Field of the Invention

The present invention relates to an evaporator control system adapted for control of one or more evaporators of a cooling system, which cooling system comprises at least one compressor for generating high pressure refrigerant gas, which high pressure re- frigerant gas is sent through condensing means for cooling and condensing into liquid refrigerant, which liquid refrigerant is sent through a pressure reduction means such as expansion valves towards one or more evaporators, from which evaporators low pres- sure refrigerant suction gas is sent through a suction pipe towards the compressor.

Background of the Invention

Control of evaporators is performed by a temperature feedback from the evaporator’s outlet to control of expansion valves. DX evaporator control is normal calculated as superheat by measuring temperature and pressure as a feedback from the outlet of the evaporator to control of an expansion valve. Overfeed systems are either pump or thermosiphon based where the refrigerant feed rate is forced by a pump or gravity driven circulation. US2013291568A discloses a system or a method for performing capacitive sensing of humidity/liquid, primarily in conductive or non-conductive liquid/gas mixtures, hav ing a control unit and at least first and second sensor electrodes, the capacity between the first and the second electrodes being measured. To measure humidity /liquid in a circulating gas/liquid mixture at least one of the sensor electrodes is formed as a tube, which is placed in the liquid/gas mixture. Based on the capacitive measurements, a calculation of at least one dataset for control of a second system is performed. The tube can be more or less filled up with liquid or gas and the electric capacity measured in pF will change as it depends on the content around or inside the tube, and if a dry gas is dr} _' there will be one value of capacity and in a situation where the gas is being replaced by liquid, the capacity value will change rapidly. Object of the Invention

It is the object of the invention to increase the system efficiency safety and cooling effect of an evaporator.

Description of the Invention

The object can be fulfilled if the evaporator control system is adapted to performs con trol of refrigerant liquid flow into evaporators based on input from one or more vapour gas quality sensors placed in the evaporator outlet or in the suction pipe, which vapour gas quality sensor measures the content of liquid refrigerant in the suction gas, which system controls the inlet of refrigerant to the evaporator based on the vapour gas quality measured in the suction pipe or at the evaporator outlet.

Hereby we can achieve that the quality of the suction gas is detected and as such it is possible to distinguish between dry and wet suction gas. By too wet suction gas there will always be a risk that liquid refrigerant is entering the compressor which can de- stroy the compressor. In a situation where the suction gas is totally dry, the cooling system has performed a superheating of the suction gas, which then reduces the effec- tiveness of the evaporator. By the pending patent application it is possible to achieve a correct filling of an evaporator and control the refrigerant feed to the evaporator in a way where a defined quality of gas is achieved at the suction outlet of the evaporator.

By a continuous regulation it is possible to perform small adjustments if the cool- ing/heating load to the evaporator is changing. By the system as disclosed, it is possi- ble by many evaporators to increase the evaporator effectiveness with some percent ages. Tests have shown that there is a power reduction in such a system which can be measured up to 20%. Signals from the quality sensor can be transmitted to a general control system for the refrigeration system. Here the system can be used for optimized control of different parameters such as valves controlling the inlet to evaporators, mo tor valves, pumps or capacity control/regulation of compressors.

In a preferred embodiment for the invention the system comprises a gas flow sensor, which gas flow sensor is adapted to measuring the gas flow in the evaporator outlet. Hereby it can be achieved that the system receives information of the gas flow from the evaporator section. The gas flow can be measured by a plurality of flow sensors, such as mass flow sensors, ultra sound sensors, vortex sensors or thermal sensors. The thermal sensors are preferred, because it is easy to place a small heating element and a thermo sensor with a small distance in the outlet from an evaporator section. By measuring the flow in each evaporator section the system can calculate the effective- ness of an evaporator. If the system is indicating a high gas flow and the gas is dry, the system can open the modulated expansion valve to increase the flow in the evapo- rator section. If no flow is indicated in an evaporator section this can indicate that no evaporation takes place in that section of an evaporator. This can indicate ice at the evaporator section and stop of the modulated expansion valve, and if more evaporator sections have reduced flow de-icing can be necessary.

In a further preferred embodiment for the invention the system can be adapted to per form simultaneous capacitive measurement of void fraction of the liquid flow rate. Hereby can be achieved that capacitive measurement with simultaneous measurement of temperature ensures greater measuring accuracy simply because it is possible here to perform a temperature compensation. The sensor is based on capacitive measure- ment principles in which two measuring electrode/conductors measure the capacity. Hereby can the ratio between vapour and liquid amount in a two-phase flow be meas- ured instantly for example without delay as a volume based void fraction measure- ment. Capacitive void fraction measurement with simultaneous measurement of the liquid flow rate ensures high accuracy and direct flow measurement measured in me- ters per second In a further preferred embodiment for the invention the system can be adapted to use ultrasound measurement to measure the velocity of the liquid in one or more tubes. Hereby can be achieved for example by use of an ultrasound transducer to measure the velocity of the liquid in the tube simply because the ultrasound is based on the dobler principle. Hereby it is possible to measure two phase flow rate.

In a further preferred embodiment for the invention the system can be adapted to per form calculation of mass flow based on the measured void fraction, the actual temper ature, liquid flow and gravity of the refrigerant. Hereby can be achieved higher accu racy compared to void fraction based on only the capacitive measurement. In combi- nation with measuring of void fraction it is possible to measure the mass flow in a tube with as two phase flow measured in kg/m3.

Mass flow measurement is highly effective if capacitive void fraction measurement with simultaneous measurement of temperature and liquid flow can ensure high meas- urement accuracy measured instantly without delay. This can be used on very de- manding applications e.g. for measuring the circulation rate on evaporators and calcu- lating the evaporator performance/output in kW. Hereby can be achieved that not only the total mass flow leaving an evaporator can be measured but it is possible also by an evaporator which is divided in a number of sections to perform measurement of mass flow from each of these sections. Hereby it is possible to investigate the operation of each section of an evaporator. For example it is possible to indicate if a section of an evaporator is ineffective because forming of ice at the outside and maybe local de- icing for a section of an evaporator could be necessary. All in all the measurement of the mass flow will increase the possibility of detecting any change in the operation of an evaporator.

In a further preferred embodiment for the invention can control of one or more expan sion valves control the inlet of refrigerant to the evaporators based on a signal gener- ated by the gas quality sensors. By replacing the thermo valve or superheat control with the vapor quality sensor, which instead of measuring superheat is measuring quality of the suction gas, it is possible to control the expansion valve in a much more effective way than based on a temperature and pressure measurement. By measuring the quality of the suction gas, it is possible to adjust the quality of the suction gas to a level where evaporation is performed totally, but without achieving superheat condi- tions. Hereby is achieved a controlled higher degree of filling of an evaporator during operation.

In a further preferred embodiment for the invention the system can be adapted to per- form control of direct expansion evaporators based on the signal from the gas quality sensor, which measure the content of liquid refrigerant in the suction gas leaving the direct expansion evaporator. Hereby it can be achieved that the gas quality sensors enable operation of a refrigeration system as a DX-system (direct expansion). The advantage of the sensor principle is a low refrigerant charge in the system compared to overfeed/flooded evaporators. A reduction in refrigerant charge by factor up to 1,000 is possible in the evaporators compared to liquid over feed or gravity fluid feed may be anticipated. By controlling the quality of the suction gas it is possible also by dry expansion evaporators to achieve a much higher yield of the evaporator, simply be- cause an optimal filling with refrigerant is possible and thereby much more effective heat transfer is performed.

In a further preferred embodiment for the invention one or more refrigerant pumps perform supply of liquid refrigerant to one or more evaporators based on signal from gas quality sensors. In larger refrigeration systems it is possible that the supply from a receiver to a plurality of evaporators is performed by a number of pumps. In such a system, the outlet from these evaporators is non-evaporated liquid sent back to the separator, where a separation is performed and where evaporated gas is returned to compressor means. Here it can be highly effective if the pumps are under control, so not only liquid refrigerant flow to the evaporators, but that the flow is reduced in a way where effective evaporation takes place. By controlling the quality of the gas leaving the different evaporators, these evaporators can operate highly effective by control of pumps or maybe by control of different valves controlling the flow into the evaporators.

In a further preferred embodiment for the invention the system can be adapted to per form control of one or more flooded evaporators based on the signal from the gas quality sensor, which measures the content of liquid refrigerant in the suction gas leaving the flooded evaporators. Hereby can be achieved that flooded evaporators can operate much more effective simply because it is possible to adjust the driving head from the receiving tank in a way where the liquid level can be adjusted in accordance with the demand for cooling. If more cooling is needed it is simply to fill up the driv- ing head so that the pumping is increased. In a situation where for example the cooling demand is decreasing, no more liquid refrigerant is added to the receiving tank and because there is a continuous evaporation, the driving head will be reduced after a short while. The system disclosed in this patent application can as such by measuring the quality of the suction gas lead to a much more effective heat transmission and es- pecially for plate heat exchangers where there normally will be circulated more refrig erant during part load operation. In a further preferred embodiment for the invention the quality sensor can perform electronic capacity calculation of the electric capacity between a first and a second sensor electrode, which first and second electrodes are placed in the suction pipe at the outlet of the evaporator. Hereby it is possible simply to measure the electric capacity in pF between two electrodes and between these electrodes the electric field will be disturbed as soon as liquid particles exist in the field. Thereby the capacity between the two sensors can indicate droplets of liquid particles when they are passing between the plates/conductors. In an electric system it is relatively easy to measure the capacity and in that way it is possible also to indicate the quality of the gas.

In a further preferred embodiment for the invention the cooling system comprises a plurality of independent evaporators, which evaporators are controlled by independent gas quality sensors controlling expansions valves that control the liquid inlet to the evaporators.

In a further preferred embodiment for the invention the invention can be disclosed as a method for operating a cooling system by an evaporator control system as disclosed in one of the claims 1-13 in the following steps of operation: a: perform measurement of the electric capacity in pF between electrodes in the quali ty sensor, b: let the evaporator control system calculate the quality of the suction gas based on the measured capacity, c: let the evaporator control system based on the quality measure the flow pattern in the evaporator outlet, d: let the evaporator control system decide if the suction gas is dry or wet, e: let the evaporator control system perform control of the liquid refrigerant inlet to the evaporator. Hereby can be achieved a highly effective control of the superheating of an evaporator and that superheat can be reduced simply because temperature is no longer measured, instead an effective measurement of the gas quality is performed.

In a further preferred embodiment for the invention the invention can be disclosed as a method for operating an evaporator control system in the following step: a: perform simultaneous measurement of void fraction of the liquid flow rate of the refrigerant, b: perform measurement of temperature of the refrigerant, c: perform pressure measurement of the refrigerant, d: perform measurement of vapour cross section and total cross section, e: calculate the mass flow in two phase flow of refrigerant.

Hereby it is possible to perform measuring of mass flow on two phase flow measured in kg/m3. Hereby it is possible to integrate the measurement of temperature and pres- sure in the same housing as the capacitive sensor. Therefore, the only extra features that are necessary in order to calculate the mass flow are to adapt an ultrasound trans- ducer outside the tube and let the software which controls the ultrasound transducer based on the dobler principle.

The technology of measuring flow velocity by ultrasound transducers is well-known. In this way it will be possible by each single sections of an evaporator to indicate the mass flow through this section. In this way any malfunction in an evaporator section will be detected. Also the capacity of the evaporator can be calculated in quite another way simply because it is able to supply a customer with mass flow data of the total mass flow through an evaporator. In a further preferred embodiment for the invention the evaporator control system can be used as riser control. In systems where for example the evaporator is placed far below a compressor, a riser is necessary in order to move the evaporated refrigerant from the refrigerator upwards, maybe several meters into the inlet of the compressor. Here it is extremely important that the gas velocity flow is sufficient to force the liq uid upwards as annular flow. The calculation and dimension of the riser pipe is often a compromise, because the load on the freezer/evaporator changed, by measuring the gas quality it is possible to control the circulation rate and keep the gas relatively dry at low load and then minimize the pressure drop in the riser pipe.

By this invention can energy optimization be possible by intelligent control of evapo- rator capacity/load on heat pumps and refrigeration systems. In connection with DX control of evaporators it is always possible to use multiple parallel circuit. The amount of fluid applied to each circuit is controlled only by one VQ sensor which means, that all inlets are regulated in dependable. In practice the zones of an evaporator will be loaded different, because of non-homogeny air circulation and ice build-up and thus reduced capacity. The new idea is to mount a little VQ capacitive sensor in the outlet of each circuit/zone and in that way control the fluid injection individually on the inlet side.

Further this invention can be used for defrost control on heat-pumps air to water types, where the energy absorber is an air-cooler placed outdoor. By this new method where single or an actively distributing valve are used, which is able to feed parallel evapora tor circuits individually. With this valve single evaporator circuits could be regularly shut off. While no refrigerant is evaporated in a closed circuit, the coil surface temper ature increases and the flow of the ambient air is sufficient to defrost this part of the evaporator. Experimental results show that under standard frost conditions the evapo rator can be kept frost-free and even under severe conditions most of the highly ineffi cient system defrosts can be avoided. Thereby system efficiency is increased signifi cantly. Description of the Drawing

Fig. 1 shows a possible embodiment for a refrigeration system.

Fig. 2 shows a multi electronic flow distributor expansion valve manifold.

Fig. 3 shows a plurality of suction tubes connected to the evaporator’s outlet.

Fig. 4 shows the measuring range DX and OVC.

Fig. 5 shows part of a refrigeration system comprising an evaporator.

Fig. 6 shows an outlet from an evaporator with a flow sensor and a vapour quality sensor.

Fig. 7 shows same embodiment as in fig. 6, but here connected to an evaporator.

Detailed Description of the Invention

Fig. 1 shows a possible embodiment for a refrigeration system 6, which comprises an evaporator control system 2. This evaporator control system 2 is connected to an evaporator 4. A compressor 8 is generating high pressure refrigeration gas in a line 10, which is connected to an oil separator 32. From here pressure line continues up to a condenser 12, where liquid refrigerant is now flowing in a tube 14 to a receiver 34. From here the refrigerant is sent to a separator 40 and further through line 42 to the evaporator 4. The evaporator 4 comprises quality sensors 26 and modulated expansion valves 38 to control the flow into the evaporator. Further is indicated several vapour quality sensors 26 in the suction outlet of the compressor 8.

In operation the compressor 8 will generate high pressure gas in the line 10, which is supplied to the oil separator 32, where oil particles are separated from the pressure gas. The pressure gas is hereafter sent further in tubing into the condenser 12, where for example air is performing a cooling of the gas, so a condensation takes place and in the tube 14 liquid high pressure refrigerant is sent towards the receiver 34. From the receiver 34 the high pressure liquid refrigerant is sent to a separator 40, which separa- tor operates as a subcooler, where relative cold suction gas is being preheated and the liquid high pressure refrigerant is being cooled to a lower temperature into the line 42. The line 42 is connected to the multi electronic liquid flow distribution expansion valve manifold 38. Here a plurality of modulated valves will be activated in order to open for liquid refrigerant to enter more evaporators sections. Each of these evapora- tor sections are then controlled by quality sensors 26, which are able to measure the quality of the suction gas and stop the modulated valve, if the gas is detected as wet.

Hereby it can be achieved that nearly optimal filling of the evaporator sections takes place. The suction gas which is leaving the evaporator sections is sent through the tubing 54 into a suction valve 36. From here the suction gas is passing the separator, where the gas and liquid is separated. Further in line 22 the suction gas is flowing to- wards the compressor 8, where one further quality sensor 26 is indicating if there should be liquid refrigerant on its way to the compressor and if liquid is detected; the sensors 26 will stop the compressor immediately. Hereby is avoided that liquid refrig erant is entering the piston compressor which would probably destroy the compressor immediately.

Fig. 2 shows a multi electronic flow distributor expansion valve manifold 38. A tube for high pressure liquid refrigerant 42 is arranged with a plurality of connections to the modulated valves 44. These modulated valves are further connected to the individual tubes for liquid supply 46, which modulated valves 44 are connected by a electronic data bus 48. This data bus 48 is by a bus interface 50 connected to the modulated valves 44.

During operation each of the modulated expansion valves 44 will be controlled by signals that are sent through the data bus 48. The valves 44 are as such operating as controllable expansion valves. Other kinds of modulations are also possible. In one situation it is of course possible to keep one of the valves totally open and in another situation totally closed. In a situation where a plurality of evaporator sections are op- erating, which number can be much higher than indicated at this figure, these evapora- tor sections are controlled independently of each other, because the inlet to the evapo- rator section is controlled by the modulated expansion valves.

Fig. 3 shows a plurality of suction tubes 52 connected to the evaporator’s outlet. The tubes 52 are connected to the suction tube 54, which is connected directly or indirectly to a suction manifold which are connected to the suction line going to the compressor. Further at fig. 3 is indicated a plurality of gas quality sensors 26, which are electrical connected to an electrical bus interface 58. This bus interface 58 is further connected to a data bus 56.

In operation the gas quality sensors will indicate if there are liquid particles in the suc- tion gas leaving the evaporator section. The gas quality sensors 26 will continuously measure the gas quality in the tubes 54 coming from the evaporator sections. As long as the gasses are dry it is possible for the modulated expansion valve 44 operating in that same evaporator to continue operation, but in a situation where one of the evapo- rator sections seems to be more or less filled up with liquid refrigerant and droplets of refrigerant is detected by the quality sensors 26, there is no need to continue filling more liquid refrigerant into an evaporator section, which is already operating with its maximum cooling effect. Therefore, also from a view of energy consumption it is highly effective to control the inlet of each evaporator section in accordance with the quality of the gas that is leaving the evaporator section.

Fig. 4 shows the measuring range DX and OVC shown in an H-log P diagram. From the figure it is clear that using the DX technology the superheat can be reduced and hereby the total energy consumption of a refrigeration system is reduced because su- perheat is only necessary in order to protect a compressor. Further the suction pressure will increase and hereby reduce the power consumption of the compressor. The extra energy that is occupied by the superheat of the refrigerant has together with the energy that is achieved during the compression also energy that has to be removed in the con- densator. The lower temperature from the compressor will lead to a more efficiency condensation.

Fig. 5 shows a part of a refrigeration system comprising an evaporator 4 (here men tioned as a freezer). The inlet to that evaporator 4 is controlled by an electronic expan sion valve 16. Further is indicated a suction pipe 22 and an evaporator outlet 28. This evaporator outlet 28 is connected to a riser 62 up to a quality sensor 26. From here the refrigerant is flowing into the suction pipe 22. A pump 30 is pumping refrigerant through the expansion valve 16. The pump 30 is controlled by a pump control 60, which pump control 60 is receiving an input from the quality sensor 26. In operation the pump will receive refrigerant from a pump-separator or condenser and increase the pressure towards the liquid control valve 16. In the evaporator 4, which could be a freezer because of the pressure in the evaporated refrigerant is able to flow through the riser 62 up to the quality sensor 26 and further into the suction pipe 22.

The traditional way of controlling a DX evaporator is to measure the suction pressure and the temperature inside or outside the suction tube of an evaporator and based on that temperature to control the inlet to the evaporator. In order to protect for example a compressor it is necessary to perform a regulation, where a superheating of the gas leaving the evaporator is performed. Often regulation is performed so that the temper ature is 10 degrees or maybe even higher in order to be sure that all the refrigerant is evaporated.

By the pending patent application it is possible to operate with a temperature very near the optimal evaporation temperature, and it is possible to utilize the entire evaporator area for cooling, on heat-pump named as energy absorber. Therefore, it is possible not only to achieve higher security for a compressor, but also to have much higher effec tive evaporators and reduce the energy consumption.

Fig. 6 shows a possible embodiment of an evaporator outlet tube 52, which is con nected to a gas flow sensor 64 and further connected to a gas quality sensor 26. Here after gas is flowing connected in a tube 54, which is a suction tube leading towards a compressor.

Hereby is achieved that not only the quality of the gas that is leaving the evaporator is measured by the gas quality sensor 26, but the velocity of the gas flow is also meas ured by the gas flow sensor 64. The gas flow sensor 64 could be a thermal based flow sensor operating in that way that a small heating element is heating one temperature sensor in combination with another temperature sensor measuring the temperature of the gas. At low gas flow is the temperature difference high, when the gas flow in crease there is a higher degree of temperature difference witch indicates a lower gas flow. In that way by calibration a relative good indication for the flow velocity can be achieved. Fig. 7 shows same features as in fig. 6, but now an evaporator 4 and an expansion valve 16 are also indicated. In operation the quality sensors 26 and the gas flow sensors 64 are together measuring the gas quality leaving the evaporator. Signals are sent to a central control system. This control system also controls the expansion valve 16. By controlling the quality of the gas leaving the evaporator 4, it is possible to totally fill the evaporator with liquid boiling refrigerant. As long as there is a flow as indicated by the gas flow sensor 64, the control system can open the expansion valve and add more refrigerant into the evaporator. As soon as the quality sensor indicates liquid particles in the gas leaving the evaporator, a command for reducing the inlet by partly or totally closing the ex- pansion valve would be given from the control system. In some situations where for example evaporation is totally ice-filled at the outside, the evaporation is not performed totally or maybe no evaporation takes place, then the gas flow sensor indicates that there is no flow in the line 52. In that situation it could be necessary also to send a command to the expansion valve 16 to let that valve close. By a system as disclosed it is possible to perform a highly effective control of evapo- rators or evaporator sections. Especially by controlling each section of an evaporator it is possible to totally fill up that evaporator with boiling refrigerant, because the quality and the flow is under control, it is possible to increase the temperature flowing in the suction line 54 towards the compressor. Therefore, the system as disclosed is increas- ing the effectivity of a cooling or heat-pump system.

Reference numerals:

Evaporator control system (2)

Evaporators (4)

Cooling system (6)

Compressor (8)

High pressure refrigerant gas (10)

Condensing means (12)

Liquid refrigerant (14)

Expansion valves (16)

Low pressure refrigerant suction gas (20)

Suction pipe (22)

Gas quality sensors (26)

Evaporator outlet (28)

Pumps (30)

Oil separator (32)

Receiver (34)

Suction valve (36)

Multi electronic flow distributor/expansion valves manifold (38) Separator (40)

Tube for high pressure liquid refrigerant (42)

Modulated valve (44)

Tube for evaporator supply (46)

Data bus (48)

Bus interface (50)

Evaporator outlet tube (52)

Suction tube (54)

Data bus (56)

Bus interface (58)

Pump control (60)

Riser (62)

Gas flow sensor (64)