TWO - PHASE FLOW : Heat transfer involving two phases such as liquid-gas is of vital importance especially in petrochemical and allied industries. The presence of a gas phase along with liquid alters not only the fluid dynamic behavior but also the heat transfer characteristics namely, the heat transfer coefficient. Two-phase flow is considered to be the common class of multiphase flow. The phases commonly involved are liquids, gases and solids and their combinations such as gas-liquid, gas-solid, liquid-liquid and solid-liquid. SHELL & TUBE HEAT EXCHANGER : The most common type of heat exchanger used in an industry contains a number of parallel tubes enclosed in a shell called as shell and tube heat exchanger. These heat exchangers are employed when a process requires large quantities of fluid to be heated or cooled. Due to their compact design these heat exchangers contain a large heat transfer area and also provide a high degree of heat transfer efficiency. Heat exchanger is an essential unit in heat extraction and recovery systems with an increasing emphasis on energy savings.