Optimization of VAV AHU Terminal Box Minimum Airflow
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Determining the optimal terminal box airflow is a complex process which is influenced by various factors, such as weather condition, supply air temperature, primary air fraction and internal load. A guideline for determination of a cost efficient minimum airflow setpoint for VAV terminal box units is drawn in this research. The most efficient optimal minimum airflow setpoint should not be a fix setting, but should be changing with zone load and ventilation requirement. A fixed minimum airflow is used in conventional control strategies. The terminal box minimum airflow required is not a constant since the supply air temperature, fresh air fraction and zone load are different. It is important to set up the minimum airflow to ensure IAQ and thermal comfort and to minimize energy consumption. Analysis has been carried out to compare how the supply air temperature, fresh air fraction and zone load affect the minimum airflow setting of an exterior zone. And 30% of design airflow is not always a good number, and may cause comfort issue or ventilation problem. If the minimum airflow is set higher than required, terminal boxes will have significantly simultaneous heating and cooling, and consume more fan power in the AHUs. If the minimum airflow is set lower than required, indoor air quality (IAQ) will be a concern. Energy saving ratio study is conducted to estimate the energy saving benefit by implementing an optimized minimum airflow.