Consider a heat exchanger consisting of 12.5 mm OD copper tubes in a staggered arrangement with transverse spacing 25 mm, longitudinal spacing 30 mm, and 9 tubes in the longitudinal direction. Condensing steam at 150°C flows inside the tubes. The heat exchanger is used to heat a stream of air flowing at 5 m/s from 20°C to 32°C. What is the average heat transfer coefficient and pressure drop for the tube bank?
GIVEN
Staggered copper tube bank with condensing steam inside tubes and air flowing over the outside
Tube outside diameter (D) = 12.5 mm = 0.0125 m
Transverse spacing (ST) = 25 mm = 0.025 m
Longitudinal spacing (SL) = 30 mm = 0.03 m
Number of rows of tubes (N) = 9
Steam temperature (Ts) = 150°C
Air velocity (Us) = 5 m/s
Air temperature: Ta,in = 20°C Ta,out = 32°C
FIND
(a) The average heat transfer coefficient (hc)
(b) The pressure drop (?p)
ASSUMPTIONS
Steady state
Thermal resistance of the condensing steam and the copper tube is negligible. Therefore, the tube
surface temperature = Ts
SKETCH
PROPERTIES AND CONSTANTS
Density (?) = 1.157 kg/m3
Thermal conductivity (k) = 0.0251 W/(m K)
Kinematic viscosity (?) = 15.9 × 10–6 m2/s
Prandtl number (Pr) = 0.71
Specific heat (c) = 1012 J/(kg K)
At the tube temperature of 150°C: Prs = 0.71
Therefore, the minimum free flow area is between adjacent tubes in a row, and the maximum air
velocity is
The Reynolds number is
(a) Applying Equation
However, this Nusselt is applicable only to tube banks of ten or more rows. Since there are only 9
rows in this case, the average Nusselt number can be estimated by multiplying the above result by 0.99
as
(b) The pressure drop is given
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