Fault Detection and Diagnostics for Rooftop Air Conditioners

60
4.2.1.4 Liquid-Line Restriction Decoupling
Figure 4-9 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 4-10 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
± 5psi
. In addition, the measurement of P 3
is not available,
so Deliverable 2.1.5 proposed two estimation techniques. Due to limited data, only the
second technique, assuming constant pressure drop across the condenser, was tested.
Figure 4-11 shows the predict pressure drop between P dis
and P
up
. It seems that a
constant pressure drop of
25psi
in the condenser can be assumed to estimate P
3.
Liquid-Line Pressure Drop (psi)
40
20
0
-20
evapfoul condfoul refleak llrestr compnv
Load level 1 Load level 2 Load level 3 Load level 4 Load level 5
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
Fault Level
Figure 4-9 Decoupling liquid-line restriction using measured pressure drop