On the issue of combating icing of steel cables
Abstract Generated abstract
This paper examines the difficulty of preventing glaze ice on twisted steel cables, where water penetrates between wires and produces a strong ice bond that resists ordinary mechanical removal. The authors argue that oils, varnishes, and similar coatings cannot reliably prevent icing, while cryohydrate substances are impractical for complete prevention but can alter ice structure by forming brine films within dendritic ice growth. Based on experiments, they propose resin and kerosene pastes containing finely powdered salts or noncorrosive solutes such as hyposulfite or sugar, which promote formation of loose, brittle ice that can detach under slight shaking or its own weight. The text gives example formulations and reports that an economical coating on a small cable could loosen centimeter-scale ice deposits for several icing cycles before requiring renewal.
Full Text
On the Issue of Combating Icing of Steel Cables
A. A. Vorob’ev and D. D. Saratovkin
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 individual steel wires, water, before freezing, can soak the cable through; thus the ice covering the cable not only adheres to its uneven surface but also penetrates it throughout. This, of course, increases the strength of the ice covering and makes it impossible to combat icing by mechanical methods of removing ice, such as chipping, scraping, etc.
Methods of combating icing by lubricating the surface of the cable may be divided into two groups:
- The lubricant completely prevents the formation of ice on the surface.
- The lubricant leads to the formation of ice that adheres weakly to the surface, is loose and easily removed either by a simple mechanical method such as chipping, by shaking the cable, or even falls off by itself after reaching a certain thickness.
Lubricating a cable with some oil that would impregnate the cable and thereby prevent water from penetrating into its interior, into the gaps between the individual wires, should reduce the strength of adhesion of the ice to the cable. However, as is known from the literature and as our investigations have shown, lubrication with any oils whatsoever cannot prevent the formation of ice on the surface. Nor do hard coatings that reduce the wettability of the coated surface (paraffin, varnishes, resins, etc.) give satisfactory results. All these coatings only reduce the surface of the cable by filling the gaps and, in this way, only weaken the forces of adhesion between the cable and the ice formed on its surface.
The use of any liquids—for example, liquid antifreezes used in aviation to combat aircraft icing—is impossible under cable-icing conditions for technical reasons, since it requires a continuous supply of the liquid being consumed.
Only such a lubricant can completely prevent the formation of ice on the surface of a cable which, either in its entirety or through one of its components, by interacting with water, would lower its freezing temperature below that of the surrounding air. Such substances may include all cryohydrates, an example of which is ordinary table salt. Coating the cable surface with a grease containing salt leads to the dissolution of the salt in the water falling on the cable and to a considerable lowering of the freezing temperature of the brine formed. In individual cases this temperature may reach \(-21^\circ\mathrm{C}\). However, elementary calculations show that, for complete protection against ice deposition, a very large quantity of salt is required, amounting to tens of percent by weight of the ice that would have been deposited if there had been no salt. This circumstance compels us to abandon the use of cryohydrates as agents that completely prevent the formation of ice.
The investigations we have carried out have so far not made it possible to obtain a lubricant that, for a prolonged time, would reliably prevent ice from forming on the surface of a cable. We believe that at present one should proceed along the path of such coatings as would cause the formation of loose, weak ice.
Coating with oils and solid substances, as indicated above, reduces the strength of adhesion of ice to the surface of the cable. However, this is insufficient, since ice can adhere firmly even to a perfectly smooth polished surface. Such a coating does not produce changes in the structure of the ice, converting it into a loose state. Such results can be obtained only by using the cryohydrates mentioned above, whose action in the present case we shall consider in greater detail.
The formation of glaze ice, as is known, occurs when supercooled water, in the form of fog or even small raindrops, falls on a surface having a temperature below 0°C. The most favorable conditions for the formation of dense transparent ice are −2°, −5°C. The crystallization of water, i.e., the growth of the ice cover in this case, obeys the usual laws of crystallization and, what is very important, ice crystals, just as in other cases, grow in the form of small dendrites, i.e., treelike formations. As investigations in the field of dendritic crystallization and the distribution of impurities in crystals have shown,¹ all extraneous substances not taking part in the construction of the crystal, such as salts dissolved in the water, etc., are forced into the space between the branches of the dendrite and solidify last, at the lowest temperature—the solidification temperature of the eutectic. If this temperature is not reached, then, consequently, the ice crystal will be a loose formation in the form of a dendrite, between the branches of which there is brine. It is clear that an extremely thin liquid interlayer is sufficient for the crystal to be able to break apart easily along this interlayer.
If the surface of the cable is covered with an ointment containing salt in such an amount as is quite insufficient to convert all the ice deposited on the cable into the liquid state, then this amount may prove entirely sufficient for there to be many small cracks in the deposited ice, filled with a salt solution. Such ice will be very brittle and, what is very important, the more brittle the closer it is to the surface on which it was deposited. Under slight shaking, this ice can easily fall off under the action of its own weight.
The investigations we have carried out in this field have shown the correctness of the arguments given and have indicated ways of developing a corresponding formulation of ointments or pastes. For practical use, a number of coating methods based on our experiments can now be proposed.
As ice-loosening salts, the cheapest and most generally available, one may propose table salt, which gives a brine that practically does not freeze down to −20°C, and saltpeter down to −12°C. The disadvantage of these salts, as of all others that dissociate in aqueous solutions, is that they can cause corrosion, rusting of the cable surface. Safe from this point of view are hyposulfite and sugar. The first gives a solution that does not freeze down to −10°C, and the second down to −6°, −7°C, i.e., solutions satisfactory from the standpoint of the conditions of glaze-ice formation.
A lubricating paste is prepared in the following manner: rosin or another similar resin is mixed with kerosene in approximately the ratio of 60% by weight rosin and 40% kerosene. Mixing should be carried out with heating, but carefully, since the mixture can easily ign—
¹ Saratovkin D. D. Distribution of impurities during crystallization. Reports of the Academy of Sciences of the USSR, 4, p. 307, 1935.
ignite. At a temperature of \(0^\circ\mathrm{C}\), this mixture has a viscosity similar to that of ordinary molasses or fresh centrifuged honey. The salt, saltpeter, hyposulfite, or sugar used is ground into a fine powder—flour. Hyposulfite is best taken anhydrous. This flour is poured into the mixture of rosin with kerosene and carefully triturated until a homogeneous paste is obtained. The finer the grains and the more homogeneous the paste, the better. The quantity of flour added may be varied depending on the desired thickness of the paste. The thickness of the paste is determined by the conditions under which it is applied to the cable. If the cable is very cold, it is better to make the paste thinner. A thicker paste may be applied to a warm cable. In general, the thickness of the paste should be similar to that of candied honey. For sugar, under conditions in which the cable temperature was \(-2^\circ\), \(-5^\circ\mathrm{C}\), we successfully used a paste of the following composition: 7 parts sugar flour to 2.5 parts rosin with kerosene.
The best results in terms of the looseness of the ice forming on the cable were obtained when using common salt and anhydrous hyposulfite, poured on in different proportions, which determined the thickness of the applied layer and consequently the consumption of paste, as well as the duration of its action.
The most economical layer is one that only fills the gaps between the wires of the cable and slightly covers the most protruding wires. A cable \(0.4\ \mathrm{cm}\) in diameter required 5–6 g of paste per meter. Such lubrication caused loosening of an ice crust up to one or two centimeters thick and, after it fell off under the action of its own weight, remained effective another two or three times. Thereafter the paste was exhausted and required replacement, i.e., a new coating of the cable.