US2156845A - Method of and apparatus for drying substances in vacuo - Google Patents

Description

May 2, 1939.

J. G. w. GENTELE 2,156,845

METHOD OF' AND APPARATUS FOR DRYING SUBSTANCES IN VACUO Filed March 22, 1938 J- G. W. Gien-ale Patented May 2, 1939 PATENT OFFICE METHOD OF AND APPARATUS FOR DRYING SUBSTANCES IN VACUO Johan Georg Wilhelm Gentele, Stockholm,

l Sweden Application March 22, 1938, Serial No. 197,485

In Austria March 27,1937

1 Claim.

My present invention refers to a method and an apparatus for drying in vacuo substances liable to change under the action of heat. Such substances are vegetables, berries, fish, eggs,

5 milk, etc.

My present invention refers to an improvement of the method and apparatus set forth in my copending application Ser. No. 41,486.

As set forth in said earlier application it has hitherto, in dryingsensitive substances lying on metallic supports, been necessary to maintain a relatively low temperature in the heating elements, such as at most 'l0-90 C. in order to avoid burning and destroying of the products to be dried when the supports are directly or indirectly heated by conducted heat. Due to the necessity of employing such low temperatures` the drying period has been very long, for instance 24-48 hours. Such a long drying period at the low temperature existing in the products to be dried in vacuo may cause great inconveniences, such as destroying of the vitamines, fermentation processes etc.

According to the invention covered by my co- 25 pending application it is possible to dry products of the kind described in a very short time, such as 1 2 hours if the same thickness of the layer of products is used as in the above mentioned example. This is due to the fact that it is possible to have the heating elements Aheated to very high temperatures, at least 200 C. and up to 600 or 800 C. or still more, Without damaging the products. The employment of such high temperature on the heating elements according to my copending application is rendered possible by preventing heat to be conducted to the metallic supports for the products, which is effected by carefully heat insulating said supports from the conductive parts of the apparatus and from the heating elements.

The heating of the metallic support and the goods lying on the same is therefore effected practically exclusively by radiant heat emanating from heating elements located above and 45 beneath each support and at a distance from the same. The heating elements which may be heated by steam, combustion gases, electric current, etc., are of substantially the same size as the supports so that said supports and the prod- 50 ucts lying on the same are uniformly heated all over their surfaces.

Applicants further investigations have shown that improved results may be obtainedif the products to be dried are uniformly heated both 55 from the upper and from the lower side. It has,

namely, been found that if the supports used are made of bright metal and these are .exposed to radiant heat from the upper and lower side the rays coming from the upper side and irnpinging on the products lying on the supports 5 will penetrate, said products so that the liquid in the upper part of the same is rapidly evaporated and expelled. The rays coming from the elements beneath the supports and impinging on the lower surface of said supports will to a great 10 extent be reflected back from the same, wherefore only a small part of the rays will be obsorbed by the support so as to heat the same. This' causes that the products are exposed to a higher temperature on the upper side than on the side 15 being in contact with the support. l

In order to avoid this inconvenience when drying substances of the kind described in vacuo applicant has proposed to treat the lower side of the metallic support and, if necessary, also the 20 upper one, so as to get more or less high ability of absorbing the heat rays. They may, for instance, be made black or dull or otherwise treated, so that the reflection of heat rays will be avoided as much as possible. 25

The metals adapted to be used as supports in applicants invention should thus be treated so as to get a high intensity of radiation. Such metals are for instance oxidised sheet iron, aluminium sheets which are blackened or made 30 dull on one or both sides so as to get a highconstant ofradiation, oxidised copper etc. Aluminium is considered to be the most convenient material since it has no chemical influence on the products to be dried. 35

By using such supports the greatest part of the rays emitted from the heating elements located below the supports Will rapidly heat the supports, so that the products lying on the same will beheated from below practically as much 40 as they are heated from the upper side, whereby the products will consequently be heated substantially uniformly from both sides In spite of the high temperature of the heating elements the supports will, due to the above mentioned heat insulation, be maintained at a relatively low temperature as long as there is some moisture in the product. Due to the fact that prac tically no heat will be conducted to the supports but the heating is effected practically exclusively by radiant heat the temperature of the products will not be higher than that which corresponds to the boiling point of the liquid contained in the products at the existing vacuum. The products are thereby safely prevented from being burnt or destroyed. I! no insulation were used heat would-be conducted to the supports from the heated walls of the apparatus in which case the supports would after a short time get substantially the same temperature as the heating elements. If, therefore, the temperature of the latter is very high, the products to be dried will be destroyed. Theheating should be interrupted before all the moisture has been removed, for instance when the content of moisture has been reduced to about 5-7%. During the last stage of the drying process it is also convenient to stepwise reduce the supply ofheat to the heating elements.

In executing the method according to applicants present invention it is convenient to use an apparatus of substantially the same construction as that shown in my zopending application. 'Ihe annexed drawing shows byway of example the same apparatus as is shown in Figs. 1 and 2 of the copending application.

Fig. 1 is a longitudinal section through the apparatus and Fig. 2 is a cross-section along the line II-II in Fig. 1.

The apparatus consists of a casing I which may be provided on the outside with a heat-insulation 2. One of the end-walls is in the form of a. door 3 which can be opened when products are to be put in or taken out and which is kept hermetically closed during the drying process. Inside the evacuated chamber A there are electrical heating elements 4, which are well-insulated, not only against electric current but also against heat so that the conduction of heat to the other parts of thev apparatus is prevented. Between the heating elements and at a suitable .distance from them the supports 5 for the products to be dried are located, said supports bey ing insulated against conducted heat from the other parts of the apparatus.

The supports may as mentioned consist of for instance oxidised sheet iron, blackened or dull, aluminium or other convenient metal with high intensity of radiation. According -to applicants present invention especially the lower side of said supports should be blackened or treated with acids or made dull in any convenient manner, but even the upper side may be made dull. By treating the lower surface or, if wanted, also the upper surface of the supports so as to get a high constant of radiation, any suitable proportion between the heat supplied from above and that supplied from below may be obtained.

For effecting the heat insulation of the supports from the wall and other heat conducting parts of the apparatus any convenient insulating material may be used, such as wood, board, wood fibres, asbestos, Bakelite or the like.

In the apparatus shown it is assumed that the heating is effected by electric current. 'I'he supports 5 and heating elements 4 are carried by a frame 30 (Fig. 2) of insulating material provided with insulated projections 9 on which said l supports and elements rest. n the outer sides of said frame there are placed electric conductors 32 with which the heating elements are connected. The heating elements may be of any convenient construction, such as plates, bands, wires. The main lines and the terminals of the elements are electrically insulated from the metal parts ofthe apparatus.

The heating elements 4 are placed at a suitable distance from thesupports for the materials to be dried so that the heat transmitted to the latter is only radiant heat.

As shown in Fig. 1, -a cooling battery I0 is placed near the one end of the evacuated chamber. This battery consists of a number of parvallel tubes (see Fig. 2) placed close alongside each other, through which cold water ilows, the temperature of the water being such that ef- .fective cooling and consequently condensation of the water vapour is caused so that the latter is quickly removed from the evaporation zone.

I2 is a. suction pipe leading to the vacuumpump I3. As this pipe leaves the apparatus on the side of the cooling batteries I0 in the evaporating chamber opposite to that on which the supports and heating elements are placed, no water-vapour escapes from said chamber into the suction pipe of the vacuum pump as it is condensed when it comes into contact with the cooling batteries. Consequently, it is possible to maintain a very high vacuum which is indicated by the gauge I4. Thus, the vacuum pump is not used for removing the water-vapour but only for maintaining the requisite vacuum.

The condensed water formed when the watervapour comes into contact with the cooling batteries is collected in a recess I5 in the bottom of the oolingzone and is thence removed through the pipe I6 to the container I1, which may suitably be graduated so that the quantity of condensed water may be measured, thus giving a. check on the progress of the drying process. On the pipe I6 there is a valve I8 and on the outlet pipe I9 a valve 26. When water is to be removed from the vessel I1 during the process, the valve I8 is closed and the valve 20 is opened.

I claim:

Apparatus for drying substances in vacuo which are liable to change under the action of heat, comprising a casing enclosing an evaporating chamber, means for creating a vacuum in said chamber, metal supports for the products to be dried in said chamber, said supports being made black or dull on at least the lower side so as to get a. high intensity of radiation, radiant heating elements in said chamber placed o n both sides of each support and at a distance from the same, means for insulating the said supports from heat conducting parts of the apparatus, and means for cooling the vapour generated from said products.

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