Trane TRG-TRC014-EN manual Single-DuctVAV Systems, Components of a VAV System, period two

period two

Components of a VAV System

notes	conditions in the space. The constant sound of the series unit, with the fan
	Most designers also believe that series units offer improved acoustical
	operating whenever the space is occupied, is generally preferred to the on-off
	sound generated by the cycling fan in the parallel unit.
	However, because the fan runs continuously whenever the space is occupied, a
	series fan-powered unit consumes more energy than a parallel fan-powered
	unit. The development of high-efficiency motors has lessoned the energy
	consumption difference. The fan in a series fan-powered unit also costs more
	than an equivalent parallel fan-powered unit because it generally requires a
	larger terminal fan.
	Fan-powered terminal units without a heating coil are typically used to provide
	tempering for those zones that require year-round cooling and have relatively
	high minimum airflow settings, such as the densely occupied interior zones of a
	building (i.e., a conference room). A fan-powered terminal unit with a heating
	coil is typically used for spaces that require seasonal cooling and heating, such
	as the exterior zones of a building. Units with heating coils are the most
	common of the fan-powered terminal units.

Single-Duct VAV Systems

	central air handler
OA	PA	55°F
90°F		[12.8°C]
[32.2°C]
	VSD
RA	single-duct
EA	VAV terminal units
80°F
[26.7°C]

Figure 33

The types of terminal units discussed so far are used in single-duct VAV systems. Single-duct VAV systems use a central return-air path that allows the air from the spaces to come back to the air handler. At the air handler, a portion of this return air is recirculated and mixed with outdoor air (introduced for space ventilation purposes). This mixture of outdoor and recirculated return air, or primary air, is then conditioned and delivered to the VAV terminal units through the supply duct system.

The remainder of the return air is exhausted from the building.

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