P. M. Silin

Surface Evaporation

(evaporation-drying).

In the practice of starch production there is encountered a special type of evaporator—the Uhland evaporator. It consists of, for example, horizontal steam tubes arranged one above another. The liquid to be evaporated is poured from a trough through a row of small openings onto the upper row of steam tubes; after washing over its surface it passes over a special side wall to the next lower tube and so on to the lowest one, from which it flows into a collecting box. To prevent steam from entering the room in which the apparatus is installed, it is enclosed by a removable wooden casing. To carry off the steam, the cavity of the casing is connected with an exhaust pipe.

The advantage of this type of evaporator is that the layer of liquid which it evaporates (its thickness is of the order of [[unclear: value]]) is small and the duration of evaporation is very short—only a few seconds. Thus the liquid does not suffer from prolonged heating, which is very important for starch production; to this are added the high productivity of such evaporation (per unit surface) and the ease of cleaning the tubes from scale (one need only remove the side walls of the casing to expose the tubes). However, in the operation of Uhland’s evaporator a major defect was discovered which compelled its removal from factories producing higher grades of starch syrup (for example, from the largest Russian starch-syrup factory, “Volzhsky,” near the pier [[unclear: “Novinka”]]). It turned out that the tubes are not washed by the liquid quite uniformly, and in those places where the layer is too thin the liquid evaporates too strongly and is overheated under the action of the high temperature of the steam. (The steam is supplied from a steam boiler at a pressure of 4–5 atm.) The syrup proves to be colored by products of the caramelization of glucose and to have changed its taste.

It appears, however, that Uhland’s evaporator can be used somewhat differently, in which case the negative aspects of its operation should disappear and, on the contrary, new positive ones should appear. For this it is necessary to make a certain change in the operation, namely: into the tubes of the evaporator, instead of steam from a steam boiler at a pressure of 5 atm. (150°), exhaust steam from a steam engine should be admitted at 100° C; in addition, the exchange of air in the casing must be intensified (a fan should be installed). The character of the evaporation will then change considerably. The liquid will not boil, since the heating steam has only the temperature of boiling. It will “evaporate” from the surface, just as water evaporates in an open vessel: partly as a result of the diffusion of water vapor into the air, partly as a result of convection. Thus this will be “surface” evaporation. The temperature of the liquid being evaporated will be below 100°, and this temperature can be regulated by admitting a greater or lesser amount of ventilation air.

The state of the air removed from the evaporator is of interest. If above a liquid whose temperature is maintained, for example, at 80° C, in an…

... upon contact with it is air, then it becomes saturated with water vapor, i.e., in it at \(80^\circ\) there will be, for each cubic meter (mixture of air and vapor), \(0.293\) kg of water vapor; the partial pressure of this vapor will be \(0.467\) atm. (see Mollier’s tables). The partial pressure of the air is \(1 - 0.467 = 0.533\) atm. Consequently, every \(0.533\) cubic meter of air will carry away with it \(0.293\) kg of water, if this air is removed; or, for 1 kg of water, \(1.82\) cubic meters of air at \(80^\circ\) will be required, or

\[ \frac{1.82 \cdot 273}{273 + 80} = 1.40 \]

cubic meters of air at a temperature of \(0^\circ\) C.

As for the distribution of temperatures in the apparatus, the following may be said about it. The temperature of the heating steam is, of course, constant—\(100^\circ\). The temperature of the air, as it passes from the lower part of the apparatus to the upper, gradually rises. The temperature of the liquid flowing from top to bottom gradually falls, since in the lower part of the apparatus it encounters cold air and, most importantly, because especially in the middle part of the apparatus intense evaporation takes place, lowering the temperature of the liquid, since here the air is not yet saturated with water vapor. In the upper part of the apparatus, the content of water vapor in the air approaches the quantity saturating it; the temperature of the air here is close (but lower) to the temperature of the liquid.

If in the upper part of the apparatus the temperature of the liquid is, for example, \(80^\circ\), and the temperature difference between the heating steam and the liquid is therefore \(100 - 80 = 20^\circ\), then the average difference of these same temperatures throughout the apparatus will be greater, since in its middle and lower parts the temperature of the liquid, owing to the intensified evaporation, will be lower than in the upper part, for example, down to \(68^\circ\), as experiments show.

The surface evaporation apparatus can be forced; for this purpose it is necessary to increase the difference between the heating-steam temperature and the liquid temperature. With a decrease in the liquid temperature, the air leaving the apparatus will have the same temperature and will be farther from the state of saturation. The difference between the temperatures of the heating steam and the liquid will increase, which will correspondingly increase the heat transfer. But the evaporator, as we shall see, will operate less economically when forced. The consumption of steam for evaporating the same quantity of water will be greater.

Some idea of the productivity of surface evaporation can be obtained from a particular example. If the liquid enters at the top and has a temperature of \(80^\circ\), and at the bottom, consequently, lower, then the mean temperature of the liquid in the evaporator may in any case be taken as no higher than \(70^\circ\) C; and the temperature difference between the heating steam and the liquid as no less than \(100 - 70 = 30^\circ\) C. The heat-transfer coefficient for such an evaporator, according to the conditions of its operation, is the same as for the rapid-current evaporators of a sugar factory, i.e., \(10\)

\[ \frac{\mathrm{Cal}}{\mathrm{mtr}^2 \times \mathrm{min} \times {}^\circ\mathrm{C}}. \]

Thus, 1 square meter of surface in 1 minute, with a temperature difference of \(30^\circ\), must transmit \(10 \cdot 30 = 300\) Cal., i.e., a square meter per hour will evaporate about 30 kg of water. For comparison, one may cite the heat-transfer capacity of 1 square meter of surface of various bodies of sugar-factory evaporators:

From this it is evident that the evaporative capacity of surface evaporation is not low; it is normal, especially if one takes into account that in it cheap low-pressure steam is used, which has a low temperature.

Surface evaporation differs from the ordinary evaporation, in essence, in that there is no boiling in it; this makes it quite original and brings it together with some kinds of drying. It is precisely for this reason that it bears the name drying as well (hence, strictly speaking, evaporation-drying). The difference from ordinary drying is that there the transfer of heat takes place directly to the material being dried, while the air only carries away the water vapors; in ordinary drying, heat is transferred to the air; the air is the carrier of heat and at the same time carries away the moisture. Further, evaporation-drying is carried out in a technically perfected form, whereas what has long been done in all chemical and pharmaceutical laboratories—namely, evaporation in a water bath—is the aim and the performance here is one and the same. Evaporation in a water bath is used when one wishes to evaporate something at a low temperature. A steam bath gives, for example, a temperature of 100°, while in the cup with the evaporated liquid the temperature is always much lower than 100°. There is no boiling here, since the vapors are freely carried away by the air circulating over the cup. Therefore such evaporation can be carried out only in a completely open cup.

The air consumption [[unclear: for the removal of evaporated water]] and the useful-action coefficient of surface evaporation depend on the temperature, the moisture content of the air, and on the degree of saturation. The more air is introduced, the more heat is spent unproductively—on heating this air to the temperature of exit from the apparatus. For example, we have already calculated that, at 80° C., per kg of evaporated water there is consumed 1.40 cubic m of air (0° C.). Taking the heat capacity of air \((C_p)\) per cubic m \(= 0.300\), we find that unproductively, in obtaining [[unclear: the vapor]], there will be expended on heating the air

\[ 1.40 \cdot 0.300 \cdot 80 = 33.6\ \mathrm{Cal}. \]

Assuming the initial temperature of the air to be \(0^\circ\). The productive [[unclear: heat]] (at \(80^\circ\) C.) went to

\[ \lambda - 60 = 631.3 - 60 = 571.3\ \mathrm{Cal}. \]

[[unclear: Thus we obtain]] that the temperature of the liquid entering for evaporation [[unclear: is]] \(60^\circ\) C. The useful-action coefficient is

\[ \frac{571.3}{571.3 + 33.6} = 0.944,\ \text{or } 94.4\%. \]

In the following table the useful-action coefficient and the air consumption are calculated for various temperatures of the outgoing air.

If air enters undersaturated with water vapor, the useful coefficient falls, since the air consumption increases inversely proportionally to its saturation coefficient. For example, at a saturation of 60% the limiting air consumption will, at, say, \(70^\circ\), be not \(2.78\), but

\[ \frac{2.78}{0.60}=4.63 \]

cubic meters per 1 kg of vapor; the heat expenditure for heating this air will correspondingly increase, and the useful coefficient will be not 90.7, but 85.4. The useful coefficients computed in this way for air saturation of 60% are placed in the last column of the table. From these figures it is evident that only in an extraordinarily forced operation—almost impossible in practice—when, per 1 kg of evaporated water,

\[ \frac{15.85}{0.60}=26.4 \text{ cu. m.} \]

of air is blown in, evaporation at \(40^\circ\) and air manages to take up only 20% of its saturation moisture content, the useful coefficient falls to 68.6%; in general, however, it remains between 80 and 90%, i.e., quite satisfactory.

The advantages of surface evaporation are as follows:

1. The possibility of rapid evaporation (in a few seconds) at a low temperature, i.e., conditions more favorable than with evaporation in a vacuum pan.

2. The low temperature of the heating steam, which also prevents the possibility of local overheating of the liquid.

3. The possibility of utilizing the exhaust steam of steam engines; consequently, cheap evaporation. Usually, in order to utilize exhaust steam for evaporation purposes, one has to use an expensive vacuum apparatus with its expensive air pump. Here, only a cheap water pump is needed.

4. [[unclear: line damaged]] of the self-heating surface [[unclear: several words]], since [[unclear: several words]].

5. Easy accessibility for cleaning scale.

6. It is possible to regulate the evaporation intensity and to force it by admitting more or less air into the evaporator.

7. Even under artisanal-factory conditions it is possible to arrange multi-stage evaporation without a vacuum and without an air pump, by making the surface evaporator the last vessel.