Fault Detection and Diagnostics for Rooftop Air Conditioners

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To handle multiple-simultaneous faults, the interactions among different faults
should be decoupled. That is, if one independent feature, which is impacted only by one
fault, can be found for each individual fault, then multiple-simultaneous faults are
decoupled. For a linear system or some special nonlinear systems, a transformation can
be found to diagonalize a transfer function matrix to decouple the system if a detailed
system physical model is available. However, to obtain such a detailed physical model
taking faults into account for a rooftop unit system is extremely difficult. Even so, an
objective is to decouple the interactions between faults. Another way to decouple the
system is to unfold the black-box representing the rooftop unit system to view it from a
microscopic point of view and find some independent features with physical meaning for
component-level faults, and isolate service faults from operation faults immediately after
service is done and when the system stops. There is an important and practical restriction
for the independence features. They should be able to be expressed as functions of lowcost
measurements such as temperature and pressure.
Overall Driving Conditions
T
aoc
, T
aie, φ
aie
Compressor Valve
Leakage
Low Refrigerant Charge
Condenser Fouling
Refrigerant Overcharge
Liquid-Line Restriction
Non-Condensable Gas
Evaporator Fouling
Disturbances
Rooftop Unit System
Compressor
Condenser
Liquid-Line
Evaporator
η
v
Reaction
m&
ca
Reaction
∆ P ll
Reaction
m&
ea
Reaction
T
evap
T
sh
T
hg
T
cond
T
sc
∆ T ea
∆ T ca
∆ P ll
Overall System State Variables
Figure 1-6 Interactions of Rooftop Unit System
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