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Part 2: Boilers: The Biggest Users of Fuel
By James Piper, P.E.
March 2012 -
While chillers are the largest users of electricity in most institutional and commercial facilities, the single biggest user of combustible fuels is the heating boiler. The primary cause of energy waste related to boilers is improperly adjusted controls, particularly in large, central heating boilers.
Complete combustion of any fuel in any boiler requires the proper mixture of air and fuel. Too much air, and the operating efficiency of the boiler drops. Too little air, and the fuel will not be completely combusted, resulting in the generation of soot that clogs heat-transfer surfaces and reduce the boiler's efficiency. To ensure complete combustion, all boilers introduce more air than they actually require. While this excess air does reduce the boiler's efficiency, it also prevents the generation of soot.
Surveys of boilers show that most operate with excess air levels of 10 percent or more. But boilers that burn natural gas can operate effectively with as little as 3 percent excess air. Oil-fired boilers do require higher levels of excess air, but most can operate with levels at 10 percent or less.
Proper adjustment of the boiler's combustion controls will limit the quantity of excess air introduced into the boiler and, as a result, increase its seasonal operating efficiency.
Automatic blowdown controls also have a direct impact on a boiler's operating efficiency, particularly larger units. All boilers accumulate deposits within their circulating water.
If these deposits are not controlled and removed on a regular basis, they accumulate on heat-transfer surfaces, reducing the boiler's operating efficiency.
Automatic blowdown controls remove a portion of the circulating water and replace it with fresh, treated water. When coupled with a water-treatment system this process helps control the accumulation of deposits within the circulating water.
To maintain efficiency, it is important that boiler operators properly adjust the rate at which they remove and replace water. Too high a blowdown rate, and energy is lost through the discharged water. Too low a blowdown rate, and the concentration of solids and their resulting deposits is too high, causing scale to form.
Part 1: HVAC Energy Efficiency Strategies
Part 3: Motors: Impact on Energy Efficiency