Sarsten, Jan A. (Millington, NJ)
Zacks, Kenneth I. (Oxshott, EN)
Myers, Michael C. (Tripoli, LY)

05/032896

06/13/1972

04/29/1970

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62/77

62/149, 62/292, 137/93, 62/114

F25B9/00; F25B45/00; G05D23/19; F25B45/00

62/77,85,114,149,242,529 73/23.1 137/90,93

O'dea, William F.

Ferguson P. D.

What is claimed is

1. A process for measuring and controlling refrigerant inventory in a multicomponent refrigeration cycle comprising:

2. The process of claim 1 wherein the step of adjusting the amount of the refrigerant comprises manually controlling flow into a line communicating between a source of refrigerant makeup and the closed cycle, when the total computed amount of refrigerant is more than said set value.

3. The process of claim 1 wherein the step of adjusting the amount of refrigerant in said cycle comprises manually controlling flow from an exhaust line communicating with said closed cycle when the total computed amount of refrigerant is less than said set value.

4. A process for controlling refrigerant inventory in a multicomponent refrigeration cycle comprising:

5. The process of claim 4 wherein the step of computing the total amount of each component comprises;

6. The process of claim 4 wherein the step of adjusting the amount of each component of said refrigerant comprises;

7. The process of claim 6 wherein the step of adjusting the amount of each component of said refrigerant is further characterized by the steps comprising;

8. The process of claim 5 wherein the amount of each of said components of said multicomponent refrigerants in the liquid phase is determined on the basis of controlled fixed liquid levels in each of said sensing points.

9. The process of claim 5 wherein the amount of each of said components of said multicomponent refrigerant in the liquid phase is determined on the basis of measured variable liquid levels in each of said sensing points.

10. In a closed refrigeration cycle apparatus comprising a refrigerant flowing across at least one compressor with at least one suction holdup volume upstream and at least one discharge holdup volume downstream of each compressor, said cycle characterized by at least one flash valve and at least one heat exchange stage downstream of said valve, said valve and said stage disposed between said discharge holdup volume and said suction holdup volume, the improvement comprising means for sensing the temperature, pressure, and composition of said refrigerant in the gaseous phase in each of said suction holdup volumes and said discharge holdup volumes, means for communicating said sensed temperature, pressure and composition, means for computing the amount of refrigerant in each of said volumes from said temperature, pressure and composition data transmitted to said computer means by said communicating means, and means responsive to said computer means for adjusting the amount of refrigerant in said apparatus.

11. The improved apparatus of claim 10 wherein said sensing means comprises a thermocouple, a pressure transducer, and a chromatograph for sensing the temperature, pressure and composition, respectively, of said multicomponent refrigerant.

12. The improved apparatus of claim 10 wherein said transmitting means comprises electrical conduits communicating said sensed data from said sensing means to said computing means.

13. The improved apparatus of claim 10 wherein said transmitting means comprises pneumatic conduits communicating said sensed data from said sensing means to said computing means.

14. The improved apparatus of claim 10 wherein said means responsive to said computing means comprises a recorder means for recording the mass of said multicomponent refrigerant in said cycle, and manual means for changing the amount of refrigerant in said system so that said amount of refrigerant is adjusted within a preset range.

15. The improved apparatus of claim 10 wherein said means responsive to said computer means comprises a controller means which controls flow through at least one control valve, said valve disposed in an inlet conduit communicating between a supply of a single component of said multicomponent refrigerant and said closed cycle, whereby said controller means opens said valve in said conduit communicating between a single component and said closed system when said computer means indicates a shortage of said component.

16. The improved apparatus of claim 10 wherein said means responsive to said computer means comprises a controller means which controls flow through at least one control valve, said control valve disposed in an exhaust conduit communicating between said closed cycle and a point outside said cycle, whereby said controller means opens at least one of said control valves when said computer means indicates an excess of refrigerant.

BACKGROUND OF THE DISCLOSURE

This invention is directed to an apparatus and process for controlling refrigeration of a closed cycle. More specifically, this invention is directed to an apparatus and process for controlling a multicomponent refrigeration cycle. Still more specifically, this invention is directed to an apparatus and method for measuring, recording and controlling the inventory of the gaseous phase of a multi-component refrigerant so as to maintain a constant inventory of refrigerant in the refrigeration circuit.

A major problem, in the prior art, relating to closed cycle refrigeration processes has been the problem of refrigeration "makeup" due to losses in the system. Such losses must be made up by additional refrigerant. However, problems arose in complex refrigeration circuits in determining whether refrigerant had been lost and how much, particularly when the load on the refrigeration cycle changed. At those times, it was often impossible for an operator to determine whether changed conditions indicated refrigerant losses, or whether such changes were due to changes in loading conditions. This problem was inherent in the prior art methods for control of refrigeration makeup. In the prior art, refrigeration makeup was usually determined as a function of compressor suction or discharge pressure. These systems worked reasonably well in those cases where the measured variable could be reliably predicted for any change in conditions. However, in complex refrigeration cycles where variable speed centrifugal compressors are employed in refrigeration cycles using multiple refrigeration stages and multicomponent refrigerants, such predictions are difficult to make and not very reliable. In such systems, changes in compressor speed, flow rate and refrigeration load cause changes to the pressure in the system and it is not possible to know to what extent the accompanying continual loss of refrigerant (or gain) was responsible.

This argument can be illustrated by the typical situation where an increased load is imposed on a closed cycle system using a centrifugal compressor. The speed of the compressor is increased. This results in a greater pressure difference across the compressor and/or a greater refrigerant flow rate. This would cause the suction pressure to decrease and the discharge pressure to increase. However, the operator cannot be sure whether some of the resultant decreased suction pressure is attributable to increased refrigerant losses. To compensate for his uncertainty, the operator would bring refrigerant makeup into the system until the suction pressure was brought up to the level existing prior to the change in loading conditions. With suction pressure restored at the now higher compressor speed, discharge pressure would increase even more.

This typical situation results in the design of compressors that must accommodate a wider range of pressures than would have been the case if the suction pressure were allowed to stay at the pressure attained with change in speed.

In the typical situation where a decreased load is imposed on a closed cycle system using a centrifugal compressor, the speed of the compressor would be decreased. This would cause the suction pressure to increase and the discharge pressure to decrease. To compensate for uncertainties in the system inventory, the operator would decrease the suction pressure by venting refrigerant.

These situations result in the need for larger refrigerant storage facilities and larger refrigerant makeup facilities than would be necessary in the case where inventory can be measured, recorded and controlled to fit system losses independent of changing processing conditions.

BRIEF SUMMARY OF THE INVENTION

The apparatus and process of the instant invention is designed to overcome the problems described above. The instant invention is directed to an apparatus and method wherein the amount of refrigerant in a refrigeration cycle is continually measured, recorded, and controlled so as to maintain a constant inventory of refrigerant. In this way refrigeration compressor costs are reduced due to easier system utilization of the entire performance range of the compressor; heat transfer and piping equipment costs are reduced as a result of lower pressure ratings for the same refrigeration requirements; refrigerant makeup facility costs are reduced as a result of a smaller makeup facility; and operating costs are reduced as a result of decreased refrigerant consumption and/or refrigerant standby storage. In accordance with the instant invention, an apparatus and method is provided for controlling refrigerant inventory in a refrigeration cycle in which the temperature and pressure of the refrigerant is sensed at major holdup locations in the cycle where the refrigerant is in the gaseous phase, as well as the percentage composition of each component of the refrigerant if it is a multicomponent refrigerant. This data is transmitted to a computer. The computer calculates the total amount of each component of the refrigerant present in the system and means responsive to the computer output are provided to adjust the amount of each component brought into the system so that the refrigerant inventory in the circuit remains constant.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood by reference to the accompanying drawings of which:

FIG. 1 is a schematic diagram of the logic of the inventory recorder controller of the instant invention;

FIG. 2 is a schematic flow diagram of the instant invention.

DETAILED DESCRIPTION

The apparatus and method of the instant invention has at its central core a computer comprising an inventory recorder and controller. FIG. 1 is an illustration of the logic upon which the computer operates. In order to better understand this logic, a review of the pertinent thermodynamic relationship is required. The number of moles of a gas in a constant volume system is characterized by

N = PV/ZRT (1)

where N is the number of moles of gas, P is pressure, V is volume, Z is compressibility factor, R is gas law constant, and T is the temperature.

It is understood that the compressibility factor Z varies with changes in temperature and pressure. However, the compressibility factor, which adjusts the ideal equation of state for non-ideal gases is taken as a constant for the particular gas, at the particular sensing location, involved. This introduces only minute errors since this factor is taken at the average value, at the particular sensing location, over anticipated conditions. It should further be appreciated that a varying compressibility factor as a function of the temperature and pressure can easily be built into the computer's logic circuit.

Since the volume, compressibility factor and the gas law constant are all constants, at the particular sensing location, the equation above can be simplified as

N = K (P/T) (2) where K = V/ZR (3)

Another relationship relates the total number of moles in a system to the sum of the individual number of moles, of each component, at each sensing location in the system. Thus,

N ₁ + N ₂ + ... + N _n = Total Moles (4)

Substituting equation (2) into equation (4) yields

K _l (P ₁ /T ₁ ) + K ₂ (P ₂ /T ₂ ) + ... + K _n (P _n /T _n ) = Total Moles (5)

Turning now to FIG. 1 which depicts the computer logic for a system comprising n sensing locations, the computer is fed temperature and pressure data from the n sensing locations in the system. This data is operated upon in Blocks 10, 10a and 10b wherein the pressure is divided by the temperature to give a valve of P./T. This data is then sent to Block 11, 11a and 11b. In Block 11, 11a and 11b, the P/T value is multiplied by K. The value K has been defined in Equation 3 above as equal to V/ZR. Of course, K is different for each sensing location due to the different effective volume and compressibility of each sensing location. Each one of these values represents the total number of moles of the individual components present in the refrigerant mixture at that sensing location. In Block 12, 12a and 12b, the individual number of moles of each component in each sensing location is determined by multiplying the value K(P/T) that enters Block 12, 12a and 12b by the percentage composition of each component at that sensing location as determined by a chromatograph monitoring that location.

Thus, assuming a three component mixture, in a three sensing location facility, the output of Block 12, 12a and 12b is (x) (K) P/T, (y) (K) P/T and (z) (K) P/T where x, y and z represent the mole fraction of the three components present in the refrigerant. Thus x ₁ K ₁ (P ₁ /T ₁ ) represents the number of moles of x in sensing location 1; y ₂ K ₂ (P ₂ /T ₂ ) represents the number of moles of y in location 2 etc. It should be appreciated that if the refrigerant were a single component Block 12 would not be present in the computer logic.

The total moles of each component in each sensing location is now combined in Block 13. This yields the total number of moles of each component of the refrigerant in the gaseous phase in the refrigeration circuit. To this sum is added the known or measured amount of refrigerant present in the liquid phase as will be described hereinafter. This sum is broken down by component, so that the results of Block 13 is the total number of moles of each refrigerant component present in the system. This result is one of the outputs of the computer. In addition, these results are inputed to Block 14 where they are totalled to yield the total number of moles in the system, which is another output of the computer. It should be appreciated that the output may be converted in mass units by multiplying each component by that component's molecular weight.

FIG. 2 is a flow diagram of the instant invention. In FIG. 2 a typical closed cycle refrigeration unit generally designated 20 is illustrated. It includes a refrigeration compressor 22. Although for convenience a single stage compressor is illustrated, the instant invention is not limited to a single stage compressor, thus two or more compressor stages may be substituted. The inlet end in communication with the compressor 22, more typically called the suction side, is designated at 32. Through conduit 32 flows the low pressure gas on its way to the compressor 22. The gas is compressed in the refrigeration compressor 22 and is discharged through conduit 34, which communicates with said compressor 22. Conduit 34 represents the discharge end or outlet of the compressor. The gas flowing through conduit 34 is cooled and at least partially condensed, in a heat exchanger 45. The pressurized fluid flows into a low pressure holdup volume 26 by way of a conduit 61.

In the preferred embodiment illustrated in FIG. 2, the low pressure holdup volume 24 is disposed upstream of the inlet or suction conduit 32. In a preferred embodiment, pressure and temperature sensors designated 28 are in communication with said volume 24. The temperature and pressure sensors 28 in FIG. 2 may utilize any convenient measurement means such as thermocouples and pressure transducers respectively. The sensors 28 are in communication with an inventory controller and recorder 21 by means of transmission means 31. In the preferred embodiment illustrated in FIG. 2, transmission means 31 comprises electronic conduits. Alternatively the transmission means 31 may comprise pneumatic conduits. In addition to the temperature and pressure sensors 28, a chromatograph 29, is in communication with the volume 24. Thus, the gas in tank 24 is analyzed in the chromatograph 29 to determine the composition of the multicomponent refrigerant therein. It should be appreciated that if a single component refrigerant were employed, there would be no need for the chromatograph 29 and it would not be included in the apparatus of the instant invention. In the case of a multicomponent refrigerant, chromatograph 29 provides a composition breakdown of the components of the refrigerant. This percentage composition data is provided to the inventory recorder and controller 21 by means of transmission means 33. In the preferred embodiment shown, transmission means 33 is an electronic conduit.

The inventory recorder and controller 21 comprises a computer connected to recording means and control means. It should be appreciated that although it is preferred to have both a recording means as well as a control means in communication with the computer, it is possible to employ the computer of the instant invention with only one of these means, in which case the system is limited to inventory recording or inventory automatic control. Any of the standard computers may be employed as part of the recorder/controller 21. Thus, an electronic analog computer, a pneumatic analog computer, a direct digital computer, or much simplified versions thereof may be employed. No matter which kind of computer is utilized, the logic illustrated in FIG. 1 is programed therein resulting in a total mole or mass output as well as an individual component molar or mass outputs.

Calculated outputs are transmitted by electrical conduit 35 to a recorder 39 which prints out these results. The recorder/controller 21 is also in electrical communication, by means of a pair of conduits 41 and 43, with a pair of control valves 36 and 38, respectively. These valves control addition and subtraction of refrigerant, respectively, into the system. Control valve 36 is disposed at point along the length of a conduit 30. Conduit 30 provides communication between a makeup facility 50 and the low pressure holdup volume 24. Although FIG. 2 shows a single facility 50, it should be appreciated that in the case of a multicomponent refrigerant, a number of makeup facilities 50 equal to the number of refrigerant components would be provided. Obviously, there would be also an equivalent number of conduits communicating between the makeup facilities and the low pressure holdup volume 24 as well as control valves of the type illustrated at 36. It should be understood that the control valve or valves 36 are responsive to a signal generated by the controller 21 and communicated to said valves 36 by means of electrical conduits 41. It should further be understood that the makeup control is on an individual component basis. Similarly, there may be a plurality of outlet control valves 38 though only one is shown for simplicity. Valve 38 is also responsive to the controller 21 by means of electrical conduit 43. Control valve 38 is disposed along the length of a conduit 37 which communicates between the low pressure holdup volume 24 and a location outside the closed cycle 40. It should be appreciated that the subtraction connection to volume 24 is illustrative and in no way limits the location or locations of such subtraction lines. The location of the subtraction lines is dictated by considerations relating to percentage composition of the stream at a particular location as will be described in greater detail hereinafter. Obviously, if a single component refrigerant is employed only a single exhaust subtraction line is used and it may be connected to any convenient point in the system. The location 40 outside the system in a preferred embodiment is a flare. That is, in those cases where the refrigerant is combustible it is burned. In those cases where the refrigerant is not a combustible mixture or where the refrigerant is to be stored, other suitable means to dispose of the refrigerant are provided. Normally, the control mode of the refrigeration cycle would be such that venting or subtraction of refrigerant is minimized for obvious reasons.

Returning now to the closed cycle 20, the discharge end of the compressor 22 is in communication with a high pressure holdup volume 26 by means of a conduit 34. Again holdup volume 26 is monitored by sensors indicated at 60 which sense temperature, pressure and composition of the stream at that location. It should be appreciated that single measuring and single transmission conduit symbols, 60 and 62, respectively, are used for convenience. Obviously, separate measuring instruments are employed and each instrument is separately connected to the computer of the inventory recorder and controller 21. In preferred embodiments thermocouples, pressure transducers and chromatographs are respectively employed to determine these properties. The temperature, pressure and composition values are signaled to the recorder-controller 21 by electrical conduits 62.

Thus, the inventory recorder and controller 21 of the instant invention receives signals from those particular sensing locations in the cycle where the bulk of the gaseous refrigerant is stored and accumulated. It should be appreciated that in a typical discharge high pressure holdup volume of the kind illustrated at 26 in FIG. 2, the refrigerant fluid, if multicomponent, is a two-phase mixture. In the preferred embodiment contemplated by the embodiment illustrated in FIG. 2 such is the case. In that case, the level of liquid is controlled (not shown). The computer of the inventory recorder and controller 21 is programmed to account for the moles of liquid in the fixed volume set by the level controller so that the total inventory of refrigerant is accounted for. Furthermore, the computer can be programmed by the use of well known solution and chemical thermodynamic relationships to determine the exact composition of the liquid phase whose volume is predetermined by the level controller. This is calculated from the data already known, that is, temperature, pressure, and composition of the gaseous phase combined with the knowledge that the liquid and gaseous phase are in equilibrium. If the liquid level is not controlled at a fixed level but allowed to fluctuate for other process reasons, the level could be measured and the output of the measurement sent to the computer of the controller-recorder 21 for liquid component calculation.

It should further be appreciated that although the embodiment illustrated in FIG. 2 only shows two holdup volumes, in more complex refrigeration cycles, there are additional holdup volumes in proportion to the number of stages of compression and/or stages of refrigeration. It should be obvious that any additional volumes employed in more complex systems would be similarly monitored and controlled by the inventory controller.

Obviously, the instant invention is not directed to precise but rather approximate refrigeration monitoring which is all that is required for most practical applications. The selection of a finite rather than infinite number of sensing locations, and the use of average properties at each of the sensing locations, results in an inventory controller that is accurate within ±5 percent in a typical practical application. Thus, precision is more than that required to adequately control changes in refrigerant inventory.

The remainder of the closed cycle 20, that is the cycle between the high pressure discharge holdup volume 26 and the low pressure suction holdup volume 24 is illustrative of a simple refrigeration system. Thus, the embodiment illustrated in FIG. 2 should not be considered limiting but is rather a minimum type of cycle.

In the instant embodiment, a first refrigerant stream flows in a conduit 42 which communicates between the liquid phase of the refrigerant contained in the discharge holdup volume 26 and a flash valve 44. The liquid refrigerant is flashed adiabatically across the valve 44. The cold refrigerant next enters a heat exchanger 50 by way of a conduit 46 which communicates between flash valve 44 and the heat exchange 50. The refrigerant is heated while cooling a stream generally indicated at 52 in the heat exchanger 50. The stream 52 represents the load that refrigeration cycle 20 is designed to cool. The heated and now gaseous refrigerant stream flows from the exchanger 50 back into the suction holdup volume 24 by means of a conduit 48.

A second refrigerant stream flows in a second conduit 53 in communication with the volume 26. Conduit 53 directs the vapor phase of the refrigerant contained in the discharge holdup volume 26 to a heat exchanger 54. In exchanger 54 the vapor phase refrigerant stream is cooled and partially condensed by a portion of the flashed refrigerant stream flowing in conduit 46. The cooled stream exiting the heat exchanger 54 enters a conduit 60 which directs the stream to a flash valve 55. The two-phase stream is flashed adiabatically across valve 55. The flashed refrigerant flows out of the valve through a conduit 56 to another heat exchanger 57. The flashed stream is heated in exchanger 57 while further cooling the load stream 52. The heat refrigerant stream is then remixed with the first refrigerant stream in conduit 46 by way of a conduit 62 which communicates between the heat exchanger 57 and conduit 46. A fraction of two refrigeration streams in conduit 46 is bypassed through a conduit 64 to provide the cooling stream in heat exchanger 54. This stream is heated therein exiting through a conduit 66. Conduit 66 directs the heated refrigeration stream back to the remainder of the refrigeration stream in conduit 48. The combined stream then enters volume 24 by way of said conduit 48.

In operation, the computer of the inventory recorder and controller 21 is programmed as previously described (see FIG. 1). One additional input into the computer program could be the desired set values for the amount of each component of refrigerant in the system. This input is omitted in the special case where the computer is used only in conjunction with a recorder as will be obvious from the following description.

Operation of the cycle 20 begins with the startup of the compressor 22. The temperature and pressure and if a multicomponent refrigerant is used, the composition, are then continually monitored at all the sensing locations in the closed cycle. The data is fed into the computer of the recorder-controller 21 where the refrigerant inventory is summed as described above taking into account the set values for the liquid fraction. This data is operated on by the computer continuously or at preset intervals. The computer output constituting the inventory of the refrigerant in the cycle 20 is printed out by the recorder 39.

Control over refrigerant inventory is maintained by one of three ways. In the first of these methods, a controller is employed. The controller of the inventory recorder-controller 21 comprises a plurality of devices responsive to the computer which are connected to said devices. The computer, continuously or during each preset time interval, compares the total molar inventory as well as the total component molar inventory, if a multicomponent refrigerant is used, with the preset values set into the program. If one or more of the components are found to be in short supply, a signal is sent by means of an electrical conduit such as that illustrated at 41 in FIG. 2 to one or more of the control valves of the type illustrated at 36. This opens the valve or valves permitting the flow of one or more components into the system. The signal is discontinued when the calculated value for the component comes up to the preset value. Alternatively, if it is found, and this is much less likely to occur, that one or more components are in oversupply in the cycle, a signal is sent to a conduit of the type illustrated at 43 to a control valve of the type illustrated at 38 to open the valve and allow the refrigerant to flow through a conduit 37 to exhaust 40. The signal is discontinued, closing the valve 38, when the calculated quantity of the refrigerant component comes within the range of the present value. It should be appreciated that although FIG. 2 illustrates an exhaust at a low pressure suction holdup volume 24, this should not be considered a limiting location for the exhaust system. Thus, exhaust lines may be disposed throughout the cycle 20. In this way it is possible to selectively exhaust refrigerant rich in one or more components. For example, if a multicomponent refrigerant were used and it was found that some of the lowest boiling components were in oversupply, a line from the top of the high pressure discharge volume 26 could be in communication with an exhaust line and that line would be opened by the controller 21 to discharge the lower boiling components which are obviously in the gaseous phase in volume 26.

The second control method encompasses the employment of the recorder of the recorder-controller 21 of the instant invention. The recorder-controller 21 records the total molar or mass composition of the refrigerant. The operator monitors the recorder in conjunction with a chromatograph, and when one or more components is in over or under supply in the cycle 20, he manually adjusts the valves to input or exhaust any of the components individually. As an example, the operator might be told to maintain refrigerant component control within 10 percent of a preset value.

A third possible method of control entails the combined use of both the manual, or recorder, method of control and the automatic or controller method of control. In this method, the system is manually controlled with an automatic control backup. This method may best be explained by illustration. In a typical operation employing this third method of control, the operator controls refrigerant inventory within a fixed range, say 10 percent. At the same time, the controller is preset to feed or remove refrigerant if the amount of refrigerant varies outside some range greater than the fixed range set for manual control, say 25 percent. This method may be preferred in some applications because of its redundancy which therefore provides greater assurance against failure.

The embodiment illustrated in FIG. 2, as stated above is illustrative of any closed cycle refrigeration system. Such a system can be used for example in a natural gas liquefaction plant where the refrigerant is a multicomponent mixture of C ₁ to C ₅ hydrocarbons and nitrogen.

It should be understood that the preferred embodiment described above in no way limits the scope of the invention. Thus, it should be understood that the apparatus and method of the instant invention may be modified without departing from the scope and spirit of the invention.