There's joy in repetition.
There's joy in repetition.
There's joy in repetition.
There's joy in repetition.

("Joy in Repetition," by Prince, 1990)

 

PTOA Segment 40: NATURALLY COOL, MAN!

The focus on process temperature-decreasing equipment continued.

Power plants and other processing facilities with a constant demand for large volumes of cooling water install natural draft Hyperbolic Cooling Towers in their cooling water systems.

For example, PTOA Readers and Students have noticed that Hyperbolic Cooling Towers are installed at nuclear power plants.

Another benefit of Hyperbolic Cooling Towers is the elimination of a fan to induce or force a mechanical draft of air through the tower.

Eliminating a fan eliminates the cost to supply power to the fan. Even better, the risk of a fan failure is eliminated in power plants...which is of paramount importance when nuclear rods need to keep cool to prevent highly exothermic nuclear chain reactions, etc.

Natural draft Hyperbolic Cooling Towers have the same common structural components found in an Induced-Draft, Cross-Flow Cooling Tower with two major exceptions:

• A tall hyperbolic tower replaces the fan and fan deck.
• The air-water flow is counter flow; air flows upward and the hot water trickles downward.

The hidden golden nugget in this PTOA segment was the focus regarding how convection currents develop.

Convection currents are fluid flow patterns caused by changes in fluid density.

The density of a fluid (gas or liquid) changes as it is heated and cooled; successive heating and cooling causes convection currents.

When a fluid is heated, its molecules get agitated by the hotter temperature and want to spread out and separate from each other. Hot molecules are less dense than cold molecules.

In any area that has a hot side and cold side (like the example shown of a room with a window and a working radiator) the fluid on the hot side...gas or liquid.. will rise up while its molecules move further and further away from each other.

Upon entering a colder area, the molecules of a fluid are no longer as agitated and will group back together. Cold molecules are more dense than hot molecules.

Because denser molecules are heavier, they flow toward the bottom of a closed container.

The operation of Hyperbolic Cooling Towers and Boilers depends upon convection currents to work.

Hyperbolic Cooling Towers would not work without the natural draft created by the change of density of air when it is heated. The molecules of the heated air want to be further away from each other and rise up and up and up.

In Boilers, water is less dense while changing into the steam phase and more dense when changing back into the water phase. The boiler would not work without the natural circulation caused by density changes between the water and steam phases.

Hey! The oceans and air are also dependent upon convection currents to work, too!

This PTOA segment covered the classifications of Cooling Towers.

Cooling tower classifications are based upon how the draft is created (natural or mechanical) and the flow pattern of the incoming cool air and hot water flow (cross flow or counter flow).

In the event a mechanical draft is in use, the sub-classification is whether or not the fan is inducting air flow or forcing air flow.

 

PTOA Segment 41: NOT WASTED!

Sometimes the need to chill down a process temperature trumps saving the money spent in utilities to make the process stream warm or hot in the first place.

Cooling Towers and Fin Fans are examples of processing equipment that inject heat into the atmosphere to get it out of a process stream pronto.

In contrast, Heat Recovery Equipment and Systems are intentionally installed to transfer thermal energy (aka heat) out of hot process streams and into process streams that need the heat.

Process Plant Owners will greatly appreciate Process Operators that understand how to convert raw materials into desired products without wasting energy.

The PTOA classified Heat Recovery Equipment as Temperature-Decreasing Equipment because the net effect is to lower the temperature of a gas stream by indirectly transferring the heat from the hot gas stream into a different process stream.

Heat Recovery Equipment and Systems are designed specifically for each processing facility and therefore do not take great instructional photos.

New Process Operators may be confused with what may appear as random piping connected to a chimney stack or running into and out of a duct.

Studying a P&ID will clarify the piping and hardware related to Heat Recovery Equipment and Systems.

Heat Recovery Equipment includes Process Stream and BFW Economizers.

Economizers/Preheaters indirectly transfer the heat of a hot gas into the process stream that needs to be heated up.

Process Stream Economizers preheat the process stream in the convection section of a fired heater before completing the heating process in the radiant section of the fired heater.

BFW Economizers preheat the BFW with hot flue gases that flow through the boiler chimney stack.

Waste Heat Boilers are part of a Waste Heat Recovery System.

A "System" is made by connecting several pieces of equipment together.

 

PTOA Segment 42: WASTE HEAT RECOVERY SYSTEMS

This PTOA Segment focussed on the role of the Waste Heat Boiler in Heat Recovery Systems.

Unlike Package Boilers, WHBs are located in the processing area because they are incorporated into the design of the plant.

WHBs can appear as separate pieces of equipment that are piped together to function as an in-line steam generator.

WHBs use the heat from a hot flowing process gas  to convert BFW into high pressure, very dry steam.

This PTOA segment compared the structure and hardware components of a WHB and package boiler.

The WHB has hardware components that equivalently function as a steam drum, mud drum, risers, and downcomers even if they are not clearly labelled as such.

The WHB does not have a stack nor a burner assembly because the BFW is boiled into steam by transferring the Heat of Vaporization from a hot gaseous process stream.

The saturated steam generated in the WHB is superheated.

The high pressure, dry superheated steam made from a WHB is suitable for use in a variable speed driver.

Recovering heat that would otherwise be wasted and converting the thermal energy into a motive force is a very efficient plant design.

 

PTOA Segment 43: SYN FULL

The conversion of Methane gas (aka Natural Gas aka CH4) into Syngas by steam reforming was featured in this PTOA segment.

Syngas is made of Hydrogen (H2), Carbon Dioxide (CO2) and usually some Carbon Monoxide (CO).

Syngas is the building block that can make many useful consumer products listed in this PTOA segment.

The Steam Methane Reforming Reaction (SMR) is very endothermic; very high temperatures are required to keep the reactions going in the direction that makes syngas.

A graphic of a Reaction Furnace in an industrial Steam Reformer depicted the flow path of the steam (H2O) and natural gas (aka CH4 and Methane) that are converted into syngas.

The graphic also showed the flow path of fuel gas and combustion air that are ignited to provide the high temperatures that must be sustained for the endothermic SMR reaction to continue.

The Syngas product and flue gases that exit the Reaction Furnace are very hot.

 

PTOA Segment 45: REFORMED...NOW WHAT?

This PTOA segment illustrated how Waste Heat Recovery Equipment and Systems are incorporated into process plants and how this hardware simultaneously decreases flue gas and process stream temperatures.

A simplified PFD of an Ammonia Plant colorfully illustrated the three sections of a Steam Reformer:

• the Reaction Furnace
• the Duct
• the Stack

The PFD also illustrated:

• How the heat from the Reaction Furnace generated by combustion is recovered in the Duct and used to preheat or superheat a total of five different process streams.
• How the heat from the flowing Reaction Furnace Syngas product is used to generate superheated steam in a WHB.

PTOA Readers and Students gained valuable PFD decoding experience tracing out the flows of the syngas, flue gas, and WHB streams.

 

©2015 PTOA Segment 00050
PTOA Deja Vu Review 2-5

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