Abstract:
A system or a method for performing capacitive sensing of humidity/liquid, primarily in conductive or non-conductive liquid/gas mixtures, having 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 capacity can be measured as it depends on the content around or inside the tube, and if a dry gas is 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.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    A refrigeration system comprising at least one compressor, which compressor delivers compressed refrigerant gas to at least one condenser, which condenser delivers liquid refrigerant to at least one flow restriction, from which flow restriction low pressure refrigerant flows to at least one evaporator, from which evaporated refrigerant flows back to the compressor, which system comprises at least one sensor for detecting liquid in the suction gas which sensor is connected to a control unit, which control unit performs control of the refrigeration system, which sensor is formed as at least one capacitive sensor, which capacitive sensor comprises at least a first and a second sensor electrode, which capacity censor at least measures the capacity between at least the first and at least the second electrode, at least one of the sensor electrodes are formed as a tube 
         [0003]    2. Description of Related Art 
         [0004]    International Patent Application Publication WO 2010/025730 A1 and corresponding U.S. Patent Application Publication 2011/0214438 disclose a method for controlling a flow of refrigerant to an evaporator arranged in a refrigeration system. The refrigeration system further comprises an expansion valve and a compressor, the expansion valve, the evaporator and the compressor being arranged in a refrigerant flow path having refrigerant flowing therein. The method comprises the steps of increasing an opening degree of the expansion valve, thereby increasing a flow of refrigerant to the evaporator sufficiently to substantially eliminate a dry zone of the evaporator, decreasing the opening degree of the expansion valve after a period of time has lapsed, and repeating the steps of increasing and decreasing the opening degree of the expansion valve. Thus, the opening degree of the expansion valve is ‘pulsated’. This causes the superheat value of refrigerant leaving the evaporator to ‘toggle’ between a zero level and a low, but positive, level. Thereby the average superheat value of the refrigerant leaving the evaporator is decreased, and the refrigeration capacity of the evaporator is utilised more efficiently. Simultaneously, it is ensured that the amount of liquid refrigerant being allowed to pass through the evaporator is kept sufficiently low to prevent damage to the compressor. 
         [0005]    European Patent Application EP 2 088 405 A2 and corresponding U.S. Pat. No. 8,146,421 concern a method and apparatus for sensing levels of insoluble fluids within a storage vessel utilizing an array of main capacitive sensors having differing geometries. The array of main capacitive sensors gives the ability to measure the levels of insoluble liquids in a vessel. Each of the main capacitive sensors include at least one pair of conductive plates capable of submersion in the at least two insoluble fluids, and the geometries of the pair of conductive plates differ and are distinct, such as in distance or in width, in each of the main capacitive sensors. In addition, the apparatus and method may include at least one reference sensor placed intermittently along the height of the vessel to provide input as to the permittivities of the insoluble fluids. 
         [0006]    Japanese Patent Application Publication JP 09 09 6618 A concerns an apparatus with sufficient accuracy for detecting state of a refrigerant, by outputting continuous electric signals proportional to the ratio of gas state and liquid state. Whereby the refrigerant state-detecting apparatus for detecting the state of a refrigerant in a circulation path. A state detection means and a detection circuit are provided in the circulation path. The state detection means has at least two sheets of flat electrodes held via a small gap to the parallel to a flow of the refrigerant, and the detection circuit takes out the change of capacitance of the state detection means in accordance with the change of dielectric constant of the refrigerant liquid as the change of voltage. 
       SUMMARY OF THE INVENTION 
       [0007]    The object of the invention is to measure phase of refrigerant (saturated/superheated). 
         [0008]    The object can be fulfilled by a system as initially described above if further modified so the tubes are measuring the liquid/gas phase of the suction gas between evaporator and compressor, which control system is based on the capacitive measurements where the system performs a calculation of at least one dataset for control of the refrigeration system. 
         [0009]    Hereby can be achieved that the tube can be more or less filled up with liquid or gas and the capacity can be measured, which capacity will depend on the content around or inside the tube, because if it is a dry gas there will be one value of capacity and in a situation where the gas is becoming filled with liquid, the capacity value will change rapidly. The capacity will change the more wet the liquid the gas will be, ending up with total submersion of the tube. In a submerged situation the capacity value can be indicated, but this value is most different form the value, when it is dry gas that is in the tube. Thereby, not only situations can be measured, but even slight change in the content of the gas can be indicated. Based on the measurement, further control values can be generated for controlling a system where gas or liquid are circulating. That could be important for example in cooling systems. 
         [0010]    The first and second censor electrodes can be formed mostly coaxial, forming a first outer electrode tube, which first outer electrode tube contains the second inner electrode tube. By coaxial tubes it is possible to measure the capacity between the tubes. Liquid or gas can flow between and inside the tubes, so an effective indication of humidity can be achieved. 
         [0011]    The first outer sensor can be part of a tube in a cooling system, which second inner tube is placed inside the outer tube and electric isolated from the outer tube, which system measures the capacity between the first outer tube and the second inner tube. Especially in cooling systems it is important to indicate for example overheating in an evaporator. In different places in a cooling system it can be very important to measure the gas and get an effective indication of the content of liquid drops in the gas. Especially the suction line towards a compressor is rather important to secure compressors against liquid hammer. 
         [0012]    The outer tube can be part of a tube connecting an evaporator with s suction inlet to a compressor for indicating liquid in the suction gas. The indication of liquid in a suction line is not only important in relation to protecting the compressor, but also in relation to achieving the most effective use of an evaporator. 
         [0013]    The system can be used for control of an expansion valve, which expansion valve controls liquid inlet to the evaporator and thereby control of the overheating of the evaporator. A very effective control of an expansion valve could be achieved by measuring the gas leaving the evaporator. In that way it should be possible to reduce the overheating of the evaporator and thereby achieve a much more effective evaporator. In traditional control of expansion valves, temperature of the gas leaving the evaporator or the pressure of the gas has been used for controlling the expansion valves. Also an electronic control system has been used for controlling the expansion valve. In all situations there is no effective protection against liquids in the gas leaving the evaporator, if not a certain overheating is allowed. Therefore, it is possible to indicate humidity in the gas leaving the evaporator. Thereby it is possible to shut down the expansion valve shortly, until the gas leaving the evaporator is dry. In that way a much more effective use of an evaporator is possible. 
         [0014]    The system can be used for control of a compressor, which control system is used as a liquid alarm, which liquid alarm stops the compressor if liquid particles are indicated in the suction gas. By measuring liquid particles in the suction gas directly before the inlet at the suction tube at a compressor, it is possible to prevent liquid particles entering the compressor. If the compressor is connected to the control system, it is possible in some situations to shut off the compressor. In other situations, if the compressor is operating with variable speed, it is possible to reduce the speed of the compressor and in that way allow liquid particles in the gas to evaporate. A safety circuit for a compressor can be rather important, even in situations where the system has never come to an actual use. The existence of the safety system can lead to a situation where a gas at a lower temperature can be used as suction gas, and in that way increase the effectivity of the compressor. It is by the safety system possible to operate with very limited superheating of the gas. 
         [0015]    This invention further concerns a method for indicating liquid ammonia in a cooling system, where at least a first measuring electrode is placed inside a cooling circuit, where the electrode are connect to a measuring circuit, where measurement of the capacity between electrode and a reference is performed, where the measured capacity is transmitted to a control system, where control of the cooling system based on the measured capacity is performed, where the capacity is measure between an inner tube and an outer tube, where measurement of liquid particles in ammonia gas flowing in the outer and inner tube towards a compressor is performed. 
         [0016]    By the method as described there can be achieved a highly effective measuring system, for example for ammonia or carbon dioxide cooling circuits. The method could in that way concern the control of a suction gas, but also level indicators. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a schematic view of a cooling system, where 
           [0018]      FIG. 2  shows a section of a suction tube, 
           [0019]      FIG. 3  shows a level indicator, 
           [0020]      FIG. 4  show a second section of a suction tube, 
           [0021]      FIGS. 5   a - 5   d  show four possible embodiments for cooling systems, and 
           [0022]      FIG. 6  show the relation between density of refrigerant and the sensor level. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIG. 1  shows a cooling system  2  which cooling system comprises a cooling media for circulating in the system. Sensors  6 . 8  are placed in the gas return line  20  which is leading from an evaporator  24  through the first sensor  6 . 8  towards the second sensor  6 . 8  further through the suction line  26  to the compressor  28 . The two sensors  6 . 8  are both connected to the control unit  10 . From the compressor  28 , the compressed refrigerant is sent to a not shown condenser where liquid refrigerant  4  is received and sent to the expansion valve  30 . From the expansion valve  30  leads a tube  32  into the evaporator  24 . 
         [0024]    In operation the first of the sensors  6 . 8  can measure the content of humidity in the gas in the line  20  leaving the evaporator. This measurement can be used for controlling the expansion valve. As long as the gas leaving the tube  20  is dry, the expansion valve can remain open. As soon as liquid is indicated in the line  20 , the expansion valve can be reduced in its opening, so further evaporation takes place. Hereby it is possible to operate the evaporator with a very limited superheating. Furthermore, before the gas enters the compressor further one detector  6 . 8  is indicated as  34 . This sensor  6 . 8  and  34  is used as a liquid alarm. If liquid is detected, the compressor might be shut down or the speed is reduced. 
         [0025]      FIG. 2  shows a sectional view of a tube section, for example the tube  20  at the  FIG. 1 . The detector is made inside the tube  20  where the detector  6  and  12  are all referring to the same component. Inside the tube  20  is indicated an inner tube which is indicated as  8 ,  14  and  22 . The inner tube  22  is kept in place by fixtures  40  where a conducting screw  42  connects the inner tube  22  electrically and where the screw  44  is an isolating screw that isolated the inner tube towards the outer tube  20 . In operation it is possible to measure the capacity between the inner tube and the outer tube. Since the flow of liquid or gas is passing through both the tubes, there is nearly no flow restriction. However, even small amounts of liquid in the circulating gas can be detected, because even small drops of liquid have a relatively high influence on the capacity that is measured between the two tubes. Therefore it is possible by the capacity measurement to indicate liquid particles in gas, and therefore adjustment of a cooling system can be performed immediately. 
         [0026]      FIG. 3  shows a system  102  for level detection. The level detector comprises an outer tube  120  and an inner tube  122 . The inner tube comprises a first section  124  and the second section  126 . The two sections are separated by isolation means  128 . The inner tube  122  is placed inside a partly open tube  132 . The inner tube and the outer tube are placed in relation to a screw  134  and a housing  134 . A measuring system  140  is connected to the lines  142 ,  144 . The line  142  is connected to the tube section  126  and the second line  144  is connected to the tube section  124 . 
         [0027]    In operation, a reference can be achieved if the section  124  is submerged in the liquid. Thereby a reference value can be calculated. The tube section  126  can hereafter perform an indication of the level of liquid around the tube. In that way it is possible to indicate the level of, for example, refrigerant in one or another tank as part of a refrigeration system, or it is possible to indicate the level of oil in a refrigeration system. The system at  FIG. 3  can be a highly effective oil indicator in refrigeration systems. The system could be placed, for example, in a submerged evaporator and there it could indicate the oil level in the evaporator. Hereby it is possible to start a pump or indicate that manual opening of a valve to tap the oil is necessary. In many modern systems the oil return is performed automatically by pumping systems. 
         [0028]      FIG. 4  shows a sectional view of a tube section, for example the tube  220  at the  FIG. 4 . The detector  206  is made inside the tube  220  where the detector  206  and  212  are all referring to the same component. Inside the tube  220  is indicated an inner tube  222 . The inner tube  222  is kept in place by fixtures  240  where a conducting screw  242  connects the inner tube  222  electrically and where the screw  244  is an isolating screw that isolated the inner tube towards the outer tube  220 . The detector  206  further comprises a second inner tube  250 , which second inner tube  250  is fixed to the outer tube  220  by conductive fixtures  252 . 
         [0029]    In operation, it is possible to measure the capacity between the inner tube and the outer tube and also between the first inner tube and the second inner tube. Hereby is the actual electric capacity increased and more accurate measurement can be achieved. 
         [0030]      FIGS. 5   a - 5   d  shows a cooling system which is divided into four different subsystems where only a compressor  328 , a condenser  340 , and a receiver  342  are common. 
         [0031]      FIG. 5   a  shows a system  302   a  which is by a high pressure line  304  connected to the receiver  342 . This line  304  is connected to an expansion valve  330   a  and further through a connection line  332   a  to an evaporator  324   a.  A suction line  320   a  is connected to a sensor  306 , 308   a  which is electrically connected to a control unit  310   a.  A suction line  326  is connects the refrigerant to the suction side of a compressor  328 . 
         [0032]    In operation, the sensor  306 , 308   a  will measure the suction gas and indicate any liquid drops that are carried in the gas. The control unit  31   a  controls the expansion valve  330   a  so by indicating any liquid drop in the sensor  306 , 308 , the expansion valve is more or less closed, or at least the flow though the expansion valve  330   a  is reduced. In this way it is achieved that the suction gas that reaches the compressor  328  is absolutely dry and free from any liquid droplets. 
         [0033]    In  FIG. 5   b  the expansion valve  330   b  is also connected to the line  304  from the receiver  342 . The expansion valve  330   b  is delivering liquid refrigerant to a tank  322  from where a line  332   b  is connected to the evaporator  324   b.  The evaporator is connected by a line  320   b  into the tank  322 . From the tank  322  there is a suction line connected to the sensor  306 , 308   b.  The sensor  306 , 308   b  is connected to a control unit  310   b  which is further connected to a liquid level indicator  323 . The control unit  310   b  is controlling the flow through the expansion valve  330   b  so that the liquid level in the tank  322  can be controlled. In this way the liquid level can be under control according to two different parameters. One parameter is the liquid level, the other is the existence of liquid drops in the detector  306 , 308   b.  In this way it can be achieved that the will always be sufficient liquid refrigerant for the evaporator  324   b,  and there will always be control of the suction gas delivered to the compressor. The use of the tank  322  has the advantage that the evaporator  324  can be fully submerged. The tank  322  operates as a liquid separator. 
         [0034]      FIG. 5   c  shows a subsystem  302   c.  Again the line  304  is connected to the expansion valve  330   c  from which expansion valve  330   c  a line  332   c  connects towards the evaporator  324   c.  The outlet of the evaporator  324   c  is via a line  320   c  connected to a senor  306 , 308   c.  The outlet of this sensor is connected to a separation tank  322 . From this separation tank evaporated refrigerant is sucked to the suction side of the compressor  328 . Liquid refrigerant is collected by a pump  325  which pump outlet is connected to the pressure line  304  that supplies liquid refrigerant towards the expansion valve  330   c.  By means of the control unit  310   c  the sensor  306 , 308   c  controls the expansion valve  330   c  and the pump  325 . 
         [0035]    Hereby a total control of the evaporator  324   c  can be achieved. At the same time it is achieved that no liquid droplets will be contained in the suction gas that reaches the compressor  328 . 
         [0036]      FIG. 5   d  shows a system  302   d  which, like the ones previously described, also receives liquid high-pressure refrigerant over the line  304 . The expansion valve  330   d  is further connected by a line  332   d  to a submerged evaporator  324   d.  A line  320   d  is connecting sensor  306 , 308   d  to the suction line  326  connected to the suction side of the compressor  328 . The sensor  306 , 308   d  is connected to a control unit  310   d  which controls the expansion valve  330   d.  In this way it is possible to close or reduce the flow through the expansion valve when droplets of liquid refrigerant are measured by the sensor  306 , 308   d.  In this way it can be ensured that the suction line towards the compressor only carries dry gas and the evaporator is completely filled with liquid refrigerant. 
         [0037]    A system as shown in  FIG. 5  could be used for different types of refrigerant where one possible refrigerant can be ammonia NH3. 
         [0038]      FIG. 6  shows a coordinate system which vertical axes indicate the density of the refrigerant flowing through a sensor, and the horizontal axis shows the signal level that can be indicated. The curve that is shown indicates that a relatively low signal level is achieved if superheated refrigerant is passing the sensor. But, as the refrigerant starts to be saturated there is a rapid increase in the signal level. That increase in signal level continues until the refrigerant phase becomes totally liquid. Indicated at  FIG. 6  is the control P-band where the refrigeration system mostly is operating where there is the area around the superheated and the saturated phase. 
         [0039]    Operating in the said P-band, it is possible to avoid liquid refrigerant in the suction line towards the compressor and in that way totally avoid compressor hamming. A further effect that is achieved is that the cooling system can operate very near the saturated system whereby the effectivity of evaporators is increasing. 
         [0040]    It is also possible to use the capacitive sensors to control humidity content in intake air for combustions processes. By using the humidity content as input for a computer system that controls the combustion process it is possible to reduce pollution and reduce fuel consumption. The system can be used for control of engines in ships, cars or aeroplanes.