<P> Heat exchangers with only one phase (liquid or gas) on each side can be called one - phase or single - phase heat exchangers . Two - phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called condensers), with the phase change usually occurring on the shell side . Boilers in steam engine locomotives are typically large, usually cylindrically - shaped shell - and - tube heat exchangers . In large power plants with steam - driven turbines, shell - and - tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which is recycled back to be turned into steam in the steam generator . </P> <P> There can be many variations on the shell and tube design . Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets . The tubes may be straight or bent in the shape of a U, called U-tubes . </P> <P> 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 . </P> <P> Surface condensers in power plants are often 1 - pass straight - tube heat exchangers (see surface condenser for diagram). Two and four pass designs are common because the fluid can enter and exit on the same side . This makes construction much simpler . </P>

Number of tubes in shell and tube heat exchanger