WO2017200471A1

WO2017200471A1 - Isotropically compressing plug screw feeder

Info

Publication number: WO2017200471A1
Application number: PCT/SE2017/050513
Authority: WO
Inventors: Stefan MELLANDER; Peter Svedberg
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2016-05-17
Filing date: 2017-05-17
Publication date: 2017-11-23
Anticipated expiration: 2018-11-17

Abstract

A plug screw feeder for transporting and compressing granular material in a transport direction and comprising a feed screw, which comprises a screw core and has a length, which is divided into a number of length sections, each of which having a length and a start and an end as seen in the transport direction, wherein the feed screw in at least one of said length sections has a first outer diameter D1 and a first pitch hi at start of said at least one length section and a second outer diameter D2 and a second pitch h2 at end of said at least one length section, and the screw core has a first core diameter d1 at start of said at least one length section and a second core diameter d2 at end of said at least one length section, characterized in that D2/D1≅d2/d1≅h2/h1 in said at least one length section.

Description

ISOTROPIC ALLY COMPRESSING PLUG SCREW FEEDER

TECHNICAL FIELD The present invention relates generally to a screw feeder for transporting material, and relates in particular to a plug screw feeder for the transport of granular material, and even more particular to an isotropically compressing plug screw feeder for the transport and compression of granular material such as wood chips.

BACKGROUND

Plug screw feeders are widely used. In the pulp industry, plug screw feeders are used for transporting wood chips or other raw materials, such as sawdust, bagasse or straw, from one part of the pulping process to another. When used in a pulping process, a plug screw feeder is typically not only used for conveying and feeding chips, but is also used for dewatering of the wood chips and/or to provide a pressure-tight plug to a pressurized zone, such as a digester. When using a plug screw feeder for the feeding of granular materials such as wood chips, a number of problems can arise. One of these problems is so-called plugging, which means that the compressed transport materials, e.g. wood fibers, get stuck somewhere along the length of the plug screw feeder, which causes a production stop. Plugging can arise when too much material is fed and forced into the plug screw feeder. Another source of plugging is when the compressed chips, which have filled a pocket in the rotor, i.e. the feed screw of the plug screw feeder, are sheared and start to co-rotate together with the rotor instead of being transported in the forward direction as intended. To overcome these problems and also to provide energy-efficient and wear-resistant plug screw feeders, many different designs have been suggested and used. However, the designing of a plug screw feeder and its components is typically an experience-based process, which furthermore is tailored to and, at least sometimes, hampered by requirements or dimensions already established and decided by the end user. The cost for producing a plug screw and a plug screw feeder is also considerable, which reduces the willingness to try and test new screw feeder designs. This means that existing plug screw feeders still suffer from drawbacks when it comes to, for example, energy consumption and the risks of shearing, co-rotation and plugging. Also, the degree of dewatering is still not satisfying for all applications in especially the pulp industry. And further, when a plug screw feeder is used to provide a pressure-tight plug to a pressurized zone, the quality and reliability of such a pressure-tight plug is always a concern, since the risk of so-called blow-back, i.e. when pressurized fluids or gases go backwards through the plug screw feeder, is something that always needs to be taken into account and minimized as far as possible.

In the U.S. Patent No. 7,143,690 to Borjesson et al, it is alleged that the compression of one volume unit in a screw compressor can be made equal in all directions if an outer diameter Di and an inner diameter di at a first axial position and an outer diameter D₂ and an inner diameter d₂ at a second axial position are chosen such that

and

where K is a compression ratio and n is a number between 2.5 and 3.5, and in the ideal case n=3. A screw compressor designed and manufactured according to these teachings is, however, not ideal.

An object of the present invention is therefore to provide an improved plug screw feeder, which is more energy efficient and which provides for enhanced dewatering. A further object is to provide an improved plug screw feeder, which reduces the risk of shearing and/or co-rotation of transport material. A still further object is to provide an improved plug screw feeder, which reduces the risk of plugging of transport material somewhere along the length of the plug screw feeder. When used in connection with a pressurized volume, the improved plug screw feeder should also provide a pressure-tight and reliable material plug, which eliminates or at least mitigates the risk of blow-back.

SUMMARY OF THE INVENTION

The above-mentioned objects are achieved with a plug screw feeder according to the independent claim. Preferred embodiments are set forth in the dependent claims. The present invention is directed to a plug screw feeder, which comprises a number of design-specific dimensions, or rather design-specific dimensional relations, regarding outer screw diameter versus inner screw diameter at beginning and end, respectively, of a feed screw section; and also regarding pitch at beginning versus pitch at end of this feed screw section. It has been realized that when both these relations are fulfilled, the effect is that the compression of the transport material in this feed screw section is equal in all directions, i.e. the compression is isotropic.

More specifically, a plug screw feeder according to the present invention is characterized by a set of dimensions, which are: outer screw diameter at the beginning (as seen in transport direction) of a feed screw section (which is called Di), outer screw diameter at the end of the feed screw section (called D₂), core diameter of the feed screw at the beginning of the feed screw section (called di), core diameter of the feed screw at the end of the feed screw section (called d₂), pitch at the beginning of the feed screw section (called hi), and pitch at the end of the feed screw section (called h₂). According to the invention, a plug screw feeder comprises a number of feed screw sections, at least one of which having a first or start diameter Di and a first or start pitch hi and a second or end diameter D₂ and a second or end pitch h₂, and comprising a screw core section having a first or start diameter di and a second or end diameter d₂, wherein D₂ divided by Di is equal to, or approximately equal to, d₂ divided by di, i.e. D₂/Di≡d₂/di, and wherein h₂ divided by hi is equal to, or approximately equal to, a nominator equal to Di times D₂ minus di times d₂ divided by a denominator equal to Di times Di minus di times di, i.e. h₂/hi≡ (DiD₂-did₂)/(DiDi-didi), which can be reduced to h₂/hi≡ d₂/di and therefore also to h₂/hi≡D₂/Di. A plug screw feeder according to the invention compresses material transported therein, and has therefore a compression ratio that is larger than one (1); and for most practical applications within the pulp industry, the compression ratio is about two (2). The feed screw is therefore preferably conical, with D₂<Di and d₂<di, and/or with h₂<hi. Thus - in contrast to the aforementioned U.S. Patent No. 7,143,690 - also the pitch of the feed screw is considered, something which has been found to be necessary to actually accomplish an isotropically compressing plug screw feeder.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be further explained hereinafter by means of non-limiting examples and with reference to the appended drawing, wherein: Fig. 1 is schematic illustration of a plug screw feeder according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A plug screw feeder according to the present invention comprises a feed screw having a length, which is divided into a number of length sections. The division into a number of length sections is arbitrary and is provided for clarity of the description and for defining the scope of the present invention. Thus, a feed screw can be divided into an arbitrary number of length sections, wherein the feed screw in at least one of these length sections fulfils the dimensional relations provided by the present invention, as will be thoroughly explained below. It is, however, preferred that most of the length sections, i.e. most of the length of the feed screw, fulfil these dimensional relations; and it is even more preferred that all, or at least essentially all, of the feed screw sections fulfil these dimensional relations. The latter corresponds to a situation when the feed screw effectively comprises only one length section, the length of which is equal to the length of the feed screw. A plug screw feeder according to the invention is compressing, i.e. it compresses material transported therein, and the plug screw feeder is therefore characterized by having a compression ratio larger than one. Fig. 1 schematically illustrates a longitudinal cross-section of a generally conical plug screw feeder 1 comprising a generally conical feed screw 2 having a length L, which in this embodiment has been divided into three length sections, A, B and C. The length sections A, B and C have different lengths LA, LB, and Lc, respectively, such that L=LA+LB+LC; and in Fig. 1 it should be appreciated the length section B is the longest length section, i.e. LB>LA and LB>LC. Below, the invention will be described with special reference to this length section B. The feed screw 2 has at start (as seen in transport direction) of length section B a first outer diameter Di and a pitch hi. Further, at the end of length section B, the feed screw 2 has a second outer diameter D₂ and a pitch h₂. The feed screw 2 comprises also a screw core 3, which at start of length section B has a first core diameter di and which at end of length section B has a second core diameter d₂. As can be seen from Fig. 1, the feed screw 2 is accommodated in and rotates in a generally conical feeder pipe 4 (which also can be referred to as a screw pipe 4), whose inner dimensions are only slightly larger than the outer dimensions of the feed screw 2. Therefore, for all practical applications, an outer diameter of a feed screw can be measured or estimated from the corresponding inner diameter of a feeder pipe in which the feed screw is accommodated and rotates.

Now, according to the invention, the dimensions (Di, di_, hi, D₂, d₂, h₂) characterizing the feed screw 2 and the screw core 3 in the length section B fulfil certain relations, to thereby provide an improved plug screw feeder 1, which solves the aforementioned problems. More particularly, the outer diameter Di of the feed screw 2 and the core diameter di of the screw core 3 as measured at start of length section B and outer diameter D₂ of the feed screw 2 and the core diameter d₂ of the screw core 3 as measured at end of length section B have been designed such that D₂/Di=ad₂/di, where 0.8<a<1.2, or more preferably 0.9<α<1.1, or even more preferably 0.95<a<1.05. Furthermore, the pitch hi of the feed screw 2 as measured at start of length section B and the pitch h₂ as measured at end of length section B fulfil the following relation h₂/hi=P(DiD₂-did₂)/(DiDi-didi), where 0.8<β<1.2, or more preferably 0.9<β<1.1, or even more preferably 0.95<β<1.05. When the plug screw feeder 1 comprises a feed screw 2 which fulfils both the relation D₂/Di=ad₂/di and the relation h₂/hi=P(DiD₂-did₂)/(DiDi-didi), with a and β as given above, it will compress transport material equally, or almost equally, in all directions, i.e. the compression is isotropic. By using the first relation with a=l, the second relation can actually be reduced to

From Fig. 1, it can be appreciated that LB<L, which means that it is not necessary that the whole length L of the feed screw 2 fulfils the relation D₂/Di=ad₂/di and the relation h₂/hi=p(DiD₂-did₂)/(DiDi-didi)=pD₂/Di, but it is preferred that at least 70 % of the length of the feed screw 2 fulfils these relations, or more preferably that 80 % of the length of the feed screw 2 fulfils these relations, or even more preferably that 90 % of the length of the feed screw 2 fulfils these two relations. In other words, LB is at least 0.7xL, or more preferably LB is at least 0.8xL, or even more preferably LB is at least 0.90xL. In the embodiment illustrated in Fig. 1 , it is the length sections A and C which do not necessarily fulfil the relation D₂/Di=ad₂/di and the relation h₂/hi=p(DiD₂-did₂)/(DiDi-didi)=pD₂/Di. In other words, the length section B, which does fulfil the relation D₂/Di=ad₂/di and the relation h2/hi=P(DiD2-did2)/(DiDi-didi)=pD2/Di, is located between two length sections that do not fulfil these two relations. It is, however, within the scope of the present invention that the length of a feed screw comprises only one length section which does not fulfil the relations D₂/Di=ad₂/di and h₂/hi=p(DiD2-did2)/(DiDi-didi)=pD₂/Di and which is located either before (as seen in transport direction) or located after a region which does fulfil these two relations. It is also possible that a feed screw comprises an intermediate length section in which the relations

and the relation

are not fulfilled, and that this intermediate length section is located between two length sections in which these two relations are fulfilled. In that case, the sum of the lengths of the length sections that do fulfil the relations

and h₂/hi=p(DiD2-did2)/(DiDi-didi)=pD₂/Di is at least 70 % of the length of the feed screw, and more preferably at least 80 % of the length of the feed screw, and even more preferably at least 90 % of the length of the feed screw. In fact, a feed screw can comprise any number of length sections in which alternately the relations

and h₂/hi=p(DiD2-did2)/(DiDi-didi)=pD₂/Di are fulfilled and are not fulfilled as long as the sum of the lengths of the length sections that do fulfil the relations

and h₂/hi=p(DiD2-did2)/(DiDi-didi)=pD₂/Di is at least 70 % of the length of the feed screw, and more preferably at least 80 % of the length of the feed screw, and even more preferably at least 90 % of the length of the feed screw. Here it should be noted that the invention relates to a plug screw feeder which compresses the material transported therein. This implies that all lengths, e.g. the length L of the feed screw 2 in Fig. 1 as well as all lengths of length sections, such as lengths LA, LB, LC, refer to "compression lengths" or "compressive lengths". A compression length or compressive length is characterized by having a compression ratio K>1, e.g. 1.5<K<3. That is, length sections of a plug screw in which material is substantially only transported without being compressed should not be taken into account when estimating the percentage of a feed screw length in which the compression is isotropic. For example, if a plug screw feeder has feed screw with a total length of 1.5 m, but 0.5 m thereof is essentially only transporting the material without compressing the material, the length - as the term is used herein - of the feed screw is 1.0 m, and according to the invention, at least 0.7 m thereof should fulfil the relations D₂/Di=ad₂/di and h₂/hi=p(DiD2-did2)/(DiDi-didi)=pD₂/Di, and preferably 0.8 m thereof should fulfil these two relations, and even more preferably at least 0.9 m thereof should fulfil these two relations.

The parameters a and β are merely introduced herein because it has been realized that also a nearly ideal plug screw feeder can be useful for many applications, i.e. a plug screw feeder in which the transported material is compressed almost but not perfectly isotropically. In other words, the parameters a and β are introduced to define and quantify a deviation from a perfectly isotropically compressing plug screw feeder. From a practical point of view, a plug screw feeder according to the present invention can alternatively be characterized by D2/Di≡d2/di≡h2/hi, where the sign≡ means "approximately equal to" or "substantially equal to". For an ideal plug screw feeder, α=β=1 and D2/Di=d2/di=h2/hi.

Further, the compression ratio K for a plug screw feeder is defined as K=Vi/V2, where Vi is the volume of matter at beginning of a length section and V₂ is the volume of the matter at end of the length section. Using the dimensions given above, the compression ratio can be expressed as K=(Di+di)(Di-dl)hi/((D2+d2)(D2-d2)h2). A plug screw feeder according to the present invention is always compressing, and has therefore always K>1. For most practical application in the pulp industry, K≡2, e.g. 1.5<K<3. For an ideal plug screw feeder K=(Di/D2)³, and with K>1 it follows that D2<Di, and consequently d2<di and li2<hi. This means than an ideal plug screw feeder is conical and has a pitch that decreases in the transport direction; and a plug screw feeder according to the invention, which fulfils the relations D₂/Di=ad₂/di and

where 0.8<<x<1.2 and 0.8<β<1.2, and more preferably 0.9<α<1.1 and/or 0.8<β<1.1, and even more preferably 0.95<a<1.05 and/or 0.95<β<1.05, in at least 70 % of the length of the feed screw is preferably also conical with inner and outer diameters that diminish in the transport direction and has a pitch that decreases in the transport direction.

Claims

1. A plug screw feeder (1) for transporting and compressing granular material in a transport direction and comprising a feed screw (2), which comprises a screw core (3) and has a length (L), which is divided into a number of length sections (A, B, C), each of which having a length (LA, LB, LC) and a start and an end as seen in the transport direction, wherein the feed screw (2) in at least one of said length sections has a first outer diameter Di and a first pitch hi at start of said at least one length section and a second outer diameter D₂ and a second pitch h₂ at end of said at least one length section, and the screw core (3) has a first core diameter di at start of said at least one length section and a second core diameter d₂ at end of said at least one length section, characterized in that D₂/Di≡d₂/di≡h₂/hi and the compression ratio K>1 in at least said one length section, and that the sum of the lengths of the length sections wherein D₂/Di≡d₂/di≡h₂/hi and K>1 is at least 70 % of the length (L) of the feed screw (2), and preferably at least 80 % of the length (L) of the feed screw (2), and more preferably at least 90 % of the length (L) of the feed screw (2).

2. The plug screw feeder (1) according to claim 1, characterized in that D₂/Di=ad₂/di and

where 0.8<<x<1.2 and 0.8<β<1.2, in at least said one length section, and that the sum of the lengths of the length sections wherein D₂/Di=ad₂/di and h₂/hi=p(DiD₂-did₂)/(DiDi-didi), where 0.8<<x<1.2 and 0.8<β<1.2, is at least 70 % of the length (L) of the feed screw (2), and preferably at least 80 % of the length (L) of the feed screw (2), and more preferably at least 90 % of the length (L) of the feed screw (2).

3. The plug screw feeder (1) according to claim 2, characterized in that 0.9<α<1.1 and more preferably that 0.95<a<1.05.

4. The plug screw feeder (1) according to claim 2 or claim 3, characterized in that 0.9<β<1.1 and more preferably that 0.95<β<1.05.

5. The plug screw feeder (1) according to any preceding claim, characterized in that the feed screw (2) is conical with D₂<Di and d₂<di.

6. The plug screw feeder (1) according to any preceding claim, characterized in that the feed screw (2) has a decreasing pitch with h₂<hi.