US2329581A - Apparatus for drying material by utilizing vapor heat - Google Patents

Description

sept- 14,

R. BERNST EIN APPARATUS FOR DRYING MATERIAL BY UTILIZING VAPOR HEAT 2 Sheets-Sheet 1 Filed June 25, 1940 Sept. 14, 1943, R. BERNSTEXN 2,329,581 I APPARATUS FOR DRYING MATERIAL BY UTILIZING VAPOR HEAT Filed June 25, 1940 2 Sheets-Shae; 2

Patented Sept. 14, 1943 APPARATUS FOR DRYING MATERIAL BY f Y-UTILIZING VAP B'HEAT V H Rudolf Bernstein, zurich, Switzerland epplicationlaun fzs, 1940,'Serial'No; 342391 I In Switzerland 0ctgberj2L-l93 7 v I l Cla im. (0134-56)" It has already been proposed to use drying apparatus with tunnel-like chambersin which the material is dried by utilizing vapor heat; The

external air can enter freely into the tunnel, and

at that place in the tunnel where it has presumably acquired the maximum moisture it is passed into a heat exchanger for the purpose of. supplying drying heat. The transmission of heat to the material to be dried occurs through the air in the tunnel which is circulated a number of times. i

Such an arrangement has the disadvantage that the quality of air which is passed over the heat exchanger through which the vapor flows and over the material to be dried is too small in comparison with the air and vapor mixture simultaneously passing through the heat exchanger to be able to transmit a sufiicient amount ofheat in the vapor to the material to be dried.

The present invention relates to a method of and an arrangement for drying material by utilizing vapor heat, whereby the aforementioned disadvantages are overcome as a result of the "drying air being brought, in .a first drying section by means of external heat and due to receiving water from the material to be dried, to

approximately such a high dew p'ointas the material to be dried will allow and being passed practically saturated through heat exchangers in. at least one further drying sectionwhile fresh air is forced over the heat exchangers and the material to be dried, said drying air being by weight at least 50 times as large as the quantity of water which, during the same time in the first drying section, is absorbed by the drying air as vapor and passed to the heat exchangers of the next drying section. v

The invention is described in connection with the accompanying drawings, in which,

Fig. 1 is a diagrammatic illustration of an apparatus consisting of two drying chambers,

' Fig. 2 is a diagrammatic illustration of an, ap-

paratus consisting of three drying chambers,

"Fig. 3' shows an arrangement with two drying chambers in vertical longitudinal section,

has-an external source of heat'l'. Fresh airenters chamber I at p; this is circulated a large number of times by the ventilator u, driven by'th'e motor-3i, and passes, mixed together with the hot-vapors in chamber I, throu gh pipe q'into tlie heat ex .changeri of chamber II and heats the air inside chamber II which is mixed with additional fresh air entering'ats, At the same time it is possible Fig. 4 shows a horizontal sectionof an arrangement with two drying chambers, v

Fig. 5 shows across section on the line IV-IV in Fig.3,

Fig.6 shows an arrangement with three drying chambers in longitudinal section, and

Fig. '7 shows a cross-'sectionon the line V+V in Fig. 6. 7 I v In Fig. l the material a tobedried isdivided between the two chambers I and II, on'eof which to supplyeach chamber I and II with fresh material and tocomplete the drying in the sa'me chamber; alternatively the material can be given a preliminary drying in either chamber I orelfl and the remainingmoisture is then extracted in chamber II or I. and air passes to the atmosphereat'r. 2

The ratio of thefquantity of air flowing inside the heat exchangers to that outside of same, which it is preferable to adhere to and'which differs very considerably from previous values, is calculated as follows:

If for instance in chamber I with the external source of heat, a practically saturated Vapor-air mixture with a dew point of C. is produced and thisis passed into the heat exchanger- 'of chamber II which is heated by vapor, the rise in temperature of the air in the drying space there can be restricted to 40 C.' if about six timesas much air is blown through it as goes through the heat exchanger during the same interval of time.

The final, mixture of; 'vapor For 1 kilogram vapor with a dew pointof 70C.

there shouldbe about 3 kilograms of air; at 40 C. however, about 20 kilograms of air, that is to. say about si point of 705C.

When it is desired to utilize the heating medium from a dew point of 70 C. down to a temperature of about-40; approximately 1'70 calories must be extracted from eachkilogram of air contained in it and this transmitted by means of the dryingair to the material to be dried; Since,

however, owing to the temperature difference which exists between the heating medium and the drying air, it is not an advantage to warm the drying air on an'average more than 15" CL, thatis lkilogr'am of this 'air absorbs during one contact with the heating elements approximately higher than beio're the quantity of drying'air must be correspondingly increasedwhen it is desired to utilize: completely the available heat.

x times as much as with a dew Since the heat content of the heating medium is mainly determined by the amount of water vapor contained therein, the necessary quantity of drytimes the quantity. I

The method described in connectionv with the example illustrated in Fig. l 'enables practicallyall the heat stored in the vapor of the drying section with the external heat source 'to be reutilized once. A further saving in external heat is possible if, in a known manner, the air-vapor Whole length of the channel. This ventilator operates like a so-called drum type blower and circulates the air in a manner indicated by the arrows in Fig. 5, so that the air whilst it passes through the channel 0 is circulated a large number of times between the material to be dried and the heat exchangers z.

In the constructional example described, the air and vapor mixture from the drying chamber 9, which is heated from an external source, after its dew point has been raised by heating as high as the material to be dried will allow (for instance forgrass 8 0 C.), is forced through the heat exchanger 1' of the second drying chamber c. In this drying chamber the air is recirculated, from the heat exchangers'i to the materialto be dried so many times by means of the mixture taken from the drying chamber I with the external heating is, as shown in Fig. 2, first through heat exchanger 12' of the drying chamber II ,from which latter the fresh air sup plyior chamber I' is taken. If for instance. the

dew points of this air is forced up to 80 Clby means of additional heat, part of itsheat which is liberatedbetween .80 C. and to C. by condensation of the water vapor contained therein can be utilized in the third drying chamber II where the material to be dried is comparatively cool.

. What has up to the present been overlooked is that in this case it is necessary to provide a very strong air circulation, because the amount of air driven through the drying chamber II. is the same as that in the heating medium in the tubes 2' and therefore considerably smaller than that in chamber II. Furthermore the temperature differences in chamber II are much smaller than in chamber 11 because it is desired to obtain here as high a dew point as possible in order to reduce to a minimum the amount of external heat required.

In the constructional example illustrated in Figs. 3-5 the material a to be dried, for instance grass, is brought in the direction of the arrow on to an endless conveyor band b whichtransports it to d where it falls down thewall e on to a second conveyor band f which transfer-s it to It, Whereit is expelled from the dryer. Onthe conveyor band 22 thematerial to be dried passes through the channel 0 which contains the heating elements 1', The conveyor band 1 moves the material to' be dried. through the channel g in which the heating elements I; are located. The channel 9 is the second drying ventilator u that the amount of recirculated air by weight is at least 50 times as great as the amount of 'vapor contained in the air and vapor mixture passing hrough th ubes 1 during the same interval of time.

g It is. permissible also to circulate the airin channel a in a similar manner, although there the. temperature difference. between heating elements and the. material to be dried may be many times greater than that in channel 0, so that, the

circulation need not be made so strong.

With the three section drying arrangement illustrated in Figs. 6 and '7, the material to be dried. l is thrown on to the conveyor band 2 in the direction shown by the arrow, where it is transported up a slope into channel 4, closed at the upper end, and from there by conveyor band '31 to the lower'end where it falls on to conveyor l0 which conveys. it through channel H, which .is ;open at both ends, to be expelled at IS. The

pp end of channel l contains the heating elements. 9 which are supplied with live steam or the like. Below these are the elongated heating boxes 6 which act'as heat exchangers and are clearly shown in Fig. 6. These boxes are fed with {channel H are powerfully cross ventilated by chamber and is heated from an external, source in the first drying chamber. This air has en tered the channelg at n and p: as dry air and is sucked in the form of moist air out of the middle of the channel 9 by means of a small ;fan 0,

which conveys it through the pipe q into the heat The heating air and condensed exchangers i. V V water flow out through the pipes t1 andtz. In addition the fan T, which is very large by comparison with fan 0, withdraws air fromthe middle of the channel a and conveys it'to the atmos- .phere; this air enters channel 0 at $1 and s2.

As can be seen from Figs. 4 and 5 the air which flows through c is strongly circulated by means of a ventilator wheelof special construction. In

channel c there is a ventilator wheel u arranged next to the conveyor b'and extending along the .drum type blowers l4 and I5, exactly in the same ,waylas channel 0 in Fig. 5. by means of inclined surfaces on the blowers or in the path of the circulating air, this latter re- At the same time ceives the required motion in a longitudinal direction through the channels. The air and vapor mixture can be drawn through the heating boxes 6 and 8 by means of the small fan l2.

The amount of free air entering and leaving the drying chambers is preferably selected to correspond With the dew point which it is desired .139 a in nd the amount of heat transmitting air is determined by the choice of the speed of circulation of same. The quantity of air driven through and its speed of circulation in the different drying chambers can. be so selected that dew points result which, due consideration being given to the available heating surfaces at each place, the average heating temperature and the circulating q an ty of dry ai enable for each drying section the intended utilization of the heat in the vap r to be hieved.

I claimf Apparatus for drying materials comprising a first drying chamber, a seconddrying chamber and a third drying chamber, a heater in the first drying chamber, a source of external heat, means for delivering heat from said external source to said heater, a first heat exchanger in the second drying chamber, a second heat exchanger in the third drying chamber, said first and second heat exchangers being connected in series, said first and second drying chambers forming an upwardly sloping channel closed at the top with the first drying chamber at, the higher level, means for passing air through said first drying chamber, means for withdrawing air and vapor mixture from said first drying chamber at its highest point and passing it through said heat exchangers, an upper conveyor band in said channel for conveying material to be dried to and through said first drying chamber, a lower conveyor band for conveying material from said first drying chamber to and through said second drying chamber, a third conveyor band for conveying material delivered by said lower'conveyor band through said third, drying chamber and means for circulating air through said second and third drying chambers, said last named means being capable of contacting at least 50 times as much air by Weight with the material to be dried as is the weight of vapor which passes from said first drying chamber to said heat exchangers in the same interval of time.

RUDOLIF BERNSTEIN.

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