Heat Exchanger Design What can we learn from considering the temperature profile over the length of the heat exchanger in addition to the end points?

Consider the Heat – T diagram: Draw T-Q profiles for cooling with changes in: Sensible heat only (Cp) Latent heat (condensation) Desuperheating+latent heat T Heat transferred (Q) or Distance along the exchanger (L) What are the equations for Q?

Expected profiles: Both profiles are linear No temperature cross Log mean temperature difference is fine for design T Heat transferred (Q) or Distance along the exchanger (L)

Another case: Same inlet and outlet temperatures as first case Temperature crossover! Log mean temperature difference is not accurate! T Heat transferred (Q) or Distance along the exchanger (L)

Now what? Partition. Calculate two temperature differences Avoid temperature crossover What about the U’s? T Heat transferred (Q) or Distance along the exchanger (L) I II Nominal values for heat transfer coefficients: gas cooling (small) liquid (bigger) condensing (very big)

What about a double pass HX? Same inlet and outlet T as in initial example Both profiles are linear Temperature cross when the fluid doubles back Log mean temperature difference must be corrected Now use F correction factors for geometry as discussed in example from last lecture. T Heat transferred (Q) or Distance along the exchanger (L)

Watch for: Changing conditions in the heat exchanger due to: Transitions (sensible heat to latent heat) Changes in composition (changes heating values!) Temperature crosses Changing U Is the use of a log mean temperature difference justified? Is partitioning of the exchanger required?

A final thought… If energy is conserved, we just rent energy as it flows through our process. If we can keep energy from crossing the boundaries of the process, we will reduce consumption…. ….$$.....reduced impact….

Need a visual tool… Plot heat duties in the process on two composite curves: Heating curve Cooling curve Note that heat duty can be moved from L to R, so organize by T profiles Where can process duties be integrated? Need at least 10 degrees C between streams T Heat transferred (Q) or Distance along the exchanger (L)

Example…