I put a spell on  you.....because you're mine!

("I Put a Spell on You," Screamin' Jay Hawkins, 1956)

 

No hocus-pocus about it, a "shell and tube heat exchanger" is often referred to as a "HEx."

Gotta admit...that nickname saves a lot of syllables!

This PTOA segment reviews the various PFD/P&ID symbols for HExes.

Not so long ago, PTOA Readers and Students learned about process industry equipment that is used to increase process temperatures (e.g., fired heaters, steam from package boilers, exothermic reactions).

Then PTOA Readers and Students were introduced to HExes and learned that HExes conserve energy; in the process of conserving energy one process stream becomes hotter while the other process stream becomes cooler.

The upcoming PTOA segments feature equipment that is used to decrease process temperatures.

The focus study on temperature-decreasing process equipment begins after the next segment that gives the answers to the DIY exercises.

 

HEx PFD and PID SYMBOLS

There are several kinds of HExes and therefore several kinds of HEx PFD and P&ID symbols.

Bottom Row: Generic HEx symbols 

The bottom row on the above heat exchanger symbol chart exhibits what are called "generic" HEx symbols; the function of the shell and tube are revealed but the actual structure of the HEx is not defined.

Generic HEx symbols are used when quickly drawing up a schematic or when the exact type of heat exchanger has yet to be determined in the processing plan.

 

The "Generic" PFD/PID HEx Heater Symbol

The generic HEx Heater symbol is on the left side of the bottom row.

A HEx Heater will have an arrow pointing downward drawn over the circle that represents the shell.

PTOA Readers and Students can find a generic HEx Heater symbol hidden in the PTOA logo!

 

 

The "Generic" PFD/PID HEx Condenser Symbol

A generic HEx Condenser symbol is shown to the right of the HEx Heater in the symbol chart ; the generic HEx Condenser will have an arrow pointing upward drawn over the circle that represents the shell. A phase change from vapor to liquid is ongoing in this exchanger.

Guess what?

The heat from the hot process stream entering a HEx Condenser is often exchanged with air.

The hot process stream leaves the HEx Condenser as a cool, condensed liquid and the air leaves warmer.

The P&ID symbol for a HEx Condenser is more like the symbol labelled "Air-Cooled Exchanger" (middle row of symbols, the second from top).

The more common process industry name for an "Air-Cooled Exchanger" is a "Fin-Fan Condenser." PTOA Readers and Student will learn more about Fin Fan Condensers very soon.

Air-Cooled Exchangers/Fin Fan Condensers are temperature-decreasing process industry equipment.

 

Kettle Reboiler Symbol (above generic HEx symbols)

PTOA Readers and Students are already familiar with the Kettle Type Reboiler symbol and Reboiler shape shown above the HEx PFD/PID symbol chart.

Kettle Type Reboilers were introduced in the PTOA Segment #23 that focussed on uses of stream.

The flow through the Kettle Type Reboiler is better defined in the symbol labelled "Reboiler" (far right column, just above the row of generic HEx symbols).

The arrows on the "Reboiler-Labelled" symbol are explained below and illustrated by the photo on the below right.

Reboiler Steam Flow:

The process arrows on the Reboiler-Labelled symbol infer that the hot fluid (steam) flows through the tube side inlet, makes two passes through the U-tube bundle, then exits the tube side outlet as condensate.

Reboiler Process Fluid Vapor and Product Flow:

The process fluid that is being reboiled enters the Kettle Reboiler via the shell inlet which is on the bottom.

Hot vapors are heated and return to the tower via the shell vapor outlet.

Hot liquid product accumulates to a level that is high enough to flow over a weir. The weir is that vertical line to the left of the tube bundle. Weirs are little dams used in process industry to manage liquid levels and flows.

The liquid flows over the weir and leaves through the product outlet, most likely heading to cooling via further heat exchange in more HExes, and finally to storage in the Tank Farm.

Ye Old Familiar 2 Pass-Tubeside (U-Tube) HEx

The symbol labelled "U-Tube Heat Exchanger" is the familiar 2-pass tube side type HEx that PTOA Readers and Students have already studied extensively and which is once again shown below.

Another example of a 2-pass tube side HEx is featured at the top of this page.

Can you find the big difference between the HEx at the top of this page and the animated HEx?  Try to figure it out before you continue reading.

Although both exchangers are 2-pass tube side designs, the shell side inlet and outlet positioning for each exchanger is different.

The differences in heat transfer that would be caused by changing the shell flow pattern are significant and will be the revisited in future PTOA Heat Transfer segments.

 

USE OF HEx SYMBOLS

PTOA Readers and Students will see the symbols in the above symbol chart appear in basic schematics like the one below.

A generic HEx Condenser is up at the top of the tower and the Kettle-Type Reboiler symbol is at the bottom of the column.

The reason for this positioning is intentional and will be revealed in future PTOA Segments on Separation Systems.

 

Take Home Messages: The verbal abbreviation for "shell and tube heat exchanger" is "HEx."

There are several  PFD and PID symbols for HExes. "Generic HEx symbols" relate the function of the HEx but do not detail which type of HEx is used.

Generic HEx symbols are used for quickly drawing up a diagram or in other cases when less information about the exact HEx design is needed.

The generic HEx symbol for Heater has a downward pointing arrow; the generic HEx symbol for Condenser has an upward pointing arrow.

Fin-Fan Condensers (aka Air Cooled Condensers) are a type of HEx used to cool down a process stream.

Reboilers are a type of HEx used to return hot vapors into the bottom of a tower and separate hot liquid product from the vapor.  Reboilers have weirs that help manage process liquid level and flow. Reboilers  often have two shell side outlets, one for hot vapors and one for liquid product.

©2015 PTOA Segment 00035
Process Industry Schematics

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