An estimation of fan performance for leaky ventilation ducts

The common approach in matching a fan to a duty is to determine the intersection between the fan curve and a single line system characteristic. The single line characteristic denotes a leak-free duct but it is common knowledge that all ducts leak to some extent. This presentation demonstrates that a leaky duct has dual characteristics. One which compensates for the increases in fan pressure and quantity caused by the leakage (which is detected by the fan), and a 'shadow' characteristic which shows the quantity delivered to the discharge end. Atkinson's equation was used to demonstrate the sensitivity of a leak-free duct to changes in duct diameter, friction factor, length and flow quantity. The concept of 'duct efficiency' was introduced to quantify the effect of leakage on the performance of a ventilation duct and demonstrate the sensitivity of a leaky duct to same changes. A spreadsheet method, which has been verified using the National Coal Board (UK) leakage design charts, was used to quantify the leakage coefficients and to establish the dual characteristics for leaky ducts. These characteristics have then been applied to a case study to assess the performance of a fan connected to 500 in of a fabric duct in an operating decline. (C) 2003 Published by Elsevier Ltd.