Introduction
In the design of microwave vacuum drying system, we utilize microwave a type of electromagnetic wave power to heat a food product inside a chamber with lowered atmospheric pressure.Use of microwave applied in a vacuum for dehydration heats the product to cause water to vaporize, without causing changes in composition. Microwave energy penetrates deeply into food products and can reduce process time by 90 percent (Decareau and Peterson, 1986).Application of microwave energy in a vacuum results in an increase in product temperature; however, the temperature rise is limited to the boiling point of the water at the lowered pressure. At a pressure of 3 kPa, free water boils at 22°C. This maintains a product temperature at a level below the temperature used under atmospheric conditions of 101 kPa.(Ostrom 1995).
This technique was first implemented in the year 1975. The microwave ovens basically works by emitting non-ionizing microwave radiation, usually at a frequency of 2.45 GHz (a wavelength of 12.24 cm), through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat, and other substances in the food absorb energyfrom the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwaves. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. (Wikipedia)
Working
The microwave is generated with the help of a magnetron. It is basically a filament made up of cathode surrounded with an inside insulated cylindrical anode maintaining cavity in between both of them. The whole system is in vacuum with powerful magnets on its ends. It is powered up by a transformer(Step-up) and a RF choke by up to 5 Kilo Volts. When powered the electron streams from cathode to anode. The magnetic field forces them to travel in curved paths in bunches like spokes of a wheel. So they are created in the gap
between the cathode and the anode known as resonating cavity. The magnetic fields helps in guiding these electrons as soon as they are threshold from cathode surface and move towards anode. The frequency and wavelength of the microwave is determined by the size of the resonating cavity. While the strength of the magnetic field controls the power output and efficiency of the magnetron.
Figure 2: Picture of a magnetron cross section
Source: (Wikipedia)
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Fig.2: The Magnetron assembly
Source: (
This magnetron is installed in a microwave power generator unit. The microwave power generator unit consist of a step up transformer. A rectifier circuit controls the power. It is further accompained by a high voltage capacitor to act as storage for providing the working voltage for the magnetron during operation. The rectifier, HV capacitior and step up transformer are hooked up with magnetron that emits the required power and frequency of microwaves. The emmited microwaves are carried from the microwave generator to the vacuum chamber with the help of wave guide. The wave guide is nothing but a precison cut tube (circular or rectangular) made up of metal like brass, silver, copper, aluminum etc. These waveguides conveys the microwave and can be used for distribution of microwaves from various points in a chamber through different types of adaptors, sections and arrangements.
Fig.3: Waveguides of different shapes and sizes
Source: (
This magnetron is installed in a microwave power generator unit. The microwave power generator unit consist of a step up transformer. A rectifier circuit controls the power In microwave ovens a stirrer is provided just infront of the waveguide to evenly distribute the microwaves on the subject. Athe stirrer is basically a metal fan on which the micowaves are incident and deviated to different angles on the subject.
Sometimes the microwaves are generated in a continuous manner by the magnetron without stopping is termed as continuous heating. While sometimes it is required for microwaves to be emmited in lapsed timing or in a pulsed manner. This depends upon the application and power with the duration of the product to be heated. The amount of heating can also be controlled by controlling the output power and by switching the power on and off in calculated time durations.
To control the power of the microwave generator a high volatge relay is used, it is also called as variable power switch. The relay controls the on/off time of the magnetron tube. It is inturn controlled by the main controller and relies on carefully timed signals from controller timer circuit.
Fig.4: Conventional house hold microwave
Source(
Material and Methods
The construction of fully operational microwave vacuum dryer works at a general frequency of 2450 MHz stated by the US government for all commercial and household microwave equipments. All types of microwave generator are available in market diffrentiated by there power output capacity. In this research, the results must be interpreted at various power outputs of say 100,300 and 500 watts of elctrical field strength. This needs to be done for obtaining multiple replications for calculation of different drying rates according to microwave power. A microwave generator of 500 watt to 1000 watt is enough for dielectric food processing. A waveguide will carry these microwaves of required wattage to the vacuum chamber. The waveguides comes in various cross-sections and are rated according to the frequency of microwaves passed through them. For example, we will require a WR-284 type of waveguides because our frquency is 2.45 GHz and the waveguide is suitable for frquency range 2.60 GHz to 3.95 GHz. The waveguide must also be compatible to the microwave generators head. In our case, the other end of waveguide is fitted in a low pressure section. It can be accomplished with the help of using a “Waveguide to pressure window” type of sectionconsisting of a glass or microwave permeable material at the vacuum chamber end. This is utilized to maintain the vacuum in the system without its leakage through the open waveguide. This can also be established by using a separate compartment for vacuumizing and heating of the product so that the vacuum remains intact.
Further, the vacuum chamber assembly comes into play. It is very simple design of a air-tight chamber with an inspection window. It consist of microwave input through waveguide from one or multiple points depending upon requirement of uniformity by proucts physical properties such as shape, size and quantity etc. In case we utilize a single point microwave input into the chamber, a stirrer can be utilized for better distribution of microwaves through out the product. It also is more cost effective by using a stirrer than using multiple point microwave entry into the system with the help of waveguide distributors and adaptors. It is therefore practiced in all house hold microwave ovens and waveguide sizes are kept as small as possible.
I am also certain about another method utilized by (changrue et al.2006) of actually using a small cylindrical chamber that will be hung inside the main microwave chamber. The vacuum will only be maintained in this inner cylinder, while the outer portion will be at normal atmospheric pressure. In this manner it will be more easier to maintain vacuum as well as microwave the subject simaltaneously.
Utilization of Vacuum Pump
The vacuum pump utilized can be connected to the main chamber with a cheap pressure control valve. The vacuum pump will either utilize a dessicator to absorb the exhaust vapors during drying operation. The same work may be accomplished by condensor when higher mass load of subject is to be dried. The pipe connecting to the chamber and chamber walls must be heated or kept at a slight higher temperature to avoid condensation of water vapor. This due to the fact that the whole microwave process is a cold process and the rise in ambient tempearture is very less. Therefore the vacuum tube may have back flow of droplets due to condensation of water vapors inside it. The capacity of vacuum pump may be decided according to the capacity of mass load of subject you want to process in a batch. Since the lab vacuum fryer has a vacuum pump and condesor unit. To economize the cost machine can utilize the same arrangement by placing the MVD equipment side by side to the vacuum fryer.
Fig.3: Schematic for Microwave Vacuum dryer
Source: (Ostorm 1995)
Instrumentation
The microwave chamber will consist of turn table or vacuum cylinder sustained by a digital weigh scale. The weight of product can be read as it decreases due to moisture evaporation during real time heating operation. This way the moisture removal rate can be calculated by observing the decrease in weight. An Infrared thermometer will be utilized to obtain temperature of the product by permanently positioning it to point at the product. The readings can be noted to analyze the change in the temperature of the subject. Multiple thermometers can be positioned on various point of the processing chamber to obtain distribution in temperature change. The ambient temperature inside the cylinder or processing chamber may be noted with the help of normal digital thermometer. The other parameter required to be noted down is the vacuum pressure which will be obtained from the vacuum control valve. The systems microwave power output and other microwave related properties as well as efficiency can be found out by doing small experiments as well as the reading through potentiometer of microwave power generator. This way all the data required for calculation of the drying kinetics of the subject can be easily calculated.
Data Collection
- The initial moisture content and weight of the product is recorded.
- The sample is placed in the drying chamber and the desired power input on the potentiometer of the microwave generator is set and other control devices and instrumentations are turned on.(the wattage usually will depend on number of replication required to be done on particular value of E; electric field energy(watt/sq. meter))
- After securing the latch of the chamber the vacuum pump is plugged and waited for the chamber to depressurize till the chamber reaches desired pressure.
- The product can be rotated for uniform distribution of microwave energy by turning on the turn table motor. (Commonly used at 1 RPM speed and very cheap to procure.)
- The microwave power generator is turned on and microwave drying commences.
- The weight on the digital scale will gradually decrease as the products moisture will be vaporized and let out of the system through vacuum pump.
- The timer will note the time of the continuing process manually.
- When the digital scale reading reaches the desired mark or percentage of moisture content the microwave generator will be turned off.
- The timer will be stopped simultaneously and the total time required for drying will be noted.
- The acquired data will be utilized for study of drying kinetics and modeling of the subjects microwave vacuum drying.
Cost estimation of various components
The cost estimation is done according to the current rates and quotation acquired from the various equipment suppliers. The cost of cheaper small miscellaneous items required for completion of the design is not being included. The microwave chamber can be fabricated in the shop itself and the preferred material to work on it is yet to be decided. I am trying to get rates for array of items required for this project and looking out for cheapest alternatives present such as ebay and aftermarket products.
Estimated time required for completion
Depending upon how fast the various components are procured. The actual fabrication will not require more than two weeks of time to complete and make it running.
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