U.S. patent application number 11/123352 was filed with the patent office on 2005-09-22 for systems and methods for head pressure control.
Invention is credited to Bean, John H. JR., Roesch, James Richard.
Application Number | 20050207909 11/123352 |
Document ID | / |
Family ID | 32926890 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050207909 |
Kind Code |
A1 |
Bean, John H. JR. ; et
al. |
September 22, 2005 |
Systems and methods for head pressure control
Abstract
The present invention relates to systems and methods for
controlling head pressure in a vapor compression system, e.g. in a
precision air conditioning system. One embodiment of the invention
provides a method for regulating working fluid flow in a vapor
compression system including a compressor. The method includes:
providing a controller; receiving signals at the controller
representative of a monitored discharge pressure in a discharge
line of the compressor; and using the controller to provide a
control signal to an actuator that controls a flow control valve
that, in turn, controls working fluid flow into the system, the
control signal being responsive at least in part to a difference
between a set point pressure and the monitored discharge
pressure.
Inventors: |
Bean, John H. JR.;
(Wentsville, MO) ; Roesch, James Richard; (St.
Peters, MO) |
Correspondence
Address: |
MINTZ, LEVIN, COHN, FERRIS, GLOVSKY
AND POPEO, P.C.
ONE FINANCIAL CENTER
BOSTON
MA
02111
US
|
Family ID: |
32926890 |
Appl. No.: |
11/123352 |
Filed: |
May 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11123352 |
May 6, 2005 |
|
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|
10382381 |
Mar 6, 2003 |
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Current U.S.
Class: |
417/297 ;
417/279 |
Current CPC
Class: |
F25B 2700/1931 20130101;
F25B 49/027 20130101 |
Class at
Publication: |
417/297 ;
417/279 |
International
Class: |
F04B 049/00 |
Claims
1. A method for regulating coolant fluid flow in a condenser of a
vapor compression refrigeration system including a compressor, the
method comprising: providing a vapor compression refrigeration
system comprising: a compressor having a discharge for compressed
refrigerant; a discharge line attached to the compressor discharge;
and a fluid cooled condenser having: a first inlet coupled to the
discharge line; a first outlet for passing working fluid to an
expansion device; a second inlet for receiving coolant from a
coolant recycling system; and a second outlet for returning coolant
to the coolant recycling system a discharge pressure sensor coupled
to the discharge line and operative to provide a discharge pressure
signal representative of the discharge pressure; a flow control
valve having an inlet for receiving coolant fluid from the coolant
recycling system and an outlet, the outlet connected to the second
inlet of the condenser, the flow control valve operative to control
the flow of the coolant fluid into the condenser; a flow control
valve actuator coupled to the flow control valve, the actuator
operative to control the flow control valve; and a controller in
communication with the discharge pressure sensor and in
communication with the actuator, the controller operative to
receive the discharge pressure signal and to control the actuator
at least in part in response to the discharge pressure signal;
receiving signals at the controller representative of a monitored
discharge pressure in a discharge line of the compressor; and using
the controller to provide a control signal to an actuator that
controls a flow control valve that, in turn, controls coolant fluid
flow into the condenser, the control signal being responsive at
least in part to a difference between a set point pressure and the
monitored discharge pressure.
2. The method of claim 1, wherein the method further comprises:
receiving a signal at the controller representative of the position
of the flow control valve and wherein the control signal is
responsive at least in part to the signal representative of the
position of the flow control valve.
3. The method of claim 1, wherein using the controller to provide a
control signal to the actuator comprises: if the controller is in a
hold position state, if the monitored discharge pressure minus the
set point pressure is above a preselected value, and if the
monitored discharge pressure is not decreasing, then entering an
opening valve state; and if the controller is in the hold position
state, if the monitored discharge pressure minus the set point
pressure is below a preselected value and if the monitored
discharge pressure is not increasing, then entering a closing valve
state.
4. The method of claim 3, wherein using the controller to provide a
control signal to the actuator further comprises: if the controller
is in an opening valve state, if the monitored discharge pressure
minus the set point pressure is below a preselected value, and if
the rate of change in the monitored discharge pressure is below a
preselected value, then entering the hold position state; and if
the controller is in a closing valve state, if the monitored
discharge pressure minus the set point pressure is above a
preselected value and if the rate of change in the monitored
discharge pressure is below a preselected value, then entering the
hold position state.
5. The method of claim 3, wherein using the controller to provide a
control signal to the actuator further comprises: if the controller
is in an opening valve state and if the monitored discharge
pressure is decreasing, then entering a pressure decreasing state;
and if the controller is in a closing valve state and if the
monitored discharge pressure is increasing, then entering a
pressure increasing state.
6. The method of claim 5, wherein using the controller to provide a
control signal to the actuator further comprises: if the controller
is in the pressure decreasing state and if the monitored discharge
pressure is increasing, then entering the opening valve state; and
if the controller is in the pressure increasing state and if the
monitored discharge pressure is decreasing, then entering the
closing valve state.
7. The method of claim 3, wherein using the controller to provide a
control signal to the actuator further comprises: when the
controller enters the opening valve state, the controller
substantially immediately signals the actuator to open the flow
control valve a preselected amount; and when the controller enters
the closing valve state, the controller substantially immediately
signals the actuator to close the flow control valve a preselected
amount.
8. The method of claim 7, wherein, while the controller is in the
opening valve state, after the controller signals the actuator to
open the flow control valve a preselected amount, the controller
waits a first off time before signaling the actuator to open the
valve further, the first off time being a function of the
difference between the monitored discharge pressure and the set
point pressure; and wherein, while the controller is in the closing
valve state, after the controller signals the actuator to close the
flow control valve a preselected amount, the controller waits a
second off time before signaling the actuator to close the valve
further, the second off time being a function of the difference
between the monitored discharge pressure and the set point
pressure.
9. The method of claim 8, wherein the first and second off times
are re-calculated regularly according to a preselected time
period.
10. The method of claim 8, wherein the first off time decreases as
the difference between the monitored discharge pressure and the set
point pressure increases, and wherein the second off time decreases
as the difference between the monitored discharge pressure and the
set point pressure increases.
11. The method of claim 10, wherein using the controller to provide
a control signal to the actuator further comprises: sending a
control signal to the actuator to set the initial position of the
flow control valve; and holding the initial position until the
controller receives a transition control signal indicating that the
compressor has been turned on.
12. The method of claim 5, wherein, using the controller to provide
a control signal to the actuator further comprises: when the
controller enters the pressure decreasing state, the controller
substantially immediately signals the actuator to close the flow
control valve a preselected amount; and when the controller enters
the pressure increasing state, the controller substantially
immediately signals the actuator to open the flow control valve a
preselected amount.
13. The method of claim 12, wherein, while the controller is in the
pressure decreasing state, after the controller signals the
actuator to close the flow control valve a preselected amount, the
controller waits a first off time before signaling the actuator to
open the valve further, the first off time being determined at
least in part by the rate at which the pressure is decreasing; and
wherein, while the controller is in the pressure increasing state,
after the controller signals the actuator to open the flow control
valve a preselected amount, the controller waits a second off time
before signaling the actuator to open the valve further, the second
off time being determined at least in part by the rate at which the
pressure is increasing.
14. The method of claim 1, wherein the controller is a
microprocessor controller.
15. The method of claim 1, wherein the method further comprises:
monitoring the actual discharge pressure using a pressure
transducer mounted on the discharge line to produce an analog
monitored discharge pressure signal.
16. The method of claim 15, wherein the method further comprises:
using an analog op-amp to convert the analog monitored discharge
pressure signal to an adjusted monitored discharge pressure
signal.
17. The method of claim 16, wherein the method further comprises:
using an analog-to-digital converter to convert the adjusted
monitored discharge pressure signal to a digital monitored
discharge pressure signal for forwarding to the controller.
18. An air conditioning unit comprising: a compressor having a
discharge for compressed refrigerant; a discharge line attached to
the compressor discharge; and a fluid cooled condenser having: a
first inlet coupled to the discharge line; a first outlet for
passing working fluid to an expansion device; a second inlet for
receiving coolant from a coolant recycling system; and a second
outlet for returning coolant to the coolant recycling system; a
discharge pressure sensor coupled to the discharge line and
operative to provide a discharge pressure signal representative of
the discharge pressure; a flow control valve having an inlet for
receiving coolant fluid from the coolant recycling system and an
outlet, the outlet connected to the second inlet of the condenser,
the flow control valve operative to control the flow of the coolant
fluid into the condenser; a flow control valve actuator coupled to
the flow control valve, the actuator operative to control the flow
control valve; and a controller in communication with the discharge
pressure sensor and in communication with the actuator, the
controller operative to receive the discharge pressure signal and
to control the actuator at least in part in response to the
discharge pressure signal.
19. The system of claim 18, wherein the discharge pressure sensor
comprises: a pressure transducer operative to provide a transducer
pressure signal representative of the pressure in the discharge
line; an op-amp coupled to the transducer and operative to convert
the transducer pressure signal to provide an amplified signal; and
an analog-to-digital converter coupled to the op-amp to convert the
amplified signal to a digital signal.
20. The system of claim 18, wherein the actuator comprises: a
feedback potentiometer operative to measure valve position and to
provide a signal representative of the valve position, and wherein
the controller is operative to receive the valve position signal
from the potentiometer and to control the actuator at least in part
in response to the valve position signal.
21. The system of claim 18, wherein the controller is further
operative to monitor a temperature control state machine to
determine cooling demand.
22. The system of claim 18, wherein the controller is a
microprocessor controller.
23. An apparatus for regulating coolant fluid flow in a vapor
compression refrigeration system, the apparatus comprising: a
compressor having a suction and a discharge for a refrigerant; a
discharge line attached to the compressor discharge; a fluid cooled
condenser having: a first inlet coupled to the discharge line; a
second inlet for receiving-coolant fluid from a conventional
coolant recycling system; a first outlet for passing working fluid
to an expansion device; a second outlet for returning coolant to
the coolant recycling system a pressure transducer coupled to the
discharge line and operative to provide a transducer pressure
signal representative of the pressure in the discharge line; an
op-amp coupled to the transducer and operative to convert the
transducer pressure signal as necessary to provide an amplified
signal; and an analog-to-digital converter coupled to the op-amp to
convert the amplified signal to a digital pressure signal; a flow
control valve having an inlet for receiving coolant fluid and an
outlet, the outlet connected to the condenser's second inlet, the
flow control valve operative to control the flow of the coolant
fluid into the condenser; a flow control valve actuator coupled to
the flow control valve, the actuator operative to control the flow
control valve, the actuator having a feedback potentiometer
operative to measure valve position and to provide a signal
representative of the valve position; and a controller in
communication with the analog-to-digital converter and in
communication with the actuator, the controller operative to
receive a digital pressure signal from the analog-to digital
converter, and to control the actuator at least in part in response
to the digital pressure signal.
24. A method for regulating coolant fluid flow in a fluid cooled
condenser of a vapor compression system including a compressor, the
method comprising: providing, providing a discharge pressure sensor
coupled to a discharge line of the compressor and operative to
provide a discharge pressure signal representative of the discharge
pressure; providing a flow control valve having an inlet for
receiving coolant fluid and an outlet, the outlet connected to an
inlet of the condenser, the flow control valve operative to control
the flow of Coolant fluid into the condenser; providing a flow
control valve actuator coupled to the flow control valve, the
actuator operative to control the flow control valve; providing a
controller in communication with the discharge pressure sensor and
in communication with the actuator, the controller operative to
receive a discharge pressure signal and to control the actuator at
least in part in response to the discharge pressure signal;
receiving signals at the controller representative of a monitored
discharge pressure in a discharge line of the compressor; and using
the controller to provide a control signal to an actuator that
controls a flow control valve that, in turn, controls coolant fluid
flow into the condenser, the control signal being responsive at
least in part to a difference between a set point pressure and the
monitored discharge pressure.
25. An apparatus for regulating coolant fluid flow, the apparatus
comprising: a vapor compression refrigeration system comprising: a
compressor having a discharge for compressed refrigerant; a
discharge line attached to the compressor discharge; and a fluid
cooled condenser having: a first inlet coupled to the discharge
line; a first outlet for passing working fluid to an expansion
device; a second inlet for receiving coolant from a coolant
recycling system; and a second outlet for returning coolant to the
coolant recycling system; discharge pressure sensor means coupled
to the discharge line for providing a discharge pressure signal
representative of the discharge pressure; flow control valve means
having an inlet for receiving coolant fluid and an outlet, the
outlet connected to the second inlet of the condenser, the flow
control valve means for controlling the flow of the coolant fluid
into the condenser; flow control valve actuator means coupled to
the flow control valve, the actuator means for controlling the flow
control valve; and controlling means in communication with the
discharge pressure sensor and in communication with the actuator,
the controlling means for receiving the discharge pressure signal
and for automatically controlling the actuator at least in part in
response to the discharge pressure signal.
26. The apparatus of claim 26, wherein the fluid cooled condenser
comprises: a coolant-cooled heat exchanger.
27. An apparatus for regulating coolant fluid flow, the apparatus
comprising: a discharge pressure sensor adapted to couple to a
discharge line of a compressor and operative to provide a discharge
pressure signal representative of a discharge pressure of the
compressor; a flow control valve having an inlet for receiving
coolant fluid from a coolant recycling system and an outlet
connected to an inlet of a fluid cooled condenser, the flow control
valve operative to control the flow of the coolant fluid into the
condenser; a flow control valve actuator coupled to the flow
control valve, the actuator operative to control the flow control
valve; and a controller in communication with the discharge
pressure sensor and in communication with the actuator, the
controller operative to receive the discharge pressure signal and
to control the actuator at least in part in response to the
discharge pressure signal.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a vapor compression system,
e.g., used for air conditioning, and more specifically to systems
and methods for controlling head pressure in a vapor compression
system.
[0002] The condensing pressure at which a condenser in a vapor
compression system operates depends upon a number of factors such
as the design conditions for which the condenser was selected, the
actual conditions at which the condenser is operating, and whether
the condenser is operating at full or partial capacity. In many
cases, the condenser operates at full capacity at all times. In
such situations, the pressure at which the condenser operates
fluctuates as a result of changes in the ambient conditions such as
outside air temperature or humidity. Because of these condensing
pressure fluctuations, refrigeration or air conditioning systems
utilizing compressors typically operate where the internal
discharge pressure of the compressor does not equal the condensing
or discharge line pressure resulting in a condition of either
"over-compression" or "under-compression".
[0003] In the under-compression case, the internal discharge
pressure is too far below the discharge line pressure. Energy is
wasted because the compressor must work against this relatively
high pressure differential. In the over-compression case, the
internal discharge pressure is too high relative to the discharge
line pressure. As a result, the condenser does not operate
efficiently because the compressor does not provide the appropriate
operating pressure to the condenser.
SUMMARY OF THE INVENTION
[0004] The present invention relates to systems and methods for
controlling head pressure in a vapor compression system, e.g. in a
precision air conditioning system. One embodiment of the invention
provides a method for regulating working fluid flow in a vapor
compression system including a compressor. The method includes:
providing a controller; receiving signals at the controller
representative of a monitored discharge pressure in a discharge
line of the compressor; and using the controller to provide a
control signal to an actuator that controls a flow control valve
that, in turn, controls working fluid flow into the system. The
control signal is responsive at least in part to a difference
between a set point pressure and the monitored discharge
pressure.
[0005] Another embodiment of the invention provides an apparatus
for regulating working fluid flow. The apparatus includes: a vapor
compression system, a discharge pressure sensor, a flow control
valve, a flow control valve actuator, and a controller. The vapor
compression system includes: a compressor having an outlet for a
working fluid; a discharge line attached to the compressor outlet;
and a condenser having a first inlet coupled to the discharge line.
The discharge pressure sensor couples to the discharge line and
provides a discharge pressure signal representative of the
discharge pressure. The flow control valve has an inlet for
receiving working fluid and an outlet. The outlet connects to the
vapor compression system. The flow control valve controls the flow
of the working fluid into the vapor compression system. The flow
control valve actuator couples to the flow control valve. The
actuator controls the flow control valve. The controller
communicates with the discharge pressure sensor and with the
actuator. The controller receives the discharge pressure signal and
controls the actuator at least in part in response to the discharge
pressure signal.
BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0006] FIG. 1 is a schematic illustration of a vapor compression
system according to one embodiment of the invention;
[0007] FIG. 2 is a state diagram for the controller of FIG. 1;
[0008] FIG. 3 is a graph of duty cycle for signals sent by the
controller of FIG. 1 as a function of head error;
[0009] FIG. 4 is a graph depicting results of the operation of one
embodiment of the system of FIG. 1; and
[0010] FIG. 5 is a graph depicting more results of the operation of
one embodiment of the system of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The present invention relates to vapor compression systems,
e.g., air conditioning systems, and more specifically to systems
and methods for electronically controlling head pressure in a vapor
compression system.
[0012] With reference to FIG. 1, an apparatus 10 according to one
embodiment of the invention includes a vapor compression system
having a compressor 14 with an inlet and an outlet; a discharge
line 16 coupled to the compressor outlet; and a condenser with a
first inlet 40 coupled to the discharge line 16, a first outlet 42
for passing working fluid to an expansion device 28, a second inlet
46 for receiving working fluid from a conventional working fluid
recycling system (not shown); and a second outlet 44 for returning
working fluid to the working fluid recycling system. The working
fluid can be one of a variety of fluids such as water or glycol.
The vapor compression system further includes a liquid line coupled
to the first outlet 42 of the condenser 22, an expansion device 28
coupled to the liquid line, an evaporator 30 coupled to the
expansion device, a fan 32 for blowing air across the evaporator
30, and a suction line coupled to the evaporator 30 and to the
inlet of the compressor 14. Embodiments of the invention use a
coolant-cooled, e.g., glycol or water, brazed plate heat exchanger
for the method of heat rejection from the refrigerant
condenser.
[0013] The apparatus further includes: a pressure sensor 18, 48, 50
coupled to the discharge line 16; a flow control valve 26 having an
inlet 52 for receiving working fluid and an outlet 54 connected to
the second inlet 46 of the condenser; an actuator 24 coupled to the
valve 26; and a controller 12 in communication with the pressure
sensor 18, 48, 50 and in communication with the actuator 24. In one
embodiment, the actuator includes a feedback potentiometer 38 for
measuring valve position and for providing a signal representative
of the valve position.
[0014] A coolant out line 56 couples to the second outlet 44 of the
condenser. A coolant in line 60 couples to the inlet 52 of the flow
control valve. The coolant out line and the coolant in line couple
to a conventional working fluid/coolant recycling system (not
shown). A bypass line 58 couples the coolant out line 56 to the
flow control valve 26. The bypass line allows the recycling system
to continue cycling fluid when the flow control valve is shut.
[0015] The pressure sensor can include a pressure transducer 18, an
op-amp 48 and an analog-to-digital (A/D) converter. In one
embodiment, the pressure sensor obtains a pressure measurement
every second. The pressure transducer 18 coupled to the discharge
line 16 provides a transducer pressure signal representative of the
pressure in the discharge line 16. The op-amp 48 coupled to the
transducer 18 converts the transducer pressure signal to an
amplified pressure signal. The A/D converter 50 receives the
amplified pressure signal and converts it to a digital pressure
signal. In one embodiment the A/D converter 50 is a conventional
A/D converter and is embedded in the controller 12.
[0016] In the illustrated embodiment, the controller 12 receives
the digital pressure signal from the A/D converter 50 and sends a
control signal 34 to the actuator, the control signal being
responsive at least in part to the digital pressure signal. The
controller 12 can also receive the valve position signal 36 from
the feedback potentiometer 38. An A/D converter 55 can convert the
valve position signal 36 to a digital signal for processing by the
controller and the controller 12 can produce a control signal 34
responsive at least in part to the valve position signal 36.
[0017] One can refer to the pressure in the discharge line as head
pressure. The present invention maintains head pressure while
reducing operation of the actuator 24 relative to current
actuator-based air conditioning systems, thus reducing the need for
repair and/or replacement of the actuator and/or valve. Embodiments
of the invention monitor head pressure relative to a predetermined
or set point head pressure. One can refer to the monitored head
pressure minus a set point pressure as head error. In one
embodiment, if the monitored head pressure is within a
predetermined range of the set point head pressure, i.e., if the
head error is below a specified level, then the system does not
change the valve position.
[0018] With reference to the controller state diagram of FIG. 2, in
the initial state the controller is in a valve closed state. In one
embodiment, the controller monitors a temperature control state
machine to determine cooling demand. Once the temperature in the
space in question increases above a selected temperature, the
controller transitions to a setting initial position state in which
the controller signals the actuator to set the valve to the initial
position. By doing so, the system starts the flow of coolant into
the compressor in preparation for operation of the vapor
compression system including operation of the compressor.
[0019] Once the system sets the initial position, the system enters
the controlling portion of the state diagram. The first state of
the controlling portion is a wait state. In one embodiment, the
controller waits for a transition control signal from the
compressor state machine that indicates that the compressor has
been started. Once the controller receives the transition control
signal from the compressor state machine, the controller
transitions to a hold position state. In one embodiment, while in
the hold position, the system monitors the head error, the
difference between the monitored head pressure and a
predetermined/set point head pressure.
[0020] If the head error is above a preselected value, e.g., 10
psi, and if the pressure is not decreasing, then the system
transitions to an opening valve state. Similarly, if the head error
is below a preselected value, e.g., -10 psi, and if the pressure is
not increasing, then the system enters a closing valve state.
[0021] Alternatively, if the controller is in the opening valve
state, if the monitored discharge pressure minus the set point
pressure is below a preselected value, e.g., 10 psi, and if the
rate of change in the monitored discharge pressure is below a
preselected value, then the controller enters the hold position
state. Similarly, if the controller is in the closing valve state,
if the monitored discharge pressure minus the set point pressure is
above a preselected value, e.g., -10 psi, and if the rate of change
in the monitored discharge pressure is below a preselected value,
then the controller enters the hold position state.
[0022] When the controller enters the opening valve or closing
valve state, the controller executes an open valve routine or a
close valve routine, respectively. In one embodiment, when the
controller enters the opening valve or closing valve state, the
controller substantially immediately signals the actuator to open
or close the valve, respectively.
[0023] One embodiment of the open valve routine is the following.
As noted above, one can refer to the monitored discharge pressure
minus a set point pressure as head error and the absolute value of
head error as Working Head Error. If the Working Head Error is
greater than 60 then the controller sets the Working Head Error to
60. If the Working Head Error is less than 10, the controller sets
the Working Head Error to 10. Then the controller looks up the "Off
Time" equation based on the working head error from Table I.
1TABLE I Working Head Error Equation Less Than Slope Intercept 20
0.0029 -0.009 30 0.0087 -0.125 40 0.0116 -0.212 50 0.0145 -0.328 60
0.0203 -0.618
[0024] The controller sets the Off Time, i.e., the time for which
the controller does not signal the actuator to open the valve, as
follows:
[0025] Off Time=0.4*((1/(Slope*Working Head Error+Intercept))-1).
The graph of the resulting duty cycle vs Working Head Error is
shown in FIG. 3. By constraining the Working Head Error to a range
of 10-60, the system constrains the duty cycle of the actuator to a
range of 2%-60%.
[0026] In one embodiment, the open (or close) valve process
calculates a new Off Time ever second.
[0027] Off Time and Valve Direction, e.g., open, are fed into a
function performed on the controller that generates a pulse of
selected length, e.g., of 0.4 seconds, on the appropriate valve
direction signal whenever the Off Time is exceeded. The controller
provides two signals to the actuator, one for closing the valve and
one for opening the valve. On entrance to the Opening Valve,
Closing Valve states, the controller sets Off Time to zero so that
the controller substantially immediately generates a pulse from the
controller to the actuator.
[0028] Similarly, one embodiment of the close valve routine is the
following. If the Working Head Error is greater than 60 then the
controller sets the Working Head Error to 60. If the Working Head
Error is less than 10, the controller sets the Working Head Error
to 10. Then the controller looks up the "Off Time" equation based
on the working head error from Table I. The controller sets the Off
Time, i.e., the time for which the controller does not signal the
actuator to open the valve, as follows: Off
Time=0.4*((1/(Slope*Working Head Error+Intercept))-1). Off Time and
Valve Direction, i.e., close, are fed into a function performed on
the controller that generates a pulse of selected length, e.g., of
0.4 seconds, on the appropriate valve direction signal whenever the
Off Time is exceeded.
[0029] Once in the opening valve state, if the head pressure is
decreasing, the controller enters the pressure decreasing state.
Similarly, once in the closing valve state, if the head pressure is
increasing, the controller enters the pressure increasing state. In
one embodiment, when the controller enters the pressure decreasing
or pressure increasing states, the controller substantially
immediately signals the actuator to close or open the valve,
respectively.
[0030] When the controller enters the pressure decreasing state,
the controller executes a pressure-decreasing pressure braking
routine. The pressure-decreasing pressure breaking routine reduces
overcompensation for head error as a result of opening the valve to
correct head error. The routine reduces such overcompensation by
closing the valve once the discharge pressure starts
decreasing.
[0031] One embodiment of the pressure-decreasing pressure breaking
routine is the following. If the monitored discharge pressure is
decreasing at a rate greater than or equal to 5 psi/sec, then the
controller sets the Off Time to 0.4 seconds. If the discharge
pressure is decreasing at a rate greater than or equal to 3 psi/sec
but less than 5 psi/sec then the controller sets the Off Time to
0.6 seconds. Otherwise, as with the opening valve routine, If the
Working Head Error is greater than 60 then the controller sets the
Working Head Error to 60. If the Working Head Error is less than
10, the controller sets the Working Head Error to 10. Then the
controller looks up the "Off Time" equation based on the working
head error from Table I. The controller sets the Off Time, i.e.,
the time for which the controller does not signal the actuator to
open the valve, as follows:
Off Time=0.4*((1/(Slope*Working Head Error+Intercept))-1).
[0032] Similarly, when the controller enters the pressure
increasing state, the controller executes a pressure-increasing
pressure braking routine. The pressure-increasing pressure breaking
routine reduces overcompensation for head error as a result of
closing the valve to correct head error. The routine reduces such
overcompensation by opening the valve once the discharge pressure
starts increasing.
[0033] One embodiment of the pressure-increasing pressure breaking
routine is the following. If the monitored discharge pressure is
increasing at a rate greater than or equal to 5 psi/sec, then the
controller sets the Off Time to 0.4 seconds. If the discharge
pressure is increasing at a rate greater than or equal to 3 psi/sec
but less than 5 psi/sec then the controller sets the Off Time to
0.6 seconds. Otherwise, as with the opening valve routine, If the
Working Head Error is greater than 60 then the controller sets the
Working Head Error to 60. If the Working Head Error is less than
10, the controller sets the Working Head Error to 10. Then the
controller looks up the "Off Time" equation based on the working
head error from Table I. The controller sets the Off Time, i.e.,
the time for which the controller does not signal the actuator to
open the valve, as follows:
Off Time=0.4*((1/(Slope*Working Head Error+Intercept))-1).
[0034] When the controller receives an off signal or a disable
signal, i.e., a signal from the compressor state machine that the
compressor has been turned off, the controller transitions to a
valve close delay state. After a preselected period of time, the
controller saves the current valve position to memory, closes the
valve and transitions to a valve closed state. The system uses the
save valve position as the initial valve position when the state
machine transitions back to the Setting Initial Position state. In
one embodiment, the controller is a microprocessor controller and
the controller has flash memory that stores the firmware for the
controller.
[0035] With reference to FIG. 3, the duty cycle for signals sent by
the controller of FIG. 1 as a function of head error is shown for
one embodiment of the invention. The graph depicted in FIG. 3 uses
the off time equation provided by Table 1. The head error is in
units of pounds per square inch. Multiplying the values marking the
Y axis by 100 gives the percentage of the duty cycle for which the
controller provides an open or close signal to the actuator. In the
illustrated embodiment, the period over which the duty cycle is
calculated is 3 minutes long and the pulse length is 0.4 seconds.
The length over which the duty cycle is calculated can vary as long
as it is several times longer than the combination of the longest
off time with the pulse length. As illustrated, the duty cycle
increases with the head error.
[0036] With reference to FIG. 4, results of the operation of one
embodiment of the system of FIG. 1 include discharge pressure in
psi, working fluid flow in gallons per minute (gpm), valve position
as a percentage of the fully open position, and suction pressure at
the compressor inlet in psi. The X axis represents time in an
hours, minutes, seconds format. The left-hand Y axis represents
valve position as a percentage of the fully open position and the
flow rate in gpm. The right-hand Y axis represents pressure in psi.
The set point pressure is 280 psi. The graph illustrates that
before the valve opens the discharge pressure is about 125 psi. As
the discharge pressure rises and falls, the valve opens and closes
in order to drive the discharge pressure to the set point. The
controller makes small adjustments in the valve position over time
to keep the discharge pressure near the set point pressure. At
approximately 10:28:00, a second compressor was turned on which
caused a disturbance in the discharge pressure. At approximately
11:00:00, an operator turned the unit off and then back on. As a
result, the valve closed and the discharge pressure dropped.
[0037] With reference to FIG. 5, more results of the operation of
one embodiment of the system of FIG. 1 include return air, supply
air, glycol outlet, and glycol inlet all in Fahrenheit. FIG. 5 also
shows the discharge pressure and suction pressure in psi as shown
in FIG. 4. The X axis again represents time in a hours, minutes,
seconds format. The left-hand Y axis represents temperature in
Fahrenheit and the right-hand Y axis represents pressure in psi. As
illustrated, the return air is generally warmer than the supply air
and the glycol outlet is generally warmer than the glycol
inlet.
[0038] Having thus described at least one illustrative embodiment
of the invention, various alterations, modifications and
improvements are contemplated by the invention including the
following: the A/D converter 50 can be embedded in the pressure
transducer 18; the controller can be implemented in hardware, e.g.,
using an application specific integrated circuit; the actuator
could be made integral to the flow control valve; and the working
fluid (e.g., the coolant) could enter the system at a location
other than at the condenser. Such alterations, modifications and
improvements are intended to be within the scope and spirit of the
invention. Accordingly, the foregoing description is by way of
example only and is not intended as limiting. The invention's limit
is defined only in the following claims and the equivalents
thereto.
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