©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 1, January 2014.
By Youming Chen, Member ASHRAE; Haitao Wang; Cary W. H. Chan; and Jianying Qin, Member ASHRAE
About the Authors
Youming Chen is a professor of Hunan University in Changsha, Hunan, China. Haitao Wang is a project engineer of Hunan University in Changsha, Hunan, China. Cary W.H. Chan is a general manager at Technical Services and Sustainability of Swire Properties Ltd in Hong Kong and Jianying Qin is an assistant building services manager of Swire Properties Ltd in Hong Kong.
After a fault detection system for nearly 1,200 VAV terminals was installed at Cambridge House in Hong Kong, the building saved 13% annually on its electrical bill. The 10-year-old office building has 36 stories with a gross floor area of 317,361 ft2 (29 484 m2). The 20th floor and the roof are mechanical floors.
The building is divided into low (ground to 25th floor) and high zones (26th to 36th floor). Two heat exchangers on the 20th floor transfer cooling energy from the low to the high zone.
Each floor in the building is served by a single duct VAV air-conditioning system. Constant air volume boxes provide 1,377 cfm (650 L/s) of fresh air for each floor. Fresh air is delivered to the air-handling unit (AHU) room of each floor by three fresh air fans. Return air is sent to the AHU room through a ceiling void.
The layout of an air-conditioning system for a typical floor is shown in Figure 1. An AHU with monitoring and control instrumentation provides an adequate fresh airflow rate, suitable supply air temperature, and supply air pressure. VAV terminals are used to maintain zone air temperature at the desired value through regulating supply air volume into the zones.
VAV terminals are likely to fail after running for several years. VAV terminal faults cause zone air temperatures out of the desired values, poor thermal comfort and more energy consumption.
The 1,186 VAV terminals are dispersed in the closed ceiling void. It is difficult to check all VAV terminals manually because of the labor involved and issues with access to tenant spaces. An investigation of VAV terminals for a similar office building took two technicians almost six months to complete. Current energy management and control systems (EMCS) do not diagnose faults of VAV terminals. But, the effective use of a fault detection and diagnosis (FDD) tool for VAV terminals can help improve thermal comfort, increase equipment service life, and reduce maintenance costs and energy consumption.
For the Cambridge House project, an online FDD tool was developed to find and identify VAV terminal faults. The FDD tool can analyze the operating data in an EMCS automatically and provide building operators with user-friendly fault information of VAV terminals.
The implementation of the FDD tool in Cambridge House covers two phases: tool development (January 2009 to December 2009) and application trial (January 2010 to present). In the application phase, many VAV terminal faults were verified and corrected because of fault reports provided by the tool.
FDD Tool for VAV Terminals
The FDD tool for VAV terminals adopts a robust fault detection and diagnosis strategy. In the strategy, a residual-based cumulative sum (CUSUM) control chart is used to detect faults. And, some expert rules derived from mass and energy balances are designed to find fault sources in VAV terminals. The FDD tool is installed in a stand-alone computer. A schematic of the FDD tool integrated with the EMCS is shown in Figure 2.
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