Page 218 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

5.2 CHAPTER FIVE

Temperature measurement finds only the level (or intensity) of the heat energy, not the
amount. There are four methods used to express temperature:
1. Degrees Fahrenheit (F) shows the temperature expressed in English units. The scale
is determined by dividing the actual temperature difference between the ice point and
steam point of water into 180 divisions. The ice point is 32°F and the steam point is 212°F.
Absolute zero is –459.7°F.
2. Degrees Celsius (C) shows the temperature expressed in metric units. The scale is deter-
mined by dividing the actual temperature difference between the ice point and steam
point of water into 100 divisions. The ice point is 0°C and the steam point is 100°C.
Absolute zero is –273.2°C.
3. Degrees Rankine (R) shows an absolute temperature scale starting at absolute zero,
using the same division units as the Fahrenheit scale. The ice point of water is 491.7°R,
the steam point is 671.7°R, and absolute zero is 0°R.
4. Degrees Kelvin (K) shows an absolute temperature scale starting at absolute zero and
uses the same division units as the Celsius scale. The ice point of water is 273.2 K, the
steam point is 373.2 K, and absolute zero is 0 K.
The Rankine and Kelvin scales were created for laboratory use and are rarely used in
facility design engineering calculations.
The quantity of heat is measured either in Btu, which is an abbreviation for British ther-
mal units, or kcal, which means kilocalorie. A less frequently used form of measurement
is the watt-hour.
One Btu is the amount of heat needed to raise 1 lb of water 1°F. One kcal is the heat needed
to raise 1 kg of water 1°C.
There are two kinds of heat flow, transient and steady state. For transient heat flow, the
temperature varies with time. For general engineering applications, the transient method is
too complex. This book will use the steady state method of calculating the heat loss through
insulation. The results of these calculations are well within the range of accuracy sufficient
for engineering purposes. Steady state heat flow is further divided into series and parallel
types. In order to simplify the calculations, the following conditions will be assumed:
1. The series concept of heat flow will be used. As the heat flows through continuous layers,
the total resistance is the sum of all of the individual layers.
2. Heat in equals heat out. There will be no accumulation of heat in the system.
3. All factors, including temperature, have reached equilibrium.
4. The temperature and heat flow of all components remain constant with time.
5. The same heat flow exists through any plane in the system.
Heat transfer, or the movement of heat energy, always flows from a higher temperature
to a lower temperature. It can occur in any one of three ways: convection, radiation, or
conduction.
Convection is a large-scale movement of liquids or gases. It cannot occur in solids.
Density differences between hot and cold fluids produce a natural gravity movement. When
a fluid is heated, it becomes less dense. The lighter fluid will move upward in the absence of
forced circulation. Heat is transferred faster when forced circulation is created by means of
a fan or pump. When a liquid or gas moves from one place to another, it takes heat energy
with it. Convection is always accompanied by conduction in the region where warmer and
colder fluids interact.
Radiant heat is similar to radio waves, but has a shorter wavelength. They are capable of
traveling through air, some solids and liquids, and vacuums. All substances at a temperature
above absolute zero will radiate heat. Radiant heat increases with temperature and is pro-
portional to the fourth power of the absolute temperature of the radiating body.

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