Questions of evaporation, whether for raising steam, or for the concentration of tanning extracts and other solutions are of considerable importance in the tanning industry, and as the same natural laws which apply to these equally govern the drying of leather, it is convenient to study the theory of the whole subject in one chapter, rather than to divide it, and place each part in a different portion of the book.

The modern conception of evaporation and vapour pressures has been described on page 75, but it will be necessary to recapitulate a little. It is a well-known fact that most liquids, if left exposed in an open vessel, gradually disappear by evaporation into the air, even at ordinary temperatures. If the vessel is heated sufficiently, the liquid “boils”; that is, bubbles of vapour are formed in it, and escape, and the evaporation is therefore much more rapid. To avoid complication, let us first imagine a liquid sealed in a glass flask, which contains no air, but which is only partially filled by the liquid. It has been pointed out that the motion of heat by which the molecules of the liquid are agitated, enables some of them to break away from the attraction by which liquid particles are held together, and pass into the form of gas or vapour, which will fill the empty part of the flask. This evaporation will, however, soon reach a limit, since the vapour cannot escape from the flask. The flying molecules of vapour produce pressure by striking the walls of the flask, while a proportion of them will strike the surface of the liquid, and again be caught and retained by its attraction; and as the pressure rises, the number of these necessarily increases till a point is reached when as many fall back and are retained (or “condensed”), as those which evaporate, and the pressure will then remain constant. The amount of the pressure will vary with the nature of the liquid, and will be the greater the more volatile it is, or, in other words, the less the power of its internal attraction. It will also increase with rising temperature, which, by increasing the velocity of motion of the molecules, renders their escape from the liquid easier, and their recapture more difficult. It will not be at all affected by the volume of vapour or the size of the flask, but so long as any liquid is present, it will depend merely upon the nature of the liquid, and the temperature. If the flask is large, more of the liquid will evaporate till the same pressure is reached. If at the outset the flask is not empty, but filled with air, it will make no difference to the pressure or quantity of the vapour in it, which will be added to that of the air, whatever that may be. If the sealing of the flask is broken so that it is open to the atmosphere, air and vapour will escape, or air will pass in, till the total pressure is equal to the atmospheric pressure outside, (about 15 lb. per square inch). As, however, the vapour in the flask is always renewed by evaporation, so that the full vapour-pressure of the liquid is maintained, the “partial” pressure (as it is called) of the air in the flask will be less than that of the outer atmosphere by the amount of the vapour-pressure, which makes up the difference. Once this balance is attained, evaporation will go on very slowly in the flask, as it can only replace the small quantity of vapour which escapes. If, however, the vapour is removed by blowing fresh air into the flask, it will rapidly be replaced in the old proportion by fresh evaporation. Thus goods in a close room will dry only very slowly, even if the temperature is high, unless the moistened air is replaced by dryer air from the outside by some effective system of ventilation. In absence of this, evaporation only becomes rapid when the temperature of the liquid is raised to its “boiling point,” that is, when the vapour-pressure becomes slightly in excess of that of the atmosphere, so that the freshly formed vapour can push out that already in the flask or chamber into the outer air, and at the same time, bubbles can be formed in the interior of the liquid by the escaping vapour. As the vapour-pressure of a liquid rises continuously with increasing temperature, and its boiling point is defined as that temperature at which it is equal in pressure to the air (or vapour) in contact with it, it is evident that the boiling point must entirely depend on the pressure. Thus the boiling point of water in a boiler at a pressure of 55 lb. per square inch above the atmosphere is 150°C., and in a partial vacuum equal to 5·8 inches of barometric pressure, is only 60° C., a fact which is made use of in the concentration of extracts and other liquids at a low temperature in the vacuum-pan. (Atmospheric pressure is taken at 30 inches or 760 millimeters of the barometer or 14·7 lb. per inch, or 1·033 kilos per square centimeter.)

If a piece of iron is placed over a powerful gas-burner, it will go on getting hotter till its temperature is nearly or quite equal to that of the gas-flame. On the other hand, a pan of water, in the same condition, once it has reached its boiling point, becomes no hotter till all the water is evaporated. It is evident that the whole available heat or energy of the gas-flame is consumed in converting the water into steam. We might convert a proportion of this energy into mechanical work, by using the steam in a steam engine; but even without this, work is actually being done by the escaping steam in raising the weight of the atmosphere, and in overcoming the attractive force which holds the particles of water together in the liquid form. It is of course known to everyone, that energy may change its form, as from heat to work, but that it cannot be destroyed, diminished or increased; and therefore the whole of the work performed in converting the water into steam is again recovered as heat when the steam is condensed. In this connection a clear distinction must be made between quantity of heat, and temperature, which in popular language are often confused. It is for instance obvious that if we mix a pound of water at boiling temperature with another pound at freezing point, the temperature is altered to 50° C., but the total quantity of heat is unchanged. It is equally clear that no change in quantity of heat takes place when 1 lb. of mercury at 100° is mixed with 1 lb. of water at 0°, though in this case, owing to the small capacity of mercury for heat, the common temperature would only be raised to about 3°. We must therefore have some measure of heat apart from the mere direct indications of the thermometer, and that most generally used is the quantity of heat required to raise 1 kilo of water 1° C. (kilogram-calorie).[180] In England the heat required to raise 1 lb. of water 1° F. is also in use as a unit. The k.-calorie is equal to 3·97 (very approximately 4) lb. × F. units. For our purpose it may be taken that 100 k.-calories of heat are required to raise 1 kilo or liter of water from freezing to boiling temperature. If, however, the water is actually frozen, we require 80 k-calories merely to melt the kilogram of ice without perceptibly raising its temperature, and when the water is raised to 100°, 536 calories of heat are still necessary merely to convert it into steam at the same temperature. To melt 1 lb. of ice requires 144 lb. × F. units, to raise it to boiling point 180 more, and to evaporate it 965 additional. The quantity of heat required for actual evaporation varies a little at different temperatures, being somewhat larger at lower temperatures, but the total heat required to raise water from the freezing point, and convert it into steam at any pressure is nearly constant, being 635 calories at atmospheric pressure, and only about 650 calories, or 1180 lb. × F. units at 50 lb. per sq. inch. The quantity of heat evolved by the combustion of 1 lb. of good coal is 13,000 to 15,000 lb. × F. units; or of 1 kilo, 7200 to 8300 k-calories, but in raising steam in a good boiler coal will only evaporate 10 times its weight of water at 100° (5360 calories or 9650 lb. × F. units), the remaining heat being lost. 1 horse-power (33,000 foot-pounds per minute)[181] in the best engines requires about 1¹/₂ lb. of coal or 15 lb. of steam per hour, but in those of worse construction may run up to many times that amount. As, even theoretically, not 20 per cent. of the total heat can be converted into mechanical work in a “perfect” engine working at 75 lb. pressure, it is often economical to use waste steam for heating or evaporation, and where this can be done profitably, the additional cost of the mechanical power is very small.

In evaporating liquids in the open pan 536 calories is required to evaporate 1 kilo of water already raised to boiling temperature, and a larger amount for salt-solutions, and it makes comparatively little difference whether this is done at 100° or at a lower temperature. Where, however, evaporation is done in vacuo, considerable economy can be effected by what are known as multiple “effects,” in which the steam from one vacuum-pan is employed to boil a second under a reduced pressure, and consequently boiling at a lower temperature. This principle can be practically applied to as many as five or six successive “effects,” the weaker liquor being usually evaporated at the highest temperature and lowest vacuum in the first “effect,” by the exhaust steam of the engine used for the vacuum pumps, while the steam from the first effect heats that of the next higher concentration, and so on. In the Yaryan evaporator (p. 339), the boiling liquid is sprayed through coil-tubes, thus exposing an enormous surface to evaporation, and the whole concentration of any given portion of liquid takes place as it passes through the apparatus, which does not, even in multiple effects, occupy more than 4 or 5 minutes; and without the temperature of the liquid ever rising above 60° or 70° C. In the case of liquids, like sugar- and tannin-solutions which are liable to chemical change from continued heating, the shortness of the time is a very great advantage. The number of effects which it is desirable to use depends greatly on the cost of fuel as compared to the largely increased cost of the apparatus. 1 lb. of coal employed in raising steam will evaporate 8¹/₂ lb. in a single-effect Yaryan, 16 lb. in a double-effect, 23¹/₂ lb. in a triple, 30¹/₂ lb. in a quadruple, and 37 lb. in a quintuple-effect apparatus.

Where liquids are evaporated in the open air at temperatures below boiling, it is advisable by some means to spread the liquid in a thin film, so as to expose a large surface, which must be continuously removed by agitation, so as to prevent the formation of a skin. A good apparatus for this purpose is the Chenalier evaporator (Fig. 92), which consists of steam-heated copper discs rotating in a trough containing the liquid, which is taken up by buckets attached to the rims of the discs, and poured over their heated surfaces. In other forms, the liquid is allowed to trickle over steam-heated pipes or corrugated plates. Such evaporators should be placed in a current of air so as to rapidly carry off the vapour formed. Their use is very objectionable for liquids, like tannin-liquors, which are injured by oxidation, and they are not nearly so economical as vacuum-pans.

The drying of leather depends on the same laws as the evaporation of liquids, but demands special consideration from its very different conditions of temperature and supply of heat. It is important to remember that evaporation cannot go on unless the vapour-pressure of the liquid to be evaporated is higher than that of the vapour in contact with it, and that air-pressure does not prevent evaporation, so that if we sweep away the stagnant vapour with dry air, evaporation will go on as quickly as in vacuo, except that the liquid cannot boil. We must also bear in mind that evaporation consumes quite as much heat at low temperatures as in a steam boiler, and that this heat must generally come from the surrounding air, the temperature of which it reduces.

The rapidity of evaporation, and the quantity of moisture which can be taken up by a given volume of air depends on the vapour-pressure, which increases with temperature. The relation between the two, and the weight of water in grams per cubic meter which can be dissolved in dry air is given in the following table. (Grams per cubic meter is practically equivalent to ounces per 1000 cubic feet. Vapour-pressure is given in millimeters of mercury of the barometer, p. 422.)

Vapour Pressure of Water.

Air is practically never dry, and in damp weather is frequently saturated with moisture to the full extent corresponding to its temperature. In England the average quantity of moisture contained in the air throughout the year is 82 per cent. of the total possible, and even in the driest summer weather it is never less than 58 per cent. So long as the water is in the form of vapour, the air remains quite clear and does not feel damp; in fogs, the air is not only saturated with moisture, but contains small liquid particles floating in it. Of course when the air is really saturated with moisture, it has no drying power whatever.

As is evident from the table, the amount of water which can be dissolved in a given volume of air rapidly increases with temperature. Air at 0° C. is only capable of containing 4·9 grams per cubic meter, or not much more than 20 per cent. of what it can contain at 25° C. It hence rapidly increases in drying power as it is warmed, and consequently the air in a warm well-ventilated drying room in winter is generally much drier, and has greater capacity for absorbing moisture than the open air in the driest summer weather. This is the principal cause of the tendency to harsh and irregular drying by the use of artificial heat; and may be remedied by a proper circulation of the air by a fan without too frequent change with the colder air outside. On the other hand the use of a little artificial heat in damp summer weather, when the air is saturated with moisture, may be quite as necessary as in winter. The amount of moisture in the air is most easily ascertained by a device known as the “wet and dry bulb thermometers.” This consists of two thermometers mounted on a board; one of which has the bulb covered with muslin, and kept moist by a lamp-wick attached to it, and dipping in a vessel of water. The temperature of the wet bulb is lowered by the heat consumed in evaporation, and the difference of its temperature from that of the dry bulb is proportionate to the drying power of the air. This may be approximately calculated in grams per cubic meter by multiplying the difference by 0·64 for Centigrade or 0·35 for Fahrenheit degrees; and if deducted from the total capacity for moisture corresponding to the temperature of the wet bulb as given in table, p. 426, will give the actual moisture in grams contained in a cubic meter of air; but for practical purposes, all that is necessary is to find by experience the temperature and difference between the wet and dry bulbs, which gives the best result for the drying required, and to maintain it as nearly as possible by regulation of the heating and ventilation. Cheap forms of the instrument are made for use in cotton-mills, where it is necessary to maintain a certain degree of moisture; or it may be improvised from two chemical thermometers which agree well together. Distilled (rain or steam) water should be used to moisten the bulb, or it will quickly become coated with lime salts, and it should be placed in a draught, or its indications will not be accurate.

It is of course obvious that not only the wet thermometer, but the wet hides or skins are cooled by evaporation, and they, in their turn, cool the air with which they are in contact, which not only becomes moistened, but is lessened in its capacity for moisture by cooling, and thus rapidly reaches a condition when it can absorb no more moisture. It is thus necessary to maintain its temperature by artificial heat, or to replace it constantly by fresh air from the outside, and which of these expedients is most economical will depend on the temperature of the air outside as compared with that which it is required to maintain. If the outside air is sufficiently warm, and not saturated with moisture, it is generally best to use it in large quantities without artificial heat, wind usually supplying the necessary motive power for its circulation. Wet goods from the pits may thus be dried to a “sammed” condition by any air which is not saturated, and above freezing point; though the drying will often be slow. For drying “off,” artificial heat is generally necessary, since the attraction of the fibre for the last traces of moisture is very considerable, and to remove it the drying power of the air must be considerably higher than that required for the evaporation of free water.[182] In drying stuffed leather a temperature must generally be maintained sufficient to keep the fats employed in partial fusion, and so permit their absorption by the leather, while at the same time the drying must be gradual, or the water may be dried out before the fats have time to take its place. This is generally best attained by the use of artificial heat, and ventilation by circulating the air by a fan without its too frequent renewal, especially in cold weather. Frequently air which has been heated and used for drying off finished goods, and so partially saturated with moisture, may be used with advantage for wet goods, or for other purposes where a more gentle drying is required. If the temperature is low outside, the amount of heat consumed in heating cold air to the temperature required may be very considerable. The weight of a cubic meter of air at 0° C. and atmospheric pressure is 1·293 kilos, and its specific heat at constant pressure is 0·2375 of that of water. Therefore to heat a cubic meter of air at ordinary pressure and temperature 1° C. will require the same amount of heat as that used to heat 0·3 kilo of water to the same extent, or in other words 0·3 of a k.-calorie. If steam-heating is used, 1 kilo of good coal burnt under the boiler should heat about 1800 cubic meters 10° C., or 1 lb. should heat 52,000 cubic feet 10° F., assuming that the condensed water is not cooled below 100° C. These seem large volumes, but if we reflect that a 48-inch Blackman fan may move 30,000 cubic feet per minute, we shall realise that the cost of coal in heating air is not inconsiderable.

We must now consider the heat consumed by the actual evaporation of the water in the leather. The actual evaporation of water already raised to 100° C. consumes 536 k.-calories, but the evaporation of water which has not previously been heated so far consumes more heat, and we may take that required at ordinary temperatures as in round numbers 600 k.-calories per kilo, or 1080 lb. × F. units per lb. Disregarding small fractions, this is equivalent to the cooling to the same temperature of an equal weight of steam in the heating pipes, and this, as we have seen, demands about ¹/₁₀ of its weight of coal for its production from water already heated to 100° C.

The cooling takes place, in the first instance, in the leather, the temperature of which is reduced like that of the wet-bulb thermometer; and this in its turn cools the air in contact with it. Thus in air-drying without artificial heat, the whole heat must be supplied by the air and the loss reduces its capacity for moisture, greatly increasing the volume required. This is not of much consequence in open-air drying, since even a light wind will supply air in enormous volume. A moderate breeze of ten miles an hour moves about 15 feet or 4¹/₂ meters per second. When, however, the air must be moved by fans, the power required becomes important. The evaporation of 1 kilo of water at summer temperature will cool about 2000 cubic meters, and that of 1 lb. 32,000 cubic feet of air 1° C.

In calculating the ventilating and heating power required in fitting up drying rooms, it is usually necessary to ascertain that required under the most unfavourable circumstances, and then add a liberal margin to cover errors and accidents. As the calculations are, in consequence of the many varying conditions, somewhat complex, it may be convenient to give as examples the quantities of air and heat required to evaporate 1 kilo (2·205 lb.) of water under different ordinary conditions, and these may serve as a basis of calculation of the drying power which must be provided for different tanneries.

1. Indifferent Open-Air Drying.—Air at 10° C. (50° F.), wet-bulb thermometer 7° C. (44·3° F.), indicating a total capacity for moisture of about 2 grm. per cubic meter; air not to be cooled beyond 7·75° C. (46° F.), leaving a residual capacity for moisture of 0·5 grm. per cubic meter. Each cubic meter will therefore take up 1·5 grm. of moisture, and as 1 kilo contains 1000 grm. we have 10001·5 = 666 cubic meters per kilo required to absorb moisture; and 6002·25° × 0·3 = 888 cubic meters reduced 2·25° to furnish the 600 cal. required for evaporation. Total air used 1554 cubic meters or 54,900 cubic feet.

2. Drying with Heat.—Outside-air at 10° saturated with moisture, heated to 20° C. (68° F.) acquires a capacity for 7·9 grm. per cubic meter. If we assume that a drying capacity of 2 grm. per meter is required to complete the drying, we have an effective capacity of 5·9 grm.

10005·9 = 170 cubic meters or 6000 cubic feet, and to heat this 10° C. will require 510 cal. Evaporation of 1 kilo will consume 600 cal. Total heat 1110 cal.

3. Drying with Heat.—Outside-air at 10° as above, heated to 25° C., giving an effective capacity for moisture of 13·5 - 2·0 = 11·5 grm. per cubic meter.

100011·5 = 87 cubic meters or 3070 cubic feet. To warm this 15° requires 391 cal.; and 600 cal. added for evaporation gives a total of 991 cal.

Comparing 2 and 3 we see that the higher temperature is more economical, where it can be allowed, than the lower, both in air and heat, though this is partly compensated by the greater loss of heat by cooling of the building, etc., which it entails.

4. Air at 0° C. heated to 20° requires about 97 cubic meters, or 3430 cubic feet of air, and a total of 1180 cal.

5. Air at 0° C. and heated to 25° C. requires 63 cubic meters or 2230 cubic feet, and a total of 1075 cal.

6. Air at -15° C. (5° F.) requires 4·5 cal. per cubic meter to raise it to 0° C., and acquires a capacity for drying of about 2 grm. per meter.

We will apply these figures to a drying room arranged with a screw-fan with a central division, or two floors, so that the air can be either circulated or replaced with fresh air from the outside at will (see Fig. 94, p. 435). Such a room with 100 feet of length clear of space required for fans, air passages, and heating pipes, and 20 feet × 8 feet in section, should hang about 800 medium butts, weighing say 12¹/₂ kilo (27 lb.) each, and when wet from the yard, containing the same weight of water. A 48-inch Blackman fan, under these conditions would probably move say 20,000 cubic feet (565 cubic meters) of air per minute, at the cost of 2 or 2¹/₂ horse-power. This, in a room of the section named, would give an average velocity of 125 feet per minute or rather under 1¹/₂ miles an hour; not at all too much to keep the air freely circulating among closely hung leather. If we assume that these butts are to be dried in a week (practically 10,000 minutes) under the conditions of No. 2, the 10,000 kilos of water they contain will require 1,700,000 cubic meters of air, or about 170 cubic meters per minute, or about ³/₁₀ of the air must be fresh every time it passes through the fan. 1 kilo of water requiring 1110 cal. must be evaporated per minute.

Under the conditions of No. 4, only 97 cubic meters of air per minute would be required, or about ⁵/₆ might be circulated without change, but the total heat required would be about the same, 1180 cal. Under the conditions of Nos. 4 and 6 some 1620 cal. per minute would be employed. It is hardly necessary to provide for the full amount of heat required by No. 6, since in this country such conditions occur but seldom, and never for more than a few days at a time, and during such a period, much less heat would suffice to carry on the drying at a slower rate, and keep out the frost.

Beside the heat required for actual drying, it is necessary to provide for that lost by the building during cold weather, and this is much more difficult to calculate. If, by arranging the outlet for moist air on the pressure side of the fan, the internal pressure of the building be kept a little lower than the outside, there can be no loss by escape of hot air, any leakage being inwards, and supplying a part of the change of air which, we have seen, is necessary. In a brick building with glass windows, the loss of heat is far less than in the old-fashioned wooden louvre-boarded structure, and where fan-drying is in constant use, the brick structure is much to be preferred. Frequent windows, with casements horizontally pivoted at the centre, will supply enough air for favourable conditions of air-drying, and when the weather is bad, resort is had to the fan. Most modern drying rooms in the Leeds district are built upon this plan. Where louvre-boarded structures must be used for fan-drying, the sides should be made as tight as possible in winter by sheets of canvas or sail-cloth nailed on, for which purpose old sails can be bought in seaport towns at reasonable rates, a few louvre-boards only being kept open for the admission of air in suitable positions.

Box, in his ‘Practical Treatise on Heat’[183] puts the loss through walls in brick buildings for a difference of 30° F. (16·6° C.) between inside and outside temperatures, at the approximate amounts shown in the following table.

Loss of Heat through Walls.

If the building is of several stories, the loss to the roof in the intermediate ones need hardly be taken into account, but if the ceiling is not tight, and open to the roof, the loss may be great, but difficult to estimate. If we consider the drying room already described, the total area of the walls and ceiling is about 4000 feet, and to maintain its temperature 30° F. above the atmosphere at 1·2 cal. per sq. foot would require 4800 cal. per hour or 80 cal. per minute, a very small amount compared to that consumed in drying.

The following table calculated from data given by Box will give some idea of the amount of steam or hot-water piping required for heating. The sizes given are for the internal diameter of the pipe, allowance being made for the increased heating surface of pipes of ordinary thickness. Small pipes are considerably more effective in proportion to their surface than large ones, and for high-pressure heating 1¹/₂ or 2-inch wrought-iron pipes are to be recommended as in many ways preferable to cast iron. The gilled or ribbed pipes now often used are also advantageous as giving a greatly increased heating surface.

Heat given by Steam-pipes.

The temperature of the air to be heated is understood to be 60° F.; at lower temperatures the quantity of heat given off by the pipes would be greater, and at higher temperatures less; the amount being approximately proportional to the difference of temperature between the air and the hot pipes. It is also important to note that the table refers to steam-pipes in still air, and that if placed in a powerful draught, (as immediately before or behind the fan), their heating effect may be at least doubled. This has not been considered in the following calculations.

Applying these figures to the estimate of 1110 calories per minute required for drying in our building, and assuming 80 calories per minute for the loss of heat through the walls, we have a total of about 71,400 calories per hour, and to obtain this would require 736 feet of 4-inch pipe at 220° F. (heated by exhaust steam) or 700 feet of 2-inch pipe heated to 300° F. by steam at 52 lb. pressure.

If we adopt the estimate of 1620 calories of No. 5 and 6, we shall require 1050 and 1000 feet of the two pipes respectively, and this covers approximately the worst conditions. We must, however, remember that these estimates are made for continuous drying during the twenty-four hours, and that if the fan and steam are only applied during a portion of this time, the supply both of air and steam must be proportionately increased, or the time of drying correspondingly lengthened.

It is very desirable, however, that the fan should be driven by a small separate engine, the steam for which will only form a small proportion of that required for heating, and of which the whole of the heat will be recovered, since even that utilised in driving the fan will again be converted into heat by the friction of the air, and will therefore cost nothing. This arrangement will enable the drying to proceed so long as the necessary steam is maintained, which in bad weather can easily be done by the night watchman. It may also be pointed out that, during a great part of the year, the goods can be dried to a “sammied” condition without heat, or in the open air, or in the case of dressing leather, a considerable part of the water can be removed by pressing or squeezing, effecting a further economy.

Fig. 93.—Blackman Fan.