Which statement best describes indirect contact heat exchangers compared to direct contact?

Which statement best describes indirect contact heat exchangers compared to direct contact?

• A. The fluids do not mix because a boundary keeps them separate
• B. The fluids mix and transfer mass
• C. They transfer heat by direct contact
• D. They rely on convection to transfer heat

Answer: (A)

Indirect contact heat exchangers keep the two fluids apart with a solid barrier, so the streams do not mix. Heat is transferred across that boundary, typically by conduction through the wall, while each fluid flows on its own side. Because there’s a boundary, there is no mass transfer between the streams, which is why the fluids remain separate. This is the defining difference from direct contact exchangers, where the fluids are allowed to come into contact and may exchange both heat and mass (changing composition or phase). The other statements describe direct contact or give an incomplete view of how heat moves in indirect systems, so they don’t capture the essential distinction.

Indirect contact heat exchangers keep the two fluids apart with a solid barrier, so the streams do not mix. Heat is transferred across that boundary, typically by conduction through the wall, while each fluid flows on its own side. Because there’s a boundary, there is no mass transfer between the streams, which is why the fluids remain separate. This is the defining difference from direct contact exchangers, where the fluids are allowed to come into contact and may exchange both heat and mass (changing composition or phase). The other statements describe direct contact or give an incomplete view of how heat moves in indirect systems, so they don’t capture the essential distinction.