Fault Detection and Diagnostics for Rooftop Air Conditioners

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Although the predefined fault level is arbitrary, the author believes that a fault
causing 20% cooling capacity degradation is worthwhile to service.
The normalized fault indicator indicates the relative severity of the individual faults.
However, the refrigerant low charge is a system-level fault and its impact on overall
system performance is not only determined by its own charge level but also other faults,
so the indicator oscillates a lot. Fortunately, data collected so far shows that this
oscillation does not change the decision of the fault detection and diagnosis method.
The normalized fault indicators are plotted using color bars. When an indicator is
larger than the threshold, 0.2, it is plotted in red, otherwise in green. According to
experience, an indicated fault level with a performance degradation less than
0.2*20%=4% is not reliable.
2.2.3 System Performance & Safety Degradation Window
Capacity and COP are usually used as criteria to indicate system performance.
The compressor is the most expensive part of the system and too much overheating
would result in safety problems such as bad lubrication and motor short-circuit. So
degradations of these three indices are plotted in this window. The capacity and COP
degradations are defined by,
normal _ value − current _ value
normal _ value
The normal value is predicted using an overall system performance model which
is built based on system manufacturer rating data. The model inputs are condenser inlet
air dry-bulb temperature and evaporator wet-bulb temperature.
The compressor overheat degradation is defined by,
The maximum
current _ T
dis
− normal _ T
max_ ∆T
dis
∆ Tdis
is found to be around 40 F by Chen’s data (2000), which
should be confirmed by more investigation. There is difficulty to predict normal values
dis
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