Method for calibrating a temperature sensor of a vapour compression system

Abstract

A method for calibrating a temperature sensor arranged in a vapor compression system is disclosed. The opening degree of an expansion device is alternatingly increased and decreased. Simultaneously a temperature of refrigerant entering the evaporator and a temperature of refrigerant leaving the evaporator are monitored. For each cycle of the opening degree of the expansion device, a maximum temperature, T.sub.1, max, of refrigerant entering the evaporator, and a minimum temperature, T.sub.2, min, of refrigerant leaving the evaporator are registered. A calibration value, ΔT.sub.1, is calculated as ΔT.sub.1=C−(T.sub.2, min−T.sub.1, max) for each cycle, and a maximum calibration value, among the calculated values is selected. Finally, temperature measurements performed by the first temperature sensor are adjusted by an amount defined by ΔT.sub.1, max.

Claims

1. A method for calibrating a temperature sensor arranged in a vapour compression system, the vapour compression system comprising a compressor, a condenser, an expansion device having a variable opening degree, and an evaporator arranged along a refrigerant path, the vapour compression system further having a first temperature sensor, S.sub.1, arranged in the refrigerant path at an inlet opening of the evaporator, and a second temperature sensor, S.sub.2, arranged in the refrigerant path at an outlet opening of the evaporator, the method comprising the steps of: alternatingly increasing and decreasing the opening degree of the expansion device between a maximum opening degree and a minimum opening degree, thereby defining a plurality of cycles of the opening degree of the expansion device, at least for a part of each cycle of the opening degree of the expansion device, monitoring a temperature of refrigerant entering the evaporator by means of the first temperature sensor, S.sub.1, and monitoring a temperature of refrigerant leaving the evaporator by means of the second temperature sensor, S.sub.2, for each cycle of the opening degree of the expansion device, registering a maximum temperature, T.sub.1, max, measured by the first temperature sensor, S.sub.1, and registering a minimum temperature, T.sub.2, min, measured by the second temperature sensor, S.sub.2, for each cycle of the opening degree of the expansion device, calculating a calibration value, ΔT.sub.1, as ΔT.sub.1=C−(T.sub.2, min−T.sub.1, max), where C is a constant, selecting a maximum calibration value, ΔT.sub.1, max, among the calibration values, ΔT.sub.1, calculated for each of the plurality of cycles of the opening degree of the expansion device, and adjusting temperature measurements performed by the first temperature sensor, S.sub.1, by an amount defined by ΔT.sub.1, max.

2. The method according to claim 1, wherein the step of alternatingly increasing and decreasing the opening degree of the expansion device is performed on the basis of measurements performed by the second temperature sensor, S.sub.2.

3. The method according to claim 2, wherein the step of monitoring a temperature of refrigerant leaving the evaporator comprises monitoring a dynamical behaviour of the temperature of refrigerant leaving the evaporator, and wherein the step of alternatingly increasing and decreasing the opening degree of the expansion device is performed on the basis of the dynamical behaviour of the temperature of the refrigerant leaving the evaporator.

4. The method according to claim 3, further comprising the step of repeating the method steps after a period of time has elapsed.

5. The method according to claim 3, wherein C corresponds to a superheat value of refrigerant leaving the evaporator during a flooded condition of the evaporator.

6. The method according to claim 2, further comprising the step of repeating the method steps after a period of time has elapsed.

7. The method according to claim 2, wherein C corresponds to a superheat value of refrigerant leaving the evaporator during a flooded condition of the evaporator.

8. The method according to claim 1, further comprising the step of repeating the method steps after a period of time has elapsed.

9. The method according to claim 8, wherein C corresponds to a superheat value of refrigerant leaving the evaporator during a flooded condition of the evaporator.

10. The method according to claim 1, wherein C corresponds to a superheat value of refrigerant leaving the evaporator during a flooded condition of the evaporator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in further detail with reference to the accompanying drawings in which

(2) FIG. 1 is a diagrammatic view of a part of a vapour compression system used for performing the method according to an embodiment of the invention, and

(3) FIG. 2 is a graph illustrating temperature variations of refrigerant during alternatingly increasing and decreasing the opening degree of an expansion device.

DETAILED DESCRIPTION

(4) FIG. 1 is a diagrammatic view of a part of a vapour compression system 1. The vapour compression system 1 comprises a compressor 2, a condenser (not shown), an expansion device 3, in the form of an electronic expansion valve (EEV), and an evaporator 4, arranged along a refrigerant path 5. A first temperature sensor 6 is arranged in the refrigerant path 5 at an inlet opening of the evaporator 4, and a second temperature sensor 7 is arranged in the refrigerant path 5 at an outlet opening of the evaporator 4. Thus, the first temperature sensor 6 measures the temperature, T.sub.1, of refrigerant entering the evaporator 4, and the second temperature sensor 7 measures the temperature, T.sub.2, of refrigerant leaving the evaporator 4.

(5) The temperature signals, T.sub.1 and T.sub.2, are communicated to a control device 8 with the purpose of controlling the opening degree of the expansion device 3 in such a manner that an optimal superheat value is obtained. Accordingly, the control device 8 is adapted to generate and supply a control signal to the expansion device 3.

(6) Furthermore, the control device 8 receives an ON/OFF signal from the compressor 2 indicating whether the compressor is operating or not. This information is also taken into account when the control signal to the expansion device 3 is generated.

(7) The first temperature sensor 6 can be calibrated in the manner described above. This will be described further below with reference to FIG. 2.

(8) FIG. 2 is a graph illustrating temperature variations of refrigerant during alternatingly increasing and decreasing the opening degree of an expansion device. The graph of FIG. 2 may, e.g., be obtained by measurements performed by the temperature sensors 6, 7 shown in FIG. 1.

(9) In the graph of FIG. 2, the opening degree of an expansion device, e.g. the expansion device 3 illustrated in FIG. 1, as a function of time is illustrated by solid line 9. It can be seen that the opening degree 9 is alternated between a maximum opening degree and a minimum opening degree, and that the opening degree 9 is switched abruptly between the maximum and minimum opening degrees. This is repeated, and a plurality of cycles of the opening degree 9 of the expansion device is thereby defined. Approximately 1½ cycle is shown in FIG. 2.

(10) While the opening degree 9 of the expansion device is alternated as described above, the temperature of refrigerant entering the evaporator and the temperature of refrigerant leaving the evaporator are measured, e.g. by means of the temperature sensors 6, 7 illustrated in FIG. 1. The dotted line 10 in FIG. 2 illustrates the temperature of refrigerant entering the evaporator, i.e. the temperature measured by means of the first temperature sensor 6, as a function of time. The dashed line 11 illustrates the temperature of refrigerant leaving the evaporator, i.e. the temperature measured by means of the second temperature sensor 7, as a function of time.

(11) In the graph of FIG. 2, the opening degree 9 of the expansion device is initially at the minimum opening degree. Accordingly, the supply of refrigerant to the evaporator is low, and the filling degree of the evaporator will therefore gradually decrease. Furthermore, the pressure of the refrigerant entering the evaporator is low. As a consequence, the temperature 10 of the refrigerant entering the evaporator decreases, as described above. Furthermore, the temperature 11 of the refrigerant leaving the evaporator increases, since an increasing part of the evaporator will contain gaseous refrigerant, and therefore an increasing part of the heat exchange taking place in the evaporator will be used for heating gaseous refrigerant, instead of for evaporating refrigerant.

(12) At a certain point in time, the opening degree 9 of the expansion device is switched to the maximum opening degree. Thereby the supply of refrigerant to the evaporator is increased significantly, thereby increasing the mass flow of refrigerant towards the evaporator and increasing the pressure of the refrigerant entering the evaporator. The increased supply of refrigerant to the evaporator furthermore causes the filling degree of the evaporator to increase.

(13) Thus, the increase in opening degree 9 of the expansion device causes the temperature 10 of the refrigerant entering the evaporator to increase. Furthermore, the increase in opening degree 9 of the expansion device causes the temperature 11 of the refrigerant leaving the evaporator to decrease. Accordingly, the temperature 10 of refrigerant entering the evaporator and the temperature 11 of refrigerant leaving the evaporator approach each other.

(14) At a later point in time, the opening degree 9 of the expansion device is once again switched to the minimum opening degree. Thereby the supply of refrigerant to the evaporator is decreased significantly, thereby decreasing the mass flow of refrigerant towards the evaporator and decreasing the pressure of the refrigerant entering the evaporator. The decreased supply of refrigerant to the evaporator furthermore causes the filling degree of the evaporator to increase towards the maximum filling degree.

(15) Thus, the decrease in opening degree 9 of the expansion device causes the temperature 10 of the refrigerant entering the evaporator to decrease. Furthermore, the decrease in opening degree 9 of the expansion device causes the temperature 11 of the refrigerant leaving the evaporator to increase. Accordingly, the temperature difference between the temperature 11 of the refrigerant leaving the evaporator and the temperature 10 of the refrigerant entering the evaporator is increased.

(16) The alternating increase and decrease in opening degree 9 of the expansion device is repeated a desired number of times, thereby defining a plurality of cycles of the opening degree of the expansion device. This results in the temperatures 10, 11 of the refrigerant entering and leaving the evaporator being alternatingly increased and decreased as described above. Accordingly, for each of the cycles of the opening degree 9 of the expansion device the temperature 10 of the refrigerant entering the evaporator reaches a maximum value, and the temperature 11 of the refrigerant leaving the evaporator reaches a minimum value. This will occur almost simultaneously, with the temperature 10 of the refrigerant entering the evaporator reaching its maximum value shortly before the temperature 11 of the refrigerant leaving the evaporator reaching its minimum value, due to the time it takes for the refrigerant to pass the evaporator. This event indicates that the evaporator has reached a flooded state or maximum filling degree, and the temperature difference between the minimum temperature 11 of the refrigerant leaving the evaporator and the maximum temperature 10 of the refrigerant entering the evaporator corresponds to the superheat value in this state.

(17) Therefore, for each cycle of the opening degree 9 of the expansion device, a calibration value, ΔT.sub.1, can be calculated as ΔT.sub.1=C−(T.sub.2, min−T.sub.2, max), where C is a constant corresponding to the superheat value in a flooded state of the evaporator, T.sub.2, min is the minimum value of the temperature 11 of the refrigerant leaving the evaporator, and T.sub.1, max is the maximum value of the temperature 10 of the refrigerant entering the evaporator. Thus, ΔT.sub.1 represents the difference between the actual superheat value and the measured superheat value, and is therefore a measure for a misreading or bad calibration of the temperature sensors relative to each other.

(18) Among the calculated calibration values a maximum value, ΔT.sub.1, max, is selected. Thereby it is ensured that the calibration value which is selected represents the situation where the evaporator is actually in a flooded state, and the calibration value thereby truly reflects the possible misreading of the temperature sensors.

(19) Finally, the subsequent temperature measurements performed by the first temperature sensor, i.e. the temperature sensor measuring the temperature 10 of refrigerant entering the evaporator, are adjusted by an amount corresponding to ΔT.sub.1, max. Thereby the superheat values which are subsequently determined based on measurements performed by means of the first and second temperature sensors will be accurate, and an accurate control of the expansion device can thereby be obtained.

(20) The embodiments of the invention described above are provided by way of example only. The skilled person will be aware of many modifications, changes and substitutions that could be made without departing from the scope of the present invention. The claims of the present invention are intended to cover all such modifications, changes and substitutions as fall within the spirit and scope of the invention.