A multi-tube heat exchanger is used in a process plant to pre-heat air before it enters a combustion chamber, using low pressure steam that flows inside the tubes and condenses. The tube bundle is configured with 6-cm outer diameter tubes in a staggered arrangement as shown in the accompanying figure. The condensing steam in the tubes maintains their outer surface temperature at 117°C, and air at 60° flows across the tube bank with a free stream velocity of 1.0 m/s. Determine the average heat transfer coefficient for air.

Question

GIVEN

• Air flow through the tube bank shown

• Air temperature (Ta) = 60°C

• Air velocity (Us) = 1 m/s

• Tube outside diameter (D) = 6 cm = 0.06 m

• Tube wall temperature (Tw) = 117°C FIND

• The average heat transfer coefficient ( ch ) ASSUMPTIONS

• Steady state

PROPERTIES AND CONSTANTS

Thermal conductivity (k) = 0.0279 W/(m

K) Kinematic viscosity (?) = 19.4 × 10–6 m2/s

Prandtl number (Pr) = 0.71 At Tw: Prs = 0.71

Answer 1

Therefore, the minimum free flow area is between adjacent tubes in a row, and the maximum air

velocity is



The Reynolds number is



Therefore, is applicable



However, this Nusselt is applicable only to tube banks of ten or more rows. Since there are only two

rows in this case, the average Nusselt number can be estimated by multiplying the above result by 0.75

as



Where f is calculated

For ST/SL=1.34 and ReD=7515 from the figure we get x=1

For ST/D= 7.5/6=1.22 and ReD=7515

f/x=0.91

Thus f =0.91