US2898435A - Heating control device - Google Patents

Description

Aug. 4, 1959 c. A. CRAFTS HEATING CONTROL DEVICE Filed April 29, 1957 INVENTOR.

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United States Patent 2,898,435 HEATING CONTROL DEVICE Cecil A. Crafts, Pasadena, Calif., assignor to Robertshaw- Fulton Controls Company, Greensburg, Pa., a corporation of Delaware Application April 29, 1957, Serial No. 655,663

7 Claims. (Cl. 219-20) This invention relates to heating control devices and more particularly to a device wherein the heating medium consists of a slurry of fusible material having a melting temperature of a value to be controlled.

At the melting temperature, the material employed as the heating medium consists of a slurry composed partly of solid material and partly of liquid. Sincethe melting point temperature is independent of the ratio of liquid to solid material, the slurry can be maintained at constant internal temperature over a wide range of heat application from the heating element. Thus, when such a slurry is employed as a heating medium, a highly regulated temperature can be obtained.

It has been found that although the temperature of the material will remain constant during variations in the liquid-solid ratio of the slurry, the conductivity of an electrically conductive material will vary with the liquid-solid ratio. It is an object of this invention to control the heat input to a slurry of conductive material employed as a heating medium by a means responsive to variations in conductivity of the slurry.

Another object of the invention is to measure the conductivity of the slurry by means of an inductance coil which acts as a primary winding of a transformer, the slurry material forming a secondary winding.

Another object of the invention is to position an inductance coil forming one arm of a bridge circuit in the slurry to sense the conductivity of the same to control the condition of bridge balance.

In the preferred embodiment of the invention, an inductance coil is wound on a core and immersed in a heating medium comprising a slurry of conductive material. The inductance coil is connected as one arm of a bridge circuit which is operative to energize a heating means for the slurry according to the degree of unbalance of the bridge circuit. The inductance coil serves as a primary winding of a transformer, and the slurry material acts as a single turn secondary winding of resistance proportional to the conductivity of the slurry. When the conductivity of the slurry changes as a result of a change in the liquid-solid ratio, the resistance of the secondary winding will change and be reflected in a change in impedance of the inductance coil to vary the degree of unbalance of the bridge circuit.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawings wherein:

Fig. l is a schematic illustration of a heating control" device embodying this invention; and v v Fig. 2 is a detail illustrating the construction of an element shown in Fig. l and illustrating schematically the operation thereof.

Referring more particularly to. Fig. 1, there is shown a heating oven or container 10 which has an inner chamber or heating space 12 and anouter annular chamber 14, the outer chamber 14 being filled with an electrically conductive material 15 which will fuse when heated to "ice a predetermined melting temperature and which possesses a high latent heat of fusion.

Referring now to the control system for the oven, a primary winding 16 of a transformer 18 is connected across a pair of line wires L1, L2 of a suitable source of alternating voltage. A secondary winding 20 of the transformer 18 is connected by a pair of conductors 22, 24 to a pair of input terminals 26, 28, respectively, of a bridge circuit indicated generally by the reference numeral 30 for impressingthe alternating voltage of the source on said input terminals.

The bridge circuit 30 is provided with a pair of output terminals 32, 34 which are connected to the input side of a power amplifier 36 bya pair of conductors 38, 40, respectively. A heating element 42 adapted to be ener gized upon unbalance of the bridge circuit is connected across the output of the power amplifier 36 by conductors 44, 46, the heating element 42 being disposed in heating relationship to the material 15.

Referring more particularly to the construction of the bridge circuit 30, this circuit includes a resistor 48 connected between the terminals 26, 32 to define one arm of the bridge, and a resistor 50 connected between the terminals 28, 32 to define another arm of the bridge. An inductance coil 52 is connected by a conductor53 between the terminals 26, 34 and a second inductance coil 54 is connected between the terminals 28, 34 by a conductor 56. The inductance coil 54 is positioned in the chamber 14 and immersed in the material 15 to be responsive to the liquid-solid ratio thereof as will later be apparent.

The inductance coil 52 forms a primary winding of a transformer 58 having a secondary winding comprising a coil 60. A variable resistor 62 is connected across the secondary winding 60. The coils 52, 54 and 60 are identical and each has the same number of turns.

Referring more particularly to Fig. 2, the inductance coil 54 maybe wound on a suitable toroidal core 64 and both are immersed in the material 15. With this arrangement, the coil 54 will act as a primary winding of a transformer, and the material 15 will form a single tum secondary winding of resistance proportional to the conductivity of the material 15. This result is illustrated schematically in Fig. 2 wherein the single turn winding formed by the material 15 is shown in dotted lines and indicated by the reference numeral 66. The resistance 68 is shown connected in the secondary winding 66 to indicate the resistance of the material 15.

The material 15 may be of any suitable fusible condutive material having a high latent heat of fusion. The container 10 is preferably made of insulating or nonconductive material or insulated from material 15 by coatings on the inner walls fo the chambers .12, 14.

When the material 15 is in the solid state and heat is applied thereto, there will be an increase in temperature of the material 15 until it reaches the fusion temperature. At the fusion temperature, the material 15will undergo a change from a solid state to a liquid state, and during this change of state a slurry of the material 15 will exist composed partly of solid material and partly of liquid material. In the slurry condition of the material 15, the fusion temperature is independent of the liquid-to-solid ratio and thus the slurry can be subjected to a wide range of heat input without affecting the temperature of the material 15. Accordingly, while the material 15 is in this slurry condition, variations in heat input will only effect the liquid-solid ratio. The conductivity of the material 15 is proportional to the resistance thereof which varies with the liquid-solid ratio.

The insulating properties of the container 10 are selected so that when a predetermined liquid-solid ratio of the material 15 exists in the slurry state thereof, a conrial 15.

dition of equilibrium is established where the power input to the heating element 42 is equal to the heat loss from the material 15. This equilibrium point may be selected at a. point where the proportions of liquid and solid material are equal.

- The resistances 48, 50 of the bridge 30 are standard resistors and the circuit 30 is in balanced condition when the impedance of the coil 54 equals the impedance of the coil 52. The impedance of the coil 54 varies with the conductivity of the material 15 as will now be apparent.

As indicated in Fig. 2, the material 15 acts as a single turn secondary winding 66 on the core 64 and has a re sistance 68 proportional to the conductivity of the mate Thus, the impedance of the coil 54 when the same is immersed in the material 15 may be determined in the following manner from the formula for the impedance' of a transformer:

where the terms are as follows:

R is the resistance of the secondary turn 66 and resistance 68 12 ,15 the resistance of the coil 54 w is the frequency of the source L1, L2

L is the inductance of the coil 54 N is the turns ratio or the number of turns in coil 54.

It will be apparent that since the resistance R54 inductance L 4, turns ratio N and the frequency w remain constant, all the terms in Equation 2 remain fixed except R swhich varies with the conductivity of the material 15. Thus, the equation may be reduced to:

BR ER, (3) CRM +D CRM +D where A, B, C, D and E are constants.

Since the only variable in the above equation is R any variation in the resistance of the material 15 will be reflected in a change in the value of the impedance of the 'coil 54.

If the coils 52, 60 are identical to the coil 54, the value of the resistance 62 appearing across the primary coil 52 will be NR However, since the turns ratio N is unity, this resistance is merely the resistance value of resistance 62.

In the case of primary coil 54, the resistance of the material 15 (winding 66 and resistance 68) which will appear across the coil 54 will be NR Since the winding 66 has a single turn, N will not be unity in this case.

Since the coils 52, 54 are identical, the constants A, B, C and D arrived at in Equation 1 will be the same for both coils Accordingly, the impedances of the coils 52, 54 will be equal and balanced when:

where N is the number of turns on the coil 54.

Adjustment of the variable resistance 62 will serve to change the value of the conductivity or resistance of the material 15 required to cause bridge balance.

In operation, the value of resistance '62 is adjusted to cause balance of the bridge circuit 30 when the material 15 is in a liquid state. Therefore, the bridge circuit 30 will normally be unbalanced during operation at the equilibrium condition where power input to the heating element 42 equals the heat loss from the material 15.

When the liquid-solid ratio of the material 15 is less than the ratio at equilibrium, the impedance of coil 54 will be less than impedance of coil 52, and the bridge 30 will be unbalanced to a large degree. At this condiexceed the heat loss from the material 15. The heat thus 4 supplied to the material in excess of the heat loss thereof will cause the slurry to become more liquid and less solid. With this resulting increase in the liquid-solid ratio of the material 15, there will be a corresponding increase in the resistance of the material 15 which is reflected in an increase in impedance of the coil 54. This increase in impedance of the coil 54 will reduce the degree of unbalance of the bridge 30, thus reducing the power input to the heating element 42. The liquid-solid ratio will continue to increase until a value of the impedance of the coil 54 is reached where the power input to the heating element 42 equals the heat loss from the material 15. At this equilibrium condition, the heat input to the material 15 will be completely dissipated in heat loss. Therefore, no further increase in liquid-solid ratio will occur, and the system will remain at equilibrium.

If, under some conditions, the liquid-solid ratio should increase above that at the equilibrium condition, the resistance of the material 15 will increase to cause an increase in impedance of the coil 54. This increase in impedance of the coil will reduce the degree of unbalance of the bridge 30 and cause a decrease in the power input to the heating element 42 to a value less than the heat loss from the material 15. Under these conditions, the heat loss in excess of the heat input will cause a decrease in the liquid-solid ratio until the equilibrium condition is re-established.

. It will be apparent that if the slurry at equilibrium condition is composed of nearly equal portions of solid and liquid material, the inherent balancing effect of the system will prevent the material 15 from changing to either completely liquid or completely solid state. If, however, the material 15 should change to a completely solid state, there will be a marked decrease in the resistance and increase in conductivity of the material 15 to cause maximum bridge unbalance. On the other hand, if the material 15 should change to a completely liquid state, the bridge circuit 30 will become balanced and the power input to the heating element 42 will be minimum. Accordingly, the system herein described will maintain the temperature in the chamber 12 substantially at the fusion temperature of the heating medium comprising the material 15.

While only one embodiment of the invention has been herein shown and described, it will be apparent to those skilled in the art that many changes may be made in the construction and arrangement of parts without departing from the scope of the invention as defined in the appended claims.

I claim:

1. In a system for controlling the temperature of a space, the combination comprising a container, a heating medium for the space positioned within said container comprising a fusible material, electrically operated means for heating said material to the fusion temperature thereof to form a'slurry of said material, an impedance bridge circuit comprising a pair of resistances and a pair of inductance coils connected as the arms of said bridge circuit, one of said coils being immersed in said material whereby said one coil acts as a primary coil of a transformer and said material acts as a secondary coil of the transformer having a resistance proportional to the liquidsolid ratio of said slurry, a third inductance coil coupled to the other of said inductance coils of said pair and defining a transformer therewith, a third resistance connected to said third coil, said bridge circuit being balanced when the resistance value of said material equals the value of said third resistance but operative to become unbalanced'in response to a change in the resistance value of said material, and means for energizing said heating means upon unbalance of said bridge circuit.

2. A device'responsive to the conductivity of a material comprising a bridge circuit, a transformer having a primary coil and a secondary coil, means for connecting said primary cell as one arm of bridge circuit,

resistance means of a predetermined value coupled to said secondary coil, an inductance coil connected as another arm of said bridge circuit and operative to cause unbalance thereof in response to a change in impedance of said inductance coil, a transformer core for said inductance coil immersed in the material whereby said inductance coil acts as a primary coil and the material acts as a secondary coil having a resistance proportional to the conductivity of the material, said resistance being adapted to change the impedance value of said inductance coil in response to variations in conductivity of said material to efiect unbalance of said bridge circuit, and means responsive to bridge circuit unbalance for varying the conductivity of the material.

3. A device responsive to the conductivity of a material as claimed in claim 2 wherein said bridge circuit comprises four impedance arms having a pair of resistance elements connected as the other two arms of said bridge.

4. A device responsive to conductivity of a material as claimed in claim 3 wherein said core is toroidal in form and has said inductance coil wound thereon.

5. In a system for controlling the temperature of a space, the combination comprising a container, a heating medium for the space positioned within said container comprising a fusible material, electrically operated means for heating said material to the fusion temperature thereof to form a slurry of said material, a normally unba1- anced impedance bridge circuit, a transformer core immersed in the material, an inductance coil connected as one arm of said bridge circuit and Wound on said transformer core to serve as a primary winding, said material being cooperative with said coreto form a secondary winding having a resistance proportional to the conductivity of said material to change the degree of unbalance of said bridge circuit upon a change in conductivity of the material, and means responsive to unbalance of said bridge circuit for energizing said heating means.

6. The combination of claim 5 wherein said trans former core is toroidal in form.

7. The combination of claim 6 wherein said bridge circuit comprises four impedance arms having a pair of resistance elements connected as the other two arms of said bridge.

References Cited in the file of this patent UNITED STATES PATENTS 1,617,360 Woodson Feb. 15, 1927 1,845,241 Cooley Feb. 16, 1932 2,086,966 Shrader July 13, 1937 2,429,819 Jordan Oct. 28, 1947 2,524,886 Colander et al. Oct. 10, 1950 2,640,089 Gilbert May 26, 1953 2,819,371 Aldrich et al. Jan. 7, 1958 OTHER REFERENCES Industrial Heating; vol. XXIII; No. 7, July 1956, pp. 1460, 1462, 1464.

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