12. In the operation of tomato scalders, a similar conclusion may be drawn. Here the steam and water do not mix; that is, intentionally, it being the purpose to have first the steam and then the cold water strike the raw material. Steam at 80 to 100 lb. pressure is supplied through a 2-in. pipe and passes through a series of perforated holes in pipes placed above and below a conveyor chain carrying the tomatoes. The bed of fruit is something like four to six inches thick, and it is expected that these steam jets will penetrate the mass and heat their outer skins in about ten seconds. Now in the same chamber in which the steam is working, and just beyond the steam jets, are jets of cold water. There is no dividing partition, and the result is a splendid condenser effect. As the water must be kept cold, more of it than otherwise necessary is turned on to counteract this effect, thereby enhancing it. Provision is made through a vent flue so that if any steam escapes both water and tomatoes it can go out through the roof.

13. Obviously, with the application of ingenuity and experiment the steam consumption of these machines might be materially lessened through radical structural changes without impairing effectiveness or capacity. On the other hand, and opposite to the case of bleachers, it is difficult to secure steam economy by nice regulation alone-for one reason because of the prejudice of operators. The writer himself has, during operation, cut down the steam valves on scalders from full opening to one-half turn, and the

girls peeling the tomatoes never knew the difference. But once let them know that the steam is reduced, and they will insist that the tomatoes are not scalded enuogh!

14. Another instance of the effectiveness of supervision: Tomatoes are dumped from baskets on to the conveyor leading to the scalder by two untutored negroes. Due to the lapses between baskets, the conveyor chain travels 5 ft., then receives a charge, then another idle 5 ft., and so on. Each basketful forms a small pyramid, 8 to 10 in. deep at the apex. The tomatoes on the outside are scalded sufficiently, but those at the thick part of the mass hardly at all, whereupon the girls handling the latter demand more steam. By the simple expedient of having the tomatoes distributed on the conveyor chain in an approximately uniformly thick bed, about 50 per cent. of steam is saved.

EXHAUST BOXES.

15. Exhaust boxes are now to be considered. In the home-made form (see Fig. 2) these are long rectangular boxes made of four planks, open at the ends, and through which a conveyor chain passes bearing the filled but uncovered cans. Drilled pipes within the box are used as steam jets playing on the cans. At the ends of the box are flues, so that when the steam is fully turned on it will not pass into the packing room, but out through the roof.

16. It is at once apparent that it is out of the question to obtain anything like a uniform temperature of the can contents in the small space of time that they are subjected to the steam, particularly if they are packed so closely and contain little liquor. Fig. 3 shows the results of some temperature measurements made by inserting a thermometer at different points. The average temperatures were obtained by calorimetric determination. It is thus seen that although it is the intention to elevate the temperature of the cans to about 160 deg., nothing like this is accomplished. If it were, the original temperature being about 100 deg., the useful heat (that is, heat actually transferred to the cans) would be 16 boiler h. p. for the conditions shown in Fig. 1. Actually, the useful heat is only 2 to 5 boiler h. p. (corresponding to temperature ranges of from 5 to 20 deg.). In Fig. I a temperature rise from 80 deg. to 120 deg, is assumed. The heat con sumed in the form of steam may be as much as anything between 20 and 30 boiler h. p., making for a thermal efficiency of about 10 to 15 per cent. Where does the difference go? Through the roof.

17. The performance of the exhaust box as regards steam economy may be materially bettered by the careful regulation of the steam supply. From trials which the writer has made, involving temperature and pressure measurements, he has found that the steam can be cut down from full on to about one-half a turn of the stop valve in some cases without materially affecting the temperature elevation. That is, the heating effect is nearly as good when the steam merely trickles through as when it pours through. To gain effectiveness, the time during which each can is subjected to the

steam must be lengthened, and the greater the time, the greater will be the machine's efficiency. This fact is being recognized by the manufacturers of modern forms of exhaust box, which are so shaped that the length of the path through them is greatly increased, thereby increasing the time of heating (to 5 and 15 min.). Such a box is shown in Fig. 4.

18. Much can be gained if the exhaust box is in part relieved of its effort by introducing whatever liquid goes with the solid part of the contents as hot as possible. As previously mentioned, after the solid material is placed in the can, either by machine or hand, liquor is added either in the form of brine, syrup or fruit juice. This liquor is previously heated, but between its heating and the time of introduction into the can it often is allowed to cool, either by halts in the procedure or through radiation from pipes and containing vessels. In every case the liquor should enter the can at about 210 deg., which high temperature may so elevate the average temperature of the packed material, especially in hot weather when the initial temperature of the solid is high, as to make the additional elevation by the exhaust box trifling.

JACKETED KETTLES.

19. Turning now to a consideration of the various packing-house units used for concentrating liquid foods and food juices, which units may be termed generally "evaporators," it will be noted that there are many different forms and types. Perhaps the most elementary form, and in some ways the most interesting, is the jacketed kettle (see Fig. 5). In shape this is the same sort of utensil as the old-time housewife used, but, of course, is larger, having a capacity of between 50 and 500 gal. It receives its heat from the steam jacket whence it gets its name. The jacket is tapped with one or more openings to receive the steam pipes and another opening for the drain pipe to carry off the condensed steam.

20. The performance of these kettles is very interesting in many ways. In the first place, it may be observed that under approximately ideal conditions their thermal efficiency may be nearly 100 per cent., the only loss being from radiation of heat from the outside of the jacket. This assumes that the condensate from the jacket is returned to the boiler through a return trap, which, however, is not always-or even often-used. Steam at 100 lb gage pressure has a temperature of 338 deg., so if steam of this pressure is used in the jacket of a kettle, it will, after condensing in the jacket, emerge from the drain pipe as water at 338 deg. If this hot water

is returned to the boiler by a return trap, none of the heat is lost (except that small amount due to radiation), and practically all of the heat given up by the steam goes to the useful purpose of evaporating water from food material.

21. There are, however, three other ways in which steam may be handled (see Fig. 6). First, the drain may be passed through an atmospheric trap. which necessitates that the water be reduced to below 212 deg. before it can be returned to the boiler. In this case the efficiency of the system is 87 per cent., and 13 per cent. of the heat consumed is wasted. Practically the same thing may be accomplished if the drain pipe is without a trap, but is supplied with a stop valve so regulated that only water will be discharged. Second, the kettle may be supplied with an atmospheric trap, or a stop valve in the drain pipe as just described, the discharge from which is not returned to the boiler. In this case, if feedwater at 70 deg, is used to make up the 212-deg. water which might have been used, the waste is 24 per cent., and the efficiency only 76 per cent. Third, the kettle may be unprovided with a trap of any kind, and the valve in the drain pipe left so wide open as to let large quantities of steam escape as well as condensate. Here it is difficult to estimate the ensuing waste, but if one is to judge by the ascending clouds of steam, it must be enormous.

22. Many operators of these and other steam-using units in the canning factory labor under the delusion that a violent commotion and an earsplitting racket are evidences of rapidity. Hence they will turn on the steam until their eys and ears are satisfied in these respects. Unless means are taken to prevent such excesses through traps and other food-proof devices, it is impossible to eliminate wastes.

23. Before leaving the subject of jacketed kettles, a word must be said about their performance as regards capacity. Judged as an evaporator suitable for high rates of evaporation, it would seem, offhand, that nothing could be cruder. There is absolutely no provision made for systematic circulation of the kettle contents, which matter of circulation is given most careful thought in the design of steam boilers. A jacketed kettle in full blast shows the most haphazard ebullition, and one would suppose that a much more effective rate of evaporation could be obtained if the circulation could be assisted. In spite of this, the fact is that this apparatus is a remark

ably quick evaporator. Results that have come to the writer's attention show, with the use of 100-lb. steam, as high as 8.5 gal. or 70 lb. of liquid evaporated per square foot per hour after the mass has come to a boil. This corresponds to about 700 B.t.u. of heat transferred per hour per square foot per degree difference of temperature, a figure comparing very favorably with the best types of feedwater heaters and condensers, which class of steam-engineering apparatus the jacketed kettle most closely resembles as regards heat transfer.

24. Jacketed kettles differently installed show very different capacities for evaporation. The question then arises: What are the factors affecting the rate of evaporation? A study of this question shows that the total amount of water a kettle can evaporate per hour may be affected chiefly by the pressure of the steam in the jacket. The rate of heat transfer is directly proportional to the difference in temperature between the boiling material (about 212 deg.) and the substance supplying the heat, that is, the steam. Now, since the temperature of steam increases with its pressure, the highpressure steam is more effective in rapidity. But if the steam pipes are too small, or if the kettle opening for steam is not large enough, there may be a considerable drop of pressure of the steam before it reaches the jacket, and a still further drop after it gets into the jacket. In consequence, and especially if the steam is initially wet, it falls in temperature and loses some of its effectiveness. In this connection it is appropriate to remark upon the prevalent packing-house custom of economizing on pipe sizes. A kettle with a bushed steam opening cannot evaporate as fast as one with an unrestricted supply. Even under the best conditions it is a matter for speculation whether or not the steam pressure in the jacket is not materially less than that in the steam pipe because of the condenser effect of the comparatively cool liquid within the kettle. At all events, much can be done to improve capacity by using carefully calculated pipe sizes, and by introducing the steam into the jacket through two or more openings instead of only one. A series of experiments with the purpose of learning the pressure within a steam jacket for different systems of piping would, it is felt, disclose facts of practical value in future design.

25. The capacity of a jacketed kettle may be much reduced by the cooking material caking to its sides. To avoid this condition, kettles are frequently equipped with mechanical stirrers, which continuously wipe the heating surface, thus keeping it clean. This action also imparts a velocity to the boiling liquid at the heating surface, which, presumably, affects the heat transfer, but just how much is not known. Similarly, the quantitative effect of high velocity of the steam in the jacket is unknown.

COIL EVAPORATORS.

26. The final limitation to capacity has to do with the area of heating surface. In this respect the jacketed kettle bears about the same relation to evaporators of refined design, such as the vacuum pans, as the first shell boiler to present-day water-tube boilers; that is, it is deficient in heatnig surface. The obvious step, then, is to add heating surface in order to increase capacity, and this has been done. The additional surface takes the form of basket or ring coils, or nests of tubes, whereby the heating surface can be tremendously increased. There is, however, an undetermined limit to the effectiveness of such additions, since they may retard the circulation of the boiling material to such an extent as to decrease instead of increase capacity.

27. Coils offer such an attractive way of forming a compact heating surface that they have come into favor in units which dispense entirely with the steam jacket. Any form of containing vessel may be used for the liquid to be concentrated, and the vessel may be made of any appropriate