And I feel like I've been here before.
Yes I feel like I've been here before.

("Deja Vu," by David Crosby of CSNY, 1970)

PTOA Segment  21: WHERE DO WE GO FROM HERE?

PTOA Readers and Students learned that the knowledge they have gained from the PTOA about the Process Variable Temperature, Process Industry Schematics and Process Industry Automation would be built upon in an upcoming Focus Study on Process Industry Temperature-Changing Equipment.

A Fired Heater was shown as an example of Temperature-Increasing Process Equipment. A Cooling Tower was shown as an example of Temperature-Decreasing Process Equipment.

A Shell and Tube Heat Exchanger was shown as an example of Temperature-Changing Process Equipment in which one process stream is cooled down via transferring heat into another process stream that needs to be heated up.

PTOA Readers and Students learned that these pieces of equipment transfer heat "indirectly," meaning the two process streams are not mixed together.

PTOA Readers and Students learned that process temperatures can be changed "directly" by chemical reactions and physical changes of state (aka phase changes).

 

PTOA Segment 22: I WANT TO TAKE YOU HIGHER

PTOA Readers and Students were introduced to the fired heater and learned its purpose, internal structural components, and process flows.

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

The identifying feature of a fired heater is a tall chimney stack. The stack creates a natural draft which makes the combustion products (aka flue gas) flow up and up and into the atmosphere.

Hot combustion products are made by igniting fuel gas and oxygen in the burners of the fired heater's firebox.

The combustion products are sometimes called 'flue gas' when they enter the chimney because a chimney is a type of flue (a duct for smoke and waste gases produced by fire).

The heat that is generated from the combustion of fuel gas and oxygen indirectly heats the process fluid that flows through the heater tubes.

The firebox is protected by a lining of refractory, a heat resistant material.

The process fluid that must be heated is divided up prior to entering the fired heater and flows through heater tubes that line both sides of the firebox.

The process fluid makes several passes through the heater tubes. After leaving the firebox, the multiple flows are recombined as a single process stream.

PTOA Readers and Students learned the ISA symbol for a fired heater and observed how the symbol hints at the process flow through the heater.

Capital "H" is used in the tag name of a fired heater. "F" can also be used and means "Furnace."

The ISA symbol for a fired heater appears in the "A" of the PTOA logo.

 

PTOA Segment 23: MORE HOT STUFF: USES OF STEAM

PTOA Readers and Students learned the industrial uses of steam.  Steam is made in "package boilers."

Domestic in-the-home boilers make hot water. The desired product of commercial and industrial sized boilers is steam.

Steam is water in the form of vapor. The temperature of boiler feed water is increased until the water changes state into its vapor form, steam.

The PTOA classifies package boilers as temperature-increasing process equipment because the uses of steam include kettle-type reboilers and steam heat tracing which indirectly heat process streams.

Other uses of steam include:

• Stripping impurities out of process streams in towers that are logically called "stripping towers."
• Producing hydrogen gas via the steam-methane-reforming reaction (SMR reaction). The SMR reaction turns methane gas (CH4) and water (H2O, in the form of steam) into carbon monoxide gas (CO) and hydrogen gas (H2).
• Spinning a steam turbine while being expanded in the device; when coupled to a compressor or electrical power generator steam is being used as a variable speed driver.
• Sanitizing and/or inerting equipment, pipes, and towers.

 

PTOA Segment 24: WHAT'S IN THE PACKAGE?

There are two kinds of boilers used in the processing industries: package boilers and waste heat boilers.

Package boilers are delivered to the process facility as a package. They are inspected annually by state boiler inspectors.

PTOA Readers and Students reviewed that the purpose of a package boiler is to change the state of Boiler Feed Water (BFW) into saturated steam, the vapor form of water.

PTOA Readers and Students learned the flow path of BFW while it changes state into steam and the hardware components within a package boiler.

The structural similarities between package boilers and fired heaters included burners, refractory-line combustion areas, process-fluid tubes in the combustion area and chimney stacks.

The operational similarities between package boilers and fired heaters included using burners to combust fuel gas and oxygen which generates heat and combustion products. The heat from combustion is indirectly transferred into process fluid flowing within tubes.

Unique structural features of a package boiler included a steam drum, a mud drum, capital D-shaped water tubes, downcomers and risers.

PTOA Readers and Students learned that the control system of a package boiler involves input from the BFW flowrate into the steam drum, the steam production flow rate, and the liquid level in the steam drum. This type of control is called 3-Element Control.

 

PTOA Segment 25: ANOTHER ONE BITES THE DUST

This important PTOA segment was dedicated to understanding the "latent=hidden" heat that is required to change the state of gases, liquids, and solids.

"Latent heat" cannot be sensed by a thermometer, hence the nickname "hidden."

A  temperature that will register on a thermometer is called "Sensible Heat."

The case study presented to distinguish latent=hidden heat from sensible heat was the common experience of bringing a pan of water to the boiling point.

When water is being boiled into steam, the sensible heat displayed as a temperature on a thermometer will remain at the boiling point of water even when much more heat is being supplied and absorbed to change the state of the water into steam.

Only after the last drop of water has been vaporized into steam will the thermometer sense ... and therefore  display as an increase in temperature ... the great increase in thermal energy that was supplied while the phase change was ongoing.

Process Operators must understand that only sensed heat is indicated during a change of state; otherwise poor operating judgment can result in damaged equipment .... and worse...injury to themselves and co-workers.

All changes of state have an associated  "latent=hidden" heat that Process Operators need to be aware of.

Changing into a "less bound together state" requires adding heat to the area that surrounds the process stream. The thermal energy (aka heat) is absorbed during the change of state:

• Changing from the solid phase into a liquid is also known as melting and requires The Heat of Fusion.
• Changing from a liquid into a gas is known as vaporizing (and evaporating) and requires Heat of Vaporization/Evaporation.

Changing into a "more-bound-together state" releases heat (aka evolves heat) into the immediate surroundings and the remaining process stream becomes cooler.

In many situations, the heat released into the immediate surroundings is not detected by a thermometer because the surrounding area is so vast that it would take much more heat release to cause a change in temperature.

For example, a lake that freezes in the fall may impact the temperature at the lake surface but will not significantly impact the ambient temperature detected on a nearby thermometer.

• Changing from a gas into a liquid is known as condensing and releases the Heat of Condensation into the immediate surroundings.

The temperature of the liquid created by condensation will be sensed by a thermometer as much colder.

The Heat of Condensation that evolves during the change of state is absorbed into the vast surroundings and may not appear to impact the local temperature.

The Heat of Condensation is the same magnitude as the  Heat of Vaporization; the difference is that the The Heat of Condensation is released into the immediate surrounding area and the Heat of Vaporization is absorbed from the immediate surrounding area (and must therefore be constantly supplied).

• Changing from a liquid into a solid is known as freezing and requires the Heat of Freezing.

The temperature of the solid created will be sensed as much colder. The Heat of Freezing is released into the immediate surrounding area which is usually too vast to cause an increase in sensed and displayed temperature.

The magnitude of the Heat of Freezing is the same as the Heat of Fusion (aka Melting); the difference is that The Heat of Freezing is released into the immediate vast surrounding area and the Heat of Fusion is absorbed from the immediate surrounding area (and must therefore be constantly supplied).

 

©2015 PTOA Segment 00046
PTOA Deja Vu Review 2-1

You need to login or register to bookmark/favorite this content.