GB2168579A - Thermostatically-controlled heater - Google Patents

Abstract

A tubular fluorescent lamp body 2 is heated by an electrically energised heater blanket 8 to raise the temperature of body 2 to a temperature at which the mercury in the lamp is vapourised, and has a metal heat sink 10 to provide a cold spot in the tube 2 on which the mercury condensers to provide a reservoir and has a thermostat 16 to control the heater temperature, there being a thermal impedance provided by nylon body 18 between the heater and thermostat 16, which also loses heat to the atmosphere, making it responsive to the atmosphere temperature, through heat sink 20. Sinks 12 and 20 project into an air-flow 14 in conduit 12. The lamp is used in xerographic copiers. <IMAGE>

Description

SPECIFICATION Thermostatically-controlled heater This invention relates to a themostatically-controlled heater, particularly for a gaseous discharge lamp, such as the low-pressure, mercury vapour, fluorescent lamps used in xerographic copiers.

With such a lamp, it is important to keep its body temperature above an optimum operating temperature, and to produce at least one 'cold spot' on the lamp body to facilitate the local condensation of mercury vapour, so as to limit variations of the lamp radiance to +5%.

In known arrangement, the lamp body loses heat directly to a heat exchanger cooled by a forced draught, and a thermostat is coupled directly to the heater blanket used to heat the lamp body and the mercury vapour contained in it. If the thermostat is designed to open at a temperature just above the maximum operating temperature of the lamp, then it has been found that the thermostat will open long before the temperature of the cold spot provided by the heat exchanger rises to within say 7"C of the lamp body temperature. This means that the lamp is effectively always running 'cold', i.e. at a temperature so far below its intended operating temperature that it gives out significantly less than its designed or desired output of light.No appreciable rise of temperature of the lamp body occurs with continued thermal cycling of the heater, so that increasing the output of the heater is not a solution to this problem.

The present invention aims at overcoming this problem by introducing a thermal delay in the path betwen the heater and the thermostat, and by providing some cooling of the thermostat, thus making operation of the thermostat sensitive to the ambient temperature.

Accordingly the present invention provides a temperature-controlled heater adapted to lose heat to the atmosphere through at least one localised heat sink, and of which the temperature is controlled by a thermostat in poor heat-conductive contact with the heater, which thermostat is adapted also to lose heat conductively to the atmosphere.

The present invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a diagrammatic view of a known form of thermostatically-controlled heater; Figure 2 is a view similar to Fig. 1 of a heater of the present invention; Figure 3 is a diagrammatic perspective view of one means for mounting the thermostat, and Figure 4 is an equivalent diagram of the Fig. 3 arrangement.

In the Figures, those elements which are the same have the same references.

A tubular fluorescent lamp body 2 has end caps 4 coupled to terminals 6 by which the lamp is energised.

Sitting on part of the lamp body 2 is an electrically-energised heater blanket 8 intended to raise the temperature of the lamp body 2 to a temperature at which the memory in the lamp is vaporised and is thus available for conducting electricity to enable the lamp to function. The blanket 8 is designed not to prevent light from falling on the platen of a xerographic copier (not shown), the amount of light being a function of the operating temperature of the lamp.

In thermal contact with the lamp body is a heat exchanger 10 in the form of a small block of metal formed with several fins which project into the interior of a conduit 12 along which flows an air stream 14. This causes the temperature of the heat exchanger 10 to be significantly below the internal temperature of the lamp body 2, so that the heat exchanger gives rise to a 'cold-spot' on the lamp body on which mercury vapour is able to condense to form liquid mercury which acts as a reservoir of fresh vapour for continued operation of the lamp.

Sitting directly on the heater blanket 8 is a thermostat 16 having an electrical contact in the circuit which energises the heater. As usual, the thermostat is adjustable, and it is normally set so as to be closed at all temperatures of the blanket below the maximum operating temperature of the lamp. Because of the direct contact of the heater with the thermostat, the thermostat is indifferent to fluctuations of the ambient temperature. Thus the thermostat tends to operate only when the heater blanket has reached the preset temperature, at which time the temperature of the lamp body itself is usually so far below the desired operating temperature that the lamp is under-run.

The system shown in Figs. 2 and 3 is intended to overcome this disadvantage. In this system, the thermostat 16 is spaced from the heater 8 by a body of nylon 18. Interposed in the heat-conducting path between the heater body and the thermostat 16 is a length of copper 20 having one end functioning as a heat exchanger by being dipped into the air stream 14 flowing along conduit 12.

The block 18 introduces thermal impedance into the heat-conducting path, and also represents a thermal capacity. The thermal delay introduced by the thermal impedance of block 18 ensures that the thermostat 16 is not opened until a significant time after the heater has reached the desired operating temperature of the lamp. This delay in the deenergisation of the heater is sufficient for the lamp body to be far closer to its desired operating temperature than in the known arrangement of Fig. 1.

The cooling of the thermostat introduced by the conduction of heat along the heat exchanger 20 renders the thermostat sensitive to the ambient temperature. The effect of this is that at low ambient temperatures the heat exchanger 20 leads away heat at a higher rate than when the ambient temperature is higher, thus increasing the time for which the blanket 8 continues to be energised.

Because of the sensitivity to the ambient temperature of the apparatus shown in Fig. 2, the heater cycles about a mean temperature which is higher when the ambient temperature is low than when it is higher, thus delivering more electrical power to the heater to compensate for the greater loss of heat, through the heat exchanger 10, to the atmosphere when the air temperature is low.

Because of the thermal impendance and capacity introduced into the heat flow path to the thermostat 16, the system is not perfectly damped. Thus the heater tends to be switched on and off as its temperature alters through a range of about 1.5"C.

For a particular form of nylon, 5mm in thickness between the heater body 8 and the heat exchanger 20, a range of heating times and temperatures such as are shown in the accompanying Table were obtained.

TABLE Sample Heater Performance Ambient First cycle Mean Mean temp ("C) Switch Peak Heater temp Heat Sink temp time (min) temp ("C) ("C) ( C) 32 3.0 86 55 34 22 2.0 89 62 30 11 5.5 93 66 25 25 From these figures, it was calculated that the optimum thickness of block 18 needed to be 5.4 mm, when made of the same material. When this was introduced, it was found that the peak temperature of the heater was raised to 120"C at an ambient temperature of 11"C, and that this peak temperature was reached after ten minutes from being switched on. At this temperature, the operation of the discharge lamp was more efficient than when the narrower block was used, and was far more efficient than with the arrangement shown in Fig. 1.

The equivalent circuit of Fig. 4 illustrates the heat flow path analogously to the flow of current through electrical components.

It will be appreciated that although the present invention has been described as having been applied to a gaseous discharge lamp, it could be used for controlling the temperature of other forms of load.

Claims (1)

1. CLAIM

   A temperature-controlled heater adapted to lose heat to the atmosphere through at least one localised heat sink and of which the temperature is controlled by a themostat in poor heatconductive contact with the heater, which thermostat is also adapted to lose heat conductively to the atmosphere.