An existing heat exchanger in the feed line to a reactor has mechanically malfun

Question

An existing heat exchanger in the feed line to a reactor has mechanically malfunctioned and must be replaced as quickly as possible to minimize plant down?time while a repair is made. You, as the section process engineer, have been given the job of determining whether a heat exchanger that is presently idle in the plant can be substituted for the malfunctioning unit and still meet the design specifications. Assume that the process stream has the physical properties of water. The stream at 100,000 lbs/hr enters the exchanger at 80F and is supposed to exit at 200F. The idle heat exchanger under consideration contains 100 tubes of 3/8

Explanation / Answer

A1 = A2 pi*di^2/4 = pi*Di^2/4 - pi*(di+2t)^2/4 di^2 = Di^2 - (di+2t)^2 20^2 = Di^2 - (24)^2 Di = 31.241 mm Thi = 70 deg. C Tci = 20 deg. C Energy balance, m_dot*c*(Thi - Tho) = m_dot*c*(Tco-Tci) (Thi - Tho) = (Tco-Tci) Reynolds number, Re = ?VD/µ Solve for diameter. Use hydraulic diameter definition, dh = 4A/p, p = wetted perimeter dh for inside pipe dh = 4*(pi*di^2/4) / pi*D dh = di Dh for outside pipe Evaluate properties at T = 45 deg. C. Dh = 4A/p Dh = 4*(pi*Di^2/4 - pi*(di+2t)^2/4) / [pi*Di + pi*(di+2t)], Do = di+2t Dh = (Di^2 - Do^2) / [Di + Do] Dh = Di - Do Reynolds for both cases can be solved, evaluate properties at T = 45 deg. C Dittus-Boelter formula, Nu_D = 0.023*Re^(4/5)*Pr^n n = 0.4 for heating and n = 0.3 for cooling Find coefficient of convection using Nusselt definition, Nu_D = hD/k h = NU_d*k / D Again use diameter, D, as Hydraulic diameter, Dh(outer pipe) and dh (inside pipe). U = 1 / [(1/hi + di/2k*ln(Do/di) + 1/ho)]

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