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NORA Oilheat Technicians Manual

stabilize the flame by vaporizing the fuel droplets more quickly. Dry-base boilers require a combustion chamber made of an insulating material such as ceramic fiber to reduce the heat loss through the base of the boiler and to prevent burning out the base. In wet-base boilers the insulating properties of the chamber are less important because the surrounding boiler water recovers the heat. Heating surface or heat exchanger The heating surfaces of the boiler are exposed to the hot combustion gases on one side and to the boiler water on the other side. Heat is conducted through these boiler surfaces from the hot gases to the water. Larger surface areas give better heat transfer. Many surfaces are designed with contours, fins, pins or surface projections to increase the outer area and improve the gas-side contact. The heat transfer surfaces must be kept clean so that good heat exchange can take place. Soot deposits on the heating surfaces act as an insulator. Baffles or turbulators Baffles and turbulators are objects placed in heating passages to redirect the gas flow for better heat transfer. In older boilers, baffles were installed at the top of some combustion chambers to improve gas contact with the heating surfaces. Some older boilers were designed for coal burning with very wide passages for the hot gases. Installing baffles or firebricks in the center of these passages forces the flow toward the boiler walls for better heat transfer. Fire-tube boilers use turbulators to prevent the flow of hot gases up through the center of the tube. Some turbulators are long narrow strips of metal that are twisted into a spiral to give a spinning motion to the hot combustion gases. Turbulators should always be put back into the tubes after the boiler is cleaned. If they are damaged or badly corroded, they should be replaced with new ones. Insulation Boilers and furnaces have thermal insulation on the outside of the heat exchanger to reduce heat loss from hot surfaces. The outer jacket or casing must be securely fastened to minimize heat loss. Boiler ratings Cast-iron and steel boilers are tested to verify heating capacity and efficiency. The Hydronics Institute publishes boiler ratings. Listings provided by the Institute show the boiler’s Btu output and its Annual Fuel Utilization Efficiency (AFUE). The AFUE is calculated based on a testing procedure specified by the US Department of Energy. The Gross Output is the total heat delivery in Btus per hour that the boiler will deliver. The NET RATING deducts a “piping and pick-up factor” equal to 15 percent of the gross output for hot water boilers. This factor takes pipe heat loss and boiler warm-up time into account. This is important to assure that the boiler will deliver adequate heating at the coldest times of the year. The NET RATING should be used for selecting a boiler. Chapter 13 Heating Systems Chapter 13—Heating Systems 13-9


NORA Oilheat Technicians Manual
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