EP3891396B1

EP3891396B1 - Apparatus and process for controlling a compressor assembly lubrication status and refrigerating machine comprising said apparatus

Info

Publication number: EP3891396B1
Application number: EP19835720.4A
Authority: EP
Inventors: Giovanni ROSSANESE; Luca Mattiello; Luigi Nalini; Willy MUVEGI
Original assignee: Carel Industries SpA
Current assignee: Carel Industries SpA
Priority date: 2018-12-04
Filing date: 2019-12-02
Publication date: 2023-06-07
Anticipated expiration: 2039-12-02
Also published as: EP3891396C0; EP3891396A1; WO2020115639A1; PL3891396T3

Description

The present invention relates to an apparatus and a process for controlling the lubrication of a compressor unit of a refrigerating machine.
Clearly, the present invention also relates to a refrigerating machine which comprises said apparatus for controlling the lubrication in a compressor unit of a refrigerating machine.
The present apparatus and process are especially suitable for controlling the lubrication in a sealed compressor unit of a refrigerating machine and, in particularly, in a single-rotor or dual-rotor rotary compressor.
In particular, the present invention is aimed at controlling the lubrication in a variable-speed compressor especially operated by a driving device provided with inverter.
With the present apparatus and process it is possible, in particular, to detect a condition where there is insufficient oil much more rapidly than conventional apparatus and processes.
In particular, the document JP2016080309 teaches providing an apparatus for controlling the lubrication of a sealed compressor unit of a refrigerating machine which comprises a capillary tube designed to cause expansion of a refrigerating fluid passing through it.
Moreover, the document EP1367259 A1 relates to a refrigerating apparatus wherein the amount of the lubricating oil in the sealed case of the compressor is detected from the difference between the temperature upstream and downstream of a pressure reducing unit. The capillary tube has an inlet connected, via valves, to a first and second tapping duct which communicate with the casing of a compressor, and an outlet which is connected to the said casing so as to introduce back into it fluid which, during use, passes through the capillary tube.
In particular, the tapping ducts are associated with the casing so as to draw off at different heights with respect to the bottom of the said casing.
Two temperature sensors are fixed to the inlet and the outlet, respectively, and are connected to a control device for calculating the difference between the temperatures detected by the two sensors.
As is known, the casing has an intake duct and a delivery duct which are designed to be connected to corresponding sections of a refrigerating circuit.
During normal operating conditions of the compressor, inside the casing there is a quantity of oil, which is mainly collected on the bottom of the casing, and the refrigerating fluid, in gaseous form, which is compressed by the compressor.
During normal operation of the compressor, the refrigerating fluid which is introduced by the action of the compressor into the refrigerating circuit conveys with it an oil fraction which crosses the refrigerating circuit so as to be finally introduced back into the casing.
Clearly, depending on the operating conditions of the refrigerating plant and of the compressor operation, the quantity of oil present in the casing of the compressor may be insufficient for suitable lubrication of the moving parts thereof.
Insufficient lubrication may result in undue overheating of the compressor and/or excessive wear of the moving parts and even seizing.
In general, insufficient lubrication results in the risk of damage to the compressor or reduces its working life and efficiency.
In accordance with the solution presented in the aforementioned patent document, in order to detect the level of the lubricating oil present in the casing of the compressor, during operation of the compressor fluid is tapped from the casing.
This fluid generally consists of a mixture of oil and refrigerating fluid.
According to the principle forming the basis of the conventional apparatus described here the greater the fraction of the refrigerating fluid tapped which passes through the capillary tube, the greater will be the temperature difference detected by the temperature sensors, owing to the throttling effect performed by the capillary tube.
The difference in temperature detected will therefore be an index of the quantity of oil drawn off at the corresponding height of the casing.
Owing to the provision of two levels of the casing from where fluid is tapped it is possible to detect at which level the tapped fluid has a small oil fraction.
This conventional solution, however, has a number of drawbacks and limitations.
In fact, normally, from the start of tapping namely when one of the tapping valves is open, it is required to wait for the apparatus to become heated before obtaining a reliable measurement of the temperature difference.
From the time of opening of the valve it may be required to wait even 5 to 10 minutes, depending on the relevant dimensions of the apparatus, in order to obtain a reliable measurement. Furthermore, it is not possible to draw off an excessive amount of fluid from the casing since, otherwise, precisely this draw-off fluid would result in the reduction, even to a critical degree, of the oil level inside the casing.
Owing to the time period of the tapping step, required to obtain a reliable measurement of the temperature difference, it follows necessarily that the frequency with which a tapping cycle may be started, with verification of the temperature difference and consequently the oil level, is equally limited.
This conventional apparatus, and its consequent operating method, are therefore relatively slow in detecting a possible critical level of the oil present in the compressor.
The delay in detection of such a critical level may result in damage to the compressor or excessive wear thereof, with a consequent reduction in its efficient working life.
The problem underlying the present invention is therefore that of increasing the rapidity of checking the lubrication of a compressor in a compressor unit of a refrigerating machine.
The task of the present invention is therefore that of solving the aforementioned problem by proposing an apparatus and a process for controlling the lubrication of a compressor unit of a refrigerating machine and a refrigerating machine comprising said apparatus which are such that a low level of lubricating oil may be detected more rapidly.
In the context of this task an object of the present invention is to propose an apparatus and a process for controlling the lubrication of a compressor unit of a refrigerating machine and a refrigerating machine comprising said apparatus which are such that the average lubrication conditions of the compressor may be detected more rapidly.
Another task of the present invention is that of developing an apparatus and a process for controlling the lubrication of a compressor unit of a refrigerating machine and a refrigerating machine comprising said apparatus which are such that the operating conditions of a refrigerating machine provided with a plurality of compressors may be improved.
This task and these and other objects which will appear more clearly below are achieved by a device and process for controlling the lubrication of a compressor unit of a refrigerating machine and by a refrigerating apparatus which comprises said device according to the attached independent claims.
Detailed characteristic features of a device and a process for controlling the lubrication of a compressor unit of a refrigerating machine and a refrigerating apparatus comprising said device, according to the invention, are described in the dependent claims. Further characteristic features and advantages of the invention will emerge more clearly from the description of at least one preferred, but not exclusive embodiment of the apparatus and process for controlling the lubrication of a compressor unit of a refrigerating machine and a refrigerating apparatus comprising said device, shown by way of a non-limiting example in the attached sets of drawings in which:

Figure 1 shows a refrigerating machine according to the present invention;
Figure 2 shows a detail of a refrigerating machine according to the present invention;
Figure 3 shows a flow diagram of a check procedure of a process according to the present invention;
Figure 4 shows a flow diagram of an estimation procedure of a process according to the present invention;
Figure 5 shows polynomial tables for calculating the mass flowrates processed by a compressor of a refrigerating machine according to the invention;
Figure 6 shows a detail of a possible embodiment of a refrigerating machine according to the present invention.

With particular reference to the said figures, 10 denotes overall a refrigerating machine which, for example, comprises:

at least one evaporation member 11;
at least one condensation member 12;
an expansion member 13, preferably with an adjustable throughput;
a compressor 15, which is preferably sealed and has a casing;
a control apparatus 16 for controlling the lubrication of the compressor 15.

According to one feature of the present invention, the control apparatus may comprise an expansion valve 17 which has an adjustable opening, an inlet 18 and an outlet 19 and is configured to expand a refrigerating fluid contained in a flow which, during use, crosses the expansion valve 17.
The inlet 18 is designed to be placed in fluid communication with the casing of the compressor 15 so as to receive a fluid flow.
The control apparatus 16 may comprise:

a first temperature sensor 20, which is thermally connected to the inlet 18 and a second temperature sensor 21 which is thermally connected to the outlet 19.

Moreover the control apparatus 16 may also comprise a control device 22 connected to the temperature sensors and to expansion valve 17 so as to drive to it.
Moreover, according to one aspect of the present invention, the control device 22 is programmed to execute a process for controlling the lubrication of a compressor unit 15 of a refrigerating machine 10, as described below.
The control apparatus will be preferably in fluid connection with the casing 15a of the compressor unit 15 at a level higher than the minimum level of lubricating oil acceptable for lubrication of the compressor unit 15.
In detail, the compressor unit 15 has an inlet duct 23 and a delivery duct 24.
The compressor unit 15, in a manner conventional per se, may have, associated with the intake duct 23, a gas-liquid separator 25 for preventing the entry 18 of liquid into said compressor unit 15. In the present description, the expression "compressor unit" is understood as meaning, generally, a single compressor (as shown in example of Figure 1) or a group of compressors (as shown in the example of Figure 2).
Each compressor may therefore have an aforementioned gas-liquid separator 25, or one which is common to a plurality of compressors which form the compressor unit 15 may be provided.
Below, for simpler description, unless otherwise indicated, reference will be made to a compressor unit consisting of a single compressor, the same description being applicable, mutatis mutandis, to a compressor unit composed of a plurality of compressors.
The gas-liquid separator 25 may have a bottom storage zone 26 where lubricating oil conveyed by the refrigerating fluid during operation of the refrigerating machine 10 may be collected.
An intake section 23a of the intake duct 23 may extend through the bottom storage zone towards the casing 15a.
A replenishment duct, not visible in the attached drawings, may connect the bottom storage zone 26 to the casing of the compressor unit 15 so as to reintroduce into the latter the lubricating oil collected inside the gas-liquid separator 25.
The replenishment duct 27 may comprise a hole formed in the intake section 23a so that the gas drawn in through the latter conveys refrigerating oil into the casing 15a.
It can therefore be understood how, by means of a control apparatus 16, according to the present invention the inlet 18 and the outlet 19 of the expansion valve 17 may be rapidly heated, it being possible to verify more rapidly and more frequently whether the temperature difference between the inlet 18 and the outlet 19 are such as to indicate a large friction of refrigerating fluid which crosses the expansion valve 17 namely that there is a small amount of lubricating oil present in the compressor.
The expansion valve 17, after being fully opened, may be set to a degree of opening which allows replenishment, by means of the replenishment duct, of the level of lubricating oil inside the compressor before full opening.
Verification of the aforementioned temperature difference may therefore be performed following said replenishment so as to prevent the action of the control device from disturbing the real operating conditions of the compressor unit 15.
It should be noted, that in the present description, the expressions "maximum opening" or "full opening", relating to the expansion valve 17, is not understood as meaning necessarily the maximum possible opening of the said valve, but is understood as meaning a maximum opening value from among those used for the expansion valve 17 in the context of the operations described here.
In other words, said expressions may be understood as meaning a predefined opening configuration depending on the relevant implementation conditions of the present invention, which may in some cases coincide with the actual maximum opening limit of the said expansion valve 17.
With particular reference to Figure 2, here the compressor unit 15 comprises a plurality of compressors which are identified by the reference numbers 151, 152 and 153.
Each may be provided with a gas-liquid separator 25 and connected to a control apparatus 16.
Preferably, these control devices will have a single common control device which, as such, is shown in Figure 2.
The outlets 19 of the control apparatus 16 will be in fluid communication with each other and in fluid communication with the intake ducts 23 of the compressors 151, 152 and 153.
Preferably, the intake ducts 23 will be preferably connected to a header 28 for obtaining an equal distribution of the return flow, from the control apparatus 16, to the compressors 151, 152 and 153.
in this case it will be possible to rebalance the level of lubricating oil in all the compressors 151, 152 and 153 by simply causing maximum opening of the expansion valves 17 of all the control apparatus 16.
In this case, in fact, the lubricant will be drained by the control devices 16 in greater amounts from those of the compressors 151, 152 or 153 in which there is greater accumulation and is redistributed between the compressors 151, 152 or 153 uniformly by means of the header 28.
The header 28 may also comprise a series of sections of the intake duct which are configured to optimize homogenization of the return flow rate among the compressors 151, 152 and 153.
The present method also relates to a process for controlling the lubrication of a compressor unit 15 of a refrigerating machine 10 which is provided with an expansion valve 17 which has an adjustable degree of opening and which has an inlet 18 and an outlet 19 and which is in fluid communication with a casing 15a of the compressor 15 so as to be passed though by a flow tapped from the latter.
Refrigerating fluid and lubricating oil for lubricating the compressor unit 15 are present inside the casing 15a.
The outlet 19 of the expansion valve 17 is connected to the intake duct of the compressor 15 so as to replenish the latter with the tapped fluid, preferably via the replenishment duct 27, for example as described above.
The process may comprise a check procedure which comprises:

a tapping operation which involves drawing a flow from the compressor and passing it through the expansion valve 17, so as to expand it;
a measurement operation which involves measuring a temperature difference which the fluid has between the inlet 18 and the outlet 19 of the expansion valve 17, while the tapping operation is in progress;
a verification operation which is positive if the modulus of the measured temperature difference, in the measurement operation, exceeds a threshold value;
a replenishment operation which involves the introduction of lubricating oil into the compressor.

The replenishment operation is carried out when the result of a verification operation is positive.
At the start of the tapping operation, a heating step may be envisaged, said step involving setting the degree of opening of the expansion valve 17 to a start-up level which corresponds to maximum opening of the expansion valve 17, so as to obtain rapid heating by the fluid tapped from the compressor, of the inlet 18 and the outlet 19 of the expansion valve 17.
The tapping operation may involve, following the heating step, a check step which involves setting the degree of opening to a check level at which the degree of opening of the expansion valve 17 is less than that of the start-up level.
The check procedure involves starting the check step, and the verification operation, not before the temperature of the inlet 18 is equal to the temperature of the oil contained in the compressor unit 15, i.e. to the discharge temperature of the compressor unit 15 or to a predefined fraction thereof.
"Discharge temperature" as well as "discharge pressure" of the compressor unit 15 are understood as meaning, respectively, the temperature and the pressure measured directly downstream of the compressor unit 15 along the delivery duct 24.
The check step may comprise a plurality of check sub-steps which are started in succession starting from a first one of the check sub-steps to an n-th check sub-step.
Each i-th check sub-step, where i varies from 1 to n, involves partially closing the expansion valve 17 so that the degree of opening of the expansion valve 17 is less than the degree of opening of the expansion valve 17 during the directly preceding check step, namely, in an (i+1)-th step of the check sub-steps, the degree of opening of the expansion valve 17 is less than that of the i-th check sub-step which directly precedes the (i+1)th check sub-step.
In particular, in Figure 3, the heating step, indicated there by the reference letter A, is indicated by way of example as the opening of the expansion valve 17 at the start-up level (identified by "@1stStep".
In this example, the heating step involves a cyclical check as to whether the temperature at the inlet 18 (indicated as "Check E2V In Temp" in Figure 3) exceeds a fraction which, in this example, is 90%, of the discharge temperature of the compressor unit 15 (indicated as "Disch T").
The check step and the verification operation may be started in any case after elapsing of a predefined time which, for example in Figure 3, is indicated as being 1 minute (message "wait 1 min").
In general during or following each of the check sub-steps the verification operation may be carried out and each (i+1)-th check sub-step may be performed only if the verification operation which was performed in the i-th check sub-step has had a positive result. The replenishing step may be performed for example only if the verification operation, performed during or following the n-th check sub-step, has a positive result.
With particular reference to Figure 3, here the n-th check sub-step is defined by the condition "Check E2V" = "@LastStep", therefore in accordance with the algorithm shown in Figure 3 for as long as there is the condition "Check E2V < @LastStep", which means that the check procedure is in the i-th check sub-step where i<n, then the check procedure passes to the next check sub-step, i.e. to i+1-th check sub-step.
This passage is shown in Figure 3 by the operation "Check E2V= @NextStep", namely partial closure of the adjustment valve to the next degree of opening.
The expansion valve 17 may be operated by a step motor.
in this case the aforementioned degree of opening may be defined by the counting of the rotation steps of the step motor.
For example, 5 degrees of opening may be envisaged such that said start-up level involves full opening of 85% of the expansion valve 17, corresponding for example to 410 steps of the step motor.
During the check sub-steps from i=1 to i=4 the opening of the regulating valve 17 may for example correspond to 350, 290, 230 and 180 steps, corresponding to opening of the valve by 73%, 60%, 48% and 37%, respectively.
The verification operation may be performed cyclically during the replenishing operation.
The replenishing operation may be terminated following a negative result of the verification operation, as indicated by way of example by the reference letter B in Figure 3 where the reference letter C indicates a possible implementation of the replenishing operation which may also comprise closing of the expansion valve 17 (indicated by "Close Check E2V") following the occurrence of a positive result of the verification operation in B and, following a predefined wait time for example of 1 minute ("wait 1 min"), the replenishment operation is terminated by means of closing of replenishing valve 29 designed to introduce lubricating oil into the casing 15a of the compressor unit 15.
Here the temperature difference measured is indicated by the expression "ΔT Read" and the threshold value is indicated by the expression "1/3 ΔT Mod".
Preferably, the threshold value is equal to the modulus of a modelled temperature difference (in Figure 3 indicated as "ΔT Mod"), or to a fraction of the latter (for example 1/3rd as in Figure 3).
The modelled temperature difference corresponds to the temperature difference which would be measured during the measurement operation if the flow tapped during the tapping operation consisted exclusively of said refrigerating fluid.
The modelled temperature difference may be equal to the difference between the temperature of the inlet 18, measured during the measurement operation, and a theoretical temperature of the outlet 19.
This may be calculated as a function of the temperature and the pressure at the inlet 18 and the pressure of the flow at the outlet 19, which is assumed as being equal to the intake pressure of the compressor, it being assumed that, via the expansion valve 17, the flow undergoes isoenthalpic transformation.
For example, the modelled temperature difference may be calculated, as for example indicated by the reference letter D in Figure 3, via the following operations:

a) calculation of the enthalpy in said isoenthalpic transformation by means of a polynomial expression of the temperature at the inlet 18 and the relative discharge pressure of the compressor, with polynomial coefficients which may be obtained by means of numerical calculation programs for data fitting for example from NIST standard tables, namely tables produced by the "National Institute of Standards and Technology";
b) calculation of the temperature modelled at the outlet 19 by means of a polynomial expression as a function of the enthalpy calculated according to point a), and the pressure for the intake duct 23 of the compressor, with polynomial coefficients which may be obtained by means of numerical calculation programs for data fitting from standard tables, for example NIST tables, namely tables produced by the US National Institute of Standards and Technology.

The check procedure may be performed cyclically, where each cycle is terminated following a negative result of the verification step.
The process according to the present invention may also comprise an estimation procedure, illustrated by way of a non-limiting example in Figure 4, which is designed to estimate a drainage index (which in Figure 4 is indicated by the expression "Tau") which represents the amount of lubricating oil which is removed from the compressor unit 15 during operation of the refrigerating machine 10.
The check procedure may be started when the drainage index exceeds a limit index.
In this case the present process may envisage calculating an operating time of the compressor where the check procedure is started when the operating time exceeds a limit time.
The drainage parameter may be cyclically calculated and the value of the drainage index in a calculation cycle will be provided by a function of the following parameters:

flowrate and speed of rotation of the compressor, in said calculation cycle;
reference flow rate and speed of rotation of the compressor;
time elapsed between a calculation cycle and the calculation cycle directly preceding said calculation cycle;
value assumed by the drainage index in said preceding calculation cycle.

Where the flow rate in the present calculation cycle may be derived by means of linear regression from flow rates calculated at predefined numbers of revolutions of the motor of the compressor unit 15 by means of a polynomial formula, for example according to the standard AHR1 571, which is a function of the evaporation temperature of the refrigerating machine 10 and the absolute discharge pressure of the compressor unit 15, with polynomial parameters characteristic of the compressor unit defined as a function of the speed of rotation of the motor rotor of the compressor unit 15.
The estimation procedure will be performed preferably only when the speed of rotation of the rotor of the compressor unit 15 reaches or exceeds a minimum speed value which, for example, may be equal to 25 rps.
In other words, the estimation procedure may always be performed when the compressor 15 is active; the control device 22 may be programmed to consider the compressor active when it reaches or exceeds said minimum speed value.
The linear regression of the mass flow rate during the current calculation cycle may be performed with reference to the maximum mass flow rate and the maximum number of revolutions or with reference to parameters in each case considered to be more representative of conditions which are potentially critical for the amount of oil discharged by the compressor 15 and introduced into the refrigeration circuit of the refrigerating machine 10.
It will likewise not be necessary, in the estimation of the drainage index, for both the parameters of the maximum mass flow rate and maximum number of revolutions to be both considered.
In fact, instead of them it is possible to consider simply the number of revolutions of the drive motor of the compressor 15 or a function thereof.
Moreover, it is possible to propose expressions of the drainage index which do not envisage the calculation of the flow rate, but which envisage the use directly of the values of the two pressures, i.e. intake pressure and delivery pressure, of the compressor unit 15. In this case these values are the effective input values for calculation of the drainage index, together with the value of the speed of rotation of the drive motor of the compressor 15.
With particular reference to Figure 4 where implementation of the aforementioned estimation procedure (indicated as "OCR Estimation Procedure") is simplified, the following steps are performed:

E - "Calc Flowrate Ref", which involves the calculation of the parameter REF Flowrate by means of the formula proposed above;
F - "Calc FlowRate @HigherSpeed FlowRate @LowerSpeed", which involves the calculation, as mentioned above, of the mass flow rate at the maximum speed and the minimum speed of the rotor of the compressor unit, namely the Flowrate Higher and Flowrate Lower parameters;
G - "Fit in linear function y=mx+q", which involves the formulation of the expression y=mx+q, as indicated above;
H - "Calc FlowRate @ActualSpeed", which consists in the calculation of the parameter Actual Flowrate at the Actual Rotor Speed;
I - "wait 5 sec", wait 5 seconds before carrying out the next operation of "calc Tau" which envisages the calculation of the parameter TauCount, as described above;
J - "Tau>Tau Limit", verifying whether the drainage index parameter TauCount exceeds the limit index "Tau Limit" which may be for example set to 900 seconds;
K - if the result of this verification is positive, the check procedure, identified by the operation "START Check", is started;
L - if the result of this verification is negative the check "Comp WorkHours > WorkHours Limit" is performed, this involving verifying whether the operating time of the compressor unit has reached a predefined maximum time limit;
M - if this verification is positive then said check procedure is started, otherwise the estimation procedure is started again from step H.

When the compressor unit 15 comprises a plurality of compressors 151, 152, 153, each provided with a control apparatus 16, the replenishment operation may comprise an equalization operation which is started when the result of the verification operation is positive for at least one of the compressors 151, 152, 153.
The equalization operation involves the opening of the expansion valves 17, preferably at the start-up level, of all the control apparatus 16 until the oil level in the compressors 151, 152, 153 is equalized.
The replenishment operation may also involve replenishment of oil from an external tank or preferably from a separator 30 if the verification operation is positive for all the compressors 151, 152, 153.
Correspondingly, the refrigerating machine 10 may comprise a separator 30 designed to separate the lubricating oil from the refrigerating fluid downstream of the delivery of the compressor unit 15, as for example shown in Figure 10.
The separator 30 may be in fluid communication with the intake duct 23 of the compressor unit 15, for example by means of an adjustable opening valve 31 or by means of a capillary tube (not shown).
Preferably, the check procedure involves monitoring the temperature of the inlet 18 and, in the event of the inlet temperature 18 falling below a limit threshold, the check procedure may envisage, according to a check based on a thermal hysteresis cycle, a temporary increase in opening of the expansion valve 17 with respect to that described above.
In an advantageous mode of implementation of the present invention, following execution of the check procedure, the next check procedure will be started assuming as degree of opening of the first check sub-step that degree of opening for which, in the preceding check procedure, the result of the verification operation was negative.
Preferably, the replenishment operation will in any case be terminated when a predefined time limit for duration of the said replenishment operation is reached; in this case, following termination of the replenishment operation, cycles of said check procedure are resumed.
It has therefore been shown how a process and an apparatus for controlling the lubrication of a compressor unit of a refrigerating machine, and a machine which comprises said apparatus, fulfil the task and achieve the objects proposed.
In particular, an apparatus and a process for controlling the lubrication of a compressor unit of a refrigerating machine and a refrigerating machine comprising said apparatus are such that a low level of lubricating oil may be detected more rapidly. Furthermore, they are able to improve the average lubrication conditions of the compressor, allowing a longer working life and efficiency.
They also improve the operating conditions of a refrigerating machine provided with a plurality of compressors, allowing rapid and simple equalization of the refrigerating oil among the compressors.
The invention thus devised may be subject to numerous modifications and variations, as long as they fall within the scope of protection of the attached claims.
For example, as a result of the replenishment operation, at least one of the verification operations for verifying the effectiveness of replenishment of the lubricating oil in said compressor unit 15 may be performed.
Moreover, for the purpose of determining the extent of protection conferred the appended claims, due account shall be taken of any element which is equivalent to an element specified in the claims. In practice, the materials used as well as the associated forms and dimensions may be varied depending on the particular requirements and the state of the art.
Where the constructional characteristics and the techniques mentioned in the following claims are followed by symbols or reference numbers, these reference numbers or symbols have been assigned with the sole purpose of facilitating understanding of the said claims and consequently they do not limit in any way the interpretation of each element which is identified, purely by way of example, by said reference numbers or symbols.

Claims

Process for controlling the lubrication of a compressor unit (15) of a refrigerating machine which is equipped with an expansion valve (17) which has an adjustable degree of opening and which has an inlet (18) and an outlet (19) and which is in fluid communication with a casing (15a) of said compressor unit (15) to be passed through by a flow tapped from the latter; within said casing (15a), refrigerant fluid and lubricating oil for lubricating said compressor unit (15) being present; the outlet (19) of said expansion valve (17) being connected to an intake duct (23) of said compressor (15) so as to replenish the latter with said tapped flow;
said process comprises a check procedure which comprises:
- a tapping operation which involves drawing a flow from said compressor and passing it through said expansion valve (17), so as to expand it;

- a measurement operation which involves measuring a temperature difference which said fluid has between the inlet (18) and the outlet (19) of said expansion valve (17);

- a verification operation which is positive if the modulus of the measured temperature difference, in said measurement operation, exceeds a threshold value;

- a replenishment operation which involves the introduction of lubricating oil into said compressor (15); said replenishment operation being carried out when said verification operation is positive.
Process according to claim 1 which, at the beginning of said tapping operation, involves a heating step which involves setting the degree of opening of said expansion valve (17) to a start-up level which corresponds to the maximum opening of said expansion valve (17), so as to obtain fast heating, by the fluid tapped from said compressor, of the inlet (18) and of the outlet (19) of said expansion valve (17).
Process according to claim 2, wherein said tapping operation involves, after said heating step, a check step which involves setting said degree of opening to a check level at which the degree of opening of said expansion valve (17) is less than that of said start-up level.
Process according to claim 3, wherein said check procedure involves starting said check step and said verification operation not before the temperature of said inlet (18) is equal to the temperature of the oil contained in said compressor unit (15) or a predefined fraction thereof.
Process according to one of the claims 3 and 4, wherein said check step comprises a plurality of check sub-steps which are started in succession starting from a first one of said check sub-steps up to an n-th one of said check sub-steps; each i-th check sub-step of said check steps, where i varies from 1 to n, involves partially closing said expansion valve (17) so that the degree of opening of said expansion valve (17) is less than the degree of opening of said expansion valve (17) in the directly preceding check step, i.e. in an (i+1)-th step of said check sub-steps, the degree of opening of said expansion valve (17) is less than that of the i-th check sub-step which directly precedes said (i+1)th check sub-step.
Process according to claim 5, wherein during or following each of said check sub-steps, said verification operation is carried out and each (i+1)-th check sub-step is performed only if the verification operation which is carried out in the i-th check sub-step has a positive result; said replenishment operation being performed only if the verification operation, carried out during or following the n-th check sub-step, is positive and wherein said verification operation is performed cyclically during said replenishment operation and said replenishment operation is terminated following a negative result of said verification operation.
Process according to one of the preceding claims, wherein said threshold value is equal to the modulus of a modelled temperature difference, or to a fraction of the latter, where said modelled temperature difference corresponds to the temperature difference that would be measured in said measurement operation if the flow tapped in said tapping operation consisted exclusively of said cooling fluid.
Process according to claim 7, wherein said modelled temperature difference is equal to the difference between the temperature of said inlet (18), measured in said measurement operation, and a theoretical temperature of said outlet (19), which is calculated as a function of the pressure and temperature of said inlet (18) and the pressure of said flow in said outlet (19), which is assumed to be equal to the intake pressure of said compressor, where it is assumed that, across said expansion valve (17), said flow undergoes an isoenthalpic transformation.
Process according to one of the preceding claims, wherein said check procedure is performed cyclically, each cycle being terminated following a negative result of said verification operation wherein said process comprises an estimation procedure suitable for estimating a drainage index which represents the amount of said lubricating oil which is removed from said compressor unit (15) during the operation of said refrigerating machine (10); said check procedure being started when said drainage index exceeds a limit index.
Process according to claim 9, which involves calculating an operating time of said compressor, where said check procedure is started when said operating time exceeds a limit time.
Process according to one of the claims 9 or 10, wherein said drainage index is calculated cyclically and the value of said drainage index in a calculation cycle is defined by a function of the intake and delivery pressures of said compressor (15) and the speed of rotation of the drive motor of said compressor (15).
Process according to claim 11, wherein said drainage index is calculated cyclically and the value of said drainage index in a calculation cycle is defined by a function of the following parameters:
- flow rate and speed of rotation of the compressor, in said calculation cycle;

- reference flow rate and speed of rotation of the compressor;

- time elapsed between a said calculation cycle and the calculation cycle directly preceding said calculation cycle;

- value assumed by said drainage index in said preceding calculation cycle.
Process according to claim 12, wherein said flow rate in the current calculation cycle is derived by means of linear regression from flow rates calculated at a predetermined number of revolutions of the motor of said compressor unit (15) by means of a polynomial formula which is a function of the evaporation temperature of said refrigerating machine (10) and of the absolute discharge pressure of said compressor unit (15), with polynomial parameters characteristic of said compressor unit (15) defined as a function of the speed of rotation of the motor rotor of said compressor unit (15); wherein said estimation procedure is performed only when the speed of rotation of the rotor of said compressor unit (15) reaches or exceeds a minimum speed value or when said compressor (15) is active.
Process according to one of the preceding claims, wherein said compressor unit (15) comprises a plurality of compressors (151, 152, 153), each provided with a control apparatus (16) and said replenishment operation comprises an equalization operation which is started when said verification operation is positive for at least one of said compressors (151, 152, 153); said equalization operation involving the opening of the expansion valves (17) of all said control apparatus (16) until the level of oil in said compressors (151, 152, 153) is equalized.
Apparatus for controlling the lubrication of a compressor unit (15) of a refrigerating machine (10) comprising:
- an expansion valve (17) having an adjustable opening, an inlet (18) and an outlet (19) and configured to expand a refrigerating fluid contained in a flow which, during use, passes through said expansion valve (17); said inlet (18) being designed to be placed in fluid communication with a refrigerating machine compressor casing (15a) so as to receive a fluid flow therefrom;

- a first temperature sensor (20) thermally connected to said inlet (18) and a second temperature sensor (21) thermally connected to said outlet (19);

- a control device (22) connected to said temperature sensors and to said expansion valve (17), so as to drive it, said control device (22) being programmed to perform a process according to one of claims 1 to 14.