Technical field
[0001] The present invention relates to a laundry dryer including a heat pump system, wherein
the refrigerant of the heat pump circuit includes a flammable refrigerant.
Background of the invention
[0002] The heat pump technology in a laundry dryer is at present the most efficient way
to dry clothes in terms of energy consumption. In a heat pump system of the laundry
dryer a process air stream flows in a closed process air stream circuit. Further,
the heat pump system includes a closed refrigerant circuit. The process air stream
is moved by a main fan, passes through a laundry chamber, which is preferably formed
as a rotatable laundry drum, and removes there water from wet clothes. Then, the process
air stream is cooled down and dehumidified in an evaporator, heated up in a condenser
and re-inserted into the laundry drum again.
[0003] The refrigerant is compressed by a compressor, condensed in the condenser, expanded
in an expansion device and then vaporized in the evaporator.
[0004] Thus, the condenser and the evaporator are components of the process air stream circuit
as well as of the refrigerant circuit. The condenser and the evaporator are heat exchangers
between the process air stream circuit and the refrigerant circuit.
[0005] Usually, the components of the heat pump system are placed in a basement of the laundry
dryer. The basement of a laundry dryer is part of a casing, which includes in addition
to the basement also walls, substantially vertically supported from the basement,
such as for instance a front wall and a rear wall, and lateral walls. In the casing,
a drum, where the laundry is introduced in order to dry the same, is rotatably supported.
In particular, the compressor, the evaporator and the condenser are arranged in said
basement below the laundry drum.
[0006] Typical refrigerants used in heat pumps are Hydroflorocarbons (HFCs), such as, for
example, R134a and R407C. However, the use of these refrigerants can adversely impact
on global warming, as they have a high Global Warming Potential (GWP), which is a
relative measure of the amount of heat trapped in the atmosphere by such refrigerants
(in gas form), compared to the amount of heat trapped in the atmosphere by a similar
mass of carbon dioxide.
[0007] Especially in the last years, the global warming problem has become increasingly
severe, and alternative refrigerants have thus been extensively studied and employed.
[0008] As also disclosed by document
EP 3 066 406 B1, hydrocarbon refrigerants, such as for example propane (R-290) and propylene (R-1270),
proved to be a good alternative for replacing the above high-GWP refrigerants in heat
pump dryers and washer-dryers appliances. These natural fluids have ideal thermal
and physical properties, besides having a negligible GWP. However, since these alternative
refrigerants are flammable and explosive, the existing regulations currently limit
the maximum charge of refrigerant in laundry, so as to prevent possible issues due
to leakages in the refrigerant circuit.
Summary of the invention
[0009] The Applicant has thus realized that, in addition to the choice of refrigerant, also
the design of the heat exchangers, i.e. namely of the evaporator and the condenser,
can severely affect energy consumption, drying efficiency and time performances. A
proper configuration of the heat exchanger(s) allows achieving several benefits, such
as maximizing the heat exchange between the refrigerant and the process air, reducing
the pressure drop both in the refrigerant and in the process air circuit, and reducing
the amount of refrigerant needed for a proper functioning of the heat pump. All these
benefits allow saving energy, improve the drying efficiency and, in general, together
with the choice of a refrigerant having a low GWP, allow the realization of a more
"ecofriendly" dryer.
[0010] Therefore, it is an object of the present invention to provide a laundry dryer with
a heat pump system having an improved design, aimed at maximizing the fraction of
refrigerant charge effectively involved in the heat exchange.
[0011] Another object of the present invention is to provide a laundry dryer with a heat
pump system which allows good performance in terms of efficiency having at the same
time a negligible impact on global warming.
[0012] According to an aspect, the invention relates to a dryer including:
∘ a treating chamber where items are introduced and treated with a process air flow;
∘ a heat pump system having a refrigerant circuit in which a refrigerant can flow,
said refrigerant circuit including a first heat exchanger where the refrigerant is
heated up, a second heat exchanger where the refrigerant is cooled off, a compressor
to pressurize and circulate the refrigerant through the refrigerant circuit, and a
pressure-lowering device; said first and/or second heat exchanger being apt to perform
heat exchange between said refrigerant flowing in said refrigerant circuit and said
process air; the refrigerant being a flammable refrigerant;
∘ wherein said second heat exchanger:
▪ is a finned tube heat exchanger comprising a tube having multiple sections one above
the other and a plurality of fins;
▪ is divided in three portions: a central portion wherein said multiple sections of
the tube are in contact with said plurality of fins, and a first and second end portions
where said tube is not in contact with the plurality of fins;
∘ and wherein a ratio between a total external volume of all sections of the tube
included in the central portion in contact with the plurality of fins and a compressor
displacement has a value higher than 28.
[0013] In the following, with the term "dryer" both drying machines which dry only as well
as combined washer-dryers are meant. In particular, also washer-dryers that wash the
laundry, spin/centrifuge it and finally tumble dry it.
[0014] The dryer includes a "treating chamber", such as a washing and/or drying chamber
(commonly called drum) where the laundry can be located in order to be washed and/or
dried; the chamber can be rotated around a chamber axis during the washing and/or
drying operations. Further, the dryer may be a front-loading dryer, which means that
the axis of rotation of the treating chamber is positioned in a horizontal manner
or slightly tilted with respect to a horizontal plane, or a top laundry dryer, where
the axis of the treating chamber is substantially vertical.
[0015] In a preferred embodiment, the dryer is a front loading laundry dryer.
[0016] The dryer preferably comprises a casing preferably including a front wall, a rear
wall, side walls, top wall and a base section or basement. The front or top wall may
comprise a user panel to command the functioning of the dryer by the user. The casing
defines the limit between the internal volume of the dryer and the outside to the
dryer. Further, preferably, the casing includes a door hinged to the casing itself,
e.g. to the front wall in case of a front loading dryer, which is openable in order
to introduce the laundry in the laundry chamber, or to the top wall in case of a top
loading dryer.
[0017] The basement has, among others, the function of housing several component of the
dryer, such as a portion of a drying air conduit, heat exchangers, a motor for rotating
the chamber, a fan, etc. Further, it has also the function of supporting some of the
walls of the casing.
[0018] The basement can be realized in any material; preferably it is realized in plastic
material. Further, the walls of the casing can also be realized in any material.
[0019] The basement is generally positioned on a floor and where it rests when the machine
is in a standard operating condition.
[0020] The basement for example may be divided in an upper and a lower shell. The upper
and lower shells define the outer boundaries of the basement, dividing a volume "inside"
of the basement and an "outside" to the basement.
[0021] In a heat pump dryer, the treating chamber is part of a process air circuit, in particular
for example a closed-loop air circuit in case of a condensed dryer or an open air
circuit in case of a vented dryer, which in both cases includes an air duct for channelling
a stream of air to dry the load. The process air circuit is connected with its two
opposite ends to the treating chamber. Hot dehumidified air is fed into the treating
chamber, flowing over the laundry, and the resulting humid cool air exits the same.
The humid air stream rich in water vapor is then fed into an evaporator of the heat
pump, where the moist warm process air is cooled and the humidity present therein
condenses. The resulting cool dehumidified air is then either vented outside the appliance
in the ambient where the latter is located or it continues in the closed-loop circuit.
In this second case, the dehumidified air in the process air circuit is then heated
up before entering again in the drying chamber by means of a condenser of the heat
pump, and the whole loop is repeated till the end of the drying cycle. Alternatively,
ambient air enters into the drum from the ambient via an inlet duct and it is heated
up by the condenser of the heat pump before entering the drying chamber. Different
circuits are known in the art in case of a washer-dryer.
[0022] The heat pump of the dryer includes a refrigerant circuit in which a refrigerant
can flow and which connects via piping a first heat exchanger or evaporator, a second
heat exchanger or condenser, a compressor and a pressure-lowering device. The refrigerant
is pressurized and circulated through the system by the compressor. On the discharge
side of the compressor, the hot and highly pressurized vapor is cooled in the condenser,
until it condenses into a high pressure, moderate temperature liquid, heating up the
process air before the latter is introduced into the drying chamber. The condensed
refrigerant then passes through the pressure-lowering device such as an expansion
device, e.g., a choke, a valve or a capillary tube. The low pressure liquid refrigerant
then enters the evaporator, in which the fluid absorbs heat and evaporates due to
the heat exchange with the warm process air exiting the drying chamber. The refrigerant
then returns to the compressor and the cycle is repeated.
[0023] In order to compress the refrigerant, the compressor includes an electric motor which
is commonly powered by a current, for example a current coming from the mains.
[0024] In order to exchange heat with the process air, the first and second heat exchangers
are positioned inside the process air circuit. The process air circuit defines a bottom
portion or a bottom part. The second heat exchanger is in abutment to the bottom part
or portion of the process air circuit. In the following, a "horizontal plane", which
is used as a reference plane, is defined as follows. The second heat exchanger touches
at least in three points the bottom part of the process air conduit. These three points
defines a plane and this plane is a horizontal plane for the reference system of the
present description. If there are more than three points of abutment between the second
heat exchanger and the bottom part/portion of the process air conduit, then the horizontal
plane is the plane that includes the majority of the points of connection.
[0025] This plane is generally really "horizontal" in the general meaning, that is, it is
parallel to the floor where the dryer is put, which is commonly accepted to be "horizontal".
However, there are situations in which the above-defined plane is not horizontal in
the common meaning of the term, for example it is tilted (i.e. it forms an angle)
with the floor where the dryer is positioned. This can happen for example because
the floor is not flat and the dryer had to be adjusted, for example using standard
provided "height-adjustable legs", to be stable on an uneven floor. Alternatively,
the heat exchanger can be indeed positioned tilted with respect to the floor.
[0026] Preferably, said second heat exchanger is located in the basement of the dryer.
[0027] In the present invention, the refrigerant used in the heat pump circuit is a flammable
refrigerant and preferably a hydrocarbon refrigerant. Although for flammable refrigerant
a maximum charge limit is set by current regulations, these refrigerants have ideal
thermal and physical properties for use in heat exchangers and, most importantly,
they have low GWPs, which means a negligible impact on global warming.
[0028] The second heat exchanger of the invention, the condenser, is a finned tube heat
exchanger comprising a tube having multiple sections one above the other and a plurality
of fins. A total length of said second heat exchanger is defined along a length direction.
[0029] Finned tube heat exchangers are the most commonly used type of heat exchangers to
transfer heat between a fluid (the refrigerant that flows inside the tube) and the
air (the drying process air that flows through the fins and outside the tubes).
[0030] Such heat exchangers typically comprise a continuous bent tube having straight portions
connected by U-bend sections, along which straight portions fins are transversally
mounted. The fins are provided with holes, or apertures, having proper shape and size
to allow to be assembled transversally along the continuous bent tube. Moreover, the
fins are suitably designed so that a contact with proper interference is ensured between
the tube and the holes of the fins. The contact between tube's portions and fins can
be random and/or sporadic, due to mounting process that may alter mechanical tolerances
and relative positioning of the tube and fins mounted thereon.
[0031] Alternatively, such heat exchangers may comprise individual straight tubes inserted
in circular holes or apertures of transversal fins, such tubes being then expanded
to provide a proper contact with interference between the tubes and the circular holes
of the fins. Ends of the straight tubes are then connected in pairs by means of short
U-bend sections, in order to ensure the continuity of the refrigerant circuit. The
U-bend sections are typically welded or soldered to the straight tubes.
[0032] In any case, there is a plurality of tube sections, all part either of the same tube
or of different separated tubes. For simplicity, the singular "tube" is used in the
following to indicate both a continuous bent tube and an assembly of tubes, the latter
comprising multiple straight tubes stacked one above substantially parallel to one
another, and connected at their ends by suitable connecting sections, such as the
aforementioned U-bend sections welded or soldered to the ends of the straight tubes.
[0033] Preferably, these tube sections are all parallel to each other. These tube's sections
may correspond to the portions of the tube which are "straight", that is extending
substantially along a single direction without bends or curves. Preferably, these
tube's sections are horizontal, that is, they are parallel to the horizontal plane.
[0034] In both the above constructions, such finned tube heat exchangers generally comprise
a central portion, for example corresponding substantially to the length of the straight
tubes' sections. This central part is that part across which process air flows and
where therefore heat exchange takes place between the refrigerant flowing in the tubes'
sections and the process air. Further, the finned tube exchangers include lateral
portions, on the two opposite sides of the central portion, comprising U-bend tube
sections connecting the straight tube portions and not involved in the heat exchange
(or minimally involved in it), as process air is not made to flow therethrough or
only minimally.
[0035] Although such lateral portions are not useful in terms of heat exchange, they are
necessary for construction reasons, as the straight tube's sections of the central
portion have to be connected to ensure continuity of the refrigerant circuit.
[0036] The minimum length of the lateral portions depends on the minimum bending radius
of the tubes, which in turns depends on size and flexibility of the material of the
tube(s), and on the space needed for welding or soldering the U-bend sections when
multiple individual straight tubes are used. According to the standard technology,
for heat exchangers the length of the lateral portions (sum of the two lengths) is
between 40 mm and 60 mm. This length is taken along a length direction, which is a
horizontal direction. Further details of the length direction are given below.
[0037] In use, the whole heat exchanger is filled with refrigerant. Therefore, a fraction
of refrigerant not involved in the heat exchange, namely the fraction of refrigerant
flowing in the lateral portions, must be taken into account when finned tube heat
exchangers are involved. This is of particular concern when flammable and explosive
refrigerants such as the aforementioned hydrocarbons are used, due to their limited
maximum charge. This maximum charge might for example be fixed in regulations.
[0038] This maximum charge limit can in turn affect the drying performances of the laundry
dryer, as the best performances of a conventional laundry dryer are typically observed
for higher values of refrigerant charge.
[0039] The reference above defined horizontal plane allows defining two orthogonal directions:
a length direction and a thickness direction. These two directions are both horizontal
directions, that is, they are parallel to the horizontal plane, and forms an angle
of substantially 90° among each other.
[0040] In the following, by "length direction", a horizontal direction substantially parallel
to a plane containing at least two of the multiple tube sections stacked one above
the other is meant to be indicated. This plane is preferably a vertical plane, that
is, a plane perpendicular to the horizontal plane. In the following, thus, "length"
means a measure taken along the length direction. The "thickness direction" is thus
automatically defined, being perpendicular to the length direction (and still horizontal).
[0041] The second heat exchanger of the invention is thus divided in three portions: a central
portion wherein said multiple sections of the tube are in contact with said fins,
and a first and second end portions where said tube is not in contact with the fins.
[0042] The lengths in the length direction of the three portions -central portion, first
and second end portions- are defined along said length direction of the heat exchanger,
and the sum of said three lengths corresponds to the total length of the heat exchanger.
[0043] Heat exchange takes place in the central portion of the second heat exchanger between
the refrigerant flowing in the multiple tube sections and the process air, flowing
transversally across the tubes and substantially parallel to the fins, the end portions
being provided only to connect said multiple tube sections of the central portion,
so as to ensure continuous flow of the refrigerant in the circuit. Such end portions
are not provided with fins since process air is not intended to be flown in these
areas, so that no heat exchange between refrigerant and air takes place in end portions.
[0044] An external volume of all sections of the tube included in the central portion (in
short, total external volume or TEV) in contact with the fins of the second heat exchanger
can be defined.
where Nt= number of sections,
Le= length of the tubes' sections in the central area,
De= external diameter of the tubes' sections.
[0045] In case the diameters is not the same for all tubes or sections, then there is a
sum of the various volumes given by tube's sections having different diameters. The
same applies in case the length is not the same for all tube's sections.
[0046] Preferably, the external diameter De is comprised between 4 mm and 10 mm.
[0047] The length of the tube's section is calculated as the length along their extension
direction. Generally, the tube's sections are straight thus their length is equal
to the extension of the tube from one end to the other. This can correspond to the
length of the central portion along the length direction if the tube's sections are
substantially horizontal (if the tube's sections extend horizontally).
[0048] The volume above is calculated in that way in case the tube's sections have a circular
cross section. If the tube's sections are not circular, then the volume is
[0049] Preferably, the number of sections Nt is comprised between 20 and 70.
[0050] Preferably, the length of the sections Le is comprised between 200 mm and 450 mm,
more preferably between 200 mm and 300 mm. Preferably the length of the tube's sections
(length of the tube in the central portion) is equal to: Le > 280 mm, preferably Le
> 300 mm, more preferably Le > 320 mm, even more preferably Le > 350 mm.
[0051] Preferably, the thickness of the tube (that is, the thickness external wall of the
tube) is comprised between 0.2 mm and 0.8 mm.
[0052] In case of a round tube, preferably the external diameter De of the tube is comprised
between 4 mm and 10 mm.
[0053] In case the diameters is not the same for all tubes, then there is a sum of the various
volumes given by tubes' sections having different diameters.
[0054] The length of the tubes' section is calculated as the length along their extension
direction. Generally, they are straight, thus the length is equal to the extension
of the tube from one end to the other. This can correspond to the length of the central
portion if the tubes' sections are substantially horizontal (the tubes' section extend
horizontally).
[0055] The external volume TEV above is calculated in that way in case the tubes are circular.
[0056] If the tube are not circular, then the volume is
Volume (total): (external area of the tube section)*Nt*Le
[0057] Furthermore, considering now the compressor of the heat pump, the compressor defines
a displacement. The compressor's displacement is a volume value, which is given by
the manufacturer of the compressor, and it is generally written in the compressor's
datasheet. For example, in case of a rotary compressor, such as in a stationary blade
rotary compressor, an eccentric or cam rotates within a chamber (also called cylinder
due to its common cylindrical shape). The rotation of the off-centre cam compresses
the gas refrigerant in the cylinder of the rotary compressor. The displacement of
this compressor is defined as the volume of the chamber (= cylinder) from which the
volume of the cam has been subtracted.
[0058] Preferably, said compressor is a rotary compressor.
[0059] Heat exchangers with higher external volume have also consequently a high internal
volume (being the two related and depending on the thickness of the tubes' walls).
[0060] Considering the case of flammable refrigerant, there is a limit in the possible amount
of refrigerant to be filled in the system. This amount is generally relatively "small"
and can be fixed by country-dependent regulations. This in turn means that using a
heat exchanger with high external volume forces the system to be "less charged" than
in case of a heat exchanger with low external volume.
[0061] For the purpose of this discussion, it is considered that the thickness of the tubes
in the heat exchanger is preferably comprised between 0.2 mm and 0.8 mm. The tubes
are preferably realized in Aluminum or in Copper or in a mixture of the two. It is
therefore understood that the external volume of the heat exchanger and its inner
volume are connected. And, thus, a higher external volume implies a higher inner volume.
[0062] The effect of a "low charged" system is that the working pressure and temperature
of the refrigerant during drying cycle increases relatively slowly and they do not
reach very high level.
[0063] Further, the refrigerant charge in the heat pump system, when the compressor is ON,
is distributed mainly in the condenser because of status of the refrigerant itself
(high pressure and in a liquid state in a portion of the heat exchangers)
[0064] It is also known that the use of a compressor with small displacement has similar
effect: the working pressure and temperature of the refrigerant during drying cycle
increases slowly and they do not reach very high level. In this way, a good care of
the fabric can be obtained because the drying can be very gentle. Further, a very
efficient performances of the dryer can be obtained as well.
[0065] In order to obtain these advantages, the proper combination of external volume (TEV)
of the condenser (only of its part which is effective in the heat exchange process,
i.e. the finned part) and compressor displacement is described by their ratio. According
to the invention, the ration is the following:
Condenser TEV / compressor displacement > 28 [cc/cc],
more preferably the above ration is > 35 [cc/cc], even more preferably > 40 [cc/cc].
[0066] Preferably, the above ratio is < 70, more preferably < 65.
[0067] Preferably, said second heat exchanger is a coil heat exchanger and the tube in said
end portions includes bends. In this embodiment, the end portions are particularly
shaped as U-bend tube sections.
[0068] Preferably, said tube has an external diameter comprised between 4 mm and 10 mm.
The upper value of such size interval is advantageously selected to limit the inner
volume of the heat exchanger so that, for a same amount of refrigerant, a higher density
of refrigerant circulating within the coil is obtained, which in turns increases the
cooling capacity of the second heat exchanger and reduces the occurrence of pressure
losses when low charges of refrigerator are involved. The lower limit, on the other
end, is provided to ensure a minimum acceptable cooling capacity of the heat exchanger.
[0069] Preferably, the dryer comprises a process air circuit including said treating chamber,
and a basement where said heat pump is located, the process air circuit including
a basement portion comprising a process air conduit where the first and second heat
exchangers are positioned, wherein said central portion of said second heat exchanger
is completely contained in said basement process air conduit. The whole central portion
of the condenser is thus used for heat exchange, using the maximum available heat
exchange surface.
[0070] Preferably, said flammable refrigerant preferably includes propane or propylene.
Propane and propylene are highly efficient natural refrigerants with the lowest levels
of harmful emissions.
[0071] Preferably, said length direction is substantially perpendicular to a main direction
of flow of said process air when passing through said second heat exchanger. In this
embodiment, the second heat exchanger has therefore a so-called "cross-flow configuration",
particularly suitable for low-pressure applications such as laundry dryers, and in
general when a large volume flow of vapor is involved and a low-pressure drop is required.
Moreover, this configuration allows a reduced size of the heat exchanger.
[0072] Preferably, the second heat exchanger defines a thickness along a thickness direction,
said thickness direction being substantially perpendicular to said length direction,
and wherein the thickness is comprised between 40 mm and 150 mm. After a certain thickness,
there are no significant improvement in the overall efficiency of the heat pump. Therefore,
the selected range is a compromise between a "small" heat exchanger so that little
refrigerant is used, and a good heat exchange.
[0073] Preferably, the displacement of the compressor is comprised between 5 cc and 12 cc.
More preferably, the displacement of the compressor is comprised between 6 cc and
9 cc. These displacements are a good compromise to obtain a proper power for the needs
of the drying cycles and the costs and size of the compressors. The compressor displacements
are within the mentioned ranges, however they are always selected so that the ratio
between the displacement and the external volume of the condenser satisfies the inequality
of the invention.
[0074] Preferably, said tube is realized in copper, aluminum or a combination of the two.
These materials have excellent thermal conductivity, besides having good properties
of thermal expansion, resistance to internal pressure, corrosion resistance and fatigue
strength.
[0075] Preferably, the total external volume of all sections of the tube included in the
central portion is comprised from 200 cc and 600 cc. These values are obtained taking
into consideration the amount of refrigerant available and the heat exchange needed.
[0076] Preferably, said first and/or second heat exchanger defines a total length along
a length direction, said end portions being at the opposite side of the central portion
along said length direction, said length direction being substantially perpendicular
to a main direction of flow of said process air when passing through said first and/or
second heat exchanger. In this embodiment, the first and/or second heat exchanger
has therefore a so-called "cross-flow configuration", particularly suitable for low-pressure
applications such as laundry dryers, and in general when a large volume flow of vapor
is involved and a low-pressure drop is required. Moreover, this configuration allows
a reduced size of the heat exchanger.
[0077] Preferably, the first and/or second heat exchanger defines a thickness along a thickness
direction, and wherein the thickness is comprised between 40 mm and 150 mm. After
a certain thickness, there are no significant improvement in the overall efficiency
of the heat pump, because pressure drops become significative. Therefore, the selected
range is a compromise between a "small" heat exchanger so that little refrigerant
is used, and a good heat exchange.
[0078] Preferably, said first and/or second heat exchanger defines a total length along
a length direction, said end portions being at the opposite side of the central portion
along said length direction, the total length being smaller than 550 mm. The total
length Lt is the sum of the central length Le plus the two lateral lengths Lc. Dryers
have commonly standard accepted dimensions to be respected and this maximum length
is optimal to use all the available space. For example, standard maximum dimensions
of a dryer are in Europe 60 cm x 60 cm (length x thickness), while commonly the basement
of the dryer is few cm smaller.
[0079] Preferably, said basement comprises an upper and a lower shell, said basement process
air conduit being formed by said upper and lower shells. An easy assembly of the machine
is obtained.
[0080] Preferably, the high pressure of the refrigerant in the stable phase of the heat
pump cycle is comprised between 19 bar and 38 bar.
[0081] Preferably, the low pressure of the refrigerant in the stable phase of the heat pump
cycle is comprised between 7 bar and 17 bar.
[0082] In addiction the bigger the exchange area of the condenser, the lower the condensation
pressure of the refrigerant. Therefore increasing the dimensions of the condenser
it is possible to obtain the same airflow at the same temperature at the outlet of
the condenser with a lower condensation pressure of the refrigerant. This is useful
for improving performances.
[0083] The pressure mentioned is measured in this way:
Low pressure of the refrigerant is measured at the inlet of the compressor, between
the evaporator and the compressor, when the cycle of the heat pump is in the stable
state.
[0084] High pressure of the refrigerant is measured at the outlet of the compressor, between
the compressor and the condenser, when the cycle of the heat pump is in the stable
state.
[0085] In case of propane as refrigerant fluid:
the high pressure in the stable phase of the cycle is preferably comprised between
19 and 32 bar, preferable from 21 to 29 bar.
[0086] The low pressure in the stable phase of the cycle is preferably comprised between
7 and 14 bar, preferable from 9 to 12 bar.
[0087] In case of propylene as refrigerant fluid:
The high pressure in the stable phase of the cycle is preferably comprised between
23 and 38 bar, preferable from 25 to 35 bar.
[0088] The low pressure in the stable phase of the cycle is preferably comprised between
8 and 17 bar, preferable from 11 to 15 bar.
[0089] The stable phase is defined as follows. The whole heat pump cycle can be divided
in a first transient phase and in a stable phase. The first transient phase starts
at the beginning of the heat pump cycle and can last up to 60% of the total duration
of the cycle, preferably up to 45%, more preferably up to 30% of the total duration
of the cycle. In the transient phase, the pressure increases gradually (single pressure
measurements can still fluctuate, but the general trend of the pressure is that of
an increase). In the stable phase, the pressure is substantially constant (also in
this case single measurements fluctuate, but the general trend is a substantially
constant value of pressure). The stable phase starts at the end of the transient phase
and may continue up to the end of the heat pump cycle. The stable phase satisfies
the following conditions during its duration.
[0090] First condition is that the stable phase contains the highest pressure of the whole
phase.
[0091] Second condition relates to its being "constant". The pressure is measured at a given
frequency, therefore, the curve of the pressure during the heat pump cycle includes
a plurality of points, one at each sampling time. These pressure values are noted
with X
i with i= 1....N, where N depends on the length of the cycle.
[0092] Within the stable phase of the heat pump cycle, taken an average of all the X
i in the stable phase, called averaged value X
aver, at least 90% of all points X
i in the stable phase are such that:
[0093] Preferably,
[0094] More preferably,
[0095] Defined the stable phase as above, then it is verified whether:
- when it comes to the high pressure (measured at the outlet of the compressor, between
the compressor and the condenser)
- when it comes to the low pressure (measured at the inlet of the compressor, between
the evaporator and the compressor)
[0096] Preferably, the amount of inflammable refrigerant contained in said heat pump refrigerant
circuit is comprised between 80 g and 300 g. More preferably, the amount of inflammable
refrigerant is comprised between 100 g and 250 g. More preferably, the amount is comprised
between 120 g and 200 g.
[0097] The amount of inflammable refrigerant might be set by regulations, which can also
be country-dependent. The amount is relatively "low" to minimize the risk of combustion.
[0098] Preferably, said tube is realized in copper, aluminum or a combination of the two.
These materials have excellent thermal conductivity, besides having good properties
of thermal expansion, resistance to internal pressure, corrosion resistance and fatigue
strength. Preferably, the tube is realized in aluminum or one of its alloys. Because
of mechanical characteristics of Cu and Al to ensure similar mechanical resistance,
the wall thickness of the Al tube is higher than Cu tube. This means that if the external
diameter of Cu and Al tube is the same, the internal diameter of Al tube is lower
than Cu one. Considering the cost of raw material, there is an economical advantage
of using Al tube instead of Cu tube heat exchangers, despite the higher amount of
material. A low internal diameter means a low inner volume of heat exchangers and
this is particularly helpful in case of flammable refrigerant where the charge quantity
is limited.
[0099] The inner diameter reduction, keeping same number of pipes' sections and length of
the heat exchangers can be achieved also by reducing the external diameter of the
tube, not only with increasing the thickness of the wall. A high external diameter
tube however increases the turbulence of the air flowing through the exchanger. This
high turbulence increases the heat exchange coefficient of the air improving the heat
amount exchanged by the exchanger.
[0100] Therefore, given an external diameter, it is better to reduce the overall inner volume
using Al pipes.
[0101] Preferably, a module of a temperature difference between a temperature of the process
air at an outlet of the second heat exchanger and the condensation temperature is
lower than 10°C. More preferably, it is smaller than 7°C, even more preferably it
is smaller than 5°C. This means that, called Tpc the temperature of the process air
at the outlet of the condenser and Tcond the temperature of condensation of the refrigerant,
the following equation is satisfied:
[0102] More preferably,
[0103] The more the exchange area of the condenser the less the temperature difference between
the air at the outlet of the condenser and the condensation temperature of the refrigerant.
Therefore increasing the dimensions of the condenser it is possible to obtain the
same airflow at the same temperature at the outlet of the condenser with a lower condensation
temperature of the refrigerant. This is useful for improving performances.
[0104] The temperature is measured at the outlet of the condenser. The following is possible:
a plane parallel to the fagade of the condenser from which the process air exits is
considered. The distance of this plane from the fagade is comprised between 0 cm and
10 cm. In the "standard" parallelepiped shape of the heat exchangers, the median of
the two sides of the condenser defining the fagade are taken on the plane. The measurement
can be taken at the intersection point (the center) or at a plurality of points (at
least 4) along the medians, these point having a distance from the intersection points
of the two medians smaller than 10 cm.
Brief description of the drawings
[0105] Further advantages of the present invention will be better understood with non-limiting
reference to the appended drawings, where:
- Fig. 1 is a perspective view of a laundry dryer realized according to the present
invention;
- Fig. 2 is a perspective view of the laundry dryer of Fig. 1 with an element of the
casing removed for showing some internal components;
- Fig. 3 is a perspective view, in a disassembled configuration, of the basement of
the dryer of Fig. 1 or Fig. 2;
- Fig. 4 is a perspective view of the basement of Fig. 3 with all elements contained
therein removed;
- Figs. 5 is a top view of the basement of Fig. 3;
- Fig. 6 is a perspective view of a heat exchanger, detail of the dryer of figure 3;
- Fig. 7 is a front view of the heat exchanger of figure 6;
- Fig. 8 is a top view of the heat exchanger of figures 6 and 7;
- Fig. 9 is a side view of the heat exchanger of figures 6 - 8;
- Fig. 10 is a simplified view of figure 9;
- Fig. 11 is a graph showing the high pressure (curve above) and low pressure (curve
below) measurements of the pressure of the refrigerant vs. time in a cycle of the
heat pump; and
- Fig. 12 is a schematic drawing of a portion of a compressor, part of the heat pump
of the dryer of figures 1 - 3.
Detailed description of one or more embodiments of the invention
[0106] With initial reference to Figs. 1 and 2, a laundry dryer realized according to the
present invention is globally indicated with 1.
[0107] Laundry dryer 1 comprises an outer box or casing 2, preferably but not necessarily
parallelepiped-shaped, and a drying chamber, such as a drum 3, for example having
the shape of a hollow cylinder, for housing the laundry and in general the clothes
and garments to be dried. The drum 3 is preferably rotatably fixed to the casing 2,
so that it can rotate around a preferably horizontal axis R (in alternative embodiments,
rotation axis may be tilted). Access to the drum 3 is achieved for example via a door
4, preferably hinged to casing 2, which can open and close an opening 4a realized
on the cabinet itself.
[0108] More in detail, casing 2 generally includes a front wall 20, a rear wall 21 and two
lateral walls 25, all mounted on a basement 24. Preferably, the basement 24 is realized
in plastic material. Preferably, basement 24 is molded via an injection molding process.
Preferably, on the front wall 20, the door 4 is hinged so as to access the drum. The
casing, with its walls 20, 21, 25, defines the volume of the laundry dryer 1. Advantageously,
basement 24 includes an upper and a lower shell portion 24a, 24b (visible in Figures
3 - 5 detailed below).
[0109] The dryer 1, and in particular basement 24 is situated generally on a floor.
[0110] Laundry dryer 1 also preferably comprises an electrical motor assembly 50 for rotating,
on command, revolving drum 3 along its axis inside cabinet 2. Motor 50 includes a
shaft 51 which defines a motor axis of rotation M.
[0111] Further, laundry dryer 1 may include an electronic central control unit (not shown)
which controls both the electrical motor assembly 50 and other components of the dryer
1 to perform, on command, one of the user-selectable drying cycles preferably stored
in the same central control unit. The programs as well other parameters of the laundry
dryer 1, or alarm and warning functions can be set and/or visualized in a control
panel 11, preferably realized in a top portion of the dryer 1, such as above door
4.
[0112] With reference to Figure 2, the rotatable drum 3 includes a mantle, having preferably
a substantially cylindrical, tubular body 3c, which is preferably made of metal material
and is arranged inside the casing 2 and apt to rotate around the general rotational
axis R. The mantle 3c defines a first end 3a and a second end 3b and the drum 3 is
so arranged that the first end 3a of the mantle 3c is faced to the laundry loading/unloading
opening realized on the front wall 20 of the casing 2 and the door 4, while the second
end 3b faces the rear wall 21.
[0113] Drum 3 may be an open drum, i.e. both ends 3a and 3b are opened, or it may include
a back wall (not shown in the appended drawings) fixedly connected to the mantle and
rotating with the latter.
[0114] In order to rotate, support elements for the rotation of the drum are provided as
well in the laundry of the invention. Such support elements might include rollers
at the front and/or at the back of the drum, as well as or alternatively a drum shaft
connected to the rear end of the drum (shaft is not depicted in the appended drawings).
In Fig. 2, for example, a roller 10 connected to the basement via a bracket 101a as
well as a roller 10 connected to the rear wall 21 via a boss 101 is depicted. Any
support element for the rotation of the drum around axis R is encompassed by the present
invention.
[0115] Dryer 1 additionally includes a process air circuit which comprises the drum 3 and
a process air conduit 18, depicted as a plurality of arrows showing the path flow
of a process air stream through the dryer 1 (see Figures 3 and 4). In the basement
24, a portion of the process air conduit 18 is formed by the connection of the upper
shell 24a and the lower shell 24b. Process air conduit 18 is preferably connected
with its opposite ends to the two opposite sides of drum 3, i.e. first and second
rear end 3a,3b of mantle 3c. Process air circuit also includes a fan or blower 12
(shown partially in Fig. 5).
[0116] The dryer 1 of the invention additionally comprises a heat pump system 30 including
a second heat exchanger (called also condenser) 31 and a first heat exchanger (called
also evaporator) 32 (see figure 3). Heat pump 30 also includes a refrigerant closed
circuit (partly depicted) in which a refrigerant fluid flows, when the dryer 1 is
in operation, cools off and may condense in correspondence of the condenser 31, releasing
heat, and warms up, in correspondence of the evaporator 32, absorbing heat. A compressor
33 receives refrigerant in a gaseous state from the evaporator 32 and supplies the
condenser 31, thereby closing the refrigerant cycle. In the following the heat exchangers
are named either condenser and evaporator or first and second heat exchanger, respectively.
More in detail, the heat pump circuit connects via piping 35 (see Fig. 3) the evaporator
32 via the compressor 33 to the condenser 31. The outlet of condenser 31 is connected
to the inlet of the evaporator 32 via an expansion device (not visible), such as a
choke, a valve or a capillary tube.
[0117] The refrigerant present in the refrigerant closed circuit of heat pump 30 is in this
preferred embodiment propane.
[0118] As depicted in figure 12, compressor 33 might be a stationary blade type rotary compressor.
The compressor 33 defines a cylindrical chamber 330 having a volume V330. In the chamber
330, an eccentric 332 is mounted, having a volume V332 and rotating by means of a
shaft 337 (for example rotated by a motor, not visible). The eccentric 332 is in contact
to a blade 331 that is mounted on a spring element 338 to slide relative to the compressor
body where the cylindrical chamber 330 is formed. The blade 331 slides due to the
action of the eccentric 332 rotation and the spring element 338 force. The blade 331
divides in a tight manner the cylindrical chamber 330 into two sub-chambers, suction
and compression chambers 333, 334 having respectively volumes V333 and V334. The suction
chamber is fluidly connected to an inlet 335 for the refrigerant, while the compression
chamber 334 is fluidly connected to an outlet 336 for the refrigerant.
[0119] During the heat pump cycle, the shaft 337 rotates and the eccentric 332 rotates eccentrically,
which causes suction work in the suction chamber and compression and discharge work
in the compression chamber. The volume of the compression chamber 334 varies depending
on the position of the eccentric and of the piston. The refrigerant therefore is compressed
by the rotation of the eccentric 332.
[0120] The compression displacement, called Vmax, is defined as:
[0121] Preferably, in correspondence of evaporator 32, the laundry dryer 1 of the invention
may include a condensed-water canister (also not visible) which collects the condensed
water produced, when the dryer 1 is in operation, inside evaporator 32 by condensation
of the surplus moisture in the process air stream arriving from the drying chamber
(i.e. drum) 3. The canister is located at the bottom of the evaporator 32. Preferably,
through a connecting pipe and a pump (not shown in the drawings), the collected water
is sent in a reservoir located in correspondence of the highest portion of the dryer
1 so as to facilitate a comfortable manual discharge of the water by the user of the
dryer 1.
[0122] The condenser 31 and the evaporator 32 of the heat pump 30 are located in correspondence
of the process air conduit 18 formed in the basement 24 (see Figure 3).
[0123] In case of a condense-type dryer - as depicted in the appended figures - where the
air process circuit is a closed loop circuit, the condenser 31 is located downstream
of the evaporator 32. The air exiting the drum 3 enters the conduit 18 and reaches
the evaporator 32, which cools down and dehumidifies the process air. The dry cool
process air continues to flow through the conduit 18 till it enters the condenser
31, where it is warmed up by the heat pump 30 before re-entering the drum 3.
[0124] It is to be understood that in the dryer 1 of the invention, an air heater, such
as an electrical heater, can also be present, in addition to the heat pump 30. In
this case, heat pump 30 and heater can also work together to speed up the heating
process (and thus reducing the drying cycle time). In the latter case, preferably
condenser 31 of heat pump 30 is located upstream the heater. Appropriate measures
should be provided to avoid the electric heater to fuse plastic components of the
dryer 1.
[0125] Further, with now reference to Figures 4 and 5, in the basement, the process air
conduit 18 includes a duct formed by the upper and the lower shells 24a, 24b, having
an inlet 19in from which process air is received from the drum 3 and an outlet 19
to channel process air out of the basement 24. Between inlet 19in and outlet 19, the
duct is formed, preferably as two single pieces joined together and belonging to the
upper and lower shell 24a, 24b, and including a first and a second portion 28 and
29. In the first portion 29 of this duct, seats are formed for locating the first
and the second heat exchangers 31, 32. Preferably, heat exchanger 31, 32 are placed
one after the other, the second heat exchanger 31 being downstream in the direction
of flow of the process air the first heat exchanger 32. Further, the second portion
28 channels the process air exiting from the second heat exchanger 31 towards the
basement outlet 19.
[0126] In the first portion 29 of the basement 24 thus the heat exchangers, and in particular
the condenser 31, are located. The condenser 31 is in contact with the lower shell
24b of the basement 24, which forms a flat portion 29a for the abutment of the condenser.
The points of contact between the basement conduit 18 and the condenser define a plane
indicated with P in figure 5. Plane P is considered the horizontal plane of reference.
In this case, considering the dryer 1 positioned on a flat floor and a flat portion
29a of the basement, the horizontal plane P is parallel to the floor and it is defined
by the standard (X,Y) coordinates. However plane P can be tilted with respect to the
floor.
[0127] Given the P plane, a vertical Z direction can be defined, so that also vertical planes,
like plane V of figure 4, can be defined as well, as planes perpendicular to plane
P.
[0128] In figures 6 - 8 a detailed representation of the heat exchanger 31 or 32 is given.
The heat exchanger 31, 32 comprises a tube or pipe 40 having an inlet 40a and an outlet
40b and including straight parallel sections, all indicated with 41, and bends, all
indicated with 42, connecting the straight parallel sections 41 among each other.
The sections 41 are one above the other, that is, some of the sections 41 lie on the
same vertical plane. The heat exchanger 31, 32 therefore defines several vertical
planes one parallel to the other(s) connecting different groups of sections 41. In
the same manner, several sections may lie on the same horizontal plane, i.e. a group
of sections lie on a plane parallel to the P plane. The heat exchanger 31, 32 therefore
defines several horizontal planes one parallel to the other connecting different groups
of sections 41. The distance between two nearest-neighbour sections 41 belonging to
the same horizontal plane is called the pitch of the row of sections in the same horizontal
plane. The distance between two nearest-neighbour sections belonging to the same vertical
plane is called the pitch of the sections in the same vertical plane. This is schematically
depicted in figure 10.
[0129] Preferably, pipe or tube 40 is realized in Aluminium. Preferably, its external diameter
is comprised between 4 mm and 10 mm.
[0130] A system of coordinates can be defined using plane P, wherein the straight sections
41 are extending along the X direction. This direction is also called the "length"
direction. The Y direction defines a "thickness" direction.
[0131] The bends 42 may be soldered connecting different sections 41 in different planes.
[0132] The sections 41 are surrounded by fins 50. The fins 50 are positioned perpendicularly
to the straight sections 41, that is, they extend along the Y direction. They also
define a pitch, that is, a distance between two nearest-neighbour fins is called the
fins' pitch.
[0133] Preferably, a pitch of the fins of the evaporator is comprised between 1.8 mm and
3.3 mm. Preferably, a pitch of the fins of the condenser is comprised between 1.4
mm and 3.3 mm.
[0134] Preferably, a pitch of the tubes of the first and/or the second heat exchanger is
comprised between 15 mm and 30 mm. Preferably, a pitch of rows of the tubes of the
first and/or the second heat exchanger is comprised between 10 mm and 30 mm.
[0135] Fins have apertures 51 in order to accommodate the sections 41 of the pipe 40. A
view of the apertures is given in the side view of figure 9. The fins 50 then are
in contact with the sections 41. There is no need to have a connection between each
fin 50 and each section 41.
[0136] As shown in detail in figures 6 - 8, each heat exchanger is therefore divided in
three parts: a central part 60 where the fins 50 are present and the tube 40 has the
straight portions 41, and two lateral portions 61, 62 at the two lateral ends of the
central portion 60 free from the fins and including the bends 42.
[0137] The central portion 60 has generally the form of a parallelepiped, with a front surface
70 which is generally a vertical surface where the process air impinges and an exit
surface 71, also vertical, from where the air exits. These surfaces 70, 71 are preferably
perpendicular to the main flow of process air (see for example figure 10). The surfaces
70, 71 are preferably rectangular.
[0138] As visible in figure 3, only the central portion 60 is located inside the process
air conduit formed in the basement 24, and more precisely in its portion 29. The lateral
portions 61, 62 are external to the conduit and are only marginally invested by the
process air.
[0139] In the frame of reference defined, the total length of the condenser 31, which is
the total length of the condenser 31 along the length or X direction, is called Lt.
This length is equal to the length of the central portion (which is generally equal
to the length of each section 41) Le plus the lengths of the two lateral portions
Lc. Assuming that the lengths of the two lateral portions are identical, then
[0140] For the condenser 31 and the evaporator, preferably Lt < 550 mm.
[0141] Further, in the Y direction, the heat exchangers 31, 32 defines a thickness t, which
is substantially the extension of the fins 50 (assuming that all fins have the same
extension) along the Y direction.
[0142] For the condenser and the evaporator, preferably 40 mm < t < 150 mm.
[0143] For both condenser and evaporator, the external volume of the central portion 60
can be calculated. This external volume is called TEV1 for the evaporator and TEV2
for the condenser. In the present embodiment, the tube 40 is substantially a cylinder
and therefore its volume is calculated by the area of a circumference multiplied by
the length of the cylinder.
where Nt= number of tubes,
Le= length of the tubes' sections in the central area,
De= external diameter of the tubes' sections.
[0144] According to the invention TEV2/Vmax > 28. The volume of the compression chamber
is thus much smaller than the total external volume of the condenser.
[0145] In normal operation, when the dryer 1 is switched on, the compressor starts and the
heat pump 30 starts its cycle. The pressure, both low and high (at the inlet and outlet
of the compressor 33, respectively), of the refrigerant starts to increase. The increase
of the pressure takes place in the so called "transient phase" of the heat pump cycle.
This behaviour is depicted in figure 11 (the upper graph is relative to pressure measurements
at the outlet of the compressor, while the lower graph is relative to measurements
of the pressure at the inlet of the compressor). At the end of the transient phase,
the stable phase begins. In the stable phase, the pressure is substantially constant,
or increases/decreases only slightly. The measurements substantially fluctuate around
a substantially constant average value. In Figure 11, in particular, the stable phase
terminates at the end of the heat pump cycle.
[0146] Considering an average X
aver of the pressures' values at the inlet/outlet of the compressor 33, these values are
preferably comprised within the following ranges: