A numerical analysis of flow maldistribution and shell side flow on heat in a shell and tube heat exchanger is presented. The flow field at the inlet and in the headers was obtained by solving conservation equations of mass and momentum by employing k-ε turbulence model. As the flow maldistribution in the header affects the heat transfer performance of the STHE, pressure drop and velocity distribution of the fluid inside the header were analyzed. Two types of headers were considered with varying header length for a Reynolds number range of 1000 to 3000. As the header length was increased to 1500 mm the flow maldistribution decreased and the static pressure was almost equal for all the tubes in case of a conical header. Also, the numerical simulations show that the conical header with 1500 mm header length has less flow maldistribution when compared to other models. The Shell side flow was modeled as a flow along a twisted tube with a diameter D and a length 30D using Catia V519. Four different models of the twisted tube with pitch varying between 4D and 5.5D were studied for a range of Reynolds number Re = 75-750. The analysis was carried out for three different wall temperatures of the twisted tube such as 343 K, 363 K and 383 K. The pressure drop increased with increase in Reynolds number, while the pressure drop and outlet fluid temperature increased with decrease in the pitch of the tube. But the convective heat transfer decreased with reduction in pitch. With a decrease in pitch, the energy transfer between the fluid and the adjacent tubes increases resulting in increased outlet fluid temperature.