Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1987-08-12
1989-04-25
Bennet, Henry A.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 7, 34 41, B01K 500
Patent
active
048234770
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for the improvement of the moisture profile of a continuous web by means of dielectric drying.
The present invention also involves an apparatus for the implementation of the method.
The product quality of several continuous web processing machines including, for instance, paper and textile web drying sections, is impaired by the uneven final moisture profile present in the web.
The problem is most recognized and quantitatively most important in the paper industry. Users require a product with an even final moisture content in order to ensure non-problematic runnability in printing, as well as to achieve a high dimensional stability of the web for accurate alignment of colors.
The unevenness of web moisture is caused by different defective functions at the wet end either before the drying section or in the dryer itself. The web defects may be generated by incorrect crowning of the press rolls, unevenness of the pressing felts or their cleaning, or by temperature differentials of the web. Persistent or random defects on the final humidity profile of the web on the dryer are caused by the edge effect related to the low humidity of the drying atmosphere and higher heat transfer factors at the edge of the web, as well as by the surface of the drying cylinder extending over the web edge, the nonsymmetrical air circulation, uneven action of the drying felts together with, for instance, the defects of condensate removal from the cylinders and asymmetrical flow of the condensate.
In order to compensate for these defects, several different auxiliary apparatus have been developed. Their operation is based on the principle of intentionally providing a functional defect which compensates for a defect of the machine, often originating from an unrecognized cause.
Rather typical examples of these means are different kinds of air blowing units, in which the atmosphere in the drying pockets is sectionally adjusted in a compensatory manner. The effect of one air blowing unit is minimal and, consequently, several units are needed for each machine; generally, as many as possible without the space limitations.
When high-velocity hoods are used, relatively few will suffice because the method of blowing air via nozzles on the paper achieves high characteristic evaporation rates and differential rates. Due to their clumsiness and high power consumption, these solutions are not very popular.
In contrast, infrared radiant heaters heated by gas or electricity with a sectionally controlled effect are very effective and popular.
In some cases, a high-frequency electromagnetic field has been used for profile improvement by locating bar electrodes cross-directionally to the web over a sufficiently large area. The dielectric constant and loss factor that determine the power absorbed as heat by the web are higher at the wetter parts of the web. Consequently, a dielectric dryer drives moisture more effectively out from the wetter parts of the web than from the dry parts when the unit is located at the dry end of the drying section. However, the disadvantage of crosswise located bar electrodes is an increase in power consumption of the dryer due to the unnecessary simultaneous drying of the dry streaks. This leads to the fact that the dryer cannot produce an even final moisture but only reduces the highest absolute and relative humidity differentials. In order to achieve any effective differential drying at all, this kind of moisture profile correcting dryer must be located at the relatively dry section of the web with a correspondingly low dielectric constant and loss factor. Naturally, this is disadvantageous with consideration to the size of the drying section. Compared to the other methods described above, however, this solution provides the advantage of disposing of any control means in the crosswise direction of the web.
The present invention relates to a method for reducing differentials of moisture profile in a web, especially a paper web, by means of high-frequency electromagnetic energy.
Th
REFERENCES:
patent: 3293765 (1966-12-01), Winkler
patent: 3470621 (1969-10-01), Bennett
Bennet Henry A.
Imatran Voima Oy
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