EP1497597B1

EP1497597B1 - Method for detecting changes in a first flux of a heat or cold transport medium in a refrigeration system

Info

Publication number: EP1497597B1
Application number: EP20030746812
Authority: EP
Inventors: Claus Thybo; Bjarne Dindler Rasmussen; Roozbeh Izadi-Zamanabad
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 2002-04-22
Filing date: 2003-04-12
Publication date: 2006-10-18
Anticipated expiration: 2023-04-12
Also published as: EP1497597A1; JP2005533230A; DK1497597T3; WO2003089854A1; AU2003226943A1; US7685830B2; DE10217975B4; US20050172647A1; AT343108T; DE10217975A1

Abstract

The invention concerns a method for detecting changes in a first flux of a heat or cold transport medium in a refrigeration system whereby the first flux is conveyed through a heat exchanger wherein occurs heat transfer form the first flux to a second flux of a coolant medium transporting heat or cold. The inventive method aims at enabling the fastest possible detection of said changes. Therefor, it consists in monitoring the first flux flowing through the heat exchanger by detecting the change in heat content of the second flux of the medium or a value derived therefrom.

Description

The invention relates to a method for detecting changes in a first media stream of a heat or cold transport medium in a refrigeration system, in which the first media stream is passed through a heat exchanger, in which a heat transfer between the first media stream and a second media stream of a heat or refrigerant he follows.
US 6,128,910 describes a method for diagnosing a refrigeration system for cooling air. In the method, physical quantities of the air passing through a heat exchanger of the plant are measured by means of a sensor arrangement (48) which is part of a measuring unit (44). The measured quantities are: air temperature, relative humidity and volume flow of the air. Based on the air temperature and the relative humidity, the enthalpy change of the air is determined on passage of the heat exchanger. The change, along with the volume flow, is used to determine reduced air flow, reduced heat transfer, as well as decreased SHR. Based on additional measurements of the refrigerant temperature in the suction line and the temperature of the liquid refrigerant between condenser and expansion valve, the refrigerant charge can be examined.
In order to explain the invention, a sales refrigerator is chosen below as an example of a refrigeration system. But it is also applicable to other refrigeration systems. In a sales freezer, such as used in supermarkets for holding refrigerated or frozen products for sale, an air stream forming the first media stream circulates in an air duct in which an evaporator is located. The evaporator is a heat exchanger to which on one side a refrigerant, that is the second medium flow, in a liquid or two-phase state (gaseous and liquid) is supplied. If the air on the other side is passed over the evaporator, there is a heat transfer from the air to the refrigerant and the air is cooled. Another example of a heat exchanger is the condenser over which Air is passed to liquefy the refrigerant. In this case, the refrigerant heat is removed.
With such a refrigeration system, one wishes to be able to determine with a certain degree of reliability whether the air flow can circulate to a sufficient extent, that is, one wishes to determine whether disturbances have occurred. Such disturbances can be caused, for example, by the fact that a fan fails, that the evaporator ices, that settle dirt in the air duct or obstruct objects such as signs or goods, the air duct and increase the flow resistance for the amount of air and thereby impede the flow of air.
Such error detection should be made as possible before the cooling capacity of the refrigeration system has decreased too much. If an error can only be detected when the temperature rises, it may already be too late for the chilled or frozen products, ie there is a risk that these products will spoil. In many cases, a disturbance of the air flow but long before damage to the cooled products means that the refrigeration system is not operated at its optimum operating point. Thus, if an error has occurred, individual components of the refrigeration system can be charged more often, which reduces their service life. This can be easily understood using the example of fans. If one of several fans fails, the remaining fan or fans can still be used to generate the cooling capacity drive required air flow through the refrigeration system. The remaining fans are loaded more often. In addition to reducing the life of the components, such as the fans, a fault has the disadvantage of increased energy consumption. The refrigeration system is not operated at its optimum operating point. Also for this reason, the detection of errors is important.
The object of the invention is to be able to recognize changes in the first media stream as early as possible.
This object is achieved in a method of the type mentioned above in that it detects the change in the enthalpy of the second medium flow or a size derived therefrom for monitoring the first medium flow flowing through the heat exchanger.
If the first medium flow is formed by an air flow, the determination of the mass of the air flowing through by measuring the air flow itself is relatively difficult. Moreover, such a measurement would also hinder the air flow, which is undesirable. Therefore, one chooses a different way: it is assumed that the air flow carries a certain amount of heat and thus has a certain energy content. The energy content can also be called enthalpy. This amount of heat is released to the refrigerant in the heat exchanger (or released from the refrigerant in the case of the condenser). If you can now capture this amount of heat, then you have a statement about how much air through the evaporator, ie the heat exchanger is performed. This statement is sufficient to detect if an error has occurred or not. The heat released by the air per time corresponds to the heat absorbed by the refrigerant per time. This balance is the basis of the method for detecting a reduced airflow in the channel. For example, you can compare this actual amount of air with a setpoint. If this actual value does not agree with the setpoint value, this is interpreted as a reduction of the airflow and one can, for example, indicate an error. This error display can be done at a relatively early stage, so long before a heavy overload of the refrigeration system has occurred or even an undesirable increase in temperature has occurred. Of course, the same procedure also applies if, instead of air, another medium, for example a liquid or a brine, is used as the first medium flow.
Preferably, to determine the change in the enthalpy of the second medium flow, a mass flow and a specific enthalpy difference of the second medium flow are determined via the heat exchanger. The specific enthalpy of a refrigerant is a matter and condition characteristic and varies from refrigerant to refrigerant, or more generally, from second media stream to second media stream. The specific enthalpy is the enthalpy per mass. However, since it is known which refrigerant is used, can be measured by measured variables such as temperatures, pressures or the like, determine the specific enthalpy of the second medium flow before and after the heat exchanger. From this, a specific enthalpy difference can be formed which, together with the mass flow, allows a statement about the enthalpy.
It is particularly preferred that, for determining the specific enthalpy difference of the second medium flow at the inlet of the expansion valve, the temperature and the pressure of the second medium flow and at the outlet of the heat exchanger, the temperature of the second medium flow and either the pressure at the outlet of the heat exchanger or the boiling temperature of the second Media flow at the entrance of the heat exchanger determined. The sensors for determining the temperature and the pressure of the second medium flow, in this case the refrigerant, are present in most cases anyway. They are needed to control the refrigeration system accordingly. It is also possible to measure the pressure of the refrigerant at the inlet and from this determine the pressure at the outlet of the heat exchanger, taking into account the pressure drop in the evaporator. Based on the measured or calculated values, it is then possible to determine the specific enthalpy with the aid of diagrams provided by the refrigerant manufacturers (so-called Log p, h diagrams). In many cases, this can also be done automatically if the corresponding relationships are stored in tables or are available via equations of state.
Preferably, a specific enthalpy difference of the first medium flow is also determined via the heat exchanger. The specific enthalpy difference of the first media stream makes it possible to relatively easily measure the mass per time of the first media stream, e.g. air, as will be shown below.
Preferably, the second medium flow is determined from a pressure difference across and the degree of opening of an expansion valve. If it is a pulse width modulated expansion valve, then the opening degree is replaced by the opening duration or the duty cycle. The mass flow of the second medium flow, for example of the refrigerant, is then proportional to the pressure difference and the opening duration. The refrigerant flow can be determined relatively easily in this way. The subcooling of the refrigerant is, however, in some cases so great that it is necessary to measure also the subcooling, because the refrigerant flow, ie the second medium flow, is influenced by the subcooling by the expansion valve. In many cases, however, one only needs to know the pressure difference and the degree of opening of the valve, because the subcooling is a fixed size of the refrigeration system, which can then be taken into account in a valve characteristic or in a proportionality constant. With "opening degree" can also be understood the opening duration in pulse width modulated valves, ie the duty cycle.
In an alternative or additional embodiment, the second medium flow can also be determined from operating data and a difference of the absolute pressures via a compressor together with the temperature of the second medium flow at the compressor inlet. The operating data is, for example, the speed of the compressor, which, together with the pressure across the compressor, allows a statement about the amount of refrigerant. For this purpose, only the knowledge of the compressor properties is required.
The first medium stream from the second medium stream and a quotient of the specific enthalpy difference of the second medium stream and the specific enthalpy difference of the first medium stream via the heat exchanger are preferably determined. As explained above, it is considered that there is an equilibrium between the amount of heat transferred from the air to the refrigerant and the amount of heat absorbed by the refrigerant from the air, that is, both quantities are the same. Put simply, the amount of heat in the air is the product of the mass flow of air through the heat exchanger and the specific enthalpy difference of the air across the heat exchanger. The amount of heat of the refrigerant is the product of the refrigerant flow, ie mass of the refrigerant per time, through the heat exchanger and the specific enthalpy difference across the heat exchanger. By a simple rule of three then the mass flow of air (or more generally: the first media stream) can be determined by the heat exchanger.
In a preferred embodiment, it is provided that one compares the first media stream with a desired value. If the actually detected, i. If the first media stream calculated from the above values does not agree with the setpoint, an error message can be generated.
In an alternative, however, it is provided that one forms a residue as the difference of a first size, which is formed from a predetermined mass flow of the first media stream and the specific enthalpy difference, and a second size, which corresponds to the change in the enthalpy of the second medium stream, and the residuum is monitored. This procedure facilitates the evaluation of the detected signals. Due to the inertia of the individual sensors, which determine temperatures, pressures and mass flow, it is possible that one can observe significant fluctuations in the signal representing the first medium flow, eg the air mass flow. These fluctuations have a relatively high frequency relative to the "inertia" of the refrigeration system. It is thus difficult to detect in such a "high-frequency" signal a trend that points to an error. By contrast, if one obtains a residual from the air mass signal, the monitoring of the residuum is much simpler and allows sufficient monitoring of the air mass flow.
In this case, it is particularly preferred that a mean value over a predetermined period of time be used as the predetermined mass flow of the first medium flow. It is assumed that one determines the mass flow in a "error-free" operation. If, during operation, deviations result from this previously determined mass flow which last for a predetermined shorter or longer period of time, then this is an indication of an error.
Preferably, with the help of the residual, an error indicator S _{i is formed} according to the following rule: $S_{i} = {\begin{cases} S_{i - 1} + s_{i} . if S_{i - 1} + s_{i} > 0 \\ 0 . if S_{i - 1} + s_{i} \leq 0 \end{cases}$

where s _{i is} calculated according to the following rule: $s_{i} = k_{1} (r_{i} - \frac{μ_{0} + μ_{1}}{2})$
wherein

i: index of a sampling time
r _i : Residual
k ₁ : Proportionality constant
μ ₀ : first reliability value
μ ₁ : second reliability value.

The first reliability value is set to zero in most cases. The second reliability value μ ₁ forms a criterion for how often one must accept a false alarm. If you have less wrong Alarms, one must accept a later discovery of a mistake. For example, if air circulation is restricted because, for example, a fan is no longer running, then the error indicator will increase with time because the periodically determined values of the residual r _{i will become} greater than zero on average. If the error indicator S _{i has reached} a predetermined size, then an alarm is triggered indicating that an error has occurred. The second reliability value is an empirical value, which, however, can be specified by the manufacturer.
Preferably, a defrost operation is initiated upon detection of a predetermined change. For example, you can initiate the defrost process when the error indicator reaches or exceeds a predetermined value. Defrosting processes can be initiated with the method if they are necessary, but the icing of the evaporator does not yet have any negative effects.
The invention will be described below with reference to a preferred embodiment in conjunction with the drawings. Herein show:

Fig. 1: a schematic view of a refrigeration system,
Fig. 2: a schematic view showing the sizes of a heat exchanger,
Fig. 3: the representation of a residuum in a first error case,
Fig. 4: the course of an error indicator for the first error case,
Fig. 5: the course of the residuum for a second error case and
Fig. 6: the representation of the error indicator for the second error case.

Fig. 1 shows schematically a refrigeration system 1 in the form of a sales freezer, as used for example in supermarkets for the sale of chilled or frozen food. The refrigeration system 1 has a storage space 2, in which the food is stored. An air duct 3 is passed around the storage space 2, i. it is located on both sides and below the storage space 2. An air flow 4, which is shown by arrows, passes after passing through the air duct 3 in a cooling zone 5 above the storage space 2. The air is then fed back to the entrance of the air duct 3, where a mixing zone 6 is located. In the mixing zone, the air stream 4 is mixed with ambient air. In doing so, e.g. replaced the cooled air that has entered the storage room 2 or otherwise disappeared into the environment.
In the air duct 3, a fan assembly 7 is arranged, which may be formed by one or more fans can. The fan assembly 7 ensures that the air flow 4 can be moved in the air duct 3. For the following description it is assumed that the fan assembly 7 drives the air flow 4 so that the mass of air per time, which is moved through the air duct 3, is constant, as long as the fan assembly 7 is running and the system is working properly.
In the air duct 3, an evaporator 8 of a refrigerant circuit is arranged. The evaporator 8 is supplied through an expansion valve 9 refrigerant from a condenser or condenser 10. The condenser 10 is supplied by a compressor or compressor 11 whose input is in turn connected to the evaporator 8, so that the refrigerant is circulated in a conventional manner. The condenser 10 is provided with a fan 12, by means of which air from the environment can be blown through the condenser 10 to dissipate heat there.
The operation of such a refrigerant circuit is known per se. The system circulates a refrigerant. The refrigerant leaves the compressor 11 as a gas under high pressure and high temperature. In the condenser 10, the refrigerant is liquefied, giving off heat. After liquefaction, the refrigerant passes through the expansion valve 9, where it is released. After expansion, the refrigerant is biphasic, ie, liquid and gaseous. The two-phase refrigerant is supplied to the evaporator 8. The liquid phase evaporates there under heat absorption, whereby the heat out the air stream 4 is removed. After the remaining refrigerant has evaporated, the refrigerant is still slightly heated and comes out of the evaporator 8 as superheated gas. Thereafter, it is fed back to the compressor 11 and compressed there.
It would now like to monitor whether the air flow 4 can flow undisturbed through the air duct 3. Disturbances can result, for example, from the fact that the fan assembly 7 has a defect and no longer promotes sufficient air. For example, one fan unit with multiple fans may fail. Although the remaining blower can still promote a certain amount of air through the air duct 3, so that the temperature in the storage chamber 2 does not rise above an allowable value. As a result, however, the refrigeration system is heavily loaded, which can result in late damage. For example, elements of the refrigeration system, such as fans, are put into operation more often. Another error case, for example, the icing of the evaporator by moisture from the ambient air, which is reflected on the evaporator.
In other words, you want to be able to permanently monitor the amount of air per time flowing through the air duct 3. The monitoring can be done quite clocked, ie at successive times, for example, have a time interval of the order of a minute. However, the determination of the mass per time of the air stream 4 with normal measuring devices relatively expensive. It is therefore used an indirect measurement by determining the heat content of the refrigerant, which has taken up the refrigerant in the evaporator 8.
This is based on the following consideration: the heat required to evaporate the refrigerant is taken up by the air in the evaporator 8, which acts as a heat exchanger. Accordingly, the following equation holds: ${\overset{•}{Q}}_{Air} = {\overset{•}{Q}}_{Ref}$

where Q̇ _{Air is} the heat actually extracted from the air per time, and Q̇ _{Ref is} the heat absorbed by the refrigerant per time. With this equation one can determine the actual value for the mass flow, ie the mass per time, for the air flowing through the air duct 3, if one can determine the heat absorbed by the refrigerant. The actual mass flow of air can then be compared with a setpoint. If the actual value does not agree with the setpoint, this is interpreted as an error, ie as a disabled airflow 4. A corresponding error message for the system can be output.
The basis for the determination of Q̇ _Ref is the following equation: ${\overset{•}{Q}}_{Ref} = {\overset{•}{m}}_{Ref} (H_{Ref . out} - H_{Ref . in})$

where ṁ _{Ref is} the refrigerant mass per time flowing through the evaporator, h _{Ref, out} is the specific enthalpy of the refrigerant at the evaporator outlet and h _{Ref, in} is the specific enthalpy at the expansion valve inlet.
The specific enthalpy of a refrigerant is a substance and state property, which varies from refrigerant to refrigerant, but can be determined for each refrigerant. The refrigerant manufacturers therefore provide so-called log p, h diagrams for each refrigerant. On the basis of these diagrams, the specific enthalpy difference can be determined via the evaporator 8. For example, to determine h _{Ref, in} such a log p, h diagram, one needs only the temperature of the refrigerant at the expansion valve inlet (T _{Ref, in} ) and the pressure at the expansion valve inlet (P _Con ). These quantities can be measured by means of a temperature sensor or a pressure sensor. The measuring points are shown schematically in Fig. 2.
In order to determine the specific enthalpy at the evaporator outlet, two measured values are needed: the temperature at the evaporator outlet (T _{ref, out} ) and either the pressure at the outlet (P _{ref, out} ) or the boiling temperature (T _{ref, in} ). The temperature at the outlet (T _{Ref, out} ) can be measured with a temperature sensor. The pressure at the outlet of the evaporator 8 (P _{ref, out} ) can be measured with a pressure sensor.
Of course, instead of the log p, h diagrams, one can also use table values, which simplifies the calculation with the aid of a processor. In many cases, the refrigerant manufacturers also provide equations of state for the refrigerants.
The mass flow rate of the refrigerant ( ṁ _Ref ) can be determined either with a flow meter. In systems with electronically controlled expansion valves operated in pulse width modulated mode, it is possible to determine the mass flow rate ṁ _Ref via the opening degree or opening duration when the pressure difference across the valve and the subcooling at the inlet of the expansion valve 10 (T _Vin ) is known. This is the case with most systems because pressure sensors are available to measure the pressure in condenser 10. Hypothermia is constant and predictable in many cases and therefore does not need to be measured. The mass flow m _Ref through the expansion valve 9 can then be calculated by means of a valve characteristic, the pressure difference, the subcooling and the opening degree or the opening duration. In many pulse width modulated expansion valves 9, it has been found that the flow m _{ref is} approximately proportional to the pressure difference and the opening duration. In this case one can determine the flow according to the following equation: ${\overset{•}{m}}_{Ref} = k_{Exp} \cdot (P_{con} - P_{Ref . out}) \cdot O D$

where P _{Con is} the pressure in the condenser 10, P _{Ref, out} the pressure in the evaporator, OD the opening time and k _{Exp is} a proportionality constant, which depends on the valve. In some cases, the subcooling of the refrigerant is so large that it is necessary to measure the subcooling because the refrigerant flow through the expansion valve is affected by the subcooling. In many cases, however, you only need the pressure difference and the degree of opening of the valve, because the subcooling is a fixed size of the refrigeration system, which can then be considered in a valve characteristic or in a proportionality constant. Another possibility for determining the mass flow rate ṁ _Ref is to evaluate variables from the compressor 11, for example the speed of the compressor, the pressure at the compressor inlet and outlet, the temperature at the compressor inlet and a compressor characteristic.
For the actual heat extracted from the air per time Q̇ _Air can be used in principle the same equation as for the heat per time that emits the refrigerant. ${\overset{•}{Q}}_{Air} = {\overset{•}{m}}_{Air} (H_{Air . in} - H_{Air . out})$

where ṁ _{Air is} the mass flow of air, h _{Air, in} the specific enthalpy of air in front of the evaporator and h _{Air, referred to} the specific enthalpy of air after the evaporator.
The specific enthalpy of the air can be calculated using the following equation: $H_{Air} = 1.006 \cdot t + x (2501 + 1 . 8th \cdot t) . [H] = kJ / kg$

where t is the temperature of the air, ie T _{Eva, in} front of the evaporator and T _{EVa, out} behind the evaporator. "x" is called the moisture ratio of the air. The moisture ratio of the air can be calculated by the following equation: $x = 0.62198 \cdot \frac{p_{W}}{p_{Amb} - p_{W}}$
Here p _{w is} the partial pressure of the water vapor in the air and p _Amb is the pressure of the air. p _Amb can either be measured or simply use a standard atmospheric pressure for this size. The deviation of the actual pressure from the standard atmospheric pressure does not play a significant role in the calculation of the amount of heat released by the air per time. The partial pressure of the water vapor is determined by the relative humidity of the air and the partial pressure of the water vapor in saturated air and can be calculated by the following equation: $p_{W} = p_{W . Sat} \cdot R H$
Here, RH is the relative humidity and Pw, sat is the partial pressure of the water vapor in saturated air. P _{w, sat} depends solely on the air temperature and can be found in thermodynamic reference works. The relative humidity RH can be measured or one uses typical values in the calculation.
If one equates the equations (2) and (4), as assumed in equation (1), then it results ${\overset{•}{m}}_{Ref} (H_{Ref . out} - H_{Ref . in}) = {\overset{•}{m}}_{Air} (H_{Air . in} - H_{Air . out})$
From this, the actual air mass flow rate ṁ _Air can be found by isolating ṁ _Air : ${\overset{•}{m}}_{Air} = {\overset{•}{m}}_{Ref} \frac{(H_{Ref . out} - H_{Ref . in})}{(H_{Air . in} - H_{Air . out})}$
This actual value for the air mass flow rate ṁ _Air can then be compared with a setpoint value and in the case of significant differences between the actual value and the setpoint, the operator of the refrigeration system can be made aware by an error message that the system is not running optimally.
In many cases, it is advisable to determine the setpoint for the air flow in a system. For example, this setpoint can be determined as an average value over a certain period in which the Plant is running under stable and faultless operating conditions. Such a period may be, for example, 100 minutes.
However, a certain difficulty arises from the fact that the signals emitted by the individual sensors (thermometers, pressure sensors) are subject to considerable fluctuations. These fluctuations may well be in opposite directions, so that one receives a signal for the size ṁ _Air , which offers certain difficulties in the evaluation. These fluctuations are a result of the dynamic conditions in the cooling system. Therefore, it may be convenient, instead of the equation (9) at regular time intervals, for example once a minute, to calculate a size, which is hereinafter referred to as "Residuum": $r = {\tilde{\overset{•}{m}}}_{Air} (H_{Air . in} - H_{Air . out}) - {\overset{•}{m}}_{Ref} (H_{Ref . out} - H_{Ref . in})$
${\tilde{\overset{•}{m}}}_{Air}$
is an estimated value for the air mass flow rate under faultless operating conditions. Instead of an estimate, one can also use a value which is determined as an average over a certain period of time from equation (9) under fault-free operating conditions.
For a plant that is flawless, the residual r should give an average of zero, although it is actually subject to significant fluctuations. Around an error characterized by a tendency of the residuum to be able to detect early, it is assumed that the value obtained for the residual r is normally distributed around an average value, irrespective of whether the installation is faultless or an error has occurred. One then calculates an error indicator S _i according to the following relationship: $S_{i} = {\begin{cases} S_{i - 1} + s_{i} . if S_{i - 1} + s_{i} > 0 \\ 0 . if S_{i - 1} + s_{i} \leq 0 \end{cases}$
where s _i can be calculated with the following equation: $s_{i} = k_{1} (r_{i} - \frac{μ_{0} + μ_{1}}{2})$
Of course, it is assumed that the error indicator S _i , ie at the first time, has been set to zero. At a later time, s _{i is taken} from equation (12) and forms the sum of this value with the error indicator S _i from an earlier point in time. If this sum is greater than zero, the error indicator is set to this new value. If this sum is equal to or less than zero, the error indicator is set to zero. In equation (12), k _{1 is} a proportionality constant. μ ₀ can be set to the value zero in the simplest case. μ ₁ is an estimated value which can be determined, for example, by generating an error and determining the average value of the residual r for this error. The value μ ₁ is a criterion for how often you have to accept a false alarm. The two μ values are therefore also referred to as reliability values.
For example, if an error occurs because a blower from the fan assembly 7 is not running, then the error indicator S _{i will become} larger because the periodically determined values of the residual r _{i become} greater than zero on average. When the fault indicator reaches a predetermined magnitude, an alarm is triggered indicating that air circulation is restricted. Increasing μ ₁ gives you fewer false positives, but it also risks discovering a bug later.
The operation of the filtering according to equation (11) will be explained with reference to FIGS. 3 and 4. In Fig. 3, the time in minutes to the right and the residue r is plotted to the right. Between t = 510 and t = 644 minutes, a blower of the fan assembly 7 has failed. This manifests itself in an increased value of the residual r. Although this increase can already be seen with reference to FIG. However, a better recognition possibility results if one looks at the error indicator S _i , the course of which is shown in FIG. 4. Here the error indicator S _{i is} plotted upward and the time t in minutes to the right. The error indicator thus increases continuously in the time between t = 510 minutes and t = 644 minutes. One can For example, when exceeding the value S _i of 0.2 x 10 ⁸ trigger an alarm.
In the time between t = 700 and t = 824 minutes, a fan of the fan assembly 7 is also stopped. The error indicator S _i continues to increase. Between these two fault conditions both blowers were active again. The error indicator S _i is thus reduced, but does not return to zero. The error indicator S _i is reliably increased in the event of an error. In the time from 0 to 510 minutes, the error indicator S _i moves in the area of the zero point. The error indicator S _i would return to zero if the system runs error free long enough. In practice, however, one will set the error indicator S _i to zero when an error has been corrected.
Figs. 5 and 6 show the development of the residual r and the development of the error indicator S _i in the case where the evaporator 8 is gradually frozen. Here, in Fig. 5, the residual r and in Fig. 6, the error indicator S _i is plotted upward, while the time t is applied to the right in minutes.
In Fig. 5, it can be seen that the mean value of the residual r gradually increases. However, it can also be seen that this increase is difficult to quantify with the necessary security for an error message. At t = 600 minutes incipient icing of the evaporator 8 occurs. Only at t = 1200 minutes could one capture such icing due to a reduced capacity of the refrigeration system.
If, for example, the limit value for the error indicator is set to 1 × 10 ⁷ , then an error would already be detected at about t = 750 minutes, ie much earlier than through a reduced performance of the system.
The method can also be used to start a defrost process. The defrost operation is started when the error indicator S _{i reaches} a predetermined size.
An advantage of this method is early detection of errors, although not more sensors are used than are present in a typical system. The faults are detected before they cause higher temperatures in the refrigeration system. Also, errors are detected before the system no longer runs optimally, taking the energy consumed as a measure.
The monitoring of the air flows at the evaporator 8 was shown. Of course, similar monitoring can also be carried out on the condenser 10. In this case, the calculations are even easier, because no humidity of the ambient air is removed when the air passes through the condenser 10. Accordingly, no water from the air condenses on the condenser 10 because it is warmer. It is disadvantageous when using the method on the capacitor 10 that two additional temperature sensors are needed to measure the temperature of the air before and after the condenser.
The method has been described for the case where the air flow is constant and an adaptation to different cooling performance requirements is achieved by the air flow is generated intermittently. However, it is in principle also possible to allow within certain limits, a variation of the air flow, if one additionally takes into account the drive power or the speed of the blower.
The method of detecting changes in the first media stream may also be used on installations that operate with indirect cooling. In such plants, one has a primary media stream in which refrigerant circulates and a secondary media stream where a refrigerant, eg brine, circulates. In the evaporator, the first medium flow cools the second medium flow. The second media stream then cools, for example, the air in a heat exchanger. One can use this method on the evaporator, but also on the air / brine heat exchanger. On the air side of the heat exchanger, the calculations do not change. The enthalpy increase, if the brine in the heat exchanger is not subjected to an evaporation process but only to an increase in temperature, can be calculated by the following formula: $Q_{K T} = c \cdot m_{K T} (T_{to} - T_{in front})$

where c is the specific heat capacity of the brine, T _after the temperature after the heat exchanger, T _before the temperature before the heat exchanger and m _{KT is} the mass flow of the brine. The constant c can be found in reference works, while the two temperatures can be measured, eg with temperature sensors. The mass flow m _KT can be determined by a mass flow meter . Of course, other options are also conceivable. Q _KT then replaces Q _Ref in the further calculations.

Claims

Method for determining changes in a first medium flow in a heat or cold carrying medium in a refrigeration system, in which the first medium flow is led through a heat exchanger, in which a heat transition between the first medium flow and a second medium flow of a heat or cold carrying medium takes place, characterised in that to monitor the first medium flow through the heat exchanger the enthalpy change of the second medium flow or a value derived from that is determined.
Method according to claim 1, characterised in that to determine the enthalpy change of the second medium flow, a mass flow and a specific enthalpy difference of the second medium flow over the heat exchanger are determined.
Method according to claim 2, characterised in that to determine the specific enthalpy difference of the second medium flow, at the inlet of the expansion valve the temperature and the pressure of the second medium flow are determined and at the outlet of the heat exchanger the temperature of the second medium flow and either the pressure at the outlet of the heat exchanger or the boiling temperature of the second medium flow at the inlet of the heat exchanger are determined.
Method according to one of the claims 1 to 3, characterised in that a specific enthalpy difference of the first medium flow over the heat exchanger is determined.
Method according to one of the claims 2 to 4, characterised in that the second medium flow is determined on the basis of a pressure difference over and the opening degree of an expansion valve.
Method according to one of the claims 2 to 4, characterised in that the second medium flow is determined on the basis of operation data and a difference of the absolute pressures over a compressor together with the temperature of the second medium flow at the compressor.
Method according to claim 5 or 6, characterised in that the first medium flow is determined on the basis of the second medium flow and a quotient of the specific enthalpy difference of the second medium flow and the specific enthalpy difference of the first medium flow over the heat exchanger.
Method according to one of the claims 5 to 7, characterised in that the first medium flow is compared with a nominal value.
Method according to one of the claims 5 to 7, characterised in that a residual is formed as a difference between a first value, which is formed by a prespecified mass flow of the first medium flow and the specific enthalpy difference, and a second value, which corresponds to the enthalpy change of the second medium flow, the residual being monitored.
Method according to claim 9, characterised in that a medium value over a predetermined period of time is used as prespecified mass flow of the first medium flow.
Method according to claim 9 or 10, characterised in that by means of the residual a fault indicator S_i is formed according to the following specification: $S_{i} = {\begin{cases} S_{i - 1} + s_{i}, wenn S_{i - 1} + s_{i} > 0 \\ 0, wenn S_{i - 1} + s_{i} \leq 0 \end{cases}$
with $s_{i} = k_{1} (r_{i} - \frac{μ_{0} + μ_{1}}{2})$

r_i: Residual

k₁: Proportionality constant

µ₀: First reliability value

µ₁: Second reliability value.
Method according to one of the claims 1 to 11, characterised in that a defrosting is started, when a predetermined change is discovered.