Civil Engineering Reference
In-Depth Information
Fig. 13.5
A basic scheme of air-handling unit
13.4.1 All-Air Systems
All-air systems may be classified as follows: variable temperature and constant
volume, constant temperature and variable volume, and variable volume and
variable temperature.
Variable temperature and constant volume systems include single-zone systems,
multi-zone with centralized blending, re-heating in each zone, dual-duct with
mixing boxes in each zone.
Constant temperature and variable volume systems are mainly dual-duct
systems, one duct with ventilation air in neutral conditions equal to those of the
space to be conditioned and the other for cooling and heating.
Variable volume and variable temperature systems are mainly dual-duct with
mixing boxes. Many combinations of the previous systems are possible.
Notice that in the re-heating mode, chilled air is supplied to each zone (for
cooling mode) and then the air is re-heated according to the temperature required
for each zone. This system consumes a great deal of energy because of the
duplication of energy use. Variable air volume, which requires supply air at
constant temperature, is based on valve control or on variable-speed fan control
(by means of a local thermostat) of the airflow supplied to each zone, to ensure the
supply only of the air needed by the real load of each zone. In this way energy
consumption is reduced.
