US3468036A

US3468036A - Method and apparatus for drying of materials

Info

Publication number: US3468036A
Application number: US696549A
Authority: US
Inventors: Josef Blaettler
Original assignee: INETA ESTABLISHMENT
Current assignee: INETA ESTABLISHMENT
Priority date: 1967-01-13
Filing date: 1968-01-09
Publication date: 1969-09-23
Anticipated expiration: 1986-09-23
Also published as: NO125205B; GB1175301A; FI48136B; DE1729411A1; DE1729411B2; DE1729411C3; FI48136C; SE348824B; AT284733B; CH469237A; FR1550816A

Description

Sept. 23, 1969 BLAETTLER 3,468,036

METHOD AND APPARATUS FOR DRYING OF MATERIALS Filed Jan. 9, 1968 4 sheets-smet 1 I NV ENT OR ATTORNEYS v wm.. il

Juif

Sept. 23, 1969 J. BLA ETTLER 3,458,036

METHOD AND APPARATUS FOR DRYING OF MATERIALS Filed Jan. 9. 1968 4 Sheets-Sheet 2 s -s E N g N .9 2 u.

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ATTORNEYJ Sept. 23, 1969 J. BLAETTLER 3,468,036

METHOD AND APPARATUS FOR DRYING OF MATERIALS Filed Jan. 9. 1968 4 Sheets-Sheet 3 MGT* BY @any y' mre?,

N @N111 mm ,il l Nm llll mll I.||l|| HH lll S II.. l lllllllllllllllllll l||+| E I m: -il 2 Il; i -li a S Sept. 23, 1969 J. BLAx-:TTLER METHOD AND APPARATUS FOR DRYING 0F MATERIALS Filed Jan. 9, 1968 4 Sheets-Sheet 4 lll .INVENTOR United States Patent O 3,468,036 METHD AND APPARATUS FOR DRYING F MATERIALS `Iosef Blaettler, Zurich, Switzerland, assignor to Ineta Establishment, Balzers, Liechtenstein, a corporation of Liechtenstein Filed Jan. 9, 1968, Ser. No. 696,549 Claims priority, application Switzerland, Jan. 13, 1967, 480/ 67 Int. Cl. F26b 3/04, 2]/08 U.S. Cl. 34-22 16 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for drying materials by means of circulating air condensation systems wherein the moisture content of the circulating air contacting the material to be dried is intermittently increased and subsequently decreased. The actual value of moisture content in the circulating air is simultaneously compared with the reference value of moisture content throughout the intermittent increase and decrease thereof. If, during an intermittent increase of the moisture content of the circulating air, the actual value of moisture content fails to reach a predetermined reference value of moisture content Within a predetermined time interval, a signal is generated and delivered to a control apparatus which is effective to decrease or lower the reference value of the moisture content. The above operation is continued until the material is sufficiently dried.

Background of the invention The present invention relates to an improved method of drying materials by means of a so-called circulating air-condensation system. Furthermore, the present invention also pertains to an improved appartus for the performance of the aforementioned inventive method.

With previously known drying techniques, for instance, for Wood and other materials, cooling of the circulating air is undertaken at the circulating-air condensation system after there has been reached the desired temperature of the chamber or compartment in which there is housed the material which is to be dried. The circulating air is cooled below the dew point by means of a cooling unit and its moisture content is reduced. Thus, after the circulating air is heated up to the previous temperature by means of a heating unit, it is able to absorb moisture from the material to be dried. This drying technique is employed for such length of time until the material to be dried, for instance wood, does not deliver any more moisture to the air. At the beginning of the drying process the wood delivers a relatively large amount of moisture to the air so that cooling of the circulating air in the cooling unit must be adjusted to a maximum value. This is most readily recognized by the fact that a characterizing line or curve which graphically portrays this operation, the so-called drying curve, runs or extends quite steeply at the beginning of the drying operation. The dryer the wood becomes, the less water is delivered to the circulating air surrounding the wood. Thus, as time progresses, the drying curve becomes atter and flatter, that is to say, it subsequently runs or extends asymptotic with regard to the null point (drying moisture, i.e. residual moisture left after drying). Thus, the danger exists that with a uniform cooling, that is to say, with uniform condensation of the circulating air during the course of the drying period, the delivery of moisture from the air to the cooling unit is greater than the delivery of moisture from the wood to the air. At this moment there occurs a so-called encasing of the wood, that is, the pores of the 3,468,036 Patented Sept. 23, 1969 "ice wood in the outer layer tend to draw together and thus prevent the removal or evaporation of the water which appears at the interior of the wood. Additionally, owing to the irregular drying of the material which proceeds much too rapidly, the stability of such material is affected in a disadvantageous manner. In the case of wood such results in the wood working upon completion of the drying operation and after such wood has already been installed in some type of building structure or has been processed into furniture. This results in considerable complaints, whether it be because the windows and doors no longer t into their frames, or because the veneers which have been glued onto the wood tend to warp and form cracks, or for any other reasons.

Summary of the invention Accordingly, it is a primary object of the present nvention to overcome the aforementioned drawbacks of the prior art techniques and constructions.

Another, more specific object of the present invention relates to an improved method and apparatus for the effective drying of materials in a highly etlicient, reliable and economical manner.

Another signicant object of the present invention relates to an improved method of, and apparatus for, the drying of materials in an extremely efficient, controlled and reliable manner, particularly for the drying of wood.

Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the inventive method is characterized by the features that the moisture content of the circulating air contacting the material to be dried is intermittently increased and subsequently decreased during the drying operation with simultaneous comparison of its reference value with its actual value, and during the reduction of the moisture content in previously determined time intervals a signal corresponding to the comparison is only then delivered to a control apparatus if after the expiration of a predetermined time the actual value of the air moisture content has not reached the reference value.

Accordingly, the quantity of cooling medium used in the cooling unit is regulated as a function of the moisture of the material to be dried.

Furthermore, the inventive apparatus for carrying out the aforementioned method is characterized by the features that a bridge circuit having a differential relay is provided for the intermittent increase and reduction of the air moisture content and a timing relay having a control mechanism is provided for the stepwise regulation of the moisture difference of the circulating air.

Brief description of the drawnigs The invention will be better understood, and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

FIGURE 1 schematically illustrates in cross-sectional view a preferred embodiment of inventive drying installation;

FIGURE 2 graphically illustrates a drying curve in order to explain the inventive method;

FIGURE 3 is a lblock diagram of the electrical apparatus; and

FIGURE 4 is a circuit diagram illustrating the differential relay as well as the time-relay provided with the reference value-control mechanism for the air moisture.

Description of the preferred embodiments Describing now the drawings, in FIGURE 1 there is illustrated a housing H providing a chamber or compartment 7 which is partially mounted in the ground at the brick work 8. Located within the compartment or chamber 7 and at 4all sides thereof is a suitable insulating material 5. This drying chamber or compartment 7 is capable of receiving or housing the material 4 which is to be dried. This material `4 is displaced into the drying chamber 7, for instance by means of suitable rails or the like which have conveniently only been schematically indicated by reference numerals 51. After completion of the drying operation this material 4 is again removed from such chamber 7. The arrows 6 schematically represent the direction of ow or circulation of the air.

The circulating air is conveyed or transported in accordance with the direction of the arrows 6 by means of a suitable ventilator or blower 2, driven by means of a standard electric motor or other appropriate drive unit. Reference numeral 1 designates a heating unit which serves to heat the circulating air to a constant pre-adjusted temperature. On the other hand, reference numeral 3 designates a suitable cooling unit or device which cools the air below the dewpoint, and therefore, removes moisture from the air. The water which condenses at the walls of the cooling unit 3 is removed by means of the discharge or outlet 9. Continuing, it should be understood thata vaporizer or evaporator 31 serves to moisten the chamber air. The generated Vapor `arrives at the air stream through the agency of a pipe or conduit 31a and the outlet portion or nozzle 3111. The purpose of this vaporizer or evaporator 31 for moistening the chamber air will be described in greater detail hereinafter.

Now, in FIGURE 1 there has only been illustrated one air circulating unit or system composed of the ventilator or blower 2, the heating device 1 and the cooling device or unit 3. However, with larger chambers or cornpartments, for instance with a chamber volume of 20 cubic meters and greater, it would be possible to use two or more such air circulation units. Smaller drying installations, for instance with a chamber volume up to five cubic meters are advantageously constructed as portable or transportable small installations which are formed as iron structures.

Referring now to FIGURE 2, it will be understood that there are illustrated therein drying curves for wood. The curve A', for instance, is for hardwood and the curve B Ifor softwood. Furthermore, along the abscissa of this graph there is plotted the drying time, and along the ordinate the percent of relative atmospheric moisture. Such atmospheric moisture is measured by means of a suitable hygrometer which is applied to the location where the circulating air has absorbed the most moisture from the material to be dried. The curve A for hardwood is represented by the connecting line for the peaks or tips of a so-called zig-zag curve. This zig-zag curve will now be explained and considered in detail.

The descending portion of the curve, namely point C along the abscissa, for instance the section k1, signifies a reduction of the relative air moisture content from 90% to 80% of the circulating air which is flowing in the direction of the arrows 6. This section k1 indicates that the cooling unit 3 is switched in and in this manner frees the circulating air of moisture until it reaches a moisture content of 80%. Thereafter, the cooling unit is switched out or the cooling work or efficiency is reduced by appropriately controlling the cooling medium, so that no moisture or only very little moisture is removed from the air, whereby the relative air moisture content again increases. This is indicated by section t1 at the curve A' in FIGURE 2. Now, as soon as the relative air moisture content has attained a value of 90%, the cooling operation undertaken by means of the cooling unit 3 is again switched in at full capacity until the relative air moisture content of the circulating air has dropped to 80%. The just described alternation between an increase of the relative moisture content (sections il, t2, etc.) and a reduction of the relative air moisture content (sections k1, k2, k3, etc.) is continued for such length of time until the relative air moisture content no longer reaches the value of This condition is indicated at section D1 along the abscissa of the graph of FIGURE 2. Then, the next reference value-stage in the control device, which will be described in greater detail in conjunction with FIGURES 3 and 4, is switched in. This will be recognized in FIGURE 2 by the fact that the total drying curve A is sub-divided into DN sections. These sections have been labeled at the top of the curve by reference numerals I to V. Each of these sections represent a particular stage. In each stage there is adjusted a predetermined reference value of the air moisture content and such is compared with the actual air moisture content prevailing in the drying chamber 7. Now, as soon as the upper Value of the predetermined relative air moisture content which has been adjusted at each stage is no longer reached, for instance 90% is stage I, the next stage, for instance stage II, is automatically switched in and provides a new upper limit of the relative atmospheric moisture content for the entire control cycle. In the new stage there again takes place the same cycles of reduction of the air moisture content (k1-kp) and increase of the moisture content of the circulating air by the material to be dried (sections tl-ts). Due to this entire process or technique which has been described above, there results the zig-zag curve up to a terminal value. At this terminal value, which is at the right hand side of FIGURE 2, the entire installation is shut down, since then a value of the relative air moisture content or humidity has been reached at which the material to be dried has been completely dried or dried to a desired degree, since a xed predetermined relationship always exists between the atmospheric moisture content and the moisture content of the material to be dried.

It will be recalled that in FIGURE 2 there is depicted the above-described curve A', for instance for hardwood, and the further curve B for soft wood. The curve B is constructed or plotted in the same manner as the curve A. Thus, it is not necessary to go into the further details of the curve B'.

Furthermore, by referring to the graph of FIGURE 2, it will be additionally recognized that the range or region B to C along the abscissa represents the heatingand vaporization period or time. This signifies that, if for instance, a stack of wood 4 is to be dried in the drying chamber 7, the air temperature must be heated to a predetermined, constantly maintained value of 50 C. for instance. Naturally, this requires quite a bit of time. The vaporization time or period, which likewise is plotted along the time axis at the interval or section B to C, is then provided if the stack of wood 4 is pre-dried wood, since by means of the so-called vaporization the dried pores `at the surface of the wood are again opened for the removal of the water from the interior of the wood cells. The heating time as well as the vaporization time is solely dependent upon the type and quantity of the material to be dried as well as upon the degree of dryness of its surface.

It should be appreciated that for materials which 4are diicult to dry it is absolutely necessary to maintain the air moisture of the compartment quite large during the heating time in order to prevent an encasing or drying out of the surface of the material. Under the expression materials which are difficult to dry there should be understood, among other things, hardwood, dough, and cement plates for instance. If dough products which are produced, for instance, by machines, such as for example spaghetti, noodles or the like, are dried, then the surface will become dry quite quickly, yet the inner portion remains moist. During the fabrication of structural elements for the pre-fabricated housing industry, for instance cement plates, there always resulted quite long drying periods since the surface of these cement plates would be dried before the inner portion could release its moisture. Only after quite a considerable period of time would the cement plates be completely dried out. The same problem exists with hardwoods. Since the surface of hardwoods is dried very quickly, the core of these woods remains moist. The present invention is therefore advantageously employed for accelerating and improving the drying of such materials.

For this purpose, and before the heating unit is placed into operation, there is additionally undertaken a moistening of the chamber air (section A-B of FIGURE 2) by means of the vaporizer 31 or equivalent structure. Now, if the chamber air has attained the moisture content of 90% (point B along the abscissa of FIGURE 2) then the vaporizer 31 is placed out of operation by the control device and at the same time the heating is switched on. If the moisture content of the air in the chamber has dropped to 80%, then the vaporizer 31 is again switched in until the moisture content has again reached 90%. This operating cycle repeats during the heating-up period (section B-C of FIGURE 2). If the material to be dried has been heated to the pre-set chamber or compartment temperature, then such will evaporate sufiicient moisture to saturate the chamber air in order to thereby prevent the previously mentioned encasing of the material undergoing the treatment.

Upon expiration of the heatingand moistening period the control switches oli the additional Vaporizer and the condensation or cooling unit is switched in (point C of FIGURE 2). From this moment on the air is intermittently demoistened and the drying operation proceeds automatically in accordance with what has been heretofore described.

Now, in FIGURE 3 there is depicted a block diagram of a preferred embodiment of inventive apparatus for carrying out the inventive method considered in connection with the explanation accompanying FIGURE 2. In FIGURE 3 there has been schematically indicated a hygrometer 11 which is operatively coupled with two bridge circuits 12 and 13. The bridge circuit or arrangement 12 acts upon a hygrometer 14. This hygrometer 14 serves to indicate the relative air moisture content prevailing in the drying chamber lor compartment 7. A further indicator is provided which can be set to a predetermined moisture value, for instance 40%. Now, if in the course of the drying process the relative air-humidity or moisture content has reached this value, then the hygrometer`14 switches the trip circuit or cutoff relay 16 through the agency of the amplifier 15. This cutoff relay or trip circuit 16 places the entire installation out of operation.

The hygrometer 11 which, as already described, is located at a suitable place within the chamber or compartment 7, is also simultaneously coupled with the bridge circuit or arrangement 13. This bridge circuit 13 is composed of a bridge arm for the reference value adjustment of the air moisture content and a bridge arm for the actual-control of the actually prevailing air moisture content or humidity. This will be explained in greater detail in connection with FIGURE 4. Continuing, it will be understood that a differential relay 17 is arranged or operably associated with the bridge circuit 13. This differential relay 17 is energized when the bridge 13 is out of balance and controls the vaporizer 31 or the cooling unit 3 through the agency of an amplifier 18 and a relay 19. Control of the cooling unit 3 is undertaken in such a manner, that in accordance with the predetermined values at the section C-D along the abscissa of the graph of FIGURE 2 in the periods k1-k4 (FIGURE 2) the cooling unit 3 is switched in and in the time periods t1-t3 the cooling unit 3 is switched out of its cooling work or capacity is reduced.

Furthermore, a time-relay 20 is connected with the relay 19 through the agency of a switching relay 30. This switching relay 30, upon the expiration of predetermined time intervals, iniiuences the branch arm for the reference-value of the bridge circuit 13 but only then actuates the reference-value stages 21 when the amplifier 18 applies an appropriate potential to the connecting line or conductor to the time relay 20 by means of the relay 19. The switching operation of the reference-value stages 21 will be recognized by referring to FIGURE 2 and is represented in the graph sections I to V. Furthermore, the amplifier 18 is coupled with a time switch 22. This time switch 22 is responsible for placing into operation and terminating the heatingand vaporization time (see the abscissa section B-C of FIGURE 2). During this entire period the time switch 22 disconnects the voltage to the amplifier 18, so that the bridge circuit 13 can not influence the cooling unit 3 -by means of the

relays

19 and 30. Only when the time switch 22 applies the voltage to the relay 30 does the actual drying operation begin as such has been indicated by the curves A and B in FIGURE 2. At this point it is also mentioned that the cut-off relay or trip circuit 16, which is arranged in the main line leading to the current supply network, represents the current supply for the amplifier 15, the amplifier 18, the time switch 22 and the amplifier 26.

Continuing, it will be understood that the amplifier 26 is located in a control circuit which is influenced by a temperature feeler 23. This temperature feeler 23 is likewise arranged in the drying chamber or compartment 7 and delivers its measurement value to the bridge circuit or arrangement 24. The temperature indicating device 25 is provided with an indicator which is influenced by the bridge circuit 24 and thus indicates the temperature prevailing in the drying chamber or compartment 7 and furthermore such temperature indicating device 25 has a control indicator which can be adjusted to a predetermined temperature value. By means of the amplifier 26 and the relay 27 it is possible with the help of this socalled -thermostatic control to switch in and switch out the heating unit 1 in such a manner that during the entire drying operation the temperature Within the drying chamber 7 is always maintained constant. As already explained heretofore, the drying temperature is dependent upon the material which is to be dried.

Turning attention now to FIGURE 4, there is depicted therein a circuit diagram of a portion of the block diagram of FIGURE 3. Once again, it will be recognized that the hygrometer 11 is electrically coupled with the bridge circuits 12 and 13 which have been schematically represented by the broken line or phantom lines boxes. The hygrometer 11 is essentially composed of a potentiometer 111 with a variable tap 112. The lower half of the potentiometer 111 provides the one arm of the bridge which is regulated in accordance with the measured air moisture content. The other variable arm of the bridge is designated by the variable resistor 131. This resistor 131 represents the maximum value which should be attained by the humidity of the air in the chamber or compartment 7. Both of the other resistors which have been designated by reference numerals 132, are iixed bridge resistors. In the balancing branch there is located a movingcoil device 17 which is constructed in known manner, and therefore, need not be considered in greater detail. Hence, when the bridge circuit 13 is balanced then the contact 171 assumes the indicated position of FIGURE 4. However, when the bridge circuit 13 is not balanced, that is to say, when the upper range or limit of the air humidity has been reached, then the contact 171 switches into engagement with the counter contact 172. This means that the relay tube 181 which in known manner may be a gasfilled tube with auxiliary discharge and a starter electrode, anode and cathode, is not ignited in the illustrated position of the contact 171, and therefore does not energize the excitation coil 191 of the relay 19.

Furthermore, it will be recognized that the switch contacts 192 and 194 are located in the illustrated position. The contact 192 couples the starter electrode of the tube 181 with a negative potential, so that ignition of the tube 181 can not occur. However, the cooling unit 3 canV only 7 then be actuated when, as already described in FIGURE 3, the time switch 22 has terminated the heatingand vaporizing period. The arm of the bridge circuit 13 which is adjustable and responsible for the reference value can also assume different values as such is indicated by the variable resistor 131. For this purpose there are arranged a series of decade stages 21H to 21V which are electrically coupled via the

switch contacts

221, 222, 223, 224 with the variable resistor 131, which provides the stage I. Switching takes place by means of a control motor or by means of a flip-Hop circuit 28. A timing relay 20 is arranged between the control device 28 and the relay 19. When the contact 193 is thrown so that it is coupled with the left countercontact, that is to say when the cooling unit 3 is switched out, then voltage is applied to the timing relay 20 by means of the contact 201. The actuated timing relay 20, after expiration of each regulated or adjusted time, closes its contact 202 so that the control device 28 is actuated. Thus, the time relay 20 always only then only actuates the stepwise switching arrangement through the agency of the control device 28 when according to the curve A of FIGURE 2 at the time period t3 the upper range of the relative air humidity, for instance 86% at stage I of FIGURE 2, has not been reached. The time t of the relay 20 can be seletcivelychanged during the entire drying operation in order to increase the moisture difference between the lower and the upper reference value. The individual resistors a, b, c, d, e provided at the stages II to V are responsible for the adjustment of the reference value of the air humidity for each individual stage. By means of the

selectors

226, 227, and so forth, it is possible to select predetermined resistors. For instance, in stage II there has been selected the resistor e and in the stage III the resistor b, This selection is already undertaken prior to the beginning of the drying operation and by means of such selection it is possible to also regulate the ascent of the drying curve. It is possible to recognize that the selection of the reference value of the relative air moisture content or humidity in the individual stages can be delivered to the control device by means of a certain or predetermined program which has already been iixed prior to the drying operation and printed upon a punch card. Depending upon how the bridge 13 is adjusted in its values by the resistors 131 and a to e of the individual decade stages 21H to 21V, the dierential relay 17 accordingly switches its switch 171 to the one side, such as 172, or to the other side, such as 173.

Simultaneous with the above-described switching of the reference value from one stage to the other, further decade stages 291 to 29V are actuated by the same control device 28. By means of these decade stages 29, which are constructed in exactly the same manner as the decade stages 22H to 22V, it is possible to change the sensitivity of the differential relay 17. The resistor selected by the decade stages 29 is connected into the current circuit of the different relay 17 Each of the resistors u to y is dimensioned in such a way that there is provided a certain and desired actual-value difference depending upon the selection which is undertaken by means of the switch. For instance, the selected resistor u has a value which corresponds to a difference of i% relative air humidity or moisture. As will be readily apparent from FIGURE 4, it is possible to adjust the desired actual value difference in each stage.

Now a switching-in operation of this type will be described. The drying operation begins at point A along the abscissa of the graph of FIGURE 2. Hereinafter reference will always be made to the course of the curve as well as to the different sections of FIGURE 2 and to the circuit arrangement of FIGURE 3. After switching in the control voltage via the trip circuit or relay 16 (FIGURE 3) the vaporizer 31 is switched in via the contact 194, relay 19 and via contact 301, and relay 30. This vaporizer 31 generates water vapor which is conducted into the drying chamber or compartment 7 and by means of which it is possible to bring about a saturation of the air up to 90% relative moisture or humidity (section V1 of FIG- URE 2). Of course, instead of the vaporizer 31 it would be possible to employ a steam valve or some other equivalent structure. Now if this air humidity has been reached (point B of FIGURE 2) then the hygrorne-ter 11 assumes a resistance position whereby the bridge circuit or arrangement 13 is detuned in such a manner that the differential relay 17 switches to the counter contact 173. The tripping relay 18 and the grid of the glow discharge tube 181 receives cathode potential, whereby such tube 181 ignites. The anode current of the tube 181 ows through the coil of the relay 19 and the contacts 191 to 194 are thrown. The vaporizer 31 is switched off. Due to closing of the contact 193 the time relay 22, via the contact 304 receives a potential together with the amplifier 26 for the heating operation. The magnet of the relay 22 closes the holding contact 222. The relay 22 is held by means of this contact 222 during the drying period. The contacts 221 are thrown, however delayed, after expiration of a previously predetermined time (point C of FIGURE 2).

After the vaporizer 31 has been switched off, the chamber air moisture or humidity drops. Due to the change of the air the resistance of the hygrometer 11 also changes together with the voltage of the bridge circuit 13 and the position of the differential relay 17. If the air humidity has dropped to then the bridge is detuned in such a manner that the differential relay 17 closes the contact 172, the tube 181 extinguishes and the relay 19 is de-energized. Now, the vaporizer 31 is again switched in by means of the Contact 194 (section V2 of FIGURE 2). This intermittent moistening of the chamber air occurs during the running of the timing relay 22 (section B-C of FIGURE 2). After the running or operation of the timing relay 22 the contact 221 is closed, the relay 30 is impressed with voltage, whereby the contacts 301 to 304 are thrown. The vaporizer 31 is continuously switched ott and the cooling unit or condensator 3 is switched in via the contact 302, together with the time-delayed relay 20 Via the contact 303. If the air humidity has reached a relative Imoisture content of 90% owing to vaporization of moisture by the material to be dried, then the bridge is detuned in such a manner that the contact 172 closes. The bridge current now ows in the opposite direction through the coil of the differential relay 17. The glow discharge tube 181 extinguishes, relay 19 is de-energized and the cooling unit 3 is switched in via the contacts 194, 302 (section k1 of FIGURE 2).

Due to separation of condensate, the air is now demoistened and indeed for such length of time until the moisture content has dropped to 80% (section k1 of FIGURE 2). Due to the change of the bridge voltage contacts 171 and 173 are thrown, the tube 181 ignites, relay 19 is energized, contact 194 is opened (the cooling unit is switched off) and contact 193 is closed. The timing relay 20 is switched in via contact 303 (section t1 of FIG- URE 2). Owing to further vaporization the air humidity again increases until reaching 90% relative moisture content. Now, contacts 171, 172 close, the tube 181 extinguishers and relay 19 is de-energized. The timing relay 20 is switched off and the cooling unit 3vis switched in via the contacts 194, 302 (section k2 of FIGURE 2). This intermittent condensation repeats for such length of time (section CD1 of stage I of FIGURE 2) until the timing relay 20 has run out or expirerd without the maximum reference value of 90% relative air humidity having been reached. The time relay 20 closes the contact 202, applies voltage to the stage switch 28 and switches to stage II (section D1-D2 of FIGURE 2). The cycle which was described in stage I repeats in the same manner, however between the maximum reference value of relative air humidity and the value of 75 relative air humidity. If the air humidity decreases further, then the control device switches the stage III (section Dz-Ds of FIGURE 2), and so fourth, until the air moisture content of humid- 9 ity which has been previously adjusted at the hygrometer 14 of FIGURE 3 has been obtained and via the amplifier 15 switches-off the cut off relay 16 and interrupts the control current circuit.

Of course, it should be understood that switching off of the control current via the cut off relay or tripping circuit 16 can also take place by means of a counting mechanism 40 depicted in FIGURE 3. This counting mechanism 4'0 records the amount of separated condensate in liters during the drying period. After the previously regulated number of liters has been condensed, then a signal is delivered to the amplifier 15 of FIGURE 3 which interrupts the control voltage from the cut off relay 16 ,and switches oftr the control current circuit.

Thus, by means of this entire control technique it is possible to attain an optimum terminal dryness for each conceivable material to be dried in accordance with its special particular properties. Even if in FIGURE 2 socalled drying curves A `and B have been depicted, this does not mean to imply that the properties of the material to be dried must be delivered to the inventive control apparatus on the basis of such graphic representations. FIGURE 2 has only been provided to improve 'the comprehensibility of the entire inventive drying technique or method. Such graphic representations were merely provided for the purpose of illustrating that it would be possible to undertake a numerical application with the inventive installation.

Even though in FIGURE 4 there has been employed by way of illustration and explanation, relays and mechanical switch contacts, it is to be expressly understood that such is not to be construed in a limiting fashion with respect to the underlying concepts of the present invention. Quite to the contrary, it would be readily possible to use electronic components which can fulfill the same purpose. Moreover, it will be apparent that various modifications and changes can be undertaken by those skilled in the art without departing from the teachings and principles of the subject invention.

Thus, while there has been shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodiment and practiced within the scope of the following claims.

Accordingly, what is claimed is:

i1. A method of drying materials by means of circulating air condensation systems, said .method comprising the steps of:

intermittently increasing and subsequently decreasing the moisture content of the circulating air contacting the material to be dried;

simultaneously comparing the actual value of moisture content in the circulating air with a reference value of moisture content; and,

delivering a signal to a control apparatus during an intermittent increase of the moisture content of the circulating air only if the actual value of moisture content in the air fails to reach the reference value of moisture content in the air within a predetermined time interval.

2. A method of drying materials as defined in claim 1, further including the steps of initially increasing the moisture content of the circulating air to a predetermined value and thereafter intermittently maintaining the moisture content of the circulating air at said predetermined value while simultaneously heating the circulating air.

3. A method of drying materials as defined in claim 1, wherein the circulating air which contacts' the material to be dried is adjusted to a predetermined temperature and wherein the intermittent reduction in its moisture content is effected by cooling the circulating air beneath its dew point as well as by subsequently heating the circulating air to said predetermined temperature.

4. A method of drying materials as defined in claim 1, wherein both the comparison magnitude between the reference value of moisture content and the actual value of moisture content during each intermittent increase and subsequent decrease of moisture content as well as the duration of said intermittent increase and subsequent decrease in moisture content are regulated to one another in such a way that the relationship between the air moisture content of the circulating air and said duration results in a drying characteristic which is inherent for the relevant material to be dried.

5. A method of drying materials as defined in claim 1, wherein said signal delivered to a control apparatus is employed for lowering said reference value of moisture content.

6. A method of drying materials as defined in claim S, further including the step of progressively reducing the quantity of cooling .medium utilized to cool the circulating air.

7. In an apparatus for drying materials, said apparatus including means for circulating air in contact with a material to be dried and means for removing moisture from said circulating air, the improvement which comprises:

means for intermittently increasing and decreasing through a predetermined moisture difference the moisture content of the circulating air contacting the material to be dried, said means including bridge circuit means and differential relay means;

time delay relay means operatively coupled with said differential relay means, said time delay relay means responding during an intermittent increase of the moisture content in the circulating air only if the moisture content fails to reach a predetermined reference value during a predetermined time interval; and

control device means responsive to said time delay relay means for the stepwise adjustment of said moisture difference of said circulating air.

8. An apparatus as defined in claim 7, wherein said bridge circuit .means includes a measuring :bridge arrangement having an indicating device for indicating the actual moisture content of the circulating air and including a terminal switch operative when said actual moisture content reaches a terminal value, said bridge circuit means further including a control bridge arrangement for comparing the actual value of moisture content in the air with a reference value; wherein said differential relay means is operatively coupled with said control bridge arrangement for continually evaluating the compared reference and actual values of the moisture content of the circulating air.

9. An apparatus as defined in claim 8, further including a hygrometer operatively coupled with both said measuring and control bridge circuit arrangements.

10. An apparatus as defined in claim 7, wherein said control device means includes stage switch means operatively coupled with said time delay relay means for controlling the reference value of the air moisture content.

11. An apparatus as defined in claim 7, wherein said control device means includes decade stage switch means for adjusting the moisture difference of the actual value of the moisture content during the intermittent increase and decrease of the moisture content.

12. An apparatus as defined in claim 10, wherein said stage switch means are manually adjustable.

13. An apparatus as defined in claim 1'1, wherein said decade stage switch means are manually adjustable.

14. An apparatus as defined in claim 7, wherein said means for intermittently increasing and decreasing the moisture content of the circulating air includes cooling means for cooling and condensing the moisture from the circulating air, and switch means operatively coupled between said differential relay means and said time delay relay means for selectively switching in said cooling means and said time delay relay means.

11 12 15. An apparatus as defined in claim 7, wherein said apparatus, said counting mechanism being operatively means for intermittently increasing and decreasing the COllpled with said cut-off relayrnoisture content of the circulating air includes a vaporizer means for increasing the moisture content of the circulating air, said apparatus further including a heating 5 References Cited UNITED STATES PATENTS mechanism for heating the circulating air. 3,148,955 9/ 1964 Nichols 34-50 16, A t S d 5 ed 'n dal' 7, f th c1 d. 3,337,967 8/ 1967 Smith 34-46 n appara us a e n l m ur er m u 2,920,398 1/1960 Liljenstrom 34-46 ing a counting mechanism for recording the amount of medium condensed out of the circulating air during the 10 KENNETH W. SPRAGUE, primary Examiner intermittent reduction of the moisture content of the US Cl XR circulating air, a cut-olf relay for switching out the entire 34 26, 50, 75