I heat up...I cool down (cool down)...
When something gets in my way I go around it.

("I Got The Music In Me," sung by Kiki Dee, 1974)

INTRO TO SHELL AND TUBE HEAT EXCHANGERS

PTOA Readers and Students are already very familiar with the shell and tube heat exchanger shown above.

Heck, PTOA Readers and Students have already analyzed a temperature control loop that worked with a shell and tube heat exchanger to control a process stream temperature. PTOA Readers and Students accomplished that feat in PTOA Segment #16 entitled "Ever Evolving ISA Symbols."

For a very good reason....saving money...the shell and tube heat exchanger would win the popularity contest as the Most Popular Piece of Processing Hardware found in any processing plant that consumes a lot of energy while turning feedstocks into desired products.

For every reactor there will be at least four kinds of shell and tube heat exchangers in the same process.  For example, the reboiler that was featured in PTOA Segment #23 that listed the uses of steam is a type of shell and tube heat exchanger.

This PTOA segment introduces the hardware and process stream flowpaths through the common two-pass shell and tube heat exchanger.

 

AN EXCHANGER ICON WORTH REMEMBERING

The graphic below is a nifty icon to keep in mind because it accurately portrays what's going on inside of a shell and tube heat exchanger. The hardware of the heat exchanger is represented by the gray square patch.

The hot, red process stream enters the top right of the icon and then flows through the exchanger. The hot process stream transfers heat until it becomes a cooler, blue process stream that exits the exchanger and then exits the icon in the top left corner.

At the same time, a cold blue process stream enters the icon in the bottom left corner and then flows through the heat exchanger.

Heat is transferred into this process stream until it becomes a hotter, red process stream that exits the exchanger and then exits the icon in the bottom right corner.

At no point do the two process streams directly mix; all the heat exchange is performed indirectly.

In summary...just like the lyric snippet above says:

One process stream heats up while another process stream cools down.

Hardware is intentionally put in the path of both process streams to enhance heat transfer by forcing the steams to either flow around or through it.

The below animated shell and tube heat exchanger graphic from Southern Heat Exchanger (SHECO) will help clarify the process stream flows through the tube side and shell side of a shell and tube heat exchanger.

 

SHELL AND TUBE FLOW PATH
AND HARDWARE NOMENCLATURE

In the real processing world, the SHECO animated shell and tube heat exchanger shown on the right would look like the real heat exchanger below.

 

Tube Side Flow Path

Assume that the yellow arrow is the colder of the two process streams that flow into the exchanger.

The colder, yellow process stream enters the bottom of the exchanger through the tube side flow inlet of the channel head and slams into a horizontal diverter plate which diverts the process flow through the lower half of a tube sheet.

The tube sheet  separates the process stream so that it flows through the separate tubes of a tube bundle.

Separating the process stream into several streams that flow through separate tubes greatly increases the surface area needed to optimize heat transfer (that's why the process flow through a fired heater is separated into separate heater tubes as well).

The yellow-arrow tubeside flow is heated while it flows the length of the exchanger tubes.

Then the yellow-arrow tube side flow makes a u-turn so that it can make another pass through the exchanger on the upper side of the tube bundle.

The process temperature of the yellow-arrow flow is much higher after completing two passes through the exchanger.

The now-much-warmer yellow process flow exits the tube bundle via the upper half of the tube sheet and then recombines in the upper part of the channel head before flowing out of the exchanger via the tube side flow outlet. 

A picture of a tube sheet and tube bundle that have been removed from the shell of an exchanger are below.

Three noteworthy things of the above picture are:

• The line at the horizontal diameter of the tube sheet shows where the diverter plate touches the tube sheet when the channel head is bolted onto the shell.
• The u-tube structure of the tube bundle is evident. The tube side flow through this exchanger makes two passes.
• The evenly spaced vertical baffles around the outside of the tube bundle force the shell side flow to swirl around the tube bundle which enhances the heat transfer between the shell side process stream and tube side process stream.

 

Shell Construction and Shell Side Flow Path

The shell of a shell and tube heat exchanger is not as interesting a piece of hardware to look at compared to the tube bundle; the shell looks like a large-diameter piece of pipe with one end closed off and has the shell inlet flange, outlet flange, and supports fabricated into it.

The photo to the left shows the original assembly of a tube bundle being inserted into a shell at the factory where this shell and tube heat exchanger was manufactured.

The simple construction of the shell compared to the more complex tube sheet and tube bundle is deceiving. The shell would be much harder to replace than a tube bundle.  Proper operation of shell and tube heat exchangers is the subject of future PTOA segments covering Stationary Equipment.

The flow path through the shell side of a shell and tube heat exchanger is as follows:

Assume that the red-arrow process stream is the hotter of the two process streams that flow into the exchanger.

The hotter, red-arrow  process stream enters the top of the exchanger through the shell side flow inlet and begins flowing through the exchanger's shell.

The vertical baffles that are attached to the outside of the tube bundle force the hotter, red-arrow shell side  process stream to swirl around the tubes in the tube bundle.

The enhanced contact of the hot process stream with the outside of the multiple tubes in the tube bundle indirectly transfers heat out of the hotter, red-arrow shell side process stream and into the multiple cooler, yellow-arrow tube side process streams.

After completing a single swirling pass, the red-arrow shell side flow exits the exchanger through the shell side flow outlet at a greatly reduced process temperature. 

Hey! This has been a lot of information! PTOA Readers and Students should take a break and then re-read this segment to fully understand the tube side and shell side flow paths through a shell and tube heat exchanger.

 

Take Home Messages: The purpose of the shell and tube heat exchanger is to cool down a process stream that is too hot by indirectly transferring its heat to a process stream that must be heated up to continue successful processing. Ergo, the shell and tube heat exchanger is both a temperature-increasing and temperature-decreasing piece of process equipment.

Process plants that require a lot of thermal energy to complete processing steps will have a wide variety of shell and tube heat exchangers.

The temperature of the exiting tube side flow will be significantly different than the temperature of the entering tube side flow.The tube side flow will flow through these parts of a shell and tube heat exchanger in the sequence listed:

• tube side flow inlet
• channel head inlet side
• horizontal diverter plate (aka horizontal baffle)
• tube sheet
• tube bundle
• channel head outlet side
• tube side flow outlet

The shell side flow will flow through the shell side inlet, be swirled around by the baffles attached to the tube bundle, and exit the shell side outlet with a significantly different temperature.

The efficiency and effectiveness of heat transfer is enhanced by increasing the contact area through which the indirect heat transfer takes place. Heat transfer in a shell and tube heat exchanger is enhanced by: 

• Separating the tube side process stream into multiple streams that flow through the multiple  tubes of a tube bundle.
• The placement of vertical baffles that cause the shell side flow to swirl around and make better contact with the outside of the tube bundle. 

 

©2015 PTOA Segment 00030
Process Industry Temperature Changing Equipment

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