Fault Detection and Diagnostics for Rooftop Air Conditioners

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has two possible causes: refrigerant is undercharged when service was done or there is a
refrigerant leakage. Therefore, low charge can be a system-level operational or service
fault.
Since the rooftop unit is under positive gage pressure system when charged, noncondensable
gases can only be introduced during service. Non-condensable gases tend to
accumulate in the condenser. Its primary impact is to increase heat transfer resistance and
results in high condensing pressures and temperatures. So, non-condensable gas is
considered to be a component-level service fault.
In summary, the characteristic of a component-level fault is that its source impact
is confined to a specific location or component and all the other impacts on the system
originate from this source impact. On the contrary, the source impact of a system-level
fault cannot be confined to a specific location or component. Operational faults usually
develop through running and occur randomly or gradually, while service faults are
introduced with service.
1.2.2 Interactions
As depicted in Figure 1-6, a rooftop unit can be represented as a black-box, which
is driven by faults, disturbances and overall system driving conditions, including T
aoc
,
T
aie, and φ
aie
, and outputs overall system state variables. It is difficult to tell which
factors contribute to the current operation state directly from overall state variables. The
SRB method uses normal state models to predict the normal operation states according to
the overall driving conditions and generates residuals to decouple the interactions
between driving conditions and faults, and further uses statistical analysis to further
decouple the actions from disturbances, but leaves the couplings among the different
faults untouched. This is the reason why the SRB FDD methods cannot handle multiplesimultaneous
faults.
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