US2934911A - Heat exchange system of compression type with air cooled or evaporative condenser and method of operating the same - Google Patents

Description

May 3, 1960 w. MICAI ETAL 2,934,911

HEAT EXCHANGE SYSTEM OF COMPRESSION TYPE wI'rH AIR COOLED OR EVAPORATIVE CONDENSER AND METHOD OF OPERATING THE SAME Filed March 28, 1956 4 2 Sheets-Sheet 1 All ' ATTORNEYS" United States Patent HEAT EXCHANGE SYSTEM OF COMPRESSIQN TYPE WITH AIR COOLED OR EVAPORATIVE CONDENSER AND METHOD OF OPERATING THE SAME William Micai and Daniel E. Kramer, Trenton, Nah, as-

signors to Kramer Trenton Company, Trenton, N.J., a corporation of New Jersey Application March 28, 1956, Serial No. 574,533

13 Claims. (Cl. 62-417) This invention relates to a heat exchange system of compression type with air cooled or evaporative condenser, and has for an object to provide such a system which embodies a refrigerant flow circuit comprising interconnected compressor, condenser, receiver, evaporator, and a refrigerant pressure reducing device between receiver and evaporator, with means and method for automatically maintaining satisfactory functioning of the system in spite of conditions under which ambient temperature at the condenser approaches or is even lower than that of the refrigerating or cooling chamber in which the evaporator is located.

Another object is to provide such a system, in which compressor operation is governed by a pressurestat, or the like, with means for insuring start of compressor operation which might be prevented by lack of rise in low side pressure in response to rise of temperature in the refrigerating or cooling chamber.

Another object is to provide such a system with means for preventing migration or flow of refrigerant from the evaporator through the low side to the condenser during intervals between compressor operation.

Another object is to provide such a system with means for maintaining adequate heat for rapid and thorough hot gas defrosting of the evaporator, and for supplying warm refrigerant from the receiver to augment the defrosting operation, Whether the ambient temperature at the condenser is higher or lower than that within the refrigerating or cooling chamber.

Another object is to provide such a system which includes means for isolating or disconnecting the condenser from the other elements of the system during the intervals between compressor operation, and for maintaining receiver pressure at the desired level during both on and off cycles of compressor operation.

Another object is to provide such a system which includes means for insuring immediate flow of refrigerant at satisfactory operating pressure from receiver to the pressure reducing device upon each resumption of compressor operation following intervals of idleness.

A further object is to provide certain improvements in the form, construction and arrangement of the several parts of the system, and in the steps of the method, whereby the above named objects, and others inherent in the invention, may be effectively attained.

In brief summary, the invention comprehends the provision of a refrigerating or air conditioning heat exchange system of the compression type, which is designed and adapted for operation with an air cooled or evaporative condenser that is exposed to outdoor atmospheric conditions, the said system being so constructed and ar ranged that, even under climatic conditions in which the condenser is subjected to ambient temperature that is higher or lower than the temperature within the chamber or space cooled by the evaporator, satisfactory operating condensing and high side pressure is maintained; migration or flow of refrigerant, during off-cycles of the compressor, from evaporator through the low side and compressor to condenser is inhibited by isolation or severance of communication with the condenser; receiver pressure is maintained in order to provide ample supply of refrigerant at operating pressure to the pressure reducing device, e.g. thermostatic expansion valve, that is in cooperative relationship with the evaporator inlet, immediately following the initiation of a compressor oncycle; adequate heat is maintained in the system for thorough and rapid evaporator defrosting; unwanted or unscheduled cessation of compressor operation or offcycles is prevented; spoilage of substances in the refrigerator or cooled chamber or space is eliminated; and constant, substantially uniform and highly eflicient operation of the system is maintained regardless of fluctuations, including extreme changes, of ambient temperature at the condenser.

Practical embodiments of the invention are shown in the accompanying drawings in which:

Fig. 1 represents diagrammatically such a system in which condensing and high side pressure are controlled by means operating on a siphonic principle, and isolation of the condenser during compressor off-cycles is effected by solenoid and check valves;

Fig. 2 similarly represents such a system in which con densing and high side pressure are controlled by means of constant pressure valves, and isolation of the condenser during compressor off-cycles is effected by constant pressure and check valves; and

Fig. 3 similarly represents such a system as shown in Fig. 2 which also includes means for reevaporating refrigerant flowing back from the evaporator to compressor intake during defrosting cycles.

In heat exchange engineering as exemplified, for instance, in the refrigerationand air conditioning industry, it is well known that in compression type systems the high side pressure is related to the condensing temperature, which latter is dependent upon the relative temperature of the coolant. sharp drop in ambient temperature at the condenser can lead to a decrease in the high side pressure of the system which lessens the functional capacity of the system by curtailing flow through the customary pressure reducing device, e.g. thermostatic expansion valve, at the inlet of the evaporator due to the reduction in pressure differential across the valve. This condition also notably restricts eflicient hot gas defrosting of the evaporator. It is also well understood that the heat transfer characteristics of evaporator and condenser are essentially opposed in that more complete flooding of the former with liquid refrigerant increases its functional capacity, while increased flooding of the latter decreases its condensing capacity. It Will therefore be evident that control of the level of liquid refrigerant within the condenser can serve correspondingly to control the high side pressure of the system and more particularly with respect to pressure at the thermostatic expansion valve or its equivalent pressure reducing device.

Advantage has been taken of this last stated factor in certain praiseworthy improvements directed to maintenance of high side pressure in the system and thoroughly satisfactory defrosting capability in spite of severe low temperature ambient to which the air cooled condenser is subjected in certain regions and certain seasons, which is of substantial commercial importance due to the fairly recent and rapid enlargement of the installing of air cooled condensers in year-round operating heat exchange systems in which the condenser is located outdoors to take advantage of the coolest ambient atmosphere in the warm While the said improvements have constituted notable ad- Patented May 3, 1960 Thus, if air is the coolant, a'

vances, they have not attained full realization of substantial perfection due to the presence of certain drawbacks or impediments. Thus, it has been experienced that, when the atmospheric temperature at the condenser drops to a degree lower than that obtaining in the chamber or space cooled by the evaporator, there is a tendency for the refrigerant liquid in the evaporator to migrate or flow through the suction conduit and the compressor into the chilled condenser during non-operating or off-cycle compressor periods. This phenomenon prevents suflicient rise in the low side pressure to restart the compressor by actuation ofits pressure switch, because the system pressure does not rise above that corresponding to the temperature of the outdoor ambient; and the result can be spoilage ofsubstances within the chamber or space designed to be cooled by the evaporator. Again, the system does not retain sufiicient heat for rapid and satisfactory defrosting of the evaporator, which is particularly true in arrangements that include heat storage means which are subject to theprovision of heat thereto by exchange from the hot gas.

discharge of the compressor.

The present invention is calculated to partake of the advantages inherent in the improvement advances above mentioned and also to afford further advances by way of obviating drawbacks and impediments to completely satisfactory operation which have been hereinabove noted.

Referring to the drawings and touching first upon Fig. l, the embodiment there shown includes a compressor 1, condenser 2, receiver 3 and evaporator 4. These parts may be of any well known or approved form but it should be noted that the condenser 2 is of the air cooled type. It and evaporator 4 are shown as fitted with the customary fan and motor units which will be understood to be connected with suitable electric circuits for activating and deactivating the fans according to the cycle of operation of the system all as is routine and thoroughly understood by operatives in this field, and requires neither illustration nor further description. The evaporator 4 is illustrated as positioned within a cooling chamber or space, two walls of which are shown and marked 5, 6; while the compressor and receiver are illustrated as positioned within a building that embraces the chamber 5, 6, and is partially represented by a wall 7 connected with a broken away outline, although they may be positioned out-of-doors.

The compressor discharge is connected by hot gas conduit 8 with the inlet of the condenser, the outlet of which latter communicates with the receiver by conduit 9. As is evident from the drawing, the conduit 9 takes the form of an inverted elongated U, one leg of the U extending upwardly from the condenser outlet 10 to the uppermost point or apex of the U, denoted by 11, while the other leg extends downwardly from the said apex to the receivef; A by-pass conduit 12 connects the compressor discharge conduit 8 with the conduit 9 at the high point 11, and in the said by-pass is fitted a regulating valve 13 of the constant outlet pressure type, which is preferably modulating and adjustable. It may be mentioned that other devices could be substituted for the valve 13 such, for instance, as a solenoid valve controlled by a pressure switch, or an automatic constant pressure expansion valve; but the modulating, adjustable constant outlet pressure valve is preferred.

The receiver 3 is connected tothe inlet of evaporator 4 by the usual liquid refrigerant supply line 14 in which is fitted, adjacent the evaporator inlet, a pressure reducnig device such as the well known thermostatic expansion valve marked 15, diagrammatically represented as controlled by the customary feeler bulb and capillary tube.

The suction line or conduit 16 establishes communication between the evaporator outlet and the compressor intake in the ordinary manner; while a defrosting conduit 17 connects the compressor hotgas discharge 8 with the evaporator inlet at a point between the evaporator and the valve 15, and is fitted with the usual solenoid valve 18.

In the operation of the parts of the system of Fig. 1 already described, the valve 13 is adjusted and set to remain closed as long as the high side pressure of the system, say adjacent the thermostatic expansion valve 15, is at or above a predetermined value, e.g. pounds per square inch. Thus arranged, the hot gas from compressor discharge flows through conduit 8 to condenser 2 and is largely liquefied therein, passing therefrom through conduit 9 to receiver 3, and from the latter through conduit 14- and valve 15 to evaporator 4, where it accompiishes the desired chilling effect and, in doing so, changes largely to the vapor phase, in which form it flows back to the compressor intake, through conduit 16, for recompression and repetition of the circuit just described. During this refrigerating cycle of the system, by-pass 12 valve 13 is closed, and the particular shape of conduit 9 has no regulating effect, merely serving as a passage for the condensed refrigerant which substantially fills it in an unbroken stream due to the siphonic down pull of the column of liquid in the longer leg which uprises from the receiver and counterbalances the down push or weight of the column of liquid in the other leg which rises from the. condenser outlet.

Upon the advent of colder weather conditions in which the ambient temperature at the condenser 2 falls to such a degree that the head or high side pressure of the system drops below the setting of valve 13, the latter will open and gas from conduit 8 will travel through bypass 12 and enter conduit 9 at its apex 11, thus interrupting the continuity of the liquid stream in the conduit and breaking its siphonic effect. This permits the liquid in the leg of conduit 9 that uprises from the condenser outlet to exert its weight pressure at the outlet and restrict outflow from the condenser, thus raising the level of liquid therein. This reduces the operative capacity of the condenser by decreasing its area of internal heat transfer surface and thus serves to elevate the head or high side pressure of the system. When the said pressure reaches the setting of valve 13, it will close and the continuity of the stream of liquid in conduit 11 will be reestablished, so that the refrigerating cycle of the system may continue in normal manner at a pressure equal to the setting of valve 13, which latter, due to its modulating characteristic, accomplishes control of the systems high side pressure without abrupt or substantial variations.

While the structural arrangement and mode of operation hereinabove described serves satisfactorily to maintain adequate high side pressure regardless of ambient atmospheric conditions at the condenser, the system thus set up is not immune to drawbacks or impediments including those heretofore explained as handicapping attainment of the desired substantially fault proof or perfection of functioning such, for instance, as the migrating of the refrigerant from the evaporator through the compressor into the cold condenser during off or nonoperating cycles of the compressor. For betterment in such respects, a solenoid valve 19 is fitted in the hot gas conduit 8, and a check valve 213 is fitted in the conduit 9 which connects condenser outlet with receiver, the said check valve being constructed freely to permit flow toward the receiver and inhibit reverse flow.

In these systems, as is well understood, the operation of the compressor is intermittent, the same being commonly governed by a pressurestat which actuates a switch, indicated generally by 21, receiving and supplying electric current through the wires shown to and cutting it off from the compressor motor; the pressurestat itself being governed by low side pressure which reflects the condition -of the refrigerating or cooling chamber in which the evaporator is located so that, when the chilling effect of the evaporator due to flow of liquid refrigerant through the thermostatic expansion valve has served to satisfy the desired temperature condition within the chamber, compressor operation is interrupted and thrown into what is known as an off-cycle, which continues until there has been a predetermined rise in temperature within the chamber and consequent rise in low side pressure, upon which occurrence the compxssor motor is again started to initiate an on-cycle of compressor operation. This control of compressor operation just described is so usual and well known as to require no further illustration nor description, but it is herein mentioned in connection with the operation of the solenoid valve 19 which is so hooked into the pressure switch electrical circuit that the solenoid valve is opened with the start of each compressor on-cycle and is closed with the start of each off-cycle.

It will thus be clear that, during on-cycles of the compressor while the system is refrigerating, the hot gas from compressor discharge will flow through condenser, receiver, thermostatic expansion valve, evaporator and thence back to compressor intake in the usual manner. When, however, a compressor off-cycle is initiated and the flow of refrigerant to condenser ceases, the pressure in the latter will fall rapidly to a degree corresponding to its ambient temperature. On the other hand, the pressure in the receiver does not fall to any substantial extent, because check valve 20 prevents any back flow of refrigerant from receiver to condenser, thus maintaining receiver pressure at a value or degree corresponding to the temperature of the liquid refrigerant therein. It will thus be seen that the closing of solenoid valve 19 at the beginning of a compressor off-cycle, coupled with the functioning of the check valve 20 during the off-cycle period, prevents migration to the condenser of refrigerant from the evaporator, which migration has hereinabove been explained as an undesirable phenomenon that can have the result of preventing actuation of the pressure switch for restarting the compressor motor according to the designed operation of the system and thus entail mal-functioning.

With reference to the means for defrosting the evaporator, it has already been mentioned that a hot gas conduit 17 leads from the compressor discharge to the evaporator inlet at a point between the evaporator and the thermostatic expansion valve, thus providing heat for the defrosting step in a manner with which those working in this field are familiar. It is the practice in such defrosting provisions to fit a solenoid valve 18, or its equivalent, in the hot gas conduit, and to hook the same into the circuit with an electric timer or the like, which at predetermined intervals serves to open the solenoid valve in the hot gas defrosting conduit and enable that operation to be accomplished; the same timing means acting to close the solenoid valve and terminate defrosting at the expiration of an established duration for which the timer has been set by the operator in charge or installing engineer. According to the present invention, the electric hook-up is such that the solenoid valve 19 closes simultaneously with the opening of the defrosting conduit solenoid valve 18 which permits the hot gas discharged from the compressor to travel directly to the evaporator for defrosting it, and the closing of valve 19 thus insures that no portion of the hot gas can flow to the condenser and deplete the supply of heat desired for defrosting. It will therefore be seen that the solenoid valve 19 positioned in the hot gas conduit 8 at a point further from the compressor than the junction of the defrosting conduit 17 with the hot gas conduit 8, performs the plural function of cooperating with the check valve 20 to isolate the condenser during off-cycles of the compressor, and insuring complete and rapid defrosting of the evaporator by preventing any flow of hot gas to the condenser during defrosting cycles. There is deemed to be no need for any showing of the electric circuit just described because such provisions for opening and closing valves in response either to clock works or to pressurestats or thermostats are so well understood and commonly practiced in this industrial field as to require no more than 6 the expression of desired result by way of teaching a suitable and appropriate embodiment.

In testing the form of the invention exhibited in Fig. 1 of the drawings, as just described, it was discovered that a certain undesirable action took place in that, during each off-cycle of the compressor, there was a tendency for the liquid refrigerant in the supply conduit 14 to flash and return to the receiver, leaving the said conduit empty or substantially so. Thus, upon the institution of the succeeding compressor on-cycle, there was no liquid in the supply conduit to feed the expansion valve and, as a result, the liquid remaining in the evaporator quickly evaporated, pulling down the low side or crank case pressure and stopping the compressor motor through the actuation of its pressure switch. However, as the evaporator temperature was fairly high, its pressure quite rapidly rose thereby elevating the low side pressure to a degree at which the pressure switch would restart the compressor motor. This series of fluctuations would be repeated until the supply line 14 refilled with liquid refrigerant and normal operation ensued. In short, the back flow of refrigerant from the supply line into the receiver engendered a series of brief on and off compressor cycles prior to restoration of normal operation, which was regarded as a feature to be disapproved and eliminated. To tlus end, and for other advantages inherent in its construction and mode of operation, themodified form of the invention illustrated in Fig. 2 was conceived and satisfactorily embodied.

Adverting to the showing of Fig. 2 in the drawings, the same reference numerals will be applied to the main elements which are characteristically like the corresponding elements in Fig. 1. Thus the compressor is denoted by 1, condenser by 2, receiver by 3, evaporator by 4, two walls of the refrigerating or cooling chamber by 5 and 6, one wall of the building structure by 7, the refrigerant supply conduit by 14, thermostatic expansion valve by 15, the suction conduit by 16 and the pressure switch by 21. As the other parts difler more or less in structural detail and/ or function, new reference numerals will be applied to them.

The hot gas conduit leading from compressor discharge to condenser inlet is marked 22, and the conduit connecting condenser outlet with the receiver is indicated by 23, which conduit, in this form of Fig. 2, omits the inverted U formation of Fig. 1. A constant inlet pressure regulating valve 24 is fitted in the discharge conduit 22 and a check valve 25, which permits flow toward the receiver and prevents reverse flow, is positioned in conduit 23. A by-pass 26 interconnects conduits 22 and 23, its junction with the former being between the compressor and the valve 24, while its junction with conduit 23 is between the receiver and check valve 25. In the by-pass 26 is positioned a constant outlet pressure valve 27. The hot gas defrosting conduit is omitted from the showing of Fig. 2 as being unnecessary for an elucidation of the inventive concept.

In the operation of the form of the invention exhibited in Fig. 2, the constant inlet pressure valve 24 will serve to regulate and maintain the pressure at the compressor head at a value corresponding to the setting or adjustment of the valve, regardless of condenser pressure; and the constant outlet pressure valve 27 will be open whenever the pressure at the receiver is below the setting or adjustment of the said valve which is sensitive to pressure at its outlet port and thus opens in response to fall in pressure below its setting. During off-cycles of the compressor, valve 24 will be closed and, as the check valve 25 prevents flow toward the condenser, the latter will be isolated or flow severedfrom the remainder of the system so that refrigerant cannot migrate from the evaporator to the condenser as hereinabove explained; while the pressure switch designed to control the on and ofiF-cycles of the compressor will remain effective.

When an on-cycle of the compressor is reestablished,

the pressure from compressor discharge will at first directly affect the receiver pressure by passage of the gas through-valve 27 because valve 24 is closed, and the consequent rise in receiver pressure which occurs after a very brief period, eg two or three seconds, functions to deliver liquid refrigerant with rapidity and energy to the expansion valve 17 for reinstating normal operation of the system without the occurrence of preliminary cycles of fluctuation in the supply conduit 14 heretofore mentioned in connection with the description of Fig. 1.

Immediately following this elevation of receiver pressure, the constant inlet pressure valve 24 is opened by the build-up of compressor head pressure, so that the hot gas begins to flow to the condenser inlet; but the check valve 25 remains closed because the pressure thereabove in conduit 23 is lower than the receiver pressure, with the result that the refrigerant flowing from compressor into the condenser raises its level in the condenser thus reducing its condensing function by gradual decrease in the area of eifectiveinternal heat exchange surface. This continues until the capacity of the condenser has been reduced to the point at which the condensing pressure plus the head of liquid in conduit 23 above check valve 25 is suflicient to open the latter and permit flow of refrigerant from compressor discharge through condenser to receiver, and thence to expansion valve and evaporator in accordance with the regular operation of the system.

It will thus be seen that this form of invention shown in Fig. 2 serves to maintain high side pressure above a predetermined minimum through valve operation, without the employment of the inverted U shaped conduit 9 of Fig. l or of the by-pass 12 connected at the apex thereof. The constant pressure valve 24 is effective to cause pressure drop due to the fact that it will throttle whenever the pressure at its inlet port is lower than its setting or adjustment. Thus, if valves 24 and 27 have corresponding settings, the throttling of valve 24 to maintain pressure at its setting value (e.g. 125 pounds per square inch), will cause refrigerant to be delivered through valve 27 to the receiver until the pressure in the latter equals 125 pounds per square inch, when valve 27 will throttle and valve 24 will open, allowing vapor from the compressor to reach the condenser and change to liquid phase. However, as no liquid can be delivered to the receiver from the condenser until pressure in the latter at least equals receiver pressure, the condenser will be flooded, as hereinabove described to lower its functional capacity until its pressure equals the 125 pounds per square inch just instanced.

It will accordingly be evident that the system of Fig. 2 automatically maintains high side pressure above a predetermined minimum; isolates the condenser during offcycles of the compressor to prevent migration of refrigerant from low side to high side; and elevates receiver pressure to normal almost instantly on reestablishment of the compressor on-cycles to promptly supply liquid under adequate pressure to the thermostatic expansion valve and feed the evaporator before drop in pressure and undesired stoppage of the compressor motor can occur.

The second modified form of the invention, set forth in Fig. 3, again has certain usual elements which may be regarded as similar to those of Figs. 1 and 2 and which, consequently, may fittingly receive the same reference numerals, i.e. the compressor 1, condenser 2, receiver 3, evaporator 4, refrigerating or cooling chamber walls and 6, building wall 7, supply conduit 14, thermostatic expansion valve 15 and the pressure switch 21.

The hot gas discharge conduit is marked 28, and fitted therein is a constant inlet pressure valve which is the same as valve 24 of Fig. 2 but is here denoted by 29 because the setup of Fig. 3 differs somewhat from that of Fig. 2. The conduit 28 is connected with the inlet of condenser 2, andthe outlet of the latter is connected with the receiver by a conduit 30 in which is positioned a check valve 31 which corresponds with valve 25 of Fig. 2. A by-pass 32 interconnects conduits 28 and 30, the

point of union of conduit 28 being between the compressor 1 and valve 29, and the junction with conduit 30 being between the receiver 3 and the valve 31'. This by-pass 32 serves the same purpose as by-pass 26 in Fig. 2, i.e. to provide for refrigerant flow from compressor discharge to receiver, and arranged therein is a constant outlet pressure valve 33 which corresponds with valve 27 of Fig. 2.

In the embodiment of Fig. 3 there is included a hot gas defrosting conduit 34 which connects the discharge of the compressor with the inlet of the evaporator at a point between the evaporator and the expansion valve 15; a solenoid valve 35 being positioned in the said conduit and adapted for operation to institute and terminate defrosting periods at predetermined times, as by the usual time clock, electric switch and circuit, not shown.

In this form of Fig. 3 there is included means for reevaporating liquid refrigerant returning from the evaporator to the compressor intake during defrosting periods, which means is fitted in the suction con-duit, here marked 36, and is like the reevaporating means disclosed in US. Patent to Israel Kramer No. 2,718,764, issued September 27, 1955. As the disclosure in the said patent is complete with detail, and this particular subject matter constitutes, of itself, no feature of the invention herein claimed, a brief outline of the same is deemed to be suflicient. It will be observed that a portion of the suction conduit 36 is formed into a coil 37 which is located within a comparatively small container or tank 38 that can conveniently be rectangular in shape and composed of steel or other appropriate material, with the points at which the conduit enters and leaves the container being rendered liquid tight, as by stuffing boxes or the like. The container 38 is largely filled with a freezable liquid 39, such as water with or without a freezing depressant, and a filler neck 40 is set in the top of the container. Interposed in conduit 35 between the evaporator and the container 38 is an adjustable automatic hold back valve 41, which is well known to this industry and the essential function of which is to control the flow of refrigerant fluid therethrough so as to reduce the pressure of the fluid to a degree not higher than that at which the valve has been adjusted to close, the valve serving, in the present case, to reduce the temperature of the refrigerant flowing therethrough sufiiciently to cause the latter to freeze some of the liquid 39 in the container 38 during defrosting periods, and, as a result thereof, revaporizing the liquid refrigerant by latent heat'derived from the freezing. The hot gas conduit 28 is formed with a coil 42 that lies in heat exchange relation with the container 38 and serves the purpose of elevating its temperature to assist in melting frozen liquid 39 and warming the same during refrigerating cycles of the system. The warming effect may be regulated by the provision of a by-pass 43 which may be supplied with any suitable form of valve 44 for opening and closing, wholly or partly, the by-pass and thereby regulating the flow of hot gas through coil 42, or bythe provision of an open by-pass which regulates the heating effect thermo-siphonically, as shown in the above named Israel Kramer Patent No. 2,718,764.

The mode of operation of the form of the invention illustrated in Fig. 3 corresponds substantially with that previously described in connection with Fig. 2, but certain additional features should be mentioned. Thus it will be noted that the constant inlet pressure valve 29 is so located that, when solenoid valve 35 is opened by its control to initiate a defrosting cycle, consequent drop in pressure at the inlet of valve 29 will cause it to close and thus insure that all, or substantially all, thehot gas emitted at the compressor discharge will travel through defrosting conduit 34 to the evaporator for a rapid and complete defrosting. It should be mentioned that, if preferred, the valve 29 could be positioned in the conduit 28 between the heating coil 42 and the condenser 2 without affecting its functional effect just described.

Adverting again to the defrosting capacity of the system shown in Fig. 3, attention is directed to the fact that, during defrosting cycles, constant outlet pressure valve 33 will be open due to the fact that the pressure on its outlet side will be less than its setting or adjustment, thereby permitting refrigerant liquid to flow through by-pass 32 and into hot gas defrosting conduit 34 with the desirable effect of accelerating the defrosting step.

Referring to both Figs. 2 and 3, the valves 27 and 33 could ,be substituted by spring loaded check valves or properly sized capillary tubes, but the adjustable constant outlet pressure valve is preferred. It should also be observed that these systems enable maintenance of desired receiver pressure and head pressure regardless of the volume of mass flow without the requirement of critical sized restrictor tubes depending upon the B.t.u. capacity of the system.

We desire it to be understood that various changes may be resorted to in the form, construction and arrangement of the several parts of the apparatus and in the steps of the method, without departing from the spirit or scope of the invention; and hence we do not intend to be limited to details herein shown or described except as the same may be included in the claims or be required by disclosure of the prior art.

What we claim is:

1. In a compression type heat exchange system having a circuit for refrigerant flow including conduit connected compressor, condenser, receiver, evaporator, refrigerant pressure reducing device in operative association with the evaporator inlet, and a control element for establishing on and off-cycles of compressor operation, refrigerant pressure actuated means for automatically preventing migration of refrigerant from evaporator and receiver to condenser during off-cycles of the compressor, and means for imposing high compressor discharge pressure on the receiver immediately following the establishing of compressor on-cycles whenever the receiver pressure is lower than its predetermined minimum normal refrigerating pressure.

2. A system as defined in claim 1, in which the means for imposing compressor discharge pressure on the receiver comprises, a by-pass conduit for providing refrigerant flow from compressor discharge to receiver, and means in said by-pass for controlling refrigerant flow therethrough which is responsive to receiver pressure.

3. A system as defined in claim 2, which also includes refrigerant fiow controlling means in the conduit between compressor and condenser that is responsive to compressor head pressure, and refrigerant flow controlling means in the conduit between condenser and receiverthat inhibits flow to the former, and in which the said by-pass conduit is connected with the conduit from compressor to condenser at a point between the compressor and the refrigerant flow controlling means in the said last named conduit.

4. A system as defined in claim 3, which also includes a hot gas defrosting conduit connecting compressor discharge at a point between the compressor discharge and the said refrigerant flow controlling means in the conduit between compressor and condenser with the evaporator.

5. In a compression type heat exchange system having a circuit for refrigerant flow including conduit interconnected compressor, condenser, receiver, evaporator, refrigerant pressure reducing device in operative association with the evaporator inlet, and a control element for establishing on and off-cycles of compressor operation, means for imposing high compressor discharge pressure on the receiver immediately following the establishing of compressor on-cycles and slightly in advance of imposing said pressure on the condenser whenever the receiver pressure is lower than the predetermined minimum normal refrigerating pressure in order to insure immediate flow of refrigerant at satisfactory operating pressure from the receiver to the pressure reducing device, said means comprising a refrigerant flow controlling device in the conduit connecting the compressor with the condenser that is automatically responsive to compressor head pressure, a by-pass conduit connected with the last named conduit at a point between the compressor and the said flow controlling device for providing refrigerant flow from the compressor discharge to the receiver, a refrigerant flow controlling device in said by-pass conduit that is automatically responsive to receiver pressure, and a refrigerant fiow controlling device in the conduit which connects condenser with receiver that permits flow toward receiver but inhibits reverse flow.

6. A system as defined in claim 5, in which the bypass connects the conduit connecting compressor and condenser with the conduit connecting condenser with receiver, and in which the refrigerant flow controlling device in the conduit connecting condenser with receiver is at a point between condenser and said by-pass conduit.

7. A system as defined in claim 6, in which the device in the conduit connecting compressor with condenser is a constant inlet pressure valve, the device in the said by-pass conduit is a constant outlet pressure valve, and the device in the conduit connecting condenser with receiver is a check valve.

8. A system as defined in claim 7, which also includes a hot gas defrosting conduit connecting the compressor discharge, at a point between the compressor and said constant inlet pressure valve and said check valve, with the evaporator.

9. A system as defined in claim 5, in which the refrigerant flow controlling device in the conduit connecting compressor with condenser is a constant inlet pressure valve, and the refrigerant flow controlling device in the said by-pass is a spring loaded check valve.

10. A system as defined in claim 9, which also includes a check valve in the conduit connecting condenser with receiver that permits flow toward receiver and prevents reverse flow.

11. In a compression type heat exchange system having a circuit for refrigerant flow including conduit connected compressor, condenser, receiver, evaporator, refrigerant pressure reducing device in operative association with the evaporator inlet, and a control element for establishing on and elf-cycles of compressor operation, refrigerant pressure actuated means for automatically preventing migration of refrigerant from evaporator and receiver to condenser during oif-cycles of the compressor, said means for preventing migration during off-cycles comprising means responsive to refrigerant pressure for isolating the condenser from communication with the remainder of the said circuit and including means in the conduit between compressor and condenser automatically responsive to compressor head pressure for preventing refrigerant flow from the former to the latter during compressor off-cycles and automatic means in the conduit between condenser and receiver for preventing flow therethrough from the latter to the former.

12. A system as defined in claim 11, in which the means in the conduit between compressor and condenser is a valve controlled by compressor head pressure to close during compressor off-cycles, and the means in the conduit between condenser and receiver is a check valve.

13. A method of automatically maintaining satisfactory functioning of a compression type heat exchange system having a circuit for refrigerant flow including conduit interconnected compressor, condenser, receiver, evaporator, refrigerant pressure reducing device in operative association with the evaporator inlet, and a control element for establishing on and off-cycles of compressor operation, regardless of ambient temperature at the condenser, which method includes the steps of automatically inhibiting refrigerant flow from the evaporator and receiver to the condenser during off-cycles of the compressor; imposing high compressor discharge pressure on the receiver immediately following the establishing of compressor on-cycles and slightly in advance of imposing said pressure on the condenser; intermittently establishing refrigerant flow directed from compressor discharge to evaporator for defrosting the latter; and automatically supplying refrigerant from the receiver to said direct flow for augmenting the defrosting effect.

References Cited in the file of this patent UNITED STATES PATENTS 1,836,072 Hull Dec. 15, 2,008,715 Hull July 23, 2,331,264 Carter Oct. 5, 1943 2,564,310 Nussbaum et a1 Aug. 14, 1951 2,610,480 Briscoe Sept. 16, 1952 2,621,051 Kramer Dec. 9, 1952 2,621,487 Warren Dec. 16, 1952 2,694,904 Lange et a1. Nov. 23, 1954 2,761,287

Malkofi Sept. 4, 1956

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