GAS OR ELECTRIC?
When it comes to furnace selection and development, choosing the right heating method can make a huge difference in the quality of your product and in the cost of operation. The choice is not always obvious, but we can guide you to the right answer based on our experience and other successful projects.
The choice between gas and electric heating for your furnace is often influenced by the rates for each energy source. In today’s market with natural gas prices near all-time lows, that may end up being the logical choice. Even so, there are some situations where electric heating is a better choice. A review of the industry-standard Available Heat Chart below shows why.
The vertical axis shows the Available Heat, or efficiency for a given set of conditions. The horizontal axis shows the Exhaust Gas Temperature, usually at or somewhat above the process temperature.
The diagonal lines represent various levels of excess air at the burner(s). A burner firing at the perfect air-gas ratio would be represented by the 0% line, and would provide the highest efficiency possible at a given temperature.
To put numbers to it, start at the 2000°F point on the bottom axis, go straight up to the 0% curve, and then read to the left axis, where the answer is approximately 45% Available Heat. This means that if your burners are perfectly tuned, the best you can ever get is 45% efficiency.
Compare that to a typical electric heating system, where the efficiency is in the 96% range, or double the efficiency of gas in this case. So depending on the relative costs of natural gas or electric power in your area, electric heating may actually be cheaper in the long run.Your process might also hold the key in the gas vs. electric consideration.
For atmosphere processes, an indirect gas-fired system isolates the products of combustion from the work by firing burners into sealed radiant tubes, or around a retort containing the work and atmosphere. Tubes and retorts reduce efficiency because they must absorb, then re-radiate the heat generated by the burners. Meanwhile the products of combustion are carried out the exhaust stacks, taking away much of the energy input. Recuperators can capture some of the waste heat, but even so, the overall efficiency of the system is typically less than 75% and perhaps as low as 20%.
An electrically-heated system can be far more efficient, especially when the elements themselves can be exposed to the atmosphere as they are in this carburizing furnace. Energy is transferred directly into the process since there are no barriers, but the keys to this approach are proper material selection and power control.
Atmospheres are often very reactive for processes like carburizing, nitriding, and Ferritic Nitrocarburizing (FNC) or Gas Soft Nitriding (GSN). That reactivity can quickly degrade an exposed element of the wrong material. Likewise, the wrong voltage and current applied to the element can lead to rapid failure in the presence of a reactive atmosphere. HTF knows the right combination for your application, based on our many successful atmosphere furnaces.
End-of-cycle cooling adds another dimension to the decision.
If the work load must be cooled in the furnace, extra metal in the form of radiant tubes or a retort must be cooled as well. Every additional ounce extends the cycle time and reduces productivity, which may increase your cost per pound of production. HTF can help you navigate the alternatives, and develop the best solution.
Summing it all up, these factors: heating efficiency and cost, process, cooling, and many others will influence the answer to the ‘gas or electric’ question – it’s not so simple!
Of course to get to the right answer for your project there are calculations to be made which involve more than the above, and we have a long track record of helping people like you. Please call us to get started.