Burner replacement on existing boilers
Dramatic efficiency gains possible
You know that your boiler is a major energy user, and you may have concerns about its efficiency. But if boilers are the right size and type for their operations, some owners are reluctant to replace them. The efficiency problem may not be the boiler itself, but the burner. Owners are discovering that they can make major improvements in operating efficiency by installing a replacement burner, with improved turndown, higher combustion efficiency, lower emissions, and precise control capability.
Role of the Burner
A report from the DOE’s Energy Efficiency and Renewable Energy Division (EERE) indicates, “The purpose of the burner is to mix molecules of fuel with molecules of air. A boiler will run only as well as the burner performs. A poorly designed boiler with an efficient burner may perform better than a well-designed boiler with a poor burner. Burners are designed to maximize combustion efficiency while minimizing the release of emissions.”
The report goes on to point out the importance of minimizing excess air in the combustion process, and the value of high turndown ratios in minimizing startup-shutdown energy losses. It also advises that in many cases, burners that were designed for oil-burning and were converted to natural gas are quite inefficient and are likely candidates for replacement.
Newer Technologies to Upgrade Existing Boiler
In a recent presentation at a Technology & Market Assessment Forum (TMAF) sponsored by the Energy Solutions Center, Hikmet Demiryol from Weishaupt Corporation discussed potential benefits of replacing older burners on boilers, taking advantage of advanced technologies. Weishaupt is a global provider of boiler burners, both as original equipment and for retrofit installations. Demiryol pointed out that the thermodynamic properties of many older boilers are still very acceptable.
He noted that a well-maintained boiler has a long life – as long as 40 years. He stated that there have not been major changes in the design of the heat exchanger in the boiler in 60 years. However, replacing the original burner with a high-efficiency modern system can result in fuel savings ranging from 5% to 40%. Demiryol said that current Weishaupt burners significantly reduce excess air at all firing rates. Lower excess air means higher combustion efficiency.
Burner Cost Low Compared to Savings
He advised that the vast majority of older boilers can be retrofitted with new burners, but a comprehensive feasibility study should be done in all cases. He emphasized that the cost of the burner, plus the cost of burner maintenance, during the lifetime of the burner represented only about 2% of the cost of the fuel that would be fired through it. Therefore, installing the most efficient possible replacement burner makes the most sense. Manufacturers such as Weishaupt offer burners in a wide range of sizes for many of the commonest brands of boilers.
Demiryol also stressed that today’s boiler burners do have improved turndown ratios. This is important because often the boiler is at low load during certain times of day, or during certain seasons of the year. If the boiler’s original burner does not have adequate turndown capability, the boiler will cycle, with consequent significant losses in boiler efficiency. Burners are available with turndown capabilities as low as 10% load, minimizing boiler cycling and improving system efficiency.
Excess Air Control Critical
A key to burner efficiency is strict regulation of the amount of excess air for combustion at all operating levels. Today’s burners – both gas-only and gas/oil – are designed to minimize energy-wasting excess air. By holding excess air levels to 4% or less, boiler efficiency is significantly improved. Many modern gas burners can hold these levels through the full turndown range. The key to excess air control is a precise damper mechanism for combustion air that maintains accurate and low excess air levels through the entire range.
Another important provider of replacement burners is Riello Burners, a global firm headquartered in Italy, with a broad international presence. According to Riello’s Jack McDonald, the company offers replacement burners for a wide range of commercial and industrial boilers. McDonald points out that the potential savings from a burner replacement can vary widely. “An average of between 5% and 15% would be expected, though I have seen as high as 30% savings.”
Oxygen Trim for Varying Conditions
He indicates that an option that is frequently being chosen is an oxygen trim control that senses O2 levels in the exhaust gas and adjusts combustion air accordingly. “We use O2 trim on units above 200 hp, as this is the area where the added expense will show benefits in terms of payback.” He notes, “O2 trim is really only useful if there are large swings in the combustion air conditions in the boiler room, variations in the fuel Btu ratings, and/or the boiler breeching has uncontrolled swings in draft.”
In any case, McDonald says, Riello burners are capable of running between 3.5% and 4.5% excess air. “This is because the mechanical cam versions of our standard burners are precisely machined and have turnbuckle-type connections so there is very little or no free play in the linkages.”
Precision Mechanical Controls
He indicates the Riello mechanical cam burners have a high repeatability, such that an initial setting will maintain that correct level for years, provided there are not changes in the combustion air temperature, fuel supply, or fouling of the burner fuel and air passages. “They should be checked at minimum yearly, though, as components can wear for a variety of reasons, causing changes in combustion.”
McDonald suggests that owners evaluate their boilers for a possible efficiency upgrade with a new burner and controls. “There are many good mechanical contractors who can provide those services. They should supply owners a detailed overview of their systems, including boiler and system efficiencies as well as potential savings both in equipment efficiencies and reductions in maintenance and downtime.”
Wide Range of Boiler Burners
Another Energy Solutions Center TMAF presenter was Pawel Mosiewicz from Industrial Combustion, a Cleaver-Brooks company. He described how his company’s line of natural and dual fuel burners cover the full range of commercial and industrial boiler markets and conserve energy through precise parallel-positioning fuel and air controls, high turndown ratios, and O2 trim to match varying fuel, air and environmental conditions. Mosiewicz also explained how modern burners can also conserve electric energy through the use of high-efficiency motors and variable frequency drives. These results are achieved while achieving NOX compliance.
Opportunities for Savings
Is it time to replace your boiler burner? At a minimum, if your burner is more than ten years old or doesn’t have the high-efficiency features of newer designs, it is appropriate to do an evaluation. Ask a qualified boiler engineer or energy efficiency expert for help in this stage. It could be that a comparatively smaller investment in a new burner could pay for itself in a few months. After that, it would begin paying you.