US3460990A - Method for cleaning objects with solvent - Google Patents

Description

Aug. 12, 1969 D. J. BARDAY 3,

METHOD FOR CLEANING OBJECTS WITH SOLVENT Original Filed Oct. 12, 1964 INVENTOR D0167, J fiacrciagy ATTORNEYS United States Patent U.S. Cl. 13431 6 Claims ABSTRACT OF THE DISCLOSURE A degreasing method in which the more volatile component of a solvent liquid mixture is selectively flash evaporated from the heat emitting section of a heat pump to generate vapor to rinse the objects washed in the solvent liquid mixture. A blanket of air vapor mixture is maintained at the top of a tank above a saturated vapor zone in turn above the solvent liquid. The air vapor blanket is maintained by continuously withdrawing some of it, condensing solvent vapor from it in the heat absorbing section of the heat pump and then returning the remaining air vapor mixture to the blanket. The condensed solvent is utilized for rinsing.

CROSS-REFERENCE TO RELATED APPLICATION This application is a division of copending application S.N. 403,142, filed Oct. 12, 1964 by this same inventor, now Patent No. 3,308,839 dated Mar. 14, 1967.

BACKGROUND OF THE INVENTION This invention relates to a method for cleaning objects by contacting them with a fluid solvent and, it more particularly relates to a vapor degreasing type of such method.

Pre-existing vapor degreasers drastically heat the liquid solvent in the wash tank to generate a vapor zone above it. This introduces contaminants into the vapor zone and promotes vapor losses, which become significant when more expensive solvents are utilized. This solvent loss is particularly serious with respect to the fiuorinated hydrocarbons and the more volatile chlorinated hydrocarbons and which have quite advantageous cleaning properties. They leave little or no residue on cleaned parts upon evaporation and are not injurious to plastics, code markings, electrical insulating materials, and the like, but they are relatively expensive. This makes it uneconomical to tolerate the normal losses occurring in industrial cleaning from vapor losses and solvent contamination. Such fiuorinated hydrocarbon solvents are trichloromonofluoromethane, trichlorotrifiuoroethane, tetrachlorodifiuoroethane, or any mixture of them. Such chlorinated hydrocarbon solvents are methyl chloroform, methylene chloride, carbon tetrachloride, or any mixture of them and the fluorinated hydrocarbon solvents. Other advantageous mixtures may contain alcohol or a relatively non-volatile chlorinated hydrocarbon such as trichloroethylene.

An object of this invention is to provide an eflicient, simple and economical method for cleaning objects by contacting them with a fluid solvent.

Another object is to provide such a method of the vapor degreasing type.

A further object is to provide such a method that minimizes solvent losses and makes it possible to economically utilize the efficient, more volatile and expensive cleaning solvents.

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SUMMARY In accordance with this invention, the vapor zone in a vapor degreaser is maintained by flash evaporating the solvent liquid from a fine spray at temperatures below the normal boiling temperature of a body of the liquid. This minimizes the carryover of contaminants from liquid into the vapor, facilitates dense saturation of the vapor zone and makes it possible to selectively evaporate the more volatile from a mixture of solvents. A more volatile and expensive solvent can thus be mixed with a less volatile and more economical wash solvent and be selectively utilized in the vapor zone and for rinsing in a single chamber unit.

The vapor zone may be effectively maintained by continuously flooding and withdrawing the generated vapor from the area above the liquid. This may be efficiently performed by evaporating a spray of wash solvent upon the heat emitting section of a heat pumping system, and by directing the withdrawn vapor over the heat absorbing section of the same system to condense the liquid from it. This productively utilizes all of the energy in the heat pumping system and provides a supply of pure solvent for rinsing purposes. The heat pumping type of vapor generating, solvent recovering and purifying system is of the type described and claimed in US. Patent 3,070,463 by this same inventor.

When a mixture of solvent of differing volatility is utilized, the more volatile component can be selectively evaporated into the vapor. This provides maximum cleaning and rinsing efiiciency in the vapor zone with a relatively small amount of more volatile and expensive solvent. Rinsing with relatively cool solvent also cools the objects below the temperature of the saturated vapor zone to condense solvent liquid upon the objects which supplements the rinsing spray.

A vapor barrier or blanket of unsaturated air may be maintained over the saturated vapor zone to isolate it from the ambient atmosphere and to abstract any drops of solvent clinging to the objects. This air blanket may be maintained by continuously withdrawing air and any absorbed vapor from above the saturated vapor zone, cooling it to condense any solvent vapor in it and returning the dry air above the vapor zone. In an apparatus open to atmosphere the dry air is returned over it at a temperature cooler than atmosphere to form a dense blanket. The rinsed objects from the vapor zone are thus completely dried in the unsaturated vapor air barrier or blanket before leaving the apparatus and all solvent is retained Within the apparatus. When the temperature of the vapor zone is higher than that of the vapor barrier or blanket, the heat absorbed from the vapor zone in the vapor barrier further unsaturates it and improves its drying efiiciency. The air withdrawn from above the vapor zone may also be advantageously cooled by the heat absorbing section of a heat pumping system and the abstracted condensate utilized for rinsing. The unsaturated vapor barrier may be condensed separately or in conjunction with the saturated vapor zone.

The heat balance of the heat pumping system may be maintained by an auxiliary cooling section for helping condense the vapor because more heat is emitted by a heat pump than is absorbed.

BRIEF DESCRIPTION OF THE DRAWINGS Novel features and advantages of the present invention will become apparent to one skilled in the art from a reading of the following description in conjunction with the accompanying drawing wherein similar reference characters refer to similar parts and in which the single figure is a diagrammatic cross-sectional view in elevation of a vapor degreasing apparatus that is one embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the figure, pump 16 draws dirty wash solvent from the bottom of tank 12 of vapor degreasing cleaning apparatus 14 through filter 18. Pump 16 discharges into solvent heating and evaporating chamber 20 by spraying solvent through perforated pipe 22. Solvent evaporating chamber 20 contains the heat emitting element 24 of a heat pumping system 26 which in cludes a refrigerant compressor 28 and a heat absorbing element 30. These elements together with an expansion valve 32 and a refrigerant receiver 34 are connected in a closed heat pumping system by piping 36. Heat emitting or rejection section 24 is, for example, the refrigerant condensing coil of a heat pumping system 26; and heat absorbing section 30 is, for example, the refrigerant evaporating coil.

Heat absorbing element 30 is enclosed within a solvent condensing chamber 38, and a vapor conduit 40 connects solvent condensing chamber 38 to a cleaning machine tank 12. Solvent evaporating chamber 20, on the opposite side of cleaning machine tank 12, is connected to tank 12 by conduit 41. The outlet from conduit 41 and the inlet to conduit 40 are at about the same elevation. The elevation controls the height of the saturated vapor zone 66 in tank 12.

Vapor produced in solvent evaporating chamber 20 is conducted by conduit 41 into cleaning machine tank 12 by distribution header 45 (laterally extended but not so shown). As saturated vapor in cleaning machine tank 12 rises to the level of vapor collecting header 47 (also laterally extended but not so shown), it spills into collecting header 47 and is conducted by conduit 40 into solvent condensing chamber 38.

Blower fan 48 induces a flow of saturated vapor into collecting header 47, and, at the same time, induces flow of a vapor-air mixture 64 from above saturated vapor zone 66. The resultant mixture of saturated vapor and vapor-air mixture is conducted by conduit 40 into solvent condensing chamber 38. Heat absorbing element 30 converts most of the vapor entering solvent condensing chamber 38 to pure liquid solvent by cooling the vapor to about 35 F. Blower fan 48 draws air, unsaturated with vapor into conduit 49 and discharges a relatively cool mixture thereof into cleaning machine tank 12 through distribution header 50 (also laterally extended but not so shown) at an elevation higher than the saturated vapor zone. A damper 51 in conduit 49 provides a means for fine adjustment of air flow back into tank 12.

The mixture is cooler than the ambient atmosphere. Thus a relatively cool and relatively dense layer of air and vapor forms a blanket 64 over the saturated vapor 66 in the machine. Blower fan 48 and distribution header 50 are designed to provide a relatively slow exit velocity of air and vapor mixture. This assures its stratification and substantially prevents the intermixing of outside air and the resultant escape of solvent vapor from the top of tank 12.

Although the air returned to tank 12 through conduit 49 and distribution header St) is saturated with solvent vapor at about 35 F., the air vapor blanket 64 itself is somewhat above 35 F. or approximately 50 F. to 60 F. Therefore, blanket 64 is not saturated with vapor but is only 60 to 70% saturated. Heat conducted into blanket 64 from the relatively warm vapor zone 66 below, the relatively warm air above and the sides of the cleaning machine account for some temperature increase and a corresponding reduction in saturation value or relative humidity. Since blanket 64 is not saturated with solvent vapor, warm parts emerging from the machine and passing through it dry promptly by evaporation of liquid from their surface.

Three factors contribute to rapid drying of parts in the air-vapor barrier zone 64. (1) Parts passing through the saturated vapor zone '66, below, are warmed by vapor condensing on the part in final rinse operation. (2) The air-vapor blanket is not saturated with solvent vapor and has the capacity to absorb additional vapor; and (3) The vapor rinse solvent is relatively volatile and has a characteristically high evaporation rate.

Solvent system cycle of operation of figure Compressor 28 and pumps 16 and 46 are started. Blower fan 48 is started. Pump 46 withdraws pure reclaimed solvent from storage tank 62 and forces the liquid through rinse spray nozzles 53. Cleaning machine tank 12 may either be empty or may contain an operating charge of relatively non-volatile wash solvent. In the first case, where a homogeneous precision solvent is used alone, liquid discharged from spray nozzles 53 enters the inlet to pump 16 as this liquid falls to the bottom of tank 12. In the second case, liquid is immediately available to pump 16.

Pump 16 supplies both the wash spray nozzles 52 and perforated pipe 22 within solvent evaporating chamber 20.

As pumps 46 and 16 force liquid solvent through spray nozzles 53 and 52 respectively, some of the liquid evaporates in chamber 20 and fills cleaning machine tank 12 with vapor up to the saturated vapor line 68. Thereafter, vapor enters collecting header 47 and is conducted by conduit 40 to heat absorbing element 30.

Heat taken up by heat absorbing element 30 in the process of condensing this vapor to liquid, is promptly transferred to heat emitting element 24 by the refrigerant pumped by compressor 28. As discharge pressure of compressor 28 increases, a pressure-operated, modulating solvent control valve 54 admits liquid solvent to perforated pipe 22.

The liquid solvent is distributed uniformly over heat rejecting element 24 and it cascades downward in a thin film to wet the entire surface of the heat rejecting element 24. The more volatile precision solvent component such as trichlorotrifluoroethane, easily flash evaporates from a mixture containing it, and the resultant vapor enters cleaning machine tank 12 through conduit 41 and distribution header 45. Less volatile solvent components, such as trichloroethylene, and soluble contaminants, such as oil, grease, flux, resins and the like, are returned to tank 12 as liquid through drain line 55.

Solvent control valve 54 admits liquid solvent to distribution header 22 and heat emitting element 24 in precisely the proper amount to maintain a constant surface temperature of heat rejecting element 24. Flow valve 54 ca be adjusted to maintain any heat rejecting temperature within the range of 75 F. to F. and possibly higher. Therefore, the heat emitting temperature can be precisely controlled to cause evaporation of a precision solvent component while less volatile components and contaminants are returned to cleaning machine tank 12 as liquid. Consequently, vapor in the machine tank 12 and vapor entering solvent condensing chamber 38 is substantially pure, homogeneous solvent of the precision cleaning type. Flash evaporation without liquid boiling eliminates entrainment of contaminated liquid droplets in the vapor.

Since a heat pumping system rejects more heat than it absorbs (in the amount of heat equivalent of electrical energy supplied to the compressor drive motor) it is inherently thermally unbalanced. Heat dissipated through conduction, convection and radiation from machine tank 12 tends to rebalance it thermally. An axially water-cooled heat absorbing element 56 is also located in the upper part of solvent condensing chamber 38 to intercept and condense some of the solvent vapor entering chamber 38. Heat absorbed by element 56 is transferred to the cooling water and is removed from the system to ositively maintain its thermal equilibrium. Furthermore, energy supplied 3 to the compressor motor is utilized to evaporate more solvent than otherwise would be produced by transferral of heat from heat absorbing element 30 to heat rejecting element 24.

A pressure-operated, modulating flow control valve 57 admits precisely the proper amount of water to auxiliary heat absorbing element 56 to maintain thermal equilibrium conditions at all times, Flow control valve 57 senses discharge pressure of compressor 28 and admits cooling water to auxiliary heat absorbing element 56 only when th temperature of heat rejecting element 24 tends to rise above the predetermined operating temperature and the discharge pressure of compressor 28 tends to rise above the predetermined Operating pressure.

A third auxiliary or supplemental heat absorbing element 58 is in a heat transfer relationship with refrigerant piping 36. Supplemental heat absorbing element 58 subcools liquid refrigerant before it is expanded within heat absorbing element 30. Heat absorbing element 58 may also function as an auxiliary refrigerant condenser during periods when precision solvent is being recovered for storage in reservoir 62, particularly toward the end of the recovery cycle. Heat absorbing element 53 may also function as an auxiliary refrigerant condenser during periods when the cleaning machine is not in use and full recovery of precision solvent is not desired. Water flow control valve 59 admits water to heat absorbing element 58 only when auxiliary cooling of refrigerant is necessary. While cleaning operation is suspended, precision solvent remains in tank 12 and the heat pumping system operates periodically to maintain the cool air blanket and prevent the loss of solvent vapor. Automatic operation is easily at tained by the use of temperatures operated and pressure operated controls of conventional type.

The selective vapor generating aspects of this provide a simple and economical method of using a mixture of a relatively expensive precision cleaning solvent, such as trichlorotrifluoroethane, and a relatively inexpensive s lvent, such as trichloroethylene. Such selective fiash evaporation also makes it possible to utilize an azeotropic mixture for wash purpose and a substantially homogeneous precision solvent for rinse purpose. For example, an excess of trichlorotrifiuoroethane can be added to an azeotropic mixture of trichlorotritluoroethane and methylene chloride or to an azeotropic mixture of trichlorotrifiuoroethane and ethyl alcohol. Vapor in the machine and vapor condenced to liquid is substantially pure trichlorotrifluoroethane while wash liquid in the machine has the compos tion of the azeotrope. The parts are rapidly dried in the unsaturated air vapor zone before they leave the machine thus eliminating solvent loss by evaporation of solvent from parts outside the machine. All of t re volatile solvent is thus purified and reclaimed for repeated use. Heat-sensitive solvents, such as methyl chloroform, may also be efiiciently used in a vapor de reasing operation Without the danger of solvent decomposition. The low temperature vapor zone also contributes to the overall efi'iciency of operation, purity of vapor and avoidance of undue solvent losses,

Cleaning cycle of operation of figure The cleaning cycle with respect to parts or assemblies moving through the apparatus shown in the figure is as follows:

A conveyor or other means, not shown, lowers parts into cleaning machine tank 12 along the right-hand side of the tank along path 63 as shown in the figure. Parts pass through the cool air-vapor blanket 64 then into the saturated vapor zone 66 and between the Wash spray nozzles 52.

The parts or objects then enter the immersion wash bath at the bottom of tank 12 and are exposed to ultrasonically agitated liquid solvent to remove adherent soils.

The parts continue through the immersion wash solvent 10 to the left-hand side of the tank shown in the figure where they emerge from Wash solvent 10 and pass through a pure liquid spray rinse from spray nozzles 53 in saturated vapor zone 66. The rinse solvent discharged through nozzles 53 is relatively cool and, consequently, the parts being cleaned are cooled to a temperature well below the temperature of the vapor in the saturated vapor zone and pure vapor promptly condenses on them to help rinse them with precision cleaning solvent.

As the parts leave saturated vapor zone 66, they are warmed up to approximately its temperature and Wet with liquid solvent. As these parts pass through the relatively cooler and unsaturated air-vapor blanket zone, a combination of effects causes all liquid to evaporate from them. The vapor so evolved is retained in the cleaning apparatus system as condensed rinsing solvent as the dry, clean parts pass out of the apparatus.

What is claimed is:

1. A method of cleaning objects by contact with a volatile solvent liquid comprising the steps of washing said objects in a solvent liquid mixture incorporating components of differing volatility, selectively flash evaporating the more volatile of said components from a fine spray of said solvent liquid mixture to generate vapor of said more volatile component, condensing part of said vapor to provide a pure supply of said more volatile solvent liquid, and rinsing said objects disposed in said vapor with said supply of said more volatile solvent liquid.

2. A method as set forth in claim 1 wherein said selective flash evaporation is accomplished by spraying said solvent liquid mixture upon the heat emitting section of a heat pumping system.

3. A method as set forth in claim 1 wherein said rinsing step is performed in a saturated vapor zone above said solvent liquid mixture.

4. A method as set forth in claim 3 wherein said saturated vapor zone substantially consists of vapor of said more volatile liquid.

5. A method as set forth in claim 4 wherein a blanket of air-vapor mixture unsaturated with said vapor of said more volat le solvent liquid is maintained above said saturated vapor zone, and said objects are passed from said vapor zone through said blanket to dry them and to retain said more volatile solvent liquid in said blanket.

6. A method as set forth in claim 5 wherein said airvapor mixture in said blanket is Withdrawn and cooled to condense said more volatile solvent liquid from it to maintain said blanket unsaturated and to recover said more volatile solvent liquid from it.

References Cited UNITED STATES PATENTS 1,892,652 12/1932 Heath 20387 XR 2,220,125 11/ 1940 Seaton 13411 2,265,762 12/1941 McKittrick et al. 203 XR 2,755,208 7/1956 Kearney 13411 2,896,640 7/1959 Randall et al 134-11 XR 2,949,119 8/1960 Smith 134-76 XR 3,030,913 4/1962 Arnold et a1 13411 XR 3,070,463 12/1962 Barday 134-11 3,074,417 1/1963 Lisowski et a1 13476 3,106,927 10/1963 Madwed 134-76 3,106,928 10/1963 Rand 134-79 MORRIS O. WOLK, Primary Examiner JOSEPH T. ZATARGA, Assistant Examiner US. Cl. X.R. 13411, 40

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