Microwave Vacuum Drying

Microwave drying is functional compared to other drying methods.Microwaves are radio waves and it operates in bands.The vacuum drying system dry from the surface inward and microwaves are inserted the every part of material at the same time.This operation called as volume heating.Additionally,the vacuum technology provides great importance to dry bulk products with poor thermal conductivity.

Figure 1:Met microwave fig drying

Drying with microwave vacuum ensures the material heated uniformly.This operation gains us 80% more energy efficiency rather than conventional gas fired dryers[1].The heat that is used in microwave is a form of dielectric heating.The electromagnetic energy is transforming into kinetic molecular energy thus the heat is generated for drying procedure.Also,the thermal sensitive products sucs as fruits and vegetables are one of the example of high-end drying applications by vacuum drying [2].Our product,MET microwave is also tested on drying many fruits and the excrements that came from water purification mainly shown in Figure1.

MET microwave vacuum drying system saves substantially time and energy.Heating operation is maden in a short time schedule then the loss in food products decreases.The amount of materials that we dried is increased compared to conventional drying as shown in Figure2. Furthermore,in our product heat efficiency can raise 40% so,the over-heating in surfaces is prevented.The microwaves could be used different packages and the multi-component foods are adequate in order to temperature profile alterations due to inward drying logic.

Fields Of Application

Microwave vacuum drying systems are mainly used in four industry area.They are semiconductor manufacturing,pharmaceutical,food and chemical industry.This markets expects process control,efficiency,flexibility,innovation and saving on time.MET microwave procures all the requirements of markets.Also,our product provides colossal convenience in order to cleaning,following of the system parts,montage and utilization.

In food industry, products like spices, snacks and other bulk products often need to be finish dried once they are processed. Microwave drying systems provide the flexibility to adjust throughout without costly heating and cooling cycles necessary for conventional drying.[3]

Figure 2:Food drying 

Figure 3:Apple drying

Basics of Microwave Drying and Dielectric Properties

As microwave heating is a form of dielectric heating, dielectric properties is thus the most important factors among all. For a substance to be microwaveable it must possess an asymmetric molecular structure, as in the case of a water molecule. The molecules of such substances form electric dipoles which, when exposed to an electric field , assume an orientation relative to the direction of that field. It is this orientation polarization that is responsible for energy generation.[2]

The essential factors to produce heat capacity in a volume element are the electric field strength of the microwave field, the frequency, and the dielectric properties of the product represented by the loss factor of the material εr’’ (equation (1)).

The loss factor is calculated as the product of the permittivity number and tangent of the loss angle δ. Both parameters depend on frequency and temperature.

P’’’ = 2 π • f • εo • εr’ • tan δ • E = 2 π • f • εo • εr’’ • E (1)

P’’’ bulk-wave energy density measured in W/m3

f operating frequency measured in Hertz

εo electrical field constant = 8.85 x 10-12 As/Vm

εr’ dielectric constant = real part of the complex permittivity number

δ dielectric loss angle measured in degrees

εr’’ dielectric loss factor = imaginary part of complex permittivity

E electric field strength measured in V/m (effective value)

The higher the loss factor of a substance is, the better the substance can be heated in a field of microwaves. Water and all aqueous substances possess a high loss factor and therefore absorb high frequent energy and microwave energy exceedingly well. Depending on their absorption behavior towards microwave irradiation, materials can be classified into three groups:

  • absorbers, e.g. water (εr’’=12 at 25°Celsius), aqueous substances
  • (practically all foodstuffs), diverse sorts of plastics
  • transparent, e.g. porcelain quartz glass (εr’’=0.0023), Teflon
  • reflectors, e.g. metal, graphite

Down to a loss factor of about εr’’ = 0.01 substances may still be heated in a microwave field. If the loss factor should be below this value, there still might be the possibility to blend in additives with higher loss factors which, however, should not change the desired properties of the substance.[2]


As compared with conventional drying methods, which mean heating from the outside to the inside by heat conduction, microwaves show so-called volume heating and have following advantages:

  • a temperature gradient directed towards the surface, i.e. temperatures inside are higher than outside, giving rise to a higher partial pressure that drives the evaporating liquid to the surface
  • consequently, the superficia l layer does not dry up completely and the surfaces remain permeable
  • the liquid evaporating inside the product is emitted through the porous structure of the solid material’s macro-capillary system, resulting in a high drying velocity
  • the heating of water and most organic solvents occurs selectively – due to the greater dielectric losses of water as compared to the product to be dried
  • swift and thorough drying of moist products with low thermal conductivity
  • static drying of thick layers without frictional losses
  • high total efficiency of energy application
  • high-speed control of the energy transport
  • short processing times, i.e. suitable for automated manufacturing.[4]


[1] http://www.cellencor.com/en/about_microwave_heating

[2] http://www.pueschner.com/downloads/publications/2007_article_mw-vacuum-drying_hk2007-short_pap.pdf

[3] https://ferriteinc.com/industrial-microwave-systems/industries-applications/food-processing/

[4] http://www.pueschner.com/en/microwave-technology/microwave-drying

By |2018-07-26T13:12:37+00:00July 17th, 2018|Categories: Uncategorised|Comments Off on Microwave Vacuum Drying