US2099085A - Superheat control for refrigeration systems - Google Patents

Description

Nov. 16, 1937. J, SHRODE 2,099,085

y SUPERHEAT CONTROL FOR REFRIGERATION SYSTEMS Filed June 8, 1936 3 lSheets-Sheet l NOV. 16, 1937. J, L, SHRODE 2,099,085

SUPERHEAT CONTROL FOR REFRIGERATION SYSTEMS Filed June 8, 1936 3 Sheets-Sheet 2 Jig. Za

gmc/rm Nov. 16, 1937. v J. sHRoDE 2,099,085

SUPERHEAT CONTROL FOR REFRIGERATION SYSTEMS Filed June 8, 1936 3 Sheets-Sheet 3 37 f '3; l/ l 4.a

Patented Nov. 16,V 1937 Sy PATENT OFFICE i SUPERHEAT CONTROL FOR REFRIGERA- TION SYSTEMS John L. Shrode, St. Louis, Mo., assignor to Alco Valve Company, Incorporated, St. Louis, Mo., a corporation of Missouri Application June 8,. 1936, Serial No. 84,203

9 Claims.

This invention relates to improvements in refrigeration systems of that type in which liquid refrigerant vaporizes and expands in an evaporator.

One of the objects of the invention is to improve the eiliciency of such systems by controlling the refrigerant in a manner that will assure the wetting of the entire surface of the evaporator without the risk of lthe liquid refrigerant being drawn over into the compressor and without the disadvantage of having the liquid refrigerant impinge on the thermostatic bulb with the result of paralyzing the expansion valve so that it does not properly respond to the refrigerating needs.

vision of means to prevent hunting or cycling of the expansion valve, which occurs when the speed of the valve does not permit an immediate response to change in the condition at the outlet of the evaporator, or where the valve response does not occur at such time that the cycling is dampened on one or more of the harmonics of the natural frequency of the system.

A further object of the invention is to provide means by which higher velocity of ow of the refrigerant through the evaporator is made possible with consequent improvement in surface eciency.

Still another object of the invention is to prol vide means for permitting the entire evaporator surface to be internally'wetted and effective for the absorption of the latent heat of evaporation of the refrigerant, and at the same time to provide a means for superheat 'control from the re- 35 frigerant gas leavingthe evaporator.

More specifically, the object of the invention is to provide a liquid and gas separator at the suction side of the system and preferably at or near the suction end voigthe evaporator and be- 10 tween the evaporator and the-'thermostatic bulb, which separator ink addition to separating liquid refrigerant `from the cgasefous *refrigerant and returning it to the' system",4 provides a' gas receiv ing chamber inf whlchthe absorbed `super-heats l from different regionsof the-evaporator'are averaged,` thus determining i for'lthe [expansion 'valve an average 'control'y factor' and` in some systems' A in jwhich'velocitiesandifl'oagd' conditions vare `such that the ygastendfs vto 'remain uniformlyfsturated throu g houi.` `v ythe3fevapbator v*limita vthe heat-exf changing vcapacity offth l*separator itself ma'yfbe dpendhd `uponf-*to ab'soijbjth upeheat neces# sary jvtotoperate' theie'irpansin'valvef Still another object of the invention is to provide the separator as described with means for Another object of the invention is the pro-l overcoming the pressure difference across the evaporator so that the separated liquid refrigerant can be returned to the evaporator from the separator.

Other objects of the invention will appear as 5 the following description of a preferred and practical embodiment thereof proceeds.

In the drawings which accompany and form a part of the following specication and throughout the several figures of which the same characters of reference have been employed to designate identical parts:

Figure 1 is a side elevation of a refrigeration system embodying the principles of the present invention;

Figure 2 is a longitudinal section of the com- -bined expansion valve and injector shown in longitudinal section at the lower end of the evaporator in Figure 1;

Figure 3 is a longitudinal section through the liquid and gas separator illustrated at the upper end of the evaporator in Figure 1;

Figure 4 is a side elevation of a modified form of the invention;

Figure 5 is a vertical section through the combined expansion valve and liquid and gas separator illustrated as an element of the refrigeration system in Figure 4;

Figure 6 is a side elevation, partly in section, showing a iiooded type evaporator with a separator above the level of liquid in the evaporator so that gravity is suiiicient to return the separated liquid refrigerant to the evaporator; and

Figure 7 is a section of view showing a vertical tank type evaporator showing theseparator built inside.

Figure 8 is a vertical section through a type of separator alternative to that shown in Figs.

3 and 5.

Figure 9 is a plan view of Figure 8 part being broken away. .j o

.Before adverting to a detailed description of the several gures, thenature .of the `invention may be best understoodby a discussion oi' the problem which it solves.' t I A1 common method for controllingl the flow oi?, liquid'refrig'erant from the high pressure side of the refrigeating apparatus to'theylovv.pressure 1T side where itZ is evaporated, is through `useyof `a thermostatic expansion valve :having fthe `valve element actuated. by the -superheat `of i thezgaseous refrigerant' leaving the evaporator.;y rif-r gr. A;

#IIt isdesirable'to installsuchaivalve atfoneend y of the evaporator and to make this valve the expansion point where the refrigerant pressure is 55 pressure of evaporation.

valve actuating devices is usually charged with a perature.

volatile substance, preferably thev same as that used as a refrigerant in the system to be controlled.

'I'he pressure in the evaporator is imposed on one side of the diaphragm-of the expansion valve while the saturated pressure of the fluid in the thermal element at the temperature to which the remote thermal bulb is subjected is imposed on the opposite side of the diaphragm. Said diaphragm is unbalanced in a direction such that the valve is normally biased toward the closed position by a spring or other loading device and in order to enable thecontrol to reach a balanced condition the pressure on one side of the diaphragm, that in the thermal element, must be increased to exactly balance the spring load and maintain the valve in the correct position.

This extra pressure is generated by the extra -temperature of the bulb when subjected to the superheated suction gas, the pressure in the bulb being that corresponding to the saturated condition of the thermostatic fluid at the bulb tem- It is obvious that with this method of control a certain proportion of the heat exchanging surface of the `evaporator 4must be devoted to the absorption of this superheat. Theoretically the evaporator may be regarded as divided into two conditions of surface. One surface extends from the evaporator inlet. that is to say, from the expansion valve K to the point where substantially all of the liquid refrigerant is evaporated, this surface being effective for the absorption of the retained heat of vaporization of the refrigerant. The rest of the evaporated surface, that is, from the point where most of the liquid is evaporated l to the point of remote bulb contact, the point of superheat control or the evaporatoroutlet, absorbs only the sensible heat required to superheat the refrigerant gas to the desired or controlled superheat.

Practically, no such line of demarcation between the saturated and superheated part of the evaporator occurs. As a matter of fact, in the majority of systems, the gas forming in the rst part of the evaporator does not remain in the saturated condition. until it reaches the line of complete evaporation near the evaporator outlet.

The flash gas formed, due to the vaporizing rei'rigerant, is in intimate contact with the metallic surface of the evaporator. It is obvious that the gas will absorb some superheat raising the temperature above the satlration point, and even though liquid refrigerant be present in the tube adjacent this gas accumulation, the low conductivity of the vaporized refrigerant will prevent the lowering of th`e temperature of the gas to the saturation point, due to heat interchange between the liquid and the vapor, if the velocity will not permit suicient time to accomplish this tempera- Ature equalization. In extra long single pass tube type vaporizers a very pronounced difficulty is experienced in establishing a line of complete evaporation due to the alternate sluggingthrough of liquid accumulations and gas pockets.

The alternate liquid slugs may surround the gas and be pushed to the evaporator outlet as a result -of the expansion ,of gas behind the liquid. By

separating the liquid slugs from the gas pockets,

'as by centrifugal separation, the gas of a lvariety -of superheat values from different parts.of the evaporator is mixed to average its superheat, and

the liquid is returned to the evaporator. This permits good average superheat control and at the same time maintains a wetted coil without the use of drying surface.

The amount of surface required for the absorption of superheat will depend on the character of the transfer surface of the refrigerant conduit, the temperature gradient between the refrigerant and the load, the total load which determines refrigerant gas velocity when evaporator tube or tank area is considered, and the character and distribution of the load.

From the above discussion it will be seen that in product cooling applications where the load is usually at a very low temperature gradient with the refrigerant temperature and where the load is commonly imposed through air with natural circulation over the evaporator, a considerable percentage of the total surface of the evaporator may be required for the absorption of the controlling superheat. Conversely, when forced draft air at higher temperature gradients with the refrigerant temperature or with liquid loads on evaporators a very small percentage of the total evaporator surface may be required for the absorption of superheat.

We must agree that the conditions of ideal eiliciency require that the entire surface of the evaporator be wetted, that the evaporation in the superheat to which the expansion valve responds should be within a very narrow range, and that little or'no liquid refrigerant should pass beyond the evaporator. Since the point of saturationnecessarily varies with load conditions on the evaporator as well as upon features previously touched upon, the only way to assure that the entire evaporator surface shall be wetted is to set the expansion valve to supply enough liquid refrigerant so that some of it will still be in liquid state when the suction end of the evaporator is reached and this implies, of course. that some of the liquid refrigerant will be entrained inthe eilluent gas. This liquid impinging upon the inner surface of the suction line at the point contacted by the thermostatic bulb, or on the bulb, itself if it is of the inserted type, causes-the eX- of response of the valve. Hunting occurs whenl the speed of the valve does not permit an immediate response to a change in evaporator outlet condition or where the valve response does not occur at such a time that-the cycling is dampened on one or more of the harmonics of the .natural frequency of the system. 4

It may be well to trace the cause of this cycling and its effect on the efliciency of the evaporator.

. When the system is started the valve opens feeding refrigerant to the evaporator. 'I'his feeding continues until the superheat at the point of remote bulb contact reaches the minimum value for which the valve is adjusted. At this point the valve closes causing the suction pressure to be reduced with an increased boiling of the refrigerant due to the lowered suction pressure. 'I'his increased boiling may cause a fcontinued carry over of liquid refrigerant even though the `75 valve has completely shut off the iiow of liquid to the coil. 'I'he valve, of course, remains closed until the entrained liquid has been passed to the compressor and enough refrigerant has been pumped from the evaporator to restore a comparatively dry condition in the gas at the point of valve remote bulb contact. At this point, the valve will again be opened by the rapid increase in temperature at the evaporator outlet and the cycle is repeated. Even in cases where the closing and opening of the valve are not complete, a variationin valve feed due to the lag of heat transfer and to the inertia of the mass of suction lines, the evaporator, and' load add to the possibility of hunting or cycling of the constant speed valve mechanism.

It has been possible to tune the valve, Athat is, match frequencies to create a dampening effect to reduce this cycling to a minimum, but consistent results require a study of each individual evaporator so that there will be no reduction in thatsurface effective for the absorption' of the latent heat of vaporization of the refrigerant during the period of valve restriction and no flood back to endanger the compressor mechanism during or immediately after the period of valve opening.

The control further provides for the precooling of part of the liquid refrigerant being circulated without the introduction of excessive flash gas to accomplish this cooling effect.

On long tubular evaporators it has been a common experience to nd during` the system shut down period that the refrigerant either drained into the vsection adjacent the point of remote bulb application by gravity or was condensed into the liquid state at the coldest point in the evaporator having evaporated from the warmer points.

When the system starts this liquid refrigerant being confined in a relatively small portion of the evaporator is not exposed to enough surface to permit the absorption of enough heat to completely vaporize. Since it cannot pass out in its liquid state without damaging the compressor and since the valve will not open until the suction gas has reached the desired superheated condition, a compressor operating on a suction pressure control switch may make several short cycles before the valve will open and permit the adequate refrigeration of the rst part of the evaporator after which the cycle is repeated. Since in the new scheme the accumulated liquid is recirculated, the rst part of the evaporator becomes effective immediately on the start of the compressor thus cutting the machine running charges into the condenser 5 and the latter is connected to the expansion valve by the conduit '6, this conduit being the normal liquid limb of the refrigeration system. In practice a receiver for a liquid refrigerant is intercalated in the conduit 6, but for the purpose of the present invention the above described schematic layout of aA refrigeration system is deemed sufficient.

The expansion valve whichI is shown in detail in Figure 2 comprises a valve 1, the stem 8 of Which is connected to a diaphragm 9 in the diamedium on the outside of the evaporator.

phragm chamber lil formed in the casing II. Aspring I2 around the valve stem normally pushes upward and biases the diaphragm in a direction .in which the valve 'I is maintained closed. When the valveis open liquid refrigerant `enters in the direction of the lower arrow and exits in the direction of the lateral arrow to the left. The venturi as shown is not a part of the normal expansion valve and its function will later be explained.

The upper side vof the diaphragm is exposed to pressure derived from the thermostatic bulb I3 shown in Figure 1, said bulb containing a vola- .tile liquid which is generally-the same liquid as constitutes the refrigerant. The bulb I3 is preferably placed at the suction end 'of the 4evaporator and responds to the temperature of the eluent refrigerant gas. The lower side of the diaphragm is subject tothe pressure of the refrigerant at the anterior end of the evaporator by way of the ports 26 and 27 which establish a through communication between the evaporator and the diaphragm chamber beneath the diaphragm. i

It is obvious that enough pressure must be developed in the bulb I3 and in the connecting tube I to overcome the pressure of the spring I2 beforethe valve I can be opened this pressure being derived from heat absorbed from the refrigerant gas. It is well understood therefore that the temperature of the refrigerant gas at this point is the sum ofthe temperature acquired by the refrigerant in doing useful refrigeration, plus an additionally absorbed temperature sumcient for developing the necessary pressure in the ther-- mostatic bulb; In other words, normally the liquid refrigerant enteringthe evaporator from the expansion valve has entirely evaporated, that is to say, reached the saturation point of its gas at a point some distance back of the suction end of' ant has the absorbent'capacity for latent heat.

In that part of the evaporator indicated within the arms of the brace b the refrigerant being in completely gaseous. state does not do any substantial refrigerative work since it cannot' absorb any more latent heat.' Heat interchange'des not take place between this gaseous refrigerant and the atmosphere of the. refrigerated chamberor whatever it may be that constitutes the outside load, but since the mass of gas in' this part of the evaporator is very small it warms up without' appreciably increasing the temperature Aof the It is clear therefore that .the section of the evaporator within the arms of the brace h may be considered as not working insofar as its refrigerative efllciency is concerned and yet it is very necessary in raising the temperaturegvof the refrigerant gas sufficiently to raise the pressure within the bulb I3 to the value at which it will overcome the spring I2 and open the valve 8. By increasing the feed of expansion valve 2,or by increasing the velocity of flow through the evaporator, the point a can be l moved to the suction end of the evaporator, butv 'in so doing, there is'danger that for a slight decrease ln the outside load liquid will be drawn out of the vevaporator through the suction conduit 3 and back to the condenser. It is known of course that the compressor will be damaged or destroyed by working against an in-compresslble medium drawback in the attempt to keep the saturation point at the eduction end of the evaporator is that some of the liquid entrained in the eiiluent gas will impinge against the bulb I3 if it be of the immersed type or against the wall of the conduit 6 against which the bulb I3 contacts, so as to keep the bulb unduly cool and keep the valve 1 shut under load conditions at which the valve should open. 'I'he evils of this have been discussed in connection with the general problem.

In order that it may be possible to continue the' evaporation of the liquid refrigerant clear to the eduction end of the evaporator and at the same time to avoid the necessity of giving over any part of the evaporator to the superheat necessaryfor boiling the thermostatic bulb up to the desired pressure, I have provided the gas and liquid separator I5 into which the evaporator immediately discharges and adjacent which the thermostatic bulb I3 is located. Figure 3 shows exemplary details of this separator and it will be noted that it comprises a casing I6 forming a bowl I"I at its lower end for the reception of the liquid and a circular baille I8 extending 4downward within the casing to a point adjacent the top of the bowl and spaced from the sides of the casing forming the annular chamber I9.

Gaseous refrigerant which may under the conditions of the problem contain entrained liquid refrigerant enters the inlet 20 which is preferably I entrained liquid against the inside wall of the casing from which the liquid trickles down into the bowl I'I. The gaseous refrigerant being thus freed from the liquid, flows around the lower edge of the circular baille I8 and is drawn up through said baille discharging by way of the mouth 2l into the suction line 3. It is thus obvious that one does not have to be precise in adjusting the expansion valve to load conditions, it being only necessary in the interest of efliciency that the adjustment be such that the last vestige of liquid will not be evaporated until the suction end of the evaporator is reached. vDue to variations in load, this will certainly mean that liquid will from time to time be drawn in through the separator, which is of no moment for it cannot possibly pass to the compressor nor can it impinge against the bulb I3 or the wall of the suction line 3 contacted by said bulb. Since the gas generated in the bulb I3 is atsaturation pressure and under the efcient conditions provided by the present invention, the gaseous refrigerant at the suction end of the evaporator is also at saturation pressure, it follows that the diaphragm 9 would be equally pressed upon by both sides in the absence of means for creating a suiiicient superheat. Such means is provided in certain types of evaporator structure in which in localized regions in the evaporator itself the gas becomes superheated through insuicient velocity to move away from the heating points fast enough to avoid the surplus head absorption. In such cases'the separator brings together the several gas bodies from the evaporator, mixes them by centrifugal force and thus averages the lsuperheat before it passes on to exert its control upon thebulb which governs the operation of the expansion valve. In those instances in which the type of evaporator is such that it can be maintained wetted from end to end without the development of superheat, the area of the Walls of the separator are preferably so designed as to provide suicient heat interchange Vbetween the 2,099,085 such as a liquid refrigerant. Perhaps the greatest gaseous refrigerant within the separator, and the outside atmosphere, to heat the gas suiiiciently to boil the liquid contents of the bulb I3 up to the desired gas pressure in the tube I4 and in the upper side of the diaphragm chamber.

Re-circulation of the separated liquid refrigerant from the bowl I1 back onto the evaporator is provided by means of the tube 22 connecting the bowl with said evaporator. Since the pressure is higher at the induction end of the evaporator than it is at the suction end, means must be provided for overcoming this pressure differential in order that the liquid from the bowl I1 may be' returned from the evaporator. This means comprises a venturi 23 into which the nozzle 24 of the expansion valve discharges, creating a sub-pressurer in the annular chamber 25 which surrounds the nozzle 24 to the rear of the ve'nturi and into which chamber the tube 22 discharges. The liquid becomes entrained in the neck of the venturi with the liquid discharging from the nozzle 24 of the expansion valve and thus is carried back into the evaporator. No claim is made to the evaporator or separator per se but only in combination with the system as illustrated and described and in respect to functions which it performs peculiar to its relationship with the several members of said system. The inlet 20 opens into the annular chamber I9 adjacent the upper part thereof impinging against the cylindrical baiiie I3 changing its direction suddenly and dropping the liquid refrigerant which it has entrained, which liquid collects in the bowl I'I. The separation of liquid is assisted by the centrifugal force engendered by the whirling of the gaseous refrigerant about the baiiie I8. Turbulence in the lower part of the chamber I9, that is to say, in the bowl I1 which receives the liquid refrigerant, is discouraged by the nature of the construction, the baille terminating above the bowl and having a wide inlet portion. It is undesirable to have turbulence within the bowl I'I, for circulation ofthe gas over the liquid in said bowl would result in a useless vaporization of' said liquid throwing an unnecessary burden upon the compressor.

In the alternative form of separator shown in Figure 8 the gaseous column with entrained liquid from the evaporator enters tangentially by way of a nozzle 28 flattened along a vertical axis, the liquid refrigerant collecting in the bottom of the separator and being returned to the system by way of the pipe 29. The cylindrical baille 3U which in this instance is the lower end of the suction pipe 3I extends substantially half way down into the separator. The Walls 32 of the separator are\ made of as small a mass as possible with consequent lessened tendency to create a temperature hold-over in the separator between the liquid slugs received by way of the nozzle 28. low conductive lining 33 which may be of synthetic resin or any other suitable heat insulating substance.

The separator 32 shown in Figure 8 may be substituted, if desired, for the separator I5 illustrated in Figure 1 and in either instance it may be desirable occasionally to drain the bowl I1 into the suction pipe 3 of the system. Figure 1 shows a tube 34 communicating with the lower portion of the bowl I1 and being connected into the pipe 3, said tube is provided with a valve 35 which may be opened to permit the gas within the separator to be drawn into the suction pipe. Of course a certain amount of liquid will be drawn in with the The separator may be lined with a suitable gas but the valve 35 may be adjusted to such an extent as to let in no more liquid than the suction pipe can evaporate before it reaches the compressor.

Figures 6 and '1 show tank type evaporators, that in Figure 6 being horizontal while that in Figure 7 is arranged vertically. In Figure 6 the suction pipe 36 leaves the top of the evaporator 31 above the liquid level therein and communicates with the separator 38. The separator has a gas outlet 39 and the remote control bulb 40' which controls the operation of the expansion valve 3l is immersed in the end of this suction pipe. The gas with entrained liquid from the pipe 36 enters the separator tangentially, whirls around the cylindrical baille 62, dropping the liquid to the bottom of the bowl and passing through the pipe39. The separator is placed at such a height that the bottom of the liquid-receiving portiorr thereof is above the liquid level in the evaporator 31. This permits gravitational return ofv liquid from the separator back into the evaporator by way of the pipe 453, said pipe is shown well lagged with insulation material t3 preventing evaporation of gas and consequent develop-` ment of b ack pressure within thepipe 33.

In Figure 7 a slightly modified form of the invention is shown in which the separator 65 is housed within the tank evaporator 66. The other elements are analogous to those shown in Figure 6. The expansion valve i1 delivers liquidrefrigerant to the evaporator by Way of the tube 18, the liquid evaporates and the gas resulting from the evaporation passes through the ports 59 and 5D in the head of the separator,

passing down into the separator dropping the entrained liquid within the bowl Vthereof and passing up through the bae l to the compressor by way of the pipe 52. The liquid drains from the separator by way of the tube 53 the lower end of which terminates short of the bottom of the evaporator.

.A slight variant of the invention is illustrated in Figures 4 and 5, instead of the expansion valve and separator being separate entities, they are combined in a unitary structure. Referring to Figure 4, the evaporator is indicated bythe reference character l and the other conventional parts of the system bear the same characters'y as that form of the invention shown in Figure 1. The combined expansion valve and separator is designated by the reference character 28. The structure-of the expansion valve is shown in Figure 5 and is quite similar to that shown in Fig- 'ure 2, the main distinction being that the evapolence and to throw liquid particles of the evaporator outward by centrifugal force against the inner wall of the separator. The liquid trickles down into the bottom or bowl 33. There is a direct connection 35 between the bowl 33 and annular chamber 25 of the venturi, the liquid from the separator being thus entrained by the liquid jet discharging from the nozzle 24 of the venturi.

In the construction shown in Figure 5, the upper `side of the diaphragm chamber is adapted to be put into communication with the thermostatic bulb I3 by the tube I4 connected to the port 36 of the casing of the expansion valve, and the lower side of the diaphragm is placed into direct communication with the anterior end of the evaporator by a tube 22 which connectedby way of a bore 31 directly with the lower side of the diaphragm chamber.

It will readily be appreciated that efliciencies higher than those possible with any other expansion method are made possible by either of these arrangements or any other equivalent arrangements which may be`devised. The control may be adjusted to maintain a completely internally wetted surface throughout the evaporator and at the same time utilize the velocity of fiow from the high side to the low side to create increased circulation in the evaporator tubes particularly when the latter are horizontal as illustrated, thus giving transfer coefficients in excess of those obtained with full-flooded control where lack of adequate circulation permits the formation of comparatively large gas pockets and dry surfaces.

The valve responding to the average superheat condition of the well mixed suction gas has much less tendency to hunt or cycle since a considerable amount of liquid may pass through the separator and veryv gradually change the average superheat of the gas. Thus it will be seen that a constant speed valve will control over a wide lrange of evaporator speeds and load conditions withoutcycling or the extraordinary care necessary in tuning the valve when liquid may hit the control point as with the present method of valve application without separation. While I have in the above description disclosed what I believe to be preferred and practical embodiments of the invention, it will be understood to those skilled in the art that the details of construction and the arrangement of parts are merely by way of example and that the invention contemplates the principles underlying all equivalent means for accomplishing the ends as herein described and covered under the terms of the appended claims.

What I claim is: f

1. Refrigeration system including an expansion valve, an evaporator and a thermostatic control device for said evaporator, serially arranged, and means between the suction end of said evaporator and said thermostatic element, preventing the temperature of refrigerant in liquid state directly affecting said thermostatic element, by arresting refrigerant in liquid form which may be entrained in the eiiluent refrigerant gas frpm said evaporator.

2. Refrigeration system including an expansion valve, an evaporator and a thermal control element for said, expansion valve, serially arranged, means between the suction end of said evaporator and said thermostatic element preventing the temperature of refrigerant in liquid state directly affecting said thermostatic element by arresting refrigerant in liquid fom which may be entrained in the eiiiuent refrigerant gas and means for returning the liquid refrigerant thus arrested to said evaporator.

' 3. Refrigeration system including an expansion valve, an evaporator and a thermal control element for said expansion valve, serially arranged, means between the suction end of said evaporator and said thermostatic element preventing the temperature of refrigerant in liquid .State directly affecting said thermostatic element static pressure bulb being adjacent the suction end of said evaporator and subject to the temperature of the eiliuent refrigerant; adiaphragm for actuating said expansion valve, a spring normally holding said expansion Yvalve closed, the side of said diaphragm opposite said spring being connected to the bulb and subjected to the pressure'thereof, the opposite side of said diaphragm being subject to the pressure within lsaid evaporator adjacent to the point of admission of liquid refrigerant from said expansion valve and a liquid and gas separator intercalated in said system between the suction end .of said evaporator and said bulb, preventing the temperature of refrigerant in liquid state directly affecting said thermostatic bulb by arresting refrigerant in liquid form which may be entrained with the effiuentrefrigerant gas, preventing it going into heat exchanging relationship with said bulb, and means for returning said liquid to said evaporator adjacent the point of admission oi' liquid from said expansion valve, said last named means including means for overcoming the pressure differential across said evaporator..

5. Refrigeration system including an expansion valve, an evaporator, and thermostatic pressure bulb, f serially arranged, said thermostatic pressure bulb being adjacent the suction end of said evaporator and subject to the temperature of the eilluent refrigerant, a diaphragm for actuating said expansion valve, a spring normally holding said expansion valve closed, the side of said diaphragm opposite said spring being connected to the bulb and subject to the pressure thereof, the opposite side of said diaphragm being subject to the pressure within said evaporator adjacent to the point of admission of liquid refrigerant from said expansion valve, and a liquid and gas separator intercalated in said system be- 4tween the suction end of said evaporator and said bulb, preventing the temperature of refrigerant in liquid state directly affecting said thermostatic bulb by arresting'refrigerant in liquid form which may be entrained with the eiiluent refrigerant gas, preventing it coming into heat exchanging relationship with said bulb, and means for returning said liquid to said evaporator adjacent to the point of admission of liquid from said expansion valve, said last named means including means for overcoming the pressure differential across said evaporator, the surface area of said separator being designed to absorb the I superheat essential for creating a pressure in said bulb suiiicient to overcome the closing spring of 'said expansion valve.

6. Refrigeration system including an evaporator, an expansion valve controlling the admission of liquid refrigerant to said evaporator, mechanical means normally holding said valve closed, fluid pressure actuated means for opening said valve, a bulb containing volatile liquid, positioned in heat exchanging jrelation to said system at a point adjacent to suction end of said evaporator, in communication with the fluid pressure actuated means, and means interposed in said system between suction end of said evaporator and said bulb `for absorbing the superheat necessary to create a bulb pressure of suicient value to overcome said valve closing means.

7. Refrigeration system, including an expan- 'sion valve, an evaporator and a thermostatic bulb, serially arranged in the order named, said thermostatic bulb being arranged in heat exchanging relationA to the suction limb of said system for controlling said expansion valve, and a mechanical means for centrifugally effecting separation of liquid and gaseous refrigerant, said means being located in said system at some point between the said expansion valve and said thermostatic bulb.

8. Refrigeration system including an expansion valve, and a thermostatic'bulb arranged in heat exchanging relation to the suction limb of s aid system for controlling said expansion valve, a separator for centrifugally effecting separation of liquid and gaseous refrigerant, said separator being located in said system at some point between said expansion valve and said thermostatic bulb, and means for returning the liquid thus separated to the evaporator at a point adjacent to said expansion valve.

9. Method of operating a refrigerating system of the type which includes an evaporator, an expansion valve, a compressor, and a bulb controlling the expansion valve, comprising varying the admission of liquid refrigerant from said expansion valve to said evaporator according to load conditions so as to keep the saturation point of the gaseous refrigerant substantially at the extreme suction end of the evaporator, separating from the gaseous refrigerant and arresting entrained liquid refrigerant at a point anterior to said bulb, bringing only the gaseous refrigerant into contact with said bulb, diverting the liquid refrigerant away from the compressor, and returning the thus arrested' liquid refrigerant to said evaporator.

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