712 |
Filed: |
July 25, 2012 |
PCT Filed: |
July 25, 2012 |
PCT NO: |
PCT/US2012/048096 |
371 Date: |
January 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61511721 |
Jul 26, 2011 |
|
|
|
Current U.S.
Class: |
165/253 ; 62/186;
62/190 |
Current CPC
Class: |
F25B 2700/2102 20130101;
F25B 2400/13 20130101; F25B 1/10 20130101; B60H 1/3228 20190501;
F25B 49/02 20130101; F25B 2400/23 20130101; F25B 2700/21152
20130101; F25B 2600/111 20130101; F25B 41/00 20130101; F25B
2700/1933 20130101; F25B 2400/072 20130101; F25B 2600/2509
20130101; F25B 2700/21163 20130101; F25B 2309/061 20130101; F25B
2600/11 20130101; Y02B 30/70 20130101; F25B 2341/0662 20130101;
F25B 2600/2513 20130101; F25B 2700/195 20130101; F25B 2700/191
20130101; F25B 2600/21 20130101; F25B 2700/21151 20130101; F25B
2600/025 20130101; F25B 2600/2501 20130101; F25B 9/008
20130101 |
Class at
Publication: |
165/253 ; 62/190;
62/186 |
International
Class: |
F25B 49/02 20060101
F25B049/02 |
Claims
1. A refrigeration system comprising: a compressor having a first
stage and a second stage; a motor driving the compressor; a heat
rejecting heat exchanger having a fan drawing ambient fluid over
the heat rejecting heat exchanger, the heat rejecting heat
exchanger including an intercooler and a gas cooler, the
intercooler coupled to an outlet of the first stage and the gas
cooler coupled to an outlet of the second stage; a flash tank
coupled to an outlet of the gas cooler; a primary expansion device
coupled to an outlet of the flash tank; a heat absorbing heat
exchanger coupled to an outlet of the primary expansion device, an
outlet of the heat absorbing heat exchanger coupled to the suction
port of the first stage; a controller for implementing a pulldown
mode for cooling a space to a control temperature within a band of
a control temperature setpoint, a control mode for maintaining the
control temperature within the band of the control temperature
setpoint and a staging logic mode for reducing system capacity
while maintaining the control temperature within the band range of
the control temperature setpoint.
2. The refrigeration system of claim 1 further comprising: a
secondary expansion device positioned between the gas cooler and
the flash tank; the pulldown mode includes controlling the
secondary expansion device to control discharge pressure at the
compressor.
3. The refrigeration system of claim 1 wherein: the pulldown mode
includes controlling the primary expansion device to control the
heat absorbing heat exchanger superheat temperature.
4. The refrigeration system of claim 1 wherein: the control mode
includes controlling drive signals to the motor to control the
control temperature.
5. The refrigeration system of claim 4 wherein: the motor is a
variable frequency drive (VFD) motor.
6. The refrigeration system of claim 1 further comprising: a
secondary expansion device positioned between the gas cooler and
the flash tank; the control mode includes controlling the secondary
expansion device to control discharge pressure at the
compressor.
7. The refrigeration system of claim 1 wherein: the control mode
includes controlling the primary expansion device to control the
heat absorbing heat exchanger superheat temperature.
8. The refrigeration system of claim 1 further comprising: a
heater; the control mode includes controlling the heater to
maintain the control temperature within the band of the control
temperature setpoint.
9. The refrigeration system of claim 1 wherein: the staging logic
mode includes modulating the fan to reduce system capacity.
10. The refrigeration system of claim 1 further comprising: an
economizer valve coupling the flash tank to an inlet of the second
stage; wherein the staging logic mode includes closing the
economizer valve.
11. The refrigeration system of claim 1 further comprising: an
unload valve coupling an outlet of the intercooler to the suction
port of the first stage; wherein the staging logic mode includes
opening the unload valve.
12. The refrigeration system of claim 1 wherein: the staging logic
mode includes modulating the fan to control the control
temperature.
13. The refrigeration system of claim 1 wherein: the controller
implements an off integrator, the off integrator tracking an amount
of error accumulated by the system when the control temperature is
below the control temperature setpoint.
14. The refrigeration system of claim 13 wherein: the off
integrator sums the difference between the control temperature and
the control temperature setpoint over time.
15. The refrigeration system of claim 14 wherein: wherein when the
sum difference exceeds an off integrator limit, the controller
implements a shut down mode.
Description
BACKGROUND OF THE INVENTION
[0001] Embodiments relate generally to refrigerant vapor
compression systems and, more particularly, to temperature control
logic for a refrigeration system.
[0002] Refrigerant vapor compression systems are well known in the
art and are commonly used for conditioning air to be supplied to a
climate controlled comfort zone within a residence, office
building, hospital, school, restaurant or other facility.
Refrigerant vapor compression systems are also commonly used in
refrigerating air supplied to display cases, merchandisers, freezer
cabinets, cold rooms or other perishable/frozen product storage
areas in commercial establishments.
[0003] Refrigerant vapor compression systems are also commonly used
in transport refrigeration systems for refrigerating air supplied
to a temperature controlled cargo space of a truck, trailer,
container or the like for transporting perishable/frozen items by
truck, rail, ship or intermodal means. Refrigerant vapor
compression systems used in connection with transport refrigeration
systems are generally subject to more stringent operating
conditions due to the wide range of operating load conditions and
the wide range of outdoor ambient conditions over which the
refrigerant vapor compression system operates to maintain product
within the cargo space at a desired temperature. The desired
temperature at which the cargo needs to be controlled can also vary
over a wide range depending on the nature of cargo to be preserved.
The refrigerant vapor compression system not only needs sufficient
capacity to rapidly pull down the temperature of product loaded
into the cargo space at ambient temperature, but also operate
efficiently at low load when maintaining a stable product
temperature during transport. Additionally, transport refrigerant
vapor compression systems are subject to vibration and movements
not experienced by stationary refrigerant vapor compression
systems.
[0004] When used to cool perishable or frozen items under
transport, a transport refrigeration system often needs to control
the temperature in the cargo space to be within a tight temperature
band. Maintaining the cargo space in the temperature band can
result in erratic operation of the refrigeration system, due to
repeatedly going above and below the temperature band. As such,
improvements in controlling the temperature of items more smoothly
would be well received in the art.
BRIEF DESCRIPTION OF THE INVENTION
[0005] According to one aspect of the invention, a refrigeration
system includes a compressor having a first stage and a second
stage; a motor driving the compressor; a heat rejecting heat
exchanger having a fan drawing ambient fluid over the heat
rejecting heat exchanger, the heat rejecting heat exchanger
including an intercooler and a gas cooler, the intercooler coupled
to an outlet of the first stage and the gas cooler coupled to an
outlet of the second stage; a flash tank coupled to an outlet of
the gas cooler; a primary expansion device coupled to an outlet of
the flash tank; a heat absorbing heat exchanger coupled to an
outlet of the primary expansion device, an outlet of the heat
absorbing heat exchanger coupled to the suction port of the first
stage; a controller for implementing a pulldown mode for cooling a
space to a control temperature within a band of a control
temperature setpoint, a control mode for maintaining the control
temperature within the band of the control temperature setpoint and
a staging logic mode for reducing system capacity while maintaining
the control temperature within the band range of the control
temperature setpoint.
[0006] These and other advantages and features will become more
apparent from the following description taken in conjunction with
the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The subject matter which is regarded as the invention is
particularly pointed out and distinctly claimed in the claims at
the conclusion of the specification. The foregoing and other
features, and advantages of the invention are apparent from the
following detailed description taken in conjunction with the
accompanying drawings in which:
[0008] FIG. 1 is a schematic diagram illustrating an exemplary
embodiment of a refrigerant vapor compression system; and
[0009] FIG. 2 is depicts exemplary control operations for the
system of FIG. 1.
[0010] The detailed description explains embodiments of the
invention, together with advantages and features, by way of example
with reference to the drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0011] FIG. 1 depicts an exemplary embodiment of a refrigerant
vapor compression system 10 suitable for use in a transport
refrigeration system for refrigerating the air or other gaseous
atmosphere within the temperature controlled cargo space of a
truck, trailer, container or the like for transporting
perishable/frozen goods. The refrigerant vapor compression system
10 is also suitable for use in conditioning air to be supplied to a
climate controlled comfort zone within a residence, office
building, hospital, school, restaurant or other facility. The
refrigerant vapor compression system could also be employed in
refrigerating air supplied to display cases, merchandisers, freezer
cabinets, cold rooms or other perishable/frozen product storage
areas in commercial establishments.
[0012] The refrigerant vapor compression system 10 is particularly
adapted for operation in a transcritical cycle with a low critical
temperature refrigerant, such as for example, but not limited to,
carbon dioxide. The refrigerant vapor compression system 10
includes a multi-stage compressor 20, a refrigerant heat rejecting
heat exchanger 40, a refrigerant heat absorbing heat exchanger 50,
also referred to herein as an evaporator, and a primary (low
pressure) expansion device 55, such as for example an electronic
expansion valve or a thermostatic expansion valve, operatively
associated with the evaporator 50, with refrigerant lines 2, 4 and
6 connecting the aforementioned components in a primary refrigerant
circuit. Primary expansion device 55 may be a stepper valve being
positionable in a multitude of operational positions.
[0013] Heat rejecting heat exchanger 40 includes a gas cooler 41
and an intercooler 43, in a heat exchange relationship with a
cooling medium, such as, but not limited to, ambient air. Fan 44
draws air over the gas cooler 41 and the intercooler 43 to remove
heat from refrigerant passing through the gas cooler 41 and/or the
intercooler 43. Fan 44 is controllable and may be a two-speed fan
operating at a first (e.g., low) and second (e.g., high) speed.
Alternatively, fan 44 is a variable speed fan having a multitude of
fan speeds controllable by electronic drive signals.
[0014] The refrigerant heat absorption heat exchanger 50 serves as
an evaporator wherein liquid refrigerant is passed in heat exchange
relationship with a fluid to be cooled, most commonly air, drawn
from and to be returned to a temperature controlled environment,
such as the cargo box of a refrigerated transport truck, trailer or
container, or a display case, merchandiser, freezer cabinet, cold
room or other perishable/frozen product storage area in a
commercial establishment, or to a climate controlled comfort zone
within a residence, office building, hospital, school, restaurant
or other facility. In the depicted embodiments, the refrigerant
heat absorbing heat exchanger 50 comprises a finned tube heat
exchanger through which refrigerant passes in heat exchange
relationship with air drawn from and returned to the refrigerated
cargo box by the evaporator fan(s) 54 associated with the
evaporator 50. The finned tube heat exchanger may comprise, for
example, a fin and round tube heat exchange coil or a fin and
micro-channel flat tube heat exchanger. Fan 54 is controllable and
may be a two-speed fan operating at a first (e.g., low) and second
(e.g., high) speed. Alternatively, fan 54 is a variable speed fan
having a multitude of fan speeds controllable by electronic drive
signals.
[0015] A heater 51 is positioned near evaporator 50 to introduce
heat to the space to be cooled in certain situations. Heater 51 may
be an electrical resistive heater capable of on-off control or
modulated control under which a multitude of heat outputs may be
achieved in response to control signals from controller 100.
[0016] The compression device 20 functions to compress the
refrigerant and to circulate refrigerant through the primary
refrigerant circuit as will be discussed in further detail
hereinafter. The compression device 20 may comprise a single
multiple stage refrigerant compressor, such as for example a scroll
compressor, a screw compressor or a reciprocating compressor,
disposed in the primary refrigerant circuit and having a first
compression stage 20a and a second compression stage 20b.
Alternatively, the compression device 20 may comprise a pair of
independent compressors 20a and 20b. In the independent compressor
embodiment, the compressors 20a and 20b may be scroll compressors,
screw compressors, reciprocating compressors, rotary compressors or
any other type of compressor or a combination of any such
compressors. In the first stage 20a of two-stage variable speed
compressor 20, refrigerant is compressed from suction pressure to
mid-stage pressure. The refrigerant is then cooled at intercooler
43 via refrigerant line 8. Then the refrigerant enters the second
stage compressor 20b via refrigerant line 16, mixed with
refrigerant from flash tank 70 or not depending on whether
economizer valve 73 is opened or closed. The refrigerant is
compressed to discharge pressure in second stage compressor 20b and
then cooled in a gas cooler 41. Motor 22 drives compressors 20a and
20b. Motor 22 may, but need not be, a variable frequency drive
(VFD) motor capable of operating at a number of speeds depending
upon operational requirements as described in further detail
herein.
[0017] Additionally, the refrigerant vapor compression system 10
includes a flash tank 70 interdisposed in refrigerant line 4 of the
primary refrigerant circuit downstream with respect to refrigerant
flow of the gas cooler 41 and upstream with respect to refrigerant
flow of the evaporator 50. A secondary (high pressure) expansion
device 65 is interdisposed in refrigerant line 4 in operative
association with and upstream of the flash tank 70. The secondary
expansion device 65 may be an electronic expansion valve or a fixed
orifice expansion device. Secondary expansion device 65 may be a
stepper valve being positionable in a multitude of operational
positions. Refrigerant traversing the secondary expansion device 65
is expanded to a lower pressure sufficient to establish a mixture
of refrigerant in a vapor state and refrigerant in a liquid state.
The flash tank 70 defines a separation chamber wherein refrigerant
in the liquid state collects in a lower portion of the separation
chamber and refrigerant in the vapor state collects in the portion
of the separation chamber above the liquid refrigerant.
[0018] Liquid refrigerant collecting in the lower portion of the
flash tank 70 passes therefrom through refrigerant line 4 and
traverses the primary expansion device 55 interdisposed in
refrigerant line 4 upstream with respect to refrigerant flow of the
evaporator 50. As this liquid refrigerant traverses the primary
expansion device 55, it expands to a lower pressure and temperature
before entering the evaporator 50. The evaporator 50 constitutes a
refrigerant evaporating heat exchanger through which expanded
refrigerant passes in heat exchange relationship with the air to be
cooled, whereby the refrigerant is vaporized and typically
superheated. As in conventional practice, the primary expansion
device 55 meters the refrigerant flow through the refrigerant line
4 to maintain a desired level of superheat in the refrigerant vapor
leaving the evaporator 50 to ensure that no liquid is present in
the refrigerant leaving the evaporator 50. The low pressure
refrigerant vapor leaving the evaporator 50 returns through
refrigerant line 6 to the suction port of the first compression
stage 20a of the compression device 20.
[0019] The refrigerant vapor compression system 10 also includes a
refrigerant vapor injection line 14. The refrigerant vapor
injection line 14 establishes refrigerant flow communication
between an upper portion of the separation chamber of the flash
tank 70 and the second stage 20b of the compressor 20. The
refrigerant vapor injection line 14 includes an economizer valve 73
that is opened under certain operational conditions as described in
further detail herein. The economizer valve 73 may be a solenoid
valve being positionable as opened or closed. Alternatively,
economizer valve 73 is a stepper valve being positionable in a
multitude of operational positions.
[0020] The refrigerant vapor compression system 10 may also include
a compressor unload bypass line 16. The discharge of first stage
20a of compressor 20 is coupled to intercooler 43 with refrigerant
line 8. Unload bypass line 16 couples the outlet of the intercooler
43 to the suction portion of first compressor stage 20a through an
unload vale 93. The unload valve 93 may be a solenoid valve being
positionable as opened or closed. Alternatively, unload valve 93 is
a stepper valve being positionable in a multitude of operational
positions.
[0021] The refrigerant vapor compression system 10 includes a
controller 100. The controller 100 controls operation of the
various flow control valves 73 and 93 to selectively direct
refrigerant flow through the refrigerant vapor injection line 14
and the unload bypass line 16, but also may control operation of
the electronic expansion devices 55 and 65, motor 22 of compressor
20, and the fans 44 and 54. As in conventional practice, in
addition to monitoring ambient conditions, the controller 100 also
monitors various operating parameters by means of various sensors
operatively associated with the controller 100 and disposed at
selected locations throughout the system. For example, in the
exemplary embodiment depicted in FIG. 1, a pressure sensor 102 is
disposed in operative association with the flash tank 70 to sense
the pressure within the flash tank 70, a pressure sensor 104 is
provided to sense the refrigerant suction pressure, and a pressure
sensor 106 is provided to sense refrigerant discharge pressure. The
pressure sensors 102, 104, 106 may be conventional pressure
sensors, such as for example, pressure transducers. Further,
temperature across the evaporator is measured by a return
temperature sensor 110 and a supply temperature sensor 112. The
temperature sensors 110 and 112 may be conventional temperature
sensors, such as for example, thermocouples or thermistors.
[0022] Embodiments of the invention include control logic
implemented by controller 100 to provide for smooth operation of
the refrigeration system 10 when maintaining temperature within a
band (e.g., perishable or frozen) for items in transport. FIG. 2 is
a flowchart of exemplary control logic implemented by controller
100.
[0023] At 200, the controller 100 determines if the control
temperature setpoint above a threshold temperature. The control
temperature setpoint is the desired temperature for the space to be
cooled. If the control temperature setpoint is below the threshold
(e.g., 14.4 degrees F./-9.7 degrees C.), then the system need not
execute control logic.
[0024] If the control temperature setpoint is above the threshold,
flow proceeds to 202 where the controller 100 determines if the
control temperature is above the control temperature setpoint plus
a band. The control temperature is the fluid, most commonly air,
temperature leaving from the refrigerant heat absorption heat
exchanger 50 as indicated by temperature sensor 112. The band is
relatively narrow (e.g., +/-0.36 degrees F./+/-0.2 degrees C.)
temperature band about the control temperature setpoint. If at 202,
the control temperature is above the control temperature setpoint
plus the band, this indicates that the space to be cooled needs to
be cooled using high capacity cooling. In this condition, flow
proceeds to 204 where a pulldown mode is entered.
[0025] In the pulldown mode 204, the goal is to cool the space
housing the able item. Controller 100 commands the frequency of VFD
drive signals to motor 22 to be held at full speed (i.e., 100%).
Secondary expansion device 65 is controlled to control the
discharge pressure at compressor 20 as indicated by pressure sensor
106 to achieve a desired discharge pressure. The desired discharge
pressure is calculated based on optimizing the system efficiency
and/or system capacity at the current operating condition. The
primary expansion device 55 is controlled to control the evaporator
50 superheat temperature. Superheat may be determined based on
temperature at the evaporator 50 and the suction pressure from
pressure sensor 104. Fans 54 and 44 are controlled by controller
100 to be set at a high speed. Controller 100 controls the
economizer valve 73 to be open or closed based on flash tank 70
pressure as indicated by pressure sensor 102.
[0026] The pulldown mode 204 continues until the control
temperature is within the band of the control temperature setpoint.
At this point, flow proceeds from 202 to control mode 206. In the
control mode 206, three control loops (e.g., PID) are used in the
refrigeration system 10. First, compressor 20 speed is adjusted to
maintain the control temperature at the setpoint. Second, discharge
pressure of compressor 20 is controlled by stepwise control of the
secondary expansion device 55 to achieve a desired discharge
pressure. The desired discharge pressure is calculated based on
optimizing the system efficiency at the current operating
condition. Third, evaporator 50 superheat temperature is controlled
by stepwise control of secondary expansion device 65.
[0027] In exemplary embodiments, the controller 100 commands the
frequency of the VFD drive signals to motor 22 to control the
control temperature of the space to be cooled. Secondary expansion
device 65 is controlled to control the discharge pressure at
compressor 20 as indicated by pressure sensor 106. The primary
expansion device 55 is controlled to control the evaporator 50
superheat temperature. Superheat may be determined based on
temperature at the evaporator 50 and the suction pressure from
pressure sensor 104. Fans 54 and 44 are controlled by controller
100 to be set at a high speed. Controller 100 controls the
economizer valve 73 to be open or closed based on flash tank 70
pressure as indicated by pressure sensor 102. Heater 51 may also be
used to keep the control temperature within the band about the
control temperature setpoint.
[0028] In order to improve system efficiency and reduce likelihood
of system shutdown, staging logic mode 208 is used to modify the
refrigeration system capacity so that the system can continue
running while meeting relatively low cooling demand. It is
desirable that the system 10 meet low cooling requirements without
shutting down, as excessive shutdown and startup results in more
erratic control temperatures. Startup also presents concerns with
compressor flooding.
[0029] The staging logic mode 208 is implemented by controller 100,
and allots control points to reduce capacity such that the
refrigerant system 10 should not shutdown, and persists the control
temperature within the band with minimal power consumption. Each
reduction in capacity is reversible and iterative and may include
one or more of the following. The speed of fan 44 may be modulated
to reduce capacity. The economizer valve 73 may be closed, and the
unloader valve 93 opened to reduce capacity. The VFD drive signal
frequency to motor 22 may be held at a VFD minimum (e.g., 15%) to
reduce capacity. Fan 44 may be modulated to control temperature.
One, all, or any number of these capacity reducing measures may be
employed to allow the refrigeration system 10 to run during periods
of low cooling demand. Further, heater 51 may also be used to keep
the control temperature within the band about the control
temperature setpoint when low capacity modes still exceed cooling
demand.
[0030] An off integrator 210 is used to determine if shutting down
the refrigeration system 10 is needed. The off integrator 210 is
implemented by controller 100 and tracks the amount of error
accumulated by the system when the control temperature is below the
control temperature setpoint. The off integrator sums the
difference between the control temperature and the control
temperature setpoint over time. If this summed difference exceeds
an off integrator limit while outside of an acceptable control
range, the refrigeration system 10 will enter a shut down mode at
212 until the control temperature exceeds the control temperature
setpoint plus the band, after which system 10 will re-enter a
startup sequence.
[0031] At 212, If the system 10 is off and the control temperature
is below the control temperature setpoint, then the system 10 will
not operate the refrigeration system, and the evaporator fan 54
will operate according to control temperature setpoint.
Additionally, if the control temperature is below the control
temperature setpoint and beyond the control range, the heater 51
may be used to control temperature.
[0032] With this control logic, the control temperature can be
smoothly maintained within the temperature band about the control
temperature setpoint, particularly when system capacity is reduced
to such low levels that the refrigerant system should shutdown
normally. The staging logic allots control points to reduce
capacity such that the refrigerant system 10 should not shutdown
and persists the control temperature within the band with minimal
power usage.
[0033] While the invention has been described in detail in
connection with only a limited number of embodiments, it should be
readily understood that the invention is not limited to such
disclosed embodiments. Rather, the invention can be modified to
incorporate any number of variations, alterations, substitutions or
equivalent arrangements not heretofore described, but which are
commensurate with the spirit and scope of the invention.
[0034] Additionally, while various embodiments of the invention
have been described, it is to be understood that aspects of the
invention may include only some of the described embodiments.
Accordingly, the invention is not to be seen as limited by the
foregoing description, but is only limited by the scope of the
appended claims.
* * * * *