Fault Detection and Diagnostics for Rooftop Air Conditioners

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Figure 2-5 Decoupling evaporator fouling fault using measured refrigerant mass flow rate
Figure 2-6 illustrates the evaporator air mass flow rate estimated using estimated
refrigerant mass flow rate. As expected, the coupling from compressor valve leakage to
evaporator fouling is not broken if refrigerant mass flow rate is estimated by compressor
map. For the same reason as condenser fouling, unilateral decoupling is sufficient for
FDD application.
Figure 2-6 Decoupling evaporator fouling fault using estimated refrigerant mass flow rate
2.1.4 Liquid-Line Restriction Decoupling
Figure 2-7 illustrates the measured liquid-line pressure drop under different fault
types with different fault and load levels. It is obvious that the liquid line pressure drop is
only influenced by the liquid-line restriction fault. The decoupling between liquid-line
restriction faults and all other faults is broken successfully.
However, it is not practical to measure the inlet and outlet pressures for FDD.
The outlet pressure P up
should be estimated using a virtual sensor. Figure 2-8 shows the
decoupling results using estimated
P
up
and measured P
3. It can be seen that the accuracy
of the
P up
estimate is within
± 5 psi . In addition, the measurement of P
3
is not available,
so Appendix 1 proposed two estimation techniques. Due to limited data, only the second
technique, assuming constant pressure drop across the condenser, was tested. Figure 2-9
shows the predict pressure drop between P
dis
and P
up. It seems that a constant pressure
drop of
25 psi in the condenser can be assumed to estimate P
3.
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