Accuracy tests for simulations of VAV dual-duct, single-zone, four-pipe fan-coil, and four-pipe induction air-handling systems

ASHRAE Transactions, July, 2006 by G.K. Yuill, J.S. Haberl, J.S. Caldwell

Four-Pipe Induction System (IU)

A schematic diagram of this system is shown in Figure 4. Outdoor air is introduced to the primary air handler at a mass flow rate sufficient to replace the exhaust air removed from the zone. The preheat coil remains off unless the outdoor air temperature drops below the setpoint, which is 45[degrees]F (7.22[degrees]C) in the cases presented. If the outdoor air temperature drops below this setpoint, the preheat coil will modulate to maintain the coil leaving air temperature at this setpoint. If the temperature after the preheat coil is such that the temperature after the fan will remain at or under the primary supply air setpoint temperature (55[degrees]F [12.78[degrees]C] in the cases presented) without additional cooling, the cooling coil remains off. Otherwise, the cooling coil modulates to maintain the primary supply air setpoint temperature after the supply fan.

Primary air is introduced into each zone's induction unit, where a secondary flow of zone supply air is induced. The total volume of supply air for each zone is constant. The setpoint for the mixed supply air for each zone changes as the space load changes. The heating and cooling coils supply heating or cooling as needed to maintain the zone setpoint temperature.

The heating and cooling coils are assumed to be in the stream of induced air returning from the room. The conditioned air is then mixed with the induction air. The specified supply airflow rate is at the specific volume of the mixed air entering the room from the induction unit.

Test Cases

Each of the mechanical systems was tested over a range of six conditions using all possible economizer cycles (for VAVDD and SZ). The six conditions are summarized in Table 1. Other fixed conditions and their applicability to the various systems are noted in Table 2.

Test Case Definitions

The sensible load for each zone was defined in this analysis as the amount of energy required to cool or heat a room's supply air from supply air temperature to room air temperature. The negative sensible loads shown in Table 1 are heating loads and the positive sensible loads are cooling loads. The latent load for each zone was defined in this analysis as the amount of energy required to vaporize the water added to the zone at 0[degrees]F (-17.78[degrees]C) plus the energy required to heat that added vapor from 0[degrees]F (-17.78[degrees]C) to room temperature.

Supply Airflow Rate. The supply volume flow rate for each zone is measured at the supply fan inlet. As the temperature and humidity preceding the supply fan change, the specific volume of that air changes. This means that the mass flow rate of supply air will change slightly from case to case.

Exhaust Airflow Rate. The exhaust volume flow rate for each zone is measured at the inlet of that zone's exhaust fan. As the temperature and humidity preceding an exhaust fan change, the specific volume of that air changes. This means that the mass flow rate of exhaust air from each zone will change slightly from case to case.