Appliance including a heat pump

Abstract

 The invention relates to an appliance (1) including a treating chamber (3) in which a process medium is introduced; a heat pump (6) having a refrigerant circuit (68) in which a refrigerant can flow, the refrigerant circuit including a first heat exchanger (61) where the refrigerant is cooled off and said process medium is warmed up, a second heat exchanger (62) where the refrigerant is heated up, a compressor (63) to pressurize and circulate the refrigerant through the refrigerant circuit, the compressor including a lubricant, and a pressure-lowering device (64). The first and second heat exchanger, compressor and pressure-lowering device are operatively connected through refrigerant circuit piping (65) into which the refrigerant is apt to flow. The refrigerant circuit piping (65) comprises at least a narrow piping portion having a narrow hydraulic diameter (D_Hnarrow), and the ratio between the narrow hydraulic diameter (D_Hnarrow) and the hydraulic diameter (D_H) of the remaining piping portions of the refrigerant circuit piping (65) is comprised between 0.4 and 0.95. The at least one narrow piping portion being located in at least one of:
- between an outlet of the second heat exchanger (62) and an inlet of the compressor (63a), or
- Between an outlet of the first heat exchanger (61) and an inlet of the pressure-lowering device (64), or
- Between an outlet of the pressure-lowering device (64) and an inlet of the second heat exchanger (62), or
- below or at the suction inlet level (L) of the compressor (63).

Description

[0001] The invention relates to an appliance and in particular to an appliance having a heat pump system.

[0002] Laundry dryers usually comprise a drying chamber arranged for containing items to be dried, a blower for circulating hot air and feeding hot air into the drying chamber, and a heating system for heating air to be fed into the drying chamber for drying the items contained therein.

[0003] Heat pump technology has been recently applied to laundry dryers in order to enhance the efficiency in drying clothes. More generally, heat pumps have been applied nowadays to a plurality of different appliances, such as dish washers, washing machines, washer-dryers and tumble dryers, to enhance the efficiency of their functioning.

[0004] In traditional heat pump dryer, the heating system comprises a refrigerant circuit in which humid hot air coming from the drum is fed so that, by means of a refrigerant fluid, the humidity contained in the hot air is made to condense and is therefore discharged, whilst hot dry air is again fed to the drum. More in detail, the air, moved by a fan, passes through the drum removing water from wet clothes, and then it is cooled down and dehumidified in a heat pump evaporator and heated up in a heat pump condenser to be re-inserted into the drum.

[0005] The refrigerating fluid is moved and compressed by a compressor.

[0006] A lubricant, usually a lubrication oil, is provided in the compressor of the refrigerant circuit to promote the safe hydrodynamic lubrication inside the same.

[0007] The lubrication oil creates a thin film within the moving parts of the compressor, such as pistons, shaft, bearings, etc. thus reducing the wear of the moving parts due to the friction.

[0008] The lubricant oil can also contribute to cool the electric motor of the compressor. This effect is important mainly in semi-hermetic or hermetic compressors in which the heat exchange with the environment is very low.

[0009] Laundry dryers of the above mentioned type are known for example from EP 1405946.

[0010] The dryer described in European patent EP 1405946 comprises a drying chamber in which items to be dried are fed, and a rotary compressor that constitutes together with the evaporator, the expansion valve and the gas cooler connected in an annular shape a refrigerant circuit. The rotary compressor is an internal middle pressure multistage compression type which uses CO₂ as a refrigerant. The rotary compressor comprises a cylindrical airtight container, and a rotary compression mechanism section which is constituted of a first rotary compression element (first stage) and a second rotary compression element (second stage) driven by a rotary shaft. The mechanical parts of the compressor are lubricated by oil fed form a reservoir by means of relevant holes to the suitable part of the compressor. The moisture contained in the air coming from the drying chamber is condensed to be discharged by the evaporator.

[0011] American patent application US 2005/0086827 describes a clothes drying machine provided with a heating pump constituted of a compressor, a heating coil, an expansion valve, and a cooling coil and capable of circulating a heat exchange medium. The items to be dried are dried by the high-temperature air heated by the heating coil, and moisture evaporated from the dried things is condensed and discarded by the cooling coil.

[0012] Also in the dryer disclosed in US 2005/0086827, a multistage compressor is provided. More in general, the heat pump of known appliances includes a refrigerant circuit in which a refrigerant can flow and which connects via piping a first heat exchanger or condenser, a second heat exchanger or evaporator, 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. 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. The refrigerant then returns to the compressor and the cycle is repeated.

[0013] In the compressor used in the known heat pump systems, some lubricant may flow from the compressor into the refrigerant circuit together with the refrigerant fluid and mix with the latter.

[0014] Depending on the type of refrigerant used in the heat pump, the lubricant oil and the refrigerant can be mutually completely miscible so that the liquid phase has only one phase and a homogeneous solution is formed between the two fluids; or the two fluids can be partially miscible so that in some proportion, they do not form a solution, but two separate liquid phases with different composition are formed at defined pressure and temperature levels. In general, the solubility increases with the pressure and decreases with the temperature level and most commonly at the outlet of the compressor they form a solution. Finally, the lubricant oil and the refrigerant may be completely immiscible, in this case two different liquid phases are formed at any temperature, pressure and composition. For the typical refrigerants used in heat pumps, refrigerant and lubricant are partly miscible in most of the cases (i.e. about 90% of the refrigerants and lubricants are partially miscible), and therefore in the following only the second and third possibilities are considered, the first one of complete miscibility being not relevant for the present invention.

[0015] A consequence of the partial miscibility (or immiscibility) of the lubricant oil and the refrigerant is that along the refrigerant circuit the two fluids may separate and the fluid flowing in the pipes can be a mixture of refrigerant and some amount of lubricant in form of droplets.

[0016] More in detail, the lubricant oil and the refrigerant may reciprocally separate in any position of the heating pump circuit, due, for example, to different operative conditions (temperature, pressure) that change the reciprocal miscibility of the two fluids. More in detail, for certain thermodynamic conditions, the refrigerant is not able to transport the separated lubricant back to the compressor, i.e. for example the refrigerant is not flowing fast enough to detach the lubricant from the piping wall and bring it back to the compressor.

[0017] Therefore, for the above reasons oil traps may be formed, and in particular the trapping can take place in any position of the refrigerant circuit.

[0018] It is necessary, however, that the mechanical parts of the compressor always remain lubricated during its functioning, and it is thus preferred that the lubricant oil returns to the latter, in order to always keep a certain amount of lubricant inside it. Moreover, the presence of the lubricant oil trapped in the heat exchangers and in the pipes of the refrigerant circuit can affect the efficiency of the system, i.e. it hinders the heat exchange between the process medium and the refrigerant.

[0019] Applicants have thus realized that the trapping of the lubricant can negatively affect the thermal efficiency of the heat pump and also the proper functioning of the compressor.

[0020] Applicants have therefore implemented a new heat pump appliance so configured to overcome the above mentioned problems.

[0021] However, the design choice of apparatuses such as domestic appliances, in particular laundry dryers, washer-dryers, washing machines and dish-washers, is heavily constrained by the limited space available and the dimensions of the heat exchangers present in the same, therefore a proper design of the piping which promotes the return of the oil to the compressor and/or a proper placing of the compressor and heat exchangers within the apparatus' casing to avoid the oil trapping is not always possible. A different approach than the choice of a different design of the refrigerant circuit is therefore implemented.

[0022] In particular, an object of the invention is to provide an appliance with heat pump that overcome the drawbacks of known appliances with heat pump.

[0023] A further object of the invention is to provide an appliance with heat pump having a simple structure and in which at the same time good thermal efficiency is assured and maintained over time.

[0024] A still further object of the invention is to provide an appliance in which the formation of oil traps is diminished or avoided.

[0025] The appliance of the invention includes a dryer, a washer-dryer, a washing machine, or a dish washer, where a process medium such as air or water is warmed up by the first heat exchanger, i.e. the condenser, and this process medium is then used also in a treating chamber. The treating chamber can be the drum of the dryer or the tub of the washing machine or washer-dryer, for example. In the treating chamber, the heated process medium is used for example to wash or dry goods such as laundry or dishes.

[0026] As mentioned, in these appliances the need of a system to minimize or avoid oil trapping is particularly felt due to the physical constraints that limit the possible geometrical and spatial configurations of the refrigerant circuit.

[0027] According to a first aspect of the invention, it is provided an appliance including:

• a treating chamber in which a process medium is introduced,
• A heat pump having a refrigerant circuit in which a refrigerant can flow, said refrigerant circuit including a first heat exchanger where the refrigerant is cooled off and said process medium is warmed up, a second heat exchanger where the refrigerant is heated up, a compressor to pressurize and circulate the refrigerant through the refrigerant circuit, said compressor including a lubricant, and a pressure-lowering device; said first and second heat exchanger, compressor and pressure-lowering device being operatively connected through refrigerant circuit piping into which said refrigerant is apt to flow;
• wherein the refrigerant circuit piping comprises at least one narrow piping portion having a narrow hydraulic diameter, and the ratio between the narrow hydraulic diameter and the hydraulic diameter of the remaining piping portions of the refrigerant circuit piping is comprised between 0.4 and 0.95; said at least one narrow piping portion being located:
  • between an outlet of the second heat exchanger and an inlet of the compressor, or
  • Between an outlet of the first heat exchanger and an inlet of the pressure-lowering device, or
  • Between an outlet of the pressure-lowering device and an inlet of the second heat exchanger, or
  • below or at the suction inlet level of the compressor.

[0028] The at least one narrow piping portion may be located alternatively or in combination in the above listed location.

[0029] The hydraulic diameter D_H of the piping is defined as:


where A is the cross sectional area of the piping and P is the wetted perimeter of the cross-section of the piping.

[0030] According to the invention, the hydraulic diameter of the narrow portion D_Hnarrow is 40% _DH ≤ D_Hnarrow ≤ 95% D_H, preferably D_Hnarrow is 40% D_H ≤ D_Hnarrow ≤ 90% D_Ha and more preferably D_Hnarrow is 55% D_H ≤ D_Hnarrow ≤ 80% D_H.

[0031] As an example, in a piping having a hydraulic diameter comprised between 7 mm and 12 mm, the narrow piping portion may have an hydraulic diameter comprised between 4 mm and 10 mm.

[0032] The shape of the cross section of the piping and its narrow portion(s) is not relevant for the present invention, and it can be squared, rectangular, circular (in this case the hydraulic diameter coincides with the internal diameter of the circle), elliptic, and so on. Preferably, the piping are made of metal, more preferably aluminum.

[0033] By providing at least a narrow piping portion, the velocity of the fluid flowing in the piping is locally increased, i.e. within the narrow portion the velocity of the fluid is higher than in the rest of the refrigerant circuit.

[0034] By increasing the velocity of the fluid (refrigerant fluid and possible lubricant oil), the refrigerant fluid will more easily drag the lubricant oil. Therefore, at the narrow piping portion, the formation of oil traps is reduced.

[0035] The flow of the lubricant oil back to the compressor is thus enhanced.

[0036] According to the invention, the narrow piping portion(s) is(are) located in positions of the refrigerant piping in which oil trapping is more probable to occur.

[0037] Applicants have found that according to tests performed in various appliances, oil trappings occur more often:

• between an outlet of the second heat exchanger and an inlet of the compressor, or
• Between an outlet of the first heat exchanger and an inlet of the pressure-lowering device, or
• Between an outlet of the pressure-lowering device and an inlet of the second heat exchanger, or
• below or at the suction inlet level of the compressor.

[0038] Therefore, as mentioned, the narrow piping portion is placed in one or more of the above locations. More than a single narrow portion may be also provided in the appliance of the invention.

[0039] Within the term fluid in the following a homogeneous solution of refrigerating fluid and lubricant oil, or a biphasic mixture of refrigerating fluid and lubricant oil, depending on the miscibility of the lubricant oil and the refrigerant fluid shall be comprised.

[0040] The fluid flow rate in the piping of the heat pump system depends on the displacement, the volumetric efficiency and the RPM of the compressor, and the temperature and the pressure level of the refrigerant at the compressor suction.

[0041] For a fixed flow rate, the velocity of the fluid in the piping varies along the piping according to the actual internal section of the piping and the actual density of the fluid:


where ṁ is the fluid flow rate, p is the refrigerant density, v is the refrigerant velocity, A is the piping section and D the hydraulic diameter of the piping, as already stated for a round piping the hydraulic diameter corresponds to the internal diameter of the piping.

[0042] As shown in the formula, the fluid velocity decreases in case of high density levels. In addition, the density is proportional to the pressure and inversely proportional to the temperature of the fluid.

[0043] Between the outlet of the condenser and the inlet of the pressure-lowering device, the pressure is relatively rather high and the temperature is relatively rather low. This gives, due to the above explained formula, a rather high density of the refrigerant, and consequently a low velocity of the same. For this reason in this portion of the refrigerant circuit oil trapping is rather probable. In particular this branch of the refrigerant circuit is the one with the highest probability of oil trapping.

[0044] As known and said previously, in a heat pump system, the compressor is responsible of the circulation of the refrigerant circuit within the circuit itself and includes the inlet for the suction of the refrigerant fluid which is then compressed and exhausted by the output. In case a portion of the refrigerant circuit, i.e. some piping, is located at a vertical level which is lower than or equal to the vertical level defined by the location of the suction inlet of the compressor, the refrigerant cannot flow into the compressor by the simple application of gravity, on the contrary a force against gravity has to be exerted in order to transport the fluid inside the compressor. In these portions of the circuit located below or at the inlet vertical level, the refrigerant fluid has to flow faster than in other circuit's portions to avoid oil trapping, because oil trapping is more likely to occur, the lubricant cannot go back by gravity to the compressor.

[0045] The terms "a vertical level lower than..." mean the following: in the normal functioning of an apparatus, a vertical axis Z and also a (X,Y) plane perpendicular to the vertical axis which is the "ground" are defined. The suction inlet for suction of the refrigerant in the compressor is located, when mounted in the dryer, at a given height along the Z axis and it is substantially the entrance to the compressor chamber from which the inlet pipe extends. Considering a plane parallel to the (X,Y) plane and which intersect the Z axis in the point in which the suction inlet is located, it defines a "suction inlet level", so that all components of the refrigerant circuit which are located below the suction inlet plane are defined as being located below the "suction inlet vertical level". In the fabrication of the appliance of the invention, however, gravity is not the solution also for those portions of the refrigerant circuit which are only "slightly" above the suction inlet plane. Indeed, for a few cm above said plane, the effect of gravity is not strong enough to force the refrigerant to flow back to the compressor and again a high velocity of the refrigerant in those portions is desired in order to avoid oil trapping.

[0046] The other two mentioned branches of the circuit, i.e. between the outlet of the evaporator and the inlet of the compressor, and between the outlet of the pressure-lowering device and the inlet of the evaporator, the risk/probability of oil trapping is lower than in the other two regions above mentioned (i.e. between the outlet of the condenser and the pressure-lowering device and below the suction inlet of the compressor), however oil trapping is still possible due to the low temperature present (as said, oil trapping is more favorable where P is high and T is low, due to a reduced refrigerant density).

[0047] As already stated, the presence of at least one narrow portion having a smaller hydraulic diameter than the remaining portions of the piping reduces the oil trapping as soon as it is formed and increases the dragging of lubricant oil by the refrigerant fluid.

[0048] With the term "length", the length of the narrow portion if a single portion is present is meant, otherwise it indicates the total length as a sum of the single lengths of the various narrow portion, in case more than a narrow portion is present in the refrigerant piping of the invention.

[0049] Preferably, the hydraulic diameter of the narrow portion is so chosen that a velocity of the fluid > 1 m/s is obtained in the latter.

[0050] At the velocity level indicated above, the refrigerant is able to drag the lubricant oil back to the compressor.

[0051] The hydraulic diameter of the piping in the remaining portions of the refrigerant circuit piping is maintained substantially constant and having a larger diameter than the narrow piping portions, so that the overall pressure drops are limited.

[0052] Indeed, a too long narrow piping portion is not preferred, the hydraulic diameter of the piping in the remaining portions of the refrigerant piping circuit is preferably kept "high" in order to limit the pressure drops within the refrigerant circuit.

[0053] The refrigerant circuit can be functionally divided in two portions, a high pressure portion which is the portion of the refrigerant circuit connecting the compressor to the pressure lowering device via the condenser, and a low pressure portion which is the portion of the circuit connecting the pressure-lowering device back to the compressor via the evaporator. The term "high" and "low" are relative terms and their meaning is that the pressure of the refrigerant in the "high pressure" portion is higher than in the "low pressure" portion.

[0054] In case the narrow piping portion is located in the high pressure portion of the circuit, downstream of the condenser, the increased pressure drop between the condenser and the lamination means, due to the presence of the narrow piping portion, can be compensated by a proper lamination means that give less pressure drop to the refrigerant. In particular, the pressure-lowering device in the appliance of the invention includes compensating means to compensate the pressure drop due to the narrow piping portions. More in detail, the pressure-lowering device includes a valve or a capillary which so dimensioned that the pressure drop between low and high pressure portions of the refrigerant circuit in the appliance of the invention is substantially the same as the pressure drop present in a refrigerant circuit without the narrow piping portion, in particular the pressure drop calculated is the one between the outlet of the condenser and the inlet of the evaporator. Therefore, the pressure-lowering device in the appliance of the invention yields a lower pressure drop than in the case without narrow piping portion.

[0055] In this way, the expansion of the fluid in the pressure-lowering device may be suitably changed in order to obtain at the output a desired pressure level for the refrigerant.

[0056] In particular, in case of a pressure-lowering device including a capillary, the capillary is either shorter or has a wider diameter than in an heat pump without the narrow piping portion. The refrigerant circuit may not include only the components already described, i.e. the compressor, the heat exchangers and the expansion devices, but can also include additional components, such as additional condenser(s) which is preferably located between the condenser and the expansion device, and/or additional evaporator(s), and/or gas liquid separator located downstream the inlet of the compressor in order to avoid the entrance of liquid into the compressor itself. Preferably, the apparatus of the invention includes an auxiliary first heat exchanger, in other words an auxiliary condenser, located between the outlet of the main condenser and the pressure-lowering device. More preferably, the secondary condenser is located below the suction inlet vertical level defined by the inlet of the compressor. Preferably, the apparatus of the inventor also includes an auxiliary second heat exchanger, i.e. and auxiliary evaporator, located between the outlet of the main evaporator and the inlet of the compressor. Preferably, this auxiliary evaporator is also located below the suction inlet level defined by the compressor.

[0057] In addition, the heat pump may also include a fan to cool off the compressor to avoid overheating during the functioning of the latter.

[0058] According to a preferred embodiment of the invention, the heat pump also includes an internal heat exchanger, apt to perform a heat exchange between the high pressure portion and the low pressure portion of the refrigerant circuit. More in detail, the internal heat exchanger includes a first piping portion of the low pressure portion and a second piping portion of the high pressure portion of the refrigerant circuit, which are brought at a given distance which allows heat exchange between the two piping portions. Preferably the first piping portion is located between the outlet of the evaporator and the inlet of the compressor, while the second portion is located between the outlet of the condenser and the inlet of the pressure-lowering device. In this way a sub-cooling of the refrigerant at the outlet of the condenser is performed, as well as a warming up of the refrigerant entering the evaporator. In this way, in a safe manner, it is possible to work all along the refrigerant circuit always at about the critical temperature where the phase transition takes place.

[0059] The invention can be understood better and implemented with reference to the attached drawings which illustrate an embodiment thereof by way of non-limiting example in which:

• Figure 1 is a perspective view of a laundry dryer according to the invention, with a portion of the casing removed;
• Figure 2 is a perspective view of an internal portion of the laundry dryer of Figure 1 in a disassembled condition;
• Figure 3 is a schematic representation of the process air circuit and of the refrigerant circuit of the dryer of Figures 1 and 2;
• Figure 4 is a schematic representation of a different embodiment of the process air circuit and of the refrigerant circuit of the dryer of Figures 1 and 2;
• Figures 5a and 5b a schematic front and lateral view, in section, respectively, of the dryer of Figure 1;
• Figures 6a and 6b are a schematic front and lateral view, in section, respectively, of the dryer of Figures 1 and 4;
• Figure 7 is a graph depicting the behaviour of the refrigerant temperature versus time in different portions of the refrigerant circuit;
• Figure 8 is a graph depicting the behaviour of the refrigerant pressure versus time in different portions of the refrigerant circuit;
• Figs. 9a and 9b are two schematic lateral views of a detail of the refrigerant circuit of the dryer of fig. 1 according to two different embodiments of the invention.

[0060] With initial reference to figs. 1 and 2, an appliance realized according to the present invention is globally indicated with 1.

[0061] As a possible appliance, a laundry dryer is described herein below, however the invention can be generalized to any appliance including an heat pump in which the first heat exchanger warms up a process medium, such as a dish-washer, washing machine, washer dryers and dryers in general. In these appliances, a treating chamber is included, where the heated process medium, such as air or water, is introduced, for example to wash clothes/dishes or to dry the same.

[0062] With the term "dryer" both clothes dryer only and washer dryer are meant.

[0063] In the following with the terms "downstream" and/or "upstream", a position with reference to the direction of the flow of a fluid inside a conduit is indicated.

[0064] Moreover, in the present context, the terms "vertical" and "horizontal" are referred to the positions of elements with respect to the appliance's position in its normal installation or functioning.

[0065] Laundry dryer 1 comprises an outer box casing 2, preferably but not necessarily parallelepiped-shaped, and a drying chamber 3 in which items to be dried, usually clothes or other garments are loaded.

[0066] The drying chamber comprises a drum 3 having the shape of a hollow cylinder that is rotatably fixed to the casing 2 and an electrical motor arranged for operating the drum 3 for rotating the latter about a rotation axis.

[0067] In the version shown, the drum 3 is rotatable about a horizontal rotation axis, nevertheless in alternative embodiments of the dryer of the invention, not shown in the Figures, the drum may rotate about a vertical axis or an axis tilted in relation to the horizontal axis.

[0068] In the casing 2, a door 3a is provided that is hinged to the casing 2 in order to be opened/closed for accessing the drum 3 the door 3a is so configured to hermetically closing the drum 3 so that safe functioning of the dryer 1 is assured.

[0069] More in detail, casing 2 generally includes a front panel 20, a rear wall panel 21 and two sidewall panel all mounted on a basement 24. Panels 20, 21 and basement 24 can be of any suitable material. Preferably, the basement 24 is realized in plastic material. Preferably, basement 24 is molded.

[0070] Preferably, basement 24 includes an upper and a lower shell 24a,24b (visible in figure 2) .

[0071] The laundry dryer 1 further comprises a fan or a blower 4 or a suitable moving device arranged for moving air inside the laundry drier 1 along an air process conduit 5, through which flows air to be used for drying the items contained in the drum 3, as better disclosed in the following.

[0072] The dryer 1 of the invention additionally comprises a heat pump 6 including a first heat exchanger called also condenser 61 and a second heat exchanger called also evaporator 62. Heat pump 6 also includes a refrigerant closed circuit 60 (schematically depicted in the picture with arrows connecting the first to the second heat exchanger and vice versa, see in detail figs. 3-5) in which a refrigerant fluid flows, when the dryer 1 is in operation, cools off and may condense in correspondence of the condenser 61, releasing heat, and evaporates, potentially even warms up, in correspondence of the second heat exchanger (evaporator) 62, absorbing heat. Alternatively, no phase transition takes place in the condenser and/or evaporator, which indicates in this case respectively a gas heater and gas cooler, the refrigerant cools off or it warms up, respectively, without condensation or evaporation.

[0073] More in detail, the refrigerant circuit 60 connects through piping 65 the evaporator 62 via a compressor 63 to the condenser 61. The cooled or condensed refrigerant arrives via a pressure lowering device, such as an expansion device 64, for example a choke, or a valve or a capillary tube, back at the evaporator 62.

[0074] The compressor 63, preferably a single-stage or multi-stage sealed compressor, preferably including a variable speed motor, includes an inlet 63a, called also suction inlet for the suction of the refrigerant, and an outlet 63b for the exhaustion of the refrigerant. Within the compressor, a lubricant reservoir (not shown) is present, in order to provide lubricant for the lubrication of the moving parts of the compressor itself. Such a lubricant may leak within the refrigerant circuit, i.e. it can be present within piping 65. The inlet 63a is located at a "suction inlet level" L (see figs. 5b and 6b where the inlet pipe level is schematically shown with a dash-dotted line and in more details figs. 9a and 9b), so that the components of the refrigerant circuit which are located below the suction inlet plane are defined as being located below the "suction inlet vertical level".

[0075] With reference now to figures 9a and 9b, two embodiments of the compressor 63 are shown in detail. Compressor 63 includes a container 63e in which a top and a bottom 63c and 63d, respectively, are defined and which encloses a compressor chamber and a lubricant chamber (both not shown in the appended drawings). The suction inlet 63a defines the suction inlet plane L depicted as a dash-dotted line in the drawings and its positioning is independent from the location of the compressor's bottom 63d. As visible in fig. 9b, the compressor 63 may also include a liquid-vapor separator 37, to avoid entrance of liquid in the compressor chamber of the casing 63e. Compressor 63 and liquid-vapor separator 37 are connected via piping 63f. It is to be understood that the suction inlet 63a defining the suction inlet plane L is always the suction inlet 63a of the compressor 63, and not the inlet 37a of separator 37. In the depicted embodiment, a portion of the piping 63f connecting separator 37 and compressor 63 is located below the suction inlet level L. Moreover, also in the compressor of fig. 9a without separator 37, the inlet pipe 63g connecting the suction inlet 63a to the refrigerant circuit 60 is located below the suction inlet level L.

[0076] The condenser 61 and the evaporator 62 of the heat pump 6 are located in correspondence of the process air conduit 5.

[0077] The dryer 1 of the invention can be a condense dryer - as depicted in the figures - where the air process conduit 5 is a closed loop circuit, and the condenser 61 is located downstream of the evaporator 62. The air exiting the drum 3 enters the conduit 5 and reaches the evaporator 62 which cools down and dehumidifies the process air. The dry cool process air continues to flow through the conduit 5 till it enters the condenser 61, where it is warmed up before re-entering the drum 3. However the dryer of the invention can also be a vented dryer, not shown, in which the process air conduit 5 includes an exhaust duct connected to the drum 3 via an aperture into which the process air enters after having passed the whole drum 3 to de-humidify the laundry.

[0078] First and/or second heat exchanger 61, 62 further preferably includes one or more heat exchanger modules 10 (shown only in fig. 2) located along the process air conduit 5, more preferably in correspondence of the basement 24 of dryer 1, as shown in fig 2 where the casing 2 and the drum 3 of the dryer 1 have been removed in order to show the heat exchangers located along the process air conduit 5. According to the embodiment shown in figs. 2, 6a and 6b, the refrigerant circuit 60 may also include an auxiliary condenser 36 to improve the drying performances (efficiency and/or drying time) for example located downstream the condenser 31 and/or, an auxiliary evaporator (not shown in the drawings) and/or, as already described and shown in figs. 9a,9b and fig. 2, the liquid-vapor separator 37 upstream the compressor 63. The auxiliary condenser is used to further lower the temperature of the refrigerant, while the auxiliary evaporator to further increase the temperature of the refrigerant.

[0079] The air process circuit 5 is a closed loop circuit. As previously discussed, the refrigerant fluid flows in the piping 65 together with some lubricant coming from the compressor 63. The lubricant can be mixed to the refrigerant or form droplets in the latter.

[0080] In the preferred embodiment, lubricant and the refrigerant are partially miscible so that, depending on the thermodynamic conditions, they may form a solution or two separate liquid phases.

[0081] The mutual solubility between lubricant oil and refrigerant fluid changes by changing the pressure and/or temperature conditions.

[0082] Therefore, in some areas of the refrigerant circuit 60, the oil present in the refrigerant fluid may be trapped in the circuit itself affecting the thermal efficiency of the heat pump system 6.

[0083] Applicants have discovered that, when oil trapping takes place in the refrigerant piping 65, it can cause an unstable behaviour of the refrigerant temperature level and/or of the refrigerant pressure level during a drying cycle.

[0084] With the term "unstable behavior", sudden variations (i.e. peaks and valleys, which are both called "extremes") of the temperature and/or pressure values in a short time interval it is meant. Preferably, the mentioned variations are from/to an upper limit to/from a lower limit, i.e. a sudden variation is a change between a maximum and a minimum of the temperature or pressure curve in a short time frame.

[0085] More in detail, Applicants have found that by monitoring - using suitable sensor(s) - the behavior of the temperature and/or the pressure of the refrigerant in preset zones of the refrigerant circuit, the oil trapping becomes visible due to the above mentioned unstable behavior of the pressure and/or temperature curve versus time. Therefore, analyzing the temperature and/or pressure fluctuations during the drying cycle, it is possible to easily detect the presence (or absence) of oil trapping in the circuit.

[0086] The results of the monitoring are shown in the graphs of Figure 7 and 8.

[0087] Figure 7 is a graph depicting the behaviour of the refrigerant temperature versus time in different portions of the refrigerant piping 65 during the drying cycle.

[0088] The upper most curve 80 shows the behavior of the temperature values of the refrigerant in the proximity of the compressor outlet 63b during the drying cycle. The second curve 81 from above shows the behavior of the temperature values of the refrigerant between the condenser 61 outlet and the pressure lowering device 64, during the drying cycle. In the same fashion, the third curve 82 from above and the lowest curve 83 show the behavior of the temperature values of the refrigerant in the proximity of the compressor inlet 63a and evaporator inlet, respectively, during the drying cycle.

[0089] As highlighted by the sketched oval line 84, there is a portion of curves 81, 82, 83 where the behaviour of the temperature versus time becomes unstable, showing a plurality of subsequent extremes one after the other for a rather long time interval. This unstable behaviour is particularly enhanced in the curves 81 and 82.

[0090] The unstable behavior above analyzed is a sign of oil trapping.

[0091] The same considerations also apply to the pressure curves as depicted in Figure 8.

[0092] Figure 8 is a graph depicting the behaviour of the refrigerant pressure versus time in different portions of the refrigerant circuit, a curve 90 is obtained with the values of the pressure upstream of the expansion device 64, while curve 91 is obtained at the outlet of the expansion device 64; in other words the two measurements are taken in the high and low pressure portions of the refrigerant circuit.

[0093] Pressure measurements, and not temperature measurements, can be made also downstream the outlet of the compressor 63. At the outlet, refrigerant and lubricant are mixed and temperature measurements will not show instability (see curve 80 of figure 7), however the pressure "waves" due to the unstable behavior caused by oil trapping are easily detected in a pressure graph.

[0094] Also in this case, the circle 92 highlights the unusual behaviour of the pressure caused by oil separation and trapping. The instability is evident in both curves 91 and 90.

[0095] In order to avoid the formation of oil trapping and/or enhancing the flowing back to the compressor 63 of the lubricant, the refrigerant fluid piping 65 comprises at least one narrow piping portion 70 having a hydraulic diameter D_Hnarrow which is, with respect to the hydraulic diameter D_H of the remaining portions of the refrigerant fluid piping 65 within the following ranges: 40% D_H ≤ D_Hnarrow ≤ 95% D_H, preferably 40% D_H ≤ D_Hnarrow ≤ 90% D_H and more preferably D_Hnarrow is 55% D_H ≤ D_Hnarrow ≤ 80% D_H. In a preferred embodiment, the narrow piping portion 70 is provided in a third branch 68 of the piping 65, arranged upstream the inlet 63a of compressor 63, i.e. between the outlet of evaporator 62 and the inlet of compressor 63, as depicted in fig. 3.

[0096] In a further embodiment, the narrow piping portion 70 is provided in a first branch 66 of the piping 65, i.e. between the outlet of the condenser 61 and the expansion device 64, as depicted in fig. 4.

[0097] In a still further version, not shown, the narrow piping portion 70 is provided in a second branch 67 of the piping 65, i.e. between the outlet of the pressure lowering device 64 and the inlet of the evaporator 62.

[0098] In the branches above mentioned, the formation of oil trapping is more likely to occur. Moreover, also in the branch(es) of the piping 65 located below the "suction inlet pipe vertical level" L, the formation of oil traps is more probable than in the other branches of the refrigerant circuit.

[0099] Moreover, in case of "low level branches", it is more difficult to drag the oil back to the compressor since also the gravity force which pushes the fluid away from the compressor has to be overcome.

[0100] It is also necessary to drag the lubricant back to the compressor 63 for allowing a good lubrication of the moving part of the latter.

[0101] Therefore, alternatively, or in addition, the narrow piping portion 70 may be located in any branch of the piping 65 arranged below the inlet pipe level 63a of the compressor 63, for example in the lower portion 65c shown in Figures 6a and 6b.

[0102] The portion 65c indeed corresponds to a portion of the piping 65 of the heat pump system 6 which is located below the suction inlet level L.

[0103] This portion can be present due to the specific construction of the heat pump system 6 which is constrained by the limited space available, or to the presence of the additional condenser 36.

[0104] In further version of the invention, more than one narrow piping portion 70 may be provided in the piping 65.

[0105] At the narrow piping portion 70, the velocity of the fluid flowing in the piping 65 is increased thus it is increased the dragging of the oil by the refrigerant and the formation of the oil traps is minimized.

[0106] An increased speed enhances the detachment of lubricant droplets from the piping 65 walls. It is thus increased the flow of the lubricant oil back to the compressor 63. Depending on the size and length of the narrow piping portion(s) 70, the pressure lowering device is configured in such a way to compensate for the pressure drop caused by the narrow piping. In particular the pressure drop between the high pressure portion of the circuit downstream the condenser, but upstream of the narrow piping portion, and the inlet of the evaporator in the low pressure portion of the circuit is substantially the same that the pressure drop calculated between the same positions in a circuit without the narrow piping portion(s).

[0107] The operation of the drying machine will now be described.

[0108] The laundry to be dried is positioned into the drum 3 and the door 3a is then closed. The laundry dryer 1 is then operated, the motor rotates the drum 3 and the blower 4 feed hot air into the drum 3 for drying the items therein provided. The hot air blown into the drum 3 warms the items to be dried to evaporate moisture. The moisture-containing process air which has dried the items flows first through the drum 3, then through the air process conduit 5 and through the condenser 61, and the evaporator 62.

[0109] At the condenser 61 and at the evaporator 62, a thermal exchange with the refrigerant fluid occurs so that at the outlet of the heat pump system 6 dry hot air is obtained, that can be fed to the drum 3 again.

[0110] The refrigerating fluid, for example, carbon dioxide, flows in the refrigerating fluid piping 65 through the condenser 61 and the evaporator 62 in the opposite directions in relation to the air. The temperature difference between the air and the refrigerating fluid allows an efficient heat exchange between the air and the refrigerating fluid.

[0111] The refrigerating fluid undergoes a phase transition from the liquid to the vapor phase due to the heat exchange with the warm process air exiting the drying chamber at the evaporator 62. The evaporated refrigerant is then supplied by means of the compressor 63 to the condenser 61, in which the refrigerant condenses again, heating up the process air before the latter is introduced into the drum 3.

[0112] The condensed refrigerant arrives at the pressure lowering device 64 and thus flows back to the evaporator 62 closing the circuit.

[0113] The refrigerant fluid is moved by the compressor 63, from which some oil can come out into the refrigerant circuit 60 and droplet of oil may thus be formed in the refrigerating piping 65. The oil can also be trapped into the piping.

[0114] Nevertheless, the presence of one or more narrowing sections 70 in the refrigerating piping 65 allows the detachment of the oil droplets from the piping's walls, removing the oil trapping, and the oil can be dragged to the compressor again.

[0115] In this way, it is much less likely that an unstable behavior of the temperature and/or pressure of the refrigerant fluid in the dryer occurs.

Claims

1. An appliance (1) including:

• a treating chamber (3) in which a process medium is introduced;

• A heat pump (6) having a refrigerant circuit (60) in which a refrigerant can flow, said refrigerant circuit (60) including a first heat exchanger (61) where the refrigerant is cooled off and said process medium is warmed up, a second heat exchanger (62) where the refrigerant is heated up, a compressor (63) to pressurize and circulate the refrigerant through the refrigerant circuit, said compressor including a lubricant, and a pressure-lowering device (64); said first and second heat exchanger, compressor and pressure-lowering device being operatively connected through refrigerant circuit piping (65) into which said refrigerant is apt to flow;

• wherein the refrigerant circuit piping (65) comprises at least one narrow piping portion (70) having a narrow hydraulic diameter (D_Hnarrow), and the ratio between the narrow hydraulic diameter (D_Hnarrow) and the hydraulic diameter (D_H) of the remaining piping portions of the refrigerant circuit piping (65) is comprised between 0.4 and 0.95;, said at least one narrow piping portion (70) being located:

- between an outlet of the second heat exchanger (62) and an inlet (63a) of the compressor (63), or

- Between an outlet of the first heat exchanger (61) and an inlet of the pressure-lowering device (64), or

- Between an outlet of the pressure-lowering device (64) and an inlet of the second heat exchanger (62), or

- below or at the suction inlet level (L) of the compressor (63).

2. The appliance (1) according to claim 1, wherein said at least one narrow piping portion (70) has a narrow hydraulic diameter (D_Hnarrow) which is comprised between 55% D_H ≤ D_Hnarrow ≤ 80% D_H with respect to the hydraulic diameter (D_H) of the remaining piping portions of said refrigerant fluid piping (65).

3. The appliance (1) according to claim 1 or 2, wherein the narrow hydraulic diameter (D_Hnarrow) of said at least one narrow piping portion (70) is so chosen that within said at least narrow piping portion (70) a velocity of said refrigerant fluid above 1 m/s is obtained.

4. The appliance (1) according to any of the preceding claims, wherein said narrow piping portion (70) is located between an outlet of the first heat exchanger (61) and an inlet of the pressure-lowering device (64) and wherein said pressure-lowering device (34) is so configured to compensate for the pressure drop caused by said narrow piping portion (70).

5. The appliance (1) of any of the preceding claims, wherein said narrow hydraulic diameter (D_Hnarrow) of said narrow piping portion (70) is comprised between 4 mm and 10 mm.

6. The appliance (1) according to any of the preceding claims, wherein the appliance (1) defines a vertical axis (Z) and plane (X,Y) perpendicular to the vertical axis (Z) and wherein the compressor (63) includes a suction inlet (63a) for suction of the refrigerant in the compressor, said suction inlet defining a suction inlet level (L) which is a plane parallel to the (X,Y) plane and which intersects the vertical axis (Z) in the point in which the suction inlet (63a) is located.

7. The apparatus (1) according to any of the preceding claims, including an auxiliary condenser (36) located between the outlet of the first heat exchanger (61) and the pressure-lowering device (64), preferably said auxiliary condenser being located below the suction inlet level (L) of the compressor (63).

8. The apparatus (1) according to any of the preceding claims, including an auxiliary evaporator located between the outlet of the second heat exchanger (62) and the inlet (63a) of the compressor (63).

9. The apparatus according to claim 8, wherein said auxiliary evaporator is located below the suction inlet level (L) of the compressor (63).

10. The appliance (1) according to any of the preceding claims, wherein said appliance (1) includes a laundry dryer, or a washer-dryer.

11. The appliance according to claim 10, wherein said second heat exchanger (62) is apt to cool off said process medium.

12. The appliance (1) according to any of the preceding claims, wherein said appliance (1) includes a washing machine or a dish-washer.

13. The appliance (1) according to any of the preceding claims, wherein said compressor (63) includes a variable speed compressor.

14. The appliance (1) according to any of the preceding claims, wherein said heat pump (6) includes a fan to cool down said compressor (63).

15. The appliance (1) according to any of the preceding claims, including an internal heat exchanger, apt to perform a heat exchange between a high pressure portion and a low pressure portion of said refrigerant circuit (60), said internal heat exchanger including a first piping portion of the low pressure portion and a second piping portion of the high pressure portion of the refrigerant circuit, first and second piping portion being at a given distance which allows heat exchange between the first and second piping portion.

Drawing