CH-2439 (RP-136) -- Effect of Vent Stack Height and Exit Velocity on Exhaust Gas Dilution PDF

The purpose of the present investigation was to determine if a simple design procedure could be developed to predict the reduction in concentration at a receptor point which would occur if either the vent stack height or its exit velocity were changed. The emphasis was on the development of a simple theory rather than attempting more general and accurate formulations which would apply to a wide variety of situations. This emphasis on simplicity is justified because the variability of the dilution process around a building introduces levels of uncertainty of two or more in predicting concentrations and any theory of stack height effects which improves on this uncertainty is an exercise in self-delusion on the part of the designer.

The results of the experiments and the simple theory will demonstrate that both the stack height and the vertical exit velocity can be combined to determine an effective stack height. This is an important concept because it allows a designer to determine a trade-off between increasing the physical height of the stack or increasing the vertical velocity from the stack.

Because the fan power necessary to exhaust a given quantity of vent gas is proportional to the square of the vent gas velocity the key question which must be answered is whether there is any significant reduction in roof receptor concentrations for reasonable vent velocities which do not require excessive fan power. If large values of exit velocity are required the necessary fan power may make it much more attractive to simply raise the height of the stack.

The results of the study reported here are part of ASHRAE Research Project 136 which considered the dilution of vented gases near buildings. Other aspects of this study are discussed by Wilson (1,2,3)