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Modeling the performance of single-duct VAV systems that use Fan Powered terminal units.

Article Excerpt
INTRODUCTION

The purpose of this study was to model the operation of the Single Duct Variable Air Volume (SDVAV) systems that used Fan-Powered Terminal Units (FPTU). The objective of this paper is to describe the system model and to serve as a reference for other papers that detail the operation of the system when series or parallel type fan powered terminal units are used.

Variable Air Volume (VAV) systems maintain zone comfort by varying the amount of primary air that is delivered to conditioned spaces. The central cooling system includes a fan and cooling coil that supply pressurized and conditioned air to the primary distribution ducts. The central fan is usually referred to as the primary fan while the air that is conditioned by the cooling coil is referred to as primary air. Primary air is supplied to the conditioned space by a VAV terminal unit that regulates the amount of primary air supplied to the zone.

Figure 1 shows a SDVAV system using five FPTU. For this project, a three zone system model was developed and used to determine a matrix of test points that were then used to perform laboratory verification of the model. After the three zone model was verified it was expanded to five zones and used to evaluate the operation of a building at five different weather locations around the United States. The three zone and five zone models used the same methodology for all calculations with the exception of error corrections that were made as a result of the laboratory verification process. The verification of the model is described in a related paper (Bryant et al. 2009).

[FIGURE 1 OMITTED]

SYSTEM MODEL OVERVIEW

The system simulation procedure began with the calculation of zone level conditions followed by the return air calculations, the introduction of fresh air, pre-heat coil process and finally the primary fan and cooling coil calculations. The zone level calculations were dependent upon the type of FPTU installed in the zone. For the purpose of describing the system model, the zone level calculations will be treated as a "black-box" model that provides known values for the parameters related to the operation of the conditioned space. Using the known space parameters, the rest of the system calculations may be performed.

After the zone-level return air calculations were completed, the mixed return-air conditions were calculated following the introduction of fresh air. Next, the air properties leaving the pre-heat coil were estimated and entering and leaving conditions for the primary fan were calculated. The properties of the air leaving the primary cooling coil were assumed to be the same as the primary air entering the SFPTU. At each step along the flow path, the temperature and moisture content of the air were calculated. In the cases where air streams were mixed, the mixed air properties were calculated.

Figure 2 shows a flow diagram that identifies the calculation sequence used in the simulation program. The following sections detail the calculation procedures used in the model.

[FIGURE 2 OMITTED]

It was not required that the inlet flow valve position at each FPTU be predicted accurately by the model for a...

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