ASME STS-1:2001 pdf free download – Steel Stacks
1.2 General
The purpose of a stack is to vent process exhaust gases to the atmosphere. The mechanical design of stacks is now controlled in part by air pollution rules and regulations. Heights and diameters are set by a balance between structural stability and function, while at the same time meeting the requirements for air pollution control dispersion of the gases to the atmosphere. The heights of steel stacks have increased to satisfy ambient air quality, and stack inlet gas temperatures have decreased as more heat energy is recovered. The importance of attention to stack heat losses has therefore increased. Stack minimum metal temperature should be held above the acid dew point of the vented gases, if possible. Stacks are being designed with many appurtenances to monitor the gases and make stack inspections.
(a) Insulation is applied to outer surface of the stack or between the shells of a dual wall stack. A thickness is selected to reduce the stack heat loss to the desired level or to provide a maximum stack exterior surface temperature. Insulation should be selected for the maximum temperature to which it will be exposed. Insulation should be held to the stack shell as recommended by the insulation manufacturer for the job conditions. When thicknesses over 1 1 ⁄ 2 in. are used, two layers should be specified so that joints can be staggered. An appropriate outer surface weather protection should be specified for external applied insulation. Metal lagging should be secured with metal bands on maximum 24-in. centers.
(b) Stack linings are used for either heat loss reduction, as a protective coating, or both. A thickness is selected for the job conditions. Specify a service temperature range. Lining reinforcing and attachments to stack shell should be per manufacturer’s recommendation.
(c) Stack surface cladding, either internal or external, will affect heat loss and should be considered in heat loss calculations.
3.2.1 Temperature/Corrosion.
The metal surface temperatures of uninsulated, unlined steel stacks may fall below flue gas dew points within the stack or at the stack outlet. The most commonly quoted stack temperature is the flue temperature at the stack inlet. It is also the most misleading because it is the metal surface temperature that is of importance. Uninsulated unlined steel stacks can have metal surface temperatures 60% or more below the flue temperatures at the stack inlet, whereas stacks with external insulation often will have metal surface temperatures that are only slightly lower than the inlet flue gas temperature. Critical corrosion temperatures are not absolute values covering all situations but present focal points for more detailed study, i.e., if stack surface temperatures fall below acid condensation dew points, external insulation and/or higher flue gas velocities could correct the situation. External insulation can be used to maintain stack surface temperature at least 50°F (28°C) above the flue gas dew point. If metal temperatures are exceeded, internal linings may be used to provide a solution.