CHE2163-Heat and Mass Transfer 2013 Heat and Mass Transfer
Heat Exchangers
1. (Incropera 11.17) A concentric tube heat exchanger of length L=2 m is used to thermally process a pharmaceutical product flowing at a mean velocity of um,c=0.1 m s−1 with an inlet temperature of Tc,i = 20◦C. The inner tube of diam- eter Di=10 mm is thin walled, and the exterior of the outer tube (Do = 20 mm) is well insulated. Water flows in the annular region between the tubes at a mean velocity um.h = 0.2 m s−1 with and inlet temperature Th,i = 60◦C. Properties of the pharmaceutical product are ν = 10 × 10−6 m2 s−1, κ = 0.25 W m−1K−1, ρ = 1100 kg m−3 and Cp = 2460 J kg−1K−1. Evaluate water properties at T ̄h = 50◦C.
(a) Determine the value of the overall heat transfer coefficient U.
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(b) Determine the mean outlet temperature of the pharmaceutical product when
the exchanger operates in the counterflow mode.
(c) Determine the mean outlet temperature of the pharmaceutical produce when the exchanger operates in the parallel-flow mode.
2. (Incropera 11.27) In a diary operation, milk at a flow rate of 25o L h−1 and a cow-body temperature of 38.6◦C must be chilled to a safet-to-store temperature of 13◦C or less. Ground water at 10◦C is available at a flow rate of 0.72 m3h−1. The density and specific heat of the milk are 1030 kg m−3 and 3860 J kg−1K−1 respectively.
(a) Determine the US product of a counterflow hear exchanger required for the chillin process. Determine the length of the exchanger if the inner pipe ha a 50-mm diameter and the overall heat transfer coefficient U = 1000 W m−2 K−1.
(b) Determine the outlet temperature of the water.
3. (Incropera 11.64) A concentric tube heat exchanger uses water, which is available at 15◦C, to cool ethylene glycol from 100 to 60◦C. The water and glycol flow rates are each 0.5 kg s−1. What are the maximum possible heat transfer rate and effectiveness of the exchanger? Which is preferred, a parallel-flow or counterflow mode of operation based on required area?
4. (Extension problem:Incropera 11.56) Saturated process steam at 1 atm is con- densed in a shell-and-tue heat exchanger (one-shell, two tube passes). Cooling water enters the tubes at 15◦C with an average velocity of 3.5 m s−1. The tubes are thin walled and made of copper with a diameter of 14 mm and length of 0.5 m. The convective heat transfer coefficient for condensation on the outer surface of the tubes is 21800 W m−2K−1.
(a) Find the number of tubes/pass required to condense 2.3 kg s−1 of steam.
(b) Find the outlet water temperature.
(c) Find the maximum possible condensation rate that could be achieved with this heat exchanger using the same water flow rate and inlet temperature.
1. a) 135 W m−2 K−1 (NuDi=5.96, NuDh=22.22, check entrance effects for internal tube), b) Tc,o = 33◦C (ε = 0.323, Cr = 0.11), c) Tc,o = 31.2◦C (ε = 0.322).
2. a) UA = 785 W K−1 (ε = 0.895,CR = 0.330,NTU = 2.842) b) Tc,o = 18.4◦C.
3. a) 1.13×105W, ε = 0.47, b) Tc,o = 40.4◦C, c)ACF/APF = 0.79 (NTUPF = 0.95,NTUCF = 0.75).
4. a) ≈ 189 (ReD = 50933,NuD = 286.5,U = 7900 W m−2 K−1,ε = 0.144,NTU = 0.155) , b) Tc,o = 27.2◦C , c) 16 kg s−1.
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