It's repetition, I'm coming back to you
Repetition, the only thing I can do
Repetition and we can see this through
It's repetition, I'm coming back to you

("Repetition," by Paul Robb of Information Society, 1989)

 

PTOA Segment 65: GO WITH THE FLOW

PTOA Readers and Students learned the differences between convective heat that is transferred into vast air and/or water heat sinks versus convective heat that is transferred between process fluids that flow through temperature-changing equipment.

Fluids that flow through pipes are confined and do not quickly lose their contained thermal energy to their surroundings.

Furthermore:

The rate of convected heat that is transferred between flowing hot and cold process streams (q/t, measured in BTU/hr or J/hr) becomes relevant because process streams are constantly flowing through temperature-changing equipment and their flow rates can be intentionally adjusted by the Control Board Operator.

The Convection Heat Transfer Rate (q/t) was defined by dividing the definition of Convection Heat Transfer (q) by time (t);  Mass Flow Rate (m/t, in pounds/hr or kilograms/hr) was simultaneously defined by dividing mass (m) by time (t) on the right side of the Convection Heat Transfer expression.

The tweaked Convection Heat Transfer expression morphed into the following expression:

q/t (in Joules/hr or BTU/hr ) =

                       C_p          *   m/t        * (Delta T)

This PTOA Segment ended with an example that detailed how the Convection Heat Transfer Rate is determined between hot and cold process fluids that flowed through a shell and tube heat exchanger.

Shell and tube HEx E1, the lead exchanger featured in the Desalter Preheater Exchanger Train (refer to  PTOA Segment 32), was chosen for the calculation example. The constantly flowing hot Top Pump Around process stream transferred heat into the constantly flowing Crude Oil.

The factor-labelling method was used to reduce the mass flow rate term (m/t) step-wise into the correct units that were needed to complete the calculation.

 

PTOA Segment 66: THE MOTHER OF ALL HEAT TRANSFER

PTOA Readers and Students learned that the third type of heat transfer, Radiation, is the original source of all heat transfer.

Just as the the sun provides the original source of thermal energy for the earth, PTOA Readers and Students began to realize that the radiant energy produced via the combustion reaction that takes place in fired heaters, boilers, and reaction furnaces provides the original source of thermal energy at industrial processing plants.

PTOA Readers and Students reviewed that conductive heat transfer is characterized by heat transferred through a physical barrier that separates hot and cold regions.

PTOA Readers and Students reviewed that convective heat transfer is characterized by the transfer of thermal energy between hot and cold flowing fluids.

PTOA Readers and Students then learned that Radiation thermal energy is characterized by electromagnetic beams which travel in straight lines in all directions from the originating hot source.

PTOA Readers and Students observed that the mathematical expression that defined Radiation Heat Transfer was complicated and that there is no need to stress about understanding it thoroughly.

PTOA Readers and Students learned that Radiation Heat Transfer logically depends upon how hot the originating source is (T= Temperature, expressed in deg K).

The mathematical expression that defined Radiation Heat Transfer introduced an absolute temperature expressed in degrees Kelvin.  This is the first use of a temperature scale other than degrees Fahrenheit or Celsius/Centigrade in the PTOA.

PTOA Readers and Students learned that a second factor that impacted Radiation Heat Transfer was the amount of surface area (A) emitting from the originating source.

Lastly, PTOA Readers and Students learned that the heat of Radiation and Convection often interface; for example, the radiated heat from a fire will diffuse into waves of convection heat.

 

PTOA Segment 67: ALL TOGETHER NOW ... SOUP'S ON!

Using the everyday example of heating up a saucepan full of soup on a stove top burner, PTOA Readers and Students reviewed the definitions and mathematical expressions for conduction, convection, and radiation heat transfer and their application in a familiar real world situation.

PTOA Readers and Students learned that the design and fabrication of the stove top burners and cookware determined how well radiated thermal energy would interface with conduction heat transfer through the cookware.

The mathematical expression for Radiation Heat Transfer introduced in the previous PTOA Segment 66 (point source radiation) was discreetly tweaked into a form that better represents commercial use of Radiation Heat Transfer.

The revised definition for Radiation Heat Transfer added a third factor to the mathematical expression; the Greek symbol σ represented how proficient the material that the burner was made out of could reflect radiated beams.

PTOA Readers and Students discovered that the only parameter of  Radiation Heat Transfer they could adjust when heating up soup was the temperature of the burner (T) via the control knob.

PTOA Readers and Students could not change the surface area of the burner (A) nor the materials used for burner fabrication (σ). Both A and σ were determined during the design phase of the stove top.

PTOA Readers and Students also discovered their ability to impact conduction heat transfer while heating up soup was likewise diminished.  The surface area (A), the barrier thickness (d), and the conductivity factor (k) were decided during the design phase of the saucepan.

Once again, PTOA Readers and Students figured out that the only option to increase the rate of conduction heat transfer to heat up the soup faster would require increasing the rate of irradiated heat transfer ... which gets back to adjusting the burner heat control knob upward.

Lastly, PTOA Readers and Students observed that their ability to impact convection heat transfer was likewise limited because the mass of soup in the saucepan as well as the specific heat capacity of the soup was determined when the can of soup was loaded into the saucepan.

PTOA Readers and Students realized that the hot temperature of the Delta T that drives convection could be manipulated by the hungry soup maker by adjusting the burner control knob upward.

Adjusting the burner control upward would increase the intensity of irradiated heat which would then increase conduction heat transfer and thus increase the temperature of the interior side of the pan which contacts the liquid soup and initiates heat transfer via convection.

Alert PTOA Readers and Students should have noticed the pattern by now; even when heating up a can of soup ... heat transfer originates with radiation.

 

©2015 PTOA Segment 00082
PTOA Deja Vu Review 3-3

You need to login or register to bookmark/favorite this content.