GB2258127A

GB2258127A - Glass tube enclosed immersion heater

Info

Publication number: GB2258127A
Application number: GB9215164A
Authority: GB
Inventors: Keith John Robert Bruce
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-07-18
Filing date: 1992-07-16
Publication date: 1993-01-27
Also published as: GB9115578D0; GB9215164D0

Abstract

A heater for liquid has a glass tube (1) within which is disposed an electrically conducting heating coil (4). The heating coil is arranged to be an easy sliding fit inside the tube such that the majority of coil turns contact the inside of the tube at a plurality of locations spaced axially and peripherally of the tube. By such contact, heat is readily transferred from the coil to the tube and, thence, into a liquid in which the tube is to be mounted. The coil (4) may be bent into a U-shape and formed over an insulating member (5). An electrical circuit (fig 2) for the heater which avoids the use of a step-down transformer compares a reference voltage with the voltage across a thermistor (TH1) of the liquid and provides an output in dependence thereon to gate a thyristor (TR1), which when gated ON applies power to the heater. <IMAGE>

Description

HEATER FOR LIOUID AND AN ELECTRIC CIRCUIT THEREFOR This invention relates to a heater for liquid and to an electrical circuit therefor.

A heater for liquid, particularly but not exclusively one known as an immersion heater, is used in numerous fields of activity. The present invention is intended for use, inter alia, in the fields of heating aquarium tanks, home or beer making, vat heating, heating chemicals for photographic processes and heating ferric chloride bars used in etching printed circuit boards.

Known heaters for liquid are formed of a glass tube which is resistant to heat, such as a tube made of Pyrex, the glass tube being closed at one end and an electrical conducting coil being inserted into the tube from the open end thereof. The electrically conducting coil is connected to a source of electricity and a current passed through the coil so that the coil, which is effectively a resistance wire, heats and the heat from the coil radiates to the glass tube and thence, when the tube is immersed in liquid, heat from the tube is emitted into the liquid. However, because in the prior art the electrically conducting heating coil is generally loosely fitted into the glass tube, so there is very little point contact between the coil and the glass tube resulting in poor conductivity from the coil to the tube.As a result, the heating coil tends to glow orange or red when electrical current passes therethrough. Such a heater is, therefore, not very efficient and when the heater is used in an aquarium so the light given off by the heated coil tends to be offensive and distracts from the otherwise calm appearance of the aquarium. Furthermore, by virtue of the fact that the coil is often heated to be red hot, so the life of the coil is reduced by continually cycling from OFF, when the coil is cold, to ON, when the coil is red hot.

An object of the present invention is to overcome the foregoing difficulties.

With the forementioned type of heater, it is usually controlled to be operated with a step-down transformer stepping down the conventional mains supply, e.g. 240V, 50 cycle, 3-phase supply to a voltage of, typically, 12V connected across the heater coil. Because mains step-down transformers tend to be bulky, the present invention also seeks to provide a more compact electrical circuit for supplying power to the heater coil.

According to a first aspect of this invention there is provided a heater for a liquid comprising a glass tube which is resistant to heat and which has a longitudinal axis, and an electrically conducting heating coil having a plurality of turns located inside the tube, such that a majority of the coil turns contact the inside of the tube at a plurality of locations spaced axially and peripherally of the tube whereby heat is readily transferred from the coil to the tube by virtue of said contact therebetween and, thence, to said liquid.

By virtue of the contact between the coil turns and the inside of the glass tube, so heat is readily transferred from the coil through the tube to the liquid and because there is improved thermal conductivity, so the coil is able to provide the same amount of heat as in the prior art, but the coil is not required to run as hot and, therefore, does not glow as in the prior art.

Preferably, said tube has a straight longitudinal axis and a circular cross-section.

Advantageously, said tube is formed of borosilicate glass having a low coefficient of expansion, such as pyrex.

In a preferred embodiment, said tube has a glass closure at one end and is open at an axially opposite end thereof to permit insert ion and subsequent connection of said coil.

Conveniently, said coil is arranged to make an even number of passes along the length of the tube and insulating means are positioned between the coil turns of each pass.

Preferably, said coil is formed into a U-shape and an electrically insulating member is positioned between the opposing limbs of said U-shape to prevent the turns of the coil short circuiting to one another.

In an alternative arrangement said insulating means is a longitudinal member having a cruciform cross-section, or a longitudinal member having an even plurality of arms of six or more.

Advantageously, the insulating member is an elongate member having a generally rectangular cross-section and a U-shaped notch is formed in an end of said elongate member position adjacent to said glass closure for locating said coil on said elongate member.

In a preferred embodiment, opposing ends of the coil are connected to an electrical cable for carrying electrical power to said coil, the connection between said coil and said cable being inside said tube and a silicone sealant is provided between the cable and the interior walls of said tube for preventing liquid entering said tube.

Advantageously, a heat shrinkable sleeve is located over the junction between the cable and the tube whereby said sleeve is heat shrunk onto the cable and the tube.

Advantageously, tie wraps are secured around the parts of the heat shrinkable sleeve over the cable and tube respectively.

Preferably, a buffer is located about said tube adjacent said one end for protecting said tube against damage.

According to a second aspect of this invention there is provided an electrical circuit for a heater including a comparator having a first input terminal connected to a reference voltage source, a second input terminal connected to a voltage bias means including a temperature sensitive resistor, and an output terminal connected to trigger a switching means, said switching means being connected to output terminals for attachment of a heating coil.

Preferably, said voltage bias means further includes a variable resistor in series with said temperature sensitive resistor, whereby the temperature sensed by said temperature sensitive resistor is adjustable by said variable resistor.

Advantageously, means are provided for connecting said electrical circuit to a source of mains power, and an indicator lamp is provided across the mains supply for indicating that power is being supplied to said circuit, and Preferably, a further indicator lamp is arranged to be connected in parallel with said output terminals for indicating power being supplied to said coil.

Conveniently, said circuit is located in a container and said output terminals are provided for connecting heating coil to the circuit and said output terminals are arranged to be readily reachable.

In accordance with a feature of this invention there is provided a combination of a heater, as in said first aspect, in combination with a circuit of said second aspect.

The invention will now be described, by way of example, with reference to the accompanying drawings in which Figure 1 shows a longitudinal cross-section of heater for liquid in accordance with this invention, Figure 2 shows a side view of a heating coil, Figure 3 shows a side view of an electrically insulating member used for preventing short-circuiting of the coil turns, Figure 4 shows an end view of the insulating member shown in Figure 3, and Figure 5 shows a circuit diagram of an electrical circuit for supplying power to the heating coil.

In the Figures like reference numerals denote like parts.

The heater for liquids shown in Figure 1 has a glass tube 1 that is blown to have a closure at one end 2 and to be open at an opposing end 3. The glass tube is made of a borosilicate glass having a low coefficient of expansion and which is resistant to heat, such a glass being, for example, Pyrex.

A helical heating coil 4 is wound as will be discussed hereinafter and the coil is bent into a U-shape and located around an electrically insulating member 5 made, for example, of mica or ceramic. The heating coil 4 may be made of 80/20 nichrome, that is having approximately 80% nickel and 20% chromium so as to provide the coil with long-life properties.

The insulating member 5 has a rectangular crosssection, as shown in Figure 4, and is elongate, as shown in Figure 3, having a generally U-shaped end 6 adjacent the closed end 2 of the tube, the U-shape end 6 being shaped so that the heating coil turns are located in position over the electrically insulating member 5 when the coil is bent into a U-shape, i.e. the U-shaped end 6 prevents the coil 4 from sliding off the member 5. The member 5 is provided so as to prevent the turns of the heating coil from shortcircuiting to one another when the coil is bent into a Ushape.

When the coil has been bent into a U-shape over the insulating member 5, the combination is inserted through the open end 3 into the tube 1. The physical size of the heating coil, when bent into a U-shape around the insulating member 5 and the internal diameter of the glass tube 1, is such that the majority of the coil turns contact the inside of the tube at a plurality of locations both axially and peripherally of the tube so that heat is readily transferred from the coil to the tube by virtue of the contact.

Respective ends of the coil 4 are connected to a two core cable 10 having the outer sheath partially removed to present insulation 11 of each of the conductors and the insulation 11 is partially removed to present the metal conductors 12 which are secured to the respective ends of the coil 4 by crimps 13.

The connection between the coil and the cable is situated just inside the open end of the tube and a silicone sealant 14 is injected into the tube between the interior of the tube and the exterior of the cable sheath for preventing ingress of liquid into the tube. A heatshrinkable sleeve 17 is located over the juncture between the cable and the tube outer surfaces and the sleeve is heat-shrunk onto the cable and tube and, in shrinking, the sleeve tends to cause a small amount of the sealant 14 to exude between the outer surface of the tube and the inner surface of the sleeve. So as to ensure tight and waterproof sealing between the sleeve and the tube, a tie wrap 15 is secured around the sleeve and tube combination and a further tie wrap 16 secured around the sleeve and cable combination.

A silicone rubber buffer ring 20 is located about the tube adjacent the closed end 2 for protecting the glass tube against damage.

So as to prevent damage to the heating coil, the heater must be immersed in liquid when power is supplied to the heating coil.

So as to be able to tightly wind the heating coil, which is initially wound to be in the form shown in Figure 2, so the wire of the heating coil is wound on a circularly cross-sectioned rod.

The wound coil length of a tightly wound heating coil is found by the following equation: 1 = K x V xd R P where 1 is the tightly wound length of the coil in millimetres, R is the resistance of the heating coil in fl/metre, V is the mains voltage, P is the required heater power in watts, d is the heating coil wire diameter in millimetres K= r# where r is the winding rod diameter in millimetres.

Thus, as an example, for a 100W heater supplied from a 240V power supply using a heating coil wound on a 2mm diameter rod, the resistance of the heating coil wire being 100 Q/metre and being of 0.127mm diameter.

1 = 10 x 1 x 240 x 0.127 = 160mm 2# 100 100 The power supply for the heating coil may be 240V, or 110V - 120V, AC, 60Hz.

The power output may be 200W, 150W, 100W, 50W.

The heater coil length, when tightly wound, may vary from 8cm to 20cm.

The external diameter of the glass tube may vary from 8mm to 12mm.

Although the heater has been described as being located in a glass tube closed at one end and open at the other end, since this is the preferred construction, the tube need not be straight but could be formed into a horseshoe-shape with both ends open and the coil disposed such that the majority of the coil turns individually touch both sides of the tube interior. Also, the tube could be open at both ends and straight so that mains cable connections are connected at both ends of the tube.

A circuit for the heater will now be described with reference to Figure 5.

A 240V AC, three-phase supply is connected with the live voltage at rail 31 and with the neutral rail connected to rail 32. A series combination of capacitor C1 and resistor R3 reduce the voltage, and the low voltage is taken from interconnection point W via rectifier D1 to a low voltage rail 33. An integrated circuit, IC1, such as a UA741, has its terminals 7 and 4 connected to the low voltage rail 33 and neutral rail 32, respectively. Pin 2 of IC1 is a reference voltage input that is derived from the junction point of potential divider resistors R2 and R4 connected between the low voltage rail and neutral rail.

The IC1 acts as a comparator and the input to be compared at terminal 3 is derived from the junction point between a resistor R1 and the series connection of a manually variable resistor VR1 and a thermistor TH1, the thermistor TH1 being arranged to be located in the liquid to be heated. The temperature to be reached is, thus, set by varying the resistance of VR1.

The output of IC1 at pin 6 is connected via the potential divider R5 and R6, having a junction point S between R5 and R6 to the gate of a thyristor TR1, which may be a model 206D. The thyristor has one electrode connected to the neutral rail 32 and another electrode connected to one terminal 51 for a load, for example, one end of the heating coil 4. The other end of the load (heating coil) is connected via a terminal 52 to the live rail 31. A load indicator neon light 53, the light being in series with a current limiting resistor R8, is connected in parallel with the load to indicate when power is being supplied to the load.

A smoothing capacitor C2 is connected between the low voltage rail 33 and the neutral rail 32 and a further neon indicator lamp is connected in series with a current limiting resistor R7 across the mains input supply to indicate when mains power is applied to the circuit.

In operation, the level of heating is sensed by thermistor TH1, the thermistor TH1 being sealed in a glass tube and located in the liquid. The level of heating is controlled by adjustment of VR1. Thus, if the water temperature cools then the resistance of TH1 will increase, thereby making the voltage at terminal 3 of IC1 go higher than at terminal 2 of IC1. Thus, the comparator produces an output voltage at terminal 6. When the voltage is of a sufficient level, so the thyristor TR1 is gated ON to applied power to the heater to heat the liquid. As the liquid temperature increases, the resistance of TH1 decreases, thus causing the voltage at terminal 3 of the comparator IC1 to decrease below the voltage at terminal 2, thereby turning the thyristor OFF and, thus, supply to the heater coil is turned OFF.

The resistors R5 and R6 are chosen so that when the comparator IC1 is OFF, a voltage below 1.8V, determined by the division ratio of voltage divider R5 and R6, is applied to the gate of the thyristor to ensure that the gate does not turn ON prematurely.

The completed circuit is formed on a printed circuit board and housed in a suitable plastics box with associated leads extending from terminals 51 and 52 to a readily reachable pair of terminals so that the heating coil may be connected thereto without removing the circuit from the box. Such a readily reachable connection may be formed by a terminal block located inside a readily removable cover or flap of the box.

The heater and circuit, therefore, above described, has the advantage that because the heater has good thermal conduction between the heating coil and the glass tube, so the heating coil does not glow when in use and the electrical circuit has the advantage of compactness and light weight.

Claims

CLAIMS:

1. A heater for a liquid comprising a glass tube which is resistant to heat and which has a longitudinal axis, and an electrically conducting heating coil having a plurality of turns located inside the tube, such that a majority of the coil turns contact the inside of the tube at a plurality of locations spaced axially and peripherally of the tube whereby heat is readily transferred from the coil to the tube by virtue of said contact therebetween and, thence, to said liquid.

2. A heater as claimed in claim 1 wherein said tube has a straight longitudinal axis and a circular cross-section.

3. A heater as claimed in claim 2 wherein said tube is formed of borosilicate glass having a low coefficient of expansion, such as Pyrex.

4. A heater as claimed in any preceding claim wherein said tube has a glass closure at one end and is open at an axially opposite end thereof to permit insert ion and subsequent connection of said coil.

5. A heater as claimed in any preceding claim wherein said coil is arranged to make an even number of passes along the length of the tube and insulating means are positioned between the coil turns of each pass.

6. A heater as claimed in claim 5 wherein said coil is formed into a U-shape and an electrically insulating member is positioned between the opposing limbs of said U-shape to prevent the turns of the coil short circuiting to one another.

7. A heater as claimed in claim 5 wherein said insulating means is a longitudinal member having a cruciform crosssection, or a longitudinal member having an even plurality of arms of six or more.

8. A heater as claimed in claim 4 wherein the insulating member is an elongate member having a generally rectangular cross-section and a U-shaped notch is formed in an end of said elongate member positioned adjacent to said glass closure for locating said coil on said elongate member.

9. A heater as claimed in any preceding claim wherein opposing ends of the coil are connected to an electrical cable for carrying electrical power to said coil, the connection between said coil and said cable being inside said tube, and a silicone sealant is provided between the cable and the interior walls of said tube for preventing liquid entering said tube.

10. A heater as claimed in claim 9 wherein a heat shrinkable sleeve is located over the junction between the cable and the tube whereby said sleeve is heat shrunk onto the cable and the tube.

11. A heater as claimed in claim 10 wherein tie wraps are secured around the parts of the heat shrinkable sleeve over the cable and tube respectively.

12. A heater as claimed in claim 4 wherein a buffer is located about said tube adjacent said one end for protecting said tube against damage.

13. An electrical circuit for a heater including a comparator having a first input terminal connected to a reference voltage source, a second input terminal connected to a voltage bias means including a temperature sensitive resistor, and an output terminal connected to trigger a switching means, said switching means being connected to output terminals for attachment of a heating coil.

14. An electrical circuit as claimed in claim 12 wherein said voltage bias means further includes a variable resistor in series with said temperature sensitive resistor, whereby the temperature sensed by said temperature sensitive resistor is adjustable by said variable resistor.

15. An electrical circuit as claimed in claim 13 or 14 wherein means are provided for connecting said electrical circuit to a source of mains power, and an indicator lamp is provided for indicating that power is being supplied to said circuit.

16. An electrical circuit as claimed in any of claims 13 15 wherein a further indicator lamp is arranged to be connected in parallel with said output terminals for indicating power being supplied to said coil.

17. An electrical circuit as claimed in any of claims 13 16 wherein said circuit is located in a container and said output terminals are provided for connecting the heating coil to the circuit and said output terminals are arranged to be readily reachable.

18. A heater for liquid substantially as herein described with reference to, and as shown in, Figures 1 - 4 of the accompanying drawings.

19. An electrical circuit substantially as herein described with reference to, and as shown in, Figure 5 of the accompanying drawings.