Ashrae Journal - December 2008 - 43

Burner Turndown Affects Efficiency Burner turndown is important for fuel savings—especially during lowload conditions. The typical burner is designed for 6:1 turndown f iring natural gas and 4:1 turndown firing oil (6:1 turndown means high-fire fuel flow is six times low-fire fuel flow). In practice, many burners operate at 3:1 turndown—meaning they light off at 33% of high-fire heat input. If 33% firing rate produces more steam (or hot water) than the plant load, the burner eventually shuts down on high steam pressure or high water temperature, and all the losses associated with post purge, standby and purge are incurred again. Frequent boiler cycling also introduces thermal shock to the boiler tubes and refractory to shorten boiler life. Because boilers tend to be oversized (typically 5% for every engineer who touches the design), it is common for boilers to cycle on and off during lowload conditions. Each time a boiler cycles off, it drafts cold combustion air during the post purge and standby periods. When the boiler is cycled on again, it must go through a purge period when more cold air is cycled through the boiler to purge the furnace of possible combustibles prior to ignition. All the heat lost to this cold air has to be recouped by the burner when it lights off again. It is not uncommon to see small boilers cycle up to 10 times per hour during low-load periods. A high-performance burner will operate safely at 10:1 turndown on gas and 8:1 turndown on oil. A turndown ratio of 10:1 means low-fire heat input is just 10% of high-fire heat input. A highturndown burner is much more likely to stay on during low-load conditions and not incur all the cycling losses of a typical burner. To determine boiler turndown, put the boiler in manual firing rate, lock it at high fire, and clock the fuel meter. Repeat the procedure with the burner at low fire. Divide the high-fire fuel flow by the low-fire fuel flow. If the burner doesn’t have a dedicated gas or oil meter, Bernoulli’s equation provides a way to approximate burner turndown when firDecember 2008 ing gas. Simply measure the gas manifold pressure (downstream of the gas flow control valve) at high fire, divide by the gas manifold pressure at low fire, and calculate the square root. The number that is left is the burner turndown. Multiple Boiler Plants Planning on going green? Finally, multiple-boiler plants can save additional fuel by installing a modern lead/lag controller. A typical system monitors one header pressure or temperature transmitter, operates the optimal number of boilers to meet the current plant load, and fires the boilers at their most efficient firing rates to minimize fuel consumption. When fuel prices were relatively inexpensive, it was common to have one boiler running in automatic and a second boiler running at low fire in case the first boiler tripped offline. A modern lead/lag controller will keep the lag boiler off, cycle it on occasionally to keep it hot, and only bring it online if the lead boiler trips offline or is not capable of handling the current plant load. Because steam and noncondensing hot water boilers are more efficient at high fire, a lead/lag controller will fire fewer boilers at the highest firing rate possible for highest efficiency. Condensing boilers are more efficient at low fire, so this strategy is reversed for condensing boilers. With the high fuel prices in the U.S., high-performance burners and controls are inexpensive compared with the price of fuel wasted by the typical boiler. Boiler owners who previously installed highperformance burners and combustion controls now can reap the benefits. Because of fuel price volatility, the old adage should apply: “Hope for the best, but plan for the worst.” Bibliography Dukelow, S.G. 1991. The Control of Boilers, pp. 74–85. Research Triangle Park: Instrument Society of America. Stultz, S.C. and J.B. Kitto. 1992. Steam. Its Generation and Use, 40th Edition, pp. 9 – 22 to 9 – 27. Lynchburg, Va.: The Babcock & Wilcox Company. White, F.M. 1986. Fluid Mechanics, 2nd edition p. 169. N.Y.: McGraw-Hill Higher Education. AERCO helps facilities become LEED certified. At AERCO, we recognize the importance of the LEED certification system and green building design and operation. In fact, as an industry leader and member of the US Green Building Council, we have written a white paper to help engineers and facility managers understand how AERCO products complement LEED-NC projects. For your FREE copy of our white paper and more details, visit www.aerco.com/leed1 Heat You Can Bank On Copyright 2008 AERCO International, Inc. www.info.hotims.com/16020-2 ASHRAE Journal 43

Ashrae Journal - December 2008

Table of Contents for the Digital Edition of Ashrae Journal - December 2008