Another one bites the dust. Another one bites the dust.
And another one gone, and another one gone.
Another one bites the dust.

("Another One Bites the Dust," by Queen, 1980)

 

THE COMMON KITCHEN CATASTROPHE

The following common kitchen catastrophe will happen to all PTOA Readers and Students during some time in their lives:

You've  put a pan of water on the stove to heat up the water....for making rice...noodles...hot cocoa.

Your cell phone rings...
or maybe there's a neighbor knocking at the door...
or maybe there are two teenagers about to rip each other apart upstairs...

whatever.

The next thing you know, you have a sauce pan that is boiled dry beyond hope, beyond help, beyond care.

Throw it out.

Now pretend that you had a do-over.

This time you have a cure for the common kitchen catastrophe that will save expensive sauce pans from boiling dry. Your invention is so clever that you can sell the device for a million dollars and retire.

You have equipped a thermometer with an alarm that will alert you when the temperature gets hotter than the 212 °F (100 °C) needed to boil water.

Guess what?

You will get the same result.

Another one bites the dust.

Throw the pan out.

The alarm would not sound until after that last smidge of water was vaporized into the atmosphere.

Only after that last smidge of water was vaporized would the temperature indicated on the thermometer zoom up and reveal how much heat the range burners had been supplying to the pan all along....alas, too late to save the pan.

What's up with that?

 

UNDERSTANDING SENSIBLE HEAT AND
THE "HIDDEN HEATS" RELATED TO
CHANGES OF STATE (AKA PHASE CHANGES)

The burners on the stove are the source of the thermal energy (also known as heat) required to do these two things in the order listed below:

• Heat the water to the boiling point temperature.
• Vaporize the liquid water into water vapor...also known as steam.

The thermometer is fully capable of sensing and displaying the gradually rising temperature of the water up to the temperature needed for boiling (212 °F/100 °C).

The temperature that is sensed and indicated on the thermometer will assure PTOA Readers and Students that the energy needed to reach the boiling point has been attained and is sustained.

So this thermal energy can be sensed, measured, and displayed as a temperature on a thermometer. Guess what this energy is called?

Sensible Heat!

Okay, that made sense.

PTOA Readers and Students need to put their thinking caps on for this next statement:

The thermometer IS NOT able to sense or detect the energy that it takes for liquid particles to free themselves up and vaporize into a steam vapor.

There's a logical name for this type of energy as well:

The Heat of Vaporization.

(Just FYI: The complete scientific name for this type of energy is the Latent Heat of Vaporization. "Latent" means "Hidden." The phrase "Heat of Vaporization" is plenty long enough and that's what PTOA will use).

Even though the Heat of Vaporization cannot be measured, detected, and displayed...it still exists.

After that last smidge of water has been vaporized the Heat of Vaporization will no longer be a happening thing because there is no more water to vaporize.

Only after the last smidge of water is vaporized into the gas/vapor known as steam will the heat being supplied by those burners again be sensed and indicated on the thermometer as an increasing temperature.

 

Thermal Energy Must Be Added To A Process
During a Change of State into a "Less-Bound-Together" Form.

A change of state/phase change that results in molecules being "less-bound-together" will require thermal energy from burners (or other sources).

The thermal energy required for a phase change into a "less-bound-together" form cannot be sensed by a thermometer or a TI.

Examples of "less-bound-together" phase changes are:

• a liquid vaporization into a gas/vapor (like in a package boiler).
• a solid melting into a liquid.

PTOA Readers and Students learned earlier in this segment that the thermal energy required to change a pot of boiling water into steam is called "The Heat of  Vaporization."

The thermal energy required to change a solid into a liquid is called "The Heat of Fusion." Too bad it is not called The Heat of Melting because that's what it is.

Once the change of state into a "less-bound-together form" has been completed, the sensed temperature on a thermometer or TI will increase.

Say what? Let's go over this one more time:

Pretend a chunk of ice has been taken out of the deep freezer at -20 deg °C ( deg -4°F) and put in a sauce pan on a kitchen range. The burner is turned on.

On the below graphic, the Y-axis shows the temperature on a thermometer (or TI) while the chunk of ice is heated up.

The X-axis shows the change in heat energy added to make the temperature on the Y-axis go up...but don't stress about understanding that now.

Eventually, the ice reaches the melting point (0 °C, 32 °F).

Once the ice starts to  melt, the temperature on the thermometer stays steady at 0 °C (32 °F), the melting point temperature. The heat from the burner is being soaked up while converting the ice into the less-bound-together form of water.

Otherwise stated, while the phase change is ongoing, the thermometer stays at 0 °C (32 °F) because the Heat of Fusion cannot be sensed.

Eventually, all the ice will become liquid water.

The water starts heating up because the thermal energy from the burners is raising its temperature. As shown in the graph, the temperature on the thermometer increases from 0 °C (32 °F) on upward.

Eventually the boiling point/vaporization point temperature of the water is reached (100 °F or 212 °C).

At that temperature, the water starts turning into the vapor/gas known as  steam (not "stream" as misspelled in the graphic).

While the phase change into steam is ongoing, the temperature on the thermometer is once again steady at the boiling point temperature of water (100 °C which is 212 °F). The heat from the burner is being soaked up while changing water into steam. This Heat of Vaporization cannot be sensed by the thermometer.

Once all the water has turned into steam, the heat from the burners will heat up all the steam present and the temperature will once again increase above the boiling point temperature (100 °C which is 212 °F). The increase in temperature will once again be shown on the thermometer.

 

Thermal Energy will Evolve From A Process
During a Phase Change into "More-Bound-Together" Form.

Who would be surprised that the same amount of thermal energy (aka heat) that is required to change into a "less-bound-together" form is released when changing into a "more-bound-together" form.

On the schematic to the left, "+Heat" means "need to add heat" while a liquid is vaporizing into a gas/vapor. "-Heat" means "heat will be evolved" while a liquid is condensing into a gas/vapor.

The thermal energy released when a gas or vapor changes into a liquid is called The Heat of Condensation.

The thermal energy released when a liquid changes into a solid is called The Heat of Freezing.

No joke it is a lot harder to imagine heat being given off while the temperature is dropping and your teeth are chattering from  the cold. But that's what's happening right at the surface where the phase change is ongoing.

 

APPLICATION OF THE ABOVE
TO PROCESS INDUSTRIES

There are many important applications of process technology using phase changes. Process Operators are often assigned to processes in which a change of state is occurring.

Common industrial process in which a change of state occurs include:

• Steam Boilers (BFW liquid vaporized into steam)
• Waste Heat Boilers (BFW liquid vaporized into steam)
• Condensate Systems (steam gas/vapor condensed into liquid condensate)
• Condensers in Distillation Systems (Process stream gases condensed into process stream liquids)
• Refrigeration Loop Systems (liquid to gas to liquid to gas to liquid...)

Process Operators  must be aware and alert that the process temperature displayed on a TI in a piece of equipment where a phase change is occurring  is only revealing the Sensible Heat at that point in the process.

 

Take Home Messages: The energy provided by the range burners of a kitchen range and by the burners of an industrial boiler must be sufficient to raise the temperature of the water to the boiling point as well as provide enough thermal energy to change the state of water into steam. 

The thermal energy needed to increase the temperature to the boiling point is called Sensible Heat. Sensible Heat can be measured by a TI.

The thermal energy needed to change the state of liquid water into steam is called The Heat of Vaporization. The Heat of Vaporization cannot be detected by a TI.

Heat must be added to a process when a change of state results in a form that is "less bound together." This change-of-state heat is "hidden"... meaning not detectable by a TI. Examples of "less-bound-together phase changes" are: 

• a liquid vaporizing into a vapor/gas which requires the Heat of Vaporization.
• a solid melting into a liquid which requires the Heat of Fusion.

Heat will be released (evolved) into a process when a change of state results in a form that is "more-bound-together." Examples of "more-bound-together phase changes" are :

• a vapor/gas condensing into a liquid  which releases The Heat of Condensation.
• a liquid freezing into a solid which releases The Heat of Freezing. 

Process Operators that work with processes in which changes of state occur must understand that TIs sense Sensible Heat. If the change of state/phase change stops, the detected temperature of the process will change one way or the other, depending upon if heat was being supplied or evolved during the phase change.

 

©2015 PTOA Segment 00025
Application of Physical Properties in Process Industry

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