AU641526B2 - Apparatus for controlling heating of fluids - Google Patents

Description

JWMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE: SPECIFICAT% ION

(ORIGINAL)

Form Class Int. Class Application Number-, Lodged.

PK 1455 30th July 1990 Complete Specification Lodged: Accepted: Published-, riority 000 eatdAr.

e 0 0 s Name of Applicant: adress of Apf.iicant "Actual inventor dd ress f or Service BLACK DECKER, INC.

Drummiond Plaza Office Park, 1423 Kirkwood Highway, Newark, Delaware, United States of America qiOHNh DUNCAN McNAIR and DAVID ALEXANDER RICHARDS WATERMARK PATENT TRADEMARK ATTORNEYS.

LOCKED BAG NO. 5, HAWTHORN, VICTORIA 3122, AUSTRALIA Complete Specification for the Invention entitled: APPARATUS FOR CONTROLLING HEATING OF FLUIDS Thie following statement is a full description of this invention, including the best method of performing it known to 2 APPARATUS FOR CONTROLLING HEATING OF FLUIDS FIELD OF INVENTION The present invention relates to apparatus for controlling heating of fluids. In particular, the present invention relates to apparatus for controlling and/or monitoring temperature of fluids being heated.

BACKGROUND ART Many fluid heating vessels, such as kettles, urns or jugs, utilise electronic controls to attain and maintain Sthe fluid in the vessel at or near a predetermined temperatuce.

The basic vessel is generally of an insulating material such as thermoplastic, thermoset plastic phenolic or porcelain china (such as old style jugs). Typically, the 15 wall thickness is about 2 5 mm.

In most heating units for the vessels, the energy Sused is electrical, and the vessel incorporates a sheathed *Ges mineral insulated element, direct (wire in water) element, dielectric (2 plate) unit, for example. Any other fvel (gas/oil) is also sometimes used.

20 In the case of a sheathed electrical element a control circuit is used in the vessel. Typically, where electronic controls are used, the actual sensor is a temperature dependent resistor (or thermistor). In this i2 case the thermistor changes its resistance in accordance with changes in the temperature around the thermistor. The output of the control is in response to the changes sensed by the thermistor and either switched on or off (proportional control) or the power supplied to heat the fluid is changed using wave chopping phase modulation, or other known techniques to increase or decrease the amount of power supplied to the heating element as required.

Fig. 1 illustrates some typical problems encountered in vessels used for the heating of fluids, for example water. Path 1 represents the temperature of fluid in a vessel when a great deal of energy is used to heat the fluid quickly. The fluid temperature rises in a short period of time and when the fluid temperature approaches boiling 3 -3point (1000) and just before point A, power is switched off from the heating element. However, the fluid temperature undesirably continues to increase into "boil over", between points A and B, before reducing to reach the desired fluid temperature (say 98 0 In the case of an urn or large fluid heating vessel, this temperature must then be maintained. Energy is wasted between points A and B.

Path 2 illustrates an alternative, where the fluid is slowly heated over a long period of time. This avoids "boil over", however, it takes far too long for the fluid to reach its desired temperature.

The problems illustrated in Fig. 1 principally stem from Time constant of the fluid temperature monitoring 15 device (thermistor); S Thermal "lag" due to indirect sensing; S* Ambient temperature changes; and oI Proximity of desired temperature to boiling point of the fluid, for example water (100 0

C).

-2 These problems appear to result for the following reasons: In fluid vessels, with a thermistor located in the fluid (direct sensing), the reaction time of the thermistor itself, on fast heating units, can cause "overshoot" in simple controls as shown in Fig path 1. Also there exists technical problems of electrically isolating the thermistor leads, as well as preventing corrosion thereof.

This generally leads to a design that to all intents, looks similar to indirect designs as described below.

As shown in Figure 2, a typical indirect sensing 30 application is shown in a water heating urn 3. The water temperature sensing thermistor 4 is pushed into a preformed pocket 5 either provided in the water heating chamber wall 6 or which is integrally moulded with the vessel wall.

Generally, a heat conducting paste 7 is used to enhance the 3 sensing of the water 8 temperature by the thermistor 4.

Thermal lag occurs due to the time required for a 4 temperature change in the fluid to be transmitted through the vessel wall 6, the paste 7 and the body of the thermistor 4 before being (electronically) sensed by the thermistor.

An ambient thermistor 9 actually reads a temperature somewhere between the temperature of the water 8 and the temperature of the air 11 which exists between the water heating chamber 6 and the outside wall 10 of the urn 3. A P.C.B. is mounted to carry ambient temperature thermistor 9 and the water temperature thermistor 4. From cold start up, the temperature around the PCB will slowly rise from room ambient temperature to steady state operating temperature, providing delays in heating the fluid as desired.

15 Water at normal atmospheric pressure boils at 100 0

C.

However, at or above 100 0 C, any energy added is dissipated as steam as no temperature increase is possible. If water 9*e temperature is desired at say 97 C, and if the heating rate is faster than the thermistor/thermal lag etc. can cope 20 with, then the water temperature will pass through 97 0

C,

through boiling point, and the fluid will boil until the thermistor catches up. This could be, typically, half a minute or more as shown in Fig. 1, path 1. This is due to the fixed "set point" by which the heater element control is 25 influenced. The "set point" is the temperature sensed at which the heating control circuit turns off or reduces heating of the fluid, during fluid heating to a desired level. More importantly, if the vessel heating control is an on/off type, then the full element wattage is applied to 30 the water after boiling and this is undesirable.

C TDS OF INVETITION An object of the present invention is t- vide a fluid heating vessel which quickly s fluid without excessive undesirable "boi r'.

35 A furt-E-5bject of the present invention is to provjeji- luid heating vessel which compensates for thermal lg in mcasurmont of temperature.

OBJECTS OF THE INVENTION An object of the present anvention is to provide a temperature control device having an adjustable s& roint which is influenced by sensed ambient temperature.

A further object of the present invention is to provide a fluid heating vessel which compensates for thermal lag in measurement of temperature.

SUMMARY OF INVENTION It has surprisingly been found that ambient temperature has effect on operating temperature preferably used in conjunction with "boil over" of a fluid heating vessel. As shown in Fig. 3, it has been observed that, if ambient temperature is normalized at 200C, and a desired water temperature is 950C, the heating controller set point should be set to switch off or reduce power to the heating element at 75°C, as measured by the water temperature sensing thermistor 4 of Fig. 2.

However, if ambient temperature is 300C, the vessel will heat water to only 900C, when the thermistor senses 750C, and if ambient temperature is the vessel will heat water to 100C ("boil over").

S 0 a o *o* oo* ooo oo* *oo •o ao o The present invention provides a temperature control device for a fluid heating vessel, said device comprising: fluid temperature sensing means adapted to provide an indication of the temperature of fluid in said vessel; control means coupled to said fluid means and adapted to regulate the supply of heat to said fluid during a heating action in accordance with an adjustable set point; and set point means adapted to provide said adjustable set point, said set point means being directly influenced by sensed ambient temperature.

The presence invention also provides a temperature control device for a fluid heating vessel, said device comprising: fluid temperature sensing means adapted to provide an indication of the tempe-rature of fluid in said vessel; control means coupled to said fluid temperature sensing means and adapted to regulate the supply of heat to said fluid during a heating action in accordance with an adjustable set point; and set point means adapted to provide said adjustable set point, said set point means being influenced by sensed ambient temperature wherein the set point is increased for an ambient temperature higher than a predetermined ambient temperature and is decreased for an ambient temperature lower than said predetermined ambient temperature to heat said fluid to a desired predetermined level irrespective of said sensed ambient temperature.

25 Preferably, the ambient temperature is sensed proximate the fluid vessel.

The present invention also provides a temperature control device for a fluid heating vessel, said device comprising: fluid temperature sensing means adapted to provide an indication of the 4.

temperature of fluid in said vessel, .4 .4 6 control means coupled to said fluid means and adapted to regulate the supply of heat to said fluid during a heating action in accordance with a set point; set point means adapted to provide an adjustable set point, said set point means being influenced by sensed ambient temperature.

In a particularly preferred arrangement the set point means comprises a second temperature sensing means adapted to sense temperature on an outer wall surface of said fluid heating vessel whereby said second temperature sensing means is influenced by both ambient temperatures and temperatures of the fluid within said vessel.

Preferred embodiments of the present invention will now be described with reference to the accompanying 15 drawings, wherein Fig. 1 shows, diagrammatically, water temperature over time as water is heated.

Fig. 2 shows a prior art water temperature control.

Fig. 3 shows, diagrammatically, the relationship 20 between final water temperature and ambient water temperature with a fixed set point control device; Fig. 4 shows a circuit diagram of one embodiment of the present invention; Fig. 5 shows one embodiment of the present *i 0 0

S

0ep 6*

S

000 4064 OSi 00 S invention; Fig. 6 shows, in more detail, the ambient thermistor located against the wall of the fluid vessel; and Fig. 7 shows, diagrammatically, the practical results of an adjustable set point with regard to heating of water.

With regard to Fig. 4, one possible circuit arrangement is disclosed foL carrying out the present invention although other circuits might also be used. In the illustrated circuit the thermistor 4 senses fluid temperature, and is compared 1,comparator 21 with an adjustable set point. The adjustable set point is provided, in this embodiment, by set resistor 20 in parallel with ambient thermistor 12. As the sensed ambient temperature is 7increased, the set point (resistances of 20 and 12) alter.

When the resistance of thermistor 4 and set point 20/12 are equal, the comparator will turn off or influence driver 22 to reduce power supplied to heater Fig. 5 shows one embodiment of the present invention as applied to a fluid heating vessel. The fluid vessel is similar to that of Fig. 2, except that the ambient thermistor 9 is replaced by set point thermistor 12. In this embodimect the thermistor 12 thermally engages the outside of the fluid heating vessel 6 so that it is influenced both by ambient temperatures and temperature of the fluid within the vessel.

Fig. 6 shows in greater detail the location of thermistor 12. The actual temperature measurement is sensed 15 inside the body of the thermistor 12 at point 12a. At this point 12a, heat is sensed from the wall 6 which in turn is influenced by the the water being heated, and point 12a is siee also influenced by the temperature of the ambient air 11 **the* which exists proximate the vessel wall 6. The temperature 20 of the fluid being heated is thus less dependent on ambient air temperature alone. Thus the measurement sensed at point 12a is realized somewhere between the actual fluid temperature in the vessel and ambient air temperature around the vessel.

S •25 As the water heats, the temperature at point 12a will be increased, thus adjusting the set point of the control circuit for the heater element.

Fig. 7 shows diagrammatically the results of the present invention. If compared with Fig. 3, it can be seen 30 that the heated water temperature will always be heated to the desired temperature (say 95 0 C) regardless of ambient temperature. In this example, if ambient temperature is 0 C, at point 12a, the temperature sensed is 53°C, resulting in a set point being reduced below 75 0 C, in order to turn the heater off earlier to attain water at 95 0

C.

Whereas, if the ambient temperature is 30 0 C, point 12a senses 57 0 C, the set point is increased above 75 0 C, and the heater is turned off later to attain water heated to 95 0

C.

8- Alternative embodiments, may include a "look-up table" for example, memory means coupled to a microprocessor in order to adjust the set point and control the fluid heating. In this alternative embodiment, ambient temperature and a fixe. set point can be sensed and provided as input to a look-u 9 table which will provide an adjusted I set point as ,utput for use by the control circuitry.

0 2

C

Claims (7)

1. A temperature control device for a fluid heating vessel, said device comprising: fluid temperature sensing means adapted to provide an indication of the temperature of fluid in said vessel; control means coupled to said fluid means and adapted to regulate the supply of heat to said fluid during a heating action in accordance with an adjustable set point; and, set point means adapted to provide said adjustable set point, said set point means being directly influenced by sensed ambient temperature.

A temperature contrul device for a fluid heating vessel, said device comprising: fluid temperature sensing means adapted to provide an indication of the temperature of fluid in said vessel; control means coupled to said fluid temperature sensing means and adapted to regulate the supply of heat to said fluid during a heating action in accordance with an adjustable set point; and set point means adapted to provide satid adjustable set point, said set point means being influenced by sensed ambient temperature wherein the set point is increased for an ambient temperature higher than a predetermined ambient temperature and is decreased for an ambient temperature lower than said predetermined ambient temperature to heat said fluid to a desired predetermined level irrespective of said sensed ambient temperature.

3. A temperature control device according to claim 2, wherein said set point S means comprises a second temperature sensing means adapted to sense temperature on an outer wall surface of said fluid heating vessel whereby said S second temperature sensing means is influenced by both ambient air temperature and temperature of the fluid within said vessel.

4. A temperature control device according to claim 2 wherein the set point is increased for an ambient temperature higher than a predetermined ambient tempera t ure and is decreased for an ambient temperature lower than set predeto rmined ambient temperature.

A temperature control device according to claim 2 or 3 characterised in that the set point is directly influenced in accordance with the sensed ambient temperature.

6. A temperature control device according to any one of claims 1 to characterised in that the set point means includes a look-up table adapted to provide said adjustable set point.

7. A temperature control device as claimed in claim 1 or 2, substantially as herein described with reference to Figures 4 to 7 of the accompanying drawings. DATED this 23rd day of June, 1993 BLACK DECKER. INC. WATERMARK PATENT TRADEMARK ATTORNEYS THE ATRIUM 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA 0. 0 C SKP/RCTS:EK[AU8141691.WPC DOC. 36] A A ABSTRACT O*se 0 t *0 S* A temperature control device for a fluid heating vessel senses ambient temperature and controls or adjusts the set point at which power is reduced to an element for heating the fluid in the heating vessel, in order to prevent boil-over of the fluid.