While water strongly absorbs microwaves for an instantaneous temperature rise, the cost of microwave heaters is generally regarded as prohibitive. A new approach to microwave heat transfer may change that perception. Based on a modular concept, the design centers on a simple element: a heat-resistant polysulfone injection-molded wave guide with inner pipe. The wave guide conducts microwave radiation emitted by the antenna of a magnetron. Only the water heats up, as the UDEL(reg) polysulfone (Amoco Chemical) offers low microwave absorption.
Test results show that 1 kW of power permits a 15C temperature increase for a water flow of 1 liter/min. Applications? Hot Drinks Vending Machine, Restaurant coffee makers, Heating of ultra-pure water for semiconductor device manufacture; medical products; chemical processes, Space heating and Hot water equipment
This artcle is dedicated to the microwave technology applied to the heating of liquids with dielectric loss such as water and beverages.
Predevelopment has been performed and the presentation files are available. It has become general and public knowledge through Internet since its introduction on May 1, 1996.
If this device is developed and marketed by someone else and if your company would only be interested in buying this product for OEM or industrial applications please send me an email at firstname.lastname@example.org and i will contact you when it will be commercially available in the United States or in Europe.
Some companies have been investigating microwave heating of ultra-pure water for semiconductor device manufacture. Most techniques have been based on heating the fluid through the walls of PFA Teflon pipes. « The biggest problem preventing commercialization has been the cost of microwave generators of the sizes required for their applications- typically > 50kW. The microwave generators alone cost approximately four times what complete heaters, based on infrared or direct-contact means, are marketed for in the U.S.A. » commented an American semiconductor equipment supplier. This problem remains in the other market segments.
The purpose of this work is to predevelop an entire new microwave heater design to overcome the above mentioned high manufacturing cost.
Regarding the applications the proposed long-term strategy is to offer a modular microwave heat transfer device as an OEM parts for the industrial equipment market ( Hot Drinks Vending Machine, ..) and then to proceed with technology transfer through joint ventures with partners in other market segments.
Classical hot water equipment is based upon an electrical or gas source. The technology used is the heating resistance submerged in water, or the heat transfer or the heating flame on a radiator with a certain water flow. These technologies are classical and the energy transfer operates from the heating element (resistance, wall transfer or radiator) to the volume of water. For specific applications one can use infra-red radiation.
It is well known that water strongly absorbs microwaves , ,  resulting in a temperature increase. By using a coil transparent to the microwaves, these will be able to heat water in an instantaneous manner.
The advantage of this system is to heat the water alone, in the volume and from a distance. The microwave activated hot water equipment will be valid for the following market segments:
Hot water equipment for domestic, office and camping.
Space heating by autonomous radiator.The hot water flow, as well as the power equipment requirement, allows one to differentiate quantitatively between these segments. As an exemple, a power of 2 KW permits a temperature increase of 30 ° C for a flow of water of 1 L / mn .
This device is particularly well suited for the volumetric heating of various liquids in an instantaneous manner. By adding a circulator it is possible to use this device as a hot water equipment with storage.
The principle can be generalized to the hot-liquid equipment market applied to liquids in restaurants, to chemical production and to medical and chemical processes.
One can propose a modular heating system which is able to receive the microwaves with a wave guide coupled to a magnetron, or to a coaxial cable for specific applications.This modular device is then applied to the liquid-heating of water, edible, chemical and medical products. The nature of the pipe material is adapted both to the liquid and the microwave radiation.
For reasons of economy and conception, it is more advantageous to manufacture a modular system of 1 KW rather than 2, 4 or 6 KW ; thus added in series or in parallel these devices will increase the temperature or the flow of the liquid. For specific industrial applications these devices will be able to be totally autonomous and thus to constitute operational heating circuits.
Microwave heat transfer.
Radiator and water-heater manufacturers, restaurant suppliers; chemical or medical products and processes.
The temperature increase is obtained by direct absorption of microwave radiation by the liquid to be heated.
The heating is obtained by convection of the liquid in contact with the heating element of a resistor or heat transfer; or by absorption of infra-red radiation.
Heating throughout the liquid volume.
Various liquids can be treated.
Space heating and hot water equipment: domestic, office, camping.
Consumption of liquids for restaurants.
Manufacturing of chemical or medical products and processes.
New installations and renewal of water- heaters and radiators, microwave heat transfer packaged applied to the other markets.
When a heating process most occurs dynamically for a various liquids, the system proposed is ideally adapted.
Transfer free of licensing fees; possible technical assistance available with an exclusive consultancy agreement.
The microwave radiation produced by the magnetron is guided to the material being treated. The guide must therefore be selected according to the loss characteristics, shape and size of the material under treatment.Thus a material which is highly reactive to microwaves requires a travelling wave guide, whereas a less reactive (low loss) material requires the use of a standing wave guide. The choice of guide is therefore correlated with the material being treated if effective microwave treatment is required . Heating water for which a strong microwave absorption occurs, requires therefore a travelling wave guide such as an elementary circular wave guide or preferably rectangular with an inner tube . This is because the penetration depth of the microwaves ie. the inverse of the absorption coefficient of water has the same order of magnitude as the wavelength of the microwaves. The proposed heat transfer is illustrated by the two figures below.
Having chosen the microwave heat transfer principle, the design of both the wave guide and the inner pipe depends upon the simplest manufacturing process. If the wave guide and the inner tube constitute a unique piece manufactured in a high performance technopolymer, a polysulfone for instance, then the design will be rectangular because of a good flow and easy heat transfer conception. Also we will use the TE10 mode,  for the electromagnetic propagation because it permits an efficient coupling for the microwave energy transfer with inherently non-radiating for the inlet and outlet water . The efficiency of the microwave water heater is entirely determined by the efficiency of the magnetron which is typicaly in the range 0.65 – 0.70.
In the case of a rectangular wave guide we can use for instance the 2.45 GHz frequency with standard dimensions WR 340 a = 86.36 mm and b = 43.18 mm, the wave guide length L being calculated. Let us consider a liquid slab of thickness e and height b placed at the center of the wave guide because the electric field is maximum at this place with the TE10 mode. If is the microwave amplitude attenuation in dB/m (decibel per meter), the power attenuation after a distance of L will be 2L then with the TE10 mode, ,  :
2L = 7.35 (2L) e dB where is the loss factor supposed temperature independent, e is in mm, L is in m. In the case of water = 8.9 at 25°C, = 6 at 40° C and = 4.2 at 55°C
For L = 0.25 m and e = 1 mm we have with 1KW and 1 Litre/mn
For an inlet water at 25°C 2 L > 22 dB the outlet temperature is 40°C
For an inlet water at 40°C 2 L > 15 dB the outlet temperature is 55°C
The corresponding volume is approximately 0.01 litre. For a best absorption at elevated temperature one can increase the pipe width e, for instance to 5 or 10 mm , the corresponding volume will be 0.05 or 0.1 litre respectively. In the termination the pipe width is increased from e to a in order to absorb the residual microwave power.
This modelisation concerns the heating of water at 2.45 GHz for which the loss factor varies as in the temperature range 25 < < 75° C, thus the heat transfer absorption coefficient varies as
where is a physical constant parameter , . Using the energy conservation equation in conjunction with the exponential decay of microwave power, a simple treatment (private notes) gives the equation :
where is the inlet temperature, is the total temperature variation from the inlet to the outlet, is the ratio / where is the temperature variation at a point distance z away from the inlet. Ignoring the absorption coefficient variation along the heat transfer, an approximate calculation can be obtained and gives the well-known equation :
where is the approximate position at ratio. Substracting eqn. (2) from eqn. (1) and dividing by eqn.(2) yields :
where , is the relative error of the approximate calculation.
For smaller values of ,
and varies linearly with .
For values of near to 1,
which is the maximum relative error.
For useful numerical calculations eqn.(2) and eqn.(3) must be ploted. Introducing the necessary volume of water which leads to the required ratio , eqn.(1) can be written as :
where by definition is the volume of a rectangular liquid slab at a point distance away from the inlet. The last equation will be valid for a liquid slab for which the height varies along the heat transfer.
By using Deby formula, the loss factor at 2.45 GHz are extrapolated from experimental values at 3 GHz, , . Fig. 1 shows a loss factor variation of about one order of magnitude in the temperature range 0 < < 100° C.
This table shows the temperature variation of loss factor at 2.45 GHz with its fitted values. A good fit to these data in the temperature range 25 < < 75° C is given by the equation :
However the fit does not hold in the temperature range 75 < T < 100° C, nor does it hold if the temperature of water is below ambiant temperature.
In the temperature range 75 < < 100° C the fit can be written as:
The form of the local energy conservation equation which governs the microwave water heating process during stady state conditions can be written as :
where and are the microwave power and the temperature of water at a distance away from the inlet, , , and are the volumetric mass, flow rate and specific heat of water respectively. However the microwave power at a distance away from the inlet is given by, ,  :
where and are the input microwave power and the heat transfert absorption coefficient respectively. For a WR 340 water heat transfer for which is the thickness of the liquid slab,
in the temperature range 25 < < 75° C where is the following physical constant :
= 7.35 230 dB.° C/m = 1.690 dB.° C/mm = 0.194 °C/mm where is in mm.
Integrating eqn.(1) gives :
where is totally absorbed along the heat transfer, and are the input and output temperatures of water respectively.
Differentiating eqn.(2) gives :
Substracting eqn.(4) from eqn.(1) yields :
eqn.(3) gives :
Substituing , eq.(5) yields :
Integrating eqn.(7) along the heat transfer gives :
is related to the process parameters by the formula :
Ignoring the absorption coefficient variation with gives the simplified equation :
where is the approximate position at ratio . Now let us consider a volume of water of thickness and height described as follows :
This design is well adapted to minimize microwave reflections on the inner pipe. The corresponding heat transfer absorption coefficient can be written as:
where is the height of the wave guide. By following the same procedure as for the rectangular slab, eqn.(7) becomes :
Integrating eqn.(11) along the heat transfer gives :
where is the necessary volume of water which leads to the required ratio . Therefore :
By definition is the volume of a rectangular liquid slab of eight at a point distance away from the inlet :
As the thickness is substantially smaller than the width of the heat transfer, varies linearly with , therefore as well as at a given ratio do not depend upon the shape of the inner pipe.
It will be noted that the volume is the appropriate parameter for describing the microwave heating process.
For a WR 340 heat transfer of height with a liquid slab of thickness , or is the physical constant which governs the microwave water heating process ,  :
= 7.35 230 dB.° C/m = 1.690 dB.° C/mm = 0.194 °C/mm where is in mm.
Using experimental data and , the value of , and can be deduced through the data given in Figs.1 and 2 in conjunction with the above formulae. is a fraction of the total temperature variation .
Fig. 1 Approximate conjugate ratio along the length of the heat tranfer.
For = 0.990 = 4.6 = 4.6v = 4.6
For = 0.999 = 6.9 = 6.9v = 6.9
Fig. 2 ratio dependence on the relative error.
For = 0.990 = = 0.78
For = 0.999 = = 0.85
is related to the process parameters with the formula :
°C = 3.79 (KW required)/GPM
For engineering purposes, some useful values of caracteristic lengths and volumes may be obtained from the model. Neglecting the thickness of the moulded wave guide, the following table gives dimensional requirements for designing with a good accuracy a 1kw WR340 heat transfer with a liquid slab of thickness e = 5 mm.
Lengths and volumes are in cm and cubic cm respectively. Having chosen = 0.99 with = 15 °C for a minimum flow rate of water of 1 litre/mn and for a microwave power of 1 kw, the minimum required length for a rectangular liquid slab of thickness = 5mm and height is approximately 30 cm. In the case of an inner pipe of height one will refer to the necessary volume of water = 60 cubic cm which leads to the required ratio = 0.99. The volume of water of 60 cubic cm is also valid for other values of as this one is substantially smaller than the width of the heat transfer. These results show that the heat transfer become inconveniently long for higher microwave power.
Thus for reasons of economy, conception and maintenance, it is more advantageous to manufacture a modular system of 1 kw rather than 2, 4 or 6 kw; thus added in series or in parallel these devices will increase the temperature or the flow of the liquid. If maintenance or repairs are required, just replace the 1KW defective modular device.
The wave guide section is chosen to ensure the microwave propagation at 2.45 GHz with the TE10 mode, as a sheilding from microwaves is necessary, , [ 8 ].
Polysulfone has the advantage of great resistance to breaking and chipping .
Polysulfone is characterised by a particular low microwave absorption and by a high resistance to ageing with a high continuous use temperature (140-160°C), .
Polysulfone is characterised by a high stability to hydrolysis at elevated temperature . For the other liquids, one will refer to the numerous experimental data . For specific applications with contamination control such as semiconductor processes, PFA or PTFE teflon® will be able to protect the liquid from polysulfone ; one can use a double-injection of teflon in the inner of the pipe. The typical properties of UDEL® polysulfone , an advanced sulfone polymer , meet the requirements. UDEL is registred trademark of Amoco Performance Products, Inc. and Amoco Chemical (Europe) S.A. Teflon is registred trademark of DuPont
Microwave heat transfer design
- Manufacture of the heat transfer in a high performance technopolymer, a polysulfone for instance, by using the injection-moulding process. In the case of more than one part, assembly of the parts by ultrasonic welding.
- Machining of the stainless steel plate which supports both the magnetron and its power supply.
- Assembly of the stainless steel plate with the wave guide by solvent fusion or by direct heat sealing.
- The shielding from microwaves is obtained by the formation of a 100-150µm Zinc projection layer on the outer surface of the wave guide using the arc-spraying method. An alternative method consists of using an electroless process such as electroless copper/nickel.
- Using current microwave technology, the microwave supply is a packaged magnetron and power supply with cooling by air, forced air or water.
1. Thorpac’s pleasing range of microwave cookware is moulded from Polysulfone which is the premier, most-proven material for such applications, as shown decisively in American markets.
2. Integral moulding of support braket, openings, mounts on Polysulfone hot-water tank cuts costs of making vending machines of hot beverages.
3. General Foods new patented institutional coffee service system, uses Polysulfone as the high strength transparent component of this new bowl. Similar coefficients of thermal expansion permits a sealed-on stainless steel bottom which provides heat resistance for direct contact with electric hot plates. The bowl is moulded and assembled by Williams Industries, Shelbyville, Indiania.
4. Polysulfone replaces brass for heavy-duty cartridge components in Grohe’s water mixing taps; by vertue of its inherent resistance to thermal shock, low coefficient of thermal expansion, dimensional stability.
5. Polysulfone is used in the body of a reusable syringe injector. Transparency and sterilizable by all methods, polysulfone replaced polycarbonate, which failed in steam exposures.The injector is about one-fourth the weight and cost of its stainless steel predecessor.
6. The translucent case of this industrial battery from Sab Nife is moulded from a flame-retardant grade of Polysulfone, to close tolerances in a straight- forward manner.
7. Many parts of Metratron milking systems of Westfalia Separator’s AG, are manufactured from Polysulfone. Transparency, impact strength and moldability, make Polysulfone the choice for a wide variety of dairy industry milk-handling applications.
8. The triple port connector of the Cardiomet 4000, a medical instrument for continuous monitoring of the blood gaz parameters, is injection-moulded from Polysulfone.The leading requirements included those of clarity, strength, stiffness, and sterilisability.
9. JG Speedfit fittings, moulded in Polysulfone make quick interconnections for plastic piping.
10. Polysulfone’s excellent surface properties are put to good use in metallised reflectors used in Zeiss-Ikon’s Perkeo Compact slide projector.
All these comments and associated images are extracted from « Designing » brochures of Amoco Performance Products with their kind permission. The above mentioned products are manufactured from UDEL® polysulfone. UDEL is registred trademark of Amoco Performance Products, Inc. and Amoco Chemical (Europe) S.A.
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- Polysulfone Design Engineering Data , Amoco Performance Product, Inc.
- K. L. Carr. Microwave Journal , July 1994.
- Collie, C. H., Ritson, D. M. and Hasted, J. B., Trans. Faraday Soc.,42A, 129, 1946.12. Buckley, F., Maryott. A, Tables of Dielectric Dispersion Data for Pure Liquids and Dilute Solutions. National Bureau of Standards, Circulation 589, 1958.
MICROWAVE HEAT TRANSFER
Exploded View – Full size
DI water heater, acid heater, Rapid control for sub-ambiant temperature.
Water heater, radiator.
Coin vending equipment, Restaurant coffee makers.
Heating of Physiologic Fluids : blood and intravenous (IV) fluids, .
In microelectronic industry the chemical generation process is based on a classical mixing of DI water and gaz which leads to a good contacting between the two phases. The purpose of mixing is to produce a high interfacial area by dispersing the gaz phase in the form of bubbles into DI water. By using the microwave heat transfer device in conjunction with a coaxial double-inlet for both DI water and gaz, there is an interest in exploring the chemical generation application.This assumption is based on a molecular mixing due to the microwave induced motion of reaction products such as ions and polar molecules.
The measurement of the outlet mass flow rate and the related total temperature variation of the heat transfer as a fonction of microwave power at a given gaz flow provide information necessary for determining the dissolution process rate and efficiency and whether or not this chemical generation device is adapted to the microelectronic market. The contribution to the total temperature variation from the chemical generation process is the heat of dissolution. The absorption of this energy can be obtained by a low inlet temperature for DI water.
This process can be generalized to the small-sized chemical reactor market applied to the in-line microwave chemical production from two liquid phases. The presence of microwaves can reduce reaction time, increase reaction yield and also supplies the heat of reaction if necessary.