US3866427A

US3866427A - Refrigeration system

Info

Publication number: US3866427A
Application number: US374701A
Authority: US
Inventors: Noel Y Rothmayer; James Crawford B Mackeand; Clark W Smith
Original assignee: Allied Chemical Corp
Current assignee: Allied Corp
Priority date: 1973-06-28
Filing date: 1973-06-28
Publication date: 1975-02-18
Anticipated expiration: 1992-02-18

Abstract

Improved means and method for determining the presence of liquid ammonia refrigerant flowing through a tubular evaporator coil of a open cycle refrigeration system and for regulating the flow of refrigerant through the tubular coil involving a vapor-liquid sensing unit disposed at an intermediate point of the tubular evaporator in heat transfer contact with refrigerant flowing through the evaporator, said vapor-liquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant.

Description

United States Patent [191 Rothmayer et a1.

1 1 Feb. 18, 1975 1 REFRIGERATION SYSTEM [73] Assignee: Allied Chemical Corporation, New

York, NY.

22 Filed: June 28,1973

21 Appl. No: 374,701

[52] US. Cl 62/7, 62/83, 62/202, 62/225, 62/503, 236/92 B [51] Int. Cl. F25b 19/00 [58] Field of Search 62/7, 83, 202, 225, 503, 62/512; 236/92 B [561 References Cited UNITED STATES PATENTS 1,969,102 8/1934 Shenton et a1 .1 (12/202 2,461,342 2/1949 Obreiter 62/83 2,534,455 12/1950 Koontz 62/202 3,405,535 10/1968 Matthies 62/202 3,478,534 11/1969 Matthies 62/225 3,680,326 8/1972 Huelle .t 62/513 3,685,310 8/1972 Fischer 3,740,961 6/1973 Fischer Primary Examiner-William F. ODea Assistant E.raminerPeter D. Ferguson Attorney, Agent, or Firm-Gerard P. Rooney [57] ABSTRACT Improved means and method for determining the presence of liquid ammonia refrigerant flowing through a tubular evaporator coil of a open cycle refrigeration system and for regulating the flow of refrigerant through the tubular coil involving a vapor-liquid sensing unit disposed at an intermediate point of the tubular evaporator in heat transfer contact with refrigerant flowing through the evaporator, said vapor-liquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant.

14 Claims, 2 Drawing Figures REFRIGERATION SYSTEM BACKGROUND OF THE INVENTION This invention relates to a refrigeration system and more particularly refers to a new and improved means and method for determining the presence of liquid refrigerant flowing through a tubular evaporator coil of the refrigeration system and for regulating the flow of refrigerant through the tubular coil.

Refrigeration systems commonly employ an evaporator in the form of a tubular coil into which is introduced a liquid refrigerant which is vaporized by absorbing heat from the surroundings of the evaporator. Conventional practice is to regulate the flow of refrigerant through the evaporator in response to the desired temperature of the compartment being refrigerated.

Effective utilization of the evaporator results when only sufficient liquid refrigerant is introduced into the tubular coil with vaporization taking place throughout the length of the coil and with substantially all vapor with little or no liquid refrigerant discharging from the tubular coil. Complete vaporization of the liquid refrigerant in the coil at an appreciable distance upstream of the terminal discharge point of the coil is inefficient utilization of that portion of the coil in which no vaporization occurs. Of greater significance perhaps is the condition where incomplete vaporization of all the liquid introduced into the evaporator occurs and a substantial amount of liquid discharges from the coil. This condition of incomplete vaporization of liquid refrigerant in the evaporator is particularly significant in open cycle systems because not only does the liquid refrigerant discharged from the evaporator represent a loss of valuable refrigerant in that cooling by evaporation in the tubular coil did not occur but also the discharge of substantial quantities of liquid refrigerant places a heavy disposal burden on the rest of the system, as for example, a burner as more fully described in U.S. Pat. Nos. 3,685,310 of Aug. 22, 1972 and 3,740,961 of June 26, 1973.

An object of the present invention is to provide a more efficient refrigeration system by means and methods of determining liquid refrigerant flowing through the tubular evaporator of the refrigeration system and regulating the flow of refrigerant therethrough to prevent discharge from the evaporator of substantial amounts of liquid refrigerant.

SUMMARY OF THE INVENTION The refrigeration system of the present invention comprises:

a. a storage tank for liquid refrigerant;

b. a tubular evaporator wherein the liquid refrigerant is vaporized by absorbing heat from the surroundings of the evaporator;

c. a conduit for conveying liquid refrigerant from the storage tank to the evaporator;

d. a conduit for conveying vaporized refrigerant from the evaporator;

e. a vapor-liquid sensing unit disposed at an intermediate point of the tubular evaporator in heat transfer contact with refrigerant flowing through the evaporator, said vapor-liquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant; and

f. means for regulating the flow of refrigerant through the tubular evaporator, acutated by said vaporliquid sensing unit.

In a more specific embodiment, the evaporator is composed of a plurality of tubular coils through which the refrigerant flows in parallel and the effluent therefrom discharges through another tubular coil and wherein the vapor-liquid sensing unit is disposed near the inlet of the tubular coil into which the effluent discharges.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing,

FIG. 1, diagrammatically illustrates an open cycle ammonia refrigeration system including a catalytic ammonia burner and having a vapor-liquid sensing unit to determine liquid refrigerant flowing through the evaporator and to regulate the flow of liquid through the evaporator to prevent discharge therefrom of substantial amounts of liquid refrigerant.

FIG. 2 illustrates a portion of the open cycle ammonia refrigeration system depicted in FIG. I wherein an alternate method of heating the vapor-liquid sensing unit is shown.

DETAILED DESCRIPTION Referring to the drawings, liquid refrigerant l is contained in storage tank 2 under autogenous pressure and the liquid is conveyed by dip pipe 3 and conduit 4 to a coil 5 located inside separator vessel 6. Although the drawing will be described with specific reference to ammonia liquid refrigerant, other liquid refrigerants are known such as S0 and fluorinated hydrocarbons. Ammonia, which has flashed or boiled to vapor in the conduit 4 is recondensed to liquid in the coil 5, which is immersed or partially immersed in liquid ammonia contained in the lower part of the separator vessel 6. The pressure in separator vessel 6 is normally maintained between 0 psig. and 10 psig. and the liquid ammonia in the vessel therefore boils at a pressure between -28 F. and -8 F. The pressure in storage tank 2 will vary dependent upon the ambient temperature which may range from 0 F. to about ll0 F. and the corresponding pressures in storage tank 2 will therefore vary from 15.7 pounds per square inch gauge to 247 pounds per square inch gauge. Separator vessel 6 is conveniently located at an elevation above storage tank 2, about 10 feet above the level of liquid refrigerant l in tank 2. As a result, the pressure in precooler coil 5 will be slightly lower than the pressure in storage tank 2 and the pressure inside precooler coil 5 will vary from about 14.2 psig to 243.5 psig. A subcooling of the liquid will therefore be produced, and this is normally between 8 and 137 difference Fahrenheit.

The liquid ammonia required to provide the pool of liquid 7 in separator 6 is initially supplied, and maintained as necessary, by a conduit 8 extending from the ammonia supply conduit 4 through valve 9, line 11 into the vessel 6. A liquid level sensing element 12 activates valve 9 to permit the flow of ammonia when required from storage tank 2 through lines 4 and 8, valve 9, line 11 to vessel 6. As previously mentioned, the pressure in vessel 6 is maintained between 0 and 10 pounds per square inch gauge which would correspond to a tem- 3 perature of the liquid ammonia in the tank of between 28 F. and -8 F.

Liquid ammonia may contain a minor amount of oil and other foreign particles. Subcooling the ammonia induces the oil to coalesce with the foreign particulate matter to form a substance which is more readily removed by filtration. Liquid ammonia subcooled in precooler coil together with coalesced foreign particulate matter, if any, is conveyed from precooler coil 5 by conduit 13 to filter 14 wherein removal of the foreign material is effected.

Subcooled liquid ammonia flows from filter 14 through line 15 to thermostatic control valve 16. This valve controls the flow of ammonia in the conduit 15 in response to the cooling requirements of the refrigerated compartment surrounding the cooling coils as sensed by temperature sensing element 17 and temperature controller 18. The ammonia then passes to the liquid feed control-valve 21 via conduit 19 and then through control restriction orifices 22 into the main cooling coils 23. The ammonia discharges from the restrictions 22 into the coils 23, 50 percent or more of the system pressure drop being developed across the restrictions. This ensures equal distribution to the ammonia flow to the several coils 23 which may, for example, be located at the front, center and rear of a truck body. Such means of ensuring distribution also provide the wherewithal to control ammonia distribution, for instance, to supply a larger flow to a cooling coil section near the rear doors of a truck, where greater refrigeration requirement might be expected than elsewhere in the body.

Ammonia from the main cooling coils 23 discharge the manifold 24 to vapor-liquid sensing unit 25. This comprises a heat conductive element and means for heating the heat conductive element, the resultant temperature of which is dependent on the rate of heat transfer therefrom. The heat conductive element may be stainless steel or aluminum or other heat conductive material which is non-corrosive and non-reactive to the refrigerant passing therethrough. A convenient method of heating the heat conductive element is by an electric resistance element connected by means of wires 26 to a source of electricity, such as a battery. Other heating means may be provided, as for example, by heat conduction in which a metal rod is connected at one end to the heat conductive element and heated at its other end by a flame or any other suitable means and heat conducted from the flame to the element. In some instances, as in trailer trucks, it may be inconvenient to provide a source of electric power. In this event, as illustrated in FIG. 2, the burner in which the spent ammonia is burned may be used as a source of heat and one end of heat conducting rod 40 may be disposed in the heated portion of the burner and the other end disposed adjacent the heat conductive element with, of course, heat being supplied by conduction through the rod. The various parts should be arranged so as to provide a relatively short distance between the burner and the vapor-liquid sensing unit. If desired,'the heat rod can be insulated to prevent loss of heat to the surroundings.

The heat conductive element of the vapor-liquid sensing unit may be disposed in the tubular evaporator in the path of and in direct contact with the flowing refrigerant. Alternatively, the vapor-liquid sensing unit may be disposed externally to the tubular evaporator in indirect heat transfer contact with refrigerant flowing through the evaporator. The heat conductive element may be bonded or clamped to the tubular evaporator which is usually fabricated of aluminum or stainless steel, both of which are good heat conductors and heat can be readily absorbed through the walls of the tube from the heated conductive element.

As illustrative, if only ammonia gas is present in the evaporator tube at the point of insertion of vapor-liquid sensing unit 25 as shown in the drawing, due to previous total evaporation of liquid ammonia in the coils 23, a heat transfer coefficient for the internally heated element would be of the order of about 10 Btu per hour per square foot per degree F and at an operating pressure in manifold 24 and sensing unit 25 of 7.5 pounds per square inch gauge, the heat conductive element would settle at a temperature of about 38 F., based on an element surface area of 0.07 square feet and an effective heat load from the internal heater of 36 Btu per hour. On the other hand, in the presence of liquid ammonia the heat transfer coefficient is of the order of 250 Btu per hour per square foot per degree F and would produce a final temperature of the sensing element of 1 1 F. This wide temperature swing of the sensing element 25 depends only on the presence or absence of liquid at the point of insertion in the manifold 24, and vapor-liquid sensing unit 25 is so arranged as to cause liquid feed control valve 21 to close off the supply of liquid to the restrictions 22 and main cooling coils 23 when liquid-ammonia is detected. Vapor-liquid sensing unit 25 may actuate liquid feed control valve 21 by any suitable means, as for example, by hydraulic means, e.g., ammonia in conduit 27 which upon being heated or cooled actuates a diaphragm in valve 21 to open or close the valve.

At the time at which liquid is detected by vaporliquid sensing unit 25, and the valve and liquid feed control valve 21 is closed, there may well be a substantial amount of unevaporated liquid ammonia remaining in the coils 23 and the conduit between valve 21 and control restriction orifices 22. Final coil 28 is therefore provided in order to make effective use of such liquid as may pass vapor-liquid sensing unit 25. Although the relative size of coil 28 may vary, good results were obtained when coil 28 had a heat transfer surface area of about one-fourth to one-third of the total heat transfer surface area of all the

coils

23 and 28. Effective utilization of coil 28 to substantially complete evaporation of liquid entering therein is obtained when vapor-liquid sensing unit 25 actuates valve 21 to regulate the flow of refrigerant through main cooling coils 23 to obtain 5 25 percent by weight, preferably 5 15 percent liquid refrigerant at the point of vapor-liquid sensing unit 25.

Although vaporization of liquid entering final coil 28 will be substantially complete at the end of coil 28, at times some liquid refrigerant will discharge with the vapor. Any liquid not evaporated in coil 28 flows together with vapor through conduit 29 into separator vessel 6, equipped with precooler coil 5, wherein the liquid is separated from the vapor. Thus, any liquid introduced through line 29 is used effectively in the precooler, and serves to reduce the amount required from valve 9. Separator vessel 6 is located typically with its vapor inlet 29 level with or below coil 28.

The operation of the system therefore results in a well defined distribution of ammonia, wherein liquid is flowing in the

cooling tubes

23 and 28 and is evaporating on the walls of these cooling tubes. The point at which the liquid evaporates to dryness is not fixed but will move towards the end of the coils 28 nearest to the vessel 6 when

valves

16 and 21 permit flow of liquid ammonia, and away from that point when either of

valve

16 or 21 is interrupting the flow of ammonia. This characteristic of the embodiment described wherein the

valves

16 and 21 are of the on-off type, as distinguished from the throttling type.

The

coils

23 and 28 can be disposed on the ceiling of a refrigerated vehicle or other compartment. An alternative embodiment, with particular advantages for smaller refrigerator trucks, as distinct from trailers or semi-trailers, is obtained by locating the

coils

23 and 28 on the inside of the front bulkhead of the vehicle, behind a partition which may be detachable for convenient access to the coils. In such an arrangement the coils 23 should be arranged above the manifold 24 and coil 28, the ammonia feed and restrictors 22 being at the top of the system and the exit from coil 28 being at the bottom. In such a layout the entry 31 to vessel 6 may be located either level with or below coil 28, or the vessel may be located above the exit of coil 28, in which case conduit 29 should be so sized as to provide hydraulic lift from coil 28 to vessel 6 of any given liquid which might flow through to the end of coil 28.

As an added precaution to prevent sudden surges of ammonia vapor, there is provided a restrictor 32 which has positive limitations of flow to the burner at the maximum desired rate of flow and with little limitation of flow at rates below the desired maximum. To this end, restrictor 32 is so sized as to have a pressure drop of about l2 pounds per square inch at a flow rate of 40 pounds per hour of ammonia which is an average rate for truck usage. A sensitive reducing valve 33 is located upstream of restrictor 32 in conduit 34 leading from separator vessel 6. Valve 33 is set to a pressure, typically 2 pounds per square inch gauge, this being the back pressure developed by the restrictor 32 and such other components as are in the further flow path of the ammonia vapor from the separator vessel 6 to the burner 35, when the flow of this vapor is 40 pounds per hour, on such other value as is chosen to be the permitted maximum flow. The valve 33 will then exercise no control over the flow until the flow is very close to the desired maximum value, typically within 0.8 pounds per hour of the 40 pounds per hour value taken as typical. As the flow tends to exceed 39.2 pounds per hour, valve 33 will tend to close and, by imposing the back pressure on the vessel 6 and coils 28 and 23 will hold the maximum ammonia flow to about 40 pounds per hour.

It is also desirable to prevent the use of refrigeration system while the doors of the refrigerator compartment are open, and for this purpose a door switch 36 may be provided. This switch will cause valve 16 to close, thus preventing the flow of ammonia to the cooling coils, whenever the doors are open, so preventing wasteful and ineffective use of ammonia. If no ammonia is supplied to burner 35 it will go out. When the compartment to be cooled is down to the required temperature, the evaporation from

coils

23 and 28, and from vessel 6 may be insufficent to maintain an adequate flame in burner 35. For this purpose some 2-3 pounds per hour ammonia are needed and this is supplied from the vapor phase of storage tank 2 through conduit 37 to a pressure reducing valve 38, and thence through a conduit 39. Conduit 39 is so sized that with a pressure of about 0.5 1 pounds per square inch gauge from valve 38 a supply of 23 pounds per hour of ammonia will be inch diameter and 48 feet long and with vapor-liquid sensing unit disposed at the inlet of coil 28 was tested first as a stationary unit and then after successful operation was disposed in the truck and tested as an in-transit refrigerating unit under varied conditions of temperature and refrigeration load. The vapor-liquid sensing unit effectively detected and maintained the liquid ammonia flowing past the sensing unit within the range of 5 15 percent liquid by actuating valve 21 which controlled the flow of liquid ammonia entering coils 23.

We claim:

1. An open cycle ammonia refrigeration system comprising:

a. a storage tank for liquid ammonia refrigerant;

c. a first conduit for conveying the liquid refrigerant from the storage tank to the evaporator;

d. a second conduit for conveying vaporized refriger-v ant from the evaporator to combusting means;

e. means for combusting the vaporized refrigerant;

f. a vapor-liquid sensing unit disposed at an intermediate point between the tubular evaporator entry and exit in heat transfer contact with the refrigerant flowing through the evaporator, said vaporliquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant; and

g. means for regulating the flow of the refrigerant through the tubular evaporator actuated by said vapor-liquid sensing unit, said means interposed in said first conduit.

2. A refrigeration system as claimed in claim 1 wherein said vapor-liquid sensing unit is disposed in the tubular evaporator in the path of and in direct contact with the flowing refrigerant.

3. A refrigeration system as claimed in claim 1 wherein said vapor-liquid sensing unit is disposed externally to the tubular evaporator in indirect heat transfer contact with refrigerant flowing through the evaporator.

4. A refrigerant system as claimed in claim 1 wherein the means for heating said heat conductive element is electrical resistance.

5. A refrigerant system as claimed in claim 1 wherein the means for heating said heat conductive element is a heated member is contact with said element whereby heat is conducted from said member to said element.

6. A refrigerant system as claimed in claim 1 wherein a member is provided a conduct heat from the combustion means to the heat conductive element.

7. A refrigerant system as claimed in claim 1 wherein the evaporator is composed of a plurality of tubular coils thru which the refrigerant flows in parallel coils and a single tubular coil through which the refrigerant discharges from the plurality of tubular coils.

8; A refrigerant system as claimed in claim 7 wherein the vapor-liquid sensing unit is disposed near the inlet of the single tubular coil into which the refrigerant discharges.

9. A refrigeration system as claimed in claim 1 wherein the means for regulating the flow of the refrigerant through the evaporator includes a liquid feed control valve disposed in the first conduit in communication with said vapor-liquid sensing unit, whereby the liquid refrigerant is prevented from enteringthe means for combusting vaporized refrigerant.

10. A refrigeration system as claimed in claim 1 including a thermostatic control valve disposed in the first conduit in communication with a temperature sensing element located in a refrigerated compartment surrounding the tubular evaporator.

11. A method for determining liquid refrigerant in a tubular evaporator of an open cycle ammonia refrigeration system wherein liquid ammonia refrigerant is va-.

porized by absorbing heat from the surroundings of the evaporator, which comprises:

a. disposing a heat conductive element in heat transfer contact with the refrigerant flowing through the evaporator;

b. heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant; and

c. relating the temperature of the heat conductive element to the proportion of liquid refrigerant flowing through the evaporator with the highest temperature indicating refrigerant substantially all in vapor and a lower temperature indicating liquid refrigerant in heat transfer contact with the heat conductive element to effect heat abstraction therefrom to lower its temperature.

12, A method as claimed in claim 11 wherein the flow of refrigerant through the tubular evaporator is regulated, actuated by variation in temperature of said heat conductive element.

13. A method as claimed in claim 11 wherein the vapor-liquid sensing unit actuates valve means to regulate the flow of the refrigerant through the evaporator to obtain 5 to 25 percent liquid refrigerant at the point of the vapor-liquid sensing unit.

14. An open cycle refrigeration system comprising:

a. a storage tank for liquid ammonia refrigerant;

c. a first conduit for conveying a liquid refrigerant from the storage tank to the evaporator;

d. a second conduit for conveying vaporized refrigerant from the evaporator to combusting means;

e. means for combusting the vaporized refrigerant;

f. vapor-liquid sensing unit disposed at an intermediate point between the tubular evaporator entry and exit in heat transfer contact with the refrigerant flowing through the evaporator such that the heat transfer surface area of an auxiliary portion of the tubular evaporator located on the discharge side of said vapor-liquid sensing unit ranges from about one-fourth to one-third of the total heat transfer area of the tubular evaporator, said vapor-liquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant; and

g. interposed in said first conduit, means for regulating the flow of the refrigerant through the tubular evaporator actuated by said vapor-liquid sensing unit.

Claims

1. An open cycle ammonia refrigeration system comprising: a. a storage tank for liquid ammonia refrigerant; b. a tubular evaporator wherein the liquid refrigerant is vaporized by absorbing heat from the surroundings of the evaporator; c. a first conduit for conveying the liquid refrigerant from the storage tank to the evaporator; d. a second conduit for conveying vaporized refrigerant from the evaporator to combusting means; e. means for combusting the vaporized refrigerant; f. a vapor-liquid sensing unit disposed at an intermediate point between the tubular evaporator entry and exit in heat transfer contact with the refrigerant flowing through the evaporator, said vapor-liquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant; and g. means for regulating the flow of the refrigerant through the tubular evaporator actuated by said vapor-liquid sensing unit, said means interposed in said first conduit.

8. A refrigerant system as claimed in claim 7 wherein the vapor-liquid sensing unit is disposed near the inlet of the single tubular coil into which the refrigerant discharges.

9. A refrigeration system as claimed in claim 1 wherein the means for regulating the flow of the refrigerant through the evaporator includes a liquid feed control valve disposed in the first conduit in communication with said vapor-liquid sensing unit, whereby the liquid refrigerant is prevented from entering the means for combusting vaporized refrigerant.

11. A method for determining liquid refrigerant in a tubular evaporator of an open cycle ammonia refrigeration system wherein liquid amonia refrigerant is vaporized by absorbing heat from the surroundings of the evaporator, which comprises: a. disposing a heat conductive element in heat transfer contact with the refrigerant flowing through the evaporator; b. heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liQuid and vapor refrigerant; and c. relating the temperature of the heat conductive element to the proportion of liquid refrigerant flowing through the evaporator with the highest temperature indicating refrigerant substantially all in vapor and a lower temperature indicating liquid refrigerant in heat transfer contact with the heat conductive element to effect heat abstraction therefrom to lower its temperature.

12. A method as claimed in claim 11 wherein the flow of refrigerant through the tubular evaporator is regulated, actuated by variation in temperature of said heat conductive element.

14. An open cycle refrigeration system comprising: a. a storage tank for liquid ammonia refrigerant; b. a tubular evaporator wherein the liquid refrigerant is vaporized by absorbing heat from the surroundings of the evaporator; c. a first conduit for conveying a liquid refrigerant from the storage tank to the evaporator; d. a second conduit for conveying vaporized refrigerant from the evaporator to combusting means; e. means for combusting the vaporized refrigerant; f. vapor-liquid sensing unit disposed at an intermediate point between the tubular evaporator entry and exit in heat transfer contact with the refrigerant flowing through the evaporator such that the heat transfer surface area of an auxiliary portion of the tubular evaporator located on the discharge side of said vapor-liquid sensing unit ranges from about one-fourth to one-third of the total heat transfer area of the tubular evaporator, said vapor-liquid sensing unit comprising a heat conductive element and means for heating said heat conductive element whereby the temperature of the heat conductive element will vary as a result of heat abstraction by refrigerant in heat transfer contact, the variation in temperature dependent on the proportion of liquid and vapor refrigerant; and g. interposed in said first conduit, means for regulating the flow of the refrigerant through the tubular evaporator actuated by said vapor-liquid sensing unit.