Over and over again my love
Over and over again with you
Over and over again my love
Over and over again with you.

("Over and Over," Neil Young, 1990)

 

PTOA Segment 68: ALL TOGETHER NOW ... RAISE PROCESS TEMPS!

The previous PTOA Segment 67 had shown PTOA Readers and Students how the three types of heat transfer ... Radiation, Conduction, and Convection ... work together to heat a pot of soup up for lunch.

This PTOA Segment focussed on showing how the three modes of heat transfer work together at the industrial level.

Fired heaters, package boilers and reaction furnaces are three types of  temperature-increasing process industry equipment specifically built to control the combustion reaction. The combustion reaction creates the radiation heat energy that is required to eventually increase the temperature of flowing fluids.

PTOA Readers and Students had already been casually introduced to temperature-increasing process industry equipment; this PTOA Segment 68 clarified that the desired, invisible product of the combustion reaction is heat in the form of radiation.

PTOA Readers and Students learned how the radiated heat  increases the exterior temperature of the tubes that line fireboxes, boilers, and reaction furnaces.

PTOA Readers and Students learned that conduction heat transfer takes place through the physical barrier of the tube wall which separates the fire side of the tube from the cooler tube interior wall.

The cooler tube interior physically touches:

• the flowing process stream in a fired heater tube.
• the flowing bfw and water vapor in a package boiler riser, downcomer, or D tube.
• the flowing gas reaction feedstocks and products in a reaction furnace tube.

This PTOA Segment did not spell it out but rather expected PTOA Readers and Students to realize that the rate of convection heat transfer into all of the flowing fluids listed above depends upon:

• the mass flow rate of the flowing fluid
• the ability of the fluid to absorb heat.
• the Delta T between the inside wall of the tube and the temperature of the fluid flowing right next to the inside skin of the tube.

Nor was the dependence upon the flowing fluid to remove heat from the temperature changing equipment emphasized.

Any impediment to flow will result in disturbing what a text would call the 'flux of heat' through the equipment.

Temperature-increasing process equipment depends upon heat to smoothly transfer from the radiation source through the physical separation between hot and cold areas and into a flowing fluid that removes the heat from the temperature-increasing equipment.

Upcoming PTOA Segments discuss impediments to 'the flux of heat' that Process Operators must be aware of.

Two other important heat-related phenomena were introduced in this PTOA Segment:

1. Metals expand (get longer) when exposed to heat and contract (get shorter) when exposed to cold.

All process facilities are built to accommodate the expansion of vessels and pipes due to the generation of hot temperatures and the eventual contraction of the same hardware due to ambient conditions and planned shutdowns.

The expansion tendency of metals will be featured topic in a future PTOA Static Equipment and Piping Focus Study Area.

2. The focus on the three types of heat transfer in a package boiler revealed the crucial role that convection plays with respect to establishing flow through a package boiler.

PTOA Readers and Students learned in PTOA Segment 57 that Fluid Flow Transport rules require a  change in pressure to exist before a fluid can flow.

Furthermore the "pressure differential" must flow from an area of high pressure area to an area of  low pressure ... analogous to a  "temperature differential" that provides the driving force for heat to flow from an area of high temperature to an area of lower temperature.

The constantly changing phases of bfw into water vapor and back into bfw generates the "pressure differential" required for flow through a package boiler; water is more dense than water vapor and the difference in densities creates the required pressure differential.

A future PTOA Fluid Flow Transport Focus Study Area will delve into the subject of pressure differentials more thoroughly.

Congratulations are in order!

Upon completing PTOA Segment 68, PTOA Readers and Students  concluded the introduction to the three types of heat transfer.

Armed with their expertise in heat transfer types, their definitions, and defining characteristics, PTOA Readers and Students were now ready to learn about operational problems that occur when heat transfer through temperature-changing equipment is impeded.

 

PTOA Segment 69: PLAYING WITH FIRE

PTOA Segment 69 provided a bridge to the topic of Radiation Heat Transfer problems.

Before learning potential impediments to Radiation Heat Transfer, PTOA Readers and Students more thoroughly studied the combustion reaction.

PTOA Readers and Students learned that:

The combustion reaction is exothermic; the generation of heat is the invisible desired product of the combustion reaction.

The reactants/feedstock of the combustion reaction are a hydrocarbon fuel and oxygen (from air).

The reactants must be exposed to an ignition source to commence combustion. The common term for an 'ignition source' is a pilot.

After combustion commences, a chemical chain reaction sustains the combustion reaction to continue.

Removal of fuel, air, ignition source, or disturbing the chemical reaction (with a fire retardant) stops the combustion reaction.

Everything has an ignition temperature so everything is a potential fuel.

In the classroom setting, Your Mentor would emphasize that the above statement includes human beings; bombs dropped on Nagasaki and Hiroshima at the end of WWII tragically vaporized many citizens instantly and the intentional process of cremation at end of life uses elevated temperatures to reduce human tissue to ash.

Currently, simple hydrocarbons are used for combustion reaction fuel; examples are methane gas, propane gas, and butane (although prior vaporization is required). A mixture of light gases used for combustion fuel is called "fuel gas."

Much heavier hydrocarbons like diesel can be used to generate industrial combustion reaction; however the burner type would be significantly different than that used for light gases.

The greater the number of carbon atoms in the hydrocarbon fuel, the more oxygen from air is required for complete combustion.

The greater the number of carbon atoms in the fuel, the more heat, water, and carbon dioxide are generated when the combustion reaction goes to completion.

The burners inside of the firebox, package boiler, and reaction furnace are where the combustion reaction takes place.

Start up instructions for each type of temperature-changing equipment burner are specific to that equipment; new Outside Process Operators must pay heed to the on-the-job oral instruction and written procedures related to starting up and shutting down heaters, boilers, and reaction furnaces.

The typical hardware of a burner and complex piping for pilot gas,fuel gas, and purge gas were shown in this PTOA Segment 69.

All heaters require establishing draft so that the combustion products and flue gases can flow upward and out of the chimney stack.

Draft is controlled by the Outside Operator via damper adjustments.

Process Operators that understand the mechanics of the combustion reaction will be able to identify and mitigate two potential problems related to creating radiant heat: Flame Impingement and Flameout.

 

©2015 PTOA Segment 00083
PTOA Deja Vu Review 3-4

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