Patent Publication Number: US-3875757-A

Title: Expansion valve for preventing hunting in refrigeration system

Description:
United States Patent Amano Apr. 8, 1975 [54] EXPANSION VALVE FOR PREVENTING 2.484.156 10/1949 Duke 62/21 I HUNTING 1 REFRIGERA&#39;HON SYSTEM 2.505.933 5/1950 Aughcy 236/92 B R26.l6l 2/1967 Leimhach 62/225 Irimur E.\&#39;uminerMeyer Perlin Attorney. Again. or Firm-Woodhams. Blanchard and Flynn [57] ABSTRACT An expansion valve to be adaptable to the refrigerating and freezing system which is designed for effectively preventing and/or eliminating pulsatory oscillation or hunting with respect to temperature and/or pressure as encountered during operation of the refrigerating and freezing system by using a specific diaphragm of minutely adjustable construction on the basis of graphic approach to analyze and meet basic physical factors in combination to cause and constitute such hunting so that such involved factors may be quickly and optimally met and lead to a physically equilibrium state of the whole refrigeration system. thereby to effectively prevent and/or eliminate such hunting.  
 10 Claims. l8 Drawing Figures [75] lnventor: Naonori Amano, Tokorozawa. Japan [73] Assignees: Kabushiki Kaisha Saginomiya Seisakusho, Tokyo; Tokyo Sanyo Electric Co., Ltd., Oizumi Gunma Pref, both of J apan [22] Filed: Jan. 17. I973 [2]] App]. No.: 324,283  
 [30] Foreign Application Priority Data Jan. I). [972 Japan r. 474106895 Dec. 5. 1972 Japan 47-12! I86 52 us. Cl. 62/]!5: 62/225: 236/92 {51] Int. Cl. F25b 4l/04 [58] Field of Search 62/222. 223, 224. 225; 236/92 [5m References Cited UNITED STATES PATENTS 2.ll 2.7l8 l2/l939 Anderson 63/225 2.297.872 lU/l942 Carter 62/2l l PATENTEDAPR ems $875,757 saw 1 or 7 FIG. 2  
 2 The second quadrant Yh The firstquadrant XIQ q I TBO X 00 1; TEO V The third quadrant Y The fourth quadrant FIG. 3  
 Superheat Reverse flow of Q the refrigerant PATENTEUAPR W5 SHEET 2 F. 7  
 FIG- 4 I) M12 M11 I! l/ I l V M l 21 M22 1, A TBo Q h J ff y TE M23 M24 FIG. 5 s erhem Reverse flow of h ihe Pefrigemm Q TBo Q T51 SuD F I 6 :4  
  Rever How of the ref h l M12 IIM I II I! [I I! 1/ 1 .T 0 n TB d T81 TE M uP M2 24 W3 M14 Reverse flow of The rva PATENTED 31975 SHLEI 3 BF 7 h \JZ G 7 K Mu (T50 Q l T81 Superheot Reverse flow of G 8 the refrigerant Q T51 TB N TE &#34;F VH3 M14 FIG 9 Superheof Reverse flow of rhe refrigerant Q 4 Ah ho Q Q1 Q T80 T8] T8 TE *AQ PATENTEDAPR 8|975 tn; 4 JI l h FIG. 10  
  INFLECTION POINTS IN FLECTION POINTS O TB TE FIG. 14  
  TB TE TE TB SSH+L SHC OSH OSHI Operating superheor 8 HC I Superheot change Static superheat EXPANSION VALVE FOR PREVENTING I-ILNTING IN REFRIGERATION SYSTEM BACKGROUND OF THE INVENTION This invention relates to an expansion valve for use in a refrigeration system. More particularly. this inven tion is concerned with a new and useful expansion valve adapted to be used in a refrigerating and freezing system. which expansion valve is specifically designed to meet and prevent occurrence of periodic instability with respect to pressure and/or temperature to be encountered during the operation of the system (hereinafter referred to as hunting&#34; in this specification).  
  When there occurs a hunting in the refrigerating and freezing system during its operation. it is a conventional practice to tighten the adjusting spring of the expansion valve in the system so as to increase a static superhcat. thereby to increase a supcrheat of an evaporator so as to stabilize a temperature at a temperature sensing bulb provided at the outlet of the evaporator. to conse qucntly diminish the periodic opcning-and-closing of the expansion valve. whereby the hunting phenomenon in the refrigerating and freezing system will be cause to cease. In such conventional practice. however. it is inevitable that the capacity of the refrigeration system will necessarily be decreased to a considerable extent.  
  When the hunting in the refrigeration system will not be put to an end even with such a series of operational procedures. it will further be required to replace the expansion valve with another one having a smaller refrigerant outlet aperture, or apply a heat sensing bulb filled with a specific gas therewithin. or else change the time constant of the response velocity of the temperature sensing bulb in response to the fluctuations of temperature. as disclosed in the Japanese Pat. publication No. 23825/l972. thereby putting the hunting phenomenon to an end.  
  On the other hand. in order to meet and eliminate the hunting phenomenon by increasing the supcrheat. while preventing the refrigerating performance of the system from decreasing past a predetermined point. such attempts are required on the part of the refrigerating system as to increase the capacity of the evaporator itself. or improve the so-called evaporator balance by properly correcting the diversion state of the refrigerant entering into the evaporator as well as the unbalanced heat load. thereby to attain the purpose of preventing the hunting phenomenon from occurring. However. since such counter-measures are not yet well established in a quantitative order. it is inevitable that the operator will at his option have to meet a hunting. when it occurs. by appropriately applying the above mentioned procedures. and if any of such procedures turn out to be useless in its effect. then any other procedures will be tried. thus resulting in such trail-and-crror circles which will undoubtedly lead to a considerable loss of labor and time.  
  At the same time. it has long been required to make the dimensions of the refrigerating system, and therefore. of the expansion valve as compact as practicably possible. as equally demanded in the other industrial fields. Moreover, there are incessantly growing requirements for more severe working conditions on such refrigeration systems which bring further factors which are likely to give rise to such hunting. Consequently, it has become more difficult to find effective measures to prevent such hunting only with the counter-measures designed to improve the balance of the evaporator as described above. and in this respect. this is the very reason why there have been such strong desires as for the realization of a device which is applicable to the refrigerating and freezing system to positively prevent such hunting from occurring in the integral system through controlling the expansion valve incorporated therein.  
 SUMMARY OF THE INVENTION In view of the above stated defects and problems which have been inherent to an expansion valve of the conventional design and construction. it would be advantageous to analyze the origin and symptoms of the hunting phenomena to be encountered during the operation of the refrigeration system. and to provide an expansion valve of novel and unique design concept for use with the refrigerating and freezing system.  
  This invention is directed essentially to meet such re quircments. which have long been neglected. On the other hand. there is a technical paper entitled Saikuru Butsurishoryo no Myakudo Genshoo&#34; (Pulsatory Phe nomena on Various Cyclic Physical Units) in the Hitachi Hyoron. Vol. 53. No. 5. 1971. for instance. which attempts to theoretically analyze hunting phenomena with respect to the evaporator system incorporating the expansion valve. This invention contemplates. aside from the viewpoint of analysis in the above mentioned paper. to analyze such hunting phenomena by way of graphic approach.  
  According to the present invention. briefly summarized by way of a typically preferred embodiment. there is provided an expansion valve for use in a refrigeration system. which comprises a diaphragm group of parts. adapted to longitudinally displace in accordance with the differential between a given pressure working on the upper diaphragm surface and that working on the under diaphragm surface. a plurality of passageways defined within the valve body. a valve group of parts. which is adapted all together to regulate the opening of the passageway so as to adjust the flow rate of the refrigerant passing thcrethrough. and an adjusting group of parts. which functions to manually minutely adjust the gap under the diaphragm so as to obtain an optimum displacement of the diaphragm.  
 BRIEF DESCRIPTION OF THE DRAWINGS The nature. principle. and details of the present invention. as well as further characteristics and advantages thereof. will become more apparent from the following detailed description with respect to a preferred embodiment of the invention when read in conjunction with the accompanying drawings, in which like parts are designated by like reference numerals. and in which:  
  FIG. 1 is a schematic wiring diagram. showing a refrigeration system including an expansion valve according to the present invention;  
  FIG. 2 is a diagrammatic graph, showing the mutual relationship between the physical factors as encountered during the operation of the refrigeration system incorporating the expansion valve shown in FIG. 1;  
  FIG. 3 is a diagrammatic graph. showing a hunting state of the expansion valve shown in FIG. 2;  
  FIGS. 4, 5, 6, 7, and 8 are respectively diagrammatic graphs. showing the results of simulation of given operating conditions;  
  FIG. 9 is a diagrammatic graph. showing the basic principle of preventing the hunting phenomenon in the refrigeration system.  
  FIG. I is a diagrammatic graph. showing the basic principle of preventing hunting phenomena which is to be applied to the expansion valve according to the invention:  
  FIGS. ll. l2. and 13 are cross-sectional views. Show ing typical examples of the expansion vahe according to the invention; and  
 FIG. HA is a fragment of FIG. ll.  
  FIGS. I4 (a) through ((1) are diagrammatic graphs. indicating schematically the basic principle shown in FIG. 10.  
 DETAILED DESCRIPTION With reference now to FIG. I, there is illustrated a schematic diagram of the refrigerating and freezing system including a compressor 28. a heat exchanger 29, an evaporator 32. and an expansion valve 31 according to this invention. The diagrammatic graph of FIG. 2 is prepared to show a hypothetical model which generally indicates the mutual relationship. such as between the characteristics of temperature and valve lift of an expansion valve. between the characteristics of valve lift and flow rate of an expansion valve. between the characteristics of flow rate of a single expansion valve and of that when installed in the refrigeration system. and the characteristics of an evaporator in the system. etc.  
  Referring to FIG. 2, the first quadrant of the graph shows the temperature vs. valve lift characteristics of one example of the expansion valve embodying the invention. wherein the X-axis indicates the temperatures of the temperature sensing bulb (TB. C) of the expansion valve and the Y-axis indicates the lifts (Ii. mm) of the valve. The second quadrant of the graph shows the lift vs. flow rate characteristics of the expansion valve. wherein the X&#39;-axis indicates the refrigerant flow rates (0. H11). and the Y-axis indicates the lift (/1. mm) of the single valve unit. The third quadrant shows the relationship between the flow rates of the single valve unit and that when installed in the refrigeration system. wherein the X&#39;-axis indicates the refrigerant flow rates (0. H11) of the single valve unit. and the Y&#39;-axis indicates the refrigerant flow rates (Q&#39;. l/h) actually entering into the evaporator. The fourth quadrant of the graph shows the characteristics of the evaporator in stalled in the refrigerating and freezing system. wherein the X-axis indicates the temperatures corresponding to the evaporation pressure (TE. C and the Y&#39;-axis indicates the refrigerant flow rates (0&#39;. l/h) actually entering into the evaporator.  
  In the diagrammatic graph of FIG. 2. all the characteristics as indicated in the first quadrant through the fourth quadrant of the graph are also functions of the pressure under which the refrigeration system is operating. However. in view of the specific design aspects of the valve which is to be operated under the super heated conditions, the pressure is considered to be a constant factor throughout the following descriptions. While. if a time lapse is to be indicated by means of a time axis (I) which is perpendicular to the paper surface. this model can also be e xpressed in terms of time lapse.  
  In FIG. 2, there is provided temperatures of the temperature sensing bulb (TB to which the lifts (h,,) of the expansion valve correspond-- to which lift (h.,)  
 the refrigerant flow rates (Q01 correspond---to which flow rates (0..) to flow rates entering into the refrigerator correspond.- to which flow rates (Q&#39;,,) the temperature corresponding to the evaporation pressure of the refrigerator (TE,,) correspond-to which temperature (TE.,) the temperature sensing bulb temperature (TB,,) corresponds. thus forming a closed loop locus in which state there is shown a stability in the operation of the refrigeration system.  
  Referring now to FIG. 3, there is shown that when a certain disturbance gives a temperature at the temperature sensing bulb (TB. there is provided an examplary cycle of hunting due to the physical factors concerned, thus representing a counter-clockwise oscillating phase of M... M. M M. and M and when this cycle of oscillation is over. the point TB. will then shift into another point TB... and after another cycle the point will now be over to TB... consequently the oscillating system will not converge infinitely.  
  This state of oscillation does not correspond to the reiated factors phenomenon of instability due to pulsa tory variations of the temperature at the temperature sensing bulb (TB) and the flow rate (Q&#39;,,). what is called hunting phenomenon as encountered in the ac tual refrigeration installation. In view of the above graphic analysis approach. it may be considered that when there occurs a convergence of the oscillation from the first quadrant through to the fourth quadrant according to this diagrammatic graph. the oscillating system will somehow reach an equilibrium state. and on the other hand. when this cycle will not converge. there takes place an unstable state which is enough to cause a hunting phenomenon of the refrigeration system.  
  In this consideration. it is possible. therefore. to analyze by way of such diagrammatic graphs whether the refrigeration system including an expansion valve will remain stable or unstable with respect to the reaction to the given disturbance.  
  The following is a simulation in attempting to know what characteristics of an expansion valve of the conventional construction will be settled down.  
  Referring now to FIG. 4, showing a simulation model in which the gradient h/TB of the temperature vs. valve lift characteristic curve is made greater in the first quadrant, it is shown that when a disturbance TB. is given to the refrigeration system. the oscillating cycle progresses in this manner; i.e.. M... M M... M... M... M therefore. the system turns out to be infinitely unstable and results in hunting.  
  Referring to FIG. 5, showing another model in which the gradient of the valve lift vs. flow rate characteristic curve Q/h is made greater in the second quadrant. it is shown that there takes place a hunting phenomenon likewise as in case of FIG. 4.  
  Now with reference to FIG. 6, which shows a model wherein the gradient li/TB of the temperature vs. valve lift characteristic curve is made more gentle in the first quadrant. it is shown that when a disturbance TB. is given to the system. this disturbance TB. is infinitely convergent to a point TB with a lapse of time. and consequently, the whole system turns out to be stable and therefore such hunting phenomenon diminishes.  
  With reference to FIG. 7, which shows a model case wherein the gradient 0/11 of the valve lift vs. flow rate characteristic curve is made more gentle, it is shown that a disturbance TB. is convergent to the point TB and the whole system therefore turns out to be stable.  
  Whilst. referring to FIG. 8. it is shown that even under a condition wherein the expansion valves shown in FIGS. 4 and 5 will result in hunting. if the characteristics of the evaporator in the refrigeration system are made better. thereby to obtain so-called well-balanced evaporator, such a hunting will be appropriately eliminated.  
  Now in summary of the above described graphic analyses referring to FIG. 9, wherein an input variable to the expansion valve is taken as ATB and an output variable of the valve to correspond thereto as A0. it may be stated on the basis of such graphic analyses that it is effective to prevent such a hunting phenomenon by maintaining the value, H AQ/ATB at or lower than a certain point (where. H is hereinafter defined as coefficient of hunting). Incidentally. the value H becomes more effective as it gets smaller. However, in view of the performance of the refrigeration system. this H value is required to have a latitude broad enough to follow up with load variations of the system. That is to say. the value H must exit within the following range; i.e.,  
  Min. H H Max. H where.  
 Min. H represents a minimum value of AQ/ATB.  
 which allows to follow up with load variations.  
 while Max. H represents a maximum value of AQ/ATB.  
 which prevents a hunting phenomenon.  
  In order to find a optimally small value of AQ/ATB in the design of an expansion value, the fraction AQ/ATB can be paraphrased as: Ah/ATB X AQ/Ah. From this paraphrase, as shown in FIGS. 6 and 7 the following two approaching methods are now more clearly con ceivable. and the inventor therefore applied:  
 Method i. To have small the gradient of Alz/ATB in the first graph while keeping the value of AQ/Ah substantially constant. or alternatively, Method II.  
 to have small the gradient of AQ/A/i in the second quadrant of the graph. while keeping the value of Ah/ATB substantially constant.  
  ln FIG. 14 (a), there is shown a case wherein the temperature vs. valve lift characteristic curve has the conventionally steep gradient. while in FIG. 14 (b). a case is shown wherein the curve has a more gentle gradient than case (a). When applying the method (i) above, if adopting case (b) instead of case (a) in an attempt to maintain an operating superheat (OSH) of the refrigeration system at an appropriate point, a static superheat (SSH) will become smaller, as small as of a minus value in the extreme, and it would be impossible to prevent a return partially in liquid state of the refrigerant to the compressor when starting the refrigeration system. thus possibly resulting in a damage ofa refrigerant compressor due to the liquid pressure to be accrued therefrom.  
  In other words. it is inevitable that there exist contradictory conditions in the design of the expansion valve which make the gradient of h/TB for the purpose of preventing the hunting phenomenon likely to contingently result in an irregular return of the refrigerant when starting the system.  
  Thus, this invention has been made with an intention to meet such contradictory conditions as described above with respect to the design of the expansion valve. As shown in FIG. 14 (0), there is provided an expansion valve which advantageously features the combination of two characteristic curves of different gradients, i.e.,  
 the expansion valve will provide such characteristics as according to the temperature vs. valve lift characteristic curve (a) of a greater static degree of superheat within the range of a given valve lift, while within a further range beyond the above mentioned range, the system will follow up with the temperature vsv valve lift characteristic curve (b) of a gentle gradient.  
  On the other hand. when applying the method ii) above. the valve according to this invention is designed in such a manner that the temperature vs. valve lift characteristic curve in the first quadrant of the graph will be allowed to accord with the curve (a). therefore, the valve lift vs. flow rate characteristic curve ofa steep gradient which corresponds to that of the characteristic curve (a) within the range of a given flow rate, while within a further range beyond the above mentioned range, the valve characteristics are designed to accord with the valve lift vs. flow rate characteristic curve of a more gentle gradient.  
  At the same time, when applying either method (i) or (ii), it is essential that the points of inflection in FIGS. 14 (c) and (:1) may optionally be located. the full aspects of which are shown in FIG. 10. Concurrently. one is free to select any gradients of the characteristic curve. as far as part 2 thereof is concerned.  
  In accordance with the above described design concept of the expansion valve. the above further advantageous features of the hunting prevention structure adaptable to the expansion valve according to this invention will more fully appear from the following detailed description, when read in conjunction with the accompanying drawings. with respect to the typical em bodiments of the invention. It is to be expressly understood, however, that the drawings are presented for the purpose of illustration only and are not in any way intended as a definition of the limits of the invention.  
  Referring now to FIG. 11 showing a preferred embodiment of the expansion valve according to the invention, there is provided an expansion valve of external constant pressure type which is designed in accordance with the method (i) i.e.. to vary the gradient of the temperature vs. valve lift characteristic curve, as described hereinbefore.  
  This expansion valve comprises the following com posite members as schematically illustrated in FIG. 1, i.e., a temperature sensing bulb 1 provided in immediate contact with the outlet of an evaporator 32, (FIG. 1) the temperature conveying gas enclosed within the bulb l communicating by means of tube 2 with a dia phragm chamber 40 which is located above the diaphragm 4 installed within the head portion of the housing 31A of an expansion valve 31, while the gas at the outlet of the evaporator 32 communicates through a thin tube 5b with the lower diaphragm chamber 5a. Inside the valve complete, there is provided the diaphragm 4 which is fixed in position by means of an upper cover plate 3 and a lower cover plate 6 through a backing plate 5, while there is provided a connecting rod 10 between the lower end of the backing plate 5 and the upper face of the flanged area of a valve 18. so that a longitudinal displacement of the diaphragm 4 may be conveyed to the valve 18.  
  Under the central portion of the backing plate 5, there is provided a resilient body such as leaf spring 7 permittingg an urging motion thereof immediately at the top surface of a stopper 9 through such components, i.e., a plurality of balls 13 and an adjusting screw 14. The valve 18 is adjustable under the force ofa coil spring 19 for adjusting the degree of superheat through a spring holder 20 by means of an adjusting screw 22, so that the valve 18 may incessantly be urged against a valve seat [7. There are protecting caps designated at numerals 24 and 27 at the lower end of the valve compiete. The refrigerant enters into the valve body from an inlet A. and after being adjusted with the flow rate thereof. flows out of an outlet B into the evaporator.  
  In accordance with the above described construction of the expansion valve, there takes place a pressure P] corresponding to the superheated gas temperature detected at the temperature sensing bulb l. on the upper surface of the diaphragm 4. etc. Consequently. there is defined an opening of the valve according to the position of equilibrium between a valve of Pl P2) X A, i.e., a difference between the gas pressure Pl above the diaphragm and the pressure P2 below the diaphragm times the area A of diaphragm. and a force F of the superheat adjusting spring 19.  
  The superheat adjusting spring [9 is adjusted with an urging force so as to provide a static superheat or static degree of superheat by means of the adjusting screw 22. The resilient body 7 such as a leaf spring or the like is preset with a clearance gap or lost motion space to be optionally selected between the lower face of the backing plate and the resilient body 7. When the above mentioned value of (PI P2) X A equals the spring force F. there is no or zero opening of the valve 18, which corresponds to the point B in H6. 14 (c). Next. as the load of the evaporator increases to a point wherein the value (Pl P2) X A is greater than the spring force F, the backing plate 5 will be lowered down, and this lowering displacement will be relayed r by means of the connecting rod so as to open the valve 18, thus admitting the refrigerant into the evaporator 32. As long as this displacement of the backing plate remains within the range corresponding to the gap 7 from the above mentioned resilient body 7, the extent of opening the valve in proportion to that displacement of the backing plate accords with the curve (B C) in FIG. 14(0).  
  When the displacement of the backing plate equals the clearance gap 7, the point where the backing plate 5 contacts the resilient body 7 may be read as the point C in the above mentioned FIG. 14 (c). When the load of the evaporator further increases, there occurs a lowering ofthe backing plate 5. However, in such an event the urging force ofthe resilient body 7 willjointly work with the spring force of the adjusting spring 19 and, therefore, the extent of displacement will accord with a curve (C D) ofa gentle gradient. in lieu of the curve (B C).  
  In other words, the opening of the valve according to the curve (C D). in comparison with that according to the curve (B C), can be made smaller with respect to the same operating degree of superheat and hence, it is possible to make a minute control on the opening of the valve.  
  In this respect, by means of selecting an appropriate load factor of the spring 19, the gradient of the curve (B C) can be obtained entirely optionally, and concurrently, the gradient of the curve (C D) may be optionally obtained by selecting a load factor of the resilient body 7 so as to conjoin with the curve (B C). The position of an inflection point (C) can also be optionally predetermined by means ofthe adjusting screw 14 which is adapted to adjust the gap between the backing plate 5 and the resilient body 7.  
  Referring now to FIG. l2, there is shown an expansion valve of exterior pressure equalizing type as a second embodiment of the present invention. In the figure, there is shown an expansion valve which is of the same construction as that of the valve according to the above mentioned first embodiment of the invention. except that there is provided a resilient body 7 such as a leaf spring between the under face and in the cavity of a valve 18 and the upper surface of a stopper 9 so that the valve 18 is urged by the spring force and may be flexibly adjusted by means of an adjusting screw 14 to an appropriate operating position thereof during the operation of the valve unit.  
  With this construction of the components, there oc curs the same function of the expansion valve as hereinbefore described in the first embodiment (FIG. 11) of this invention, i.e., the opening of the valve is adjustably determined according to a position wherein a force (Pl P2) X A comes in equilibrium with a spring force F, where P1 is a pressure corresponding to the temperature of superheated gas working on the upper surface of the diaphragm, P2 is a pressure working on the under surface of the diaphragm, and A is the area of the diaphragm.  
  Various aspects of the valve opening function are shown in FIGS. 14 (a) through (d), and the selection of the gradients in the valve characteristics may like wise be obtained as hereinbefore described.  
  Turning now to FIG. 13, there is shown an expansion valve of exterior pressure equalizing type as a third embodiment of the invention, which is worked out by applying a method to vary the gradient of the valve lift vs. flow rate characteristic curve described hereinbefore in the method (ii).  
  ln this drawing figure, such components of the valve as a temperature sensing bulb l, a thin tube 2, an upper cover plate 3, a lower cover plate 6, a diaphragm 4, and an exterior pressure equalizing tube Sb are all identical with those shown in the first embodiment in FIG. 11.  
  In the valve according to this embodiment of the invention, by the function of the valve 18 provided within the communicating hole, there occurs an inflow of the refrigerant from an inlet A through an outlet B to the evaporator. There is provided a through hole in the central portion of the valve 18, and the valve 18 is in contact with a stopper 9 by means of a plurality of rods 18c, downwardly extending to loosely pass through an upper spring holder 20b. There is provided a coil spring 19 for adjusting a degree of superheat between the above mentioned upper spring holder 20b and a lower spring holder 20a, and the valve 18 is incessantly urged against a valve seat 17 under the spring force of a spring 19, which may be adjusted to an optimal force by means of an adjusting screw 22. Also, there is provided a ball valve 18b between the through hole of the valve 18 and the upper spring holder 20b so as to adjust an opening in the through hole, and at the same time the valve 18 is urged against the valve seat 17 by an auxiliary spring 18s. On the other hand, there is provided a backing plate 5 in contact with the under surface of the diaphragm 4, which backing plate is relayed with the upper spring holder 20b by means of a con necting rod 10 provided between the under surface of the backing plate 5 and the upper surface of the upper spring holder 20b. so thatt&#39;he longitudinal displacement of the diaphragm may be immediately conveyed to the spring 19.  
  Furthermore. the positions of the abovementioned stopper 9 and an adjusting rod I31: may be optionally selected by means of an adjusting screw 14 so as to provide a gap 01. while there is provided a gap B between the under surface of the upper spring holder and the upper surface of the stopper 9.  
  la the above mentioned construction of the expansion valve. when a differential force of Pl P2) X A equals with a differential force of Fl F2); where (PI P2) is a differential between a pressure P] working on the upper surface of the valve diaphragm and corresponding to the temperature at the temperature sensing bulb and a pressure P2 from the outlet of the evaporator working on the under surface of the diaphragm, A is the area of the diaphragm. Fl and F2 are the spring forces of the spring 19 and the auxiliary spring 18x. re spectively; there is no opening either between the valve 18 and the valve seat 17 or between the through hole 18a of the valve 18 and the ball valve l8!) and. therefore. there occurs no flow of the refrigerant from the inlet A to the outlet B.  
 The above mentioned point ofzero&#34; opening of the valve corresponds to the point in FIG. 14 (d). When there occurs an increase in the load on the evaporator, therefore. with (P1 P2) X A (Fl F2). the backing plate 5 will come downwardly. and this displacement is relayed to the upper spring holder b through the connecting rod 10, thus pushing down the upper spring holder 20!), compressing the adjusting spring 19, and thereby the valve 18, together with the stopper 9 being connected by the connecting rod 18c, will downwardly follow the displacement of the spring 19, while leaving the ball valve 18!) to close the through hole 18:! under the urging force of the auxiliary spring I8s. whereby the previously closed communicating hole of the valve seat 17 opens, and consequently, the gap at between the stopper 9 and the adjusting rod 13a decreases. When there takes place a further increase in the load on the evaporator, so far as this increase is within a range which corresponds to the gap a. there occurs an increase of the opening between the valve 18 and the valve seat 17. thereby to increasingly correspond the inflow of the refrigerant to that increased opening. This state may be read as the curve (0 N) in FIG. 14(d).  
  When the displacement of the backing plate 5 becomes equal to the gap 0:. the contact pointbetween the adjusting rod 13a and the stopper 9 may be read as the point N in FIG. 14((1).  
  When there occurs a further increase in the load on the part of the evaporator, further lowering motion of the stopper 9 is prevented by the adjusting rod 130. Thus, the valve 18 does not come down any further, and only upper spring holder 20b comes downwardly within a range which corresponds to the gap [3. In the meantime, the ball valve 1817 comes down following the lowering motion of the upper spring holder 20!). thereby to open the through hole 18a. whereupon there will be additional refrigerant flow according to the opening of the through hole 18a and to the opening between the valve 18 and the valve seat 17.  
  However, since this addition of the inflow of the refrigerant from the opening of the through hole 180 is rather small in comparison with that of the opening between the valve 18 and the valve seat 17, it becomes possible to practice a minute control of the refrigerant flow rate with respect to a given operating superheat or operating degree of superheat. This aspect may be read on the curve (M N) in FIG. I4 (d).  
  The location of the inflection point N may be optionally selected by adjusting the gap or by means of the adjusting screw 14, while the gradient of the curve (N M) may be optionally determined by selecting the diameter. or cross-sectional area of the through hole 180. and the adjusting range of refrigerant flow rate may likewise be obtained by adjusting the gap B.  
  Thus. in the valves constructed according to the embodiments of FIGS. I1 and 12, which valves are planned to operate according to the diagram illustrated in FIG. 14 (c). the pressure differential as imposed on the diaphragm 4 causes movement thereof. which through the rod 10 causes a corresponding opening movement of the valve IS. The opening of the valve 18 thus occurs in a substantially linear manner in proportion to the pressure differential existing across the diaphragm 4. Thus. as the pressure differential on the diaphragm 4 increases. the displacement of the valve 18 also substantially proportionately increases. substantially as illustrated by the line BC in FIG. 14 (c). However. when the diaphragm 4 and the valve I8 have been moved to a position so that the clearance gap associ ated with the spring 7 is completely eliminated. which point corresponds to the point C illustrated in FIG. 14 (c). then an increased resistance is imposed on the valve due to the combination of the two springs acting together. Thus. the pressure differential imposed on the diaphragm must thus be substantially larger in order to cause further displacement of the valve 18 away from its closed position.  
  With respect to the embodiment of FIG. 13, the valve 18 is again moved away from its closed position in a manner which is substantially directly proportional to the pressure differential imposed across the diaphragm since the opening movement of the valve 18 is opposed by the difference in the spring force imposed by the springs 18s and 19. This initial opening movement of the valve thus substantially corresponds to the line ON illustrated in FIG. 14 (d). However. when the valve 18 has been moved to a sufficient extent so as to completely eliminate the gap between the stopper 9 and the adjusting member 130. which point corresponds to the point N in FIG. 14 (d), then further movement of the valve 18 is prevented but. as the pressure differential on the diaphragm increases, the holder 20b and the ball 18b move downwardly so as to open the hole so that the area of the valve opening continues to increase. This latter mode of operation corresponds to the line MN in FIG. 14 (d).  
  As fully described on typical three embodiments of the present invention, this invention is essentially intended for providing a new and useful expansion valve on the basis of a series of graphic approaches so as to detect the basic problems of unstable and cyclic state or hunting phenomenon with respect to temperature and pressure as encountered during the operation of the refrigeration system, which has long been inevitable and inherent to such system including an expansion valve. By virtue of such advantageous features as hereinbefore fully described, the expansion valve according to this invention incorporates an efficient diaphragm construction therewithin which can be adapted to versatilely meet or rather prevent the above mentioned hunting phenomena from occurring, and it now becomes possible either in the design or the operation of the refrigerating and freezing system to prevent such hunting from occurring by firstly selecting an appropriate point of inflection, then by adapting the normal op erating range of the system to a curve range of gentle gradient, and furthermore, such hunting phenomena can be continuously met and eliminated through operational adjustments during the operation of the system. In addition to the above mentioned advantageous feature, this expansion valve according to the invention affords a remarkable utility in the application to the re frigeration system. since a static degree of superheat of the evaporator can be taken as great as practicably possible. any accidental back flovv ofthc refrigerant under the liquid state to the compressor can positively be prevented. when starting the refrigeration system.  
  The invention has been described in an illustrative manner and it should be understood that the terminology which has been used herein is intended to be only in the nature of words ofdescription rather than of limitation, Obviously. many other modifications and varia&#39; tions of the present invention are possible in light of the above teachings.  
  What is claimed is: l. A method for preventing hunting with respect to temperature and pressure, without reducing static superheat, in a refrigeration system of the kind including an evaporator. a temperature sensing device at the outlet ofthe evaporator for sensing evaporator outlet ternperature, an expansion valve responsive to said temperature sensing device for controlling flow of refrigerant gas to the evaporator, and wherein Ah represents variation in expansion valve opening, ATB represents the temperature variation sensed by the temperature sensing device, and A0 represents variation in the flow rate through the expansion valve, comprising:  
 controlling opening of said expansion valve to con form with the ratio AQ/ATB AQ/Ah Ah/ATB during opening movement of said expansion valve;  
 setting the ratio AQ/ATB k a first value. for expan sion valve opening movement between closed and predetermined part open positions. so as to provide a desired static superheat;  
 setting the ratio AQ/ATB k a second value, for expansion valve opening movement beyond said pre determined part open position,  
 and limiting k; to a value less than k and which prevents said hunting.  
  2. An expansion valve assembly for use with a refrigeration system. comprising:  
 housing means including first and second ports;  
 valve means movably mounted on said housing means for controlling flow between said first and second ports, said valve means having a two part opening range bounded by a closed position and an open position and divided by a preselected part open position;  
 actuator means operatively connected to said valve means and responsive to a fluid pressure for causing movement of said valve means in an opening direction through said positions; and  
 biasing means coacting with said valve means for imposing on said valve means a closing force increasing (l) at a first rate as said valve means opens from said closed position to said preselected part open position and (2) at a second rate which is different from said first rate as said valve means opens beyond said preselected part open position, so as to provide a closing force inflection point at said preselected part open position, said biasing means comprising first resilient means for providing one said rate in one part of said range and second resilient means coactive with said first resilient means in the other part of said range for providing the other said rate, and means defining a lost motion space in series with said second resilient means for rendering movement of said valve means independent of said second resilient means in said one part of said opening range.  
  3. A valve assembly according to claim 2, wherein said first resilient means is disposed in continuous en gagement with said valve means for imposing a restoring force thereon causing said valve means to be normally urged in the closing direction, said second resilient means being disposed for imposing a restoring force on said valve means only after said valve means has been displaced in an opening direction past said preselected part open position.  
  4. A valve assembly according to claim 3, wherein the restoring forces generated by said first and second resilient means both act in a closing direction of the valve means and said lost motion space lies between said second resilient means and a surface movable with said valve means and actuator means.  
  5. A valve assembly according to claim 2, wherein said first resilient means coacts with said valve means. said first resilient means continuously imposing a restoring force on said valve means tending to urge same in the closing direction, said second resilient means acting on said valve means and normally tending to urge said valve means in the opening direction, said second resilient means exerting an opening force on said valve means only as said valve means is moved between said preselected part open and closed positions.  
  6. A valve assembly according to claim 5, wherein said valve means includes a first movable valve member disposed for coaction with a valve seat which is stationarily provided on said housing means, said second resilient means coacting on said first valve member for normally tending to move same in an opening direction, said valve means including a valve opening passing through said first valve member and a second movable valve member disposed for closing said valve opening, said first resilient means continuously urging said second valve member in a closing direction for closing said valve opening, the resilient urging of said first resilient means also continuously tending to urge said first valve member in a closing direction, and including means blocking opening of said first valve member beyond said part open position. said lost motion space lying intermediate said blocking means and a portion of said actuator means.  
  7. An expansion valve assembly for use with a refrigeration system, comprising:  
 housing means including first and second ports,  
 valve means movably mounted on said housing means for controlling flow between said first and second ports;  
 actuator means operatively connected to said valve means and responsive to a fluid pressure for causing movement of said valve means in an opening direction, said actuator means comprising a flexible diaphragm chambered in said housing and shiftable in response to opposing pressures applied to opposite sides thereof and a connecting member coupled between said diaphragm and valve means for urging said valve means in an opening direction in response to a not pressure on one side of said diaphragm; and  
 biasing means coacting with said valve means for urging same in a closing direction whereby said valve means is normally maintained in a closed position. said biasing means imposing a restoring force on said valve means. which restoring force increases at a first rate as said valve means is moved away from said closed position through a predetermined distance. said biasing means imposing another restoring force on said valve means as it is moved away from said closed position beyond said predetermined distance, which latter restoring force increases at a second rate which is different from said first rate. said biasing means comprising first and second bias springs and means interconnecting said first bias spring with said valve means for continuously urging said valve means in said closing direction. said second spring being responsive to opening of said valve means beyond said predetermined distance for additionally resisting opening of said valve means and means adjustably determining a lost motion space in the path between and connecting said second spring and valve means. said lost motion space being open and decreasing in size during valve means travel from the valve closed position to said predetermined opening distance and being absent during further opening of said valve means.  
  8. A valve assembly according to claim 7 in which said second spring is disposed in opposition to the other side of said diaphragm, and said lost motion space is disposed between said second spring and said diaphragm.  
  9. A valve assembly according to claim 7 in which said second spring is disposed adjacent said first spring and said lost motion space is disposed between said second spring and said valve means.  
  10. An expansion valve assembly for use with a refrigeration system, comprising:  
 housing means including first and second ports;  
 valve means movably mounted on said housing means for controlling flow between said first and second ports;  
 actuator means operatively connected to said valve means and responsive to a fluid pressure for causing movement of said valve means in an opening direction. said actuator means comprising a flexible diaphragm chambered in said housing and shiftable in response to opposing pressures applied to opposite sides thereof and a connecting member coupled between said diaphragm and valve means for urging said valve means in an opening direction in response to a not pressure on one side of said diaphragm; and  
 biasing means coacting with said valve means for urging same in a closing direction whereby said valve means is normally maintained in a closed position. said biasing means imposing a restoring force on said valve means, which restoring force increases at a first rate as said valve means is moved away from said closed position through a predetermined distance. said biasing means impos ing another restoring force on said valve means as it is moved away from said closed position beyond said predetermined distance. which latter restoring force increases at a second rate which is different from said first rate. said biasing means comprising first and second bias springs. a spring holder opera tively sandwiched between said connecting memher and said first spring and movable in a valve opening direction by said connecting member and in a valve closing direction by said first spring. said valve means including a first valve member movable in an opening direction for permitting flow between said first and second ports and stopper means fixed to said first valve member for move ment therewith, said spring holder being loosely disposed between said first valve member and stopper means for establishing a first lost motion space between said spring holder and stopper means. an adjusting rod carried by said housing means in backing relation to said stopper means and spaced therefrom by a second lost motion space for preventing further opening movement of said first valve member after opening movement of said valve means beyond said predetermined distance. said second spring engaging said first valve member for urging same in an opening direction, said valve means further including a passage through said first valve member and a second valve member sandwiched between said spring holder and first valve member for normally closing said passage. said spring holder being movable during opening movement of said valve means beyond said predetermined distance to close said first lost motion space and remove said second valve member from closing contact with said passage and thereby further open said valve means. whereby initial opening of said valve means is assisted by said second spring but resisted by said first spring and said further opening of said valve means is resisted by said first spring but not llJlflLlfllCCd by said second spring.