U.S. patent application number 13/180785 was filed with the patent office on 2013-01-17 for temperature control in a refrigerated transport container.
This patent application is currently assigned to A.P. MOLLER - MAERSK A/S. The applicant listed for this patent is Janneke Emmy De Kramer-Cuppen, Leijn Johannes Sjerp Lukasse. Invention is credited to Janneke Emmy De Kramer-Cuppen, Leijn Johannes Sjerp Lukasse.
Application Number | 20130014527 13/180785 |
Document ID | / |
Family ID | 47518129 |
Filed Date | 2013-01-17 |
United States Patent
Application |
20130014527 |
Kind Code |
A1 |
Lukasse; Leijn Johannes Sjerp ;
et al. |
January 17, 2013 |
TEMPERATURE CONTROL IN A REFRIGERATED TRANSPORT CONTAINER
Abstract
Disclosed is a system for and a method of controlling
temperature within a refrigerated transport container (1), the
refrigerated transport container (1) comprising at least a
transport volume (45), a control unit (7), and a cooling space
(41), one or more evaporator fans (10) providing an air flow
through the cooling space (41), where air passing through the
cooling space passes at least a return air temperature sensor (5),
a cooling unit (16), and a supply air temperature sensor (25),
wherein the method comprises controlling unmeasured temperatures in
the transport volume (45) within a temperature range adjacent to a
setpoint or target temperature (Tset), using two or more transport
volume temperature indicators, where the indicators are based on at
least measured supply air temperature and/or measured return air
temperature. In this way, control of unmeasured temperatures in the
transport volume is provided that enables improved control over
temperatures of the loaded perishable produce thereby reducing the
rate of quality loss of the transported produce.
Inventors: |
Lukasse; Leijn Johannes Sjerp;
(Ede, NL) ; De Kramer-Cuppen; Janneke Emmy;
(Bennekom, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lukasse; Leijn Johannes Sjerp
De Kramer-Cuppen; Janneke Emmy |
Ede
Bennekom |
|
NL
NL |
|
|
Assignee: |
A.P. MOLLER - MAERSK A/S
Kobenhavn K
DK
|
Family ID: |
47518129 |
Appl. No.: |
13/180785 |
Filed: |
July 12, 2011 |
Current U.S.
Class: |
62/129 ;
62/89 |
Current CPC
Class: |
F25D 2700/123 20130101;
F25D 17/06 20130101; F25D 29/003 20130101 |
Class at
Publication: |
62/129 ;
62/89 |
International
Class: |
F25D 17/06 20060101
F25D017/06; F25B 49/00 20060101 F25B049/00 |
Claims
1. A method of controlling temperature within a refrigerated
transport container (1), the refrigerated transport container (1)
comprising at least a transport volume (45), a control unit (7),
and a cooling space (41), one or more evaporator fans (10)
providing an air flow through the cooling space (41), where air
passing through the cooling space passes at least a return air
temperature sensor (5), a cooling unit (16), and a supply air
temperature sensor (25), wherein the method comprises: controlling
unmeasured temperatures in the transport volume (45) within a
temperature range adjacent to a setpoint or target temperature
(Tset), using two or more transport volume temperature indicators,
where the indicators are based on at least measured supply air
temperature and/or measured return air temperature.
2. The method according to claim 1, wherein the at least two
transport volume temperature indicators are one or more selected
from the group consisting of: current and/or recent supply air
temperature (Tsup), or a function thereof, current and/or recent
return air temperature (Tret), or a function thereof, an estimator
for temperature (Tcold) in a coldest spot of the transport volume
(45), one or more estimators for temperatures (Twarm) in one or
more warmer spots in the transport volume (45), where upon
activation of the controller (7), the estimators are initialized
using: current and/or recent return air temperatures Tret and/or,
current and/or recent supply air temperatures Tsup and/or, earlier
estimates if available and/or a history of power supply to the
cooling unit within a predetermined period of time.
3. The method according to claim 2, wherein the estimator for
temperature (Tcold) in a coldest spot of the transport volume (45)
estimates temperature (Tcold) in a coldest spot of the transport
volume (45) based on current and/or recent supply air temperatures
(Tsup) and one or more previous estimates of the temperature
(Tcold) in a coldest spot of the transport volume (45), and/or the
one or more estimators for temperatures (Twarm) in one or more
warmer spots in the transport volume (45) estimates temperatures
(Twarm) in one or more warmer spots of the transport volume (45)
based on current and/or recent supply air temperatures (Tsup),
current and/or recent return air temperatures (Tret), and one or
more previous estimates for temperatures (Twarm) in one or more
warmer spots in the transport volume (45).
4. The method according to claim 1, wherein the method comprises:
using an estimator for temperature (Tcold) in a coldest spot of the
transport volume and one or more estimators for temperatures
(Twarm) in one or more warmer spots of the transport volume (45),
and controlling a weighted average of these estimators to the
temperature setpoint (Tset).
5. The method according to claim 3, wherein the method comprises:
constraining the estimator for temperature (Tcold) in the coldest
spot to a minimum constraint and/or a maximum constraint.
6. The method according to claim 1, wherein the method comprises:
using supply air temperature (Tsup) or a time-averaged function
thereof, and return air temperature (Tret) or a time-averaged
function thereof as indicators for the coldest and the warmest
temperature in the transport volume, respectively, and controlling
a weighted average of the supply air temperature and the return air
temperature to the temperature setpoint entered into the control
unit.
7. The method according to claim 6, wherein the method comprises:
constraining the supply air temperature, or a time-averaged
function thereof, to a minimum constraint and/or a maximum
constraint.
8. The method according to claim 1, wherein the method comprises:
controlling by a slave-controller the supply air temperature or a
time-averaged function thereof to a supply air temperature setpoint
(Tset_slave), and adjusting the supply air temperature setpoint
(Tset_slave) as a function of a temperature setpoint (Tset) and a
measured return air temperature by a master-controller (203).
9. The method according to claim 8, wherein the adjustment of the
supply air temperature setpoint is made such that the weighted
average of the supply air temperature and the return air
temperature substantially equals the temperature setpoint
(Tset).
10. The method according to claim 8, wherein the method comprises
constraining the supply air temperature setpoint (Tset_slave) to a
minimum constraint and/or a maximum constraint.
11. The method according to claim 4, wherein the value for the
minimum constraint and/or the maximum constraint is dependent on
the temperature setpoint and/or the time elapsed since activation
of the controller (7).
12. The method according to claim 1, where the refrigerated
transport container is not a transport container but another type
of refrigerated space in connection with a cooling unit.
13. A system for controlling temperature within a refrigerated
transport container (1), the refrigerated transport container (1)
comprising at least a transport volume (45), and a cooling space
(41), one or more evaporator fans (10) providing an air flow
through the cooling space (41), where air passing through the
cooling space passes at least a return air temperature sensor (5),
a cooling unit (16), and a supply air temperature sensor (25),
wherein the system comprises a control unit (7) adapted to: control
unmeasured temperatures in the transport volume (45) within a
temperature range adjacent to a setpoint or target temperature
(Tset), using two or more transport volume temperature indicators,
where the indicators are based on at least measured supply air
temperature and/or measured return air temperature.
14. The system according to claim 13, wherein the at least two
transport volume temperature indicators are one or more selected
from the group consisting of: current and/or recent supply air
temperature (Tsup), or a function thereof, current and/or recent
return air temperature (Tret), or a function thereof, an estimator
for temperature (Tcold) in a coldest spot of the transport volume
(45), one or more estimators for temperatures (Twarm) in one or
more warmer spots in the transport volume (45), where upon
activation of the controller (7), the estimators are initialized
using: current and/or recent return air temperatures Tret and/or,
current and/or recent supply air temperatures Tsup and/or, earlier
estimates if available and/or a history of power supply to the
cooling unit within a predetermined period of time.
15. The system according to claim 14, wherein the estimator for
temperature (Tcold) in a coldest spot of the transport volume (45)
estimates temperature (Tcold) in a coldest spot of the transport
volume (45) based on current and/or recent supply air temperatures
(Tsup) and one or more previous estimates of the temperature
(Tcold) in a coldest spot of the transport volume (45), and/or the
one or more estimators for temperatures (Twarm) in one or more
warmer spots in the transport volume (45) estimates temperatures
(Twarm) in one or more warmer spots of the transport volume (45)
based on current and/or recent supply air temperatures (Tsup),
current and/or recent return air temperatures (Tret), and one or
more previous estimates for temperatures (Twarm) in one or more
warmer spots in the transport volume (45).
16. The system according to claim 13, wherein the controller (7) is
adapted to: use an estimator for temperature (Tcold) in a coldest
spot of the transport volume and one or more estimators for
temperatures (Twarm) in one or more warmer spots of the transport
volume (45), and controlling a weighted average of these estimators
to the temperature setpoint (Tset).
17. The system according to claim 15, wherein the controller (7) is
adapted to: constrain the estimator for temperature (Tcold) in the
coldest spot to a minimum constraint and/or a maximum
constraint.
18. The system according to claim 13, wherein the controller (7) is
adapted to: use supply air temperature (Tsup) or a time-averaged
function thereof, and return air temperature (Tret) or a
time-averaged function thereof as indicators for the coldest and
the warmest temperature in the transport volume, respectively, and
control a weighted average of the supply air temperature and the
return air temperature to the temperature setpoint entered into the
control unit.
19. The system according to claim 18, wherein the controller (7) is
adapted to: constrain the supply air temperature, or a
time-averaged function thereof, to a minimum constraint and/or a
maximum constraint.
20. The system according to claim 13, wherein the controller (7) is
adapted to: control by a slave-controller the supply air
temperature or a time-averaged function thereof to a supply air
temperature setpoint (Tset_slave), and adjust the supply air
temperature setpoint (Tset_slave) as a function of a temperature
setpoint (Tset) and a measured return air temperature by a
master-controller (203).
21. The system according to claim 20, wherein the adjustment of the
supply air temperature setpoint is made such that the weighted
average of the supply air temperature and the return air
temperature substantially equals the temperature setpoint
(Tset).
22. The system according to claim 20, wherein the controller (7) is
adapted to constrain the supply air temperature setpoint
(Tset_slave) to a minimum constraint and/or a maximum
constraint.
23. The system according to claim 16, wherein the value for the
minimum constraint and/or the maximum constraint is dependent on
the temperature setpoint and/or the time elapsed since activation
of the controller (7).
24. The system according to claim 13, wherein the refrigerated
transport container is not a transport container but another type
of refrigerated space in connection with a cooling unit.
Description
[0001] Disclosed is a method of and a system for controlling
temperature within a refrigerated transport container, or other
refrigerated storage spaces.
BACKGROUND
[0002] Temperature in a refrigerated transport container, or
another kind of refrigerated storage space, is typically controlled
within a temperature range adjacent to a setpoint or target
temperature (forth referred to as setpoint temperature or
setpoint). The refrigerated transport container may for example
comprise an insulated enclosure divided in a cooling space and a
transport volume. Typically, the transport volume is loaded with
perishable produce such as meat, vegetables and fruit, etc. The
setpoint temperature is then typically chosen to reduce quality
degradation of the perishable produce.
[0003] The cooling space may e.g. be separated from the transport
volume by a panel equipped with one or more openings to allow a
return air flow from the transport volume into the cooling space
and a supply air temperature flow from the cooling space into the
transport volume.
[0004] The air flow through the cooling space typically passes at
least a return air temperature sensor, a device for reducing the
temperature of the passing air, e.g. a cooling unit or system, and
a supply air temperature sensor. In such systems, the return air
temperature sensor typically measures the temperature of air
returning from the transport volume while the supply air
temperature sensor measures the temperature of air supplied to the
transport volume.
[0005] Temperature control protocols may selectively control a
cooling unit coupled to the refrigerated transport container in
order to maintain the setpoint temperature inside the refrigerated
transport container.
[0006] One typical type of a cooling unit or refrigeration unit
used in refrigerated storage transport containers is based on the
so-called vapour compression refrigeration cycle. This cycle
comprises at least a compressor, a condenser, an expansion device,
an evaporator and a capacity regulating device. The compressor
sucks refrigerant vapour from the evaporator and compresses the
refrigerant vapour which subsequently flows to the condenser at
high pressure. The condenser ejects its heat to a medium outside
the refrigerated transport container while condensing the
refrigerant vapour. The liquefied refrigerant then flows to the
expansion device in which a refrigerant pressure drops. The low
pressure refrigerant then flows to the evaporator where the
refrigerant evaporates while extracting the required heat from the
refrigerated transport container.
[0007] Other typical cooling units or refrigeration units used in
refrigerated transport containers may be different.
[0008] Temperatures in the transport volume are typically
unmeasured. In a steady state operation, measured supply air
temperature may normally be a fairly accurate representative of a
coldest temperature in the transport volume. In the steady state
operation, measured return air temperature may usually be a
reasonable representative of average temperature in the transport
volume. In the steady state operation, a warmest temperature in the
transport volume is usually a little higher than return air
temperature, but remains unknown and e.g. depends on the way the
cargo is stowed inside the container.
[0009] For frozen commodities, typically shipped at setpoints below
-10.degree. C. and usually around -20.degree. C., it is especially
important that produce temperature is not too far above setpoint.
Therefore for setpoints below -10.degree. C., it is common practice
to control a measured return air temperature closely to the
setpoint.
[0010] For chilled commodities, typically shipped at setpoints
above -10.degree. C., both too high and too low produce
temperatures are undesirable. The adverse effect of too high above
setpoint is fairly obvious; that is the whole reason why
refrigeration is applied. However being too low below setpoint,
chilled commodities may actually suffer as well. Some chilled
commodities are susceptible to chilling injury, e.g. like bananas
turning grey in home fridges.
[0011] Furthermore, many chilled commodities are susceptible to
freezing injury, which especially becomes an issue when sensitive
commodities like grapes are shipped at setpoints just above their
freezing point.
[0012] Traditionally, refrigerated transport containers used to be
stuffed with produce which was already pre-cooled to a temperature
close to setpoint, so transport volume temperatures were always
more or less in the steady state condition.
[0013] The current practice however, is that ever more containers
are stuffed with warm produce right after harvest, whereby it is up
to the container's cooling unit to reduce produce temperature from
stuffing temperature to a temperature range adjacent to the
setpoint temperature. In the banana trade for example, it is now
standard operations procedure to load uncooled bananas of around
25.degree. C. in containers operating at a setpoint of about
13.5.degree. C. In these non-steady state conditions, return air
temperature becomes a poor indicator of the warmest temperature
inside the transport volume.
[0014] Typically, the warmest temperature converges a lot slower to
a temperature range adjacent to a setpoint temperature than return
air temperature.
[0015] In view of the increasing number of warmly-stuffed
containers, there is a need to effectively and efficiently
manipulate measured supply and return air temperature in order to
ensure that actual transport volume temperatures reside as much as
possible and as quickly as possible within a desired temperature
range adjacent to a setpoint temperature.
SUMMARY
[0016] It is an object to provide a temperature control for a
refrigerated transport container more advanced than just
controlling either return air temperature or supply air temperature
to a setpoint. The temperature control ensures that a larger
portion of the transport volume temperatures resides in a desired
temperature range adjacent to a setpoint temperature during a
larger part of the transport time.
[0017] A first aspect relates to a method of controlling
temperature within a refrigerated transport container, the
refrigerated transport container comprising at least a transport
volume, a control unit, and a cooling space, one or more evaporator
fans providing an air flow through the cooling space, where air
passing through the cooling space passes at least a return air
temperature sensor, a cooling unit, and a supply air temperature
sensor, wherein the method comprises: [0018] controlling unmeasured
temperatures in the transport volume within a temperature range
adjacent to a setpoint or target temperature, using two or more
transport volume temperature indicators, where the indicators are
based on at least measured supply air temperature and/or measured
return air temperature.
[0019] Average produce temperature within the refrigerated
transport volume typically lies somewhere in-between the supply air
temperature and a few degrees above the return air temperature due
to temperature gradients within the transport volume.
[0020] An advantage of controlling unmeasured temperatures in the
transport volume, instead of just supply or return air temperature,
within a temperature range adjacent to a setpoint or target
temperature (Tset), is that this improves control over temperatures
of the loaded perishable produce.
[0021] The motivation for transporting perishable commodities in
refrigerated transport containers is that their quality loss
depends on temperature. Moreover, the rate of quality loss
deteriorates at suboptimal temperatures. [0022] Controlling
temperatures in the transport volume helps to reduce the rate of
quality loss. Especially in pulldown situations, occurring in
warmly-stuffed containers, the advantage may be significant because
then the difference between produce temperature and either supply
or return air temperature is largest.
[0023] In one embodiment, the at least two transport volume
temperature indicators are one or more selected from the group
consisting of: [0024] current and/or recent supply air temperature,
or a function thereof, [0025] current and/or recent return air
temperature, or a function thereof, [0026] an estimator for
temperature in a coldest spot of the transport volume, [0027] one
or more estimators for temperatures in one or more warmer spots in
the transport volume, where upon activation of the controller (e.g.
when the cooling unit powers up), the estimators are initialized
using: [0028] current and/or recent return air temperatures Tret
and/or, [0029] current and/or recent supply air temperatures Tsup
and/or, [0030] earlier estimates if available and/or [0031] a
history of power supply to the cooling unit within a predetermined
period of time (e.g. the last 24 hours or so).
[0032] Temperatures in the transport volume are unmeasured and
therefore cannot be controlled directly. The use of transport
volume temperature indicators, correlated to temperatures in the
transport volume, advantageously enable indirect control over
temperatures in the transport volume, more than just controlling
return or supply air temperature to a setpoint.
[0033] The estimators may e.g. be initialized or re-initialized
after a power cut or powering down based on the latest estimate
made just before the power cut or power down happened e.g. taking
into account the duration of the power cut. One example may e.g. be
that the initial estimate after power is established again is equal
to the estimate at the power cut or power down plus a factor (e.g.
0.1.degree. C./h) times the duration of the period (h) of time
without power.
[0034] In one embodiment, the estimator for temperature in a
coldest spot of the transport volume estimates temperature in a
coldest spot of the transport volume based on current and/or recent
supply air temperatures and one or more previous estimates of the
temperature in a coldest spot of the transport volume, and/or the
one or more estimators for temperatures in one or more warmer spots
in the transport volume estimates temperatures in one or more
warmer spots of the transport volume based on current and/or recent
supply air temperatures, current and/or recent return air
temperatures, and one or more previous estimates for temperatures
in one or more warmer spots in the transport volume.
[0035] The estimator for temperature in a coldest spot of the
transport volume may e.g. be an estimator whose change is based on
a function of current and/or recent supply air temperatures and one
or more previous estimates of the temperature in a coldest spot of
the transport volume.
[0036] The estimator for temperatures in one or more warmer spots
in the transport volume may e.g. be an estimator whose change is
based on a function of the current and/or recent supply air
temperatures, current and/or recent return air temperatures, and
one or more previous estimates for temperatures in one or more
warmer spots in the transport volume.
[0037] When to-be-controlled states of any dynamic process are
unmeasured, the use of estimators for that states advantageously
offer the possibility to have some degree of control over those
states. Temperatures in the transport volume are unmeasured, yet
some degree of control becomes possible by using estimators for
temperature (Tcold) in a coldest spot of the transport volume and
one or more estimators for temperatures (Twarm) in one or more
warmer spots in the transport volume. The estimators could for
example be mathematical filters mapping available information on
current and/or recent supply air temperature and current and/or
recent power supply to the rate of temperature change at the
coldest and one or more warmer locations in the transport volume.
These filters could be tuned using earlier collected experimental
measurements of trajectories of supply air temperature and
temperature in the coldest and one or more warmer locations in the
transport volume.
[0038] In one embodiment, the method comprises: [0039] using an
estimator for temperature in a coldest spot of the transport volume
and one or more estimators for temperatures in one or more warmer
spots of the transport volume, and controlling a weighted average
of these estimators to the temperature setpoint (e.g. plus an
offset, where the offset maybe zero).
[0040] Controlling a weighted average of an estimate for
temperature (Tcold) in a coldest spot of the transport volume and
one or more estimators for temperatures (Twarm) in one or more
warmer spots of the transport volume offers an important advantage
over just controlling supply or return air temperature to setpoint:
it controls a true representative of produce temperature to
setpoint.
[0041] In one embodiment, the method comprises: [0042] constraining
the estimator for temperature in the coldest spot to a minimum
constraint and/or a maximum constraint.
[0043] Including maximum and minimum constraints advantageously
helps to avoid the exceeding of temperature limits that are
critical to produce quality. Especially important are the limits in
chilled mode below which chilling injury or freezing injury may be
inflicted, or the limit in frozen mode above which the carried
commodity may start to thaw. A well-known example of chilling
injury is the dull grey coloration of bananas stored in home
fridges. The risk of freezing injury especially exists for all
fruit stored at temperatures just above their freezing point (for
example the pale brown coloration of grapes and their stems).
[0044] In one embodiment, the method comprises: [0045] using supply
air temperature or a time-averaged function thereof, and return air
temperature or a time-averaged function thereof as indicators for
the coldest and the warmest temperature in the transport volume,
respectively, and [0046] controlling a weighted average of the
supply air temperature and the return air temperature to the
temperature setpoint entered into the control unit (e.g. plus an
offset, where the offset maybe zero).
[0047] In calculation of the weighted average, the weight of supply
air temperature may differ from the weight of the return air
temperature.
[0048] Controlling a weighted average of an estimate for
temperature (Tcold) in a coldest spot and an estimate for
temperature (Twarm) in a warmest spot of the transport volume
offers an important advantage over just controlling supply or
return air temperature to setpoint: it controls a true
representative of produce temperature to setpoint. Supply air
temperature (Tsup) or a time-averaged function thereof, and return
air temperature (Tret) or a time-averaged function thereof, are not
the most advanced estimators for the coldest and the warmest
temperature in the transport volume, but the advantage is that they
are straightforwardly available in any refrigerated transport
container.
[0049] In one embodiment, the method comprises: [0050] constraining
the supply air temperature, or a time-averaged function thereof, to
a minimum constraint and/or a maximum constraint.
[0051] Including maximum and minimum constraints advantageously
help to avoid the exceeding of temperature limits that are critical
to produce quality, as explained above.
[0052] In one embodiment, the method comprises: [0053] controlling
by a slave-controller the supply air temperature or a time-averaged
function thereof to a supply air temperature setpoint, and
adjusting the supply air temperature setpoint as a function of a
temperature setpoint and a measured return air temperature by a
master-controller.
[0054] This is to some extent an alternative implementation of the
embodiment controlling a weighted average of the supply air
temperature and the return air temperature to the temperature
setpoint with similar advantages.
[0055] An additional advantage of using the master-slave concept is
the possibility to use the master controller to make the supply air
temperature setpoint any possible function of current and/or
recently measured return air temperature and to also shape the
dynamics of the response of supply air temperature to changes in
return air temperature.
[0056] In one embodiment, the master-controller adjusts the supply
air temperature setpoint such that the weighted average of the
supply air temperature and the return air temperature substantially
equals the temperature setpoint (e.g. plus an offset, where the
offset maybe zero).
[0057] In calculation of the weighted average the weight of supply
air temperature may differ from the weight of the return air
temperature.
[0058] This advantageously combines the advantages provided by the
master-slave concept as used in the preceding embodiment with the
advantage of controlling a weighted average of an easily available
estimate for temperature (Tcold) in a coldest spot and an easily
available estimate for temperature (Twarm) in one or more warmer
spots of the transport volume, which is the control of a true
representative of produce temperature to setpoint.
[0059] In one embodiment, the method comprises [0060] constraining
the supply air temperature setpoint, as adjusted by the
master-controller, to a minimum constraint and/or a maximum
constraint.
[0061] Including maximum and minimum constraints advantageously
helps to avoid the exceeding of temperature limits that are
critical to produce quality, as explained above.
[0062] In one embodiment, the value for the minimum constraint
and/or the maximum constraint is dependent on the temperature
setpoint and/or the time elapsed since activation of the
controller.
[0063] Making maximum and minimum constraints dependent on the
temperature setpoint and/or the time elapsed since activation of
the controller advantageously increases flexibility to tailor the
constraints to the actual need. At for example a setpoint of
-20.degree. C. a maximum constraint should be close to setpoint,
because for frozen commodities it is only important that produce
temperatures stay below a certain level. At for example a setpoint
of 0.degree. C. a minimum constraint should be close to setpoint to
avoid freezing injury, while a maximum constraint might be more
tolerant. The time elapsed since activation of the controller
correlates to lowest temperature in the transport volume. Therefore
for example in a warmly-stuffed container with grapes right after
activation of the controller at power-up a supply air temperature
multiple degrees C. below the freezing point will not freeze the
grapes, while later on that risk increases. So a minimum constraint
tightening over time may be appropriate.
[0064] In one embodiment, the refrigerated transport container is
not a transport container but another type of refrigerated space in
connection with a cooling unit. This could for example be an item
of refrigerated road transport equipment, a reefer ship, or any
type of stationary cold storage room.
[0065] A second aspect relates to a system for controlling
temperature within a refrigerated transport container, the
refrigerated transport container comprising at least a transport
volume, and a cooling space, one or more evaporator fans providing
an air flow through the cooling space, where air passing through
the cooling space passes at least a return air temperature sensor,
a cooling unit, and a supply air temperature sensor, wherein the
system comprises a control unit adapted to: [0066] control
unmeasured temperatures in the transport volume within a
temperature range adjacent to a setpoint or target temperature,
using two or more transport volume temperature indicators, where
the indicators are based on at least measured supply air
temperature and/or measured return air temperature.
[0067] The embodiments of the system correspond to the embodiments
of the method and have the same advantages for the same
reasons.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] Preferred embodiments of the invention will be described in
more detail in connection with the appended drawings, in which:
[0069] FIG. 1 schematically illustrates a simplified longitudinal
cross-sectional view of a refrigerated space in the form of a
refrigerated transport container;
[0070] FIG. 2 schematically illustrates a block diagram
representing a so-called master-slave controller according to one
embodiment;
[0071] FIG. 3 presents a computer simulation output schematically
illustrating a setpoint (Tset) entered into a controller and
temperature trajectories for a temperature of the supply air flow
(Tsup), a temperature of the return air flow (Tret) and a warmest
produce temperature (Twarm) in the transport volume in a situation
where Tsup is controlled to the entered Tset;
[0072] FIG. 4 presents another computer simulation output
schematically illustrating a setpoint (Tset) entered into a
master-controller and temperature trajectories for a temperature of
the supply air flow (Tsup), a temperature of the return air flow
(Tret), a warmest produce temperature (Twarm), and a
slave-controller's setpoint (Tset_slave) adjusted by a master
controller;
[0073] FIG. 5 schematically illustrates measurements collected in a
real transport container where temperature is controlled like in
FIG. 3;
[0074] FIG. 6 schematically illustrates measurements collected in a
real transport container where temperature is controlled like in
FIG. 4.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0075] FIG. 1 schematically illustrates a simplified longitudinal
cross-sectional view of a refrigerated space in the form of a
refrigerated transport container.
[0076] Shown is one example of a refrigerated transport container
1, or another type of refrigerated storage space, comprising at
least a transport volume 45, a control unit 7, and a cooling space
41. The cooling space 41 may be situated inside an insulated
enclosure of the transport container 1 and may (as shown) be
separated from the transport volume 45 by a panel or the like
equipped with one or more openings to allow a return air flow 50
into the cooling space 41 and a supply air flow 55 out of the
cooling space 41.
[0077] The air flow through the cooling space may be maintained by
for example one or more evaporator fans 10 or one or more other
units providing a similar function. On its way through the cooling
space 41, air successively passes at least a return air temperature
sensor 5, the one or more evaporator fans 10, a cooling unit or
system 16 (or one or more other units with a similar function)
reducing the temperature of the passing air, and a supply air
temperature sensor 25.
[0078] In this kind of system, the return air temperature sensor 5
measures the temperature of air returning from the transport volume
(forth denoted Tret), while the supply air temperature sensor 25
measures the temperature of air supplied to the transport volume
(forth denoted Tsup).
[0079] Unmeasured temperatures in the transport volume (45) are
controlled by the controller (7) to be within a temperature range
adjacent to a setpoint temperature (Tset) using two or more
transport volume temperature indicators, where the indicators are
based on at least measured supply air temperature and/or measured
return air temperature. As a result the temperature control is more
advanced than just controlling supply or return air temperature to
a setpoint Tset, like in traditional chilled respectively frozen
mode operation. For example the average temperature of the supply
air temperature Tsup may temporarily be allowed to be below the
setpoint Tset in order to speed up the pulldown of procude
temperatures in the transport volume.
[0080] The controller (7) may e.g. comprise a master-slave
controller setup as explained in connection with FIG. 2 or its
functionality could be provided in another fashion.
[0081] Further aspects and variations will be explained further in
the following.
[0082] FIG. 2 schematically illustrates a block diagram
representing a so-called master-slave controller according to one
embodiment. In this embodiment, the process 217 represents
temperature dynamics within a refrigerated transport container (see
e.g. 1 in FIG. 1). Though each location in the refrigerated
transport container has its own temperature 219, only two of them
are measured: a Return air Temperature Sensor 5 measures the return
air temperature Tret 213 and a Supply air Temperature Sensor 25
measures the supply air temperature Tsup 209.
[0083] This block diagram represents a so-called master-slave
controller 200 according to one embodiment where an entered
setpoint Tset 201 generally is first processed in a master
controller 203 that based on Tset 201 and Tret 213 manipulates or
derives a second or modified setpoint Tset_slave 205. The
difference between the modified setpoint Tset_slave 205 and supply
air temperature Tsup 209 is then received by the slave controller
207, which then aims to minimize this difference, effectively
controllingTsup 209 to the modified setpoint Tset_slave 205 by
adjusting the amount of heat absorbed by the cooling unit (see e.g.
16 in FIG. 1) in a cooling space of the refrigerated transport
container, which in this schematic representation may be regarded
to be part of the process 217.
[0084] In the present embodiment, the user's setpoint Tset 201 is
treated as a setpoint to a master controller 203 where the master
controller 203 manipulates the slave setpoint Tset_slave 205. The
slave controller 207 then controls the supply air temperature Tsup
209 to the slave setpoint Tset_slave 205. The slave setpoint
Tset_slave 205 deliberately deviates from the master setpoint Tset
201 with the objective to control the average of Tsup 209 and Tret
213 to the setpoint Tset 201. By allowing the average Tsup 209 to
be below Tset 201 instead of controlling it to Tset, a larger
portion of the temperatures 219, including produce temperatures, in
the container will be in a temperature range adjacent to setpoint
Tset 201 and will be so quicker.
[0085] When a controller (see e.g. 7 in FIG. 1) initiates, for
example when the unit powers up, Tset_slave 205 may be initialized
as a function of Tset 201 and Tret 213, for example according to
Tset_slave=Tset-0.5.times.(Tret-Tset). This lowers, in this
specific example, the modified or effective supply air temperature
setpoint with half the difference between the temperature of the
return air and the normal setpoint. It is to be understood that
other suitable initializations may be used. What is significant is
that the modified or effective supply air temperature setpoint
Tset_slave is lowered initially in proportion to the difference
between return air temperature and setpoint Tset.
[0086] Following this initialization Tset_slave 205 may then be
updated by the master controller 203 at the beginning of each
subsequent cycle e.g. according to:
Tset_slave(k+1)=max(Tset_slave_min;
(1-0.2.times.tcycle/60).times.Tset_slave(k)+0.2.times.tcycle/60.times.(2.-
times.Tset-Tret(k)))[.degree. C.],
where k designates the k-th cycle, tcycle=duration of the preceding
cycle [minutes], T.sub.ret(k)=return air temperature averaged over
the k-th cycle [.degree. C.], Tset_slave(k)=slave setpoint during
the k-th cycle, and Tset_slave_min=a lower constraint on
Tset_slave, meant to avoid freezing or chilling injury and e.g.
given by Tset_slave=Tset-1.degree. C.
[0087] In the equation above, a cycle is a predefined period of
time, which may be constant or may be defined otherwise. For
example, in systems with on/off controlled compressors it may be
defined as a period of time from one start of a compressor until
its next start.
[0088] The preceding equation helps to control the average of Tsup
and Tret to Tset. This can be seen by observing that a control
objective `average of Tsup and Tret=Tset` is equivalent to
`(Tsup+Tret)/2=Tset` is equivalent to `Tsup=2.times.Tset-Tret`. If
we assume that Tsup=Tset_slave, something the slave-controller may
take care of, then `Tsup=2.times.Tset-Tret` is equivalent to the
control objective `Tset_slave=2.times.Tset-Tret`. A very simple
implementation of this, is to program the master controller
according to:
Tset_slave(k+1)=max(Tset_slave_min; 2.times.Tset-Tret(k))[.degree.
C.]
However, any high-frequent fluctuation in Tret(k) is just passed on
to Tset_slave(k+1). This could then result in undesired
high-frequent oscillations in Tset_slave. To avoid this behaviour,
a low pass filter is added. One example of a simple low-pass filter
is a linear difference equation of the type
Tset_slave(k+1)=(1-smoothing factor).times.Tset_slave(k)+smoothing
factor.times.Tret(k), which is used in the preceding paragraph,
using a `smoothing factor=0.2.times.tcycle/60`.
[0089] FIG. 3 schematically illustrates a computer simulation with
a setpoint (Tset) 301 entered into a controller and temperature
trajectories for a temperature of the supply air flow (Tsup) 302, a
temperature of the return air flow (Tret) 303 and a warmest produce
temperature (Twarm) 304 in the transport volume.
[0090] In this situation Tsup 302 is controlled to the entered Tset
301. This reflects a traditional approach to temperature control in
chilled mode operation. It could be achieved by a control set-up as
depicted in FIG. 2 where the master controller just sets Tset_slave
to Tset 301, although a more natural implementation would then be
to omit the master controller and just feed the difference between
Tset 301 and Tsup 302 to the slave controller (which then in effect
becomes a master controller or the only controller for this
purpose).
[0091] In traditional frozen mode operation, Tret 303 would be
controlled to Tset 301. In that situation, the temperature pulldown
would proceed at maximum cooling capacity until the curve of Tret
303 reaches setpoint, regardless how much Tsup 302 undershoots the
setpoint Tset 301.
[0092] FIG. 3 illustrates the traditional approach in chilled mode
operation, i.e. operation at setpoints above -10.degree. C. In real
shipments the warmest produce temperature Twarm 304 in the
transport volume is normally unmeasured, but the computer
simulation shows a realistic pattern.
[0093] FIG. 4 shows a computer simulation with simulated
trajectories for temperature Tsup 302, Tret 303, Twarm 304
resulting from entering the setpoint Tset 301 into a
master-controller, which then manipulates the slave-controller's
setpoint Tset_slave 305. The slave-controller's setpoint Tset_slave
305 is adjusted by the master controller, that based on Tset 301
and Tret 303 manipulates the setpoint Tset_slave 305 (constrained
to Tset_slave.gtoreq.Tset-1) with the objective to control the
average of Tsup 302 and Tret 303 to Tset 301, while the slave
controller aims to minimize the difference between supply air
temperature Tsup 302 and its adjusted supply air temperature
setpoint Tset_slave 305.
[0094] This master-slave controller is an implementation of the
embodiment depicted in FIG. 2 with the master-controller executing
the algorithm as described in relation to FIG. 2.
[0095] Comparing FIG. 3 and FIG. 4 illustrates that a faster
temperature pulldown, i.e. a faster approach of the temperature to
the setpoint, is achieved due to the master-slave control in FIG.
4, while yet maintaining control over Tsup 302. For example after 2
days in FIG. 3, Twarm 304 is still 6.7.degree. C., while in FIG. 4
Twarm 304 then is already down to 6.degree. C. This is achieved by
allowing supply air temperatures Tsup 302 colder than Tset 301. In
general this means an increased risk of chilling injury. However
the period of coldest Tsup 302 typically occurs in the beginning of
the pulldown when temperatures in most locations in the transport
volume are still above Tset 301. Consequentially the risk of
inducing chilling injury is very limited while the benefit of
faster pulldown is clear, namely less quality degradation due to
too high temperatures (i.e. the whole idea of applying
refrigeration).
[0096] In frozen mode operation the master-slave concept may be
used for example to limit the undershoot of Tsup 302 during
temperature pulldown like in FIG. 4. This would for example offer
the advantage of some energy saving at the expense of a slightly
slower pulldown of warmest temperature Twarm 304 in the transport
volume.
[0097] FIG. 5 and FIG. 6 show the trajectories of Tsup 302 and Tret
303 registered during two test shipments. It concerns two
refrigerated transport containers making the same journey
simultaneously. The containers both carry a cargo of warmly-stuffed
citrus. The high initial cargo temperature causes high return air
temperatures during the initial days of the voyage.
[0098] FIG. 5 shows the trajectories of Tsup 302 and Tret 303
registered in a container where Tsup 302 is controlled to Tset 301,
like in the simulation in FIG. 3. Note that the persistent
0.2.degree. C. offset between Tsup 302 and Tset 303 in FIG. 5 is a
consequence of a difference between the supply air temperature
recorder sensor used to record the temperature measurements and the
supply air temperature controller sensor (not shown; see e.g. 5 in
FIG. 1).
[0099] FIG. 5 schematically illustrates a setpoint Tset 301 entered
into a controller and temperature trajectories for a temperature of
the supply air flow Tsup 302, and a temperature of the return air
flow Tret 303. Like in FIG. 3, the supply air temperature Tsup 302
is controlled to the entered Tset 301. FIG. 5 does not contain the
warmest produce temperature Twarm, as e.g. shown in FIG. 3, as in
real shipments this is unknown.
[0100] FIG. 6 displays the recorded Tsup 302 and Tret 303 in a
container controlled according to the concept shown in FIG. 2 and
simulated in FIG. 4. It schematically illustrates a setpoint Tset
301 entered into a controller and temperature trajectories for a
temperature of the supply air flow Tsup 302, and a temperature of
the return air flow Tret 303. FIG. 6 does not contain the warmest
produce temperature Twarm as this is not known in real
shipments.
[0101] FIG. 6 illustrates how the master controller, deriving
Tset_slave, e.g. as described in connection with FIG. 2, responds
to the high initial Tret 303 by reducing Tset_slave (not shown, but
approximately equal to Tsup 302) to its lower bound Tset 301 minus
1.degree. C. Consequentially the pulldown of Tret 303 is faster.
Later on, Tret 303 comes ever closer to Tset 301, while the master
controller gradually rises Tset_slave with the objective to control
the average of Tsup 302 and Tret 303 to Tset 301.
[0102] In FIG. 6, a minor jitter is observable on Tsup 302. This is
caused by the on/off control method implemented in the slave
controller, with the excitations of Tsup 302 smoothened again to a
large extent by displaying hourly averaged values of Tsup 302 in
FIG. 6.
[0103] In both charts (FIG. 5 and FIG. 6) the rise of Tsup 302 and
Tret 303 up to 8-9.degree. C. around 091220-00 is typically the
result of a few hours without electric power supply, during which
the container was moved from land to ship. Other power off periods
due to unknown factors occur in FIG. 5 around 091228-12 and in FIG.
6 around 091224-00. Also in both charts. minor spikes of about
1.degree. C. are visible in Tret 303 in a regular frequency. These
are due to so-called defrosts, marked on the horizontal axis with
little cubes 306 on the horizontal axis. In FIG. 5 these occur more
or less once a day, in FIG. 6 less frequent. During a defrost
period, a defrost control algorithm, e.g. implemented in the same
control unit (7 in FIG. 1), overrules the temperature controller,
stops cooling, stops the evaporator fans (10 in FIG. 1) and
supplies heat to the cooling unit (16 in FIG. 1) in order to remove
frost formed on the cooling unit. Once the defrost controller
terminates the defrost, the evaporator fans resume the air
circulation and the temperature controller resumes temperature
control.
* * * * *