Carbon dioxide gas at 1 atmosphere pressure it to be heated from 25°C to 75°C by pumping it through a tube bank at a velocity of 4 m/s. The tubes are heated by steam condensing within them at 200°C. The tubes have 10 mm OD, are in an in–line arrangement, have a longitudinal spacing of 15 mm and a transverse spacing of 17 mm. If 13 tube rows are required, what is the average heat transfer coefficient and what is the pressure drop of the carbon dioxide?
GIVEN
• In-line tube bank: condensing steam inside, CO2 outside
• CO2 temperatures
? In: (Tg,in) = 25°C
? Out: (Tg,out) = 75°C
• CO2 velocity (Us) = 4 m/s
• Steam temperature (Ts) = 200°C
• Tube outside diameter (D) = 10 mm = 0.01 m
• Longitudinal spacing (SL) = 15 mm = 0.015 m
• Transverse spacing (ST) = 17 mm = 0.017 m
• Number of tubes (N) = 13
FIND (a) The average heat transfer coefficient (hc) (b) The CO2 pressure drop (?p)
ASSUMPTIONS
• Steady state
• The thermal resistances of the condensing steam and the tube walls are negligible (tube wall temperature = Ts)
SKETCH
The maximum CO2 velocity is
The Reynolds number is
(a) The Nusselt number for this geometry is obtained from
(No correction is needed since N > 10)
(b) From
The pressure drop coefficient (f) is contained in
For SL/D = 1.5 and ReD = 104, from f/x ? 0.5
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