Temperature
Temperature scales are a way of describing how hot a substance is.
A lump of matter contains energy. There are many forms of energy,
one of them is Kinetic energy and measuring temperature is a way
of measuring how furiously the molecules contained in a lump of
substance are moving about. This molecular activity causes what
we perceive as the temperature of an object. A refrigeration mechanic
must be able to deal with temperatures in various scales. Traditionally
the English system has been used (Fahrenheit degrees) and a whole
series of familiar capacity measurements like Horse Power, BTU's,
Tons, and PSI have been the norm. However the metric system which
is supposed to be easier to work with is encroaching in many locations.
In both systems there are standard and absolute temperature scales.
Try playing with the above temperature converter. Type a value in
any one of the input boxes and click on the Convert Button. Here
are several interesting values to try: -40 �F, 0 �R, 373 �K, 21
�C
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Fahrenheit
The Fahrenheit temperature scale was developed by no less than Fahrenheit
himself back in the early 1700's. It was based on scientifically
observable occurrences such as human body temperature and melting
ice. Those points were assigned arbitrary values which made sense
at the time. The newly created number scale was widened for easier
reading and when boiling water was measured at 212 degrees, Fahrenheit
changed the value of freezing water from 30 to 32 degrees to achieve
the more attractive scale of 180 degrees between water's freezing
and boiling points. There are 180 degrees in 1/2 of a circle and
this was a temptation too great to resist.
Celsius
In theory the Celsius scale should be much easier to work with.
It is based on calling the freezing point of water zero and the
boiling point of water 100. There are therefore 100 degrees between
those 2 points. The Celsius temperature scale is also referred to
as the "Centigrade" scale. Centigrade means "consisting of or divided
into 100 degrees. I wonder what a comfortable room of 70� F would
be in Celsius? If you don't happen to have a conversion calculator
at your disposal you can always rely on the following 2 formulae:
Kelvin
Scientists use the Kelvin scale, which is based on the Celsius scale,
but has no negative numbers. Instead of basing it's zero point on
the freezing point of water, it bases it's zero point on Absolute
Zero. which is the theoretical temperature where all heat has
been removed from a substance. Hence any amount of heat added creates
a positive temperature. Negative numbers can mess up a scientist's
mathematical calculations. You will find that in refrigeration,
we too must use absolute temperature scales for some things. At
Absolute Zero scientists believe that molecular motion would stop.
Rankine
Rankine is the English version of an absolute temperature scale.
Add 460 degrees to Fahrenheit temperatures to obtain the Rankine
temperature. Input 0� in the Rankine box on the calculator above
and you will see why.
Heat
Temperature is a qualitative measurement. Heat is a quantitative
measurement. The temperature "quality" of a object describes how
hot it is but not the total amount of heat it actually contains.
Here's a silly example which makes clear the distinction. Let's
say we have two blocks of iron. One is a mere cubic inch, the other
is 10 feet cubed. We heat each of them to 150� F and you verify
this with some sort of thermometer. They both have the same temperature
but do they both contain the same amount of heat? When you throw
the little cube in your swimming pool nothing noticeable happens
to the temperature of the pool water but when you toss in the huge
iron chunk the pool water can be measured to rise somewhat over
time. If there was a noticeable amount of heat transfer from the
large chunk of iron but not from the small chunk of iron then surely
the large chunk contained more heat than the small one even though
they were at the same temperature. The temperature of an object
is a reflection of the kinetic energy of the atoms or molecules
that make it up. Fast molecules = high kinetic energy = high temperature.
On the other hand heat represents the total amount of kinetic energy
in an object. Heat is measured in BTU's. Recall that 1 BTU is the
amount of heat required to change the temperature of 1 Lb. of water
through 1� F. So it would take 2 BTU to raise the temperature of
2 Lb. of water through 1� F. And it would take 30 BTU to raise the
temperature of 3 Lb. of water by 10� F. BTU's (or their metric counterparts)
are the fundamental unit of heat used in the refrigeration industry.
Larger quantities of heat are described with the term Ton.
12,000 BTU = 1 Ton. A building might have a 3 Ton Air Conditioning
system which would be equivalent to 36,000 BTUH.
Specific Heat
Different substances have different heat holding capabilities
and thermal properties. Just because 1 Lb. of water will change
precisely through 1� F when 1 BTU is applied to it does not necessarily
mean that the same thing will happen with 1 Lb. of copper or 1
Lb. of steel or 1 Lb. of ice cream. There is a need to be able
to specify those differences and the method utilized is to compare
all substances to water. Water is given a specific heat value
of 1. This means that it that 1 BTU is required to change the
temperature of 1 Lb. of water through 1� F. The specific heat
of water can also be described in the metric system. The metric
specific heat of water is 1 calorie per gram per degree Celsius.
This value also works out to 1. In other words it would take 1
calorie of heat to raise the temperature of 1 gram of water through
1 degree Celsius. Specific heat is a dimensionless quantity. It
is purely a number having no unit of measurement associated with
it. In Refrigeration specific heat values are used to calculate
capacity requirements for refrigerating known quantities of product.
For example one might need to be able to select refrigeration
equipment capable of cooling 5000 Lb. of beef from 55� F to -20�
F. A calculation like that must take into consideration the fact
that the specific heat of a substance usually is different above
and below it's freezing point.
Latent Heat
Latent Heat is the heat given up or absorbed by a substance as it
changes state. It is called latent because it is not associated
with a change in temperature. Each substance has a characteristic
latent heat of fusion, latent heat of vaporization, latent heat
of condensation and latent heat of sublimation.
Sensible Heat
Sensible Heat is associated with a temperature change, as opposed
to latent heat. This is so-called because it can be sensed by humans.
If the air in a building was to be heated from 60 �F to 70 �F only
sensible heat would be involved. However, if the air in a building
was to be cooled from 80 �F to 70 �F and humidity was to be removed
from the air at the same time, then both sensible and latent heats
would be involved.
Insulator
Electrical wires are coated with an insulating material so electricity
stays in the conductor (wire). Thermal insulation on the other hand
tries to keep heat from transferring. Thermal insulation does not
stop heat transfer, it only slows down the rate of transfer. The
greater the amount and quality of insulation, the greater the insulating
effect and the slower is the thermal transfer. There is insulation
inside cooler and freezer walls and in the perimeter walls of conditioned
spaces. If fiberglass batting is being used it should be noted that
the glass fibers are actually a poor insulator. It is the tiny pockets
of trapped air in-between the fibers that actually are responsible
for the insulating effect.
Conductor
The chart below shows the specific heat values of several materials.
Notice the very small specific heat value that copper has. This
means it would take a mere .093 BTU to raise 1 Lb. of copper through
1 degree. Copper has a bigger temperature change for the same heat
input compared to many other materials. Copper transfers heat readily
and would not make a very good insulator, it conducts heat too well.
The smaller the specific heat number, the better of a conductor
a material is. You can see why heat transfer devices like evaporators
and condensers are made from materials like aluminum and copper.
| Material |
Specific Heat (Btu/Lb./�F) |
| Water |
1.00 |
| Air |
.24 |
| Aluminum |
.22 |
| Iron |
.12 |
| Copper |
.093 |
| Concrete |
.23 |
| Glass |
.20 |
| White Pine |
.67 |
| Ice |
.50 |
| Rock |
.20 |
Pressure
Pressure is what occurs when a force is applied over an area. More
specifically, pressure is the ratio of the force acting on a surface
to the area of the surface. The equation for pressure represents
this rather straightforwardly; P=F/A This equation means that Pressure
equals Force divided by Area. Let's look at a couple of very simple
examples. As is demonstrated in the sketch below, the same weight
can exert completely different pressures depending on how much surface
area it is spread out over. Note that when you multiply FT by Lbs
you get a unit called FT Lb. (pronounced Foot Pounds) This is a
legitimate unit of pressure however in refrigeration we use pounds
per square inch not pounds per square foot. This is abbreviated
to PSI. Just as with temperature, pressure has many different scales
that can be used and can be described with the English system or
the Metric system. The remainder of this book will be referring
to the English system of measurements. We seldom deal with gravitational
forces as shown in the diagram although it is an important concept
to be aware of. Rather, we deal with the pressures and temperatures
of gases and that is what the next section is all about.
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