1: Introduction to Microwave
Microwave is one of radio wave, and radio wave is one of electromagnetic wave.
Since the electromagnetic wave is spread by the interaction of electric field and magnetic field, it can also be spread in a vacuum.
Electromagnetic wave is a wave that has two elements, such as wavelength and frequency. Wave length is about the length of top to top of the wave, frequency is number of waves that appears in a second.
The velocity of electromagnetic wave in a free space, such as vacuum and air, is constantly 300,000km per second regardless of frequency. Wave length equals to this divided by frequency.
As shown in Figure 1, electromagnetic wave is called differently depends on frequency. It is used in various applications according to their characteristics. Light is also a kind of electromagnetic wave. The electromagnetic waves that are less than 3000GHz, is classified as radio wave. Radio wave that has frequency of 300MHz to 300GHz(wave length of 1m to 1mm) is known as microwave.
Microwave has been applied to communication, radio telescope for astronomy, radar surveilance system, and also to GPS positioning system known as car navigation system. Heating is another application of microwave.
2: About the frequencies that can be used for Microwave heating device
ITU (International Telecommunication Union) would assign the available frequencies of radio wave that depends on use. It is finally decided by the laws of each countries.
Regarding microwave that has frequency of 300MHz to 300GHz (wave length of 1m to 1mm), ITU has allocated frequencies for the purpose of industrial, scientific, medical use, as shown in Table 1.
433.92MHz has been recognized as ISM frequency in some countries, in the first region (Europe).
915MHz has been recognized as ISM frequency in the second region (North and South America)
ISM frequencies that can be used worldwide is the ISM frequency of 2450MHz or higher.
Table 1: ISM frequency of microwave band
On the other hand, there is a law with strict limits to regulate leakage of radio waves in order to
avoid interference with telecommunication failure. However, for example, the Radio Law doesn't
regulate the limit of radio leakage for the ISM frequency of 2450MHz band.
2450MHz band Magnetron
(output 2kW water-cooled type)
Therefore, microwave device that uses this frequency band (called ISM equipment) better be designed to meet the safety limits.
In contrast, devices that use radio frequencies other than the ISM frequency need a large-scale
countermeasurement for radio leakage to meet the regulation of Radio Law, such as, radio wave
shields for equipment installation room, or for entire building.
This is why various industrial heating equipment is using ISM frequency, including microwave oven at home.
The 2450MHz is the most popular among ISM frequency bands
due to not only being usable in any countries in the world, but
also existence of a microwave oscillator tube shown in Figure 2.
The relatively inexpensive magnetron (Output: 300W to 10kW)
that is compact built, light weight, and permanent magnet
attached, is also a huge contribution to market expansion
of the 2450MHz band.
3: The principle of microwave heating
You may have experienced that you could not have seen the BS broadcasting TV channel when it is raining hard. The most efficient frequency of the microwave absorption of water is said to be around 18GHz.
The frequency of microwave oven is 2.45GHz (2450MHz). And the frequency of BS broadcasting TV is around 12GHz. You can probably understand the reason for not being able to see the BS broadcasting TV in hard rain by now. It is that BS airwaves have been absorbed by the rain. Power of BS airwaves is weak, so rain will not be heated. But in principle, rainwater is heated by the absorbing power from BS airwaves.
In this chapter we explain about, "The principle of microwave heating", "Microwave power absorbed by the dielectric", "The depth of microwave penetration into the dielectric", and "Dielectric properties of the dielectric".
(1) The principle of microwave heating
IEC(International Electrotechnical Commission) defines, Microwave heating is to heat dielectric materials through mainly their molecular motion and their ionic conduction by the action of electromagnetic waves of 300MHz to 300GHz.
The principle of the microwave heating is very difficult. And it's not easy to explain, but the following will give you a rough idea.
"For the vibration of microwave field, for example, when permanent electric dipole in the dielectric materials follows the oscillation of the microwave field slightly later, i.e., to the change of microwave field, in case permanent electric dipole changes with phase lag, this phase delay is the resistance to change of microwave field. Then, the permanent electric dipole will be heated by this resistance." Briefly, "The permanent electric dipole is forced to vibrate while resisting and genarates heat by this action."
Followings are explanation with figures:
The principle of microwave heating described in using Figure
Figure 3 shows the structure of water molecules that are discussedas representatives of the permanent dipole.
Water consists of two hydrogen atoms and one oxygen atom. It doen't have electric charge as a whole, a oxigen atom is bounding with two hydrogen atoms at an angle of 104.5°. Those atoms take a little charge of each minus(-) and plus(+) to form a dipole.
Then as shown in Figure 4, when there is no external electric field, it has set a balance. But when placed in external electric field, dipole will turn towards the electric field.
For example, when the water is irradiated with radio wave, it means to give a electric field for alternating. In case of microwave oven, vibration of 2450 million times plus and minus to be replaced per second.
Figure 5 shows a case where a too much lower frequency of radio wave is irradiated to the permanent dipole of water. In this case, the permanent dipole will immidiately follow the directions of electric field. So in this case, water doesn't generate heat.
On the other hand, Figure 6 shows a case where a too much higher frequency of radio wave is irradiated to the permanet dipole. In this sice, since electric field changes its direction too fast, dipole won't be able to follow. Then, water does not genarate heat in this case also.
In contrast to these, Figure 7 shows a case where moderate frequency of radio wave is irradiated to the permanet dipole. In this case, the permanent dipole changes a little behind the electric field. During the time delay, water is absorbing energy from radio wave and genarates heat. And, this moderate frequency is the microwave.
(2) Microwave heating formula and the dielectric properties of materials
From the above description, you might think that microwave can only heat dielectric materials. However, metal such as stainless steel, metal oxides such as iron sand can also be heated in microwave.
And, even for metal powder, microwave penetrates inside while heating. However, for the metal plate, most of microwave will be reflected instead of penetrating.
In this chapter, instead of explaining about the metal, "The microwave power absorbed by the dielectric", "The depth of microwave penetration into the dielectric", "Dielectric properties of the dielectric" are explained.
(A) The microwave power absorbed by the dielectric (theoretical formula)
Equation 1 shows microwave power P1 absorbed by the dielectric in a theoretical formula.
Relative permittivity of the dielectric material εr and dielectric loss angle of the dielectric material tanδ show characteristics of material (dielectric). In Equation 1, multiplication of relative permittivity εr and dielectric loss angle tanδ is known as the dielectric loss factor (also referred to as loss factor). This factor shows the level
of microwave power absorption by the material.
On the other hand, E in Equation 1 is the electric field strength which will be determined at the design of applicator in device as well as at conditions of the dielectric in the applicator. So, there is no way to find out the actual electric field strength in the dielectric. To find the microwave power absorbed by the dielectric, it can be explained by the calculation described in calories in (B).
(B) The microwave power absorbed by the dielectric (calculation formula fo calculating calories)
In the applicator, a vessel with liquid w [ g ] (at initial teperature T1 [℃]) is irradiated at microwave power PA [ W ] for t [ s ]. Then, the liquid becomes T2 [℃]. And, the specific heat of the liquid is C [ J /(kg･K)].
Then, microwave power [ W ] absorbed by the liquid is given by Equation 2. And, the heating efficiency is given by Equation 3.
For example, when the liquid is water, by using the specific heat of water 4180 [ J /(kg･K)], absorbed microwave power of water can be calculated.
(C) The depth of microwave penetration into dielectric
The microwave penetration depth from the surface of the dielectric to inside is defined as depth that is 50% of the surface power.
And, the formula for the penetration depth is given by Equation 4.
(D) Dielectric properties of materials
We have explained that relative permittivity of the dielectric material εr and dielectric loss angle of the dielectric material tanδ show characteristics of material (dielectric) at section (A).
Figure 8 is a characteristic diagram showing the relative permittivity εr and the dielectric loss angle tanδ of various materials. As a result, roughly materials on up-right which has a large absorption of microwave's penetration depth is shallow, and materials on below-left which has a little absorption of microwave's penetration depth is deep.
4: The characteristics of microwave heating
There are some characteristics of microwave heating that other heating methods (canventional heating method) do not have. The following are the features.
・Rapid heating, Select heating
・High heating efficiency, Rapid response and temperature control
・Heating uniformity, Clean energy
・Good working and operating environment
Will describe each categories below
(1) The internal heating
As shown in Figure 9, microwave will reach the object to be heated at the same speed of light. Then it enters into the object as a wave, and by getting absorbed, the object generates heat. Therefore as shown in Figure 10, microwave heating is internal heating.
(2) Rapid heating
As shown in Figure 10, in conventional heating, the object's temperature rises by spreading heat energy from the surface to inside (external heating) On the other hand, by microwave heating, the object will generate heat on their own by the penetration of the microwave. Not necessary to consider about the heat conduction. That is why rapid heating is possible by microwave. Although the object has to be large enough for the microwave to penetrate, the smaller objects will also be heated from inside as the depth of microwave penetration. As shown in Figure11, compare to the conventional heating which is heated from the surface, microwave will still be faster for the small objecs.
(3) Selected heating
Microwave heating, as shown in Figure 8, there is a difference in microwave power absorption depending on the material.
For example in Figure 8, a borosilicate glass is sold as microwavable glass container. When this glass is heated with water in it, only water gets heated. That's because microwave power absorption of the glass is ignored since there is only 3000/1 of water.
Therefore, as shown in Figure 12, when a good material of container is selected, microwave can only heat the object and heat efficiency improves substantially.
(4) High heating efficiency
Microwave penetrates into the object at the speed of light. And the object to be heated generates heat on their own. You get high heating efficiency because no need to consider the heating losses of air inside the heating furnace.
(5) Rapid response and temperature control
Microwave penetrates into the object at the speed of light. And the object to be heated generates heat on their own. So it allows rapid response. For examle, you can start and stop the heating instantly. In addition, by the adjustment of microwave output, you can control the amount of heat energy generated inside the heated object. Therefore as shown in Figure 13, you can instantly respond to the temperature changes of the object to keep the temperature setting.
(6) Heating uniformity
Each part of the heated object generates heat, so even for those objects with complicated shape, it can be heated relatively uniform. To keep the heating unifomity, stirrer, turntable, and belt conveyor is used for heating blur related to wave length.
(7) Clean energy
Microwave doesn't require a medium, because it propagates only by changes of electric fields and magnetic fields. It can propagate in a vacuum. It reaches the object and penetrates without heating the air. The heated object genarates heat by absorbing microwave energy to convert it to heat energy. Therefore, it can be said as clean energy because it doesn't heat the air during process.
(8) Good operation and work environment
Conventional heating requires a heat source, and the temperature rises not only of heated object, but also of heat source and the heating furnace. So the temperature of room equiped heating furnace goes high because of radiant heat. This is a operation and work environment issue. On the other hand, microwave heating only uses electricity to generate heat of the object. The temperature of the object only rises, not the furnace. And there is no radiant heat, so it's possible to keep operational and good working environment.
5: The basic structure of microwave power application apparatus and microwave device
Electric devices that generates microwaves are magnetrons, klystrons, gyrotrons.
Magnetron is comparatively inexpensive and it can generate a large power. Magnetron is a type of vacuum tube that has been also used for the household type microwaves. In micro Denshi, we primarily use the 2450MHz band magnetron. We manufacture microwave power application apparatuses that has 300W to 300kW outputs.
(1) The basic structure of microwave power application apparatus
In Figure14, shows the basic structure of microwave power application apparatus. Microwave that is generated by magnetron built inside the Generator ①(Oscillator), Transferring Waveguide⑥, Isolator②, Power Monitor③, Waveguide⑥, EH-Tuner④, then reaches the Applicator⑤ to heat the object to be heated inside the Applicator.
In here, microwave that is oscillated by the Generator is called the traveling wave (or incident wave). On the other hand, microwave that is reflected by the Applicator is called the reflected wave (or reflection wave) And the microwave power consumed within the Applicator, is same value as reflected wave deducted by the traveling wave. ( Strictly speaking, micorwave power consumption from EH Tuner to the Applicator )
(2) The functions of each microwave devices
①Generator (Oscillator)： A device that oscillates microwave. Generator is connected to a waveguide (the waveguide standard: WRJ-2/WRI-22, Flange:BRJ-2/FUDR22) has opened nozzle. The microwave which is generated by magnetron propagates in waveguide to be emitted from the nozzle.
Therefore, as shown in Figure 14, operating tests must be done after the connection of microwave devices to the Applicator. Otherwise, it is dangerous.
②Isolator： A device, sends traveling wave directly to the Applicator, and absorbs reflected wave by built-in dummy load to avoid returning back to the Generator.
Reflected wave which occurs by rotation of the Turntable and the Stirrer. Isolator can reduce the influence of reflected wave fluctuation. Without this fluctuation, magnetron can continue stable operation. In other words, Isolator functions to protect magnetron
③Power Monitor： A device that monitors traveling and reflected waves of microwave power propagating through the rectangular waveguide. Must be careful when reflected wave becomes large, there is an increase in error.
Micro Denshi's Power Monitor has been devised to show the power accurately, even when magnetron is driven by other power supply.
④EH-Tuner： There are two kinds of tuners, which are three stub and EH. EH-Tuner is recommended for easy adjustment. By adjusting E- or H-tuner, changes the phase and magnitude of microwave reflection at the tuner section It is also adjustable to set the display value of reflected power to zero by adjusting E- or H- tuner.
This means that, by adjusting E- or H- tuner, generates a same magnitude reverse phase wave to counter the reflected wave. And as a result, the reflected wave has been denied.
When the reflected power wave value is zero on the display of Power Monitor, power consumption of after tuner to inside applicator is maximized. This condition is called "The matching"
⑤Applicator： A heating tank that heats the object placed inside by irradiation of microwave. Depending on the application, ther are a variety of shapes, such as batch type, conveyor type, waveguide type, etc.
Micro Denshi's applicator has been devised to minimize the reflection caused at the junction of waveguide.
⑥Waveguide： Microwave (electromagnetic wave) propagates in the interaction of electric and magnetic fields. Microwave is transmittable when metal pipe with cross section is used.
In general, for the microwave heating equipment, 2GHz standard rectangular waveguide of rectangular cross section is used. ( Standard waveguide: WRJ-2/WRI-22, Flange: BRJ-2/FUDR22)
(3) The matching adjustment of applicator with stirrer.
The principle of the matching described in the (2)④, is under the conditions where there is no stirrer in the applicator. In contrast, in Figure 14, for example, when there is a stirrer (stirrer fan) or turntable inside the applicator, depends on the rotation, reflecting position and magnitude differs. Here are some explanation of the matching in this case.
For example, when only stirrer is rotaing inside the applicator, according to the rotation, the value of reflected power indicated on display changes greatly. In this case, adjust EH-tuner to minimize the reflected power on display. Then, the unadjustable reflected power goes to the isolator to be absorbed by the dummy load in it.
In Figure 14, describing reflection in orange dotted line, becomes thinner at the EH-Tuner. And it will be absorbed by the dummy load of the Isolator. Figure 14 shows the matching in this case.
In addition, when the reflected power is large to pass through Power Monitor, there will be a significant errors. In case of controlling the traveling wave by detecting reflected wave, better to use the dedicated devices to calculate the accurate reflection power.