Freezing-Points of Dilute Alcohol. _________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Alcohol. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 4.8 | 0.9916 | -2.0 | +28.4 |
| 11.3 | 0.9824 | 5.0 | 23.0 |
| 16.4 | 0.9761 | 7.5 | 18.5 |
| 18.8 | 0.9732 | 9.4 | 15.1 |
| 20.3 | 0.9712 | 10.6 | 12.9 |
| 22.1 | 0.9689 | 12.2 | 10.0 |
| 24.2 | 0.9662 | 14.0 | 6.8 |
| 26.7 | 0.9627 | 16.0 | 3.2 |
| 29.9 | 0.9578 | 18.9 | -2.0 |
|_______________|___________________|__________|__________|
Freezing-Points of Dilute Glycerin. _________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Glycerin. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 10 | 1.024 | -1.0 | +30.2 |
| 20 | 1.051 | 2.5 | 27.5 |
| 30 | 1.075 | 6.0 | 21.2 |
| 40 | 1.105 | 17.5 | 0.5 |
| 50 | 1.127 | 31.3 | -24.3 |
|_______________|___________________|__________|__________|
Freezing-Points of Calcium Chloride Solutions. _________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| CaCl_2. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 6 | 1.05 | -3.0 | +26.6 |
| 8 | 1.067 | 4.3 | 24.3 |
| 10 | 1.985 | 5.9 | 21.4 |
| 12 | 1.103 | 7.7 | 18.1 |
| 14 | 1.121 | 9.8 | 14.4 |
| 16 | 1.140 | 12.2 | 10.0 |
| 18 | 1.159 | 15.2 | 4.6 |
| 20 | 1.170 | 18.6 | -1.5 |
|_______________|___________________|__________|__________|
Calcium chloride will probably be procured in the solid state, but it can be purchased as a concentrated solution, being sold under the name of "calcidum" [Footnote: This proprietary German article is a liquid which begins to solidify at -42° C. (-43.6° F.), and is completely solid at -56° C. (-69)° F.). Diluted with one-third its volume of water, it freezes between -20° and -28° C. (-4° and-l8.4° F.). The makers recommend that it should be mixed with an equal volume of water. Another material known as "Gefrierschutzflüssigkeit" and made by the Flörsheim chemical works, freezes at -35° C. (-3° F.). Diluted with one-quarter its volume of water, it solidifies at -18° C. (-0.4° F.); with equal parts of water it freezes at -12° C. (10.4° F.). A third product, called "calcidum oxychlorid," has been found by Caro and Saulmann to be an impure 35 per cent. solution of calcium chloride. Not one of these is suitable for addition to the water used in the generating chamber of an acetylene apparatus, the reasons for this having already been mentioned.] for the protection of gasholder seals. Glycerin itself resembles a strong solution of calcium chloride in being a viscid, oily-looking liquid; and both are so much heavier than water that they will not mix with further quantities unless they are thoroughly agitated therewith. Either may be poured through water, or have water floated upon it, without any appreciable admixture taking place; and therefore in first adding them to the seal great care must be taken that they are uniformly distributed throughout the liquid. If the whole contents of the seal cannot conveniently be run into an open vessel in which the mixing can be performed, the sealing water must be drawn off a little at a time and a corresponding quantity of the protective reagent added to it. Care must be taken also that motives of economy do not lead to excessive dilution of the reagent; the seal must be competent to remain liquid under the prolonged influence of the most severe frost ever known to occur in the neighbourhood where the plant is situated. If the holder is placed out of doors in an exposed spot where heavy rains may fall on the top of the bell, or where snow may collect there and melt, the water is apt to run down into the seal, diluting the upper layers until they lose the frost- resisting power they originally had. This danger may be prevented by erecting a sloping roof over the bell crown, or by stirring up the seal and adding more preservative whenever it has been diluted with rain water. Quite small holders would probably always be placed inside the generator-house, where their seals may be protected by the same means as are applied to the generator itself. It need hardly be said that all remarks about the dangers incidental to the freezing of holder seals and the methods for obviating them refer equally to every item in the acetylene plant which contains water or is fitted with a water-sealed cover; only the water which is actually used for decomposing the calcium carbide cannot be protected from frost by the addition of calcium chloride or glycerin--that water must be kept from falling to its natural freezing-point. From Mauricheau-Beaupré's experiments, referred to on page 106, it would appear that a further reason for avoiding an addition of calcium chloride to the water used for decomposing carbide should lie in the danger of causing a troublesome production of froth within the generator.
It will be convenient to digress here for the purpose of considering how the generators of an acetylene apparatus themselves should be protected from frost; but it may be said at the outset that it is impossible to lay down any fixed rules applicable to all cases, since local conditions, such as climate, available resources, dimensions, and exposed or protected position of the plant-house vary so largely in different situations. In all important installations every item of the plant, except the holder, will be collected in one or two rooms of a single building constructed of brick or other incombustible material. Assuming that long-continued frost reigns at times in the neighbourhood, the whole of such a building, with the exception of one apartment used as a carbide store only, is judiciously fitted with a heating arrangement like those employed in conservatories or hothouses; a system of pipes in which warm water is kept circulating being run round the walls of each chamber near the floor. The boiler, heated with coke, paraffin, or even acetylene, must naturally be placed in a separate room of the apparatus-house having no direct (indoor) communication with the rooms containing the generators, purifiers, &c. Instead of coils of pipe, "radiators" of the usual commercial patterns may be adopted; but the immediate source of heat should be steam, or preferably hot water, and not hot air or combustion products from the stove. In exposed situations, where the holder is out of doors, one branch of the flow-pipe should enter and travel round the seal as previously suggested. Most large country residences are already provided with suitable heating apparatus for warming the greenhouses, and part of the heat may be capable of diversion into the acetylene generator-shed if the latter is erected in a convenient spot. In fact, if any existing hot-water warming appliances are already at hand, and if they are powerful enough to do a little more work, it may be well to put the generator-building in such a position that it can be efficiently supplied with artificial warmth from those boilers; for any extra length of main necessary to lead the gas into the residence from a distant generator will cost less on the revenue account than the fuel required to feed a special heating arrangement. In smaller installations, especially such as are to be found in mild climates, it may be possible to render the apparatus-house sufficiently frost-proof without artificial heat by building it partly underground, fitting it with a double skylight in place of a window for the entrance of daylight, and banking up its walls all round with thick layers of earth. The house must have a door, however, which must open outwards and easily, so that no obstacle may prevent a hurried exit in emergencies. Such a door can hardly be made very thick or double without rendering it heavy and difficult to open; and the single door will be scarcely capable of protecting the interior if the frost is severe and prolonged. Ventilators, too, must be provided to allow of the escape of any gas that may accidentally issue from the plant during recharging, &c.; and some aperture in the roof will be required for the passage of the vent pipe or pipes, which, in certain types of apparatus, move upwards and downwards with the bell of the holder. These openings manifestly afford facilities for the entry of cold air, so that although this method of protecting generator-houses has proved efficient in many places, it can only be considered inferior to the plan of installing a proper heating arrangement. Occasionally, where local regulations do not forbid, the entire generator-house may be built as a "lean-to" against some brick wall which happens to be kept constantly warm, say by having a furnace or a large kitchen stove on its other side.
In less complicated installations, where there are only two distinct items in the plant to be protected from frost--generator and holder--or where generator and holder are combined into one piece of apparatus, other methods of warming become possible. As the reaction between calcium carbide and water evolves much heat, the most obvious way of preventing the plant from freezing is to economise that heat, i.e., to retain as much of it as is necessary within the apparatus. Such a process, clearly, is only available if the plant is suitable in external form, is practically self-contained, and comprises no isolated vessels containing an aqueous liquid. It is indicated, therefore, rather for carbide-to- water generators, or for water-to-carbide apparatus in which the carbide chambers are situated inside the main water reservoir--any apparatus, in fact, where much water is present and where it is all together in one receptacle. Moreover, the method of heat economy is suited for application to automatic generators rather than to those belonging to the opposite system, because automatic apparatus will be generating gas, and consequently evolving heat, every evening till late at night--just at the time when frost begins to be severe. A non-automatic generator will usually be at work only in the mornings, and its store of heat will accordingly be much more difficult to retain till nightfall. With the object of storing up the heat evolved in the generator, it must be covered with some material possessed of the lowest heat-conducting power possible; and the proper positions for that material in order of decreasing importance are the top, sides, and bottom of the plant. The generator may either be covered with a thick layer of straw, carpet, flannel, or the like, as is done in the protection of exposed water- pipes; or it may be provided with a jacket filled with some liquid. In view of the advisability of not having any organic or combustible material near the generator, the solid substances just mentioned may preferably be replaced by one of those partially inorganic compositions sold for "lagging" steam-pipes and engine-cylinders, such as "Fossil meal." Indeed, the exact nature of the lagging matters comparatively little, because the active substance in retaining the heat in the acetylene generator or the steam-pipe is the air entangled in the pores of the lagging; and therefore the value of any particular material depends mainly on its exhibiting a high degree of porosity. The idea of fitting a water jacket round an acetylene generator is not altogether good, but it may be greatly improved upon by putting into the jacket a strong solution of some cheap saline body which has the property of separating from its aqueous solution in the form of crystals containing water of crystallisation, and of evolving much heat in so separating. This method of storing much heat in a small space where a fire cannot be lighted is in common use on some railways, where passengers' foot-warmers are filled with a strong solution of sodium acetate. When sodium acetate is dissolved in water it manifestly exists in the liquid state, and it is presumably present in its anhydrous condition (i.e., not combined with water of crystallisation). The common crystals are solid, and contain 3 molecules of water of crystallisation--also clearly in the solid state. Now, the reaction
NaC_2H_3O_2 + 3H_2O = NaC_2H_3O_2.3H_2O
(anhydrous acetate) (crystals)
evolves 4.37 calories (Berthelot), or 1.46 calorie for each molecule of water; and whereas 1 kilo. of water only evolves 1 large calorie of heat as its temperature falls 1° C., 18 grammes of water (1 gramme-molecule) evolve l.46 large calorie when they enter into combination with anhydrous sodium acetate to assist in forming crystals--and this 1.46 calorie may either be permitted to warm the mass of crystals, or made to do useful work by raising the temperature of some adjacent substance. Sodium acetate crystals dissolve in 3.9 parts by weight of water at 6° C. (43° F.) or in 2.4 parts at 37° C. (99° F.). If, then, a jacket round an acetylene apparatus is filled with a warm solution of sodium acetate crystals in (say) 3 parts by weight of water, the liquid will crystallise when it reaches some temperature between 99° and 43° F.; but when the generator comes into action, the heat liberated will change the mass of crystals into a liquid without raising its sensible temperature to anything like the extent that would happen were the jacket full of simple water. Not being particularly warm to the touch, the liquefied product in the jacket will not lose much heat by radiation, &c., into the surrounding air; but when the water in the generator falls again (after evolution of acetylene ceases) the contents of the jacket will also cool, and finally will begin to crystallise once more, passing a large amount of low-temperature heat into the water of the generator, and safely maintaining it for long periods of time at a temperature suitable for the further evolution of gas. Like the liquid in the seal of an isolated gasholder, the liquid in such a jacket will last indefinitely; and therefore the cost of the sodium acetate in negligible.
Another method of keeping warm the water in any part of an acetylene installation consists in piling round the apparatus a heap of fresh stable manure, which, as is well known, emits much heat as it rots. Where horses are kept, such a process may be said to cost nothing. It has the advantage over methods of lagging or jacketing that the manure can be thrown over any pipe, water-seal, washing apparatus, &c., even if the plant is constructed in several separate items. Unfortunately the ammonia and the volatile organic compounds which are produced during the natural decomposition of stable manure tend seriously to corrode iron and steel, and therefore this method of protecting an apparatus from frost should only be employed temporarily in times of emergency.