U.S. patent application number 15/109521 was filed with the patent office on 2016-11-10 for a method for controlling a supply of refrigerant to an evaporator based on temperature measurements.
The applicant listed for this patent is DANFOSS A/S. Invention is credited to Roozbeh Izadi-Zamanabadi.
Application Number | 20160327322 15/109521 |
Document ID | / |
Family ID | 49949538 |
Filed Date | 2016-11-10 |
United States Patent
Application |
20160327322 |
Kind Code |
A1 |
Izadi-Zamanabadi; Roozbeh |
November 10, 2016 |
A METHOD FOR CONTROLLING A SUPPLY OF REFRIGERANT TO AN EVAPORATOR
BASED ON TEMPERATURE MEASUREMENTS
Abstract
A method for controlling a supply of refrigerant to an
evaporator (2) of a vapour compression system (1), such as a
refrigeration system, an air condition system or a heat pump. The
opening degree of the expansion valve (3) is controlled on the
basis of an air temperature, T.sub.air, of air flowing across the
evaporator (2), and in order to reach a reference air temperature,
T.sub.air, ref. The opening degree is set to the calculated opening
degree, overlaid with a perturbation signal. A temperature signal,
S.sub.2, representing a temperature of refrigerant leaving the
evaporator (2) is monitored and analysed. In the case that the
analysis reveals that a dry zone of the evaporator (2) is
approaching a minimum length, the opening degree of the expansion
valve (3) is decreased. This provides a safety mechanism which
ensures that liquid refrigerant is prevented from passing through
the evaporator (2).
Inventors: |
Izadi-Zamanabadi; Roozbeh;
(Sonderborg, DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DANFOSS A/S |
Nordborg |
|
DK |
|
|
Family ID: |
49949538 |
Appl. No.: |
15/109521 |
Filed: |
December 16, 2014 |
PCT Filed: |
December 16, 2014 |
PCT NO: |
PCT/EP2014/077904 |
371 Date: |
July 1, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25B 2500/28 20130101;
F25B 2600/2513 20130101; F25B 2500/19 20130101; F25B 2700/21171
20130101; F25B 49/02 20130101; F25B 2600/21 20130101 |
International
Class: |
F25B 49/02 20060101
F25B049/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 2014 |
EP |
14151107.1 |
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 circuit, the method comprising the steps of:
obtaining a temperature, T.sub.air, of air flowing across the
evaporator, controlling an opening degree of the expansion valve,
on the basis of the obtained temperature, T.sub.air, and in order
to reach a reference air temperature, T.sub.air, ref, of the air
flowing across the evaporator, providing a perturbation signal, and
setting the opening degree of the expansion valve to the controlled
opening degree, overlaid with the perturbation signal, monitoring a
temperature signal, S.sub.2, representing a temperature of
refrigerant leaving the evaporator, analysing the temperature
signal, S.sub.2, and decreasing the opening degree of the expansion
valve in the case that said analysis reveals that a dry zone of the
evaporator is approaching a minimum length.
2. The method according to claim 1, wherein the step of analysing
the temperature signal, S.sub.2, comprises obtaining a rate of
change of the temperature signal, S.sub.2, and wherein the step of
decreasing the opening degree comprises decreasing the opening
degree of the expansion valve in the case that an absolute value of
the rate of change of the temperature signal, S.sub.2, reaches a
maximum value.
3. The method according to claim 1, wherein the step of analysing
the temperature signal, S.sub.2, comprises the steps of:
identifying a component of the temperature signal, S.sub.2,
corresponding to the perturbation signal, comparing the identified
component of the temperature signal, S.sub.2, to the original
perturbation signal, and determining whether or not the dry zone of
the evaporator is approaching a minimum length, based on said
comparison.
4. The method according to claim 3, wherein the step of comparing
comprises determining a distortion of the identified component of
the temperature signal, S.sub.2.
5. The method according to claim 1, wherein the step of analysing
the temperature signal, S.sub.2, comprises identifying one or more
statistical components of the temperature signal, S.sub.2.
6. The method according to claim 1, wherein the perturbation signal
is a sinusoidal type signal.
7. The method according to claim 1, wherein the perturbation signal
is a relay type signal.
8. The method according to claim 1, wherein the temperature,
T.sub.air, is a temperature of air flowing towards the
evaporator.
9. The method according to claim 1, wherein the temperature,
T.sub.air, is a temperature of air flowing away from the
evaporator.
10. The method according to claim 1, wherein the temperature,
T.sub.air, represents a weighted value of a temperature of air
flowing towards the evaporator and a temperature of air flowing
away from the evaporator.
11. The method according to claim 1, further comprising the step of
performing a pull down process in the case that the temperature,
T.sub.air, of air flowing across the evaporator is above a
predefined upper threshold value.
12. The method according to claim 11, wherein the step of
performing a pull down process comprises the steps of: opening the
expansion valve to a maximum opening degree, monitoring a
temperature signal, S.sub.2, representing a temperature of
refrigerant leaving the evaporator, analysing the temperature
signal, S.sub.2, and decreasing the opening degree of the expansion
valve in the case that said analysis reveals that an absolute value
of a rate of change of the temperature signal, S.sub.2, has reached
a maximum value.
13. A method for controlling a supply of refrigerant to an
evaporator of a vapour compression system during a pull down
process, 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 circuit, the
method comprising the steps of: opening the expansion valve to a
maximum opening degree, monitoring a temperature signal, S.sub.2,
representing a temperature of refrigerant leaving the evaporator,
analysing the temperature signal, S.sub.2, and decreasing the
opening degree of the expansion valve in the case that said
analysis reveals that an absolute value of a rate of change of the
temperature signal, S.sub.2, reaches a maximum value.
14. The method according to claim 2, wherein the perturbation
signal is a sinusoidal type signal.
15. The method according to claim 3, wherein the perturbation
signal is a sinusoidal type signal.
16. The method according to claim 4, wherein the perturbation
signal is a sinusoidal type signal.
17. The method according to claim 5, wherein the perturbation
signal is a sinusoidal type signal.
18. The method according to claim 2, wherein the perturbation
signal is a relay type signal.
19. The method according to claim 3, wherein the perturbation
signal is a relay type signal.
20. The method according to claim 4, wherein the perturbation
signal is a relay type signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is entitled to the benefit of and
incorporates by reference subject matter disclosed in the
International Patent Application No. PCT/EP2014/077904 filed on
Dec. 16, 2014 and European Patent Application No. 14151107 filed on
Jan. 14, 2014.
TECHNICAL FIELD
[0002] The present invention relates to a method for controlling a
supply of refrigerant to an evaporator, in particular to an
evaporator which forms part of a vapour compression system, such as
a refrigeration system, an air condition system or a heat pump.
According to the method of the present invention, the supply of
refrigerant to the evaporator can be controlled in a manner which
provides a desired target temperature in a refrigerated or heated
volume, while preventing liquid refrigerant from entering the
suction line, and based solely on 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 the 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 then 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] In order to obtain the superheat value of refrigerant
leaving the evaporator, the temperature as well as the pressure of
the refrigerant leaving the evaporator is normally measured. The
pressure sensor required in this case introduces the risk that the
pressure sensor falls out or malfunctions, thereby making it
impossible to measure the superheat value until the pressure sensor
is restored. Furthermore, the pressure sensor introduces the risk
of leaks in the system.
[0006] 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
[0007] It is an object of embodiments of the invention to provide a
method for controlling a supply of refrigerant to an evaporator, in
which the supply of refrigerant is normally controlled to provide a
predefined target temperature in a refrigerated or heated volume,
while a safety mechanism prevents that liquid refrigerant reaches
the compressor.
[0008] It is a further object of embodiments of the invention to
provide a method for controlling a supply of refrigerant to an
evaporator during a pull down process, in which a fast pull down is
ensured, while it is prevented that liquid refrigerant reaches the
compressor.
[0009] According to a first aspect 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 circuit, the method comprising the steps of: [0010]
obtaining a temperature, T.sub.air, of air flowing across the
evaporator, [0011] controlling an opening degree of the expansion
valve, on the basis of the obtained temperature, T.sub.air, and in
order to reach a reference air temperature, T.sub.air, ref, of the
air flowing across the evaporator, [0012] providing a perturbation
signal, and setting the opening degree of the expansion valve to
the controlled opening degree, overlaid with the perturbation
signal, [0013] monitoring a temperature signal, S.sub.2,
representing a temperature of refrigerant leaving the evaporator,
[0014] analysing the temperature signal, S.sub.2, and [0015]
decreasing the opening degree of the expansion valve in the case
that said analysis reveals that a dry zone of the evaporator is
approaching a minimum length.
[0016] 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.
[0017] 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.
[0018] The method according to the first aspect of 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.
[0019] According to the method according to the first aspect of the
invention a temperature, T.sub.air, of air flowing across the
evaporator is initially obtained. This may preferably be done by
means of one or more temperature sensors arranged in an air passage
across the evaporator. The temperature, T.sub.air, could, e.g. be
the temperature of air flowing towards the evaporator, the
temperature of air flowing away from 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. This will
be described in further detail below. In any event, T.sub.air
represents a temperature prevailing in a refrigerated volume
arranged near the evaporator. Accordingly, T.sub.air reflects a
cooling need of the refrigerated volume.
[0020] Next, an opening degree of the expansion valve is controlled
on the basis of the obtained temperature, T.sub.air, and in order
to reach a reference air temperature, T.sub.air, ref, of the air
flowing across the evaporator. As described above, T.sub.air
reflects a temperature prevailing in the refrigerated volume, and
thereby a cooling need of the refrigerated volume. The reference
air temperature, T.sub.air, ref, is a target temperature, which it
is desired to obtain in the refrigerated volume. Thus, by comparing
the obtained temperature, T.sub.air, to the reference air
temperature, T.sub.air, ref, it can be revealed whether the
prevailing temperature in the refrigerated volume is close to or
far away from the desired target temperature. In the case that the
prevailing temperature is far away from the target temperature,
further cooling is highly needed, and the supply of refrigerant to
the evaporator should be such as to provide as much cooling as
possible. Similarly, in the case that the prevailing temperature is
close to the desired target temperature, the need for further
cooling is somewhat lower, and the supply of refrigerant to the
evaporator can be controlled in a manner which provides less
cooling, but which instead ensures a low energy consumption.
[0021] Thus, under normal circumstances the supply of refrigerant
to the evaporator is controlled solely in a manner which ensures
that a desired target temperature in the refrigerated volume is
obtained.
[0022] It should be noted that increasing the opening degree of the
expansion valve results in an increase in the supply of refrigerant
to the evaporator, and decreasing the opening degree of the
expansion valve results in a decrease in the supply of refrigerant
to the evaporator.
[0023] Next, a perturbation signal is provided, and the opening
degree of the expansion valve is set to the controlled opening
degree, overlaid with the perturbation signal. Thus, the opening
degree of the expansion valve fluctuates around a mean value, which
represents the controlled opening degree, i.e. the opening degree
which is dictated by the obtained temperature, T.sub.air. The
fluctuations are determined by the perturbation signal, and may,
e.g., be sinusoidal, of a relay type, or of any other suitable
type. This will be described in further detail below. In the
present context the term `perturbation signal` should be
interpreted to mean a signal which varies on a time scale, which is
significantly shorter than the time scale on which the controlled
opening degree of the expansion valve varies.
[0024] Then a temperature signal, S.sub.2, representing a
temperature of refrigerant leaving the evaporator is monitored.
This may, e.g., be done using a temperature sensor arranged in the
refrigerant path immediately after the outlet of the evaporator.
Thus, the temperature signal, S.sub.2, represents a relative value
of the superheat value of the refrigerant leaving the evaporator.
The monitored temperature signal, S.sub.2, is analysed.
[0025] Finally, the opening degree of the expansion valve is
decreased in the case that the analysis of the monitored
temperature signal, S.sub.2, reveals that a dry zone of the
evaporator is approaching a minimum length.
[0026] In the present context the term `dry zone of the evaporator`
should be interpreted to mean a part of the evaporator containing
only gaseous refrigerant. A dry zone of a long length thereby
indicates that liquid refrigerant is evaporated in the evaporator
well before reaching the evaporator outlet, while a dry zone of a
short length indicates that liquid refrigerant is present along a
substantial part of the evaporator. Accordingly, when the dry zone
of the evaporator approaches a minimum length, then the boundary
between the mixed liquid/gaseous refrigerant and the purely gaseous
refrigerant is approaching the outlet of the evaporator. As
described above, when this boundary reaches the outlet of the
evaporator there is a risk that liquid refrigerant is allowed to
pass through the evaporator, and thereby there is a risk that
liquid refrigerant reaches the compressor, thereby causing damage
to the compressor. Therefore, when the dry zone of the evaporator
approaches a minimum length, the supply of refrigerant to the
evaporator must be decreased in order to avoid this situation.
[0027] Whether or not the dry zone of the evaporator is approaching
a minimum length can be established in a number of ways. It has
been found by the inventors of the present invention that when the
dry zone of the evaporator approaches a minimum length, the
behaviour of the temperature signal, S.sub.2, changes in a
significant manner. Thus, when analysing the temperature signal,
S.sub.2, signs of these changes may be detected. For instance, the
inventors of the present invention have found that if the opening
degree of the expansion valve is slowly increased, then the
temperature of the refrigerant leaving the evaporator will decrease
abruptly when the opening degree of the expansion valve reaches a
level, where the supply of refrigerant to the evaporator is
sufficient to reduce the dry zone of the evaporator to the minimum
length. This may be regarded as an `unstable region`. If the
opening degree is increased even further, there is a significant
risk that liquid refrigerant is passed through the evaporator. This
may be regarded as a `critical region`.
[0028] Thus, the step of analysing the temperature signal, S.sub.2,
may comprise obtaining a rate of change of the temperature signal,
S.sub.2, and the step of decreasing the opening degree may comprise
decreasing the opening degree of the expansion valve in the case
that an absolute value of the rate of change of the temperature
signal, S.sub.2, reaches a maximum value, such as a global or a
local maximum. As described above, the temperature signal, S.sub.2,
decreases abruptly, when the unstable region is entered. Thus, when
the absolute value of the rate of change of the temperature signal,
S.sub.2, reaches the maximum value, it can be concluded that the
unstable region has been entered, and that the dry zone of the
evaporator is therefore approaching the minimum length. The actual
maximum value is not a fixed or unique value, but may change
depending on the operating point. However, an extreme of the signal
will be reached, since the curve defines a saddle point, and it is
this saddle point, which indicates that the unstable region has
been entered.
[0029] Before reaching the unstable region, the signal, S.sub.2,
follows a concave curve, in the middle of the unstable region there
is a saddle point, and from the unstable region until the
evaporator is completely flooded, the signal, S.sub.2, follows a
convex curve. On the concave part of the curve, the rate of change
of the signal is negative and becomes smaller the closer it gets to
the saddle point. At the saddle point, the rate of change of the
signal, S.sub.2, reaches its minimum. Hence, by computing the
minimum of the rate of change of the signal, S.sub.2, the saddle
point, which represents the centre of the unstable region, can be
identified. As the expansion valve is largely open while this
process is being carried out, it is obvious that the dry zone of
the evaporator approaches its minimum length. Accordingly, the
opening degree of the expansion valve must be decreased, at this
point, in order to avoid entering the critical region.
[0030] As an alternative, the step of analysing the temperature
signal, S.sub.2, may comprise the steps of: [0031] identifying a
component of the temperature signal, S.sub.2, corresponding to the
perturbation signal, [0032] comparing the identified component of
the temperature signal, S.sub.2, to the original perturbation
signal, and [0033] determining whether or not the dry zone of the
evaporator is approaching a minimum length, based on said
comparison.
[0034] The component of the temperature signal, S.sub.2, could,
e.g., be variations in the temperature signals, S.sub.2, which
corresponds to the variations in the opening degree which are
defined by the perturbation signal and/or specific frequency
components of the signal. For instance, in the case that the
perturbation signal is a sinusoidal signal, the component could,
e.g., be a frequency component with substantially the same
frequency as the sinusoidal perturbation signal, or with a
different frequency. For instance, the component could be a
frequency component which is a sum of several sinusoidal
signals.
[0035] Comparing the identified component of the temperature
signal, S.sub.2, to the original perturbation signal, reveals in
which manner the perturbations applied to the opening degree of the
expansion valve affects the monitored temperature signal, S.sub.2.
The comparison could be an actual comparison between the
perturbation signal and the identified component. Alternatively, it
could be a comparison between corresponding characteristics of the
two signals, such as frequency and/or amplitude.
[0036] It has been found by the inventors of the present invention
that the manner in which the perturbations applied to the opening
degree of the expansion valve affects the monitored temperature
signal, S.sub.2, changes significantly, when the unstable region is
entered, and the dry zone of the evaporator is therefore
approaching a minimum length. If signs of such significant changes
are detected during the analysis of the temperature signal,
S.sub.2, it can therefore be concluded that the dry zone of the
evaporator is approaching a minimum length, and accordingly the
opening degree of the expansion valve must be decreased in order to
prevent that liquid refrigerant reaches the compressor.
[0037] For instance, in the case that the identified component of
the perturbation signal is a main frequency, then the temperature
signal, S.sub.2, may contain the main frequency as well as one or
more additional frequency components, e.g. corresponding to
harmonics of the main frequency. Performing a Fast Fourier
Transform (FFT) of the temperature signal, S.sub.2, will result in
a number of parameters, corresponding to the additional frequency
components. The sign of these parameters will change when the
saddle point, as described above, is reached, i.e. when the
unstable region is reached. Thus, when a change in sign of the
parameters is detected, the opening degree of the expansion valve
must be decreased in order to avoid that liquid refrigerant reaches
the compressor.
[0038] The step of comparing the identified component of the
temperature signal, S.sub.2, to the original perturbation signal
may comprise determining a distortion of the identified component
of the temperature signal, S.sub.2. In some cases the distortion of
the component may change significantly when the unstable region is
entered. Thus, when such changes are detected, it can be concluded
that the dry zone of the evaporator is approaching a minimum
length, and that the opening degree of the expansion valve must
therefore be decreased in order to prevent that liquid refrigerant
reaches the compressor. The distortion could, e.g., include that
the perturbation signal is a perfect sinusoidal signal, while the
identified component is a fluctuation of the temperature signal,
with a frequency which may be similar to the frequency of the
sinusoidal perturbation signal, but which is not a perfect
sinusoidal signal. As an alternative, the distortion could be a
combination of several frequencies that are multipliers of the
original perturbation signal's frequency.
[0039] As another alternative, the step of analysing the
temperature signal, S.sub.2, may comprise identifying one or more
statistical components of the temperature signal, S.sub.2. The
statistical components could, e.g., include a mean value, a
variance, etc., of the signal. Or the statistical component could
include other descriptors of the probability distribution in the
temperature signal, S.sub.2. For instance, when the temperature
signal, S.sub.2, approaches the unstable region, the variance of
the temperature signal, S.sub.2, increases. Similarly, when the
temperature signal, S.sub.2, moves away from the unstable region,
the corresponding variance tends to decrease significantly.
[0040] The perturbation signal may be a sinusoidal type signal. In
this case the opening degree of the expansion valve fluctuates in a
substantially sinusoidal manner about the opening degree value
which is dictated by the temperature, T.sub.air, of air flowing
across the evaporator. The frequency of the sinusoidal perturbation
signal may be recognised in the monitored temperature signal,
S.sub.2.
[0041] As an alternative, the perturbation signal may be a relay
type signal. In this case the opening degree of the expansion valve
fluctuates in a relay-like manner, or as a square signal, about the
opening degree value which is dictated by the temperature,
T.sub.air, of air flowing across the evaporator.
[0042] As another alternative, the perturbation signal may be of
any other suitable kind, preferably a periodical signal, e.g. a
triangular signal.
[0043] The temperature, T.sub.air, may be a temperature of air
flowing towards the evaporator. According to this embodiment, the
opening degree of the expansion valve is controlled on the basis of
a temperature which is prevailing in air in a refrigerated volume,
before the air is passed across the evaporator and thereby cooled.
It can be assumed that this temperature varies relatively slowly,
since it represents the temperature in the entire refrigerated
volume.
[0044] As an alternative, the temperature, T.sub.air, may be a
temperature of air flowing away from the evaporator. According to
this embodiment, the opening degree of the expansion valve is also
controlled on the basis of a temperature which is prevailing in air
in a refrigerated volume. However, in this case the temperature is
measured in air which has just passed across the evaporator, and
which has therefore just been cooled by the evaporator.
Accordingly, this temperature will not only reflect the temperature
prevailing in the entire refrigerated volume, but will also reflect
the instantaneous cooling power of the evaporator, since a high
cooling power will reduce this temperature. Thus, according to this
embodiment the instantaneous cooling power of the evaporator is
taken into account when controlling the opening degree of the
expansion valve.
[0045] As another alternative, the temperature, T.sub.air, may
represent a weighted value of a temperature of air flowing towards
the evaporator and a temperature of air flowing away from the
evaporator. According to this embodiment, the instantaneous cooling
power of the evaporator is also taken into account when controlling
the opening degree of the expansion valve. However, in this case
the impact on the controlled opening degree is smaller than in the
embodiment described above.
[0046] The method may further comprise the step of performing a
pull down process in the case that the temperature, T.sub.air, of
air flowing across the evaporator is above a predefined upper
threshold value. If the temperature, T.sub.air, exceeds the
predefined upper threshold value, it may be assumed that the
difference between the actual air temperature, T.sub.air, and the
target temperature or reference temperature, T.sub.air, ref, is
relatively large, i.e. that T.sub.air is significantly higher than
T.sub.air, ref. In this case it may be necessary to reduce the
actual air temperature, T.sub.air, quickly, in order to be able to
reach T.sub.air, ref within a reasonable time period. This may be
obtained by performing a pull down process in this case. In the
present context the term `pull down process` should be interpreted
to mean a process which applies a maximum, or at least very high,
cooling power in order to pull down, or reduce, the air temperature
inside the refrigerated volume quickly. It may, e.g., be relevant
to perform a pull down process when the system is initially
started, or when new products have been positioned in the
refrigerated volume.
[0047] The step of performing a pull down process may comprise the
steps of: [0048] opening the expansion valve to a maximum opening
degree, [0049] monitoring a temperature signal, S.sub.2,
representing a temperature of refrigerant leaving the evaporator,
[0050] analysing the temperature signal, S.sub.2, and [0051]
decreasing the opening degree of the expansion valve in the case
that said analysis reveals that an absolute value of a rate of
change of the temperature signal, S.sub.2, has reached a maximum
value.
[0052] Opening the expansion valve to a maximum opening degree
ensures that the evaporator is filled as fast as possible, and
thereby it is ensured that a maximum cooling power is provided.
However, this also includes a risk that liquid refrigerant is
allowed to pass through the evaporator, and potentially reach the
compressor.
[0053] Therefore a temperature signal, S.sub.2, representing a
temperature of refrigerant leaving the evaporator is monitored and
analysed, as described above. In the case that the analysis reveals
that an absolute value of a rate of change of the temperature
signal, S.sub.2, has reached a maximum value, the opening degree of
the expansion valve is decreased.
[0054] As described above, an abrupt decrease in the rate of change
of the monitored temperature signal, S.sub.2, indicates that the
unstable region has been entered, and that the dry zone of the
evaporator is therefore approaching a minimum length. Accordingly,
this indicates that there is a risk that liquid refrigerant is
allowed to pass through the evaporator if the maximum opening
degree of the expansion valve is maintained, and therefore the
opening degree of the expansion valve must be decreased in order to
avoid this situation.
[0055] Thus, according to this embodiment, an efficient pull down
process is provided, while it is ensured that liquid refrigerant is
not allowed to reach the compressor.
[0056] According to a second aspect the invention provides a method
for controlling a supply of refrigerant to an evaporator of a
vapour compression system during a pull down process, 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 circuit, the method comprising the steps
of: [0057] opening the expansion valve to a maximum opening degree,
[0058] monitoring a temperature signal, S.sub.2, representing a
temperature of refrigerant leaving the evaporator, [0059] analysing
the temperature signal, S.sub.2, and [0060] decreasing the opening
degree of the expansion valve in the case that said analysis
reveals that an absolute value of a rate of change of the
temperature signal, S.sub.2, reaches a maximum value.
[0061] It should be noted that a person skilled in the art would
readily recognise that any feature described in combination with
the first aspect of the invention could also be combined with the
second aspect of the invention, and vice versa. Thus, the remarks
set forth above are equally applicable here.
[0062] The pull down process of the second aspect of the invention
has already been described in detail above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] The invention will now be described in further detail with
reference to the accompanying drawings in which
[0064] FIG. 1 is a graph illustrating a monitored temperature,
S.sub.2, as a function of opening degree of an expansion valve,
[0065] FIG. 2 is a diagrammatic view of a part of a vapour
compression system for performing a method according to a first
embodiment of the invention,
[0066] FIG. 3 is a diagrammatic view of a part of a vapour
compression system for performing a method according to a second
embodiment of the invention, and
[0067] FIG. 4 is graph illustrating opening degree of an expansion
valve and monitored temperatures of a vapour compression system,
while performing a method according to an embodiment of the
invention.
DETAILED DESCRIPTION
[0068] FIG. 1 is a graph illustrating a monitored temperature,
S.sub.2, of refrigerant leaving an evaporator of a vapour
compression system, as a function of opening degree of an expansion
valve which controls the supply of refrigerant to the
evaporator.
[0069] It can be seen that when the opening degree of the expansion
valve is relatively small, the monitored temperature, S.sub.2, of
the refrigerant leaving the evaporator is relatively high, close to
a temperature, T.sub.air, of ambient air. Furthermore, the
monitored temperature, S.sub.2, remains almost constant when the
opening degree of the expansion valve is increased. This indicates
that the liquid part of the refrigerant, which is supplied to the
evaporator, is evaporated well before it reaches the outlet of the
evaporator. Accordingly, the superheat value of the refrigerant
leaving the evaporator can be assumed to be relatively high, and
the risk of liquid refrigerant passing through the evaporator is
very low.
[0070] As the opening degree of the expansion valve is increased
further, the monitored temperature, S.sub.2, decreases
significantly and abruptly towards an evaporating temperature,
T.sub.e, i.e. the temperature at which the refrigerant evaporates
at the pressure prevailing in the refrigerant, or the dew point.
Thus, when the monitored temperature, S.sub.2, approaches the
evaporating temperature, T.sub.e, this is an indication that the
superheat value is approaching zero. This is an indication that the
dry zone of the evaporator is approaching a minimum length, and
that the risk of liquid refrigerant passing through the evaporator
is increasing.
[0071] The region where the monitored temperature, S.sub.2,
decreases abruptly may be referred to as an `unstable region`. When
monitoring and analysing the temperature, S.sub.2, entering this
region can be detected, e.g. by monitoring the rate of change of
the temperature signal, and identifying an absolute maximum value
of the rate of change, since the rate of change will be large and
negative. However, entering the unstable region may be detected in
other ways, as described above.
[0072] The region where the rate of change of the monitored
temperature, S.sub.2, once again decreases, and the temperature,
S.sub.2, comes very close to the evaporating temperature, may be
referred to as a `critical region`, because this is the region
where there is a high risk that liquid refrigerant is allowed to
pass through the evaporator, and thereby there is a risk that
liquid refrigerant may reach the compressor.
[0073] Thus, it is desirable to control the opening degree of the
expansion valve in such a manner that the critical region is not
entered. According to the present invention, this may be obtained
by reducing the opening degree of the expansion valve when it is
detected that the unstable region is entered. When this occurs, the
critical region will be reached if the opening degree of the
expansion valve is increased further. Therefore it can be prevented
that the critical region is entered, if the opening degree of the
expansion valve is reduced when the unstable region is entered.
[0074] It is noted that since the evaporating temperature, T.sub.e,
depends on the pressure prevailing in the refrigerant, it will
normally not be sufficient to measure the temperature, S.sub.2, of
refrigerant leaving the evaporator, and compare the measured
temperature to a fixed evaporating temperature. This is why, in the
method of the present invention, the temperature signal, S.sub.2,
is monitored and analysed, e.g. deriving the rate of change of the
temperature signal, in order to detect when the unstable region is
entered.
[0075] FIG. 2 is a diagrammatic view of a part of a vapour
compression system 1 for performing a method according to a first
embodiment of the invention. The vapour compression system 1
comprises an evaporator 2 arranged in a refrigerant circuit 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 circuit for controlling the supply of refrigerant to
the evaporator 2.
[0076] The vapour compression system 1 further comprises a number
of temperature sensors. A first temperature sensor 4 is arranged in
the refrigerant circuit 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.
[0077] 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.
[0078] 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.
[0079] The temperature signals, S.sub.3 and S.sub.4, measured by
the second temperature sensor 5 and the third temperature sensor 6
are supplied to a sensor selection unit 7. The sensor selection
unit 7 selects whether to apply one of the temperature signals,
S.sub.3 and S.sub.4, when controlling the expansion valve 3, or to
apply 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 represents an air temperature, corresponding to the
selection performed by the selection unit 7. The temperature
signal, T.sub.air, is supplied to a control unit 8, which is
arranged to control an opening degree of the expansion valve 3.
[0080] A reference air temperature, T.sub.air, ref, is also
supplied to the control unit 8. The reference air temperature,
T.sub.air, ref, represents a reference or target temperature which
is desired in the air flowing across the evaporator 2.
[0081] The control unit 8 compares the temperature signal,
T.sub.air, to the reference air temperature, T.sub.air, ref, and
calculates an opening degree of the expansion valve 3, based on
this comparison. The opening degree of the expansion valve 3 is
selected in such a manner that the opening degree ensures a supply
of refrigerant to the evaporator 2, which causes the air
temperature, T.sub.air, to approach the reference air temperature,
T.sub.air. ref. Thus, the control unit 8 controls the opening
degree of the expansion valve 3 on the basis of the selected air
temperature, T.sub.air, and in order to reach the reference air
temperature, T.sub.air, ref.
[0082] The temperature signal, S.sub.2, measured by the first
temperature sensor 4 is also supplied to the control unit 8.
Thereby, the temperature of refrigerant leaving the evaporator 2
may also be taken into account when the opening degree of the
expansion valve 3 is calculated by the control unit 8.
[0083] When the control unit 8 has calculated an opening degree of
the expansion valve 3 as described above, the control unit 8
applies a perturbation signal to the calculated opening degree. In
the embodiment illustrated in FIG. 2, the perturbation signal is a
relay like perturbation signal. The resulting signal is supplied to
the expansion valve 3, and the opening degree of the expansion
valve 3 is controlled to be the calculated opening degree, overlaid
with the perturbation signal.
[0084] Thus, under normal circumstances, the opening degree of the
expansion valve 3, and thereby the supply of refrigerant to the
evaporator 2, is controlled on the basis of the air temperature,
T.sub.air, in order to obtain the reference air temperature,
T.sub.air, ref, but overlaid with the perturbation signal.
[0085] However, the temperature signal, S.sub.2, measured by the
first temperature sensor 4, is also supplied to an analysing unit
9. The analysing unit 9 analyses the temperature signal, S.sub.2,
in particular with respect to the rate of change of the temperature
signal, S.sub.2. The result of the analysis is supplied to a safety
logic unit 10. The safety logic unit 10 monitors the rate of change
of the temperature signal, S.sub.2, and in the case that an
absolute value of the rate of change of the temperature signal,
S.sub.2, reaches a maximum value, the safety logic unit 10 sends a
signal to the control unit 8, requesting that the opening degree of
the expansion valve 3 is decreased. In response to this signal, the
control unit 8 decreases the opening degree of the expansion valve
3.
[0086] As described above, when the rate of change of the
temperature of refrigerant leaving the evaporator 2, decreases
abruptly, this is a sign that the unstable region has been entered,
and that there is a risk of entering the critical region, if the
opening degree of the expansion valve 3 is not decreased.
Therefore, the safety logic unit 10 in this manner ensures that it
is efficiently prevented that liquid refrigerant is allowed to pass
through the evaporator 2 and reach the compressor.
[0087] FIG. 3 is a diagrammatic view of a part of a vapour
compression system 1 for performing a method according to a second
embodiment of the invention. The vapour compression system 1 of
FIG. 3 operates in a manner which is similar to the operation of
the vapour compression system of FIG. 2, and the operation of the
vapour compression system 1 will therefore not be described in
detail here.
[0088] The vapour compression system 1 of FIG. 3 further comprises
a first bandpass filter 11, through which the selected temperature
signal, T.sub.air, is passed, along with the reference air
temperature, T.sub.air, ref, to a control unit 12. The control unit
12 could, e.g. be a proportional integral (P1) regulator. The
output of the control unit 12 is supplied to a summation unit
13.
[0089] The vapour compression system 1 of FIG. 3 also comprises a
second bandpass filter 14, through which the temperature signal,
S.sub.2, measured by the first temperature sensor 4 is passed
before being supplied to the summation unit 13.
[0090] The summation unit 13 is further provided with a reference
temperature signal, S.sub.2, ref, representing a target or
reference temperature for refrigerant leaving the evaporator 2.
[0091] Passing the temperature signals, T.sub.air and S.sub.2,
through bandpass filters 11 and 14 ensures that only temperature
signals within a desired frequency band are applied for controlling
the opening degree of the expansion valve 3. It should be noted
that the bandpass filters 11 and 14 could conveniently be realized
in the control units 12 and 8.
[0092] Based on the signals supplied thereto, the summation unit 13
provides an input signal to the control unit 8. The input signal
reflects the comparison between the selected air temperature,
T.sub.air, and the reference air temperature, T.sub.air, ref,
provided by the control unit 12, as well as a comparison between
the measured temperature signal, S.sub.2, and the reference
temperature, S.sub.2, ref, which is performed by the summation unit
13.
[0093] Based on the input signal, the control unit 8 calculates an
opening degree of the expansion valve 3, essentially as described
above. The calculated opening degree is supplied to a summation
unit 15. A perturbation unit 16 generates a perturbation signal and
supplies this to the summation unit 15. The summation unit 15 then
defines an opening degree of the expansion valve 3 as the
calculated opening degree overlaid with the perturbation signal. In
the embodiment of FIG. 3, the perturbation signal is a sinusoidal
signal.
[0094] The safety mechanism provided by the analysing unit 9 and
the safety logic unit 10 operates essentially as described above
with reference to FIG. 2, except that it may apply alternative ways
of detecting that the unstable region has been entered. Such
alternative ways have already been described above.
[0095] FIG. 4 is graph illustrating opening degree of an expansion
valve and monitored temperatures of a vapour compression system,
while performing a method according to an embodiment of the
invention. The vapour compression system could, e.g., be the vapour
compression system of FIG. 2 or the vapour compression system of
FIG. 3.
[0096] The graph of FIG. 4 illustrates how the opening degree 17
varies as a function of time, and how various temperatures measured
in the vapour compression system react to the variations of the
opening degree 17. It should be noted, that in FIG. 4 the opening
degree 17 is shown without the overlaid perturbation signal for
clarity. Graph 18 represents the temperature of refrigerant leaving
the evaporator, i.e. corresponding to the temperature signal,
S.sub.2, described above. Graph 19 represents the temperature of
air flowing towards the evaporator, i.e. corresponding to the
temperature signal, S.sub.3, described above. Graph 20 represents
the temperature of air flowing away from the evaporator, i.e.
corresponding to the temperature signal, S.sub.4, described above.
Graph 21 represents the evaporating temperature, i.e. the
temperature at which the refrigerant evaporates in the evaporator.
This temperature varies in dependence of the pressure prevailing in
the refrigerant. Finally, graph 22 represents the reference air
temperature, T.sub.air, ref.
[0097] It can be seen from FIG. 4, that initially the temperatures
18, 19 and 20 are all relatively high. In particular, the air
temperatures 19, 20 are both significantly higher than the
reference air temperature 22, and the temperature 18 of refrigerant
leaving the evaporator is significantly higher than the evaporating
temperature 21. This is due to the fact that the vapour compression
system has recently been switched on after having been switched off
for a period of time, and indicates that a large cooling effect is
required in order to reach the reference air temperature 22.
Furthermore, the superheat value of refrigerant leaving the
evaporator is relatively high, and therefore the risk of liquid
refrigerant passing through the evaporator is very low.
[0098] As a consequence, a pull down process is initiated. This
includes opening the expansion valve to a maximum opening degree,
while monitoring the various temperature signals 18, 19, 20. It is
clear from FIG. 4, that this causes the measured air temperatures
19, 20 to decrease rapidly. Furthermore, the temperature 18 of
refrigerant leaving the evaporator decreases and approaches the
evaporating temperature 21, i.e. the superheat value of the
refrigerant leaving the evaporator decreases towards zero.
[0099] After a while, an absolute value of the rate of change of
the temperature 18 of refrigerant leaving the evaporator reaches a
maximum value. This can be seen in FIG. 4 as an abrupt decrease in
the temperature 18. As described above, this is an indication that
the unstable region has been entered, and therefore, in response
thereto, the opening degree 17 of the expansion valve is decreased
to a minimum value. Thereby the pull down process is terminated,
and a system identification period is entered. It is clear from
FIG. 4, that the temperature 18 is indeed approaching the
evaporating temperature 21, at the time where the opening degree 17
is decreased to the minimum value.
[0100] During the system identification period, the opening degree
17 of the expansion valve is switched between the maximum value and
the minimum value, while the temperatures 18, 19, 20 are monitored.
It can be seen, that each time the temperature 18 of refrigerant
leaving the evaporator decreases abruptly, in the manner described
above, the opening degree 17 is switched from the maximum value to
the minimum value. One of the objectives of the system
identification period is to identify the current operating point of
the system.
[0101] After a while the system identification period is
terminated, and a normal control period is initiated. During the
normal control period, the opening degree 17 of the expansion valve
is controlled on the basis of the temperature 20 of air flowing
away from the evaporator, and in order to reach the reference
temperature 22. However, a safety process is also applied, which
ensures that the opening degree 17 of the expansion valve is
decreased to the minimum value in the case that it is detected that
the unstable region has been entered, e.g. by means of an analysis
of the rate of change of the temperature signal 18. In the
situation illustrated in FIG. 4, the temperature 18 of refrigerant
leaving the evaporator stays well above the evaporating temperature
21 during the entire normal control period. Thus, the unstable
region is not entered, there is no risk of liquid refrigerant
passing through the evaporator, and the safety process is therefore
not applied.
[0102] 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.
* * * * *