Civil Engineering Reference
In-Depth Information
When the ventilation rate is high, systems allowing heat recovery from exhaust
air can be installed (see the example in Sect.
13.7
).
Economizer systems are used to mix external air with return air; for instance in
cooling mode return air at 24
C (75
F) can be mixed with cold outside air to
maintain input air at 10-15
C (50-60
F) in a cooled space. If the outside
temperature is around the preset value, 100 % outside air can be used. Thus the
system works in the so-called free cooling mode. In hot seasons, the integration of
outside air is kept as low as possible.
13.7
Practical Examples
Example 1
Heat recovery from exhaust air from buildings and inlet air entering
buildings
Industrial laboratories, such as those in pharmaceutical and chemical factories,
generally require high ventilation rates: 10-15 volumes of outside air which enters
the building and must be heated or cooled to maintain the required temperature and
relative humidity.
This example illustrates the energy saving by a group of exchangers (air to air
with a water-glycol solution as intermediate fluid) which recovers the energy
content of the exhaust outlet air in the cold season and of inlet air in the hot season.
The flow of the air without and with recovery equipment is represented in
Figs.
13.6
and
13.7
. Boiler-fed steam/air exchangers do all the heating. If recovery
exchangers are installed, the system is modified as in the figures. There are two
operation modes:
- Cold season mode (see Fig.
13.6
):
Exchangers are installed to recover sensible and sometimes also latent heat from
the exhaust outlet air. The heat is released to a water-glycol solution, which
circulates by means of pumps, and exchanges heat with the inlet air that
otherwise would have to be heated by boiler-fed steam/air exchangers.
- Hot season mode (see Fig.
13.7
):
The inlet air passes through a cooling dehumidification section, followed by a
re-heating section where boiler-fed steam/air exchangers are installed. The
recovery system extracts heat from the total inlet air and transfers it to a part
(40 %) of the air leaving the dehumidification section, thus reducing energy
consumption for re-heating the latter. The total inlet air is cooled; thus, the
consumption of the compressor refrigerating plant is reduced.
- Intermediate season mode:
No recovery occurs and all heat is supplied by boiler-fed steam/air exchangers.
The energy saving in cold season and hot season modes is calculated by
considering only (1) the energy which the HVAC system (boiler plant and
compressor refrigerating plant) would have required without recovered heat
and (2) the additional consumption due to the new system. The calculations
are summarized in Table
13.2
.
Economic evaluation is shown in Table
19.4
.
