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Exploring the subtleties of Process Design ### Chemical and Process Engineering Resources     Category: Fluid Dynamics Question: How can I determine the choked or sonic flow in an orifice? Keywords: v1i17,choked,choke,sonic,flow,gas,orifice,calculate,determine,calculation Answer: Choked flow (also known as sonic flow) occurs when the ratio of the upstream gas pressure to the downstream gas pressure is equal to or greater than [(k + 1) / 2]k / ( k - 1) , where k is the specific heat ratio (Cp / Cv). For many gases, k ranges from about 1.1 to about 1.4, and so choked gas flow usually occurs when the upstream pressure is about twice the downstream pressure. The equation for choked flow is: Q = C A (g k d PU )1/2 [2 / (k + 1)](k +1) / (2k - 2) If the ratio of upstream to downstream gas pressure is less than [(k + 1) / 2]k / ( k - 1),then the flow is non-choked (i.e., sub-sonic) and the equation for non-choked flow is: Q = C A (2 g d PU)1/2 [k / (k - 1)]1/2 [(PD / PU)2/k - (PD / PU )(k + 1) / k ]1/2where: Q = mass flow, lb/s C = discharge coefficient (usually about 0.72) A = orifice hole area, ft2 g = 32.17 ft/s2 gravitational acceleration k = Cp / Cv = (specific heat at constant pressure) / (specific heat at constant volume) d = gas density, lb/ft3, at upstream pressure PU = upstream pressure, lbs/ft2 PD = downstream pressure, lbs/ft2

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