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)

1 p

2 vap

3 1 2

Q mC T

Q m H

Q Q Q

–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)

1 1 lm,1 2 2 lm,2Q U A T U A T

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…