period two

Components of a VAV System

notes

the respective duct system to the modulation devices in the dual-duct VAV

terminal units.

 

While this single-fan configuration requires only one air handler, it is very

 

complicated to control efficiently. In this example, the economizer is controlling

 

the mixed air temperature to 55°F [12.8°C], thus saving the energy to operate

 

the cooling coil. However, the heating coil must warm the air from 55°F [12.8°C]

 

to the 105°F [40.6°C] primary air temperature. If the economizer was not

 

activated, the mixed air temperature would be 75°F [23.9°C], requiring less

 

heating energy, but now the cooling coil must operate. Optimizing the energy

 

use of a single-fan, dual-duct system requires a very complicated control

 

system and does not operate as efficiently as a two-fan, dual-duct system.

Two-Fan, Dual-Duct VAV System

OA

55°F

PA

55°F [12.8°C]

40°F

[12.8°C]

 

 

[4.4°C]

 

 

 

cooling

 

dual-duct

air handler

 

 

 

VAV

heating

 

terminal

 

units

air handler

 

 

RA

PA

105°F [40.6°C]

EA

 

 

75°F

 

Figure 38

[23.9°C]

 

Dual-duct systems can be very energy efficient when the mixing of cool and heated air is avoided and two supply fans are used – one for heating, the other for cooling. This arrangement eliminates the inefficient or wasteful reheating of mixed air in a single-fan, dual-duct system and allows an airside economizer to provide cool primary air whenever possible.

At the cooling air handler, a portion of the return air is recirculated and mixed with outdoor air (introduced for space ventilation purposes). This mixture of outdoor and recirculated return air is then cooled and delivered as cool primary air to one of the modulation devices in the dual-duct VAV terminal units through the cooling supply duct system.

Air delivered by the heating air handler contains only recirculated air, no outdoor air. This air is heated and delivered as warm primary airflow to the other modulation device in the dual-duct VAV terminal units through the heating supply duct system.

The remainder of the return air is exhausted from the building. The cooling air modulation device typically has a minimum setting to accommodate the ventilation requirements of the space.

TRG-TRC014-EN

25

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Trane TRG-TRC014-EN manual Two-Fan, Dual-Duct VAV System
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TRG-TRC014-EN specifications

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