EP3084065A1

EP3084065A1 - Appliance having a heat pump for treating articles

Info

Publication number: EP3084065A1
Application number: EP13811965.6A
Authority: EP
Inventors: Filippo BELLOMARE; Alessandro DALLA ROSA
Original assignee: Electrolux Appliances AB
Current assignee: Electrolux Appliances AB
Priority date: 2013-12-20
Filing date: 2013-12-20
Publication date: 2016-10-26
Anticipated expiration: 2033-12-20
Also published as: WO2015090431A1; PL3084065T3; EP3084065B1

Abstract

The present invention relates to an articles treatment appliance (1), in particular for treating laundry or tableware, having a heat pump system (5), the heat pump system having a refrigerant loop. The appliance (1) comprises: an articles treatment chamber (2) for treating articles using a medium; a first heat exchanger (6) for heating a refrigerant; a second heat exchanger (7) for cooling the refrigerant and heating the medium; a refrigerant expansion device (14) arranged in the refrigerant loop between the second heat exchanger (7) and the first heat exchanger (6), and a compressor (8) arranged in the refrigerant loop between the first heat exchanger (6) and the second heat exchanger (7). The refrigerant is a blend comprising carbon dioxide (CO2) and at least one hydrocarbon (HC).

Description

APPLIANCE FOR TREATING ARTICLES

The present invention relates to appliances for treating articles, and is in particular directed to appliances for treating laundry or tableware, like laundry washers, laundry washers/dryers, dishwashers.

BACKGROUND ART

Appliances for drying laundry, like laundry dryers (tumble dryers) and laundry washers/dryers, generally comprise a drying chamber for accommodating therein the laundry to be dried. A heated and dehumidified drying medium, typically air, is guided through the drying chamber. Upon passing through the drying chamber and laundry, the heated and dehumidified drying medium takes up humidity and at the same time cools down. The drying medium then exits the drying chamber, thereby discharging humidity from the drying chamber and the laundry.

In order to improve the energy efficiency of such appliances, it is known to use heat pumps. In this way, residual heat from the drying medium exiting the drying chamber can be extracted therefrom and transferred again to the drying medium before it re-enters the drying chamber.

The drying medium is cooled down and dehumidified and then heated up in the heat pump system and finally reinserted again into the drying chamber.

The heat pump system typically comprises a refrigerant flowing in a closed-loop refrigerant circuit constituted by a first heat exchanger, a compressor, a second heat exchanger and an expansion device. The first heat exchanger cools and dehumidifies the drying medium leaving the drying chamber while the second heat exchanger heats up the drying medium. The refrigerant flows in the refrigerant circuit where it is compressed by the compressor and expanded in the expansion device. The second heat exchanger which is arranged immediately downstream of the compressor is subjected therefore to higher pressure levels with respect to the first heat exchanger which is arranged immediately downstream of the expansion device.

The temperatures of the drying medium and the refrigerant are strongly correlated to each other.

Traditional refrigerants like halogenated refrigerants, such as HydroChloroFluoroCarbons (HCFC) and HydroFluoroCarbons (HFC) (like R134a and R407C), are known to cause damages to the environment, inter alia by their ozone-destroying properties and most of all by their high Global Warming Potential (GWP), which is the measure of how much a given mass of greenhouse gas is estimated to contribute to global warming.

Recently the international protocols have limited the use of halogenated refrigerants.

Synthetic fluids with lower environmental impact has been introduced, such as HydroFluoroH olefins (HFOs).

Nevertheless, said synthetic fluids are still partially toxic and still have an impact on the environment, in particular in terms of recycling process.

Other heat pumps of known type use carbon dioxide (CO₂) as refrigerant. Carbon dioxide is a natural refrigerant and has advantageously a lower impact on the environment in terms of both ozone-destroying properties and GWP.

A first drawback of appliances with heat pump systems using carbon dioxide (CO₂) as refrigerant is linked to the high critical pressure (73,773 bar) and the low critical temperature (30,978 °C) of such refrigerant, in particular when the heat pump system is used in laundry dryers.

In laundry dryers, in fact, the drying medium (drying air) which leaves the second heat exchanger and enters the drying chamber is heated up to a temperature about 60°C, i.e. quite higher than the critical temperature of the refrigerant (30,978 °C).

This means that the refrigerant in the second heat exchanger is in a supercritical state, i.e. with temperature and pressure above the respective critical temperature (30,978 °C) and pressure (73,773 bar).

The heat pump, in particular the second heat exchanger, is therefore affected by high pressure levels and the components of the heat pump itself must be opportunely designed to support high pressure levels, thus requiring expensive components.

High pressure levels, then, reduce the reliability of the system and increase risks of failure.

Heat pump system working with high pressure levels and pressure above the critical pressure, then, typically requires a back pressure valve arranged downstream of the second heat exchanger which controls the pressure of the refrigerant in order to maintain it within a safety range.

This complicates the heat pump design and increases the size and manufacturing costs.

Furthermore, intervention of the back pressure valve causes pressure losses of the refrigerant which is detrimental to the efficiency of the heat pump.

Moreover, a heat pump system where the refrigerant is in a supercritical state at least in the second heat exchanger, i.e. with pressure levels above the critical pressure, shows a low efficiency.

EP2053159 discloses the use of propane (R290) as refrigerant. Propane can be considered an alternative to halogenated refrigerants, thanks to its low environmental impact in terms of GWP. Nevertheless, a main drawback of propane is its high flammability and the system requires dedicated protection against fire hazard.

The main object of the present invention is therefore to overcome said drawbacks.

In particular, it is one object of the present invention to provide appliances for treating articles, in particular appliances for treating laundry or tableware, having an increased energy efficiency.

It is another object of the present invention to provide appliances for treating articles, in particular appliances for treating laundry or tableware, having an at least reduced environmental impact.

It is a further object of the present invention to provide appliances for treating articles, in particular appliances for treating laundry or tableware, having reduced manufacturing costs.

A further object of the present invention is to provide appliances for treating articles, in particular appliances for treating laundry or tableware, ensuring the safety requirements.

DISCLOSURE OF INVENTION

The Applicant has found that an energy-efficient refrigerant with low environmental impact and that can be expediently used, being particularly efficient due to its thermodynamical properties, in heat-pump article treatment appliances, particularly appliances for treating laundry or tableware, is a fluid which is a blend comprising carbon dioxide and at least one hydrocarbon.

In a first aspect the present invention relates, therefore, to an articles treatment appliance, in particular for treating laundry or tableware, having a heat pump system, the heat pump system having a refrigerant loop, the appliance comprising:

an articles treatment chamber for treating articles using a medium;

a first heat exchanger for heating a refrigerant;

a second heat exchanger for cooling the refrigerant and heating the medium; a refrigerant expansion device arranged in the refrigerant loop between the second heat exchanger and the first heat exchanger, and

a compressor arranged in the refrigerant loop between the first heat exchanger and the second heat exchanger,

wherein the refrigerant is a blend comprising carbon dioxide and at least one hydrocarbon.

Preferably, the refrigerant is a blend that allows obtaining a subcritical thermodynamic cycle.

Advantageously, the temperature of the refrigerant during the isobaric condensation phase in the second heat exchanger is lower than the critical temperature of the refrigerant.

The percentage in weight of said carbon dioxide in the refrigerant is comprised between 10% and 70%, preferably comprised between 20% and 60%, preferably comprised between 20% and 40%, preferably comprised between 25% and 35% and more preferably equal to 30%.

In a preferred embodiment of the invention, the at least one hydrocarbon is preferably one of the following hydrocarbons: Propane, Butane, Isobutane, Ethane or Propene.

In a preferred embodiment of the invention, the refrigerant is a blend comprising carbon dioxide and R44 la.

Preferably, the difference between the critical temperature of the refrigerant and the temperature of the medium at the second heat exchanger outlet is comprised between +40°C and -40°C, preferably between +30°C and -30°C, more preferably between +20°C and -20°C.

In further preferred embodiments of the invention, the critical temperature of the refrigerant is equal or higher than the temperature of the medium at the second heat exchanger outlet.

Preferably, the difference between the critical temperature of the refrigerant and the temperature of the medium at the second heat exchanger outlet is comprised between +40°C and 0°C, preferably between +30°C and 10°C, preferably between +25°C and 15°C, more preferably equal to 20°C.

In further preferred embodiments of the invention, the critical temperature of the refrigerant is lower than the temperature of the medium at the second heat exchanger outlet.

Preferably, the critical temperature of the refrigerant is comprised between 100°C and 34°C, preferably between 90°C and 40°C, preferably between 80°C and 60°C, preferably between 75°C and 65°C more preferably equal to 70°C.

In a preferred embodiment of the invention, the percentage in weight of the carbon dioxide in the refrigerant is comprised between 25% and 35% , the difference between the critical temperature of the refrigerant and the temperature of the medium at the second heat exchanger outlet is between +25 °C and 15°C and the critical temperature of the refrigerant is between 75°C and 65°C.

In a more preferred embodiment of the invention, the percentage in weight of the carbon dioxide in the refrigerant is substantially equal to 30%, the difference between the critical temperature of the refrigerant and the temperature of the medium at the second heat exchanger outlet is substantially equal to +20°C and the critical temperature of the refrigerant is substantially equal to 70°C.

Preferably, in the second heat exchanger and during the isobaric condensation phase, the temperature of the refrigerant decreases from a higher temperature (dew temperature) to a lower temperature (bubble temperature).

Preferably, in the first heat exchanger and during the isobaric evaporation phase, the temperature of the refrigerant increases from a lower temperature to a higher temperature (dew temperature).

Preferably, the first heat exchanger is adapted for cooling the medium. In this case, preferably, the appliance is a laundry dryer.

In a preferred embodiment of the invention, the nominal first heat exchanger inlet temperature of the medium is about 40° C at the least.

More preferably, the nominal first heat exchanger inlet temperature of the medium is comprised between 100 °C and 50°C, preferably between 70°C and 50°C, preferably between 60°C and 50°C, preferably between 60°C and 55°C, more preferably equal to 58°C.

Preferably, a nominal second heat exchanger outlet temperature of the medium is 100 °C at the most.

More preferably, the nominal second heat exchanger outlet temperature of the medium is comprised between 100 °C and 50°C, preferably between 70°C and 50°C, preferably between 60°C and 55°C, more preferably equal to 58°C.

Preferably the appliance comprises a closed-loop circuit wherein the medium circulates.

In a preferred embodiment of the invention, the medium comprises washing water for washing the articles.

In a further preferred embodiment of the invention, the medium comprises drying air for drying said articles.

In a preferred embodiment of the invention, the appliance comprises a carbon dioxide detector.

Advantageously, the leakage of the refrigerant may be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be made clearer by reading the following detailed description of exemplary and non- limitative embodiments thereof, referring to the following drawing figures, wherein:

- Figure 1 schematically shows a heat pump laundry dryer according to an embodiment of the present invention;

- Figure 2 shows the temperature-entropy diagram of a refrigerant according to a first preferred embodiment of the present invention;

- Figure 3 shows the temperature-entropy diagram of a refrigerant according to another preferred embodiment of the present invention;

- Figure 4 shows the temperature-entropy diagram of a refrigerant according to another preferred embodiment of the present invention;

- Figure 5 is an isometric view of the heat pump laundry dryer, with one lateral wall removed, and

- Figure 6 shows in exploded view a basement of the laundry dryer of Figure 5 configured for accommodating the heat pump.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 schematically shows a heat pump laundry dryer 1 according to an embodiment of the present invention. It is pointed out that although in the following description a heat pump laundry dryer is considered, this choice is merely exemplary, because the present invention applies generally to any appliance for treating articles, in particular for treating laundry or tableware, like laundry washers, laundry washers/dryers, laundry dryers, dishwashers equipped with a heat pump in heat-exchange relationship with an article treatment medium that can be a washing medium or a drying medium.

The heat pump laundry dryer 1 comprises a drying chamber 2, preferably a rotatable drum. In operation, the drying drum 2 accommodates wet laundry 3 to be dried. For adequately drying the laundry 3, a drying medium 4, such as air, in particular comprising ambient air, is circulated through the drying drum 2 via a drying medium circuit, which preferably forms a closed-loop circuit.

For drying the laundry 3, the drying medium 4 heated to a temperature of 100°C at the most and thereby having a comparatively low relative humidity is fed into the drying drum 2 and impinges the wet laundry 3. As a consequence, humidity of the wet laundry 3 is absorbed by the drying medium 4 thereby drying the laundry 3. As the laundry 3 in the drying drum 2 generally has a temperature lower than drying medium 4 temperature entering the drying drum 2, the drying medium 4 also cools down, for example to temperatures of about 40°C.

After having passed through the drying drum 2, the drying medium 4, having a comparatively high relative humidity, exits the drying drum 2 and is further cooled down to condense excess humidity therefrom. After that, the drying medium 4 is recirculated through the drying drum 2. Before re-entering the drying drum 2, the drying medium 4 is heated up again, thereby reducing its relative humidity.

More preferably, the drying medium 4 is heated to a temperature comprised between 100 °C and 50°C, preferably between 70°C and 50°C, preferably between 60°C and 50°C, preferably between 60°C and 55°C, more preferably at a temperature equal to 58°C.

For dehumidifying and reheating the drying medium 4, the heat pump laundry dryer 1 comprises a heat pump system or unit 5. The heat pump unit 5 exemplarily comprises a refrigerant evaporator 6 and a refrigerant liquefier 7.

The heat pump unit 5 further comprises a compressor 8 interconnected between the refrigerant evaporator 6 and the refrigerant liquefier 7. A refrigerant evaporator outlet 9 is connected to a compressor inlet 10 and a compressor outlet 11 is connected to a refrigerant liquefier inlet 12.

A refrigerant liquefier outlet 13 is connected via a throttling element 14, a capillary for example, to a refrigerant evaporator inlet 15.

By the heat pump unit 5, heat is transferred from the refrigerant evaporator 6 to the refrigerant liquefier 7.

A refrigerant circulates in the heat pump unit closed circuit, is heated up at the refrigerant evaporator 6 and cooled down at the refrigerant liquefier 7.

In the refrigerant liquefier 7, in particular, the refrigerant is cooled down and also condensed (liquefied).

The relatively low temperature of the refrigerant at the refrigerant evaporator 6 is used to cool down the drying medium 4 so as to condensate humidity, i.e. to dehumidify the drying medium 4 exiting the drying drum 2.

The elevated temperature of the refrigerant at the refrigerant liquefier 7 is used to reheat the drying medium 4 which in turn is then fed to the drying drum 2 for drying the laundry 3.

In further embodiments, the drying medium may not form a closed-loop. In this case, for example, the drying medium may be conveyed to the refrigerant liquefier 7 from outside, then conveyed into the drying drum 2, from the drying drum 2 conveyed to the refrigerant evaporator 6 and finally expelled to the outside.

The heat pump unit 5 may further comprise auxiliary heat exchangers for further optimizing energy efficiency. For example, an auxiliary refrigerant evaporator and an auxiliary refrigerant liquefier may be provided. The number of auxiliary refrigerant heat exchangers can be varied from one to nearly any arbitrary number. An auxiliary refrigerant evaporator may be used to speed up the heat-up phase of the heat pump dryer and a refrigerant liquefier may be used to balance the excess of energy of the heat pump dryer.

The direction of refrigerant flow is indicated in Figure 1 by small arrows, whilst the flow of the drying medium 4 is indicated by larger and broader arrows. The heat pump laundry dryer 1 may comprise a fan 16 adapted to and designed for circulating the drying medium 4 within the heat pump laundry dryer circuit. In connection with the heat pump laundry dryer 1 described so far, the Applicant has found that, both for energetic efficiency and for environmental compatibility, it is of considerable advantage to use a refrigerant that is a blend comprising carbon dioxide C0₂ and at least one hydrocarbon HC.

The Applicant has found that a suitable mix of carbon dioxide and at least one hydrocarbon is a blend that allows obtaining a subcritical thermodynamic cycle, as will be described in detail hereinafter in particular with reference to preferred embodiments of the invention.

In particular, the Applicant has found that the temperature of the new refrigerant during the isobaric condensation phase in the refrigerant liquefier is lower than the critical temperature Tk of the refrigerant.

The Applicant has found to be of particular advantage the use of a suitable mix of carbon dioxide and at least one hydrocarbon wherein the percentage in weight of carbon dioxide is preferably comprised between 10% and 70%, preferably comprised between 20% and 60%, preferably comprised between 20% and 40%, preferably comprised between 25% and 35% and more preferably equal to 30%. Preferably, a suitable mix of carbon dioxide and at least one hydrocarbon is a blend wherein the hydrocarbon is preferably one of the following hydrocarbons: Propane (R290), Butane (R600), Isobutane (R600a), Ethane (R170) or Propene (R1270).

It is clear that the mixture may comprise only one of said hydrocarbons or, alternatively, a combination of two or more of said hydrocarbons.

A particularly preferable blend comprises 30% of carbon dioxide and 70% of Propane (R290).

Another preferred blend comprises 30% of carbon dioxide and 70% of other hydrocarbons mixed together, wherein the weight ratio composition of the hydrocarbons may be, for example, 25 : 15 : 10 : 10 : 10 of Propane : Butane : Isobutane : Ethane : Propene, respectively.

In a further preferred embodiment, for example, the refrigerant may be a blend comprising carbon dioxide and R441a, wherein R441a is a blend composed by Ethane (C₂H₆), Propane (C₃H₈), Buthane (C₄H₁₀) and Isobuthane (2-metil Propane) (C₄H₁₀).

Advantageously, the refrigerant according to the invention is composed by natural fluids, so it is more eco-friendly than both HFCs and HFOs of known type. The refrigerant of the invention does not contain, for example, Halogen atoms such as CI (Chlorine) and F (Fluorine) but contains only C (carbon) and H (hydrogen) atoms.

Advantageously, the natural fluids of the refrigerant according to the invention does not require particular operations in terms of recycling process.

Advantageously, the GWP of the refrigerant according to the invention is close to zero. The use of natural fluids in the refrigerant according to the invention, then, allows the use of mineral lubricant oils dispersed therein.

It is known to use lubricant oils dispersed in the refrigerant in a heat pump unit in order to guarantee the correct functioning of the compressor.

In the refrigerant of known type, such as HFCs and HFOs, the lubricant oils used are composed by synthetic oils, while mineral lubricant oils cannot be used since they are not soluble.

In the refrigerant of the invention, on the contrary, natural mineral lubricant oils can be used since they are soluble in natural fluids. Environmental impact is advantageously therefore further reduced.

In the refrigerant of the invention, anyway, synthetic oils may also be still used. Furthermore, the new blend comprising carbon dioxide and at least one hydrocarbon shows a lower critical pressure and a higher critical temperature compared to the refrigerant using exclusively carbon dioxide.

The refrigerant of the invention, therefore, may work in a subcritical thermodynamic cycle, as said above. The heat pump unit 5 is therefore affected by lower pressure levels and the components of the same heat pump unit 5 may be opportunely designed to support lower pressure levels. This leads to the use of cheaper components and therefore to reduced manufacturing costs.

Lower pressure levels, then, increase the reliability of the system and reduce risks of failure.

Advantageously, then, thanks to lower pressure levels the heat pump unit 5 of the invention does not require a back pressure valve as in known system. In particular in the heat pump unit 5, and more particularly in the refrigerant liquefier 7, the refrigerant is in subcritical state, i.e. with pressure levels below the critical pressure. Therefore, the use of a back pressure valve arranged downstream of the refrigerant liquefier 7 as in known system, is avoided.

This simplifies the design of the heat pump unit 5, limits its size and reduces the manufacturing costs.

Furthermore, pressure losses caused by a back pressure valve are avoided and therefore the efficiency of the heat pump unit 5 is higher than that of known system.

Furthermore, the subcritical thermodynamic cycle shows a higher efficiency than a transcritical cycle.

Another advantageous characteristic of the refrigerant comprising carbon dioxide and at least one hydrocarbon is that it is a zeotropic blend. Pure fluids or azeotropic blends, are characterized by the fact that the temperature stays at a constant level during isobaric change of phase (evaporation and condensation). Zeotropic blends are instead characterized by the fact that the temperature does not remain constant during the isobaric change of phases (it increases during the evaporation phase and it decreases during the condensation phase). The temperature at which the evaporation and condensation phases occur depends on the refrigerant pressure and it is called "saturation temperature". In case of azeotropic fluids, the temperature of the saturated liquid (that is called "bubble temperature") and the temperature of the saturated vapour (that is called "dew temperature") are the same. In case of zeotropic blends, the bubble temperature is lower than the dew temperature. The difference between these two values (dew temperature minus bubble temperature) is called "glide".

In a heat pump, the presence of a certain level of glide allows a better matching between the drying medium and the refrigerant temperature profiles at the refrigerant evaporator and at the refrigerant liquefier. The temperature difference between the drying medium and the refrigerant decreases in case of zeotropic refrigerant, then the efficiency of the heat pump unit increases.

The zeotropic nature of the refrigerant comprising carbon dioxide and at least one hydrocarbon distinguishes this fluid from other fluids which does not have glide (for example the R134a fluid) or which has a low glide level (for example

R407C fluid).

Figure 2 shows the temperature-entropy diagram relating to the refrigerant thermodynamic cycle in the case of a preferred embodiment of the refrigerant comprising a percentage in weight of carbon dioxide substantially equal to 30% and a percentage in weight of propane substantially equal to 70%; the entropy (S, in [kJ/kg K]) is on the abscissa, while the temperature (T, in [°C]) is on the ordinate. SLC denotes the saturated liquid curve and SVC denotes the saturated vapour curve. On the same diagram the temperature profile of the drying medium 4 (dashed line) is also shown (only the temperature value is relevant; the entropy values refer only to the refrigerant).

Hereinafter, the term "drying air" will also be used as a synonym of the term "drying medium".

The cycle depicted in solid line (from state 1 to state 2, from state 2 to state 3, from state 3 to state 4 and from state 4 back to state 1) represents the thermodynamic cycle of the refrigerant; in particular, state 1 is the state of the refrigerant at the refrigerant liquefier inlet 12, state 2 is the state of the refrigerant at the refrigerant liquefier outlet 13, state 3 is the state of the refrigerant at the refrigerant evaporator inlet 15 and state 4 is the state of the refrigerant at the refrigerant evaporator outlet 9.

In the cycle here described, preferably, a degree of superheating is maintained at the refrigerant evaporator outlet 9, as illustrated by the line "SH" in figure 2. This preferably assures that no refrigerant in liquid form enters the compressor 8. In different embodiments, nevertheless, superheating may not be present.

Analogously, in preferred embodiments a degree of subcooling may be preferably maintained at the refrigerant liquefier outlet 13.

In respect of the drying air 4, Ta is the temperature of the drying air 4 upon leaving the drum 2 and entering the refrigerant evaporator 6 (point A in the diagram), Tb is the temperature of the drying air 4 after having passed through the refrigerant evaporator 6 and before passing through the refrigerant liquefier 7 (point B), and Tc is the temperature of the drying air 4 after having passed through the refrigerant liquefier 7 before re-entering the drum 2, i.e. at the refrigerant liquefier outlet 13 (point C).

In the diagram also the critical temperature Tk of the refrigerant is also depicted. From the diagram, it can be appreciated that the temperature of the refrigerant does not remain constant during the refrigerant transitions of phase, i.e. a certain level of glide is present. In particular in the refrigerant liquefier 7 and during the isobaric condensation phase, the temperature of the refrigerant from a high temperature Tdl (dew temperature) decreases to a lower temperature Tbl (bubble temperature). Analogously, in the refrigerant evaporator 6 and during the isobaric evaporation phase, the temperature of the refrigerant from a low temperature Tb2 increase to a higher temperature Td2 (dew temperature).

From the diagram it can be appreciated that the refrigerant of the invention works in a subcritical thermodynamic cycle, since the temperature of the refrigerant during the isobaric condensation phase in the refrigerant liquefier 7 is lower than the critical temperature Tk of the refrigerant.

It can be further appreciated from the diagram that there is a good matching between the temperature of the refrigerant and that of the drying air 4. "Good matching" means that the course of the temperature of the drying air 4 passing the refrigerant liquefier 7 (from B to C) is close to the course of the temperature of refrigerant in the same refrigerant liquefier 7 (from Tdl to Tbl) and also the course of the temperature of the drying air 4 passing the refrigerant evaporator 6 (from A to B) is close to the course of the temperature of refrigerant in the same refrigerant evaporator 6 (from Tb2 to Td2).

Advantageously, the drying air 4 can be heated up at a higher level, or the average condensation temperature can be lower in the refrigerant liquefier, and the drying air 4 can be cooled down at a lower level, or the average evaporation temperature can be higher in the refrigerant evaporator. This leads to a higher heating and cooling capacity, or to a lower power consumption keeping the same thermal capacity (the closer the condensation and evaporation temperature levels, the higher the efficiency of the heat pump system).

Furthermore, from the diagram it can be appreciated that the difference between the critical temperature Tk of the refrigerant and the temperature Tc of the drying air 4 at the refrigerant liquefier outlet 13 (i.e. Tk-Tc) is positive. Such difference Tk-Tc is about 20°C.

Figure 3 shows the temperature-entropy diagram relating to the refrigerant thermodynamic cycle in the case of a refrigerant comprising a low concentration of carbon dioxide (namely the percentage in weight of carbon dioxide is substantially equal to 10%).

Also in this case, it can be appreciated that the temperature of the refrigerant does not remain constant during the refrigerant transitions of phase, i.e. a certain level of glide is present. In particular in the refrigerant liquefier 7 and during the isobaric condensation phase, the temperature of the refrigerant from a high temperature T'dl (dew temperature) decreases to a lower temperature T'bl (bubble temperature). Analogously, in the refrigerant evaporator 6 and during the isobaric evaporation phase, the temperature of the refrigerant from a low temperature T'b2 increase to a higher temperature T'd2 (dew temperature).

Also in this case, it can be further appreciated from the diagram that there is a good matching between the temperature of the refrigerant and that of the drying air 4. The same advantages above described are therefore achieved.

Furthermore, from the diagram it can be appreciated that the difference between the critical temperature T'k of the refrigerant and the temperature T'c of the drying air 4 at the liquefier outlet 13 (i.e. T'k-T'c) is positive. Such difference T'k-T'c is about +40°C.

Figure 4 shows the temperature-entropy diagram relating to the refrigerant thermodynamic cycle in the case of a refrigerant comprising a high concentration of carbon dioxide (namely the percentage in weight of carbon dioxide C0₂ is substantially equal to 70%)

Also in this case, it can be appreciated that the temperature of the refrigerant does not remain constant during the refrigerant transitions of phase, i.e. a certain level of glide is present. In particular in the refrigerant liquefier 7 and during the isobaric condensation phase, the temperature of the refrigerant from a high temperature T 'dl (dew temperature) decreases to a lower temperature T"bl (bubble temperature). Analogously, in the refrigerant evaporator 6 and during the isobaric evaporation phase, the temperature of the refrigerant from a low temperature T"b2 increase to a higher temperature T"d2 (dew temperature). Also in this case, it can be appreciated that the refrigerant of the invention works in a subcritical thermodynamic cycle, since the temperature of the refrigerant during the isobaric condensation phase in the refrigerant liquefier 7 is lower than the critical temperature T 'k of the refrigerant.

Furthermore, from the diagram it can be appreciated that the difference between the critical temperature T 'k of the refrigerant and the temperature T 'c of the drying air 4 at the refrigerant liquefier outlet 13 (i.e. T"k-T"c) is negative. Such difference T"k-T"c is about -40°C.

The Applicant has found that a suitable mix of carbon dioxide and at least one hydrocarbon is a blend that allows the difference between the critical temperature Tk, T'k, T 'k of the refrigerant and the temperature Tc, T'c, T 'c of the drying air 4 at the refrigerant liquefier outlet 13, to be comprised between +40°C and - 40°C, preferably between +30°C and -30°C, more preferably between +20°C and -20°C.

Also, the Applicant has found that a suitable mix of carbon dioxide and at least one hydrocarbon is a blend that allows the critical temperature Tk, T'k of the refrigerant to be equal or higher than the temperature Tc, T'c of the drying air 4 at the refrigerant liquefier outlet 13.

In this case, preferably, the difference between the critical temperature Tk, T'k of the refrigerant and the temperature Tc, T'c of the drying air 4 at the refrigerant liquefier outlet 13 is comprised between +40°C and 0°C, preferably between +30°C and 10°C, preferably between +25°C and 15°C, more preferably equal to 20°C.

Furthermore, the Applicant has found to be of particular advantage that the critical temperature Tk, T'k, T"k of the refrigerant is comprised between 100°C and 34°C, preferably between 90°C and 40°C, preferably between 80°C and 60°C, preferably between 75°C and 65°C more preferably equal to 70°C.

Figure 5 is an isometric view of an exemplary laundry dryer 1. The laundry dryer 1 comprises a cabinet 700, having lateral walls (one of which has been removed in the drawing) and housing the drying drum 2 and the heat pump unit 5.

The heat pump unit 5 is for example housed in an appliance basement 705, which is shown per se in Figure 6. The basement 705 is for example a shell composed of two half-shells 805 and 810 designed to match each other so that, when matched, they define inside them a space for accommodating the heat pump unit parts, like the refrigerant evaporator 6, the refrigerant liquefier 7, the compressor 8, the capillary 14 and passageways for the drying air 4.

In a preferred embodiment of the invention, not illustrated, the laundry dryer may also be provided with a carbon dioxide detector (or sensor).

For example, infrared gas sensors (NDIR) or chemical gas sensors may be used. This sensor advantageously detects the presence of carbon dioxide in case of refrigerant leakage.

In case of refrigerant leakage, carbon dioxide will leak out in advance with respect to the remaining part of the refrigerant which comprises one or more hydrocarbons. This is due to the higher volatility of the carbon dioxide compared to that of the hydrocarbons.

The sensor, therefore, will immediately detect the carbon dioxide leakage before the hydrocarbons leakage occurs and will preferably and advantageously send an alarm signal.

The alarm signal may be opportunely elaborated from the central processing unit of the laundry dryer in order to take proper actions to avoid risks.

In particular, the processing unit will take proper actions to reduce/eliminate the risk of fire and/or explosion of leaking hydrocarbons.

It should be noted that in case the laundry dryer is not equipped with a carbon dioxide detector, the risk of fire and/or explosion of leaking hydrocarbons is in any case reduced with respect to the case of a refrigerant totally composed of hydrocarbons. The presence of a percentage of carbon dioxide in the blend advantageously mitigates the flammability of hydrocarbons, thanks to the lower quantity of hydrocarbons (HCs).

On the contrary, know system totally composed of hydrocarbons (HCs) requires dedicated protection system against any fire hazard.

It has thus been shown that the present invention allows the set objects to be achieved. In particular, it makes it possible to obtain a appliance for treating articles, in particular an appliance for treating laundry or tableware, having an increased energy efficiency.

Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the present invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention. All such changes and modifications are intended to be included within the scope of the invention as defined by the appended claims.

Claims

1. Articles treatment appliance (1), in particular for treating laundry or tableware, having a heat pump system (5), the heat pump system having a refrigerant loop, the appliance (1) comprising:

an articles treatment chamber (2) for treating articles using a medium;

a first heat exchanger (6) for heating a refrigerant;

a second heat exchanger (7) for cooling the refrigerant and heating the medium; a refrigerant expansion device (14) arranged in the refrigerant loop between the second heat exchanger (7) and the first heat exchanger (6), and

a compressor (8) arranged in the refrigerant loop between the first heat exchanger

(6) and the second heat exchanger (7),

wherein said refrigerant is a blend comprising carbon dioxide (C0₂) and at least one hydrocarbon (HC).

2. The appliance (1) of claim 1, wherein said refrigerant is a blend that allows obtaining a subcritical thermodynamic cycle wherein the temperature of said refrigerant during the isobaric condensation phase in said second heat exchanger

(7) is lower than the critical temperature of said refrigerant.

3. The appliance (1) of any one of the preceding claims, wherein the percentage in weight of said carbon dioxide (C0₂) in said refrigerant is comprised between

10% and 70%, preferably comprised between 20% and 60%, preferably comprised between 20% and 40%, preferably comprised between 25% and 35% and more preferably equal to 30%.

4. The appliance (1) of any one of the preceding claims, wherein said at least one hydrocarbon (HC) is preferably one of the following hydrocarbons: Propane,

Butane, Isobutane, Ethane or Propene.

5. The appliance (1) of any one of the preceding claims, wherein the difference between the critical temperature (Tk; T'k; T"k) of said refrigerant and the temperature (Tc, T'c, T"c) of said medium (4) at the second heat exchanger outlet (13) is comprised between +40°C and -40°C, preferably between +30°C and -30°C, more preferably between +20°C and -20°C

6. The appliance (1) of any one of the preceding claims from 1 to 4, wherein the critical temperature (Tk, T'k) of said refrigerant is equal or higher than the temperature (Tc, T'c) of said medium (4) at the second heat exchanger outlet (13).

7. The appliance (1) of claim 6, wherein the difference between the critical temperature (Tk, T'k) of said refrigerant and the temperature (Tc; T'c) of said medium (4) at the second heat exchanger outlet (13) is comprised between +40°C and 0°C, preferably between +30°C and 10°C, preferably between +25°C and 15°C, more preferably equal to +20°C.

8. The appliance (1) of any one of the preceding claims from 1 to 5, wherein the critical temperature (T"k) of said refrigerant is lower than the temperature (T"c) of said medium (4) at the second heat exchanger outlet (13).

9. The appliance (1) of any one of the preceding claims, wherein the critical temperature (Tk; T'k; T"k) of said refrigerant is comprised between 100°C and

34°C, preferably between 90°C and 40°C, preferably between 80°C and 60°C, preferably between 75°C and 65°C more preferably equal to 70°C.

10. The appliance (1) of any one of the preceding claims, wherein the percentage in weight of said carbon dioxide (C0₂) in said refrigerant is comprised between 25% and 35% , wherein the difference between the critical temperature (Tk) of said refrigerant and the temperature (Tc) of said medium (4) at the second heat exchanger outlet (13) is between +25°C and 15°C and wherein the critical temperature (Tk) of said refrigerant is between 75°C and 65°C.

11. The appliance (1) of any one of the preceding claims, wherein said refrigerant is a blend comprising carbon dioxide (C0₂) and R441a.

12. The appliance (1) of any one of the preceding claims, wherein in said second heat exchanger (7) and during the isobaric condensation phase, the temperature of said refrigerant decreases from a higher temperature (Tdl ; T'dl; T"dl) to a lower temperature (Tb 1 ; T ' b 1 ; T " b 1 ) .

13. The appliance (1) of any one of the preceding claims, wherein in said first heat exchanger (6) and during the isobaric evaporation phase, the temperature of said refrigerant increases from a lower temperature (Tb2; T'b2; T"b2) to a higher temperature (Td2; T'd2; T"d2).

14. The appliance (1) of any one of the preceding claims, wherein the first heat exchanger (6) is adapted for cooling the medium and wherein said appliance is preferably a laundry dryer (1).

15. The appliance (1) of claim 14, wherein a nominal first heat exchanger inlet temperature of the medium is about 40°C at the least.

16. The appliance (1) of any one of the preceding claims, wherein said medium comprises washing water for washing said articles or drying air for drying said articles.

17. The appliance (1) of any one of the preceding claims, wherein a nominal second heat exchanger outlet temperature of the medium is 100°C at the most.

18. The appliance (1) of any one of the preceding claims, comprising a closed- loop circuit wherein the medium circulates.

19. The appliance (1) of any one of the preceding claims, comprising a carbon dioxide detector for detecting a refrigerant leakage.