SE543857C2 - Method and apparatus for defibrating wood chips, and a return steam pipe for such an apparatus - Google Patents

Abstract

The invention relates to a method and an apparatus for defibrating wood chips comprising a preheater and a defibrator, said preheater comprising a wood chips inlet and a wood chips outlet, the defibrator comprising a wood chips inlet and a wood fiber outlet, the wood chips outlet of the preheater is connected to said wood chips inlet of said defibrator via a feeder pipe comprising at least one wood chips feeding arrangement for feeding wood chips to said defibrator, a return steam pipe is arranged between said feeder pipe and said preheater, characterized in that said return steam pipe is provided with a cleaning pipe for providing a gas into said return steam pipe.

Description

METHOD AND APPARATUS FOR DEFIBRATING WOOD CHIPS, AND A RETURNSTEAM PIPE FOR SUCH AN APPARATUS Technical field of the Invention The present invention relates to a method and apparatus for defibrating wood chips and a return steam pipe for such an apparatus.

Background of the Invention When def1brating wood chips in a defibrator, steam is generated from the moisture in thewood chips. Some of that moisture may attempt to travel backwards against the wood flow.To facilitate evacuation of steam and to avoid conflict with wood flow going in the oppositedirection, the wood chips are fed into the defibrator with a ribbon feed screw that allowssteam to go backwards in the center while the heavier wood chips are fed forward in theperiphery (ribbon). To evacuate the steam travelling backwards in the center of the ribbonscrew a retum steam pipe at a feeder pipe in which the ribbon screw is provided is connectedto a top portion of a preheater in which the wood chips are heated to a temperature at or above 150 °C.

However cleaver this construction is by evacuating the steam generated in said defibratorback to the preheater to allow more stable feeding of wood chips into the defibrator, it onlyworks if the pressure in the ribbon feed screw is higher than that of the top of the preheaterplus the pressure drop in the retum steam pipe. During certain conditions this may not be the case and during such conditions said retum steam pipe may be clogged.

The problem with a defibrating process in which the retum steam pipe is clogged is that thedefibration process needs to be shut down in order to clean said retum steam pipe. Such ashut-down will result in a low yield from the def1brating process as well as manhours in service which will reduce the profitability from the process.

Object of the Invention The present invention aims at obviating the afore-mentioned problem. A primary object ofthe present invention is to provide an improved apparatus for def1brating wood chips which has a higher yield than prior art apparatuses. Another object of the present invention is to provide an improved apparatus for defibrating Wood chips Which require less process-mode dependent service than prior art apparatuses.

Summarv of the Invention According to the invention at least the primary object is attained by means of the apparatushaving the features defined in the independent claims. Preferred embodiments of the present invention are further defined in the dependent claims.

According to a first aspect of the present invention it is provided an apparatus for defibratingWood chips comprising a preheater and a defibrator, said preheater comprising a Wood chipsinlet and a Wood chips outlet, the defibrator comprising a Wood chips inlet and a Wood fiberoutlet, the Wood chips outlet of the preheater is connected to said Wood chips inlet of saiddefibrator via a feeder pipe comprising at least one Wood chips feeding arrangement forfeeding Wood chips to said defibrator, a retum steam pipe is arranged between said feederpipe and said preheater, Wherein said retum steam pipe is provided With a cleaning pipe forproviding a gas into said retum steam pipe and Wherein said cleaning pipe is provided insaid retum steam pipe closer to the feeder pipe than said preheater. An advantage of thisembodiment is that clogging of the retum steam pipe may be prohibited regardless of the process settings and/or process conditions of the defibration apparatus.

According to the invention said cleaning pipe is provided in said retum steam pipe closer tothe feeder pipe than said preheater. The advantage of this feature is that the cleaning pipe isprovided in the close to an inlet of said retum steam pipe for allowing cleaning of the full length of the same.

In various example embodiments of the present invention said Wood chip inlet of saidpreheater is arranged in a top portion of said preheater and said Wood chip outlet is arrangedin a bottom portion of said preheater. The advantage of these embodiments is that thetemperature, moisture and/or added chemical substances may be more evenly provided tothe full contents of Wood chips since all Wood chips are, at least during the movement fromthe top to the bottom of said preheater, exposed to the same atmosphere. This Will enhance the process speed of sufficiently pretreated Wood chips for the later defibrating step.

In various example embodiments of the present invention said cleaning pipe is at least partlyprovided inside said return steam pipe. The advantage of these embodiments is that suchinsertion of the cleaning pipe into said return steam pipe may cause so-called venturi effects in the retum steam pipe which may enhance the cleaning properties.

In various example embodiments according to the present invention said gas provided tosaid cleaning pipe is steam. And in embodiments of the invention, the steam is fresh steam.The advantage of these embodiments is that the added cleaning component is the same as the base component supposed to be running in said retum steam pipe.

In various example embodiments according to the present invention the cleaning pipe isprovided with a controllable valve arrangement. The advantage of these embodiments is thatsaid valve may be used to adjust the amount of cleaning component, the duty time of cleaningand introduce cleaning component pressure waves in order to further increase the cleaning properties.

In various example embodiments of the present invention said retum steam pipe is connectedto a top portion of said preheater. The advantage of these embodiments is that the top portionof the preheater is supposed to be free from wood chips so that the steam easily can be recirculated.

In various example embodiments according to the present invention said wood chips feedingarrangement in said feeder pipe a ribbon screw feeder. The advantage of these embodimentsis that a feeding of wood chips into the defibrator is not disturbed by the backwardly travelling flow of steam.

In various example embodiments according to the present invention a screw feeder isarranged in said preheater for feeding wood chips from said preheater to said ribbon screwfeeder. The advantage of these embodiments is that the ribbon screw feed may be fed with a constant flow of wood chips as long as wood chips is present in the preheater.

In various example embodiments according to the present invention said retum steam pipe is arranged between said preheater and said ribbon screw on said feeder pipe. The advantage of these embodiments is that the return steam pipe is arranged at the rear of the ribbon feed screw in order to recirculate the steam generated in the defibrating unit.

In various example embodiments according to the present invention said portion of saidcleaning pipe provided inside of said return steam pipe is essentially in parallel With thereturn steam pipe. In embodiments of the invention, the cleaning pipe is provided in thecenter of the return steam pipe. The advantage of these embodiments is that the gas provided by said cleaning pipe is assisting the floW in the return steam pipe.

In various example embodiments according to the present invention at least 3 cm of saidcleaning pipe is provided inside said return steam pipe. The advantage of these embodimentsis that a suff1cient amount of said cleaning pipe inserted into the retum steam pipe may createventuri effects Which may yet further improve the cleaning properties. HoWever, one Wantsto keep the insertion as little as possible since the volume of the retum steam pipe not affected by the gas stream emanating from be cleaning pipe may still be clogged.

In various example embodiments of the present invention a cross-section area of saidcleaning pipe is less than half of the cross-section area of the retum steam pipe. Theadvantage of these embodiments is that the amount of gas emanating from the cleaning pipemay be suff1ciently high but not limiting the retum floW of steam in the retum steam pipe.For this reason, said cleaning pipe must have a suff1ciently smaller cross-sectional area thanthe retum steam pipe hoWever not so small that the floW from said cleaning pipe is not enough from cleaning purpose.

In another aspect of the present invention it is provided a method for def1brating Wood chipscomprising the step of: a. preheating the Wood chips is a preheater, b. feeding the Wood chips from said preheater to a defibrator in a feeder pipe, defibrating the Wood chips in said defibrator, d. retuming steam from the feeder pipe to the preheater in a retum steam pipe, and e. providing extemal gas to said retum steam pipe via a cleaning pipe, Wherein saidcleaning pipe is provided in said retum steam pipe closer to the feeder pipe than said preheater.

In yet another aspect of the present invention it is provided a return steam pipe configuredfor being connectable in a first end to a wood chips feeding arrangement provided betweena preheater and a def1brator in an apparatus for defibrating wood chips and in a second endto a preheater of the apparatus for defibrating wood chips, wherein said return steam pipe isprovided with a cleaning pipe for providing a gas into said return steam pipe, wherein saidcleaning pipe is provided in said return steam pipe closer to said first end connectable to the wood chips feeding arrangement than said second end connectable to said preheater.

Further advantages with and features of the invention will be apparent from the following detailed description of preferred embodiments.

Brief description of the drawings A more complete understanding of the abovementioned and other features and advantagesof the present invention will be apparent from the following detailed description of preferredembodiments in conjunction with the appended drawings, wherein: Fig. 1 is a schematic side view of an example embodiment of a defibrating apparatusaccording to the present invention, Fig. 2 is an enlarged view of a portion of a first example embodiment of a retum steam pipeaccording to the present invention, and Fig. 3 is an enlarged view of a portion of a second example embodiment of a retum steam pipe according to the present invention.

Detailed description of preferred embodiments of the invention Fig. 1 depicts a schematic side view of an example embodiment of a defibrating apparatus100 according to the present invention. The def1brating apparatus comprising a preheater130 and a def1brator 150. Wood chips 140 to be defibrated by the defibrating apparatus 100may first enter a steaming bin 142. Wood chips 140 from the steaming bin 142 may beinserted into the preheater by a screw feeder 132, which in the depicted embodiment isprovided at the bottom of the steaming bin 142. Wood chips 140 from the steaming bin 142may enter a wood chips inlet 131 at a top portion of the preheater 130.

The steaming bin 142 essentially provides for driving out air trapped in the interior of the wood chips and increasing the water contents of the wood chips. This may be performed by exposing the wood chips to an increased humidif1ed atmosphere. The steaming bin 142 mayregulate the humidity by inserting steam from a steam pipe 127. A Valve 126 may open end close depending on the level of humidity in the steaming bin.

In the preheater 130 wood chips may be heated to about 175 °C. It is well known in the artthat a significant drop in the power required to fiberize wood chips is required when thetemperature of the wood chips exceeds 150 °C. The preheating may be performed in anenvironment of constant temperature and pressure. The preheating time is deterrninate bythe heating time of the wood chips needed for lignin in the middle lamella to reach a glasstransition point. A therrnal softening of lignin may allow for a relatively easy mechanical separation of f1bers in the def1brator 150.

As depicted in figure 1, the preheater 130 may be provided with steam from a steam pipe125. Steam pipe 125 may be provided with a regulated valve 124 for deterrnining the amountand time said steam may enter the preheater. Steam may also be provided at a top portion ofthe preheater 130 via steam pipe 129 which is provided with a controllable valve 128. Aconveyor screw 170 at the bottom of the preheater 130 feeds the wood chips from thepreheater to a ribbon screw 152 via a feeder pipe 139. The conveyor screw 170 in thepreheater feed the wood chips in the form of a dense or semi dense plug through the feederpipe 139. This plug comprising soft wood chips and water. The plug of wood chips will prohibit escape and insert of steam from and into the preheater via the feeder pipe 139.

When defibrating or refining wfood chips in a. defibrator or refiner, steam is generated fromthe rnoisture in the chip, Some of that moisttire must travel backtvards against the tvood flovv.To facilitate evacuation and to avoid conflict wfith wood fltrw going forvvaird, the wfootl chipsare fed into the defiliratoi' wlith a hollotv/rilvlion feed screw* 152 that allows steam to gobackwards in the center tvhilc the heavier chips a fed foiwfvard in the pefiplieiy (ribbon). Toevacuate the steam traveling liackxvaids in the center of” the ribbon feed screw l52 a returnsteam pipe 110 at the back portion of the ribbon feed screw feeder giipe 139 is connected tothe top; of the preheater 130. Steam flows/ring in the reverse direction *with respect to the woodchips in the ribbon feed screw 152 is tbrced to travel via the ieturn steam pipe l l0, arrangedat tlie back portion of the ribbon feed screw' 152, to the preheater instead of entering thepreheater 130 *Jia the screw' feed pipe since the screw feeder pipe is ibloclted wdtlt a inoving plug of wfood. chips.

Hotvevfei' clever this construction is evacuating steain to allow' more stable feeding of woodchips, it only work if the pressure in the rihlion feed screw' 152 is liigher than that of the topof the preheater låt) plus the presstire drop of the return steani-/fhalance pipe lll). Duringcertain coiiditioris this may :not he the case and sometimes the differential pressure over thereturn steam or balance pipe is not high enough to ensure enough fioyv and/or speed to makethis steam evacuation stable or sinooth. In such cases there may be a need for cleaning tliereturn steam or balance pipe llt). By inserting fiesh steam into the return steam (balance)pipe lll) in an iiiserted smaller Cleaning pipe lZÛ, 126” a so-called Venturi effect can becreated facilitating flow in it regardless of the arnount of steam generated bertweeii the discsin the defibrator 150. Consequentlyg the flow velocity in the return steam (balance) pipe llt)can be controlled (by the arnount of gas, such as steam or fresh steam inserted) avoiding the low velocitfy sitiiations that cause the clogging and/or build-tips.

With the Cleaning piiie 126 iirovided inside the return steam pipe l ll), under-pressure maybe created at the inlet of said return steam pipe. This iinder-pressure at the inlet of said returnsteam pipe allows for :forcing (venturi effect) steam generated at the defiliratoi' 156 into thereturn steam pipe although the over-pressure in the return steam pipe is small compared tothe top of the preheater where the return steam iwipe is ending. The venturi effect is increasedWith an increased flow of steam in the cleaning pipe. With the venturi effect the clogging of the return steam pipe may be prohilvited for the full length of the return steam pipe.

As an example, as gas or steam is ej ected from the cleaning pipe, it mixes with the steam inthe return steam pipe. Imagine that the gas or steam from the cleaning pipe mixes with fourtimes as much steam in the return steam pipe. As momentum is conserved, this cloud ofsteani/gas, now five times as much as came out of the cleaning pipe, is now moving at onefifth the speed of the flow from the cleaning pipe. All that gas/steam moving in an upwarddirection of the return steam pipe requires replacement steam to be pulled in from the inletof the return steam pipe. In this way a suction at the inlet of the return steam pipe is created by inserting a flow of gas inside the return steam pipe by said cleaning pipe.

Moving five times as much gas/steam may seem efficient, but the kinetic energy of themoving gas or steam is a function of velocity squared. Thus, we may have five times as much gas or steam moving, but the kinetic energy per volume of air is now l/25th of what it was in the flow of gas or steam from the Cleaning pipe, and the total kinetic energy is one f1fthof what we started with. The Venturi effect in this case may be useful to suck up steam fromthe feeder pipe 139 that otherwise shouldn't go through the retum steam pipe. At least aportion of the Cleaning pipe 120” provided inside said return steam pipe may beeoneeiitrically arranged in the return steam pipe. ln various example eniboolinierit the returnsteam pipe has a cross sectional area which is several times larger than the cross-sectionalarea. of the portion of the cleaiiiiig pipe 120” provided inside said return steam pipe 110. Asa further development, adding measuring devices on the return steam or ibalance pipe maygive a. better understanding of the environment around it and, eventually, controlling theVelocity of the flow' may be a way of controllirig the pressure of the ribhori (feed) screw.With this possibility and increased understanding of the pressure balance, a new more stablecontrol of the samer, might lead to a more Smooth/stable feediiig of chips into the defilvrator or refiner. in various exanrpale ernhodirnents the cleaning pipe 120 is provided at said retum steam pipe110 closer to the feeder pipe 139 (at the back portion of the ribbon feed screw 152) than saidpreheater 130. The cleaning pipe 120 may be in arranged in close vicinity to a back side ofsaid ribbon feed screw 152. The cleaning pipe 120 may be inserted into the retum steam pipe110. The inserted length of said cleaning pip 120 in said retum steam pipe is denoted by120”. The inserted length may vary in between 1 cm to 10 cm. In various exampleembodiments said cleaning pipe 120 is having a cross sectional area being less than half thecross-sectional area of the retum steam pipe 110. In various example embodiments theinserted length 120” of said cleaning pipe 120 into said retum steam pipe 110 may be 1-5 %of the total length of the retum steam pipe 110. The cleaning pipe 120 can have a constantcross section throughout the portion 120” being inside said retum steam pipe 110.Altematively said cleaning pipe 120 may have a smaller cross section at the outlet compared to the cross section in the beginning of the portion being inside said retum steam pipe 110.

The pressure of the gas in said cleaning pipe 120 is higher than the pressure in the retumsteam pipe 110. The pressure of the gas in said cleaning pipe may be constant or vary. Incertain circumstances high pressure pulses may be inserted by the cleaning pipe into theretum steam pipe for breaking loose clogged material inside said retum steam pipe. The wood chip inlet 131 of said preheater 130 may be arranged in a top portion of said preheater 130 and said wood chip Outlet 134 may be arranged in a bottom portion of said preheater 130.

The gas provided to said Cleaning pipe may be steam and can preferably be fresh steam.However other gases may be used which will not affect the process. One may for instanceadd acid to the steam. The acid may increase the cleaning properties which in addition may be a process component in the preheater 130.

The retum steam pipe 110 is with its first end 114 attached to the feeder pipe 139 comprisingthe ribbon feed screw and with a second end 112 attached to a top portion of the preheater130. A desired flow of direction of steam in said retum steam pipe 110 is from said first end114 to said second end 112. A flow of gas in said cleaning pipe 120 is therefore in the samedirection for assisting the flow of steam in said retum steam pipe 110, i.e., the cleaning pipe120 is attached to said retum steam pipe 110 in the vicinity of where said retum steam pipe110 attaches to said feeder pipe 139 such as their flow will assist each other in a direction to the top of the preheater 130.

The ribbon feed screw 152 is providing wood chips to the defibrator 150 Via a wood chipsinlet 151 of said defibrator 150. In said defibrator 150 the wood chips are tumed into fiberby grinding the wood chips in between to grinding discs with Variable distance to each other.From an outlet 154 of said defibrator 150 comes wood fiber in a suitable form for inter alia making paper.

The cleaning pipe 120 may be provided with a controllable valve arrangement 122. Thecontrollable valve arrangement may be used for causing the above-mentioned pressurepulses and/or for controlling the pressure and time said gas is flowing in said cleaning pipe120. The portion 120°of said cleaning pipe 120 provided inside of said retum steam pipe 110can essentially be in parallel with the retum steam pipe 110. Preferably the portion 120” of said cleaning pipe 120 is provided in the center of the retum steam pipe 110.

In figure 1 it is depicted a common gas inlet 175 for providing gas or steam to the cleaningpipe 120, steam pipe 127 to the steaming bin 142 and steam pipe 125 to the preheater 130.

However, in various example embodiments said cleaning pipe, steam pipe 125 and steam pipe 127 may be connected to separate steam generating units, preferably for generating fresh steam.

A method for def1brating wood chips may comprising the steps of: preheating the wood chipsis a preheater 130, feeding the wood chips from said preheater to a def1brator in a feederpipe, retuming steam from the feeder pipe to the preheater in a retum steam pipe andproviding extemal gas to said retum steam pipe via a cleaning pipe for allowing free passagein said retum steam pipe. The provision of gas may be constant with a constant pressure andflow. The gas may be pure steam or steam blended with a suitable component for assisting the defibrating process.

In figure 2 it is depicted an enlarged view of a portion of an inventive retum steam pipe 110according to the present invention. The bottom portion is to be connected to the feeder pipe139 and the upper portion is to be connected to the preheater 130. At the bottom portion thecleaning pipe 120 is inserted into the retum steam pipe 110. The portion of the cleaning pipe120” inside said retum steam pipe is depicted to be essentially directed in the same manneras the retum steam pipe in order to create a flow of steam from the cleaning pipe whichessentially coincides with the flow of direction of steam in the retum steam pipe 110. Theflow of steam in figure 2 is from the bottom to the top in the cleaning pipe 110 and in the retum steam pipe 120.

Figure 3 depicts an enlarged view of a portion of a second example embodiment of a retumsteam pipe according to the present invention. The difference compared to figure 2 is thatthe cleaning pipe 120 is provided at the retum steam pipe further away from the inlet of saidretum steam pipe. The inserted portion 120' of the cleaning pipe 120 is in parallel with thesurrounding retum steam pipe. Preferably the cleaning pipe 120 is arranged in the center ofthe steam pipe 110. Although the cleaning pipe 120 is further away from the inlet of theretum steam pipe 110, the suction force when introducing a stream of gas/steam from thecleaning pipe 120 into the retum steam pipe may be enough to direct steam from the backportion of the ribbon feed screw 152 into the retum stem pipe 110 and forward said steam to the top portion of the preheater 130. 11 The return steam pipe 110 according to the present invention may be sold as a spare part.Said return steam pipe 110 may be retrofitted to old types of defibrating apparatuses for improving the reliability.

Feasible modifications of the Invention The invention is not limited only to the embodiments described above and shown in thedrawings, which primarily have an illustrative and exemplifying purpose. This patentapplication is intended to cover all adjustments and variants of the preferred embodimentsdescribed herein, thus the present invention is defined by the wording of the appended claimsand the equivalents thereof Thus, the equipment may be modified in all kinds of ways within the scope of the appended claims.

Throughout this specification and the claims which follows, unless the context requiresotherwise, the word “comprise”, and variations such as ““comprises” or “comprising”, willbe understood to imply the inclusion of a stated integer or steps or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims (25)

Clainis

1. An apparatus (100) for defibrating Wood chips (140) coniprising a preheater (130)and a defibrator (150), said preheater (130) con1prising a Wood chips inlet (131) and a Woodchips Outlet (134), the defibrator (150) con1prising a Wood chips inlet (151) and a Wood fiberOutlet (154), the Wood chips outlet (134) of the preheater(130) is connected to said Woodchips inlet (151) of said defibrator (150) via a feeder pipe (139) coniprising at least one Woodchips feeding arrangen1ent (152) for feeding Wood chips (140) to said defibrator (150), areturn steani pipe (110) is arranged between said feeder pipe (139) and said preheater (130),characterized in that said return stean1 pipe (110) is provided With a cleaning pipe (120,120°) for providing a gas into said return steam pipe (110), Wherein said cleaning pipe (120,120') is provided in said return steam pipe (110) closer to the feeder pipe (139) than saidpreheater (130).

2. The apparatus according to clain1 1, Wherein said Wood chip inlet (131) of saidpreheater (130) is arranged in a top portion of said preheater (130) and said Wood chip outlet(134) is arranged in a bottom portion of said preheater (130).

3. The apparatus (100) according to any one of the preceding clain1s, Wherein said cleaning pipe (120, 120°) is at least partly provided inside said return steani pipe (110).

4. The apparatus (100) according to any one of the preceding clain1s, Wherein said gas provided to said cleaning pipe (120) is stean1.

5. The apparatus (100) according to any one of the preceding clain1s, Wherein the cleaning pipe (120, 120') is provided With a controllable valve arrangenient (122).

6. The apparatus (100) according to any one of the preceding clain1s, Wherein said return steani pipe (110) is connected to a top portion of said preheater (130).

7. The apparatus (100) according to any one of the preceding clain1s, Wherein said Wood chips feeding arrangenient (152) in said feeder pipe (139) is a ribbon screw feeder.

8. The apparatus (100) according to claim 7, wherein a screw feeder (170) is arrangedin said preheater (130) for feeding wood chips (140) from said preheater (130) to said ribbonscrew feeder (152).

9. The apparatus (100) according to claim 7, wherein said return steam pipe (110) is arranged between said preheater (130) and said ribbon screw (152) on said feeder pipe (139).

10. The apparatus (100) according to claim 3, wherein a portion of said cleaning pipe(120, l20') is provided inside of said return steam pipe (110) and essentially in parallel withthe return steam pipe (110).

11. The apparatus (100) according to claim 3, wherein a portion of said cleaning pipe(120, l20') is provided inside of said retum steam pipe (110) and provided essentially in the center of the retum steam pipe (110).

12. The apparatus (100) according to claim 11, wherein at least 3 cm of said cleaning pipe (l20°) is provided inside said retum steam pipe (110).

13. The apparatus (100) according to any one of the preceding claims, wherein a cross-section area of said cleaning pipe (120”) is less than half of the cross-section area of the retum steam pipe (110).

14. The apparatus (100) according to any one of the preceding claims, wherein a flow ofgas from said cleaning pipe (120, 120') is configured for assisting the flow of steam in the retum steam pipe (110) from the feeder pipe (139) to the preheater (130).

15. The apparatus (100) according to any one of claims 1-14, wherein said preheater(130) further comprising at least one steam inlet (125, 129) for providing steam generated extemal to said apparatus into said preheater.

16. A method for defibrating wood chips comprising the steps of:a. preheating the wood chips is a preheater,b. feeding the wood chips from said preheater to a defibrator in a feeder pipe, c. defibrating the wood chips in said defibrator, and d. retuming steam from the feeder pipe to the preheater in a return steam pipe,characterized in that the method further comprises the step of: e. providing external gas to said return steam pipe via a cleaning pipe, wherein saidcleaning pipe is provided in said return steam pipe closer to the feeder pipe than said preheater.

17. The method according to claim 16, further comprising the steps of: f. feeding wood chips from said feeder pipe to said defibrator by means of a ribbon feedscrew, which ribbon feed screw allowing steam generated in said defibrator to retum in abackward direction compared to the wood chips in said ribbon feed screw and thereby increasing a pressure of steam in said feeder pipe and said retum steam pipe.

18. The method according to claim 16 or 17, wherein said extemal gas provided to said cleaning pipe is steam.

19. The method according to any one of claim 16 or 18, wherein a flow of gas from saidcleaning pipe is assisting the flow of steam in the retum steam pipe from the feeder pipe to the preheater.

20. A retum steam pipe (110) conf1gured for being connectable in a first end to a woodchips feeding arrangement provided between a preheater and a defibrator in an apparatus fordef1brating wood chips and in a second end to a preheater of the apparatus for defibratingwood chips, characterized in that said retum steam pipe is provided with a cleaning pipefor providing a gas into said retum steam pipe, wherein said cleaning pipe is provided in saidretum steam pipe closer to said first end connectable to the wood chips feeding arrangement than said second end connectable to said preheater.

21. The retum steam pipe according to claim 20, wherein said cleaning pipe is at least partly provided inside said retum steam pipe.

22. The retum steam pipe according to any one of claims 20-21, wherein the cleaning pipe is provided with a controllable valve arrangement.

23. The return steam pipe according to claim 21, Wherein a portion of said Cleaning pipe is provided inside of said return steam pipe and essentially in parallel With the return steam pipe.

24. The return steam pipe according to claim 23, Wherein a portion of the cleaning pipe is provided at least 3 cm into said retum steam pipe.

25. The retum steam pipe according to any one of claim 20-24, Wherein an outlet of saidcleaning pipe is directed in said retum steam pipe in a direction towards said second end connectable to said preheater.