Stack diameter

4 Stack Diameter. The exit diameter of the stack is 5 ft. This is based on the exhaust volume of the gases and the desired exit velocity. 

Qeneral. Constraction., The stack is constructed o f reinforced concrete. There is a ladder extending the entire height of the stack. Light ning arresters are. .provided at the top of the stack. Aircraft obstruction warning lights are not provided. 

U) lane, J. A , Argon Activity Reactor Cooling Air, ORNL CF 49-l 1-180, November 17, 1949.  

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Table 2. Number of Stack Diameters from Flow Disturbance ...

Distance from stack, number of stack diameters... 

Aerodynamic Downwash Should the stack exit velocity be too low as compared with the speed of the crosswind, some of the effluent can be pulled downward by the low pressure on the lee side of the stack. This phenomenon, known as stack-tip downwash, can be minimized by keeping the exit velocity greater than the mean wind speed (i.e., typically twice the mean wind speed). Another way to minimize stack-tip downwash is to fit the top of the stack with a flat disc that extends for at least one stack diameter outward from the stack. 

Percent of stack diameter from inside wall to traverse point)... 

Stack diameter or (length + width)/2 (m) Number of velocity measurement points... 

Gas velocihes can also be measured with anemometers (rotating vane, hot wire, etc.), from visual observations such as the velocity of smoke puffs, or from mass balance data (knowing the fuel consumption rate, air/ fuel ratio, and stack diameter). 

Holland Ah = (1.5V,d -H 0.04QJ/U where Ah — plume rise (m), Vj = stack exit velocity (m/s), d = stack diameter (m), = heat emission rate (kcal/s), U = stack top wind speed (m/s) Highly empirical. Requires stack testing confirmation on case-bycase basis... 

For stack diameters up to 7.5 m the stack height is normally 15 m above grade. The bottom of the stack is elevated to allow air for combustion to enter. The minimum clearance between the bottom of the stack and grade is either 1.8 m or 0.3 D, whichever is greater. 

If W, > Wa, it will be necessary to modify the burner layout in order to provide more air flow area per jet. If necessary, the stack diameter must be... 

We shall now provide a second example to illustrate step-by-step calculations. In this example a flare stack is estimated to be 80% efficient in combusting HjS off-gas. The total off-gas through the stack is 400,000 kg/hr, of which 7.0 weight percent is H2S. The physieal stack height is 250 m, the stack diameter is 5.5 m, and the stack emission velocity is 18 m/s. The stack emission temperature is 15°C. The meteorological conditions may be described as a bright sunny day with a mean wind speed of 3 m/s. 

Sulfur dio.vide is being emitted from a stack at a rate of 50 sefs, and the prevailing wind velocity is 15 ft/s. The stack e. it gas velocity is 60 ft/s. The stack diameter is 3.5 feet. Estimate tlie physical stack height necessary to meet an air pollution standard of 5 ppiiiv nia.xinimii SO2 (30 min average concentration) GLC. 

In engineering terminology, the stack draft often is expressed in pounds per second, since this quantity is invariable for different atmospheric pressures and effluent densities. The effluent velocity (V2) varies considerably with such values, however, for any fixed discharge rate and stack diameter. 

The EPA Method 1 (see Appendix B for a complete list of EPA Methods) recommends that both sample and velocity measurements be made at least 8 internal diameters downstream of any flow disturbance (e.g., elbow, duct contraction, or expansion) and at least 2 internal diameters upstream from any flow disfurbance. If this is not possible, then measurements may be made as close as 2 internal diameters downstream and 0.5 internal diameters upstream of disturbances, but more measurements across the duct cross section are needed. Figure 7.5 shows the recommended minimum number of traverse (sampling) points for measuring either the gas sample composition or the gas velocity where no particulates are in the gas stream. The figure shows that at least 8 or 9 measurement points are recommended for internal stack diameters between 12 and 24 in. where the measurement location is at least 8 internal diameters downstream and 2 internal diameters upstream from any upsfream disturbances. Por internal stack diameters greater than 24 in. in internal diameter, at least 12 sampling points are recommended if fhe measuremenf location is af leasf 7... 

The minimum distance to a disturbance determines the minimum number of recommended fraverse points. For example, if the stack diameter is 18 in. in internal diameter, the gas stream contains no particulates, and the sample location is 10 internal diameters downstream from the closest downstream disturbance but only 1 internal diameter upstream from fhe closesf upsfream disturbance, then the distance to the upstream disturbance determines the minimum number of fraverses. According to Figure 7.5, only 8 or 9 fraverses would be needed based on fhe downsfream disfance (10 diameters) to a disfurbance buf af leasf 16 fraverses are recommended because fhe sample location is only 1 infernal diameter upstream from the location. 

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