Why do skirting boards get damp, how to dry them, how to increase insulation. Method for electroosmotic drying of paper cable insulation How to dry a telephone cable

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Despite the fact that the drying and impregnation operation is extremely important for obtaining the proper quality of the cable, the methods of drying and impregnation vary greatly among different factories. Prof. Whitehead, who published in 1928 his research on cable drying and impregnation, which he began on behalf of the American Institute of Electrical Engineers, says that in American factories he found the widest variations in this regard, namely from six days of drying under high vacuum and with pre-drying in air until there is no drying at 20 hours. boiling in a hot impregnating mass and under reduced pressure. The same diversity is observed in Europe, and here the Heaver’s method, used at the English Glover’s plant, stands out, as already mentioned above. All this indicates a lack of uniformity in understanding the meaning of the process and its course and a relatively small experimental study of it.
It is known that the quality of a dielectric very much depends on the presence of moisture in it, so its complete removal is very important. Before drying, the cable insulation contains a lot of moisture, which takes a very long time to remove without taking special measures. N. Mailer gives the following simple calculation on this matter:
Cable 35 kV, 395 m.n. with a length of 1,000 tons, the paper weighs 2,000 kg, which at 7% humidity gives a water content in the cable of 140 kg. If such a cable is placed in a vacuum apparatus with a volume of 8 m3 and dried with a current of dry air at 20 ° C, then the volume of the vacuum apparatus must be changed 1000 times, provided that the air removed each time is completely saturated with moisture. The need for such a large volume of dry air during natural drying indicates the need to use artificial measures during drying: heat and vacuum. However, both have their drawbacks: high vacuum greatly complicates heat transfer from the walls of the boiler to the cable; the amount of steam contained in a given volume of a vacuum apparatus is less at low pressure than at high pressure; rapid evaporation causes the cable temperature to drop quickly, making drying difficult. Therefore, the usual, or, as the British say, “routine” drying method basically consists in the fact that the cable immersed in a vacuum apparatus is first heated at atmospheric pressure and at open lid boiler using steam passed into the coil or boiler jacket. This heating lasts from several hours to 2-3 days at a temperature of 110-120 C, and the time is set according to production experience or laboratory testing. After such heating, the boiler is closed with a lid and a vacuum is created in it, at which drying continues at the same temperature of 110 - 120 ° C. For the most part, a vacuum of about 90-95% is given, but new modern installations reach pressures of up to 5 mm and even up to 2 mm rt. Art., and for especially high-voltage cables, using laboratory-type mercury pumps, a higher vacuum is achieved. At such high vacuums, it is necessary to use an impregnating mass welded under vacuum, since otherwise it foams heavily when entering the boiler.
Both during the heating process and during the drying process, not all cable elements increase their temperature equally. As measurements show, the copper core of the cable reaches a temperature of 100-110 ° C only after a very long time of continuous drying, about a day or more; in 5-6 hours. this temperature reaches a value of only 60-80 ° C. Sometimes vacuum drying is interrupted by the introduction of dry gas (air or preferably carbon dioxide), thereby increasing the temperature of the core, and then a vacuum is applied again: this is the so-called push drying. It must be borne in mind that when the vacuum is interrupted, the temperature of water evaporation increases, so the drying of the cable also stops. Currently, instead of jerk drying, heating of the cores is often used. electric shock, which greatly speeds up the drying process. Such heating is always carried out with direct current, because with alternating current a very high voltage of the current source is required due to the high inductive resistance of the cable being dried. Generally speaking, accelerating the drying process is beneficial not only in terms of better use of equipment and saving steam that heats the vacuum dryer, but also in terms of improving the quality of insulation, since paper can be damaged during prolonged heating. Current drying is usually not economically profitable, since it absorbs a large amount of energy, but there are still reasons to use it if there is not a sufficient number of vacuum devices or if they want to shorten the process.
For low voltage cables voltage up to 3 kV, and sometimes up to 6 kV, the drying process is often completely omitted and replaced by cooking a cable, usually preheated by current, in a hot mass. The moisture " cooking method" is removed during the cooking process. This method has some economic advantages, but no technical advantages it does not improve the quality of the cable. When using the cooking method, it is recommended to preheat the cable using electric current or another method, since otherwise the cold cable lowers the temperature of the impregnating mass too much and thereby complicates the cooking process.
When making cables for very high voltages, before the end of drying, the vacuum apparatus is sometimes filled with carbon dioxide, which is then evacuated. The purpose of this operation is to replace, on the one hand, the chemically active oxygen of the residual air with neutral carbon dioxide, and on the other hand, to reduce internal voids in the cable, since carbon dioxide dissolves much more in the impregnating mass than air, which entails represents a reduction in the original voids.
The process of drying and impregnating the cable is usually carried out in the same boiler to avoid contact of the cable with air, because dry cable is very hygroscopic. The hot impregnating mass is sucked in due to the vacuum prevailing in the boiler. The temperature of the suction mass is usually on the order of 115-135 ° C, and according to N. Mflller even 140 ° C. Such a high temperature of the impregnating mass is necessary, since at the end of drying the temperature of the copper core does not reach 100 ° C, and since penetration mass through paper stops at about 80 ° C, then at a lower temperature of the incoming mass there can easily be a risk of under-impregnation of the cable, since the mass should cool especially strongly near the relatively cold copper core and the adjacent insulation layers. The second circumstance that necessitates the high temperature of the impregnating mass is that in order for the mass to penetrate into all the pores of the paper, a hot mass is needed when its viscosity is sufficiently low.
In order to obtain good and deep impregnation, the process of sucking the mass into the boiler must be quite slow and last at least 1-2 hours. If the suction proceeds quickly, then there will be a lot of air in the cable, because it is impossible to achieve an absolute vacuum in the boiler. In addition, the impregnating mass entering the vacuum apparatus foams strongly, since at reduced pressure the gases dissolved in it begin to come out of it, but with slow impregnation, some of these gases are removed using suction pumps. B good installed installations for impregnation of high-voltage cables, the impregnating mass is degassed and kept under vacuum to prevent the reverse dissolution of gases in it and to prevent oxidation; such a mass no longer foams when impregnated. Sometimes the mass is stored under nitrogen, which has a low solubility coefficient.
In order to improve impregnation, it is sometimes carried out in jerks, changing vacuum to pressure; further details of this impregnation method will be given later when describing the control of drying and impregnation. Sometimes during impregnation, pressure increased by 3-4 at is used in order to drive the impregnating mass into the cable. In order to allow such impregnation, Krupp boilers are designed for this increased pressure. Practice, however, has not fully justified this method, as will be seen below, and it has now been abandoned almost everywhere.
The cable should be impregnated as completely as possible to ensure good dielectric and thermal properties of the cable. Since the impregnating mass has a very high coefficient of thermal expansion, the cable must be cooled before applying the lead sheath. Good practice for high-voltage cables is to cool the cable so that the temperature of the cooled cable is 4-5 ° C above the ambient temperature, and cooling below the ambient temperature is not allowed to avoid the deposition of moisture from the environment on the cable.
We begin the description of the drying and impregnation process and equipment with a description of the production of oil-rosin impregnating mass. This mass is cooked either in the same vacuum apparatus in which the cable is impregnated, or, more conveniently, in special boilers. In fig. 207 shows one of these boilers from Rot, this boiler has a diameter of 4.2 m, is heated by a coil and is equipped with a stirrer that makes 30 rpm. Such boilers are usually loaded with rosin first, and then oil is added. Cooking is carried out under steam heating for several hours at a temperature of about 120 ° C until all the rosin is dissolved in the oil and its foaming, which depends on the release of vapors and moisture, stops. The impregnating mass for high-voltage cables is boiled under vacuum in order to eliminate the dissolution of gases in it and prevent oxidation. The freshly brewed mass should usually stand for several days in order to allow the hydroxy acids contained in the rosin to fall out of solution, otherwise they may fall out in the cable insulation over time. Sometimes cable factories perform contact purification of oil using bleaching clays. Oil filtration through conventional filters is also often used to eliminate mechanical impurities.

Both of these types of drying are approximately equally common, only drying on drums in the vast majority of cases is carried out in vertical rather than horizontal boilers, as shown in Fig. 210. The relative advantages and disadvantages of drum and basket drying are as follows:

Fig. 207. Boiler for cooking impregnating mass from Rot.
The cables are supplied for drying and impregnation either wound on iron drums, onto which they are received from three-phase machines, or in so-called iron baskets, into which they are rewound from the drums. Drying of cables on drums is shown in Fig. 208, which shows three drums with cables prepared for drying in a horizontal kettle and connected to each other and with special terminals for drying by electric current. The basket view is shown in Fig. 209, which shows a holey basket converted into a blind one.

Fig. 208. Drying cables on drums in horizontal boilers.

When drying in a basket, the cable must be rewound into the basket at least once from the receiving drum, and in this case the cable goes into the lead press “against the feathers,” i.e. top layer paper applied with a positive overlap, the paper may lift up into the press.

Fig. 209. Basket for drying and impregnating cables.
The advantages of drying in baskets are that the basket can be made blind, that is, without holes, open only at the top, which allows the cable to be cooled not in a vacuum apparatus, but in a special room, which greatly increases the use of vacuum apparatuses, with on the one hand, and allows the cable manufacturing process to be carried out without contact of the uncooled cable with air, on the other hand.

Fig. 210. Drying scheme in a vertical boiler.

When drying on a drum, there is no need to rewind the cable, but it becomes almost inevitable that the cable will be transferred by air after impregnation into special cooling tanks, since otherwise the use of equipment for drying and impregnation will be negligible. In addition, it is very difficult to crimp thin cables from drums, since a lot of force is required to rotate the drum in a thick, cold mass. Then, with the commonly used drying and impregnation equipment, the cables on the drums must be turned on edge before drying.
Vacuum dryers can be divided into the following three types: vertical boilers, horizontal boilers and drying ovens. The diagram of a vertical boiler is shown in Fig. 210, here inside the boiler there is a dotted line depicting a drum with a cable immersed in the boiler. The diagram of a horizontal boiler is shown in Fig. 211, such a boiler is opened by moving a carriage with a boiler lid mounted on it; this boiler is completely unsuited to receiving baskets. In fig. 212 shows a view of a Krupp drying cabinet; this cabinet is equipped turntables, on which baskets with cables are placed. Such cabinets are only suitable for drying cables, and the cable must be rewound in baskets.
For impregnation of power cables, the most common type of boiler is the vertical boiler. Modern boilers for very high voltage cables they are built very large, namely to receive baskets up to 3 and 4 liters in diameter, but for ordinary needs they are limited to boilers for baskets with a diameter of 2-2.5 m. Usually one boiler includes from two to three baskets. These boilers can also be used for drying on drums. The great convenience of this type of boiler is that during impregnation, with the lid open, you can observe the state of the mass mirror and, based on its condition, judge whether the impregnation has ended or not, since after the end of impregnation, no gas bubbles or moisture should be released from the mass. These boilers are heated either by a steam coil or a steam jacket. Boilers with a steam jacket are more expensive than boilers with a coil, but they are better since the coils often get upset. In addition, with a jacket it is easier to clean the boiler; you can use superheated steam, which is beneficial. A further advantage of the jacket is that it makes it easier to cool the boiler by running cold water into it.

Fig. 211. Drying scheme in a horizontal boiler.
In America, it is customary to use oil instead of steam to heat boilers. The objections raised against the use of oil, however, are that oil is flammable; distillation products developing from it require a special device for removal; When the oil is cooled, very high pressure must be applied at the beginning of the process, which greatly increases the cost of installation.
Horizontal boilers for the production of power cables are used very rarely, and essentially they are not suitable for this purpose, because they have the following main disadvantages:

Fig. 212. Drying cabinet from Fr. Krupp, Grusonwerk.

During impregnation, the mass is greedily absorbed by the cable, and the mirror of the impregnating mass quickly decreases, due to which the upper part of the drum with the cable may be under-impregnated if the mass is not collected during the process itself, which is very inconvenient.

2. Since a boiler filled with mass cannot be opened, the mass has to be drained from the boiler in a hot state, which has a detrimental effect on the quality of the cable.
The first of these shortcomings, however, can be quite easily eliminated by installing special reservoirs with impregnating mass on top of the boiler, from where its consumption is replenished. The disadvantage of horizontal boilers is that it is more difficult to maintain cleanliness around them than around vertical boilers. The generally accepted opinion is that vertical boilers are more suitable for the production of power cables, horizontal ones - for the production of telephone cables, and cabinets - for drying telephone cables of small diameter, which should also be dried in baskets.
A typical diagram of a drying-impregnation device is shown in Fig. 213. Here A is an iron drum with a cable; B - vacuum apparatus; C - vacuum pump; D - tank with impregnating mass; E - surface condenser for water vapor sucked from the cable.
IN production conditions cable drying control consists of observing the inspection window of the condenser, through which you can see whether the sucked steam is condensing or not.

Fig. 213. Diagram of a drying and impregnation device for cables impregnated with a viscous mass.
The drain valve at the condenser also makes it possible to monitor the drainage of condensation water and roughly judge the stage of the process, however, both of these methods are very primitive and do not allow precise definition process. Currently, to establish a typical drying and impregnation regime, there are several methods based on measuring the electrical characteristics of the cable during drying and impregnation. The first report on the use of such a method was made by W. A. Del Mag in 1924. According to this report, measurement during drying and impregnation was used in American cable factories electrical capacitance cable using alternating current. Direct current was not used, since with it the measurement results fluctuate very much due to inevitable temperature fluctuations and due to significant electrical absorption.

Fig. 214. Change in cable capacitance during drying and impregnation according to W. A. Del Mag
The nature of the change in capacity over time according to W. A. Del Mag is shown in Fig. 214. As can be seen from this figure, at the beginning of the process the capacitance increases very strongly, apparently partly due to the increase in the temperature of the cable, and partly due to sweating of the cable. Then the capacity begins to fall, and after some time it becomes constant. The moment when the capacity became constant corresponds to
obviously at the end of the drying process. When the mass is introduced into the boiler, i.e., at the beginning of impregnation, the cable capacitance first increases very quickly, then the increase slows down, and finally, the capacitance becomes constant, which corresponds to the end of impregnation. It should be noted that in FIG. 214 the scale for the size of the container during impregnation is taken to be several times smaller than for drying.

Fig. 215. Change in cable capacitance during impregnation according to P. Junius’y.
Of the several subsequent reports on the development of methods for controlling drying and impregnation by electrical changes, the work of P. Junius, produced at the German cable factory Hackethal Draht u, deserves mention. Kabelwerke. Junius recorded capacitance versus time curves using K. W. Wagner’s bridge alternating current tone frequency. The most interesting are his observations on the impregnation process. He showed especially clearly the effect of pressure shocks on the degree of impregnation. In fig. 215 shows, according to Junius, the dependence of the electrical capacitance on the impregnation time, and it is clear that when impregnated under vacuum, the capacitance increases relatively slowly, which indicates a gradual increase in the degree of impregnation. When pressure is applied to the vacuum apparatus by admitting atmospheric air, the container immediately jumps upward, which indicates compression air bubbles in the cable.
When vacuum is applied again, the capacity value again drops, but not to its previous value. Repeated pressure shocks again increase the capacity to a certain constant limiting value. The degree of gap between the capacitance limit and the vacuum capacitance indicates the degree of cable evacuation.
It should, however, be pointed out that the ionization curve given by P. Junius for the cable for which the curve in Fig. 215, had no inflection point.
This method of studying drying and impregnation provides a criterion by which P. Junius evaluates some artificial methods used in the cable impregnation process. Some factories try to raise the ends of the impregnated cable so high that they come out of the impregnating mass during impregnation. By this they try to prevent the penetration of mass from the ends of the cable, because then the degree of impregnation of the cable can be judged from the cut end. P. Junius considers this removal of the ends to be harmful, because when the boiler is opened, the impregnating mass is pressed into the cable under the influence of external pressure, and with the ends of the cable coming out of the mass, at the same pressure, air will be pressed into the cable through the ends.
Another artificial method is that during impregnation, pressure is applied to the boiler at certain intervals so that the mass penetrates better into the paper layers. P. Junius does not consider this method to have great advantages, since when the pressure is released, the mass is expelled from the layer of paper by the pressure of air bubbles compressed in the cable insulation. P. Junius offers the following method of rational impregnation:
On the cable (without a lead sheath) located in the impregnation boiler, a coupling with a tight fit is put on one end to create a vacuum inside the cable; This coupling is connected to a special powerful vacuum unit. When the boiler is closed, the cable is evacuated both through the coupling and through the boiler.

Fig. 216. Scheme of oil impregnation of a filled cable according to E. F. Nuezel’io.
Electrical testing is a very time-consuming procedure that can only be applied to type testing. Currently, there are ways to control the degree of cable drying by passing air and steam sucked from the boiler through indicators that chemically indicate the presence or absence of water vapor.

Fig. 217. Diagram of oil impregnation of a filled cable at the Sevkabel plant.

Let us also dwell on the features of drying and impregnation of oil-filled cables. As mentioned above, these cables are dried (or rather, dried) after applying a lead sheath, so the equipment for drying these cables is significantly different from conventional ones. In fig. 216 shows a diagram of the connection of devices for impregnating an oil-filled cable, given by E. F. Nuezel’eM)