Hood Entry Coefficients of Compound Exhaust Hoods

Abstract

A traditional method for assessing the flow rate in ventilation systems is based on multiple readings of velocity or velocity pressure (VP) (usually 10 or 20 points) taken in ductwork sections located away from fittings (> seven × diameters of straight duct). This study seeks to eliminate the need for a multiple-point evaluation and replace it with a simplified method that requires only a single measurement of hood static pressure (SP_h) taken at a more accessible location (< three × diameters of straight duct from the hood entry). The SP_h method is widely used for the assessment of flow rate in simple hoods. However, industrial applications quite often use compound hoods that are regularly of the slot/plenum type. For these hoods, a “compound coefficient of entry” has not been published, which makes the use of the hood static pressure method unfeasible. This study proposes a model for the computation of a “compound coefficient of entry” and validates the use of this model to assess flow rate in two systems of well-defined geometry (multi-slotted/plenum and single-slotted/tapered or “fish-tail” types). When using a conservative value of the slot loss factor (1.78), the proposed model yielded an estimate of the volumetric flow rate within 10% of that provided by a more comprehensive method of assessment. The simplicity of the hood static pressure method makes it very desirable, even in the upper range of experimental error found in this study.

Keywords:

• compound hoods
• entry coefficients
• entry loss factors
• industrial ventilation
• local exhaust
• slotted/plenum hoods

ACKNOWLEDGMENTS

I would like to recognize the contribution to this study of Christopher Davis and Lacy Casteel, who were responsible for collecting the experimental data, and Pamela Osborn for her editorial support.