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Gas and Electric Comparisons

What is Electric Heating?
Electric process heating is made up of several different technologies.

Electric vs. Gas Radiant Heating
Gas and electric radiant heaters and burners are used in a wide variety of industrial applications. Major applications using both heating methods are listed below.

  • Drying Ovens - Similar performance. Gas sometimes favored where air movement in oven is required.
  • Finish & Powder Coat Curing - Similar performance.
  • Food Processing (Baking, Toasting, etc.) - Similar performance.
  • Metals Heat Treating, including Atmosphere Furnaces - Gas radiant tubes have largely replaced electric elements because of higher heat densities - faster heat up, higher production rates.
  • Plastics Forming - Electric favored where complex temperature zoning is required or heat densities are very low. Gas catalytic making inroads into low density applications.
  • Pulp & Paper - Gas favored for bulk water removal, electric for cross-web profiling.
  • Textile Drying - Similar performance. Fastest cool down characteristics may determine selection.
  • Vacuum Heat Treating - Electric nearly universal, although gas-fired units are beginning to enter market.

Some electric equipment is more efficient, some isn't; but what matters to the customer is the cost of operation.

Electricity costing only $0.01 per kilowatt hour is equivalent to natural gas at $2.93 per million Btu. In most areas, electricity costs at least 4 or 5 cents per kWh, equivalent to a whopping $11.72 to $14.65 per million Btu.

Because gas is usually much less expensive than electricity, gas-fired equipment usually costs less to operate, even if its efficiency may be lower.

Energy costs alone shouldn't be used to determine which technology to select. Assuming both technologies satisfy the customer's technical requirements, an analysis to determine the cost of ownership or total cost per unit of production will give a much clearer picture of which equipment is best for their needs.

Gas - Electric Efficiency Comparison
Use this chart to figure how efficient gas-fired equipment has to be to match the energy cost of competitive electric equipment.

Gas - Electric Efficiency Comparison: Example
Your customer pays 5.5 cents per kWh for electricity, including demand charges, and $6.00 per million Btu for gas. Enter the chart at 5.5 cents/kWh and move up to the $6 line, then over to the right to get an efficiency factor of 0.37.

The manufacturer of the electric equipment claims 75% efficiency. If the gas equipment is more efficient than:

75% X 0.37 = 28%, it will cost less to operate.

Electric Radiant Heating
Electric radiant heating covers a wide range of processes and temperatures.

The common thread is that electric current is passed through a resistance element, heating it. That element than transfers heat to the process by direct radiation or by convection, via a stream of moving air.

Direct Electric Resistance Heaters
Direct electric resistance heaters operate by passing a low voltage current directly through the piece to be heated. The work piece is, in effect, its own heating element; and heating rate is extremely rapid.

Direct resistance heaters are an alternative to induction heaters or gas furnaces for heating steel for forging, forming and upsetting. They are best suited for high production volumes of small work pieces of constant size.

Electric Radiant Heating Elements - 1
Examples of electric infrared heaters for low temperature applications.

Despite the wide differences in configuration, all these heaters have one thing in common - they generate heat by passing an electric current through a resistance wire.

Electric Radiant Heating Elements - 2
Examples of electric radiant elements for high temperature applications.

Despite wide differences in configuration, all these heaters have one thing in common - they generate heat by passing an electric current through a metal or ceramic resistance element.

Radio Frequency (RF) and Microwave Heating
Certain materials contain dipolar molecules, like water and certain organic chemicals, which oscillate in the presence of a high frequency alternating current electrical field. The molecules' oscillations generate heat inside the product, heating it rapidly and uniformly.

Microwave and RF heating are normally used for "niche" applications which are difficult to heat properly by conventional methods, and they are often used in conjunction with those methods to improve their performance.

Food, pharmaceutical and wood products drying, adhesives curing and plastics sealing are examples of applications that have used RF and microwave heating.

Does Electric Make It Easier to Create Specific Temperature Profiles?
In continuous applications, where the temperature profile is developed along the length of the oven or furnace, generally not.

However, electric elements may have the edge in applications like plastic thermoformers, where an array of heating elements radiate to a fixed object or surface, and variable temperature profiles are needed to accommodate variations in the product.

The limitation of gas isn't in the heater size - heaters as small as 6" square are available - but in the need for individual temperature and safety controls for each burner module. This often makes the equipment complex and more costly than electric.

The Components of a Temperature Control System - 1
Whether the energy source is electricity, gas, or coal, all automatic temperature control systems have certain basic components:

The Components of a Temperature Control System - 2
The same types of temperature sensors and controllers can be used on gas or electric systems. The difference is in the energy input regulating device.

Electric Heating Systems Use:

  • Mechanical Contactors
  • Mercury Relays
  • Saturable Core Reactors
  • Solid State Relays
  • SCRs (Silicon Controlled Rectifiers)

Gas Heating Systems Use:

  • Solenoid Valves
  • High-Low Control Valves
  • Modulating Control Valves

Electric Heating Power Controllers
All electric power controllers are essentially on-off devices. The amount of power they send to the heating process is determined by the percentage of time they are on.

The sensitivity with which they can hold process temperatures depends on how quickly they can switch on and off.

Temperature Control Sensitivity
The ability of an on-off heat or power regulating device to track closely to the process temperature depends on how quickly it can cycle from on to off and back again.

If the on-off cycle is short,
there will be little or no
noticeable variation in
temperature.
 
If the on-off cycle is long,
the operating temperature
will cycle above and below
the setpoint. This is
sometimes called
"sawtoothing".

Firing Rate Control Devices for Gas Burners - On-Off or High-Low
The two most common devices used for on-off or high-low control of air and gas flow to burners are:

Their ability to hold a steady process temperature depends on how quickly they can cycle from high to low and back again.

Minimum Cycle Times of Gas and Electric On-Off Power Controllers
Approximate minimum on-off cycle times, seconds:

The shorter the cycle time, the more closely the system can match the desired temperature without sawtoothing. For most processes, sawtoothing will be negligible with any controller having a cycle time of 5 seconds or less.

Gas Heating Firing Rate Controllers
In addition, gas firing rate controllers can be proportional devices. Proportional controls regulate the firing rate one of two ways...

1. By matching the control valve position to the
input needed (position or current proportioning),
so there's no cycling.
2. By adjusting the percentage of time the valve is at the high input position (time proportioning).

Minimum Resolution of Gas Proportioning Power Controllers
Time-proportioning controllers are basically on-off devices, so their minimum cycle times are the same as if they're used in the on-off mode.

The sensitivity with which a position proportioning control motor and valve can hold a fixed temperature depends on the number of position steps the motor takes between its limits of travel and the length of time required to take each of those steps. Modern control motors take from 50 to 300 steps in as little as 6 to 8 seconds.

Induction Heating and Melting
If metals are placed inside a strong magnetic field from an alternating current, electrical currents will be "induced" (generated) inside the metal. As those currents try to flow against the metal's electrical resistance, internal heat will be generated. If the field is strong enough, the metal can be heated or even melted.

Induction Heating is Faster than Gas Furnace Heating

Each Method has its Strengths and Weaknesses
Induction is most efficient and productive heating bars whose cross-sections are simple and uniform (rounds, squares or round-cornered squares).

Efficiency and temperature uniformity also benefit if the diameter or thickness of the bar is close to the coil diameter (good coupling) and matched to the electrical frequency. Drastic changes in stock size require changing the inductor coil and operating frequency. This impairs productivity.

Gas furnaces have the edge with stock of non-uniform cross-section and where stock sizes are changed frequently.

Over the Years, These Claims Have Been Made for Induction Heating Induction Heating:

  • Is nearly 100% efficient.
  • Is faster than gas heating.
  • Produces less oxidation of the steel.
  • Uses less manpower.
  • Is easier to work near than a gas furnace.
  • Is versatile.

Induction heating is not even close to 100% efficiency. Sizable amounts of energy are lost to cooling water and as stray magnetic fields, in addition to switch gear and transformer losses. Typical system efficiencies for forging bar heaters:

Compared to Conventional Furnaces, Induction Heating Produces Less Scale because it Heats the Steel More Quickly
Steel begins to scale (oxidize) rapidly above 1600°F; so the quicker it can be heated to forging or rolling temperature, the less scale will form.

Some of the recently-developed rapid heating gas furnaces do not heat as fast as induction, but their scale formation rates are nearly as low.

 

Induction heaters are relatively cool
and quiet. But many modern ceramic
fiber-insulated furnaces have such
low heat loss and noise levels that
with their doors closed, it is difficult
to tell they are even operating.
Induction heaters produce strong magnetic
fields in their immediate vicinity. There is
now concern that in poorly shielded
installations, these fields may have long-term
health effects. The National Institute of
Occupational Safety and Health (NIOSH) is
studying this phenomenon.

Principal Induction Applications

Induction Melting
Two types of induction melters are commonly used:

  • Coreless Furnaces used mostly for melting
  • Channel Furnaces used mostly for holding and refining

Induction melting furnaces are efficient, especially at the high temperatures, produce less metal oxidation than gas-fired furnaces, and their magnetic fields stir the metal, aiding temperature and composition uniformity.

For melting iron, few gas furnaces can compete with them for efficiency. For melting brass and bronze, gas crucible furnaces compete well.

Unlike gas furnaces, induction melters can be used for melting metals under a vacuum.

Electric Arc Melting Furnaces
Electric Arc Melting Furnaces are used mostly for melting steel, especially in mini-mills. There are no directly competitive gas melting furnaces.

However, to raise productivity, oxy-natural gas auxiliary burners contribute 20 - 40% of the total heat input on many modern arc furnaces. On a 100 megawatt furnace, this is a gas consumption of 68 to 137 Mcf!


Induction and Flame Hardening

Induction hardening is a specialized form of
heat treating, where localized areas of parts,
such as gear teeth or cam surfaces, are
preferentially hardened. To succeed, the part
surface must be heated very quickly, while the
rest of the piece remains relatively cool.
Flame hardening, with high intensity
oxy-natural gas flames, produces
similar results.

Immersed Electrode Melting Furnaces
Immersed electrode melting furnaces are used primarily for salt bath heat treating and dip brazing.

The molten salt completes the circuit between the electrodes. Electromagnetic currents circulate the salt throughout the tank, improving heat transfer and temperature uniformity.

Furnaces like these can be heated by gas-fired immersion tubes, although salt circulation may not be as thorough and the tubes occupy more tank volume than the electrodes.

For salt temperatures above 1500°F, immersion tube materials and sizing have to be done carefully to avoid premature failure.

Externally-heated salt pots have roughly equal performance on gas or electricity.

It is the material handling system feeding and unloading the heater or furnace, not the method of heating, that determines manpower requirements. Induction heater manufacturers have long taken advantage of this.