Page 543 - Mechanical Engineers' Handbook (Volume 4)

532 Indoor Environmental Control

heating in winter and cooling in summer. The humidity control is present in the form of
humidification and dehumidification, but not always. Recently, air cleaners became cheaper
and, as a result, are installed more widely for their potential to remove dust and other
potential allergens. Overall, residential HVAC primarily controls the indoor air temperature.
For commercial buildings, HVAC systems are more sophisticated and may control many
parameters, such as pressure, temperature, humidity, carbon dioxide levels, and other gaseous
or particulate contaminant concentrations.

1.1 Moist Air Parameters
HVAC systems supply treated outdoor air to building spaces. The outdoor air conditions
depend on location, elevation, and time of day. When designing an HVAC system, engineers
use outdoor air parameters for U.S. standard atmosphere (NASA, 1976). The standard at-
mosphere contains the following components specified by volume fraction: 78.084% nitrogen
(N ), 20.948% oxygen (O ), 0.934% argon (A), 0.031% carbon dioxide (CO ), 0.003% other
2
2
2
minor gases. These components form a gas mixture called dry air. The atmospheric air, in
addition to the dry air, includes water vapor and different gaseous and particulate contami-
nants. The standard design of HVAC systems accounts for the atmospheric air as a binary
mixture of dry air and water vapor called moist air. Both of these moist air components are
considered to obey the ideal gas law:
For dry air: p v RT (1)
a
aa
For water vapor: p v RT (2)
v
vv
where p is the gas partial pressure, v is the specific gas volume, T is the gas absolute
temperature, and R is the specific gas constant (R 287 J/(kg K); R 462 J/(kg K)).
v
a
The ideal gas law proves to be an excellent approximation for the real gas behavior of
both moist air components. The following moist air parameters are commonly used in the
design of HVAC systems: pressure p, dry bulb temperature T , wet bulb temperature T WB ,
DB
dew point temperature T , humidity ratio W, relative humidity , enthalpy i, and specific
dp
volume v.
The atmospheric pressure is the first consideration when designing an HVAC system.
The standard barometric pressure at sea level is 101.325 kPa, which linearly decreases with
the elevation based on the following equation (ASHRAE, 2001):
p 101.325(1 2.25577 10 5 H) 5.2559 [kPa] (3)
where p is the barometric pressure in [kPa] and H is the elevation in [m].
and
The total barometric pressure is a sum of the partial pressures of the dry air p a
water vapor p based on the Gibbs-Dalton’s law for ideal gases:
v
p p p v (4)
a
The standard atmosphere also has defined standard air temperatures for different ele-
vations, but when designing an HVAC system a more detailed temperature distribution is
taken into account. The design temperature varies with geographic location and it is tabulated
for most of the United States, Canada, and other world locations (ASHRAE, Chapter 27,
2001). In fact, two different temperatures, dry bulb temperature and wet bulb temperature,
are specified for each location. To distinguish these two temperatures, it is necessary to
introduce a condition called the saturation of moist air. Saturation is a condition of moist air
that occurs because the moist air can contain only a limited amount of water vapor. When
saturation occurs, the excess water vapor condenses on nearby surfaces. By definition, the