I want to take you higher...
Baby, baby, baby light my fire...
I want to, I want to, I want to...
I want to take you higher
Boom laka-laka-laka, Boom laka-laka-laka

("I Want To Take You Higher,"
by Sly and the Family Stone, 1969)

 

INTRODUCTION TO THE FIRED HEATER

The fired heater is a common piece of temperature-increasing process equipment.

PTOA Readers and Students will learn much more about fired heater internals, safe operations, permitted operations, and troubleshooting in future PTOA segments.

 

Visual Aids of Fired Heaters

The picture to the left shows three fired heaters. Small fired  heaters are sometimes  called furnaces.

The picture to the right shows a much bigger fired heater in the twilight, all lit up in by its safety lighting.

PTOA Readers and Students can easily recognize fired heaters in a processing facility because they all have a tall chimney called a stack.

 

Simplified Schematic of a Fired Heater

The schematic of a fired heater to the right reveals that fired heaters and natural gas kitchen ranges have something in common: burners with flames!

The rectangular part of the fired heater is called the firebox (but unfortunately is not labelled so in the schematic).

The little circles in the firebox schematic represent the process fluid being heated up while flowing through the heater's tubes.

The internal walls of the firebox are lined inside with several inches of refractory.

Refractory is a highly heat-resistant and insulating material that has to be carefully cured when installed... like cement. Without refractory,the walls of the firebox would burn down.

The stack of the fired heater is very important architecture. A home fireplace would not work without a chimney and a fired heater would not work without a stack.

Just like the chimney of a home hearth or wood stove, the stack is much more narrow than the body of the firebox. Hot combustion products composed of hot water vapor (H20), carbon monoxide (CO) and carbon dioxide (CO2) are forced to flow faster in a more narrow space. This faster flow of combustion products helps to create "draft" or "upward flow" through the heater.

Hot stagnant gases stalled in a heater and near a flame would cause a big ka-boom (laka-laka-laka)!

 

Visual Aid of Firebox Heater Tubes

Schematics of fired heaters are very confusing to new Process Technology students...and they should be!

Prior to entering the heater, the process flow is sent through a manifold that evenly divides and distributes the process stream flow to both sides of the heater. After being heated, the exiting flows merge again into one (much hotter) process stream.

So the little circles on the internal walls of the heater schematic above represent the heater tubes shown in the photo to the right.

Prior to entering the heater's firebox, the process stream was divided up into two separate streams to flow through the heater tubes that line both walls of the heater box.

The process fluid flows to the end of one heater tube and then takes a u-turn to flow in the opposite direction.

In Process Operator jargon, each time the process fluid flows to a different level in the heater, the process flow is "taking another pass through the heater." Ergo, each level of heater tubes is called "a pass."

 

The Exact Purpose of all Fired Heaters

Right now is a great time for PTOA Readers and Students to start figuring out what a piece of industrial equipment does by noticing the change in process variables that enter and exit the equipment.

The "feedstock" to a fired heater is a process fluid that is not at the desired higher temperature.

The "product" that leaves a fired heater is the same process fluid only now the process temperature is at a targeted hotter temperature.

The purpose of the fired heater is to make absolutely certain that a target process temperature is attained before the process fluid flows into the next piece of processing equipment. 

 

ISA P&ID Symbol for a Fired Heater

The below schematic is a simplified process flow diagram (PFD) for benzene production.

PTOA Readers and Students can find an ISA P&ID symbol for a fired heater with the tag name H-101 (Heater 101) in the PFD.

The zig-zaggy lines that make a backward "Z" in H-101  represent the process flow as it flows through the heater tubes inside H-101.

Note that this schematic infers that the process flow enters the heater at the bottom part of the heater and then exits higher up.

PTOA Readers and Students can visualize the real process flow being divided up and delivered to both sides of the heater. Then the process fluid flows upward, making multiple passes, until it exits the heater and is combined as one much hotter process stream.

The PFD also shows how air and fuel are charged separately to the bottom of the heater where the burners would be located. Each burner has a pilot light to ignite the fuel (which is logically called "fuel gas").

The combustion of fuel gas and the oxygen from the air generates hot combustion products...just like the heat you feel coming from a campfire.

The hot combustion products never directly mix with the process stream; the process temperature is increased indirectly by the mechanics of heat transfer.

Guess what?

The combustion products from all heater stacks contribute to global warming and anyone that tells you differently is unaware, not very smart, and/or possibly lying.

(Your fireplace chimney also contributes to global warming but it is not industrial-sized nor is it operating 24/7 throughout the year)

The dilemma is that the world still needs benzene, the product made from the process.

Therefore, Process Operators and Control Board Operators must prioritized operating fired heaters efficiently and within permitted levels to minimize damage to the environment.

 

The Purpose of H-101 Illustrated

The benzene plant PFD is repeated below for ease of reference:

Recall that the purpose of a fired heater is to absolutely guarantee that a target process temperature will be attained before the process flow enters the next piece of processing equipment.

The arrowheads on the PFD show PTOA Readers and Students that a process stream labelled "4" enters fired heater H-101 and a process stream labelled "6" leaves it.

The PFD arrowheads reveal that process stream #6 exits H-101 and then flows into R-101.

R-101 is a Reactor.  Reactors chemically change the feedstock; the chemical composition of the product that flows out of R-101 is very different than the feedstock that flowed into it.

Conclusions:

The purpose of H-101 is to make certain the reaction temperature needed for R-101 is attained.

The process temperature must have to be at a target temperature for the reaction that makes benzene to occur in R-101.

If the reaction temperature of the process stream is not reached prior to flowing into the reactor, then the desired product of benzene will not be optimally produced.

 

 

Take Home Messages:  The purpose of a fired heater is to raise the process temperature to the target temperature needed for the next piece of processing equipment to perform its job successfully.

This introduction to fired heaters included the following fired heater nomenclature that PTOA Readers and Students need to know and understand:

stack, draft, firebox, burners, refractory, heater tubes, pass, fuel gas, combustion products.

The first letter for the ISA tag name of a fired heater is "H" for Heater or "F" for Furnace. A furnace is a small fired heater.

The ISA P&ID symbol for a fired heater is easy to identify because it has a stack (an ISA P&ID symbol for a fired heater appears in the PTOA logo). The symbol gives the P&ID reader a hint where the process stream enters and exits the fired heater. 

The combustion products from fired heaters contribute to global warming. Process Operators and Control Board Operators must operate fired heaters efficiently and within permitted levels to minimize environmental damage.

PTOA Readers and Students can determine the purpose of a piece of processing equipment by noticing the difference in process variables (Temperature, Pressure, Flowrate, and Level) between the entering and exiting process streams.

©2015 PTOA Segment 00022
Process Industry Temperature Changing Equipment

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