US3392542A - Hot gas defrostable refrigeration system - Google Patents

Description

y 6, 1968 o. J. NUSSBAUM 3,392,542

HOT GAS DEPROSTABLE REFRIGERATION SYSTEM Filed Oct. 14, 1966 5 Sheets-Sheet 1 INVENTOR O-r'ro I- NussBAum BY m wsm a mmuk wows.

ATTORNEYS July 16, 1968 o. J. NUSSBAUM 3,392,542

HOT GAS DEFROSTABLE REFRIGERATION SYSTEM Filed Oct. 14, 1966 5 Sheets-Sheet 2 E KTER NAL EQ\)AL1ZEB L LINE iigg I8 l I l l 1 RECEIVE-e EXTERNAL /EQUAL\ZEP uNE 4Q EECEI v52.

INVENTOR O'r-ro I NUSS BAUM wasm iwunnsw ATTORNEYS ca/va F4 y 6, 1968 o. J. NUSSBAUM 3,392,542

HOT GAS DEFROSTABLE REFRIGERATION SYSTEM Filed Oct. 14, 1966 5 Sheets-Sheet 3 I 0 UL D. &

Conn FKJ/V INVENT OR 61"!0 INussBAuM BY masawft wufinkficfiikmmci ATTORNEYS United States Patent 3,392,542 HOT GAS DEFROSTABLE REFRIGERATION SYSTEM Otto J. Nnssbaum, Atlanta, Ga., assignor to Larkin Coils, Inc., Atlanta, Ga., a corporation of Georgia Filed Oct. 14, 1966, Ser. No. 586,815 8 Claims. (Cl. 62-196) ABSTRACT OF THE DISCLOSURE A hot gas defrostable refrigeration system wherein the conduit means connecting the condenser with the evaporator includes a single conduit extending between the high side and the low side of the system serving as a combined liquid and hot gas line conducting condensed refrigerant to the evaporator during the refrigeration cycle and conducting gaseous refrigerant Without any liquid refrigerant intermixed therewith to the evaporator during the defrost cycle, together with suitable valve means,

the hot gaseous refrigerant being delivered to the evaporator at suitable temperature and pressure during the defrost cycle to cause defrosting without any condensation of refrigerant in the evaporator.

The present invention relates in general to refrigeration systems, and more particularly, to an improved refrigeration system having a hot gas defrosting system incorporated therein for periodic defrosting of the evaporator by admission of hot gaseous refrigerant thereto from the compressor, without providing the usual separate hot gas line.

Although, hot gas defrost systems are generally recognized as being the most eflicient and fastest available defrost systems, the general acceptance is still hampered by the fact that by comparison with other defrosting systems, hot gas defrost systems are more costly to install because they require the refrigeration contractor to run a separate by-pass line from the condensing unit to the evaporator. Such hot gas defrosting systems generally supply the hot gaseous refrigerant from the discharge or high side of the compressor to the evaporator by way of a separate branch conduit or by-pass line which is connected to the usual conduit from the high pressure side of the compressor to the condenser. It is a common characteristic of any such hot gas defrost systems that during the defrost operation, the liquid line extending from the condenser to the evaporator, in a receiverless system, or from the receiver to the evaporator in a receiver system, is unused because flow to the evaporator goes through the hot gas by-pass line and flow from the evaporator occurs through the suction line. Thus, in such hot gas defrost systems, while the cost-increasing separate hot gas line for bypassing the hot gas to the evaporator is used during the defrost operation, that portion of the liquid line extending from the condensing unit to the evaporator remains idle throughout the period of the defrost operation.

It is therefore an object of the present invention to provide a novel refrigeration system having a hot gas defrost system incorporated therein, wherein the liquid line extending from the condensing unit to the evaporator, which normally conveys the condensed liquid refrigerant to the evaporator during normal refrigeration operation, is utilized as a hot gas line during the defrost operation to convey hot gaseous refrigerant directly from the compressor to the evaporator, thus eliminating the need for the installation work attendant to the provision of a separate by-pass line for hot gaseous refrigerant.

Another object of the present invention, is the provision of a novel refrigeration system having a hot gas 3,392,542 Patented July 16, 1968 defrosting system incorporated therein, utilizing the same conduit line from the condensing unit to the evaporator for the conduction of liquid refrigerant during the refrigeration cycle and for the conduction of hot gaseous refrigerant during the defrosting cycle.

Another object of the present invention, is the provision of a novel refrigeration system having a hot gas defrost system incorporated therein, of the character described in the immediately preceding paragraph, wherein the need for oversizing the combined liquid and hot gas defrost line extending from the condensing unit to the evaporator is avoided.

Another object of the present invention is the provision of a novel refrigeration system having a hot gas defrost system incorporated therein, wherein the system is arranged so that the defrost pressure control valve may be incorporated as part of the condensing unit and a hot gas defrost solenoid valve may be provided in the evaporator assembly, thus facilitating the provision of a packaged system without requiring separate installation by a refrigeration contractor of a separate valved by-pass line from the condensing unit to the evaporator.

Other objects, advantages, and capabilities of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings illustrating severalpreferred embodiments of the invention.

In the drawings:

FIGURE 1 is a diagrammatic view of a refrigeration and hot gas defrosting system, embodying the present invention in a receiverless system;

FIGURE 2 is a fragmentary diagrammatic view of a modification of the system of FIGURE 1 for a receiver system, repeating only the illustration of those components of the system of FIGURE 1 necessary to an undertanding of the location of the receiver and its associated components in that system;

FIGURE 3 is a diagrammatic view of a modified refrigeration and hot gas defrosting system embodying the present invention, employing head pressure control features utilizing a heated receiver;

FIGURE 4 is a fragmentary diagrammatic view representing a modification of FIGURE 3 and employing a heated receiver in the head pressure control system; and

FIGURES 5 and 6 are schematic control wiring diagrams disclosing examples of control circuits which may be used with the systems of the present invention.

The invention will be specifically described in conjunction with the type of hot gas defrosting method disclosed in the Shrader Patent No. 3,098,363 or the White Patent No. 2,688,860 as an aid to understanding the preferred forms of the invention, wherein the formation of liquid in the evaporator during the defrost operation is prevented by controlling the pressure of the hot gas entering the evaporator so that the saturation temperature of the refrigerant will be below that of the surrounding evaporator tube and frost. In such a system, no condensation takes place in the evapoartor during the defrost operation so that no supplementary devices such as re-evaporators are necessary to prevent passage of liquid refrigerant to the compressor. However, the hot gaseous refrigerant delivered to the evaporator surrenders only sensible heat to the tube surface for defrosting, as distinguished from the latent heat which is surrendered by the more conventional systems where condensation takes place in the evaporator, although the temperature of the superheated gaseous refrigerant entering the evaporator during the defrosting cycle is considerably higher than what would be the case in the latent heat systems, so that the greater temperature difference between the hot gas and the coil surface results in more intensive heating of the evaporator during defrost.

Referring particularly to the system illustrated in FIG- URE 1, the refrigeration system comprises a compressor of conventional construction, having a suction intake or low side 11 and a discharge or high side 12. The condenser discharge is' connected through conduit 13 to a junction 14, such as a T fitting, an extension conduit 15 of the discharge conduit 13 extending to the condenser 16, such for example, as an air-cooled condenser disposed at an exterior location having the usual fan associated therewith. The outlet end of the condenser 16 is connected by conduit 17 having a check valve 18 therein, which extends to a junction point 19 and has a liquid line 20 extending therefrom to the inlet end of the evaporator 21, the conduit 20 serving as the combined liquid line and hot gas defrost line in a manner to be hereafter described in detail. The liquid line 20 has a liquid solenoid valve 22 therein and a branched pair of paralleled conduits near the evaporator 21, indicated by reference characters 23 and 24, either one of which may be a continuation of the liquid line 20 and the other may be a parallel conduit connected at both ends to the liquid line 20 at spaced points along the same, one of which conduits 23, 24 contains a thermostatic expansion valve 25 for regulating the flow of liquid refrigerant to the evaporator during the refrigeration cycle, and the other of which contains a defrost solenoid valve 26. The evaporator outlet is connected by the usual suction line 27 to the suction intake or low side 11 of the compressor 10. A branch conduit 28 extends from the junction point 14 of the discharge conduit 13 to an outlet pressure responsive control valve 30, the outlet end of the valve being connected by conduit 31 to the junction point 19 of the line 17 extending from the outlet of the condenser 16.

In the refrigeration cycle of the system, refrigerant is discharged by the compressor 10 through the conduits 13, 16 to the condenser 16. The outlet pressure responsive control valve 30 is set to remain closed as long as the pressure at its outlet is above the predetermined defrosting pressure, so that during the refrigeration cycle this valve 30 is closed and no gaseous refrigerant flows through the conduits 28, 31. From the condenser 16, the liquid refrigerant flows through check valve 18 in conduit 17 and through the liquid line 20, the liquid solenoid valve 22 which is open during the refrigeration cycle, and the thermostatic expansion valve 25, to the evaporator 21. The defrost solenoid valve 26 in the parallel section 23 of the liquid line is closed during the refrigeration cycle. From the evaporator 21, vaporized refrigerant returns to the compressor 10 through the suction line 27 and the cycle repeats.

When it is desired to defrost the evaporator, the defrost operation is initiated by a suitable timer and the system is caused to undergo three separate phases of the defrost cycle. During the first phase, the liquid solenoid valve 22 is closed and the defrost solenoid valve 26 remains closed while the fan on the evaporator 21 is stopped. The effect of closure of the liquid solenoid valve 22 while continuing operation of the compressor will be to rapidly evacuate the liquid line 20 between the solenoid valve 22 and the evaporator of all residual liquid refrigerant. Termination of this phase may either be timed or it may be signaled by a reduction in the evaporator pressure to a selected level, for example, about 15 p.s.i. gauge. The second phase of the defrost operation starts with opening of the defrost solenoid 26 and the liquid solenoid valve 22. The evaporator fan for evaporator 21 remains off. This will cause a very sharp drop in pressure at the outlet of the outlet pressure sensitive control valve 30, as a result of which the valve 30 opens and discharge gas from the compressor 10 will be diverted directly through line 28, valve 30 and line. 31 into the liquid line 20. At the same time, the condenser fan motor is stopped, for example, by a high side pressure switch sensing the sharp drop in pressure which occurs at this point, or by switch means responding to the previously mentioned reduction in evaporator pressure to the selected level. The hot gaseous refrigerant thus flows through the line 28, valve 30 and line 31, and thence through liquid line 20 and its open valves 22 and 26, to the evaporator 21. Here, the highly superheated gas is de-superheated, surrrendering heat to the frost on the evaporator surface and returns to the compressor 10 through the suction line 27, where the cycle is again repeated until defrost is complete. Completion of defrost is signaled by a rise in temperature of the suction line 27 near the outlet of the evaporator 21. This signal may conveniently be sensed to close the defrost solenoid 26, thus initiating the third phase of the defrost operation wherein the evaporator fan for evaporator 21 remains off, and the liquid solenoid valve 22 may be closed, if desired, to permit reduction of the pressure and temperature in the evaporator to the normal operating level. This third phase is terminated when the temperature of the suction line 27 at the outlet of the evapoartor 21 is reduced back to its normal operating level, at which point the evapoartor fan is started and the liquid solenoid valve 22, if closed during the third phase, is opened. During the third phase of the defrost cycle, a rise of pressure at the outlet of the outlet pressure responsive control valve 30 will have occurred, causing that valve to close, so that refrigerant will again resume its normal pass through discharge line 13, 15, condenser 16, check valve 18, and liquid line 20 to the evapoartor 21.

Operation of the refrigeration cycle is terminated when the thermostat sensing the temperature in the space served by the refrigeration system is satisfied, signaling the liquid solenoid valve 22 to close, With the system otherwise in the refrigeration mode, the pressure at the suction inlet of the compressor 10 will be rapidly reduced by this closure of the liquid solenoid valve 22, causing the conventional low pressure switch of the compressor 10 to stop operation of the compressor until that thermostat again calls for cooling and opens the liquid solenoid valve 22.

It should be noted that the outlet pressure sensitive control valve 30 performs no function during refrigeration because it will always be closed as long as the condenser functions as such.

Operation of the timer motor used for defrost may be in such a manner that starting of the timer is permitted only during periods when the space thermostat is calling for cooling and the timer is prevented from operating when the space thermostat is in a satisfied condition. This will prevent a defrost from being initiated while the compressor 10 is not in operation. This prevents unnecessary defrost operations, because if the space thermostat is in satisfied condition, no frost is accumulated on the evaporator and consequently, no defrost will be needed.

FIGURE 2 discloses a system similar to FIGURE 1 but incorporating a liquid receiver, such as would be used for larger systems in which the condenser will not be capable of holding the total refrigerant charge during pumpdown. The use of the receiver dictates the need for an additional receiver solenoid valve at the receiver outlet, which is closed during defrost while the liquid solenoid and defrost solenoid are both open. In this modified arrangement employing the receiver indicated by the reference character 35, a receiver inlet branch conduit 36 is connected from the liquid line 20 at a point near the upstream end thereof adjacent the junction point 19 to the receiver 35 and a receiver outlet conduit 37 in the form of a branch line is connected between the receiver and the liquid line at a point spaced somewhat downstream from the point of connection between the conduit 36 and the liquid line. The receiver outlet conduit 37 has a receiver solenoid valve 38 therein. The defrost cycle in this system of FIGURE 2 is very similar to that of FIGURE 1, in that when the defrost operation is initiated by the timer, the liquid solenoid valve 22 is closed during the first phase while the defrost solenoid valve 26 remains closed, and the fan on the evaporator 21 is stopped. The liquid line 20 is thereupon rapidly evacuated of all liquid refrigerant, and the second phase of the defrost operation is initiated by opening the defrost solenoid valve 26 and opening of the liquid solenoid valve 22, with simultaneous closing of the receiver solenoid valve 38. This causes a very sharp drop in pressure at the outlet of the outlet pressure sensitive control valve 30, opening this valve to permit flow of discharge gas from the compressor through the discharge line 13, by-pass conduit 28, control valve 30, and line 31, and thence through the liquid line and open valves 22 and 26 to the evaporator 21, where the highly superheated gas is de-superheated, surrendering heat to the frost on the evaporator surface to melt the same.

The de-superheated gas returns to the compressor 10 through the suction line 27 where the cycle is again repeated until defrost is complete. The rise in temperature of the suction line 27 near the outlet of the evaporator 21 signaling completion of defrost serves to close defrost solenoid valve 26 while the evaporator fan remains stopped, to permit reduction of the pressure and temperature in the evaporator to the normal operating level. Valve 22 may also be closed during this terminal phase of the defrost cycle. When the temperature of the suction line 27 at the outlet of the evaporator 21 is reduced back to its normal operating level, the evaporator fan is started and the liquid solenoid valve 22, if closed during the third phase, is opened. The receiver solenoid valve 33 in the receiver outlet line 37 may be opened at the initiation of the third phase of the defrost cycle when the defrost solenoid valve 26 is closed, or it may be opened at the completion of the third phase of the defrost cycle. Upon reopening of the receiver solenoid valve 38, closing of the defrost solenoid valve 26 and opening of the liquid solenoid valve 22, the refrigeration system resumes the normal operating cycle for refrigeration.

FIGURE 3 illustrates an embodiment of the basic refrigeration and hot gas defrost system of the two preceding embodiments employing the combined liquid and hot gas line 20, in conjunction with a head pressure control system using a heated receiver. In the embodiment of FIGURE 3, the components of the system corresponding to those described in the preceding embodiments are identified by the same reference characters as previously used. The receiver 35 in the FIGURE 3 embodiment, however, is a heated receiver, having for example, an internal electric heater therein, identified by the reference character 40, which is controlled by a suitable switch, such as a pressure sensitive switch, responsive to receiver pressure or receiver temperature, in accordance with the principles disclosed in the earlier US. Patent No. 3,238,- 737 granted Mar. 8, 1966, to Raymond M. Shrader et al., and owned by the assignee of this application, so as to elevate the temperature, and consequently the pressure, in the receiver 35 when the receiver pressure falls below a selected level. Thus, means are provided to energize the heater 40 so as to heat the liquid refrigerant in the receiver 36 to elevate the vapor pressure in the receiver to a desired degree whenever the head pressure, or other criteria sensed, indicates that excessive capacity of the condenser 16 is lowering head pressure. The receiver inlet line 36 extending from the downstream end of the condenser outlet line 17, has a normally open solenoid valve 41 therein, and the condenser outlet line 17 below the check valve 18 therein is connected at the junction with the receiver inlet line 36 by a by-pass line 42 having a check valve 43 therein, which connects to the receiver outlet line 37. The junction of the by-pass line 42 and receiver outlet line 37, above the usual manual valve in the latter, is connected to the liquid line 20, which serves as the combined liquid and hot gas line, similar to the liquid line of the preceding embodiments, having the liquid line solenoid valve 22 therein. The hot gas by-pass line by-passing the condenser 16 during the defrost cycle, and composed of line 28, outlet pressure responsive valve 30 and line 31 connects to the liquid line 20 in the FIG- URE 3 embodiment downstream of the liquid solenoid valve 22, between the latter and the paralleled thermostatic expansion valve 26 and defrost solenoid valve 26. The outlet pressure responsive control valve 30 in this embodiment is also equipped with an external equalizer line indicated at 45, coupled to the suction line 27 near the compressor intake 11, to enable the control valve 30 to respond directly to compressor suction pressure.

In the summer mode of operation of the FIGURE 3 system, when the ambient conditions are appropriate for ordinary eflicient operation of the refrigeration system, the solenoid valve 41 is open and the condensed liquid refrigerant in the line 17 delivered by the condenser 16 passes through the open valve 41 and receiver inlet 36 through the receiver 35 and hence through the receiver outlet line 37 and combined liquid and hot gas line 20 to the thermostatic expansion valve 25, for delivery to the evaporator 21 and evaporation therein in the normal manner. In the winter operation of the system during seasons when low ambient conditions occur which might undesirably reduce head pressure, the solenoid valve 41 is closed so that the receiver 35 is connected to the system only by the one line and a T connection whereby automatic heating of the receiver 35 will boost head pressure and vary condenser capacity for efiicient expansion valve operation as ambient conditions require.

The operation of the system disclosed in FIGURE 3 is similar to that of FIGURE 1 insofar as the defrost cycle is concerned. In the first phase following initiation of defrost, the liquid solenoid valve 22 is closed and the defrost solenoid valve 26 remains closed while the fan of the evaporator 21 is stopped. The combined liquid and hot gas line 25 is thus evacuated of all residual liquid refrigerant. The defrost solenoid valve 26 is then opened and liquid solenoid valve 22 remains closed, initiating the second phase, while the evaporator fan remains off. The resultant sharp drop in pressure at the outlet of the control valve 30 opens the control valve 3t) to permit discharge gas to be diverted through lines 28, 31 directly into the combined liquid and hot gas line 20, and thence through the valve 26 to the evaporator 21. When the defrost is completed, the third phase is initiated wherein the defrost solenoid valve 26 is closed while the avaporator fan remains stopped for a short period and the liquid solenoid valve 22 remains closed to permit reduction of the pressure and temperature in the evaporator to the normal operating level.

The FIGURE 4 system is very similar to that of FIG- URE 3, employing a heated receiver 35 like the FIG- URE 3 embodiment, but having a slightly simplified head pressure control arrangement. In the system of FIGURE 4, the receiver outlet line 37 is retained as the single receiver connection with the condenser outlet line 17 and the combined liquid and hot gas line 20, the solenoid valve 41 and the receiver inlet line 36 being eliminated. The liquid solenoid valve 22 is located between the downstream end of the condenser outlet line 17 and the outlet line 31 connected to the outlet of the control valve 30 as in the FIGURE 3 embodiment. However, in this version of FIGURE 4, the downstream pressure sensitive control valve 30 has a shut-off solenoid valve 50 installed on its inlet side to permit operation on a pumpdown cycle; in other words, the control valve 30 is prevented from feeding hot gas to the combined liquid and hot gas line 20 when the liquid solenoid valve 50 closes in response to a signal from the thermostat. The control valve 30 and solenoid shut-off valve 50 may be combined into a single device which is commercially available. A distinctive property of these systems herein above described is that they never run out of heat during defrost as some other methods now presently commercially available do. The reason is that the defrosting circuit is completely self-contained and no refrigerant is either added or taken away from the defrosting circuit. Furthermore, the compressor is the sole source of heat on which the system relies and in view of the fact that the compressor always operates during defrost, it provides an inexhaustible source of heat. Since no condensation takes place during defrost in the particular versions herein described, only vapor circulates in the defrost cycle so that the refrigerant charge required for the defrosting circuit is minimal.

As previously described, however, the present systems may also be used with defrost methods involving condensation of hot gaseous refrigerant in the evaporator, so long as a source of heat is included in the circuit between the evaporator and the compressor intake during the defrost cycle to ensure that no liquid refrigerant reaches the compressor.

FIGURE illustrates schematically one control Wiring circuit which may be used for the systems of FIG- URES l or 2, the solenoid valves in the control wiring circuit of FIGURE 5 corresponding to solenoid valves of the refrigeration circuit of FIGURES l and 2 being designated by the same reference characters used in FIG- URES 1 and 2. As shown in the control wiring circuit of FIGURE 5, the timer motor 60 is connected in series with the space thermostat 61 so that the timer-motor operates only when the thermostat 61 is closed. Therefore, the defrost operation can start only when the thermostat 61 is closed, and consequently, the system is in operation. This is particularly desirable in cold climates because if defrost is initiated with the compressor olf, considerable time will elapse until a suflicient head pressure is generated to produce defrost. The timer motor 60 operates a double throw switch 60A having normally closed contact 60A and normally open contact 60A, and a normally closed single throw switch 608. During the refrigeration cycle, the switches 60A and 60B are in the positions shown in FIGURE 5, wherein the supply circuit to liquid line solenoid valve 22 is completed through contact 60A of switch 60A and space thermostat 61, maintaining the valve 22 open as long as the refrigerated space needs cooling. At the same time, the evaporator fan motor 21A is energized through switch 608 as long as defrost termination thermostat 62 is in the cold position, indicating that the evaporator coil is on the refrigeration cycle.

When defrost is initiated, switch 60A shifts to open contact 60A, closing valve 22, and to close contact 60A". However, hot gas solenoid valve 26 is not energized, because of open pressure switch P, util the evaporator has been evacuated, whereupon the pressure switch P closes and energizes the relay coil 65 and solenoid valve 26 to open the valve 22. This signals completion of the first phase and commencement of the second phase of the defrost cycle. Energizing of the relay coil 65 closes its contacts 65A to maintain the coil 65 energized even though the pressure switch P breaks contact after a short period of time, and thus maintaining hot gas solenoid valve 26 energized and therefore in open condition. Simultaneous closure of a second set of contacts 65B of coil 65 also re-energizes and opens liquid line solenoid valve 22, and normally closed contacts 65C of relay coil 65 in the supply circuit for the condenser fan opens to stop the condenser fan. When the thermostat 62 senses that the evaporator coil has been heated up to a predetermined tem perature, as indicated by the thermostat 62 assuming the hot position, a timer release solenoid coil 65 is energized, which returns the switches 60A and 60B to the refrigeration position. This recloses solenoid valve 26 for the third phase of the defrost cycle, during which operation of the compressor, while the evaporator fan is stopped, reduces the pressure and temperature of the evaporator to the normal operating level, signaled by the return of the thermostat 62 to the cold position, which restarts the evaporator fan 21A and resumes normal refrigeration. The pressure switch P is of the type having a wide differential, and is set so that it Will break when the evaporator pressure or suction pressure exceeds the maximum which the compressor can safely tolerate. Therefore, the pressure switch P performs the double function of a defrost control and a high back pressure safety control. The cut-in point of the pressure switch must be above the setting of the conventional low pressure cutout of the system, so that the system will not be stopped after initiation of defrost.

FIGURE 6 shows a similar control wiring circuit which may be used with the system of FIGURE 4, the single throw switch 60B controlled by timer 60 in this case opening the circuit to liquid line solenoid valve 22 to close this valve throughout the first and second phases of the defrost cycle and the double throw switch 60A de-energizing the evaporator fan motor 21A throughout the entire defrost cycle. Following evacuation of the liquid line 20, and evaporator 21, during the first phase of defrost, pressure switch P closes, energizing hot gas solenoid valve 26 and valve 50 to open the conduit 23 by-passing the expansion valve 25 and open valve 50 in the inlet line to valve 30. Closure of valve 50 during the first or pumpdown phase of defrost therefore ensures no by-passing of hot gaseous refrigerant through lines 28, 31 and valve 30 until liquid line 20 and the evaporator have been evacuated. As with the earlier described circuit of FIGURE 5, making of the hot contact of defrost termination thermostat 62 energizes the release solenoid coil 64 to return switches 60A and 60B to the refrigeration position, reclosing valves 26 and 50, but leaving the evaporator fan motor 21A de-energized for the third phase of the defrost cycle until thermostat 62 returns to the position closing its cold contact.

In each of these systems, it will be recognized that a pressure signal is utilized, sensed by a pressure switch P, to indicate proper evacuation of liquid from the refrigerant circuit downstream of the liquid line solenoid valve 22 before the second phase of the defrost cycle begins wherein the hot gaseous refrigerant is admitted to the evaporator. If mere timed delay for the evacuation phase were to be employed, this would establish a fixed time period for an occurrence which requires a time period varying with ambient temperature and evaporator frost load.

If liquid return to the compressor is to be positively prevented, all liquid must be removed from the defrost circuit prior to opening of the defrost solenoid valve 26, since the quantity of liquid in the circuit will vary with factors of ambient temperature and evaporator frost load. Removal of liquid from the defrost circuit prior to opening of the solenoid valve 26 can be positively assured only by reducing the saturation pressure in the circuit to a predetermined value ensuring evacuation of the liquid, preferably sensed by some pressure sensing means.

While several preferred examples of the present invention have been particularly shown and described, it is apparent that various modifications may be made therein within the spirit and scope of the invention, and it is desired, therefore, that only such limitations be placed on the invention as are imposed by the prior art and set forth in the appended claims.

What is claimed is:

1. A hot gas defrostable refrigeration system having a refrigeration cycle and a defrost cycle, comprising a compressor having discharge and suction sides, the system including a highside portion wherein higher pressures prevail and a lowside portion wherein lower pressures prevail, a condenser in said highside portion coupled to said discharge side to receive refrigerant gas from the compressor during the refrigeration cycle, an evaporator in said lowside portion coupled to said suction side for return of refrigerant vapor from the evaporator to the compressor, conduit means connecting said condenser with said evaporator for flow of condensed refrigerant to the evaporator during the refrigerant cycle and for flow of gaseous refrigerant without intermixture of any liquid refrigerant therewith during said defrost cycle including a single conduit extending between the highside and lowside of the system serving as a combined liquid and hot gas line conducting condensed refrigerant to the evaporator during the refrigerant cycle and conducting gaseous refrigerant to the evaporator during the defrost cycle, means for delivering condensed refrigerant from said condenser to said single conduit during said refrigeration cycle, bypass conduit means for delivering hot gaseous refrigerant from said discharge side to said single conduit in by-passing relation to said condenser during said defrost cycle, and first and second valve regulated conduit sections for conducting refrigerant from said single conduit to said evaporator respectively through said second conduit section during said refrigeration cycle and through said first section by-passing said second section during said defrost cycle, said bypass conduit means including an outlet pressure sensitive valve responsive to pressure conditions reflected from said evaporator through said single conduit when said first valve regulated conduit section is open for maintaining said valve closed during the refrigerant cycle and for opening the same during a selected portion of the defrost cycle.

2. A hot gas defrostable refrigeration system as defined in claim 1 wherein additional valve means controlling said single conduit are provided adjacent the end thereof nearest the condenser to close the single conduit during a selected initial portion of the defrost cycle for evacuating the single conduit of liquid refrigerant before admission of hot gaseous refrigerant to said single conduit.

3. A hot gas defrostable refrigeration system as defined in claim 2, wherein said means for delivering hot gaseous refrigerant from the compressor discharge side to said single conduit comprises a branch conduit having an upstream end communicating with said discharge side and a downstream end in communication with said single conduit and having said outlet pressure sensitive control valve intermediate said ends automatically responsive to pressure conditions at said downstream end to close said branch conduit throughout the refrigeration cycle and to automatically open said branch conduit during the defrost cycle responsive to pressure reduction in said single conduit during the defrost cycle for admitting hot gaseous refrigerant to said single conduit for delivery to the evaporator.

4. A hot gas defrostable refrigeration system as defined in claim 2, wherein said first conduit section includes a first electrically controlled valve, said valve means controlling said single conduit being a second electrically controlled valve and said system including electrical control means responsive to selected events to initiate a defrost cycle having three phases; the electrical control means having means for closing both said first and second valves during the first of said phases, means responsive to evaporator pressure conditions for opening both said first and second valves and establishing the second of said phases only when the sensed pressure signifies that the evaporator has been evacuated, and means responsive to evaporator temperature to reclose said first and second valves when the evaporator temperature reaches a selected level and initiate said third phase, said evaporator having fan means, and said control means including said means maintaining said fan means deactivated throughout said three phases.

5. A hot gas defrostable refrigeration system as defined in claim 1, wherein said first conduit section has a solenoid valve therein, and electrical control means for closing said solenoid valve throughout the refrigeration cycle to cause refrigerant to flow from said single conduit through said second conduit section to said evaporator and for opening said solenoid valve to admit hot gaseous refrigerant to said evaporator through said section first conduit section during selected portions of the defrost cycle.

6. A hot gas defrostable refrigeration system as defined in claim 1, wherein said means for delivering hot gaseous refrigerant from the compressor discharge side to said single conduit comprises a branch conduit having an upstream end communicating with said discharge side and a downstream end in communication with said single conduit and having said outlet pressure sensitive control valve intermediate said ends automatically responsive to pressure conditions at said downstream end to close said branch conduit throughout the refrigeration cycle and to automatically open said branch conduit during the defrost cycle responsive to pressure reduction in said single conduit during the defrost cycle for admitting hot gaseous refrigerant to said single conduit for delivery to the evaporator.

7. A hot gas defrostable refrigeration system as defined in claim 1, wherein said conduit means connecting the condenser with the evaporator includes a receiver adjacent the condenser, and means communicating the receiver with the condenser and the end of said single conduit nearest the condenser to receive condensed refrigerant from the condenser and to supply refrigerant to said single conduit.

8. A hot gas defrostable refrigeration system having a refrigeration cycle and a defrost cycle, comprising a compressor having discharge and suction sides, a condenser coupled to said discharge side to receive refrigerant gas from the compressor during the refrigeration cycle, an evaporator coupled to said suction side for return of refrigerant vapor from the evaporator to the compressor, conduit means connecting said condenser with said evaporator for flow of condensed refrigerant to the evaporator during the refrigerant cycle including a single conduit spanning the major portion of the distance between the condenser and evaporator serving as a combined liquid and hot gas line conducting condensed refrigerant to the evaporator during the refrigerant cycle and conducting gaseous refrigerant to the evaporator during the defrost cycle, means for delivering condensed refrigerant from said condenser to said single conduit during said refrigeration cycle, means for delivering hot gaseous refrigerant from said discharge side to said single conduit in hy-passing relation to said condenser during said defrost cycle, means having an expansion device leg and a valved leg for conducting refrigerant from said single conduit to said evaporator through said expansion device leg during said refrigeration cycle and through said valved leg by-passing the expansion device during said defrost cycle, additional valve means controlling said single conduit adjacent the end thereof nearest the condenser to close the single conduit during a selected initial portion of the defrost cycle for evacuating the single conduit of liquid refrigerant before admission of hot gaseous refrigerant thereto, said means for delivering hot gaseous refrigerant from the compressor discharge side to said single conduit comprising a branch conduit having an upstream end communicating with said discharge side and a downstream end in communication with said single conduit and having an outlet pressure sensitive control valve intermediate said ends automatically responsive to pressure conditions at said downstream end to close said branch conduit throughout the refrigeration cycle and to automatically open said branch conduit during the defrost cycle responsive to pressure reduction in said single conduit during the defrost cycle for admitting hot gaseous refrigerant to said single conduit for delivery to the evaporator, said valved leg including a first electrically controlled valve, said valve means controlling said single conduit being a second electrically controlled valve and said system including electrical control means responsive to selected events to initiate a defrost cycle having three phases; the electrical control means having means for closing both said first and second valves during the first of said phases, means responsive to evaporator pressure conditions for opening both said first and second valves and establishing the second of said phases only when the sensed pressure signifies that the evaporator has been evacuated, and means responsive to evaporator temperature to reclose said first and second valves when the evaporator temperature reaches a selected level and initiate said third phase, said evaporator having fan means, and said control means including said means References Cited UNITED STATES PATENTS 3/1963 Marlo. 7/1967 Watkins 62-278 XR MEYER PERLIN, Primary Examiner.

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