Patent Application
20100206506
The present invention relates to a machine for production of a fibrous web, with a forming section in which the web is formed from a pulp suspension on a fabric and with a Yankee or another drying unit, where at least one further rotatable dryer with a hood is located after the forming section and before the Yankee or another drying unit. The invention also relates to a process for the production of a web performed with the machine according to the invention.
An example of a fibrous web is tissue paper. Tissue paper or tissue is the collective term for a large number of paper grades. It includes, for example, the household hygiene sector, with such grades as facial wipes, paper handkerchiefs, napkins, toilet tissue and kitchen roll. The characteristic feature of high-quality tissue grades is a sheet structure that is not too highly compacted, thus providing high specific volume or high bulk. This results in such positive properties as greater softness, higher water absorption capacity and higher water absorption speed.
In the production process for tissue a pulp suspension is prepared accordingly in a manner already known before being fed through a headbox and onto a forming wire to form a pulp web; this process is also referred to as forming or sheet formation. Very different types of forming unit are known, such as the Fourdrinier former, suction breast roll, crescent former, twin wire former, and so on. The pulp web is already dewatered during forming, and after this by using vacuum and suction equipment, for example, suitable dewatering foils, but also presses, such as press rolls or pressing belts. The dry content obtainable with these dewatering devices is limited, which is why the pulp web subsequently also undergoes thermal drying in a dryer section. This drying process operates with hot air or steam, for example, or with steam-heated rolls round which the pulp web runs. Other drying methods, however, are also conceivable, using infrared sources or microwaves. In many cases, drying by means of a Yankee has prevailed. This is a steam-heated, rotating cylinder with a smooth surface and the pulp web runs over a part of this cylinder without any supporting fabric. When it leaves the surface of the Yankee, the pulp web is scraped off (creped) by the aid of a doctor. As a result of this scraping process the pulp web is pushed together, thus increasing the thickness and softness of the pulp web. This is particularly desirable for tissue. Following the drying process, the pulp web is wound onto a reel or is further processed.
A process of this type and a corresponding device with a sheet formation zone, a press zone, and a Yankee is described in DE 10 2005 054 510 A1, for example. A process of this kind, however, generally cannot produce top-grade tissue.
A similar process is disclosed in US 2005 166418 AA. Here, a further dryer, a “boost dryer”, is included before the Yankee. The tissue web, a permeable conveying belt, a condensation fabric, and an impermeable pressing belt run round this steam-heated, additional dryer. The water from the tissue web condenses in the condensation fabric and can be removed from it as soon as the condensation fabric has separated again from the tissue web, i.e. after the “boost dryer”. The disadvantage of this drying arrangement is its very complex design due to the large number of belts.
EP1749934 A1 also discloses an additional drying unit before the Yankee. Here, the web undergoes pre-drying by means of a TAD drum operating according to the through-air drying principle. This type of through-air dryer has very high energy consumption and a complex design.
The invention is thus based on the objective of presenting an improved machine for the production of a fibrous web that has a simple design and can produce for instance very high value tissue. It is intended, in particular, to increase the absorption capacity, the bulk or specific volume, and the softness of the tissue web produced.
This objective is achieved with a machine in which a dryer located before a Yankee or another drying unit like a through-air dryer or a combination of a Yankee and a through-air dryer has an imperforate (gas-tight) surface and a burner to directly heat the gas-tight surface.
It is possible to provide the burner inside the dryer. Heating the dryer with an internal burner holds the advantage that higher surface temperatures can be achieved than when using conventional steam-heated dryers. The surface temperature here can be up to 400° C.
With this tissue manufacturing process, webs with all possible basis weights down to a very low basis weight of 7 g/m2 and less can be produced.
It is an advantage when the dryer is wrapped by a fabric, felt, foraminous member or other belt which produces pressure on the web to overcome the vapor pressure of the escaping water vapor and hold the web in continuous contact with the dryer. The fabric, felt, foraminous member or other belt produces 700 to 14 000 Pa pressure on the web.
A dryer hood can be provided as a simple extractor hood or an enclosed hood to collect the steam generated. The dryer hood is preferably designed as a steam collecting hood.
It is an advantage if the dryer has a diameter of 1 to 5 m because dryers of this kind can easily be integrated into existing tissue machines.
It is favorable if the dryer surface is defined by a cylinder shell with a wall thickness of 5 to 80 mm. A wall thickness of this kind guarantees adequate stability with good heat transfer.
For the drying process it is an advantage if the cylinder shell is made of a material with thermal conductivity equal or greater than 25 W/(K*m), for example it can be made of steel, cast iron, copper, aluminium, or alloys containing at least one of these metals.
In an advantageous embodiment the burner is designed as a gas burner, particularly a natural gas burner. It is also conceivable, however, that the burner is designed as an oil burner. In this case the heat can be transferred by radiation, by convection, by combustion gas, or by a combination of these methods.
In a further advantageous embodiment the surface of the dryer has a non-stick coating. This non-stick coating prevents the tissue web from sticking to the surface of the dryer. This non-stick effect can be achieved by spraying on chemicals.
In order to achieve a good drying effect it is expedient for the tissue web to have a wrap angle of at least 270° around the dryer, preferably 330°, when in operation. This large angle of wrap makes optimum use of the hot dryer surface for drying purposes.
The invention also relates to a suitable process for production of tissue, in which a tissue web is formed from a pulp suspension on a fabric in a forming section and this tissue web is then dried on a final drying unit, with an additional, intermediate drying step for the tissue web being implemented before the web reaches the final drying unit. According to the invention the additional drying step is performed using a dryer heated by a burner and with a gas-tight surface. Here, the steam generated during drying is preferably removed via a steam collecting hood.
The steam generated in the steam collecting hood is preferably fed to further processes, being used for example to heat a Yankee hood or a Yankee. The thermal energy contained in the steam from the steam collecting hood can thus be used in further process steps, resulting in an energy saving for the entire process.
Similarly, the exhaust gas from the burner can be used to pre-heat fresh air for the burner.
In a preferred embodiment the entire dewatering process for the fibrous web can be conducted without a press section. If the press section is omitted, a particularly soft web can be produced.
Of course, it is also conceivable, for example, to carry out the additional drying step with several dryers according to the invention in order to improve drying.
Aspects of the invention will be set forth in greater detail with reference to the accompanying drawing, in which:
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The main body of the dryer preferably takes the form of a rotating drum having cylinder shell 10 made of a material with good thermal conductivity, for example steel, cast iron, aluminium, copper, or a metal alloy. The dryer 5 is preferably surrounded by a hood 6 that can be designed as a steam hood and used to remove the steam generated. In this example the steam is then fed through a steam duct 11 to the Yankee 4 heating system or to the Yankee hood 12.
In the illustrated embodiment, the forming fabric 3 carries the tissue web to and runs around the dryer 5 in such a way that the web 1 to be dried comes into direct contact and co-rotates with the gas-tight surface 7 of the cylinder shell 10. The fabric 3 is permeable at least for steam.
During operation, a pulp suspension is fed through a headbox 13 and sprayed over the entire machine width between the wire 14 and the fabric 3. A tissue web 1 then forms on the fabric 3 and is guided directly over the rotating dryer 5 for drying purposes. The surface temperature of the dryer 5 is between 150° C. and 400° C. Here, the gas-tight surface 7 of the dryer 5 is preferably provided with a non-stick coating that prevents the tissue web 1 from sticking. The steam generated at the dryer 5 by the drying process escapes through the fabric 3 into the steam hood 6, from where it can be fed through a steam duct 11 to the drying unit 4, in this case to a Yankee and/or the Yankee hood in order to make use of the waste heat. The exhaust air from the burner can be used to pre-heat the fresh air for the burner.
After the dryer 5, the tissue web 1 is transferred to the Yankee and the drying process continues. A doctor blade 15 then scrapes the dried tissue web 1 off the Yankee 4. A subsequent winder reels the finished tissue web.
It should be appreciated that the invention encompasses other embodiments in which, for example, a TAD drum alone or a TAD drum and a Yankee are used in addition to the dryer 5. Also, more than one intermediate dryer can be used, and the fabric for holding the web against the shell surface need not be the forming fabric.
