WO1995012794A1

WO1995012794A1 - Method and apparatus for drying

Info

Publication number: WO1995012794A1
Application number: PCT/SE1994/001029
Authority: WO
Inventors: Per Erik Gustafsson; Tore LÖÖF
Original assignee: Per Erik Gustafsson
Priority date: 1993-11-05
Filing date: 1994-11-02
Publication date: 1995-05-11
Also published as: SE9303658D0

Abstract

The present invention concerns a method and a device for drying moisture-containing objects using microwave energy. The aim of the present invention is to gain full control over the drying process in a tunnel dryer and also to optimize the process so that the drying takes place as quickly as possible without the material being damaged. According to the invention, a step-by-step treatment of the material to be dried is achieved in a particular way. The drying equipment is built up of the required number of standardised modules (1, 2, 3), each of which is provided with one or more energy generators (8, 9, 10). Two adjacent modules are joined by means of a reconditioning part (4, 5), the length of which is adapted to suit the requirements of the process. In immediate connection with the outlet from each drying module certain parameters related to the drying are measured in the objects being dried. These parameters can be, for example, surface temperature and moisture content. Both the energy supply in one or more drying modules and the conditions in the reconditioning module can be adjusted with the aid of these parameters.

Description

Method and apparatus for drying

This invention relates to means and method for drying of moisture containing goods. The term moisture includes water, solvent and other liquids.

In various branches of the industry there is often a need to dry goods and substances as one step in various processes. Generally, drying means removal of water vapour or other vaporized solvents. Usually, drying takes place in certain insulated places and by means of heated air which also functions as a heat carrier. The process may be in batches or continuous in so-called throughflow dryers, where the goods is transported through the drying space on a conveyer of some kind.

Direct microwave or high frequency energy heating has during the latest years been used instead of the energy consuming method of heat carrying air. The use of microwave energy, however, requires a polar molecule of the goods or the substance which shall be effected, i.e. removed from a solid material. Water molecules are polar. Therefore water is suitable for being affected by e.g. microwaves. The drying process may be said to be removal of water molecules from a piece of goods or from a surface.

Microwaves are elctro-magnetic waves with a length between 1 -1000 mm. For the above mentioned purposes two frequences are used, namely 2450 and 900 mHz. The ability to affect water molecules directly has given microwaves as a source of energy a great importance e.g. for the drying of waterbased colours and glue, drying of wood etc.

Various kinds of drying spaces, cavities and through-put dryers are available for treatment with microwave energy. In order to prevent the microwaves from passing out of the cavities to the surroundings, there are different methods of closing inlet and outlet openings. In order to generate microwave energy there are complete energy generators which can be applied to cavities. These can also be so made that at the same time they bring about a climate conditioning.

One difficult problem by drying with microwave energy is to control the supply of energy relative to the demand of each moment. The reason for this is that the total effect is channeled to the polar molecules present in the treatment chamber. For instance, when drying of water based colour on a piece of wood, which in itself contains a certian amount of water molecules, the energy primarily goes to places having the highest concentration of water molecules, thus to the layer of paint. Hereby as a first choice the water molecues of the paint layer are heated and vaporized, but as these are gradually removed from the layer, there is an increasingly faster heating of the water molecules in the piece of wood. At worst this may lead to damage of the goods, e.g. by cell bursting or as the utmost consequence breakdown of the cells. In order to prevent this, the energy supply must be carefully controlled and matched with the demand. This causes problems especially by throughflow dryers, where the material to be dried is transported through the drying cavity at a fixed speed.

One purpose of the present invention is to give full control over the drying process in a throughflow dryer and to optimize the process, so that drying takes place as fast as possible without the material being damaged.

According to the invention there is achieved in a special way a stepwise treatment of the material to be dried. The drying equipment comprises a number as required of standardized modules, each having one or more transmitters. Two adjacent modules may be united with a reconditioning module, the length of which is adapted to the need of the process. The control of the energy supplied as well as of the reconditioning process is controlled by at least one drying parameter which is measured of the object after the immediately preceding drying steps. Preferably, the setting of the different process parameters is automatic and this is calculated by means of a computer. The method according to the invention may be used for different kinds of dryers, such as conveyer dryers for single objects which pass through drying modules placed on a conveyer and intermediate reconditioning modules onto vertical dryers, where vertically arranged conveyers take objects, e.g. wood, in a vertical direction through the drying space which increases the distance of conveying.

The invention will be described more in detail below with reference to the annexed figures.

Figure 1 shows a throughflow dryer with three drying modules and two intermediate and one final reconditioning module.

Figure 2 shows a vertical dryer for timber, also having three drying modules and two intermediate and one final reconditioning module.

The dryer plant shown in figure 1 comprises three dryer modules 1, 2 and 3. Between these there are reconditioning sections 4 and 5 and the dryer plant ends with a reconditioning section 6, comprising several modules. The goods which shall be dried is intended to be positioned on conveyer bands 7, which run continuously through the whole of the dryer plant. Each dryer module has a unit 8, 9 and 10 including microwave transmitters. The reconditioning modules have inlet openings 11 and outlet openings 12 for air. The cooling air obtained from the transmitters 8, 9 and 10 may, if suitable, be used in the reconditioning sections or as additional energy in the initial cavity.

The plant shown in figure 2 comprises two dryer modules 15 and 17 and one reconditioning module 16. In each of these there is a vertical conveyer, by means of which the objects 13 to be dried, e.g. wood, are conveyed in vertical direction through the dryer module so that the dwell time therein is extended. Between the dryer modules transportation is done by means of a conveyer band 7. Each dryer module has a number of transmitters 8, inlet channels 11 and outlet channels 12 for air.

Adjacent to the outlet from each dryer module certain drying parameters of the dried objects are measured. These parameters can for example be surface temperature and moisture content. By measurement and comparison of the temperatures of the in- and outflowing air, the energy consumption for evaporation can be calculated. By means of these parameters the energy supply in one or more dryer modules and the conditions of the reconditioning module are controlled. In the reconditioning modules air is blown through and the parameters controlled are primarily the temperature och moisture of the air. By splitting up the drying process in several steps in this way and by conditioning the dried objects in between under control of the measured values, the temperature, moisture content and other properties of the dried objects are evened out and the problems of local overheating previously mentioned can be avoided. When drying wood one can hereby avoid damages such as cracks, deformation and collapse of cells. In all situations the method and means of the invention gives drying with the highest possible supply of energy and thereby temperature and speed of drying. Measurement and control of the temperatures of the dryied objects can be done by contactless measuring. By splitting up the process in a number of separately controlled steps, extra heat may be supplied initially in the drying process when the objects to be dried are wood of low temperature and i other similar situations.

Claims

1) Method by drying of moisture containing objects by means of microwaves in a throughflow dryer, characterized therein, that drying takes place in several steps in separate modules, whereby the energy supplied in one step is controlled by at least one drying parameter which has been measured on the object after the preceding step and which also controls a reconditioning, i.e. cooling of the object between the drying steps.

2) Method according to claim 1, characterized therein, that the measured parameter is the surface temperature of the object.

3) Device for drying of moisture containing objects by means of microwave treatment in a throughflow dryer, characterized therein, that it comprises several consecutive modules

(1, 2, 3), each being a selfcontained cavity, whereby the energy supplied is separately controlled for each module, and between the modules positioned reconditioning sections (4, 5), where the objects can be cooled and arrangements for measurement of a parameter of the object which is dependant upon the drying and for controlling of the next following drying step in dependence of the value of the measured parameter.