GB2121259A - Electric heating mantles - Google Patents

Abstract

An electric heating mantle for small vessels includes a cup-shaped moulded body or former 4 supporting on its inner surface a sort liner 6 and heating element 5. A thermal switch 9 or 9' is mounted adjacent the external surface of the body 4 to sense the temperature thereat and controls the supply of current to the heating element to avoid excessive power dissipation and overheating of the mantle and a vessel being heated. <IMAGE>

Description

SPECIFICATION Electric heating mantles This invention relates to electrical heating mantles for supporting and heating small vessels such as laboratory retorts and flasks.

Such mantles, usually known as laboratory mantles, generally comprise an outer, generally cuplike casing, a cup shaped body of insulating material and a heater assembly, typically formed as a glass fibre woven cloth having an insulated, electrical resistance heating element attached to or otherwise incorporated in the cloth.

The cup-shaped body closely supports the heater assembly, which is shaped so that its inner concave surface is closely adapted to the flask or retort which is to be heated.

Laboratory mantles of this form are relatively slow to heat up because of thermal inertia. For example, a 500 ml flask, fuily filled with water, needs a 180 watt element to boil the water in 22 minutes. Once the water has boiled, it is for the user to switch off the heater and remove the flask.

Otherwise the element continues to dissipate 1 80 watts and the water starts to boil off, possibly boiling dry and then the flask may be destroyed.

The present invention aims at the provision of a laboratory mantle designed to obviate or reduce these problems.

In accordance with the invention, there is provided an electric heating mantle for supporting and heating small vessels, comprising a heating element supported by a cup-shaped body of electrical insulating material, and including a thermally responsive switch means for controlling the supply of current to the element mounted adjacent the external surface of the said body to sense the temperature thereof.

Some forms of laboratory mantle in accordance with the invention are described below, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a half-section of a mantle; and Figure 1 A is a scrap-section showing a possible modification; and Figures 2, 3 and 4 show some possible alternative forms of electrical control circuits.

The laboratory mantle shown in Figure 1 includes an outer metallic casing consisting of a cup-shaped base 1, a cylindrical side wall 2 and a top cover 3 in the form of a sheet metal spinning which is secured to the upper edge of a side wall and spun over to form a top retaining flange 3A.

Within the casing there is supported an electrically insulated, moulded body, or former 4, typically of resin impregnated ceramic fibres, a soft liner 6 of soft, needled glass fibres and a heater assembly 5 comprising a woven glass fibre carrier incorporating an electrical resistance heating element, the heater assembly being of well-known construction per se.

The former, soft liner and heater assembly are firmly held in position by the tension of a toroidal coil spring 7 which surrounds the lower portion of the former 4 and engages a number of flexible retaining arms 8, causing them to bear upwardly and inwardly on the former so as to clamp the former, the soft liner and the heater assembly against the underside of the retaining flange 3A.

Typically, three or six arms are provided, equispaced about the circumference of the former.

The above described construction is believed to be novel in itself.

The mantle incorporates a thermal switch 9 positioned adjacent the external surface of the former controlling the supply of current to the heating element.

In Figure 1 two possible positions for the switch are shown diagrammatically at 9 and 9', the switch preferably being held against the former by the adjacent retaining arm 8. Alternatively, a separate attachment means may be provided, or the switch 9 may be securely housed in a recess 4A in the external surface of the former, as shown in Fig. 1 A. The switch may be held in place, for example, by adhesive, cement or press fit.

In each case, the switch 9 is held in heattransfer contact with the exterior of the former at a position determined empirically, where the rate of change of temperature corresponds with that at the centre of the flask or retort. The heat gradient across the soft liner and former is substantially constant, typically in the region of 250C, so that when the water temperature reaches 1 000C, the temperature at the switch is 750C.

In the example illustrated in Figure 1, an adjustable electronic controller 11 is mounted in the base of the casing and has a thumbwheel 12 for adjusting the setting of the controller.

As shown in Figure 2, the switch 9, which is normally closed, is connected in series with the heating element 13, rated at, say, 1 80 watts and the controller 11 is connected in parallel with the switch, across its fixed contacts (or equivalent).

Thus on starting up a boil, the switch 9 passes 240V AC to the element 1 3 and the controller is by-passed. When a predetermined temperature is sensed by the switch, say 750C using the exemplary figures given above, the switch opens and the controller passes 0 to 80% of the power according to its setting. For example, if 60 watts are required to maintain the boil the controller passes 1/3 of the full power to the element.

In the control arrangement illustrated diagrammatically in Figure 3, the switch 9 is connected in parallel with a diode 14 acting as a half-wave rectifier, so that on opening of the thermal switch, the supply to the element is dropped from 240V AC to 1 2or, quartering the power consumption and heating effect of the element.

Another variant is illustrated in Figure 4, in which the element is split into two parts 1 3A, 1 3B, which may or may not be of equal wattage, and the thermal switch 9 on opening cuts out one part 1 3B of the heating element, the other part 1 3A continuing to operate at its full wattage.

Thermal switches and controls as described above may alternatively be set to detect a temperature higher than the boiling point of the liquid being heated to protect the equipment against over-heating, for example due to excessive or complete boiling off.

The thermal switches employed may be of various known types, but we presently prefer to use a thermally tripped electromechanical switch.

It will be appreciated by those skilled in the art that by reducing the power supplied to the element after the required maximum temperature has been reached, the element is thermally understressed and its useful iife thereby prolonged.

Claims (6)

1. An electric heating mantle for supporting and heating small vessels, comprising a heating element supported by a cup-shaped body of electrical insulating material, and including a thermally responsive switch means for controlling the supply of current to the element mounted adjacent the external surface of the said body to sense the temperature thereof.

2. A mantle according to claim 1, wherein the said switch means is arranged to operate, in response to a predetermined minimum temperature being sensed, to reduce to a predetermined level the power supplied to the heating element.

3. A mantle according to claim 1 or 2, wherein the said body is of moulded construction, and has a soft lining against which the element is supported.

4. A mantle according to claim 1, 2 or 3, wherein the said switch means is held against the external surface of the body by a mechanical clamping arrangement which also holds the body and the element in place within an outer casing.

5. A mantle according to claim 1, 2 or 3, wherein the external surface of the body has a localised recess in which the said switch means is securely located.

6. An electric heating mantle substantially as herein described with reference to the accompanying drawings.