GB2193110A - Solvent cleaning apparatus - Google Patents

Abstract

A method of solvent cleaning of articles wherein articles to be cleaned are introduced into a cleaner containing a solvent, the contaminated solvent is supplied to a still for purification, the pure condensate of the still is returned to the cleaner to achieve a solvent circulation, and the rate of flow of condensed solvent to the cleaner is used to effect control over the heat input to the still. The invention further resides in a solvent cleaning apparatus comprising a cleaner (13) wherein a solvent is used to displace contaminants from articles to be cleaned, a still (14, 15) incorporating heating means (18) for heating contaminated solvent within the still (14, 15) and a condenser (15) for condensing solvent vapour, a first flow path (19) whereby condensate from the still condenser (15) is returned to the cleaner (13), a second flow path whereby contaminated solvent from the cleaner (13) flows to the still (14, 15), and means (22) monitoring the flow of pure solvent in said first flow path (19) for effecting control over the heat input to the still (14, 15) from said heating means (18) in accordance with the flow in said first flow path (19). <IMAGE>

Description

SPECIFICATION Solvent cleaning apparatus This invention relates to apparatus for cleaning articles by use of a solvent, the apparatus being of the kind including a still for purifying the solvent.

Known forms of solvent cleaning apparatus include those in which the still is an integral part of the cleaning unit, and those where the still is a free-standing unit linked by inlet and discharge pipes to the cleaning unit. A variety of different heat input arrangements for the still are known, and include electrical heating, heating by means of a heat exchanger where the heating medium is steam or hot water, and the use of the heating coil of a heat pump. The still includes a condenser for condensing solvent vapour generated by the heat input to the contaminated solvent in the still, and the pure condensate is supplied to the cleaning unit.As the solvent in the cleaning unit becomes contaminated by contaminants washed from the articles being cleaned the flow of pure solvent from the still to the cleaning unit continuously purges the solvent in the cleaning unit and the purged, contaminated solvent flows back to the still for purification. There is thus a continuous cycle. The design of the heating arrangement of the still and the design of the condenser of the still are such that in normal operating conditions the rate of circulation of solvent between the still and the cleaning unit will be optimised in relation to the intended through-put of articles to be cleaned. It is known that the rate of vapour loss from such apparatus rises in accordance with a rising distillation rate.Thus in an apparatus set-up for a given through-put of articles to be cleaned, the heat input to the still will be set to give a desired distillation rate (and therefore a desired solvent circulation rate) in relation to the intended throughput of articles. If the article through-put is lower than the design value there will be an "unnecessary" loss of solvent by virtue of the vapour loss associated with the set heat input. The same problem arises during batch cleaning when a cleaned batch has been removed and before a new batch is input.Alternatively if the heat input has been set for a lower work rate then there will be insufficient solvent circulation (and therefore insufficient purging of contaminated solvent) and/or insufficient vapour to permit an efficient vapour pre-wash when the demanded work rate rises, for example when placing a new batch in the cleaning unit. Vapour loss is a significant problem both in view of the cost of the solvent and in view of the risk of pollution, by solvent vapour, of the ambient atmosphere.

The problem is most marked with steam or hot water heat exchanger heating in view of the difficulty of designing such apparatus to give a specified heat output. The problem also exists, but to a lesser extent, where the heat source is a heat pump, and to an even lesser extent where an electrical heater is utilized.

Such heating devices can of course be designed more easily to give an accurate specified heat output. In all of the known forms of apparatus the heating device of the still has a manual control device whereby initial setting up of the apparatus is effected, and of course normally the heating devices will be set during installation to give the optimum flow rate of solvent with minimum solvent loss. It is not intended that the manual control devices are utilized during operation of the apparatus thus the aforementioned problems can arise where ambient conditions or other external influences change.

It is an object of the present invention to provide a method of solvent cleaning, and a solvent cleaning apparatus wherein the aforementioned problems are minimized.

In accordance with a first aspect of the present invention there is provided a method of solvent cleaning of articles wherein articles to be cleaned are introduced into a cleaner containing a solvent, the contaminated solvent is supplied to a still for purification, the pure condensate of the still is returned to the cleaner to achieve a solvent circulation, and the rate of flow of condensed solvent to the cleaner is used to effect control over the heat input to the still.

In accordance with a second aspect of the present invention there is provided a solvent cleaning apparatus comprising a cleaner wherein a solvent is used to displace contaminants from articles to be cleaned, a still incorporating heating means for heating contaminated solvent within the still and a condenser for condensing solvent vapour, a first flow path whereby condensate from the still condenser is returned to the cleaner, a second flow path whereby contaminated solvent from the cleaner flows to the still, and means monitoring the flow of pure solvent in said first flow path for effecting control over the heat input to the still from said heating means in accordance with the flow in said first flow path.

Conveniently, said means monitoring pure solvent flow includes a turbine flow meter.

Alternatively said means monitoring pure solvent flow is a digital means.

Desirably said digital means includes a pure solvent reservoir and a float switch responsive to the solvent level in the reservoir.

One example of the invention is illustrated in the accompanying drawings wherein; Figure 1 is a diagrammatic representation of a cleaning apparatus, Figure 2 is a circuit diagram of part of the electrical circuit of the apparatus illustrated in Figure 1, Figure 3 is a diagrammatical representation of an alternative control arrangement to that illustrated in Figure 2, Figure 4 a view similar to Figure 3 of a modification, and, Figure 5 is a view similar to Figure 1 of a modification.

Referring first to Figures 1 and 2 of the drawings, the cleaning apparatus comprises a hollow housing 11 divided internally by a transverse wall 12 to define a washing bath 13 and a boiling sump 14. The bath 13 is filled to the level of the top edge of the wall 12 with an organic solvent (for example Arklone or Genklene LV), and since as will be described later clean solvent flows into the bath 13 then the level of solvent in the bath 13 is maintained at the level of the top of the wall 12 which constitutes a weir over which solvent flows into the sump 14. The upper end of the housing 11 is open, and around the upper end is positioned one or more cooling coils 15 constituting a condenser. Beneath the coils 15 and extending around the periphery of the housing is a continuous trough 16 for collecting condensate dripping from the coil or coils 15.

Within the base of the washing bath 13 there is provided an agitator 17 conveniently an ultra-sonic device whereby articles to be cleaned, immersed in the bath 13, are subjected through the solvent to ultra-sonic vibration. The ultra-sonic vibration enhances the solvent action in effecting cleaning of articles in the bath 13. Beneath the sump 14 is a heater 18 whereby the solvent in the sump 14 is boiled to produce vapour subsequently condensed by the coils 15. The heater 18 can take a number of different forms all of which are known. For example, the heater 1#8 may be an electrical heater, a heat exchanger the heating fluid of which is steam or hot water, or the heat coil of a heat pump.

When articles are washed in the bath 13 the solvent in the bath 13 becomes contaminated, and by virtue of the flow of clean solvent into the bath 13 contaminated solvent is purged from the bath over the weir constituted by the wall 12 and into the sump 14.

The heater 18 causes boiling of the contaminated solvent within the sump, and solvent vapour passes upwardly within the housing and is condensed by the coils 15. Condensate collected #by the trough 16 flows through a conduit 19 to a conventional water separator 21 wherein any condensed water mixed with the solvent is removed. Pure solvent passing from the water separator 21 returns to the bath 13 and thus there is a circulation of solvent. When the apparatus is operating in an equilibrium state then the heat input to the boiling sump is such that the amount of solvent vapour evolved is below the maximum capacity of the condenser 15, and thus there is minimal loss of solvent from the apparatus.

However, in the event that the equilibrium is disturbed, for example by a decrease in the quantity of articles being cleaned the distillation rate will rise (since fewer articles are being heated) and thus there will be a corresponding rise in the amount of vapour evolved and the vapour loss from the apparatus. The increase in vapour evolved will generate an increase in the flow of condensate back to the bath 13. Such a occurrence is disadvantageous inter alia in that expensive solvent is lost from the system, and in that the ambient atmosphere may be polluted by the solvent vapour.

In order to prevent such an occurrence, as shown in Figure 1, the flow of pure condensate from the water separator 21 to the bath 13 is monitored by a flow monitoring device 22 and the output signal from the flow monitoring device 22 is utilized to control the heater 18. In Figure 1 a control arrangement is illustrated at 23. The flow monitoring device 22 and the control arrangement 23 can take a wide variety of different forms. However, in the example illustrated in Figures 1 and 2 the flow monitoring device 22 is a turbine device the speed of rotation of the turbine being proportional to the flow of condensate there through. The turbine drives an electrical generator, and the d.c. voltage output of the generator is thus proportional to the flow of solvent.

As illustrated in Figure 2 the voltage output from the device 22 is compared by a comparator 24 with a reference voltage determined by a manually adjustable potentiometer 25.

When the voltage output from the device 22 exceeds the reference voltage for a period of time, conveniently sixty seconds, the comparator 24 produces an output which operates an electromagnetic relay 26. The relay 26 effects switching of a solenoid operated valve 27 in the heating fluid flow line to the heat exchanger constituting the heater 18. It will be recognised that the time delay is effected by the capacitor/resistor arrangement 28. In the event that the flow rate monitored by the device 22 exceeds the predetermined value (as set by the potentiometer 25) the solenoid valve 27 is caused to close thus cutting off the flow of heating fluid to the heat exchanger. The sump 14 thus cools and the amount of vapour being evolved is reduced.

When the consequential flow of pure condensate through the monitoring device 22 has fallen below the predetermined value for the necessary period of time then the relay 26 is de-energized thus opening the solenoid valve 27 to re-establish heating. It will be recognised that where the heating device 18 is an electrical heater then the solenoid valve 27 can be dispensed with and the relay 26 can control the flow of electrical current to the heater directly. Similarly, where the heater 18 is a heat pump then the solenoid valve 27 will be replaced by a device effecting similar con trol over the operation of the heat pump.

Thus should the distillation rate rise the heat input will be reduced thereby reducing the distillation rate and consequently reducing the rate of solvent loss.

It will be understood that a turbine flow meter as described above is an analogue device. However, in place of the turbine meter a digital device as illustrated in Figure 3 could be utilized.

Figure 3, which is diagrammatic representation, illustrates a digital flow monitoring device comprising a reservoir 31 into which pure condensate from the water separator 21 flows. The flow from the water separator 21 flows into the reservoir 31 by way of conduit 19, entering the reservoir 31 above the solvent level therein to avoid problems arising from back pressure in the system, but below the vapour level in the reservoir 31 so as to minimise vapour loss from the reservoir. The reservoir 31 has an outlet 32 -connected through a manually adjustable restrictor 33 to the washing bath 13. One wall of the reservoir 31 carries a float operated electrical switch 34 which in effect monitors the level of pure solvent in the reservoir 31.The setting of the restrictor 33 is chosen to ensure that there will be a level of solvent within the reservoir 31 in the normal operating conditions of the apparatus. In the case where the heater 18 is a heat exchanger then the float operated switch 34 controls a solenoid operated valve 27 which, as described above, can switch on or off the flow of heating fluid to the heater 18. As the flow of condensate from the condenser 15 increases the level of condensate (pure solvent) collecting in the reservoir 31 will rise since the flow from the condenser 15 will exceed the flow rate of the restrictor 33. The float of the switch 34 will thus rise and a point will be reached at which the switch 34 closes thus energizing the solenoid valve 27 to cause to the valve 27 to close.Closure of the valve 27 cuts off the flow of heating fluid to the heater 18 thus allowing the sump 14 to start to cool. Consequent upon cooling of the sump 14 the amount of vapour evolved, the rate of vapour loss, and thus the flow of condensate into the reservoir 31 will be reduced and the level in the reservoir 31, will fall thus permitting the switch 34 to open so re-establishing the flow of heating fluid through the solenoid valve 27 to the heater 18. It will be recognised that the digital system is considerably more simple than the analogue system described with reference to Figure 2 and thus for many applications is the preferred arrangement.It will also be recognised that where the heater 18 is an electrical heater then the switch 34 may effect direct control over the current flow to the heater, although where the switch 34 is of a low electrical rating it may be preferred to use the switch 34 to control operation of an electro-magnetic relay in turn controlling flow of electrical current to the heater. As mentioned above, where the heater 18 is a heat pump then the solenoid valve 27 will be replaced by an equivalent device controlling operation of the heat pump in accordance with the on or off setting of the switch 34.

In the apparatus described with reference to Figures 1 and 2 there is provided, within the control circuit, a time delay arrangement 28.

An equivalent arrangement could be provided in the apparatus described above in relation to Figure 3 and the modificationm thereof to be described with reference to Figure 4. However, it will be recognised that a similar effect can be achieved by careful choice of the volume and shape of reservoir 31 and the hysteresis of the float switch 34.

The time constant imposed on the systems described herein is desirable to accomodate short term fluctuations in distillation rate which will occur normally in use. A preferred operating mode is one in which the heat input to the sump is increased very rapidly as the average distillation rate falls and the heat input is reduced slowly when the average distillation rate (solvent return flow rate) rises. Such an operating mode would allow rapid response to the input of a new batch of articles to be cleaned but would avoid reduction in the distillation rate arising from fluctuations in the vapour evolved during cleaning of a batch of articles.Moreover for the period immediately following removal of a batch of cleaned articles a high distillation rate would be maintained assuming rapid purging of the contaminated solvent in the cleaning bath whereafter the rate would fall, thus minimising vapour loss, until a new batch of articles is introduced to the bath.

In the modification illustrated in Figure4 the water separator 21 and the float switch 34 are combined. Condensed solvent and water enter a reservoir 131 by way of conduit 19 and water floating on the solvent is discharged by way of conduit 121. Solvent flows beneath internal partition 136 into a weir chamber 137 where it can overflow weir 138 into a float chamber 138. The float of a float operated switch 134 monitors the level of solvent in the chamber 138 as described with reference to switch 34 in Figure 3. Solvent exists the chamber 138 for return to the washing bath by way of a conduit 139 equipped with a variable restrictor (not shown) equivalent to the restrictor 33 in Figure 3. A vapour vent 140 of the reservoir 131 returns solvent vapour to the washing bath at a point below the vapour level therein.

Figure 5 illustrates a modification in which the still for purifying the solvent and the washing bath wherein articles are cleaned are two separate and freestanding units linked by first and second solvent flow paths in the form of conduits. Thus, there is provided a cleaning unit 11 a consisting of a washing bath 13 containing solvent and incorporating an ultrasonic agitator 17 and condenser coils 15a.

The still unit 1 1b comprises a boiling sump 14 having a heater 18 which may take any of the forms described above and being equipped, adjacent its upper end, with a condenser coil or coils 15 the condensate from which is collected in a peripheral trough 16. A conduit 19 defines a first solvent flow path from the trough 16 through a water separator 21 and a flow monitoring device 22 to the bath 13 of the unit 1 lea. A second conduit 35 interconnects the bath 13 of the unit 11 a with the sump 14 of the unit Ilb. It can be seen that the conduit 35 opens into the unit 11 a part way up the wall thereof, and thus defines the maximum level of solvent within the bath 13.Solvent contaminated by the contaminants washed from the articles in the bath 13 flows under gravity through the conduit 35 into the boiling sump 14. The level of the trough 16 is higher than the maximum solvent level in the bath 13 and solvent flows through the conduit 19 to enter the bath 13 of the unit 1 1a adjacent the bottom of the bath. Thus although the units 1 lea and 1 1b are separate and freestanding units the mode of operation of the modification illustrated in the Figure 5 is identical to that described in relation to Figure 1, and the various alternatives described in relation to Figure 1 are applicable to the modification illustrated in Figure 5.It will be recognised however, that if desired rather than relying upon a gravity circulation the arrangement illustrated in Figure 5 could incorporate pumping means in one or both flow paths to achieve the necessary circulation of solvent.

The various arrangements described with reference to Figure 1 exhibit a further advantage over the known equivalent arrangements over and above the advantages described previously. This additional advantage arises in a situation where the operating mode includes the step of hanging cold articles to be washed above the washing bath 13 before immersing them in the washing bath. It will be recog nised that in the cambined apparatus of the kind illustrated in Figure 1 the region above the bath 13 is filled with hot solvent vapour as- is the region above the sump 14. Thus when a cold article to be washed is positioned above the bath 13 solvent will condense on the cold article and will run under gravity over the surface of the article thus effecting a preliminary washing (pre-wash) prior to the main washing which takes place during immersion of the article in the bath.

Condensed, and now contaminated solvent will drip from the articles into the bath 13, but because condensation is taking place on cold articles positioned over the bath the amount of the condensate collected from the coils 15, and thus flowing through the conduit 19 will be reduced. When operating in this mode the conventional system fails to achieve an optimum circulation of solvent since the amount of pure solvent being returned to the sump 13 is reduced by the amount condensing on the cold articles. Thus purging of the contaminated solvent in the bath 13 is reduced, and the main washing of the articles by immersion in the bath 13 may be less effective since there is a higher concentration of contaminants already in the bath 13.However, in the various forms of apparatus described above in relation to Figure 1 the reduction in pure solvent returning through the conduit 19 (consequent upon condensation on the cold articles positioned above the bath 13) will indicate to the control system that insufficient condensate is being returned and will thus cause increased operation of the heater 18 causing more solvent to be evolved, and causing the return flow through the conduit 19 to be maintained at its optimum level. Thus the apparatus automatically adjusts to operation in the mode where articles are first positioned in the vapour for preliminary washing so maintaining the optimum circulation rate of solvent in the system.

It will be recognised therefore that in prior art apparatus wherein there is no control over the heater 18 during operation of the apparatus then the apparatus is initially adjusted to achieve equilibrium between the heater and the condenser in the expected operating conditions. By comparison the apparatus described above in relation to Figures 1 to 4 effects control over the operation of the heater in accordance with the return flow of pure solvent to the washing bath. Thus the apparatus described above can accommodate changes occuring during operation both in terms of ambient and other external conditions, and changes in the mode of operation and/or the rate at which articles to be cleaned are introduced into the apparatus.

Claims (11)

1. A method of solvent cleaning of articles wherein articles to be cleaned are introduced .into a cleaner containing a solvent, the contaminated solvent is supplied to a still for purification, the pure condensate of the still is returned to the cleaner to achieve a solvent circulation, and the rate of flow of condensed solvent to the cleaner is used to effect control over the heat input to the still.

2. A solvent cleaning apparatus comprising a cleaner wherein a solvent is used to dis place contaminants from articles to be cleaned, a still incorporating heating means for heating contaminated solvent within the still and a condenser for condensing solvent va pour, a first flow path whereby condensate from the still condenser is returned to the cleaner, a second flow path whereby contami nated solvent from the cleaner flows to the still, and means monitoring the flow of pure solvent in said first flow path for effecting control over the heat input to the still from said heating means in accordance with the flow in said first flow path.

3. Apparatus as claimed in claim 2 wherein said means monitoring pure solvent flow includes a turbine flow meter.

4. Apparatus as claimed in claim 2 wherein said means monitoring pure solvent flow is a digital means.

5. Apparatus as claimed in claim 4 wherein said digital means includes a pure solvent reservoir and a float switch responsive to the solvent level in the reservoir.

6. A method of solvent cleaning substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

7. A method of solvent cleaning substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

8. A method of solvent cleaning substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

9. Solvent cleaning apparatus substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

10. Solvent cleaning apparatus substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

11. Solvent cleaning apparatus substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.