Gustafsson, Per Erik (Stjärnvägen 26, Skövde, S-541 55, SE)
|1.||Method for drying with use of microwaves characterized in that drying occurs partly by means of microwaves, and partly by means of dehumidified air that is divided in a first and a second flow, of which the first is used for cooling of the microwave generators by means of which the microwaves used for drying are generated. Drying occurs in several such steps and the performed work for dehumidification is detected and used for controlling the energy supplied to the microwave generators in the following step.|
|2.||Method according to requirement 1 characterized in that the second part of the airflow is heated before it is used for drying.|
|3.||Method according to requirement 1 or 2 characterized in that the partial flows are mixed before they are used for drying.|
|4.||Method according to any of the previous requirements characterized in that the air used for drying, fully or partially, is recirculated for dehumidification.|
|5.||Method according to any of the previous requirements characterized in that the performed work for dehumidification is detected and used for controlling the energy that is supplied to the microwave generators.|
|6.||Device for drying with use of microwaves and air characterized in that it contains modules for cooling and dehumidifying of air.|
|7.||Device according to requirement 6 characterized in that it also contains modules with microwave generators in addition to channels for the conduction of the circulating air.|
The present invention refers to a method and device for drying using microwaves.
Microwaves used for drying objects cause humidity diffusion from the object's interior towards its surface, from which the air will be transmitted to the ambient atmosphere, which in most cases releases air. For the latter part of the process, microwaves are not adequate, due to a risk of local overheating, among other reasons. Other methods for drying have thus been combined with microwave drying. For example, using the heated cooling air from the magnetrons for this purpose, re-circulation of air becomes possible. Such a device is described in the Swedish patent number 518979. It is also known through the published patent application WO 00/43717 that during batchwise drying, a part of the circulating quantity of air is dehumidified. Partition of airflows is not controlled and the described method and device result in the fact that large quantities of energy are cooled away without resulting in any benefit.
The purpose of the present invention is to show the method and device for drying using microwaves as well as a complementary method for achieving the most effective drying possible. Through the invention, it will also become possible to reach a high efficiency of microwave generators.
Microwaves cause humidity diffusion in a material or in a substance. The humidity diffusion occurs when warmer air moves towards cooler areas, or outwards from within, towards a cooler surface. The humidity that is diffused towards the surface must be ventilated away; the best effect is achieved if airflow with low relative humidity blows towards the surface and absorbs the humidity. Too high a temperature in the air restricts the humidity diffusion, while too low a temperature has the result that the air cannot absorb humidity to a sufficient extent. In many cases, the best result is achieved with air that has a temperature close to room temperature, and that contains as little water as possible.
According to the invention, airflow is used, preferably re-circulating, which is continuously cooled down with help of a cooling battery. The water in the air is condensed out. The airflow is then divided into two parts, of which one part cools electrical or microwave components and thus heats up somewhat. The second part of the airflow passes a condenser battery where the air temperature may be adjusted. Both these airflows are then merged and pushed into a treatment space in which the goods for drying are placed. The airflow may thus be controlled with respect to temperature, speed, and water content. The heat energy, which is formed at the dehumidification process, is partially used in a condenser battery in the equipment. The excess is further transferred by means of a heat exchanger to an external cooler or condenser. The amount of cooling water that passes by the heat exchanger is controlled using a three-way valve, which is controlled by sensors in the airflow.
A drying installation especially built for a certain purpose is seldom appropriate for a change of product or product range. Reconstruction and adaptation to new conditions often becomes complicated and expensive. With the dryer used in the present invention, adaptation becomes significantly easier, because it is composed of modules that may be added, removed, or exchanged. All function modules are equal in dimension. A drying module contains a pressure chamber in its upper part from which the air may be pushed down into a pressure box in the treatment space through a perforation. The pressure box is constituted by supplying the treatment space with an under ceiling of microwave transparent, long and narrow cassettes, the longer sides of which have guiding plates for the air. These form adjustable slits when the position of the cassettes in relationship to each other is changed. The entering air may be accelerated to the desired speed in those slits.
Vertically adjustable reflector plates may be placed under a conveyor belt in a continuous dryer. With the help of these plates, the penetration depth of the microwaves, in e.g. a disc, may be controlled and reduced. It is also possible to use external inlet air with climate treatment instead of circulating air.
The controlled partition of the airflow gives the possibility of controlling and adjusting the process in order to reach high efficiency and good energy economy. When the need for dehumidification increases, the compressor must work harder. The work performed by the compressor, instantaneous compressor power, may be detected and used for controlling the amount of energy, which will be supplied through the microwave generators. If the compressor is working at its full capacity, more energy will not be supplied to the microwave generators, because this would only result in the heating up of the circulating air and the lessening of the drying effect due to the increased water content in the circulating air.
A device according to the invention, as well as the designs of such a device have the characteristic properties given in the patent claims.
In the following, the invention shall be closer described in connection to the example in a preferred design that is shown in appended drawings.
Figure 1 shows a flow chart according to the invention.
Figure 2 shows a device for drying according to the invention.
Figure 3 shows, partly in cross-section, parts of the device according to figure 2.
The objects to be dried are transported into treatment space 1 , where the supply and the removal after drying may occur batchwise or continuously. In most cases, a continuous form is preferred, whereby the drying goods are transported on a conveyor belt, into, through, and out of the treatment space. In this space, drying occurs partly through microwaves and partly through passing airflow. The microwaves are generated by magnetrons 2 and are directed to the object that will be dried in a known manner. The used drying air is returned through a channel 14 to a dehumidification aggregate 5. This may also be supplied with additional air quantities from the outside in order to compensate for air lost by, for example, leaking through the inlet and outlet openings of the treatment space. The re-circulated air quantity is, however, significantly larger than the additionally supplied quantity.
In the dehumidification aggregate 5, the air is dehumidified. Here also, its temperature may be lowered below the dewpoint. The humidity, which is removed, is drained as water through a drain duct 6. The airflow, driven by a fan 4, is divided after the dehumidification into two parts. One of the flows is transported through a channel 10 to the magnetrons and is used as cooling air for them. Through cooling, improved efficiency and extended life of the magnetrons are achieved. The second part of the flow is transported through a duct 11 to a battery 3 in which the air is heated to a preset temperature. A cooling and heating medium is circulated between the dehumidification aggregate 5 and the heat exchanger 3. Any excess heat is extracted from a heat exchanger 12 through the duct 13. The circulating medium is returned to the dehumidification aggregate 5 through a compressor 15. In the battery 3, part of the airflow is heated to preset temperature and transported further through a channel 7. The two parts of the airflow from channels 7 and 8 are then collected in a common flow in channel 9 and supplied to the treatment space 1. Thus, a mainly closed circulation is formed.
A device for drying according to the invention is preferably built up of modules as shown in figures 2 and 3. A conveyor belt 16, for the transport of drying goods through the device, passes through all the modules. The circulating air is filtered, cooled and dehumidified in the two basic modules 17, 18, which also contain fans 20 and is afterwards divided into two flows. A various number of drying and microwave modules 19 with magnetrons 21 are connected to the basic modules. In this way, it is possible to simply make an installation with desired capacity and other properties.
An example of a dryer with three sections and a control system for this dryer is as follows. Since the drying occurs gradually in stages with a step in each section, a drying installation of this type may hence also be called a step dryer. Each step consists of a number of modules. Two basic modules contain fans and filters, in addition to a compressor, a cooling and condenser battery, and a heat exchanger. A number of microwave modules may be connected to the basic modules, each equipped with a number of microwave generators. The dehumidification systems of the basic modules contain an evaporation battery where the air is cooled and the
condensation is drained, a compressor and two condenser batteries are connected in series. One of these condenser batteries preheats the circulating air to a preset temperature and the other condenser deals with the excess heat through water- cooling. The water is cooled in turn externally by a cooling-water cooler, which keeps the water temperature constant. A three-way control valve is present in the external cooling water circuit.
In order to create and maintain an optimal drying process, each step or section in the dryer will evaporate the amount of water allowed by the material and the dryer capacity. In order to reach this, the drying process must be controlled in every step; relevant parameters are the air temperature and the relative humidity. In the first section, a number of magnetrons are connected in a predetermined manner. When the water evaporation occurs in the goods to be dried, then the temperature in the condenser batteries increases. Since the temperature of the air coming out from one of the condenser batteries is set to a constant value, the other condenser must deal with the excess heat by having water flow through. The water flow is controlled by means of the three-way valve. The section must be adjusted so that the three-way valve does not open entirely, since in that case the section would not manage to cool away the condensation heat of the water, which would result in a temperature increase in the condenser batteries, which, in turn, would result in an over-pressured guard in the compressor unit being released. Thus, it is important not to connect more magnetrons than may be managed by the compressor capacity. The same condition applies also to the subsequent section. In the third section, there are a number of special, continuous generators by means of which the magnetron power may be continuously controlled. The control occurs towards a predefined rh-value (relative humidity), such that, if the rh-value drops, then the microwave power increases so that more water is driven out of the material to be dried. If the rh-value exceeds the predefined maximum value, then the dryer is unbalanced and more magnetrons must be connected in the earlier sections. The entire process is controlled by a master control unit.