Thermal vacuum purification that consists of two stages is deemed to be one of the most effective methods of transformer oil drying and degassing. The first stage is required for intensive foaming of oil under vacuum, when gases and vapours are emitted and form the oil film surface.
At the second stage, the foam formation subsides little by little and consequently reduces the gas and water outlet velocity. Normally there is no more than between 10 to 40% of impurities left in the oil. It is desirable to increase the interface surface between oil and gas phases and at the same time reduce the layer thickness.
For many years, many facilities used machines where transformer oil was spread in a thin layer over a surface. Such kind of equipment included degassifiers with Raschig rings, flat plates and others. In such systems, the velocity of water and gas outlet depended on a gas diffusion coefficient, the oil’s density and viscosity as well as its thickness.
There also existed some degassing units where water and gases were removed by mechanical atomizers that spray oil into a special vacuum chamber. There has also been an ultrasonic atomizer that made it possible to reach a dispersion of spraying at the level of 10 microns. The main disadvantage of these units was deemed to be a relatively low velocity of water and gas outlet from the processed oil. Therefore, there have been more attempts to increase the velocity of impurities removeal during drying and degassing. Particularly, there has been an atomizer that was developed that ensures a thin-film oil spraying (the film thickness is several microns). This innovation has made it possible to increase the velocity of gas and water remval by 20 liters oer minute on each atomizer.
Ultrasonic cavitation is another method of degassing and drying that is characterized by a relative velocity of impurities removal. A special ultrasonic vibrator is put into the oil that is under vacuum. This unit favors the formation of vacuum micro-bubbles. Water and gases from their membranes are emitted inside a bubble. Once they get onto the oil surface, these bubbles absorb water and gases.
Todays, in the hi-tech market there are different units operating according to several different approaches. The thermal vacuum purification CMM-C unit, offered by GlobeCore is deemed to be the most optimal equipment in terms of quality to price ratio. This unit allows equipment owners to work with oils with a viscosity of no more than 70 cSt at 50ºС. In addition, CMM-C units may be used for topping up high voltage bushings.
Oil processed on GlobeCore equipment has the following characteristics:
- mass moisture content no more than 10 g/t;
- filtration fineness – 5 microns; and
- residual gas content – 0.2%.