Duct logic is the process of planning for duct thermal movement, combined with planning to minimize duct dust dropout.
Ducts move with changes in internal temperature. Ducts are assumed to have the same temperature as their internal gasses, which may be up to 900 °F. If the internal duct temperature exceeds 900 °F, refractory lining is used to minimize the duct surface temperature. At 900 °F, ducts may grow approximately 1/2 inch per 10 feet of length. This movement must be carefully planned for, with cloth (or metal) expansion joints at each equipment flange, and one joint per each straight section of ductwork.
Sloping ductwork at or above the duct dust angle of repose will minimize dust buildup. Therefore many ducts carrying high dust loads slope at 30 degrees, or higher.
Famous quotes containing the word logic:
“The logic of worldly success rests on a fallacy: the strange error that our perfection depends on the thoughts and opinions and applause of other men! A weird life it is, indeed, to be living always in somebody elses imagination, as if that were the only place in which one could at last become real!”
—Thomas Merton (19151968)