Civil Engineering Reference
In-Depth Information
the mixture, and space thermostat setting. In this system a variable frequency drive
(VFD) is used to modulate the air handling unit supply fan. Supply fan then pro-
vides a total air fl ow equal to the airfl ow quantity required (in each instant) to the
sum of all the terminal unit's primary airfl ows. This primary air then will be mixed
with some of the plenum air through the terminal unit constant volume fan as needed
before being discharged to the room (Terminal units are also equipped with hot
water reheat coils or electrical strips that are designed for either heating or dehu-
midifi cation of the air).
The sources of energy consumption in this type of systems are air handling unit
supply fan motor input power, return fan motor input power, hot water pumps or
electric preheat strips (if it is used for freeze protection instead of hot water coil) in
air handling unit and hot water pumps or electric reheat strip (if it is used for dehu-
midifi cation or heating instead of hot water coil). Duct air leakage, heat transfer
from ducts, and effi ciency loss in water coils are other sources that can indirectly
increase the energy consumption of the system. The small energy consumed by the
low-voltage power provided for control valves and dampers is the other source of
energy consumption (Khazaii
2012
).
8.12.5
Variable Air Volume System with Series
Fan-Powered Units
This system (Fig.
8.8
) is made of a central variable volume supply fan that supplies
conditioned air to all the room's series fan-powered VAV terminal units through
medium pressure ducts. Each room will receive a constant quantity of air. Quantity
and temperature of the mixture of the air which is delivered to each room is designed
based on the room sensible load, the quantity and temperature of the outdoor air in
the mixture, and space thermostat setting. In this system a variable frequency drive
(VFD) is used to modulate the air handling unit supply fan. Supply fan then pro-
vides a total air fl ow equal to the airfl ow quantity required (in each instant) to the
sum of all the active terminal unit's primary airfl ows. This primary air then will be
mixed with some of the plenum air and a constant fl ow will be delivered to the
space, any time room requires conditioning. (Terminal units are also equipped with
hot water reheat coils or electrical strips that are designed for either heating or
dehumidifi cation of the air). This type unit is usually used for special spaces such as
conference rooms that require large quantity of air at all time due to the space
relatively large occupancy.
The sources of energy consumption in this type of systems are air handling unit
supply fan motor input power, return/exhaust fan motor input power, terminal unit fan
motor input power, hot water pumps or electric preheat strips (if it is used for freeze
protection instead of hot water coil) in air handling unit, and hot water pumps or
electric reheat strips (if it is used for dehumidifi cation or heating instead of hot water
coil). Duct air leakage, heat transfer from ducts, and effi ciency loss in water coils are
other sources that can indirectly increase the energy consumption of the system.
