U.S. patent application number 15/528206 was filed with the patent office on 2017-11-16 for a method for controlling a supply of refrigerant to an evaporator including calculating a reference temperature.
The applicant listed for this patent is DANFOSS A/S. Invention is credited to Roozbeh Izadi-Zamanabadi.
Application Number | 20170328617 15/528206 |
Document ID | / |
Family ID | 52146114 |
Filed Date | 2017-11-16 |
United States Patent
Application |
20170328617 |
Kind Code |
A1 |
Izadi-Zamanabadi; Roozbeh |
November 16, 2017 |
A METHOD FOR CONTROLLING A SUPPLY OF REFRIGERANT TO AN EVAPORATOR
INCLUDING CALCULATING A REFERENCE TEMPERATURE
Abstract
A method for controlling a supply of refrigerant to an
evaporator (2) of a vapour compression system (1) is disclosed.
During a system identification phase an opening degree (12) of the
expansion valve (3) is alternatingly increased and decreased, and a
maximum temperature difference, (S.sub.4-S.sub.2).sub.max, between
temperature, S.sub.4, of air flowing away from the evaporator (2)
and temperature, S.sub.2, of refrigerant leaving the evaporator (2)
is determined. During normal operation, the supply of refrigerant
to the evaporator (2) is controlled by calculating a reference
temperature, S.sub.2,ref, based on the monitored temperature,
S.sub.4, and the maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, determined during the system
identification phase. The supply of refrigerant to the evaporator
(2) is controlled in order to obtain a temperature, S.sub.2, of
refrigerant leaving the evaporator (2) which is substantially equal
to the calculated reference temperature, S.sub.2,ref.
Inventors: |
Izadi-Zamanabadi; Roozbeh;
(Sonderborg, DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DANFOSS A/S |
Nordborg |
|
DK |
|
|
Family ID: |
52146114 |
Appl. No.: |
15/528206 |
Filed: |
October 8, 2015 |
PCT Filed: |
October 8, 2015 |
PCT NO: |
PCT/EP2015/073232 |
371 Date: |
May 19, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25B 2500/19 20130101;
F25B 41/062 20130101; F25B 49/02 20130101; F25B 2500/26 20130101;
F25B 2600/2513 20130101; F25B 2700/21173 20130101; F25B 2700/21175
20130101 |
International
Class: |
F25B 41/06 20060101
F25B041/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2014 |
EP |
14197575.5 |
Claims
1. A method for controlling a supply of refrigerant to an
evaporator of a vapour compression system, the vapour compression
system comprising at least one evaporator, at least one compressor,
at least one condenser and at least one expansion valve arranged in
a refrigerant path, the method comprising the steps of: initiating
a system identification phase, in which an opening degree of the
expansion valve is alternatingly increased and decreased, during
the system identification phase, monitoring a temperature, S.sub.2,
of refrigerant leaving the evaporator, and a temperature, S.sub.4,
of air flowing across the evaporator, at a position where the air
is flowing away from the evaporator, and determining a maximum
temperature difference, (S.sub.4-S.sub.2).sub.max, between the
monitored temperatures, upon completion of the system
identification phase, controlling a supply of refrigerant to the
evaporator by: monitoring the temperature, S.sub.2, of refrigerant
leaving the evaporator, and the temperature, S.sub.4, of air
flowing away from the evaporator, calculating a reference
temperature, S.sub.2,ref, based on the monitored temperature,
S.sub.4, of air flowing away from the evaporator and the maximum
temperature difference, (S.sub.4-S.sub.2).sub.max, determined
during the system identification phase, and controlling the supply
of refrigerant to the evaporator, based on the calculated reference
temperature, S.sub.2,ref, and in order to obtain a temperature,
S.sub.2, of refrigerant leaving the evaporator which is
substantially equal to the calculated reference temperature,
S.sub.2,ref.
2. The method according to claim 1, wherein the step of calculating
a reference temperature, S.sub.2,ref, comprises calculating a mean
temperature, S.sub.4, of the temperature, S.sub.4, of air flowing
away from the evaporator, during a predefined previous time
interval.
3. The method according to claim 2, wherein the reference
temperature, S.sub.2,ref, is calculated as:
S.sub.2,ref=S.sub.4-(S.sub.4-S.sub.2).sub.max+.DELTA., wherein
.DELTA. is a constant safety value.
4. The method according to claim 1, wherein the step of controlling
the supply of refrigerant to the evaporator is performed using a
proportional integral (PI) regulator.
5. The method according to claim 4, further comprising the steps
of, during the system identification phase, determining one or more
system dynamic parameters, supplying the system dynamic parameters
to the proportional integral (PI) regulator, and designing the
proportional integral (PI) regulator in accordance with the system
dynamic parameters.
6. The method according to claim 1, wherein the step of controlling
the supply of refrigerant to the evaporator further comprises the
steps of: obtaining an air temperature, T.sub.air, of an air flow
across the evaporator, comparing the obtained air temperature,
T.sub.air, to a reference value, T.sub.air,ref, and controlling the
supply of refrigerant to the evaporator on the basis of the
comparing step, as well as on the basis of the calculated reference
temperature, S.sub.2,ref.
7. The method according to claim 2, wherein the step of controlling
the supply of refrigerant to the evaporator is performed using a
proportional integral (PI) regulator.
8. The method according to claim 3 wherein the step of controlling
the supply of refrigerant to the evaporator is performed using a
proportional integral (PI) regulator.
9. The method according to claim 2, wherein the step of controlling
the supply of refrigerant to the evaporator further comprises the
steps of: obtaining an air temperature, T.sub.air, of an air flow
across the evaporator, comparing the obtained air temperature,
T.sub.air, to a reference value, T.sub.air,ref, and controlling the
supply of refrigerant to the evaporator on the basis of the
comparing step, as well as on the basis of the calculated reference
temperature, S.sub.2,ref.
10. The method according to claim 3, wherein the step of
controlling the supply of refrigerant to the evaporator further
comprises the steps of: obtaining an air temperature, T.sub.air, of
an air flow across the evaporator, comparing the obtained air
temperature, T.sub.air, to a reference value, T.sub.air,ref, and
controlling the supply of refrigerant to the evaporator on the
basis of the comparing step, as well as on the basis of the
calculated reference temperature, S.sub.2,ref.
11. The method according to claim 4, wherein the step of
controlling the supply of refrigerant to the evaporator further
comprises the steps of: obtaining an air temperature, T.sub.air, of
an air flow across the evaporator, comparing the obtained airs,
T.sub.air, to a reference value, T.sub.air,ref, and controlling the
supply of refrigerant to the evaporator on the basis of the
comparing step, as well as on the basis of the calculated reference
temperature, S.sub.2,ref.
12. The method according to claim 5, wherein the step of
controlling the supply of refrigerant to the evaporator further
comprises the steps of: obtaining an air temperature, T.sub.air, of
an air flow across the evaporator, comparing the obtained air
temperature, T.sub.air, to a reference value, T.sub.air,ref, and
controlling the supply of refrigerant to the evaporator on the
basis of the comparing step, as well as on the basis of the
calculated reference temperature, S.sub.2,ref.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a National Stage application of
International Patent Application No. PCT/EP2015/073232, filed on
Oct. 8, 2015, which claims priority to European Patent Application
No. 14197575.5, filed on Dec. 12, 2014, each of which is hereby
incorporated by reference in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a method for controlling a
supply of refrigerant to an evaporator. More particularly, in the
method of the invention the supply of refrigerant is controlled on
the basis of a reference temperature, which has been calculated on
the basis of temperature measurements.
BACKGROUND
[0003] Vapour compression systems, such as refrigeration systems,
air condition systems or heat pumps, normally comprise at least one
compressor, at least one condenser, at least one expansion device,
e.g. in the form of expansion valves, and at least one evaporator
arranged along a refrigerant path. Refrigerant circulates the
refrigerant path and is alternatingly expanded and compressed, and
heat exchange takes place in the condensers and the evaporators.
Expanded refrigerant enters the evaporators in a mixed state of
gaseous and liquid refrigerant. As the refrigerant passes through
the evaporators, it evaporates while exchanging heat with a
secondary fluid flow, such as an air flow, across each evaporator.
In order to utilise the potential refrigerating capacity of a given
evaporator to a maximum extent, it is desirable that liquid
refrigerant is present along the entire length of the evaporator.
On the other hand, it is undesirable that liquid refrigerant passes
through the evaporator and into the suction line, since it may
cause damage to the compressors if liquid refrigerant reaches the
compressors. It is therefore desirable to control the supply of
refrigerant to the evaporators in such a manner that, in a given
evaporator, the boundary between mixed phase refrigerant and
gaseous refrigerant is exactly at the outlet of the evaporator.
[0004] In order to obtain this, the superheat of the refrigerant
leaving the evaporators is often measured and/or calculated. The
superheat is the difference between the temperature of the
refrigerant leaving the evaporator and the dew point of the
refrigerant leaving the evaporator. A low superheat value, thus,
indicates that the temperature of the refrigerant leaving the
evaporator is close to the dew point, while a high superheat value
indicates that the temperature of refrigerant leaving the
evaporator is significantly higher than the dew point, and that a
significant part of the evaporator therefore contains gaseous
refrigerant. In the part of the evaporator which contains gaseous
refrigerant, the heat transfer between the ambient and the
refrigerant flowing in the evaporator is significantly lower than
in the part of the evaporator which contains a mixture of gaseous
and liquid refrigerant. Therefore the overall efficiency of the
evaporator is reduced when a significant part of the evaporator
contains gaseous refrigerant. It is therefore attempted to control
the supply of refrigerant to the evaporator in such a manner that
the superheat value is maintained at a small, but positive,
level.
[0005] WO 2012/052019 A1 describes a method for controlling a
supply of refrigerant to an evaporator, in which the SH=0 point can
be determined purely on the basis of a measured temperature signal.
A component, such as an expansion valve, a fan or a compressor, is
actuated in such a manner that a dry zone of the evaporator is
changed. A temperature signal, representing a temperature of
refrigerant leaving the evaporator is measured and analysed, e.g.
including deriving a rate of change signal. Then a temperature
value where a gain of a transfer function between the actuated
component and the measured temperature signal drops from a maximum
value to a minimum value is determined. The determined temperature
value is defined as corresponding to a zero superheat value
(SH=0).
SUMMARY
[0006] It is an object of embodiments of the invention to provide a
method for controlling a supply of refrigerant to an evaporator, in
which a reference temperature can be calculated dynamically.
[0007] It is a further object of embodiments of the invention to
provide a method for controlling a supply of refrigerant to an
evaporator, in which an optimal superheat of refrigerant leaving
the evaporator can be obtained, even if operating conditions
change.
[0008] The invention provides a method for controlling a supply of
refrigerant to an evaporator of a vapour compression system, the
vapour compression system comprising at least one evaporator, at
least one compressor, at least one condenser and at least one
expansion valve arranged in a refrigerant path, the method
comprising the steps of: [0009] initiating a system identification
phase, in which an opening degree of the expansion valve is
alternatingly increased and decreased, [0010] during the system
identification phase, monitoring a temperature, S.sub.2, of
refrigerant leaving the evaporator, and a temperature, S.sub.4, of
air flowing across the evaporator, at a position where the air is
flowing away from the evaporator, and determining a maximum
temperature difference, (S.sub.4-S.sub.2).sub.max, between the
monitored temperatures, [0011] upon completion of the system
identification phase, controlling a supply of refrigerant to the
evaporator by: [0012] monitoring the temperature, S.sub.2, of
refrigerant leaving the evaporator, and the temperature, S.sub.4,
of air flowing away from the evaporator, [0013] calculating a
reference temperature, S.sub.2,ref, based on the monitored
temperature, S.sub.4, of air flowing away from the evaporator and
the maximum temperature difference, (S.sub.4-S.sub.2).sub.max,
determined during the system identification phase, and [0014]
controlling the supply of refrigerant to the evaporator, based on
the calculated reference temperature, S.sub.2,ref, and in order to
obtain a temperature, S.sub.2, of refrigerant leaving the
evaporator which is substantially equal to the calculated reference
temperature, S.sub.2,ref.
[0015] In the present context the term `vapour compression system`
should be interpreted to mean any system in which a flow of fluid
medium, such as refrigerant, circulates and is alternatingly
compressed and expanded, thereby providing either refrigeration or
heating of a volume. Thus, the vapour compression system may be a
refrigeration system, an air condition system, a heat pump,
etc.
[0016] The vapour compression system comprises at least one
evaporator, at least one compressor, at least one condenser, and at
least one expansion valve. Thus, the vapour compression system may
comprise only one of each of these components, or the vapour
compression system may comprise two or more of any of these
components. For instance, the vapour compression system may
comprise a single compressor, or it may comprise two or more
compressors, e.g. arranged in a compressor rack. Similarly, the
vapour compression system may comprise only one evaporator, or it
may comprise two or more evaporators. In the latter case each
evaporator may be arranged to provide refrigeration for a separate
refrigerated volume. The separate refrigerated volumes may, e.g.,
be separate display cases of a supermarket. In any event, each
evaporator is preferably connected to a separate expansion valve
which controls the supply of refrigerant to that evaporator,
independently of the refrigerant supply to the other evaporators.
Furthermore, an evaporator unit may comprise a single section, or
two or more sections which may be connected in series or in
parallel.
[0017] The method according to the invention is related to control
of the supply of refrigerant to a single evaporator, via the
corresponding expansion valve. However, this evaporator may very
well be arranged in a vapour compression system comprising one or
more additional evaporators, in which case the supply of
refrigerant to these additional evaporators is controlled
separately.
[0018] According to the method of the invention a system
identification phase is first initiated. During the system
identification phase an opening degree of the expansion valve is
alternatingly increased and decreased, i.e. the opening degree of
the expansion valve is modulated or pulsated. In the present
context the term `system identification phase` should be
interpreted to mean a time period, in which various characteristics
of the vapour compression system, such as dynamics of the vapour
compression system, can be identified, in order to take such
characteristics into account when operating the vapour compression
system.
[0019] During the system identification phase, a temperature,
S.sub.2, of refrigerant leaving the evaporator, and a temperature,
S.sub.4, of air flowing across the evaporator, at a position where
the air is flowing away from the evaporator, are monitored. The
temperature, S.sub.2, is related to the superheat of the
refrigerant leaving the evaporator. The temperature, S.sub.4, is a
temperature of air flowing across the evaporator after heat
exchange has taken place with refrigerant flowing through the
evaporator, i.e. the air has been cooled by the evaporator.
Accordingly, the temperature, S.sub.4, reflects a temperature
prevailing in a refrigerated volume, as well as the instantaneous
cooling power of the evaporator, since a high cooling power will
reduce this temperature.
[0020] The temperatures, S.sub.2 and S.sub.4, may, e.g., be
measured by means of temperature sensors arranged at appropriate
positions.
[0021] Based on the monitored temperatures, S.sub.2 and S.sub.4, a
maximum temperature difference, (S.sub.4-S.sub.2).sub.max, between
the monitored temperatures is determined. The alternating increases
and decreases of the opening degree of the expansion valve during
the system identification phase causes the monitored temperatures
to alternatingly increase and decrease. However, the variations in
the temperature, S.sub.2, of refrigerant leaving the evaporator are
expected to be larger than the variations in the temperature,
S.sub.4, of air flowing across the evaporator. Thereby the
difference between the monitored temperatures, S.sub.2 and S.sub.4,
varies across a period of increasing and decreasing the opening
degree of the expansion valve. For each period, the maximum
difference between the monitored temperatures is determined.
Subsequently, the maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, is determined as the maximum value of
the temperature differences determined for each of the periods.
[0022] Accordingly, the maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, provides information regarding how far
the temperature, S.sub.2, of refrigerant leaving the evaporator is
from an optimal superheat value, under the given circumstances, and
thereby information regarding how much further the temperature,
S.sub.2, can be lowered without risking flooding of the evaporator,
i.e. that liquid refrigerant is allowed to pass through the
evaporator. During the system identification phase the evaporator
will be nearly flooded in several attempts or periods. The rational
is to attempt to move the operating point to its expected, cold,
region. When the evaporator is nearly flooded the temperature,
S.sub.2, of refrigerant leaving the evaporator approaches a level
near the dew point temperature. In this case it provides a good
estimate of the dew point temperature. At the same time the
temperature, S.sub.4, of air flowing away from the evaporator
approaches its lowest possible level. The distance between the
lowest S.sub.4 value and the lowest S.sub.2 value in the same
period provides an estimated measure for the expected temperature
difference between the refrigerant dew point and the S.sub.4
measurements when the evaporator is almost filled with liquid
refrigerant, and the refrigerated space is cooled down. This
distance enables us to define a setpoint for the temperature,
S.sub.2, of refrigerant leaving the evaporator, which is
appropriately close to the dew point temperature, while avoiding
flooding of the evaporator.
[0023] Upon completion of the system identification phase, the
supply of refrigerant to the evaporator is controlled in the
following manner.
[0024] The temperature, S.sub.2, of refrigerant leaving the
evaporator and the temperature, S.sub.4, of air flowing across the
evaporator in a direction away from the evaporator are monitored,
essentially as described above.
[0025] Then a reference temperature, S.sub.2,ref, is calculated,
based on the monitored temperature, S.sub.4, and the maximum
temperature difference, (S.sub.4-S.sub.2).sub.max, which was
determined during the system identification phase. Accordingly, the
reference temperature, S.sub.2,ref, is calculated dynamically
during operation of the vapour compression system, including the
control of the supply of refrigerant to the evaporator. Thus, if
the monitored temperature, S.sub.4, changes, then the calculated
reference temperature, S.sub.2,ref, changes accordingly. The
temperature, S.sub.4, is an appropriate parameter for this purpose,
because, in steady state operation of the vapour compression
system, the temperature, S.sub.4, is the coldest signal, and
variations in the temperature, S.sub.4, are directly correlated to
the dew point temperature of refrigerant leaving the evaporator.
Thus, if the dew point temperature of refrigerant leaving the
evaporator increases, then the temperature, S.sub.4, of air flowing
away from the evaporator also increases. Similarly, of the dew
point temperature of refrigerant leaving the evaporator decreases,
e.g. due to the suction pressure being lowered, then the
temperature, S.sub.4, of air flowing away from the evaporator also
decreases.
[0026] Furthermore, since the reference temperature, S.sub.2,ref,
is calculated based on the maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, it also reflects how much further the
temperature, S.sub.2, of refrigerant leaving the evaporator can
safely be decreased, as described above.
[0027] The step of calculating a reference temperature,
S.sub.2,ref, could, e.g., be performed using a calculator, which
may, e.g., be or form part of a controller or a micro
processor.
[0028] Finally, the supply of refrigerant to the evaporator is
controlled, based on the calculated reference temperature,
S.sub.2,ref, and in order to obtain a temperature, S.sub.2, of
refrigerant leaving the evaporator which is substantially equal to
the calculated reference temperature, S.sub.2,ref. Thereby it is
ensured that the supply of refrigerant to the evaporator is
controlled in such a manner that a safe level of superheat of the
refrigerant leaving the evaporator is maintained, without having to
calculate the superheat value or measure the pressure of the
refrigerant leaving the evaporator.
[0029] The step of calculating a reference temperature,
S.sub.2,ref, may comprise calculating a mean temperature, S.sub.4,
of the temperature, S.sub.4, of air flowing away from the
evaporator, during a predefined previous time interval. According
to this embodiment, a mean or average value of the temperature,
S.sub.4, is obtained during a time interval of a suitable length,
and the obtained mean value is used for calculating the reference
temperature, S.sub.2,ref, instead of the instantaneously measured
temperature, S.sub.4. Thereby, the temperature value applied when
the reference temperature, S.sub.2,ref, is calculated represents a
typical temperature prevailing in the air flowing across the
evaporator, at a position where the air is flowing away from the
evaporator, and any fluctuations or variations on a short time
scale are not taken into account.
[0030] The predefined previous time period could, e.g., simply be a
time period of a given length which has lapsed immediately before
the current point in time. As an alternative, the predefined
previous time period could be a previous night period or a previous
day period. In this case, one mean temperature may be applied
during the day time and another mean temperature may be applied
during the night time.
[0031] The reference temperature, S.sub.2,ref, may, in this case,
be calculated as:
S.sub.2,ref-S.sub.4-(S.sub.4-S.sub.2).sub.max+.DELTA.,
wherein .DELTA. is a constant safety value.
[0032] According to this embodiment, the reference temperature,
S.sub.2,ref, is calculated as the difference between the mean
value, S.sub.4, of the temperature of the air flowing away from the
evaporator, and the maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, which was determined during the system
identification phase. This will, in principle, provide a reference
temperature, S.sub.2,ref, which corresponds to a zero superheat
value (SH=0). However, in order to provide a safety margin for the
controlled temperature of refrigerant leaving the evaporator,
thereby ensuring that the superheat value of the refrigerant
leaving the evaporator does not become critically low, thereby
risking that liquid refrigerant passes through the evaporator, the
constant safety value, .DELTA., is added. Thus, according to this
embodiment, the supply of refrigerant to the evaporator is
controlled in such a manner that an optimum superheat value of the
refrigerant leaving the evaporator is obtained.
[0033] The step of controlling the supply of refrigerant to the
evaporator may be performed using a proportional integral (PI)
regulator. This is a standard component, and it provides a very
easy way of controlling the vapour compression system. The
calculated reference temperature, S.sub.2,ref, and the measured
temperature, S.sub.2, of the refrigerant leaving the evaporator are
supplied to the PI regulator, and based thereon the supply of
refrigerant to the evaporator is controlled in such a manner that
the supply of refrigerant is increased in the case that the
measured temperature, S.sub.2, is higher than the reference
temperature, S.sub.2,ref, and the supply of refrigerant is
decreased in the case that the measured temperature, S.sub.2, is
lower than the reference temperature, S.sub.2,ref.
[0034] As an alternative, other classical regulators could be
applied.
[0035] The method may further comprise the steps of, during the
system identification phase, determining one or more system dynamic
parameters, supplying the system dynamic parameters to the
proportional integral (PI) regulator, and designing the
proportional integral (PI) regulator in accordance with the system
dynamic parameters. According to this embodiment, the PI regulator
is designed in accordance with actual information regarding the
system dynamics. This ensures an accurate control of the supply of
refrigerant to the evaporator.
[0036] The system dynamic parameters may, e.g., include various
time constants, time delays and/or gains of the vapour compression
system.
[0037] The step of controlling the supply of refrigerant to the
evaporator may further comprise the steps of: [0038] obtaining an
air temperature, T.sub.air, of an air flow across the evaporator,
[0039] comparing the obtained air temperature, T.sub.air, to a
reference value, T.sub.air,ref, and [0040] controlling the supply
of refrigerant to the evaporator on the basis of the comparing
step, as well as on the basis of the calculated reference
temperature, S.sub.2,ref.
[0041] According to this embodiment, the supply of refrigerant to
the evaporator is further controlled in order to obtain a desired
temperature of the air flowing across the evaporator. This will
typically correspond to a temperature of air inside a refrigerated
volume. Accordingly, the supply of refrigerant to the evaporator is
controlled in order to obtain a desired temperature inside a
refrigerated volume.
[0042] The air temperature, T.sub.air, may be the temperature of
air flowing towards the evaporator, i.e. before the air is cooled
the evaporator, the temperature of air flowing away from the
evaporator, i.e. immediately after the air has been cooled by the
evaporator, or a weighted value of the temperature of air flowing
towards the evaporator and the temperature of air flowing away from
the evaporator. Taking the air temperature into account in this
manner ensures that the opening degree of the expansion valve, and
thereby the supply of refrigerant to the evaporator, is varied in a
smooth and continuous manner.
[0043] The supply of refrigerant to the evaporator may be
controlled using a single PI regulator taking the temperature of
refrigerant leaving the evaporator, as well as the air temperature
into account.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] The invention will now be described in further detail with
reference to the accompanying drawings in which
[0045] FIG. 1 is a diagrammatic view of a part of a vapour
compression system for performing a method according to an
embodiment of the invention,
[0046] FIG. 2 is a graph illustrating opening degree of an
expansion valve and various temperature measurements during
operation of a vapour compression system using a method according
to an embodiment of the invention, and
[0047] FIG. 3 is a block diagram illustrating a method according to
an embodiment of the invention.
DETAILED DESCRIPTION
[0048] FIG. 1 is a diagrammatic view of a part of a vapour
compression system 1 for performing a method according to an
embodiment of the invention. The vapour compression system 1
comprises an evaporator 2 arranged in a refrigerant path along with
one or more compressors (not shown) and one or more condensers (not
shown). An expansion valve 3 is also arranged in the refrigerant
path for controlling the supply of refrigerant to the evaporator
2.
[0049] The vapour compression system 1 further comprises a number
of temperature sensors. A first temperature sensor 4 is arranged in
the refrigerant path after the outlet of the evaporator 2.
Accordingly, the first temperature sensor 4 measures a temperature
signal, S.sub.2, which represents the temperature of refrigerant
leaving the evaporator 2.
[0050] A second temperature sensor 5 is arranged in a secondary air
flow across the evaporator 2, at a position before the air reaches
the evaporator 2. Accordingly, the second temperature sensor 5
measures a temperature signal, S.sub.3, which represents the
temperature of air flowing towards the evaporator 2. It should be
noted that, for the purpose of performing the method according to
the invention, the second temperature sensor 5 could be
omitted.
[0051] A third temperature sensor 6 is arranged in the secondary
air flow across the evaporator 2, at a position after the air has
passed the evaporator 2. Accordingly, the third temperature sensor
6 measures a temperature signal, S.sub.4, which represents the
temperature of air flowing away from the evaporator 2.
[0052] During a system identification phase, an opening degree of
the expansion valve 3 is alternatingly increased and decreased.
Simultaneously, the temperature signals, S.sub.2 and S.sub.4, are
monitored and supplied to a reference temperature calculator 7. In
the reference temperature calculator 7 a maximum temperature
difference, (S.sub.4-S.sub.2).sub.max, is determined, based on the
measured temperature signals, S.sub.2 and S.sub.4. This will be
described in further detail below with reference to FIG. 2. The
reference temperature calculator 7 may, e.g., be or form part of a
controller or a micro processor.
[0053] When the system identification phase has been completed, the
temperature signal, S.sub.4, is monitored and supplied to the
reference temperature calculator 7. The reference temperature
calculator 7 then calculates a reference temperature, S.sub.2,ref,
based on the monitored temperature, S.sub.4, and the maximum
temperature difference, (S.sub.4-S.sub.2).sub.max, which was
determined during the system identification phase. The reference
temperature, S.sub.2,ref, may, e.g., be calculated as:
S.sub.2,ref=S.sub.4-(S.sub.4-S.sub.2).sub.max+.DELTA.,
where S.sub.4 is a mean value of S.sub.4 obtained during a
predefined previous time interval, and A is a constant safety
value. The reference temperature, S.sub.2,ref, calculated in this
manner represents a temperature of refrigerant leaving the
evaporator 2, which provides an optimum superheat of the
refrigerant.
[0054] The calculated reference temperature, S.sub.2,ref, is
supplied to a proportional integral (PI) regulator 8. Furthermore,
the temperature signal, S.sub.2, is monitored and supplied to the
PI regulator 8. Based thereon the PI regulator 8 generates a
control signal for the expansion valve 3. In response to the
control signal, the expansion valve 3 adjusts the opening degree,
and thereby the supply of refrigerant to the evaporator 2, in order
to obtain a temperature, S.sub.2, of refrigerant leaving the
evaporator 2, which is substantially equal to the calculated
reference temperature, S.sub.2,ref.
[0055] Furthermore, the temperature signals, S.sub.3 and S.sub.4,
are supplied to a sensor selection unit 9. The sensor selection
unit 9 selects whether to select one of the temperature signals,
S.sub.3 and S.sub.4, as an air temperature, T.sub.air, being
representative for a temperature prevailing inside a refrigerated
volume, or to select a weighted value of the two temperature
signals, S.sub.3 and S.sub.4. The selection may, e.g., be based on
the availability of sensors 5 and 6, or on the choice of the
installer. Based on the selection, a temperature signal, T.sub.air,
is generated, and T.sub.air is supplied to an air tracking unit
10.
[0056] A reference air temperature, T.sub.air,ref, is also supplied
to the air tracking unit 10. The reference air temperature,
T.sub.air,ref, represents a reference or target temperature which
is desired inside the refrigerated volume, i.e. in the air flowing
across the evaporator 2.
[0057] The air tracking unit 10 compares the temperature signal,
T.sub.air, to the reference air temperature, T.sub.air,ref, and
generates a signal which is supplied to the PI regulator 8. The
generated signal indicates how close the actual air temperature,
T.sub.air, is to the reference air temperature, T.sub.air,ref.
Thus, when the PI regulator 8 generates the control signal for the
expansion valve 3, it takes into account that adjustments to the
opening degree of the expansion valve 3 may also be required in
order to obtain a desired temperature of the air flowing across the
evaporator 2. Accordingly, the opening degree of the expansion
valve 3, and thereby the supply of refrigerant to the evaporator 2,
is controlled in order to obtain a reference temperature,
S.sub.2,ref, of refrigerant leaving the evaporator 2, as well as in
order to obtain a reference temperature, T.sub.air,ref, of air
flowing across the evaporator 2, inside a refrigerated volume.
[0058] The vapour compression system 1 is further provided with
safety logic 11 in order to detect possible errors or safety
issues. For instance, the safety logic 11 may detect if there is a
risk of flooding the evaporator 2, i.e. if the superheat value of
refrigerant leaving the evaporator 2 is approaching zero.
[0059] FIG. 2 is a graph illustrating opening degree of an
expansion valve and various temperature measurements during
operation of a vapour compression system using a method according
to an embodiment of the invention. The vapour compression system
could, e.g., be the vapour compression system illustrated in FIG.
1.
[0060] The graph of FIG. 2 illustrates three different phases of
control of the vapour compression system, i.e. a pull-down phase, a
system identification phase, and a normal control phase.
[0061] Prior to the pull-down phase the vapour compression system
has been inoperative for a period of time. Therefore all
temperatures, S.sub.2, S.sub.3 and S.sub.4, prevailing in the
vapour compression system have equalized at substantially the same
temperature level.
[0062] At initiation of the pull-down phase, the opening degree 12
of the expansion valve is set to a maximum value, in order to fill
the evaporator as quickly as possible, thereby driving the measured
temperatures downwards. The temperature signal, S.sub.2, represents
the temperature of refrigerant leaving the evaporator, the
temperature signal, S.sub.3, represents the temperature of air
flowing towards the evaporator, and the temperature, S.sub.4,
represents the temperature of air flowing away from the evaporator,
as described above with reference to FIG. 1.
[0063] It can be seen from FIG. 2 that, during the pull-down phase,
the temperature, S.sub.4, of air flowing away from the evaporator
initially decreases faster than the other measured temperature,
S.sub.2 and S.sub.3. This is due to the fact that the evaporator is
cooling the air flowing across the evaporator in an efficient
manner. The temperature, S.sub.3, of air flowing towards the
evaporator and the temperature, S.sub.2, of refrigerant leaving the
evaporator also decrease, but at a slower rate.
[0064] At a certain point in time the temperature, S.sub.2, of
refrigerant leaving the evaporator decreases drastically. This is
an indication that liquid refrigerant is present throughout almost
the entire length of the evaporator, and that a zero superheat
situation is approaching. Accordingly, the pull-down phase is
ended, and the system identification phase is initiated.
[0065] During the system identification phase the opening degree 12
of the expansion valve is alternatingly increased and decreased
between a maximum value and a minimum value. Furthermore, the
temperature signals, S.sub.2, S.sub.3 and S.sub.4, are
monitored.
[0066] It can be seen from FIG. 2 that the alternating increases
and decreases of the opening degree 12 of the expansion valve
results in significant decreases and increases of the temperature,
S.sub.2, of refrigerant leaving the evaporator. This is due to the
fact that the evaporator is almost filled with liquid refrigerant,
and variations in the opening degree 12 of the expansion valve, and
thereby in the supply of refrigerant to the evaporator, thereby
significantly affects the temperature, S.sub.2, of refrigerant
leaving the evaporator.
[0067] The temperature, S.sub.4, of air flowing away from the
evaporator also varies during the system identification phase,
since the air is cooled by the evaporator, and the temperature,
S.sub.4, of air flowing away from the evaporator thereby depends on
the evaporating temperature. However, the variations in the
temperature, S.sub.4, of air flowing away from the evaporator are
smaller than the variations in the temperature, S.sub.2, of
refrigerant leaving the evaporator.
[0068] Finally, the temperature, S.sub.3, of air flowing towards
the evaporator simply continues to decrease during the system
identification phase, with only insignificant variations.
[0069] For each period of increasing and decreasing the opening
degree 12 of the expansion valve, the temperature difference,
(S.sub.4-S.sub.2), between the temperature, S.sub.2, of refrigerant
leaving the evaporator and the temperature, S.sub.4, of air flowing
away from the evaporator is monitored, and a largest value of the
temperature difference is determined. Upon completion of the system
identification phase a maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, is determined as the largest of the
temperature differences, (S.sub.4-S.sub.2), determined during the
periods of increasing and decreasing the opening degree 12 of the
expansion valve. The maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, is supplied to a reference temperature
calculator, and used for calculating a reference temperature,
S.sub.2,ref, in the manner described above.
[0070] The maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, provides information regarding how far
the temperature, S.sub.2, of refrigerant leaving the evaporator is
from an optimal superheat value, under the given circumstances, and
thereby information regarding how much further the temperature,
S.sub.2, can be lowered without risking flooding of the evaporator,
i.e. that liquid refrigerant is allowed to pass through the
evaporator. A reference temperature, S.sub.2,ref, for the
refrigerant leaving the evaporator, which is calculated on the
basis of the maximum temperature difference,
(S.sub.4-S.sub.2).sub.max, thereby takes this information into
account.
[0071] Finally, the normal control phase is initiated. During this
phase the opening degree 12 of the expansion valve is controlled in
order to obtain a temperature, S.sub.2, of refrigerant leaving the
evaporator, which is substantially equal to the reference
temperature, S.sub.2,ref. The reference temperature, S.sub.2,ref,
is continuously calculated, on the basis of the maximum temperature
difference, (S.sub.4-S.sub.2).sub.max, which was determined during
the system identification phase, and on the basis of the monitored
temperature, S.sub.4, of air flowing away from the evaporator.
[0072] FIG. 3 is a block diagram illustrating a method according to
an embodiment of the invention, e.g. for controlling the vapour
compression system of FIG. 1.
[0073] An air temperature, T.sub.air, and a reference air
temperature, T.sub.air,ref, are supplied to a comparator 13 of an
air tracking unit 10, e.g. in the manner described above with
reference to FIG. 1. The output of the comparator 13 is passed
through a filter 14, and a gain 15 is applied to the signal before
it is supplied from the air tracking unit 10 to an integral part of
a proportional integral (PI) regulator 8.
[0074] Furthermore, a reference temperature calculator 7 calculates
a reference temperature, S.sub.2,ref, of the refrigerant leaving
the evaporator. The reference temperature, S.sub.2,ref, is
calculated essentially in the manner described above, and is
supplied to a comparator 16, where it is compared to a measured
temperature, S.sub.2, of refrigerant leaving the evaporator.
[0075] The output of the comparator 16 is supplied to the PI
regulator 8. Based on the input received from the air tracking unit
10 and the reference temperature calculator 7, the PI regulator 8
generates a control signal for an expansion valve, in order to
control the opening degree of the expansion valve in such a manner
that the temperature, S.sub.2, of refrigerant leaving the
evaporator is substantially equal to the calculated reference
temperature, S.sub.2,ref, and in such a manner that an air
temperature, T.sub.air, which is equal to the reference air
temperature, T.sub.air,ref, is obtained. The input received from
the air tracking unit 10 ensures that the opening degree of the
expansion valve is controlled in a smooth manner.
[0076] While the present disclosure has been illustrated and
described with respect to a particular embodiment thereof, it
should be appreciated by those of ordinary skill in the art that
various modifications to this disclosure may be made without
departing from the spirit and scope of the present disclosure.
* * * * *