I'm so hot for you, I'm so hot for you
I'm so hot for you and you're so cold!

("She's So Cold," by M. Jagger & K. Richards, 1979)

 

PTOA Readers and Students should recognize the shell and tube heat exchanger icon featured above because it was introduced in PTOA Segment 30.

PTOA Readers and Students have learned that a Temperature Differential...aka Delta T....aka ΔT... is required for heat transfer to take place.

The Delta T is provided by the two process streams that flow through the exchanger.

The hot process stream shown entering the upper right of the icon (indirectly) transfers its heat into the cold process stream shown entering the lower left of the icon.

The two prominent red arrows that point downward are drawn to emphasize that the heat is transferred from the hot process stream into the cold process stream.

The two red arrows DO NOT infer that a physical linkage exists between the two process streams; the heat transfer is accomplished indirectly.

The hot process stream exits the exchanger at a lower process temperature than it had when it entered the HEx.

The cold process stream exits the exchanger at a higher process temperature than it had when it entered the HEx.

Although they are related, "heat" and "temperature" are not the same and their differences will be clarified in this PTOA Segment.

 

"HEAT" IS NOT THE SAME AS "TEMPERATURE"

Temperatures are Measured in Degrees

PTOA Readers and Students learned way back in the first PTOA Segment that "Temperature" is a sense of hotness or coldness.

Distinguishing a "hot temperature" from a "cold temperature" has been possible since the sun introduced the concept of "hot."

Without sunshine the earth would just be very very very cold.

Just 300 years ago, human beings developed instruments called "thermometers."

The humans made a scale so that everybody could have a  universal understanding of hotness and coldness called "temperature" expressed in degrees °F (Fahrenheit scale) and °C (Celsius/Centigrade scale).

 

Heat and the Units of Heat Measurement

PTOA Readers and Students recently learned that heat is flowing thermal energy.

There is no handy-dandy picture of heat to upload into this PTOA Segment like a picture of a thermometer can be shown to represent temperature.

Just like the wind, heat definitely exists but is camera-shy.

Heat is measured in:

• British Thermal Units (BTUs) or
• Joules.

The definition of a BTU was established by experiment.

A BTU is the amount of thermal energy it takes to change the temperature of:

a) one pound of water (a defined mass of water)

b) from 58.5 deg F to 59.5 deg F (specifying the temperature also specifies the density of the water)

c) while conducting the experiment in an environment that is exactly at one atmosphere of pressure.... the same pressure that PTOA Readers and Students living at sea level feel on their skin every day.

A few more experiments determined that a BTU is equal to 1055 Joules.

So a BTU is somewhat over a thousand times bigger than a Joule.

Otherwise stated:

A BTU is 1.055 times bigger than a  KiloJoule or KJ (because "Kilo" and "K" mean "times 1000").

 

BTU Calculator Link

The below link to a BTU calculator is provided to help PTOA Readers and Students get a real world idea of how much heat a BTU represents.

BTU Calculator

The two examples featured in the BTU Calculator include the following statements:

• One very well-insulated 1,000-square-foot home in Boston needs around 24,000 BTUs to heat in winter.
• One 2,000-square-foot-home with normal insulation conditions in Washington, D.C., needs a 48,000 BTU air conditioner for adequate cooling in summer.

These statements are not accurate because they are missing a time factor.

Unfortunate as it may be, the omission is a great segue into the next topic... Heat Rate.

 

GOTTA TALK ABOUT HEAT (TRANSFER) RATE

PTOA Readers and Students know that heat is always in transit, always moving.

However, knowing that "something is flowing" is not nearly as helpful as knowing "how much stuff is flowing over a unit of time"...like a minute or an hour or a year or a millennium.

"BTUs per (one) hour" is a verbal expression for a Heat  Rate.

The phrase clearly describes how many BTUs are transferred within the time interval of one hour.

Process industry schematics will use "BTU/hr" as short hand for the units of Heat Rate.

PTOA Readers and Students should note that the time factor is always written in the denominator.

Ergo ....

... the verbalized expression "per (one) hr" is written " /hr."

The backslash represents the word "per."

In summary:

The term Rate used verbally and in written procedures indicates that a time interval is involved.

Here are two common process industry examples:

"Slowly increase the flow rate to 100,000 Barrels/day."

"The exchanger is designed to transfer 480,000 BTU/hr."

 

TEMPERATURE DOES NOT NEED HEAT TO EXIST
BUT HEAT CANNOT EXIST WITHOUT TEMPERATURE

Temperature can exist.... be experienced... without heat.

Just look at the thermometer closest to you and you will instantly be informed about the local  ambient temperature that you are experiencing.

On the other hand,

Heat is totally dependent upon not one but two temperatures to exist.

PTOA Readers already know that the difference in temperatures is the Temperature Differential (aka Delta T and ΔT).

PTOA Readers and Students already know that Delta T provides the driving force for heat to transfer from hot to cold.

Once temperature equilibrium has been reached between the two temperatures....

the Temperature Differential disappears...

and heat no longer exists.

However, as long as no phase changing is going on...

the equalized temperature can be sensed and observed!

Heat needs two temperatures to exist but temperature does not need heat to exist.

 

DIY!

Fred will help PTOA Readers and Students better understand the difference between temperature and heat.

Fred lives in a coastal town that borders the northern Pacific ocean.

Situation One:

Fred likes to drink cold beer with his friends at his favorite tavern, the Leopard Markovinikov.

For the comfort of the patrons, the tavern's temperature is kept at a steady 70 °F throughout the year.

Fred also likes to talk a lot.

Fred orders a frosty mug of beer and starts chatting with his buddies.

By the time Fred stops babbling and takes a sip of his beer, he discovers that his frosty mug is no longer frosty but has warmed up.

Question 1: Why did Fred's cold beer warm up?

Question 2: What temperature did Fred's cold beer warm up to?

Situation Two:

Fred drinks too much beer and engages in poor decision making.

Fred decides to go sailing  even though he is drunk. He falls into the 50 °F saltwater.

Question 3: Does Fred die of hypothermia because:

• the cold water surrounding Fred's body reduces his temperature below that needed to sustain life?

or

• The Delta T established between Fred's 98.6 °F body temperature and the 50 °F sea temperature causes Fred's body heat to flow into the cold water in an effort to equalize the two temperatures?

Question 4: In the event you answered that Fred dies because the heat flows from his body to equalize with the temperature of the sea water...why doesn't the temperature of the ocean increase?

 

Situation 3:

A week later, Fred's body is pulled up as by-catch by the crew of the Fishy-Wish in a purse seine.

Question 5: What temperature is Fred's body?

 

 

TAKE HOME MESSAGES: This PTOA Segment featured the differences between temperature and heat.

Temperature is easy to measure on a thermometer and is measured in degrees Fahrenheit or Celsius/Centigrade.

Heat is not as easy to measure; the amount of heat transfer is determined from experimental observations which created definitions and relationships generally accepted to be accurate. 

The process-industry size units of heat are BTUs and Joules.

Since heat is thermal energy in transit, the magnitude of thermal energy moving  is often expressed over the time duration of the thermal energy movement ... hence, "Heat Rate."

The word "Rate" means "per unit time." An expression of "rate" will have a unit of time is written in the denominator and will verbally be express "per hour"  "per second"  "per year" etc.

Heat Rates express the amount of heat transferred over a period of time. 

Examples of Heat Rates are:

48,000 BTUs/hr which is also 45.5 J/hr .

Temperature can exist without heat but heat requires two temperatures to exist.

 

©2015 PTOA Segment 00059
PTOA Heat Transfer Focus Study Area

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