The invention relates to vapour cleaning plant, that is plant in which a suitable solvent, usually a chlorinated solvent which boils at a temperature substantially below the boiling point of water, is vaporised and allowed to condense on cold manufactured components to dissolve any grease, oil, tar and other soluble contaminants on the components and to flush off any other soiling substances such as dirt, swarf and sand.

Such a plant comprises basically a tank with a sump at its bottom for liquid solvent, a heater in the sump for vaporising the solvent and above the sump a grid on which may rest baskets containing the components to be cleaned. The selected solvent has a vapour substantially- heavier than air and tends therefore to accumulate in the tank above the sump displacing air from the tank. The atmosphere in the tank is thus substantially vaporised solvent. Condensed solvent falls from the components being cleaned back into the sump with greases and oil dissolved in it and also flushes off soiling substances.

In commonly used plant of this kind, various methods are employed to restrict escape of solvent vapour through the open tank top. Thus a condenser coil is commonly provided at the tank top, cold water or other refrigerant being circulated through the coil to condense vapour rising above a given level. Also, there is usually provided rim extraction means comprising ducting around the tank top and a fan to create a curtain of fast flowing air over the mouth of the tank whereby any small amounts of vapour which may pass the condenser are entrained and ducted away.

It is common in practice for the plant to be used only intermittently and, because of the relatively long "heat-up" time, to be left in an operational condition for long periods of non-use, e.g. during lunch and tea breaks and when loading and unloading workpieces to be cleaned or when no workpieces are available for treatment. As a result, not only is more energy used than is necessary to keep the solvent up to working temperature due to excessive solvent evaporation, but also large amounts of energy are used to maintain adequate circulation of refrigerant through the condensor coil. Also heat extracted by the condenser is wasted.

The Invention provides heater control means in a vapour cleaning plant whereby energy economy may be achieved, the control means comprising a temperature-sensitive sensor or a plurality of such sensors disposed at a preselected level, or preselected levels respectively, in the tank and connected to control heating of the liquid solvent to maintain substantially a desired top level of the solvent vapour In the tank, and to boost heating of the solvent on fall of the top level below its desired value.

In a preferred arrangement, the control means comprise a pair of sensors positioned at respective levels in the tank and connected to comparator means for controlling heating to maintain a vapour top level between the sensors, and a third sensor positioned at a level between the pair of sensors and connected to override the operation of the comparator control to initiate additional heating of the liquid solvent when the vapour falls below the third sensor.

When the plant Is idle, the pair of sensors with the comparator means operate to maintain a selected temperature

differential between the pair of sensors so that only small amounts of heat energy are required to maintain the plant in readiness for use, but when, for instance cold components to be cleaned are placed in the tank, the vapour top level immediately falls due to vapour condensation on the components and the third sensor operates to override the control by the comparator means to allow ⁷ sufficient additional and rapid heating to restore the vapour top level. As compared with the commonly used plant in which full heating is maintained and in which escape of vapour is prevented by condensation on the condenser coil, the control effects a substantial saving in energy.

The sensors will be temperature-sensitive devices such as temperature-sensitive diodes or other solid state devices, or thermo-electric devices, such as thermistors and thermocouples. Two of the sensors may be connected in a comparator circuit controlling idling operation of the heater(s) and the third is arranged to bring in full heating of the solvent when required.

According to an important feature, as a result of the heating control, there Is provided operational vapour cleaning plant without a condenser coil at the top of the tank, so substantially reducing the plant cost and cost of operation.

In such plant, when the tank walls are Insulated and become hot, there is a tendency for a thin layer of vapour to creep up the hot walls of the tank beyond the sensors. To avoid escape of such vapour, it is arranged that the rim extraction air is ducted over the top of the walls of the tank to cool them thereby to effect condensation of the layer of vapour.

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The operational characteristics of plant having a condensing coil can be improved by provision of control means of this invention.

One form of vapour cleansing plant of this invention is illustrated by way of example in the accompanying drawings in which:-

Fig. 1 Is a sectional elevation,

Fig. 2 is a partial plan partly in section,

Fig. 3 is a block diagram showing a suitable form of the control, and

Fig. 4 shows a simplified control.

The illustrated plant comprises an open-topped tank 10, the walls of which will be suitably heat Insulated.

The tank near its bottom has a work-supporting grid- 11, below which there Is a liquid solvent sump 12 in which there is a heater 13, shown as an electric immersion heater. Other forms of heating may be employed as known for vapour cleansing plant. Below the grid 11 and above the sump 12, there is a collector tray 14 for receiving condensed solvent falling from the components being cleansed, and the grid and tray are angled slightly so that the collected solvent with any soil flows into a space 15. The space 15 Is separated by- a wall 16 from a water-separating space 17 which In turn is separated by a weir 18 from the solvent sump 12.

At a suitable position there is mounted on a wall of the tank 10 a panel for temperature sensitive control equipment comprising three sensors 20, 21, 22 at different levels. The sensors 20, 22 are connected to comparator means which, when operative, control the operation of heater 13 so as to maintain in the tank vapour atmosphere

with its top substantially at the levels of the sensors so that substantially a prescribed temperature is sensed by sensor 21. The setting of the controls will depend on the solvent In use.

The sensor 21 is set so that, so long as the prescribed temperature and thus the desired top level of the vapour atmosphere is maintained, heat supply is controlled by the comparator means sufficient to make up heat losses from the tank walls, but that, on fall of the temperature sensed by sensor 21, due for instance to cold workpieces being placed in the tank, the comparator circuit is overridden and full heating is switched on under the action of sensor 21.

In the illustrated construction, the condensor coil usually provided at the top of vapour cleaning plant is not employed, and to avoid loss of solvent due to creep of solvent vapour along the hot walls of the tank, the following arrangement is used.

Around the rim of the tank and forming its mouth, there is provided a duct 23 with slots 24 in its wall, the duct 23 being connected by downwardly-directed ducts 25 over the outer surface of the tank walls to an extraction manifold 26 which has a mounting 27 for an extraction fan. The fan in use creates a fast-flowing curtain of air over the tank mouth, the air flowing through slots 24 Into duct 23 and then through ducts 25, so cooling the tank walls, into the extraction manifold 26. The resulting condensate runs down the tank walls into channels 28 and thence into space 15. The channels 28 also provide supports for the grid 11.

Referring to Fig. 3, there is shown in block diagram form one control arrangement as utilised with the three sensors 20, 21 and 22.

The sensors 20, 22 are connected through respective amplifiers 30 to an amplifier 31 In such manner as to obtain an output signal representative of the difference in the temperatures sensed by sensors 20, 22. This signal is fed through an amplifier 32 to a comparator 33 in which the signal is compared with a signal from ramp generator 35.

Also fed to the comparator 33 is a signal from a comparator 37 which signal is dependent upon the actual temperature sensed by sensor 21. The ramp generator 35 and comparator 37 are connected to the comparator 33 through a gate 34.

The output of comparator 33 controls operation, j≥.g. of relays 36, effective to determine the heat supply to the apparatus.

The operation Is as follow's:-

So long as the sensor 21 senses a temperature corresponding substantially that of the boiling point of the de-greasing liquid, there is a zero output from comparator 37 and control of the relays 36 is by the signal from the ramp generator 35 compared with that from amplifier 32. The result is that, depending on the difference of temperatures sensed by sensors 20, 22, shorter or longer periods of heat input occur such as to tend to maintain a required temperature difference. The power to the relays may either be under burst or phase power control by choice of the timing periods in the ramp generator.

When, however, the sensor 21 senses a temperature substantially below the boiling point of the de-greasing liquid, a signal is generated by comparator 37 which opens the gate 34 to prevent a signal reaching the comparator 33

from the ramp generator, and affects the operation of the comparator so that its output to heater control 36 operates to initiate full continuous heating.

In the control of Fig. 4, the sensors 20, 22 are omitted and the signal which is compared with that of the ramp generator is derived from a pre-settable device 38.

It will be appreciated that with the apparatus above described controlling vapour level electronically effects substantial energy savings w^hich, depending on the use of the apparatus, can be between 30% and 60%. Substantial energy savings can also be achieved by fitting the forms of control to existing apparatus having water cooling coils since the water requirement is greatly reduced. Further in all forms of the apparatus vapour losses can be markedly reduced.