Boilers Design and Components #1

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Boilers Design and Components #1

Boilers have two sections, the combustion section and the heat transfer section. The combustion section is the space in which the fuel-air mixture burns. Figure 10.1 shows a commercial boiler with the combustion chamber at the bottom. In this boiler, the base is insulated, but the top and sides of the combustion chamber are heat transfer surfaces. The proportion of air significantly influences the efficiency. If there is excess air, it is heated as it goes through the boiler, carrying heat with it up the chimney. Too little air will cause poor combustion, usually producing noxious combustions products and, in the extreme, may cause extra expense by allowing unburnt fuel through the boiler and up the chimney.

The second section of the boiler is the heat-transfer section. This section comprises the two upper spaces in the illustration shown below, where the hot gases pass right to- left and then left-to-right, before exiting to go up the flue. In large boilers, the heat transfer section will be fabricated of cast iron sections that are bolted together, or of welded steel plate and tubes. In smaller, particularly domestic, boilers, the heat-transfer section may be fabricated from copper, aluminum or stainless steel sheet. Boilers can be designed for any fuel: electricity, gas, oil, or coal are the most usual. In this age of recycling and sustainability, there is also an initiative to use urban and manufacturing waste as fuel.

In all boilers, there is a need to modulate, or adjust, the heat input. Gas and oil burners may be cycled “on” and “off.” The longer the “on” cycle, the greater the heat input. With the “on-off” cycle, the water temperature or steam output will vary up and down, particularly at low loads. This may not matter, but the efficiency improves and cycling effect is much reduced by having a burner with “high-low-off” cycles.

On larger units, a modulating burner will usually be installed that can adjust the output from 100% down to some minimum output. The burner modulation range is called the “turn-down ratio,” which is the ratio between full “on” and the lowest continuous operation. A burner that can operate at anywhere from 100% output down to 10% output has a 10:1 turn-down ratio. With a modulating burner, efficiency increases as the output drops. This increase in efficiency is due to the increase in the ratio of heat-exchanger surface-area to heat-input as the output, or firing rate, is reduced.

 

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