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Ideally, the fan in any combustion process will supply just enough air to completely burn all the fuel, and no more. This will help keep heated, but unused, air from going up the stack. Actually, this idea is approachable with gas burners but impractical with wood- or coal-fired combustion. Thus, nearly all air volume requirements for combustion processes are calculated to include some margin of excess air. Insufficient air volume will result in wasted fuel and excessive particulate along with potentially explosive gases in the exhaust system. From this viewpoint it is better to include excess air volume.
The combustion of coal and most fuel oils will release sulfur fumes into the flue gas. If a wet scrubbing or cleaning apparatus is used, water vapor will combine with the sulfur to form sulfuric acid. This can place severe constraints on the fan types available to handle this highly corrosive gas stream. For this very reason, flue-gas-desulfurization equipment is designed into the pollution control systems of many combustion processes. Another alternative to reduce the potential for sulfuric acid in the exhaust system is to mix lime or crushed limestone in a fluidized bed combustion process so the lime will neutralize the sulfur and stabilize the pH of the exhaust gases.
The amount of air required for theoretically perfect combustion is based on the portion of the combustible substances carbon (C), hydrogen (H2), oxygen (O2), and sulfur (S) contained in fuel. These are the only combustibles found in common fuels. In practice, the combustion system designer should determine the actual air volume requirements and the excess air margin based on an analysis of the fuel in question. The fan selection for the application should be capable of meeting both conditions with good efficiency, economy, and stability. Whenever possible, the actual condition should represent the most efficient point of operation for the fan selected for the application.
To reduce volume and pressure to meet the actual design or reduced load conditions, inlet dampers or variable frequency drives are used. Variable speed offers the most efficient means of performance reduction, although the initial cost and equipment maintenance is greater than that of dampers. These must be evaluated on an individual job basis to determine whether the power savings will offset the greater initial price differential and added maintenance costs. The criteria for selecting the fan motor is usually specified per job. Often, the motor is sized to handle the hot operating conditions so the fan can be dampered for low load periods such as start-up or shut-down. This reduces the dampering, or turndown, range required under actual conditions.
Fans used in combustion processes, whether forced or induced draft, should be capable of meeting the following minimum requirements:
1) The fan pressure curves should be stable throughout the entire operating range of the system. Certain fans, such as most radials, pressure blowers, and airfoil fns, are stable from wide-open to completely closed-off to offer the broadest possible control range.
2) The fan and all its components should be designed to meet even the test block condition without passing through the first critical frequency of the rotating parts. A common specification calls for the fan shaft’s first critical speed to be 125% of the maximum operating speed.
3) The entire fan assembly should be rugged to withstand industrial service. Catalogs or drawings should contain complete material specifications.
4) Whenever possible the entire fan, motor, and drive assembly should be factory assembled, aligned, and test run to ensure smooth operation. The fan manufacturer should be capable of test running complete assemblies.
Induced-draft fans have further special requirements:
5) Where fan airstream temperature exceeds 300°F., the fan should include a shaft cooler and the bearing base should be separated from the fan housing.
6) The fan should be selected to handle the maximum particulate loading.