METHOD AND APPARATUS FOR CONTROLLING REFRIGERANT FLOW IN A REFRIGERATION SYSTEM

Abstract: A water chiller refrigeration system is disclosed having a refrigerant expansion valve which is controlled by an electric digitally operated stepper motor. The system includes a first temperature sensor for generating an electrical analog signal corresponding with the saturated refrigerant temperature in the evaporator and a second temperature sensor for generat­ ing an electrical analog signal corresponding to the tempera­ ture of refrigerant entering the compression means of the compressor after having passed in heat exchange relation with the compressor motor. A microcomputer receives the generated temperature signals and provides a digital output signal to the stepper motor to rapidly and precisely adjust the refrigerant flow from the condenser to the evaporator in accordance with the actual superheat of refrigerant entering the compression means.

Description: METHOD AND APPARATUS FOR CONTROLLING REFRIGERANT FLOW IN A REFRIGERATION SYSTEM Background of the Invention This invention relates to refrigeration systems wherein the flow of refrigerant from the refrigerant condenser to the refrigerant evaporator is governed by an adjustable expansion valve which is responsive to a temperature condition of the refrigerant to be compressed. Refrigeration systems must be designed so as to prevent liquid refrigerant from entering the compressor cylinders along with refrigerant vapor because this condition, known as "slugging" or "flooding", can result in serious damage to the compressor. Ideally, the refrigerant expansion valve controls the passage of refrigerant to the evaporator so that all of the refrigerant is evaporated prior to leaving the evaporator. Prior refrigeration systems have frequently controlled the flow of refrigerant through the expansion valve by sensing the refrigerant pressure or equivalent temperature and the temperature of superheated refrigerant leaving the evaporator. This difference in temperature is known in the art as "superheat", and prior systems have usually been designed to require a large safety margin of superheat in the refrigerant leaving the evaporator so as to avoid the slugging condition previously referred to. A large safety margin is required because of previously unaccounted for system variables affecting the actual superheat of refrigerant entering the compressor cylinders and due to the practical difficulties in sensing low refrigerant superheats and the sluggishness of the response of prior expansion valves to changes in refrigerant superheat. This safety margin is provided by utilizing excess evaporator heat exchange surface to assure complete evaporation of the refrigerant or, in other words, by restricting the flow of refrigerant to less than optimum for the heat exchange surface that is present in the system, thereby restricting the capacity of the system for a given energy input. In many refrigeration systems the refrigerant vapor leaving the evaporator is passed in heat exchange relation with the compressor motor prior to entering the compression section of the compressor so as to cool the motor and to further assure that any liquid refrigerant which accidentally may have reached the compressor due to sluggish control system response is evaporated prior to entering the compressor cylinders. Passing the refrigerant over the compressor motor adds additional superheat to the refrigerant entering the cylinders of the compressor. In prior systems, this additional amount of superheat has not been taken into consideration in adjusting the expansion valve because it is not reflected in the temperature of refrigerant leaving the evaporator, which is the common way in which superheat is sensed. Summary of the Invention In accordance with the present invention the flow of refrigerant from the condenser to the evaporator is governed by an expansion valve which is controlled in response to the superheat of refrigerant entering the compression section after having passed in heat exchange relation with the compressor motor. This arrangement makes possible more efficient utilization of the evaporator surface because a greater amount of refrigerant may be passed to the evaporator, or a lessor amount of evaporator surface may be designed for a given capacity system, because the superheat may be sensed with greater accuracy and the superheat added to the refrigerant by the compressor motor is taken into consideration in controlling the quantity of refrigerant supplied to the evaporator. In the preferred embodiment, the superheat of the refrigerant is sensed at a point just prior to the entrance of the refrigerant into the cylinders of a reciprocating compressor by a thermistor which provides an electrical analog signal to a microcomputer that rapidly adjusts the position of the expansion valve by providing a digital output signal to a stepper motor which incrementally controls the position of the valve and flow of refrigerant to the evaporator. This arrangement provides very rapid response to changes in superheat so that a relatively low safety margin of superheat may be maintained without danger of slugging the compressor, and a consequent further improvement in capacity and efficiency of the system. Brief Description of the Drawings Figure 1 is a schematic illustration of a refrigeration system, and its associated control, embodying the present invention. Figure 2 is an illustration, partly in cross-section, of a refrigeration compressor having a temperature sensor at a preferred location for measuring superheat. Figure 3 is a flow chart illustrating the basic logic embodied in a microprocessor based control which embodies the present invention. Description of a Preferred Embodiment This invention will be described with reference to a refrigeration system, commonly called a water or brine chiller, which uses an air cooled condenser, a reciprocating compressor and an evaporator such as a chiller vessel for directly expanding refrigerant in heat exchange with water or brine being chilled. However, it will be understood that the invention is equally applicable to heat pumps, or to machines whose primary purpose is to provide heating or which utilize liquid cooled condensers or other types of hermetic compressors. Also, while the invention will be described with respect to a direct expansion evaporator vessel for chilling water or brine on the exterior of the heat exchange tubes therein, the system may employ a flooded evaporator having the refrigerant on the outside of the tubes or an evaporator for directly cooling air or other fluids. Furthermore, a system of the type described may, in practice actually employ one or more chiller vessels, compressors, and condensers arranged in parallel or staged refrigerant circuits to provide the desired refrigeration or heating capacity. Also, the invention will be described with reference to the preferred microprocessor based control system driving an electrically actuated stepper valve motor, but it will be understood that mechanical, electrical, pneumatic or other controls may be used within the scope and spirit of the invention. Referring particularly to Figure 1, there is illustrated a refrigeration system 1 having a control 2. Refrigeration system 1 comprises a reciprocating compressor 4, an air cooled condenser 6 having a motor driven fan 12 for passing air over the tubes of the condenser, refrigerant expansion valve 8 controlled by motor 9, and an evaporator 10. Evaporator 10 comprises a cylindrical vessel having a shell 14 with tube sheets 18 and 19 disposed adjacent the ends thereof supporting a plurality of heat exchange tubes 16. Tube sheet 18 is spaced from one end of the evaporator vessel to form a refrigerant header 20. Tube sheet 19 is spaced from the other end of the evaporator vessel and together with horizontal partition 23 forms a refrigerant inlet header 21 and a refrigerant outlet header 22. One or more internal baffles 26 are usually provided within shell 14 for directing the water or brine to be chilled in a desired path within the interior of the vessel for effective heat transfer with heat exchange tubes 16. In operation, refrigerant vapor is withdrawn from refrigerant outlet header 22 of evaporator 10 through suction passage 30 by compressor 4. The refrigerant vapor is compressed in compressor 4 and passed through hot gas passage 31 into condenser 6 where the refrigerant is condensed to a liquid by heat exchange with a cooling medium such as air. The liquid refrigerant passes through liquid passage 32, having expansion valve 8 therein, into the refrigerant inlet header 21 of evaporator 10. The expanded low pressure refrigerant flows from header 21 through a portion of heat exchange tubes 16 into refrigerant header 20, from which it enters the remaining heat exchange tubes 16 and passes to refrigerant outlet header 22. The refrigerant passing through heat exchange tubes 16 evaporates therein and cools the water admitted into the evaporator vessel through warm water inlet passage 28. The water which has been cooled in the evaporator vessel then passes to a desired location through chilled water outlet passage 29 to provide cooling at a desired location. Control system 2 comprises, in its preferred form, a microcomputer 34 having suitable microprocessor, memory, input/output and power switching devices to control a digitally controllable electric stepper motor 9, which in turn incrementally adjusts the opening and closing of expansion valve 8 to incrementally control the flow of refrigerant from condenser 6 to evaporator 10. Microcomputer 34 acquires analog temperature input signals from thermistors 36 and 38 and processes those input signals to generate a digital output signal to actuate stepper motor 9. A suitable microcomputer controlled expansion valve and associated stepper motor are more fully described in our copending application serial number (Attorney Docket No. 5414). Thermistor 36 is a part of evaporator probe assembly 40 which is located in the refrigerant inlet header of evaporator 10, or some other suitable location for sensing the saturated refrigerant temperature in the evaporator. Probe assembly 40 comprises a probe 41 containing thermistor 36 which is suitably inserted via a bushing and seal assembly 44 into refrigerant inlet header 21 to sense the temperature of refrigerant entering the evaporator. This temperature corresponds to the saturation temperature of the expanded refrigerant, prior to complete evaporation of the liquid refrigerant within heat exchange tubes 16. Thermistor 38 is a part of compressor probe assembly 42 which senses a refrigerant temperature at a desired location within the shell of compressor 4. Referring now to Figure 2, compressor 4 is shown in more detail. Compressor 4 may be of the hermetic or semi hermetic variety, and includes a compressor housing 58 such as a cast or drawn shell. Housing 58 encloses an electric motor means 60 which is illustrated as being of the induction type, and a compression means 62 which is illustrated as being of the reciprocating type. Electric motor means 60 has a plurality of stator windings 66 and a squirrel cage rotor member 68 connected to a rotor shaft 70 for imparting torque to crank shaft 72 which is journaled in bearings 71 supported by housing 58. Compression means 62 comprises one or more cylinders 76 within which are disposed a like number of pistons 74. Pistons 74 are connected to crank shaft 72 by connecting rods 73 which reciprocate the pistons withinthe cylinders to compress refrigerant vapor. Refrigerant vapor is admitted to the cylinders through refrigerant suction valves 78 and is discharged from the cylinders through refrigerant discharge valves 80 associated with each of the cylinders. A refrigerant suction inlet passage 84 is provided to admit refrigerant into the compressor from suction passage 30 and a compressed refrigerant vapor outlet passage 86 is provided to discharge compressed refrigerant into hot gas passage 31 of the refrigeration system. Cold refrigerant gas from evaporator 6 passes into compressor 4 from suction passage 30 through refrigerant vapor inlet 84, and passes in heat exchange relation with motor 60 to cool the motor. The refrigerant vapor passing in heat exchange relation with motor 60 absorbs heat from the motor and then passes through suction manifold 88 into the cylinders of compression means 62. Suction manifold 88 provides a refrigerant vapor passage which lies downstream of electric motor 60 and upstream of suction valves 78 and compressor cylinders 76. The refrigerant vapor entering suction manifold 88 has therefore absorbed additional superheat from motor 60 prior to entering cylinders 76 of the compression means. The refrigerant vapor entering cylinders 76 is compressed by reciprocation of pistons 74 and the compressed refrigerant then passes through refrigerant discharge valves 80 into refrigerant outlet passage 86, from which it passes through hot gas passage 31 to condenser 6. Compressor probe assembly 42 is preferably located in suction manifold 88 and comprises a suitable bushing and seal assembly 45 having temperature probe 43 extending into suction manifold 88. Thermistor 38 is located in probe 43 and senses the temperature of the superheated refrigerant vapor passing downstream of motor means 60 after having passed in heat exchange relation with motor 60 and prior to entry of the refrigerant vapor into cylinders 76 of compression means 62. At this point, the refrigerant vapor in thoroughly mixed and substantially homogeneous so that an accurate indication of superheat may be obtained. Probe 43 may be alternatively located at any other suitable location within compressor shell 58 so that it senses the temperature of refrigerant vapor which has been superheated by heat exchange with motor 60 prior to its entry into compression means 62. In the preferred embodiment of this invention thermistors 36 and 38 generate analog electrical temperature signals which are processed by microcomputer 34 to provide a digital output to an electric stepping motor which incrementally controls the position of expansion valve 8. Alternatively any temperature sensitive resistance or other element may be employed instead of the thermistors to sense the required temperatures or pressures equivalent thereto. Figure 3 illustrates a flow diagram of a basic program for accomplishing the signal processing. In step 101 the temperature sensed by thermistor 36, which corresponds to refrigerant saturation temperature in the evaporator, is read and recorded as temperature tl. In step 102 the temperature sensed by thermistor 38 is read and recorded as superheated refrigerant temperature t2. In step 103 the absolute present position of the expansion valve is recalled from memory and inputted into the microprocessor. In step 104 the superheat of the refrigerant entering the compression means, sensed by thermistor 38, is computed by subtracting tl from t2. In step 105 a suitable valve opening and closing algorithm is computed to generate a valve change or error signal for opening and closing of expansion valve 8. The particular algorithm may be imperically determined as a function of the desired superheat to be maintained and the characteristics of the refrigeration system and of the valve and its associated stepper motor. For example, it may be desired to maintain approximately 150F of superheat in the refrigerant entering the compression means of the system and therefore, t2 - tl 15C should desirably equal zero. For values of t2 - tl 15C which differ from zero the deviation may be adjusted by various sytem functions to derive a valve change signal for opening or closing the valve to the desired position to restore the superheat to 150F It will be appreciated that various characteristics of the system can be taken into consideration in generating the valve change signal, including the non-linear flow characteristics of the valve, and the rate at which it is desired that the valve adjust the refrigerant flow for a given deviation from the desired superheat. Also, other sensed parameters of the system may be supplied to the microcomputer to override or modify the valve change signal derived from the sensed superheat, such as an excess or insufficient system pressure, an excess system temperature, excess motor current, system load changes and other conditions which may require a modification of the refrigerant flow. In step 106 the system logic computes the new expansion valve position which is desired from the present valve position recalled in step 103 and the valve change signal derived from step 105. For example, stepper motor 9 may be a, bipolar electric stepper providing 760 steps between the completely closed valve position and the completely open valve position. At some point in the operation of the refrigeration system the valve may be completely closed to establish a zero reference position for the valve. Thereafter the microcomputer remembers the absolute position of the valve as each valve change signal is applied to the stepper motor to open or close the valve from its previous position. In step 107 the logic computes the number of valve motor steps which should be added or subtracted from the current position of the valve in order to provide the desired refrigerant flow to the evaporator. This output signal which is generated by the microprocessor is then applied to a suitable output driver logic stage 108 which applies a digital output signal to pulse the appropriate windings of stepper motor 9 to cause it to incrementally open or close expansion valve 8 by the desired number of steps. While a digitally controllable electric stepper motor controlled expansion valve is illustrated as being the preferred embodiment of this invention, expansion valve 8 may be adjusted by any suitable valve actuator means coupled with an appropriate control system responsive to the sensed superheat condition. However, an electric stepper motor controlled expansion valve is particularly advantageous because of the precision and rapidity of its response and because it is especially well adapted to control by a microcomputer which may set a precise desired position of the valve in accordance with all sensed and determined system characteristics. However, mechanical, electrical, pneumatic and other systems of control may. be utilized to control the position of the expansion valve in response to a superheat signal which is derived from refrigerant passing from the motor means into the compression means of the refrigerant compressor, so as to take into account the superheat added by heat exchange of the refrigerant with the compressor motor. By taking into account the superheat added to refrigerant vapor by the compressor motor it is possible to operate the refrigeration system with a smaller amount of superheat in the refrigerant leaving the evaporator without the danger of refrigerant liquid entering the compression means where it may cause damage. This is advantageous because it is possible to design the evaporator with a smaller amount of heat exchange surface due to the fact that less superheat is required to be added by the evaporator. Furthermore, much more accurate control of the refrigerant flowing through the system is possible by sensing the superheat at the critical location in the system between the motor and compression sections of the compressor. Previously, the amount of superheat required to be provided by the evaporator was more or less of an approximation based on assumptions concerning the safe superheat of refrigerant entering the compressor rather than the actual superheat of refrigerant entering the compression section. To ensure safe operation of the system, a very large safety factor was necessary, resulting in the system generally operating with greatly excess superheat under most conditions. Applicants invention enables this superheat to be more precisely determined and the refrigerant flow quickly corrected in the event of any abnormal or other changes in operating conditions which causes the superheat of the refrigerant entering the compression means to deviate from that anticipated by design assumptions. Consequently, the system may be designed to operate safely with a much smaller margin of excess superheat. Furthermore, it is difficult to reliably sense or measure low values of superheat. Consequently, the measurement of superheat at the outlet of the evaporator, which may desirably be on the order of less than 5 F, further contributes to the necessity of a large safety factor to prevent flooding of the compressor with refrigerant liquid. However, in accordance with Applicant's invention the superheat of the refrigerant is sensed at a point within the compressor where it enters the compression section, at which point the superheat is relatively high, as on the order of 150F, and it can be easily measured with satisfactory accuracy. This ability to more accurately sense the actual superheat enables a further reduction in the necessary superheat safety factor. By use of the preferred digitally controlled expansion valve, the changes in refrigerant flow may be accomplished with far greater accuracy and speed than prior systems, thereby further enabling a lower margin of safety in the superheat of the refrigerant leaving the evaporator. Consequently, this invention enables the evaporator heat exchange surface to be significantly reduced, or a given evaporator to safely provide a greater refrigeration capacity for a given energy input than previous sytems, due to the precision and speed with which refrigerant flow in the system is controlled. That, in turn, enables the refrigeration system to have a lower initial cost, a higher capacity and increased energy efficiency. It will be appreciated that this invention may be otherwise embodied within the scope of the following claims.

Claims: CLAIMS What is claimed is: 1. A refrigeration system comprising: (A) an evaporator for evaporating liquid refrigerant to provide cooling; (B) a condenser for condensing refrigerant vapor; (C) a compressor for compressing refrigerant vapor formed in the evaporator and for passing the compressed refrigerant vapor to the condenser, said compressor including a housing, compression means disposed within said housing for compressing refrigerant vapor, motor means disposed within said housing for driving said compression means, refrigerant passage means within said housing for passing refrigerant vapor through said housing in heat exchange relation with said motor means to said compression means, to cool said motor means prior to passing into the compression means; (D) an adjustable refrigerant expansion valve for controlling flow of refrigerant from the condenser to the evaporator; and (E) control means for adjusting the refrigerant expansion valve in response to a function of the temperature of the refrigerant passing from said motor means into said compression means. 2. A refrigeration system as defined in claim 1 wherein the control means includes a temperature sensor disposed in said housing at a location in said refrigerant passage means for sensing the temperature of the refrigerant entering said compression means. 3. A refrigeration system as defined in claim 1 wherein the control means comprises means for adjusting the refrigerant expansion valve in response to a function of the superheat of the refrigerant entering said compression means. 4. A refrigeration system comprising: (A) an evaporator for evaporating liquid refrigerant to provide cooling; (B) a condenser for condensing refrigerant vapor; (C) a compressor for compressing refrigerant vapor formed in the evaporator and for passing the compressed refrigerant vapor to the condenser, said compressor including a housing, compression means disposed within said housing for compressing refrigerant vapor, motor means disposed within said housing for driving said compression means, refrigerant passage means within said housing for passing refrigerant through said housing in heat exchange relation with said motor means to said compression means, to cool the motor means prior to passing into the compression means; (D) an adjustable refrigerant expansion valve for controlling flow of refrigerant from the condenser to the evaporator; ; (E) control means for adjusting the refrigerant expansion valve in response to a function of the temperature of the refrigerant passing from said motor means into said compression means, said control means comprising: (1) a first temperature sensor disposed at a location in said system for sensing a first temp erature corresponding with the saturation temperature of refrigerant in said evaporator; (2) a second temperature sensor disposed in said housing at a location for sensing a second temperature corresponding with the temperature of the refrigerant passing through said housing at a location between said motor means and said com pression means; and (3) signal processing means connected for providing a superheat control signal to said expansion valve which is a function of the difference between said second and said first sensed temp eratures; and (F) a valve motor connected to said adjustable refrigerant expansion valve for adjusting the refrigerant expansion valve in response to the superheat control signal generated by said signal processing means. 5. A refrigeration system as defined in claim 4 wherein: (A) said first and second temperature sensors comprise temperature senstive elements for providing first and second analog electrical temperature signals; (B) said signal processing means comprises a microcomputer programmed and connected to provide a digital superheat control signal to said valve motor in response to said first and second analog temperature signals; and (C) said valve motor comprises a digitally controllable electric stepper motor which is actuated in response to the digital superheat control signal generated by said microcomputer, to incrementally adjust the refrigerant expansion valve in response to the digital superheat control signal. 6. A method of controlling the flow of refrigerant in a refrigeration system having a condenser, an adjustable refrigerant expansion valve, an evaporator and a compressor, wherein the compressor includes a compressor housing enclosing a compressor motor and a compression means driven by the compressor motor, which comprises the steps of: (A) passing refrigerant from the evaporator in heat exchange relation with the compressor motor to cool the motor prior to passing the refrigerant into the compression means; (B) sensing the temperature of refrigerant passing through the compressor housing from the compressor motor into the compression means; (C) generating a control signal which is a function of the sensed refrigerant temperature; and (D) controlling the passage of refrigerant from the condenser to the evaporator of said refrigeration system by adjusting the refrigerant expansion valve in response to said control signal. 7. A method of controlling the flow of refrigerant in a refrigeration system as defined in claim 6, wherein the step of generating a control signal comprises generating a control signal which is a function of the superheat of refrigerant passing from the compressor motor into the compression means. 8. A method of controlling the flow of refrigerant in a refrigeration system as defined in claim 6, wherein: (A) the step of generating a valve control signal comprises generating a digital control signal; and (B) the step of controlling the passage of refrigerant from the condenser to the evaporator comprises incrementally adjusting the refrigerant expansion valve in response to the digital valve control signal. 9. A method of controlling the flow of refrigerant in a refrigeration system having a condenser, an adjustable refrigerant expansion valve, an evaporator and a compressor, wherein the compressor includes a compressor housing enclosing a compressor motor and a compression means driven by the compressor motor, which comprises the steps of: : (A) passing refrigerant from the evaporator in heat exchange relation with the compressor motor to cool the motor prior to passing the refrigerant into the compression means; (B) sensing a first temperature in said system corresponding with the saturation temperature of refrigerant in the evaporator, and generating a first temperature signal which is a function of said first temperature; (C) sensing a second temperature within the compressor housing which corresponds with the temperature of refrigerant passing from the compressor motor into the compression means, and generating a second temperature signal which is a function of said second temperature; (D) generating a valve control signal which is a function of the difference in said second and first temperature signals; and (E) controlling the passage of refrigerant from the condenser to the evaporator of the refrigeration system by adjusting the refrigerant expansion valve in response to the valve control signal. 10. A method of controlling the flow of refrigerant in a refrigeration system as defined in claim 9, wherein: (A) the step of generating a superheat control signal comprises generating a digital superheat control signal; and (B) the step of controlling the passage of refrigerant from the condenser to the evaporator comprises incrementally adjusting the refrigerant expansion valve in response to the digital superheat control signal.

Inventors: LORD, RICHARD G., SCHEDEL, JOHN W.
Applicants: CARRIER CORPORATION
Application year: 1984
Main classification: G05D23
IPC codes: G05D23, F25B1, F25B41