GB2150273A - Refrigeration systems - Google Patents

Abstract

A refrigeration system includes a pressure-responsive switch 100, sensitive to pressures in a refrigerant supply line 22, 24, 111 extending from a condenser 20 to an evaporator or evaporators 32, 34 or in a receiver 28 communicating with the supply line 24, which switch 100 operates to open a normally closed solenoid valve 56, to dump hot gas from a compressor discharge line 18, into the receiver 28. The switch 100 acts whenever pressure falls below a predetermined minimum value. The refrigeration system can include both the pressure switch 100 and a sensing device 46a arranged to detect "flashing" in the refrigerant supply line and to act on the same valve 56, with the switch 100 adapted to override the sensing device 46a whenever the pressure in the refrigerant supply line drops below the minimum pressure for which the switch 100 is preset, without regard to whether flashing has begun to develop downstream from the switch 100. <IMAGE>

Description

SPECIFICATION Refrigeration systems The present invention relates, generally, to refrigeration systems, and has an application in such systems utilized for refrigerating display cases in supermarkets. In a more particular sense, the present invention relates to refrigeration systems having controls operating automatically to sense a drop in pressure in the supply of liquid refrigerant between the condenser and the evaporator or evaporators, and, more specifically, to sense when the pressure drops below a minimum accepted value in the liquid supply line extending from the condenser, and in the receiver communicating with that line.

In commercial refrigeration systems of the type described, the typical installation incorporates a liquid supply line, extending from the outlet side of the condenser to supply refrigerant to the evaporators. Commonly, a receiver, either of the surge or pass-through type, is in communicating relation to the liquid supply line. It is known that in order to supply liquid refrigerant to the evaporators in a manner conducive to optimum refrigerating efficiency, the pressure within the liquid supply line should not be allowed to drop below an acceptable minimum.

To maintain pressure in the refrigerant supply line, a control line is typically connected between the top portion of the receiver and the compressor discharge line. The compressor or compressors discharge hot gas, and under controlled conditions this hot gas can be caused to flow into the top portion of the receiver, to increase the pressure in the receiver and hence in the liquid supply line. In this way, the head pressure in the liquid supply line is elevated to a required, acceptable minimum.

Normally, a valve is utilized in the receiver pressure control line extending from the compressor discharge line. Typically, this valve has been sensitive to pressures on its outlet side to open and permit the flow of hot gas to the receiver.

Many patents have been issued, relating to the pressurizing of the receiver by hot gas taken from the compressure discharge line and reference is made by way of example to the present Applicants' U.S. Patents Nos. 3,905,202,4,012,921 and and 4,231,229, all of which disclose a receiver pressure control line extending from the compressor discharge line to the top portion of the receiver.

In the present Applicants' U.S. Patent No.

4,328,682 receiver pressure is controlled in a somewhat different fashion from that disclosed in the above referred to patents. In the last named patent, "flashing" in the refrigerant supply line, upstream from the expansion valves, is detected by a sensing device, which, through a power circuit or module, operates a normally closed solenoid valve in a receiver pressure control line extending from the compressor discharge line to the top portion of the receiver. When such a condition is detected by the sensing device, the signal produced thereby is operative to open the valve to dump hot gas from the compressor discharge line into the top portion of the receiver, whereby to elevate the receiver pressure and hence the pressure in the refrigerant supply line, until the flashing condition is terminated.

These devices are fully efficient for their intended applications, but it has been found desirable to further address the problem of a dropping of head pressure in the refrigerant supply line and receiver below an acceptable minimum, to assure that the pressure in this area of the refrigerating system will be at or above a value required for efficient refrigeration.

In the above-mentioned U.S. Patent No. 4,328,682, thus, head pressure is elevated to whatever extent is necessary to eliminate "flashing". The present invention, as distinguished from that of U.S. Patent No.4,328,682, has as its main object the elevation of head pressure to a minimum acceptable value predetermined as important to the proper functioning of the system. This is done without regard to the presence or absence of conditions conducive to flashing (although it is more than likely that bringing the head pressure up to a minimum acceptable value will aid in controlling such conditions).In other words, the present invention is proposed to override the sensing devices referred to above, if they have been incorporated in the system, and will keep the head pressure up to a predetermined minimum even if flashing has not yet occurred.

According to the present invention there is provided a refrigeration system which essentially includes at least one each of the following; namely a compressor, a condenser, a receiver having top and bottom portions, and an evaporator, and in which there is a compressor gas discharge line that extends from the compressor to the condenser, a refrigerant supply line through which refrigerant flows from the condenser to the evaporator, the refrigerant supply line and the receiver together comprising a refrigerant supply means in which a head pressure is to be developed for assuring a pressurized supply of refrigerant to the evaporator, the refrigerant supply line being in communication, at a location between the condenser and the evaporator, with the bottom of the receiver whereby the pressure within the refrigerant supply line will be responsive to the pressure existing in the top portion of the receiver, a head pressure control line connected between the top portion of the receiver and the compressor gas discharge line, an expansion valve in the refrigerant supply line, and a refrigerant return line extending from the evaporator to the compressor, and which further comprises: a) a pressure switch continuously sensitive to pressures within the refrigerant supply means and connected to a source of electrical power; and b) normally closed valve means connected in the head pressure control line to control flow therethrough, said valve means being in an electric circuit with the pressure switch and the source of electrical power and being opened, as a result of the response of the pressure switch to a drop in head pressure within the refrigerant supply means below a minimum value predetermined as necessary for efficiently supplying refrigerant to the evaporator, for dumping gas from the compressor gas discharge line into the top portion of the receiver through the head pressure control line to thereby increase the pressure in the top portion of the receiver, said valve means being held open for that period of time necessary to elevate the head pressure in the refrigerant supply means sufficiently to assure a continued supply of refrigerant to the evaporator at an efficient head pressure.

Summarized briefly, the invention may take various forms, depending on whether or not the system incorporates a surge receiver or, alternatively, a pass-through receiver. In one form of the invention, in which a surge receiver is used, the invention operates in combination with the sensing device and power module and solenoid valve disclosed in the above-mentioned U.S. Patent No.

4,328,682. In this form, a pressure switch is connected in pressure-sensitive relation to the refrigerant supply line, between the condenser and receiver, and is connected to a power circuit or module, common to the sensing device of U.S. Patent No.

4,328,682. The pressure switch, when it senses that pressure in the refrigerant supply line has dropped below an acceptable minimum, operates through the power module to open the solenoid valve, thus to dump hot gas from the compressor discharge line to the top portion of the receiver until the head pressure within the refrigerant supply line rises to an acceptable minimum, thereby de-activating the switch and permitting the solenoid valve to revert to its normally closed condition.

In this arrangement, the pressure switch is adapted to override the sensing device utilized for the purpose of detecting flashing in the refrigerant supply line. In other words, the sensing device can detect flashing, and can operate to open the solenoid valve through the power circuit, to increase pressure in the receiver until the flashing condition disappears. However, the pressure switch of the present invention may operate to maintain the solenoid valve in open position even though the flashing has disappeared, should the pressure switch sense that the head pressure in the refrigerant supply line still has not been elevated to a value or level found desirable for optimum refrigerating efficiency.

In other circumstances, even though flashing may not have developed, the pressure switch will operate if it senses that pressure has dropped below the minimum acceptable value in the refrigerant supply line, to open the solenoid valve and keep it open for as long as it is necessary to maintain a flow of hot gas into the top portion of the receiver and raise the head pressure to the acceptable minimum value.

In a second form of the invention, the device is operative to sense pressures within the top portion of a surge receiver, in a system in which a sensing device is omitted from the refrigerant supply line. In these circumstances, the pressure switch acts to open the solenoid valve in the control line extending from the compressor discharge line to the top portion of the receiver, responsive to a pressure drop within the receiver below an acceptable minimum found necessary for the maintenance of proper head pressure in a refrigerant supply line that bypasses but is in communication with the receiver.

In a third form, the invention is applied to a passthrough receiver, with the pressure switch being in pressure-sensing relation to the top portion of the receiver, and being connected to a power circuit and to the above-mentioned solenoid valve.

In all these forms of the invention, a check valve is used in the drop leg from the condenser in place of an inlet pressure regulating (IPR) valve heretofore employed. The pressure switch and check valve combine to discharge the functions of the IPR valve, though in a far more positive and trouble-free manner, since the IPR valve often fails to hold its settings with a desired precision. Since the check valve prevents reverse flow up the drop leg and the pressure switch closes to control head pressure downstream from the check valve, the need for an IPR valve, which closed or modulated to build up inlet pressure between it and the condenser, is dispensed with.

In order that the invention may be better understood, the following detailed description of preferred embodiments thereof is made, by way of example, with reference to the accompanying drawings, in which: Figure 7 is a schematic representation of a refrigerating system in which the pressure-sensitive switch is mounted on the refrigerant supply line, in association with a sensing device, power circuit, and solenoid valve already provided for the purpose of detecting and eliminating flashing in the refrigerant supply line; Figure 2 is a schematic view showing a second form of the invention, in which the pressure-sensing device is mounted directly upon a receiver of the surge type, and in which a sensing device is not used; Figure 3 is a schematic view in which the invention is applied to a system utilizing a pass-through receiver; and Figure 4 is an electrical block diagram showing a typical wiring arrangement for the invention when utilized with the flash gas sensing device of Figure 1.

In the form of the invention shown in Figure 1, elements common both to the present invention and to the system disclosed in U.S. Patent No. 4,328,682, have been designated by the same reference numerals both in the patent and in the present application.

The disclosure of U.S. Patent No. 4,328,682 is, it may be noted, incorporated herein by reference.

Briefly, for the purposes of the present application it is sufficient to note that in common with the system disclosed in U.S. Patent No. 4,328,682, the present invention, in the form of Figure 1, includes a plurality of compressors 10, 12, 14 connected in parallel to discharge compressed gaseous refrigerant through a common header 16 into a compressor discharge line 18 which passes the hot gas to a condenser 20 desirably cooled by ambient air, and of sufficient capacity to condense the entire output of the three compressors. Condenser 20 delivers liquid refrigerant through a condenser drain line 22 to liquid line 24. A surge receiver 28 communicates at its bottom with liquid line 24, through a connecting line 30.Liquid line 24 is connected, through a portion of line 111,to a series of evaporators connected in parallel, as shown at 32,34.

The condenser drain line 22 (also referred to herein as "the drop leg 22"), liquid line 24 and the portion of line 111 together make up the refrigerant supply line.

The evaporators 32,34 are provided at their inlet ends with expansion valves 36, 38 respectively.

Refrigerant entering the evaporators through the expansion valves from the liquid line flows back to the compressors through return line 42a, 42b which are in communication with a common return header 44 communicating with suction line 45 common to the several compressors.

The invention is not limited to a system utilizing multiple compressors and/or multiple evaporators. It may be employed advantageously in systems utilizing one or more compressors, as well as one or more evaporators. Typically, however, a commercial installation in a supermarket will be as shown, utilizing a multiplicity of compressors connected in parallel, and a bank of evaporators which, in the same system, would also be in parallel.

Aflashing sensor has been illustrated in Figure 1, and has been generally designated 46a.

In the Figure 1 embodiment of the invention, separate sensing devices 48a, 486 have been mounted to sense the condition of liquid refrigerant entering the evaporators 32, 34 respectively through evaporator inlet lines 49a, 49b.

The sensing devices, in the illustrated embodiment, comprise sight glasses, which are mounted directly in the respective evaporator inlet lines.

Liquid refrigerant flowing to the expansion valves thus passes through the sight glasses, immediately upstream ofthe respective expansion valves.

The refrigerant, at this location, is desirably in a fully liquid form, to provide a solid column of liquid at the inlet to each thermostatic expansion valve, that is to say, a liquid seal at each valve location.

Maintenance of the liquid seal at the location of each expansion valve is, however, not always achieved during the normal operation of a refrigerating system of the character illustrated and described.

In particular, flashing takes place at one or more expansion valves, not infrequently. When this happens, the integrity of the liquid seal is broken, in that instead of a solid column of liquid being presented to the expansion valve, there is, instead, a vaporliquid combination.

The refrigerant, when the condition known as flashing occurs, loses its normal clarity, that is to say, from a condition in which it has the visual characteristics of clear water, it volatilizes to a cloudy appearance and will reflect any light ray directed toward it.

At this point, it may be noted that there can be additional evaporators, as well as additional sensors associated with the additional evaporators, as illustrated schematically in Figure 1. For the purposes of the present disclosure, however, the invention will be described as though there were only two evaporators and two associated sensing devices, one for each of them.

Designated at 56 is a solenoid valve, which controls flow through a head pressure control line 58 communicating between the compressor discharge line 18 and the top of the surge receiver 28. Between the surge receiver and the solenoid valve, there is provided in the present instance a conventional check valve 60. This is arranged to prevent any flow from the receiver to the compressor discharge line, in the event the receiver pressure should exceed that in the compressor discharge line in a situation in which the solenoid valve 56 happens to be open.

Electrical circuits in which the operation of electrical devices, in this case the solenoid valve 56, is controlled by means of one or more phototubes, are in and of themselves well known. These so-called "electric eye" circuits are thus illustrated in a very basic, schematic way herein. In Figure 1, sight glasses 48a, 48b are respectively disposed in positions to intercept light beams directed from light souces 52a, 52b to phototubes 50a,50b respectively.

The phototubes are separately connected by leads 54a, 54b respectively to a conventional power circuit 63, connected to a source of electrical power 64 and controlling the operation of the solenoid valve 56 through leads 62. In the typical "electric eye" power circuit, the signal resulting from the action of light impressed upon the cathode of the phototube is amplified within the typical electric eye power circuit shown, to operate a relay or the like within the power circuit, to control the flow of current from the power source 64 to the solenoid 56.

Referring to evaporator 34 by way of example, if refrigerant is flowing through the sight glass 48b in a fully clear, liquid condition, maintaining a good liquid seal at the expansion valve 38, there is no interruption of the light beam between light source 52b and phototube SOb. In these circumstances, the phototube is energized to impress a signal on the power circuit 63 illustrated in Figure 1. Assume the valve 56 is of the type in which it is normally closed when de-energized. The appropriate relay or other equivalent means of circuit 63 in this event may be normally energized in the presence of the signal from phototube 50b, to hold the circuit open between the source of power and the solenoid.

This would be the normal, efficient operation in a condition in which there is no flashing at the inlet side of any of the expansion valves. In these circumstances, the head pressure is high enough, within the liquid line 24 and hence in the evaporator inlet lines 49a, 49b,to maintain a good liquid seal at the expansion valve locations. There would thus be no reason to artificially maintain the head pressure at these locations any higher than necessary to maintain a good seal and, consequently, good and efficient operation of the evaporators.

If the head pressure should fall, no problems result at the location of the expansion valves unless and until the pressure falls to the point where it produces liquid flashing within one or more of the sight glasses. When this happens, as indicated above, the solenoid valve is opened immediately and by causing the flow of hot gas from the discharge line 18 into the receiver, produces added pressure upon the liquid within the receiver and hence upon the liquid refrigerant flowing through the liquid line 24 and the evaporator inlet lines. The valve remains open, and the head pressure within the liquid line rises, only to the extent necessary to eliminate the flashing condition and restore the desired condition in which there is a solid column of liquid at the inlet side of the affected expansion valve or valves.

The connection of the phototubes to the power circuit would, of course, in Figure 1 be such as to cause the solenoid valve to be opened when flashing is detected in any of the evaporator inlet lines. It is not necessary that flashing occur in all the lines before the valve opens.

The structure described above has been fully disclosed in U.S. Patent No. 4,328,682 and in and of itself does not constitute part of the presentinven- tion. In the form of Figure 1, however, the present invention operates in cooperation with the flashing sensors.

In this connection, it has been common, in refrigerating systems of the type described, to utilize, in the drain line 22, which is the part of the refrigerant supply line extending from the outlet side of the condenser an inlet pressure responsive ("IPR") valve. Such a valve with be found in all of the systems of the patents identified above. Its function has been to close or modulate responsive to pressure drops below an acceptable value at the outlet side of the condenser. Usually, it has been adjusted to respond to a predetermined inlet pressure so as to assure the desired condensing pressure in condenser 20 and produce at least partial flooding thereof under outdoor temperature conditions requiring throttling of the valve.

In the embodiments of the present invention herein described, this inlet pressure responsive valve is omitted. There is provided, instead, only a check valve 104, permitting flow only in a direction from the condenser toward the evaporators.

In accordance with the present invention, there is provided a pressure switch 100. In and of itself, switches of this type are well known. Typically, they can be mounted on any conduit through which there is fluid flow under pressure, and can be adjusted to sense drops in pressure below a predetermined acceptable, selected value. Under these circumstances, a pressure drop below the acceptable value results in closing of the switch contacts. Typically, a switch of this type incorporates a diaphragm which deflects according to the sensed pressures to close or open the contacts of the switch.

In the illustrated example, leads 102 extend from the switch to the power circuit 63 aiready provided in the refrigerating system. Within the power circuit, the closing of the pressure switch contacts is operative to open the normally closed solenoid valve 56, so that hot gas is dumped from the compressor discharge line 18 to the top portion of the receiver 28.

This pressurizes the receiver 28. The pressure continues to rise in the top portion of the receiver, as gas continues to flow through the now open solenoid valve 56, until it is above the acceptable minimum. The elevation of the pressure in the top portion of the receiver creates pressure against the liquid in the bottom portion thereof, which in turn is transmitted to that part of the refrigerant supply line denoted herein as liquid line 24, in the area of the pressure switch 100. As a result, the pressure switch 100 opens, thus opening the circuit to the solenoid valve 56, and permitting the solenoid valve to revert to its normally closed condition.

It may be observed that the arrangement illustrated is one in which the pressure switch 100 acts in conjunction with the sensing device disclosed in U.S. Patent No. 4,328,682, in a way to assure that there will be neither undesirable flashing in the area of the expansion valves, our a drop in head pressure within the refrigerant supply line below an acceptable minimum. Eitherthe sensing device or the pressure switch 100 is operable to open the same solenoid valve, to dump hot gas into the top portion of the receiver. Both may, indeed, be operating at the same or in overlapping periods of time. Whenever the pressure switch 100 operates however, it will continue to hold the solenoid valve open for as long as it is necessary to elevate the head pressure within the refrigerant supply line to a minimum value found desirable for optimum refrigerating efficiency.At the same time, there is no danger of the head pressure in the refrigerant supply line being elevated to an excessive value, that is, a value found unnecessary for full refrigerating efficiency and hence productive of an undesirable expenditure of energy.

In Figure 4, there is disclosed a typical electrical circuit that may be provided when the pressure switch 100 is used in conjunction with a sensing device of the type illustrated in Figure 1.

Referring to Figure 4, it is seen that extending from the contacts of the pressure switch 100 are leads 102.

One of these leads may be connected directly to one of the leads 62 extending from solenoid valve 56.

The other lead 62 may be connected directly to one side of the source of electrical power 64. The other lead 102, extending from the other contact of switch 100, is connected to the other side of the source of electrical power.

In this way, whenever the pressure switch 100 closes, it will cause the solenoid valve 56 to open, without regard to whether or not the sensing devices 50a, sob, have detected flashing conditions and have sent one or more signals to open the solenoid valve.

Assuming that the solenoid valve 56 has indeed been called upon by one or more of the sensors 50a, 50b to open to pressurize the receiver, it will not close if the pressure responsive switch still senses that the pressure in the refrigerant supply line remains below the acceptable minimum value for which the switch 100 has been preset.

In the circuit illustrated in Figure 4, either or both of the sensing devices, 50a, 50b can also operate to open the solenoid valve 56, by acting upon the power module 63, to close switch contacts 120 within said module. As a result, power will flow from one side of the circuit through lead 122, closed switch contacts 120, and one lead 62 to the solenoid valve 56, returning from the solenoid valve 56 through the other lead 62 to the other side of the power circuit.

The check valve 104 serves an important function.

Were it not present, and assuming that the receiver is pressurized by hot gas flowing into it through control line 58, refrigerant might be forced out of the receiver, and up the drop leg 22 to the condenser, since there would be less restriction to free flow in that direction than through the expansion valves.

The check valve 104 acts in cooperation with the pressure switch 100, however, in such a way that the switch 100 closes, and with the check valve 104 blocking reverse flow up the drop leg 22, an elevation in head pressure to an acceptable minimum value will be transmitted to the portion of the liquid line 24 downstream from the check valve 104, where it is needed.

It will be understood that the refrigerant supply line and the receiver may be appropriately regarded as constituting, together, a refrigerant supply means generally designated 113. This means includes the drain line or drop leg 22, check valve 104, liquid line 24, receiver 28, connecting line 30 and the portion 111 of the refrigerant supply line that is disposed between the receiver and expansion valves.

Referring now to Figure 2, there is here disclosed an embodiment of the invention in which the sensing devices have been omitted, while retaining, however, the solenoid valve 56 and its associated check valve 60 in the receiver pressure control line 58. A sight glass 48 may be retained for visual observation purposes.

In this embodiment of the invention, the pressure switch has been designated 106, and is mounted directly upon the top portion of the receiver, in pressure-sensing relation to the upper portion of the receiver 28, into which hot gases flow through line 58 from the compressor discharge line 18.

In this embodiment, switch 106 includes leads 108, 110 extending from the respective switch contacts.

Lead 108 extends to one side of a source of electrical power. Lead 110 extends to one contact of solenoid valve 56. Extending from the other contact of the solenoid valve is a lead 112 extending to the other side of the source of electrical power.

Switch 106 is normally open. However, assuming that the switch senses a drop in pressure within the top portion of the receiver below an accepted minimum value, the switch contacts will close. As a result, a circuit is closed through the solenoid valve 56. Valve 56 is thus caused to open, to dump hot gas from the compressor discharge line 18 into the top portion of the receiver. The hot gas continues to flow into the receiver 28, as long as it is necessary to pressurize the receiver to an extent found sufficient for assuring proper refrigerating efficiency. As soon as this value is reached, the switch 106 opens, causing the valve 56 to revert to its normally closed condition.

In the embodiment of the invention shown in Figure 2, it is possible to incorporate the flashing sensors shown in Figure 1, and their associated circuitry. In other words, it is possible to mount the switch 106 on the receiver itself, rather than on the refrigerant supply line, while otherwise utilizing the flashing sensor devices and circuitry of Figure 1.

In the embodiment of the invention shown in Figure 3, instead of a surge receiver there is illustrated a pass-through receiver 114. In this embodiment, again, the drop leg 22, that is, the line extending from the outlet side of the condenser to supply liquid refrigerant to the evaporators, is again provided only with a check valve 104, ratherthan a IPR valve such as in, for example, U.S. Patent No.

4,328,682.

Since a pass-through receiver is used, the drop leg 22 includes a portion 24 extending into the bottom area of receiver 114. Liquid refrigerant is drawn from the receiver through a correspondingly placed inlet end of the second portion 116 of the refrigerant supply line. This liquid is supplied to the evaporators for refrigerating purposes.

In this embodiment of the invention, again, there is provided a switch 106 sensitive to pressures directly within the top portion of the receiver. Also provided is the circuitry shown in Figure 2, including leads 108,110 connecting the switch 106 and solenoid valve 56 in series with a source of electrical power.

Again, the device shown in Figure 3 could utilize the sensing device circuitry shown in Figure 1, should this be so desired, in which event the circuit illustrated schematically in Figure 4 would be provided.

In all embodiments of the invention, the pressure switch 100 or 106 acts in the same way, that is, whenever the head pressure as sensed in the refrigerant supply means (that is, either in the refrigerant supply line or in the top portion of the receiver) drops below a minimum acceptable value, switch 100 or 106 closes a circuit through the normally closed solenoid valve 56, causing said valve to be energized to an open condition, so that hot gas will be dumped from the compressor discharge line into the top portion of the receiver, pressurizing the receiver for as long as necessary to raise the head pressure to an acceptable minimum value. As soon as this minimum value is reached, the solenoid valve is de-energized, because the switch will sense the elevation of the pressure to an acceptable level, and will open automatically as a result of the sensed, acceptable head pressure.

In Figure 3, the refrigerant supply means has been generally designated 1 13a and includes the drop leg 22 from the condenser, check valve 104, refrigerant supply line 24, the continuation 116 of the refrigerant supply line, and the receiver 114.

Claims (13)

1. A refrigeration system which essentially includes at least one each of the following; namely, a compressor, a condenser, a receiver having top and bottom portions, and an evaporator, and in which there is a compressor gas discharge line that extends from the compresserto the condensor, a refrigerant supply line through which refrigerant flows from the condenser to the evaporator, the refrigerant supply line and the receiver together comprising a refrigerant supply means in which a head pressure is to be developed for assuring a pressurized supply of refrigerant to the evaporator, the refrigerant supply line being in communication, at a location between the condenser and the evaporator, with the bottom of the receiver whereby the pressure within the refrigerant supply line will be responsive to the pressure existing in the top portion of the receiver, a head pressure control line connected between the top portion of the receiver and the compressor gas discharge iine, an expansion valve in the refrigerant supply line, and a refrigerant return line extending from the evaporator to the compressor, and which further comprises: a) a pressure switch continuously sensitive to pressures within the refrigerant supply means and connected to a source of electrical power; and b) normally closed valve means connected in the head pressure control line to control flow therethrough, said valve means being in an electric circuit with the pressure switch and the source of electrical power and being opened, as a result of the response of the pressure switch to a drop in head pressure within the refrigerant supply means below a minimum value predetermined as necessary for efficiently supplying refrigerant to the evaporator, for dumping gas from the compressor gas discharge line into the top portion of the receiver through the head pressure control line to thereby increase the pressure in the top portion of the receiver, said valve means being held open for that period of time necessary to elevate the head pressure in the refrigerant supply means sufficiently to assure a continued supply of refrigerant to the evaporator at an efficient head pressure.

2. A refrigeration system according to Claim 1, wherein the receiver is of the surge receiver type and is by-passed by the refrigerant supply line, the system including a connecting line extending from the bottom of the receiver to the refrigerant supply line.

3. A refrigeration system according to Claim 1, wherein the receiver is of the pass-through type, the refrigerant supply line including first and second portions extending from the condenser to the receiver and from the receiver to the evaporator respectively.

4. A refrigeration system according to any one of Claims 1 to 3, wherein the pressure switch is mounted on the receiver in pressure-sensitive relation to the top portion of the receiver.

5. A refrigeration system according to any one of Claims 1 to 3, wherein the pressure switch is mounted on and is sensitive to pressure within the refrigerant supply line.

6. A refrigeration system according to Claim 5, wherein the location at which the pressure switch is mounted on the refrigerant supply line is between the condenser and the receiver.

7. A refrigeration system according to Claim 6, further including a check valve mounted in the refrigerant supply line between the condenser and the pressure switch and permitting flow in the refrigerant supply line only in a direction from the condenser toward the receiver.

8. A refrigeration system according to Claim 3 or Claim 4, further including a check valve mounted in the refrigerant supply line between the condenser and the receiver and permitting flow in the refrigerant supply tine only in a direction from the conde nsertoward the receiver.

9. A refrigeration system according to either Claim 7 or Claim 8, wherein the check valve constitutes the sole control of refrigerant flow between the condenser and the receiver.

10. A refrigeration system according to any one of the preceding claims further including a sensing device for detecting flashing of the refrigerant in the refrigerant supply line upstream from the expansion valve but downstream from the receiver, said sensing device, in the absence of flashing, normally being in a non-operating mode, said device switching to a second, operating mode upon but not before the occurrence of said flashing, said valve means being opened by the sensing device upon operation thereof to its second position to provide a mode of dumping gas from the compressor discharge line into the top portion of the receiver through the head pressure control line to increase the head pressure in the refrigerant supply line responsive to the detection of the flashing, said valve means being held open, when actuated as a response to the sensing device in its second operating mode, only for a period of time necessary to elevate the head pressure in the refrigerant supply means sufficiently to eliminate the flashing condition.

11. A refrigeration system according to Claim 10, wherein the pressure switch is arranged to override the sensing device in actuating the valve means.

12. A refrigeration system according to either Claim 10 or Claim 11, wherein the valve means, sensing device and the pressure switch are all electrically actuated and the source of electrical power is common to all of them.

13. A refrigeration system substantially as hereinbefore described with reference to the accompanying drawings.