GB2209768A - Vapour/liquid solvent degreasing plant - Google Patents

Abstract

The plant comprises an open-topped container 20 provided with means e.g. a condensing coil 21 to control the escape of vapour, and a vapour generator 22 comprising a supply of liquid solvent 23 and a heater 24 therefor. The open area of container 20 is minimised to prevent excessive loss of vapour. Means are provided for introducing liquid solvent into container 22 to immerse the work to be cleaned held e.g. in a basket (2, Fig. 3). Such means may comprise solvent holding tanks 26, 27, 28 for solvent at different temperatures and/or states of cleanliness, with pumps 29, inlet manifold 30 and spray jets 32 for introducing liquid solvent into container 22, and with return line 36 provided with control valve means 37 and siphon means or a further pump for returning solvent to the appropriate holding tank. Condensate collecting means 39 and line 40 are provided for returning condensate to the clean solvent holding tank 28. Container 20 may also be provided with ultrasonic generators 43 and selected holding tanks may be provided with heaters 33. <IMAGE>

Description

DEGREASING AND CLEING PLANT This invention relates to vapour degreasing and cleaning plant and particularly relates to plant using vapour degreasing followed by several stages of liquid solvent cleaning and rinsing cycles.

Fig. 1 shows a knows plant to consist of multiple tans wlti a stole enclosure 1. Work to be cleaned is lower in a basket 2 inte a first tank or are 3 wherein a liquid 4, such as halogenated hydrocarbon cleaning solvent is heated to boiling point by heaters 5 to generate a cleaning vapour 6. After vapour cleaning/ degreasing the basket 2 is, typically, sequentially transferred to a second tank 7, containing hot liquid solvent heated by heaters 8; a cool rinse tank 9, containing liquid solvent at ambient temperature; an ultrasonic cleaning tank 10 containing cool liquid solvent agitated by ultrasonic transducers 11; and a final cool rinse tank 12, containing cold distilled liquid solvent.

Other arrangements of tanks can be employed and the whole plant is completed by conventional vapour condense coils 13 about the top of the enclosure and a water separator 14 in tne return condensate line to tank 12.

Haloqenated hydrocarbons have the benefit that the are an extremely aooc cleaning solvent, are non-flammable and have a relatively low toxicity. However, standards of industrial hygiene are becoming stricter and contamination of the environment by halogenated hydrocarbons, caused by diffusion losses from the tank surface and also by "drag-out" on the parts is becoming subject to stricter controls.

In order to process a basket load of work, it is clear that the surface area of the cleaning tank will need to be of sWz. a size that the basket can be accepted into the tank aperture. Diffusion losses will be proportional to the surface area of the tank and it is obvious that using the above-described conventional multi-tank cleaning system, the tank surface area is increased by a multiple of the number of tanks and, in turn, the diffusion losses are increases by the same factor.

Other disadvantages of the multi-tank system are that the system is inflexible, cleaning cycles are limited to the various tanks provided within the enclosure; which are clearly not readily changed, so that variations in cleaning cycles are limited to omission of use of a particular tank or tanks or the order in which the tanks are uses. Contaminants, debris and the like remove fro the work bv the cleaning process accumulates in the bottoms or sumps of the various tanks.

is an objet o the present invention to provide a method of and apparatus for cleaning that overcomec the above stated disadvantages.

According to the present invention, a method of cleaning comprises the steps of: (i) loading of a work to be cleaned into a container, provided with vapour control means about the container top; (ii) generating a solvent cleaning vapour to clean the work; (iii) introducing liquid solvent into the container to submerge the work; (iv) removing liquid solvent from the container; ana, (v) unloading the cleaned work from the container.

In a preferred embodiment of the present invention, steps (iii) and (iv) are sequentially repeated for liquid solvent maintained at different temperatures.

5 this means, the work does not have to be transferred from tank-to-tank for cleaning processes, the cleaning vapour and cleaning liquid solvent can be sequentially introduced to and removed from the container; i.e. the work remains stationary and the various cleaning processes are brought t the work.

Also according to the present invention, cleaning appartus comprises: (i) a container provided with vapour control means about the container top; (il a so solvent vapour generator connected to the container; (ii on or more tanks for liquid solvent; (iv) means to convey liquid solvent from the or a given tank to the container; and, (v) means to remove liquid solvent from the container to the or the respective given tank.

In a preferred embodiment of the present invention, the vapour control means is a cooling condense coil about tne to of the container; liquid solvent is houses in a separate tank or tanks maintained at different temperatures provided with means, such as a pump, to deliver the liquid solvent to the container via an inlet pipe connected between the pump and the container and a valve to permit liquid solvent to return to the tank via an outlet pipe connected between the container and the tank.

With multiple tanks, each tank has a pump connected to an inlet manifold for the container and each tank has a control valve connected to an outlet manifold for the container; whereby liquid solvent can drain by gravity from the container to the respective tanks. A control system is connected to the pumps and control valves to manage the apparatus and pump appropriate liquid solvent into and permit flow of liquid solvent out of the container. Conveniently, a filter can be provided in the outet fro the container. Also a path-way is provided for vapour to be conveyed fro the container to t cr eacn tan; and f ro the tanks bosk to the vapour generator; thereby to maintain the vapour throughout the system and exclude air. Individual tanks can be provided with heaters so that hot liquid solvent can be pumped to the container; also the container may have its own heaters and ultrasonic generators.

Conveniently, the work to be cleaned is housed in a basket within a container, the basket remains stationary and pumping of the liquid solvent into the container induces considerable agitation which mechanically aids cleaning and penetration of liquid solvent into very tir.ç crevices within a worn. Also, because liquid solvent is being pumped at relatively high speed, contaminants are scoured from the surface of the work; by the aforesaid introduction of filtersinto the system, the liquid solvent can be continuously cleaned of solid particles while the work is immersed in the liquid.

Provision of a distillation facility also means that soluble contaminants can be continuously removed.

The invention is particularly applicable to the electronics industry and has been found effective with cleaning printed circuits boards containing surface mounts devices where the space between the rear surface of the device and the main surface of the printed circuit board iE extremely snarl and wnere contaminants are particularly difficult to remove.

Any number of cleaning stages can be utilized and the whole process is particularly suited to automatic micro-processor control, with pressure and temperature sensors ensuring that each part of the process is maintained within preset parameters to ensure that a given cleaning cycle has been properly adhered to.

Manual operator involvement is minimised, it only being necessary to place a work-filled basket into the container and to remove a basket with cleaned work at the end of the cleaning cycle. This reduces exposure of the operator to the possibility of solvent contamination ana also ensures that the system can be controlled to very precise time schedules.

Automatic handling equipment can be employed to raise and lower the basket into and out of the container and again this is particularly suited to computer control.

The advantages given by the present invention are: (i) single open container reducing solvent losses when compared to conventional multi-tank dip systems; (11) the machine is designed to ensure the maximum of operator safety by reducing solvent losses and reducing the exposure o the operator to solvent; iii; automatic control 0 the ceaning process ensures consistent results; (iv) the design of the system allows a very complicated cleaning cycle or solvent-based process to be carried out automatically and is particularly suitable for integration into modern manufacturing techniques; (v) very efficient cleaning of parts to the highest of standards is simply obtained; (vi) the process is very flexible, any number of tanks can be fitted; (vii) any form of heating can be employed; (viii) any form of cooling can be employed; (ix) combined heating and cooling systems, particularly those used in reverse cycle refrigeration techniques, are very suitable for use with apparatus according to the present invention; (x) any non-flammable halogenated hydrocarbon can be used.

The above and other features of the invention are illustrated, by way of example, in the drawings wherein: Fig. 2 is a diagram of cleaning apparatus in accordance with the present invention; Fig. 3 is a diagram of a simple combined container and vapour generator or use with the present invention; an Figs 4a ana 4 are c;ac-ams of anotner form of vapour generator ane con taine for use with the present invention.

As shown by Fig. 2, an open-topped container 20 has a condense coil 21 to control and prevent vapour from escaping through the open top of the container, a vapour generator 22 is attached to one side of the container 20; the vapour generator comprising a supply of liquid solvent 23 heated by a heater 24 to boiling point, generate vapour flowing over a weir 25 into the container 20.

A series of tanks 26, 27, 28 each have a pump 29 connected to an inlet manifold 30 itself connected by a pipe 31 leading into the bottom of the container 20; the bottom portion of pipe 31 having a series of upwardly directed spray jets 32. Tank 26 is provided with a heater 33. By this means the liquid solvent in either tank 26, tank 27 or tank 28 can be introduced under pressure to the container 20 in an example, tank 26 contained hot liquid solvent, tank 27 cool liquid solvent for rinsing and tank 28 a final rinse cool liquid solvent.

Liquid solvent is removed from container 20 by means of pump, magnetic control valves and gravity flow, or a syphon having an activator powered by electricity, hydraulics or pneumatics, all controlled by a tank control circuit (none shown). Whatever means are used to remove line so vent it ro tne container; an outlet pipe 34, naving an in-line filter 35, is connected to an outlet manifold 36 and thence via control valves 37 to each tank 26, 27, 28.

Solvent vapour produced by generator 22 condenses on the condensing coil 21 and is collected in a condensate channel 38 from where it is directed to a water separator or dryer 39 and, thereafter, flows through line 40 to final rinse tank 28, fro where it overflows to tank 27 and tank 2, as indicated bv the lines and arrows in the figure.

The storage tanks are constructed as a single unit 41 which has a freeboard above the divider between each tank to permit liquid solvent to overflow tftirough the tanks, gradually cleaning the system, and back to the sump 23 or vapour generator 22 by line 42. This direction of flow results in the cleanest liquid solvent being stored in final rinse tank 28, with increasing contamination from tanks 27 and 26 respectively. However the constant flow of liquid solvent gradually cleans the whole system.

The first stage of operation of the apparatus is conventional in that, using a halogenated hydrocarbon solvent, heated vapor is formed which is heavier than air and is at a temperature higher than the average ambient temperature. Parts to be cleaned are at ambient terperature an are placed into the vapour atmosphere in container 20 which allows solent to condense on the surface of the parts; this removes oil, grease, resins and like contaminants that are soluble in the solvent.

This contamination s retained within the sump of container 20. The whole system is periodically cleaned because dissclved contaminants and dirt are removed each cycle through pape 34. As the solvent flow is counter current, i.e, water separator 9 by line 40 to tank 28 and then to tanxe 2, 25 ano f inally returning to the boiling sump 23 of a vapour generater 22; this acts as a still for the system and, as previously mentioned, it is the sump of container 20 which Will need cleaning from time to time.

Typ@eally t sour within container 20 is regulated by means of controi of condensing coils, thermostatic control o neat source 24 with additional air cooling and maltiple temperature sensors (not shown) arranged within the container and integrated into a control circuit.

Tn above solvent condensation process does not remove solid contaminants and immersion into liquid solvent with mechanical or other agitation is employed to remove solid contaminating particles.

The invention is also applicable to other chemical processes using similar apparatus for example phosphating and chromating of metal surfaces and the use of halogenated hydrocarbon based mixtures for stripping paints, plastics and the like.

Once the vapour cleaning stage is finished, hot liquid solvent is then pumped from storage tank 26 (where it has been heated by heater 23) and passes into the bottom of container 20 through inlet pipe 31, which, as described above, is fitted with sprays 32 or perforations, the container is permitted to fill rapidly to immerse the work and the liquid is then permitted to flow continuously for a period of time at high speed over the surface of the parts of the work to be cleaned, introducing scouring and scrubbing thereof. The liquid solvent is recycled through the tank 26 via the pump 29, line 31 and returning via line 34, through the filter 35 and the valve 37.

In order to introduce extra cleaning, container 20 can be evacuated allowing contaminated liquid to drain by gravity fro crevices or holes and the cycle can then be recommenced. This can be carried out any number of times to improve the cleaning cycle. When the cycle fro hot liquid tans. 26 is completed, liquid solvent is then Se 'ectec from the next tank 7 for rinsing or other purposes and then fror tank 28 and so or through. the number of tanks included in the systen.Each tank contains liquid solvent at varlous stages of cleanliness and at temperatures which can be present to meet the requisite conditions.

Ultrasonic transmitters 43 can be fitted to the base of the container 20 or to Its side walls and these can be activated at suitable times by the control system.

1 liquids can ne contin@ously filtered either throucn a central filtration system, such as filter 35 in return pipe 34 described above, or, for example, using a separate filter In the return to each tank.

Once a solvent cleaning cycle has been started, at no time is the work being cleaned exposed to air; unless this is required. The offset vapour generator 22 is generating vapour continuously and even when liquid solvent has been pumped from the container 20 it will remain filled with solvent vapour In many cases, this may be required to prevent oxidation of contaminants between the cleaning or process cycles and it also helps to reduce solvent losses by preventing solvent laden air from being removed from the machine.

In an example for the removal of resin flue from printed circuit boards, an azeotropic mixture of 113 trichlorotriifluoroethane with an alcohol, typically Methanol, (e.c. Du Pont Freon TMS [Trade Mark])was used as a solvent. Typ@cally. a cleaning cycle with such a solvent comprise: @@ lowering cas@es into vapour; (ii) continuously circulation hot liquid solvent from tank 26 to remove most of the flux into solution using the temperature of the liquid solvent and the mechanical action of the moving liquid solvent.When cleaning closely packed surface mount devices, container 20 will be drained and refilled several times from hot liquid solvent tank 26; (lli) on compietion of the hot liq@@@ solvent cleaning cycle, coo rinse liquid solvent is pumpe@ into container 20 fro tank 27; ultrasonic cleanina usIng transmitters 43 can be employed at this stage in the cycle. uo ultrasonic cleaning is employed to remove very small particle trapped in crevices in the printed circuit board or the devices mounted thereon; (ivi a final cleaning rinse using liquid solvent pumped from tank 28 completes the immersion part of the cleaning cycle;; (v) after the final rinse, and the pumping of solvent back into tank 28, the printed circuit boards are at a cool temperature and are allowed to stay in solvent vapour for a final rinse where vapour condenses onto the surf aces of the printed circuit boards giving then a final clean in a completely clean liquid i.e. the recondensed vapour, thereafter the boards are dried by bein@ heated to vapour temperature for the solvent at which p@int condensation ceases.

Fig 3 is a diagram of a container 20 to be used for automatic machine loading of a basket 2 of work to be cleaned. In automatic loading, the freeboard height (ref: FH) need only be low and a simple design of combined container and vapour generator can be employed. Solvent is boiled in the sump 44 of the container by means of a heater 45, generated vapour condensing on the work in the basket 2 and vapour being prevented fromtleaving the container by means of the condense coils 21.

Figs. 4a and 4b show an alternative form, generally similar to that shown in Fig. 2, with a separate vapour generator 22 connected by a weir 25 to the cleaning container 20 and a first set of condense coils 21 near the top of the container.

Extra condense coils 46 are employed near the base or sump of the container 20. When lower condense coil 46 is turned on the vapour height is lowered giving a high freeboard FH. Once the basket has been placed in the machine and a lid (not shown) closed, the lower condense coil is switched off 2Xd vapour will rise past the basket to the height of the usual upper condense coil 21, reducing the freeboard height. As an alternative to condense coils1 temperature sensors can be employed operating to regulate heat input from heaters 24 to the boiling liquid 23 and thus control the upper vapour level.

In a further, unillustrated, embodiment an airlock is fitted above the container 20. The airlock has an outer, sealed, door through which a basket can be passed to be supported by a lowering mechanism. The outer door is closed and an inner, sealed, door system, directly above the container, is opened and the lowering mechanism is operated to lower the basket into the container 20. At the end of the cleaning operation, the basket is returned to the airlock by the lowering mechanism and the inner door system is closed. Solvent laden air is extracted from the airlock by a fan or the like and passed to a solvent absorption system wherein solvent is extracted from the air by a molecular sieve, activated carbon, cold trap or the like. Recovered solvent could be returned to the cleaning apparatus. By this means the cleaning apparatus is wholly sealed from atmosphere and effectively no solvent is permitted to escape from the apparatus into atmosphere for contamination.

The further advantage of such an airlock system for a fully automatic cleaning apparatus is that, as opposed to the high flow air extraction systems required by conventional cleaning apparatus, only the small volume of the airlock needs to be evacuated and this much reduces the size of air extraction plant, solvent absorption cleaner and the like, with consequent cost savings in manufacture and operation of the apparatus.

Claims (1)

1. CLAIMS:

   1. A method of cleaning comprising the steps of: i) loading work to be cleaned into a container, provided with vapour control means about the container top; ii) generating a solvent cleaning vapour to clean the work; iii) introducins a liquid solvent into the container to submerge the work; iv) removing liquid solvent from the container; and, v) unloading the cleaned work from the container.

   2. A method as claimed in claim 1 and comprising the further steps of sequentially repeating steps iii) and iv) of claim 1 for liquid solvent maintained at different temperatures.

   mto A method as claimed in claim I or claim 2, wherein liquid solvent is continuously passed from a storage tank into the container and back to the storage tank.

   4. Cleaning apparatus comprising: i) a container provided with vapour control means about the container top; ii) a solvent vapour generator connected to the container; iii) one or more tanks for liquid solvent; iv) means to convey liquid solvent from the or a given tank to the container; ano, v) means to remove liquid solvent from the container to the or the respective given tank.

   5. Apparatus as claimed in claim 4 wherein the vapour control means is a cooling condense coil about the top of the container.

   6. Apparatus as claimed in claim 4 or claim 5, wherein each tank is provided with pumping means to deliver liquid solvent to the container via an inlet pipe connected between the pumping means and the container and valve means to permit liquid solvent to return to the tank via an outlet pipe connected between the container and the tank.

   kpF,araruc as claimed in claim 8 and-provided with multiple tanks for liquid solvent and means to heat liquid solvent in one or more of the tanks, wherein each tank has a pumping means connected to an inlet manifold for the container and each tank has valve means connected to an outlet manifold for the container.

   8. Apparatus as claimed in claim 6 or claim 7 wherein a filter is provided between the container outlet and the or each tank.

   9. Apparatus as claimed in any of claimed 4 to 8 wherein a channel is provided within the container to collect condensed solvent and a pipe-line is provided to convey condensed solvent to the or one of the tanks.

   10. Apparatus as claimed in any of claims 4 to 9 wherein the top of the container is closed by an airlock, the interior of which is connected to an air extractor feeding a solvent absorption system; airtight doors being provided between the airlock and the interior of the container and between the airlock and atmosphere.

   11. A method of cleaning substantIally as described with reference to Figs. L 2, R and 4b c the drawings.

   12. Apparatus for cleaning substantially as described with reference to or as shown by Fig. 2 or Fig. 3 or Figs. 4a and 4b of the drawings.