CA2216257C - Absorption over-concentration control - Google Patents

Abstract

An over-concentration control system for use with an absorption machine of the type having either a single, double and triple effect cooling and heating cycle, which uses lithium bromide in solution with water as the operating liquid. The operating liquid being characterized by a concentration indicative of the quantity of lithium bromide dissolved in water, and by a phase diagram having a crystallization boundary that defines the combinations of concentration and temperature which correspond to a condition of saturation in the solution. A sensing means is provided at a predetermined location within the system which is responsive to the depth of said solution for generating a concentration signal indicative of the concentration of the liquid. A temperature sensor is also provided for generating a temperature signal indicative of the temperature of the liquid. Further means are provided which are responsive to the temperature signal and the concentration signal for calculating a representation of the absorption cycle of the machine which may be plotted on a phase diagram for the lithium bromide system. The representation includes a plurality of critical state points which are defined by predetermined respectivecombinations of concentration and temperature. Means are also provided for comparing the actual concentration and temperature of the liquid to concentrations and temperatures which lie on the said crystallization boundary for lithium bromide for generating a difference signal. Control means are provided which are responsive to the magnitude of the difference signal for changing the operating state of said machine as necessary to prevent the liquid from reaching a combination of concentration and temperature that lies on the crystallization boundary.

Description

ABSORPTION OVER-CONCENTRATION CONTROL

This invention relates generally to a control system for an absorption liquid chiller, and more specifically to an analog sensor to measure the concentration of lithium bromide in the system.
Absorption systems operate with a variety of refrigerant/absorbent pairs, one of which is water/lithium bromide. The concentration of the absorbent is constantly ch~nging from low to high coLlcc,ll,alions depending upon which vessels the solution is occupying and the conditions at which the chiller is being controlled to operate. The lithium bromide solution can change from a liquid state to a solid state under certain conditions. This solid state condition is known as cryst~lli7~tion.
When cryst~lli7~tion occurs in an absorption chiller, the chiller is not able tofunction properly and it usually l~uilcs a significant and costly effort to correct the problem.
Over-conce~ lion in absollltion systems becomes more of a concern as the amount of refrigerant that boils out of the solution increases. The typical method of monitoring this process is to l"onitor the refrigerant level in the evaporator sump.
When the level of the refrigerant reaches a certain point, a discrete level float switch will close and cause the ~ pliate co~,~live actions to take place. This is a reactive type of control algo,ilh,l" and is predetçnnined by the height of the level switch. The float cannot anticipate when too much refrigerant is being removed from the solution prior to the single trip point of the switch.
It is the.efore an object of the present invention to provide an improved absol~tion refrigeration system.
This object is achieved in a method and dppal~lusaCCOIding to the preambles of the claims and by the fc~lules of the char~ctçri7ing parts thereof.
In order to o~e~olllc the problems of the prior art described above, the present invention is directed to the use of an analog-type level switch that canrespond to the ch~nging refrigerant level in the evaporator. This ch~nging level is a direct indication of the weak solution concentration leaving the absorber sump.
Once this conccll~lalion is known, along with other measured temperatures, the absorption cycle can be accurately calculated. Once the cycle is known and related to the fluid properties, the point at which cryst-qlli~tion occurs can be monitored and compared to the current opc~ g conditions. If the operating conditions approach the cryst-qlli7~tion concentrations, corrective action is taken to reduce the lithium bromide concentration and protect the chiller. Through the use of a microprocessor, the chiller can operate in a proactive manner by keeping the machine away from cryst~lli7-qtion rather than simply reacting to high lithium bromide concentrations as is recently now done in the prior art. With this type of control, an absorption unit theoretically should never crystallize (except for an extended power failure or mechanical failure).
For a fuller undersPn(ling of the nature and object of the invention, reference should be made to the following detailed description of a plcfcllcd mode of practicing the invention, read in connection with the acco~ ying drawings, in which:
FIG. la is a sch~m~tic illustration of an analog level switch mech-q-ni~m suitable for use in the present invention.
FIG. lb is a sçh~ tic illustration of a switch and resistor located at the break away section illustrated by the circle on the support shaft of the switch shown in FIG. 1 a.
FIG. 2 is a schPlnqtic. diagram for lithium brornide in water with a plot of thesolution cycle for a typical chiller including the cryst~q-lli7~tion line.
FIG. 3 is a s~h~m-q-tic illustration of an embodiment illustrating the flow through a double effect chiller system.
FIG. 4 re~ nls the equilibriurn diagram for lithium brornide in water.
An absorption chiller uses water as the refrigerant in vessels ...~ ed under a deep vacuurn. The chiller operates on the simple principle that under low absolute pres~ulc (vacuum), water absorbs heat and vaporizes (boils) at a collc~ndingly low tcl~lp~ e. For example, at the very deep vacuurn of 0.25 in.
(6.4 mm) of mercury absolute p~s~ulc, water boils at the relatively low ternperature of 40 F (4 C). To obtain the energy required for this boiling, it takes heat from, and therefore chills, another fluid (usually water). The chilled fluid then can be used for cooling purposes.

To make this cooling process continuous, the refrigerant vapor must be removed as it is produced. To accomplish this, a solution of lithium bromide salt in water is used to absorb the water vapor. Lithium bromide has a high affinity forwater, and absorbs it in large quantities under the right conditions. The removal of the refrigerant vapor by absorption keeps the machine ples~ule low enough for the cooling vaporization to continue. However, this process dilutes the solution andreduces its absol~tion capacily. Therefore the diluted lithium bromide solution is pumped to separate vessels where it is heated to release (boil offl the previously absofl,cd water. Relatively cool con~1en~ing water from a cooling tower or othersource removes enough heat from this vapor to condense it again into liquid for reuse in the cooling cycle. The concel,llated lithium bromide solution is then returned to the original vessel to continue the absGI~tion process.
Fig. 3 illustrates the flow through a double effect chiller system 30. The major sections of the chiller m~chine are contained in several vessels. A large lower shell 32 contains the evaporator and absorber sections 34 and 36, res~eclively. The evaporator and absorber are positioned as side by side in units. In the evaporator section, the refrigerant water v~olizes and cools the chilled water for the air conditioning or cooling process. In the absorber, vaporized water from the ev~po~lor is absorbed by lithium bromide solution.
Another vessel which is positioned above the evaporator/absorber assembly is the high-stage generator 38. Here, approximately half of the diluted solution from the abso~ is heated and reconc~ aled to recover slightly over half of the water previously absoll,ed.
An additional vessel is also positioned above the evaporator/abso,l,el assembly and contains the low-stage generator 40 and cond~l~ser 42. The other half of the diluted solution is heated and reconcentrated in the low-stage generator by high t~lllp~ e water vapor from the high-stage generator. The water vapor released from the solution in this process is cond~n~ed to liquid in the condenser section.
This chiller embodiment also has: two solution heat exchangers 44 and 46 and a steam cond~ te heat çxc.~ ger 48 to improve operating economy; an external purge system to m~in~in machine vacuum by the removal of noncon~çncables; hermetic pumps 50 and 52 to circulate the solution and refrigerant; and various operational, capacity, and safety devices to provide automatic, reliable machine performance. A capacity valve 64 controls the heat input to the chiller. Additional hardware and components which are normally associated with the chiller system include a drain trap 56, relief valve 58, t~ pcidlute sensor 62, temperature controller TC and level control device LCD.
The arrows in Fig. 3 indicate the direction of flow through the system.
The above described absol~.lion chiller is typical of the absorption chiller ~ chine to which the present invention is applicable. A more complete description of this machine and other typical chillers are set forth in Start-Up, Operation, and Maintenance Instructions, Double-Effect Hermetic Absorption Liquid Chillers, Catalog No. 531-607, published by Carrier Corporation which is incorporated herein by reference. It should be understood that this invention also applies to single effect and the various multi-effect absorption cycles.
In an embodiment of the present invention, an analog level switch 10 is mounted in the evaporator overflow box 54 of a chiller as illustrated in Fig. 3. Fig. 1 is an enlarged view of switch 10. As illu~llated in Fig. 1, the ~is~ labeled "A" is a known parameter. As the float 12 travels over ~ict~nce "A" along a hollow shaft 20 the exact position of the float is detennined A series of reed switches 14 and resistors 16 which are positioned within a cylindrical core member 22 contained within shaft 20 are activated by a set of magnets 18 in the float, act like a potentiometer and change the output voltage which is l~ ...illed through electrical lead wires 24 to mic,o~ cessor 60. The voltage that is m~u,ed can be directly tr~ncl~ted to a conce.lll~tion using the appropriate c~lc~ tions. The level switch must be initially calibrated when the unit is installed. There are two cignifi~nt reasons for this calibration: 1) No two units are identical, the refrigerant volume varies depending on shell sizes and unit size, 2) There are two styles of abso,l,er/c;va~lator shells (overtunder and side by side) which have different refrigerant level relationships. Level switch 10 is available as a col,lpollent from IMO Industries under the tracl~nQrne XT Series Level Trans,lliller.
The units are calibrated when the service technician "trims" or adjusts the refrigerant charge in the unit. The unit is brought to a 50% nominal load condition and stabilized. The tech~ician takes a weak solution sample from the absorber sump and measures the concentration using a hydrometer. The technician then measures the voltage of the level switch and records it into the control algoli~n contained in microprocessor 60. The technician then runs the m~hine to 100% nominal load condition and repeats the procedure. This calibration sets two points on the voltage/concentration curve that fully defines the specific opc~ g parameters for that particular unit.
In order to verify this conce~t, a chiller was opelaled at various conditions while recording actual weak solution collccnllalions (these measulc,llents were made using a hydrometer) and the voltage signal from the level switch. A mathematicalrelationship is determined from this data.
The results from this testing indicated that for a given concentration, the voltage would always be the same, independent of the conditions at which the chiller was opela~ g. Knowing that a specific voltage has a direct relationship to conccllll~lion, the entire o~e.~dting cycle ofthe chiller was accurately plotted starting from the voltage rea(1ingc Further testing was carried out and illc~ tcd into newly developed control algo~ilhllls. These new algolilLIlls are capable of calc~ ting the cc,~ lion of the lithium bromide at any state point with good ac~ul~y. Fig. 2 isa ~~ tic diagram which illustrates a typical chiller cycle. The numbered points on the chart collcs~ond to the lithium bromide solution as it travels throughout the chiller. Fig. 4 lcprcsents the equilibrium diagram for lithium bromide in water. The solution cycle is illustrated by plotting it on a basic equilibrium diagram for lithium bromide in solution with water. The diagram (Fig. 2) can also be used for p~.rullll~lce analyses and troubleshooting.
The left scale on the diagram indicates solution and water vapor plc~sules at equilibrium conditions. The right scale in~lic?tçs the corresponding saturation (boiling or con-l~cin~) temperatures for both the refrigerant (water) and the solution.
The bottom scale represents solution concentration, c~rcssed as a weight ~e.~ clll~ge of lithium bromide by weight in solution with water. For example, a lithium bromide concentration of 60% means 60% lithiurn bromide and 40% water by weight.
In Fig. 4, the curved lines running diagonally left to right are solution tell~eldlurb lines (not to be confused with the horizontal saturation temperature lines). The single curved line beginning at the lower right represents the cryst~lli7~t;0n line. At any combination oftemperature and concentration to the right of this line, the solution will be cryst~lli7ed (solidify) and restrict flow. The slightly sloped lines extending from the bottom of the diagram are solution-specific gravity lines. The concentration of a lithium bromide solution sample can be cletennined by measuring its specific gravity with a hydrometer and reading its solution temperature. Then, plot the intersection point for these two values and read straight down to the percent lithium brornide scale. The collc~ondirlg vapor p,~ule can also be det~mined by reading the scale straight to the left of the point, and its saturation tclllpc~dtule can be read on the scale to the right.

Plotting the Solution Cycle An absol~tion solution cycle at typical full load conditions is plotted in Fig.
2 from Points 1 through 13. These values will vary with different loads and Gp~raling 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 inte~
and cannot be measured.
Point 2 leplesbl,ls the diluted (weak) solution after it leaves the absorber andbefore it enters the low-tcln~,e.alulc heat ~Ych~nger. This includes its flow through the solution pump. This point can be measured with a solution sample from the pump discharge.
Point 3 reples~llt~ the weak solution leaving the low-tcLu~l~lulc heat exchanger. It is at the same concentration as Point 2, but at a higher tcll.p~,lalule after gaining heat from the strong solution. This tblllp~ laLule can be mea~ulcd.
Point 4 l~cpfe3ents the weak solution leaving the drain heat exchanger. It is atthe same concentration as Point 3, but at a higher tcl~clal-uc after gaining heat from the steam condensate. This temperature can be measured. At this point, the weak solution first flows through the level control device (LCD) valve and then it is split, with approximately half going to the low-stage generator, and the rest going on to the high-temperature heat exch~nger.
Point S represents the weak solution in the low-stage generator after being preheated to the boiling temperature. The solution will boil at tempelal~cs and concentrations collc~)ollding to a saturation temperature established by the vapor condçncing temperature in the condenser. This condition is intemal and cannot bemeasured.
Point 6 represents the weak solution leaving the high-temperature heat exchanger and entering the high-stage generator. It 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 tenll,el~lule. The solution will boil at teL~ dlureO and concentrations colle~onding to a saturation tempçrature established by the vaporcon~lçncing tel~ldlulc in the low-stage generator tubes. This condition is internal and cannot be mea~ur~d.
Point 8 represents the strong solution leaving the high-stage generator and cntP~ing the high-tell-l,-,.alule heat eY~ . after being lwonce~lld1ed by boiling out refrigerant. it can be plotted approximately by rnP~c... ;.-g the tempelalules of the leaving strong solution and the con-lPnced vapor leaving the low-stage generatortubes (saturation temperature). This condition cannot be meaOu~od accurately.
Point 9 l~l~,selllO the strong solution from the high te~e~ heat exch~ge- as it flows between the two heat exchangers. It is the same concelllldlion as Point 8 but at a cooler t~ pcldlule after giving up heat to the weak solution. The tcmp~,.alulc can be measured on those models which have separate solution heat ~Ych~ngers.
Point 10 rcpleOelllO the strong solution leaving the low-stage generator and entering the low-t~n~ lule heat exchanger. It is at a weaker concentration than the solution from the high-stage generator, and can be plotted ~pr~ximately by measuring the telnpc~ cs of the leaving strong solution and vapor condPnc~te (saturation temperature). This condition cannot be measured accurately.

Point 11 represents the ~llixlule of strong solution from the high-temperature heat exch~nger and strong solution from the low-stage generator as they both enter the low-temperature heat exchanger. The temperature can be measured on those models which have separate solution heat exchangers.
Point 12 represents the combined strong solution before it leaves the low-te,,,pclal~lre heat exchanger after giving up heat to the weak solution. This condition is intern~l and cannot be measured.
Point 13 represents the strong solution leaving the low-temperature heat exchanger and entering the abso,l,cl spray nozzles, after being mixed with some weak solution in the heat exchanger. The telllp~,ldlulc can be measured, but theconcentration cannot be sampled. After leaving the spray no771es, the solution is somewhat cooled and concentrated as it flashes to the lower pres~u,e of the absorber, at Point 1.
The following describes how the state points in Fig. 2 are obtained. Point 2 is defined by conc~ dlion from the level sensor in conju"clion with direct solution temperature measu,en,ent.
The re~ig~ lt level sensor voltage is calibrated at a first startup of the m~thine to acc~lely establish the relationship between the refrigerant level in the t;vapol~tor and the collc~ aliorl of the lithium bromide solution in the absorber.
This shall be done by taking a solution reading at a low and high con~ntration level and associated refrigerant level sensor voltages. The conc~ dlion shall then be interpolated and extrapolated ~c~...,.;.~g a linear relationship between the two points.
Note that the relatio~l~ip b~,lwwn refrigerant level and the voltage is inverse, i.e., for an increasing level there is a decl~cillg voltage input.
Point 2~ is at the same conce,ltldion as Point 2 but at a saturation te~ )c;ldl~re defined by the refrigerant telnp. rd~ . The re~in-ler of the points are calculated by the use of state point equations, crysPlli7~ion line equations, additional sensor information, conc~.,l,dion balances, and mass balances. Points 9X
and 14X are defined by use of cryst~lli7~tion line equation at the solution tempe,~lures of points 9 and 14 les~ ;lively. These calculations are standard calculations which can be easily carried out by those versed in the art.

_9_ Concentration Control O erride and Fault Protection From the above calculation, CONC9 and CONC14 shall be used for overriding the capacity control routine or generating a non-recycle shutdown if the concentration of the lithiurn bromide should become too high. The concentration protection shall consist of an inhibit threshold, a close threshold, and a safety shutdown threshold (points IN, CD and SS, respectively in Fig. 2) for each calculated concellllalion (CONC9 and CONC14). When the calculated concentration exceeds the inhibit threshold, the capacity valve 64 shall be inhibited from opening until the calculated conce~ a~ion drops below the inhibit thresholdminus 0.5 percent. If the calculated concentration exceeds the close threshold, the capa~ily valve 64 shall be closed until it is below the inhibit threshold minus 0.5 percent co~c~ lion. If the c~lc~ te~l concentration exceeds its associated safety shutdown threshold then a non-recycle shutdown with dilution cycle shall be initiated.
The conce~ àtion thresholds associated with each point are as follows:
Point Inhibit Close Fault/Shutdown (% 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 calc~ tion will protect the m~cl~ine and show the usefulness of the invention during operation. In the advent of a power loss, nonnal shutdown is not possible. The invention provides for the storage of data prior to power loss.
This data is COI~al'~Xl to data taken at restoration of power and is used to detçrmine if the solution is cryst~lli7e~1 and if it is safe to restart the m~r~line.

Calculate Projected Crys~D~ Dtion Solution Temperatures TSOL9X = CrysPlli7~tion Line Equation (CONC9X) TSOL14X = Cryst~1li7~tion Line Equation (CONC14X) Calculate Differences and Solution Temperature DIFF9 = TSOL9 - TSOL9X
DIFF13 = TSOL13 - TSOL14X
If (DIFF9 < DIFF13) then .
TSOL9S = TSOL9 - DIFF9 TSOL13S = TSOL13 - DIFF9 Else TSOL9S = TSOL9 - DIFF13 TSOL13S = TSOL13 - DIFF13 Power Loss Determination for Dilution Cycle If ((TSOL9 < TSOL9S) or (TSOL13 < TSOL13S) ) then Alarm State Else If ((TSOL9 < TSOL9S + 25) or (TSOL13 < TSOL13S + 25)) then Power Loss Dilution Cycle = TRUE
Else Power Loss Dilution Cycle = FALSE

The purpose of the above-described invention is not only to prevent over-concentration of the lithium bromide solution in an absorption m~c~line but also to take preventive measures and attempt to m~int~in m~c~ine operation should the CoL~cc~ alion exceed "Normal" opelalillg conditions, thus avoiding unnecess~y ",;..~ e shutdowns. This is accomplished by first detennining the critical statepoints on the ~.,;..-,hi~e opcldliilg cycle. Typical state points of the two stage operating cycle are shown on the previous chart. Two state points 9 and 14 are det~rnined by temperature and pleS~iule sensors located on the m~clline used in conj u.l.;lion with an analog refrigerant level sensor. The level sensor is calibrated during the m--c~ine startup to be a direct indicator of weak solution concentration.
The level sensor has a voltage output which is directly related to refrigerant level.
The refrigerant level is directly related to weak solution concentration.
Two voltage readings are taken corresponding to two or more weak solution collccllllalions. This data is entered into a microprocessor control system. This develops a relationship that will be used to deterrnine weak solution concentration at any opelalil~g condition.
Now other state points can be calculated that will be used to calculate the two designated critical points 9 and 14. These t vo critical points are compaled to points 9X and 14X which are the points where lithium bromide crysti lli7es. Three pre-determined points are established between the critical points (9 and 14~ and the point where lithium bromide crystallizes (9X and 14X at constant lithium bromide temp~ ules). If state points 9 and 14 reach the first predet~rmined point then the machine's capacity control valve is inhibited from opening, indicated in Fig. 2 by point "IN". If the second point "CD" is reached the capacity control valve closes until the critical points move away from the cryst~lli7~tion line. If the third point "SS" is reached, the m~hine will undergo a "SAFEl~' shutdown and go into a dilution cycle.
It is also possible to calculate and display "absorber loss" with the information gathered by the sensors and with the equation used to calculate the state points. Absorber loss is the difference between the refrigerant t~l,.peralure and the lithium bromide saturation temperature in t_e absorber. This difference, defined in degrees Fahrenheit is an indication of m~chine pelr,, . ~ e A further advantage of the present invention is the capability to store data in the event of a power loss to determine n~ hine re~linçss when the power is restored.

Claims (6)

1. An over-concentration control system for use with an absorption machine of the type having one of a single, double and triple effect cooling and heating cycle, and of the type which uses an operating liquid comprising a solution of an ionicsolute in a refrigerant solvent, said operating liquid being characterized by a concentration indicative of the quantity of said solute dissolved in said solvent, and by a phase diagram having a crystallization boundary that defines the combinations of concentration and temperature which correspond to a condition of saturation in said solution, characterized by:
means response to the depth of said solution at a predetermined location in the machine for generating a concentration signal indicative of the concentration of said liquid;
a temperature sensor for generating a temperature signal indicative of the temperature of said liquid;
means responsive to said temperature signal and said concentration signal for calculating a representation of the absorption cycle of said machine which may be plotted on said phase diagram, said representation including a plurality of critical points defined by predetermined respective combinations of concentration and temperature;
means for comparing the actual concentration and temperature of said liquid to concentrations and temperature of said machine which may be plotted onsaid phase diagram, said representation including a plurality of critical pointsdefined by predetermined respective combinations of concentration and temperature;
means for comparing the actual concentration and temperature of said liquid to concentrations and temperatures which lie on said crystallization boundary and for generating a difference signal; and means responsive to the magnitude of said difference signal for changing the operating state of said machine as necessary to prevent said liquid from reacting a combination of concentration and temperature that lies on said crystallization boundary.

2. The system of claim 1 in which the operating liquid comprises a solution of lithium bromide in water.

3. The system of claim 2 in which the means responsive to the depth of the solution comprises an analog switch which contains a float device.

4. An over-concentration control system for use with an absorption machine of the type having one of a single, double and triple effect cooling and heating cycle, and of the type which uses a water solution of lithium bromide as the operating liquid, said operating liquid being characterized by a concentration of lithium bromide in the water, and by a phase diagram having a crystallization boundary that defines the combinations of lithium bromide concentration and temperature which correspond to a condition of saturation in said solution, characterized by:
means responsive to the depth of said solution at a predetermined location in the machine for generating a concentration signal indicative of the lithium bromide concentration of said liquid;
a temperature sensor for generating a temperature signal indicative of the temperature of said liquid;
means responsive to said temperature signal and said concentration signal for calculating a representation of the absorption cycle of said machine which may be plotted on said phase diagram, said representation including a plurality of critical state points defined by predetermined respective combinations of concentration and temperature;
means for comparing the actual concentration and temperature of said liquid to concentrations and temperature of said machine which may be plotted onsaid phase diagram, said representation including a plurality of critical pointsdefined by predetermined respective combinations of concentration and temperature;
means for comparing the actual concentration and temperature of said liquid to concentrations and temperatures which lie on said crystallization boundary and for generating a difference signal; and means responsive to the magnitude of said difference signal for changing the operating state of said machine as necessary to prevent said liquid from reaching a combination of concentration and temperature that lies on said crystallization boundary.

5. The system of claim 4 in which said predetermined location is contained within the evaporator section of the absorption machine.

6. The system of claim 5 in which the means responsive to the depth of the solution comprises an analog level switch which generates a voltage which is converted into a concentration signal for the lithium bromide contained in the solution.