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. Radio Frequency versus Microwaves?
7. 2,450 MHz versus 915 MHz?
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 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 IMS planar 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.
IMS 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 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. 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, IMS 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, IMS 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. 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
usually varies from 12 to 24 months. |
In addition to the applications above, IMS planar 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 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 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. 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 typically operate between 10 and
30 MHz with wavelengths of 100 to 35 feet (30 to 10 meters). In
comparison, units designed and built by Industrial Microwave Systems
use frequencies between 460 and 2,450 MHz with corresponding wavelengths
between 24 and 4 inches (60 to 10 cm). Generally speaking, the
efficiency of power utilization is far lower in a 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 6 inches (15 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.
For planar applications requiring belt widths in excess of 40” (100
cm), where edge-to-edge uniformity is essential, control of microwave
energy is superior to RF. Low moisture levels and high production
belt speeds, such as those encountered in the textile industry,
are far better suited to IMS microwave heating due to their characteristics
of control and response time respectively.
7. 2,450 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 2,450 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 95% using IMS technology so that the
total system efficiency usually exceeds 80%. This compares
with 55 to 70% total system efficiency obtainable from 2,450
MHz generators. |
| The depth of penetration of microwave energy
at 915 MHz is about three times as great as that at 2,450 MHz. |
| With their higher total system efficiencies,
915 MHz heaters and dryers tend to have lower running costs
than comparable 2,450 MHz units. |
| One 100 KW 915 MHz generator will be about
50% cheaper than seven 15 KW 2,450 MHz units. |
| The low power 2,450 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 2,450 MHz system is considerably smaller than those used
in 915 MHz units. This makes them suitable for small-scale
installations. |
| 2,450 MHz is efficient where fast product
expansion is required, such as dry frying of starch-based foods. |
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 generated by infrared heat (up to 1500 deg
F or 815 deg C) may be used to remove moisture 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.
When IR is used in low power mode in combination with microwave
heating, it is possible to reduce the microwave power input to
lower surface moisture and improve product quality. Conversely,
high penetration of IR can actually increase surface moisture levels.
IR heating can be produced using either gas fired or electric
powered generators. With either system, a disadvantage of IR is
the high cost of energy compared to other types of heating, including
microwaves. A second issue is that the glass IR tubes are not always
acceptable in many process applications.
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 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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