Drying and gas or vapor contact with solids – Material treated by electromagnetic energy – Infrared energy
Patent
1998-07-13
1999-08-03
Bennett, Henry
Drying and gas or vapor contact with solids
Material treated by electromagnetic energy
Infrared energy
F26B 334
Patent
active
059309148
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention concerns a method for drying a moving web material, in which method infrared radiation is directed at the material to be dried and in which method the moving web material is passed through the radiation one of an infrared radiator while the web material to be dried absorbs radiation into itself, in which method the radiation produced by at least one first infrared radiator and the radiation produced by at least one second infrared radiator are applied to the moving web material to be dried, said radiators being fitted in the vicinity of one another, and the wavelength of the maximum intensity of the radiation of said first infrared radiator being shorter than the wavelength of the maximum intensity of the radiation of said second infrared radiator, in which case, in the drying process, the spectrum of the overall radiation is optimal in view of the absorption spectrum of the material to be dried, and in which method the first infrared radiator is placed at one side of the web material and the second infrared radiator at the opposite side.
The invention also concerns a device for drying a moving web material, which device is fitted to direct infrared radiation at the moving web material to be dried, and which device comprises at least one first infrared radiator and at least one second infrared radiator, which are fitted at the vicinity of one another, and the wavelength of the maximum intensity of the radiation of the first infrared radiator being shorter than the wavelength of the maximum intensity of the radiation of said second radiator, and in which device the first infrared radiator is placed at one side of the web material and the second infrared radiator at the opposite side.
BACKGROUND OF THE INVENTION
In paper and textile industries and also in other fields of industry, a moving web material is dried. In the production and finishing of paper, there are a number of stages at which drying has to be carried out by means of a method not contacting the web, for example by drying by means of radiation.
The infrared radiator devices currently used for drying of a web material consist of high-temperature quartz-tube radiators or of gas-operated medium-wave radiators. The wavelength range of a high-temperature short-wave radiator is substantially 0.5 . . . 5.0 .mu.m, while the peak is at about 1.2 .mu.m. When a thin web is dried, the short-wave radiation penetrates through the web, because the absorption coefficient of the material is, as a rule, poor in the wavelength range between 0.5 .mu.m and 2.0 .mu.m, as the absorption peak is in a range substantially higher than 2 .mu.m. Thus, the emission peak of the radiator and the absorption peak of the web material do not coincide. However, with a high-temperature short-wave radiator, a high power density per unit of area is achieved. The power density may be up to 450 kW per sq m, in which case the radiation energy absorbed into the web is higher than 130 kW per sq m. Power densities of said order are required in an attempt to obtain quick drying, which is again necessary, for example, in a process of coating of paper.
The wavelength range of medium-wave infrared radiators is substantially 1.5 .mu.m . . . 6.0 .mu.m. The wavelength corresponding to the maximum intensity is placed approximately between 2.0 .mu.m and 3.0 .mu.m. One of the points of absorption maximum of the water to be evaporated is situated within said interval. At said interval, the absorptivity of cellulosic fibres is also good. Out of the reasons mentioned above, the radiation efficiency of the radiation of a medium-wave radiator is high, about 40-60%, whereas the corresponding efficiency with short-wave infrared radiators, i.e. with a high-temperature radiator, is about 30-35% when drying of thin web materials is concerned. When the thickness of the material is increased, the efficiency of absorption becomes higher especially for the short-wave radiators.
The maximum power density attainable with medium-wave infrared radiators is 60 . . . 75 kW per sq
REFERENCES:
patent: 4773167 (1988-09-01), Jacobi, Jr.
patent: 4882852 (1989-11-01), Kautto
patent: 5010659 (1991-04-01), Treleven
Alexandersson Borje
Johansson Leif
Bennett Henry
Gravini Steve
Infrarodteknik AB
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