|Fig. 1. Bottom-loading furnace A with damaged and missing brickwork|
The stand-alone performance of a well-adjusted modern furnace, at best, is only 30-40%. However, other common inefficiencies can easily reduce this efficiency to just 15-20% because the furnace is operated with less than an ideal heating cycle, because it has old and deteriorated thermal insulation, and because its heating system is in disrepair. With such large inefficiency, there is plenty you can do to easily improve it by as much as 50%, which represents a huge financial opportunity for you and is the focus of this article.
When figuring the payback of a repair or efficiency gain, it is perfectly acceptable if you calculate your efficiency gains based on the lifetime of the investment. Your own management typically wants to know what the payback is, so use this method of calculation.
Contemporary management recognizes that best practices compel them to support energy-conservation initiatives within their company. I cannot think of a company that does not have a profit-making goal. Whether we call it profit making or energy savings, it is still the same idea. For some reason, the human spirit enjoys engaging in games they can win. Therefore, consider developing a couple of well-chosen goals and metrics that measure your progress toward those goals. You have probably heard the old adage, “if you can’t measure it, you cannot control it.”
|Fig. 2. Bottom-loading furnace A with heat-seal brick replaced|
Where Do You Look for Higher Energy Efficiency?
If you don’t quite know where to find efficiency gains in your furnace area, here are some tips where to look for “low-hanging fruit.” In our 46 years of building and repairing industrial furnaces, we have seen plenty of examples of best and worst practices. Because the range of industrial heating applications varies widely among readers of this magazine, some of these tips may not apply to you, but please continue reading to discover tips that can help your business become more competitive and financially stronger, not to mention greener.
Tip #1: Insulation Repair
We choose to discuss this first because it applies universally to so many companies. This is one of the most common forms of deferred repair work that we observe. Missing or damaged refractory allows excess heat leakage that the heating system must compensate for by overworking, thus raising your energy cost. Furthermore, heat leaks often mean cold-air infiltration into the furnace and upset temperature uniformity, thus lowering the yield of heat-treated product, which can also damage the equipment causing even more expensive repair.
Pictured in Figures 1, 3 and 5 are classic examples of furnace interiors urgently in need of refractory repairs. Figures 2, 4 and 6 show furnace interiors with excellent refractory repair.
The before and after story behind Figures 1 and 2 is that temperature uniformity became so bad in this furnace that there was no choice but to repair the brickwork. Otherwise, the furnace could not be used. This damage had been tolerated for several months, during which time it put the surrounding steel furnace structure at risk of being warped due to excessive heat losses. More importantly, it was ruining the temperature uniformity within the furnace, not to mention having to adjust its heating system to input greater heat to offset the cold air leaking into the furnace. The most expensive consequence of this damaged brickwork was the high energy inefficiency that was tolerated for several months before it was finally repaired.
|Fig. 3. Front-loading furnace B with broken and missing insulation|
Tip #2: Conversion from Brick to Fiber InsulationIf you operate a brick-lined batch furnace that heats to about 2100°F in 8 hours, soaks for 2 hours and then cools, you are a prime candidate for conversion to a fiber-insulated furnace. The justification for this is found in a comparison of the heat loss and heat storage between the two choices of refractory insulation constructions.
For sake of example, assume your furnace has an interior volume of 6 x 6 x 6 feet. A furnace of this description insulated with 9 inches of brick will have double the heat storage per square foot of furnace lining compared with a fiber-insulated furnace. It is no surprise that the brick-lined furnace needs a heating system nearly twice as large compared to the fiber-insulated furnace. Which furnace would you rather pay the heating bill for?
With the higher heat storage, a brick-lined furnace will also effectively take twice as long to cool down, obstructing your ability to turn the furnace around every 24 hours. Using ceramic-fiber insulation can so substantially lower your energy bill, it is an alternative that warrants your serious consideration even before your brick-lined furnace insulation is on its “last leg.”
|Fig. 4. Front-loading furnace B restored back to efficient heating condition|
Tip #3: Test and Adjust Your Burner SystemIf your business uses a fuel-fired furnace, you probably know what type burner adjustment, control and furnace atmosphere (chemically) are most desirable for the particular material you heat (e.g., on-ratio, excess air or excess fuel).
The manufacturer of your burners can most likely provide fuel-capacity curves stating heating capacities for various airflow rates for a particular fuel and operating temperature. Armed with this data and a digital monometer, you can set the correct gas and air settings on your burners accurately and easily. After setting the desired burner performance and furnace-atmosphere adjustments, it is possible to confirm them by measuring your flue-gas oxygen and carbon dioxide levels. Equipment needed to perform these burner and flue-gas measurements is not expensive, and if it is used periodically, you can be assured that your company is operating its furnaces in a fuel-efficient manner.
The benefit of planning and achieving the correct gas/air ratio is quickly evident by understanding the graph (Fig. 7) showing available heat for differing rates of excess air assuming a 2000°F flue-gas temperature. Looking at Fig. 7, you can see that if you meant to be processing on-ratio (0% excess air) with 60°F combustion air but find from analysis of your flue gas that you are operating with 50% excess air, you can determine from this chart that the fuel your furnace is burning is only utilizing 28% available heat rather than 46% (e.g., for 0% excess air). In effect, your furnace is consuming 65% more fuel than necessary. At today’s average national fuel cost of $3.50 MCF for a continuous process that consumes 1,000 ft3 of gas per hour, this amounts to burning $39,000 of annual profitability.
One other important adjustment to your furnace’s combustion system concerns the operating pressure within your furnaces, which is controlled by the exhaust damper setting. Fuel-fired furnaces should be fired with a modestly positive pressure measured at hearth level (e.g., <0.1 inch W.C.). This modestly positive pressure within a furnace when the burners are on high fire prevents cold air from infiltrating into the furnace and prevents the heating inefficiency described when the flue gas chart was explained.
|Fig. 5. A furnace needing repair of all insulation surrounding the door opening and of heating elements that were starting to fall loose|
Tip #4: Test and Service Your Electric Heating System
Electric heating systems are quite different than burner heating systems. The principal difference is that electric heating is often passive without vigorous convection.
If your furnace has metallic heating elements, it is vital that you inspect and verify all elements are drawing correct amperage and appear to have the same color of orange, red or yellow once they reach a 1400-1800°F temperature with visible color. A more technical way to test them without heating the furnace is to use a volt/ohm meter to test each element for continuity and measure their ohmic resistance. The important thing is to inspect and test your elements, and the need to replace faulty elements is obvious.
Heating elements that are falling out or off of the mounting supports (Fig. 5) should be resupported or replaced by the same means they were designed to be supported.
Elements that do not heat are more harmful than you may think. The remaining good elements then have to work that much harder to produce additional wattage to achieve the setpoint temperature. As fewer elements work harder, it can stress and burn out the working heating elements that remain.
|Fig. 6. Gas-fired, car-bottom furnace with newly installed ceramic-fiber wall and roof insulation|
Tip #5: Optimize Your Heating Cycle Have you considered altering your heating cycle to shorten its length of time? This is more feasible than you may realize. We often see batch-furnace operations where we believe the heating cycle can be optimized by shortening certain segments of the cycle, as long as temperature uniformity can be maintained, with the result being that the overall cycle time is shortened.
To approach this scientifically, one person should be in charge that possesses a reasonably broad knowledge of the specific heating process and material they are processing. Here are some examples.
Customer A started to have trouble heating their product as fast as they had in the past. Because they did not have time to conduct test firings to develop a more informed decision, they instead slowed down their furnace heating rates enough to achieve the required temperature uniformity. They never changed back nor did they ever attempt to test and solve their original problem.
Customer B has heated their product very slowly for years in an old, inefficient furnace because it could not heat any faster. If they tried to heat more rapidly, its poor temperature uniformity was intolerable. Then they purchased a new furnace with excellent temperature uniformity, but they began to heat it on the same slow cycle as their old furnace.
How do you know if present heating and cooling cycles are optimum for high production and high yield for your type of product?
- You can perform your own planned trial-and-error heating tests if you have the time.
- You can hire an outside consultant to mastermind the process development. This will undoubtedly cost money you can weigh against the value of the improvement you expect.
- A less-expensive approach, if you have the time, is to contact a knowledgeable furnace builder with experience heating your particular type of product. You may have to call and talk to several furnace builders until you find someone with the specialized knowledge and experience you seek. Our company offers free non-proprietary advice if it is something we have firsthand knowledge of. If we don’t know, we will recommend another expert.
Heating faster with less product loss is much more energy efficient. Therefore, we challenge you to look critically at your present heating cycles and to ask yourself if the heating and cooling cycles are optimum and equal to the best practices in your industry.
|Fig. 7. Available heat chart provided courtesy of North American Combustion Handbook, volume 1, third edition, page 63.|
Set a Corporate Goal
A corporate goal will create a culture within your company that pursues energy conservation. One large company took a proactive approach directed at reducing its carbon footprint. This action reportedly resulted in a 30% reduction of carbon emissions per unit of revenue. Can you imagine how much newfound profit this amounted to?
For this company, it became easier to be green in terms of decreasing the size of their carbon footprint. They also greened up corporate profitability as well as social image. “It’s easy being green.” IH
For more information: Contact Charlie Birks, sales manager, Keith Company, 8323 Loch Lomond Drive, Pico Rivera, CA 90660; tel: 800-545-4567; fax: 562-949-3696; e-mail: C.Birks@KeithCompany.com; web: www.KeithCompany.com
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