Civil Engineering Reference
In-Depth Information
between the total dispersion surface (ground floor, ceiling, external surfaces with
their windows) and the building's volume.
The colder the average outdoor temperature, the lower the total building coeffi-
cient
c
d
(typical values range from 0.35 to 1 W/m
3
K) must be; the higher the
dispersion surface/volume ratio, the higher the
c
d
coefficient.
The
n
v
coefficient generally does not exceed unity if referred to the
building gross volume, so that average values of (
c
d
+ 0.35
n
v
) ranging from 0.7 to
1.3 W/m
3
K can be assumed for the total load calculation. Notice that for particular
situations, the
n
v
coefficient can be even higher than 10.
Latent heat can be introduced in the simplified relationship (
Q
) by using the
difference between the total enthalpy of the outside and inside air-water mixture
(see Sect.
13.2
).
13.4
Typical HVAC Systems
In HVAC systems the heating mode is generally operated by means of boilers, heat
pumps, and recovered heat; occasionally, depending on local conditions, electri-
cally heated systems can be used.
HVAC systems for cooling are basically refrigeration systems and evaporation
systems.
The refrigerant evaporates through an evaporator that can be an air/refrigerant or
water/refrigerant exchanger. If the exchanger is placed in the room to be cooled, air
is cooled directly when it moves across the coils. Condensation can be by water
(with cooling towers) or by air.
In evaporation HVAC systems, only pumps are required to move water inside
the circuit and to force contact with air.
Heat pumps, as individual units or centralized plants, provide both heating and
Auxiliary equipment comprises AHUs (air-handling units), pumps and fans, and
control systems.
HVAC systems can be classified as (1) all-air systems, (2) all-water systems,
(3) air-water systems, and (4) individual units. Class (1) systems serve the
conditioned space primarily by means of a network of ducts. Class (2) systems
are based on a piping network which delivers energy as water to distributed units,
typically fan coil units. Class (3) systems are a combination of the previous
systems. Class (4) systems are based on independent equipment.
Figure
13.5
shows a basic scheme of an air-handling unit where the main energy
uses (electricity for fans, cold and hot water, steam) are pointed out. Notice that the
share among them depends on the operating parameters, indoor and outdoor.
Electricity is always the most important part of AHU energy consumption.
