US 3359749 A
Description (OCR text may contain errors)
Dec. 26, 1967 HOWLAND E A 3,359,749
DIFFERENTIAL CONTROL DEVICE Filed June 17, 1965 INVENTOR.
LELAND L.HOWLAND BY HAROLD E MC CLURE mromvns VN ON United States Patent 3,359,749 DIFFERENTIAL CONTROL DEVICE Leland L. Hon/land, Minneapolis, and Harold E. Mc- Clure, Wayzata, Minn, assignors to Thermo King Corporation, Minneapolis, Minn, a corporation of Delaware Filed June 17, 1965, Ser. No. 464,630 4 Claims. (Cl. 62140) This invention relates to improvements in the control of a system involving differential factors. In general, the invention is concerned with a control device that is used under conditions where a differential in factors, such as pressure, is subject to change or variation. More particularly, the invention is concerned with the control of the defrosting of a refrigerant heat exchanger used in a sysem where the rate or volume of air which is forced through the heat exchanger is not constant, but is subject to variation.
Within the general field of refrigeration, there exists certain areas of application Where the rate of refrigeration and of air flow vary considerably. As an example, in the industry of transporting perishable products, the enclosures in which the products are stored, or the vehicles used for the transportation of perishables, are often subject to changing temperature and weather conditions; and moreover, it is often desirable to provide refrigeration equipment that requires only a minimum of attention. For this reason, some refrigeration systems used in this industry may be of a type which operate continuously, but at two different rates. Such a system provides, on demand, :a high rate of heat exchange and air flow until the demand is satisfied, when the system operates at a lower rate of heat exchange and air flow until the next demand for refrigeration occurs. Such a system is disclosed in US. Patents 2,477,377 and 2,992,541, both assigned to the assignee of this application.
In such a system, as referred to above, the air which is circulated into contact with the refrigerant heat exchanger will generally contain moisture that congeals on the surface of the heat exchanger, and thus impedes the flow of air between the conduits and the fins that extend between said conduits. Removal of the congealed moisture is effected by defrosting, which may be accomplished either by the application of external heat, or a modification of the refrigerant circuit to obtain heat from the compressed refrigerant gases.
As frost or congealed moisture collects on the surface of the heat exchanger, it will impede the flow of air between the coils and fins that form the heat exchanger. Unless this condition is corrected by defrosting, the layer of frost will build up to a point where little if any air can flow through the heat exchanger. Since such air is generally forced through the heat exchanger by means of a fan or blower, as the layer of frost accumulates, a substantial differential in air pressure will be developed on the opposite sides of the heat exchanger, and this differential may be utilized to actuate the defrosting means. However, such a system will not be fully operative when the rate or volume or air flow is subject to change, for the reason that the pressure differential between the opposite sides of the heat exchanger does not remain relatively constant when the rate at which the air is forced against one side of the heat exchanger is subject to substantial variation. Thus, when the air is forced against the heat exchanger at a high rate, the impedance of the structural elements of the heat exchanger, even when relatively frost free, will create a greater pressure differential than would occur when the air is tnaveling at a lower rate, even though at the lower rate the passages of the heat exchanger are partially clogged with frost or ice.
In the present invention there is provided a control device which responds to pressure differential, and which is effective even when such differential is subject to change. Thus, the control is responsive to a first differential when the force of the air or other flow is relatively low, and to another differential when the force of air or other flow is relatively high.
An object of the invention is to provide a control device which is adapted to operate on a variation in differential between two factors, and particularly where one of the factors is subject to variation.
Another object is to provide a control device embody ing a diaphragm which responds to a differential in pressure, together with means for controlling the movement of the diaphragm to prevent a responsive movement thereof when, under certain conditions the differential is changed.
A further object is to control the defrosting of a refrigerant heat exchanger under conditions where the rate of air flow with respect to the heat exchanger is subject to variation as between a low rate of air flow and a high rate of air flow.
Other and further objects may become apparent from the following specification and claims, and in the drawings in which:
FIG. 1 is a cross sectional view of a diaphragm operated control device;
FIG. 2 illustrates a modification of a portion of the structure shown in FIG. 1; and
FIG. 3 is a schematic view of a refrigeration unit, together With a control device of the type disclosed in either FIG. 1 or 2, for controlling the defrosting of the refrigerant heat exchanger.
Referring now to the several views of the drawing, the invention will be described in detail.
Referring to FIG. 1, general reference numeral 10' indicates in its entirety a control device for controlling the operation of a mechanism in response to a differential in factors, such as a difference in pressure. Reference character 12 designates a diaphragm chamber within which is located a flexibly movable diaphragm 14. A conduit 16 communicates with the interior of chamber 12 on one side of the diaphragm 14, and extends from what is potentially a higher source of pressure, while on the other side of diaphragm 14, a conduit 18 joins chamber 12 with a potentially lower source of pressure. Mounted on the exterior of chamber 12 is an electric switch 20 provided with an actuator 22 disposed within the interior of chamber 12 and in contact with the diaphragm 14 to be actuated by the movement of said diaphragm.
A solenoid 24 is mounted on a bracket 26 that is secured to chamber :12, and the solenoid is made adjustable with respect to bracket 26 by a pair of fastening means 28, 28a, that extend through slotted portions, not shown, in bracket 26. The solenoid 24 contains a movable armature 30 that carries a depending pin 32, which engages a coil spring 34 mounted on the surface of diaphragm 14. The armature 30 and its depending pin 32 are made adjustable with respect to the spring 34 and diaphragm 14 by means of an adjusting screw 36 carried by bracket 26. In this figure, the armature 3 0 and its associated parts are shown in the position they assume when the inductance coil of solenoid 24 is de-energized.
Referring now to FIG. 2, the portion of the control device 10 shown here is substantially identical to that disclosed in FIG. 1, except that in this figure the armature designated at 30a is also shown in the position it assumes when the inductance coil of solenoid 24a is deenergized. A coil spring 38 is disposed between armature 30a and an adjusting screw 36a carried by bracket 26a. In this disclosure, when the coil of solenoid 24a is en- 3 ergized, the armature 34in moves upwardly against spring 38.
Referring now to FIG. 3, is disclosed a refrigeration system composed of a compressor 46, whose high pressure or discharge side is connected to a conduit 42 that extends to a condenser 44. A receiver 46 is connected to the outlet end of condenser 4-4. A conduit 48 extends from an opposite end of receiver 46 to a refrigerant evaporator 50, and the flow of refrigerant fiuid to the evaporator 50 is controlled by an expansion valve 52 under the influence of a temperature sensitive element 54, that is in thermal contact with a portion of evaporator 56. The outlet end of the evaporator 50 is connected by a conduit 56 to the low pressure side of compressor 40.
To defrost the evaporator Sit, a hot gas line 58 extends from conduit 42 to evaporator 50 at a point adjacent the inlet end of the evaporator, and the flow of fluid through conduit 58 is controlled by a valve 6t), under the influence of an electromagnetic actuator 62. It should be understood that the evaporator 55 may be defrosted by other conventional means under the control of a similar electromagnetic device.
Compressor 40 is driven by a prime mover 64, which may be either an internal combustion engine, or a twospeed electric motor. The prime mover 64 operates either at a low speed or at a higher speed under the control of a speed control device 66 which is actuated by an electromagnetic device 68 under the influence of a thermostatic control device 70 having a temperature sensitive portion 71.
The evaporator 50 is provided with a multiplicity of heat conducting fins 72, and is located within an enclosure 74, which would normally be insulated. A fan or blower 76 is provided for circulating the air through the evaporator within the enclosure 74. The fan 76 is illustrated as being driven by the prime mover 64- through a mechanical connection 78, although it should be understood that if desired, fan 76 can also be driven by an independent prime mover, provided said prime mover is also adapted to operate at a low speed, and high speed.
Conduit 16, which is an open ended tube, has its outer open end extending to enclosure 74 between fan 76, and refrigerant heat exchanger 50, at a point which may be regarded as a high pressure area. Conduit 18 is, likewise, an open ended tube, and has its outer open end extending into enclosure 74 on a side of the heat exchanger t) opposite from fan 76, or in an area which may be regarded as a low pressure area.
A battery 80 forms a source of electrical power, and is connected to a conductor 82 that extends to a conductor 84. Conductor 84 extends to the thermostatic control device 70, and to one pole of switch 20. A conductor 86 extends from the thermostatic control device 70 to the electromagnetic device 68, and a branch conductor 88 extends from conductor 86 to solenoid 24. A conductor 90 extends from the other pole of switch 20 to the electromagnetic device 62 that serves to operate valve 6t).
The operation of the invention, in conjunction with a refrigeration system, will now be described. The purpose of the refrigeration system is to maintain a predetermined temperature within the enclosure 74, and to all practical extent, this is accomplished in a conventional manner. Assuming that the prime mover 64 is in continuous operation, and is continuously driving compressor 40 and fan 76 at either a low speed, where only minimal refrigeration is required, or at a higher speed when there is a demand for refrigeration within the enclosure '74. The refrigerant fluid which is discharged from the compressor 40 passes through the conduit 42 to the condenser 44, and thence to the receiver 46. Liquid refrigerant flows from the receiver 46 through conduit 48 to evaporator 5t under control of the expansion valve 52 whose operation is controlled under the influence of the temperature sensitive portion 54. The refrigerant vapors t formed within the evaporator are returned to the compressor 40 through conduit 56.
Since the fan or blower 76 is in continuous operation, it is circulating air within the enclosure 74 into contact with the refrigerant heat exchanger'composed of evaporator 5t and its heat conducting fins 72. Assuming that said air contains moisture, the latter will congeal on the cold surfaces of coil 5% and fins 72, and will thus progressively impede the flow of air through the heat exchanger, and thus require periodic removal by defrosting of coil 50.
The control device it) is adapted to control the actuation of the electromagnetic device 62, and thus valve in response to a differential in pressure on the opposite sides of the heat exchanger 5t as reflected by the pressure conducted to chamber 12 on the opposite sides of diaphragm 14 by conduits 16 and 118. To adjust the control device illustrated in FIG. 1, which is normally a factory adjustment, the solenoid 24 is actuated and then adjusted on the bracket 26 by means of the adjusting screws 28 and 28a to a point relative to the spring 34 and diaphragm M, where fiexure of the diaphragm occurs at a predetermined high pressure which would exist when the fan 76 is operating at high speed, and the flow of air through the heat exchanger St? has been reduced to about 50% of its normal flow. This restricted flow would amount to about one inch in a water gauge. The armature 30 in its unattracted condition is also adjusted with respect to spring 34 by the set screw 36 to permit the llexure of diaphragm 14 when the fan 76 is operating at its lower speed, and the pressure differential amounts to approximately 0.4 of an inch in a water gauge.
The adjustment of the control device, shown in FIG. 2, is identical to that disclosed with respect to the control device in FIG. 1, except that in this modification, the armature 30a is biased in its unattracted position by spring 38, and when solenoid 24a is energized, the armature 30a moves upwardly against the bias of spring 33.
Assuming now that the refrigeration system is in operation with the prime mover 64 and the fan 76 operating at. the lower speed, there will be a minimal amount of refrigeration being supplied to the space "74, and when the frost layer on the heat exchanger 50 and its fins 72 build up to a point where the differential in pressure transmitted by the tubes 16 and 18 are such that the diaphragm 14 flexes against the bias of spring 34 to move the actuator 22 to a switch closing position, the switch 2t) closes a circuit to the electromagnetic device 62 to initiate defrosting. When this occurs, hot compressed gas from the compressor 40 flows through the conduit 58 to the evaporator, and thence back to the compressor to produce heating of the evaporator 5t} and its fins 72 to melt the frost or ice thereon. When the pressure differential on opposite sides of the evaporator 50 is reduced by the removal of the frost, the diaphragm 14 moves back to its normal position, thereby opening switch Ztl and de-energizing the electromagnetic device 62, permitting resumption of refrigeration. When desired, the switch 20 can be used in a circuit where the termination of defrosting and the resumption of the refrigeration cycle is controlled by a thermal switch that responds to the temperature of the refrigerant heat exchanger.
Generally speaking, when the refrigerant heat exchanger composed of evaporator 50 and its fins 72 become coated with frost or ice, there is relatively little heat exchange with the air in enclosure 74, and the temperature Within the enclosure may rise. When this condition occurs, the control device 70, through its sensing element 71, will demand additional refrigeration, and the thermostat will energize the electromagnetic device 68 to cause the control device 66 to increase the speed of the prime mover 74. Concomitant with the energization of the control device 68, the solenoid 24 is also energized, driving the armature 30 outwardly to place additional pressure on the coil spring 34 to impede the flexing of diaphragm 14 until the setting thereof, representing the greater differential, has been exceeded. If, under these conditions, there is sufiicient frost or ice on the heat exchanger to substantially impede the flow of air therethrough, the greater force of the air being discharged by blower 76 will be conveyed through the conduit 16 to chamber 12 and thus force the diaphragm 14 to move against the bias of spring 34, even though the latter is under additional stress of the activated armature 30, until the actuator 22 is moved to close switch 20 and thereby close a circuit to the electromagnetic device 62 to open valve 60 and thereby admit hot gas to the evaporator to melt the ice on the heat exchanger 50. As the frost or ice on the heat exchanger is melted by the defrosting action, the pressure differential will diminish and eventually permit the diaphragm 14 to move away from switch actuator 22 to open the circuit, terminating defrosting and permitting the refrigeration action to resume.
With the arrangement shown in FIG. 2, there would be certain modifications in the structure and electrical system, and operation of the system disclosed in FIG. 3. In this arrangement, electromagnetic device 68 and the solenoid 24a would be energized by control 70 when the prime mover 64 is in its low speed operation, and would be de-energized when the prime mover is in its high speed operation. The relationship of coil spring 38 to coil spring 34 is such that when solenoid 24a is deenergized, a relatively high differential would be required to flex the diaphragm 14, and energization of solenoid 24a during the low speed operation of the prime mover 64 would cause the armature 30a to move against the bias of spring 38 and thereby diminish the pressure on spring 34. In all other respects, defrosting of the refrigerant heat exchanger would occur in the manner as previously described.
The principal advantage of this invention is that it provides differential control of a mechanism, such as the defrosting of a refrigerant system under two conditions, such as when the air is being circulated at a low rate and pressure, but also providing adequate control when the pressure differential is changed, as by the circulation of air at a higher rate or pressure, and thus compensating for the change in differential.
In a refrigeration system where air is circulated through a refrigerant heat exchanger at different rates, depending on the demand, the present invention provides automatic defrosting under either condition, and will generally provide rapid defrosting when air flow through the refrigerant heat exchanger is substantially impeded.
The present invention will also find utility in other fields where a fluid is passed through a restricted area, such as a filtration system, and wherein modification of the system is required when flow is substantially impeded.
The invention is not restricted to a refrigeration system, nor to the specific refrigeration system disclosed in the drawing, and the invention is defined in the terms of the appended claims.
1. A control device, comprising a controlling member, a diaphragm which is subject to movement relative to said controlling member, means for imposing separate forces on opposite sides of said diaphragm, resilient means coacting with said diaphragm in opposition to one of said forces, and a motor means which is energized with the increase of said one of said forces and cooperating with said diaphragm in opposition to the increase of said one force.
2. A control device, comprising a controlling member, a diaphragm which is subject to movement relative to said controlling member, means for imposing separate forces on opposite sides of said diaphragm, resilient means coacting with said diaphragm in opposition to one of said forces, and motor means which is energized with the increase of said one force coacting with said resilient means in opposition to the increase in said one force.
3. In a refrigeration system embodying a refrigerant heat exchanger, a blower which is operative at a low speed and a higher speed circulating air relative to said heat exchanger, means for controlling the defrosting of said heat exchanger; the improvement of a control device for controlling said defrosting control means when said blower is operative at said higher speed, comprising a controlling member operatively connected to said defrosting control means, a diaphragm operatively associated with said controlling member, means for imposing on the opposite sides of said diaphragm the air pressure existing on opposite sides of said heat exchanger, and a motor which is energized when said blower is circulating air at said higher rate coacting with said diaphragm in opposition to the increase in pressure on one side of said diaphragm.
4. In a refrigeraton system embodying in combination, a refrigerant heat exchanger, control means for defrosting said heat exchanger, a blower for circulating air relative to said heat exchanger at a low speed and a higher speed, control means for controlling the speed of said blower, means operatively associated with said defrosting control means and sensitive to a differential in pressure on opposite sides of said heat exchanger for initiating defrosting on a change in said differential when said blower is operating at one of said speeds, and means actuated by said speed control means for changing a factor in the actuation of said defrosting control means when said blower is operating at said other speed.
References Cited UNITED STATES PATENTS 2,147,678 2/ 1939 Smith 62225 X 2,216,589 10/ 1940 Grooms -62209 X 2,975,611 3/ 1961 Pietsch 62-140 X 2,992,542 7/1961 Arthur 62-440 X 3,004,399 10/1961 Keller 62140 MEYER PERLIN, Primary Examiner.