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Try micro wave technology

time:2008-08-28 08:48:19  View:        

1. How does microwave compare to conventional heating?
In conventional or surface heating, the process time is limited by the rate of heat flow into the body of the material from the surface as determined by its specific heat, thermal conductivity, density and viscosity. Surface heating is not only slow, but also non-uniform with the surfaces, edges and corners being much hotter than the inside of the material. Consequently, the quality of conventionally heated materials is variable and frequently inferior to the desired result.
Imperfect heating causes product rejections, wasted energy and extended process times that require large production areas devoted to ovens. Large ovens are slow to respond to needed temperature changes, take a long time to warm up and have high heat capacities and radiant losses. Their sluggish performance makes them slow to respond to changes in production requirements making their control difficult, subjective and expensive.
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 electro-magnetically, not as a thermal heat flux. Therefore, the rate of heating is not limited and the uniformity of heat distribution is greatly improved. Heating times can be reduced to less than one percent of that required using conventional techniques.

2. What are the advantages?

Because volumetric heating is not dependent on heat transfer by conduction or convection, it is possible to use microwave heating for applications where conventional heat transfer is inadequate. One example is in heterogeneous fluids where the identical heating of solids and liquids is required to minimize over-processing. Another is for obtaining very low final moisture levels for product without over-drying. Other advantages include:
Microwaves generate higher power densities, enabling increased production speeds and decreased production costs.
Microwave systems are more compact, requiring a smaller equipment space or footprint.
Microwave energy is precisely controllable and can be turned on and off instantly, eliminating the need for warm-up and cool-down.
Lack of high temperature heating surfaces reduces product fouling in cylindrical microwave heaters. This increases production run times and reduces both cleaning times and chemical costs.
Microwaves are a non-contact drying technology. One example is the application of dryers in the textile industry, which reduce material finish marring, decrease drying stresses, and improve product quality.
Microwave energy is selectively absorbed by areas of greater moisture. This results in more uniform temperature and moisture profiles, improved yields and enhanced product performance.
The use of industrial microwave systems avoids combustible gaseous by-products, eliminating the need for environmental permits and improving working conditions.


3. What are the disadvantages?
Historically, the primary technological drawback to using microwave energy for industrial processing has been the inability to create uniform energy distribution. If uniform energy distribution is not present, wet regions of the target material are underexposed, and other regions are overexposed. This is analogous to the hot spots and cold spots generated in your microwave oven at home when heating or defrosting food like a potato or frozen chicken.
Severe overexposure of non-uniform energy distribution may provide excessive focus of heat build up resulting in burnt material or a fire hazard. The uniformity of distribution designed into IMS microwave equipment overcomes this problem.
Another disadvantage is the depth of penetration achievable using microwave energy. This is a function of microwave frequency, dielectric properties of the material being heated and its temperature. As a general rule, the higher the frequency, the lower the depth of penetration.

4. What about safety?
Using patented applicator design geometries and a unique slotted choking mechanism, IMS technology reduces microwave leakage from system entry and exit points to virtually non-detectable levels for both their planar and cylindrical heating systems. This poses no threat of electro-magnetic radiation to the health and safety of equipment operators.
Heaters and dryers operate at a twenty times higher level of electromagnetic emission safety than that specified by the FDA for domestic microwave systems. As a further precaution, all control systems are supplied with safety interlocks and leakage detectors that shut down power instantaneously in the event of equipment malfunction or misuse.

5. What about economics?
A common misconception is that microwave heating is always more expensive than heating by conventional techniques. The actual answer depends on the application. In some cases, microwaves can be 50% more efficient than conventional systems, resulting in major savings in energy consumption and cost.
When used as a Pre-Dryer in combination with conventional gas or oil heated air dryers, microwave systems allow overall production capacities to be increased by 25 to 93%. This is because the pre-dryer performs three functions, namely:
Removes residual moisture.
Preheats moisture to the evaporative temperature.
Equalizes the moisture level of product to the conventional dryer.
With current energy costs, the return on capital invested in an IMS pre-dryer can vary from 12 to 24 months.
When used as a Post-Dryer in combination with conventional gas or oil-heated dryers, microwave systems are disproportionately more efficient than conventional dryers at achieving final moisture levels of less than 20%. This is because the lower the moisture level, the more difficult it is to drive moisture from the center of material to the surface by conventional heat conduction and convection processes. A post-dryer provides:
Uniformity of moisture control and surface temperature of the final product.
Higher production efficiency due to increased process speeds.
Improved product quality resulting from reduced surface temperatures, compared with conventional post-dryer designs.
Return on capital invested in an post-dryer usually varies from 12 to 24 months.
In addition to the applications above, units are often used as Stand-Alone Dryers. These may be the most economical solution where minimal equipment floor space or footprint is available for a new application, or when expansion of existing production facilities would require building modifications to accommodate a conventional drying system.
In the case of Liquid Heating, the production cost of providing sensible heat transfer from microwave energy is approximately one third higher than using steam in a conventional heat exchanger. However, this is offset by several factors, including:
The reduced capital investment in steam boilers, steam trains, condensate collection and water treatment plant.
The ability to use high power densities enables microwave heaters to substantially increase production rates.
Uniform energy distribution minimizes fouling depositions in even the most viscous products. This is particularly important with thermally sensitive materials such as chemical polymers, food ingredients, nutraceuticals, biotech products & pharmaceuticals.
For multiphase food products, hold times are greatly reduced using high temperature heaters, as the equilibration of hot and cold spots is virtually instantaneous. This is because the difference in their temperatures is minimal, unlike conventional heaters. Smaller hold tubes also reduce capital investment and operating costs for system pumps.
With volumetric heating of multiphase products, solids loadings of 70% or higher can be processed since the carrier fluid is not used as the primary heat delivery medium.
The shorter residence times achievable with microwave heating improve product quality. Compared to conventional heating, IMS heated food products tend to retain a higher percentage of flavors and nutrients.
Before designing any microwave heating or drying system for a customer, IMS prepares an economic study based on the required process specifications. Contact us for further details.

6. What about maintenance?
In addition to downtime for cleaning and inspection, conventional dryers and heat exchangers need periodic servicing with an expensive inventory of parts and a highly trained labour force. Apart from periodic examination for wear on the belt of a planar system or the tube in a cylindrical heater, the only part that requires maintenance on an IMS system is the magnetron. In the event of a malfunction or misuse through incorrect operation, this can be replaced and often repaired.
Although the operating life of a 915 MHz commercial magnetron can be greater, IMS recommends that the magnetron be replaced after 8,000 hours of operation. This translates to a maintenance cost of about US$1 per operating hour.
Low power 2,450 MHz magnetrons cannot be repaired, but larger units usually can be. A typical operating life for magnetrons at this frequency is 6,000 hours, although some vendors limit their warranty to 6 months or 500 hours.

  

 
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