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Factors of influencing microwave heating
2017-02-14
     1.Frequency.
    Higher frequency makes the heat more quickly in microwave heating area.As the higher frequency with the shorter wavelength,it lead to the smaller penetrate ability.2450MHz is heating faster than 915MHz does,but 915MHz penetrates deeper than the former.Therefore,in order to achieve uniform heating to the thicker material should use less frequency.On the contrary,we can choose higher frequency to increase the heating speed to thinner material.In addition,the frequency can also affect the dielectric loss coefficient.Coefficient of pure water at room temperature in 2450MHz is greater than the double of 915MHz.
 
     2.Dielectric properties of materials.
    Different medium have different relative dielectric constant and loss tangent. The relative dielectric coefficient and loss tangent bigger than the general one.Therefore,in general case,the greater moisture content of materials the greater loss dielectric.Some materials coefficient go down when temperature rises,then it appears so-called automatic balancing.This balance of microwave heating is more evenly and to avoid weakness of overheating.However,the temperature rose when some materials were heating,it come out with risen medium loss coefficient and then we call vicious circle.The ice dielectric coefficient is 3.2 with its loss coefficient is 0.001 in 2450MHz.However,the water is 80 with 0.2 in the same situation.So if the melt water can not be drained quickly as both the dielectric coefficient and loss coefficient are much larger than ice when heating frozen food,the energy will being absorbed by water rather than ice at last so the ice can not be heating which affect the thawing speed.
    The following table will show the relationship of dielectric coefficient and medium loss coefficient of beef,pork,patatoes and carrots in 915MHz and 2450MHz.(25℃)
  
Food type Frequency  MHz dielectric coefficient loss coefficient
Beef 915 65 27
2450 61 17
Pork 915 59 26
2450 58 16
Patatoes 915 65 19
2450 64 14
Carrots 915 73 20
2450 72 15
 
    From the table we can see that there is little relationship between dielectric coefficient but it has greatly effective of frequency and medium loss coefficient,such as the dielectric coefficient is much smaller in 2450MHz than medium loss coefficient in 915MHz.
 
    3.Electric field strength.
    Electric field strength is associated with microwave heating power.Heating speed is fast with big power and electric strength.
 
    4.The density of the material.
    Material density affects not only the MW heating,but also the dielectric properties,thereby indirectly affecting the thermal effect of MW.The bigger material density the slower warming.
The air density is ideal for MW penetrating and its relative dielectric coefficient is also small,about 0.1.In addition,it is hard to be heated if there is much air in the materials which cause the smaller relative dielectric coefficient.High density materials(such as bone)have poor absorption on MW and the reflection effect on it.When heating poultry and steaks with bone,it will appear a cold zone near the bone.The part of this zone is ripe after others.
 
    5.Specific heat capacity of MW.
    Food is often multicomponent mixtures consisting of various raw materials.Different component have different specific heat capacity,which will have a different Specific heat capacity.Different components have different dielectric properties,so it has a different ability to absorb MW power.Small Specific heat capacity materials temp. rise faster,so when component of food are heated by MW should have a good comparison of heat capacity to control.Then this will be enable to make the various components of the velocity synchronization.Above these factors are interrelated,such as energy-saving properties of materials and specific heat capacity,not only under the influence of temp.,salt concentration in the food.The heating speed becomes faster with smaller penetration depth as the content of salt.Therefore,it should be carefully taken into account in selecting and mastering the control conditions in practical work.

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