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Debottlenecking Shell and Tube Heat Exchangers

<p>Your shell and tube heat exchanger has served you well.  But maybe your heat transfer needs have grown and you&#39;re just not getting enough heating, cooling, condensing, or evaporation these days.  Don&#39;t send your shell and tube heat exchanger to scrap pile just yet, it may only need a partner to help out.</p>

Compact heat exchangers can be an excellent option to debottleneck shell and tube heat exchangers of all types and sizes.  This is especially true when space is limited.  

Why Are Compact Heat Exchangers a Good Choice for Debottlenecking?

Plate heat exchangers are fundamentally different from shell and tube heat exchangers in that they allow for is known as a "temperature cross". Figure 1 will help explain:

Attached Image: temp-cross.gif
Figure 1: Comparing Shell and Tube with Plate Heat Exchangers

As you can see on the left, a typical shell and tube duty shows no "temperature cross". The outlet of the cold side generally has a 8-10 °F "approach" to the hot side outlet temperature. Also, notice the typical service U-value (or overall heat transfer coefficient). On the right, a typical plate heat exchanger duty is shown. Notice the deep temperature cross which allows the cold side outlet temperature to get close to the hot side inlet temperature. A plate heat exchanger can perform what would take 4-6 shell and tube units in series, in a single unit. Also, notice the much higher service U-values that are possible with a plate heat exchanger.

Plate heat exchangers are able to perform these temperature crosses thanks to their purely countercurrent flow:

Figure 2: Plate Heat Exchanger Flow Pattern

So, let's consider an example. Let's say that you have a shell and tube acting as a boiler down heat recovery exchanger. Currently the shell and tube takes 200 GPM of blow down and cools it from 200 to 160 °F by heating the incoming boiler feed water (150 GPM) from 60 to 113 °F. Installing a plate heat exchanger downstream of the shell and tube unit, could further heat the incoming feed water from 113 to 150 °F using the 160 °F blow down water. That an additional 2.8 MBtu/h of heat recovery in a relatively small heat exchanger that may look something like this:

Attached Image: phe.gif
Figure 3: Approximate Size of Plate Heat Exchanger Required for Example

Other Examples of Debottlenecking

Other examples of using plate heat exchangers to debottleneck other processes include the use of a welded plate heat exchanger to further cool a hot oil stream (Figure 4), using a plate evaporator as a booster for a calandria style evaporator (Figure 5), and installing a plate condenser to either remove a bottleneck or as a secondary condenser on existing units (Figure 6).

Attached Image: shell-tube-cp-side-by-side.gif
Figure 4: Hot Oil Cooler Debottlenecking
Attached Image: alfa-vap-boost.gif
Figure 5: Boosting an Existing Evaporator
Attached Image: alfacond.gif
Figure 6: Plate Condenser for More Capacity

So, if you have a heat exchanger bottleneck in your plant or building, consider one of these methods to give your old workhorse some help. There are many creative ways to add heat transfer capacity to an existing duty while still saving floor space and your existing equipment.