Refrigeration & Air Conditioning Technology, 8e - 28

Section 1 Theory of Heat

Figure 1.25 This gauge reads 50 psig. To convert this gauge reading to
psia, add 15 psi (atmospheric pressure) to 50 psig to get 65 psia.

or reference point. If you want to know what the absolute
pressure is, you must add the atmospheric pressure to the
gauge reading. For example, to convert a gauge reading of
50 psig to absolute pressure, you must add the atmospheric
pressure of 14.696 psi to the gauge reading. Let us round off

Figure 1.26 Digital gauges. The compound gauge on the left reads pressures above and below atmospheric pressure. The high-pressure gauge on
the right will read pressures up to 800 psi. The center gauge is a vacuum
gauge, which is used for system evacuation (see Unit 8, "Leak Detection,
System Evacuation, and System Cleanup," for more on this topic).

Gauges provided by Appion Inc. Photo by Eugene Silberstein

28

14.696 to 15 for this example. Then 50 psig 1 15 5 65 psia
(pounds per square inch absolute), figure 1.25. New generation gauges are of the digital variety, figure 1-26.

SUMMaRy
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Thermometers measure temperature. Four temperature scales are Fahrenheit, Celsius, Fahrenheit absolute
(Rankine), and Celsius absolute (Kelvin).
Molecules in matter are constantly moving. The higher the
temperature, the faster they move.
The British thermal unit (Btu) describes the quantity of
heat in a substance. One Btu is the amount of heat necessary to raise the temperature of 1 lb of water 1°F.
The transfer of heat by conduction is the transfer of heat
from molecule to molecule.
The transfer of heat by convection is the actual moving of
heat in a fluid (vapor state or liquid state) from one place
to another.
Radiant heat is a form of energy that does not depend on
matter as a medium of transfer. Solid objects absorb the
energy, become heated, and transfer the heat to the air.
Sensible heat causes a rise in temperature of a substance.

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Latent (or hidden) heat is heat added to a substance
that causes a change of state and does not register on a
thermometer.
Specific heat is the amount of heat (measured in Btu)
required to raise the temperature of 1  lb of a substance
1°F. Substances have different specific heats.
Pressure is the force applied to a specific unit of area. The
atmosphere around the earth has weight and therefore
exerts pressure.
Barometers measure atmospheric pressures in inches of
mercury. Two of the barometers used are the mercury and
the aneroid.
Gauges have been developed to measure pressures in
enclosed systems. Two common gauges used in the airconditioning, heating, and refrigeration industry are the
compound gauge and the high-pressure gauge.

REVIEW QUESTIoNS
1. Temperature is defined as
A. how hot it is.
B. the level of heat.
C. how cold it is.
D. why it is hot.

2.
3.
4.
5.

State the standard conditions for water to boil at 212°F.
List four types of temperature scales.
Under standard conditions, water freezes at _____°C.
Molecular motion stops at _____°F.



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
Refrigeration & Air Conditioning Technology, 8e - ii
Refrigeration & Air Conditioning Technology, 8e - iii
Refrigeration & Air Conditioning Technology, 8e - Contents
Refrigeration & Air Conditioning Technology, 8e - v
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