Electrical Energy Utilisation & Electric Transaction

Electrical Energy Utilisation and Electric Transaction

Electric Heating

Heating is required for domestic purposes such as cooking and space heating, and for industrial purposes such as melting, hardening, tempering, case-hardening, drying and welding. Practically all heating requirements can be met by suitable electric heating equipment. Electric heating offers advantages over solid-fuel or liquid-fuel heating (coal, oil, gas) in controllability, cleanliness and, in many cases, economy and safety.

Heating-element materials

The material chosen for a heating element must possess:

• High resistivity - to generate heat at usable current levels.
• High melting point - to operate at elevated temperatures without melting.
• Low temperature coefficient of resistance - for predictable performance with temperature change.
• Oxidation resistance - to withstand operating temperatures without rapid deterioration.
• Mechanical strength and ductility - to be formed into required shapes and to resist vibration.

Common heating-element alloys for low and medium temperatures are:

• Nickel-chromium alloy (Ni 80% - Cr 20%).
• Nickel-chromium-iron alloy (Ni 65% - Cr 15% - Fe 20%). The addition of iron reduces manufacturing cost and slightly reduces the temperature at which oxidation begins.

Methods of electric heating

Electric heating methods are classified by how electrical energy is converted and delivered to the material to be heated.

1. Resistance heating

Resistance heating relies on the I2R loss (Joule heating) when current passes through a resistive element. It is widely used for heat treatment of metals (annealing, hardening), drying, baking, stoving, and domestic and commercial cooking. Using wire resistance elements, temperatures up to about 1,000 °C can be obtained in ovens.

Two principal implementations:

• Direct resistance heating: current is passed through the workpiece itself. This method has high efficiency and is used in applications such as salt-bath furnaces and electrode boilers for heating water.
• Indirect resistance heating: current is passed through a separate heating element; the produced heat is transferred to the charge by conduction, convection, and/or radiation. Typical uses are immersion heaters, resistance ovens, domestic cookers and many heat-treatment furnaces.

2. Infrared or radiant heating

In infrared (radiant) heating, heat from incandescent or specially designed radiant elements is directed towards the workpiece as electromagnetic radiation. Advantages include:

• Rapid heating.
• Compact heating units.
• Flexibility in localised heating.
• Good safety and low contamination risk.

Applications: paint stoving, drying of radio cabinets and wooden furniture, preheating of plastics prior to moulding, softening thermoplastic sheets and drying of paper, textiles and pottery where moisture content is low.

3. Arc heating

Arc heating uses the intense temperatures of an electric arc (typically 3,000-3,500 °C depending on electrode and arc conditions). Two principal forms:

• Direct arc furnaces: arc struck between electrode(s) and the charge; heat is transferred by conduction and radiation directly to the material. Common in steel production.
• Indirect arc furnaces: arc struck between electrodes, and heat is transferred to the charge mainly by radiation. Used for melting non-ferrous metals.

4. Induction heating

Induction heating induces eddy currents in a conductive body by alternating magnetic fields from a coil. The eddy currents generate heat in the body; heating power depends on the induced voltage and the electrical resistance of the charge. Induction heating is most effective for metals because they present the required conductivity and often low resistance; higher frequency and higher flux density improve coupling and surface heating. Typical applications include surface hardening, forging, brazing and induction melting.

5. High-frequency eddy-current heating

In high-frequency eddy-current heating, the workpiece is placed inside a coil carrying high-frequency current. An alternating magnetic field induces strong eddy currents in the workpiece; the resulting I2R loss heats the surface and near-surface regions. For magnetic materials, hysteresis losses add to heating. Applications include surface hardening, annealing and soldering.

6. Dielectric (high-frequency) heating

Dielectric heating places the material between electrodes connected to a high-frequency AC source. Heat is produced within the material due to polarisation losses (dipole rotation and ionic conduction), so heating is volumetric and often more uniform. Applications: preheating plastic preforms, wood lamination (gluing), baking of foundry cores, diathermy (medical), sterilisation and various textile and food-processing operations.

Electric Welding

Welding is the process of joining two metal pieces by melting and coalescing their surfaces, with or without filler metal, usually aided by heat and sometimes pressure. Electric methods of welding use electrical energy as the principal heat source.

Classification of welding processes

The main welding processes used in general engineering are:

• Gas welding - oxyacetylene, oxyhydrogen and air-acetylene.
• Resistance welding - butt, spot, projection, seam and percussion welding.
• Arc welding - carbon arc, metal arc, gas metal arc, gas tungsten arc, atomic hydrogen arc, plasma arc, submerged arc, flux-cored arc and electroslag welding.
• Thermit welding (exothermic welding with aluminium-iron oxide reaction).
• Solid-state welding - friction, ultrasonic, diffusion and explosive welding.
• Newer/high-energy methods - electron-beam welding and laser welding.

Resistance welding

Resistance welding is a group of processes where a high electric current is sent through the metal pieces in contact so that local Joule heating at the contact areas melts and fuses the parts. The process is rapid, suited to mass production and allows simultaneous application of pressure to aid joint formation. Common forms:

1. Butt welding

Two types: upset butt welding (parts pressed together and heated by current) and flash butt welding (use of flashing to remove surface impurities followed by upsetting). Flash-butt welds approach the parent metal strength in static loading and are commonly used for chain links, rail ends and axles.

2. Spot welding

Spot welding joins sheets at discrete points by pressing them between copper electrodes and passing a high current for a short time. Spot welds are quick and suited to sheet metal fabrication (e.g. automotive panels). They are neither airtight nor watertight and are limited by electrode force and current capacity (typically plate thickness up to 10-12 mm for many machines).

3. Projection welding

Projection welding is a variation of spot welding where localised projections on one workpiece concentrate current and force, producing controlled welds at those projections. Advantages include higher throughput, improved electrode life and better finished appearance. Commonly used for stamped or punched parts.

4. Seam welding

Seam welding produces a continuous weld made by a series of overlapping spot welds using rotating wheel electrodes. It yields leak-proof joints and is widely used for welding fuel tanks, tubes, containers, refrigerators and other sheet-metal assemblies.

5. Percussion welding

Percussion welding is a fast process using a short high-energy current impulse (often from a capacitor discharge) to establish a rapid arc and fuse the contact zone. Heating is localised and very rapid, reducing heat-affected zones in surrounding material.

Arc welding

Arc welding uses the heat of an electric arc struck between an electrode and the workpiece (or between two electrodes) to melt and join metals. The arc is established by touching then withdrawing the electrode to create a controllable gap; the arc temperature and energy melt the workpiece and filler, forming a fused joint on solidification. Arc welding is typically a non-pressure process (no external compressive force is required).

Typical welding arc voltages are 20-40 V. Welding currents range from a few tens of amperes for sheet work up to several kiloamperes for heavy automatic welding.

1. Carbon arc welding

Uses a carbon electrode to maintain the arc. Two variants exist: with and without flux. The fluxed method prevents oxidation and is used for ferrous metals; the non-fluxed method is used for some non-ferrous metals. DC supplies are commonly used. Applications include sheet steel, copper alloys and aluminium work.

2. Metal arc welding

Here the consumable electrode is a metal rod of essentially the same composition as the workpiece; the electrode melts to provide filler metal. Both AC and DC can be used. Typical open-circuit voltages are 50-60 V for DC and 70-100 V for AC depending on process and electrode.

3. Atomic hydrogen, inert-gas and gas-shielded processes

Processes such as atomic hydrogen arc welding (hydrogen gas passed through arc between tungsten electrodes), gas tungsten arc welding (GTAW/TIG), and gas metal arc welding (GMAW/MIG) use shielding gases to protect the molten weld from atmospheric contamination and produce high-quality, ductile welds. They are widely used for stainless steels, aluminium and other non-ferrous alloys.

Electric welding equipment

Welding power sources may be DC or AC type:

• DC welding sets: may be generator-based (differential-compound wound DC generator with drooping V-I characteristic) driven by a prime mover, or rectifier-fed (dry-type rectifier such as selenium rectifiers used with multiphase, high-leakage transformers).
• AC welding sets: step-down transformers (single-phase or three-phase) providing low open-circuit voltages (80-100 V) and high currents. Current control techniques include magnetic shunts, series reactors (choke coils), tap changing in the primary, and series resistance (with reduced efficiency).

Illumination

Illumination is the result of light falling on surfaces. Although commonly used interchangeably, light denotes the radiant energy causing visual sensation, while illumination denotes the amount of that light incident on a surface.

Definitions and units

• Light: radiant energy from a source that produces visual sensation; often measured in lumen-hours for energy over time.
• Luminous flux: total light power emitted by a source per unit time, measured in lumens (lm).
• Luminous intensity: luminous flux per unit solid angle in a given direction; measured in candela (cd) or lm/steradian.
• Lumen: the unit of luminous flux. One candela uniformly radiating into one steradian produces one lumen.
• Candela (cd): SI unit of luminous intensity; defined by a specified value related to a black-body radiator at a reference temperature.
• Illuminance (illumination): luminous flux incident per unit area on a surface; measured in lux (lm/m2) or metre-candles.
• Mean spherical candle power (MSCP): average candle power over all directions and planes from the source.
• Lamp efficiency: lumens emitted per watt of input (lm/W).
• Specific consumption: input power per unit average candle power (W/cd).
• Brightness (luminance): luminous intensity per unit projected area of a surface; measured in nits (cd/m2). A larger unit is the stilb (cd/cm2).
• Glare: brightness in the field of vision causing annoyance, discomfort or reduced visibility.
• Space-height ratio: ratio of horizontal spacing between adjacent lamps to their mounting height above the working plane.
• Utilisation factor: ratio of total lumens reaching the working plane to total lumens emitted by the lamp.
• Maintenance factor: ratio of maintained illuminance under normal working conditions to initial illuminance when the system is new and clean; always less than unity.
• Depreciation factor: inverse of the maintenance factor; ratio of initial metre-candles to ultimate maintained metre-candles; always greater than unity.

Laws of illumination

• Inverse-square law: For a point source, illuminance on a surface is inversely proportional to the square of the distance between the source and the surface (E ∝ 1/d2), provided the source can be approximated as a point.
• Lambert's cosine law: Illuminance at a point on a surface is proportional to the cosine of the angle between the surface normal and the direction of the incident light (E ∝ cos θ).

Types of electric lamps

Common lamp types with typical characteristics and applications:

1. Arc lamps

Arc lamps sustain an electrical discharge between electrodes and give intense light. Examples include carbon arc and flame arc lamps; historically used for cinema projectors and searchlights. Typical luminous efficiencies are modest (carbon arc ≈ 12 lm/W). Colour quality and control issues led to displacement by discharge lamps for many applications.

2. Incandescent lamps

Incandescent or filament lamps operate by heating a filament (tungsten, carbon historically) in an evacuated or inert gas-filled bulb until it glows. Filament materials must have high melting point, low vapour pressure, high resistivity and adequate mechanical strength. Typical operating temperature for tungsten filaments is around 1,800-2,500 °C depending on design; average efficiency for tungsten lamps is about 10 lm/W. Light output falls gradually with lamp ageing; typical depreciation might be ~15% over useful life.

3. Gas-filled incandescent lamps

To permit higher filament temperatures (and improved efficiency) without rapid evaporation, bulbs are filled with inert gases such as argon with small amounts of nitrogen, or in special lamps krypton. Krypton gives better performance but is costly; argon is commonly used.

4. Gaseous discharge lamps

Discharge lamps excite gas or vapour to emit light, often with higher efficiencies and different spectral characteristics than incandescent lamps. Two classes:

• Discharge lamps that emit the discharge colour directly, e.g. sodium-vapour, mercury-vapour and neon lamps.
• Fluorescent lamps that use ultraviolet radiation from a discharge to excite phosphors coating the tube, which then emit visible light.

(a) Sodium-vapour lamps

Sodium vapour lamps contain metallic sodium and a starting gas; high-pressure types operate at elevated temperatures (several hundred °C) and are enclosed in protective outer envelopes to conserve heat. Practical luminous efficacies are high (approx. 40-50 lm/W for high-pressure sodium lamps). They are widely used for street lighting and floodlighting, though spectral colour rendition can be limited in low-pressure types (monochromatic yellow).

(b) High-pressure mercury-vapour lamps

These lamps give a greenish-blue light with moderate efficiency (≈ 30-40 lm/W). Colour rendition is limited; high-pressure mercury lamps are produced in ratings such as 250 W and 400 W for use on 200-250 V AC supplies.

(c) Mercury-iodide (metal-halide) lamps

Adding metal iodides fills spectral gaps and improves colour rendering. These lamps offer higher efficacies (often 75-90 lm/W depending on design) and are suitable for floodlighting, industrial lighting and sports arenas.

(d) Neon lamps

Neon lamps are cold-cathode discharge lamps filled with neon (often with small amounts of other gases). They produce coloured light (neon: reddish-orange) and are used as indicators and for advertising (neon tubing). Small neon lamps may consume only a few watts.

(e) Fluorescent tubes

Fluorescent lamps are efficient (typical overall system efficacies ≈ 40 lm/W or higher depending on ballast and phosphor). A low-pressure mercury discharge produces ultraviolet radiation; phosphor coatings convert this to visible light. Typical tube construction includes argon at low pressure and a small quantity of mercury vapour, electrodes coated with electron-emissive material, a starter and a choke (ballast) to limit current and provide the starting impulse. A capacitor may be connected to improve power factor. Normal life is typically several thousand hours (e.g. ~7,500 h under standard test conditions), with life depending on switching frequency and operating conditions.

(f) Compact fluorescent lamps (CFLs)

CFLs are fluorescent lamps designed to replace incandescent lamps in fixtures designed for screw-in bulbs. They use folded or spiral tubes with an integrated ballast in the base. Compared to equivalent incandescent lamps, CFLs use roughly one-fifth to one-third of the power and last 8-15 times longer. CFLs contain small quantities of mercury, which requires careful end-of-life collection and disposal.

Electrolytic Processes

Electrolytic processes use electrical energy to drive chemical reactions (electrolysis). They are essential for metal extraction and refining (aluminium, copper, zinc, magnesium, sodium), manufacture of chemicals (caustic soda, chlorine, hydrogen, potassium permanganate), electrodeposition (electroplating, electroforming, electrotyping), and rebuilding worn parts by electrodeposition.

Fundamental relation of electrolysis

The mass of substance deposited (or liberated) at an electrode during electrolysis is proportional to the total electric charge passed. The relation is:

m = Z I t

where:

• m = mass deposited (kg)
• I = current (A)
• t = time (s)
• Z = electrochemical equivalent of the substance (kg/C)

The electrochemical equivalent Z can be obtained from the molar mass and the number of electrons exchanged in the reaction. In terms of molar quantities:

Z = M / (F · n)

where:

• M = molar mass of the substance (kg/mol)
• F = Faraday constant ≈ 96485 C/mol
• n = number of electrons transferred per ion (valency)

Using this relation permits calculation of material consumption or deposition rates for a given current and time.

Power supplies and practical considerations

Electrolytic cells require direct current and low cell voltages. Small electroplating operations typically use currents in the range 100-200 A at 10-12 V. Large-scale extraction, refining and chemical manufacture require very large DC supplies and substantial energy consumption. Practical design of electrolytic plants considers cell voltage, current density, electrode area, mass transport (agitation), electrolyte composition and temperature, anode/cathode materials, and cell arrangement for uniform deposition and efficient energy use.

Applications

• Metal extraction: aluminium by Hall-Héroult electrolysis from molten aluminium oxide; sodium and magnesium by molten-salt electrolysis.
• Refining: electrolytic refining of copper, gold, silver and nickel to high purities.
• Electroplating: deposition of chromium, nickel, copper, silver and gold for corrosion resistance, wear resistance and decorative finishes.
• Electrochemical manufacture: chlor-alkali process for caustic soda and chlorine; electrolytic production of perchlorates and permanganates.

Safety, Efficiency and Selection Criteria

When selecting an electrical heating, welding, illumination or electrolytic process, consider:

• Process requirements: required temperature, heating rate, localisation of heat, atmosphere control and material compatibility.
• Energy efficiency: how much of the electrical energy is converted to useful heat or chemical change and how losses are minimised.
• Control and repeatability: ease of regulating power, temperature and duty cycle for consistent results.
• Safety and environmental impact: electric shocks, fumes, gases, ultraviolet radiation, mercury content in lamps, and disposal considerations (mercury in CFLs, for example).
• Capital and operating costs: equipment cost, maintenance, electrode consumables, and energy cost.

Summary

This chapter reviewed principal methods for converting electrical energy into heat (resistance, radiant, arc, induction, eddy-current and dielectric heating), summarised electric welding processes and equipment, defined illumination concepts and lamp types, and outlined electrolytic processes including the fundamental electrolysis relation m = Z I t and the meaning of electrochemical equivalent Z. Selection of a process depends on the technical requirements, energy efficiency, operational control, safety and cost considerations.