Ashrae Journal - December 2008 - (Page 39) Boiler Size/Type 300 hp Firetube 600 hp Firetube 40,000 lb/h Watertube Annual Fuel Cost at 25% Capacity Factor Gas $325,000 $650,000 $1.3 million Oil $775,000 $1.55 million $3.1 million Boiler Cost $110,000 $155,000 $550,000 Equipment Capital Cost Burner Cost $15,000 $35,000 $70,000 Controls Cost $3,000 –1 0,000 $5,000 – 15,000 $7,000 – 50,000 Table 1: Boiler fuel costs vs. equipment costs. Reducing stack temperature and raising feedwater temperature are done at the Typical Loss as Boiler Loss Explanation Percentage of Boiler boiler level by specifying a high-efficienHeat Input cy boiler with an economizer. If a steam boiler does not have an economizer, it Energy Required to Heat Combustion Air Dry Gas Loss 8% – 12% can be retrofitted in the field. According From Ambient to Stack Temperature to Kenneth McKelvy of Babcock and Energy Required to Heat Water Formed Wilcox, the rule of thumb is every 40°F Hydrogen Loss From Combustion of Fuel From Ambient 4% – 7% to Stack Temperature (22°C)decrease in stack temperature equates to a 1% increase in boiler effiRadiation Loss Heat Lost to Boiler Surroundings 0.25% – 2% ciency. A typical economizer can increase Energy in Unburned Fuel or Carbon Monboiler efficiency by 4% to 6%. Unburned Carbon 0% – 0.5% oxide That Exits The Stack Using an air heater to raise the combustion air temperature is commonplace Energy Required to Heat Water in the Moisture in Air Combustion Air From Ambient To Stack 0.1% – 0.4% with utility boilers but is almost never Temperature done with packaged boilers. The necessary ductwork is expensive and can Table 2: Typical boiler losses. affect the low-NOx performance of the burner. Adding air preheaters to packaged boilers is usually Dry Gas Loss CP(Air) x (TS – TA) * (1 + Excess Air%) x MTheoretical Air = considered to be impractical. However, a little-used “trick of CP(H O) x (Enthalpy of Steam at 1 psi and TStack – the trade” called an air stack can increase boiler efficiency. An Hydrogen Loss = Enthalpy of Water at TAmbient)/100 air stack is simply ductwork run from the combustion air inlet CP(H O) x H2O in Air x (TStack – TAmbient) Moisture in Air = to the top of the boiler room, where the air may be as much as 20°F (11°C) warmer. This inexpensive, purely mechanical Value determined from table maintained by the Radiation Loss = American Boiler Manufacturers Association boiler modification will increase boiler efficiency in enclosed boiler rooms. Table 3: Boiler loss nomenclature. 2 2 Low Excess Air Burners Save Fuel One of the most cost-effective fuel-saving strategies for a packaged boiler is specifying/retrofitting to a low excess air burner. The typical standard burner offered by the boiler manufacturer has a guarantee of 15% to 20% excess air operation at high fire only. (5% excess air is approximately 1% stack oxygen.) Because ambient air is 79% inert (nitrogen, argon, and water vapor), it’s important to minimize the amount of fuel wasted heating mostly inert air. A high-performance burner will deliver 12.5% to 15% excess air performance from midfire to high fire. This lower excess air performance over a much larger boiler load will deliver significant fuel savings because less fuel will be wasted heating mostly inert combustion air. Many burners are not tuned in the field to meet published excess air guarantees, and many boiler owners do not hold the manufacturers responsible for achieving these guarantees— especially with low-NOx burners. When aggressive emission guarantees are made for NOx or carbon monoxide, excess air and burner turndown guarantees are sometimes sacrificed. December 2008 Boiler owners often cannot operate a boiler if the emissions are out of compliance. However, the only penalty for operating a boiler with higher than guaranteed excess air, or lower than guaranteed turndown, is additional fuel consumption. Although fuel consumption is an important consideration, it is less urgent than the threat of government regulators shutting down a boiler because of an air permit violation. Real-World Boiler Operation Even for burners that are operating within their guaranteed emissions, the technician doing the boiler tuneup should maintain an “excess air cushion” to accommodate higher ambient temperatures, variations in fuel pressure, and other unexpected changes that affect burner stoichiometry. To reduce the amount of “excess air cushion,” the new burner should be equipped with a combustion control system that can compensate for changes in ambient conditions (Figure 1). It is important to understand that boiler manufacturer efASHRAE Journal 39
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