Chapter 7 shell tube heat exchanger

CHAPTER SHELL TUBE HEAT EXCHANGERS Technical requirements • Overall heat transfer coefficient • Pressure drop • Heat transfer area • Operating under temperature and pressure design • Structure and leakage Fluid selection • Maximum of density, heat capacity, thermal conductivity, latent heat • Melting point, boiling point, phase are suitable to operation condition • Minimum viscosity • Flammability, corrosion, hazard, purity Fluid arrangment Gas flow Gas flow Mixed – Unmixed flow Unmixed – Unmixed flow Fluid velocity High velocity, high number, high heat transfer coefficient also, high pressure drop as well Fluids Appropriate velocity Low viscosity liquids (water, alcohol…) 0.5 ÷ 3.0 High viscosity liquids (oil, glycol, glycerine…) 0.2 ÷ 1.0 Flue gas ÷ 10 Air 12 ÷ 16 Compressed air 15 ÷ 30 Saturated steam 30 ÷ 50 Superheated steam 30 ÷ 75 ⁄ Shell and Tube Configuration Fluid in outer tube (1 pass) Fluid in inner tube (1 pass) Fluid in inner tube (1 pass) Fluid in outer tube (1 pass) Double pipe heat exchanger Shell-and-Tube Heat Exchangers Shell-and-Tube Heat Exchangers Configuration Rear header Fluid in shell (1 pass) Fluid in tube (2 passes) Shell Front header Tube bundle Codes and Standards • Boiler and Pressure Vessel Code (ASME) Section VIII: Confined Pressure Vessles Section II: Materials Section V: Non Destructive Testing • British Master Pressure Vessel Standard (BS 5500) Data collection • Design pressure = = and temperature + 1.72 + 14℃ Tube bundle vibration • High flow rate and pressure drop • Avoid the cross flow • Promote longitudinal flow Testing • American Institute of Chemical Engineers’ Standard Testing Procedure for Heat Exchanger DESIGN CONSIDERATIONS Heat transfer Pressure drop Physical size Cost Example Water at the rate of 68 ⁄ is heated from 68℃ to 75℃ by an oil having a specific heat of 1.9 ⁄ ℃ The fluids are used in a counterflow double pipe heat exchanger, and the oil enters the exchanger at 110℃ and leaves at 75℃ The overall heat transfer coefficient is 320 ⁄ ℃ Calculate the heat exchanger area Example Instead of the double pipe heat exchanger of Example 1, it is desired to use a shell–and–tube exchanger with the water making one shell pass and the oil making two tube passes Calculate the area required for this exchanger, assuming that the overall heat transfer coefficient remains at 320 ⁄ ℃ Example Water at the rate of 30000 ⁄ℎ is heated from 100℉ to 130℉ in a shell–and–tube heat exchanger On the shell side one pass is used with water as the heating fluid, 15000 ⁄ℎ entering the exchanger at 200℉ The overall heat transfer coefficient is ⁄ℎ 250 ℉, and the average water velocity in the diameter tubes is 1.2 ⁄ Because of space limitations, the tube length must not be longer than Calculate the number of tube passes, the number of tubes per pass, and the length of tubes, consistent with this restriction Example A heat exchanger like that shown as Figure is used to heat an oil in the tubes c = 1.9 ⁄ ℃ from 15℃ to 85℃ Blowing across the outside of the tubes is steam that enters at 130℃ and leaves at 110℃ with a mass flow of 5.2 ⁄ The overall heat transfer coefficient is 275 ⁄ ℃ and for steam is 1.86 ⁄ ℃ Calculate the surface area of the heat exchanger Gas flow Example Investigate the heat transfer performance of the exchanger in Example if the oil flow rate is reduced in half while the steam flow remains the same Assume remains constant at 275 ⁄ ℃ Example Complete the Example using the effectiveness method Example The heat exchanger of Example is used for heating water as described in the example Using the same entering–fluid temperatures, calculate the exit ⁄ water temperature when only 40 of water is heated but the same quantity of oil is used Also calculate the total heat transfer under these new conditions Example CHAPTER SHELL TUBE HEAT EXCHANGERS