The following article on degasification was taken from sources by Enervac Corporation
The exposure of oil to vacuum alone or use of a spray pattern is not sufficient to remove all soluble gas in a single pass through a vacuum chamber. The fallacy of the atomisation of oil in vacuum, lies in a phenomena that in vacuum, in absence of any matter, the oil sprayed under pressure tends to accept a form with minimum external surface, a sphere, instead of the intended thin film, maximum surface exposure.
R.M.Russek expresses this phenomenon in his very comprehensive paper on Treatment of Paper-Insulated Cables as follows: –
The removal of water and gases takes place in a degasification plant or degasser. In the classical degasser the oil was heated to reduce its viscosity to a suitable value and was sprayed through nozzles against the walls of the degassing chamber, maintained under vacuum.
If one spraying cycle was insufficient the cycle was repeated until satisfactory results were obtained. Today this method of degassing is not considered satisfactory, because it is appreciated that the breaking up of the oil into small droplets, occluding vapour and gases, increases the surface tension of these bubbles and make their final disintegration very difficult. The vacuum becomes ineffective and the whole process leads to incomplete degasification, and several passes are not completely effectual.
The modern procedure is based on exposure to vacuum of a thin oil layer, spread over a large area. Under these conditions the oil quickly achieves a state of equilibrium with the evacuated atmosphere of the degassing column. To complete this procedure, the presence of millions of sharp ends of fibreglass contributes to the evolution of the gas from the oil and to break up of the forming gas bubbles. It is common knowledge that when boiling liquids, gas bubbles, later vapour bubbles, always form on some irregular or sharp point in the beaker or kettle. The smooth surfaces leads to overheating of the liquid over the boiling point. The sharp points of the fibreglass fibres accelerate the evolution of the gas from the liquid, by forming nuclei of the degassing and with the help of a relative motion of oil film against the nuclei, assist in breaking the surface of the forming gas bubbles, thereby releasing the gas.
The Enervac process for degasification and vacuum dehydration of electrical insulating oils employs all the best factors favourable in reaching the solubility equilibrium line in a single pass. The theoretical curve of solubility is a straight line. The efficiency of any degasification depends on how close to the theoretical equilibrium line one can get in a single pass at a given vacuum level. If two or three passes are required to reach gas content of 0.2 percent at 500 micron pressure, the efficiency of the system leaves much to be desired.
ENERVAC`S UNIQUE ACCELERATOR ELEMENT.
GLASS FIBRE DEGASSING ELEMENTS both extend the oil surface and present millions of sharp ends to the flow of the oil and each creates a point of release.
There are several methods employed for removal of soluble gases from oils. In all cases it is necessary to expose a thin film of oil to a vacuum for a certain length of time. It has been proven that agitation of the oil and the presence of a number of sharp points will accelerate the evolution of gas from the oil. Enervac utilises a unique chemically- inert fibreglass accelerator cartridge in the vacuum chamber.
This performs the following functions: –
First their in- depth design structure allows any free water to be rapidly separated from the oil by coalescence even before it reaches the evaporation stage.
Second, the millions of glass fibres 3-10 micron diameter provide a large total surface for exposure of a thin oil film to vacuum.
Third, the millions of sharp points of the glass fibres all in random direction promote a point of fast release of the gases and vapours from the oil.
Fourth, the element acts as a fine filter removing solid contaminants. These cartridges are easily replaced and disposable.