Technical field of the invention

The present invention relates to a diagnostic method for checking a defrost operation in a refrigeration system.

In particular, this diagnostic method allows to accurately and reliably detect any causes of inefficiency or malfunctioning of the defrost operation of the evaporator of a refrigeration system, such as in particular, a deep freezer, for example intended to contain retail products and installed in a supermarket.

State of art

During the operation of a refrigeration system, the thermal exchange surface of its cold exchanger, or evaporator, which is exposed to the environment to be refrigerated, tends to become covered with ice due to the condensation and subsequent freezing of the vapour contained in the air of that environment. The ice layer tends to thermally isolate the thermal exchange surface of the evaporator from the environment, reducing the efficiency of the refrigeration system.

To obviate this drawback, it is known to perform a cyclical defrost operation of the thermal exchange surface of the evaporator by heating this surface, for example by means of electrical resistances, in order to completely melt the ice formed on this surface. The evaporator is deactivated during the defrost operation.

The defrost operation is monitored by a controller of the refrigeration system by means of a defrost probe designed to detect the temperature of the thermal exchange surface. When the defrost operation is completed, the controller reactivates the evaporator, thus resuming the refrigeration action.

Particularly in the case of deep freezers used for storing perishable goods, for example food products, the need is strongly felt to keep these goods at a temperature that is always lower than a given safety temperature. Therefore, the defrost operation must be performed in such a way as to limit the heat transferred to the evaporator as much as possible and must also have a duration which is as limited as possible in order to avoid an unacceptable increase in temperature (given that as mentioned, the refrigeration action is interrupted during the defrost operation). Furthermore, after the defrost operation, a so-called "pull-down" step is envisaged, i.e. a sudden lowering of the temperature, which involves the activation of the evaporator at maximum cooling capacity to bring it back as quickly as possible to optimal working conditions and to restore the temperature of the compartment to be refrigerated.

A possible positioning error of the defrost probe, for example too close to the electrical defrost resistances, as well as a possible decalibration or malfunctioning of the defrost probe, can compromise the quality and effectiveness of the defrost operation. Too long or too short intervals between two successive defrost operations can compromise the efficiency of the refrigeration system. For example, the defrost operation can be incomplete or infrequent, and therefore compromise the heat exchange at the thermal exchange surface of the evaporator. Conversely, the defrost operation can be too long or too frequent, causing too high temperature peaks at the thermal exchange surface, and therefore requiring a considerable increase in the cooling action to compensate for the excessive heating, and consequently leading to an excessive consumption of power.

Summary of the invention

The object of the present invention is to improve the defrost efficiency of a refrigeration system, while allowing the identification of any malfunctions or incorrect configurations of the machine which could compromise the efficiency of the defrost operation.

This and other objects are fully achieved according to the present invention thanks to a diagnostic method for checking a defrost operation as defined in attached independent claim 1.

Thanks to this diagnostic method, it is possible to identify, in a simple but at the same time reliable way, conditions of incorrect execution of the defrost operation, whether due to malfunctions of the defrost probe or of the electrical resistances, to an incorrect positioning of the defrost probe, or to an incorrect setting of the defrost parameters (such as the time interval between two consecutive defrost operations, the duration of the defrost operation, the defrost operation end temperature, etc.), and also to propose - depending on the cause that generated an abnormal operating condition - one or more possible corrective actions aimed at eliminating this cause. The diagnostic method of the present invention also allows achieving a reduction in energy consumption, both during the defrost operation and in normal operation, since it ensures that the evaporator is always efficient. Furthermore, a better operating quality of the refrigeration system is obtained, with the operating temperatures always being kept within the optimal ranges, and therefore a better quality of the food products is ensured in the event the refrigeration system is a deep freezer for the preservation of food products.

Preferred ways of implementing the method according to the present invention are the subject of the dependent claims, the content of which is to be understood as an integral part of the following description.

The characteristics and advantages of the present invention will become evident from the following detailed description, provided purely by way of non-limiting example with reference to the attached drawings.

Brief description of the drawings

Reference will be made in the following detailed description of the invention to the figures of the attached drawings, in which:

- Figure 1 is a graph showing the trend over time of the temperature detected by the defrost probe of the evaporator of a refrigeration system during a defrost operation;

- Figure 2 is a normalized histogram of the temperature frequencies during the defrost operation of Figure 1;

- Figure 3 is a graph showing an example of a time trend of the temperature detected by the defrost probe of the evaporator of a refrigeration system in the case of a defrost operation carried out correctly;

- Figures 4 to 9 are graphs showing examples of the time trend of the temperature detected by the defrost probe of the evaporator of a refrigeration system in the case of defrost operations carried out incorrectly; and

- Figures 10 and 11 show a block diagram of the diagnostic method for checking the defrost operation of the evaporator of a refrigeration system according to an embodiment of the present invention.

Detailed description

As explained in the introduction to the present description, during the operation of a refrigeration system, the thermal exchange surface of the evaporator tends to become covered with ice. To melt the ice layer thus formed, the refrigeration system controller cyclically controls a defrost operation of the evaporator, which is performed by heating, for example by means of electrical resistances, the thermal exchange surface of the evaporator so as to completely melt the layer of ice formed on this surface. The defrost operation is monitored by the controller by means of a defrost probe which is mounted in the coldest part of the evaporator, in contact with the thermal exchange surface thereof.

Figure 1 of the attached drawings shows the typical time trend of the temperature measured by the defrost probe during a defrost operation in the case of a defrost probe functioning correctly and correctly positioned. For the purposes of the present invention, the expression "during the defrost operation" is to be understood as during the time interval in which heat is supplied to the thermal exchange surface of the evaporator, for example by the activation of electrical resistances.

Due to the effect of the heat supplied to the thermal exchange surface of the evaporator, for example by means of electrical resistances, the temperature detected by the defrost probe initially increases (time interval between the instants indicated in the diagram with to and ti), and then remains substantially constant at a given value (indicated with T _melt ) for a certain time interval (between the instants indicated in the diagram with ti and t2), and then increases again until the end of the defrost operation (time interval between instants t ₂ and t ₃ ).

The time trend of the temperature therefore shows an inflection point at the temperature Tmeit in the interval between the time instants ti and t ₂ , during which the phase transition of the ice from the solid to the liquid phase takes place. This temperature (hereinafter referred to as the "reference temperature") represents the temperature measured by the defrost probe when the ice on the thermal exchange surface begins to melt. In the event of a possible offset error of the defrost probe, the reference temperature T _melt will not be equal to the melting temperature of the ice, i.e. 0° C.

Therefore, it is possible to determine the possible offset error of the defrost probe by detecting the temperature value at which the time trend of the temperature has an inflection point.

The example shown in Figure 1 shows that the inflection point of the temperature trend is positioned at -6.5° C, which means that the defrost probe in question has an offset error of -6.5° C. In this case, the temperature of -6.5° C will therefore be considered as the reference temperature Tmeit.

As shown in Figure 2, the detection of the inflection point in the time trend of the temperature measured by the defrost probe, and therefore the determination of the offset error of the probe, can take place by constructing the normalized histogram of the temperature frequencies recorded by the probe at each defrost operation. The presence of an evident peak (value for example higher than 0.6) indicates the temperature value measured by the probe (in this example the temperature of -6.5° C) at which the ice melts.

In the event that the peak of the histogram is not clear, the construction of a second histogram is advantageously provided, the classes of which are offset by half of their amplitude. In this way it is possible to detect the melting temperature if it straddles two classes of the first histogram, and therefore if two close peaks, but of lower value, were present in this histogram.

Preferably, the offset error of the defrost probe is determined on a daily basis by examining all the defrost operations of the day in question and considering the average of the inflection point temperatures detected during these operations. For an inflection point to be considered valid, and therefore its temperature to be considered for the purpose of determining the offset error, the following two conditions must occur simultaneously:

- there must be a single peak with a value not lower than 0.6 in the normalized histogram; and

- the non-normalized histogram must count at least 4 temperature samples at the peak. The second condition reported above therefore means that the inflection point must last at least 4 minutes to be considered valid in the case of temperature sampling at intervals of, for example 1 minute.

If it is not possible to determine the offset error on the basis of the data of the day in question, the last value detected in the defrost probe memory is searched. If this search is unsuccessful, a null offset error, and therefore a reference temperature value T _meit equal to 0° C, is assumed.

The diagnostic method can therefore include a step of verifying the correct configuration of the refrigeration system, which in the event of a positive result, consents to the possibility of verifying the correct execution of each individual defrost operation, while in the event of a negative result, provides a fault signal for requesting the intervention of an operator in order to restore the correct configuration of the refrigeration system.

In this verifying step, the aforementioned reference temperature T _meit is used (which as said, represents the offset error of the defrost probe) and the temperatures of the refrigeration system are considered when it is operating, i.e. when there is a passage of coolant gas in the evaporator, and therefore a defrost operation is not in progress.

More precisely, this verifying step is carried out by taking into consideration the following temperatures in addition to the reference temperature Tmeit of the defrost probe: - the evaporation temperature T _evp , i.e. the temperature of the coolant gas at the evaporator inlet (this temperature is typically not detected by a temperature probe, rather is obtained on the basis of the pressure measured by a pressure sensor in the evaporator),

- the temperature T _def of the defrost probe, and

- the operation temperature T _reg of the refrigeration system (i.e. the temperature of the air at the evaporator outlet).

The following positive constant values are also used to carry out this verifying step:

- the constant value ei _, lower than 1, for example between 0.1 and 0.3, having the physical meaning of the minimum acceptable difference between the defrost temperature T _def and the evaporation temperature T _evp ,

- the constant value ε _2, lower than 1, but higher than ei, having the physical meaning of the difference between the defrost temperature T _def and the evaporation temperature T _evp beyond which the refrigeration system can be considered correctly configured,

- the constant value ε ₃ , higher than 1 , for example between 1 and 2, having the physical meaning of the minimum acceptable difference between the operation temperature T _reg of the refrigeration system and the defrost temperature T _def , and

- the constant value ε ₄ , higher than 8 ₃ , for example between 2 and 3, having the physical meaning of the difference between the operation temperature T _reg of the refrigeration system and the defrost temperature T _def beyond which the refrigeration system can be considered correctly configured.

The constant values ε ₁ , ε ₂ , ε ₃ and ε ₄ can be fixed arbitrarily, either manually or by further automated analyses, to adapt the method to the particular refrigeration system to be controlled, even if it is preferable for them to be included in the range examples indicated above.

Considering a number n of samples acquired with the refrigeration system in operation (i.e. when defrost operations are not in progress), in order to evaluate if the refrigeration system is configured correctly and suitable for the remainder of the diagnosis procedure, the following two performance indices S _de and S _rd are calculated: where "ramp (v, p ₁ , P ₂ )" means a function that returns a minimum value equal to 0 for v < p ₁ , a maximum value equal to 1 for v >p2, and a linearly increasing value from 0 to 1 as v increases in the range between p ₁ and p2.

S _de is a score that assumes a real value between 0 and 1 and indicates how much the difference between the defrost temperature T _def and the evaporation temperature T _evp respects the expected conditions. The evaporation temperature T _evp must be the lowest temperature inside the refrigeration system.

S _rd is a score that assumes a real value between 0 and 1 and indicates how much the difference between the operation temperature T _re g of the refrigeration system and the defrost temperature T _def respects the expected conditions. The defrost temperature T _def must always be lower than the operation temperature T _reg when the refrigeration system is in operation (i.e. far from the defrost operation steps).

The lower of the two indices S _de and S _rd calculated on a daily basis is used as a compliance index l _c of the refrigeration system.

If the value of the compliance index of the refrigeration system is higher than or equal to a threshold value C, then consent is given to the analysis of the individual defrost operations which took place on the same day for which the compliance of the system refrigerator was evaluated. If, on the other hand, the value of the compliance index of the refrigeration system is lower than the threshold value C, then the analysis of the individual defrost operations of that day is not performed. The sensitivity with which the refrigeration system is considered compliant for the analysis of the individual defrost operations depends on the threshold value C. It has been empirically found that a threshold value C comprised between 0.3 and 0.4 is optimal for correctly filtering refrigeration systems not compliant for analysis.

This step of verifying the correct configuration of the refrigeration system is preferably carried out on a daily basis, i.e. considering all the samples acquired (for example with a frequency of one every minute) throughout the day. If the compliance index of the refrigeration system is lower than the threshold value C, for example due to malfunctioning or incorrect positioning of the defrost probe, the diagnosis process is interrupted and leads to the generation of a fault signal which informs the operator about the fault found.

The diagnostic process therefore includes a verification step of each individual defrost operation in order to determine whether this operation has been performed correctly or not and, if not, to report the presence of a fault and preferably indicate one or more possible corrective actions to remedy this fault. This verification step is performed by checking the time trend of the temperature detected by the defrost probe during the defrost operation and verifying the respect of a series of conditions, as illustrated below.

To verify the correct execution of the single defrost operation, two performance indices are considered, Smelt and Smax, which depend respectively on the ratio rt _m-d = t _melt / t _def between the time interval t _melt in which the defrost temperature T _def is equal to the reference temperature Tmeit (time interval t ₁ -t ₂ in the diagram in Figure 1) and the total duration t _def of the defrost operation, and the maximum value T _max reached by the defrost temperature T _def during the defrost operation. As explained in detail below, these performance indices must be higher than a given constant value, which means, with regards to the index Smelt, that the ratio rt _m-d must be higher than a given minimum threshold value, and with regards to the index Smax, that the maximum temperature T _max must be higher than a given minimum threshold but lower than a given maximum threshold value.

Preferably, the indices Smelt and Smax are calculated according to the following formulas:

Smelt ⁼ ramp(rtm-d, τ1, τ2), and

Smax = trapezoid(Tmax, 01, 02, 03, 04), where "trapezoid (v, p ₁ , p ₂ , P ₃ , P ₄ )" means a function that returns a minimum value equal to 0 for v <p ₁ or v >p ₄ , a maximum value equal to 1 for v comprised between P ₂ and p ₃ , a linearly increasing value from 0 to 1 as v increases in the range between p ₁ and P ₂ , and a linearly decreasing value from 1 to 0 as v increases in the range between P ₃ and P ₄ , and where: τ1 is a positive constant value in the order of decimal places, for example between 0 and 0.1 or 0.15, and represents the minimum threshold that the ratio t _melt / t _def must reach (in the sense that values of this ratio which are lower than ti denote a potentially superfluous defrost operation as it is performed despite the low quantity of ice to melt),

T ₂ is a positive constant value in the order of decimals, but in any case higher than ti (for example equal to 0.2 or 0.25), and represents the value of the ratio t _melt / t _def beyond which the ice melting time compared to the total duration of the defrost operation is considered adequate, θ ₁ is a first temperature value, heuristically chosen for example in the range between 2° C and 3° C, θ ₂ is a second temperature value, heuristically chosen for example in the range between 5° C and 6° C, 03 is a third temperature value, heuristically chosen for example in the range between 14° C and 16° C, and

04 is a fourth temperature value, heuristically chosen for example in the range between 20° C and 22° C.

The index Smelt, whose value varies between 0 and 1, indicates whether the time interval in which the ice melted during the defrost operation under analysis was long enough. This index will be maximum (equal to 1) in the case of ratio t _melt / t _def higher than the constant value t _å.

The index Smax, whose value varies between 0 and 1, will be maximum (equal to 1) when the maximum temperature T _max during the defrost operation is comprised between 0 ₂ and 03. During the defrost operation, the maximum temperature T _max must exceed a given minimum value to ensure that all the ice inside the evaporator has melted, but at the same time is not to exceed a given maximum value to avoid unnecessary waste of energy as well as to avoid thermal shock to the food stored in the deep freezer in the event that the refrigeration system in question is a deep freezer.

The defrost operation is considered correctly performed, and therefore no fault signal is generated, if the time trend of the defrost temperature T _def during the defrost operation shows an inflection point at the reference temperature Tmeit and if both the following conditions are met:

Smelt > γ ₁ , and S _max > γ ₂ , where γ ₁ and γ ₂ are positive constant values lower than 1.

This means on the one hand that the ratio t _melt / t _def must be higher than a given minimum threshold value τ _min , which in the case of the Smelt index calculated with the aforementioned "ramp" function, will be equal to , and on the other hand that the maximum temperature T _max must be comprised between a minimum threshold value 0min and a maximum threshold value 0 _max , which in the case of the index S _max calculated with the aforementioned "trapezoid" function, will be respectively equal to

For simplicity, the following description considers the case in which the two constant values γ ₁ and γ ₂ are equal to the same value g, in particular equal to 0.66.

An example of a time trend of the defrost temperature T _def in the case of a defrost operation which satisfies the above-mentioned requirements, and which is therefore considered by the present diagnostic method to be correctly performed, is shown in Figure 3.

Figures 4 to 6 show respective examples of the time trend of the defrost temperature T _def in the case of defrost operations performed incorrectly, for which the diagnostic method according to the invention therefore provides for the generation of a fault signal. In all these examples, the time trend of the defrost temperature T _def shows an inflection point at the reference temperature T _melt , but at least one of the performance indices S _max and S _melt is lower than the constant value g defined above.

More specifically, Figure 4 refers to a defrost operation in which Smelt < γ. This situation indicates a scarce presence of ice in the evaporator before the start of the defrost operation. In fact, there was an ice melting phase, as shown by the fact that the defrost temperature trend T _def shows an inflection point, but this phase was short-lived as the ice melted very quickly. In this case therefore, the defrost operation was performed without any need for it, with consequent useless consumption of electricity. Reprogramming the time of the defrost operations is recommended as a corrective action in this case, with an increase in the time interval between two consecutive operations.

Figure 5 shows an example of a defrost operation in which Smax < g and T _max < T _melt + θ ₂ . This situation indicates the possibility that the ice inside the evaporator has not completely melted. In this case, the recommended corrective actions are checking the evaporator for any ice and if there is any, the forced execution of an additional defrost operation. Additionally or alternatively, increasing the duration t _def of the defrost operation is recommended.

Figure 6 shows an example of a defrost operation in which S _max < γ and T _max ≥ T _melt + θ2. This situation indicates that the ice inside the evaporator has completely melted, but that the maximum temperature has reached too high a value, for example due to an incorrect configuration of the defrost parameters (such as the duration of the defrost operation), thus leading to an unnecessary consumption of electricity. In this case, the recommended corrective actions are the reduction of the duration t _def of the defrost operation and the setting of a lower temperature at the end of the defrost operation.

Figures 7 to 9 show respective examples of the time trend of the defrost temperature T _def in the case of defrost operations carried out incorrectly, for which the diagnostic method according to the invention therefore provides for the generation of a fault signal. The time trend of the defrost temperature T _def does not show an inflection point at the reference temperature Tmeit in any of these examples, and therefore the performance index Smelt is lower than the constant value g defined above. More specifically, Figure 7 refers to a defrost operation in which T _max < Tmeit + qi. This situation indicates that the defrost operation was not able to completely melt the ice inside the evaporator due to the failure to reach a temperature sufficiently higher than the reference temperature T _melt due, for example to the fact that the defrost operation was too short and/or to a malfunction of the electrical resistances. In this case, the recommended corrective actions are the forced execution of an additional defrost operation, after checking the actual presence of residual ice inside the evaporator, and in addition or alternatively, increasing the duration t _def of the defrost operation.

Figure 8 relates to a defrost operation in which T _max ≥ T _melt + qi. This situation indicates a possible absence of ice in the evaporator prior to the start of the defrost operation, but in any case the achievement of a defrost temperature high enough to guarantee the absence of residual ice in the evaporator at the end of the defrost operation. As in the case described above with reference to Figure 4, the recommended corrective action also in this case is reprogramming the time of the defrost operations with an increase in the time interval between two consecutive operations.

Finally, Figure 9 shows an example of a defrost operation in which S _max < g and T _max > Tmeit + 02. This situation indicates a possible absence of ice in the evaporator prior to the start of the defrost operation, together with the achievement of a maximum temperature which is too high, and therefore an unnecessary consumption of electricity. As in the case described above with reference to Figure 6, the recommended corrective actions also in this case are the reduction of the duration t _def of the defrost operation and the setting of a lower temperature at the end of the defrost operation.

Finally, Figures 10 and 11 show (divided into two parts) the flow chart of an implementation example of the diagnostic method described above, in which all the steps described above are envisaged.

However, these steps may not all be envisaged or implemented. For example, assuming that the refrigeration system is correctly configured and functioning correctly, the aforementioned step of verifying the correct configuration of the refrigeration system (indicated as " step 2" in the part of the flow chart shown in Figure 10) might not be carried out. In this case, the verification step (indicated as " step 3" in Figures 10 and 11) would be carried out at each defrost operation, aiming to identify the presence of an inflection point in the time trend of the defrost probe temperature, as well as to verify how the maximum value of the defrost temperature during the defrost operation relates in relation to the reference temperature Tmeit, in order to assess whether the defrost operation has been performed correctly or not and, in this second case, generate a fault signal and recommend one or more corrective actions according to the type of fault found. In a preferred embodiment of the method however, the step of verifying the correct configuration of the refrigeration system is advantageously carried out and meeting the aforementioned condition for the compliance index value of the refrigeration system not to be lower than the threshold value C constitutes a necessary precondition for the subsequent verification step of the correct execution of the defrost operation to be carried out. In other words, if the aforementioned condition for the compliance index value of the refrigeration system not to be lower than the threshold value C is not met, then the verification step of the correct execution of the defrost operation is not carried out, rather a fault signal is generated which invites the operator to check the correct configuration of the refrigeration system, in particular the correct positioning of the defrost probe and/or its correct functioning.

As for the step (indicated as " step 1" in the part of the flow chart shown in Figure 10) for calculating the offset error of the temperature probe, i.e. calculating the reference temperature T _meit , this can also be optional. In fact, in the case the possible offset error of the temperature probe is known with certainty and precision, it is possible to use this value to verify in which relationship the maximum temperature measured by the defrost probe during a defrost operation occurs with respect to the reference temperature Tmeit. In a preferred embodiment of the method however, this calculation step is advantageously performed in such a way as to ensure the reliability of the aforementioned checking and verification steps, since these both use the reference temperature Tmeit and are therefore affected by a possible offset error of the defrost probe.

The diagnostic method described above is advantageously performed by a special software installed in the controller of the refrigeration system or in a remote control system connected remotely with the refrigeration system and communicating with it through a communication network, in particular over the Internet.

In light of the description provided above, the advantages that can be achieved with a diagnostic method the object of the present invention are evident.

The method allows a way to simply and reliably identify conditions of the incorrect execution of the defrost operation, whether due to malfunctions of the defrost probe or of the electrical resistances, to an incorrect positioning of the defrosting probe or to an incorrect setting of the defrost parameters (such as the time interval between two consecutive defrost operations, the duration of the defrost operation, the temperature at the end of the defrost operation, etc.), while also proposing - depending on the cause that generated an abnormal operating condition - one or more possible corrective actions aimed at eliminating this cause. The method also makes it possible to achieve a reduction in energy consumption, both during the defrost operation and during normal operation, since it ensures that the evaporator is always efficient. Furthermore, a better operating quality of the refrigeration system is obtained, with the operating temperatures always being kept within the optimal ranges, and therefore a better quality of the food products is ensured in the event the refrigeration system is a deep freezer for the preservation of food products.

The present invention has been described here with reference to a preferred embodiment thereof. It is to be understood that other embodiments may be provided which share the same inventive core with the one described here, as defined by the scope of protection of the claims set out below.