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Dissertation
Accuracy and stability of VAV box control at low flows
Doctor of Philosophy (Ph.D.), Drexel University
Apr 2012
DOI:
https://doi.org/10.17918/etd-3847
Abstract
Variable air volume (VAV) systems with direct digital controllers (DDC) have been widely adopted in Heating, Ventilating and Air Conditioning (HVAC) systems of commercial, industrial, and large residential buildings, because they provide better energy efficiency and occupant comfort. Normally, a VAV terminal unit defines a minimum airflow rate to satisfy the space ventilation requirement and/or the proper operation of a terminal heating coil. However, if the embedded airflow sensor becomes inaccurate, and the designed minimum airflow rate is less than the minimum controllable airflow rate, then a series of problems could happen, such as a lack of ventilation, uneven control of airflow, reduced damper and operator life, and energy waste. This study aims at 1) identifying the strong factors that cause these inaccuracy and instability issues, and the relationship between the strong factors and the performance of the airflow sensor, controller, and terminal unit system through systematically designed laboratory and field tests; and 2) based on the findings from the testing results, identifying a practical solution to improve the VAV airflow measurement accuracy at low flow conditions. Five factors, namely, inlet condition, VAV damper position, airflow rate, controller type/brand, and controller ambient temperature are identified as strong factors that affect the accuracy of VAV airflow measurement. It is found that system balancing at a low airflow rate (i.e. 560 fpm) is an effective method to improve overall VAV airflow measurement accuracy in the laboratory test. However, system balancing is not effective at improving the VAV airflow measurement during the field test due to the limitations of available reference airflow measurement in a real building. An alternative solution, namely, flow conditioner is therefore optimally designed and evaluated using both computational fluid dynamics (CFD) modeling approach and laboratory testing. The tested prototype, a 60%-porosity K-lab/Laws plate, effectively reduces the VAV airflow reading errors in the laboratory tests from ±8~±40% to be always within ±5% of the reference airflow reading when positions immediately before the VAV box inlet.
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Details
- Title
- Accuracy and stability of VAV box control at low flows
- Creators
- Ran Liu - DU
- Contributors
- Jin Wen (Advisor) - Drexel University (1970-)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University; Philadelphia, Pennsylvania
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- Civil (and Architectural) Engineering [Historical]; College of Engineering (1970-2026); Drexel University
- Other Identifier
- 3847; 991014632200004721