Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 204

(a) (b)

Figure 6.9 Waveguide bends: H bend (a) and E bend (b).

In most industrial heating applications, “lat-face” flanges are the most
cost-effective choice, whereas “choke”-type flanges are more popular for
military and communication applications. Most commercial waveguide
components are made of aluminum because of its low cost and good per-
formance. If heat loss is of major concern, copper waveguides may be more
appropriate, although their costs usually outweigh the slightly lower heat
loss advantage. Where sanitation is of high importance, such as in the food-
processing industry, stainless steel waveguides may be used.
It is common in microwave transmission lines that the direction of trans-
mission changes once or more times from the original direction of the
launch waveguide. Waveguide bends are used to change the direction of the
waveguide transmission system. H bend and E bend are two types of com-
mon waveguide bends used to change the direction of the transmission line
in the horizontal and vertical directions, respectively (Figure 6.9).
An illustrative example showing how the various components of the
microwave power transfer unit and microwave generation unit are config-
ured for a typical industrial microwave heating system used in the sepa-
ration of adhering mortar from RCA is shown in Figures 6.10 and 6.11.
Figure 6.10 shows the magnetron being used to generate microwaves using
the high voltage supplied at its filaments by the filament transformer. As
can be seen, the system shown in Figure 6.10 uses a circulator and water
load to dissipate the reflected power and thereby protect the magnetron.
Figure 6.11 shows a typical transmission line, including a dual directional
coupler to measure both the forward and the reverse power, a straight sec-
tion rectangular waveguide, a waveguide bend to change the transmission
direction, and an autotuner to maximise power absorption by the load
placed in the multimode applicator (RCA processing chamber).