The mechanism for drying with microwave energy is quite different from that of standard drying. In standard drying, heat is transferred to the surface of the material by conduction, convection or radiation, and into the interior of the material by thermal conduction. Moisture is initially flashed far from the surface, and therefore the remaining water diffuses to the surface. This is often a slow method in standard drying, and the diffusion rate is limited, requiring high external temperatures to get the temperature variations needed.
In standard or surface heating, the process time is limited by the speed of the heat flow into the body of the material from the surface as determined by its heat, thermal conductivity, density and viscosity. Surface heat isn’t only slow, however also non-uniform with the surfaces, edges and corners being a lot of hotter than the inside of the material. Consequently, the quality of conventionally heated materials is variable and frequently inferior to the desired result.
Microwaves aren’t kind of heat, however rather types of energy that are manifested as heat through their interaction with materials. Microwaves at first excite the outer layers of molecules. The inner part of the material is warmed as heat travels from the outer layers inward. Most of the moisture is vaporized before leaving the material.
If the material is very wet and the pressure inside rises rapidly, the liquid will be removed from the material due to the difference in pressure. This creates a sort of pumping action, forcing liquid to the surface, often as vapour. The result is very rapid dryingwithout the need to overheat the atmosphere, and perhaps cause case hardening or other surface overheating phenomena.
Mechanism of Heating
Microwave energy doesn’t heat the room — only the required material — without harmful greenhouse gas emissions from the heat supply. The energy is principally absorbed by a wet material placed in the cavity. Water, fat and sugar molecules in food materials absorb energy from the microwave in a method called electromagnetism, a phenomena related to electric and magnetic fields, and their interactions with one another, and with electric charges and currents.
Microwaves are electromagnetic waves having a wavelength (peak-to-peak distance) varying from 1 millimetre to 1 meter. 1GHz = 1,000 MHz Frequency of microwaves lies between 0.3 and 3.0 GHz. A domestic microwave operates at 2,450 MHz (a wavelength of 12.24 cm). Approximately 90 percent of American homes have microwave ovens. Industrial/commercial microwave systems typically operate at 900 MHz (a wavelength of 32.68 cm). This range allows more efficient penetration of the microwave through the material.
Microwave heating is most effective on liquid water, and far less on fats and sugars and frozen water where the molecules are not liberal to rotate. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast to grilling that depends on infrared radiation, or the thermal convection of a convection oven, that deposits heat shallowly at the food surface.
Energy Savings
There was a time when electrically powered systems were considered too costly to operate for high-power applications. Today, with the rising costs of oil and gas, this is no longer necessarily true. There is significant interest today in the potential energy saving from a microwave heating system. The offsets to the current cost of electricity include the increased speed of drying, the direct coupling of energy into the material and possible lower drying temperatures.
Processors that use steam, gas or electric heating systems are aware of the high costs of running these systems. Consider the energy-saving benefits of microwave heating vs. conventional heating:
- Slashed energy consumption by up to 50 percent.
- Reduced man-hours and downtime involved in cleaning.
- Minimized ancillary equipment, such as heated jackets, boiling pans and heating vessels.
- Smaller heating and/or cooking footprint.
- Less warm-up and cool-down time.
- No heating of the room, only the material.
Economics of Microwave Systems
Several criteria for successful microwave drying systems are related to reduce cost. Cost saving may be realized through:
- Energy savings.
- Faster batch processing.
- Operational efficiencies.
- Increased throughput.
- Labour reduction.
- Reduction in heat load in the plant.
- Reduced maintenance costs.
- Reduction in product fouling.
- Less floor space needed.
In addition, with the challenges of the current economic climate, as well as growing concerns about carbon footprints, many manufacturers are looking for ways to reduce operating costs and cut down on energy consumption. Conversely, with microwaves, heating the volume of a material at substantially the same rate is possible. This is known as volumetric heating. Energy is transferred through the material electromagnetically, not as a thermal heat flux. Therefore, the rate of heating is not limited and the uniformity of heat distribution is greatly improved. The microwaves heat the product at the surface and deep into the product until the microwave energy is absorbed.
A common misconception is that microwave heating is often costlier than heating by standard techniques. Of course, the actual answer depends on the application. In some cases, microwaves can be used 50 % more efficiently than conventional systems, leading to major savings in energy consumption and price. Imperfect heating causes product rejections, wasted energy and extended process times that need large production areas dedicated to ovens. Giant ovens are slow to respond to required temperature changes, take a long time to heat up, and have high heat capacities and radiant losses. Their sluggish performance makes them slow to respond to changes in production necessities, creating their control tough, subjective and costly.
Applications, Functions & Industries
Our equipment is capable of performing multiple applications, including heating, drying, tempering, cooking on multiple-stage recipes that thaw, cook and simmer in the same vessel aided by programmable process control. Possibly eliminate some permits and do away with your troublesome boilers with costly insurance.
Microwave technology is a viable commercial alternative to traditional cooking and heating methods, and is now being seriously considered by many food, drink and pharmaceutical producers. Our equipment is ideal for cooking foodstuffs, such as gravies, sauces, chilies, soups, syrups, eggs, puddings, fruit juice, salad dressings, canned and packaged fruits and vegetables and much more.
Uniformity of heating with precise temperature control is also vital to several different industries, as well as chemicals, plastics, minerals, textiles and a variety of different industrial processes.
We at KERONE is have a team of experts to help you with your need of microwave and convectional based technology from our wide experience. For any query write us at [email protected]