Frequently Asked Questions

1. How does microwave compare to conventional heating?
2. What are the advantages?
3. What are the disadvantages?
4. What about safety?
5. What about economics?
6. 2450 MHz versus 915 MHz?
7. Radio Frequency versus Microwaves?
8. Infrared versus Microwaves?
9. What about maintenance?


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 fluids containing particulates 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. IMS planar dryers in the textile industry 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 foods 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 unique choking mechanisms, 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 electromagnetic radiation to the health and safety of equipment operators.  

IMS heaters and dryers are designed to operate at a level of electromagnetic emission that is twenty times more stringent than the IEEE/ANSI standard. As a further precaution, all IMS 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 and local utility costs.  In some cases, microwaves can be 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, IMS microwave systems allow overall production capacities to be increased by 25 to 33%.  This is because the pre-dryer:

Preheats moisture to the evaporative temperature.
Can help to equalize the moisture level of product before the conventional dryer.

With current energy costs, the return on capital invested in an IMS pre-dryer has been as low as 12 to 24 months.

When used as a Post-Dryer in combination with conventional gas or oil-heated dryers, IMS microwave systems are 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.  An IMS 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 IMS post-dryer has been as low as from 12 to 24 months.

In addition to the applications above, IMS planar units are 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 Heating a pumpable fluid using an IMS Cylindrical Heating System, the production cost of providing sensible heat transfer from microwave energy is totally dependent on the local cost of utilities than can vary by a factor of five throughout North America.  As a rough guide, this cost is expected to be 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 IMS 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 commercial microwave heating or drying system, IMS recommends working with its customer to prepare a customized Economic Value Analysis based on the actual process specifications required.  Contact Us for further details.


6. 2450 MHz versus 915 MHz?

915 MHz generators can provide up to 100 KW from a single magnetron. Although the cost is similar, the largest commercial 2450 MHz units available use 30 KW magnetrons.
915 MHz generators lose about 15% efficiency in producing electromagnetic energy from electric power. However, the conversion of that energy into useful heating or drying is often greater than 97% using IMS technology so that the total system efficiency usually exceeds 82%. This compares with 55 to 70% total system efficiency obtainable from 2450 MHz generators.
The depth of penetration of microwave energy at 915 MHz is about three times as great as that at 2450 MHz.
With their higher total system efficiencies, 915 MHz heaters and dryers tend to have lower running costs than comparable 2450 MHz units.
One 100 KW 915 MHz generator will be about 50% cheaper than seven 15 KW 2450 MHz units.
The low power 2450 MHz magnetrons developed from the proliferation of domestic microwave ovens are inexpensive and readily available. This makes them ideal for low flow capacity R & D applications.
The size of magnetrons and wave-guides for a 2450 MHz system is considerably smaller than those used in 915 MHz units. This makes them suitable for small-scale installations.

 


7. Radio Frequency versus Microwaves?

Radio frequency (RF) and microwaves are forms of electromagnetic energy but differ in operating frequency and wavelength.  Both are allocated specific bands of operation by international governments.  In the USA, these are monitored and controlled by the Federal Communication Commission or FCC.
Industrial radio frequencies operate between approximately 7 and 41 MHz with wavelengths of about 141 to 24 feet (43 to 7.3 meters). In comparison, units designed and built by Industrial Microwave Systems use frequencies of 915 and 2450 MHz with corresponding wavelengths of 13 and 5 inches (33 and 12 cm). Generally speaking, the efficiency of power utilization is far lower in an RF generator than a microwave unit, although the initial capital cost per kW of power output is higher.
Selection of RF or microwave heating will depend on product physical properties and required process conditions for a particular application. Where penetration depth in excess of 2 inches (5 cm) is required and control of uniformity of heating is not a major issue, radio frequency offers a good solution. However, where uniformity of drying and moisture control is essential, an IMS microwave dryer is the obvious answer.

8. Infrared versus Microwaves?

Infrared (IR) lies between visible light and microwaves in the electromagnetic spectrum.  As an alternative to hot air, the high operating temperatures and heat intensity generated by infrared heating tubes (up to 1500 deg F or 815 deg C) may be used to remove moisture rapidly from the surface of a product while keeping it brown or crispy.  This is particularly useful in such applications as drying or baking in the food industry.

Heat production is achieved by molecular friction of the water molecules in microwave heating whereas infrared energy is absorbed and converted into heat.  The depth of penetration into a product is usually much higher with microwaves and the extent of heating is dependent on moisture content in microwave heating while surface characteristics dictate the absorption of infrared radiation.

IR heating can be produced using either gas fired or electric powered infrared tubes.  With either system, a disadvantage of IR is that the extremely high temperature of the IR tubes is unacceptable in many process applications.  Not only are they hot while running, but also require a cool down period when turned off.


9. 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 labor 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 easily be replaced in less than thirty minutes.  In many cases, a magnetron can be repaired or refurbished for about two-thirds the cost of a new unit. 

Although most vendors limit their warranty to 2,500 hours, the operating life of a 915 MHz 100 kW commercial magnetron can be greater. IMS recommends that the magnetron be replaced annually or after 8,000 hours of operation, whichever is sooner.  This translates to a maintenance cost of about US$1.50 per operating hour at maximum power output.

Low power 2450 MHz magnetrons cannot be repaired, but larger units usually can be.  A typical operating life for magnetrons at this frequency is 6,000 hours.