May, 1921
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
453
Tunnel Dryers By Grahame B. Ridley HEINEMAN-PEARSON Co., RIALTO BUILDING, SANFRANCISCO, CALIFORNIA
There are many types of dryer t h a t may be considered as forms, or variations, of tunnels, but for the purpose of this discussion the term will be limited t o those dryers in which the material on trays is moved progressively in one direction through a tunnel supplied with a current of heated air which is introduced a t one end and removed a t the other end. I n this type all the heat used in drying is assumed t o be supplied by the moving air which also removes all the moisture evaporated. The movement of the air is dependent on the difference of pressure a t t h e two ends of the tunnel, and its direction is always from the hot end t o the cold end. This form of dryer is typical of those used in many of the larger fruit dehydrating plants, and, owing to low labor costs with large capacities, i t seems t o be rapidly superseding other types for this work. THE AIR SUPPLY
The air is usually heated in one of three ways: by steam coils, hot air furnaces, or direct heat. S T E A M HEAT-Steam is the most expensive from the standpoint of both initial cost and thermal efficiency, but is subject t o very exact regulation and thermostatic control. I n some cases i t is possible t o utilize steam t h a t would otherwise be wasted in some kindred process, as, for instance, where a dehydrating plant is operated in connection with a cannery. H O T AIR FURNACE-HOt air furnaces usually consist of a furnace from which the products of combustion are carried through a multiplicity of tubes over which the air t o be heated is drawn or blown. Sometimes the process is reversed, and the products of combustion surround the tubes through which the air t o be heated is drawn. Both of these types may be likened t o a steam boiler without any water in it, and are adapted only t o fairly low temperatures, unless constructed of material especially selected t o meet the requirements, as the danger of destruction from high temperatures and the accompanying high rate of oxidation is very great. D I R E C T HEAT-The use of direct heat is the most economical method of heating the air, but is dependent on a furnace in which complete combustion may be secured. Some very interesting work has recently been done in this line, and some of t h e largest commercial plants are now using this principle in the drying of fruit. Furnace thermal efficiencies of over 90 per cent are obtained, and repairs and replacements are negligible. rah-s-The heated air is forced through the tunnel by a fan, or fans, which take the form of a suction €an a t the cold end, or a pressure fan a t the hot end; or a f a n a t each end may be used. I n the case of a suction fan any leakage into the tunnel, such as t h a t caused by opening the tunnel t o take out a car, allows cold air t o rush in and reduces the temperature in the tunnel. This makes i t advisable in copmercial
installations, where a suction fan is used, t o provide air locks large enough for a n operator and car. Where a pressure fan is used, if the door a t t h e hot end of the tunnel is opened hot air rushes out and there is a reduction of the air velocity through the tunnel, but no lowering of the temperature. Where this type of fan is used it is not customary t o provide air locks, and the labor cost of handling the cars in and out of the tunnel is reduced. OPERATION O F TUNNEL DRYERS
T R A Y S A N D cARs-The material t o be dried is usually spread on trays, which are stacked on cars with sufficient space between the trays t o allow of the passage of the requisite amount of air. Sometimes the trays are moved through the tunnel on slides or rollers and transferred t o and from cars a t the ends. The cars are guided in the tunnel by tracks, and in some cases a track system is laid throughout the plant. I n other plants the cars have caster wheels and may be moved anywhere on a concrete floor. This allows of greater flexibility and a saving of space, especially where drying occupies a period of 24 hrs. and loading and unloading is completed in a shorter period. I n plants having a small capacity the cars are rushed through the tunnels by hand, but in larger plants they are often moved by a chain conveyor, which is motordriven through a clutch. I n the design of large plants the handling of the material during the processes preliminary and subsequent t o drying must be carefully considered, as frequently these processes cost more t o carry out than the actual drying. TIME I N T E R V A L B E T W E E N R E M O V A L O F CARS
I n a tunnel dryer handling a uniform material under ideal conditions the operation becomes purely mechanical, and the tunnel should be loaded a t all times with the same amount of material, which will vary from a condition of maximum moisture content a t one end t o the condition of desired final moisture content a t the other. It is evident t h a t , under these conditions, whenever a car of dried material is taken out a car of wet material should be put in a t the other end, and all the cars in the tunnel advanced one position. The time interval separating the taking of the cars out of the tunnel will be dependent on the number of cars in the tunnel and the length of the drying time. Fig. 1 has been prepared t o show this relation. If a specific example is taken, such as a tunnel containing eight cars and a drying time of 12 hrs., i t will be seen, by following up the line for 24 hrs. t o its intersection with the sloping line marked “12 hrs. drying time,” t h a t sixteen cars should come out in 24 hrs. and the time interval between cars will be 1.5 hrs., as is shown above the figure 12 denoting the drying time. Similarly, if the tunnel should be operated for only 12 hrs. i t will be seen t h a t eight cars will be taken out and the time interval between cars will remain the
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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
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length of time. This is probably best expressed in pounds per hour, and is dependent on the amount of material dried in a given time and t h e amount of moisture taken out. I n the fruit industry i t is a trade custom t o express this in terms of the ratio between the weight of the wet fruit and t h e weight of the dry fruit. This is called the wet-dry ratio, and t h e weight of t h e dry fruit is taken in all cases as 1. The nomograph in Fig. 6 has been prepared t o show the relation between these factors. Expressing the moisture removed in terms of a percentage of t h e original weight of the material is probably a more convenient mode of specifying the conditions taking place during drying. CALCULATION O F AMOUNT O F AIR R E Q U I R E D
Hours
F/qumon shphqhnesure dry/+ hmes /hhours ~;lpfirnbustdon&v~ Twnne/ copcrc;fy
FLQ.TUNNEL LOADINGDIAGRAM POR DETERMINATION O R TIME INTERVAL (HEADWAY)BETWEEN ENTERINGCARS, OUTPUTOF TUNNEL PER 24 HRS., DRYXNG TIMES, ETC.
Since all the heat used for evaporation is obtained from the air, t h e use of this in doing t h e work of evaporation will result in a drop in the temperature of t h e air which is a n exact function of the amount of water evaporated and may be calculated. Taking the weight of 1 cu. f t . of air a t 60" F. as 0.0761 lb. and t h e specific heat of air a t constant pressure as 0.2375, the amount of heat needed t o raise 1 cu. f t . of air 1 " F. is equal t o 0.0761 X 0.2375, or 0.01807 B. t. u., and conversely 1 cu. f t . of air dropping l o F. will release 0.01807 B. t. u.
same. This chart may also be useful in showing the number of cars needed t o keep up a continuous operation. If i t is assumed t h a t the trays are loaded during a n 8-hr. period, i t is evident t h a t enough loaded cars must be provided t o supply the tunnel during the 16 hrs. when no loading takes place. It will be seen from the chart t h a t during 16 hrs. eleven cars will be put into the tunnel, so t h a t a minimum of nineteen cars must be provided. Fig. 2 shows, in slightly different form, a condition of irregular loading a s compared t o the ideal condition. This chart also shows t h e number of cars in the tunnel a t a n y one time and the proportion of overload and underload as compared t o t h e normal load. A chart of some such form as this, made up from day t o day on the job, is of great assistance in determining the effects of variations in the operating conditions, a s i t gives a graphic record of each individual car. CAPACITY O F T U N N E L
The holding capacity of a tunnel is usually based on the number of square feet of tray area multiplied by the load per square foot, and the output per 24 hrs. depends on the drying time and is usually stated in tons of wet material. The nomographs in Figs. 3, 4, and 5 may be used for the rapid determination of these quantities. A straight line through selected points on any two scales will intersect the third scale a t a point indicating the third factor. WET-DRY RATIO
Before making any determinations of t h e amount of air and heat needed under any contemplated conditions of drying, i t is necessary t o know t h e amount of water t h a t must be evaporated in some given
FIG.2
Assume a condition where the atmospheric air has a temperature of 60" F. and is t o be heated t o 160' F., a t which temperature i t enters the tunnel from which i t is exhausted a t a temperature of 120" F. If the material enters t h e tunnel a t the cold end, t h e temperature a t which the evaporation of the moistuce takes place will vary from the entering
May, 1921
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
6
455
do
20 iI g O
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- 30
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15
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8
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2.5
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6 0
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temperature of t h e material a t G O " F. t o a temperature which may approach 160' F. in the case of material dried t o a point beyond which no further evaporation is possible. T h e heat needed t o evaporate 1 lb. of water a t 60' is 1058 B. t. u., and the heat needed t o raise 1 lb. of water from 60' t o 160' and evaporate i t a t t h a t temperature is 1102 B. t. u. Assuming the mean value of 1080 B. t. u. as t h e average amount of heat needed t o evaporate 1 lb. of water, the number of cu. f t . of air needed will be
~
0.01807
or 60,000 cu. f t . ,
in round numbers, dropping 1O F. While this is the actual amount of air needed t o evaporate t h e water, a n additional amount of air must be supplied t o furnish t h e heat required t o raise t h e temperature of t h e material and t h e trays and cars t o t h e temperature a t which they are discharged from the tunnel. Continuing the above example, if it is desired t o evaporate 900 lbs. of water a n hour, or 15 lbs. per rnin., with a drop in temperature of 40°, the air required for evaporation alone will equal
60'ooo or 22,500 cu. ft. per min., and with 40 a wet-dry ratio of 4 : 1, 14.4 tons of wet material, or 3.6 tons of dry material, will be handled in 24 hrs., or 300 Ibs. per hr. Assuming a weight for t h e cars and trays needed t o carry this quantity of material of 400 lbs., and a n average specific heat of the material, cars, and trays of 0.3, the amount of heat needed t o raise this mass t o the hot end temperature of 160" will be 700 X 100 x 0.3, or 21,000 B. t. u. per hr. If the material is discharged a t t h e cold end, t h e amount of heat needed will be 700 X 60 X 0.3 or 12,600 B. t. u. per hour. This is equivalent in the first case t o 350 B. t. u. per min., and in t h e second case t o 210 B. t. u. per min. Since 15 X 1080, or 16,200 B. t. u . , are supplied by 22,500 cu. f t . per rnin.,
350 x 22,500, 350 B. t. u. will be supplied by 16,200 or 486 cu. f t . per min. However, the full temperature drop of 40" is not available in this case, and the mean temperature drop of 20' may be taken instead, thus requiring twice t h e amount of air, or 972 cu. f t . per min., and the amount of air required per pound of water evaporated with 1' drop in temperature will
be 60,000
+
972 22,500 601000 or 62,592 cu. f t .
In
the second case t h e full drop of 40' is available, since, if t h e material, trays, and car are heated above the outlet temperature of 120', they will return the heat in cooling t o t h a t temperature, and the additional air needed will be 292 cu. f t . per min. The nomograph in Fig. 7 has been prepared t o show the relation of the temperature drop t o t h e volume of air used, but it must be borne in mind t h a t the volume of air used is based on its weight a t 60" F., and t h a t this volume must be corrected for the temperature a t which evaporation actually takes place. I n the above example this will become 27,500 cu. f t . per min. a t 160" and 26,700 cu. f t . per min. a t 120°, or in other words, 75,000 cu. f t . of air should be allowed in order t o evaporate 1 lb. of water a t 160' F. with a drop of 1" F. under the particular conditions assumed. It is often more convenient in making calculations of air requirements a t varying temperatures t o work with pounds of air until the final results are reached, and then transpose t o cubic feet. RATE O F EVAPORATION
Evaporation is due t o a difference in pressure between the moisture in the material and t h e surrounding atmosphere, a n d the rate of evaporation is a function of this difference. The rate of evaporation is affected by temperature, humidity, velocity
T H E J O U R N A L OF I N D U S T R I A L AATD E N G I N E E R I N G C H E M I S T R Y
456
of air flow, or barometric pressure only in so far as they determine t h e difference in pressure. I n order t o cause evaporation t h e pressure of t h e moisture in the material must be greater t h a n t h a t of t h e surrounding atmosphere. When they are equal evaporation ceases, and if t h e pressure of t h e surrounding atmosphere is greater t h a n t h a t of t h e moisture i n t h e material, the material will acquire moisture instead of losing it. I n tunnel dryers of t h e type under consideration, evaporation is caused b y bringing a current of heated air into contact with t h e material t o be dried. This air has a certain capacity for absorbing moisture due t o its not being saturated, and is effective in proportion t o this capacity. The pressure causing evaporation of water is vapor pressure, and saturated vapor has known pressures for each temperature which may be found published in steam tables and expressed in inches of mercury. The vapor pressure of partially saturated air may be found from various formulas, of which t h e following by Professor Ferrel is probably the best known:
f
=
f' - 0.000367P(t - t ' )
( + z2) 1
the temperature of the dry bulb in F. the temperature of wet bulb in F. the actual vapor pressure in the air in inches of mercury. f' = the maximum vapor pressure present at the wet bulb temperature t'. P = the barometric pressure in inches of mercury.
where t
=
t' = f=
If F be the maximum vapor pressure a t the dry bulb
f
temperature t , then the relative humidity is -. F If no air were present, t h e condition of t h e moisture could be likened t o t h a t of steam saturated a t a ternperature and pressure corresponding t o t h e dew point and superheated t o t h e temperature of the dry bulb and t o a pressure corresponding t o F. However, owing t o t h e presence of t h e air, the temperature of t h e wet bulb rises above t h e dew point, and t h e effective head is reduced by an amount equal t o t h e differeace between the vapor pressure, f', and t h e vapor pressure of the moisture at dew point, f. This may be expressed as an effective head equal t o : F -f(7-f) or F -f' I n many materials the moisture is held partly as free water on the surface or between t h e cells of t h e material, and partly as water more intimately combined with t h e cell structure. Under certain conditions of operation, a curve of the drying rate of some materials will show a distinct break when the free water is evaporated and t h e water in mechanical combination alone is left. This break is accompanied by a rise in t h e temperature shown by a thermometer having t h e bulb immersed i n t h e material. Until this point is reached, t h e thermometer in t h e material will show a temperature very close t o t h e temperature of the wet bulb thermometer in t h e air, and i t is reasonable t o suppose t h a t , as long as evaporation is not forced t o a point beyond t h e ability of t h e
Vol. 13, No. 5
material t o part with its free water, t h e temperature of t h e material will be t h a t of t h e wet bulb thermometer, and its vapor pressure will be t h a t of saturated vapor a t t h a t temperature, and t h a t evaporation will take place a t t h a t temperature. When t h e attempted rate of evaporation is greater t h a n t h a t a t which t h e material can give u p its moisture, t h e temperature of t h e material will rise above t h a t of t h e wet bulb thermometer, and t h e condition may be likened t o t h a t obtaining in a closed t a n k from which t h e rate of flow is controlled by a vent. The reduced head due t o this condition is shown by t h e increased temperature, and finally becomes zero when t h e temperature of t h e material equals t h e temperature of t h e dry bulb thermometer and drying ceases. Vapor pressures are directly dependent on temperatures and absolute pressures for any given substance, and since, in a tunnel dryer, all 8 drying may be con9 sidered as being IO carried on a t atc 300 mospheric pressure, the variations i n - li - 15 vapor pressure are $: due t o changes of 6. - 't g-no temperature. Since z : 2- 2 a definite amount 2. tL25 of heat is necesb- 4 2: sary for t h e evapt- 3 5;m oration of water, i t $: c -35 follows t h a t many calculations of dryFORHUL I\: c4000 ing conditions ,may be worked out in two ways: one from t h e point of vapor pressures, and one from the Fro. 6 point of t h e heat utilized. VCihile these two methods are interrelated, still i t is often possible t o use one t o check the other. VI
AIR VELOCITY
I n the above, t h e question of the velocity of t h e air has been neglected, i t being assumed t h a t the moisture evaporated was removed as soon as the vapor was formed. This would be t h e case with a n infinite velocity; but on t h e other hand, if there were no movement of t h e air, i t would become saturated and t h e temperature of t h e material would rise until equilibrium was established. Unless t h e velocity of t h e air is considerable, i t is probable t h a t there is a condition of varying saturation from t h e surface of t h e material t o the main air stream and a corresponding decrease in t h e effective vapor-pressure head between t h e moisture in t h e material and the atmosphere immediately in contact with it. This is similar t o t h e skin effect surrounding boiler tubes, and t h e action of increased velocity in securing greater heat transference is similar in both cases. I n order for full advantage t o be taken of
May, 1821
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERIhTG C H E M I S T R Y
1:
457
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50
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80
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FIG.7
FIG.8
high velocities, it is necessary t h a t the air be brought into close contact with the surface of the material being dried, and t h e trays should not be spaced farther apart t h a n is necessary t o secure the passage of the requisite amount of air a t the desired velocity. When determining drying rates under varying conditions, the air velocity should be kept constant until the other variables are investigated. I n the ordinary tunnel dryer, the velocity is substantially constant a t any position, but decreases from t h e hot t o t h e cold end as t h e volume of air is reduced by cooling. T h e usual velocities employed are from 300 t o 1000 f t . per min., and i t appears t h a t above the latter figure the effect of increased velocity becomes less marked, and in most cases is not worth the cost of the power required t o produce it. I n order t o obtain uniform temperatures over the cross-section area of the tunnel, a certain velocity is needed, because, if the velocity is too low, convection currents and other disturbances will cause wide variations in temperature, resulting in uneven drying. It is also important t h a t the full area of t h e tunnel be occupied by t h e trays and trucks. The air shows a wonderful facility for taking the easiest route and will by-pass around the trays instead of going between them if i t is given any chance. Some heat is expended in raising the temperature of the water vapor from t h e temperature a t which it is evaporated t o the temperature of the surrounding air. THE DRYING TEMPERATURE
The advisable temperature for drying is generally determined b y some characteristic of the material. Most materials in their finally dried condition have a limiting temperature which cannot be exceeded without deterioration taking place. Some materials have a definite rate of evaporation which may not be exceeded without injury, and others show distinct
T;ro. 9
variations of condition when dried a t different rates. This is particularly noticeable with some fruits which, when dried slowly, tend t o darken and acquire t h e leathery skin characteristic of sun-dried fruit, but when dried rapidly preserve the original color and texture of the fresh fruit t o a marked degree. The commercial tendency is naturally t o hasten the drying in order t o increase the output of the plant and reduce the equipment needed, and in many cases this also tends t o produce the best product. Some materials must be started a t a fairly low temperature and high relative humidity and brought up slowly t o the temperature of evaporation. These materials are characterized by poor heat and moisture transference qualities, and, if they are put into a hot, dry atmosphere, the surface dries rapidly, while t h e center of the material remains cool and moist. A high vapor pressure is produced a t the surface, which tends t o drive the moisture both t o the surrounding atmosphere and also toward the center of the material where the moisture is under lower vapor pressure owing t o its lom7er temperature. This still further aggravates the condition and may cause a hard shell, or coating, of dry material t o form around the still nioist interior. This is similar t o the action of searing a steak and is known as "case hardening." It effectually prevents further drying, unless the material is subjected t o an atmosphere of high relative humidity for a considerable time. Hardwood lumber is typical of this class of material. Where the material is finished a t the hot end of the tunnel i t is not safe t o have the temperature of the air higher t h a n the temperature t h a t the material can stand without injury, unless evaporation is not nearly completed. If the material is finished a t the cold end, somewhat different conditions result. Many materials will stand a higher temperature when moist than when dry.
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T H E J O U R A T A L O F I N D U S T R I A L 4ATD E N G I N E E R I N G C H E M I S T R Y
Also, in most materials the rate of evaporation is greatest when they contain the most moisture, and their temperature will not rise above t h a t of the wet bulb until drying is partially completed, so the difference between the temperature of the material and that of the air will be greater a t the hot end of the tunnel if drying is started a t t h a t end. It is evident from this t h a t , for a limiting maximum temperature of the material, the allowable temperature a t the hot end of the tunnel may be greater if the material is entered a t t h a t end and finished a t the cold end. This, of course, means more rapid drying, and if the temperature drop through the tunnel is made suitable t o the material i t is often possible t o keep the temperature of the material practically constant throughout the drying operation. When the material may be heated t o a higher temperature in its moist condition than in its dry condition, a still greater saving in the time of drying may be made by a higher entering air temperature. When the material is entered a t the hot end, the temperature drop through t h e tunnel must be regulated t o suit the requirements of the material and where a small temperature drop is used, i t is probable t h a t t h ? air will not be brought t o a n economical point of saturation, and it then becomes necessary t o recirculate a portion of it. Recirculation is also used for the purpose of regulating the humidity with regard t o the requirements of the material. For materials which stand a higher temperature in a moist atmosphere than i n a dry one, the added humidity a t the cold end is a n advantage. It must be remembered t h a t with high humidities the temperature of the dew point is raised, and a condition often occurs where the material put into the tunnel has a temperature lower than the dew point. I n this case n o drying takes place until the temperature of t h e material is raised above the dew point temperature, and during the warming-up process moisture may condense on the material, in which case the temperature of the air will rise owing t o the releasing of the latent heat in the vapor. This addition of moisture is less marked when the material is entered a t the hot end, as the warming-up process is hastened by the higher temperature, but in any case may be serious with some materials. This is especially SO with fruit which often condenses enough moisture t o form serious dripping, which washes off the juice and the sugar contained in it, and deposits a thick sirup on the floor of the tunnel and on the trucks and trays. This is a loss of the most valuable part of the fruit, and is best avoided by preheating in a n atmosphere of sufficiently low dew point. THERMAL EFFICIENCY
The following observations were made a t a plant successfully using direct heat. I n this plant the fuel used had a Baum6 gravity of 31.8, weighing 7.22 lbs. per gal. I t s heat value was 19,875 B. t. U . per Ib. The quantity of fuel used during t h e 2.5 hrs. of the test was 28 gal., equal t o 1.35 Ibs. per min. The air was heated 87" from a temperature of 64" F. t o a temperature of 151" F., a t which temperature the weight would be 0.065 Ib. per cu. f t . The actual
Vol. 13, KO.5
air flow, as estimated by Pitot tube readings, checked by anemometer readings, was 18,400 cu. f t . per min., a t the higher temperature. The total number of cubic feet of air per minute possible t o heat with this 1.35 X 19,875 or 20,000 weight of fuel is 0.2375 X 0.065 X 87 cu. f t . per min., and the thermal efficiency of the furnace was 18'400 or 92 per cent. Other tests on this 20,000 and similar installations have shown efficiencies ranging from 92 t o 98.5 per cent. It is very difficult t o arrive a t the determination of the actual air flow, and, where efficiencies are as high as those shown, a difference of 2 per cent of the air flow means 1 per cent of the efficiency, but there can be no question but t h a t direct heat is a most efficient way of heating the air, when it is correctly applied. I n some systems of using direct heat i t is necessary t o use a high gravity and therefore expensive fuel, but other systems operate satisfactorily with Diesel fuel oil, and it seems probable t h a t even straight crude oil may be used eventually. Where t h e cost is not prohibitive, electricity is the ideal method of providing direct heat. The nomograph in Fig. 8 shows the relation between various furnace efficiencies and fuel costs. While the furnace efficiency is of interest, it is often desirable t o know the overall thermal efficiency of the dryer. During operation this is most easily arrived a t by taking a period of 24 hrs., or longer, and subtracting the weight of t h e dried material from t h e weight of the wet material t o get the weight of t h e water evaporated, and then comparing this with the number of gallons of fuel used. This relation is shown by the nomograph in Fig. 9, which must be corrected t o conform t o t h e actual fuel used. OPERATION O F A F R U I T DEHYDRATING PLANT
The method of operation of a fruit dehydrating plant, on prunes for instance, is subject t o local conditions, but in general the fruit is brought t o the plant in what are known as "lug boxes," which hold from 40 t o 50 lbs. The gross weight is taken when coming in, and the net weight is determined b y weighing the outgoing boxes, or using the average of a number of them, t o secure the tare weight. The boxes are unloaded on t o a platform, or directly on t o a roller conveyor. The fruit is emptied into the dipper from the lug boxes, and the empty boxes are returned t o the unloading platform where they are picked up by the teams and returned t o the orchards. I n the dipper the prunes are plunged into a tank of boiling lye which removes the waxy bloom and checks the skin with a number of small cracks, without which drying is almost out of the question. I n t h e larger plants this dipping is done in a machine having a n endless draper belt conveyor which carries the prunes through a tank of lye heated with stSam coils and then through a tank of cold running water. After coming out of this tank the prunes are passed under cold water sprays, which further cleanse them. This machine handles from 4 t o 5 tons of fruit an hour,
T H E J O U R N A L OF IrVDGSTRIAL A N D Eh'GINEERING C H E M I S T R Y
RIay, 1921
a n d discharges on t o a combined grader and shaker feeder which sorts the prunes into two grades according t o size, and discards the small, immature fruit as culls. The fruit is discharged from the shaker feeder directly on t o the trays where a slight additional spreading is done by hand, and the trays areloaded by hand on t o the tunnel trucks. The trucks are placed in the tunnels by hand and are moved through the tunnels by a chain conveyor. Whenever a truck of dried fruit is taken out of the tunnel, the whole string of trucks is moved forward one position and a truck of wet fruit put in a t the other end, thus keeping the tunnel always loaded t o capacity. The trucks holding the loaded trays of dried fruit are moved b y hand t o a hopper discharging into the boot of a n elevator which carries the fruit t o the second story, whence i t is distributed by belt conveyors, or wheelbarrows, t o the storage bins. The trays, after being emptied into the hopper, are put on a roller conveyor which carries them direct t o the discharge end of the shaker feeder where they are reloaded. The empty trucks are returned t o t h e loading point b y hand. It will be seen t h a t , by this method, the equipment of trays and trucks makes a continuous circuit, but t h a t the wet fruit and dry fruit are handled a t different points on the circuit and are kept separated. Also, the fruit in the lug boxes and the dried fruit storage bins is kept away from the part of the plant where active operations are in force. I t is essential t h a t in any plant of this kind, storage space be provided t o take care of interference in the normal cycle of operation. Storage room must be provided for the incoming fruit which may arrive faster than i t can be handled, and for the outgoing lug boxes which may accumulate. Trucks may be loaded faster t h a n they can be put into the tunnels and may come out of the tunnels faster t h a n they can be unloaded. Trays may be emptied faster t h a n they can be loaded, and for all these conditions space must be provided. This question of space becomes still more important when part of t h e operation, such as dipping, takes place during only a portion of the day, while drying is continued throughout the 2 4 hrs. COST O F OPERATION
I n calculating the cost of drying any given material, i t is best t o bring all labor costs t o a basis of hours of labor per ton of dry material, and all other costs, such as t h e cost of fuel, power, and material, t o a similar basis per unit of cost. The following example may be considered typical of this method: Here i t is assumed t h a t a fruit product, such as prunes, is t o be dried in a dehydrating plant having the following characteristics: 6400 sq. ft. 4 tunnels, each having a tray area of .... Total tray area ............ 25,600 sq. f t . Tray load per s .. 3 Ibs. Drying t i m e . . ..... 18 hrs. 2.25 : 1 Wet-dry ratio, t . Maximum temperature a t hot end ...... 160' F. Allowable temperature drop, 50' Temperature a t cold e n d . . 110' F. 60 per cent Overall thermal efficiency.. Wet capacity in Ibs. per hr. per sq. ft.. 0 . 1 7 (Fig. 3)
..
.. .. ........... ............. ............
....
4.59
Wet fruit capacity of plant in tons per 24
hrs ................................
52 (Fig. 4)
Dry fruit capacity of plant in tons per 24
.........................
city of each tunnel in tons . uit dried in each tunnel in 21
23.1 9 . 6 (Fig. 5 )
., .
13
Tons of dry fruit output from each tunnel in 24 h r s . . Pounds of water evaporated per hour, per
5.77
.........................
....................... 600 (Fig. 6) r tunnel on basis of 1 lb. of water evaporated by 70,000 cu. ft. dropping 1'. ....................... 14,000 cu. f t . per min. (Fig. 7 ) Tons of wa 4 hrs., 52-23.1, . . . . . . 28.9 Gallons of f overall efficiency of 60 per cent and fuel cnpable of evaporating 135 lbs. of water 714 (Fig. 9) per g a l . .
...........................
The fruit will all be brought t o the plant during 12 hrs. Taking the capacity of the dipper as 4 . 5 tons of wet fruit per hour, it is evident t h a t i t can handle the total requirements of the plant in 21 hrs. Drying will be continuous for 24 hrs. Unloading the trays may be continuous, or intermittent. The distribution of the labor on the job will vary widely under different managements and different types of labor. As an instance of this latter point, it has been found t h a t , owing t o the small stature of Japanese laborers, four men are required t o stack trays a t a height of 7 feet, while the same work can be done easily by two tall white men. For purposes of illustration the labor may be distributed as follows: CLASS Weigher.. ...........................
Number of Men 1
Period Worked
Total Hours
Distributing to bins. Furnaces and boiler. Superintendent.. TOTALHOURS
......... 1 ............
On a daily capacity of 52 wet tons and 2 3 . 1 dry tons, this is equal t o 6.38 hrs. per wet ton and 14.37 hrs. per dry t o n . . The fuel used in the furnaces has been estimated as 714 gal., t o which must be added t h a t used by the boiler furnishing steam t o heat the dipper, say, 100 gal., or a total of 814 gal., equal t o 15.65 gal. per wet ton and 35.2 gal. per dry ton. The power used will be t h a t needed t o drive the four fans and the furnace blowers for 2 4 hrs., the dipper for 12 hrs., and the tunnel conveyors and elevator for short periods. I n addition, lights will be needed for some 12 hrs. This may total some 1000 kw. hrs. per 24 hrs., equal t o 19.2 kw. hrs. per wet ton, or 43.27 kw. hrs. per dry ton. Lye for dipping will run about 10 lbs. per wet ton, or 2 2 . 5 lbs. per dry ton. I n addition t o the above, there will be some expense for water and other incidentals and for repairs. To the above costs of productive operation, must be added the overhead and fixed charges. On plants '
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t h a t are operated on a single, seasonal product where t h e drying of t h e year’s crop may have t o be completed in a month or six weeks, these charges become very heavy when prorated on a tonnage basis, and every effort should be made t o extend t h e drying period as far as possible throughout t h e year on other products. SU
MARY
Tunnel dryers may be built suitable for drying any material t h a t can be handled economically on trays. They are particularly suitable for handling large quantities of fairly uniform material under conditions of progressive, continuous operation. They are cheap t o construct and economical t o operate. A successful tunnel dryer must have means for varying t h e temperature of t h e air through any range t h a t may be required. It must be arranged t o allow of regulating t h e humidity of t h e air with exactness. The velocity of t h e air should be under complete control and recirculation of any desired portion of it should be provided for. The dryer should be considered only in its relation t o t h e rest of t h e plant, and t h e handling of t h e material before and after drying should receive close attention. T h e tendency of commercial practice is t o hasten drying as much as possible, and this can often be done without injury t o t h e material, and in some cases with distinct benefit, when all t h e conditions of t h e problem are known. Materials t h a t will stand a
Vol. 13, No. 5
higher temperature when wet t h a n when d r y can generally be dried most rapidly when they enter t h e tunnel a t t h e hot end, as t h e temperature of t h e cold end then becomes t h e limiting temperature, and a higher average temperature may be maintained. The highest thermal efficiencies are obtained when t h e air is discharged at t h e highest relative humidity t h a t t h e condition of t h e product will permit. It is usually more economical t o use a large quantity of air a t high velocity with recirculation, t h a n t o use a small quantity of air with low velocity and no recirculation. I n closing, it may be stated t h a t there is need of more exact information as t o t h e behavior of various materials under different conditions occurring in drying. Little seems t o be known, even b y established manufacturers, in regard t o t h e characteristics of t h e products t h a t they t u r n out, and in almost every problem of design considerable leeway must be allowed t o take care of contingencies t h a t cannot be foretold. This adds t o t h e expense of t h e installation, which, t o a great extent, could be avoided by more accurate knowledge of t h e premises on which t h e solution of t h e problem must be based. NoTE-The
paper on “Vacuum Drying” b y Charles
0. Lavett and D. J. Van Marle was not received in time for inclusion in this report, and will be printed in a later number of THISJ O U R N A L .
ADDRESSES AND CONTRIBUTED ARTICLES The Immediate Needs of Chemistry in America’ By William J. Hale D o w CHEMICAL COMPANY, MIDLAND, MICHIGAN
Both university men and industrial men have depicted many essentials necessary for the success of young chemists. A good rigorous training is always to be encouraged for those who seek a chemical future. Further, if we start this training in early childhood, all the better; simple thinking with clear deductions makes for better faculties in later days. Our elementary schools and high schools may stimulate the scientific spirit when once aroused, but mcxe than likely they will not, amidst the deluge of diversified de7 dion to things utterly foreign to mental advancement. Thus, a mathematical course easily surpasses in value the sum total of all other subjects taught in our schools;no matter whether scientific or unscientific be the student’s interests, his mental makeup is incomplete until he has had this training. During my experience in teaching, I found the greatest number of freshmen more deficient in this field than in any other. Of course their use of English is pathetic, but this slowly improves through influence of educational environment. As a result, I have become thoroughly convinced that mathematics makes for the greatest good to students of our primary and secondary schools. How far they should pursue this subject in college and university naturally will depend on their future aims in life. Let us grant, then, without argument that a rigorous early training constitutes a firm foundation for the best chemical training a t the university. *Address delivered before the Society of the Sigma X i at Purdue University, LaFayette, Ind., February 17, 1921; also presented before the Division of Dye Chemistry at the Blst Meeting of the American Chemical Society, Rochester, X. Y.,April 26 to 29, 1921.
The young men of the universities pursuing courses in chemistry or chemical engineering have commanded, next in order, the chief attention of our many lecturers on this general subject. Some have told us of the advantages accruing from a purely scientific course of study; others have told us of the immense advantages which fall to those pursuing a more utilitarian course, such as chemical engineering. I qhall hesitate here no longer than to remark that little difference does it make what course a young man takes so long as he knows well the fundamental principles of his science, and cognate sciences, and can readily apply this knowledge when occasion demands. The great majority of young chemists graduating from our universities select some position with an industry where chemists are essential or nearly so. In those instances where the “nearly so” variety obtains, you may consider the young chemist as acting in all capacities at once. In general, however, the young men are placed directly in the research divisions or in the analytical laboratories. The varied training of these chemical neophytes forbids any serious discussion as to just what they are best fitted for. They occupy, so to speak, the same relative position as freshmen entering college. Though the proverbial rough edges and apron strings of the verdant freshmen are long since removed, there have appeared anew certain oddities in our graduate which now must be corrected; such, for example, as the experimental niceties, the more or less sanctified professorial customs of procedure, and the textbook overdrapes. The first condition is soon remedied when he finds himself working in vessels too large for the fine balances; the second is removed more slowly,
