On the Issue of Combating Icing of Wires and Cables

A. A. VOROB’EV

Icing of a surface is often accompanied by disruption of the normal operation of installations, and sometimes also leads to accidents. In connection with this, in recent years the study of this phenomenon and the development of methods for preventing icing and combating it have acquired considerable interest for the electric-power industry, aviation, communications, and others [1].

In practice, the following three types of icing should be considered: 1) pure transparent ice—glaze ice, 2) a matte ice formation, called rime, and 3) hoarfrost—a light snow-like icy deposit.

The first type—pure ice, also called glaze ice—is formed as a result of the freezing of comparatively large drops of supercooled water falling onto a surface at a temperature slightly below zero (usually −2° to −5°C). These drops, striking the surface, do not freeze immediately and not entirely; before turning into ice, they as it were spread out around the point of impact, and therefore the ice that forms has a smooth surface.

The second type—rime—an opaque, milky-white, rough deposit, has a granular structure. Ice accretions of this type have a lower specific gravity than pure ice (0.5 and even less). This type of icing appears in clouds or in fog consisting of highly supercooled minute water droplets at temperatures from −2° to −26°C, i.e., at temperatures still lower than those at which pure ice forms.

The third type of icing—hoarfrost—is a light ice formation that arises when a surface cooled to a temperature below zero enters an atmosphere filled with supersaturated vapors; these vapors then deposit on the surface in the form of a thin snow-like coating.

When seeking practical measures for combating icing, only the first two types should be considered, since hoarfrost, in all cases of icing encountered in practice, is not of special interest as a very light and weakly adhering deposit. As for the first two types, pure transparent ice—“glaze ice”—always presents the greatest danger. Both the rate of its build-up and the strength of its adhesion to the iced surface are greater than in the case of rime formation. Measures for combating glaze ice should also be effective for combating rime.

The methods used to combat glaze ice on wires and cables may be divided into two groups:

a) methods for preventing icing (preventive methods),
b) methods for removing the ice crust before it has reached a dangerous thickness.

Ice formation can be prevented by maintaining the temperature of the surface in contact with the aqueous fog above zero, and also—

by covering it with a layer of so-called antifreezes, i.e., liquids that mix with the water falling on the surface and lower its freezing temperature.

Of considerable interest, for example, is the development of methods for combating icing of vertically suspended cables.

Raising the surface temperature of a long cable relative to the ambient temperature can be accomplished by electrical heating. However, this method of preventing icing has substantial shortcomings. First of all, it requires a return conductor, which entails expenditures of conductor material and certain structural difficulties. In addition, there is the nonuniformity of energy consumption, which depends strongly on wind speed. Thus, the considerable consumption of energy for heating the cable and the rapid increase in its expenditure with increasing wind speed, as well as the need for a power-supply installation, present substantial difficulties for the use of electrical heating as a measure to prevent icing of a cable.

We proposed, and together with colleagues studied, the following three methods of combating ice accretion:

a) destruction and removal of ice accretion by means of mechanical oscillations;

b) a physico-chemical method of transforming ice into a mechanically weak state, when the ice crust can fall off under its own weight. This is achieved by coating the surface of the wires with a suitable lubricant;

c) prevention of icing of wires during their corona discharge.

Laboratory investigations showed the effectiveness of these methods [2].

The ice cover can be removed from wires by exciting mechanical oscillations in them. During oscillations, there will be a considerable concentration of energy at the antinodes, which can lead to destruction of the ice cover. Pieces of ice that have lost adhesion to the surface will be thrown off from it during the oscillations. A change in the distribution of mass along the oscillating body will cause a shift of nodes and antinodes, which will lead to destruction of the ice at new points, etc. [3].

An experimental study carried out by the author jointly with N. B. Bogdanova gave certain positive results. The experiments were conducted on wires of diameter 0.3–2.5 mm and length 1–23 m, coated with an oil-rosin compound of various viscosities. The wire was fastened in a vertical position and tensioned with a weight. Transverse oscillations of short wavelength, excited in the wire by a bow, an electromagnet, sharp blows, a heavy body, etc., lead to destruction of the coating layer and to its rebounding in pieces over the entire surface of the wire.

Torsional oscillations arising when a wire twisted through a certain angle is unwound were accompanied by destruction of the compound layer and its rebounding. The wire was twisted by placing it between two planes, which were tightened by means of screws. When twisted through a certain angle, the wire slipped in the planes, which was accompanied by its striking the surfaces of the clamp and by the occurrence of strong oscillations. With the aid of a Federsen mirror the oscillations were observed on a screen. It was noticed that higher-order oscillations, caused by the impact during slipping of the wire in the planes of the clamp, were superimposed on the fundamental oscillations.

Experiments with steel cables of various cross sections showed that when transverse oscillations were excited in them, the layer of compound was destroyed and rebounded. In these experiments, destruction of the coating was observed only over a small length of cable near the vibrator. Torsional oscillations of cables,

naturally, it was not possible to induce it. The supply of energy necessary for destroying the coating layer in the given cross-section of the wire by exciting transverse and torsional oscillations in it is limited by the damping of the oscillations and by the elasticity of the medium.

The formation of ice on the surface of steel cables differs from other cases of icing in that, owing to the structure of the cable itself, which is twisted from separate steel wires, water, before freezing, can soak the cable through and through and, in this way, the ice covering the cable will not only adhere to its uneven surface, but will also permeate it entirely. This, of course, increases the strength of the ice covering and makes it more difficult to combat icing by mechanical methods, such as striking, scraping, etc.

Together with Docent Saratovkin we found and studied several methods of combating icing by coating the surfaces of bodies with suitable compositions.

If a voltage above the critical value is applied to a wire subject to the danger of icing, then drops of rain, particles of fog, and ice crystals, having become charged in the corona sheath, will be repelled from the surface of the coronal wire and, under certain conditions, will be completely unable to settle on it. It is known, for example, that in electrofilters suspended particles become charged, acquire a charge of the same sign as that of the coronal wire, and, moving toward the opposite electrode, settle on it. Only a small number of the most massive particles settle on the coronal wire itself.

If the effect of charging water drops with a charge of the same sign as the charge of the wire takes place, then, as the voltage is raised above the critical corona voltage on the wire, moisture will settle in ever smaller quantities. At a certain voltage the wire, while in a humid atmosphere, should prove to be dry. Investigations carried out by us jointly with N. B. Bogdanova [4] confirmed these assumptions. Experiments under low-temperature conditions, when icing could be observed, showed that an increase in voltage above the critical value is accompanied by a decrease in the thickness of the ice crust that forms. At a certain voltage above the critical value, the formation of ice is not observed at all.

The study of the question of icing of surfaces has led us to the conclusion that it is not yet possible to propose an effective and practically acceptable universal method of combating this phenomenon. In each individual case, from among a number of possible methods of combating icing, one must choose the most expedient.

The methods proposed by us and considered in this article for preventing icing of metallic surfaces and for combating ice may find practical application in certain specific cases.

CITED LITERATURE

1. N. V. Lebedev. Combating Icing of Aircraft. State Publishing House, Moscow—Leningrad, 1939.

2. Report on the work “Icing of Wires and Cables and Combating It.” Transport-Energy Institute of the West-Siberian Branch of the Academy of Sciences of the USSR. N. B. Bogdanova, Brodyanskii, Prof. A. A. Vorob’ev, Docent Zavadovskaya E. K., Docent D. D. Saratovkin. Tomsk, 1944.

A. A. Vorob’ev, N. B. Bogdanova. Author’s application No. 417 and 613, BRIZ, NKES.

1. A. A. Vorob’ev and N. B. Bogdanova, ZhTF, vol. XV, issue 8, pp. 574–576, 1945.

2. N. B. Bogdanova. Investigation of Moisture Deposition on Coronal Wires. Dissertation for the academic degree of Candidate of Technical Sciences. Tomsk, Polytechnic Institute, Transport-Energy Institute of the West-Siberian Branch of the Academy of Sciences of the USSR, 1946.