WASTE HEAT RECOVERY SYSTEMS
The Steam Drum is above the Process Gas Cooler in this Waste Heat Boiler. The Process Gas flowing through the cooler transfers the Heat of Vaporization into the BFW jacket of this heat Exchanger.
Water's bubbling, it's b..b..b..bubbling
bubbling bubbling bubbling bubbling
like it's coming to a boil.
("Steam," by Peter Gabriel, 1992)
FOCUS ON WASTE HEAT RECOVERY SYSTEMS
Waste Heat Recovery Systems are composed of pieces of Waste Heat Recovery Equipment strung together for the unified purpose of transferring the thermal energy (aka heat) out of a hot process gas stream pronto instead of wasting the energy by sending it into the atmosphere.
The design of a Waste Heat Recovery System depends upon the process technology used at the processing plant.
The design of a Waste Heat Recovery System is so site-specific to each plant that there are not good visual aids to accompany this PTOA segment!
THE NIFTY-NESS OF WASTE HEAT BOILERS (WHBs)
Waste Heat Boilers (WHBs) are in-line boilers that indirectly transfer the heat from a very hot gas into a BFW jacketed heat exchanger.
The hot process gas must be cooled down prior to flowing into the next piece of processing equipment.
The hot gas flows just once through the jacketed exchanger vessel and still has sufficient time to vaporized the BFW because of its high temperature (1600 °F which is 871°C).
So A LOT OF HEAT is transferred pronto from that hot gas!
Enough heat to supply the Heat of Vaporization into the BFW and generate large volume of saturated steam.
Superheated steam is generated in the superheaters of Waste Heat Recovery Systems.
PTOA Readers and Students learned in PTOA Segment #23 that superheated steam can be used in a variable-speed driver to rotate a compressor or turbine.
Wait a minute!
Are you telling me that heat that would otherwise be wasted is indirectly transferred from a hot process gas and ultimately used to create a motive force...like for a variable speed driver?
No kidding!
The heat from a flowing process gas stream can be used to generate superheated steam from boiler feed water (BFW).
The superheated steam can be used to drive a compressor or turbine that can generate electrical power!
Since the process gas stream needed to be cooled down for the next step anyway...isn't a WHB a nifty design scheme to conserve energy?
COMMON WHBs AND PACKAGE BOILER COMPONENTS
Package Boilers are located in a Utility Building.
Waste Heat Boilers are located in the processing area because they are just one piece of equipment incorporated into a Waste Heat Recovery System.
In some processing facilities the separate components of a Waste Heat Boiler can even look like they are totally non-related processing equipment.
Only the astute Process Operator that has walked-out the P&ID will understand how the integrated but separate pieces of equipment work together as the WHB in a Waste Heat Recovery System.
The Waste Heat Recovery System that incorporates a WHB will also include a "Mud Drum Surrogate" and Steam Drum that are functional equivalent to a Mud Drum and Steam Drum of a Package Boiler.
This Waste Heat Boiler has many of the same functioning components of the water-tube Package Boiler design.
As shown in the picture to the left, the Steam Drum and "Mud Drum Surrogate" can be attached by piping but not enclosed in the same housing.
The various parts of the WHB are also not necessarily labelled as "Steam Drum" or "Mud Drum Surrogate" or "Riser" or "Downcomer" as in a Package Boiler.
The Steam Drum might be labelled as a Vessel (V-1001 for example).
Hot Gas Cooler on the Bottom and Steam Drum on the Top of this WHB.
The "Mud Drum Surrogate" could very likely be labelled something like Process Gas Cooler but could also be labelled as a random heat exchanger (E-1001 for example).
Whatever it is called on the PFD and PID, the "Mud Drum Surrogate" aka Process Gas Cooler (aka randomly numbered exchanger) in a WHB is a BFW-jacketed heat exchanger.
The Steam Drum is always physically located above the "Mud Drum Surrogate"/Process Gas Cooler/Exchanger.
What may appear as random piping between these two pieces of equipment perform functionally as Risers and Downcomers.
Hot process gases flow once-through the interior of the BFW-jacketed exchanger.
Steam is generated in the jacket and flows up to the Steam Drum via "legs" or "pipes" that function as risers.
BFW is returned to the steam jacketed exchanger in piping that functions as downcomers.
NO STACK NO BURNERS IN A WASTE HEAT BOILER
Unlike Package Boilers, WHBs do not have stacks or burners.
The hot process gas is already way too hot when it flows into the Process Gas Cooler; the hot temperature of the process gas was generated courtesy of a fired heater located somewhere upstream to the WHB.
No stack means no additional investment in burners or fuel gas is needed to generate a large volume of very hot and dry superheated steam.
TAKE HOME MESSAGES: Waste Heat Recovery Systems are specific to each processing facility. In fact, they are so specific to each plant that there are not great photos to include as PTOA visual aids!
Waste Heat Boilers are comprised of separate pieces of process industry equipment that work together to generate high pressure steam in a Waste Heat Recovery System.
The Waste Heat Boiler design has many of same functional features found in the water-tube Package Boiler design; however, the equipment may not be labelled "Mud Drum," "Steam Drum," "Riser," or "Downcomer."
A WHB does not have burners nor a stack.
Steam is made by transferring the Heat of Vaporization from a flowing hot process gas into BFW.
High Pressure Superheated Steam is generated from the saturated steam product made in the WHB by flowing the stream through a superheater.
The hot and dry superheated stream can be used as a variable speed driver to rotate equipment and/or produce electrical power.
©2015 PTOA Segment 00043
Process Industry Temperature-Changing Equipment
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