The shell is closed at both ends by the tubesheets. These normally have a considerable thickness and are penetrated by the tubes. The tube-to-tubesheet joints must be hermetically sealed to avoid fluid leakage from one side of the tubesheet to the other. One possible joint method is the welded construction indicated in the detail of Fig. 6-1.
The tubesheets are bolted to the heads, which act as collectors and distributors of the fluid circulating inside the tubes. This fluid enters the exchanger through a head nozzle and goes into and through the tubes up to the opposite head, where it exits. The other fluid goes into the shell through an inlet nozzle, fills all the space surrounding the tubes, and moves toward the outlet nozzle, where it exits. So both fluids are separated by the tubes wall, which constitute the heat transfer area . If N is the number of tubes of the heat exchanger, L is the length of the tubes, and Do is the external diameter of the tubes, the heat transfer area will be