Refrigeration & Air Conditioning Technology, 8e - 18

18

Section 1 Theory of Heat

If we have a Celsius temperature that we want to convert
to a Fahrenheit temperature, we can use the following formula:
°F 5 (1.8 3 °C) 1 32°
For example, if we had a Celsius temperature of 20°C, we can
determine the equivalent Fahrenheit temperature by plugging
the Celsius value into the formula to get:
°F 5 (1.8 3 °C) 1 32°
°F 5 (1.8 3 20°C) 1 32°
°F 5 36° 1 32°
°F 5 68°
So, 20°C 5 68°F

Figure  1.5 (A) A Fahrenheit and Rankine thermometer. (B) A Celsius
and Kelvin thermometer.
FAHRENHEIT

RANKINE

CELSIUS

KELVIN

212 8F

6728R

1008C

373 K

WATER FREEZES 328F
08F

4928R
4608R

08C

273 K

WATER BOILS

If we have a Fahrenheit temperature that we want to convert
to a Celsius temperature, we can use the following formula:
°C 5 (°F 2 32°) 4 1.8
For example, if we had a Fahrenheit temperature of 50°F, we
can determine the equivalent Celsius temperature by plugging the Fahrenheit value into the formula to get:
°C 5 (°F 2 32°) 4 1.8
°C 5 (50°F 2 32°) 4 1.8
°C 5 18° 4 1.8
°C 5 10°
So, 50°F 5 10°C
Up to this point, temperature has been expressed in
everyday terms. It is equally important in the HVAC/R
industry to refer to temperature in engineering and scientific
terms. Performance ratings of equipment are established
using absolute temperatures. Performance ratings allow
for easy comparison among equipment produced by
different manufacturers. The Fahrenheit absolute scale is
called the Rankine scale (named for its inventor, W. J. M.
Rankine), and the Celsius absolute scale is known as the
Kelvin scale (named for the scientist Lord Kelvin). Absolute
temperature scales begin where molecular motion starts;
and use 0 as the starting point. For instance, 0 on the
Fahrenheit absolute scale is called absolute zero or 0°
Rankine (0°R). Similarly, 0 on the Celsius absolute scale
is called absolute zero or 0 Kelvin (0°K), figure 1.5. The
Fahrenheit/Celsius and the Rankine/Kelvin scales are used
interchangeably to describe equipment and fundamentals
of this industry.

1.3 INTRoDUCTIoN To HEaT
The laws of thermodynamics can help us to understand what
heat is all about. The first law of thermodynamics states that
energy can be neither created nor destroyed, but can be converted from one form to another. This means that most of
the heat the world experiences is not being continuously created but is being converted from other forms of energy, like

MOLECULAR
-4608F
MOTION STOPS

0°R

(A)

-2738C

0K

(B)

fossil fuels (gas and oil). This heat can also be accounted for
when it is transferred from one substance to another.
Temperature describes the level of heat with reference
to absolute zero, which is the temperature at which there
is no heat present in a substance and all molecular motion
has stopped. The term used to describe the quantity of heat
or heat content is known as the British thermal unit (Btu),
which indicates how much heat energy is contained in a
substance. The rate of heat transfer can be determined by
considering the time it takes to transfer a certain amount
of heat energy. Air-conditioning and heating equipment is
rated in Btu/h, where the "h" represents "hour." An airconditioning system that is rated at 24,000  Btu/h has the
capacity to remove 24,000  Btu of heat energy from the
structure every hour.
The Btu is defined as the amount of heat required to raise
the temperature of 1 pound (lb) of water 1°F. For example,
when 1 lb of water (about 1 pint) is heated from 68°F to 69°F,
1 Btu of heat energy is absorbed into the water, figure 1.6.
When a temperature difference exists between two substances,
heat transfer will occur. The next three sections in this unit
discuss the three types of heat transfer, namely, conduction,
convection, and radiation. Temperature difference is the driving force behind heat transfer. The greater the temperature
difference, the greater the heat transfer rate. Heat flows naturally from a warmer substance to a cooler substance. Rapidly moving molecules in the warmer substance give up some
of their energy to the slower-moving molecules in the cooler



Refrigeration & Air Conditioning Technology, 8e

Table of Contents for the Digital Edition of Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
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Refrigeration & Air Conditioning Technology, 8e - Contents
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