Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.11
Kelvin (�K) in the Celsius system and in degrees Rankine (�R) in the Fahrenheit
system. Absolute zero or zero degrees in either system is determined by considering
the theoretical behavior of an ideal gas, and for such a gas,
PV� a mRTa or Pav�RT a
(13.8)
where Pa � absolute pressure on the gas, psf
V � volume of the gas, ft3 v � specific volume of the gas, ft3 /lb � reciprocal of the gas density
m � mass of the gas, lb
Ta � absolute temperature
R � universal gas constant
For a gas under constant pressure, the absolute temperature theoretically will be
zero when the volume is zero and all molecular motion ceases. Under these conditions,
the absolute zero temperature has been determined to be nearly �273�C
and �460�F. Therefore,
Kelvin temperature �K � Celsius temperature � 273� (13.9)
Rankine temperature �R � Fahrenheit temperature � 460� (13.10)
In the Rankine system, the universal gas constant R equals 1545.3 divided by
the molecular weight of the gas. For air, R � 53.4, and for water vapor, R � 85.8.
13.2.6 Latent Heat
The sensible heat of a substance is associated with a sensible change in temperature.
In contrast, the latent heat of a substance is always involved with a change in state
of a substance, such as from ice to water and from water to steam or water vapor.
Latent heat is very important in HVAC calculations and design, because the total
heat content of air almost always contains some water in the form of vapor. The
concept of latent heat may be clarified by consideration of the changes of state of
water.
When heat is added to ice, the temperature rises until the ice reaches its melting
point. Then, the ice continues to absorb heat without a change in temperature until
a required amount of heat is absorbed per pound of ice, at which point it begins
melting to form liquid water. The reverse is also true: if the liquid is cooled to the
freezing point, this same quantity of heat must be removed to cause the liquid water
to change to the solid (ice) state. This heat is called the latent heat of fusion for
water. It is equal to 144 Btu and will convert 1 lb of ice at 32�F to 1 lb of water
at 32�F. Thus,
Latent heat of fusion for water � 144 Btu/lb (13.11)
If the pound of water is heated further, say to 212�F, then an additional 180 Btu
of heat must be added to effect the 180�F sensible change in temperature. At this
temperature, any further addition of heat will not increase the temperature of the
water beyond 212�F. With the continued application of heat, the water experiences
violent agitation, called boiling. The boiling temperature of water is 212�F at
atmospheric pressure.
With continued heating, the boiling water absorbs 970 Btu for each pound of
water without a change in temperature and completely changes its state from liquid
at 212�F to water vapor, or steam, at 212�F. Therefore, at 212�F,