In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two-phase, shell-and-tube heat exchangers which typically have U-tubes. They are used to boil water recycled from a surface condenser into steam to drive a turbine to produce power. Most shell-and-tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes in one end of each tube and out the other. 
In Offshore Industries, Water Makers are made up of U tube Heat Exchangers. To Maintain the Lower Pressure Drop in High Tube Length. 

A tube bundle heat exchanger with baffles is disclosed wherein a tube bundle is housed within an inner casing which is acted upon from both the inside and the outside by a heating medium

A multiple tube bundle heat exchanger includes axially opposite tube plates, a housing, and a plurality of tube bundles disposed between the tube plates. Each of the tube bundles is an independent tube bundle subassembly forming an integral unit having at least one heat exchanger tube with two axial tube ends.

In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two-phase, shell-and-tube heat exchangers which typically have U-tubes. They are used to boil water recycled from a surface condenser into steam to drive a turbine to produce power. Most shell-and-tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes in one end of each tube and out the other. 
In Offshore Industries, Water Makers are made up of U tube Heat Exchangers. To Maintain the Lower Pressure Drop in High Tube Length. 

A tube bundle heat exchanger with baffles is disclosed wherein a tube bundle is housed within an inner casing which is acted upon from both the inside and the outside by a heating medium

A multiple tube bundle heat exchanger includes axially opposite tube plates, a housing, and a plurality of tube bundles disposed between the tube plates. Each of the tube bundles is an independent tube bundle subassembly forming an integral unit having at least one heat exchanger tube with two axial tube ends.