There are many variations in the design of the housing and tube. Typically, the end of each tube is connected to a plenum (sometimes referred to as a water tank) through a hole in the tube sheet. The tube can be straight or curved into a U shape, called a U-shaped tube.
In a nuclear power plant called a pressurized water reactor, a large heat exchanger called a steam generator is a two-phase shell-and-tube heat exchanger that typically has a U-shaped tube. They are used to boil water recovered from the surface condenser into steam to drive the turbine to generate electricity. Most shell and tube heat exchangers are 1, 2 or 4 channel designs on the tube side. This refers to the number of times the fluid in the tube passes through the fluid in the housing. In a single pass heat exchanger, fluid enters one end of each tube and exits the other end.
The surface condenser of a power plant is usually a 1-way straight tube heat exchanger (see surface condenser in the figure). Two and four pass designs are common because fluids can enter and exit on the same side. This makes construction much easier.
There is usually a baffle to guide the flow through the shell side so that the fluid does not short through the shell side, leaving an inefficient low flow. These are usually attached to the tube bundle rather than the housing so that the bundle is still removable for maintenance.
Counter current heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams. Many companies however do not use two pass heat exchangers with a u-tube because they can break easily in addition to being more expensive to build. Often multiple heat exchangers can be used to simulate the counter current flow of a single large exchanger.