Fault Detection and Diagnostics for Rooftop Air Conditioners

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as an independent feature only for very large liquid-line restriction faults. However, lowcost
is an advantage of using temperature drop as the feature.
To detect small liquid-line restriction faults, the pressure drop could be used.
However, pressure measurements are relatively expensive. Therefore, an approximate
scheme should be developed. Figure 1-10 illustrates the relevant components and state
variables.
P 3
T cond
P cond
P 2
900
Drier/Filter
T up
P up
T 3
COND
T 2
P [psia]
500
200
100
T 3 P 3
T up P up
T sat
P sat
P 2
T 2
EXP
T 4
P 4
EVAP
P 1
50
30
T 4 P
P 1
4
T 1
80 100 120 140 160 180
T evap
P evap
T 1
h [Btu/lb m ]
Figure 1-10 Vapor Compression Cycle Illustration
Using the nomenclature defined in Figure 1-10, the pressure drop across the
filter/dryer is
∆Pll = P3
− P up
P
3
can be approximated very well by P cond
= P sat
( T cond
) if T
cond
can be measured properly
because the condenser pressure drop resulting from liquid refrigerant flow is small (less
than 3 psi). The key point for this technique is placement of the condenser temperature
sensor, because a non-saturated temperature would result in large estimation error,
especially if a superheated temperature were measured. A relatively safe approach is to
place the temperature sensor at the point where different condensing circuits are
combined, where the refrigerant is almost never superheated unless the refrigerant charge
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