U.S. patent application number 12/894680 was filed with the patent office on 2012-04-05 for method and system for temperature control in refrigerated storage spaces.
Invention is credited to Janneke Emmy DE KRAMER-CUPPEN, Leijn Johannes Sjerp LUKASSE.
Application Number | 20120079840 12/894680 |
Document ID | / |
Family ID | 45888641 |
Filed Date | 2012-04-05 |
United States Patent
Application |
20120079840 |
Kind Code |
A1 |
LUKASSE; Leijn Johannes Sjerp ;
et al. |
April 5, 2012 |
METHOD AND SYSTEM FOR TEMPERATURE CONTROL IN REFRIGERATED STORAGE
SPACES
Abstract
The present invention relates to a method of controlling air
temperature within a refrigerated storage space based on a
temperature-error integral of supply air discharged into the
storage space. Another aspect of the invention relates to a
refrigerated storage space comprising a corresponding temperature
control system.
Inventors: |
LUKASSE; Leijn Johannes Sjerp;
(Ede, NL) ; DE KRAMER-CUPPEN; Janneke Emmy;
(Bennekom, NL) |
Family ID: |
45888641 |
Appl. No.: |
12/894680 |
Filed: |
September 30, 2010 |
Current U.S.
Class: |
62/115 ;
62/159 |
Current CPC
Class: |
F25D 2700/123 20130101;
F25D 2400/02 20130101; F25D 2317/04111 20130101; F25D 17/042
20130101; F25D 29/00 20130101 |
Class at
Publication: |
62/115 ;
62/159 |
International
Class: |
F25B 29/00 20060101
F25B029/00 |
Claims
1. A method of controlling air temperature within a refrigerated
storage space, the method comprising steps of: circulating return
air from the refrigerated storage space through a cooling unit to
provide a flow of cooled supply air at a supply air temperature,
discharging the flow of cooled supply air into the refrigerated
storage space to control air temperature inside the refrigerated
storage space, measuring the supply air temperature by a first
temperature sensor, computing a temperature-error integral of the
supply air based on a difference over time between the supply air
temperature and a reference temperature, adjusting the supply air
temperature based on the temperature-error integral such that a
time average of the supply air temperature substantially equals the
reference temperature.
2. A method of controlling air temperature according to claim 1,
wherein the reference temperature comprises an adjusted setpoint
temperature or a setpoint temperature.
3. A method of controlling air temperature according to claim 1,
comprising a further step of: circulating the return air from the
refrigerated storage space through a heating unit to provide a flow
of heated supply air at the supply air temperature.
4. A method of controlling air temperature according to claim 1,
comprising a step of: controlling respective operational states of
the heating unit or the cooling unit or both to adjust the supply
air temperature.
5. A method of controlling air temperature according to claim 1,
comprising a step of: switching operational states of the cooling
unit between exclusively ON and OFF states to make the adjustment
of the supply air temperature.
6. A method of controlling air temperature according to claim 5,
comprising a further step of: regulating the cooling capacity of
the cooling unit by a capacity regulating device coupled to vapour
compression refrigeration cycle, containing a compressor and an
evaporator.
7. A method of controlling air temperature according to claim 6,
wherein the cooling capacity of the cooling unit is regulated by a
step of: controlling a refrigerant flow rate between the evaporator
and the compressor by a suction valve.
8. A method of controlling air temperature according to claim 1,
comprising further steps of: comparing the temperature-error
integral of the supply air with a first integral error threshold
and a second integral error threshold, changing an operational
state of at least one of the cooling unit, the heating unit and
internal fans if the temperature-error integral exceeds the first
integral error threshold or falls below the second integral error
threshold.
9. A method of controlling air temperature according to claim 8,
wherein a difference between the first integral error threshold and
the second integral error threshold is set to a value between
20.degree. C.*minutes and 200.degree. C.*minutes.
10. A method of controlling air temperature according to claim 8,
comprising further steps of: setting the operational state of the
cooling unit to active for at least a first minimum time period if
the temperature-error integral exceeds the first integral error
threshold, setting the operational state of the heating unit to
active for at least a second minimum time period if the
temperature-error integral is below the second integral error
threshold.
11. A method of controlling air temperature according to claim 8,
wherein: the first integral error threshold is set to a value
between 50 and 200.degree. C.*minutes; and the second integral
error threshold is set to a value between -100 and -10.degree.
C.*minutes.
12. A method of controlling air temperature according to claim 1,
comprising a step of: adjusting the reference temperature as a
function of a setpoint temperature and a temperature of the return
air measured by a return air temperature sensor.
13. A method of controlling air temperature according to claim 12,
wherein the adjustment of the reference temperature is made such
that the average of the supply air temperature and the return air
temperature substantially equals the setpoint temperature.
14. A method of controlling air temperature according to claim 12,
comprising a step of: limiting the reference temperature to a lower
temperature limit dependent on the setpoint temperature, and
optionally a further step of: limiting the reference temperature to
an upper temperature limit.
15. A method of controlling air temperature according to claim 8,
comprising steps of, during circulation periods, comparing the
temperature-error integral to a first heating threshold, maintain
internal fans at a first preset speed if the temperature-error
integral is below the first heating threshold.
16. A method of controlling air temperature according to claim 15,
comprising a further step of: comparing the temperature-error
integral to a second heating threshold smaller than the first
heating threshold, maintain the internal fans at a second preset
speed if the temperature-error integral falls below the second
heating threshold; wherein the a second preset speed is higher than
the first preset speed.
17. A method of controlling air temperature according to claim 5,
comprising further steps of: comparing a duration of a previous
circulation period with a circulation time threshold t.sub.ct,
maintain the speed of the internal fans at a maximum speed during a
current circulation period if the previous circulation period was
smaller than the circulation time threshold t.sub.ct.
18. A method of controlling air temperature according to claim 17,
wherein the circulation time threshold t.sub.ct depends on a change
of the temperature-error integral during the previous circulation
period.
19. A method of controlling air temperature according to claim 1,
wherein the temperature-error integral is computed at regular time
intervals such as time intervals smaller than 1 minute, preferably
smaller than 10 seconds such as about every second.
20. A method of controlling air temperature and relative humidity
according to claim 1, comprising further steps of: simultaneous
heating and cooling the supply air at a first speed setting of the
internal fans when a measured relative humidity (RH) of the air
inside the transport volume is higher than a first humidity
threshold derived from the setpoint value of the relative humidity
(RH.sub.set), setting a second fan speed of the internal fans
during circulation periods when a measured relative humidity (RH)
of the air is higher than a second humidity threshold derived from
the setpoint value of the relative humidity; wherein fan speed in
the first fan speed is lower than fan speed at the second fan speed
setting.
21. A method of controlling air temperature and relative humidity
according to claim 20, comprising further steps: setting the
operational state of the heating unit to ON during circulation
periods when a measured relative humidity (RH) of the air is higher
than a third humidity threshold derived from the setpoint value of
the relative humidity.
22. A refrigerated storage space comprising: a refrigerated volume
for housing a commodity load, a cooling unit configured to receive
return air from the refrigerated volume and generate a flow of
cooled supply air at a supply air temperature, an air flow passage
coupled to the refrigerated volume to discharge the supply air
therein and control air temperature within the refrigerated volume,
a first temperature sensor adapted to measuring the supply air
temperature, a temperature control system adapted to: computing a
temperature-error integral of the supply air based on a difference
over time between the supply air temperature and a reference
temperature, adjusting the supply air temperature based on the
temperature-error integral such that a time average of the supply
air temperature is substantially equal to the reference
temperature.
23. A refrigerated storage space according to claim 22, wherein the
reference temperature comprises an adjusted setpoint temperature or
a setpoint temperature.
24. A refrigerated storage space according to claim 22, comprising
a heating unit configured to supply a flow of heated supply air at
the supply air temperature from the return air from the
refrigerated storage space.
25. A refrigerated storage space according to claim 24, wherein the
temperature control system is adapted to: controlling respective
operational states of the heating unit or the cooling unit or both
to adjust the supply air temperature.
26. A refrigerated storage space according to claim 25, wherein the
temperature control system is adapted to: switching operational
states of the cooling unit between exclusively ON and OFF states to
make the adjustment of the supply air temperature.
27. A refrigerated storage space according to claim 22, wherein the
temperature control system is adapted to: adjusting the reference
temperature as a function of a setpoint temperature and a
temperature of the return air measured by a return air temperature
sensor.
28. A refrigerated storage space according to claim 27, wherein the
temperature control system is adapted to adjust the reference
temperature such that the average of the supply air temperature and
the return air temperature substantially equals the setpoint
temperature.
29. A refrigerated storage space according to claim 22, comprising
a refrigerated container wherein the refrigerated volume comprises
a transport volume of the refrigerated container.
Description
[0001] The present invention relates to a method of controlling air
temperature within a refrigerated storage space based on a
temperature-error integral of supply air discharged into the
storage space. Another aspect of the invention relates to a
refrigerated storage space comprising a corresponding temperature
control system.
BACKGROUND OF THE INVENTION
[0002] It is important to maintain the temperature of perishable
produce held in refrigerated storage spaces at a desired or
setpoint temperature. The setpoint temperature is chosen to keep
the perishable produce such as meat, vegetables and fruit, at
correct temperature to avoid quality degradation. It is known in
the art to apply temperature control protocols that selectively
control operational states of heaters, cooling devices and internal
fans such as evaporator fans of cooling units, coupled to the
refrigerated storage space, to maintain a setpoint air temperature
inside the refrigerated storage space. The refrigerated storage
space may for example comprise a transport volume of a refrigerated
container.
[0003] The typical cooling unit or refrigeration unit used in
refrigerated storage spaces 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 storage space
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 storage space. A capacity regulating device,
which may comprise a suction modulating valve, controls the cooling
capacity of the cooling unit. Cooling capacity is the amount of
heat absorbed by the evaporator per unit of time. A typical
characteristic of vapour compression refrigeration cycles is that
their energy efficiency reduces in part-load operation, i.e.
whenever the compressor continues to be driven or active while the
capacity regulating device reduces the cooling capacity.
[0004] EP 2 116 798 A1 discloses a refrigeration system which has
an energy saving operation mode performing a first action in which
a compressor and an internal fan are driven while the cooling
capacity of an evaporator is regulated. In a second action, when
the blow-off side air temperature in a cold storage is kept at a
set value in the first action, the cooling capacity of the
evaporator is increased to lower the blow-off side air temperature
to a lower limit temperature of a desired temperature range
containing the set value and the compressor and the internal fan
are then stopped. In a third action in which, when the blow-off
side air temperature after the second action rises to an upper
limit temperature of the desired temperature range, the first
action is restarted.
[0005] Despite various attempts to lower energy consumption of the
cooling units, substantial amounts of energy are still consumed in
today's refrigerated storage spaces. Consequently, there is a
continuing need for providing temperature control algorithms and
systems with improved energy efficiency to reduce energy costs and
reduce CO.sub.2 emissions from cooling or heating the refrigerated
storage spaces.
[0006] In one embodiment, the present invention provides an
improved energy saving methodology for controlling the respective
operational states or modes of heating units, cooling devices and
internal fans of existing cooling units of refrigerated storage
spaces. The method allows temperature control systems of existing
refrigerated storage spaces to benefit from the invention without
any need for hardware replacements or modifications. The improved
temperature control methodology may advantageously be implemented
as embedded control software executed on a microprocessor of a
temperature control system associated with the refrigerated storage
space to improve energy efficiency of existing heating units,
cooling units and internal fans or circulation fans. Consequently,
the present invention may conveniently, but not exclusively, be
implemented by a software update of existing embedded control
software or program code of the temperature control system.
SUMMARY OF INVENTION
[0007] A first aspect of the invention relates to a method of
controlling air temperature of a refrigerated storage space, the
method comprising steps of: [0008] circulating return air from the
refrigerated storage space through a cooling unit to provide a flow
of cooled supply air at a supply air temperature, [0009]
discharging the supply air into the refrigerated storage space to
control the air temperature inside the refrigerated storage space,
[0010] measuring the supply air temperature by a first temperature
sensor, [0011] computing a temperature-error integral of the supply
air based on a difference over time between the supply air
temperature and a reference temperature, [0012] adjusting the
supply air temperature based on the temperature-error integral such
that a time average of the supply air temperature substantially
equals the reference temperature.
[0013] The time average of the supply air temperature preferably
deviates with less than +/-0.5.degree. C., or more preferably with
less than +/-0.2.degree. C., from the reference temperature under
steady-state operation of a temperature control system or algorithm
comprising the present methodology of controlling air temperature
of the refrigerated storage space.
[0014] The refrigerated storage space may comprise various types of
stationary or transportable refrigerated compartments or spaces
such as spaces of household freezers and refrigerators, cold
storage houses and refrigerated containers.
[0015] In accordance with the present invention, the temperature of
the cooled supply air or supply air is adjusted based on the
temperature-error integral, which is an integral over the
difference between the supply air temperature and the predetermined
reference temperature. The use of the supply air temperature-error
integral allows wider fluctuation of instantaneous supply air
temperature while yet providing accurate control of average supply
air temperature over time. In contrast, traditional temperature
control algorithms or schemes seek to maintain the instantaneous
supply air temperature at setpoint temperature, possibly within
upper and lower temperature limits. The wider fluctuations of the
instantaneous supply air temperature afforded by the present
temperature control methodology or scheme enables exclusively
switching operational states of the cooling unit or device between
ON and OFF while maintaining accurate control over the
time-averaged supply air temperature. The exclusive switching of
operational states of the cooling unit between ON and OFF, while
maintaining accurate control over the time-averaged supply air
temperature such as within the above-mentioned temperature
deviations, may be provided by pure integral control (1-control)
based adjustment of the supply air temperature based on a current
value of an error signal derived from, or constituted by, the
temperature-error integral.
[0016] In the present specification, the ON state of the cooling
unit or device means it operates at, or close to, its maximum
capacity, i.e. about 100% capacity such as at more than 85% of its
maximum cooling capacity, even more preferably above 90 or 95% of
its maximum cooling capacity. The ON operation at, or close to, the
maximum capacity of the cooling unit avoids energy inefficient
part-load cooling. The ON state is furthermore preferably placed at
a percentage of the maximum cooling capacity where operation is
highly efficient. This percentage may vary between specific cooling
units but typically lies in the above-mentioned range between
85-100% of the maximum cooling capacity.
[0017] Despite the allowed wider fluctuations of the instantaneous
supply air temperature, average supply air temperature is
accurately controlled. The temperature of the commodity load, which
typically comprises perishable produce, situated within the
transport volume of the container is maintained within tight limits
or bounds due to thermal inertia of the commodity load despite the
wider fluctuations of the instantaneous supply air temperature.
[0018] The reference temperature may comprise, or be set to, an
adjusted setpoint temperature or a setpoint temperature of the
refrigerated storage space. The adjusted setpoint temperature is
preferably derived from the setpoint temperature and possibly one
or more additional temperature variables of the temperature control
algorithm.
[0019] The present methodology for air temperature control within
the transport volume preferably comprises controlling both the
cooling unit and a heating unit. Accordingly, in one embodiment,
the present methodology comprises a further step of: [0020]
circulating the return air from the refrigerated storage space
through the heating unit to provide a flow of heated supply air at
the supply air temperature. This embodiment enables the present
temperature control methodology to provide a flow of both cooled
supply air and heated supply air, depending on the circumstances,
to maintain the commodity load at a desired temperature across a
wide range of external environmental temperatures outside the
refrigerated storage space.
[0021] In the present specification, the ON state of the heating
unit or device means it operates at, or close to, its maximum
capacity, i.e. about 100% capacity such as at more than 85% of its
maximum heating capacity, even more preferably above 90 or 95% of
its maximum heating capacity. Furthermore, the ON state is
preferably placed at a percentage of the maximum heating capacity
where operation is highly efficient. This percentage may vary
between specific types of heating units.
[0022] The computation of the temperature-error integral of the
supply air is preferably performed by the temperature control
system operatively coupled to the refrigerated storage space. The
temperature control system may reside in a dedicated cooling unit
mounted to a wall section of the refrigerated storage space.
Alternatively, certain parts of the temperature control system may
be situated remotely and coupled to control operation of the
cooling unit, heating unit etc through a wired or wireless
communications interface.
[0023] The temperature control system may comprise a microprocessor
operating according to a set of embedded program instructions or
embedded software. The embedded software may be adapted to receive
and process supply air temperature data provided by the first
temperature sensor and determine appropriate control actions of the
cooling unit and/or heating unit to adjust the supply air
temperature in a desired direction to reach the desired air
temperature inside the refrigerated storage space. The
temperature-error integral is preferably computed at regular time
intervals such as time intervals smaller than 1 minute, preferably
smaller than 10 seconds such as about every second.
[0024] In accordance with a preferred embodiment of the invention,
the method of controlling air temperature comprises a step of:
[0025] adjusting the reference temperature as a function of the
setpoint temperature and a temperature of the return air measured
by a return air temperature sensor or second temperature sensor.
Hereafter this adjusted reference temperature or adjusted setpoint
temperature is denoted by T.sub.set.sub.--.sub.quest. The setpoint
temperature is normally identical to a displayed setpoint
temperature of the refrigeration unit of the refrigerated storage
space. By adjusting the reference temperature as function of the
setpoint temperature and the temperature of the return air, the
inventors have demonstrated improved quality preservation of the
perishable produce by improved produce temperature control. This
has been achieved because 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.
[0026] In one embodiment, the adjustment of the reference
temperature is made such that the average of the supply air
temperature and the return air temperature substantially equals,
preferably within +/-0.2 degree C., the setpoint temperature unless
upper and lower limits for the reference temperature prohibit this.
The average produce temperature is thereby maintained close to the
setpoint temperature which represents the desired commodity load
temperature.
[0027] The method of controlling air temperature according to the
invention preferably comprises a step of controlling respective
operational states of the heating unit or the cooling unit or both
to adjust the supply air temperature. As previously mentioned, the
respective operational states of the heating and cooling units are
preferably switched between ON and OFF. In one embodiment, the
method comprises a step of exclusively switching operational states
of the cooling unit between ON and OFF to make the adjustment of
the supply air temperature. This embodiment is particularly
advantageous since it avoids time periods of inefficient part-load
refrigeration, leading to markedly improved energy efficiency.
[0028] In the present specification, the ON state of a heating or
cooling unit means the unit in question operates at, or close to,
its maximum capacity. In certain embodiments, the cooling unit
and/or the heating unit may only possess a fixed number of discrete
operational states such as two, three or four etc. In other
embodiments, the operational state of the cooling unit and/or the
heating unit may be continuously variable between for example 0%
(OFF) and the maximum capacity.
[0029] The cooling unit may comprise a compressor coupled to an
evaporator with a capacity regulating device regulating the cooling
capacity of the cooling unit. In one such embodiment the compressor
is coupled to the evaporator via the capacity regulating device,
such as a suction valve, mounted on a fluid connection between the
compressor and the evaporator. In such an embodiment, the present
methodology may comprise a step of: [0030] controlling a
refrigerant flow rate between the evaporator and the compressor by
a capacity regulating device to adjust a cooling capacity of the
evaporator. In a particularly advantageous embodiment, the
operational state of the capacity regulating device is such that
the refrigeration capacity is substantially maximized during time
periods, preferably all time periods, where the compressor is ON,
i.e. operating at, or close to, its maximum capacity.
[0031] According to yet another preferred embodiment, the
temperature-error integral of the supply air is controlled to stay
within upper and lower bounds or limits as defined by first and
second integral error thresholds, respectively. The method
comprising further steps of: [0032] comparing the temperature-error
integral of the supply air with the integral first integral error
threshold and the second integral error threshold, [0033] changing
an operational state of at least one of the cooling unit, the
heating unit and the internal fans if the temperature-error
integral exceeds the first integral error threshold or falls below
the second integral error threshold.
[0034] The operational state of the cooling unit is controlled such
that the cooling unit preferably is activated if the first integral
error threshold is exceeded indicating the average supply air
temperature over a preceding time period, such as a time period of
a current cycle, was too high. Likewise, the heating unit is
preferably activated if temperature-error integral falls below the
second integral error threshold indicating that average supply air
temperature over the past time period was too low.
[0035] Experimental investigations by the present inventors
indicate that the first integral error threshold may be set to a
value between 50 and 200.degree. C.*minutes (the unit being
.degree. C. times minutes) and the second integral error threshold
set to a value between -100 and -10.degree. C.*minutes for typical
refrigerated storage spaces. A difference between the first
integral error threshold and the second integral error threshold
may be set to a value between 20 and 200.degree. C.*minutes to have
sufficient, but not too much, bandwidth or distance between cooling
and heating operation.
[0036] The methodology of controlling air temperature-error
integral to stay between the first and second integral error
thresholds may comprise further steps of: [0037] setting the
operational state of the cooling unit to active for at least a
first minimum time period if the temperature-error integral exceeds
the first integral error threshold, [0038] setting the operational
state of the heating unit to active for at least a second minimum
time period if the temperature-error integral is smaller than the
second integral error threshold.
[0039] The active operational state of the cooling unit is
preferably ON at all times and the active operational state of the
heating unit preferably ON at all times as well. Each of the first
and second minimum time periods may be set to a value between 1 and
10 minutes such as around 5 minutes to avoid unnecessary wear and
tear of the compressor and electrical contactors.
[0040] According to another preferred embodiment, the method of
controlling air temperature comprises a step of: [0041] limiting
the reference temperature to a lower temperature limit dependent on
the setpoint temperature. Optionally, the reference temperature may
also be limited to an upper temperature limit. Limiting how low the
reference temperature can drop relative to the setpoint temperature
is very helpful in avoiding chilling or freezing damage to the
commodity load caused by possible prolonged drops of the supply air
temperature. For some setpoint temperatures the lower temperature
limit is to be substantially equal to the setpoint temperature. For
other setpoint temperatures the lower temperature limit is set to a
value between 0.1 and 2.0.degree. C. below the setpoint
temperature.
[0042] In another embodiment, the method of controlling air
temperature comprises steps of, during circulation periods,
comparing the temperature-error integral to a first heating
threshold and maintaining the internal fans at a first preset speed
if the temperature-error integral is below the first heating
threshold. In the present specification, the term "circulation
period" means time periods where the operational state of the
heating unit is OFF and the operational state of the cooling unit
is OFF. The internal fans may be configured to operate at a number
of discrete preset speed settings such as OFF, Low and High with a
predefined speed ratio between the Low and High speed settings such
as a ratio of 2 or 3 or more. The OFF, Low and High speed settings
may for example correspond to air flow rates of 0, 3000 and 6000
m.sup.3 per hour, respectively. The first preset speed may in this
situation be either Low or High. By forcing the internal fans to
run at the first preset speed if the temperature-error integral
lies below the first heating threshold, the temperature control
system or algorithm only activates the heating unit if fan energy
does not suffice to supply required heat energy. Fan energy is an
advantageous heating source due to its double effect contributing
with both heating of the supply air and mixing of the air inside
the refrigerated storage space.
[0043] In the present specification each of the discussed fan speed
settings of the internal fan or fans may be provided by joined
operation of all internal fans present in the refrigerated storage
space. Different fan speed settings may be achieved by changing the
actual speed of one or several individual fan(s) or by turning a
certain number of fans ON or OFF.
[0044] This embodiment may comprise a further step of: [0045]
comparing the temperature-error integral to a second heating
threshold smaller than the first heating threshold, [0046] maintain
the internal fans at a second preset speed if the temperature-error
integral is below the second heating threshold; the second preset
speed is higher than the first preset speed. In the above described
situation with a set of discrete fan speeds, the second preset
speed may be High if the first preset speed is Low. In other
embodiments, the internal fans speed may be adjustable over a
continuous speed range and the second preset speed set to any speed
higher than the first preset speed. One advantage of using multiple
heating thresholds is a maximization of the ratio air flow divided
by energy input because physics dictate that air flow generated by
a fan is a linear function of the cube of its power draw.
[0047] In another embodiment, the present methodology comprises
further steps of: [0048] comparing a duration of a previous
circulation period with a circulation time threshold t.sub.ct,
[0049] maintain the speed of the internal fans at a maximum speed
during a current circulation period if the previous circulation
period was smaller than the circulation time threshold t.sub.ct.
This embodiment ensures effective air circulation within the
refrigerated storage space during the current circulation period if
a previous circulation period was relatively short, i.e. below the
circulation time threshold t.sub.ct. Such relatively short previous
circulation periods may indicate a relatively large net heat load
on the refrigerated storage space which makes it important to
provide high air circulation to limit or decrease air temperature
distribution differences within the refrigerated storage space. In
the present specification, the "net heat load" means the sum of
heat ingress into the refrigerated storage space and autonomous
heat production of the commodity load within the refrigerated
storage space.
[0050] According to a further refinement of the above-mentioned
embodiment, the circulation time threshold t.sub.ct depends on a
change of the temperature-error integral during the previous
circulation period.
[0051] A number of embodiments of the present invention are
advantageously configured to control relative humidity (RH) of the
air inside refrigerated storage spaces in addition to controlling
the air temperature. In one embodiment the methodology comprises
further steps of: [0052] simultaneously heating and cooling the
supply air at a first speed setting of internal fans when a
measured relative humidity (RH) of the air inside the transport
volume is higher than a first humidity threshold derived from the
setpoint value of the relative humidity (RH.sub.set), [0053]
setting a second fan speed of the internal fans during circulation
periods when a measured relative humidity (RH) of the air is higher
than a second humidity threshold derived from the setpoint value of
the relative humidity; [0054] wherein the first fan speed setting
is a lower fan speed than the second fan speed setting. The first
fan speed setting may be Low and the second fan speed setting High.
The lower speed of the first fan speed setting is preferred because
it increases a dehumidification capacity of a refrigeration unit
controlled by the temperature control system.
[0055] In another embodiment, the above embodiment with switching
between the first and second fan speed settings may comprise
further steps of: [0056] setting the heating unit to an ON state
during circulation periods when a measured relative humidity of the
air is higher than a third humidity threshold derived from the
setpoint value of the relative humidity. By increasing heat
production during circulation periods as outlined above, the
duration of the circulation period is shortened by evoking earlier
cooling and therefore also earlier dehumidification.
[0057] A second aspect of the invention relates to a refrigerated
storage space comprising a refrigerated volume for housing a
commodity load. A cooling unit is configured to receive return air
from the refrigerated volume and generate a flow of cooled supply
air at a supply air temperature and an air flow passage is coupled
to the refrigerated volume to discharge the supply air therein and
control air temperature within the refrigerated volume. A first
temperature sensor is adapted to measuring the supply air
temperature. A temperature control system is adapted to computing a
temperature-error integral of the supply air based on a difference
over time between the supply air temperature and a reference
temperature. The temperature control system is additionally adapted
to adjusting the supply air temperature based on the
temperature-error integral such that a time average of the supply
air temperature substantially equals the reference temperature.
[0058] The return air from the refrigerated volume may be conveyed
to the cooling unit or the heating unit through a second air flow
passage. The second air flow passage may in certain embodiments be
located close to a ceiling portion of the refrigerated storage
space. A second or return air temperature sensor may be provided
for determining the return air temperature. As previously
mentioned, the computation of the temperature-error integral of the
supply air is preferably performed by a temperature control
algorithm executed by the temperature control system operatively
coupled to the refrigerated storage space. The temperature control
system may comprise a microprocessor operating according to a set
of embedded program instructions or embedded software to execute
the temperature control algorithm. Alternatively, the temperature
control system may comprise dedicated computation hardware such as
programmable logic or hardwired arithmetic and logic circuit blocks
configured to execute the required computational steps of the
temperature control algorithm.
[0059] As previously mentioned, the refrigerated storage space
preferably comprises a heating unit configured to supply heated
supply air at the supply air temperature by circulating the return
air from the refrigerated storage space. The temperature control
system may be adapted to controlling respective operational states
of the heating unit or the cooling unit or both to adjust the
supply air temperature.
[0060] To maximize cooling energy efficiency of the cooling unit,
the temperature control system may be adapted to exclusively
switching operational states of the cooling unit between ON and OFF
states to make the adjustment of the supply air temperature. This
may be achieved by switching operational states of a compressor of
the cooling unit between exclusively ON and OFF states so as to
avoid energy inefficient part-load operation of the compressor.
[0061] According to an advantageous embodiment, the temperature
control system of the refrigerated storage space is adapted to
adjust the reference temperature as a function of a setpoint
temperature and a temperature of the return air measured by a
return air temperature sensor. In this embodiment, the temperature
control system is preferably further adapted to adjust the
reference temperature such that the average of the supply air
temperature and the return air temperature substantially equals the
setpoint temperature. By controlling the average of the supply air
temperature and the return air temperature to the setpoint
temperature, improved produce quality preservation can be achieved
due to improved produce temperature control. The present inventors
have exploited the fact that average produce temperature in the
refrigerated transport volume is closer to the mean of the supply
and return air temperature than to the supply air temperature.
[0062] The temperature control system of the present refrigerated
storage space may naturally be further adapted or refined to
comprise any of the previously described functions or features in
accordance with the first aspect of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] A preferred embodiment of the invention will be described in
more detail in connection with the appended drawings, in which:
[0064] FIG. 1 is a simplified side cross-sectional view of a
refrigerated container in accordance with a preferred embodiment of
the invention,
[0065] FIG. 2 is a state diagram illustrating respective
operational modes of a cooling unit, a heating unit and internal
fans as function of a temperature-error integral of supply air
discharged into a transport volume of the refrigerated container in
accordance with the preferred embodiment of the invention,
[0066] FIGS. 3 and 4 are flow charts illustrating program steps
executed by a microprocessor-implemented temperature control
algorithm of a temperature control system of the refrigerated
container in accordance with a preferred embodiment of the
invention,
[0067] FIG. 5 illustrates logic rules applied by the temperature
control algorithm for controlling internal fans speed,
[0068] FIG. 6 comprises a series of graphs illustrating
experimentally recorded values of various key variables of the
temperature control algorithm under high net heat load conditions
in accordance with the preferred embodiment of the invention,
[0069] FIG. 7 comprises a series of graphs illustrating the same
variables as FIG. 6 but under low net heat load conditions,
[0070] FIG. 8 comprises a series of graphs illustrating the same
variables as FIG. 6 but under conditions where heating is required;
and
[0071] FIG. 9 is a graph illustrating experimentally recorded
values of produce temperature versus supply air temperature.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0072] In FIG. 1, the refrigerated container 1 comprises a frontal
section having a refrigeration unit 40 housing multiple, usually
two or three, variable speed internal fans 10 (depicted
schematically as a single fan), heating unit or element 20 and a
cooling unit 15. A load or cargo section 30 of the refrigerated
container 1 comprises a commodity load comprising a plurality of
stackable transport boxes 35 arranged within a transport volume 45
such as to leave appropriate clearance at a ceiling and a floor
structure for air flow passages above and beneath the commodity
load. The frontal section comprises a refrigeration unit 40
comprising return air temperature sensor 5 adapted to measure a
temperature of a return air flow 50 comprising air that has been
circulated through the transport volume 45. Another temperature
sensor 25 is adapted to measuring a supply air temperature of
heated or cooled supply air 55 discharged into the transport volume
45 through an air flow passage. A temperature control system (not
illustrated) comprises a programmed microprocessor which controls
respective operational states of the variable speed internal fans
10, the heating unit 20 and the cooling unit 15 in accordance with
a temperature control algorithm defined by a set of microprocessor
program instructions. The temperature control system additionally
comprises a user interface, for example a LCD display, where an
operator or ship technician can enter or modify certain parameter
values of the temperature control algorithm such as a setpoint
temperature of the refrigerated container 1. The operation of the
temperature control algorithm is explained in detail below with
reference to FIG. 2 and FIGS. 5 & 6.
[0073] The state diagram 200 of FIG. 2 schematically illustrates
how switching between control modes or states of the present
temperature control algorithm is performed in a preferred
embodiment as function of a temperature-error integral of the
supply air discharged into a transport volume of the refrigerated
container. Arrow 202 points in direction of increasing values of
the temperature-error integral (TEI).
[0074] The state or domain diagram 200 comprises a number of
temperature-error integral thresholds or limits in-between
individual control states 204, 206, 208, 210 and 212. A first
threshold, TEI_heat_stage.sub.--3_lim, constitutes a lower integral
error threshold such that the heating unit is switched ON if a
current value of the temperature-error integral falls below this
threshold. In the upper portion of the state diagram 200 in-between
control states 204 and 206, a first threshold, TEI_max_cool,
constitutes a first or upper integral error threshold such that the
cooling unit is switched ON if a current value of the
temperature-error integral exceeds this upper threshold.
[0075] Three intermediate states, where the operational states of
both the cooling unit and the heating unit are OFF, are located in
abutment in-between the upper control state 204, cooling, and the
lowermost control state 212, heating. These three intermediate
states comprise two heating states, control states 208 and 210, and
a circulation state 206. In the control states 208 and 210, the
heating unit resides in operational state OFF and the internal fans
are exploited to both supply heat to the supply air and add
circulation to the air inside the transport volume. In the control
state 208 the internal fans are set in the Low speed operational
state while the internal fans speed is set to High speed in the
control state 210 reflecting a requirement for higher heat
production due to the decreasing value of the TEI as indicated by
the arrow 202.
[0076] In the circulation control state 206, the internal fans may
be switched between three different operational states having
different fan speeds such as Off, Low and High. In this embodiment
internal fans speed is switched between High, Low and Off during
circulation periods. The speed of the internal fans is maintained
at a maximum or High speed during a current circulation period if
the previous circulation period was smaller than the circulation
time threshold t.sub.ct. Otherwise the fan speed is kept at Low
speed at the start of the circulation period and subsequently
enters a fan speed cycling program during which: [0077] the fan
speed is reduced by one step such as from High to Low speed after
maintaining the fan speed for its predetermined maximum duration
setting. An additional condition for switching from Low to Off is
that the previous circulation fan speed was High. [0078] The fan
speed is increased by one step after maintaining the fan speed for
its predetermined maximum duration setting. An additional condition
for making a change from Low to High speed is that the previous
circulation period fan speed was Off. [0079] The fan speed is
reduced by one step if the fan speed was increased a preset time
period ago such as 10 or 5 minutes, while changes to the return air
temperature Tret in that preset time period fell within
predetermined upper and lower limits or bounds. [0080] The fan
speed is increased by one step if the change of the return air
temperature Tret since the start of the current fan speed setting
fell outside predetermined upper and lower limits or bounds.
[0081] This above set of rules implies that the fan speed setting
of the internal fans always changes speed setting one step at a
time, so never from Off to High or vice versa. The above-described
algorithm or rules for switching between fans speed setting during
the circulation period is schematically illustrated by FIG. 5.
[0082] In the present embodiment, the setting of the heating
thresholds between the individual states 204, 206, 208, 210 and 212
are:
TEI_max_cool=90.degree. C.*min,
TEI_heat_stage.sub.--1_lim=0.degree. C.*min,
TEI_heat_stage.sub.--2_lim=-10.degree. C.*min,
TEI_heat_stage.sub.--3_lim=-30.degree. C.*min.
[0083] The variables used in describing the present embodiments of
the temperature control algorithm are defined below in Table 1.
TABLE-US-00001 TABLE 1 Definition of variables of the temperature
control algorithm. Variable/acronym Description Tset_quest
[.degree. C.] Reference temperature to which cycle-averaged supply
air temperature is controlled cycle_length [min] A length of a
current cycle Cycle Period of time used in the temperature control
algorithm. In usual cooling operation it is the time elapsed
between two consecutive cooling compressor starts. Tret_avg
[.degree. C.] Average return air temperature during last completed
cycle TEI [.degree. C.*min] Supply air temperature-error integral,
i.e. integral over Tsup minus Tset_quest. Tsup [.degree. C.] Supply
air temperature Tret [.degree. C.] Return air temperature Tset
[.degree. C.] Current temperature setpoint .DELTA.Tset_quest_min
Tset_quest_min = Tset + .DELTA.Tset_quest_min is a lower bound on
[.degree. C.] Tset_quest. This bound may esp. be hit during high
heat load. .DELTA.Tset_quest_max Tset_quest_max = Tset +
.DELTA.Tset_quest_max [.degree. C.] is an upper bound [.degree. C.]
on Tset_quest. This bound may esp. be hit during large negative
heat load, like may occur when Tset = +30.degree. C. in Canadian
winters. TEI_max_cool [.degree. C.*min] See FIG. 2. Cooling
switches ON if .intg. t 0 t ( T sup ( .tau. ) - Tset_quest ( .tau.
) ) d .tau. > TEI_max _cool . ##EQU00001## It will only switch
off again when min. on-time has passed by and .intg. t 0 t ( T sup
( .tau. ) - Tset_quest ( .tau. ) ) d .tau. .ltoreq. TEI_max _cool .
##EQU00002## TEI_heat_stage3_lim [.degree. C.*min] See FIG. 2.
Heating switches on when .intg. t 0 t ( T sup ( .tau. ) -
Tset_quest ( .tau. ) ) d .tau. < TEI_heat _stage3 _lim .
##EQU00003## It will only switch off again when min. on-time has
passed by and .intg. t 0 t ( T sup ( .tau. ) - Tset_quest ( .tau. )
) d .tau. .gtoreq. TEI_heat _stage3 _lim . ##EQU00004## TEI_max
[.degree. C.*min] To avoid integral wind-up the value of TEI is
limited to the range TEI_min .ltoreq. TEI .ltoreq. TEI_max. TEI_min
[.degree. C.*min] To avoid integral wind-up the value of TEI is
limited to the range TEI_min .ltoreq. TEI .ltoreq. TEI_max.
t.sub.ct [min] circulation time threshold.
[0084] In the present embodiment of the invention, the calculated
supply air temperature-error integral TEI is bounded to the
interval [TEI_min, TEI_max]. TEI(t) [.degree. C.*min] is calculated
in time discrete format by:
TEI_now=(Tsup-Tset_quest)*ts.
TEI=max(TEI_min,min(TEI_max,TEI+TEI_now));
where ts is a sampling interval or time period.
[0085] Experiments have revealed that the sampling time period, ts,
preferably should be less than 10 seconds such as about 1
second.
[0086] A starting value for TEI is determined anytime the
temperature control algorithm has not been operating. This
condition will occur following algorithm power-up. In these cases,
the internal fans are first run at High speed for 15 seconds and
then an initial value of TEI is calculated using:
TEI=max(TEI_min,min(TEI_max, 40*(Tret--Tset_quest)+30); wherein
the anti-integral windup precautions max( . . . , min( . . . , . .
. )) avoid the integral from getting excessively large.
[0087] In the present embodiment of the invention the reference
temperature is an adjusted setpoint temperature Tset_quest. The
setpoint adjustment is made such that an average of the supply air
temperature Tsup and a measured return air temperature Tret
substantially equals a setpoint temperature Tset. More
specifically, in the present temperature control algorithm, a
cycle-averaged supply air temperature Tsup is controlled to the
adjusted setpoint temperature Tset_quest.
[0088] The cycle is defined as a period of time starting at an end
of a previous cycle and ending when one of the following three
conditions applies: [0089] 1. Cooling unit switches ON, [0090] 2.
Heating unit switches OFF, [0091] 3. Last cycle ended more than 1
hour ago
[0092] During each cycle, the cycle length and the average of the
return air temperature are updated at regular time intervals. These
values are used to calculate Tset_quest at the start of the next
cycle. At the start of the first cycle after power-up Tset_quest is
set to Tset. Following this initialization, Tset_quest is
calculated at the beginning of each subsequent cycle according to
the equations below:
Tset_quest_new=(1-0.2*cycle_length/60)*Tset_quest+0.2*cycle_length/60*(T-
set-(Tret_avg-Tset));
Tset_quest=max(Tset+.DELTA.Tset_quest_min;min(Tset+.DELTA.Tset_quest_max-
;Tset_quest_new)).
[0093] Please refer to the definition of the above variables in
Table 1.
[0094] The flowchart extending across FIGS. 3 and 4 provides a
simplified summary of the above-described operation of the present
temperature control algorithm or algorithm during each call to the
temperature control algorithm. The algorithm starts in step 301 and
proceeds to step 303 wherein the value of the temperature-error
integral TEI is updated using the current supply air temperature
and Tset_quest. In step 303 also the average return air temperature
Tret_avg is updated using a current return air temperature.
[0095] In step 305 the algorithm checks whether all respective
minimum ON or OFF time periods associated with the heating unit,
cooling unit and internal fans speed have expired. Suitable minimum
ON and OFF times are generally in the range between 1 and 10
minutes such as about 5 minutes but may be adjusted based on
specific characteristics of various components of the refrigeration
unit.
[0096] If these minimum ON or OFF time periods have not expired,
the algorithm proceeds to step 319 and maintains current
operational states of the heating unit, cooling unit and internal
fans. Thereafter, the algorithm proceeds to step 401. On the other
hand if all minimum ON or OFF time periods have expired, the
algorithm proceeds to step 307 and tests whether the current value
of the temperature-error integral TEI exceeds the upper bound or
first error threshold TEI_max_cool of the temperature-error
integral. If the current value of the temperature-error integral
TEI exceeds the upper bound it indicates that the average supply
air temperature through the current cycle is getting too high.
Therefore, the operational state of the cooling unit is switched to
ON in step 321 and the state of the heating unit is set to Off. The
internal fans speed is also switched to, or maintained at, High
speed setting. The High speed setting during cooling is
advantageous for several reasons, one of them being less
dehumidification and therefore less weight loss to the commodity
load. After step 321, the algorithm proceeds to step 401.
[0097] On the other hand if the current value of the
temperature-error integral TEI is smaller than the upper bound, the
algorithm either switches the operational state of the cooling unit
to OFF, or maintains an already existing OFF state, in step 309.
The algorithm proceeds to step 311 and tests whether the current
value of the temperature-error integral TEI is smaller than the
lower bound or second integral error threshold TEI_heat_stage3_lim
of the temperature-error integral. If the current value of the
temperature-error integral TEI is smaller than the lower bound it
indicates that the average supply air temperature through the
current cycle is getting too low. Therefore, the algorithm proceeds
to step 323 and switches the operational state of the heating unit
to ON. The internal fans speed is also switched to, or maintained
at, the High setting so as to add heat and homogenise or minimize
temperature variations within the transport volume. After step 323,
the algorithm proceeds to step 401 with the effect described
below.
[0098] On the other hand if the current value of the
temperature-error integral TEI is larger than the lower bound, the
algorithm proceeds to step 313 and sets or switches the operational
state of the heating unit to Off and proceeds to step 315. In step
315 it is evaluated whether the previous circulation period was
shorter than the circulation time threshold t.sub.ct. If true (Y),
the algorithm proceeds to step 325 and sets the internal fans speed
to High, after which the algorithm proceeds to step 401.
[0099] On the other hand if in step 315 it is evaluated that the
previous circulation period was not (N) shorter than the
circulation time threshold t.sub.ct, the algorithm leaves the fan
speed to the circulation period's fan cycling program in step 317.
The algorithm proceeds to step 317 and determines the appropriate
internal fans speed or state (i.e. OFF, Low, High) by applying the
logic rules governing internal fans state during circulation
periods as outlined above in connection with the description of
FIG. 2. After setting the appropriate internal fans speed, the
algorithm proceeds to step 401 (refer to FIG. 4).
[0100] In step 401, the algorithm checks or determines whether or
not the current cycle has been completed. This is done by
evaluating the three logic rules or conditions outlined before and
determining if one of these conditions applies. The algorithm
proceeds to step 405 if the algorithm determines that the cycle has
been completed and computes an updated value of the adjusted
setpoint temperature Tset_quest based on setpoint temperature Tset
and the average return air temperature Tret_avg.
[0101] The value of the average return air temperature Tret_avg is
thereafter skipped and preparations are made for calculation of the
average return air temperature in the upcoming cycle. The algorithm
proceeds to its ending in step 407.
[0102] If the algorithm in step 401 determines that the current
cycle has not been completed, a current value of the adjusted
setpoint temperature Tset_quest is maintained in step 403. After
that the algorithm proceeds to its ending in step 407. Thereafter
the algorithm proceeds to await a next call of a control algorithm
at step 301. The next call will typically occur after a certain
delay period, i.e. the sampling time interval minus computation
time.
[0103] FIG. 6 is series of graphs 601, 603 and 605 illustrating
experimentally recorded values of selected variables of the
above-described temperature control algorithm under high net heat
load conditions of the refrigerated container.
[0104] Graph 601 shows temperature values in .degree. C. on the
y-axis for the adjusted setpoint temperature Tset_quest (long
dotted line), the return air temperature Tret (short dotted line)
and the supply air temperature Tsup (full line). The x-axis unit is
time in minutes.
[0105] Graph 603 shows corresponding (to graph 601) values of the
computed temperature-error integral (TEI) in units of .degree.
C.*min on the y-axis and time in minutes on the y-axis. The full
line represents values of the TEI and the horizontal dotted line
represents a value of 90.degree. C.*min for a first or upper
integral threshold error TEI_max_cool of the temperature-error
integral.
[0106] Finally, graph 605 shows corresponding (to graphs 601 and
603) operating states of the cooling unit, heating unit and
internal fans where the states are indicated on the y-axis. The
respective ON states of the cooling unit and heating unit are
indicated by the value "1" and OFF states as the value "0". For the
internal fans, the High setting or state (maximum fan speed) is
indicated as "2", the Low setting as "1" and the OFF setting as
"0".
[0107] As illustrated, the supply air temperature on graph 601
varies considerably between 0.3.degree. C. and -3.5.degree. C.
while the return air temperature varies considerably less between
about between 0.2.degree. C. and -0.2.degree. C. The low
variability of the return air temperature is caused by thermal
inertia of the produce in the transport volume 45 (of FIG. 1). The
adjusted setpoint temperature, Tset_quest (long dotted line), is
kept constant at about -1.5.degree. C.
[0108] By comparing the value of the supply air temperature on
graph 601 and the ON periods of the cooling unit on graph 605, the
sudden drop or increase of supply air temperature in response to
switching the cooling unit between Off and ON states is
evident.
[0109] By inspection of the TEI curve on graph 603 and the ON
periods of the cooling unit on graph 605, it is indicated how a TEI
value above the 90.degree. C.*min upper bound on the TEI,
TEI_max_cool, leads to activation of the cooling unit which stays
ON until the flow of cooled supply air has caused the TEI to drop
below the upper bound (and the minimum ON time has passed).
Consequently, the present temperature control algorithm does not
activate the cooling unit based on certain preset limits or bounds
on the supply air temperature or return air temperature but instead
activates the cooling unit based on limits or constraints placed on
the TEI.
[0110] The activation of the cooling unit on graph 605 also shows
that the operational state of internal fans remains High (state
"2") for the entire depicted time period while the heating unit
remains OFF as expected in view of the high net heat load. The fan
speed remains High during this short circulation period because the
duration of the circulation period is shorter than the circulation
time threshold t.sub.ct.
[0111] FIG. 7 is series of graphs 701, 703 and 705 illustrating
experimentally recorded values of selected variables of the
above-described temperature control algorithm under low net heat
load conditions of the refrigerated container. The individual
curves of these graphs correspond to those of FIG. 6. The main
difference to the graphs of FIG. 6 is the length of the circulation
periods, i.e. time periods where both the heating unit and the
cooling unit are in OFF state, as evidenced by the relative scaling
of the time axes and the curves of graphs 605 and 705 which
indicate respective operational states of the heating unit, cooling
unit and internal fans under the different net head load
conditions. During these circulation periods in FIG. 7, the
internal fans speed starts cycling between Low and Off according to
the earlier described rules for controlling the fan speed during
circulation periods. Time periods where the operational state of
the cooling unit is ON are always accompanied by a High speed
setting of the internal fans in the present embodiment of the
invention.
[0112] FIG. 8 is series of graphs 801, 803 and 805 illustrating
experimentally recorded values of selected variables of the
above-described temperature control algorithm under environmental
conditions where heating is required to maintain the commodity load
at or close to the adjusted setpoint temperature. Consequently, the
coherent nature of the present temperature control algorithm
capable of supplying heated or cooled supply air as required is
demonstrated. As indicated by curve 813 on graph 803, a second or
lower integral error threshold is set to a value -10.degree.
C.*minutes. When the TEI curve 809 falls below this lower integral
error threshold 813, the heating unit is switched ON as depicted on
graph 805 which as expected leads to increasing supply air
temperature as evidenced by the supply air temperature curve 807 of
graph 801. As shown on graph 803, the increasing supply air
temperature starting at about t=9 minutes leads to an increasing
value of the TEI after a short delay period. The internal fans
speed is maintained High during the entire time period while the
operational state of the cooling unit is OFF for the entire time
period since no cooling is required.
[0113] FIG. 9 is a graph illustrating experimentally recorded
values of produce temperature, on curve 901, versus supply air
temperature, depicted on curve 903, measured at the same position
in the transport volume of the refrigerated container where the
supply air enters. It is evident that produce temperature is kept
within very tight limits of about +/-0.1.degree. C. despite
considerable variation of the supply air temperature from about 1.5
to 6.4.degree. C. Thus, demonstrating that the thermal inertia of
the produce suffices to annihilate air temperature fluctuations of
this frequency. The present temperature control system exploits
this thermal inertia to maintain highly accurate control over
produce temperature by controlling the temperature-error integral
so as to stay within the upper integral error threshold and the
lower integral error threshold.
* * * * *