US 3168805 A
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Description (OCR text may contain errors)
United States Patent Ofiice 3,158,805 Patented Feb. 9, 195
This invention relates to power elements of the type which comprise a casing having an internal chamber and a piston arranged to move into and out of the casing in accordance with variations in chamber volume.
1 In such power elements the internal chamber is com- 'monly charged with a material having a high coeflicient of thermal expansion, such as wax, so that when the material isheated (as by an electric resistance heater) the piston moves forward out of the casing; when the expansion material is cooled, as by the ambient atmosphere, the piston returns into the casing under the influence of the external load against which the piston is working. The load can be any device which it is desired to control by mechanical motion, as for example a liquid valve.
One object of the present invention is to provide a power element wherein the forward piston stroke is obmwtained with a relatively small heat input, thereby neces- 2 turn transmits the pressure to a squeezable rubberplug 18 which is arranged above a piston 20.
The resistance heater may be varied as to design. However, as shown, the heater includes two pin-like terminals 19 and 21, and a generally cylindrical coil of resistance wire 23 wrapped therearound so that one end of the wire connects directly with terminal 19 and the other end extends along the wall of casing 17, as at 25, before connecting with terminal 21. Each terminal pin is preferably provided with a sleeve-like insulator 27.
A-s semi-schematically shown in the drawing, power element 10 is mounted atop a liquid valve 22 which is provided with an inlet chamber 24, an outlet chamber 26, and a port 28. The piston of the power element is arranged to engage the diaphragm seal which is interconnected with a stem 32 and poppet valve element 34;
Thus,lthermal expansion of material 12 causes a downis in circuit with a first on-off control switch .35,and a the expansion material is conditioned to undergo rapid relatively high temperature, whereby the subsequent cooling cycle is accomplished by a relatively large temperature differential between the expansion material and the ambient atmosphere.
Other objects of this invention will appear from the following description and appended claims, reference being had to the accompanying drawings forming a part of \this specification wherein like reference characters designate corresponding parts in the several views.
In the orawmgs: 1 FIGURE 1 is a sectional view taken through a powe second limit switch 37. Switch 37 includes a first terminal 38, a second terminal 40, and a switch blade 42 having a portion thereof in registry with-the shoulder formed by dielectric washer 44. With such an arrangement, when heater 14 is energized the resultant expansion of material 12 causes the piston to be moved downwardly until the shoulder at 44 engages the blade 42 and thus interrupts the circuit between terminals 38 and '40, therebyde-energizing the heater. With switch 35 in a closed condition the piston 20 will cycle rapidly a very short distance under the control of limit switch 37.
As will be evident, when the heater is de-energized by switch 35 the ambient atmosphere surrounding the power element casing 17 acts to cool material 12 and thus permits the power element piston 20 to return to its illustrated position under the influence of spring 36 and/or the liquid pressure in inlet chamber 24.
In the illustrated power element the expansion material 12 .may consist of dichlorodifluoromethane 7(CCl F which is a common material obtainable, for example, fromthe' Du Pont Company under the tradename Freon 12. As indicated in FIG. 1, the CCl F is initially charged into chamber 15 in a liquid condition so as to occupy onlya fractional part of the chamber 15 volume.
At one atmosphere pressure the CCl F boils. at approxielement of this invention and showing same in position for opening and closing a conventional liquid valve;
FIG. 2 is a chart plotting the pressure-enthalpy characteristic of the expansion material utilized in the FIG. 1 power element.
Beforeexplaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is tobe understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.
Referring in greater detail to the drawings, there is shown a power element 10 having a charge of thermally expansible material 12 which is arranged to be heated by an electric resistance heater 14, whereby to develop a fluid pressure on the flexible diaphragm 16. Diaphragm 16 in mately -21f F., and the charging operation is therefore preferably carried out at this temperature or slightly therebelow. Preferably the charging operation is carried out in a manner to exclude air from chamber 15.: This may be accomplished either by charging under vacuum or at atemperature high enough to allow the formation of CCl F vapor in the casing. For example, with cup element 27 disposed in an upright position dissociated from the other components, liquid CCI F may be sprayed into the cup element to produce a mild vaporizing action therein. The CCI F vapor is heavier than air so that it sits in the cup and excludes the air from the cup interior. After charging the cup element may be assembled with the other casing parts to form the completed casing.
CCI F is of interest as a power element expansion material because it has a relatively lowspecific heat, relatively low critical temperature, and relatively large pressure change per unit temperature change at temperatures near its critical temperature. Another material having similar advantageous properties is methyl alcohol.- w
areaeos which tend to separate the molecules. The kinetic energy 7 forces increase with increasing temperature, and when they predminate over the intermolecular attractive forces the material is vaporized. At relatively low temperatures pres sure may be applied to force the molecules toward one another in spite of the kinetic energy forces; thus the pressure brings the molecules close enough together so that the intermolecular attractive forces can hold. the material in V a liquid condition. However when the temperature is increased. above a critical value it is impossible by the application of high pressures to overcome the kinetic energy forces; the kinetic energy forces predominate as though the intermolecular forces were not present. For this rea-' son, when the material approaches or reaches its critical temperature each'unit temperature change is potentially capable of producing a relatively large free kinetic energy change and large pressure change. In the present invention this characteristic is utilized to produce a power element having improved operating efficiency.
As before stated, one suitable expansion material which lends itself to operation near its critical temperature is CCl2F In FIG. 2 there is shown a'chart which plots the pressure against enthalpy for CCl F It will be seen that when the material is in :a saturated liquid condition, at relatively low temperatures each unit temperature change is accompanied by a' relatively small pressure change, whereas in higher. temperature ranges each unit temperature change effects a relatively large pressure change.
i In the lower temperature ranges, as for example from 80 'F. to 120 F., the rate of pressure change is relatively low, being in the neighborhood of about 2 p.s.i. per degree F. change. In the temperature range from 200 to the critical temperature of 233.6 the pressure changes from about 430 p.s.i. to about 597 p.s.i. which represents an average rate of pressure change of about p.s.i. per degree F. At temperatures above the critical temperature the rate of pressure change per unit temperature change is relatively high, being in the neighborhood of p.s.i. per degree F. change or higher.
I havefound that if the power element is fully charged with 1iquid'CCl F the thermal expansion results in volumeincrease while the material is still in a relatively low temperature condition; operation is then on pure liquid expansion where the piston motion per unit temperature change is relatively small. However, I have noted thatif power element chamber is only partially charged with liquid CCl' F the substantiallyvacant or vapor space in the chamber will absorb the initial low temperature expansion so that pressure development will be delayed until the material is in a relatively high temperature condition.
By thus delaying the pressure development I am able to" charged into chamber 15 is dependent-on several factors, including the nature of the material, the required pressure to be applied on diaphragm 16, and the required increase in chamber volume necessary for the desired piston stroke. When working with CCl F- and a power element needing an internal pressure of 1,000 p.s.i. and a twenty percent increase in chamber volume, I have found thatchamber 15 may be initially charged about one-half to two-thirds full with liquid CCl F Such a charge leaves sufficient 'vacant space in the chamber so that the operating stroke of the piston takes place in a'temperature range somewhat above 200 R, which is the preferred operating range.
In one experimental arrangement the power element was constructed to have a piston diameter of about onequarter inch and a piston stroke of about two-tenths inch.-
The piston was provided with an external load ofabout thirtyfive pounds, and the frictional resistance offered by plug 18 was about fifteen pounds; thus the total load was about fifty pounds. The internal pressure in chamber 15 necessary to produce a satisfactory piston movement 'is approximately 1,000 p.s.i.
To evaluate the performance of this experimental power element the element was charged with differing quanti:
ties of CCl F- and measurements taken of the temperatures and piston stroke times. The temperatures were taken with thermocouples applied to the outer surface of cup 27".
In power elements having internal resistance heaters the piston stroke time is different in the same element; 'depending on how frequently the element is cycled For example, if the element is cycled for a one minute period during every fifteen operational minutes, the walls of the element will tend to rise only a few degrees above room temperature throughout each cycle, and the piston will have a relatively slow outstroke and fast return stroke; this allows the electrically-produced heat to be easily dissipated through the power element walls. It on the other hand the element is cycled substantially continuously the'walls takes place when heater 14'is de-energized; this promotes V a rapidreturn stroke of the piston. A rapid return stroke is also promoted by the fact that a relatively small quantity of material'I12 is present in chamber 15. Thus, with small amounts of material 12 less" heat is needed to be of the element'will tend to have a higher average temperamm which tends to promote a relatively fast outstroke and slow return stroke. In the case of the described experi mental. power element the piston stroke times were meas ured both with the element cycled only infrequently and with the element cycled continuously.
With the power element charged with liquid CCl F to about forty-one percent of the chamber volume the cold element cy'cledout in about fifteen seconds and back in about five seconds; after a period of continuous cycling the piston outstroke time dropped to about eight seconds, and the return stroke time increased to about twelve seconds; The piston started its outstroke 'at'about 233 F.,
and completed its outstroke at about 250 F.
With the power element charged with liquid CCl F to about fifty-eight percent of the chamber volume. the cycle times were about the same as' with the forty-one percent charge. However the temperatures were somewhat reduced, ranging from 222 F. at the start of the piston outstroke to about 240 F. at completion of the piston outstroke.
With the power element charged with liquid CCI F to aboutrs'eventy-one percent of the chamber volume the cycle times were considerably increased to values inexcess of thirty seconds. The piston began its outstroke at about 166. F. and completed its outstroke at about 190 F.
With the power element charged with liquid CCl F to about seventy-eight percent of the chamber volume the piston stroke times were further increased, especially on the cooling stroke. On the heating stroke the piston began its movement at about F. Of the various charges tested to fifty-eight percent change appears to be the most'satisfactory from the standpoint of quick operating times and safe oper'ational temperatures. The forty-one percent charge attains afr'elatively high temperature which could .in some arrangements cause breakdown of the rubber parts and/or decompose cially true since the temperatures within the CCl F are r somewhat higher than those registered on the power element outer surface. The seventy-one percent and seventyeight percent charges are believed unsatisfactory for many applications because of their long operating times. On the basis of experimental evidence a partial charge of fifty percent to sixty-five percent appears to be most practical.
The partially charged CCl F element is believed to represent considerable improvement over known constructions. For example, wax-charged elements and watercharged elements have under generally similar loads required relatively long piston forward stroke times in the neighborhood of one minute, and return stroke times of thirty seconds to one minute or more. Oil-charged elements have in certain instances had response times varying from one minute to three or four minutes. In such 'elements the usable expansions have been obtained at temperatures appreciably below the critical temperature of the charged material, and itwas therefore not possible to achieve the high internal pressures and response times which characterize the operation of thede'scribed element.
During the description reference has been made particularly to thematerial CCI F This material is particularly useful because it has a relatively low critical temperature and low specific heat. It is contemplated however that other materials might be employed while still practicing the invention. Onematerial which is of some interest is methyl alcohol.
It is contemplated that the invention can be utilized a in other power element constructions than the one specifically shown in the drawing. Forexample, it is possible to locate the heated portion of the element remote from the diaphragm-plug portion, in which case the heated portion can be provided with a capillary tube connection to the diaphragm chamber portion. Such an arrangement is'advantageous in achieving low diaphragm temperatures. As previously noted, the inventionhas as its principal objects the provision of a power element which can be operated with a relatively small heat input and which attains a relatively high operating temperature and pressure near or above the critical temperature and pressure,
. whereby to promote a rapid piston forward stroke and a rapid piston return stroke under high load conditions, By utilizing the concepts of this invention it is believed that these objects have been achieved. i
What is claimed is: I
.1. In a heat-actuated power element having a chamberforming'casing, a loaded power member extending from said casing, and a thermal expansion substance within the chamber: the improvement comprising the thermal expansion substance filling only a predetermined fractional A substance having an expansion up to its critical temperature which produces a chamber pressure less than said predetermined pressure, whereby substantial motion is produced only as the critical temperature is approached.
2. In a heat-actuated power element having a chamberforming casing, an electric heater within said casing for heating the chamber interior, a loaded piston extending from said casing, and a thermal expansion substance within the chamber; the improvementcomprising the utiliza 6 a tion of dichlorodifluoromethane as the thermal expansion substance, said substance having a mass which causes it to occupy approximately sixty percent of the chamber volume when in a liquidcondition at its normal boiling point; the piston having a loading which requires an internal chamber pressure of approximately 1,000 p.s.i. for the attainment of outward piston motion, whereby energization of the electric heater effects piston movement only after the chamber temperature is above 200 F.
3. In a heat-actuated power element having a chamberformin casing, a loaded power member extending from said casing, and a thermal expansion substance within the chamber: the improvement comprising the thermal expansion substance occupying between and 65% of the chamber volume when in a liquid condition at its normal boiling point; the power member having a loading which prevents substantial power member motion until the chamber pressure reaches a predetermined value; and the expansion liquidsubstancehaving an expansion up to its critical temperature'which produces a chamber pressure lessthan said predetermined pressure, whereby substantial motion is produced only as the critical temperature is approached. f
4. In a heat-actuatediaower element having a chamberforming casing a loaded power member extending from said casing, and a thermal expansion substance withinthe chamber: the improvement comprising the thermal exforming casing, a loaded power member extending from said casing, a thermal expansion substance within the chamber, an electricalresistance heating element within the chamber for causing the expansion substance to move the power member in one direction, and electrical switch means controlling the heating element in response to said one direction movement of the power member whereby to permit the expansionsubstance to cool and allow the loading to return the power member in the opposite direction: the improvement comprising the thermal expansion substance filling only a predetermined fractional portion of the chamber in a liquid condition when the power element is below its operating temperature range;
the power member having a loading which prevents substantial power member motion until the chamber pressure reaches a predetermined value; and the expansion liquid substance having an expansion up to its critical temperature which produces a chamber pressure less than the predetermined pressure, whereby substantial motion is produced only as the critical temperature is approached.
References Cited in the tile of this patent UNITED STATES PATENTS 2,548,708 Dickey Apr. 10, 1951 2,976,675 7 Bonner Mar. 28, 1961 3,098,358"
Paschke luly 23, 1963 Holmes Nov. 10, 1942