Oh say, can you see...***

...just by looking at the outside of a shell and tube heat exchanger if the hot process fluid is the one flowing into the tubeside (or wait a minute....is it shellside?) ...which means the cold process fluid must be flowing into the shellside (no...no... I meant tubeside)?

The hot or cold process fluid could be entering either the shellside or the tubeside of shell and tube heat exchanger.

There are a few rules about shell and tube heat exchanger design that help predict which type of fluid would be flowing through the shellside or tubeside. A future PTOA segment will list these helpful hints.

Fortunately, the Process Flow Diagram (PFD) and the Piping and Instrumentation Diagram (P&ID) are resources that can be relied upon to resolve which process fluid is a-coming and a-going through the shells and tubes of a shell and tube heat exchanger.

***("The Star-Spangled Banner," lyric by Francis Scott Key in 1814 and melody from an English drinking song written by John Stafford Smith).

 

SHELL AND TUBE HEAT EXCHANGER
ISA PFD SYMBOLS

The generic ISA symbol for a shell and tube heat exchanger is to the right.

• The circle of the symbol represents the shell side flow through the exchanger.
• The zig-zag line inside of the circle represents the tube side flow through the exchanger.

 

Analyzing a Shell and Tube Heat Exchanger Graphic

After reading just 30 PTOA segments, PTOA Readers and Students can expertly interpret the process flow graphic of the shell and tube heat exchanger shown below.

PTOA Readers and Students understand that a process fluid enters the tube side of the heat exchanger on the left and then exits on the right.

Is this process stream hotter or colder when it exits the exchanger?

To answer that question,  PTOA Readers and Students can observe that steam is flowing into and out of the exchanger shell.

PTOA Readers and Students already know that steam is used for indirect heating (and they know how the steam is made...in a package boiler).

Conclusion:

The thermal energy (heat) of the steam is being transferred into the cooler tube side process fluid.

The steam that flows through the shell side indirectly heats up the tube side flow and then exits the exchanger with less thermal energy; therefore, the process stream exits the tube side of the exchanger much warmer. 

 

TAKE ANOTHER LOOK
AT THE BENZENE PLANT PFD

Quick Review of Known Benzene Plant PFD Features

PTOA Readers and Students will recognize the below PFD of a benzene plant which was first presented in the focus on fired heaters.

The fired heater is H-101;  the PFD  label key at the top of the PFD identifies H-101 as the Feed Heater.

Process stream #4 is heated up in H-101 and then exits H-101 as the much hotter process stream #6 that flows into Reactor R-101.

A quick glance over the PFD reveals that six shell and tube heat exchangers are in the benzene plant.The exchangers are labelled E101 through E106 and they look like the ISA symbol at the right.

PTOA Readers and Students should find this process equipment in the PFD if they have not yet done so: H-101, R-101, and six shell and tube heat exchangers labelled E101, E102, E103, E104, E105, and E106.

Focus on Feed Preheater E-101

Before flowing into H-101, process Stream #4 flows out of shell and tube Exchanger 101 (E-101). The PFD label key at the top of the page identifies E-101 as the Feed Preheater.

Aha! The label is a big hint to PFD readers regarding the purpose of E-101:

E-101 preheats the feed to the reactor before the final heating step to reach a target process temperature occurs in the fired heater.

The combined feed entering E-101 is made of toluene oil and hydrogen gas.

The Toluene feedstream (Stream #2) is made from fresh Toluene (Stream #1) and recycled Toluene (Stream #11) from the bottoms of the Benzene Column (T-101) located way on the right side of the PFD.

These toluene streams are mixed together in V-101 (the Toluene Strorage Drum)  and then pumped to the exchanger via P101A or the backup pump, P102B (the Toluene Feed Pumps).

Go ahead and confirm the above statements by tracing out the lines. Take all the time you need. Your Mentor will wait until you are done.

Ready? Okay...

The Hydrogen feedstock is made from Stream #5 and fresh  Hydrogen (Stream #3...and this hydrogen was undoubtedly was made in a hydrogen plant).

Go ahead and confirm the above statements by tracing out the lines. Your Mentor will wait some more.

Okay! Well done!

PTOA Readers and Students should notice that the combined Toluene + Hydrogen feedstream flows into the shell side of E-101.

Since E-101 is labelled a Feed Preheater, the  shell side flow is entering E-101 colder and will exit hotter.

This PFD does not clarify the exact name of the hot process stream that flows into the tube side of E-101 (the zig-zag lines in the symbol). This hot fluid heats up the combined Toluene + Hydrogen feedstock that flows into the shell side of E-101.

There is a hint that the heating process stream comes from the "hps" (an unspecified "high pressure separator"). Whatever it is, this stream enters E-101 on the tube side hot and exits the exchanger tube bundle colder.

 

Focus on Reactor Effluent Cooler E-102

The reactor effluent (Stream #9) flows out of R-101 and into the shell side of E-102. The PFD label key identifies E-102 as the Reactor Effluent Cooler. This label is a big hint that the reactor effluent enters E-102 hotter and leaves cooler.

What process fluid is being used to exchange heat with the reactor effluent?

The process fluid that enters the tube side of E-102 is  "cooling water."  PFD readers know this because a "CW" is written on the drawing near either the E-102 tube side inlet or outlet (the PFD reader cannot distinguish the tube side inlet from the outlet in this PFD).

Cooling water is an auxiliary utility generated at processing facilities specifically to be used as a cooling fluid in shell and tube heat exchangers. 

The cooling water flows into the E-102 tube side cold and exits the E-102 tube bundle much warmer.

Great job!

PTOA Readers and Students have learned how to determine the duty of shell and tube heat exchangers while learning how to interpret a PFD!

 

Take Home Messages: The ISA PFD symbol for a shell and tube heat exchanger is a circle with a zig zag line through it. The circle represents the shell side of the exchanger; the zigzag line represents the tube bundle.

Hot process streams or cold process streams can enter the shell side or tube side of an exchanger.

PFDs and PIDs clarify which process streams will exit a shell and tube heat exchanger hotter or colder.

The following PFD/PID hints help decode which of the process streams in a shell and tube heat exchanger are getting hotter and colder: 

• • The name of the equipment found in the PFD label key.
  • Cooling water will always enter an exchanger cold and leave hotter.
  • Steam will always enter an exchanger as the hot stream and will exit the exchanger with less thermal energy (aka heat).

Tracing out process lines on a PFD takes a lot of concentration!

 

©2015 PTOA Segment 00031
Process Industry Schematics

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