GB2163328A - Tank heater - Google Patents

Abstract

A heater (2), having a glass tube (4) containing a heating element (5) and a thermostat transducer (7) in contact with the glass tube (4), is contained within a brass tube (3). There is a small clearance, typically 0.2mm, between the glass tube (4) and the brass tube (3) such that heat can transfer from the element (5) to a liquid surrounding the brass tube (3) and the thermostat transducer (7) can register the temperature of this liquid. The brass tube (3) has a flange (24) for fitting the tank heater (1) into a tank (20) to extend horizontally parallel to and at a small separation from a bottom of the tank. <IMAGE>

Description

SPECIFICATION Tank Heater The present invention relates to a tank heater and to a tank with a tank heater.

Photographic processing chemicals are sensitive to their temperature. If their temperature is not correct good quality prints, for instance, will not be obtained.

Presently available heaters for processing tanks are not as convenient as they might be.

For instance, one tank heater operates in the manner of an electric cooker hob with a heating coil-placed below the tank. This coil becomes so hot that it is advisable, if the tank is stood on a table, to place an insulating layer of asbestos between the heater and the table for fear of burning the table.

Various thermostatic heaters of the domestic water heating "immersion heater" type are commercially available, but these are generally of too high a wattage, at least for the purposes envisaged in the development programme in which this invention was made.

Too high a wattage results in too large a temperature over-shoot despite operation of the thermostat. Furthermore the thermostats in these heaters tend not to be sufficiently accurate.

Suitably low wattage heaters do exist for heating fish tanks of the type which are used in the house for pet tropical fish. Although one such heater has thermostatic temperature control, it is not suitable for use in photographic processing tanks in the manner in which it is intended to be used in fish tanks, as will be explained.

Such a heater is herein referred to as a heater of the type defined, and comprises a tubular glass jacket closed at one end, a resistive heating element contained within the glass jacket in one portion thereof and a thermostatic temperature control circuit in another portion of the glass jacket. The control circuit has a temperature sensitive transducer which is normally in contact with the glass of the jacket.

This heater is intended to be used with its normally tubular glass jacket aligned vertically with the heating element below the control circuit. The heater is intended to be immersed sufficiently for the transducer to be below the water level. However, the heater cannot be totally immersed since the upper end of the tube is not watertightly sealed.

This heater is not suitable for use in photographic processing tanks firstly because having a glass jacket it is susceptible of damage in the processing environment which is much more hostile than the pet fish environment.

Secondly, if used in the manner intended for a fish tank inadequate circulation in the processing tank will occur because, despite the heating element being disposed at the bottom of the tube, the temperature sensitive transducer is above the element. In this position the transducer does not register the temperature of a flow of unheated water but rather a flow of water heated by the element. Thirdly, in the specific processing tank developed in the above mentioned programme a number of inner processing tanks are contained within an outer water jacket. The inner tanks have top rims effectively covering the water in the outer tank preventing access for the heater.Finally, it is not possible to overcome the last two problems either by submerging the heater and aligning it horizontally because as mentioned the "upper" end is not watertightly sealed or by inserting it through the side of the tank due to difficulty in forming a reliably watertight seal at the tank side onto the glass.

In an effort to provide an improved tank heater and an improved tank with a tank heater, it was surprisingly found that if a heater of the type defined was enclosed in a closely fitting metal tube its thermostatic performance was still adequate despite an unavoidable small air gap between the glass and the metal tube.

According to one aspect of the invention there is provided a tank heater comprising a heater of type defined in combination with a closely fitting metal tube having a closed end, the heater being accommodated in the metal tube in use with the metal tube extending into liquid to be heated.

Preferably the metal tube is of a metal which is a good conductor of heat such as brass or copper.

It has been found that a clearance of .1 mm to .4mm between the glass jacket and the metal tube produces a satisfactory heater. A lower clearance is not practical due to manufacturing tolerance in the dimensions of the glass.

Preferably the metal tube has a sealed-on cap whereby the heater is held captive in the metal tube. Although it is envisaged that the cap may be watertight with a power lead to heater suitably sealingly fitted in the cap, whereby the tank heater may be totally immersed, it is preferred that the metal tube has a flange for sealing extension of the tube into a tank from a side thereof.

The flange may conveniently be provided with a temperature monitoring temperature sensitive transducer. It also provides a convenient earth connection position.

According to another aspect of the invention there is provided a tank having a tank heater of the first aspect sealingly extending into the tank from a side thereof.

The tank heater is preferably oriented horizontally.

Preferably the tank is a compound tank having one or more inner tanks arranged in a water-jacket providing outer tank, with the tank heater extending into the waterjacket space. The or each inner tank is advantageous provided with an upper rim covering the water-jacket space to prevent items being dropped into the waterjacket space and to conserve heat. The outer tank may be of plastics material but is preferably of stainless steel.

To help understanding of the invention a specific embodiment thereof will now be described with reference to the accompanying drawing in which: Figure 1 is a cross-sectional side view of a tank heater according to the invention.

Figure 2 is a cross-sectional side view on a smaller scale on the line ll-ll in Fig. 3 of the tank heater fitted in a tank, and Figure 3 is a cross-sectional end view on the line Ill-Ill in Fig. 2 of the tank.

Referring first to Fig. 1, the tank heater 1 comprises a heater 2 of the type defined contained within a brass tube 3 with a nominal clearance of .2mm between the glass tube 4 of the heater 2 and the brass tube 3. The heater 2 includes a resistive heating element 5 under control of a thermostat circuit 6 with a temperature sensitive transducer 7 in contact with the glass. The brass tube 3 has a sweated-in end plug 8 and a sweated-on collar 9. A cap 10, having a lead access aperture 11, captivates the heater 2 by being glued to the collar 9. Through the aperture 11 pass a power lead 1 2 including an earth connection led to the collar at 1 3 and a temperature monitor lead 14 connected to a temperature monitoring temperature sensitive transducer 1 5 accommodated in a bore in the collar.

The heater 2 will normally be preset to a required temperature, although optionally a temperature setting rheostat 1 6 in an end fitting of the heater 2 may be accessible via an aperture 1 8 in the cap 1 0. Normally the aperture 1 8 will be closed in use, for instance with solder.

Figs. 2 and 3 show the tank heater 1 installed horizontally in a tank 1 9 comprising an outer-water-jacket-tank 20 containing three processing tanks 21. The tank heater 1 is held captive by a nut 22 threaded on the collar 9 sealingly captivating a pair of washers 23 and the side of the outer tank 20 between itself and a flange 24 on the collar 9. The processing tanks have legs 25 allowing circulation of the water-jacket water below the processing tanks. The tanks 21 also have upper rims 26 with down-turned lips. The rims prevent access to the water-jacket, in particular preventing accidental water immersion of paper in a paper carrier which was intended to be placed in one of the processing tanks 21.

The tanks 20 and 21 are of stainless steel.

A 200 watt heater 2 can maintain a filled water-jacket tank 20 having dimensions of 585mm long X 1 75mm across X 460mm deep, with filled processing tanks 21 at a constant 34"C within 0.5 .

The invention is not intended to be restricted to the details of the above described embodiment. For instance, to provide for adjustment of the thermostatic setting of the heater, a rheostat cquivalent to the rheostat 1 6-may be provided at a position remote from the tank with a lead passing out through aperture 11.

Claims (11)

1. A tank heater comprising a tank heater of the type defined and a closely fitting metal tube, in which the heater of the type defined is accommodated with a small clearance.

2. A tank heater as claimed in claim 1, wherein the metal tube has a closed end.

3. A tank heater as claimed in claim 1 or claim 2, wherein the metal tube is of brass or copper.

4. A tank heater as claimed in claim 1 or claim 2 or claim 3, wherein the said clearance is between 0.1 and 0.4mm.

5. A tank heater as claimed in any one of the preceding claims, wherein the metal tube has a flange for sealing extension of the tube into a tank.

6. A tank heater as claimed in claim 5, wherein the flange is provided with a temperature-mon itoring temperature-sensitive transducer.

7. A tank heater as claimed in claim 5 or claim 6, wherein the heater of the type defined has a temperature setting rheostat in an end fitting, and wherein the metal tube has an end cap secured to the flange for protecting the said heater and having an aperture, sealed in use, for providing access to the rheostat.

8. A tank including a tank heater, the tank heater being as claimed in any one of the preceding claims and sealingly extending into the tank through a side thereof.

9. A tank as claimed in claim 8, wherein the tank heater is orientated substantially parallel with the intended free surface of liquid to be contained in the tank.

1 0. A tank as claimed in claim 8 or claim 9, wherein the tank is a compound tank having one or more inner tanks intended to extend into a jacket of liquid contained in the tank, the tank heater extending into a space for the liquid jacket.

11. A tank as claimed in claim 10, wherein the or each inner tank has legs supporting it above a bottom of the tank, the tank heater being arranged between the tank bottom and bottom(s) of the inner tank(s).

1 2. A tank as claimed in claim 10 or claim 11, wherein the or each inner tank has an upper rim having downturned drip lips.

1 3. A tank heater substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawing.

1 4. A tank substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawing.