Safety, cost and choosing electric heat

Electric heat is clean at the point of use, easy to control and quiet, and it needs no flue, gas supply or combustion air. It is also, per megajoule, usually more expensive than gas, and it brings electrical hazards of its own. Choosing between electric and gas heating, and running electric equipment safely and economically, means looking at the whole picture rather than one number: the energy price, the efficiency at the point of use, the demand charges, the equipment, the maintenance, the process and the people.

This chapter sets out the main hazards of electric heating equipment, costs a year of the curing oven’s operation, compares it with gas, and works through a demand-charge surprise on the coating business’s first electricity bill after the oven was installed.

Electrical safety

Electric heating equipment carries large currents at mains voltage, often in hot, dusty and sometimes damp conditions that degrade insulation. The main electrical hazards are electric shock from live parts or from a casing made live by an insulation fault, and burns or fire from overheated conductors and connections. The controls follow from earlier chapters:

• installation and repair only by licensed electricians, to AS/NZS 3000 and the equipment maker’s instructions;
• protective earthing of all metal casings, so that an insulation fault trips the protection rather than making the casing live;
• residual current protection where required, to protect people from shock (Chapter 4);
• isolation and lock-out before any work on heaters or terminals, with the circuit proved dead (Chapter 4);
• insulation resistance testing of elements and wiring during maintenance (Chapter 1);
• thermographic inspection of switchboards and terminal boxes to find hot connections (Chapters 2 and 4).

Safe Work Australia’s model code of practice on managing electrical risks in the workplace sets out the duties and controls in general terms, and jurisdictions’ electrical safety regulators set the licensing and installation requirements.

Burns and hot surfaces

Ovens, furnaces and induction work produce surfaces and parts hot enough to cause severe burns: oven doors and frames at well over 100 °C, furnace loads at 650 °C, induction-heated bars at 900 °C, all of which may look no different from cold ones. Guarding, insulation of accessible surfaces, warning signs, heat-resistant gloves and clothing, tools for handling hot parts, and safe set-down areas for hot work all address the hazard. Hot parts left to cool in walkways, or on benches where someone else will pick them up, are a common cause of burns, so cooling areas are defined and marked.

Fire and fume

Electric heaters remove the combustion hazards of gas, but not fire. Overheated elements can ignite dust, fluff or debris in heater boxes and ducts; ovens can ignite combustible residues on parts and racks; and some coatings and processes release vapours that are flammable in the oven atmosphere. Oven makers design ventilation, purging and interlocks for the coatings their ovens are intended for, and those requirements must be kept when processes change. Regular cleaning of heater boxes and ducts, the airflow and over-temperature interlocks described in Chapters 3 and 9, and fire extinguishers suited to electrical fires near the equipment all form part of the controls. Fumes from curing coatings are removed by the exhaust (Chapter 6), which must be kept running and maintained.

Figure 1Safety checks for the coating business's electric heating equipment

• Avoid: Isolators lockable and labelledeach oven, furnace and induction heater
• Avoid: Over-temperature limits testedsix-monthly, by tripping them
• Avoid: Airflow and cooling-water interlocks testedoven fan and induction coil
• Avoid: Insulation resistance and thermographyat each electrical service
• Avoid: Heater boxes and ducts cleanedquarterly, or sooner if dusty
• Do: ? Process changes reviewednew coatings checked against the oven maker's requirements

Maintenance as a safety measure

Electric heating equipment that is maintained stays safe; equipment that is not slowly becomes dangerous. Elements age and sag towards their supports and casings (Chapter 5); terminals loosen with heating and cooling (Chapter 2); insulation in hot terminal boxes hardens and cracks; dust collects in heater boxes; interlocks are bypassed during repairs and forgotten (Chapter 3). A maintenance schedule that inspects elements and supports, retightens and checks terminals, tests insulation resistance, cleans heater boxes and ducts, and functionally tests every interlock and limit turns those slow failures into routine findings. The coating business’s schedule is set from the oven and furnace makers’ recommendations, and every finding is recorded, so that a trend, such as one zone’s elements failing more often than the others, is noticed and investigated.

The cost of a year’s curing

This is an illustration. The curing oven uses about 286 kWh on a typical production day (Chapter 1), including its warm-up. Over 250 days, that is about 71,500 kWh a year, which at 30 cents per kilowatt-hour, excluding GST, costs about A$21,450.

That is the energy charge only. Many business electricity tariffs also include a demand charge, based on the highest power drawn in each billing period, and supply and network charges that do not depend on usage. The demand charge can be a large part of the bill for equipment like an oven, which draws its full power at start-up.

Electric or gas?

The price of energy per megajoule is the starting point of any comparison. At 30 cents per kilowatt-hour, electricity costs 0.30 ÷ 3.6 ≈ 8.3 cents per MJ, all of which becomes heat inside the oven. Natural gas at an illustrative A$25 per gigajoule costs 2.5 cents per MJ, but a gas burner loses part of its heat up the flue; at an overall efficiency of about 80% for a directly fired oven, the useful heat costs about 2.5 ÷ 0.8 ≈ 3.1 cents per MJ.

Figure 2A year of curing oven heat, electric and gas

	Electric	Gas, about 80% efficient
Useful heat a year	about 257 GJ (71,500 kWh)	about 257 GJ
Energy bought	71,500 kWh	about 322 GJ of gas
Price per useful MJ	about 8.3 cents	about 3.1 cents
Energy cost a year	about A$21,450	about A$8,040

On energy price alone, gas wins clearly for this oven, by about A$13,400 a year. But the decision does not rest on energy price alone:

• Equipment and installation. A gas oven needs a gas supply, burners, a flue and gas safety systems; an electric oven needs a large electrical supply. Either may be expensive, depending on what the site already has.
• Demand charges. An electric oven can raise the site’s peak demand significantly; a gas oven does not.
• Process. Electric heat is clean, with no combustion products in contact with the coating, and is easy to control precisely, which matters for some coatings and heat treatments.
• Maintenance and safety. Gas burners need regular servicing and gas safety checks; electric heaters need element replacement and electrical testing.
• On-site generation and emissions. A business with rooftop solar can supply some daytime electric heating from its own generation, and the emissions of electric heat depend on how the electricity is generated; gas combustion produces carbon dioxide on site.

The coating business chose electric heating because its building had no gas supply and connecting one was expensive, because its rooftop solar offset part of the oven’s daytime use, and because its customers valued the clean curing. It accepted the higher energy price knowingly, having costed it, and set out to control the demand charge, as the failure below shows. A decision made that way can be revisited sensibly if prices, the site or the process change; a decision made on habit cannot.

Reading a business electricity tariff

Before costing any electric heating equipment, it is worth reading the tariff it will run on, because business tariffs vary widely in structure. Common elements include:

• an energy charge per kilowatt-hour, sometimes different at peak, shoulder and off-peak times;
• a demand charge per kilowatt or kilovolt-ampere of the highest demand in a period, often measured over half-hour intervals;
• a supply or service charge per day, independent of use;
• network and environmental charges, which may be included in the others or listed separately.

Time-of-use energy prices make it worth running flexible heating loads, such as a furnace program or a hot-water tank, in cheaper periods. Demand charges make it worth staggering start-ups and limiting simultaneous peaks. Supply charges make it worth combining equipment on one connection where possible. An engineer who costs an oven at a single energy price and ignores the rest can be wrong by a large margin, in either direction.

Saving electric heat

The heat balance in Chapter 6 showed where an oven’s energy goes, and the savings follow the same order whatever the energy source: full, continuous production to spread the warm-up; exhaust set to the required rate and no higher; lighter racks; quick batch changes; and sound insulation and seals. Electric equipment adds a few savings of its own: timed and staged start-ups to limit demand (Chapter 9), solid-state switching for steadier temperatures and longer element life, and running flexible loads when on-site solar is generating or energy prices are lowest. A sub-meter on each major heater makes the savings measurable, which is what keeps them in place after the enthusiasm of the first month has passed.

The view from 1895

Houston and Kennelly costed electric heat carefully. They gave electricity prices of about 8 cents per kilowatt-hour in large quantities and 15 cents in small, worked out that boiling a gallon of water in an electric kettle cost about 4 cents once the kettle’s efficiency was allowed for, and described the uses electric heat had already found: small heaters and cooking, welding, and furnaces making products, such as carborundum and calcium carbide, that no fuel-fired furnace could make. Those first uses are the clue to electric heat’s place in industry ever since. It has rarely been the cheapest heat per unit of energy, but it is often the best heat for the job: clean, precisely controlled, delivered exactly where it is wanted, and able to reach temperatures and atmospheres that combustion cannot. The coating business’s decision weighed exactly those qualities against the price.

A failure: the first bill

This is an illustration. The coating business’s first electricity bill after the oven was installed included a demand charge far larger than expected. The tariff charged an illustrative A$15 per kilowatt per month on the highest half-hour demand in the month, and the meter data showed a peak of about 95 kW, reached at 7 a.m. on several days, when the oven, the tempering furnace and the air compressor all started together. At A$15 per kilowatt, that peak cost about 95 × 15 ≈ A$1,425 for the month, on top of the energy charge.

The engineer’s analysis showed that the peak came entirely from simultaneous start-up. Once warm, the oven drew about 36 kW and the plant’s combined demand stayed below 60 kW. The fix was control and scheduling, not new equipment: the oven controller was programmed to start on a timer at 6 a.m. with its warm-up staged at 45 kW (Chapter 9), finishing before the shift arrived; the tempering furnace was scheduled to start after the oven reached temperature; and the compressor’s start was left to its own controls. The next month’s peak was about 70 kW, saving about (95 − 70) × 15 ≈ A$375 a month, about A$4,500 a year, with no new equipment and an oven ready when the shift began.

Bringing it together

Each chapter of this book has applied one or two relationships to one part of the coating business’s electric heating. Joule’s law gave the heaters’ power and the cables’ losses. Resistivity gave the elements’ length. The heat balance gave the oven’s demand and warm-up. Radiation explained the furnace. Penetration depth explained induction. Control decided temperature accuracy, element life and peak demand. And the costs tied them together. The same relationships, and the same habit of writing down where the heat goes, apply to any electric heating equipment a business might buy, run or fix.

Exercise: the tempering furnace’s energy

The tempering furnace runs three programs a week, each averaging about 4 hours at an average power of about 6 kW. Calculate its annual energy use and energy cost at 30 cents per kilowatt-hour. Then estimate the cost of the same useful heat from a gas furnace at A$25 per GJ with an overall efficiency of about 70%.

The furnace uses about 3 × 52 × 4 × 6 ≈ 3,744 kWh a year, costing about 3,744 × 0.30 ≈ A$1,123, excluding GST. The same useful heat, 3,744 kWh or about 13.5 GJ, would need about 13.5 ÷ 0.70 ≈ 19.3 GJ of gas, costing about 19.3 × 25 ≈ A$481.

The gas furnace would save about A$640 a year in energy, but it would need a gas supply the building does not have, burners, a flue and gas safety maintenance, and it would bring combustion products into contact with the parts unless it were indirectly fired. For a furnace used three times a week, the energy saving is small beside those costs, which is why the business kept the furnace electric.