Engineers' Handbook of Industrial Microwave Heating
A complete guide, this book presents industrial microwave heating from an engineering base and integrating the essential elements of microwave theory and heat transfer with practical design, application and operational issues.
Inspec keywords: microwave heating; heating; radar
Other keywords: microwave technology; engineers handbook; heating materials; radar; microwave energy; industrial microwave heating; industrial processes
Subjects: Process heating; Industrial and medical applications of microwaves; Radar equipment, systems and applications
For radio-interference limitation, the field intensity allowed out-of-band is of the order of 100 μV/m, corresponding to a power-flux density of 2.6 x 10-11 W/m, a level close to the noise limit of most antenna-receiver systems. These quantified field intensities and power-flux densities are given to provide a perspective on the magnitude of signals encountered in electroheat systems, discussed in detail in later chapters.
Heating of a dielectric material by a microwave field is a process which, for purity, should be discussed in detail at molecular level, and the engineer interested in the mechanism in depth is referred to the many texts (e.g. Von Hippel, 1954; Frohlich, 1958; Metaxas and Meredith, 1983) on the subject. Usually, however, it is sufficient to know that there are two primary heating mechanisms, one in which the dielectric material behaves like a poor electrical conductor, having a finite resistivity measurable at DC, and which is usually substantially constant as the frequency extends upwards into the microwave region.
Early in the development of industrial-microwave-heating technology, it became clear that conventional heat transfer within the workload, and to its surroundings, could have a very important effect on the process, and equipment and process operation must be designed accordingly. In addition to the elementary heat balance-that the energy, of all kinds, injected into the workload must equate with its change in thermal energy plus the energy transferred to its surroundings-no electroheat system can be evaluated and quantified properly without due attention to the heat and mass transfer in the workload during and following exposure to volumetric heating. Heat flow occurs within the workload through thermal conduction resulting from temperature gradients imparted during volumetric heating; and also from the surface to the surroundings via convection, radiation and conduction due to temperature difference between the surface and immediate environment.
In this chapter we first consider basic concepts of microwave power transmission, introducing the essential parameters commonly used, familiarity with which is vital to a proper understanding of the technology. Transmission in coaxial, parallel-plate and open-wire lines in the elementary transverse electromagnetic (TEM) mode is then discussed, before waveguides are introduced. There follows a discussion on waveguide practice, with descriptions and characteristics of the components commonly used in an industrial microwave-heating installation.
Multimode ovens are overwhelmingly the most commonly used form of microwave heating applicator. Mechanically they are very simple, essentially comprising a closed metal box with accessories, and so they are easy to make. The popularity of the multimode oven arises not only from its mechanical simplicity, but from its extraordinary diversity in its ability to process a very surprisingly wide range of workloads, in both size and electrical properties.
This chapter considers the class of microwave-heating applicators which essentially have a broadband, nonresonant frequency response. They are usually terminated in an absorbing load which dissipates the residual power not usefully dissipated in the workload, and in a well designed system the proportion of the input power in the terminating absorber is less than 10%. The principal feature is that the impedance match 'seen' from the generator is always favourable under normal operation (i.e. nonfault conditions), whether or not the workload is present. If the input match has VSWR < 1.3 under all nonfault conditions, these applicators can be fed direcdy from the magnetron generator (i.e. without a circulator) without instability, especially if a switch-mode power supply is used with a high-speed reverse-power detector for rapid response to faults.
Except for plasma generation, resonant systems have not been widely used in production industry because of the difficulties of control. As the workload properties and other parameters of the system change with temperature and wear, both the cavity resonant frequency and the generator frequency drift, and power transfer falls. It is necessary to retune to restore the correct operation point, and in most cases this is required so frequently as to demand an automatic system. In the past, automatic frequency control has been difficult to accomplish reliably in a normal production environment, but this situation has now changed with the advent of microprocessor control systems. It is worth observing that plasma generation is a recent and important development of microwave heating in a very high-technology application where production is in the hands of highly qualified operators who readily understand frequency-control systems.This chapter is a review of the basic principles of resonant-system operation with particular emphasis on practical features. Design procedures are adequately described elsewhere (Metaxas and Meredith, 1993).
Continuous-flow, conveyorised applicators necessarily have open apertures for the passage of workload through the oven. Choke tunnels are provided at the entrance and exit to reduce the high field strength in the oven to an acceptable value at the open ends. The objectives of the choke tunnel are threefold.
Most industrial microwave heating systems demand power in excess of 10 kW, often extending into the range 100 kW to 1 MW. The magnetron is overwhelmingly superior in satisfying the above needs compared with the klystron or with solid-state power sources. Although progress is being made in solid-state power supplies, they are far short of the power-output requirement and are very expensive, and will not be considered further.
This chapter reviews the power supplies required to drive microwave generators, and it is devoted especially to supplies for magnetrons as they are overwhelmingly the most widely used microwave power sources in industrial heating.
In summary, the network analyser comprises a precision microwave generator which can be swept in output frequency over a preset range and a sensitive measurement receiver capable of accurate analysis of phase and amplitude of a reflected and/or transmitted wave relative to the forward wave from the generator.
Large microwave-heating installations can rarely be tested under full-scale production conditions in-house at the equipment manufacturer's premises. This may be because of the logistics or cost of arranging for provision of the appropriate workload, or available space, or limitation of available electrical power. Testing must therefore be a two-stage procedure in which the manufacturer tests all the items prior to shipment, followed by testing to the operational specification on site. Testing presupposes that a formal specification exists against which the tests can be made. The existence of the equipment-performance specification is a vital contractual necessity for both purchaser and vendor, and its preparation must be done with the most extreme care to ensure that all relevant expectations of the customer are properly expressed, and are within the realisable performance of the manufacturer's equipment.
Industrial microwave equipment comprises many systems which must be assessed for safety. Apart from the high profile of microwave exposure, there are high voltages (medium and EHT), hot surfaces and steam, and mechanical power drives with both slow- and fast-moving machinery. All these represent a hazard, but no more so than for other more conventional machinery; and the microwave component, correctly handled, is probably the least risk. It often raises anxiety and fear in people who are unfamiliar with it; they cannot see it, and because it is sometimes classified as radiation there is a confused feeling of risk of a radioactive nature.
The range of applications of microwave heating in industry is so wide that it is not possible to make a universal assessment of the overall financial implications. This appendix is intended to serve as an 'aide memoire' listing the parameters which have been found important in affecting economic viability.
A full and carefully considered specification is an essential prerequisite to a contract for the supply of industrial plant, especially so for microwave equipment where the technology is probably unfamiliar to the intending purchaser. It must be carefully set out and presented without specialised technical jargon so that both parties have a clear understanding of all the aspects of the equipment supply; there must be no surprises at any stage.
The following is a list of words and phrases commonly used in the industrial RF and microwave vocabulary, for the assistance of those new to the technology.
The above parameter values are used by food technologists in quantifying the properties of heat treatment processes for foods.
The simple program below gives the total number of modes in an empty rectangular multimode cavity, and their eigenvalues (mode numbers), having their resonant frequencies within a bandwidth of ± r %. The cavity size is normalised to the free-space wavelength. Both TE and TM modes are included.
Many definitions quantifying the moisture content of materials are in use, and it is very important to specify which is used in specifying it. Some definitions are peculiar to a particular industry or process, while others are more appropriate to the quantity of water present. For example 'percentage solids' is used for slurries and liquids because it shows directly the quantity most of interest, but moisture content 'dry-weight basis' is used for nearly dry materials. All the definitions are interrelated, but their numerical values for a specific case are very different. It is therefore imperative to define moisture content precisely for performance specification.
