U.S. patent application number 13/180834 was filed with the patent office on 2013-01-17 for humidity control in a refrigerated transport container with an intermittently operated compressor.
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 | 20130014522 13/180834 |
Document ID | / |
Family ID | 47518126 |
Filed Date | 2013-01-17 |
United States Patent
Application |
20130014522 |
Kind Code |
A1 |
Lukasse; Leijn Johannes Sjerp ;
et al. |
January 17, 2013 |
HUMIDITY CONTROL IN A REFRIGERATED TRANSPORT CONTAINER WITH AN
INTERMITTENTLY OPERATED COMPRESSOR
Abstract
There is a growing need for dehumidification control in
refrigerated transport containers with an intermittently operated
compressor. The disclosed system and method of dehumidification
control, controlling the speed of one or more evaporator fans and a
heating unit, meets that need using one or more dehumidification
need indicators and one or more condensation indicators. More
specifically a system for and method is disclosed, comprising:
controlling the operation of the one or more evaporator fans (10)
for dehumidification purposes according to: if a need for
dehumidification is determined using at least one dehumidification
need indicator and if at least one condensation indicator indicates
that the evaporator (16) is sufficiently cold respectively too warm
for condensation or deposition of water vapor on its external
surface, then setting or maintaining the speed of the one or more
evaporator fans (10) to a reduced speed (LOW) in order to increase
a condensation or deposition rate respectively to an increased
speed (HIGH) in order to increase heat load to the evaporator (16),
respectively. In other embodiments the system and method controls
the heating unit as well.
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: |
47518126 |
Appl. No.: |
13/180834 |
Filed: |
July 12, 2011 |
Current U.S.
Class: |
62/93 ;
62/126 |
Current CPC
Class: |
Y02B 40/32 20130101;
F25D 17/042 20130101; F25D 2400/02 20130101; Y02B 30/70 20130101;
Y02B 40/00 20130101; Y02B 30/743 20130101; F25B 2600/112 20130101;
F25D 11/003 20130101; F25D 2317/04111 20130101 |
Class at
Publication: |
62/93 ;
62/126 |
International
Class: |
F25D 17/06 20060101
F25D017/06; F25B 49/00 20060101 F25B049/00 |
Claims
1. A method of dehumidification control in a refrigerated transport
container (1), the refrigerated transport container (1) comprising
at least one heating unit (20), a transport volume (45), a cooling
unit (40), and a control unit, wherein the cooling unit (40)
comprises at least a compressor (6) and an evaporator (16)
comprising one or more evaporator fans (10), the heating unit (20)
is situated downstream of the evaporator (16), where the compressor
(6) is intermittently operated between a first active state and a
second less active state, wherein the method comprises: controlling
the operation of the one or more evaporator fans (10) for
dehumidification purposes according to if a need for
dehumidification is determined using at least one dehumidification
need indicator and if at least one condensation indicator indicates
that the evaporator (16) is sufficiently cold for condensation or
deposition of water vapor on its external surface, then setting or
maintaining the speed of the one or more evaporator fans (10) to a
reduced speed (LOW) in order to increase a condensation or
deposition rate, and/or if a need for dehumidification is
determined using at least one dehumidification need indicator and
if at least one condensation indicator indicates that the
evaporator (16) is too warm for condensation or deposition of water
vapor on its external surface, then setting or maintaining the
speed of the one or more evaporator fans (10) to an increased speed
(HIGH) in order to increase heat load to the evaporator (16).
2. The method according to claim 1, wherein the at least one
dehumidification need indicator is one or more selected from the
group of: a difference between current or recent relative humidity
(RH) and relative humidity setpoint (RHset), or a function thereof,
a time elapsed since a most recent start of operating in
dehumidification mode, and that the evaporator fans (10) were
operating at a reduced speed (LOW) the last time the compressor (6)
was in the first active state.
3. The method according to claim 1, wherein the at least one
condensation indicator is one or more selected from the group of:
that the compressor is in the first active state, that a supply air
temperature is colder than a return air temperature, that the
supply air temperature is colder than a setpoint temperature, a
measured temperature of at least a part of an external surface of
the evaporator (16) is below dewpoint, and a function of a boiling
temperature of the refrigerant inside the evaporator is below
dewpoint.
4. The method according to claim 1, wherein the method comprises
the step of controlling the operation of the one or more evaporator
fans (10) for dehumidification purposes by setting or maintaining
the speed of the one or more evaporator fans (10) to be a reduced
speed (LOW) thereby increasing condensation or deposition if: the
compressor (6) is in the second less active state, the evaporator
fans (10) were operating at a reduced speed (LOW) the last time the
compressor (6) was in the first active state, and a supply air
temperature is colder than a return air temperature, and/or, the
compressor (6) is in the first active state and the difference
between current relative humidity (RH) and relative humidity
setpoint (RHset) is determined to be above a second predetermined
dehumidification need threshold (RHlim2), and/or the compressor (6)
is in the first active state, a time elapsed since a most recent
start of operating in dehumidification mode is less than a
predetermined period of time.
5. The method according to claim 1, wherein the method comprises
the step of controlling the operation of the one or more evaporator
fans (10) for dehumidification purposes by setting or maintaining
the speed of the one or more evaporator fans (10) to be an
increased speed (HIGH) thereby increasing heat load to the
evaporator if: the compressor (6) is in the second less active
state and if the difference between current relative humidity (RH)
and relative humidity setpoint (RHset) is above a first
predetermined dehumidification need threshold (RHlim1) and if at
least one condensation indicator indicates that the evaporator (16)
is too warm for condensation or deposition of water vapor on its
external surface.
6. The method according to claim 1, wherein the method comprises
controlling the operation of the heating unit (20) for
dehumidification purposes by activating the heating unit (20)
thereby increasing heat load to the evaporator (16) when a need for
dehumidification is determined using at least one dehumidification
need indicator.
7. The method according to claim 6, wherein the heating unit (20)
is activated if: the compressor (6) is in the first active state
and the difference between current relative humidity (RH) and
relative humidity setpoint (RHset) is determined to be above the
second predetermined dehumidification need threshold (RHlim2),
and/or the compressor (6) is in the first active state, and a time
elapsed since a most recent start of operating in dehumidification
mode is less than a predetermined period of time, and/or the
compressor (6) is in the second less active state, the heating unit
was active at the end of a last period the compressor (6) was in
the first active state and the difference between current relative
humidity (RH) and relative humidity setpoint (RHset) is determined
to be above the second predetermined dehumidification need
threshold (RHlim2), and/or the compressor (6) is in the second less
active state, and the difference between current relative humidity
(RH) and relative humidity setpoint (RHset) is determined to be
above a third predetermined dehumidification need threshold
(RHlim3).
8. The method according to claim 1, wherein the first predetermined
dehumidification need threshold (RHlim1) is lower than or equal to
the second predetermined dehumidification need threshold
(RHlim2).
9. The method according to claim 7, wherein the second
predetermined dehumidification need threshold (RHlim2) is lower
than or equal to the third predetermined dehumidification need
threshold (RHlim3).
10. The method according to claim 1, wherein the state of heating
unit, compressor and evaporator fans may only change if one or more
minimum duration times for being in a given state for the heating
unit (20) and/or the compressor (6) and/or the evaporator fans (10)
have elapsed.
11. The method according to claim 1, wherein the method is only
performed if the temperature is within specification by a
predetermined margin.
12. A method according to claim 1, wherein the refrigerated
transport container is not a transport container but another type
of refrigerated space in connection with a refrigeration unit.
13. A system for dehumidification in a refrigerated transport
container (1), the refrigerated transport container (1) comprising
at least one heating unit (20), a transport volume (45), a cooling
unit (40), and a control unit, wherein the cooling unit (40)
comprises at least a compressor (6) and an evaporator (16)
comprising one or more evaporator fans (10), the heating unit (20)
is situated downstream of the evaporator (16), where the compressor
(6) is intermittently operated between a first active state and a
second less active state, wherein the system comprises a processing
unit adapted to: control the operation of the one or more
evaporator fans (10) for dehumidification purposes according to if
a need for dehumidification is determined using at least one
dehumidification need indicator and if at least one condensation
indicator indicates that the evaporator (16) is sufficiently cold
for condensation or deposition of water vapor on its external
surface, then setting or maintaining the speed of the one or more
evaporator fans (10) to a reduced speed (LOW) in order to increase
a condensation or deposition rate, and/or if a need for
dehumidification is determined using at least one dehumidification
need indicator and if at least one condensation indicator indicates
that the evaporator (16) is too warm for condensation or deposition
of water vapor on its external surface, then setting or maintaining
the speed of the one or more evaporator fans (10) to an increased
speed (HIGH) in order to increase heat load to the evaporator
(16).
14. The system according to claim 13, wherein the at least one
dehumidification need indicator is one or more selected from the
group of: a difference between current or recent relative humidity
(RH) and relative humidity setpoint (RHset), or a function thereof,
a time elapsed since a most recent start of operating in
dehumidification mode, and that the evaporator fans (10) were
operating at a reduced speed (LOW) the last time the compressor (6)
was in the first active state.
15. The system according to claim 13, wherein the at least one
condensation indicator is one or more selected from the group of:
that the compressor is in the first active state, that a supply air
temperature is colder than a return air temperature, that the
supply air temperature is colder than a setpoint temperature, a
measured temperature of at least a part of an external surface of
the evaporator (16) is below dewpoint, and a function of a boiling
temperature of the refrigerant inside the evaporator is below
dewpoint.
16. The system according to claim 13, wherein the processing unit
is adapted to control the operation of the one or more evaporator
fans (10) for dehumidification purposes by setting or maintaining
the speed of the one or more evaporator fans (10) to be a reduced
speed (LOW) thereby increasing condensation or deposition if: the
compressor (6) is in the second less active state, the evaporator
fans (10) were operating at a reduced speed (LOW) the last time the
compressor (6) was in the first active state, and a supply air
temperature is colder than a return air temperature, and/or, the
compressor (6) is in the first active state and the difference
between current relative humidity (RH) and relative humidity
setpoint (RHset) is determined to be above a second predetermined
dehumidification need threshold (RHlim2), and/or the compressor (6)
is in the first active state, a time elapsed since a most recent
start of operating in dehumidification mode is less than a
predetermined period of time.
17. The system according to claim 13, wherein the processing unit
is adapted to control the operation of the one or more evaporator
fans (10) for dehumidification purposes by setting or maintaining
the speed of the one or more evaporator fans (10) to be an
increased speed (HIGH) thereby increasing heat load to the
evaporator if: the compressor (6) is in the second less active
state and if the difference between current relative humidity (RH)
and relative humidity setpoint (RHset) is above a first
predetermined dehumidification need threshold (RHlim1) and if at
least one condensation indicator indicates that the evaporator (16)
is too warm for condensation or deposition of water vapor on its
external surface.
18. The system according to claim 13, wherein the processing unit
is adapted to control the operation of the heating unit (20) for
dehumidification purposes by activating the heating unit (20)
thereby increasing heat load to the evaporator (16) when a need for
dehumidification is determined using at least one dehumidification
need indicator.
19. The system according to claim 18, wherein the heating unit (20)
is activated if: the compressor (6) is in the first active state
and the difference between current relative humidity (RH) and
relative humidity setpoint (RHset) is determined to be above the
second predetermined dehumidification need threshold (RHlim2),
and/or the compressor (6) is in the first active state, and a time
elapsed since a most recent start of operating at increased
condensation or deposition rate is less than a predetermined period
of time, and/or the compressor (6) is in the second less active
state, the heating unit was active at the end of a last period the
compressor (6) was in the first active state and the difference
between current relative humidity (RH) and relative humidity
setpoint (RHset) is determined to be above the second predetermined
dehumidification need threshold (RHlim2), and/or the compressor (6)
is in the second less active state, and the difference between
current relative humidity (RH) and relative humidity setpoint
(RHset) is determined to be above a third predetermined
dehumidification need threshold (RHlim3).
20. The system according to claim 13, wherein the first
predetermined dehumidification need threshold (RHlim1) is lower
than or equal to the second predetermined dehumidification need
threshold (RHlim2).
21. The system according to claim 19, wherein the second
predetermined dehumidification need threshold (RHlim2) is lower
than or equal to the third predetermined dehumidification need
threshold (RHlim3).
22. The system according to claim 13, wherein the state of heating
unit, compressor and evaporator fans may only change if one or more
minimum duration times for being in a given state for the heating
unit (20) and/or the compressor (6) and/or the evaporator fans (10)
have elapsed.
23. The system according to claim 13, wherein the processing unit
is adapted to only perform dehumidification if the temperature is
within specification by a predetermined margin.
24. A 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 refrigeration unit.
Description
[0001] The present invention relates to a method of and a system
for dehumidification control in a refrigerated transport container
thereby controlling relative humidity within a refrigerated storage
space with an intermittently operated compressor.
BACKGROUND OF THE INVENTION
[0002] Temperature in a refrigerated storage space is controlled
within a temperature range adjacent to a setpoint or target
temperature. The refrigerated storage space may for example
comprise a transport volume of a refrigerated transport container.
The setpoint temperature is chosen to keep perishable produce such
as meat, vegetables and fruit or other, at correct temperatures to
avoid quality degradation.
[0003] In many shipments of perishable goods, not only should the
temperature be controlled, but the humidity should be decreased as
well to protect the carried commodity from mould growth and/or
condensation. Humidity in a refrigerated storage space is then
controlled within a humidity range adjacent to a setpoint or target
absolute or relative humidity. Many refrigerated transport
containers are equipped to both control temperature and decrease
humidity.
[0004] Decreasing humidity typically requires reducing the lowest
dry-bulb temperature of air on its way through the cooling unit
further below its dewpoint temperature. This will increase the rate
of condensation or deposition of water vapour at this coldest
location. This coldest location typically is the evaporator's
external surface or a part of that. There are normally two
condensation or deposition rate increasing factors: 1) lowering the
coldest temperature of refrigerant inside the evaporator and 2)
reducing the air flow rate over the cold evaporator in order to
increase the residence time of air on the evaporator surface in
order to allow the air temperature to come closer to the
refrigerant temperature.
[0005] One way of lowering the coldest temperature inside the
evaporator is to increase the heat load of the evaporator, because
simultaneously the temperature controller will then reduce the
temperature of the refrigerant inside the evaporator, with the aim
not to disturb the supply air temperature. The joint effect is
reducing the temperature of air leaving the evaporator without
increasing the supply air temperature. Increasing the heat load of
the evaporator may be done by turning on a heating unit (situated
downstream of the evaporator).
[0006] Another way, may be to decrease the heat absorbing area of
the evaporator e.g. by physically shutting off a part of the
evaporator or by increasing the so-called superheat.
[0007] The above two condensation or deposition rate increasing
factors are traditional approaches to increase the dehumidification
capacity. See for example patent specifications U.S. Pat. No.
4,003,729, U.S. Pat. No. 4,182,133 and patent application
publication US 2011/0041539 A1.
[0008] In refrigerated transport containers with a continuously
running compressor, a common way of decreasing humidity is 1) to
increase the heat supplied by the heating unit, and 2) to reduce
the air flow over the evaporator by reducing the evaporator fan
speed to a relatively low speed.
[0009] However, using an intermittently operated compressor as
opposed to using a continuously running compressor in order to save
energy presents a new situation for dehumidification control. An
intermittently operated compressor may switch between a first
active state, e.g. ON, MAX or near MAX, and a second less active
state, e.g. LOW, near OFF or OFF. The two factors of increasing
condensation or deposition rate of water vapour described above are
not working so straightforwardly in relation to an intermittently
operated compressor. As one example, for dehumidification purposes
it has been found that the decrease of the fan speed should only be
done when there is a need to dehumidify and the temperature of the
external area of the evaporator is below the dewpoint temperature.
For a continuously running compressor, the temperature of the
external area of the evaporator will be below the dewpoint
temperature most of the time, but for an intermittently operated
compressor that is usually only the case when the compressor is in
the first active state.
[0010] A further issue is how to indicate a need for
dehumidification when using an intermittently operated compressor
as an intermittently operated compressor naturally will introduce
minor temperature variations where the temperature variations will
cause variations in relative humidity. So simply the difference
between current relative humidity and a relative humidity setpoint,
an otherwise classical and reliable dehumidification need indicator
in systems with a continuously running compressor, is no longer an
optimal dehumidification need indicator in systems with an
intermittently operated compressor.
SUMMARY
[0011] Generally, the disclosed humidity controller controls
dehumidification in refrigerated transport containers with an
intermittently operated compressor by manipulating the speed of
evaporator fans and possibly the state of a heating unit situated
downstream of the evaporator. The controller uses dehumidification
need indicators and condensation indicators. Dehumidification need
indicators are indicators indicating whether it is desirable to
reduce relative humidity. An example of a classically used
dehumidification need indicator is the difference between current
relative humidity and a relative humidity setpoint. Condensation
indicators are used advantageously to determine if the evaporator
surface or a part of it is sufficiently cold to increase the rate
of water vapour condensation or deposition on the evaporator's
external surface by reducing the evaporator fans speed.
[0012] A first aspect relates to a method of dehumidification in a
refrigerated transport container, the refrigerated transport
container comprising at least one heating unit, a transport volume,
a cooling unit, and a control unit, wherein the cooling unit
comprises at least a compressor and an evaporator comprising one or
more evaporator fans, the heating unit is situated downstream of
the evaporator, where the compressor is intermittently operated
between a first active state (e.g. ON, MAX, or near MAX) and a
second less active state (e.g. OFF, near OFF, or MIN), wherein the
method comprises: controlling the operation of the one or more
evaporator fans for dehumidification purposes according to [0013]
if a need for dehumidification is determined using at least one
dehumidification need indicator and if at least one condensation
indicator indicates that the evaporator is sufficiently cold for
condensation or deposition of water vapor on its external surface,
then setting or maintaining the speed of the one or more evaporator
fans to a reduced speed (LOW) in order to increase a condensation
or deposition rate, and/or [0014] if a need for dehumidification is
determined using at least one dehumidification need indicator and
if at least one condensation indicator indicates that the
evaporator is too warm for condensation or deposition of water
vapor on its external surface, then setting or maintaining the
speed of the one or more evaporator fans to an increased speed
(HIGH) in order to increase heat load to the evaporator.
Intermittent compressor operation is energetically attractive
because it may avoid operation in inefficient part-load conditions.
An advantage of the current dehumidification method is that it
offers a possibility to combine dehumidification, i.e. operation at
a purposely increased condensation or deposition rate, with
intermittent compressor operation. Reducing the fan speed if at
least one condensation indicator indicates that the evaporator is
sufficiently cold for condensation or deposition of water vapor on
its external surface instantaneously increases the condensation or
deposition rate. Increasing the fan speed if at least one
condensation indicator indicates that the evaporator is too warm
for condensation or deposition of water vapor on its external
surface has multiple positive effects. Namely: [0015] increased
heat load to the container will shorten the duration of periods
where the compressor is in the less active state, and hence the
periods where the evaporator is too warm for condensation or
deposition of water vapor on its external surface. Doing so,
advances the time the condensation or deposition rate can be
increased again. [0016] Dehumidification is typically applied to
protect the shipped produce from mould growth. There are two
factors contributing to reducing mould growth: reduce relative
humidity (i.e. dehumidify) and increase air flow. This fan speed
increase contributes to both anti-mould factors. [0017] As a
welcome side-effect it improves the mixing of air thus reducing
temperature distribution in the transport volume.
[0018] In one embodiment, the at least one dehumidification need
indicator is one or more selected from the group of: [0019] a
difference between current or recent relative humidity (RH) and
relative humidity setpoint (RHset), or a function thereof, [0020] a
time elapsed since a most recent start of operating in
dehumidification mode, and [0021] that the evaporator fans (10)
were operating at a reduced speed (LOW) the last time the
compressor (6) was in the first active state (this signifies that a
last non-circulation mode was a dehumidification mode).
[0022] By dehumidification mode is to be understood an operation
mode in which the compressor is in the first active state, heating
unit is on, and evaporator fans speed is low. This mode on
purposely increases the rate of condensation or deposition in order
to reduce relative humidity.
[0023] By non-circulation mode is to be understood an operation
mode in which at least either the compressor is in the first active
state or the heating unit is on.
[0024] Using dehumidification need indicators helps tailoring the
amount of condensation or deposition to the need. Dehumidification
is a very energy-consuming process. The dehumidification need
indicators are used to stop the dehumidification process when the
need has been met, while resuming dehumidification when
necessary.
[0025] In one embodiment, the at least one condensation indicator
is one or more selected from the group of: [0026] that the
compressor is in the first active state, [0027] that a supply air
temperature is colder than a return air temperature e.g. plus an
offset, where the offset may be zero, [0028] that the supply air
temperature is colder than a setpoint temperature e.g. plus an
offset that may be zero, [0029] a measured temperature of at least
a part of an external surface of the evaporator is below dewpoint,
and [0030] a function of a boiling temperature of the refrigerant
inside the evaporator is below dewpoint (the boiling temperature
may readily be derived from pressure measurements and knowledge of
the known refrigerant properties).
[0031] Some of these condensation indicators are standard available
in the controller of the cooling unit. They can be used
advantageously to predict if reducing the evaporator fans speed
actually increases the rate of water vapour condensation or
deposition on the evaporator's external surface.
[0032] In one embodiment, the method comprises the step of [0033]
controlling the operation of the one or more evaporator fans for
dehumidification purposes by setting or maintaining the speed of
the one or more evaporator fans to be a reduced speed (LOW) thereby
increasing condensation or deposition if: [0034] the compressor is
in the second less active state, the evaporator fans were operating
at a reduced speed (LOW) the last time the compressor was in the
first active state, and a supply air temperature is colder than a
return air temperature, and/or, [0035] the compressor is in the
first active state and the difference between current relative
humidity (RH) and relative humidity setpoint (RHset) is determined
to be above a second predetermined dehumidification need threshold
(RHlim2), and/or [0036] the compressor is in the first active
state, a time elapsed since a most recent start of operating in
dehumidification mode is less than a predetermined period of
time.
[0037] In the above embodiment the situation described in the first
bullet increases the condensation or deposition rate for
dehumidification if there is a need to dehumidify, by exploiting a
cold evaporator when the compressor is in the second less active
state. This condition typically applies during the minutes just
after the compressor transitioned from the first active state to
the second less active state. The second situation increases the
condensation or deposition rate for dehumidification if there is a
need to dehumidify, by exploiting a cold evaporator due to the
compressor operating in the first active state. The third situation
increases the condensation or deposition rate for dehumidification
if relative humidity was too high less than a predetermined period
of time ago. This makes the dehumidification method robust to
variations in relative humidity that will occur naturally due to
the temperature variations induced by the intermittently operated
compressor.
[0038] In one embodiment, the method comprises the step of [0039]
controlling the operation of the one or more evaporator fans for
dehumidification purposes by setting or maintaining the speed of
the one or more evaporator fans to be an increased speed (HIGH)
thereby increasing heat load to the evaporator if: [0040] the
compressor is in the second less active state and if the difference
between current relative humidity (RH) and relative humidity
setpoint (RHset) is above a first predetermined dehumidification
need threshold (RHlim1) and if at least one condensation indicator
indicates that the evaporator is too warm for condensation or
deposition of water vapor on its external surface.
[0041] Increasing evaporator fans speed if there is a need to
dehumidify while no possibility to instantaneously increase the
rate of condensation or deposition has multiple advantages, as
mentioned before.
[0042] In one embodiment, the method comprises controlling the
operation of the heating unit for dehumidification purposes by
activating the heating unit thereby increasing heat load to the
evaporator when a need for dehumidification is determined using at
least one dehumidification need indicator.
[0043] The advantage of activating the heating unit is that it
increases the heat load to the evaporator, which in turn forces the
temperature controller to increase the percentage of time the
compressor operates in the first active state in order to properly
maintain temperature. The increased percentage of time the
compressor operates in the first active state increases the
percentage of time the evaporator is sufficiently cold to increase
the condensation or deposition rate.
[0044] In one embodiment, the heating unit is activated if: [0045]
the compressor is in the first active state and the difference
between current relative humidity (RH) and relative humidity
setpoint (RHset) is determined to be above the second predetermined
dehumidification need threshold (RHlim2), and/or [0046] the
compressor is in the first active state, and a time elapsed since a
most recent start of operating in dehumidification mode is less
than a predetermined period of time, and/or [0047] the compressor
is in the second less active state, the heating unit was active at
the end of a last period the compressor was in the first active
state (signifying that the last non-circulation mode was a
dehumidification mode) and the difference between current relative
humidity (RH) and relative humidity setpoint (RHset) is determined
to be above the second predetermined dehumidification need
threshold (RHlim2), and/or [0048] the compressor is in the second
less active state, and the difference between current relative
humidity (RH) and relative humidity setpoint (RHset) is determined
to be above a third predetermined dehumidification need threshold
(RHlim3).
[0049] This is one advantageous implementation where multiple
dehumidification need indicators, and combinations of those, are
used to decide on activating the heating unit. The first situation
activates the heating unit when the compressor is in the first
active state (a condensation indicator) and the difference between
measured relative humidity (RH) and its setpoint (RHset), one of
the dehumidification need indicators, is above a predetermined
threshold value (RHlim2). The second situation activates the
heating unit while the compressor is in the first active state (a
condensation indicator), regardless of actual relative humidity,
just because relative humidity was too high less than a
predetermined period of time ago (also a dehumidification need
indicator).
[0050] This makes the method robust to variations in relative
humidity that will occur naturally due to the temperature
variations induced by the intermittently operated compressor. The
third situation activates the heating unit while the compressor is
in the second less active state because two indicators indicate a
need for dehumidification: at the end of the latest period the
compressor was in the first active state there still was a need to
dehumidify, and also now the difference between measured relative
humidity (RH) and its setpoint (RHset), one of the dehumidification
need indicators, is above a predetermined threshold value (RHlim2).
The fourth situation activates the heating unit while the
compressor is in the second less active state, regardless of past
conditions, if relative humidity is far too high. This is
implemented by evaluating if the difference between measured
relative humidity (RH) and its setpoint (RHset), one of the
dehumidification need indicators, is above a predetermined
threshold value (RHlim3).
[0051] In one embodiment, the first predetermined dehumidification
need threshold (RHlim1) is lower than or equal to the second
predetermined dehumidification need threshold (RHlim2).
[0052] An advantage of RHlim1<RHlim2 is that in this way it is
ensured that at the slightest need for dehumidification
(RH>RHlim1) then the first action will be to increase the speed
of the evaporator fans, with the multiple advantageous effects as
mentioned before. Only if the dehumidification need grows
(RH>RHlim2) the control will call upon the actions of fan speed
reduction and activating the heating unit.
[0053] In one embodiment, the second predetermined dehumidification
need threshold (RHlim2) is lower than or equal to the third
predetermined dehumidification need threshold (RHlim3).
[0054] An advantage of RHlim2<RHlim3 is that only when relative
humidity is far too high (RH>RHlim3) as a last option the
control calls upon the least energy-efficient step of activating
the heating unit while the compressor is in the second less active
state.
[0055] In one embodiment, the state of heating unit, compressor and
evaporator fans may only change if one or more minimum duration
times for being in a given state for the heating unit and/or the
compressor and/or the evaporator fans have elapsed.
[0056] The primary reason for introducing these minimum durations
is to avoid overly frequent state changes as a means to protect
unit hardware, including compressor lubrication and contactor
wear.
[0057] In one embodiment, the method is only performed if the
temperature is within specification by a predetermined margin.
[0058] The advantage of the above precondition is that it ensures
that temperature control prevails over humidity control. In
refrigerated transport and storage proper temperature control is
always the first priority.
[0059] In one embodiment, the refrigerated transport container is
not a transport container but another type of refrigerated space in
connection with a refrigeration unit. This could e.g. be an item of
refrigerated road transport equipment, a reefer ship, or any type
of stationary cold storage room.
[0060] A second aspect relates to a system for dehumidification in
a refrigerated transport container, the refrigerated transport
container comprising at least one heating unit, a transport volume,
and a cooling unit, a control unit, wherein the cooling unit
comprises at least a compressor and an evaporator comprising one or
more evaporator fans, the heating unit is situated downstream of
the evaporator, where the compressor is intermittently operated
between a first active state and a second less active state,
wherein the system comprises a processing unit adapted to: [0061]
control the operation of the one or more evaporator fans for
dehumidification purposes according to [0062] if a need for
dehumidification is determined using at least one dehumidification
need indicator and if at least one condensation indicator indicates
that the evaporator is sufficiently cold for condensation or
deposition of water vapor on its external surface, then setting or
maintaining the speed of the one or more evaporator fans to a
reduced speed (LOW) in order to increase a condensation or
deposition rate, and/or [0063] if a need for dehumidification is
determined using at least one dehumidification need indicator and
if at least one condensation indicator indicates that the
evaporator is too warm for condensation or deposition of water
vapor on its external surface, then setting or maintaining the
speed of the one or more evaporator fans to an increased speed
(HIGH) in order to increase heat load to the evaporator.
[0064] 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
[0065] Preferred embodiments of the invention will be described in
more detail in connection with the appended drawings, in which:
[0066] FIG. 1 schematically illustrates a simplified longitudinal
cross-sectional view of a refrigerated space in the form of a
refrigerated transport container;
[0067] FIG. 2 illustrates a so-called Mollier diagram for humid air
with a dehumidification cycle illustrated (1->2->3);
[0068] FIG. 3 is a state diagram illustrating respective
operational modes of a compressor, a heating unit and one or more
evaporator fans as a function of a temperature error integral
(TEI);
[0069] FIGS. 4 and 5 are a flow chart illustrating steps executed
by a microprocessor-implemented dehumidification control algorithm
or program of a control system of a refrigerated transport
container,
DESCRIPTION OF PREFERRED EMBODIMENTS
[0070] FIG. 1 schematically illustrates a simplified longitudinal
cross-sectional view of a refrigerated space in the form of a
refrigerated transport container. Shown in FIG. 1 is one example of
a refrigerated transport container 1 comprising a frontal section
having a cooling unit or system 40 and a load/cargo section or
transport volume 45. The transport volume 45 of the refrigerated
transport container 1 comprises a commodity load e.g. comprising a
plurality of stackable transport cartons or crates 35 arranged
within the 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.
[0071] The cooling unit 40 in this example comprises a so-called
vapour compression refrigeration circuit and a cooling space 41.
The refrigeration circuit at least comprises a compressor 6, a
condenser 7 with one or more condenser fans 9, an expansion device
8 and an evaporator 16 with one or more evaporator fans 10. The
compressor 6 and the condenser 7 with the one or more condenser
fans 9 are situated outside the insulated enclosure of the
transport container 1.
[0072] The cooling space 41 is situated inside the insulated
enclosure of the transport container 1. The cooling space 41 may
(as shown) be separated from the transport volume 45 by a panel
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. The air flow through the cooling space is
maintained by one or more evaporator fans 10. On its way through
the cooling space 41, air successively passes the one or more
evaporator fans 10, the evaporator 16, and a heating unit 20. The
cooling space 41 may also comprise a relative humidity sensor 2,
usually situated upstream of the one or more evaporator fans 10,
and at least a return air temperature sensor 5 and/or a supply air
temperature sensor 25, and may further comprise a defrost
termination temperature sensor or the like 17 measuring the
evaporator surface temperature.
[0073] Other typical cooling units or refrigeration units used in
refrigerated transport containers may be different.
[0074] A control system (not illustrated) comprises one or more
programmed microprocessors, which controls respective operational
states of the variable speed evaporator fans 10, the heating unit
20, and the compressor 6 in accordance with a control algorithm
defined by a set of microprocessor program instructions. The
control system may additionally comprise a user interface, for
example a LCD display, where an operator or ship technician can
enter or modify certain parameter values of the control algorithm
such as a setpoint temperature of the refrigerated transport
container 1, etc.
[0075] The one or more evaporator fans 10 may be configured to
operate at a number of discrete preset speed settings and may have
a given speed setting selected from the group of: a first (referred
to as HIGH throughout the specification) speed setting with a
predetermined first speed, a second (referred to as LOW throughout
the specification) speed setting with a predetermined second speed,
and a third (referred to as OFF throughout the specification) speed
setting with a predetermined third speed, where the first speed is
greater than the second speed and the second speed is greater than
the third speed.
[0076] In one embodiment, a predefined speed ratio between the HIGH
and LOW speed setting is a ratio of at least about 1.5 and may be 2
or 3 or more.
[0077] The OFF, LOW and HIGH speed settings may for example
correspond to air flow rates of about 0, about 3000 and about 6000
m.sup.3 per hour, respectively, but may depend on the specific
refrigerated transport container and/or refrigeration unit or
system used.
[0078] Throughout the present specification, each of the discussed
fan speed settings of the evaporator fan or fans may be provided by
joined operation of all or some of the evaporator fans present in
the refrigerated transport container. Different fan speed settings
may be achieved by changing the actual speed of one or several
individual evaporator fan(s) or by turning a certain number of fans
ON or OFF.
[0079] In the present specification, the first active state of the
intermittently operated compressor 6 means it operates at more than
10%, preferably more than 90%, of its maximum capacity. In the
second less active state of the intermittently operated compressor
6 it operates at less than 10%, preferably at or about 0%, of its
maximum capacity. An intermittently operated compressor 6 may
reside in the second less active state for temperature control
purposes for more than 20% of the time with more than 2
interruptions per hour by the first active state.
[0080] In a similar way, in the present specification, the ON state
of the heating unit 20 means it operates at more than 10%,
preferably more than 90%, of its maximum heating capacity. In the
OFF state the heating unit 20 operates at less than 10%, preferably
at or about 0%, of its maximum heating capacity.
[0081] In certain embodiments, the compressor 6 and/or the heating
unit 20 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 compressor 6 and/or the heating unit 20
may be continuously variable between for example 0% (OFF) and the
maximum capacity.
[0082] Currently, there exists no efficient means of controlling
dehumidification capacity in refrigerated containers with an
intermittently operated compressor.
[0083] It is to be understood that an intermittently operated
compressor induces temperature variations, which induce relative
humidity variations. Hence there is a need for better criteria for
assessing a need for increased dehumidification (dehumidification
need indicators) in relation to an intermittently operated
compressor than simply observing that the current relative humidity
is above a target relative humidity, as is otherwise done. All
possible criteria within the spirit of assessing the need for
increased dehumidification are within the scope of the
invention.
[0084] Relative humidity in a refrigerated space, like a
refrigerated transport container 1, may depend on the setpoint
temperature, ambient conditions, amount of fresh air exchange, heat
load, and/or moisture production by the cargo.
[0085] Moisture is typically removed by condensing on the
evaporator 16 or frosting on the evaporator 16 and subsequent
defrosting. Usually in refrigerated transport containers 1 without
dehumidification, relative humidity (RH) will be higher than 90%.
If a lower RH is required then humidity control is needed.
[0086] FIG. 2 shows a Mollier diagram for humid air. It illustrates
the relation between dry bulb temperature in .degree. C. (vertical
axis), absolute humidity in g water vapor per kg air (horizontal
axis on top of FIG. 2) and relative humidity in %. Relative
humidity is depicted by the curved lines, the right-most line
presents saturated air at 100% RH, the one-but-right-most line
presents 90% RH, and so on. Note that the term temperature, if not
other specified, actually means dry bulb temperature. The dewpoint
temperature of a volume of air is the temperature to which it must
be reduced before condense starts to form. For any air condition in
FIG. 2 the dewpoint is assessed by moving vertically down to
RH=100% curve. In FIG. 2 the arrows from point 1 to 2 to 3
illustrate the dehumidification process taking place in cooling
space 41.
[0087] In refrigeration transport containers 1, the principle of
dehumidification is to first sub-cool the return air flow 50 at the
evaporator 16 and subsequently reheat the air before it is supplied
to the transport volume 45 again. In the example in FIG. 2, a
return air flow 50 at (temperature, RH)=(+3.degree. C., 85%) (point
1 in FIG. 2) is first cooled at the evaporator 16 from point 2 to
the dewpoint temperature of approximately +1.degree. C. as
illustrated by the vertical line down from point 1 in FIG. 2.
[0088] Subsequent cooling, and thus dehumidification, proceeds
(askew line towards point 2) till air leaves the evaporator 16
(point 2 in FIG. 2). In that process, the absolute humidity of the
air is reduced from about 4.0 to about 2.8 g/kg (horizontal axis at
the top of FIG. 2). Finally the heating unit 20 reheats the
dehumidified air to point 3 in FIG. 3 at (temperature,
RH)=(+1.degree. C., 70%).
[0089] FIG. 3 is a state diagram for temperature control with an
intermittently operated compressor. Shown is a state diagram
illustrating respective operational modes of a compressor
(cooling), a heating unit and one or more evaporator fans as a
function of a temperature error integral TEI. Here TEI is the
integral over measured supply air temperature minus temperature
setpoint. TEI is just one possible temperature-related parameter on
which intermittent control of cooling and heating could be
based.
[0090] The state diagram 200 of FIG. 3 schematically illustrates
how switching between control modes or states of cooling, heating,
and/or air circulation within a refrigerated transport container
may be performed as a function of TEI.
[0091] Arrow 202 points in direction of increasing values of TEI.
The state or mode diagram 200 comprises a number of TEI thresholds
or limits in-between individual control modes 204, 206, 208, 210,
and 212. A first threshold, TEI_heat_stage.sub.--3_lim, constitutes
a lower threshold where the heating unit is switched ON and the
cooling is (maintained) OFF, i.e. the compressor is in the second
less active state, if TEI falls below this threshold.
[0092] In the upper portion of the state diagram 200 in-between
control modes 204 and 206, an additional threshold, TEI_max_cool,
constitutes an upper threshold where the compressor is in the first
active state while the heating unit is in an OFF state if TEI
exceeds this upper threshold.
[0093] Three intermediate modes 206, 208, and 210, where the
operational states of the compressor is the second less active
state and the heating unit is OFF, are located in abutment
in-between the upper control mode 204 (cooling) and the lowermost
control mode 212 (heating via the heating unit). These three
intermediate modes comprise two heating modes, control modes 208
and 210, and a circulation mode 206.
[0094] In the control modes 208 and 210, the heating unit resides
in operational state OFF and the one or more evaporator fans are
exploited to both supply heat to the supply air and add circulation
to the air inside the transport volume.
[0095] In the control mode 208, the one or more evaporator fans are
set in the LOW speed operational state while the speed of the one
or more evaporator fans is set to HIGH speed in the control mode
210 reflecting a requirement for higher heat production due to the
decreasing value of TEI.
[0096] In the circulation control mode 206, tightly controlling the
internal air circulation, the evaporator fans may be switched
between three different operational states having different fan
speeds such as OFF, LOW and HIGH during circulation periods.
[0097] In the present specification, the term "circulation mode"
means time periods where the operational state of the heating unit
is OFF and the operational state of the compressor is in the second
less active state.
[0098] Please note, the circulation mode 206 shown in FIG. 3
illustrates control between three possible speeds of the one or
more evaporator fans, while other fan speed control schemes may use
two states (LOW and HIGH) during normal operation while yet other
schemes may use more than three possible speeds.
[0099] As one example, the thresholds between the individual states
204, 206, 208, 210, and 212 may be:
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.
[0100] FIG. 4 and FIG. 5 is a flow chart illustrating steps
executed by a microprocessor-implemented dehumidification control
method or algorithm or program of a control system of a
refrigerated transport container.
[0101] The flow chart shown in FIGS. 4 and 5 provides one example
of the operation of a dehumidification control algorithm 400,
including the operation of one or more evaporator fans (see e.g. 10
in FIG. 1) and a heating unit (see e.g. 20 in FIG. 1).
[0102] The algorithm starts in step 402 and proceeds to step 404
where it is tested whether all minimum duration times for being in
a given state for the heating unit (20) and/or the compressor (6)
and/or the evaporator fans (10) have elapsed, i.e. whether it
should be allowed to change state of the various functional units
like evaporator fans 10, heating unit 20, and/or compressor 6.
[0103] If the test in step 404 is no (N), the method proceeds back
to the starting step 402, as no change of state is allowed.
[0104] If the test in step 404 is yes (Y), the method proceeds to
step 406 where it is checked whether the temperature is in-range,
i.e. sufficiently close to the temperature setpoint.
[0105] Testing whether the temperature is within spec or in-range
may e.g. be done by testing whether the TEI is between certain
predetermined minimum and maximum thresholds (TEI_min_in_range and
TEI_max_in_range). Alternatively, the test can be whether the
supply or return air flow temperature, or a function thereof, is
within a predetermined temperature range adjacent to a setpoint
temperature, where the function may also take into account the
temperatures observed recently, e.g. during the last 30
minutes.
[0106] If test 406 is no, the method proceeds back to the starting
step 402, thus giving priority to temperature control, as
dehumidification might be at the expense of the needed temperature
control. This ensures that dehumidification is performed only when
the temperature is under adequate control. In refrigerated
transport containers, adequate control of the temperature is
normally the most important parameter.
[0107] If test 406 is yes, the method proceeds to step 408 where it
is tested whether the temperature is below a certain measure or
margin below the maximum in-range threshold (TEI_max_in_range).
Dehumidification control may cause a rise in temperature,
especially when nearly the maximum cooling capacity is needed to
maintain temperature within the in-range region. Therefore starting
dehumidification while temperature is just below the maximum
in-range threshold is to be avoided. This may e.g. be done by
testing whether the TEI is below the predetermined maximum in-range
threshold (TEI_max_in_range) minus a predetermined margin, e.g.
10.degree. C.*min.
[0108] If the test of step 408 is no, the method proceeds back to
the starting step 402, as there is no temperature safety margin for
performing dehumidification.
[0109] If the test of step 408 is yes, the method proceeds to step
410 where it is tested whether the compressor is ON, i.e. in the
first active state (a condensation indicator). If the compressor is
in the second less active state (i.e. the test in step 410 gives
no), the method proceeds to step 418, which will be explained
later. If the compressor is in the first active state, the method
proceeds to step 412, where it is tested whether the difference
between a current relative humidity RH and the relative humidity
setpoint RHset (a dehumidification need indicator) is larger (or
equal to or larger) than a predetermined second dehumidification
need threshold or value (RHlim2). The second dehumidification need
threshold may e.g. be +3%. Other thresholds may be used in other
embodiments. What is significant is that the second RH threshold is
above the RH setpoint but to a small or lesser degree than a third
RH threshold as explained later.
[0110] A current relative humidity RH may e.g. be obtained using a
relative humidity sensor e.g. located in a return air flow (see
e.g. 2 in FIG. 1).
[0111] If the test of step 412 is yes then the relative humidity is
above the RH setpoint by a certain degree (in the specific example
by more than 3%) and the air is too humid and the method proceeds
to step 414 where a dehumidification mode, i.e. an operation where
dehumidification is performed in one way or another, is entered or
maintained. Entering dehumidification mode includes for example
that the heating unit is turned ON and the speed of the one or more
evaporator fans is set to or maintained at LOW to perform
dehumidification. Both heating and fan speed reduction have an
advantageous effect in relation to performing dehumidification, as
explained earlier. Other ways of performing dehumidification may be
contemplated, e.g. by decreasing the heat absorbing area of the
evaporator by physically shutting off a part of a split evaporator
or by increasing the so-called superheat. Increasing the superheat
fills a larger share of the evaporator tubes with gas, thus
effectively leaving a smaller part of the evaporator for heat
absorption. After step 414, the method proceeds to the starting
step 402.
[0112] If the test of step 412 is no, the method proceeds to step
416 where it is tested whether the method has been in a
dehumidification mode for less than a predetermined period of time,
e.g. 30 minutes (a dehumidification need indicator). If yes, the
method proceeds to step 414 where a dehumidification mode is then
maintained. This is to ensure that if a dehumidification mode is or
has been active within a certain period of time it is not abandoned
too soon and thereby can have its full or a larger effect even
though the test of step 412 in the mean time has become no, i.e.
the difference between the current relative humidity RH and the
relative humidity setpoint RHset has become less (or equal to or
less) than the predetermined second dehumidification need threshold
threshold (RHlim2).
[0113] If the test in step 416 is no, i.e. the method has been in
dehumidification mode for more than 30 minutes, and temperature
control has still requested compressor ON as assessed in step 410,
then the dehumidification control method leaves the control of fan
speed and heating unit to the temperature controller and proceeds
to the starting step 402.
[0114] Reverting back to step 410 in the case of no, the method
proceeds to step 418 where it is tested whether the heating unit is
ON for temperature control purposes. The test may e.g. be made by
checking whether the heating unit is ON and
TEI<TEI_heat_stage.sub.--3_lim (FIG. 3). If yes, the control of
fan speed and heating unit is left to the temperature controller
and the dehumidification control method proceeds back to the
starting step 402. If no, the method proceeds to step 420 (in FIG.
5) where it is tested whether the last non-circulation mode was a
dehumidification mode (a dehumidification need indicator).
[0115] If the test in step 420 is yes, extra dehumidification may
be welcome and the method proceeds to step 426, which will be
explained later.
[0116] If the test in step 420 is no, the method proceeds to step
422, where it is tested whether the difference between a current
relative humidity RH and the relative humidity setpoint RHset (a
dehumidification need indicator) is larger (or equal to or larger)
than a predetermined first dehumidification need threshold or value
(RHlim1). The first RH threshold may e.g. be minus 2%. Other
thresholds may be used in other embodiments. What is significant is
that the first RH threshold is below the second RH threshold.
[0117] If the test in step 422 is no, the relative humidity is
distinctly lower than requested and dehumidification is not to be
performed and the method proceeds to the starting step 402 without
changing any states for dehumidification purposes.
[0118] If the test in step 422 is yes, the method proceeds to step
424, where the fan speed of the one or more evaporator fans is set
to or maintained at HIGH before proceeding to step 430. Setting or
maintaining fan speed at HIGH will generate heat that will evoke an
earlier need to transition the compressor state to its first active
state and therefore earlier dehumidification.
[0119] Reverting back to step 420 in the case of yes, the method
proceeds to step 426 where it is tested whether the evaporator
surface is colder than dewpoint temperature (a condensation
indicator). If yes, the method proceeds to step 428 setting or
maintaining the fan speed of the of the one or more evaporator fans
at LOW before proceeding to step 430. In the first period after a
compressor abandoning the first active state the evaporator surface
temperature may still be below dewpoint and condensation or
deposition on the evaporator might continue and it may be possible
to exploit this for dehumidification. The steps 426 and 428 enable
taking advantage of this by setting the fan speed to LOW.
[0120] The dewpoint temperature may e.g. be derived via a relative
humidity sensor (see e.g. 2 in FIG. 1) and a temperature sensor
(see e.g. 5 in FIG. 1) located in the return air flow (see e.g. 50
in FIG. 1), and the evaporator surface temperature may be measured
using for example a defrost termination temperature sensor (see
e.g. 17 in FIG. 1). Evaporator surface temperature colder than
dewpoint temperature might also be assumed based on other
indicators, for example: when supply air temperature is below
return air temperature then apparently the evaporator is colder
than the return air flow and it may be assumed that its surface
temperature then is below dewpoint temperature.
[0121] If the test of 426 is no, signifying that the temperature of
the evaporator surface cannot be used for dehumidification
purposes, the method proceeds to step 422 (explained already).
[0122] In step 430, regardless of whether the method arrived from
step 424 or 428, it is tested whether the difference between a
current relative humidity RH and the relative humidity setpoint
RHset (a dehumidification need indicator) is larger (or equal to or
larger) than the predetermined second dehumidification need
threshold or value (RHlim2). This step corresponds to step 412.
[0123] If the test in step 430 is no, the method proceeds to the
starting step 402 with the result that the fan speed of the of the
one or more evaporator fans has been set to HIGH (in step 424) or
to LOW (in step 428), and the heating unit remains OFF.
[0124] If the result of the test in step 430 is yes, the method
proceeds to step 436 where it is tested whether the last
non-circulation mode was a dehumidification mode (a
dehumidification need indicator). The test in step 436 corresponds
to the test in step 420. Again, if the test in step 436 is yes,
extra dehumidification may be welcome and the method proceeds to
step 434 turning the heating unit ON, after which the method
proceeds to the starting step 402. The reason for making the test
again is that step 430 may be entered from both the yes and the no
branch of step 420. It is to be understood that the test need not
actually be made again but the result of the test in step 420 may
be stored.
[0125] If the test in step 436 is no, the method proceeds to step
432 where it is tested whether the difference between a current
relative humidity RH and the relative humidity setpoint RHset (a
dehumidification need indicator) is larger (or equal to or larger)
than a predetermined third dehumidification need threshold or value
(RHlim3). The third RH threshold may e.g. be +15%. Other thresholds
may be used in other embodiments. What is significant is that the
third RH threshold is larger than the second RH threshold.
[0126] If the test in step 432 is yes, signifying that the air is
far more humid than requested, the method proceeds to step 434
where the heating unit is turned ON before proceeding to the
starting step 402. Turning the heating unit ON while the compressor
is in the second less active state raises temperature rapidly and
hence advances the moment the compressor transitions to the first
active state and hence enforces an earlier possibility to re-enter
dehumidification mode (compressor in first active state, heating
unit ON, fan speed LOW).
[0127] If the test in step 432 is no, the method proceeds to the
starting step 402 with the result that the fan speed of the of the
one or more evaporator fans has been set to HIGH (in step 424) or
to LOW (in step 428) and that the heating unit remains OFF.
* * * * *