Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Compressor Control for Heat Transfer System
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to of U.S. Patent Application No. 13/837,119
filed March 15,
2013, the contents of which are incorporated herein by reference.
Field
[0001] The present invention relates to heat transfer systems.
Background
[0002] It is known to employ energy exchange technologies in order to, for
example,
recover excess heat energy from an air-conditioning system to provide energy
to heat water.
Many examples of such heat-exchange technologies came about in the early 1980s
which
reflect the end of the energy crises of the 1970s. It is interesting to note
that these heat-
exchange technologies have not been generally adopted.
[0003] Existing solutions do not provide precise and robust control
adequate for heat
recovery systems, given that waste-heat recovery typically has large
temperature gradients of
the kind unforgiving to poor control.
Summary
[0004] According to one aspect of the present invention, a heat transfer
system includes a
compressor for circulating a working fluid, the compressor having an inlet and
an outlet. The
compressor is operable at a controllable operating capacity. The system
further includes a
condenser connected to the outlet of the compressor, an electrically
controlled valve
positioned to receive working fluid from the outlet of the condenser, an
evaporator connected
between an outlet of the electrically controlled valve and the inlet of the
compressor, a
discharge pressure sensor located between the outlet of the compressor and the
inlet of the
electrically controlled valve, a condenser temperature sensor positioned to
measure a
temperature at the condenser, and a controller connected to the compressor,
the discharge
pressure sensor, and the condenser temperature sensor. The controller is
configured to adjust
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the operating capacity of the compressor to maintain output of the condenser
temperature
sensor at a condenser temperature set point. The controller is further
configured to reduce the
operating capacity of the compressor when output of the discharge pressure
sensor indicates
that a maximum operating pressure of the compressor has been exceeded. The
maximum
operating pressure has a saturation temperature higher than the condenser
temperature set
point.
[0005] The controller can be configured to shut down the compressor when
the output of
the discharge pressure sensor indicates that a shutdown pressure has been
exceeded, the
shutdown pressure having a saturation temperature higher than the saturation
temperature of
the maximum operating pressure.
[0006] The system can further include a discharge temperature sensor
located at the outlet
of the compressor. The controller can be configured to incrementally close the
electrically
controlled valve when output of the discharge temperature sensor falls below a
minimum
discharge superheat temperature determined from output of the discharge
pressure sensor.
[0007] The system can further include a subcooler connected between the
condenser and
the electrically controlled valve.
[0008] The condenser can be configured to receive flow of potable water to
be heated.
[0009] The evaporator can be configured to receive flow of waste-heat
bearing fluid.
[0010] According to another aspect of the present invention, a method of
controlling a
heat transfer system includes determining a condenser temperature at a
condenser connected
with an electrically controlled valve, an evaporator, and a compressor for
circulating a working
fluid. The compressor is operable at a controllable operating capacity. The
method further
includes adjusting the operating capacity of the compressor to maintain the
condenser
temperature at a condenser temperature set point, and reducing the operating
capacity of the
compressor when a discharge pressure of the compressor indicates that a
maximum operating
pressure of the compressor has been exceeded. The maximum operating pressure
corresponds
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to a saturation temperature that is higher than the condenser temperature set
point. The
method further includes returning to adjusting the operating capacity of the
compressor to
maintain the condenser temperature at the condenser temperature set point
after the
discharge pressure of the compressor returns to below the maximum operating
pressure.
[0011] The method can further include shutting down the compressor when the
discharge
pressure sensor indicates that a shutdown pressure has been exceeded. The
shutdown pressure
has a saturation temperature higher than the saturation temperature of the
maximum
operating pressure.
[0012] The method can further include incrementally closing the
electrically controlled
valve when a compressor discharge temperature falls below a minimum discharge
superheat
temperature.
[0013] The method can further include receiving flow of potable water to be
heated at the
condenser.
[0014] The method can further include receiving flow of waste-heat bearing
fluid at the
evaporator.
[0015] According to another aspect of the present invention, a heat
transfer system
includes a compressor for circulating a working fluid. The compressor has an
inlet and an outlet
and is operable at a controllable operating capacity. The system further
includes a condenser
connected to the outlet of the compressor. The condenser is configured to
receive flow of
water to be heated. The system further includes an electrically controlled
valve positioned to
receive working fluid from the outlet of the condenser, and an evaporator
connected between
an outlet of the electrically controlled valve and the inlet of the
compressor. The evaporator is
configured to receive flow of waste-heat bearing fluid. The system further
includes a discharge
pressure sensor located between the outlet of the compressor and the inlet of
the electrically
controlled valve, a condenser temperature sensor positioned to measure a
temperature of the
flow of water to be heated, and a controller connected to the compressor, the
discharge
pressure sensor, and the condenser temperature sensor. The controller is
configured to adjust
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the operating capacity of the compressor to maintain output of the condenser
temperature
sensor at a condenser temperature set point, except when output of the
discharge pressure
sensor indicates that a maximum operating pressure of the compressor has been
exceeded, in
which case the controller reduces the operating capacity of the compressor.
Brief Description of the Drawings
[0016] The drawings illustrate, by way of example only, embodiments of the
present
invention.
[0017] FIG. 1 is a diagram of a heat transfer system according to an
embodiment of the
present invention.
[0018] FIG. 2 is a pressure-enthalpy chart for the working fluid and the
heat transfer
system.
[0019] FIG. 3 is a block diagram of control logic of the controller.
[0020] FIG. 4 is a block diagram of decision logic of the controller.
[0021] FIG. 5 is a diagram of a heat transfer system according to another
embodiment.
[0022] FIG. 6 is a diagram of a heat transfer system according to another
embodiment.
[0023] FIG. 7 is a block diagram of control logic of the controller
according to another
embodiment.
[0024] FIG. 8 is a graph showing an example of compressor control.
Detailed Description
[0025] FIG. 1 shows a heat transfer system 10 according to an embodiment of
the present
invention. The heat transfer system may be known as a heat pump, refrigeration
loop, or
similar. The heat transfer system provides precise and robust control,
particularly when used in
waste-heat recovery and water heating for human use.
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[0026] The heat transfer system 10 includes a compressor 12, a condenser
14, an
electrically controlled expansion valve 16, and an evaporator 18 connected
together in a loop
by fluid conducting piping. A working fluid is provided to the system 10. The
working fluid can
include refrigerants, such as R-134a, R-12, R-124a, R-401a, R-404a, R-409A, R-
414A, or similar.
[0027] The compressor 12 is a screw-type compressor that circulates the
working fluid in
the system 10. The compressor 12 has an inlet for receiving working fluid in a
low-pressure
vapor state, and an outlet for discharging compressed working fluid as a high-
pressure vapor. In
other embodiments, the compressor is another kind of compressor.
[0028] The condenser 14 has an inlet connected to the outlet of the
compressor 12, and
has an outlet that feeds the electrically controlled valve 16. The condenser
14 can be
configured to receive water or other fluid to heat. In this embodiment, cold
water 22 flows into
the condenser 14 and leaves the condenser 14 as hot water 24. For example,
cold water 22
arrives at between 10 and 55 degrees Celsius and is heated to hot water 24 at
between 40 and
70 degrees Celsius. Other temperatures are also possible. These example
temperatures are
conducive to heating water for residential or hotel use for cleaning, washing,
cooking, or
bathing. Cold water 22 may be potable and may originate from a municipal
supply, from a re-
circulating hot water tank, from a boiler feed line, or similar.
[0029] The electrically controlled valve 16 is positioned to receive at its
inlet condensed
working fluid from the outlet of the condenser 14. The electrically controlled
valve 16 may be
known as an ETX valve. The electrically controlled valve 16 can include a
stepper motor and
gear assembly configured to position a pin in the port through which working
fluid flows, so as
to incrementally open or close the port to increase or decrease flow of
working fluid. The
electrically controlled valve 16 creates a controllable pressure drop in the
working fluid,
thereby expanding the working fluid into a mixed vapor-liquid state at its
outlet. Control of the
valve 16 controls the pressure drop and thus the exiting quality, temperature,
and pressure of
the working fluid.
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[0030] The evaporator 18 is connected between the outlet of the
electrically controlled
valve 16 and the inlet of the compressor 12. The evaporator 18 can be
configured to receive a
heat-bearing medium, such as water, an alternative liquid, or a gas. In this
embodiment, waste-
heat bearing fluid 26, such as that available from air-conditioning systems,
enters the
evaporator 18 and discharges its heat to the working fluid, before leaving the
evaporator 18 as
cooled fluid 28. The temperature of the arriving waste-heat bearing fluid 26
may be between
about 10 and 50 degrees Celsius. Other temperatures are also possible.
[0031] The system 10 may further include a subcooler 32 connected between
the
condenser 14 and the electrically controlled valve 16. Flow of working fluid
through the
subcooler 32 may discharge heat to very cold water 34, having a temperature
below the
temperature of the cold water input to the condenser 14. Warmed water exiting
the subcooler
32 may be fed into the condenser 14 as cold water 22.
[0032] The system 10 further includes a suction pressure sensor 42 located
between the
outlet of the electrically controlled valve 16 and the inlet of the compressor
12. In this
embodiment, the suction pressure sensor 42 is located near the inlet of the
compressor 12. The
specific location of the suction pressure sensor 42 can be varied, provided
that the pressure
drop expected between the location of the suction temperature sensor 44 and
the compressor
12 is taken into account.
[0033] The system 10 further includes a suction temperature sensor 44
located at the inlet
of the compressor 12.
[0034] The system 10 further includes a controller 50 connected to the
suction pressure
sensor 42, the suction temperature sensor 44, and the electrically controlled
valve 16. The
controller 50 can include a processor, memory, input interface, and output
interface. The
controller 50 is configured to adjust the electrically controlled valve 16 to
maintain output of
the suction pressure sensor 42 and the suction temperature sensor 44 at levels
above a
saturation point of the working fluid.
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[0035] FIG. 2 shows a pressure-enthalpy chart for the working fluid. No
specific working
fluid is depicted. However, the chart applies to at least those working fluids
mentioned herein.
lsothermals are shown in dashed line.
[0036] The controller 50 is configured to adjust (e.g., incrementally open
or close) the
electrically controlled valve 16 to maintain output of the suction temperature
sensor 44 at a
suction superheat temperature 62. To achieve this, a suction superheat set
point 64 is set
above the saturation point of the working fluid. Maintaining the suction
superheat temperature
62 to be at the suction superheat set point 64 can prevent the evaporator 18
from overheating
the working fluid, which may detrimentally affect output of the compressor 12
and cost
compressor power, and may also prevent under-heating the working fluid, which
can
advantageously prevent liquid-state working fluid from entering the compressor
12.
[0037] The controller 50 determines compressor inlet saturation temperature
from output
of the suction pressure sensor 42 and subtracts the determined saturation
temperature from
the output of the suction temperature sensor 44 to determine the actual
suction superheat
temperature 62. The controller 50 employs a suction superheat control loop to
maintain the
suction superheat temperature 62 at the suction superheat set point 64 by
controlling the
electrically controlled valve 16. Example values for the suction superheat set
point 64 include 3-
degrees Kelvin, and similar values above saturation suitable for a safety
margin above
saturation. The suction superheat set point 64 is a differential temperature
relative to the
saturation temperature and so can be expressed in relative units such as
Celsius or Fahrenheit
or absolute units such as Kelvin or Rankine.
[0038] In operation, when the heat input from the waste-heat bearing water
26 decreases,
the system 10 may tend to output lower temperature working fluid at the
evaporator 18, which
may bring the working fluid exiting the evaporator 18 towards a saturated
state. The risk of
saturation at the compressor inlet is reduced or prevented by the controller
50 maintaining the
suction superheat temperature 62 at the suction superheat set point 64.
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[0039] The controller 50 may also be configured to incrementally close the
electrically
controlled valve 16 to maintain the output of the suction pressure sensor 42
to below a
maximum suction pressure 65. This can advantageously maintain the suction
pressure below
the suction pressure limit of the compressor, particularly when the
temperature of waste-heat
bearing water 26 is relatively high. The maximum suction pressure 65 can be
expressed in units
of pressure or as a maximum saturation temperature, with output of the suction
pressure
sensor 42 being converted to saturation temperature to allow comparison.
[0040] Referring back to FIG. 1, the system 10 can further include a
discharge pressure
sensor 46 located between the outlet of the compressor 12 and the inlet of the
electrically
controlled valve 16. In this embodiment, the discharge pressure sensor 46 is
located near the
outlet of the compressor 12. The specific location of the discharge pressure
sensor 46 can be
varied, provided that the pressure drop expected between the location of the
discharge
pressure sensor 46 and the outlet of the compressor is taken into account. The
system 10 can
further include a discharge temperature sensor 48 located at the outlet of the
compressor 12.
[0041] The controller 50 can be further configured to incrementally close
the electrically
controlled valve 16 to maintain output of the discharge pressure sensor 46 and
the discharge
temperature sensor 48 at levels above saturation of the working fluid.
[0042] Referring again to FIG. 2, the controller 50 is configured to
incrementally close the
electrically controlled valve 16 to maintain output of the discharge
temperature sensor 48 at a
discharge superheat temperature 66 that is above a minimum discharge superheat
temperature 68. This can advantageously maintain the discharge superheat,
particularly on
start-up when the system 10 is cold or when the suction pressure is high and
the discharge
pressure is low. Operating the compressor 12 too close to saturation at
discharge can result in
liquid-state working fluid entering the lubricating oil system of the
compressor 12. This problem
is particularly evident in semi-hermetic screw-type compressors, which permit
working fluid to
enter the oil separator and may allow working fluid to cool significantly at
discharge. Thus, the
compressor discharge is controlled by maintaining the discharge superheat
temperature 66 at
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least a minimum discharge superheat temperature 68 amount above the saturation
point of
the working fluid.
[0043] The controller 50 determines compressor discharge saturation
temperature from
output of the discharge pressure sensor 46 and subtracts the determined
saturation
temperature from the output of the discharge temperature sensor 48 to
determine the actual
discharge superheat temperature 66. The controller 50 employs a discharge
superheat control
loop to maintain the discharge superheat temperature 66 at above the minimum
discharge
superheat temperature 68 by incrementally closing the electrically controlled
valve 16. Example
values for minimum discharge superheat temperature 68 include 20- 25 degrees
Kelvin, and
similar values above saturation sufficient to prevent working fluid from
cooling excessively
inside the compressor 12 where it may contaminate lubricating oil and reduce
the service life of
the compressor 12. The minimum discharge superheat temperature 68 is a
differential
temperature relative to the saturation temperature and so can be expressed in
relative units
such as Celsius or Fahrenheit or absolute units such as Kelvin or Rankine.
[0044] Discharge pressure of the compressor 12 can be allowed to float
based on control
using the suction superheat set point 64, maximum suction pressure 65, and the
minimum
discharge superheat temperature 68. The controller 50 is thus configured to
adjust the
electrically controlled valve 16 to maintain evaporator 18 pressure as high as
practical, while
not exceeding the suction pressure limit of the compressor 12 and also while
preventing
saturated working fluid from condensing in the oil separator of compressor
12..
[0045] It can be seen from FIG. 2 that the system 10, when applied to waste
heat recovery
for heating residential or hotel water, operates at a relatively high end of
the thermodynamic
cycle for the working fluid. This allows efficient use of commonly available
and safe working
fluids to recover waste heat.
[0046] FIG. 3 illustrates control logic resident in the controller 50. The
control logic can
implement the methods and other techniques described herein. As such, the
control logic may
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take the form of a specialized computer program, a group of parameters
inputted into a
preprogrammed control routine, or the like.
[0047] Output from the suction pressure sensor 42 is converted to a
saturation
temperature 82 at the inlet of the compressor 12. The can be performed with
reference to a
lookup table 84 that stores relationships between saturation pressures and
saturation
temperatures for the working fluid. The measured suction temperature from the
suction
temperature sensor 44 is reduced by the suction saturation temperature 82 to
arrive at the
actual suction superheat temperature 62.
[0048] The actual suction superheat temperature 62 and the suction
superheat set point
64 are provided as inputs to a suction superheat control loop 88 whose output
is a suction
superheat valve command 90 for adjusting the electrically controlled valve 16.
The suction
superheat valve command 90 is a change in valve position that brings the
actual suction
superheat temperature 62 towards the suction superheat set point 64. The
suction superheat
set point 64 can be inputted by an operator of the system 10. The actual
suction superheat
temperature 62 and the suction superheat set point 64 can be expressed as true
temperatures
on a standard scale (e.g., 25 degrees Celsius) or as temperatures relative to
saturation
temperature (e.g., 5 degrees Celsius or Kelvin, or by convention "5K"). It is
expected that such
an incremental change in the valve 16 position is an incremental opening or
closing of the valve
16.
[0049] Similarly, output from the discharge pressure sensor 46 is converted
to a saturation
temperature 92 at the outlet of the compressor 12 with reference to the lookup
table 84. The
measured discharge temperature from the discharge temperature sensor 48 is
reduced by the
discharge saturation temperature 92 to arrive at the actual discharge
superheat temperature
66. The actual discharge superheat temperature 66 and the minimum discharge
superheat
temperature 68 are provided as inputs to a discharge superheat control loop 96
whose output
is a discharge superheat valve command 98 for adjusting the electrically
controlled valve 16.
The discharge superheat valve command 98 is an incremental change in the valve
16 position
that keeps the actual discharge superheat temperature 66 above the minimum
discharge
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superheat temperature 68. It is expected that such an incremental change in
the valve 16
position is an incremental closing of the valve 16. The minimum discharge
superheat
temperature 68 can be inputted by an operator of the system 10. The superheat
temperatures
66, 68 can be expressed in a standard scale (e.g., 80 degrees Celsius) or as
relative
temperatures (e.g., 20K).
[0050] An operator-adjustable maximum suction pressure 65 and the output of
the suction
pressure sensor 42 are taken as inputs to a suction pressure control loop 102,
which outputs a
suction pressure valve command 104 representing an incremental change in the
valve 16
position that keeps the measured suction pressure below the maximum suction
pressure 65. It
is expected that such an incremental change in the valve 16 position is an
incremental closing of
the valve 16.
[0051] The control loops 88, 96, 102 may each be PI, PID, or P feedback
control loop that
provides error output representative of an incremental valve opening or
closing value. In this
embodiment, the control loops 88, 96, 102 are PI feedback control loops.
[0052] Decision logic 106 determines which of the valve commands 90, 98,
104 to send to
the electrically controlled valve 16 as the actual valve command 108. In this
embodiment, the
decision logic 106 selects the valve command 90, 98, 104 that requests the
largest increment of
closing. If no valve command 90, 98, 104 requests an incremental closing of
the valve 16, then
the control logic selects the suction superheat valve command 90. This results
in the ignoring of
any incremental open requests from the discharge superheat valve command 98
and the
suction pressure valve command 104. In other words, the controller 50
incrementally adjusts
the valve 16 based on the suction superheat set point 64, unless the discharge
superheat
temperature 68 falls below its minimum 68 or the suction pressure 42 exceeds
its maximum 65,
in which case the controller 50 incrementally closes the valve 16 by the
maximum closing
increment requested. That is, the suction superheat control loop 88 controls
the valve 16,
unless incremental valve closing is requested by either or both of the suction
pressure control
loop 102 and the discharge superheat control loop 96, in which case control of
the valve is
passed to the control loop 102, 96 or 88 requesting largest incremental amount
of valve closing.
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[0053] FIG. 4 illustrates an example embodiment of the decision logic 106,
assuming that
incremental close commands are represented by negative values and incremental
open
commands are represented by positive values. The lowest value out of the
suction superheat
valve command 90, the suction pressure valve command 104, and the discharge
superheat
valve command 98 is selected at 120. If the lowest value is not negative, as
determined at 122,
then the value of the suction superheat valve command 90 is taken, at 124, as
the output valve
command 108. If the lowest value is negative, then, at 126, the lowest value
is taken as the
output valve command 108 to control the valve 16 to incrementally close.
[0054] The control process illustrated in FIGs. 3 and 4 repeats in real
time or near real time,
as the system 10 operates.
[0055] The controller 50 may further provide for an alarm shutdown if any
of the sensors
42 ¨48 detects an abnormal condition on one of the control loops.
[0056] FIG. 5 illustrates another embodiment of a heat transfer system 130
according to
the present invention. The system 130 is similar to the system 10 and only
differences will be
discussed in detail. For description of other features and aspects of the
system 130, description
of the system 10 can be referenced, with like numerals identifying like
components.
[0057] The heat transfer system 130 uses two of the heat transfer systems
10, one a low-
pressure system 134 to provide initial heating to water and another a high-
pressure system 136
to provide further heating to the water.
[0058] The evaporators 18 may each receive waste-heat bearing fluid 26 and
output cooled
fluid 28. The subcoolers 32 may be fed in parallel with very cold water 32,
which is warmed at
22 and then fed through the low-pressure system's condenser 14 before being
fed through the
high-pressure system's condenser 14, so that the water is progressively
heated. Heated water
24 exits the first condenser 14 and further heated water 138 exits the second
condenser 14.
[0059] A controller 132 controls operation of the low-pressure system 134
and the high-
pressure system 136. The systems 134, 136 may use different working fluids and
may be
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controlled at different pressures and temperatures. However, the principles of
control are the
same as discussed above.
[0060] The controller 132 operates using the teachings discussed herein for
the control 50.
That is, the controller 132 references the compressor suction temperature and
pressure for
each system 134, 136 to adjust the respective electrically controlled valve 16
to maintain the
working fluid at the inlet of each of the compressors 50 at a respective
suction superheat set
point. At the same time, the controller 132 may reference compressor suction
pressure for
each system to incrementally close the respective electrically controlled
valves 16 to maintain
each suction pressure to below a respective maximum suction pressure. Further,
the controller
132 may control the discharge temperature and pressure for each system 134,
136 to adjust
the respective electrically controlled valve 16 to keep the working fluid at
the outlet of the
compressor 50 above a respective minimum discharge superheat temperature.
[0061] The suction superheat set points, the maximum suction pressures, and
the
minimum discharge superheat temperatures may be different or the same for each
of the low-
pressure system 134 and the high-pressure system 136. For example, the suction
superheat set
points and the minimum discharge superheat temperatures may be the same in
both the low-
pressure system 134 and the high-pressure system 136, while different maximum
suction
pressures may be used for the systems 134, 136. Other examples are also
contemplated.
[0062] FIG. 6 illustrates another embodiment of a heat transfer system 150
according to
the present invention. The system 150 is similar to the system 10 and only
differences will be
discussed in detail. For description of other features and aspects of the
system 150, description
of the system 10 can be referenced, with like numerals identifying like
components. In addition,
the system 150 can be used for each of the high- and low-pressure systems 136,
134 of FIG. 5.
[0063] The system 150 includes a condenser temperature sensor 49 located at
the
condenser 14. In this embodiment, the condenser temperature sensor 49 is
located at the
outlet of the condenser 14 in or near the flow of heated water 24. In other
embodiments, the
condenser temperature sensor 49 is located at other locations, such as inside
the water-side of
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the condenser itself or in the flow of heated water 24 downstream of the
condenser 14. Any of
these locations, as well as other locations, can be considered as at the
condenser 14.
[0064] The controller 152 references output of the condenser temperature
sensor 49 to
adjust the operating capacity of the compressor 12 to maintain the temperature
measured by
the condenser temperature sensor at a condenser temperature set point.
Further, the
controller 152 is also configured to reduce the operating capacity of the
compressor 12 when
the output of the discharge pressure sensor 46 indicates that a maximum
operating pressure of
the compressor 12 has been exceeded. The maximum operating pressure
corresponds to a
saturation temperature that is higher than the condenser temperature set
point. Further, the
compressor 12 may be shut down when output of the discharge pressure sensor 46
indicates
that a shutdown pressure has been exceeded. The shutdown pressure corresponds
to a
saturation temperature that is higher than the saturation temperature of the
maximum
operating pressure. In the present embodiment, the controller 152 is able to
keep the output
of the discharge pressure sensor 46 below the shutdown pressure until the
compressor reaches
it minimum capacity, at which point the controller 152 may no longer be able
to maintain the
output of the discharge pressure sensor 46 below the shutdown pressure without
turning the
compressor off. In other embodiments, the electrically controlled valve 16 can
be closed to
reduce the compressor capacity in conjunction with or without the compressor's
12 internal
capacity control.
[0065] The controller 152 can also be configured as discussed elsewhere
herein to, for
example, incrementally close the electrically controlled valve 16 when output
of the discharge
temperature sensor 48 falls below the minimum discharge superheat temperature
68,
incrementally close the valve 16 when the maximum suction pressure 65 is
exceeded, and
adjust the valve 16 so that the suction superheat temperature 62 tracks the
suction superheat
set point 64.
[0066] FIG. 7 illustrates control logic resident in the controller 152. The
control logic can
implement the methods and other techniques described herein. As such, the
control logic may
take the form of a specialized computer program, a group of parameters
inputted into a
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preprogrammed control routine, or the like. Control logic of the controller
152 can be
combined with control logic of the controller 50 (FIG. 3).
[0067] Output of the condenser temperature sensor 49 and a condenser
temperature set
point 162 are provided as inputs to a water-heating control loop 164 whose
output is an adjust
capacity command 166 for adjusting the operating capacity of the compressor
12. The adjust
capacity command 166 represents a change in compressor operating capacity that
brings the
measured condenser temperature towards the set point 162. For example, if the
measured
condenser temperature rises above the set point 162, then the compressor
capacity is reduced.
Conversely, if the measured condenser temperature drops below the set point
162, then the
compressor capacity is increased. The condenser temperature set point 162 can
be inputted by
an operator of the system 150. The output of the condenser temperature sensor
49 and the
condenser temperature set point 162 can be expressed as true temperatures on a
standard
scale (e.g., 60 degrees Celsius).
[0068] In this embodiment, a maximum operating pressure for the compressor
12 can be
inputted as a saturation temperature operating limit 168. Output from the
discharge pressure
sensor 46 is converted to the discharge saturation temperature 92 with
reference to the lookup
table 84, which stores relationships between saturation pressures and
saturation temperatures
for the working fluid. The discharge saturation temperature 92 and the
saturation temperature
limit 168 are used as inputs by a compressor back-off control loop 170, which
is configured to
output an adjust capacity command 172 to keep the discharge saturation
temperature 92
below the saturation temperature limit 168. In other embodiments, the
compressor back-off
control loop 170 operates on pressure values directly.
[0069] In addition, a shutdown pressure for the compressor 12 can be
inputted as a
shutdown saturation temperature 174. The discharge saturation temperature 92
and the
shutdown saturation temperature 174 are used as inputs by a compressor
shutdown check 176
which is configured to output a shutdown command 178 when the compressor 12
maximum
safe pressure has been exceeded. In other embodiments, the compressor shutdown
check 176
operates on pressure values directly.
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[0070] The control loops 164, 170 may each be PI, PID, or P feedback
control loop that
provides error output representative of an incremental change in compressor
capacity. The
compressor shutdown check 176 can include a logical comparison that acts on
inputted values
and outputs a resulting value.
[0071] Decision logic 180 selects one of the adjust capacity commands 166,
172 and the
shutdown command 178 to send to the compressor 12 as the actual compressor
command 182.
In this embodiment, the decision logic 180 is as follows. Presence of a
shutdown command 178
always results in the compressor 12 being shut down. Otherwise, the adjust
capacity command
172 from the back-off control loop 170 is selected if it represents the
greatest amount of
reduced capacity for the compressor 12, while the adjust capacity command 166
from the
water-heating control loop 164 is selected at all other times. In other words,
assuming that
reducing capacity commands have negative values and increasing capacity
commands are
positive, if no shutdown command 178 is present, then the decision logic 180
selects the lowest
negative value from the control loops 164, 170. If no negative value is
available, then the
decision logic 180 selects the positive value from the water-heating control
loop 164. Thus, the
compressor 12 is operated according to the water-heating control loop 164,
unless further
reduced capacity is demanded by the back-off control loop 170, and both of
these control
schemes are overridden by the shutdown check 176.
[0072] FIG. 8 shows an example of operation of the system 150 as controlled
by the
controller 152 when the system 150 is used for water heating.
[0073] Initially, the temperature of hot water 24 output by the condenser
14, as measured
by the condenser temperature sensor 49, tracks the set point 162 of, for
example, 60 degrees
Celsius. The discharge saturation temperature 92 of the compressor 12 is
somewhat higher
(e.g., 64 degrees Celsius) at this time due to the temperature approach at the
hot end which, is
present to a larger or smaller degree in all heat exchangers, and the
compressor 12 is running
at full capacity.
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[0074] After operating at steady state for a time, a disturbance
occurs, such as a normal
reduction in flow rate of cold water 22 into the condenser 14. Consequently,
the hot water
temperature 49 rises. The water-heating control loop 164 responds by reducing
the compressor
capacity. However, the hot water temperature 49 continues to rise. As a
result, the compressor
discharge saturation temperature 92 rises to exceed the saturation temperature
limit 168 (e.g.,
68 degrees Celsius). In response, the compressor back-off control loop 170
commands the
compressor 12 to further reduce capacity until the discharge saturation
temperature 92 is
below the saturation temperature limit 168, thereby avoiding compressor
shutdown at the
shutdown saturation temperature 174 (e.g., 72 degrees Celsius).
[0075] A short time later, the flow rate of cold water 22 into the
condenser 14 increases,
and the temperatures 92, 49 drop enough so that control of the compressor 12
is returned to
the water-heating control loop 164.
[0076] As can be seen, controlling the compressor 12 in this way
prevents shutdown of the
compressor 12. Further, use of the compressor back-off control loop 170
permits the operation
of system 150 over a wider range of inlet conditions for cold water 22. By
enabling system 150
to operate in this manner, the number of starts and stops of compressor 12 are
reduced. This is
advantageous as it provides more consistent temperature of hot water 24 and
extends the life
of compressor 12. Other advantages will be apparent to those skilled in the
art.
[0077] In view of the above, it should be understood that the
control techniques and
systems described herein are precise, robust, and efficient, and particularly
well suited for
, control of heat transfer systems used for waste heat recovery to
heat water for human use in
cooking, cleaning, bathing and other activities.
[0078] While the foregoing provides certain non-limiting example
embodiments, it should
be understood that combinations, subsets, and variations of the foregoing are
contemplated.
The monopoly sought is defined by the claims.
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