WO2004082858A1

WO2004082858A1 - Parts cleaning

Info

Publication number: WO2004082858A1
Application number: PCT/EP2004/003018
Authority: WO
Inventors: Kenneth Stig Lindqvist; Jan Hamrefors
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2003-03-21
Filing date: 2004-03-22
Publication date: 2004-09-30
Anticipated expiration: 2005-09-21

Abstract

The invention relates to a method to introduce an additive into a washing chamber (12) of a C02 dry cleaning system. The additive is transferred from an additive reservoir (18) into a C02 transfer line (11) connecting a C02 storage tank (4) with said washing chamber (12). Gaseous C02 from said C02 storage tank (4) is introduced into said CO2 transfer line (11) and said additive is propelled into said washing chamber (12) by said gaseous C02.

Description

Specification

Parts Cleaning

The invention relates to a method to introduce an additive into a washing chamber of a C0₂ dry cleaning system wherein said additive is transferred from an additive reservoir into a CO₂ transfer line connecting a CO₂ storage tank with said washing chamber.

Dry-cleaning using liquid carbon dioxide is known as an environmentally friendly cleaning technique with favourable cleaning properties which can be used to remove contaminants from garments or textiles as well as from metal, machinery, workpieces or other parts. It is further known that the cleaning performance of carbon dioxide dry- cleaning can be improved by the addition of detergents, surfactants or other additives.

There are several possibilities for the introduction of such additives: For example, the additives can be premixed in CO₂ and stored in high-pressure cylinders. But that requires a complicated manufacturing process and the filling of the cylinders has to be carried out at a specialty gas plant by a person with gas filling experience. Further, there might occur changes in the concentration of the additive in the C0₂ during use of the mixture.

Instead of premixing the additive into CO₂, other carrier fluids, for example gaseous ■ nitrogen, may be used. However, the disadvantages with respect to the filling procedure and the distribution of the mixtures remain the same.

US patent 6,129,451 discloses another method to add a surfactant or a detergent to a carbon dioxide dry cleaning system. The additive is pumped from a supply source into a mixing reservoir having a capacity of 2 iitres. During pumping of the additive the vent valve of the mixing reservoir is open to the atmosphere. Liquid carbon dioxide is then pumped into the reservoir and thoroughly mixed with the detergent. The mixture is transferred to the washing chamber under the force of a liquid carbon dioxide pump and dispersed into the washing chamber. r

A thorough mixing of the additive with the liquid CO₂ requires that the maximum solubility of the additive in CO₂ is known. That means that in the method according to US 6,129,451 the necessary capacity of the mixing reservoir may vary from additive to additive. Further, according to US 6,129,451 the mixing reservoir is open to the atmosphere when the additive is pumped into it, meaning that a low pressure pump can be used which is cheaper than a high pressure pump. However, since in the next process step liquid C0₂ is injected into the mixing reservoir without preceeding pressurisatiαn of the reservoir to a pressure above the triple point of CO₂, there is a certain risk of dry ice formation.

Therefore, it is an object of the invention to provide a method to introduce an additive into a CO₂ dry cleaning system.

This object is achieved by a method to introduce an additive into a washing chamber of a CO₂ dry cleaning system wherein said additive is transferred from an additive reservoir into a C0₂ transfer line connecting a CO₂ storage tank with said washing chamber, wherein gaseous CO₂ from said C0₂ storage tank is introduced into said CO₂ transfer line and wherein said additive is propelled into said washing chamber by said gaseous CO₂.

CO₂ dry cleaning is typically carried out at pressures above 35 bars up to more than 70 bars. The injection of the additives directly into the pressurized washing chamber thus requires a high pressure pump. Therefore, according to a preferred embodiment of the invention during the transfer of the additive into the C0₂ transfer line the pressure within the CO₂ transfer line does not exceed 20 bars, preferably does not exceed 10 bars. It is then possible to use a low pressure pump to pump the additive out of the additive reservoir.

The CO₂ transfer line advantageously comprises a segment of increased diameter, a bulb or a kind of vessel and the additive is fed into that segment or very close to that segment. For example the CO₂ transfer line may comprise a kind of bulb or a vessel with a CO₂ inlet, an additive inlet and an outlet for CO₂ or additive propelled by CO₂. The segment of increased diameter is not aimed for mixing the additive with CO₂, but provides a sufficient volume in order to assure that enough additive can be pumped into the CO₂ transfer line by using a low pressure pump or can be pushed into the CO₂ transfer line by an excess pressure in the additive reservoir. The capacity of that segment or bulb should be high enough that the pressure in it does not exceed a certain maximum pressure, which for example is determined by the maximum pressure of the low pressure pump or the excess pressure in the additive reservoir. It is apparent for the man skilled in the art that the orientation of the bulb can be chosen in several ways. But preferably the bulb is vertically oriented with the inlet for the additive and the C0₂ near or at the bottom of the bulb.

It is further advantageous to depressurize the washing chamber prior to injecting the additive into it. Depressurization means that the pressure is lowered to a value below the excess pressure in the additive reservoir or below the maximum pressure that can be achieved by the low pressure pump which is used to pump the additive from the additive reservoir into the CO₂ transfer line. Due to the pressure difference between the C0₂ transfer line and the washing chamber the additive can then be easily pushed into the washing chamber by gaseous C0₂.

For example, after a wash cycle, the C0₂ transfer line or the bulb contain a pressure of typically 17-20 bar. By opening the C0₂ transfer line to the washing chamber for only a couple of seconds, prior to the injection of the additive into the C0₂ transfer line or the bulb, it is assured that the bulb and the C0₂ transfer line are ending up at ambient or even lower pressure, depending on the pressure within the washing chamber.

In a preferred embodiment the washing chamber does not contain liquid C0₂ during the injection of the additive into the washing chamber. In that case first the additives are ^■ blown into the washing chamber by gaseous C0₂ and then liquid CO₂ is introduced into the chamber. Prior to entering liquid C0₂ the C0₂ transfer line, all additives should be pushed out of the transfer line in order to avoid freezing of the additives which might lead to clogging of the CO₂ transfer line.

But the invention can also be used when the washing chamber is already filled with liquid CO₂ and there is only need to add detergents or when the washing chamber is partly filled with liquid CO₂. In this case the pressure in the washing chamber must of course not exceed the pressure of the gas that is used to propel the additive into the washing chamber, for example the pressure in the washing chamber must not exceed . the pressure available in the CO₂ storage tank. Preferably the gaseous CO₂ used as propellant is withdrawn from the head space of. the liquid C0₂ storage tank which is designed for replacing losses during the washing cycle with liquid C0₂. Since the CO₂ gas reserve in the CO₂ liquid storage tank is often . small, the pressurization of the washing chamber to a pressure above the triple point is preferably not made by using gas from that storage tank, but the washing machine will have to provide gas that can be used to pressurize the washing chamber.

Instead of the liquid CO₂ storage tank which is designed for replacing losses during the washing cycle with liquid C0₂ and which has a pressure of about 17 to 20 bar, it is also possible to use gas from the internal high pressure working tank of the dry-cleaning machine as propellant.

Preferably, there is no deliberate mixing of the additive and the C0₂ prior to entering the washing chamber. Of course when the gaseous CO₂ propels the additive into the washing chamber some mixing of CO₂ and additive occurs. But according to the invention the major part of the mixing takes place in the washing chamber. For that reason it is advantageous to have a rotating drum or a rotating basket within the washing chamber which creates agitation of the liquid CO₂.

The agitation in the washing chamber could be improved^' by the use of baffles or other agitating means, preferably located on the outside of the basket. Flexible baffles, for example made of teflon, or baffles designed like brushes may be used.

It is preferred to spray the additive through a nozzle into the washing chamber, that is through an outlet that represents a restriction compared to the overall diameter of the C0₂ transfer line. Such a nozzle should be designed for the additive injection as well as for the transfer of liquid C0₂ into the washing chamber. To avoid the risk of clogging, for example by particles of frozen additive, it might also be advantageous to use a CO₂ transfer line with a straight tube end without any restriction in diameter.

Preferably the outlet of the CO₂ transfer line, which is for example provided with a nozzle, is designed to flow rates of 5 to 20 litres/minute of additive, preferably of flow rates between 10 and 15 litres/minute. A reasonable time to spray the required amount of additive into the washing chamber would be less than 10 seconds. Preferably the injection time corresponds to the amount of detergent that shall be injected for each given wash situation. In that case a certain injection time and thus the amount injected result in a desired detergent concentration in the actual bath.

As mentioned above it is preferred to have a rotating basket perforated with holes within the washing chamber. The distance between the outer surface of the rotating drum and the inner walls of the washing chamber is typically about 20 to 50 mm. It has been found that the additive should be sprayed onto the outside of the rotating drum in order to thoroughly mix the additive and the liquid CO₂. Otherwise the additive might be introduced into some dead ends within the washing chamber which will never be reached by liquid C0₂, or where agitation will not be sufficient to mix the detergent with the C0₂.

The position of the spray nozzle respectively the CO₂ transfer outlet which is used to introduce the additive into the washing chamber should be optimized. It has been found that the degree of mixing of the additive and the liquid C0₂ within the washing chamber depends on several parameters, for example the spraying angle, the position of the additive outlet, the tubing sizes and the design of the spray nozzle. Advantageously the spraying direction should be perpendicular to the basket surface and the spraying point, that is the additive outfet, should be located in the lower half of the basket.

Preferably the additive is sprayed at a four or five o'clock position onto the basket if the basket is rotated clockwise.

In case a motor to rotate the basket is located within the washing chamber it is preferred to provide the additive outlet with a kind of hood to avoid spraying possibly harmful additives onto the motor.

In one embodiment of the invention there is only one additive reservoir which contains a mixture of detergent, anti-static agent and perfume. But it is also possible to have different reservoirs for different additives. For example, when cleaning the objects in a two-bath cycle with a wash bath and a rinse bath, it is advantageous to use only detergents in the wash bath and different additives,- e.g. perfume, anti-static, antibacterial agents, anti-flammables or impregnating/water-proof agents in the rinse bath. Independent of the cleaning method the additive must be soluble in CO₂, must not be hazardous and the detergent should have good cleaning properties. Further the additives should preferably be in the liquid state at room temperature and should not become solid when in contact with CO₂ at temperatures of about -15°C.

The invention is in particular useful to transfer an additive into the washing chamber and to spray the parts to be cleaned with that additive before liquid carbon dioxide is filled into the washing chamber.

According to this embodiment an additive or a cleaning fluid is sprayed over the parts placed in the washing chamber of the washing machine. The additive fluid, which is in particular a detergent or a co-solvent, does not only take away some dirt or impurities from the parts, but also wets the surface of the parts as a concentrate. Thus all the strength of the additive fluid is used to brake up or dissolve the impurities. The dirt removed from the parts is then in general transferred to the still. The washing chamber is partly or completely filled with liquid carbon dioxide and the parts are cleaned in that liquid carbon dioxide bath which may also contain some additional chemicals.

By removing as much dirt as possible in this inventive manner, that is by spraying with . an additive, especially a cleaning fluid, the consumption of detergent in the subsequent cleaning step in the liquid carbon dioxide bath is reduced. Further the re-deposit of dirt which has already been removed from other parts is minimized. A lower dirt concentration is reached at an earlier stage of the cleaning process which improves the overall cleaning result.

This pre-cleaning is preferably useful in cleaning parts or components of glass, metal, plastics, silicone and electronic components of different shape and condition. In cleaning garments the amount of detergent used will normally be so low that if the detergent is sprayed as a concentrate on the garment it will be uneven distributed over the surface, and this might lead to an uneven cleaning result. But in certain cases the inventive pre-cleaning may also be useful in cleaning garments or textiles.

It has been proven advantageous to spray liquid carbon dioxide onto the parts before filling the washing chamber with liquid carbon dioxide. That pre-cleaning step removes dirt and particles from the parts in an easy way. Thus it is assured that less dirt is in the washing chamber when it comes to the subsequent cleaning step in the liquid carbon dioxide bath.

The cleaning performance can further be improved by rotating or moving the parts to be cleaned relative to the jet spray of the cleaning fluid. In that respect it is advantageous to provide a rotatable basket within the washing chamber and to place the parts into that basket. When the cleaning fluid is injected into the washing chamber the basket and consequently the parts within the basket are rotated in order to subject all surfaces of the parts to the cleaning fluid.

Instead of or in addition to rotating the basket it is also possible to rotate or move the nozzle which is used to spray the cleaning fluid over the parts. Further it is preferable to provide several nozzles or outlets for the cleaning fluid arranged in different positions relative to the washing chamber. The cleaning fluid is then sprayed from different directions over the parts. It is in particular advantageous to use only one nozzle at one time and to switch between different nozzles.

This pre-washing or pre-spotting step significantly improves the overall cleaning performance. The pre-washing step is not only useful in a one-bath-cleaning process but also in a two-bath process. In the two-bath process the parts are subsequently cleaned in a first bath and a second bath of liquid carbon dioxide with draining of the liquid carbon dioxide between the two steps. As described above prior to the cleaning in the first bath the parts can be sprayed with that cleaning fluid. Then, a first cleaning step in liquid carbon dioxide, with possible additives, is carried out. After that first cleaning step the liquid carbon dioxide is drained from the washing chamber. Before the washing chamber is re-filled with liquid carbon dioxide in order to perform the second cleaning step, a cleaning fluid, preferably a chemical, is sprayed over the parts. Of course when more than two baths are used the inventive injection of an additional cleaning fluid can be carried out between any of the cleaning steps in those baths.

After spraying of the cleaning fluid over the parts there is in general not only liquid carbon dioxide but also a detergent or another additive introduced into the washing chamber. Such additives are preferably pumped into the washing chamber by means of a high pressure pump. The washing chamber is filled with. liquid carbon dioxide until a predetermined level is reached. Then a high pressure pump is started and the additive is injected into the liquid carbon dioxide.

The amount of additive can be reduced if the liquid carbon dioxide is stirred within the washing chamber, for example by a propeller, jet streams, rotation of a basket or ultrasonic agitation. The additive is thus easier diluted in the liquid carbon dioxide and is faster and even distributed to all parts in the washing chamber. The contact between concentrated particles of additive and the parts is also prevented which otherwise could harm the surface of the parts. Preferably the liquid carbon dioxide is stirred during, the injection of the additive and during the following cleaning operation. Thus the additive will not sink down to the bottom of the washing chamber and the additive can be utilised to a high degree. The dilution of the additive in the liquid carbon dioxide can be further improved by choosing a proper location of the nozzle which is used to spray the additive into the washing chamber. If the parts are sensitive to the concentrated additive the additive could be directly sprayed into liquid C0 in the washing chamber in order to be diluted as fast as possible.

It has been found that even if the basket rotates when the bath of liquid carbon dioxide is drained from the washing chamber to the still, dirt or particles could be left on the surface of the parts, in particular when the parts are of complicated shape. Therefore, it is preferred to add a washing step wherein the parts are washed with pure liquid carbon dioxide. The parts could either be sprayed with pure liquid carbon dioxide or the washing chamber is re-filled with liquid carbon dioxide.

Further it has been found advantageous to clean the inner surface of the washing chamber in the same manner, that is by spraying liquid CO₂ on it. A preferable procedure would be to drain the liquid carbon dioxide out from the washing chamber . and at the same time to clean the washing chamber by spraying liquid C0₂ onto the inner surfaces of the washing chamber. Then the parts are sprayed with pure liquid CO₂ with simultaneous draining of any overflow of liquid CO₂. Finally the washing chamber is de-pressurized and the parts are unloaded.

Different types of chemicals can be added to the parts after the cleaning steps in the liquid carbon dioxide bath. The chemicals can be injected into the liquid carbon dioxide bath or sprayed on the surface of the parts after the liquid carbon dioxide is drained from the washing chamber.

If the additional chemicals are injected into liquid carbon dioxide it is preferred to have less liquid carbon dioxide in the washing chamber than during the cleaning step. But the amount of liquid carbon dioxide should be sufficient to reach all parts cleaned in the washing chamber.

Preferably chemicals like lubricants, impregnating or anti-static agents are added. It is an advantage if the chemicals have a poor solubility in liquid carbon dioxide since then a higher percentage of the chemicals will add to the parts. These chemicals shall either be added to the surface of the parts or wet through the material of the parts. In general it is important that the chemical is more or less equally distributed on the surface of the parts or in the material. If a chemical has a good solubility in liquid carbon dioxide it might be worth to re-use the chemical by using an additional pressure vessel in which the mixture of liquid carbon dioxide and the chemical is stored from one cleaning cycle to another.

Normally the parts are below room temperature when they are taken out of the washing chamber. There is a certain risk that water could condense at the surface of the parts and damage the parts. This is preferably avoided by heating the parts when they are still in the washing chamber.

That heating can be carried out by an external heating system. For example, after the cleaning step in the liquid carbon dioxide bath the washing chamber is emptied and de- pressurized. When the pressure reaches a certain value, for example 10 bar, gaseous carbon dioxide from the washing chamber is externally heated, if needed filtered and then returned into the washing chamber in order to heat up the parts. Instead of heating gaseous carbon dioxide from the washing chamber it is also possible to transfer another hot dry gas into the washing chamber. That gas could be nitrogen, compressed air or any other suitable gas.

Further, the gas in the washing chamber and thus the parts can be heated by indirect heat exchange with a hot medium. A hot fluid, for example water or steam, is transferred into a heat exchanger located within the washing chamber. The hot medium can be circulated through the heat exchanger or pass it only once.

The parts can also be heated by an internal heating system. Gas is taken from the cleaning machine, for example from the still, and compressed in order to increase its temperature and then transferred into the washing chamber or through a heat exchanger placed in the washing chamber. The gas can also be taken from the washing chamber during the de-pressurization phase. Then additional heat is added to the carbon dioxide gas before passing it through a heat exchanger in the washing chamber. The additional heat added to the gas can for example result from a compressor and/or by passing the gas through a heat exchanger in which heat from an external source is added to the gas. The gas could also be taken from the still and in the same way as above be used to heat the parts in the washing chamber.

The parts have normally a positive or negative charge when they are unloaded from the washing chamber after the cleaning process. Thus particles or dirt can easily contaminate the surface of the parts. A cost-efficient way to remove the static charge from the parts is to purge the washing chamber by a ionised gas after the washing chamber has been de-pressurized. The ionised gas, for example air, is injected by nozzles or in any other suitable way. The ionised gas does not only remove the static charge but also takes away particles from the parts which may have been collected during the cleaning process.

A preferred cleaning cycle would comprise the following steps: 1. loading the parts into the washing chamber

2. cleaning of the washing chamber with liquid carbon dioxide

3. Pre-washing the parts with a spray of chemicals

4. Vacuum pumping or purging the washing chamber with gaseous carbon dioxide

5. Pressurisation of the washing chamber with gaseous carbon dioxide to a pressure above about.5 bar

6. Transfer of liquid carbon dioxide into the washing chamber in order to compensate for the losses in the washing machine

7. pressurisation of the washing chamber, for example to a pressure of about 17 -bar

8. pre-washing the parts with liquid carbon dioxide and / or chemicals 9. fill-up of the washing chamber with liquid carbon dioxide 10. feeding of chemicals into the washing chamber

11. cleaning the parts in the bath of liquid carbon dioxide

12. draining of the liquid carbon dioxide from the washing chamber

13. post-cleaning the parts with pure liquid carbon dioxide

14. post-treatment with chemicals

15. de-pressurisation of the washing chamber

16. heating of the parts

17. de-ionization of the parts

18. unloading the parts

It will be apparent to one skilled in the art that there are many ways to combine the specific cleaning steps mentioned above and that some of the above steps, in ^"particular steps 2, 3, 8, 10, 13, 14, 16 and 17, are only optional and may be omitted in certain cases.

The invention as well as further details and preferred embodiments of the invention are disclosed in the following description and illustrated in the accompanying drawings, in which

figure 1 is a schematic drawing of the inventive detergent injection system, and figures 2 and 3 show preferred embodiments.

The C0₂ dry cleaning system shown in the figure is of modular design. It comprises a supply unit 1 , a detergent injection system according to the invention 2 and the washing machine 3. Each of the modules 1 , 2, 3 essentially only contains equipment belonging to its own function.

Supply unit 1 includes all parts of the system which are necessary for the transfer of CO₂, in gaseous as well as in liquid form. For sake of simplicity in the figure only a CO₂ storage vessel 4 is shown and all necessary safety features have been omitted in module 1. For example module 1 has to contain safety features that will protect module 1 , which is a low pressure unit, from back flow from module 3, which is a high pressure unit. Storage vessel 4 contains C0₂ in liquid phase 5 and in the gaseous state 6. A C0₂ gas transfer tube .7 is connected to the head space 6 of storage vessel 4. The flow of CO₂ gas can be controlled by means of gas valve 8. Liquid CO₂ can be withdrawn from . storage vessel 1 via liquid transfer tube 9 which is provided with valve 10. Of course, the valves 8 and 10 could be replaced by a 3-way valve that can be turned between gas and liquid inlet, thus providing gas or liquid on its outlet to transfer line 11.

C0₂ transfer line 11 connects gas transfer tube 7 as well as liquid transfer tube 9 with the washing chamber 12 of the washing machine 3. Within a portion 13 the inner diameter of transfer line 11 is increased to form a kind of bulb 13 with a volume of about one litre. Washing chamber 12 comprises a^'perforated basket 14 which is rotated in a clockwise direction. The end of C0₂ transfer line 11 is provided with a spray nozzle 15 which is directed to the outside of the perforated basket 14. Spray nozzle 15 is located in the lower half of rotating basket 14 and the angle between the vertical and the spray direction is between 30° and 60°, preferably about 45°. The spray direction is perpendicular to the outside of basket 14. Upstream of spray nozzle 15 transfer line 11 is provided with a valve 16 which is part of the washing machine and is a high pressure ball valve, controlled and actuated by the washing machine^'.

A three-way-valve 17 is interposed in transfer line 11 between C0₂ storage vessel 4 and bulb 13. An additive reservoir 18 contains a detergent or a mixture of detergents, anti-static and anti-bacterial agents and perfume. Three-way-valve 17 interconnects with the additive reservoir 18 by means of an additive transfer tube 19 and a hose 20. A low pressure pump 21 is interposed between the additive reservoir 18 and hose 20.

Further, additive transfer tube 19 or hose 20 comprise a safety relief valve 23 and a check valve 24 which are designed to handle a back pressure burst from the washing chamber 12 which might occur when 3-way valve 17 is in a position where it connects hose 20 with bulb 13 or if it risks leaking between hose 20 and bulb 13 independently of its position. Additional safety relief valves of appropriate size and design may need to be added. Also, if the pressure in the system downstream the low pressure pump 21 is too high, i.e. if the pressure in high pressure hose 20 is too high for the low pressure pump 21 to overcome, the additives will be able to flow back through a by-pass line 25 situated parallel to the low pressure pump 21. The sequence of operation of the detergent injection system is as follows: The objects to be cleaned, for example garments or parts, are loaded into the basket 14 of the washing machine 3. Then the door of the washing chamber 12 is closed.

Three-way-valve 17 is turned to open the way from the additive reservoir 18 to transfer line 11.

After the previous wash cycle, the bulb 13 and transfer line 11 contain a pressure of typically 17-20 bar, because the transfer that has. been made during that previous cycle left said pressure in the system: Either, a full transfer was made with injection of detergent into the washing chamber, followed by liquid CO₂ transfer from tank 4 and followed by purging of the lines with gaseous CO₂ with a pressure of 17-20 bar. Or only a detergent injection took place, for example if the machine did not need additional C0₂, which also left 17-20 bar in bulb 13.

By opening the valve 16 for only a couple of seconds, prior to the injection of additives into the bulb 13, it is assured that the bulb 13 and transfer line 11 are vented into the washing chamber 12, thus ending up at ambient or even lower pressure, given that the washing chamber 12 is at ambient pressure or has been evacuated.

Then valve 16 is closed and detergent is injected into the bulb 13. Low pressure pump 21 is started and additive is pumped from the additive reservoir 18 into transfer line 11 and in particular into bulb 13. Prior to starting pump 21 as well as during the pumping procedure the pressure in transfer line 11 is controlled by means of a pressure indicator 22. When the correct amount of additive is pumped into the bulb 13 low pressure pump 21 is stopped. The pressure increase measured by the pressure indicator 22 is directly related to the amount of additive pumped into the bulb 13 and thus can be used to determine the amount of additive in the bulb 13.

But it is also possible to use a liquid level indicator within the additive reservoir 18 or a flowmeter in order to measure the amount of additive transferred into transfer line 11. Finally, knowing the flowrate of the pump 21 or the volume per pump strike, if a piston pump is used, the amount of additives injected could be determined by measuring the pumping time or by counting the number of pump strikes. During the injection of additives into the bulb 13, the pressure within the bulb 13 must be below the maximum pressure of the low pressure pump 21. Further, in order to save time it is advantageous to evacuate and then pressurize washing chamber 12 during additive injection into bulb 13. Valve 16 is closed during that period. The pressurization of the washing chamber 12 should be made to above the triple point of C02, but not higher than the pressure available from the storage tank 4 or whatever gas source is used for propelling the additives.

Next, three-way-valve 17 is turned to open the connection between the liquid CO₂ storage vessel 4 and transfer line 11. Valve 10 is closed and valve 8 is opened.

Gaseous CO₂ flows from the head space 6 of storage vessel 4 into transfer line 11.

Then washing chamber valve 16 is opened for about 10 seconds. By means of the ^~ gaseous CO₂ the additive is propelled through transfer line 11 into the washing chamber 12 and sprayed on the outer surface of rotating basket 14. When the additive transfer is completed washing chamber valve 16 is closed.

Supply unit 1 is then used to transfer liquid CO₂ into washing chamber 12, but only the amount of CO₂ corresponding to losses due to venting after the previous wash cycle(s). Therefore, gas valve 8 is closed and valves 10 and 16 are opened. Liquid C0₂ is pushed by the over pressure in head space 6 of C0₂ storage vessel 4 through transfer line 11 into washing chamber 12. When enough C0₂ to make up for losses has been transferred into the washing chamber 12 from storage tank 4, the rest of the C0₂ needed for washing is filled into the washing chamber 12 from the internal high pressure storage tank of the washing machine itself. When there is enough liquid C0₂ . in the washing chamber 12, the cleaning process can be started.

As described above, the amount of liquid CO taken from tank 4 is only aimed to make up for losses during the previous wash cycle(s). The rest of the liquid CO₂ which is required for the bath is taken from the internal working tank (not shown in the figure) of the washing machine.

It is obvious for a person skilled in the art that several parts of the equipment used with the invention are preferably controlled by any kind of electrical or pneumatical means. For example, valves 8, 10, 16 and 17 and pump 21 may be controlled by a control unit. Figure 2 shows an embodiment where the additive transferred to the washing chamber 101 is used for pre-cleaning the parts to be cleaned.

The additive is transferred to the washing chamber 101 as follows: Chemical supply 102 with chemical 103 is not directly connected to the washing chamber 101 but to a chemical cylinder 121. By means of pump 104 a portion of the chemical 103 is pumped from chemical supply 102 into chemical cylinder 121. Chemical cylinder 121 is then pressurized by gaseous carbon dioxide via gas line 122. It is also possible to omit pump 104 and to first de-pressurize chemical cylinder 121 and to use the pressure difference between the chemical supply 102 and the chemical cylinder 121 to transfer the chemical 103 into chemical cylinder 121.

The pressure difference between chemical cylinder 121 and washing chamber 101 is used to push chemical 103 via line 123 to washing chamber 101. At the end of line 123 a closed circular pipeline 124 is provided which comprises several nozzles 125. Each nozzle 125 is provided with a valve 126. The chemical 103 is sprayed over the parts to be cleaned by switching between the nozzles 125. Thus it is possible to spray different surfaces of the parts.

Instead of switching between different nozzles it is also possible that the parts within the washing chamber 101 are rotated, for example by placing the parts into a rotatable basket within the washing chamber 101. The rotation of the parts is carried out at the same time when the chemical 103 is sprayed into the washing chamber 101. The chemical 103 wets the surface of the parts and removes impurities from the parts.

The chemical 103 is subjected to the parts for a certain time dependent on the desired . quality of purity and the status of the parts. Then washing chamber 101 is pressurized with gaseous carbon dioxide and partly or completely filled with liquid carbon dioxide. Additional chemicals may also be added at this stage. Then the parts are cleaned in the liquid carbon dioxide bath. When the cleaning operation is finished the liquid carbon dioxide is drained from washing chamber 101 via a bottom filter 107 into still 108. Between the filter 107 and the still 108 a valve 109 is provided which can be used to either keep the chemicals 103 under the whole cleaning cycle or to dump the chemicals 103 to the still 108 before liquid C0₂ is filled into the washing chamber 101. Figure 3 shows another embodiment of the invention. Chemical 103 is pumped into a chemical vessel 131 by means of a pump 104. Then valves 134 and 135 are opened. Line 140 is connected to a gaseous carbon dioxide supply, for example gaseous carbon dioxide is taken from the washing machine. Valve 141 is put in the open position. Valve 142 is closed at this stage. Gaseous carbon dioxide under pressure flows from line 140 via line 143 into the chemical vessel 131 and pressurizes chemical, vessel 131. Thus the chemical 103 is pushed from chemical vessel 131 via line 137 and nozzle 106 into washing chamber 101.

When the spraying is finished the pressure in chemical vessel 131 is equalized with the pressure in washing chamber 101. For that reason valve 141 is closed and valve 142 opened. After equalizing the pressures washing chamber 101 is drained into chemical vessel 131 by opening valve 136. Any dirt drained from washing chamber 101 into chemical vessel 131 which is heavier than the chemical 103 is collected in the bottom portion 139 of chemical vessel 131 and taken out as waste 132.

In another alternative not shown in the figures two chemical vessels are provided, one to supply chemical for spraying over the parts, the other to collect used chemical.

Instead of or in addition to pre-spotting the parts with a chemical, for example a detergent, it is also advantageous to spray the parts with liquid carbon dioxide prior to cleaning them in a bath of liquid carbon dioxide.

Claims

1. Method to introduce an additive into a washing chamber of a C0₂ dry cleaning system wherein said additive is transferred from an additive reservoir into a CO₂ transfer line connecting a C0₂ storage tank with said washing chamber, characterized in that gaseous CO₂ from said CO₂ storage tank (4) is introduced into said CO₂ transfer line (11) and that said additive is propelled into said washing chamber (12, 101 ) by said gaseous C0₂.

2. Method according to claim 1 , characterized in that during said transfer of said additive into said C0₂ transfer line (11) the pressure within said C0₂ transfer line

(11 ) does not exceed 20 bars, preferably does not exceed 10 bars.

3. Method according to any of claims 1 or 2, characterized in that said C0₂ transfer line (11) comprises a segment (13, 121 , 131) of increased diameter and that said additive is transferred into said segment (13, 121 , 131).

4. Method for cleaning parts in a washing chamber according to any of claims 1 to 3, characterized in that said additive is sprayed on said parts before said washing chamber (12, 101) is at least partly filled with liquid carbon dioxide.

5. Method according to claim 4 characterized in that said parts are sprayed with a chemical (103), in particular with a detergent or a co-solvent.

6. Method according to any of claims 4 or 5 characterized in that said parts are rotated during said spraying of said additive (103).

7. Method according to any of claims 4 to 6 characterized in that said additive (103) is subsequently sprayed through several nozzles (125).

8. Method according to any of claims 4 to 7 characterized in that a first and a second cleaning operation in liquid carbon dioxide is carried out and that prior to said first and / or said second cleaning operation said parts are sprayed with said additive (103).

. Method according to any of claims 1 to 8 characterized in that after said parts have been cleaned in said liquid carbon dioxide said parts are once more cleaned in pure liquid carbon dioxide.

10. Method according to any of claims 1.to 9 characterized in that said washing chamber (101) is sprayed with liquid carbon dioxide.

11. Method according to any of claims 1 to 10 characterized in that after said parts have been cleaned in said liquid carbon dioxide an additional chemical, in particular an impregnating or anti-static agent or a lubricant, is added to said parts.

12. Method according to any of claims 1 to 11 characterized in that said parts are heated before they are unloaded from said washing chamber (101 ).

13. Method according to any of claims 1 to 12 characterized in that an ionised gas is introduced into said washing chamber (101) before said parts are unloaded from said washing chamber (101 ).