US3896558A - Method of controlling drying of timber in a closed room - Google Patents

Method of controlling drying of timber in a closed room Download PDF

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Publication number
US3896558A
US3896558A US413131A US41313173A US3896558A US 3896558 A US3896558 A US 3896558A US 413131 A US413131 A US 413131A US 41313173 A US41313173 A US 41313173A US 3896558 A US3896558 A US 3896558A
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Prior art keywords
humidity
drying chamber
timber
ventilation
drying
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Expired - Lifetime
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US413131A
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English (en)
Inventor
Anders Lovgren
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Alfred Lovgren Byggnads AB
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Alfred Lovgren Byggnads AB
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B21/00—Arrangements or duct systems, e.g. in combination with pallet boxes, for supplying and controlling air or gases for drying solid materials or objects
    • F26B21/06—Controlling, e.g. regulating, parameters of gas supply
    • F26B21/08—Humidity
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B2210/00—Drying processes and machines for solid objects characterised by the specific requirements of the drying good
    • F26B2210/16—Wood, e.g. lumber, timber

Definitions

  • ABSTRACT A method of controlling the drying of timber which comprises placing the timber in a closed drying chamber, sensing its humidity and comparing it to a reference value, initiating ventilation of the chamber when a certain difference relationship exists between the sensed humidity and said reference value, discontinuing ventilation when another difference relationship exists, effecting further ventilation when a certain difference relationship exists between the sensed humidity and said reference value, determining the rate at which actual humidity in the drying chamber changes per unit of time during the interval between the cessation of ventilation and initiation of ventilation, and changing the reference value when the rate determined falls outside a certain range.
  • the artificial or controlled drying of sawn timber usually takes place in travelling dryers, in which bundles of timber of substantially the same dimensions are advanced in steps through zones of different air condition, or in chamber dryers, in which the timber is arranged stationary and the condition of the surrounding air is changed, for example by injection of heated air until the drying of the timber is completed. Irrespective of whether this change in condition is brought about by constant or discontinued injection of heated air, or in another way, it is to be controlled so that the timber in the drying chamber is not subjected to high internal shrinkage stresses, which a.o. would result in crack formation as, for example, in the case of too rapid drying, or to fungus attacks (discolourization or mould).
  • the drying process in a chamber dryer heretofore was controlled either by program control according to a predetermined time and climate procedure, i.e. by predetermining the allowable drying time and accordingly controlling the drying climate, or by controlling the dehumidifrcation obtained in the timber in the chamber, i.e. by continuously measuring the moisture content of the timber and proceeding with the drying until the timber shows a predetermined moisture content.
  • program control i.e. by predetermining the allowable drying time and accordingly controlling the drying climate
  • the dehumidifrcation obtained in the timber in the chamber i.e. by continuously measuring the moisture content of the timber and proceeding with the drying until the timber shows a predetermined moisture content.
  • a further disadvantage of the known methods of controlling timber drying in closed rooms or chambers is that the methods cannot be applied to the drying of wood species with a high cracking tendency, for example birch.
  • the object of the present invention is to develop a method of controlling the drying of timber in a closed chamber or another space, for example in a chamber dryer, which does not involve the disadvantages of the known methods and renders possible an optimum drying rate with respect to the design of the drying chamber and the nature of the timber, and a total drying time in full adaptation to the circumstances, i.e. the amount of water bound in the timber, the timber density, structure a.s.o., and which method thereby renders it possible to maintain the highest possible relative humidity in the chamber and thereby eliminates the requirement of soaking, i.e. wetting the timber during drying, which is necessary for effecting a rapid drying with the known methods of controlling timber drying.
  • the invention relates to a device for carrying out the method. This is achieved in that the method and device according to the invention show the characterizing features defined in the claims.
  • FIG. 1 shows a block diagram for an embodiment of the device according to the invention
  • FIG. 2 shows two curves a and b, which illustrate the relation between relative humidity in the drying chamber and the time at the method and device according to the invention and, respectively, the relation between the temperature of the air surrounding the timber and the time during a drying process.
  • a suitable drying temperature in the drying chamber is achieved relatively rapidly and maintained substantially constant until the moisture content in the timber is reduced to a predetermined value, as shown in a schematic manner by the curve b in FIG. 2.
  • the said predetermined value which at a given temperature corresponds to a definite value of the relative humidity in the drying chamber, is fed into a drying program, which also includes the upper allowable value for the relative humidity in the chamber, for example above which value a continuous ventilation of the chamber always will take place.
  • a transmitter 1 for relative humidity senses the relative humidity in the drying chamber shown in the drawings and emits a signal, which is proportional to the read relative humidity, to an amplifier 2, which has an output voltage proportional to the relative humidity.
  • the amplifier is connected to a comparator 3 with an upper and lower hysteresis level at its input for comparing the actual value, i.e. the relative humidity in the chamber, with an upwardly and downwardly adjustable nominal value, which thus must not be greater than the value for the highest allowable relative humidity in the drying chamber.
  • the comparator 3 effects ventilation of the drying chamber, i.e.
  • the relative humidity is lowered by one or more fans 8, which ventilation continues until the humidity in the drying chamber falls below the nominal value valid at the moment in question minus the difference between said nominal value and the lower hysteresis level of the comparator.
  • the ventilation stops and thereby rises due to evaporation from the timber the humidity in the drying chamber until the actual value entering the comparator again amounts to the upper hysteresis level of the comparator, i.e. to the nominal value valid at the moment in question plus the difference between said nominal value and the upper hysteresis level.
  • the time passing between each ventilation is measured and supervised by a programmable chronometer 4 with two outputs x and, respectively, y and with data input means 5 indicating an upper and a lower limit for the time between each ventilation.
  • this predetermined time interval is exceeded, i.e. when it takes a longer time to achieve the humidity in the chamber corresponding to the valid nominal value than indicated by the programmed time interval, the chronometer 4 emits a pulse, for example a one, at the output x to a programmable nominal value switch 6, and in a corresponding manner a pulse is emitted, for example a one, at the output y when said time interval is fallen below.
  • the chronometer 4 does not emit a pulse at any of its outputs x, y.
  • the programmed actual value switch 6 has an input x, which is connected to the output x of the chronometer, and an input y which is connected to the output y of the chronometer, and is programmed so as at pulses at the input x, to decrease in steps the nominal value, i.e. the reference voltage, by a predetermined degree, for example by 5% per step, and, respectively, at pulses at the input y, to increase the nominal value by a predetermined degree, for example by 5% per step.
  • the comparator 3 is informed on this new nominal value and thereafter compares the actual value with the new nominal value a.s.o. until the program has arrived at the lowest nominal value, which can be set separately, for example in the setting means designated by 7, and corresponds to the predetermined final value of the moisture content in the timber.
  • the diffusion rate is measured and controlled by studying the increase in the relative or absolute humidity of the air per time unit. This is effected by measuring the time from the stop of the ventilation to its new start, i.e. the time 2 (see the curve A in FIG. 2) which it takes for the actual value voltage proportional to the relative humidity in the chamber to rise from the lower hysteresis level of the comparator 3 to its upper hysteresis level.
  • the timber in the drying chamber dries, its capacity of maintaining the relative humidity in the drying chamber decreases, and ventilation need not be carried out as often, i.e. the time passing between each ventilation increases.
  • this time exceeds a certain value t the programmed reference voltage or the nominal value is stepped down one step, which may correspond to 5% as indicated above, but of course also other percentages can be taken into consideration.
  • a new nominal value for the relative humidity in the drying chamber is obtained and thereafter maintained as long as possible in the chamber.
  • the program is, as mentioned, reversible so that at too high an increase in the diffusion rate the program steps back one or more steps, thereby increasing the nominal value for the relative humidity and, thus, decreasing the diffusion rate.
  • the method according to the invention further renders a total drying time, which is fully adjusted to the prevailing circumstances as regards the amount of water bound 1' the timber, the timber sity, structure a.s.o.
  • a method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber,
  • step (a) feeding the signal generated in step (a) to a comparator means having an upper and lower hysterisis level at its input and comparing the signal generated in step (a), which represents the actual humidity in the drying chamber, with an upwardly and downwardly adjustable nominal value for humidity which must not be greater than the value for the highest allowable humidity in the drying chamber, c. initiating ventilation of the drying chamber when the actual value of the humidity exceeds the nominal value valid at the moment in question plus the difference between said nominal value and the upper hysterisis level of said comparator means,
  • step (g) changing the aforesaid nominal value when the time measured in step (g) falls outside a certain time interval.
  • a method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber,
  • a method of controlling the drying of timber which comprises a. placing the timber in a closed drying chamber,
  • step (b) varying the magnitude of the reference value used in step (b) when the rate determined in step (c) falls outside a certain range.
  • An arrangement for drying timber comprising a. a drying chamber adapted to contain timber,
  • comparator means for comparing the value sensed by said sensor means with a reference value
  • program means for varying the magnitude of the reference value utilized by the comparator means when the rate determined by the means of (f) falls outside a certain range.
  • means (f) comprises a chronometer which indicates the upper and lower limit for the time between the stopping and starting of the ventilator and is operatively connected to a reference value switch in the program means, which in turn is operatively connected to the comparator means so as to be able to transmit thereto differing reference values.
  • program means includes a means for measuring the change in humidity for a predetermined period of time and a means for actuating a reference value switch cacomparator means.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Drying Of Solid Materials (AREA)
  • Control Of Non-Electrical Variables (AREA)
US413131A 1972-11-08 1973-11-05 Method of controlling drying of timber in a closed room Expired - Lifetime US3896558A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
SE1445872 1972-11-08

Publications (1)

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US3896558A true US3896558A (en) 1975-07-29

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US413131A Expired - Lifetime US3896558A (en) 1972-11-08 1973-11-05 Method of controlling drying of timber in a closed room

Country Status (9)

Country Link
US (1) US3896558A (enrdf_load_stackoverflow)
JP (1) JPS532499B2 (enrdf_load_stackoverflow)
CA (1) CA997034A (enrdf_load_stackoverflow)
CH (1) CH564746A5 (enrdf_load_stackoverflow)
DD (1) DD109438A5 (enrdf_load_stackoverflow)
FR (1) FR2205988A5 (enrdf_load_stackoverflow)
GB (1) GB1441284A (enrdf_load_stackoverflow)
IT (1) IT1001659B (enrdf_load_stackoverflow)
NO (1) NO138544C (enrdf_load_stackoverflow)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4356641A (en) * 1980-12-15 1982-11-02 Armstrong World Industries Kiln control system
US4953298A (en) * 1989-02-24 1990-09-04 Wagner Electronic Products, Inc. Kiln controller
WO2002090855A1 (en) * 2001-05-10 2002-11-14 Edwards Systems Technology, Incorporated Absolute humidity sensor to control drying equipment
GB2382125B (en) * 2001-10-02 2005-06-08 Raymond John King Drying cabinet
WO2006104447A1 (en) 2005-04-01 2006-10-05 Coldbay Ab Method for drying wood
US7748137B2 (en) * 2007-07-15 2010-07-06 Yin Wang Wood-drying solar greenhouse

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2604245B1 (fr) * 1986-09-19 1988-12-23 Gautreau Jean Paul Procede de sechage de produits hygroscopiques de fortes epaisseurs, notamment du bois.

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1328662A (en) * 1919-12-17 1920-01-20 Jr Frederick K Fish Process of drying lumber
US1413018A (en) * 1920-09-28 1922-04-18 Fujino Kakuji Apparatus and process for drying wood
US3744144A (en) * 1971-04-23 1973-07-10 H Weis Automated controls for lumber drying kiln

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1513727A (en) * 1922-04-10 1924-10-28 Humidity Control Company Drying-control apparatus
FR1171195A (fr) * 1956-04-13 1959-01-22 Perfectionnements apportés aux procédés pour sécher du bois dans une chambre de séchage
CH469237A (de) * 1967-01-13 1969-02-28 Ineta Establishment Verfahren und Einrichtung zum Trocknen von Gütern
DE1930210A1 (de) * 1969-06-13 1971-01-07 Bayer Ag Verfahren und Vorrichtung zur Trocknung von Stoffen in Vakuumtrocknungsanlagen auf eine vorausbestimmbare Materialfeuchte

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1328662A (en) * 1919-12-17 1920-01-20 Jr Frederick K Fish Process of drying lumber
US1413018A (en) * 1920-09-28 1922-04-18 Fujino Kakuji Apparatus and process for drying wood
US3744144A (en) * 1971-04-23 1973-07-10 H Weis Automated controls for lumber drying kiln

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4356641A (en) * 1980-12-15 1982-11-02 Armstrong World Industries Kiln control system
US4953298A (en) * 1989-02-24 1990-09-04 Wagner Electronic Products, Inc. Kiln controller
WO2002090855A1 (en) * 2001-05-10 2002-11-14 Edwards Systems Technology, Incorporated Absolute humidity sensor to control drying equipment
GB2382125B (en) * 2001-10-02 2005-06-08 Raymond John King Drying cabinet
WO2006104447A1 (en) 2005-04-01 2006-10-05 Coldbay Ab Method for drying wood
EP1864066A4 (en) * 2005-04-01 2014-04-16 Coldbay Ab METHOD FOR DRYING WOOD
US7748137B2 (en) * 2007-07-15 2010-07-06 Yin Wang Wood-drying solar greenhouse

Also Published As

Publication number Publication date
DD109438A5 (enrdf_load_stackoverflow) 1974-11-05
CA997034A (en) 1976-09-14
NO138544B (no) 1978-06-12
GB1441284A (en) 1976-06-30
DE2355930B2 (de) 1976-04-22
JPS5082653A (enrdf_load_stackoverflow) 1975-07-04
NO138544C (no) 1978-09-20
JPS532499B2 (enrdf_load_stackoverflow) 1978-01-28
CH564746A5 (enrdf_load_stackoverflow) 1975-07-31
FR2205988A5 (enrdf_load_stackoverflow) 1974-05-31
IT1001659B (it) 1976-04-30
DE2355930A1 (de) 1974-05-09
AU6219373A (en) 1975-05-08

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