Sensible Heat and Latent Heat – Fundamentals Module #9

sensible heat latent heatDear Steve – name the two most confusing heat transfer concepts ever attempted to be taught to you by the military?  Great question.  Sensible and latent heats.  Most confusing thing EVAR!  I have had so many people try to explain them to me.

I still barely know what sensible heat is, let alone latent heat.  I am actually kind of excited to put this post up today because that means I get to re-visit the very confusing, to me, engineering principles of both latent and sensible heats.  It’s been so long that I believe there are 8 earlier posts on the fundamentals, this is continuing along that path.

We’ve talked about heat and getting it to do work, the various laws.  Now we are going to keep going by looking at these two ideas.

Getting your brain around the ideas of both sensible heat and latent heat is really important for people looking to make careers in engineering, hvac, refrigeration, combustion.  It is important to at least have heard of the fundamental principles of why things, like substances, behave the way they do.  And how man has harnessed the power to use them to do work.


What Are They?

sensible heatSensible heat and latent heat are terms often used to indicate the effect that the flow of heat has on a substance.

The flow of heat from one substance to another normally shows up as a temperature change in each substance. The hotter substance becomes cooler, the cooler substance becomes hotter. However, the flow of heat is not reflected in a temperature change in a substance that is in the process of changing from one physical state (think solid, liquid, or gas) to another.

Here We Go

When the flow of heat IS reflected in a temperature change, we say that sensible heat has been added to or removed from the substance. Essentially this is heat that can be sensed or felt.

When the flow of heat IS NOT reflected in a temperature change, but IS REFLECTED in the changing physical state of a substance, we say that latent heat has been added or removed.  Seems rather simple right now.

Any flags go up for you?  Don’t worry they didn’t for me.   Maybe some of you bright ones that have been following along realized that wait, he’s talking about sensible heat and latent heat as though there are two different types of heat to consider.

It’s important to keep the following points in mind:

  1. heat is the movement (flow) of thermal energy;
  2. all adding and removing heat really means is that we are providing temperature differentials so thermal energy can flow between substances; and
  3. when we talk about sensible heat and latent heat, we are talking about two different kinds of effects that can be produced by heat, not about two different types of heat.


High School Chemistry 101 Real Quick

The three basic physical states of all matter are solid, liquid, and gas. The physical state of a substance is closely related to the distance between molecules. As a general rule, the molecules are closest together in solids, farther apart in liquids, and farthest apart in gases – (think density here – which is more dense, a solid or gas.)

Back to the confusing stuff

When heat flow to a substance is NOT REFLECTED in a temperature increase in that substance, the energy is being used to increase the distance between the molecules of the substance and to change it from a solid to a liquid or from a liquid to a gas.

what is sensible heatYou might say that latent heat is the energy price that must be paid for a change of state from solid to liquid or from liquid to gas. The energy is not lost. It is stored in the substance as internal energy. And now I understand it.

The energy price is only on loan or credit as it will at some point be paid back when the substance changes back from gas to liquid or from liquid to solid, since heat flows from the substance during these changes of state.

Of course every substance has it’s own values.  Water boils and freezes at different temperatures compared to say other substances like alcohol or gasoline.  Basically different amounts of thermal energy would be required to change states of different substances.

What is a BTU?

A British thermal unit.

Both internal energy and heat is measured using the British thermal unit (btu).

1 btu is the thermal energy required to raise the temperature of 1 pound of pure water to 1°F.

Another unit in which thermal energy maybe measured is the calorie. The calorie is the amount of heat required to raise the temperature of 1 gram of pure water 1°C. One btu equals 252 calories. Your air conditioning unit or furnace for example have btu ratings.


Example

Water is often the standard as it is the source of life.  It is often used and sometimes will be set with a value of 1.00 and then other substances will be measured in comparison to it.

Look at this chart:

water sensible heat chart

If we start with 1 pound of ice at 0°F, we must add 16 btu to raise the temperature of the ice to 32°F.

We call this adding sensible heat.

To change the pound of ice at 32°F to a pound of water at 32°F, we must add 144 btu (the latent heat of fusion). No change in temperature will occur while the ice is melting. After all the ice has melted, however, the temperature of the water will be raised as additional heat is supplied.

If we add 180 btu—that is, 1 btu for each degree of temperature between 32°F and 212°F, the temperature of the water will be raised to the boiling point. To change the pound of water at 212°F to a pound of steam at 212°F, we must add 970 btu (the latent heat of vaporization).

After all the water has been converted to steam, the addition of more heat will cause an increase in the temperature of the steam. If we add about 44 btu to the pound of steam that is at 212°F, we can super heat it to 300°F.

The same relationships apply when heat is being removed.

The removal of 44 btu from the pound of steam that is at 300°F will cause the temperature to drop to 212°F. As the pound of steam at 212°F changes to a pound of water at 212°F, 970 btu are released. When a substance is changing from a gas or vapor to a liquid, the heat that is given off is latent heat of condensation.

Notice, however, that the latent heat of condensation is exactly the same as the latent heat of vaporization. The removal of another 180 btu of sensible heat will lower the temperature of the pound of pure water from 212°F to 32°F. As the pound of water at 32°F changes to a pound of ice at 32°F, 144 btu are given off without any accompanying change in temperature. Further removal of heat causes the temperature of the ice to decrease.

Conclusion

I know writing this post about sensible heat and latent heat sure helped me understand them.  I have probably been taught these two things at least 10 times.  I have probably read about them on my own at least another 50.  Writing it out slowly, thinking about it line by line, and how it would be easiest to present, really made me finally grasp a huge chunk more of understanding it.  Hope you go something out of it as well.  Let me know – leave a comment, add some more.

 

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