SMEDING, Simon Franciscus (32 Marnixstraat, GW Leeuwarden, GW Leeuwarden, NL-8913, NL)
| C L A I M S 1. Method for detecting a change in a liquid level (41) of a liquid refrigerant (40) in an evaporator (18) and/or controlling a liquid level of a liquid refrigerant (40) in an evaporator (18), which evaporator (18) is provided with: - a heat exchanger (42) for supplying heat to the liquid refrigerant (40) in order to produce a vaporous refrigerant (45), in which the vaporous refrigerant (45) is situated above the liquid level (41) and is saturated, - a supply (19) for supplying the liquid refrigerant, - a discharge (20) for discharging the vaporous refrigerant, - a temperature sensor (51) which is fitted at a level in the evaporator (18), in which the temperature sensor (51) measures a temperature, and in which the temperature measured by the temperature sensor (51) is compared to a reference temperature, and in which at a first point in time the liquid level (41) of the liquid refrigerant (40) is situated above the temperature sensor (51) and it is detected that the temperature measured by the temperature sensor (51) is substantially equal to the reference temperature, and in which after the first point in time vaporous refrigerant (45) is discharged via the discharge (20) of the evaporator (18) and liquid refrigerant (40) evaporates in the evaporator (18), so that the liquid level (41) in the evaporator (18) drops, and in which an amount of the liquid refrigerant (40) adheres to the temperature sensor (51) when the liquid level (41) drops below the temperature sensor (51) and the temperature sensor (51) cools down as a result of evaporation of the adhering liquid refrigerant and it is detected that the temperature measured by the temperature sensor (51 ) is lower than the reference temperature. 2. Method according to claim 1 , in which the reference temperature is determined by an average per unit time of the temperature measured by the temperature sensor (51). 3. Method according to claim 1 , in which the temperature sensor (51) is a first temperature sensor (51), and in which the evaporator is provided with a second temperature sensor (52) which is fitted below the level of the first temperature sensor (51), and in which the second temperature sensor (52) measures a second temperature, and in which the reference temperature is determined by the second temperature. 4. Method according to one of the preceding claims, in which a signal is emitted if it is detected that the temperature measured by the temperature sensor (51) is lower than the reference temperature. 5. Method according to claim 4, in which the signal is emitted after it has been detected for at least 2 seconds that the average per unit time of the temperature measured by the temperature sensor (51) is at feast 1°C lower than the reference temperature. 6. Method according to claim 4 or 5, in which the signal comprises a control signal, and in which the control signal controls the supply of liquid refrigerant (40) to the supply of the evaporator. 7. Method according to one of the preceding claims, in which liquid refrigerant (40) is supplied to the evaporator (18) in order to increase the liquid level (41) after it has been detected that the temperature measured by the temperature sensor (51) is lower than the reference temperature as a result of evaporation of the adhering liquid refrigerant (40). 8. Method according to claim 6 or 7, in which liquid refrigerant (40) is supplied, at least until it is detected that the temperature measured by the temperature sensor (51 ) is substantially equal to the reference temperature. 9. Method according to one of the preceding claims, in which the evaporator (18) is provided with a further temperature sensor (54) which is fitted above the level of the temperature sensor (51), and in which a further temperature is measured using the further temperature sensor (54), and in which at a further point in time the liquid level (41) of the liquid refrigerant (40) is below the temperature sensor (51), and in which after the further point in time liquid refrigerant is supplied with a temperature which is higher than the temperature of the liquid refrigerant in the evaporator, and in which, if the liquid level (41) rises above the temperature sensor (51 ), it is detected that the temperature measured by the temperature sensor (51) is higher than the temperature measured by the further temperature sensor (54), and in which the liquid refrigerant is supplied until it is detected that the temperature measured by the further temperature sensor (54) is substantially equal to the temperature measured by the temperature sensor (51). 10. Method according to one of the preceding claims, in which the heat exchanger is provided with a heat exchange line (23) having an inlet (21) and an outlet (22), and in which a fluid flows from the inlet (21) to the outlet (22), and in which the fluid becomes colder during evaporation of the liquid refrigerant (40) by the extraction of heat from the fluid which flows through the heat exchange line (23). 11. Method according to one of the preceding claims, in which the heat exchanger (42) is partially immersed in the liquid refrigerant. 12. Method according to one of the preceding claims, wherein the temperature sensor 5 (51) is not heated. 13. System comprising an evaporator (18) which is provided with: - a liquid refrigerant (40) which defines a liquid level (41) in the evaporator (18), - a heat exchanger (42) for supplying heat to the liquid refrigerant (40) in order to 10 produce a vaporous refrigerant (45), in which the vaporous refrigerant (45) is situated above the liquid level (41) and is saturated, - a supply (19) for supplying the liquid refrigerant, - a discharge (20) for discharging the vaporous refrigerant, - a temperature sensor (51) for measuring a temperature, 15 characterized in that the temperature sensor (51) is fitted in the evaporator (18) at a level, and the system comprises a device (53) for detecting a change in the liquid level (41) of the liquid refrigerant (40) in the evaporator (18), which device (53) is connected to the temperature sensor (51) in order to receive the temperature measured by the temperature sensor (51), 20 and which device (53) is configured to detect when the temperature measured by the temperature sensor (51) is lower than a reference temperature. 14. System according to claim 13, in which the device (53) is provided with a calculating device for calculating an average per unit time of the temperature measured by the 25 temperature sensor (51 ), and in which the reference temperature is determined by the average calculated by the calculating device. 15. System according to claim 13, in which the temperature sensor (51) is a first temperature sensor (51) for measuring a first temperature, and in which the evaporator is 30 provided with a second temperature sensor (52) for measuring a second temperature, in which the second temperature sensor (52) in the evaporator (18) is fitted below the level of the first temperature sensor (51), and in which the reference temperature is determined by the second temperature. 35 16. System according to one of claims 3-15, in which the device (53) is configured to emit a signal if the temperature measured by the temperature sensor (51) is lower than the reference temperature. 17. System according to claim 16, in which the device (53) is configured to emit the signal after it has been detected for at least 2 seconds that the average per unit time of the temperature measured by the temperature sensor (51) is at least 1°C lower than the 5 reference temperature. 18. System according to claim 16 or 17, in which the supply (19) of the evaporator is provided with a valve (91), and in which the signal comprises a control signal for opening the valve (91) in order to supply a liquid refrigerant to the evaporator (18). 10 19. System according to claim 18, in which the device (53) is configured to emit a second control signal for closing the valve (91) in order to end the supply of a liquid refrigerant to the evaporator (18) if the temperature measured by the temperature sensor (51) is substantially equal to the reference temperature. 15 20. System according to claim 18, in which the supplied liquid refrigerant has a higher temperature than the liquid refrigerant in the evaporator (18), and in which the evaporator (18) is provided with a further temperature sensor (54) for measuring a further temperature, which further temperature sensor (54) is fitted above the level of the temperature sensor (51), 20 and in which the device (53) is connected to the further temperature sensor (54) for receiving the further temperature, and in which the device (53) is configured to emit a second control signal for closing the valve (91) in order to end the supply of liquid refrigerant to the evaporator (18) if the temperature measured by the further temperature sensor (54) is substantially equal to the temperature measured by the temperature sensor (51). 25 21. System according to one of claims 13-20, in which the heat exchanger (42) comprises a bottom surface (43) and a top surface (44), and in which the temperature sensor (51) is fitted at a level between the bottom surface (43) and the top surface (44). 30 22. System according to one of claims 13-21 , in which the system comprises no heating element for heating the temperature sensor (51). 23. Sorption cooling system comprising: - a reactor (3) which is provided with a sorbent, a refrigerant, and a heat exchange 35 line (8) which extends through the sorbent and the refrigerant in the reactor (3), which reactor (3) is configured so as to alternately carry out adsorption and desorption of the sorbent in the reactor (3), which reactor (3) comprises a supply (4) and a discharge (5) for vaporous refrigerant, - a condenser (10) which is provided with a supply (11) for vaporous refrigerant and a discharge (12) for liquid refrigerant, in which the supply (1 ) of the condenser (10) is connected to the discharge (5) of the reactor (3), - a system according to one of claims 12-22, in which the discharge (20) of the evaporator (18) is connected to the supply (4) of the reactor (3), and in which the discharge (12) of the condenser (10) is connected to the supply (19) of the evaporator (18). |
The invention relates to a method for detecting a change in the liquid level of a liquid refrigerant in an evaporator and/or a method for controlling a liquid level of a liquid refrigerant in an evaporator.
US5782131 discloses a level sensor for detecting a change in the level of liquid refrigerant in an evaporator. The evaporator is provided with a heat exchanger having heat exchanger tubes. The evaporator contains liquid refrigerant. Above the level of the liquid refrigerant, a vaporous refrigerant is present. The liquid refrigerant extracts heat from water which flows through the heat exchanger tubes, thus rendering the water colder. By supplying heat to the liquid refrigerant, the liquid refrigerant evaporates, resulting in a decrease in the liquid level. In order to keep the liquid refrigerant at a minimum desired liquid level, the evaporator is provided with a level sensor.
The level sensor is arranged in the evaporator in such a way that the bottom end of the level sensor is situated just above the top heat exchange line. The level sensor is provided with a heating element and several thermistors which are arranged one above the other and are connected in series. The heating element increases the temperature of the thermistors. When the liquid level increases, the liquid refrigerant comes into contact with the bottom thermistor of the level sensor. Since the heat transfer of the liquid refrigerant is greater than that of the vaporous refrigerant, the bottom thermistor cools down. As the liquid level rises further, the thermistors arranged above the latter are immersed in liquid refrigerant. The change in the resistance of the thermistors is a measure for the liquid level in the evaporator. However, this level sensor is relatively inaccurate.
It is an object of the invention to provide an improved method for detecting a change in the liquid level of a liquid refrigerant in an evaporator.
This object is achieved according to the invention by a method for detecting a change in the liquid level of a liquid refrigerant in an evaporator, in particular a drop in the liquid level, which evaporator is provided with:
- a heat exchanger for supplying heat to the liquid refrigerant in order to produce a vaporous refrigerant, in which the vaporous refrigerant is situated above the liquid level and is saturated,
- a supply for supplying the liquid refrigerant,
- a discharge for discharging the vaporous refrigerant,
- at least one temperature sensor which is fitted at a level in the evaporator, for example a desired minimum liquid level, in which the temperature sensor measures a temperature, and in which the temperature measured by the temperature sensor is compared to a reference temperature, and in which the liquid level of the liquid refrigerant is situated above the temperature sensor at a first point in time and it is detected that the temperature measured by the temperature sensor is substantially equal to the reference temperature, and in which vaporous refrigerant is discharged via the discharge of the evaporator and liquid refrigerant evaporates in the evaporator after the first point in time, so that the liquid level in the evaporator drops, and in which an amount of the liquid refrigerant adheres to the temperature sensor when the liquid level drops below the temperature sensor and the temperature sensor cools down as a result of evaporation of the adhering liquid refrigerant and it is detected that the temperature measured by the temperature sensor is lower than the reference temperature.
The liquid refrigerant determines the liquid level in the evaporator. The vaporous refrigerant above the liquid level is substantially completely saturated, for example the vaporous refrigerant is 95-100% saturated. The vaporous refrigerant is discharged via the discharge of the evaporator. By means of the heat exchanger, heat is extracted from a product to be cooled and supplied to the liquid refrigerant. As a result thereof, the liquid refrigerant evaporates and the liquid level in the evaporator drops. The temperature sensor is fitted at a vertical level in the evaporator, for example a minimum desired liquid level.
With the method according to the invention, the temperature measured by the temperature sensor is compared to a reference temperature. The reference temperature corresponds to the temperature of the liquid refrigerant in the evaporator. At the first point in time - the initial state - the temperature sensor is immersed in the liquid refrigerant and the temperature measured by the temperature sensor is found to be substantially equal to the reference temperature. When the liquid level drops below the temperature sensor, the temperature sensor comes to lie free above the liquid level, and an amount of liquid refrigerant adheres thereto.
With the above-described evaporator according to US5782131, the liquid level drops as a result of the liquid refrigerant boiling and vaporous refrigerant being formed. When the liquid level drops below the level sensor, it is also possible for an amount of the liquid refrigerant to adhere to the thermistors. The heating element heats the thermistors, so that the adhering liquid refrigerant will evaporate. However, as the heating element supplies heat to the thermistors and heat is extracted from the thermistors due to evaporation of the adhering liquid refrigerant, it is not possible or hardly possible to predict, whether the temperature of the thermistors will rise or drop when the liquid level drops below the level sensor. Therefore, a drop in the liquid level cannot be detected. According to US5782131 , an increase in the liquid level to above the level sensor is only detected when supplying a liquid refrigerant. The thermistors are initially surrounded by the vaporous refrigerant. When the liquid refrigerant is supplied and reaches the thermistors, the thermistors measure a drop in temperature. It is therefore not known from US5782131 that the temperature sensor cools down as a result of evaporation of the adhering liquid refrigerant and neither that it is found that the temperature measured by the temperature sensor is lower than the reference temperature.
According to the invention, a change in the filling level is detected by detecting a drop in the liquid level below the level sensor. Due to the adhering liquid refrigerant evaporating from the surface of the temperature sensor, the temperature measured by the latter drops. If it is found that the temperature measured by the temperature sensor is lower than the reference temperature, this means that the liquid level has dropped below the temperature sensor. The measured temperature difference is relatively large as heat of evaporation is extracted from the adhering liquid refrigerant. As a result thereof, the measurement of the change in the liquid level is particularly accurate.
A further advantage is that the temperature sensor for carrying out the method according to the invention can be especially compact and inexpensive. No heating element is required. The temperature sensor requires only a limited installation space. In addition, the reliability is high, since the temperature sensor does not comprise any moving parts. Also, the temperature sensor is hardly susceptible to dirt. Furthermore, the temperature sensor can withstand various pressures, temperatures and chemicals.
It should be noted that a level sensor for a cryogenic medium in a container is known from EP1039271. The container is filled with the cryogenic medium which is partly in the liquid phase and partly in the gas phase. Two temperature sensors are installed in the container in order to determine the filling level of the container. The first temperature sensor is at a desired filling level. The second temperature sensor is fitted so far below the latter that it is ensured that the second temperature sensor is surrounded by liquid. The temperature in the medium in the liquid phase is lower than in the gas phase. If the first temperature sensor is surrounded by medium in the liquid phase, the temperature difference between the temperatures measured by the temperature sensors is small. If, however, the first temperature sensor is surrounded by medium in the gas phase, the first temperature sensor has a higher temperature than the second temperature sensor which is situated below the latter. The temperature difference is compared to a reference value. In case of a temperature difference which is smaller than the reference value, the filling position is at the level of the first temperature sensor or above. If the temperature difference is greater than the reference value, the liquid level is below the first temperature sensor. However, in this case, no use is made of evaporation of adhering liquid from the surface of the first temperature sensor. In addition, the container according to EP1039271 is not an evaporator. it should furthermore be noted that a level sensor for oil in a container is known from DE 10136058. The heated oil is held in the container and determines a filling level. Air is present above the filling level of the oil. The air has a lower temperature than the heated oil. In the container, a level sensor with temperature sensors is fitted. The temperatures measured therewith can be used to deduce whether the associated temperature sensor is surrounded by the heated oil or the colder air. However, no evaporation of oil from the surface of the temperature sensor occurs. In addition, the container according to
DE 10136058 is not an evaporator.
it is possible that the reference temperature is determined by an average per unit time of the temperature measured by the temperature sensor. In this case, the reference temperature is calculated. The calculated reference temperature is compared to the current temperature measured by the temperature sensor. The reference temperature is, for example, the average of the temperature measured by the temperature sensor calculated over a period of at least 20 or at least 30 seconds. The reference temperature can be determined and compared to the current temperature measured by the temperature sensor using software.
It is also possible for the temperature sensor to be a first temperature sensor, in which the evaporator is provided with a second temperature sensor which is fitted below the level of the first temperature sensor, and in which the second temperature sensor measures a second temperature, and in which the reference temperature is determined by the second temperature. In this case, the reference temperature is measured. The two temperature sensors are spaced apart at different vertical levels in the evaporator. At the first point in time - the initial state - the first and second temperature sensors are both immersed in the liquid refrigerant and the first and second temperature are found to be substantially equal. When the liquid level drops below the first temperature sensor, the liquid level remains above the second temperature sensor. By means of the second temperature sensor, the temperature of the liquid refrigerant is continuously measured. An amount of liquid refrigerant adheres to the first temperature sensor which has come to lie free above the liquid level. By evaporating the adhering liquid refrigerant from the surface of the first temperature sensor, the temperature measured by the latter drops, if the first temperature is found to be lower than the second temperature, this means that the liquid level has dropped below the first temperature sensor.
According to the invention, a signal can be emitted if it is detected that the
temperature measured by the temperature sensor is lower than the reference temperature. The signal indicates that the temperature measured by the temperature sensor is lower than the reference temperature. The liquid level has then dropped to just below the level of the temperature sensor. It is possible for the signal to be emitted after it has been measured for at least 2 seconds that the average of the temperature measured by the temperature sensor per unit time is at least 1°C lower than the reference temperature. During operation, temperature fluctuations will occur over time. The temperature fluctuations are caused by, for example, the boiling of the liquid refrigerant. As a result of the temperature fluctuations, the temperature sensor may for a (very) short period of time indicate a slightly lower
temperature, while the temperature sensor is still situated below the liquid level. The temperature fluctuations are smaller than the drop in temperature which occurs upon evaporation of adhering water from the surface of the temperature sensor. If, however, the temperature per unit time which is measured by the temperature sensor is on average at least 1°C colder than the reference temperature for at least 2 seconds, it is clearly possible to infer therefrom that the temperature sensor has come to lie above the liquid level and the signal is emitted.
If the reference temperature is determined by the second temperature measured by the second temperature sensor, the signal is, for example, emitted after it is measured for at least 2 seconds that the average of the first temperature per unit time is at least 1°C lower than the average of the second temperature per unit time.
The invention also relates to a method for controlling the liquid level of a liquid refrigerant in an evaporator, comprising a method for detecting a change in the liquid level of a liquid refrigerant in an evaporator as described above, in which the detection that the temperature measured by the temperature sensor is lower than the reference temperature is used to control the liquid level. Liquid refrigerant can be supplied to the evaporator in order to increase the liquid level after it has been detected that the temperature measured by the temperature sensor is lower than the reference temperature by evaporation of the adhering liquid refrigerant. For example, the signal which is emitted if it is detected that the temperature measured by the temperature sensor is lower than the reference temperature comprises a control signal, in which the control signal controls the supply of liquid refrigerant to the supply of the evaporator.
In this case, it is possible for liquid refrigerant to be supplied at least until it is detected that the temperature measured by the temperature sensor is substantially equal to the reference temperature. This means that the liquid refrigerant has again risen above the temperature sensor. By means of the control signal, the liquid level of the liquid refrigerant can be adjusted to a desired level in the evaporator.
It is possible for the evaporator to be provided with a further temperature sensor which is fitted above the level of the temperature sensor, for example at a desired maximum liquid level, and in which a further temperature is measured using the further temperature sensor, and in which the liquid level of the liquid refrigerant is below the temperature sensor at a further point in time, and in which liquid refrigerant is supplied after the further point in time with a temperature which is higher than the temperature of the liquid refrigerant in the evaporator, and in which, if the liquid level rises above the temperature sensor, it is detected that the temperature measured by the temperature sensor is higher than the temperature measured by the further temperature sensor, and in which the liquid refrigerant is supplied until it is detected that the temperature measured by the further temperature sensor is substantially equal to the temperature measured by the temperature sensor. If the first and possibly the second temperature sensor are below the liquid level, while the further temperature sensor is fitted above the latter, the first and possibly the second temperature sensor are immersed in the liquid refrigerant, while the further temperature sensor is surrounded by the vaporous refrigerant. The temperature of the liquid and vaporous refrigerant is substantially equal. During replenishment of the evaporator, liquid refrigerant is supplied. If the liquid refrigerant being supplied has a higher temperature than the liquid refrigerant which is already present in the evaporator, the temperature of the liquid refrigerant in the evaporator rises. The first and possibly the second temperature sensor then measure a higher temperature than the further temperature sensor. If the further temperature rises while liquid refrigerant is being supplied, and becomes substantially equal to the first and possibly the second temperature, this means that the liquid level has risen to the level of the further temperature sensor. The supply of liquid can then be stopped.
The absolute pressure in the evaporator is, for example, between 5-25 mbar. At such a low pressure, the liquid level in the evaporator can be accurately monitored by means of the above-described method. The invention can obviously also be used at other pressures, such as a pressure above atmospheric pressure.
It is possible for the heat exchanger to be provided with a heat exchange line having an inlet and an outlet, and in which a fluid flows from the inlet to the outlet, and in which the fluid becomes colder during evaporation of the liquid refrigerant by the extraction of heat from the fluid which flows through the heat exchange line. The fluid which is becoming colder is the cold product.
It is possible for the heat exchanger, at least at the first point in time, to be partially immersed in the liquid refrigerant. The liquid level of the liquid refrigerant is located between the bottom side and the top side of the heat exchanger. For example, the heat exchanger has a top surface which is substantially flat and or runs substantially horizontally. The liquid level is located below the top surface. In addition, the heat exchanger may have a bottom surface which is substantially flat and/or runs substantially horizontally. The liquid level stays above the bottom surface. The heat exchanger may constantly be partially surrounded by the liquid refrigerant. The invention also relates to a system comprising an evaporator which is provided with:
- a liquid refrigerant which defines a liquid level in the evaporator,
- a heat exchanger for supplying heat to the liquid refrigerant in order to produce a vaporous refrigerant, in which the vaporous refrigerant is situated above the liquid level and is saturated,
- a supply for supplying the liquid refrigerant,
- a discharge for discharging the vaporous refrigerant,
- at least one temperature sensor for measuring a temperature.
According to the invention, the temperature sensor is fitted in the evaporator at a level, for example at a desired minimum liquid level, and the system comprises a device for detecting a change in the liquid level of the liquid refrigerant in the evaporator, which device is connected to the temperature sensor in order to receive the temperature measured by the temperature sensor, and which device is configured to detect when the temperature measured by the temperature sensor is lower than a reference temperature. The device is able to determine when the temperature sensor cools down, that is to say when the temperature drops below the reference temperature. The device may be configured to emit a signal if the temperature measured by the temperature sensor is lower than the reference temperature.
In an embodiment, the device is provided with a calculating device for calculating an average of the temperature measured by the temperature sensor per unit time, in which the reference temperature is determined by the average calculated by the calculating device. In this case, the reference temperature is a calculated temperature. The calculating device is, for example, a computer comprising software for determining the reference temperature.
It is possible for the temperature sensor to be a first temperature sensor for measuring a first temperature, in which the evaporator is provided with a second temperature sensor for measuring a second temperature, in which the second temperature sensor in the evaporator is fitted below the level of the first temperature sensor, and in which the reference temperature is determined by the second temperature. In this case, the reference temperature is a measured temperature.
In an embodiment, the device is configured to emit the signal after it has been detected for at least 2 seconds that the average of the temperature measured by the temperature sensor per unit time is at least 1°C lower than the reference temperature. The reference temperature is for example a calculated time-averaged temperature of the average of the second temperature per unit time. In an embodiment, the supply of the evaporator is provided with a valve, in which the signal comprises a control signal for opening the valve in order to supply a liquid refrigerant to the evaporator.
In this case, it is possible for the device to be configured to emit a second control signal in order to close the valve so as to end the supply of liquid refrigerant to the evaporator if the temperature measured by the temperature sensor is substantially equal to the reference temperature.
The device comprises, for example, a control device which can emit a control signal to the valve in order to open or close the latter. If the temperature measured by the temperature sensor decreases with respect to the reference temperature, the liquid level has dropped below said temperature sensor. The control device then sends a control signal to the valve in order to open the valve, so that liquid refrigerant flows into the evaporator. The liquid level rises and when the temperature sensor is immersed in the liquid refrigerant, the temperature measured by the temperature sensor becomes equal to the reference temperature again. The control device then sends, for example, a second control signal to the valve in order to close the valve.
It is possible that the liquid refrigerant being supplied has a higher temperature than the liquid refrigerant present in the evaporator, and that the evaporator is provided with a further temperature sensor for measuring a further temperature, which further temperature sensor is fitted above the level of the temperature sensor, for example at a desired maximum liquid level, and in which the device is connected to the further temperature sensor for receiving the further temperature, and in which the device is configured to emit a second control signal for closing the valve in order to end the supply of liquid refrigerant to the evaporator if the temperature measured by the further temperature sensor is substantially equal to the temperature measured by the temperature sensor. The rise in the liquid level associated with the supply of a liquid refrigerant to the evaporator will in this case continue past the level of the first temperature sensor and only stops when the level of the further temperature sensor is reached.
The temperature sensors are located at different vertical positions. The difference in height between the first temperature sensor and the second temperature sensor is, for example, between 1-10 mm. The difference in height between the first temperature sensor and the further temperature sensor may, for example, also be between 1-10 mm. Thus, changes in the liquid level can be detected accurately.
It is possible for the first temperature sensor and/or the third temperature sensor to be provided with a capillary element for retaining an amount of liquid refrigerant. This improves the adhesion of liquid refrigerant. The capillary element is, for example, a small piece of cotton which is provided around the temperature sensor. The invention furthermore relates to a sorption cooling system comprising:
- at least one reactor which is provided with a sorbent, a refrigerant, and a heat exchange line which extends through the sorbent and the refrigerant in the reactor, which reactor is configured so as to alternately carry out adsorption and desorption of the sorbent in the reactor, which reactor comprises a supply and a discharge for vaporous refrigerant,
- a condenser which is provided with a supply for vaporous refrigerant and a discharge for liquid refrigerant, in which the supply of the condenser is connected to the discharge of the reactor,
- a system as described above, in which the discharge of the evaporator is connected to the supply of the reactor, and in which the discharge of the condenser is connected to the supply of the evaporator.
The invention will now be described in more detail with reference to the attached drawing, in which:
Fig. 1 shows a process diagram of a first embodiment of a sorption cooling system according to the invention;
Fig. 2 shows a process diagram of a second embodiment of a sorption cooling system according to the invention;
Fig. 3 shows a diagrammatic cross-sectional view of the evaporator of the sorption cooling system shown in Fig. 1;
Fig. 4 shows a diagrammatic cross-sectional view of a second embodiment of an evaporator for the sorption cooling system shown in Fig. 1;
Fig. 5 shows a diagrammatic cross-sectional view of a third embodiment of an evaporator for the sorption cooling system shown in Fig. 1.
The sorption cooling system 1 shown in Fig. 1 comprises a reactor 3, a condenser 10, an evaporator 18, a heat source 26, a heat emitter 28 and a valve system 30. The sorption cooling system 1 uses heat from the heat source 26 to generate cold.
The reactor 3 contains a sorbent with bonded refrigerant. In this exemplary embodiment, the sorbent is silica gel and the refrigerant is water. Silica gel is highly hygroscopic, that is to say it absorbs water. In the completely saturated state, silica gel can absorb approximately 35 % by weight of water. Of course, other combinations of a sorbent and refrigerant are also possible. The reactor 3 has a supply 4 for supplying water vapour from the evaporator 18 and a discharge 5 for discharging water vapour to the condenser 10. A heat exchange line 8 extends through the silica gel with bonded water in the reactor 3. The heat exchange line 8 is connected to the valve system 30.
The condenser 0 comprises a supply 11 for supplying water vapour from the reactor
3. The discharge 5 of the reactor 3 and the supply 11 of the condenser 10 are connected to one another by means of a vapour passage 92. In the vapour passage 92, a vapour valve 96 is fitted. The condenser 0 is provided with a heat exchange line 15 for transporting a cool liquid, such as cool water. In the condenser 10, the supplied water vapour condenses, after which the water (condensate) leaves the condenser 10 via a discharge 12.
The discharge 12 of the condenser 10 is connected to a supply 19 of the evaporator 18 via a return line 90. In the return line 90, a condensate valve 91 is fitted. The evaporator 18 comprises a heat exchange line 23 having an inlet 21 and an outlet 22. A fluid, such as water, flows through the heat exchange line 23. This fluid transfers heat to the water
(condensate) supplied via the supply 19. This produces water vapour which leaves the evaporator 18 via a discharge 20. The water vapour returns to the supply 4 of the reactor 3 via a vapour passage 93. In the vapour passage 93 between the discharge 20 of the evaporator 18 and the supply 4 of the reactor 3, a vapour valve 96 is also fitted.
The cooling process using the sorption cooling system 1 works according to a batchwise process - the reactor 3 is configured to alternately carry out adsorption and desorption of the sorbent in the reactor 3. Initially, the silica gel in the reactor 3 contains, for example, approximately 10 percent of bound water, while the temperature is approximately 30°C. Since the refrigerant circuit does not contain any gases other than the water vapour, the pressure is caused by the water vapour pressure. As a result of the heating of the silica gel, the pressure gradually increases until the water vapour pressure above the silica gel is higher than the vapour pressure at the temperature in the condenser 10. The pressure in the reactor 3 rises, for example, to 60 mbar, while the pressure in the condenser 0 is 50 mbar. Now water vapour will flow to the condenser 10 via the vapour valve 96 and the silica gel in the reactor 3 will heat up further while giving off water vapour (desorption).
When the silica gel contains, for example, only 3 percent of bound water, the silica gel is cooled. In this case, the pressure drops to a pressure which is lower than the pressure in the evaporator 18. The absolute pressure in the evaporator 18 is, for example, between 5-25 mbar. Water vapour originating from the evaporator 10 flows to the reactor 3 via the vapour valve 96 and is absorbed in the silica gel (adsorption). The absorption of water continues until the silica gel again contains, for example, approximately 10 percent bound water at a temperature of approximately 30°C.
With the sorption cooling system 1 from Fig. 1 , during the cooling phase of the silica gel in the reactor 3, water vapour from the evaporator 18 is attracted and the water
(condensate) supplied via the supply 19 evaporates in the evaporator 18. In this case, heat is extracted from the cold fluid which flows through the heat exchange line 23 of the evaporator, that is to say the temperature of the cold fluid drops. The temperature of the cold fluid is below the ambient temperature, for example between 5-15°C, for example 10°C. The cold fluid, for example cold water, forms the cold product of the sorption cooling system 1. The refrigerant - in this exemplary embodiment water/water vapour - circulates in a refrigerant circuit of the sorption cooling system 1. In the refrigerant circuit of the sorption cooling system, a pressure of, for example, 0.1-60 mbar prevails. In order to alternately cool and heat the reactor 3 with the silica gel and the water bound thereto, a coolant circuit is provided. The coolant circuit comprises the valve system 30, the heat source 26 and the heat emitter 28. The valve system 30 is configured to alternately supply warm water and cool water to the reactor 3.
A second embodiment of a sorption cooling system is illustrated in Fig. 2. Similar or identical parts are denoted by the same reference numerals. The sorption cooling system 1 shown in Fig. 2 comprises a second reactor 73 which is filled with silica gel and water bound thereto. Analogous to reactor 3, the second reactor 73 comprises a supply 74 and a discharge 75 for water vapour. A heat exchange line 78 extends through the silica gel in the second reactor 73. The condenser 10 comprises a second supply 16 which is connected to the discharge 75 of the second reactor 73 via a vapour passage 94. In the vapour passage 94 between the second supply 16 of the condenser 10 and the discharge 75 of the second reactor 73, a vapour valve 96 is provided. As a result of condensation of water vapour in the condenser 10, water (condensate) is formed which flows out of the condenser 10 via the discharge 12. The water (condensate) is supplied to the supply 19 of the evaporator 18 via the return line 90 and the condensate valve 91.
In the evaporator 18, the water (condensate) supplied via the supply 9 can be evaporated by causing a fluid to flow through the heat exchange line 23. The evaporator 18 has a second discharge 25 for discharging water vapour. The second discharge 25 is connected to the supply 74 of the second reactor 73 by means of a vapour passage 95. In the vapour passage 95, a vapour valve 96 is provided.
The sorption cooling system shown in Fig. 2 has a second refrigerant circuit in which the refrigerant - in this exemplary embodiment water/water vapour - can circulate. The operation of the sorption cooling system with the second refrigerant circuit of the second reactor 73 is identical to that described above with reference to the first exemplary embodiment shown in Fig. 1. With the sorption cooling system illustrated in Fig. 2, the batch processes in the first and second refrigerant circuits are operated in opposite phases in order to produce cold continuously.
The evaporator 18 of the sorption cooling system shown in Fig. 1 is illustrated in more detail in Fig. 3. The evaporator 18 is substantially heat-insulated. The evaporator 18 is for example covered externally with a heat-insulating material (not shown). In the evaporator 18, a heat exchanger 42 is provided which has a bottom surface 43 and a top surface 44. In this exemplary embodiment, the heat exchanger 42 is block-shaped, with the bottom surface 43 and the top surface 44 being substantially flat and running substantially horizontally. In this exemplary embodiment, the height of the heat exchanger 42 is approximately 5-50 mm. The heat exchange line 23 runs through the heat exchanger 42.
The evaporator 18 is partially filled with liquid refrigerant 40. The liquid refrigerant 40 determines a liquid level 41 in the evaporator. Above the liquid level 41 , there is vaporous refrigerant 45 which results from evaporation of the liquid refrigerant 40. The vaporous refrigerant 45 is substantially completely saturated.
The heat exchanger 42 is provided with a structure having a surface for forming a thin film of liquid refrigerant thereon (not shown). In this exemplary embodiment, the heat exchanger 42 comprises fins (not shown) which extend from the bottom surface 43 to the top surface 44. The desired minimum liquid level is substantially in the middle between the bottom surface 43 and the top surface 44. Liquid refrigerant is brought up via the fins and forms a thin liquid film on the fins. When heat is supplied via the heat exchange line 23, the thin liquid film can easily evaporate.
The evaporator 18 comprises a level sensor for the liquid level 41. When the liquid level drops below a desired minimum liquid level, it is more difficult for the liquid refrigerant 40 to evaporate. For example, the formation of a thin liquid film of liquid refrigerant on the fins is then insufficient.
In the exemplary embodiment shown in Fig. 3, the level sensor comprises two temperature sensors 51,52. At the location of the desired minimum liquid level, a first temperature sensor 51 for measuring a first temperature is provided. Below the first temperature sensor 51 , a second temperature sensor 52 for measuring a second
temperature is arranged. The second temperature is a reference temperature. The difference in height between the first temperature sensor 51 and the second temperature sensor 52 is, for example, between 1-10 mm.
The temperature sensors 51 ,52 are connected to a device 53. The device 53 is configured to receive the first and second temperatures of the temperature sensors 51,52 and can determine when a drop in the first temperature with respect to the second temperature occurs. In this exemplary embodiment, the device 53 is configured as a control device which is connected to the condensate valve 91. The control device 53 is configured to control the condensate valve 91.
The operation of the level sensor shown in Fig. 3 in the evaporator 18 is as follows. Initially, the evaporator 18 is filled with liquid refrigerant beyond the first temperature sensor 51 which is situated at the desired minimum level. During production of cold, fluid flows - the product to be cooled - through the heat exchange line 23 of the evaporator 8. In this case, heat is transferred to the liquid refrigerant 40 which changes into vaporous refrigerant 45, resulting in a drop in the liquid level 41. At the same time, saturated vaporous refrigerant 45 is discharged via the discharge 20 of the evaporator 18. The discharged saturated vaporous refrigerant 45 flows to the reactor 3.
In the device 53, the first and second temperatures are continuously compared to one another. Initially, the first and second temperature sensors 51,52 are both immersed in the liquid refrigerant 40 and the device 53 determines that the first and second temperature are substantially equal. If the liquid level drops below the first temperature sensor 51 , the liquid level 41 initially stays above the second temperature sensor 52. An amount of liquid refrigerant 40 adheres to the first temperature sensor 51 which comes to lie free above the liquid level 41. By evaporating the adhering liquid refrigerant 40 from the surface of the first temperature sensor 51 , the temperature measured by the latter drops. If the device 53 detects that the first temperature is lower than the second temperature, this means that the liquid level 41 has dropped below the first temperature sensor 51. In that case, the device 53 emits a signal.
In this exemplary embodiment, the device 53 is configured such that the signal is only emitted after it has been detected for at least 2 seconds that the average of the first temperature per second is at least 1°C lower than the average of the second temperature per second. As a result of the boiling of the liquid refrigerant 40, temperature fluctuations occur over time. This means that the first temperature sensor 51 can for a (very) short time indicate a slightly lower temperature than the second temperature sensor 52, while the first temperature sensor 51 is still below the liquid level 41. However, if the first temperature which is measured by the first temperature sensor 51 on average per second is for at least 2 seconds at least 1°C colder than the second temperature which is measured by the second temperature sensor 52 on average per second, it is substantially certain that the first temperature sensor 51 has come to lie above the liquid level 41.
The emitted signal may be an alarm signal or another signal. In this exemplary embodiment, the device 53 configured as a control device sends a control signal to the condensate valve 91 in order to open the condensate valve 91. As a result thereof, the evaporator 18 is replenished with liquid refrigerant, so that the liquid level 41 in the evaporator 18 rises. When the liquid level 41 has risen to such a degree that the first temperature sensor 51 is immersed again, the device 53 can detect that the first and second temperature have become equal again. The device 53 then sends a second control signal to the condensate valve 91 in order to close the condensate valve 91. In this manner, the liquid level 41 of the liquid refrigerant 40 in the evaporator 18 can be controlled.
Fig. 4 shows a second embodiment of the level sensor in the evaporator 18. Identical or similar parts therein are referred to with the same reference numerals. The second embodiment has only one temperature sensor 51, that is to say compared to the embodiment illustrated in Fig. 3, the second temperature sensor 52 has been omitted. The temperature measured by the temperature sensor 51 is not compared to a measured reference temperature, but to a calculated reference temperature. In the exemplary embodiment shown in Fig. 4, the device 53 is provided with a calculating device for calculating an average per unit time of the temperature measured by the temperature sensor 51. The average calculated by the calculating device forms the reference temperature. Apart from that, the operation is identical to that described above with reference to Fig. 3.
If the single temperature sensor 51 has come to lie above the liquid level 41, the device 53 configured as a control device can send a control signal to the condensate valve in order to open the condensate valve 91. As a result thereof, the evaporator 18 is replenished with liquid refrigerant, so that the liquid level 41 in the evaporator 18 rises. If the liquid level 41 rises to such a degree that the single temperature sensor 51 becomes immersed again, the single temperature sensor 51 measures a relatively quick rise in temperature. The device 53 then sends, if desired with a time delay, a second control signal to the condensate valve 91 in order to close the condensate valve 91.
Fig. 5 shows a third embodiment of the level sensor in the evaporator 18. Identical or similar parts are denoted therein by the same reference numerals. The evaporator 18 shown in Fig. 5 is provided with a third temperature sensor 54 for measuring a third temperature. The third temperature sensor 54 is connected to the device 53. The device 53 continuously receives a third temperature which is measured by the third temperature sensor 54.
The third temperature sensor 54 is fitted at a desired maximum level. If the liquid level were, for example, higher than the heat exchanger 44, the fins of the heat exchanger 44 would be completely surrounded by liquid refrigerant. In that case, no thin liquid film would be formed on the fins, which would adversely affect evaporation. The desired maximum level is between the first temperature sensor 51 and the top surface 44 of the heat exchanger 42. The third temperature sensor 54 is fitted above the level of the first temperature sensor 51. The difference in height between the first temperature sensor 51 and the third temperature sensor 54 is, for example, between 1-10 mm.
The operation of the level sensor shown in Fig. 5 in the evaporator 18 is as follows. As is the case with the level sensor illustrated in Fig. 3, the device 53 may determine that the liquid level 41 has dropped below the first temperature sensor 51 if the first temperature decreases with respect to the second temperature. The device 53 then sends a control signal to the condensate valve 91 in order to open the condensate valve 91 , so that liquid refrigerant flows into the evaporator 18.
in this exemplary embodiment, the supplied liquid refrigerant is condensate from the condenser 10, and is hotter than the liquid refrigerant 40 which has already been introduced into the evaporator 18. If it is assumed that the first and second temperature sensors 51 ,52 are below the liquid level 41 , while the third temperature sensor 54 is above the latter, then the first and second temperature sensor 51 ,52 are immersed in the liquid refrigerant 40, while the third temperature sensor 54 is surrounded by the vaporous refrigerant. The temperature of the liquid and vaporous refrigerant is substantially equal.
When replenishing the evaporator 18 with liquid refrigerant at a higher temperature, the temperature of the liquid refrigerant 40 in the evaporator 18 rises. The first and second temperature sensor 51 ,52 then indicate a higher temperature than the third temperature sensor 54. If the third temperature rises when supplying a liquid refrigerant and becomes substantially equal to the first and second temperature, this means that the liquid level 41 has risen to the level of the third temperature sensor 54. The device 53 thus allows the liquid level 41 to rise until the third temperature is equal to the first temperature. Only then does the device 53 send the second control signal to the condensate valve 91 in order to close the condensate valve 91. According to Fig. 5, the evaporator 18 is therefore replenished up to the desired maximum level.
The exemplary embodiments illustrated in Figs. 3-5 only comprise one discharge 20 for discharging a vaporous refrigerant, but may be provided with a second discharge 25 to make the evaporators suitable for use with the sorption cooling system from Fig. 2. Apart from that, the parts and operation of the level sensors from Figs. 3-5 can remain unchanged.
The invention is not limited to the exemplary embodiments illustrated in the figures. The person skilled in the art can make various modifications without departing from the scope of the invention. For example, the first temperature sensor and/or the third
temperature sensor may be provided with a capillary element for retaining a liquid refrigerant. The capillary element is, for example, a small piece of cotton which is provided around the temperature sensor. When the liquid level drops below the first and/or third temperature sensor, a relatively large amount of liquid refrigerant adheres to the capillary element.
Evaporation thereof leads to a relatively significant cooling which can be detected clearly.
It is furthermore possible for a further temperature sensor which corresponds to the third temperature sensor 54 from Fig. 5 to be added to the embodiment illustrated in Fig. 4. In other words, the second temperature sensor 52 may be omitted in Fig. 5. The liquid level 41 is then controlled with respect to the desired minimum level using a single temperature sensor 51 , while the further temperature sensor 54 ensures that the liquid level 41 does not rise above the desired maximum level.
It is also possible for a fourth temperature sensor to be fitted in the evaporator for measuring a fourth temperature (not shown). The fourth temperature sensor is provided, for example, below the third temperature sensor 54. If the liquid level drops below the third temperature sensor, the device measures a cooling down of the third temperature sensor with respect to the fourth temperature sensor, in this case, the operation is the same as described above for a drop in the liquid level below the first temperature sensor. As a result thereof, it is possible to monitor when the liquid level drops below the desired maximum level. Obviously, the evaporator may comprise yet more temperature sensors.