Thermodynamics
ENERGY, WORK, AND HEAT
Another type of heat is called latent heat. Latent heat is the amount of heat added to or removed
from a substance to produce a change in phase. When latent heat is added, no temperature
change occurs. There are two types of latent heat. The first is the latent heat of fusion. This
is the amount of heat added or removed to change phase between solid and liquid. The second
type of latent heat is the latent heat of vaporization. This is the amount of heat added or
removed to change phase between liquid and vapor. The latent heat of vaporization is sometimes
called the latent heat of condensation.
Different substances are affected to different magnitudes by the addition of heat. When a given
amount of heat is added to different substances, their temperatures increase by different amounts.
The ratio of the heat (Q) added to or removed from a substance to the change in temperature
(DT) produced is called the heat capacity (Cp) of the substance. The heat capacity of a substance
per unit mass is called the specific heat (cp) of the substance. The subscript p indicates that the
heat capacity and specific heat apply when the heat is added or removed at constant pressure.
(1-17)
Cp
Q
DT
cp
Q
mDT
cp
q
DT
where:
Cp
=
heat capacity at constant pressure (Btu/°F)
cp
=
specific heat at constant pressure (Btu/lbm-°F)
Q
=
heat transferred (Btu)
q
=
heat transferred per unit mass (Btu/lbm)
m
=
mass (lbm)
DT
=
temperature change (°F)
One lbm of water is raised 1°F and one Btu of heat is added. This implies that the specific heat
(cp) of water is one Btu/lbm-°F. The cp of water is equal to one Btu/lbm-°F only at 39.1°F.
By rearranging Equation 1-17 we obtain Q = mcpDT, which is used to calculate latent heat. By
substituting mass flow rate in lbm/hr,
, for m, we obtain
. This equation is used
m
Q
mcpDT
to calculate heat transfer in Btu/hr and will be useful in later chapters.
Rev. 0
Page 21
HT-01
