National Oilheat Research Alliance 9 The spray pattern of the oil droplets must match the air pattern created by the burner. Flame retention burners use high speed burner motors and air pattern shaping to create the high static air pressure required to form the high velocity air swirl and internal recirculation needed for clean, efficient combustion. Heat exchangers also need time, temperature and turbulence. The more time the combustion gases spend in the heat exchanger, the more heat will be transferred to the living space. The hotter the temperature of the gases, the quicker the heat transfer will take place, and the more turbulent the flow of the gases through the exchanger, the more they will sweep the walls of the heat exchanger, providing better heat transfer. Excess Air heat loss is a function of the temperature and volume of the combustion gases. As the volume of combustion gases increases, the speed at which they travel through the heat exchanger increases. The faster they travel, the less time the heat exchanger has to strip heat from the gases. The more excess air we add to the flame–the more combustion gases will flow through the heat exchanger. To do this requires the gases to flow quickly, reducing the time they are in contact with the heat exchanger. As a result, more hot gases go up the flue raising the net stack temperature and losing more energy. This lowers the steady state efficiency. All the excess air is taking heat that could be used in the house and sending it up the chimney. Air that enters the burner at 65°F from the building leaves the top of the chimney at anywhere from 200 to 600°F. The less air up the chimney, then, the better the efficiency. That is why sealing up all excess air leaks into the heat exchanger and proper air adjustment of the burner are so important for improving efficiency. It is also why draft regulators need to be properly adjusted. Only -.01 to -.02"wc over the fire draft is needed—just enough to slowly move the combustion gases through the heat exchanger. Over fire draft higher than -.02"wc dramatically increases both on and off-cycle losses. In some buildings, the chimney suction is so strong, two draft regulators are required to get the over fire draft down to -.02"wc. Draft regulators are a compromise. It is true that air is being pulled from the building up the chimney, but that is much better than pulling the gases through the heat exchanger too quickly. A little lost air at room temperature is a lot better than losing 300 to 600 degree combustion gases. Figure 6. Figure 6 Heat Transfer in a Boiler or Furnace Heat to Building Cool Return from Building
NORA Advanced Efficiency
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