Method for determining a health status of an oil circuit of an oil-injected compressor

Abstract

According to an embodiment, a method for determining a health status of an oil circuit (121) of an oil-injected compressor (102) for circulating oil as a lubricant and cooling agent past a compressor element is disclosed, comprising iteratively performing the steps of measuring (200) an ambient temperature, an ambient pressure, and / or an ambient humidity level; measuring (201) an injection temperature at a location (130) before the oil has flowed past the compressor element; and an outlet temperature at a location (140) after the oil has flowed past the compressor element; estimating (213) the injection temperature and the exhaust temperature using a model descriptive of the oil circuit (121) and based on the values from the first group; calculating a deviation (214) between the measured (212) and estimated (213) injection temperature, between the measured (212) and estimated (213) exhaust temperature; and wherein the health status (216) is determined based on these deviations.

Description

METHOD FOR DETERMINING A HEALTH STATUS OF AN OIL CIRCUITOF AN OIL-INJECTED COMPRESSORTechnical Field

[0001] The present invention relates to a method for determining a health status of an oil circuit of an oil-injected compressor.State of the art

[0002] A compressor is a mechanical machine, designed to provide a gas, such as ambient air, under a higher pressure, compressed air, for applications in industrial processes and / or the medical sector. Depending on the required pressure, the desired application, a desired result, and other preconditions, one can choose from a variety of compressor technologies, such as an axial compressor versus a centrifugal compressor, as well as an oil-free versus an oil-lubricated compressor.

[0003] An oil-lubricated screw compressor, also called an oil-injected screw compressor, is a compressor that is lubricated with oil for lubrication of the various mechanical parts of the compressor. In addition to the compressor elements, bearings and gears can also be lubricated.

[0004] The oil also has a cooling function. The compressed air is cooled by means of a cooling liquid, usually oil, in the compression space between the screws. The oil then circulates in a closed system, the oil circuit, between an oil drum, an oil cooler, and the compressor elements, in this case screws, and is mixed with the air beforecompression. This allows to control the operating temperature of the compressor.

[0005] After compressing the air, mixed with the oil, said oil is separated from the compressed air in an oil separator. The compressed air then continues to flow through an aftercooler and then to an air tank to provide it to a user.

[0006] The separated oil is then pumped back into the oil circuit via the oil separator. This can be done by means of an oil pump, but also by the compressor elements themselves through a pressure-driven injection.

[0007] A closed oil circuit construction of an oil-injected compressor then comprises an oil cooler, if necessary an oil pump, a gearbox, compressor elements, and an oil separator. Furthermore, a bypass channel may be present between the oil separator and the oil cooler, wherein a portion is diverted to the oil cooler, and a portion is returned directly into the oil circuit without flowing through the oil cooler. The portion that is diverted is then controlled by a thermostat. Said thermostat then receives warm oil from the oil separator, which is then mixed with cooled oil from the oil cooler, based on a certain mixing fraction to achieve a desired temperature. The cooled oil then helps to increase the efficiency of the compressor, as well as to lubricate the compressor elements and bearings, as mentioned above. The viscosity must be sufficiently low such that sufficient flow of the oil can be guaranteed. The thermostat then further ensures that condensate is prevented from forming, which can happen when the temperature reaches the dew point of moist air. Condensate can lead to accelerated aging of the oil and also of the metal of the compressor.

[0008] Note that a different structure is also possible. However, to guarantee a proper operation of the compressor, it must be understood that a proper operation of the oil circuit is essential in any construction. However, as with any mechanical machine, some form of degradation will inevitably occur over time due to, among other things, the presence of rotating parts, contamination in the ambient air around the machine, being subject to large temperature differences, the loss of oil and other internal or external influences that can prevent and / or deteriorate the proper functioning of the compressor.

[0009] Important components of the oil circuit are the oil cooler and the thermostat because the degradation thereof has a direct impact on the temperature of the oil. A poorly functioning oil cooler and / or a poorly functioning thermostat will lead to excessively high operating temperatures, which has an impact on the aging of the oil and mechanical components. In addition, the machine will be shut down in the event that operating temperatures become too high to prevent self-ignition of the oil. On the other hand, a poorly functioning thermostat can also lead to operating temperatures that are too low. This in turn can lead to the formation of condensate, which can lead to chemical degradation of the oil, as well as degradation of the material, for example metal, from which the oil cooler is made.

[0010] Traditionally, a maintenance plan is drawn up based on the predetermined allowable operating hours of the compressor. After the number of permissible operating hours has expired, oil is then preventively refilled and / or replaced. However, this does not take into account the actual condition of the compressor and the environment in which the compressor is installed. In addition, there is also an increasing use of high-quality synthetic oils that typically have a longer lifespan. It is therefore becoming increasingly important to be able to make a good estimate of the available oil volume in the oil circuit. This can potentially scale up the intervals between changing and / or refilling, which then has the advantage of reducing maintenance costs. However, this requires proper monitoring of the compressor. It should further be understood that proper monitoring of the compressor may also comprise monitoring other aging processes, such as degradation of the oil quality or other components in the oil circuit.

[0011] Therefor, there is a need for a method for determining the technical condition or health status of an oil circuit of an oil-injected compressor.

[0012] It is therefore an object of the present invention to provide a method for monitoring a compressor and supporting apparatuses of such compressor.Summary of the Invention

[0013] According to the present invention, the above-identified objective is achieved by providing, according to a first aspect of the invention, a computer-implemented method according to the first claim, for determining a health status of an oil circuit of an oil-injected compressor, the oil circuit comprising an oil cooler, an oil separator, and a thermostat, the thermostat configured to mix oil from the oil separator with oil from the oil cooler, based on a predefined mixing fraction, the oil circuit configured to circulate oil as a lubricant and cooling agent past a compressor element of the compressor, the method comprising iteratively performing the steps of:- measuring values of quantities from a first group comprising an ambient temperature, an ambient pressure, an ambient humidity level, a rotational speed of the compressor element, a rotational speed of a fan of the oil circuit, if present, and / or a setting position of an active thermostat, if present ;- measuring values of quantities from a second group comprising: o a first temperature, the injection temperature, at a location before the oil has flowed past the compressor element; and o a second temperature, the outlet temperature, at a location after the oil has flowed past the compressor element;- estimating values of quantities from the second group using a model descriptive of the oil circuit and based on the values from the first group;- calculating a first deviation between the measured and estimated injection temperature, and a second deviation between the measured and estimated exhaust temperature; and wherein the health status is determined on the basis of the first and the second deviation.

[0014] The measuring step is understood to mean quantitatively entering the values of a quantity obtained from one or more observations, recordings or sampling at a specific measuring location by means of suitable measuring instruments, such as sensors for expressing an observed quantity in a number with a relevant unit that can be compared with other values of the same quantity.

[0015] The estimation step is understood to mean determining the values of quantities that are part of the second group on the basis of the values of quantities that are part of the first group, using a scientific and / or mathematical model representative of a technical process and / or apparatus with as input values of the quantities that are part of the first group, and as output the values of the quantities of the second group that must be determined and therefore estimated on the basis of one or more calculations.

[0016] The oil circuit is an oil circuit as known in the state of the art and suitable for an oil-injected compressor, preferably a screw compressor. In a simple embodiment, it comprises an oil cooler to cool the oil before it flows past the compressor elements. "Flowing" also comprises "flowing", "passing", or another term that makes it clear that the oil circulates in the circuit, where it is then injected, for example, into the compressor chamber as mist. The term "flowing" therefore comprises atomizing the oil to circulate it further. The oil circuit further comprises an oil separator after the compressor elements, i.e. in the circuit after the oil has flowed past the compressor elements. Said oil separator is configured to extract the oil from the compressed air, as known according to the state of the art. Furthermore, the oil circuit can optionally also comprise an oil pump to circulate the oil in the circuit. Alternatively, in the absence of an oil pump, the oil is circulated through the circuit by the compressor elements themselves based on pressure-driven injection.

[0017] The oil circuit further comprises a thermostat. The thermostat is configured to mix warm oil from the oil separator based on a certain mixing fraction with cooled oil from the oil cooling system to obtain a desired temperature of the oil to inject into the compressor element. The thermostat can be of the active or passive type.

[0018] An active thermostat has an actively controlled valve that is controlled such that at a certain mixing fraction, oil from the oil separator is mixed with oil from the oil circuit such that a predefined temperature is reached for the injection temperature. With a passively controlled valve, the valve will be controlled mechanically using a bimetal, and this will result based on a set mixing temperature in the injection temperature as long as the temperature of the cooled oil is lower than the desired mixing temperature.

[0019] When the oil circuit comprises an active thermostat and oil separator, accordingto an embodiment, the thermostat is configured to mix the oil from the oil separator with oil from the oil cooler based on the mixing fraction. Here the method further comprises the step of measuring a setting position of the active thermostat, and wherein the health status is further determined on the basis of the setting position.

[0020] When the oil circuit comprises a passive thermostat and oil separator, the method according to an embodiment further comprises the step of measuring a third temperature, the cooling temperature, at a location after the oil has flowed past the oil cooler, and the health status is further determined based on the cooling temperature. Note that said embodiment can also be applicable to an active thermostat when an effective setting position cannot be measured.

[0021] As a result, an unexpected increased injection temperature can be directly linked to a poorly functioning thermostat. In other words, a problem with the thermostat can be distinguished in this way from a problem with the oil cooler and / or a too low flow rate, mass or volume in the oil circuit. With an active thermostat, it is necessary to know the real position of the thermostat valve, or as mentioned above if this is not possible by measuring the temperature of the cooled oil. Since this is not possible with a passive thermostat, the temperature of the cooled oil will be measured here.

[0022] The basic idea is that oil flows from the oil separator, to the oil circuit, through the compressor elements, and again to the oil separator, such that the oil serves as a lubricant and cooling agent for the oil-injected compressor as known in the state of the art. It should also be understood that there are other components along the oil circuit, such as a gearbox, a thermostat as explained above, and that the oil also flows past one or more bearings of an engine.

[0023] According to the method, the temperature of the oil will be measured at various positions along the oil circuit to infer or determine the health status of the oil circuit.

[0024] The first location where the temperature is measured is at a location before the oil has flowed past the one or more compressor elements. The temperature measured here is further referred to as the injection temperature. It should further be understood that, for reasons of practicality, said location may also be a location as close aspossible to the point where the oil is introduced into the one or more compressor elements.

[0025] The second location where the temperature is measured is at a location after the oil has flowed past the one or more compressor elements. Said temperature is further referred to as the exhaust temperature. Again, for reasons of practicality, said location may be a location close to the point where the oil leaves the one or more compressor elements.

[0026] Due to the thermodynamic process of compression of air or another gas by the compressor elements into compressed air or compressed gas, said air or gas will heat up. By allowing the oil to flow past the compressor elements during said compression, said generated heat can be absorbed by the oil. It should therefore be understood that the temperature of the oil, before it flows through the compressor elements, determines the amount of heat that can be absorbed.

[0027] Furthermore, the expected values of a second group of variables, such as the injection temperature and the exhaust temperature respectively, will be estimated on the basis of a stationary model. Said model is a scientific and / or mathematical model and is representative of the compressor in combination with the oil circuit. The model is, for example, a physical or multi-physical model comprising a set of equations and / or empirical relationships describing the different physical components of the compressor and the oil cooler and dependent on each other by means of one or more common quantities.

[0028] Since the model is representative of the compressor, or more specifically the oil circuit, the expected injection temperature and outlet temperature can be estimated on the basis thereof, in combination with the ambient temperature. This can be further explained as follows. The model comprises the value of the nominal power of the compressor, allowing to deduce what the generated heat will be due to compression during a nominal regime. Furthermore, the model will also comprise the cooling capacity of the oil circuit which corresponds to the heat that can be removed, also at a nominal regime. If the temperature of the environment in which the compressor is installed is also taken into account, it can then be deduced what the expected injectiontemperature and outlet temperature are. In other words, according to said embodiment these temperatures can be determined if it is assumed that the compressor is running at nominal regime. To increase the accuracy of the estimates, they are then further determined on the basis of other quantities from the first group indicative of the operating conditions and / or environmental conditions. These variables are an ambient temperature, an ambient pressure, an ambient humidity level, a rotational speed of the compressor element, a rotational speed of a cooling circuit fan, if present, and / or a setting position of an active thermostat, if present.

[0029] By these disclosed steps of the method according to the invention, it should be understood that, in this way, it is indirectly determined what the increase in the oil temperature is when it passes past the compressor elements, and therefore what is the heat absorbed during compression.

[0030] However, instead of merely analysing the temperature increase of the oil when it passes past the compressor elements, the inventors have found that these measurements can be used to deduce the health status of the oil circuit, and more specifically the oil flow rate, the oil mass, and the oil volume in the circuit, as well as the degree of clogging of the oil cooler and whether or not the thermostat is functioning correctly.

[0031] The oil cooling system is connected to the oil circuit via a primary circuit such that the oil flows through said primary circuit. Said primary circuit therefore forms an integral part of the oil circuit when the oil cooling system is connected to it. Furthermore, the oil cooling system has a secondary circuit through which a cooling agent flows to extract heat from the oil flowing in the primary circuit and to transfer said heat to the environment. The primary and secondary circuits are separated from each other in the sense that the oil cannot be mixed with the cooling agent, whereby the combination of the two forms a heat exchanger. Furthermore, a variety of cooling media can be used for the secondary circuit. According to an embodiment, the cooling agent is the ambient air around the compressor, whereby the cooling agent then flows through the secondary circuit via a forced flow with the aid of a fan. The oil cooling system also has an air side along which said ambient air flows.

[0032] However, the air side of the oil cooling system can become clogged by dust particles and other airborne contaminants. Said phenomenon is also called clogging, and the extent to which said phenomenon occurs is further referred to as the degree of clogging.

[0033] Furthermore, a predefined mass or volume of oil should be present in the oil circuit to guarantee its function as a lubricant and cooling agent in optimal conditions. Said mass or volume then corresponds to a certain flow rate that flows through the oil circuit. If the flow rate is lower than a predefined value, which in turn corresponds to a too low mass or volume, the function of the oil circuit can no longer be guaranteed. As already mentioned above, based on the disclosed steps, the oil flow rate, oil mass, and / or oil volume can then also be determined, and thus the health status of the oil cooling system.

[0034] As will be explained further, the inventors have found that, based on the steps stated above, a distinction can also be made between the three aspects of the health status, namely a distinction between a too low oil flow, oil mass and / or oil volume, a blockage of the air side of the oil cooler, and a poorly functioning thermostat.

[0035] The advantage of this is that the cause of a poorly functioning oil circuit can be reported prior to an intervention. Too low an oil flow rate, oil mass or oil volume requires a different form of intervention compared to a cooling system with a high degree of blockage or a poorly functioning thermostat, such that the intervention can be optimized through such reporting.

[0036] According to an embodiment of the invention, the estimation further comprises estimating one or more time-dependent quantities from a third group indicative of a thermal inertia of the oil circuit and / or a transit time of oil through the oil circuit, wherein the model further comprises a differential equation and / or a convolution comprising a time-dependent status variable comprising one or more quantities from the third group, and wherein the values of quantities from the second group are further estimated on the basis of the estimated values from the third group. The variables from the third group are an oil temperature at the outlet of the oil separator, a mass flow of the oil, an inlet pressure of the oil at an inlet of the oil cooler, and / or an inlet temperature ofthe oil cooler.

[0037] By supplementing a stationary model with one or more time-dependent quantities from this third group, the thermal inertia of the oil circuit and / or the transit time of the oil through the oil circuit and the various components is taken into account. The quantities of the second and third groups will then be estimated at successive points in time using an update equation. The frequency or time step at which the estimation and updating takes place, in turn, depends on an expected change over time in the respective quantity or quantities. This allows the number of calculations to be tailored to the physical aspect. For quantities from the second group, the result of these estimates is then compared with the measured values of quantities from that second group.

[0038] According to an embodiment, measuring values of quantities from a second group further comprises measuring a third temperature, the outlet temperature of the oil cooler, and estimating further comprises estimating the third temperature, and calculating further comprises calculating a third deviation between the measured and estimated third temperature, and wherein the health status is further determined based on the third deviation.

[0039] The determination of the health status can therefore be further refined by, on the one hand, estimating the exhaust temperature, and on the other hand, estimating said temperature, and wherein the difference between said measurement and said estimate is then taken into account to determine the health status.

[0040] According to an embodiment, the thermostat is an active thermostat and the method further comprises the step of measuring a setting position of the active thermostat, and measuring the third temperature, the outlet temperature of the oil cooler, is done on the basis of measuring the setting position.

[0041] In other words, measuring the temperature can be done indirectly because there is a relationship between the setting position of the active thermostat and the outlet temperature of the oil cooler. The setting values are then translated to said temperature and further used as a measurement.

[0042] According to a second aspect of the invention, there is disclosed a data processing system comprising a processing unit configured to perform the method of the first aspect of the invention.

[0043] According to a third aspect of the invention, there is disclosed a computer program product containing computer-executable instructions for performing the method of the first aspect when said program is run on a computer.

[0044] According to a fourth aspect of the invention, there is disclosed a computer- readable storage medium containing the computer program product of the third aspect.

[0045] According to a fifth aspect of the invention, there is disclosed a compressor comprising the data processing system of the second aspect of the invention.

[0046] According to a sixth aspect of the invention, there is disclosed a method for determining a health status of an oil circuit of an oil-injected compressor, the oil circuit comprising an oil cooler, an oil separator, and a thermostat, the thermostat configured to mix oil from the oil separator with oil from the oil cooler, based on a predefined mixing fraction, the oil circuit configured to circulate oil as a lubricant and cooling agent past a compressor element of the compressor, the method comprising iteratively performing the steps of:- measuring values of quantities from a first group comprising an ambient temperature, an ambient pressure, an ambient humidity level, a rotational speed of the compressor element, a rotational speed of a fan of the oil circuit, if present, and / or a setting position of a thermostat;- measuring values of quantities from a second group comprising: o a first temperature, the injection temperature, at a location before the oil has flowed past the compressor element; and o a second temperature, the outlet temperature, at a location after the oil has flowed past the compressor element;- estimating values of quantities from the second group using a model descriptive of the oil circuit and based on the values from the first group;- calculating a first deviation between the measured and estimated injection temperature, and a second deviation between the measured and estimated exhaust temperature; and wherein the health status is determined on the basis of the first and the second deviation.

[0047] According to an embodiment, the estimation further comprises estimating one or more quantities from a third group indicative of a thermal inertia of the oil circuit and / or a transit time of oil through the oil circuit, wherein the model further comprises a differential equation and / or a convolution comprising a time-dependent status variable comprising one or more quantities from the third group, and wherein the values of quantities from the second group are further estimated on the basis of the estimated values from the third group.

[0048] The quantities from the third group comprise one of the group of an oil temperature at the outlet of the oil separator, a mass flow of the oil, an inlet pressure of the oil at an inlet of the oil cooler, an inlet temperature of the oil cooler.

[0049] According to an embodiment, the health status is further determined using a regression analysis of which the first and second deviation are explanatory quantities, and the health status is a dependent quantity.

[0050] According to an embodiment, a regression analysis model comprises one of the group of support vector machine, neural network, non-linear regression comprising analytical equations, decision trees, random forests, and / or gradient boosting methods.

[0051] According to an embodiment, the health status comprises a degree of clogging of the oil cooler and / or an oil flow rate lower than a predefined value and / or comprises a failing thermostat.

[0052] According to an embodiment, the estimation is further carried out on the basisof one or more setting parameters of the compressor.

[0053] According to an embodiment, the setting parameters comprise one or more of the group of a pressure, a flow rate, a humidity level, a power, a rotation speed.Brief description of the drawings

[0054] The invention will be further illustrated with reference to the figures, wherein

[0055] Fig. 1 schematically illustrates an air-cooled oil-injected screw compressor and associated peripherals; and

[0056] Fig. 2 schematically illustrates the steps for determining the health status of the air-cooled oil-injected screw compressor according to an embodiment of the disclosed invention;

[0057] Fig. 3 schematically illustrates a method for estimating time-dependent quantities; and

[0058] Fig. 4 illustrates differences between measured data and estimated values of the injection temperature and the exhaust temperature respectively;

[0059] Fig. 5 schematically illustrates steps for determining a health status of an oil circuit of an oil-injected compressor; and

[0060] Fig. 6 provides an overview of the different quantities per group.Detailed description of the embodiments

[0061] The present invention will be described with respect to certain embodiments and with reference to certain drawings, but the invention is not limited thereto and is determined only by the claims. The drawings described are only schematic and nonlimiting. In the drawings, the size of certain elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and relative dimensions do not necessarily correspond to actual practical embodiments of the invention.

[0062] Furthermore, the terms first, second, third and the like are used in the description and in the claims to distinguish between similar elements and not necessarily to describe a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention may be practiced in sequences other than those described or illustrated herein.

[0063] In addition, the terms "above", "below", "over", "under" and the like in the description and claims are used for illustrative purposes and not necessarily to describe relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein may be employed in orientations other than those described or illustrated herein.

[0064] Furthermore, the various embodiments, although referred to as "preferred embodiments", are to be construed as exemplary means of carrying out the invention rather than as a limitation on the scope of the invention.

[0065] The term “comprising”, used in the claims, should not be construed as being limited to the means or steps set forth below; the term does not exclude other elements or steps. The term should be interpreted as specifying the presence of the mentioned features, elements, steps or components referred to, but does not exclude the presence or addition of one or more other features, elements, steps or components, or groups thereof. The scope of the expression “a device comprising means A and B” should therefore not be limited to devices consisting only of components A and B. The meaning is that, with respect to the present invention, only components A and B of the device are listed, and the claim is further construed to also include equivalents of these components.

[0066] Fig. 1 schematically illustrates an air-cooled oil-injected screw compressor. In said schematic representation, four circuits are distinguished, being an air circuit 120, an oil circuit 121 , a water circuit 123, and a circuit 122 through which a mixture of air and oil flows. However, it should be understood that this represents an ideal situation, since in a practical embodiment, for example, the circuit 120 may still contain residual particles of oil after compression, as will be explained later.

[0067] The operation can be described as follows. At the inlet 101 , ambient air is drawn into the compressor 102 to compress it. As will be explained further, said ambient air is mixed with oil before it is compressed with compressor 102. After the compressor 102, an oil separation vessel 104 is present from which oil is pumped into the oil circuit 121 through the compressor element from compressor 102. This is called pressure- driven injection. The thermostat 112 determines the temperature of the oil that is injected into the engine 113 and associated gearbox and then into the compressor element of the compressor 102. The thermostat 112 receives warm oil directly from the oil separation vessel 104 and oil cooled by an oil cooler 110. The thermostat 112 further determines the mixing fraction between hot and cold oil to obtain the desired temperature of the oil injected into the compressor 102. The cooled oil from the oil cooler 110 serves to increase the efficiency of the compressor element, as well as to lubricate the compressor element and bearings. The viscosity must remain sufficiently high such that sufficient flow of the oil is guaranteed. The thermostat 112 must also prevent condensate from forming by keeping the temperature above the dew point of water. The presence of condensate can lead to accelerated aging of the oil and metal of the compressor due to chemical interactions.

[0068] There are three main degradation mechanisms that often occur and therefore should be monitored. A first degradation mechanism is a poorly functioning cooler package, being the air cooler 107 and the oil cooler 110. A second degradation mechanism is a reduction in the oil level. A third degradation mechanism is a poorly functioning or failing thermostat. The latter means that a desired or set mixing fraction does not correspond to the actual mixing fraction.

[0069] A failing cooler package 107, 110 can be associated with increasing cloggingor with a failing fan 111. The clogging of the cooler package 107, 110 mainly concerns the accumulation of dirt or dust on the air side of the coolers 107, 110. A reduction in the oil level occurs because a small fraction of oil is allowed through the oil separating element 105. Said oil carryover will also increase as the oil separation element 105 becomes clogged and / or contaminated. In addition, leaks can also occur in the system that can cause additional oil losses. A maintenance plan for an oil-injected compressor is based on the lifespan and availability of oil in the machine. Topping up and / or replacing the oil is done according to the state of the art on the basis of operating hours and therefore does not take into account the actual loss of oil from a particulate machine. Due to the increasing use of high-quality synthetic oils that remain suitable as a cooling agent for a longer period of time, compared to conventional oils, it is therefore important to make a good estimate in a timely manner of the amount of oil still available in the oil circuit 121. Scaling up the intervals between changing and / or refilling the oil has the advantage that maintenance costs are reduced. This therefore requires a good monitoring.

[0070] A poorly functioning thermostat means that the mixing fraction for mixing the portion of the heated oil that is diverted from the bypass channel to the oil cooler and the cooled oil from said oil cooler will be incorrectly set relative to the desired temperature. In other words, the oil will either have too high or too low a temperature compared to the desired one.

[0071] The oil circuit 121 serves to provide the compressor element of the compressor 102 with a cooling and lubricant. Consequently, an increase in the exhaust temperature measured at measuring point 140 can be considered a primary symptom indicating a problem with the oil circuit 121 , given that the air, drawn in at the inlet 101 is within specified specifications of the compressor 102 measured at measuring point 130. There are three main forms of degradation as a result of which the outlet temperature measured at measuring point 140 deviates from a value expected during normal operation of compressor 102.

[0072] On the one hand, the temperature of the injected oil may be too high or too low, on the other hand, the flow rate of the injected oil may be too low. An injection temperature that is too high or too low can be measured directly, but in relation to astandard value under the same operational condition. In other words, a deviation must be determined using a model.

[0073] An oil flow rate that is too low cannot be measured directly, but can be deduced based on the temperature measurements at measuring points 130 and 140.

[0074] With reference to Fig. 2, which schematically illustrates the steps for determining the health status of the air-cooled oil-injected screw compressor 102, it will be further explained how the above-mentioned degradations can be derived.

[0075] In a first step 200, the ambient temperature, the ambient pressure and the ambient humidity are measured and stored in module 210 for further processing. In a second step 201 , which can take place simultaneously with step 200, the temperature at location 130 is measured as well as the temperature at location 140. The first temperature is the injection temperature, the second temperature is the outlet temperature. These values are then stored in module 211 for further processing. The injection temperature and the exhaust temperature are then estimated in module 213 based on the measured values in step 200.

[0076] For example, the injection temperature can be estimated from the temperature at the separator, the cooling air temperature, the oil flow rate and the fan rotation speed. The outlet temperature can then be estimated from the injection temperature, the inlet temperature, the oil flow rate, and the rotational speed of the compressor elements.

[0077] Between modules 212 and 213, the measured values and the estimated values can then be compared with each other to determine a first deviation 214 between the measured and estimated injection temperature 130 and a second deviation 215 between the measured and estimated exhaust temperature 140. After that, a health status can be determined on the basis of these two deviations 214 and 215, for example expressed in a value in module 216.

[0078] Estimating the values of the injection temperature and the exhaust temperature can be done by explicitly taking into account the time dependence of one or morequantities, for example using a differential equation. The first time derivative of the temperature at the outlet of the oil separator then depends, for example, on the rotational speed of the compressor elements, the pressure at the outlet, and the difference between the outlet temperature and the temperature at the outlet of the oil separator.

[0079] Fig. 3 schematically illustrates a method for estimating such quantities, which may be the temperature at the outlet of the oil separator and / or a temperature at a location in the oil circuit that is difficult to measure. These values can then be processed in module 310 to a value 220, which then serves as input for module 213.

[0080] In the absence of unexpected obstructions in the oil circuit 121 , an increased injection temperature can be linked to a poorly functioning thermostat 112 or cooler package 107, 110. Too low an oil flow can be linked to a too low oil level in the oil separation vessel 104.

[0081] To distinguish a problem with the thermostat 112 from a problem with the cooler package 107, 110, the actual position of the thermostat valve 112 can be measured, as well as the temperature of the cooled oil, i.e. at position 150.

[0082] With reference to Fig. 4, it can then be deduced what the problem is. Fig. 4 illustrates differences between measured data and estimated quantities, wherein the difference between the measured and estimated injection temperature 130 is illustrated on the horizontal axis, and the difference between the measured and estimated exhaust temperature 140 is illustrated on the vertical axis. This concerns, in zone 401 , the problem of too little oil in the oil circuit 121 , and in zone 402, a blockage of the cooler package 107, 110.

[0083] With reference to Fig. 2 wherein it is illustrated that, based on the two deviations 214 and 215 also illustrated in Fig. 4, a health status is determined, for example expressed as a value in module 216.

[0084] Fig. 5 illustrates a scheme for assessing the health status of the oil circuit 121. After the measurements and estimates 500 of the temperatures, said status can bedetermined. When the difference 501 between the estimation and measurement of the exhaust temperature 130 is smaller than a predefined value, it is deduced that the oil circuit 121 shows no deviations and therefore works optimally 502. If said difference 501 is larger, then in step 503 the difference between the measurement and the estimate of the injection temperature 140 is examined. If said difference 503 is smaller than a predefined value, this means that the oil level is too low 504 and an instruction can be given to refill the oil level 505. If said difference 503 is greater than a predefined value, the difference 506 between the measurement and estimation of the oil temperature at the outlet 150 of the oil separator is taken into account. When said difference 506 is smaller than a predefined value, it means that the thermostat is failing 507. Otherwise, it means that the cooler package is failing 508 due to increasing clogging.

[0085] Finally, Fig. 6 is an overview of the different quantities per group. The first group 600 comprises quantities that are measured, but not estimated. These comprise a compressor element rotational speed, an oil circuit fan rotational speed, if present, an ambient temperature, an ambient pressure, an ambient humidity level, and a compressor outlet pressure. The second group 601 comprises quantities that are measured and estimated. The quantities from said group comprise an outlet temperature of the oil after the element, an injection temperature of the oil before the element, and an outlet temperature of the oil at the oil cooler. The group 602 comprises quantities that are only estimated and therefore not measured. These comprise an oil temperature at the outlet of the oil separator, a mass flow of the oil, an inlet pressure of the oil at an inlet of the oil cooler, and an inlet temperature of the oil cooler. Furthermore, the outlet temperature of the oil after the element can be timedependent, indicated by reference 604, as well as the inlet temperature of the oil cooler, indicated by reference 605. Reference 603 therefore indicates time-dependent quantities. Reference 603 therefore points to time-dependent quantities, and forms, as the union of references 604 and 605, the third group of quantities.

Claims

Claims

1. - A computer-implemented method for determining a health status of an oil circuit (121) of an oil-injected compressor (102), the oil circuit (121) comprising an oil cooler (110), an oil separator (104), and a thermostat (112), the thermostat (112) configured to mix oil from the oil separator (104) with oil from the oil cooler (110), based on a predefined mixing fraction, the oil circuit (121) configured to circulate oil as a lubricant and cooling agent past a compressor element of the compressor (102), the method comprising iteratively performing the steps of:- measuring (200) values of quantities from a first group comprising an ambient temperature, an ambient pressure, an ambient humidity level, a rotational speed of the compressor element, a rotational speed of a fan of the oil circuit, if present, and / or a setting position of a thermostat;- measuring (201) values of quantities from a second group comprising:• a first temperature, the injection temperature, at a location (130) before the oil has flowed past the compressor element; and• a second temperature, the outlet temperature, at a location (140) after the oil has flowed past the compressor element;- estimating (213) values of quantities from the second group using a model descriptive of the oil circuit (121) and based on the values from the first group;- calculating a first deviation (214) between the measured (212) and estimated (213) injection temperature, and a second deviation (215) between the measured (212) and estimated (213) exhaust temperature; and wherein the health status (216) is determined on the basis of the first (214) and the second (215) deviation.

2. The computer- implemented method according to claim 1, further comprising the step of:- reporting the health status of the oil circuit.

3. The computer-implemented method according to any one of the preceding claims, wherein the estimating (213) further comprises estimating one or more quantities from a third group (220) indicative of a thermal inertia of the oil circuit and / or a transit time of oil through the oil circuit, wherein the model further comprises a differential equation and / or a convolution comprising a time-dependent status variable comprising one or more quantities from the third group, and wherein the values of quantities from the second group are further estimated on the basis of the estimated values from the third group.

4. - The computer- implemented method according to claim 3, wherein the quantities from the third group comprise one of the group of:- an oil temperature at the outlet of the oil separator (104);- a mass flow of the oil;- an inlet pressure of the oil at an inlet of the oil cooler;- an inlet temperature of the oil cooler.

5. - The computer-implemented method according to any one of the preceding claims, wherein the health status (216) is further determined using a regression analysis of which the first (214) and second (215) deviation are explanatory quantities, and the health status (216) is a dependent quantity.

6. - The computer- implemented method according to claim 5, of which a regression analysis model comprises one of the group of support vector machine, neural network, non-linear regression comprising analytical equations, decision trees, random forests, and / or gradient boosting methods.

7. - The computer-implemented method according to any one of the preceding claims, measuring (201) values of quantities from a second group further comprising:- a third temperature, the oil cooler outlet temperature (150). and wherein the estimating further comprises estimating the third temperature, and the calculating further comprises calculating a third deviation between the measured and estimated third temperature, and wherein the health status is further determined based on the third deviation.

8. - The computer- implemented method according to claim 7, wherein the thermostat is an active thermostat (112), the method further comprising the step of:- measuring a setting position (305) of the active thermostat (112); and wherein measuring the third temperature takes place on the basis of measuring the setting position (305).

9. - The computer-implemented method according to any one of the preceding claims, wherein the health status (216) comprises: a degree of clogging of the oil cooler (110), and / or an oil flow rate lower than a predefined value and / or comprises a failing thermostat.

10. - Computer-implemented method according to any one of the preceding claims, wherein the estimation is further carried out on the basis of one or more setting parameters of the compressor (102).

11. - Computer- implemented method according to claim 10, the adjustment parameters comprising one or more of the group of a pressure, a flow rate, a humidity level, a power, a rotational speed.

12. - A data processing system, comprising a processing unit configured to perform the method of any one of the preceding claims.

13. - A computer program product containing computer-executable instructions for performing the method of any one of claims 1 to 11, when said program is run on a computer.

14. - A computer-readable storage medium containing the computer program product of claim 13.

15. - A compressor (102) comprising the data processing system of claim 12.Method for determining a health status of an oil circuit of an oil-injected compressor

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