Pressurized water is often available at elevated temperatures and may be used for space heating or industrial process applications. In such cases it is customary to use a tube bundle in which the water is passed through the tubes, while air is passed in cross flow over the tubes. Consider a staggered arrangement for which the tube outside diameter is 16.4 mm and the longitudinal and transverse pitches are SL = 34.3 mm and ST-31.3 mm. There are seven rows of tubes in the airflow direction and eight tubes per row. Under typical operating conditions the cylinder surface temperature is at 70°C, while the air upstream temperature and velocity are 15°C and 6 m/s, respectively. Determine the air-side convection coefficient and the rate of heat transfer for the tube bundle. What is the air-side pressure drop? SOLUTION Known: Geometry and operating conditions of a tube bank. Find: 1. Air-side convection coefficient and heat rate. 2. Pressure drop Schematic: 34.3 mm Water tube D-16.4 mm Sr 31.3 mm T, = 70°C 7-7,-15°C 13 OO O O O O O Row 1 Assumptions: 1. Steady-state, incompressible flow conditions. 2. Negligible radiation effects 3. Negligible effect of change in air temperature across tube bank on air properties Properties: Table A4, air (To-15°C): ? = 1.217 kg/m3, Cp = 1007 J/kg . K u-14.82 x 10-6 m, s, k-o.0253 w/m . K Pr-o.710. Table A4, air (T-70°C): Pr- o.7o1. Table A.4, air (T- 43°C): 17.4 x 106m2/s, k-0.0274 W/m K, Pr 0.705.
Repeat Example 7.7 for a more compact tube bank in which the longitudinal and transverse pitches are SL = ST = 20.5 mm. All other conditions remain the same.