Building Design and Construction Handbook - Merritt - Ventech!

13.8 SECTION THIRTEEN
to a material or substance, there will be an increase in average velocity of its
molecules or electrons, with an increase in their kinetic energy. Likewise, as heat
is removed, there will be a decrease in the average molecular velocity and, therefore,
also the electron or molecular kinetic energy.
A thermometer is used to measure the degree of heat in a substance or material.
The thermometer includes an appropriate graduated scale to indicate the change in
temperature of the substance. The change in temperature as read on a thermometer
is a measure of heat transferred to or from the substance. A unit of temperature is
called a degree and is equivalent to one graduation on the scale.
By convention, the scale is an interval scale. The Celsius thermometer is a metric
system of measuring temperature; 0�C is assigned to the temperature at which water
freezes and 100�C to the temperature at which water boils at normal atmospheric
conditions. Hence, on a Celsius thermometer, there are 100 intervals or graduations,
called degrees, between the freezing and boiling temperatures. Each interval or
degree is called 1 Celsius degree.
In the Fahrenheit system, 32�F is used to designate the freezing temperature of
water and 212�F the boiling temperature at normal atmospheric pressure. Hence,
on the Fahrenheit scale, a degree is equal to 1 ⁄180 of the distance on the scale
between the freezing and boiling temperatures. Conversion formulas used for each
scale are as follows:
13.2.2 Thermal Capacity and Specific Heat
�F � 1.8 � �C � 32 (13.1)
5 �C � ⁄9(�F � 32) (13.2)
The thermal capacity of a substance is indicated by the quantity of heat required
to raise the temperature of 1 lb of the substance 1�F. In HVAC calculations, thermal
capacity is usually expressed by the British thermal unit (Btu).
One Btu is the amount of heat that is required to increase the temperature of 1
lb of water 1�F at or near 39.2�F, which is the temperature at which water has its
maximum density. Conversely, if 1 Btu is removed from 1 lb of water, its temperature
will be reduced by 1�F.
Various quantities of heat will produce changes of 1�F per pound of substances
other than water. Thus, thermal capacity is entirely dependent on the specific heat
of the substances.
The specific heat of a substance is the ratio of the heat content or thermal
capacity of a substance to that of water. And by definition, the specific heat of
water is unity.
It is customary in HVAC calculations to use specific heat in lieu of thermal
capacity, because of the convenience of using the Btu as a unit of heat quantity
without conversions. Specific heats of air and some common building materials are
shown in Table 13.1. Data for other substances may be obtained from tables in the
‘‘ASHRAE Handbook—Fundamentals,’’ American Society of Heating, Refrigerating
and Air-Conditioning Engineers. An examination of Table 13.1 indicates that
the specific heat of these materials is less than unity and that, of all common
substances, water possesses the largest specific heat and the largest thermal capacity.