US2505581A - Means for drying gas in electrical apparatus - Google Patents

Description

April 25, 1950 M. UNGER MEANS FOR DRYING GAS IN ELECTRICAL APPARATUS Filed March 26, 1945 2 Sheets-Sheet 1 Inventor Magn us Uns er,

His Attorneg.

Patented Apr. 25, 1950 MEANS FOR DRYING GAS IN ELECTRICAL APPARATUS Magnus Unger, Pittsfield, Mass., assignor to General Electric Company, a corporation of New York Application March 26, 1945, Serial No. 584,823

2 Claims. (Cl. 17414) I have found, however, that even after careful predrying some moisture still remains absorbed in the fibers of the solid insulation. In hermetically-sealed apparatus of the type in which the solid insulation is immersed in liquid insulation, with a gas cushion above the liquid level, there is danger that the remaining moisture will in time be driven out of the solid insulation by heat and will eventually be diffused in the form of water vapor in the gas cushion where it can condense out in drops on the inside of the cover or casing during sudden lowering of the ambient temperature. Such drops falling into the liquid insulation may produce disastrous results.

In accordance with this invention there is provided a predetermined amount of hygroscopic material in the gas cushion for cleaning up any traces of water vapor which may appear. This material i preferably arranged in a special manner so that effective changes in gas cushion pressure due to thermal expansion and contraction of the liquid insulation, or to differences in temperature of the gas on either side of the hygroscopic material caused by changes in ambient temperature,.wiil force the gas through the hygroscopic material so as to increase its effectiveness in absorbing moisture. Preferably also, additional means is provided for preventing contact of the insulating liquid with the hygroscopic material.

Such hygroscopic material will effectively dry the gas which is introduced above the liquid level so that it will not be so important to predry this gas before it is introduced and in addition water vapor which enters as a result of leaks or the removal of the manhole cover for inspection purposes will be absorbed.

An object of the invention is to provide a new and improved electric induction apparatus.

Another object of the invention is to provide a new and improved desiccator system for hermetically-sealed electric apparatus.

The invention will be better understood from the following description taken in connection with the accompanying drawings and it scope will be pointed out in the appended claims.

In the drawings Fig. 1 is a sectional view of an embodiment of the invention; Fig. 2 is a modification in which the effectiveness of the hygroscopic material is increased by mounting it so that the gas is forced back and forth through it and in which this material is arranged so that it is more difficult for the insulating liquid to come in contact with it; Figs. 3 and 4 are modifications of Fig. 2; Fig. 5 is another modification in which the air dryer is connected to an expansion tank; Fig. 6 is a modification of Fig. 5 in which the gas is more effectively forced through the moisture absorbing material; Fig. 7 is a further modification in which a coil condenser is used for preventing vaporized insulating liquid from coming in contact with the hygroscopic material; Fig. 8 is another modification of Fig. 1; Fig. 9 is a view showing the relation between the desiccator of Fig. 1, the tank, the air cushion and the insulating liquid; and Fig. 10 illustrates a modification in which ambient temperature changes alone cause circulation of the gas through the hygroscopic material.

Referring now to the drawings and more particularly to Fig. 1, there is shown therein a portion of a cover I for a tank which may, for example, be the tank 2 of a transformer, as shown in Fig. 9. The tank is partially filled with an insulating liquid 3, such as pure mineral oil or aromatic chlorinated hydrocarbon, above which is a gas cushion 4. The gas may be either air or an inert gas, such as nitrogen. Inserted in an opening in the cover is a wire mesh basket 5 which is supported by a perforated bottom plate 6 and a bolt 1 which is fastened to a cap or manhole cover 8 which in turn is fastened to a flange 9. The flange 9 may be welded to the cover I, and the cap or cover may be bolted to the flange with a suitable gasket l0 between the flange and the cover. Inside the mesh basket is a predetermined amount of hygroscopic material I l, a preferred material being silica gel because it has different colors when wet and dry.

Silica gel will absorb about 40 per cent of its own weight of water, and according to the law of vapor pressure, as the water in the gas cushion is being absorbed the emulsified water in the liquid will vaporize into the cushion until all water is absorbed. One-half pound of silica gel should be sufllcient for about 1000 gallons of wet liquid insulation and gas cushion, or roughly, a 2000 kilowatt transformer. The silica gel can readily be examined by removing the basket from the tank. It is blue when dry and pink when wet so that the amount of moisture which is absorbed can quickly be determined by inspection, and further, the material can be quickly reconditioned by heating it a short time at about 150 C.

In Fig. 2 the silica gel II is mounted approximately in the middle of a housing 12, it being supported by perforated means, such as finely divided mesh screen l3 which prevents the silica gel from falling through it, which fine screen is in turn supported by a coarser screen ll. This housing I2 is mounted on top of the tank through a short connection it having a flange it which is bolted or otherwise fastened to a matching flange on the tank. A glass window ll is provided in the side of the casing so as to make it easy to inspect the silica gel. Also, a filling plug I8 is provided in the top of the casing. For draining the desiccator of any excess moisture a fluid or drain valve I 9 is provided in its bottom, and for preventing the entrance of foreign matter and moist air into the tank when the desiccator is removed for reconditioning or for replacement a shut-off valve 20 is provided in the flange 9 on the transformer top.

It will be observed in Fig. 2 that there is a gas space 2| on the opposite side of the silica gel from the gas cushion which is above the liquid in the tank. Therefore, as the liquid level rises and falls due to thermal expansion, as indicated in Fig. 9, the equalizing of the as pressure on both sides of the silica gel due to the changes in gas cushion pressure will force the gas to and fro through the silica gel, thus causing more of the gas to come in contact with the silica gel and rendering it more effective for absorbing moisture. Furthermore, it will be seen that it is relatively dii'ilcult for the liquid in the tank to come in contact with the silica gel because it would have to pass upward through the throat connection l5, About the only time there is danger of the liquid coming in contact with the silica gel is when the transformer is bein shipped. Under these conditions acceleration and deceleration of the tank will cause the liquid to slosh around and therefore it is likely to splash against the basket shown in Fig. 1 but it would have much less chance of entering the desiccator shown in Fig. 2 because of the restricted passage l which if necessary can be packed with material like fibre glass or covered with another wire screen which retards liquids but lets gas through freely. It will be understood, of course, that the insulating liquid will also tend to be absorbed by the silica gel and consequently this will impair the ability of the silica gel to absorb water.

In case the desiccator shown in Fig. 2 extends too far above the top of the tank it can be arranged as shown in Fig. 3, while another arrangement which gives an intermediate maximum height between those provided in Figs. 2 and 3 is shown in Fig. 4.

In the case of large power transformers, shipping clearances sometimes limit their overall height so that it is necessary to make the height of the air cushion as small as possible, but as this may result in excessive gas pressures, an expansion tank is provided on the side of the transformer so that actually the air cushion has a larger volume than the space in the main tank top of the main tank 2. This expansion tank is shown as being provided with a diaphragm 28 which divides it into upper and lower sections, and the desiccator or silica gel dryer 24 has upper and lower connections 25 and 28 which communicate respectively with the upper and lower sections of the expansion tank. In this manner changes in gas pressure will cause the gas to pass through the dryer so as to equalize the pressure in the two sections of the expansion tank, The two compartments of the expansion tank are provided with drain valves 21 and 20 and also a vacuum pressure gauge 28 is attached to the upper compartment of the expansion tank. The desiccator 24 shown in Fig. 5 may either be a glass cylinder or it may be provided with a glass window for making inspection of the silica gel easy.

In order to cause more of the gas to pass through the hygroscopic material the construction shown in Fig. 6 may be used. In this figure the expansion tank is not divided into sections and the outlet tube or pipe 30 from the top of the main tank communicates directly with one side or port of the dryer 24, the other port of the'dryer being connected to the expansion tank. In this way all of the gas interchanged between the main tank and the expansion tank must pass through the dryer, whereas in Fig. 5 only the gas interchanged between the upper and lower sections of the expansion tank pass through the dryer.

Preferably the pipe 30 is connected to the bottom of the dryer and a sump 3| with a drain valve 32 is provided for draining oif any excess liquid, such as condensed vapor from the liquid insulation in the main tank.

In order more positively to prevent vaporized insulating liquid from reaching the drying material the pipe 30 may be coiled as shown in Fig. 'l with the connection leading to the gas cushion in the main tank at the bottom and the connection leading to the dryer being at the top so that condensate forming in the coils will run back in the main tank. It should be understood that insulating liquid, especially volatile insulating liquid like askarel, vaporizes at a substantially higher temperature than water so that it will condense out before water vapor will condense out so that while the coil shown in Fig. 7 prevents vapors from the liquid insulation from reaching the dryer, it does not prevent water from reaching the dryer.

A more rugged construction than that shown in Fig. l and a simpler construction than that shown in the other figures is illustrated in Fig. 8 in which the hygroscopic materials is enclosed in a steel cylinder 33 fastened by any suitable means, such as by welding to the underside of a cap or manhole cover which is fastened to the tank cover in the same manner as in Fig. 1. This cylinder is closed at the bottom by a steel plate 34 having drilled openings which are covered by double fine screen 35. Supports Ii fastened to the bottom cover support first a coarse screen 31 and then a fine screen 38 on which is spread the drying material II and an ample air space 2| is left above the drying material for the same purpose as described in connection with Fig. 2.

For observing the condition of the drying material a gasketed plug I9 is inserted in the cover 8. Another effect which causes circulation or movement of the gas through the hygroscopic scopic material from the gas which communicates directly with the gas cushion, being more exposed to the surrounding air, will follow the temperature of the latter more closely than the gas in the cushion. Consequently, differential pressures will be set up due to changing ambient temperature and this will also cause an interchange of gas between the two sides of the hygroscopic material. This efiect also takes place in Fig. 6 as the temperature of the gas in the expansion tank 22 will follow changes in ambient temperature more readily than will the gas in the cushion at the top of the main tank 2.

In the modification shown in Fig. 10 a desiccator such as is shown in Fig. 2 is combined with a heat exchanger or condenser coil 39. Thus, for example, one end of the coil 39 is connected to the top of the gas cushion through a second opening 40 in the main tank 2 and the other end of the condenser coil is connected to the top of the desiccator as by removing the plug 18 shown in Fig. 2.

The operation of Fig. 10 is as follows: If the ambient temperature should fall the gas in the coil 39 will cool more rapidly than the gas in the main tank 2 or the gas in the desiccator i2 because of the relatively large surface of the coil 39. Consequently, a. gas circulation will be set up in the direction of the arrows in Fig. 10, thus drawing gas from the gas cushion up through the desiccant II. If the ambient temperature increases, then the circulation will be in the opposite direction.

It will be observed that in Fig. 10 the circulation is solely responsive to changes in ambient temperature and that changes in gas cushion pressure resulting from expansion and contraction of the insulating liquid in the tank will have practically no efiect on the circulation of gas through the hygroscopic material.

While there have been shown and described particular embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a hermetically-sealed tank, electric apparatus within said tank, said apparatus having solid insulation, liquid insulation within said tank in which said apparatus and its solid insulation are submerged, a gas cushion in said tank above the level of said liquid, a hermetically-sealed gas expansion tank mounted at the side of. said first-mentioned tank, a, desiccator having communicating upper and lower chambers, a quantity of hygroscopic material in the upper chamber, the lower chamber constituting a sump, a drain valve at the bottom of said sump, a conduit connected between the top of said upper chamber and said expansion tank, and a second conduit connected between said gas cushion and said desiccator intermediate its upper and lower chambers.

2. In combination, a main hermetically sealed tank, induction apparatus within said tank, said apparatus having solid insulation, liquid insulation within said tank in which said apparatus and its solid insulation are submerged, a gas cushion in said tank above the level of said liquid, a gas expansion tank mounted alongside said main tank and substantially equal in height to the main tank, an opening near the center of the expansion tank, a container of hygroscopic material mounted below said opening, conduit means for connecting the top of said container to said opening, an opening in the top of said main tank, a tubular helical condenser mounted adjacent said opening in said main tank with the axis of said condenser vertical, conduit means interconnecting the lower end of said condenser to said opening in said main tank and conduit means connecting the top of said condenser to the bottom of the container of said hygroscopic material.

MAGNUS UNGER.

REFERENCES CITED The following references are of record in the

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