At Hydro Quebec, James Bay, sits a large body of water. In raw heat, it is equivalent to some 2,500 tons of coal per hour, ready to be converted to electricity.


There is a great head of water -- and plenty of it -- at James Bay on the north shore of Quebec. In raw heat, it is equivalent to some 2,500 tons of coal per hour, ready to be converted to electricity. You are in Boston, Mass. How fast can you have some? Right away.

The turbo-generators, transformers and rectifiers there put out 15,000 MW at bipolar +/-450 KV DC for transmission to Montreal; DC because it avoids those massive wasteful capacitance currents in the lines that AC would cause. Near Montreal it is converted back to AC and distributed throughout Canada and the northeastern United States at the voltage of your choice.

This is a massive first stage of refinement. Now what kind of heat or energy do you want?

Here is just a small selection of creative ways of further refinement and adaptation.

With electric ovens, furnaces, water heaters, space heaters, plastics machinery and radiant heaters, all you need is some kind of resistance element. Some important differences in how the heat is applied are realized by your choice of heater material. It could be a nickel/chrome alloy, silicon carbide, molybdenum disilicide, tungsten-in-quartz and so on, depending on the process, the environment and the temperature you want to achieve. Then you’ll need some means of modulating the power; typically you would use a contactor or SCR. You’ll likely need a temperature sensor and controller -- yet another stage of refinement; precise temperature and its timing profile.

That deals with delivery by convection and radiation. Not good enough? You might want to deliver heat directly into your product using its own resistance as a heater; this is called ohmic heating. Examples are glass melters, salt baths and food processors for cooking or sterilizing. Here you could stick three electrodes through the tank, directly into the product, then apply and modulate three-phase AC power.

If you want to put heat deep into metal without waiting for surface heat to penetrate, the answer is electromagnetic induction heating.

If you are melting metal, you may be lucky and find that normal AC at 50 or 60 Hz works well enough. In other applications, the shape, size and metal resistivity will usually dictate some other frequency, some expensive conversion equipment and an applicator. This is a copper coil arrangement, sometimes with a ferromagnetic yoke, designed to put the magnetic field where it is most effective. Often, the applicator will be a coiled copper tube carrying cooling water.

Surface hardening of steel is a common application for induction heating, where the frequency, power density and coil geometry are chosen to limit heat penetration to a specific area and skin depth of the metal.



The Ionized State

This requires a power supply of several hundred volts to initiate ionization in the gas-filled space between some kind of electrode and the workpiece. Then, some form of current limiting is used as the high voltage required to initiate the discharge is no longer required.

At the low power level, ionization can take the form of a glow discharge at 0.1 to some 50 mA. Application examples are the CO2 laser, plasma chemistry and surface treatments, etching, assisted chemical and physical vapor depositions, ion implantation, oxidation and sputtering.

Abnormal Glow Discharge. Here you encounter currents of some 0.5 to 50 A. Applications include carburizing, nitriding, arc furnace or plasma torch.

Arc Discharge. Here currents range from 1 to some 10,000 A. Typical applications are, energy sources for the Nd:YAG and Ruby lasers, plasma arcs for cutting, welding or spraying, plasma furnaces and arc furnaces for metal melting.

Radio Frequency Drying

In a radio frequency drying system, the RF generator creates an alternating electric field between two electrodes. The material to be dried is conveyed between the electrodes, where the alternating energy causes polar molecules in the water to continually re-orient to face opposite poles -- much the same way magnets move in an alternating magnetic field. The energy losses of this movement cause the water in the material to rapidly heat throughout the material's entire mass.

Applications are in textiles, post-baking of biscuits, paper converting and board making. The radio frequencies reserved for industrial use by the Federal Communications Commission are 13.56 MHz , 27.12 MHz and 40.68 MHz. It is important that the frequency remains within tolerance or be attenuated so as not

to interfere with radio communications. Units of 50 to 900 kW have been used. Modern design techniques produce RF dryers that operate at 40 MHz. These higher frequency systems do the same drying work as 27 or 13 MHz dryers, but at 20 percent to 60 percent lower voltage, respectively. This permits the RF drying system to process materials at very low moisture levels without the arcing problems encountered with dryers at lower frequencies.

Microwave Systems. Frequency here is 2,450 MHz for up to 30 kW and 915 MHz for higher power and efficiency -- 100 kW from a single magnetron.

This is used where volumetric heating is required and heat transfer by conduction or convection is not possible. Industrial applications include drying, food tempering, preheating for rubber vulcanization, lumber pultrusion at 800 kW, gluing and many more.

You will be no stranger to non-uniform microwave heating, unless you stay out of the kitchen. This can be minimized by geometric design of the applicator and field pattern. PH

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