EP2832266B1

EP2832266B1 - Method of controlling a horizontal refrigerated display cabinet

Info

Publication number: EP2832266B1
Application number: EP14177245.9A
Authority: EP
Inventors: Giuseppe Giomo
Original assignee: Epta SpA
Current assignee: Epta SpA
Priority date: 2013-08-02
Filing date: 2014-07-16
Publication date: 2018-04-04
Anticipated expiration: 2034-07-16
Also published as: ITPD20130222A1; ES2675821T3; EP2832266A1

Description

Field of application

The present invention relates to a method of controlling a horizontal refrigerated display cabinet.
The term "refrigerated cabinet" refers to cabinets at medium temperature, i.e. cabinets suitable to ensure product storage temperatures close to zero. In particular, with reference to the UNI EN 23953 standard, the cabinets which the present invention relates to belong to the climate class M0 with temperatures between -1°C and +4°C, class M1 with temperatures between -1°C +5°C, or class M2 with temperatures between -1°C and +7°C.
The horizontal refrigerator cabinet which the present invention relates is in particular of the type for assisted service or self-service.
The horizontal refrigerator cabinet which the present invention relates to may be of the independent type with a built-in condenser group/compressor (cabinet known in the jargon as "plug-in") or of the type with a remote condenser group/compressor. In the latter case, the remote group may be dedicated or shared with other cabinets and installed for example in a central plant. State of the art
Generally, as shown in Figure 1, a horizontal, refrigerated display cabinet B for assisted service or self-service comprises a shelf F on which the foodstuffs are placed, possibly contained in trays, and an evaporator E normally placed on the rear side of the cabinet. By means of fans (not shown) a flow of air A is created through the evaporator which, cooled, is then forced towards the products on display inside the cabinet to then be recirculated to the evaporator again.
Operatively, the air temperature is kept at specific values in order to be able to display the foodstuffs within a predefined temperature range in compliance with current legislation. To maintain such temperatures, the refrigerator cabinet is provided with one or more controller devices (electromechanical or electronic) and a sensor to measure the temperature of the air flow. Once the set temperature has been reached, the controller device stops the flow of refrigerant gas to the evaporator and re-activates it only when the measurement sensor measures a temperature higher than the predetermined set temperature ΔT. In other words, the cabinets are regulated by On/Off cycles. The ΔT value is normally about 1-2°C; lower values may result in switching on and off too frequently.
To prevent the evaporator from reaching temperatures which are too low, causing excessive drying of the air and thus of the product displayed, a valve (usually mechanical) is placed downstream of the evaporator which limits the evaporation pressure of the refrigerant at the evaporator to a predefined constant value. Such pressure valve must be calibrated at pressure values which take into account the maximum thermal loads conceivable (temperatures of the store, external irradiations, summer period, handling of the product during sale, machinery operating in the vicinity of the display cabinet, etc...) to prevent the product from warming to above the permitted temperatures.
Refrigerated cabinets according to the state of the art are known from AU 697 909 B2 or US 6 298 673 B1 .
The traditional horizontal, refrigerator cabinets described above have some drawbacks.
The temperature adjustment of the cabinet with On/Off cycles causes fluctuations of the air temperature (as shown schematically in Figure 2a attached, where the On phases are indicated with a and the Off phases with b), and thus in the temperature of the products displayed. These temperature fluctuations, which have values of 2-3°C compared to the set temperature, determine small thermal shocks in the products displayed which over time have a negative impact on the characteristics of said products.
In the ON phases the air normally reaches sub-zero temperatures, as shown in Figure 2b. The air flowing through the evaporator thus deposits water on it in the form of crystals. The quantity of water deposited will be greater the lower the temperature of the evaporator is. In addition to the formation of ice there is thus a drying effect of the air, with reduction of the relative humidity (RH%). The air sent to the products thus has lower relative humidity values and thus tends to remove moisture from the products displayed, drying them. This adversely affects the product characteristics resulting in weight losses thereof which in some cases may even be substantial, especially in the case of meats. The deterioration of the quality of the product is visible.
The formation of ice on the evaporator increases with time. The presence of ice reduces the through section of the air, resulting in a variation of the air flow between when the evaporator is free and when it has deposits of ice on it. This causes not only a reduction of the heat exchange, but also a different distribution of the air inside the cabinet with non-uniformity in the cooling of the displayed products.
Defrosting phases of the evaporator are therefore regularly provided for in the existing refrigerator cabinets (in Figure 2a a defrosting phase is indicated with c), during which the refrigeration cycle is interrupted for a time sufficient to allow the detachment of the ice from the surface of the evaporator. This causes, however, inevitable rises in temperature of the air, and thus of the products, to the detriment of the quality of storage.
Refrigerator cabinets are provided with electric resistors which are arranged in contact with the evaporator and are activated in the defrosting phase to accelerate the same. This way however there is the disadvantage of supplying heat inside the cabinet, reducing energy efficiency. In addition, after the defrosting cycle, in order to quickly restore the temperature of the display cabinet and dispose of the heat input from the electric resistor, the On phase is particularly long (see Figure 2a phase denoted by d) and the evaporator particularly cold, increasing the phenomena described above, in particular of drying of the air.
To compensate for the reduction of relative humidity (RH%) of the air, some refrigerator cabinets are equipped with an air humidification system. The use of a humidifier to increase the relative humidity, however, is expensive and can cause the increase of the bacterial load.

Presentation of the invention

Consequently, the purpose of the present invention is to eliminate or at least attenuate the drawbacks of the prior art mentioned above, by providing a method of controlling a horizontal, refrigerated display cabinet which makes it possible to reduce the temperature fluctuations in the displayed products and at the same time which is operatively simple to manage.
A further purpose of the present invention is to provide a method of controlling a horizontal, refrigerated display cabinet, which makes regular defrosting of the evaporator unnecessary.
A further purpose of the present invention is to provide a method of controlling a horizontal, refrigerated display cabinet, which makes restoration of the humidity unnecessary.

Brief description of the drawings

The technical characteristics of the invention according to the aforesaid purposes can be seen clearly from the contents of the following claims and the advantages thereof will be more clearly comprehensible from the detailed description below, made with reference to the appended drawings showing one or more embodiments by way of non-limiting examples, wherein:

Figure 1 shows a schematic view of a horizontal, refrigerated display cabinet for assisted service or free access purchases;
Figure 2a shows a schematic diagram of a typical time trend of the temperature of the air leaving the evaporator in a horizontal, refrigerated display cabinet of the conventional type;
Figure 2b shows an example of the time trend of the temperature of the air leaving the evaporator (graph T1) and of the temperature of the evaporator surface (graph T2) in a horizontal, refrigerated display cabinet of the conventional type;
Figure 3 shows a simplified diagram of the refrigeration circuit of a horizontal, refrigerated display cabinet with a dedicated condenser group/compressor;
Figure 4 shows a simplified diagram of the refrigeration circuit of a horizontal, refrigerated display cabinet of the type with a remote condensing group/ compressor;
Figure 5 schematically shows a typical time trend of the temperature of the air leaving the evaporator in a horizontal, refrigerated display cabinet and
Figure 6 shows a schematic diagram of a typical time trend of the temperature of the air leaving the evaporator (graph T) and the evaporation pressure (graph P) in a horizontal, refrigerated display cabinet.
Figure 7 shows an example of a time trend of the temperature of the air leaving the evaporator (graph T3), of the evaporation pressure (graph P) and the temperature at the surface of the evaporator (graph T4) in a horizontal, refrigerated display cabinet.

Detailed description

According to a general embodiment of the invention, the control method applies to a horizontal, refrigerated display cabinet (in particular of the type for assisted service or self-service) comprising at least an evaporator forming part of a steam compression cooling circuit and ventilation means suitable to generate an air flow that circulates between the evaporator and the display compartment of the cabinet itself.
According to the invention, the evaporator is always kept active with continuous generation of cooling capacity. In other words, regardless of the fact that the cabinet is of the independent type with built-in condensing group/ compressor, or the type with remote condensing group/compressor, the refrigerant fluid is always made to flow to the evaporator. The control method according to the invention thus adopts an approach opposite to the traditional ones, which instead provide for on/off cycles of the evaporator or the entire refrigeration circuit.
Maintaining continuous functioning of the evaporator does not mean that the evaporator cannot be turned off for short periods. The continuity of evaporator activation is functional to the control of the cabinet. In other words, according to the invention the deactivation of the evaporator is not functional to the control of the cabinet and to the regulation of its operation.
The control method according to the invention further comprises the following operating steps:

a) fixing a set temperature for the air temperature leaving the evaporator;
b) measuring over time the temperature of the air flow leaving the evaporator;
c) measuring over time the evaporation pressure of the refrigerant fluid at the evaporator,

The temperature of the air flow leaving the evaporator is an indirect measure of the external thermal loads which the refrigerator cabinet is subject to. In fact, being the operating conditions of the evaporator equal, the temperature of the air flow leaving the evaporator varies only if the temperature of the environment to be cooled changes, i.e. of the display compartment of the cabinet and of the products displayed in it.
The evaporation pressure of the refrigerant fluid is an indirect measure of the temperature of the refrigerant fluid in the evaporator. In fact, in the evaporator (in conditions of liquid-vapour equilibrium) a given pressure corresponds to a single temperature. Adjusting the evaporation pressure thus means adjusting the temperature of the refrigerant and therefore the temperature on the surface of the evaporator also.
According to the invention, the control of the refrigerated display cabinet is conducted by continuously regulating the evaporation pressure of the refrigerant fluid at the evaporator, depending on the measured temperature values of the air, so as to have the air flow leaving the evaporator at a temperature stable at the set value in regular functioning conditions of the cabinet.
"Stable temperature at the set value" means that the measured temperature of the air flow leaving the evaporator may differ from the set value by not more than ± 0.5°C, preferably not more than ± 0. 2°C, and even more preferably not more than ± 0.1°C.
"Regular functioning conditions of the cabinet" means the conditions in which the cabinet is when it is put in conditions of normal operation with the flow of air leaving the evaporator at the set temperature. The regular functioning conditions of the cabinet are different from the "transitory phases" at the moment of turning on the cabinet when the air flow is initially at ambient temperature and has not yet been brought to the regular functioning conditions with the air flow at the set temperature.
Operatively, the method thus provides for a continuous adaptation to the external conditions of the cabinet, which may be defined as "dynamic adaptation".
According to the invention in regular functioning conditions of the cabinet a step d) is performed of maintaining the temperature of the air leaving the evaporator at the set temperature by continuously regulating the evaporation pressure (if necessary) of the refrigerant fluid at the evaporator depending on the measured temperature values of the air.
According to the invention in regular functioning conditions, having reached a situation of equilibrium between the heat removed from the cabinet and the heat provided to the cabinet by the surrounding environment - in the absence of external heat loads - the pressure is kept stable at an equilibrium value.
If no significant disturbances of such equilibrium occur, the evaporation pressure and consequently also the surface temperature of the evaporator itself are thus maintained substantially constant or in any case subject to greatly limited fluctuations, as shown in Figures 5, 6 and 7.
In the first place, this leads to a substantial reduction in the temperature fluctuations of the air flow leaving the evaporator, with beneficial effects on the products stored in the refrigerator cabinet.
Furthermore, as described further below, maintaining the pressure (and therefore also the surface temperature of the evaporator) at a stable equilibrium value prevents pressure fluctuations related to the refrigerant circuit from affecting the functioning of the cabinet.
Again, the stability of the evaporation pressure and temperature at the surface of the evaporator significantly reduces phenomena of ice deposits on the evaporator and thus reduction of the relative humidity of the air leaving said evaporator.
Unlike the present invention, in traditional systems with on/off cycles the evaporation pressure is never adjusted continuously, but is fixed at a value that takes into account the maximum thermal loads conceivable in the normal operation of the cabinet. In conventional systems, therefore, the system regulates the temperature by cyclically turning off or on the refrigerant circuit depending on whether too much heat is being removed or accumulated. This way very large, cyclical fluctuations, such as those shown in Figures 2a and 2b are imposed on the temperature at the surface of the evaporator, and thus of the temperature of the air flow. Such fluctuations are further amplified by delays in intervention of the system related to the overall inertia of the refrigerant circuit both when turned off and on.
According to the invention in regular functioning conditions of the cabinet, during the maintenance step d), in the presence of consistent external heat loads which disturb the situation of equilibrium (such as for example the opening of the front glass panel of the cabinet), the evaporation pressure is regulated by progressively reducing the value as the measured temperature value of the air increases, and vice versa, so as to restore a situation of equilibrium.
Preferably, the pressure regulation in the presence of external thermal loads which perturb the situation of equilibrium is carried out following predefined trends.
In particular, such trends may also provide for a temporary stabilisation step of the pressure at a minimum intervention value. As shown schematically in the example in Figure 6, such temporary stabilisation step is preceded by a step of reducing the pressure and may be followed by a step of increasing the pressure. Such increase step is aimed at achieving a new situation of equilibrium at an evaporation pressure as high as possible, being the size of evaporator equal, compatibly with the need to maintain the air temperature leaving the evaporator stable at the set value and thus compatibly with the temperature values of the displayed product, as required by current legislation. This may happen in the case in which the thermal load which has disrupted the previous equilibrium is eliminated and the system can thus return to stabilising itself at a higher evaporation pressure, possibly (but not necessarily) at the same pressure as the previous equilibrium.
Advantageously, when the cabinet is in transitory functioning conditions - i.e.when the air temperature is at ambient temperature or in any case at temperatures above the set value - a step e) is conducted of reaching the set temperature for the air leaving the evaporator by regulating the evaporation pressure depending on the measured air temperature.
In particular, the regulation of the evaporation pressure may be conducted by following a predefined trend, depending on the measured temperature of the air flow leaving the evaporator.
Preferably, during the step e) of reaching the set temperature the evaporation pressure is regulated by progressively increasing the pressure value as the air temperature measured decreases, starting from a predefined minimum initial pressure value. Such minimum pressure value is chosen so as to avoid shocks to the products in terms of drying. Indeed, it is preferable to cool the products as gently as possible, starting from pressure and temperature values at the evaporator as high as possible. The pressure is then kept stable at an equilibrium value when the temperature has reached the set value. An example of this trend is illustrated schematically in the example in Figure 6, where the step e) of reaching the set temperature is indicated with e, while the maintenance step d) is indicated by d.
Advantageously, the regulation of the pressure depending on the measured temperature is carried out in steps. An example of reaching an equilibrium is reported below. In regular functioning conditions the temperature of the air flow leaving the evaporator is measured every fraction of a second. If the temperature measured is more than 0.1°C higher than the set temperature (set), the pressure is decreased by 0.1Psi (thereby reducing the temperature of the evaporator); after a fraction of a second the operation is repeated until the set temperature is reached, and an equilibrium condition thus achieved. In the opposite case in which the temperature is more than 0.1°C below the desired temperature, the same loop is performed in reverse (thus increasing the temperature of the evaporator). The function of the valve being that of maintaining a defined pressure in the manner described above, even when there are variations in pressure in the system, these will be compensated by the valve.
According to a particularly preferred implementation of the invention, the evaporator is sized in such a way that - keeping the evaporator constantly active- the refrigerant capacity is sufficient to keep the temperature of the products positioned in the display compartment of said cabinet according to regulations while operating with evaporator temperatures above zero.
This way ice is prevented from forming in the evaporator in regular functioning conditions (i.e. in the prevailing operating conditions of the cabinet).
This makes regular defrosting of the evaporator unnecessary, with all the possible benefits in terms of functioning continuity of the refrigerated cabinet.
In the initial step of turning on the cabinet (transitional) or in the presence of particularly strong thermal loads (for example, opening of the front panel of the cabinet) it may happen that the temperature on the surface of the evaporator temporarily falls below 0°C and thus that ice deposits on the evaporator. These are however short, temporary situations which are overcome by subsequent and more permanent regular functioning situations in which the temperature of the evaporator is above 0°C.
During such regular functioning phases of the cabinet, the ice that may have formed on the evaporator would be eliminated in a short time, without the need for a specific defrosting step.
Thanks to the present invention it is therefore possible to expand the refrigerant fluid at pressures and therefore also at temperatures above zero.
Being the size of evaporator equal, this would not be possible if a traditional on/off regulation were adopted. In fact, in this case, in the on phases it would be necessary to make the evaporator operate at a temperature below zero to compensate the off phases.
The fact of operating in regular functioning conditions with evaporator temperatures above zero also makes it possible to eliminate or significantly mitigate the drying of the air inside the refrigerated cabinet. The problems of reducing relative humidity which afflict the traditional systems are thus substantially resolved without the need to use air humidifiers. All this improves the quality and shelf life of the products displayed in the cabinet.
According to the invention, the step b) of measuring the temperature of the air flow leaving the evaporator over time is performed by means of at least one temperature sensor installed in the path of the air flow downstream of the evaporator and the step c) of measuring the evaporation pressure of the refrigerant fluid at the evaporator over time is performed by means of a pressure transducer installed in the refrigeration circuit downstream of the evaporator.
According to the invention the evaporation pressure is regulated by means of a regulation valve installed in the refrigerant circuit downstream of the evaporator. Preferably, such pressure regulating valve is of the electronic type.
According to the invention the regulation of the evaporation pressure- both in step e) of reaching the set value and the maintenance step d)- is controlled by a programmable electronic control unit which receives the signals of the temperature sensor and pressure transducer in input and operates the pressure regulating valve depending on the pressure and temperature values measured.
Operatively, the step b) of measuring over time the temperature of the air flow leaving the evaporator is repeated continuously at predefined time intervals. Preferably, such predefined time intervals are regular. Advantageously, the value of such intervals is between a few milliseconds and a few minutes. Preferably, values below one second give a much more accurate adjustment
Operatively, the step c) of measuring over time the evaporation pressure of the refrigerant fluid at the evaporator is continuously repeated at predefined time intervals. Preferably, the pressure measurement is carried out simultaneously with the temperature measurement. These predefined time intervals are regular. Advantageously, the value of such intervals is between a few milliseconds and a few minutes. Preferably, values below one second give a much more accurate adjustment. The shorter the time interval between one measurement and the next, the smaller the difference between the set value and the measured temperature of the air leaving the evaporator and therefore the greater the stability of the temperature of the air flow at the set value.
Preferably, the step e) of reaching the set temperature is conducted before the step d) of maintaining the set temperature and with a view to the latter.
A horizontal refrigerated display cabinet, in particular of the type for assisted service or self-service - controllable with the method described above - has been globally denoted by reference numeral 1 in the attached figures.
According to a general embodiment, illustrated in Figures 3 and 4, the cabinet 1 comprises:

at least one evaporator 2 belonging to a steam compression refrigeration circuit 10;
ventilation means suitable to generate a flow of air A which circulates between the evaporator 10 and the display compartment of said cabinet;
at least one temperature sensor 3 installed in the path of the air flow A downstream of the evaporator 2 to measure the temperature of the air flow leaving the evaporator.

The refrigerated cabinet 1 comprises a pressure regulating valve 4 installed in the refrigerant circuit 10 downstream of the evaporator 2. The pressure regulating valve 4 is preferably of the electronic type.
The refrigerated cabinet 1 comprises a pressure transducer 5 installed in the refrigerant circuit downstream of the evaporator 2 (preferably upstream of the regulating valve 4).
The refrigerated cabinet 1 comprises a programmable electronic control unit 6 which receives the signals generated by the temperature sensor 3 and by the pressure transducer 5 in input and operates the pressure regulating valve 4 depending on the pressure and temperature values measured.
As already said, the refrigerant circuit 10 is of the vapour compression type. In the diagrams of Figures 3 and 4, the expansion valve is denoted by number 7 and the condensing group/compressor by number 8, connected fluidically to the evaporator 2.
The horizontal refrigerator cabinet 1 can in particular be of the independent type with built-in condensing group/ compressor (cabinet known in the jargon as "plug-in"), as illustrated in the diagram in Figure 3.
The horizontal refrigerator cabinet 1 can in particular be of the type with remote condensing group/ compressor. As illustrated in the diagram in Figure 4, the remote condensing group/compressor can be shared with other cabinets (indicated by reference numeral 11) and be installed in a central plant
The invention permits several advantages to be achieved, in part already described.
The control method of a horizontal refrigerated display cabinet according to the invention first of all makes it possible to reduce the temperature fluctuations in the products displayed, with benefits in terms of quality of storage of said products.
The control method according to the invention is also operatively simple to manage. The invention in fact replaces the traditional control through On/Off cycles of the evaporator based on the prediction of the maximum thermal loads conceivable with a dynamic type controlling, based on continuous operation of the evaporator and on a continuous measurement of the actual heat loads which the cabinet is subject to. According to the invention, the evaporator is made to operate at the highest expansion/ evaporation temperature possible, being the size of evaporator equal, compatibly with the temperature values of the product displayed, as required by legislation. Compared to conventional systems, only a pressure regulating valve, a pressure detector (transducer) and an appropriate controller are needed.
Thanks to the fact that the method according to the invention provides for operating - in conditions of equilibrium- at a stable evaporation pressure, pressure fluctuations related to the refrigeration circuit (for example in the case of a shared condensing group/ compressor) are prevented from affecting the functioning of the cabinet.
The method of controlling a horizontal refrigerated display cabinet according to the invention makes it possible to expand the refrigerant at the evaporator at positive temperatures in regular functioning conditions. This substantially eliminates the phenomena of ice formation and thereby makes regular defrosting of the evaporator unnecessary. As a result, the adoption of electric resistors for defrosting the evaporator also becomes entirely unnecessary.
Thanks to the fact that the formation of ice at the evaporator is reduced to a marginal phenomenon, there are also benefits in terms of greater uniformity in the time distribution of the cooled air in the display compartment.
The stabilisation of the pressure at values which permit the expansion of the refrigerant at positive temperatures in regular functioning conditions also makes it possible to eliminate or significantly mitigate the drying of the air by the evaporator to the benefit of the products displayed in the cabinet.
The invention thus conceived thereby achieves the intended objectives.
Obviously, its practical embodiments may assume forms and configurations different from those described while remaining within the sphere of protection of the invention.
Furthermore, all the parts may be replaced with technically equivalent parts and the dimensions, shapes and materials used may be varied as required.

Claims

Method of controlling a horizontal refrigerated display cabinet, said cabinet comprising at least one evaporator (2) belonging to a steam compression refrigerant circuit (10) and ventilation means suitable to generate a flow of air which circulates between the evaporator (2) and the display compartment of said cabinet, said method being characterised by the fact that it keeps the evaporator (2) constantly active with generation of cooling capacity and that it comprises the following operating steps:
a) fixing a set temperature for the air temperature leaving the evaporator (2);

b) measuring over time the temperature of the air flow leaving the evaporator (2);

c) measuring over time the evaporation pressure of the refrigerant fluid at the evaporator (2);
wherein the control of the cabinet is conducted by continuously regulating the evaporation pressure of the refrigerant fluid at the evaporator (2) depending on the measured temperature values of the air so as to have the air flow leaving the evaporator (2) at a temperature stable at the set value in regular functioning conditions of the cabinet (1),
wherein the step b) of measuring the temperature of the air flow leaving the evaporator (2) is performed by means of at least one temperature sensor (3) installed in the path of the air flow downstream of the evaporator (2), the step c) of measuring the evaporation pressure of the refrigerant fluid at the evaporator (2) being performed by means of a pressure transducer (5) installed in the refrigeration circuit (10) downstream of the evaporator (2),
the evaporation pressure being regulated by means of a regulation valve (4) installed in the refrigerant circuit (10) downstream of the evaporator (2), said valve (4) preferably being electronic,
the regulation of the evaporation pressure being controlled by a programmable electronic control unit (6) which receives the signals of the temperature sensor (3) and pressure transducer (5) in input and operates the pressure regulation valve (4) depending on the measured pressure and temperature values,
wherein in regular functioning conditions of the cabinet a step d) is performed of maintaining the temperature of the air leaving the evaporator at the set temperature by continuously regulating the evaporation pressure of the refrigerant fluid at the evaporator depending on the temperature measured values of the air,
wherein in regular functioning conditions of the cabinet,
having reached a situation of equilibrium between the heat removed from the cabinet and the heat provided to the cabinet by the surrounding environment - in the absence of external heat loads - the pressure is kept stable at an equilibrium value,
wherein in regular functioning conditions of the cabinet, during the maintenance step d), in the presence of external heat loads which disturb the situation of equilibrium between the heat removed from the cabinet and the heat provided to the cabinet by the surrounding environment, the evaporation pressure is regulated by progressively reducing the pressure value as the temperature value of the air measured increases, and vice versa, depending on the air temperature measured, so as to restore a situation of equilibrium.
Method according to claim 1, wherein the evaporator is sized in such a way that - keeping the evaporator constantly active - the refrigerant capacity is sufficient to keep the temperature of the products positioned in the display compartment of said cabinet according to regulations by operating with evaporator temperatures above zero.
Method according to one or more of the previous claims, wherein in transitory functioning conditions of the cabinet - when the air temperature is at ambient temperature- a step e) is conducted of reaching the set temperature for the air leaving the evaporator by regulating the evaporation pressure depending on the measured air temperature.
Method according to claim 3, wherein during the step e) of reaching the set temperature the evaporation pressure is regulated by progressively increasing the pressure value as the measured air temperature decreases, starting from an initial predefined minimum pressure value, the pressure then being maintained stable at an equilibrium value when the temperature has reached the set value.
Method according to claim 4, wherein the regulation of the evaporation pressure is conducted following a predefined trend depending on the air temperature.