If I drink this beer,
I'll probably have two or three, if I know me
I'll be digging up the past like it's money in the backyard.

("If I Drink This Beer," J.Singleton & B. Tursi, approx 2014)

 

WASTE HEAT RECOVERY EQUIPMENT & SYSTEMS

Let's step back for a BIG PICTURE MOMENT:

This PTOA Focus Study Area continues to feature industrial process equipment that changes the process variable "Temperature."

To be more specific...these last few PTOA segments have featured the following process equipment that is used to decrease the process temperature:

 

Shell and Tube Heat Exchangers...

 

 

 

Fin Fan Condensers...

 

 

 

Cooling Towers and Cooling Water Systems.

 

 

Okay. The BIG PICTURE MOMENT is over.

PTOA Readers and Students have already learned that Shell and Tube HExes conserve thermal energy (otherwise known as heat) by swapping heat between streams that need to be heated up with those that need to be cooled down.

However, Fin Fan Condensers and Cooling Towers inject the heat that they have indirectly removed from process streams into the surrounding ambient air.

The heat removed by Fin Fans and Cooling Towers allows the process temperature of a process stream to decrease as necessary to upgrade raw materials into desired products.

Yet the removed thermal energy (aka heat) can never be recaptured and used in the process again. Otherwise stated:

The heat is wasted.

Anytime that thermal energy (aka heat) is wasted, money that was used to purchase fuel for the fired heaters is being burned.

Therefore, a variety of Waste Heat Recovery Equipment and/or Systems will be installed at processing facilities to help reduce the amount of wasted dollars spent on utilities.

Waste Heat Recovery Equipment and Systems are chosen and modified for each processing facility and for that reason do not make very good visual aids for instruction.

The PTOA will focus on three types of the many kinds of Waste Heat Recovery Equipment:

• BFW Economizers
• Process Stream Preheaters
• Waste Heat Boilers

BFW Economizers and Process Stream Preheaters are Waste Heat Recovery Equipment located in the stacks of package boilers and fired heaters, respectively.

BFW Economizers and Process Stream Preheaters are featured in this PTOA Segment.

Waste Heat Boilers are part of a Waste Heat Recovery System that links multiple pieces of Waste Heat Recovery Equipment together to ingeniously transfer the heat from a process gas into BFW to generate superheated steam.

The superheated steam is made by indirectly transferring the Heat of Vaporization from a very hot flowing process gas stream into boiler feed water (BFW).

Waste Heat Boilers (WHBs) are featured in an upcoming series of PTOA Segments.

The purpose of Waste Heat Recovery Equipment and Systems  is to conserve energy by using heat that would otherwise be wasted ... and simultaneously decrease a process gas temperature.

 

BFW ECONOMIZERS

PTOA Readers and Students already know that a Package Boiler converts BFW into steam.

The BFW must be heated up to the boiling point temperature of water (212 °F, 100 °C).

Doesn't it just make economic sense to preheat the BFW by running the BFW piping through the chimney stack of the boiler?

"Economizer" is the perfect name for a piece of Waste Heat Equipment that economically preheats BFW destined for use in the package boiler by indirectly transferring heat from flue gases that must be cooled down before they exit the boiler's chimney stack.

The picture to the left shows BFW entering the multiple passes of an Economizer coil that is in the chimney stack of a Package Boiler.

The entering BFW is labelled "Water Inlet." The BFW is heated while flowing through the Economizer and exits much hotter (labelled "Water Outlet").

The hot flue gas flows over the exterior of the coil and indirectly transfers heat into the BFW.

The below graphic clarifies how the Economizer is piped into the Package Boiler stack.

The BFW from the Feedwater Tank (blue line) is pumped to the Economizer that is located in the chimney. The BFW exits the Economizer hotter (red line) and flows into the Steam Drum of the Package Boiler.

The real-life Economizer photo below illustrates that the BFW Economizer reduces the temperature of the exiting flue gases by 125 °C (225 °F) while simultaneously preheating the BFW to an unspecified temperature.

 

The above picture is proof positive that Waste Heat Recovery Equipment does not take great pictures to use as visual aids for instruction!

 

PROCESS STREAM PREHEATERS 

PTOA Readers and Students have already been introduced to Fired Heaters and will learn more about them in future PTOA segments that focus on Stationary Equipment and Operations.

A schematic of several configurations of fired heaters is below.

PTOA Readers and Students should notice that the "Radiant Tubes" are located in the (not labelled) "Radiant Section" of the firebox which  is just above the burner flame.

The "Convection Tubes" are located in the (not labelled) "Convection Section" that is nearest the chimney stack.

Convection Section temperatures are hotter than Radiant Section temperatures.

Process Stream Preheaters take advantage of the heat that would be lost out the stack by running process stream piping through this hot section which effectively preheats the process stream.

The below graphic illustrates how a feedstock called Feed Oil flows through the Convection Section of the Fired Heater, then flows through the Radiant Section.

The temperature of the exiting flue gases (aka combustion products) would be decreased several hundred degrees and the money spent in natural gas or fuel gas to fire the burners would not be wasted.

Even better...the reduced temperature of the combustion products helps mitigate global warming!

Boiler Feed Water (BFW) piping can also be wrapped around the exterior of a fired heater stack to generate low pressure (50 psig) steam.

 

TAKE HOME MESSAGES: Waste Heat Recovery Equipment and Systems serve the duo purposes of cooling down hot process gas streams and conserving thermal energy (aka heat).

 Waste Heat Recovery Equipment and Systems are specific to each processing facility.

Three types of Waste Heat Recovery Equipment are:

• BFW Economizers
• Process Stream Preheaters
• Waste Heat Boilers.

BFW Economizers are located in the stacks of package boilers and preheat the BFW that is destined to flow into the package boiler. 

Process Stream Preheaters preheat a process stream by directing the flow through the hot convection section of a fired heater prior to completing the the heating of the process stream in the radiant section of the fired heater.

Besides conserving energy for the Plant Owner, both BFW Economizers and Process Stream Preheaters also benefit the environment by reducing the temperature of hot flue gases that flow into the atmosphere.

Waste Heat Boilers are part of a steam-generating Waste Heat Recovery System that cools down a hot process gas stream via indirectly transferring the Heat of Vaporization into BFW destined to flow into a WHB. 

WHBs are featured in the next PTOA Segment. 

©2015 PTOA Segment 00042
Process Industry Temperature Changing Equipment

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