Repeating, Repeating
I'm burnt out, burnt out on everything
Repeating over again
Burn out, burnt out on everything
Will this end?

("Repeating, Repeating," The Juliana Theory, 2003)

PTOA Segment 73: NOT TOO MUCH ... NOT TOO LITTLE ... JUST RIGHT!

PTOA Segments 70 and 72 had introduced PTOA Readers and Students to problems that result when insufficient oxygen (from air) is available to complete the combustion reaction in fired heaters, package boilers, and reaction furnaces.

Oxygen-deficiency combustion problems include flame impingement, soot buildup, emissions of the global warming gas carbon monoxide, and the potential of blowing up a firebox due to flameout.

Nobody would fault PTOA Readers and Students to logically conclude that opening up on air registers to flood the firebox with plenty of excess combustion air would remedy all the combustion-related problems.

This PTOA Segment 73 explained the problems that too much combustion air causes. PTOA Readers and Students learned:

All fuel gas contains a tidge of sulfur; the sulfur combines with excess oxygen to form the air pollutant sulfur dioxide. Sulfur dioxide is an acid gas that showers down on earth as acid rain.

Sulfur dioxide is also a lung irritant.

Industrial combustion is accountable for 69% of the sulfur dioxide detected in air.

Air is composed of 21% oxygen and 78% nitrogen.

Excess air means there is plenty of excess nitrogen to form a class of air pollutants called nitrogen oxides.

Nitrogen oxides are lung irritants that also contribute to acid rain.

Furthermore, some nitrogen oxides react with sunlight and airborne light hydrocarbons to form ground-level ozone.

Ground-level ozone is the bad kind of ozone, not the ozone layer that protects the earth from the sun's radiant energy.

Ground-level ozone contributes to the formation of smog.

Industrial combustion is accountable for almost a third of nitrogen oxides (NOx)detected in air.

In the USA, the Environmental Protection Agency (EPA) requires the use of Low-NOx burners to reduce the formation of nitrogen oxides.

Control Board and Outside Process Operators are on the front line with respect to keeping the air quality in their neighborhood as clean as possible.

This PTOA Segment 73 also detailed the difference between the terms "combustion products" and "flue gases" which had been used in many previous PTOA Segments.

The desired product of the completed combustion reaction is heat; the byproducts of the completed combustion reaction are carbon dioxide and water vapor.

Flue gases will also include gaseous components made of sulfur and oxygen (SOx), oxides of nitrogen (NOx), and carbon monoxide (CO is formed when the fuel gas is does not complete the combustion reaction).

PTOA Readers and Students also learned that too much combustion air results in significantly inefficient heater operations:

Heaters that are operated with too much excess air will have a "flip-flopped" temperature profile; the temperatures in the upper radiant and convection sections will be warmer than the temperatures in the lower parts of the firebox.

The temperatures become flip-flopped because the volume of air that whooshes up into the air registers is too much for combustion and does not have sufficient time to warm up in the radiant section before it is drafted upwards into the upper radiant and convection sections.

The convection section is designed to provide 30% of the total heat transfer into the flowing process fluid.

The "flip-flopped temperature profile" means that the heat transfer duty within the convection section unsustainably exceeds 30% and the radiant section is not being used effectively.

Thirty cents of every dollar spent at processing facilities is used to create and maintain target temperatures.

Twenty cents of that thirty cents (66%) is spent on fuel to attain target process temperatures. Plant Owners expect Process Operators to optimize the oxygen needed for the combustion reaction which means:

Not too much ... not too little ... just the right amount of oxygen (from air).

In the USA, Excess Oxygen Analyzers are located in the stack and their readings are brought into the control panel in the Control Room.Control Board Operators use Excess O₂ intelligence to control the primary source of combustion air.

An "Analyzer Reading" is the fifth type of Process Variable mentioned in the PTOA (the main four process variables are Temperature, Pressure, Level, and Flow).

ISA uses the tag letter "A" to denote devices used in analyzing control loop.

When Outside Process Operators open peepholes to check for hot spots and flame impingement, they temporarily increase the air content (hence, oxygen content) in the firebox.

Outside Process Operators also impact combustion air volume when changes are made on the air register of a burner.

Air can also enter through any unintentional opening or leak in the heater structure. Inability to control Excess O₂ and/or strange, unexplained flip-flops in the temperature profile through the firebox would hint that a firebox has a structural leak.

 

PTOA Segment 74: HERE ... THERE ... AND EVERYWHERE

PTOA Segment 74 showed PTOA Readers and Students how the management of heat transfer is prevalent throughout each processing facility and not just limited to temperature-changing equipment.

As stated above, thirty cents of every processing dollar is spent creating and maintaining the target temperatures that are needed to convert feedstocks into desired products.

Pipes that are insulated reduce the rate of conductive heat transfer from the flowing thermal energy contained within a pipe to the temperature of the atmosphere that surrounds the pipe.

Pipes that are left intentionally bare (e.g., the return header to a cooling tower and the finned tubes in fin fan heat exchangers) inform the new Process Operator that the intention is to waste the thermal energy (aka heat) contained in the flowing fluid via conduction into the heat sink provided by the surrounding atmosphere.

Otherwise stated, an uninsulated line indicates the upcoming processing step requires cooling the process fluid so why not let The Universe help lower the temperature?

This PTOA Segment 74 included a focus on how the mechanics of conduction and convection heat transfer work together to cool process fluids that flow through the temperature-decreasing equipment known as a fin fan (aka air cooled heat exchanger).

Separating the process fluid into multiple streams and putting fins on the tubes increases heat transfer surface area (A) through very thin fins (d).

The driving force Delta T for heat transfer in a fin fan is provided by the hot fluid flowing through the tubes and the temperature of the surrounding ambient air heat sink.

The thermal heat radiated from the finned tubes is then convected into the atmosphere by forced-air or induced-air fans.

Conduction heat transfer through a fin fan can be impeded by debris clogging the tubes of a fin fan just like soot clogs the convection section tubes in a firebox.

Otherwise stated, the debris coats the finned tubes with a layer of low conductivity material (k) which effectively increases the diameter of the finned tube (d) as well as diminishes the total surface area available for conduction heat transfer(A).

Finned tubes can be cleaned in place or may have to be removed bank by bank to a cement slab specifically made for hydro cleaning.

Convection heat transfer through fin fans can be impeded by leaky tubes.

Leaky tubes must be plugged and ... logically ... plugged tubes decrease the mass flow rate (m/t) capacity through the fin fan.

Most fin fans are sized to allow sufficient mass flow rate (m/t) for heat transfer even with some tubes temporarily plugged.

When too many tube are plugged, the reduced mass flow rate (m/t) of the process fluid that requires cooling becomes too low for effective convection heat transfer and the desired low process temperatures are not attainable.

Leaky tubes are often caused by corrosion caused by chemical deposits.

Many chemicals are intentionally injected into processing lines for specific purposes (pH control, corrosion inhibitors, etc).

When chemical injection rates are not adjusted for process stream flow rate variances, corrosive deposits in the thin fin fan tubes can result in tube leakage.

Control Board Operators must alert Outside Process Operators about process flowrate changes so that the Outside Operator can make the correct adjustment on chemical injection rates.

©2015 PTOA Segment 00085
PTOA Deja Vu Review 3-6

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