A sample text widget

Etiam pulvinar consectetur dolor sed malesuada. Ut convallis euismod dolor nec pretium. Nunc ut tristique massa.

Nam sodales mi vitae dolor ullamcorper et vulputate enim accumsan. Morbi orci magna, tincidunt vitae molestie nec, molestie at mi. Nulla nulla lorem, suscipit in posuere in, interdum non magna.

Episode 048 – Heat ,Thermodynamics, and Insulation – Part 3

In this episode we review the difference between heat and temperature, state the laws of thermodynamics and how heat is transferred and then finally move on to insulation.

To further explain the difference between heat and temperature, I make the analogy of two tanks as show in the following two pictures. In the first picture two tanks of similar size are connected.  The first tank is completely full and the second one is completely empty but once the valve is opened they reach equilibrium with both tanks as the same level.

 

 

 

 

 

 

 

 

 

In the second picture the 2nd tank is much smaller than the 1st.  When the valve is opened the levels again reaches equilibrium, but the levels are much higher than in the 1st example.

In these two analogies, level is similar to temperature, and volume of the water is similar to heat within each object.

 

 

 

 

 

 

 

 

 

Heat always flows from a higher temperature object to a lower temperature object.  There are 4 laws of thermodynamics.

  • The zeroth law.  If two objects are at thermal equilibrium and an a third object is at equilibrium with one of the other two then all three are at thermal equilibrium.
  • The First Law:  Energy cannot be created or destroyed but can only be changed from one type to another.  (There ain’t no free lunch law.)
  • The 2nd Law:  When changing from one type of energy to another energy is always lost in the form of wasted heat.  (The lunch man wants a profit also.)
  • The 3rd law:  There is a zero energy state called absolute zero.  (not very useful in day-to-day things, but very useful when calculating energy usage.)

We then reviewed the three ways heat can be transferred from a hot object to a cool object.  These are:

  • Conduction… The reason a pot holder must be used with my skillet.
  • Radiation… Why the sun feels good on a cool day.
  • Convection – The circulation and flow in a liquid or gas.

Insulation slows heat energy transfer but slowing one or more of the conduction methods described above.   The best insulator known is a Dewar (Thermos bottle).  It slows radiation by having reflective surfaces.  Conduction and convection are reduced by having a vacuum in the space between two bottles.

insulation in the walls of conventionally constructed houses in the USA prevents convection within the spaces between the studs by forming a maze and “trapping air” in small spaces between fibers.

The following is not on the audio:

The R value of insulation is a measure of the resistance to heat flow.  This is most easily understood by looking a 1/R = U which is a measure of heat conductivity.

In the US, U is in BTU/hr / (Sq ft * deg F) temperature difference between the surfaces.

In the rest of the world U is Watts/(Square Meter * deg C)

When calculating the heat load in a room the R values of the outside walls are calculated.  This is converted to a U value and then multiplied by the wall area and the temperature difference.   The same calculations are performed for the doors and windows and all the heat loads are added.   Additional heat load assumptions are made about heat sources within the room, people, lights and electrical equipment, as well as assumptions about air infiltration.  This is then used to calculate the duct size and/or the size of the heater.

The details of these calculations can be found in a book called “Modern Refrigeration and Air Conditioning” by Althouse, Turquest, and Bracciano.

Gary

 

 

Print Friendly

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>