Let's work together ...
Come on, come on ...
Let's work together

("Let's Work Together," by Canned Heat, 1970)

 

RADIATION, CONDUCTION, & CONVECTION HEAT TRANSFER
IN THE PROCESS INDUSTRIES

PTOA Readers and Students recently learned how

• Radiation
• Conduction
• and Convection

are all needed to work together when heating up soup for lunch.

The exact same heat transfer modes required to heat soup up for lunch are also required to attain target process temperatures in temperature-increasing process industry equipment.

Understanding the principles of heat transfer in process industry is important.

Process Operators that possess a fundamental understanding of how heat transfer works will be more able to recognize when the temperature-changing equipment is working optimally or needs an adjustment.

Process Operators that understand heat transfer will also understand why specific materials are chosen in the design and manufacture of temperature-changing equipment.

More importantly, they will understand the limits of the materials of construction which will result in a more safe operating environment.

The temperature-changing equipment used for cooling or swapping heat will rely upon just two of the three forms of heat transfer to get the job done, Conduction and Convection.

The Mother of All Heat Transfer, Radiation, will be present whenever and wherever an industrial process requires a target temperature to be attained and maintained for processing success.

The industrial equipment that incorporates all three types of heat transfer for the purpose of attaining a process temperature are:

 

 

• Fired Heaters.
• Reaction Furnaces.
• Package Boilers.

 

FIRED HEATERS, REACTION FURNACES, AND BOILERS

 

Fired Heaters

A graphic comparing three Fired Heater designs is featured at the top of this PTOA segment.

Note that all designs have a specified areas in which predominantly Radiation Heat Transfer and Convection Heat Transfer occur.

The photo to the left shows the inside of an operating  "Cabin Style" Fired Heater.

The view is the same that an Outside Process Operator would have while performing his/her rounds.

Cabin Style Fired Heaters have rows of horizontally hanging tubes on their opposite walls.

Whichever style of Fired Heater, radiant heat from the burners is conducted through the heater tubes.

The fluid that flows through the tubes is heated via convection to the target process temperature.

Voila!

Just like heating up hot soup for lunch in a saucepan, the process fluid that flows through the tubes in the firebox is heated to a target temperature via the three methods of heat transfer.

 

Reaction Furnace

A Reaction Heater/Furnace like the one shown in the graphic below likewise incorporates all three methods of heat transfer to perform its intended industrial process function.

In a Reaction Heater the burners are situated at the top of the firebox and the powerful flames blast downward.

PTOA Readers and Students that are reading the PTOA Segments in sequential order as intended already know that the tubes of a Reaction Heater are stuffed with catalyst.

The gas feedstock/reactants that flow through the tubes are chemically changed by the catalyst.

The process temperature required to generate syngas exceed 2000 °F.

Radiant Heat is conducted through the tubes and provides the required high reaction temperature via convection to the flowing gas feedstock/reactants.

All Process Operators must be aware that the term "process technology" includes understanding why the materials of fabrication were chosen to build process equipment as well as the operating limits of the materials chosen for equipment fabrication.

The Reaction Heater operates at such an extreme process temperature that it provides an excellent study to tie the principles of heat transfer to the selection of materials used to manufacture the tubes.

The Reaction Heater tubes must withstand extreme process temperatures around 2800 °F (1538 °C) .

Does your wood house make weird cracking noises during the winter when it gets really cold and again when the ambient temperature warms back up?

The groaning is explained by the contraction and expansion of the wood when the ambient temperature drops down into an extremely cold range and then warms up again.

All materials expand when heated and contract when cold.

The materials that Reaction Heater tubes are made from must have a low 'linear thermal expansion tendency' because, at 2800 °F, the room to accommodate a large expansion tendency would require a very tall heater ... and Reaction Heaters are tall enough because the reaction tubes inside must be long enough to provide enough time for the reaction to occur!

The Reaction Heater tubes must also have a decent capability to conduct heat at extremely high temperatures.

PTOA Readers and Students already know that the capability to conduct heat is characterized as "k," the thermal conductivity of the metal that the tubes are made of.

Lastly, the material that the Reaction Heater tubes are made of must exist in quantities that make it economically feasible to fabricate the tubes.

Wow! There are a lot of factors to consider when choosing materials to build temperature-increasing equipment that operates continually in extreme temperature ranges.

The below process is used to develop the shortlist of possible heater tube materials and deduce the optimal choice:

1. The ability of a material to withstand very high process temperatures for long periods of time and have a decent conductivity ability means the material must be a metal.
2. The requirement to exist in commercial quantities and not expand too much at high heat leads to the choice of titanium to fabricate Reaction Heater tubes.

Titanium costs seventeen times as much as the carbon steel tubes used in the typical process fluid fired heater. Replacing titanium reaction tubes on an unplanned emergency basis is a cost Plant Owners want to avoid.

Process Operators that understand how heat transfer equipment works will be more knowledgeable with regard to operating  an energy intensive process unit such as a Reaction Heater ... and greater awareness results in avoiding unplanned shutdowns.

 

Package Boilers

PTOA Readers and Students are very familiar with the form and function of a Package Boiler shown in the cutaway to the right.

The flames from the burner provide Radiant Heat that is conducted through the risers, downcomers and D tubes that connect the mud drum and steam drum.

The Radiant Heat that is conducted through the tubes is transferred by convection into the BFW that is flowing through them.

The increase in temperature initiates a phase change from water into saturated steam (water vapor).

Just like boiling water in a covered pot or heating soup in a saucepan, the heat transferred into the flowing BFW causes the water particles to become agitated and separate from each other.

The vapor that is created will change back into the water phase once it moves further away from the firebox and into the steam drum.

The convection currents that result in the boiler are created by the changing density between the heavier water phase and the lighter vapor phase.

These changes in density provide the pressure differential that the Universe requires to exist before a fluid  ... like steam and BFW ... can flow.

 

TAKE HOME MESSAGES: All three types of heat transfer will be required when a process requires attaining and sustaining a target process temperature.

Radiation, the Mother of All Heat Transfer, is required to create the thermal energy that is subsequently transferred by conduction and convection.

The three pieces of Process Industry Equipment that use all 3 types of heat transfer are:

• Fired Heaters
• Reaction Furnaces/Reaction Heaters
• Package Boilers (but not Waste Heat Boilers!)

 

©2015 PTOA Segment 00068
PTOA Heat Transfer Focus Study Area

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