(This article courtesy of Webster Combustion)
Obviously, the topic of fuel savings in commercial boilers is not a new one. However, there are some sharp differences in the way “savings” are presented to potential end users when this topic is discussed or used as a selling point. There are those who talk only about fuel efficiency and those who look beyond fuel to achieve total energy savings.
Some boiler burner manufacturers tout high efficiency from fuel savings achieved through high turndown combined with low excess air. And in cases where the process, or boiler load, requires large swings then high turndown is very important. The less cycling (turning off and on) your boiler has to do, then the more money you will save and the lower your maintenance costs will be.
And is low excess air important? Of course it is! High excess air means you are wasting money heating up all that “excess” air. Reducing excess air across your entire turndown range, down to as low as 10% of the firing rate, sounds like a great plan. Where do we sign up? But there are trade-offs in achieving this objective.
Usually, the only way to achieve low excess air across a high turndown range, 10 to 1 for example, is to have a high pressure drop across the burner combustion head. And the only way to achieve that high pressure drop is to use a larger combustion air fan and motor. With these requirements comes the tradeoff, as using the larger fan and motor increases both the initial cost of the equipment and the operating cost that comes in the form of higher electrical usage.
Consulting engineers responsible for specifying high efficiency boiler burners should be aware that fuel efficiency is only a small part of the whole story. In fact, a much greater savings can be achieved through reduction of motor horsepower to achieve the same burner firing rate. While the excess air reduction isn’t quite as high, the total energy savings resulting from O2 and motor horsepower reduction is significantly greater.
Let’s look at the story of two burners, Burner A and Burner B applied to a 600HP scotch-marine boiler. Both burners have a 10 to 1 turndown ratio to reduce cycling, but have the following operating characteristics:
Burner A maintains 3% O2 levels from 100% down to 10% of the firing rate using a 25HP blower motor.
Burner B maintains 3% O2 levels from 100% down to 20% of the firing rate using a 15HP blower motor.
So, the only difference in fuel savings between the two burners is at the very lowest firing rates, below 20% of the high fire rate. Therefore, the fuel savings for Burner A only occur when the burner is firing at 10% of the high fire rate. Even if we make the unrealistic assumption that Burner A fires at the 10% rate twenty-four hours per day, three-hundred sixty-five days per year, the fuel savings would only be roughly $350 per year. Obviously, the burner will not run at this low firing rate for such a long period, so this savings figure is drastically over-stated.
On the other hand, because Burner B is using a much smaller motor, the electrical savings over the same time period and operating parameters would equate to approximately $7,000. So the Total Energy Savings is approximately $6,650 in favor of Burner B, and that number does not include the higher initial cost of Burner A, or the fact that Burner A will not run at the 10% firing rate all day, every day, and therefore will likely not achieve even the $350 in fuel savings.
Not all burner manufacturers offer a high turndown, low motor horsepower burner series. One such manufacturer achieves this by using a high-swirl firing head technology with low pressure drop across the burner combustion head, coupled with a highly efficient in-line combustion air fan to provide superior mixing of the fuel and air and lower its motor horsepower requirements. This combination allows the burner to operate with low excess air across a large operating range. Less excess air means improved fuel efficiency, and high efficiency combined with high turndown reduces heat losses in the boiler that can result from cycling when a burner is stopping and starting. This type of burner will provide fast payback and the best possible return on its investment by virtue of total energy savings, not just fuel savings.
All applications can benefit from burners that help improve total energy savings. In particular, hospitals, schools and universities are especially well suited and represent the types of facilities that often depend on limited funding and, or must make the most of their energy budgets. Professional equipment representatives and manufacturers can provide payback analysis and a comparison of the proposed combustion equipment based on O2 levels, turndown capabilities, and motor HP.