Trane TRG-TRC014-EN manual Two-Fan, Dual-DuctVAV System, Components of a VAV System, period two

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.

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