Ashrae Journal - December 2008 - (Page 38) Understanding Fuel Savings In the Boiler Room Photo 1: A boiler room at Southern Connecticut State University. The control cabinets feature multiple loop controllers, vertical draft indicator, and a tank gage and leak detection system. By David Eoff T he crude oil peak price of $147 per barrel in July 2008 demonstrated that fuel oil prices going forward will be volatile and subject to extreme price spikes. Boiler owners and plant engineers need to understand boiler efficiency and what can be done to save fuel in the typical boiler room. Table 1 illustrates the amount of fuel a typical small boiler consumes, compared with the capital cost of the boiler, burner, and controls. The data in Table 1 are based on natural gas at $10 per MMBtu and heating oil at $3.63 per gallon ($0.97 per L). It becomes apparent that when operated at a 25% capacity factor, a boiler will consume at least twice its capital cost in fuel every year. An oil-fired boiler will consume at least four times its cost in fuel every year. Because of their relatively low capital cost and high impact on boiler efficiency, burner and combustion control retrofits offer boiler owners the quickest paybacks based on energy savings. 38 ASHRAE Journal Understanding Boiler Heat Losses To better understand the effect of burner and controls performance on efficiency, we need to have an understanding of boiler heat losses. Using typical packaged boiler operating data, boiler efficiency calculated according to the American Society of Mechanical Engineers (ASME) PTC 4.1, Power Test Codes for Steam Generating Units, yields typical boiler losses (Table 2). ASME PTC 4.1 includes instructions for calculating boiler efficiency by the direct method comparing steam output versus heat input. It also calculates boiler efficiency by the indirect (by losses) method where individual boiler losses are ashrae.org calculated and totaled. The “by losses” method requires only stack temperature and oxygen content and can easily be calculated with a portable combustion analyzer or built into the combustion controls. ANSI Standard Z21.13 outlines efficiency calculations for hot water boilers. The loss mechanisms are similar. Table 3 details the equations used to calculate three of the largest boiler losses to illustrate how these losses can be affected by boiler/burner performance. The three equations in Table 3 for different boiler losses have several elements in common, suggesting a common strategy for reducing boiler losses, including: • Reducing stack temperature; • Minimizing excess air levels; • Raising boiler feedwater temperature; and • Raising combustion air temperature to the burner. About the Author David Eoff is national sales manager at Preferred Utilities, Danbury, Conn. December 2008 http://www.ashrae.org
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