MXPA97007790A - Absorc overcontrolling control - Google Patents

Abstract

An overconcentration control system for use with an absorption machine of the type that has a heating cycle and cooling effect either individual, double or triple, which uses lithium bromide in solution with water, as the operating liquid . The operating liquid is characterized by a concentration indicative of the amount of lithium bromide dissolved in the water, and by a phase diagram that has a crystallization limit that defines the combinations of concentration and temperature, which correspond to a condition of saturation in the solution. Detection means are provided at a predetermined location within the system, which respond to the depth of the solution to generate a concentration signal indicative of the liquid concentration. A temperature sensor is also provided to generate a temperature signal indicative of the liquid temperature. In addition, means are provided which respond to the temperature signal and to the concentration signal to calculate a representation of the absorption cycle of the machine, which can be represented in a phase diagram for the lithium bromide system. The representation includes a plurality of critical state points, which are defined by predetermined respective combinations of concentration and temperature.

Description

ABSORPTION ABSORPTION CONTROL DESCRIPTION OF THE INVENTION This invention relates generally to a control system for an absorption liquid cooler, and more specifically to an analogous detector for measuring the concentration of lithium bromide in the system. The absorption systems operate with a variety of refrigerant / sorbent pairs, one of which is water / lithium bromide. The concentration of the absorber is constantly changing from low to high concentrations, depending on which vessels are occupying the solution and the conditions to which the cooler is being controlled for the operation. The lithium bromide solution can change from a liquid state to a solid state under certain conditions. This solid state condition is known as crystallization. When crystallization occurs in an absorption chiller, the chiller is not able to function properly and usually requires significant effort and cost to correct the problem. Overconcentration in the absorption systems becomes an aspect of greater interest as the amount of coolant that boils in the solution increases. The typical method to verify this procedure is to check the coolant level in the evaporator manifold. When the coolant level reaches a certain point, a discrete level float switch will close and appropriate corrective actions will be presented. This is a type of algorithm control reaction and is predetermined by the height of the level switch. Flotation can not be anticipated when too much coolant is being removed from the solution before a single point of travel of the switch. Therefore, it is an object of the present invention to provide an improved absorption cooling system. This object is achieved in a method and apparatus in accordance with the preambles of the claims and through the various aspects of their characterization parts. In order to overcome the problems of the prior art described above, the present invention is directed to the use of a level switch of analogous type which can respond to the level of coolant change in the evaporator. This level of change is a direct indication of the concentration of weak solution leaving the absorber collector. Once this concentration is known, along with other measured temperatures, the absorption cycle can be exactly calculated. Once the cycle is known and related to the properties of the fluid, the point at which the crystallization occurs can be verified and compared with actual operating conditions. If the operating conditions reach the crystallization concentrations, the correction action is taken to reduce the concentration of lithium bromide and protect the cooler. Through the use of a microprocessor, the cooler can operate in a pro-active manner, keeping the machine away from crystallization instead of simply reacting to high concentrations of lithium bromide as is recently realized in the art. With this type of control, theoretically, an absorption unit should never crystallize (except during an extended power failure or mechanical failure). For a complete understanding of the nature and subject of the invention, reference should be made to the following detailed description of a preferred mode of practice of the invention, read in conjunction with the accompanying drawings, in which: FIGURE 1 is a schematic illustration of an analogous level switch mechanism, suitable for use in the present invention. FIGURE lb is a schematic illustration of a switch and resistor located in the interruption section illustrated by the circle in the switch support arrow shown in FIG.

FIGURE 2 is a schematic diagram for bromide lithium in water with a graph of the solution cycle for a typical cooler including the crystallization line. FIGURE 3 is a schematic illustration of a mode illustrating flow through a double acting cooler system. FIGURE 4 represents the equilibrium diagram for lithium bromide in water. An absorption cooler uses water as the refrigerant in vessels maintained under a depth vacuum. The cooler operates on a simple principle that under the absolute low pressure (vacuum), the water absorbs heat and vaporizes (boils) at a correspondingly low temperature. For example, at a very deep vacuum of 6.54 mm (0.25 inches) of absolute pressure mercury, the water boils at a relatively low temperature of 4 ° C (40 ° F). To obtain the energy required for this boiling, heat is taken from, and therefore cooled, from another fluid (usually water). Then, the cooled fluid can be used for cooling purposes. To make this cooling procedure continuous, the refrigerant vapor must be removed as it is produced. To achieve this, a solution of lithium bromide salt in water is used to absorb water vapor. Lithium bromide has a high affinity, and absorbs it in large quantities under correct conditions. The removal of the refrigerant vapor by absorption keeps the pressure of the machine low enough, so that the vaporization of cooling continues. However, this procedure dilutes the solution and reduces its absorption capacity. Therefore, the diluted lithium bromide solution is pumped into separate containers where it is heated to liberate (boil) the previously absorbed water. The relatively cold condensation water from a cooling tower or other source removes enough heat from this steam to condense it back to liquid for reuse in the cooling cycle. The concentrated lithium bromide solution is then returned to the original container to continue the absorption procedure. Figure 3 illustrates a flow through a double effect cooling system 30. The main sections of the chiller are contained in several containers. A large lower protection 32 contains the evaporator and absorber sections 34 and 36, respectively. The evaporator and the absorber are collaterally placed in units. In the evaporator section, the cooling water vaporizes and cools the cooling water of the air conditioning or cooling process. In the absorber, the evaporated water from the evaporator is desorbed by the lithium bromide solution. Another container, which is placed above the evaporator / absorber assembly, is the high stage generator 38. Here, approximately half of the diluted solution of the absorber is heated and reconcentrated to recover slightly more than half of the water previously absorbed . An additional container is also placed above the evaporator / absorber assembly and contains the low stage generator 40 and the condenser 42. The other half of the diluted solution is heated and reconcentrated in the low stage generator by means of high water vapor. High stage generator temperature. The water vapor released from the solution in this process is condensed to liquid in the condenser section. This mode of the cooler also has: two solution heat exchangers 44 and 46 and a steam condensate heat exchanger 48 to improve the economy of the operation; an external purge system to maintain the vacuum of the machine through the removal of non-condensable products; hermetic pumps 50 and 52 to circulate the solution and the refrigerant; and various operational, capacity and safety devices to provide automatic and reliable operation of the machine. A capacity valve 64 controls the heat input to the cooler. Additional hardware and components, which are normally associated with the cooling system, include a drain trap 56, a safety valve 58, a temperature sensor 62, a TC temperature controller and an LCD level control device. The arrows in Figure 3 indicate the direction of flow through the system. The absorption chiller described above is typical of absorption chiller machines, to which the present invention is applicable. A more complete description of this machine and other typical chillers is established in Start-Up, Operation, and Maintenance Instructions, Double-Effect Hermetic Absorption Liquid Chillers, Catalog No. 531-607, published by Carrier Corporation, which is incorporated here for reference. It should be understood that this invention also applies to an individual effect and absorption cycles of several multiple effects. In one embodiment of the present invention, an analog level switch 10 is mounted in the overflow box of the evaporator 54 of a cooler as illustrated in Figure 3. Figure 1 is an enlarged view of the switch 10. As illustrated in Figure 1, the distance marked "A" is a known parameter. As the float 12 travels over the distance "A" along a hollow arrow 20, the exact position of the float is determined. A series of reed switches 14 and resistors 16, which are placed inside a cylindrical core member 22, contained within the arrow 20 are activated through a group of magnets 18 on the float, act as a potentiometer and change the output voltage, which is transmitted through electrical conductor wires 24 to the microprocessor 60. The voltage that is measured, can be directly translated to a concentration using the appropriate calculations. The level switch must be initially calibrated when the unit is installed. There are two important reasons for this calibration: 1) Neither of the two units are identical, the volume of refrigerant varies depending on the sizes of the protection and the size of the unit, 2) There are two styles of absorber / evaporator protections (for up / down and collaterally), which have different coolant level ratios. The level 10 switch is available as a component of IMO Industries under the commercial name of XT Series Level Transmitter. Units are calibrated when the service technician "balances" or adjusts the refrigerant charge in the unit. The unit is brought to 50% of the rated load condition and stabilized. The technician takes a sample of weak solution from the sump of the absorber and measures the concentration using a hydrometer. The technician then measures the voltage of the level switch and registers it in the control algorithm contained in the microprocessor 60. Then, the technician operates the machine at a 100% nominal load condition and repeats the procedure. This calibration sets two points in the voltage / concentration curve that totally define the specific operating parameters for the particular unit. In order to verify this concept, a cooler was operated at various conditions, while real concentrations of weak solution were recorded (these measurements were made using a hydrometer), and the voltage signal from the level switch. A mathematical relationship is determined from these data. The results of this test indicated that for a given concentration, the voltage could always be the same, regardless of the conditions to which the cooler was operated. Knowing this, a specific voltage has a direct relation to the concentration, the total operation of the cooler cycle is plotted exactly starting from the voltage readings. Another test was carried out and incorporated into newly developed control algorithms. These new algorithms are able to calculate the concentration of lithium bromide at any point of state with good accuracy. Figure 2 is a schematic diagram illustrating a typical cooler cycle. The points numbered in the graph correspond to the lithium bromide solution as it travels through the cooler. Figure 4 represents the equilibrium diagram for lithium bromide in water. The solution cycle is illustrated by plotting it in a basic equilibrium diagram for lithium bromide in solution with water. The diagram (Figure 2) can also be used to perform analyzes and to investigate faults. The left scale in the diagram indicates the vapor pressures of water and solution at equilibrium conditions. The right scale indicates the corresponding saturation temperatures (boiling or condensation) for both the refrigerant (water) and the solution. The bottom scale represents the concentration of the solution, expressed as a weight percentage of lithium bromide by weight of the solution with water. For example, a concentration of lithium bromide of 60% means 60% lithium bromide and 40% water by weight. In Figure 4, the curved lines running diagonal from left to right are the temperature lines of the solution (which should not be confused with the horizontal saturation temperature lines). The only curved line that begins in the lower right part represents the crystallization line. In any combination of temperature and concentration to the right of this line, the solution will be crystallized (solidified) and flow restricted. The slightly inclined lines extending from the bottom of the diagram are solution-specific gravity lines. The concentration of a sample of lithium bromide solution can be determined by measuring its specific gravity with a hydrometer and reading its solution temperature. Next, graph the intersection point for these two values and read towards the percentage scale of lithium bromide. The corresponding vapor pressure can also be determined by reading the straight scale to the left of the point and its saturation temperature can be read in the scale to the right. Graphing the Solution Cycle A cycle of absorption solution at typical full load conditions is plotted in Figure 2 from Points 1 to 13. These values will vary with different loads and / or operating conditions. Point 1 represents the strong solution in the absorber, as it begins to absorb water vapor after being sprayed from the absorber nozzles. This condition is internal and can not be measured.

Point 2 represents the diluted (weak) solution after it exits the absorber and before it enters the heat exchanger at a low temperature. This includes its flow through the solution pump. This point can be measured with a sample of solution from the discharge of the pump. Point 3 represents the weak solution that comes out of the low temperature heat exchanger. It is at the same concentration as in Point 2, but at a higher temperature after gaining heat from the strong solution. This temperature can be measured. Point 4 represents the weak solution that comes out of the drain heat exchanger. This is the same concentration as Point 3, but at a higher temperature after gaining heat from the steam condenser. This temperature can be measured. At this point, the weak solution first flows through the level control device (LCD) and then separates, approximately half goes to the low stage generator, and the rest goes to the high temperature heat exchanger. Point 5 represents the weak solution in the low-stage generator after being pre-heated to the boiling temperature. The solution will boil at temperatures and concentrations that correspond to a saturation temperature established by the condensing temperature of steam in the condenser. This condition is internal and can not be measured. Point 6 represents the weak solution that comes out of the high temperature heat exchanger and enters the high stage generator. This is at the same concentration as' Point 4, but at a higher temperature after gaining heat from the strong solution. This temperature can be measured. Point 7 represents the weak solution in the high stage generator after being preheated to the boiling temperature. The solution will boil at temperatures and concentrations that correspond to a saturation temperature established by the vapor condensation temperature in the low-stage generator tubes. This condition is internal and can not be measured. Point 8 represents the strong solution that comes out of the high-stage generator and enters the high-temperature heat exchanger after being reconcentrated by boiling the refrigerant. This can also be plotted approximately by measuring the temperatures of the strong solution coming out and the condensed steam coming out of the low-stage generator tubes (saturation temperature). This condition can not be measured accurately.

Point 9 represents the strong solution of the high temperature heat exchanger as it flows between the two heat exchangers. This is at the same concentration at Point 8, but at a colder temperature after bringing heat to the weak solution. The temperature can be measured in these models, which have separate solution heat exchangers. Point 10 represents the strong solution that comes out of the low-stage generator and enters the low-temperature heat exchanger. This is at a weaker concentration than the high-stage generator solution, and can be graphically approximated by measuring the temperatures of the strong solution coming out and the steam condensate. (saturation temperature). This condition can not be measured accurately. Point 11 represents the mixing of a strong solution of the high temperature heat exchanger and the strong solution of the low stage generator as both enter the low temperature heat exchanger. The temperature can be measured on those models that have separate solution heat exchangers. Point 12 represents the combined strong solution before it exits the low temperature heat exchanger after drawing heat into the weak solution. This condition is internal and can not be measured.

Point 13 represents the strong solution that comes out of the low temperature heat exchanger and enters the nozzles of the absorber, after being mixed with some of the weak solution in the heat exchanger. The temperature can be measured, but the concentration can not be measured. After leaving the spray nozzles, the solution is a little cooled and concentrated as one proceeds to the lowest pressure of the absorber, at Point 1. The following describes how the state points are obtained in Figure 2. Point 2 is defined by the concentration of the level detector together with the temperature measurement of the direct solution. The refrigerant level detector voltage is calibrated at the first start of the machine to establish exactly the relationship between the refrigerant level in the evaporator and the concentration of the lithium bromide solution in the absorber. This should be done by taking a solution reading at a level of high and low concentrations and associated refrigerant level detector voltages. The concentration must then be interpolated and extrapolated assuming a linear relationship between the two points. Note that the relationship between the refrigerant level and the voltage is inverse, that is, for an increase level there is a reduction voltage input.

Point 21 is at the same concentration as Point 2, but at a saturation temperature defined by the temperature of the refrigerant. The rest of the points are calculated through the use of state point equations, crystallization line equations, additional detector information, concentration equilibria, and mass equilibria. The 9X and 14X points are defined by the use of the crystallization line equation at the solution temperatures of Points 9 and 14, respectively. These calculations are normal calculations that can be easily performed by those skilled in the art. Control Override Concentration and Fault Protection From the above calculation, CONC9 and CONC14 should be used to override the capacity control routine or generate an interruption without recirculation, if the concentration of lithium bromide should be too high. The concentration protection should consist of an inhibition threshold, a closed threshold, and a safety interruption threshold (points IN, CD and SS, respectively in Figure 2), for each calculated concentration (C0NC9 and CONC14). When the calculated concentration exceeds the inhibition threshold, the capacity valve 64 must be inhibited from its opening until the calculated concentration falls below the inhibition threshold minus 0.5 percent. If the calculated concentration exceeds the receding threshold, the capacity valve 64 must be closed until it is below the inhibition threshold minus 0.5 percent concentration. if the calculated concentration exceeds its associated safety interruption threshold, then an interruption without recirculation must be initiated with a dilution cycle. The concentration thresholds associated with each point are as follows: Closed Inhibition Point Fault / Interruption (% CONC.) (% CONC.) (% CONC.) CONC9 CONC9X-1.5% CONC9X-1.0% CONC9X-0.5% CONC14 CONC14X-1.5 % CONC14X-1.0% CONC14X-0.5% The above calculation will protect the machine and show the utility of the invention during the operation. In the case of a loss of energy, a normal interruption is not possible. The invention provides data storage before the loss of energy. These data are compared with the data taken in the restoration of energy and are used to determine if the solution is crystallized and if it is safe to restart the machine. Calculate Projected Crystallization Solution Temperatures TSOL9X = Crystallization Line Equation (CONC9X) TS0L14X = Crystallization Line Equation (CONC14X) Calculate Differences and Solution Temperature DIFF9 = TS0L9 - TS0L9X DIFF13 = TS0L13 -TS0L14X Yes (DIFF9 <DIFF13) then TSOLS = TS0L9 - DIFF9 TS0L13S = TS0L13 - DIFF9 Other TS0L9S = TS0L9 - DIFF13 TS0L13S = TS0L13 - DIFF13 Determination of Energy Loss for the Dilution Cycle If ((TSOL9 <TSOL9S) or (TSOL13 <TSOL13S)) then Alarm Status Other Yes ((TSOL9 <TSOL9S + 25) or (TSOL13 <TSOL13S + 25) then Dilution Cycle of Energy Loss = TRUE Another Energy Loss Dilution Cycle = FALSE The purpose of the invention described above is not only to avoid overconcentration of the lithium bromide solution in an absorption machine, but also to take preventive measures and try to maintain the operation of the machine if the concentration exceeds the "normal" operating conditions, thus avoiding unnecessary interruptions of the machine, this is achieved by first determining the critical state points at the machine's operating site, the typical state points of the two-cycle operation. stages are shown in the previous diagram The two status points 9 and 14 are determined by temperature and pressure sensors located on the machine used together n an analogous refrigerant level detector. The level detector is calibrated during the start of the machine to be a direct indicator of the concentration of weak solution. The level detector has a voltage output, which is directly related to the coolant level. The level of refrigerant is directly related to the concentration of weak solution. Two voltage readings are taken that correspond to two or more concentrations of weak solution. This data is entered into a microprocessor control system. This develops a relationship that will be used to determine the concentration of weak solution in any condition of the operation. Now, other state points can be calculated which will be used to calculate the two typical points designated 9 and 14. These two critical points are compared with points 9X and 14X, which are the points where lithium bromide crystallizes. Three predetermined points are established between the critical points (9 and 14) and the point where the lithium bromide is crystallized (9X and 14X at constant temperatures of lithium bromide). If the state points 9 and 14 reach the first predetermined point, then the capacity control valve of the machine is inhibited for its opening, indicated by Figure 2 for the "IN" point. If the second point "CD" is reached, the capacity control valve closes until the critical points move away from the crystallization line. If the third point "SS" is reached, the machine will suffer a "SAFETY" interruption and go to a dilution cycle. It is also possible to calculate and present "the loss of the absorber" with the information gathered by the detectors and with the equation used to calculate the state points. The loss of the absorber is the difference between the temperature of the refrigerant and the saturation temperature of the lithium bromide in the absorber. This difference, defined in degrees centigrade is an indication of the operation of the machine. A further advantage of the present invention is the ability to store data in the case of an energy loss to determine the readings of the machine when the energy is restored.

Claims (6)

1. CLAIMS 1. An overconcentration control system for use with an absorption machine of the type having a cooling cycle and heating of individual, double and triple effect, and of a type which utilizes an operating liquid comprising a solution of an ionic solute and a refrigerant solvent, the operating liquid is characterized by a concentration indicative of the amount of the solute dissolved in the solvent, and by a phase diagram that has a crystallization limit that defines the concentration and temperature combinations , which correspond to a condition of saturation in the solution, characterized in that it comprises: means that respond to the depth of the solution to a predetermined location in the machine for the generation of a concentration signal indicative of the concentration of liquid; a temperature sensor for generating a temperature signal indicative of the temperature of the liquid; means responsive to the temperature signal and the concentration signal to calculate a representation of the absorption cycle of the machine, which can be plotted in the phase diagram, the representation includes a plurality of critical points defined by predetermined respective combinations of concentration and temperature; means for comparing the actual concentration and temperature of the liquid at concentrations and temperatures of the machine, which can be plotted in the phase diagram, the representation includes a plurality of critical points defined by the respective predetermined combinations of concentration and temperature; means for comparing the actual concentration and temperature of the liquid at concentrations and temperatures that lie at the crystallization limit and for generating a difference signal; and means responsive to the magnitude of the difference signal to change the operating state of the machine as necessary to prevent the liquid from reaching a combination of concentration and temperature that is at the crystallization limit.
2. 2. The system according to claim 1, characterized in that the operating liquid comprises a solution of lithium bromide in water.
3. The system according to claim 2, characterized in that the means that respond to the depth of the solution comprise an analogous switch containing a float device.
4. 4. An overconcentration control system for use with an absorption machine of the type that has a cooling cycle and heating of individual, double and triple effect, and of a type which uses a solution of lithium bromide, such as liquid of operation, such operating liquid is characterized by a concentration of lithium bromide in water, and by a phase diagram having a crystallization limit that defines the concentration and temperature combinations of lithium bromide, which correspond to a condition of saturation in the solution, characterized in that it comprises: means that respond to the depth of the solution to a predetermined location in the machine for the generation of a concentration signal indicative of the concentration of lithium bromide in the liquid; a temperature sensor for generating a temperature signal indicative of the temperature of the liquid; means responsive to the temperature signal and the concentration signal to calculate a representation of the absorption cycle of the machine, which can be plotted in the phase diagram, the representation includes a plurality of points of critical states defined by predetermined respective combinations of concentration and temperature; means for comparing the actual concentration and temperature of the liquid at concentrations and temperatures of the machine, which can be plotted in the phase diagram, the representation includes a plurality of critical points defined by the respective predetermined combinations of concentration and temperature; means for comparing the actual concentration and temperature of the liquid at concentrations and temperatures that are at the crystallization limit and for generating a difference signal; and means that respond to the magnitude of the difference signal to change the operating state of the machine as necessary to prevent the liquid from reaching a combination of concentration and temperature that is within the crystallization limit.
5. 5. The system in accordance with the claim 4, characterized in that the predetermined location is contained within the evaporator section of the absorption machine.
6. 6. The system in accordance with the claim 5, characterized in that the means that respond to the depth of the solution comprise an analog level switch that generates a voltage, which is converted to a concentration signal for the lithium bromide contained in the solution. SUMMARY OF THE INVENTION An overconcentration control system for use with an absorption machine of the type having a heating cycle and cooling effect whether individual, double or triple, which uses lithium bromide in solution with water, as the operating liquid. The operating liquid is characterized by a concentration indicative of the amount of lithium bromide dissolved in the water, and by a phase diagram that has a crystallization limit that defines the combinations of concentration and temperature, which correspond to a condition of saturation in the solution. Detection means are provided at a predetermined location within the system, which respond to the depth of the solution to generate a concentration signal indicative of the liquid concentration. A temperature sensor is also provided to generate a temperature signal indicative of the liquid temperature. In addition, means are provided which respond to the temperature signal and to the concentration signal to calculate a representation of the absorption cycle of the machine, which can be represented in a phase diagram for the lithium bromide system. The representation includes a plurality of critical state points, which are defined by predetermined respective combinations of concentration and temperature. Means are also provided for comparing the actual concentration and temperature of the liquid with concentrations and temperatures that are in the crystallization limit for lithium bromide to generate a difference signal. Control means are provided that respond to the magnitude of the difference signal to change the operating state of the machine as necessary, to prevent the liquid from reaching a combination of concentration and temperature that is within the crystallization limit.