Overview of ..
This part of the Energy Efficiency Manual shows you how
to save energy in induction air handling systems. Induction systems
are a form of single-duct reheat system, which is notoriously wasteful
because it uses heating energy to warm up air that has been previously
cooled. During low-load conditions, much more energy may be cancelled
by reheat than actually enters the space to provide cooling or heating.
The primary air is cooled by a coil in the air handling unit. The
temperature in individual spaces is regulated by a reheat coil in
the terminal unit, which is controlled by the space thermostat.
The high pressure requirement increases fan power, even though
the central fan delivers less volume. Also, induction systems are
designed to operate with low primary air temperatures. This may
require lower chilled water temperatures, which reduces the efficiency
of the chiller.
The most visible distinguishing feature of an induction system
is the terminal unit. Induction terminal units have no fans. Air
movement through coils in the terminal unit is induced by high-pressure
air, called “primary” air, that comes from a central
air handling unit. The primary air is passed through an array of
nozzles in the terminal unit that create a venturi effect, or vacuum.
The vacuum recirculates air from the space through the terminal
unit coil. The space air, called “secondary air,” mixes
with the primary air and is discharged into the space.
Induction terminal units commonly are packaged in enclosures that
are similar in size and appearance to fan-coil units. They may also
be installed in ceiling plenums. The coils are almost always hydronic.
The temperature of the coil may be controlled by throttling the
water through the coil, or a damper may be installed in the terminal
unit to bypass air around the coil.
If the terminal unit coils are designed to change between heating
and cooling, the system will have much less reheat energy waste
than if the coils are designed only for heating. When the space
requires cooling, both the coil in the air handling unit and the
coils in the terminal units cool the air. Reheat does not occur
under these circumstances. However, when a space requires heating,
the coil in the terminal unit must cancel the cooling energy in
the primary air. The amount of reheating that is unavoidable depends
on the diversity of the space loads. If some spaces have a high
cooling load at the same time that other spaces have a heating load,
reheating of the primary air will occur in the spaces that require
Terminal units that have changeover coils require effective drainage
of the moisture that condenses on the coils during cooling operation.
The design of an induction system with changeover terminal unit
coils may assume that dehumidification is accomplished by the coil
in the air handling unit, rather than by the coil in the terminal
unit. However, conditions can still occur that cause moisture to
condense on the coils. The energy conservation measures presented
here include the methods for avoiding excess reheat, which can be
done with inexpensive temperature reset controls and other methods.
You will learn to apply temperature setback to induction systems,
along with techniques for maintaining air distribution efficiency.