REFRIGERATED DISPLAY CABINET WITH A SMART DEFROST SYSTEM

Abstract

 Refrigeration display cabinet (100) comprising a compartment (101), at least one door (102) for providing access to the compartment (101), a refrigeration system (103) comprising an evaporator (130), a defrost system (104) of the evaporator (130), a control unit (2) configured to operate the defrost system (104), temperature detection means (3) to detect an ambient temperature (Ta), door open detection means (4) and a timer (5) to detect a door open time (t_do), and an algorithm (6) configured to receive the ambient temperature (Ta) and the door open time (t_do) to calculate a maximum door open time (t_def), after which the defrost system (104) is to be operated.

Description

[0001] The present invention relates to a refrigerated display cabinet with smart defrost system.

[0002] Refrigerated display cabinets have a refrigeration system comprising an evaporator that cools the air that circulates in a refrigerated compartment of the refrigerated display cabinet that contains the products to be cooled and displayed. A door on the refrigerated display cabinet provides access to the refrigerated compartment.

[0003] During the use of the refrigerated display cabinet, moist air enters the refrigerated compartment each time the door that provides access to the refrigerated compartment is opened. Therefore, the air passing through the evaporator contains moisture in the form of vapor droplets. When the moist air touches the evaporator, it undergoes a solidification process, changing from the vapor state to the solid state of ice. As this situation persists, the space between the fins of the evaporator will be covered with frost and ice. Such a situation reduces the heat exchange of the evaporator.

[0004] Therefore, on a periodical basis, the frost must be removed from the evaporator by means of a defrost cycle. There are different types of defrost systems, but all of them work by giving up heat to the evaporator to melt the frost and the ice formed on the evaporator.

[0005] Based on experiments conducted in the test room, in which an average number of openings and closings of the refrigerated display cabinet is simulated, the operator can decide how often to defrost the evaporator. On the contrary, the duration of the defrost cycle is determined by a probe that monitors the temperature of the evaporator and terminates the defrost cycle when all the ice has melted.

[0006] Nevertheless, while actually using the refrigerated display cabinet, the local situations may be different from the test situations, and the number and timing of door openings is different from the case established by the test standards. By way of example, on the day the stores are closed, the refrigerated display cabinet is never opened, but defrost cycles are carried out on a periodical basis as established in the test conditions, as if door openings were to take place.

[0007] US5564286A discloses a refrigerator according to the preamble of claim 1.

[0008] US2005/120727A1 discloses a freezer with defrost indicator.

[0009] US5678416A discloses a method and an apparatus for controlling a refrigerator in normal state and in overload state.

[0010] The purpose of the present invention is to eliminate the drawbacks of the prior art by providing a refrigeration display cabinet with defrost system of the evaporator that is efficient, effective and reliable.

[0011] Another purpose is to provide such a refrigerated display cabinet with defrost system of the evaporator that is practical, versatile, and easy to implement and realize.

[0012] These purposes are achieved in accordance with the invention with the features of the appended independent claims.

[0013] Advantageous embodiments of the invention appear from the dependent claims.

[0014] Further features of the invention will appear clearer from the following detailed description, referring to a purely illustrative and therefore non-limiting embodiment, illustrated in the appended drawings, wherein:

Fig. 1 illustrates three sectional views according to sectional planes A-A, B-B and C-C, respectively, of a refrigerated display cabinet loaded with test packages (Tylose) for a test;

Fig. 2 is a graph illustrating the temperature trend over time for each test package contained in the refrigerated display cabinet of Fig. 1, during the test;

Fig. 3 is a psychrometric diagram of the moist air;

Fig. 4 is a diagram illustrating a polynomial function of the maximum door open time as a function of the ambient temperature;

Fig. 5 is a diagram illustrating a discretization of the ambient temperature as a function of the time;

Fig. 6 is a table illustrating the parameters used by an algorithm to calculate a period between two defrosts;

Fig. 7 is a block diagram illustrating the operation of the refrigeration display cabinet according to the invention; and

Fig. 8 is a block diagram of the algorithm contained in the control cabinet of the refrigerated display cabinet according to the invention.

[0015] With reference to Figs. 1 and 2, a performance test of a refrigerated display cabinet according to the standard UNI EN 23953 is described.

[0016] During the test, the refrigeration display cabinet is loaded with specific products comprising test packages named Tylose. Temperature probes are placed in the refrigerated display cabinet in accordance with the standards.

[0017] The refrigerated display cabinet is placed in the climatic chamber under the climatic conditions of the designated class, for example, in class 7 at 35°C and 75% relative humidity, which set the maximum environmental conditions for operation.

[0018] After a period of stability lasting at least one day, the door open test is begun. With the light on, each door of the refrigerated display cabinet is opened for a time period of 3 minutes. At the conclusion of such a period, cyclic door openings begin, simulating the picking of a product by the user. In accordance with the standard, each door is opened for 6 seconds, 6 times per hour, for 12 hours. At the end of the 12 hours, the doors are closed and the light is turned off for another 12 hours.

[0019] With reference to Fig. 2, the temperatures of the test packages during the door open test get hot and should never exceed -15°C. To prevent the test packages from exceeding -15°C, a defrost cycle (corresponding to the vertical line in the first part of the graph) is carried out to remove the frost from the evaporator and restore the optimal operating condition of the evaporator to generate cold.

[0020] According to the experimental data, in climate class 7 (Temperature = 35 °C and relative humidity = 75%), keeping the door of the refrigerated display cabinet open, it was necessary to run a defrost cycle after a maximum door open time (t_def) equal to t_def = 396 seconds. Taking such a maximum door open time (t_def) into account as the maximum value in climate class 7, one can extrapolate the maximum door open times for the other climate classes based on the amount of water contained in the room air.

[0021] Fig. 3 illustrates a psychrometric diagram of the humid air. From the psychrometric diagram of Fig. 3, the water content for the reference climate conditions can be identified. In the case of climate class 7, there is a water content of 28 grams. In the case of climate class 4, there is a water content of 16 grams, and in the case of climate class 3, there is a water content of 12 grams.

[0022] Knowing that with 28 grams of water the maximum door open time is t_def = 396 seconds, and knowing the water contents for climate classes 4 and 3, by direct proportionality it is possible to calculate the maximum door open times in climate classes 4 and 3.

[0023] For climate class 4, the maximum door open time is given by the following proportion:

[0024] For climate class 3 the maximum door open time is given by the following proportion:

[0025] The results are shown in Table 1 below.

Table 1

Climate class	Ambient temperature (°C)	Room humidity (%)	Water (g)/ air (kg)	Open time (s)
7	35	75	28	396
4	30	55	16	693
3	25	60	12	924

[0026] With reference to Fig. 4, by means of a second-degree polynomial function (y =f(x)), staring from the conditions in Table 1, a law linking the maximum door open time to the ambient temperature can be derived.

[0027] In a diagram in which the abscissae (x) are the temperature (T) and the ordinates (y) are the time (t), the polynomial function f(x) can be obtained by interpolation of three points having the following Cartesian coordinates:

P1 (25; 924)

P2 (30; 693)

P3 (35; 396)

[0028] Points P1, P2, P3 represent temperature and maximum door open time in the case of climate classes 3, 4, and 7, respectively.

[0029] From such interpolation with a second-degree polynomial (second-degree spline), the following function is obtained:

wherein

A = -1.32

B = 26.4

C = 1089

[0030] The function f(x) is used to calculate the maximum door open time based on the ambient temperature.

[0031] Based on such function f(x), an operation algorithm is implemented to decide how often the defrost cycle of the evaporator is to be started.

[0032] The basic principle is that defrosting occurs according to the total door open time and to the ambient temperature at which the refrigerated display cabinet operates. After the refrigerated display cabinet is turned on, the first defrost cycle will occur at a preset time. At the end of the first defrost cycle, the door opening time will be counted by adding up all the opening times of the door. If the ambient temperature does not change, the maximum time is obtained from the polynomial function f(x).

wherein:

t_def is the maximum time that must elapse before running a defrost cycle, starting from the previous defrost cycle, at a set ambient temperature.

T_amb is the ambient temperature.

N_D is the number of doors of the refrigerated display cabinet.

[0033] Since the ambient temperature will not remain stable and will vary according to the climate conditions of the day, with a lower temperature in the morning and a higher temperature in the afternoon, or according to whether the air conditioning is turned on or off, the ambient temperature (T_amb) must be discretized by intervals (sample time interval e.g. one hour). As shown in Fig. 5, within a sample time interval, the average value of temperatures in such an interval is taken as temperature value.

[0034] Then the time interval that must elapse between two defrost cycles is recalculated each time, depending on the ambient temperature, taking into account the total door open time.

[0035] In order to provide an explanation of the calculation algorithm, it is necessary to define some quantities:

t_c = sample time interval for the discretization of the ambient temperature;

t_er = total real door open time;

T_amb.c = average ambient temperature in the sample time interval (t_c);

t_do = door open time in the sample time interval (t_c);

t_cedo = equivalent door open time recalculated on the new room condition of the average ambient temperature T_amb.c. The equivalent door open time t_cedo is calculated by multiplying the total door open time elapsed until the previous sample interval but recalculated at the new ambient temperature value t_Tedo for the ratio of the maximum time t_def calculated in the current sample time interval to the maximum time t_def-1 calculate in the previous sample time interval:

t_Tedo= Total equivalent door open time, e.g. the addition of the door open time t_do in the current sample time interval and the equivalent door open time t_cedo calculated from the t_Tedo value in the previous sample time interval:

[0036] The defrost cycle will be run when the total equivalent door open time (t_Tedo) exceeds the maximum time (t_def) within which the defrost cycle is to be carried at the ambient temperature (T_amb.c).

[0037] By way of example, Fig. 6 illustrates a Table 2 that simulates a potential operating condition of a refrigerated display cabinet with one door, wherein the sample period (sample time (t_c) is set at 1 hour. At the end of the eleventh hour, the refrigerated display cabinet starts the defrost cycle because the total equivalent door open time (t_Tedo) exceeds the maximum time (t_def) calculated at the temperature of 26°C in accordance with the polynomial function (1).

[0038] Referring to Fig. 7, the refrigerated display cabinet (100) according to the invention comprises:

• a compartment (101) suitable for containing products to be refrigerated and displayed;
• at least one door (102) for access to the compartment (101);
• a refrigeration system (103) comprising an evaporator (130) for cooling the compartment (101) by means of a flow of cold air passing through the evaporator (130);
• a defrost system (104) suitable for defrosting the evaporator (130); and
• a control unit (2) configured to operate the defrost system (104).

[0039] According to the invention, the refrigeration display cabinet (100) further comprises:

• temperature detection means (3) arranged in the space outside the compartment (101) to detect a ambient temperature (Ta) of the spacer in which the refrigerated display cabinet is located;
• door open detection means (4) associated with said at least one door (102) of the refrigerated display unit to detect the opening of the door;
• a timer (5) associated with said door open detection means (3) to detect a door open time (t_do); and
• an algorithm (6) contained in said control unit (2).

[0040] The algorithm (6) is configured to:

• receive said ambient temperature (Ta) and said door open time (t_do) and calculate a maximum door open time (t_def),
• calculate a total equivalent door open time (t_Tedo) by adding up the door open times (t_do + t_cedo),
• compare said total equivalent door open time (t_Tedo) with said maximum time (t_def),
• operate the defrost system (104), when said total equivalent door open time (t_Tedo) exceeds said maximum time (t_def).

[0041] With reference to Fig. 8, the algorithm (6) comprises a polynomial function (1) that receives the ambient temperature (Ta) as input and accordingly outputs said maximum door open time (t_def) after which the defrost system (104) is to be operated.

[0042] Such a polynomial function (1) is obtained:

• by experimentally detecting a first maximum door open time at a first temperature, at a first relative humidity and at a first water content of the air in the space, so as to obtain a first point having the first temperature and the first maximum time as coordinates,
• by calculating a second maximum open door time at a second temperature and at a second relative humidity of the space, by direct proportionality of the water content, so as to obtain a second point having the second temperature and the second maximum time as coordinates,
• by calculating a third maximum open door time at a third temperature and at a third relative humidity of the space, by direct proportionality of the water content, so as to obtain a third point having the third temperature and the third maximum time as coordinates, and
• interpolating the three points obtained with a second degree polynomial.

[0043] The algorithm (6) comprises a comparator (7) that compares the total equivalent door open time (t_Tedo) obtained by adding up the door open times (t_do; t_cedo), with said maximum door open time (t_def) obtained from the polynomial function (1).

[0044] When the total equivalent door open time (t_Tedo) exceeds the maximum time (t_def), the control unit (2) operates the defrost system (104) to start a defrost cycle.

[0045] The algorithm (6) comprises a discretization and average block (8) that discretizes the ambient temperature (Ta) into sample time intervals (t_c), taking the average value of the temperatures in the sample time interval as average ambient temperature value (T_amb.c). Such an average ambient temperature value (T_amb.c) is sent to the polynomial function (1) to calculate the maximum time (t_def) in each sample time interval.

[0046] The algorithm (6) comprises a multiplier and divider block (9) that multiplies the door open time (t_do) by the ratio of the maximum time (t_def ) calculated in the current sample time interval to the maximum time (t_def-1) calculated in the previous sample time interval, and outputs an equivalent door open time (t_cedo) given by the following formula;

[0047] The algorithm (6) comprises an adder block (10) that adds the door open time (t_do) in the current sample time interval and the equivalent door open time (t_cedo) calculated from the value of the total equivalent door open time (t_Tedo) in the previous sample time interval, and outputs said total equivalent door open time (t_Tedo) given by the following formula:

[0048] The software of the algorithm (6) is of parametric type and thus allows for a variation of some parameters in order to adapt it to the refrigerated display cabinet (100) or improve the accuracy level of the algorithm (6).

[0049] The modifiable parameters are:

t_dm = minimum defrost time between two successive defrost cycles. Regardless of the result of the algorithm, the control unit (2) will not operate the defrost system (104) as long as the time between two defrost cycles exceeds such a value;

t_dM = maximum defrost time between two successive defrost cycles. Regardless of the result of the algorithm, the control unit (2) operates the defrost system (104) when the time between two defrost cycles exceeds such a value;

t_c sample time interval for event discretization;

A quadratic coefficient of the polynomial function (1),

B linear coefficient of the polynomial function (1), and

C ordinate at the origin of the polynomial function (1).

[0050] Equivalent variations and modifications may be made to the present embodiment of the invention, within the scope of a person skilled in the art, but still within the scope of the invention as expressed by the appended claims.

Claims

1. Refrigerated display cabinet (100) comprising:

- a compartment (101) suitable for containing products to be refrigerated and displayed;

- at least one door (102) for providing access to the compartment (101);

- a refrigeration system (103) comprising an evaporator (130) to cool the compartment (101) by means of a flow of cold air that passes through the evaporator (130);

- a defrost system (104) suitable for defrosting the evaporator (130); and

- a control unit (2) configured to operate the defrost system (104),

- temperature detection means (3) arranged in the space outside the compartment (101) to detect an ambient temperature (Ta) of the space in which the refrigerated display cabinet (100) is located;

- door open detection means (4) associated with said at least one door (102) of the refrigerated display cabinet to detect when the door is opened;

- a timer (5) associated with said door open detection means (4) to detect a door open time (t_do); and

- an algorithm (6) contained in said control unit (2);

wherein said algorithm (6) is configured to:

- receive said ambient temperature (Ta) and said door open time (t_do) and calculate a maximum door open time (t_def),

- calculate a total equivalent door open time (t_Tedo) by adding up the door open times (t_do; t_cedo)

- compare said total equivalent door open time (t_Tedo) with said maximum time (t_def),

- activate the defrost system (104) when said total equivalent door open time (t_Tedo) exceeds said maximum time (t_def);

characterized in that

the algorithm (6) comprises:

- a polynomial function (1) that receives the ambient temperature (Ta) as input and accordingly outputs said maximum door open time (t_def), and

- a comparator (7) that compares said total equivalent door open time (t_Tedo) with said maximum door open time (t_def) obtained from the polynomial function (1);

wherein said polynomial function (1) is obtained by:

- experimentally detecting a first maximum door open time at a first temperature, at a first relative humidity and at a first water content of the air in the space, so as to obtain a first point having the first temperature and the first maximum time as coordinates,

- calculating a second maximum door open time at a second temperature and at a second relative humidity of the space, by direct proportionality of the water content, so as to obtain a second point having the second temperature and the second maximum time as coordinates,

- calculating a third maximum door open time at a third temperature and at a third relative humidity of the space by direct proportionality of the water content, so as to obtain a third point having the third temperature and the third maximum time as coordinates, and

- interpolating the three points obtained with a second degree polynomial.

2. The refrigerated display cabinet (100) according to claim 1, wherein the algorithm (6) comprises a discretization and average block (8) that discretizes the ambient temperature (Ta) into sample time intervals (t_c), taking the average temperature value in the sample time interval as average ambient temperature value (T_amb.c); said average ambient temperature value (T_amb.c) being sent to the polynomial function (1).

3. The refrigerated display cabinet (100) according to claim 2, wherein the algorithm (6) comprises

- a multiplier and divider block (9) that multiplies the door open time (t_do) by the ratio of the maximum time (t_def) calculated in the current sample time interval to the maximum time (t_def-1) calculated in a previous sample time interval, and outputs an equivalent door open time (t_cedo) given by the following formula:

- an adder block (10) that adds the door open time (t_do) in the current sample time interval and the equivalent door open time (t_cedo) calculated from the value of the total equivalent door open time (t_Tedo) in a previous sample time interval and outputs said total equivalent door open time (t_Tedo) given by the following formula:

4. Process for operating a defrost system (104) of an evaporator (130) of a refrigerated display cabinet (100) having a compartment (101) suitable for containing products to be refrigerated and displayed and at least one door (102) for providing access to the compartment (101); the process comprising the following steps:

- detecting an ambient temperature (Ta) of the space wherein the refrigerated display cabinet (100) is located,

- detecting a door open time (t_do) during which said at least one door (102) of the refrigerated display cabinet is open,

- using said ambient temperature (Ta) and said door open time (t_do) to calculate a maximum door open time (t_def),

- calculating a total equivalent door open time (t_Tedo) by adding the door open times (t_do; t_cedo),

- comparing said total equivalent door open time (t_Tedo) with said maximum time (t_def),

- activating the defrost system (104) when said total equivalent door open time (t_Tedo) exceeds said maximum time (t_def),

- using a polynomial function (1) which receives the ambient temperature (Ta) as input and accordingly outputs said maximum door open time (t_def), and

- comparing said total equivalent door open time (t_Tedo) with said maximum door open time (t_def) obtained from the polynomial function (1);

wherein said polynomial function (1) is obtained by:

- experimentally detecting a first maximum door open time at a first temperature, at a first relative humidity and at a first water content of the air in the space, so as to obtain a first point having the first temperature and the first maximum time as coordinates,

- calculating a second maximum door open time at a second temperature and at a second relative humidity in the space, by direct proportionality of the water content, so as to obtain a second point having the second temperature and the second maximum time as coordinates,

- calculating a third maximum door open time at a third temperature and at a third relative humidity in the space, by direct proportionality of the water content, so as to obtain a third point having the third temperature and the third maximum time as coordinates,

- interpolating the three points obtained with a second degree polynomial.

5. The process according to claim 4, comprising the step of discretizing the ambient temperature (Ta) into sample time intervals, taking the average temperature value in the sample time interval as average ambient temperature value (T_amb.c); said average ambient temperature value (T_amb.c) being sent to the polynomial function (1).

6. The process according to claim 5, comprising the following steps:

- multiplying the door open time (t_do) by the ratio of the maximum time (t_def) calculated in the current sample time interval to the maximum time (t_def-1) calculated in a previous sample time interval, and outputting an equivalent door open time (t_cedo) given by the following formula:

- adding the door open time (t_do) in the current sample time interval and the equivalent door open time (t_cedo) calculated from the total equivalent door open time (t_Tedo) in a previous sample time interval and outputting said total equivalent door open time (t_Tedo) given by the following formula:

Drawing