Civil Engineering Reference
In-Depth Information
partial pressure and constant humidity ratio so that a new temperature (point 3) is
reached with a relative humidity less than 100 %.
In order to avoid re-heating, which is an energy-consuming operation, dehumid-
ification systems can bring the moist air mixture near the comfort zone instead of
reaching saturation.
Humidification is implemented when it is necessary to increase the mois-
ture content of the air.
It can be achieved by steam or by water injection. In the first case, the
humidity ratio and the dry-bulb temperature will increase. In the latter
the temperature at the outlet will be lower than at the inlet.
Cooling by evaporation can be done in hot dry climates. This is accomplished by
spraying water into the air or by forcing contact with water. Because of the low
humidity of the inlet air, part of the water evaporates, thus lowering air temperature
and increasing relative humidity. If the inlet relative humidity value is small, the
final humidity can be considered comfortable. No energy is added, so this phenom-
enon can be considered an adiabatic process (like humidification by water injec-
tion). As the constant enthalpy curves of the mixture are close to those of constant
wet-bulb temperature, the process occurs at roughly constant wet-bulb temperature,
as is shown by psychrometric charts.
Adiabatic mixing of two moist airstreams may occur when hot and cold streams
are mixed or when exhaust air or external air is added.
13.3 Heating and Cooling Load Calculation
The design of HVAC systems requires a detailed analysis of cooling and heating
loads. The choice among different systems depends on many factors such as
building layout and occupancy, installation and maintenance requirements, capital
and operating cost, energy cost, and production requirements.
The first step in designing an HVAC system is the calculation of heating and
cooling loads.
To calculate the heating load, heat transmission losses through walls, glass,
ceilings, floors, and other surfaces must be considered together with infiltration
losses. Required indoor and outdoor temperatures must obviously be taken into
account. In addition, the energy required to warm outdoor air to the space
temperatures must be considered. The sum of all these terms represents the total
heating load.
The internal heat sources (sensible and latent heat produced by people, motors,
and machinery) may affect the size of the heating system and its operating modes.
In industrial plants (manufacturing, data center, others) and commercial buildings,
internal heat sources can supply a significant portion of the heating requirements.
