Capturing heat from the furnace
Regenerators: Another Approach
Another potential tool for salvaging a significant amount of the heat energy from a furnace exhaust is the regenerator. Although less commonly used than recuperators, regenerators are still widely used in high temperature furnaces such as glass and steel reheat furnaces. A regenerator uses two or more vessels or vessel sections containing a high thermal conductivity matrix. The matrices may be ceramic or metallic. Exhaust passes through the matrix giving up a large portion of its heat before discharging.
After the matrix is heated, the stream is mechanically directed to another section or vessel and incoming combustion air is drawn through the hot section and heated. The regenerator alternates the hot and cold flows so heat recovery is continuous. The advantage of a regenerator over a recuperator is that it presents a much larger amount of heat exchange surface to the hot exhaust flow.
A disadvantage is that the heat content of the incoming air has some variation, making precise combustion control more difficult. This can be reduced by faster cycle times or the use of multiple vessels at various stages of cooling. Another consideration is that some exhaust products inevitably remain in the vessel to combine with incoming combustion air.
In the Future
There are still a good many plants with major energy reduction potential, and the level of adoption is uneven. Russ Chapman from Firebridge notes that larger companies are starting to benchmark their large plants against each other, and against the competition, in terms of sustainability of the plants. However he feels that a single-minded focus on short term profitability sometimes means a lack of incentives for long-term energy saving projects.
As an example he cites a Tier 2 automotive manufacturer that has a plant with a monthly energy bill of $200,000 per month. He feels this company could reduce that bill by 25%. However the plant is on its third plant manager in three years, and no action is taking place in making improvements. Clearly, energy savings are not always enough.
Global Competition Driven
Chapman notes that smelting in the copper mining industry is an example of an industry starting to move in this direction. “It’s because they are competing worldwide and are comparing key performance indicators, making efforts to get in line.”
Asked if effective heat recovery can lower greenhouse gas emissions from industrial furnace applications, Chapman replies, “Absolutely! Energy reduction equals emission reduction. The only inhibitor is scale.” He explains, “The cost of retrofitting little furnaces is disproportionately higher than one operating at, say, 45 MMBtu/hr or larger, so the economic case is more difficult to make.”
Taking the Step
Unquestionably, heat recovery strategies have huge potential for reducing energy bills and plant emissions.
Jim Roberts from Eclipse notes, “Sometimes the cost looks staggering, but remember that most furnaces have a very active running schedule, so the paybacks, even at low gas costs, can be very fast. Even if paybacks are in the two-to-three year range, the long-term savings are worth it.”
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Annual Salary Survey
After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.
The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.