Vapour degreasing is the process of cleaning parts by condensing solvent vapours on workpieces. It uses the hot vapours of a chlorinated or fluorinated solvent to remove soils, particularly oils, greases, and waxes.
A vapour degreasing unit consists of a steel tank with a heated solvent reservoir, or sump, at the bottom and a cooling zone near the top. Sufficient heat is introduced into the sump to boil the solvent and generate hot solvent vapour. Because the hot vapour is heavier than air, it displaces the air and fills the tank up to the cooling zone. The hot vapour is condensed when it reaches the cooling zone, thus maintaining a fixed vapour level and creating a thermal balance. The temperature differential between the hot vapour and the cool workpiece causes the vapour to condense on the workpiece and dissolve the soil.
The soils removed from the workpieces usually boil at much higher temperatures than the solvent, which results in the formation of essentially pure solvent vapours, even though the boiling solvent may be quite contaminated with soil from previous work parts.
In vapour degreasing, the parts are heated by condensation of the solvent vapours to the boiling temperature of the degreasing solvent, and they dry instantly as they are withdrawn from the vapour zone.
Solvent stability is usually controlled by the addition of stabilizers when the solvent is manufactured. Most common chlorinated solvents all require stabilizers to perform successfully in vapour degreasing.
The simplest form of degreasing system uses the condensation of solvent vapour only. If the workpiece contains blind holes or recesses that are not accessible to the vapour, or if the soil cannot be removed by the vapour, a spray stage may be added. The system then consists of vapour – spray – vapour stages. Other techniques include warm liquid-vapour, boiling liquid/warm liquid-vapour systems. Also ultrasonic cleaning can be used in conjunction with the vapour degreasing process.
When a ferrous metal is vapour degreased, organic films are usually removed, and the metal is highly susceptible to atmospheric corrosion. When precision steel parts with a high surface finish (e.g. antifriction bearings) are being degreased and complete rust prevention is desired, rust proofing by flushing or immersion should be included as an integral part of the degreasing system.
Boil degreasing (German: Abkochentfettung)
In a bath a cleaning agent, usually a hydrocarbon or a chlorinated hydrocarbon, sometimes a neutral or alkaline aqueous cleaner is heated up to boiling point and the parts are suspended in that boiling liquid. Fats and waxes are melting and additionally the continuous movement of the bubbles causes a dispersing of the dirt. Very often this method is used as a preceding step in a vapour degreaser. In other processes energy consumption is too high. Also particulates can be removed. Components should be heat shock resistant.
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