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ABSTRACT
Single duct variable air volume systems often use series and parallel fan powered terminal units to control the air flow in conditioned spaces. A laboratory verified model of single duct variable air volume systems was developed that used series and parallel fan terminal units where the fan speeds were controlled by either silicon controlled rectifiers or electronically commutated motors.
The single duct variable air volume simulation results showed that there was no significant difference between the annual energy used by parallel systems with silicon controlled rectifier controlled motors when they were compared to parallel systems that used electronically commutated motors. The study included the simulation of the operation of the same facility at five weather locations around the United States.
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INTRODUCTION
Figure 1 shows a typical single-duct variable air volume (SDVAV) system that uses fan-powered terminal units (FPTUs) to control the flow of conditioned air supplied by the primary fan. The FPTU has a small fan that is mounted either in series or in parallel with the primary system fan. Figure 2 shows a schematic of a typical parallel FPTU. The unit is called a parallel FPTU because the air flow path of the terminal fan is parallel to the air flow path of the primary fan.
In Figure 1 the terminal unit fan induces air from the return air plenum into the mixing chamber while primary air flows into the mixing chamber. The mixed air stream leaves the mixing chamber and is then supplied to the zone.
First generation FPTUs had constant speed fans where the fan motor speed was regulated with a silicon controlled rectifier (SCR) controller. Once the fan speed was set during the installation and commissioning process, it was operated as a constant speed on or off device. Manufacturers have begun to make available electronically commutated motors (ECM) in FPTUs which allow the speed of the fan to vary as the load in the zone changes. ECM motors also use about one-third of the power of a similar sized SCR controlled motor but they cost more than conventional single speed motors (Edmondson et. al. 2011). The parallel unit fan only operates at low primary air flow conditions when heat is added to maintain space...