Fault Detection and Diagnostics for Rooftop Air Conditioners

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Table 1-3 also lists the refrigerant quality for different refrigerant charge levels
with an ambient temperature of 30 o C
. From Table 1-3, it can be seen that refrigerant
overcharge would raise the non-condensable FDD sensitivity, whereas refrigerant
undercharge would reduce the non-condensable FDD sensitivity. Even at an unrealistic
worst case of 20% of nominal charge, this feature would provide sufficient FDD
sensitivity.
Table 1-3 χref
and ∆ T
cond,
abnorm
with compressor off and ambient temperature=30 o C and
1% non-condensable gas
Charge Level 20% 40% 60% 80% 90% 100% 110% 120% 140%
χ 0.384 0.169 0.098 0.062 0.030 0.040 0.033 0.026 0.016
ref
∆ T
cond,
abnorm( o C ) -0.90 -2.04 -3.32 -3.36 -6.90 -8.62 -10.3 -13.3 -21.6
T
cond,
abnorm
In summary, although refrigerant charge has some influence on the sensitivity of
∆ to detect and diagnose a non-condensable gas fault, the influence would not
change the fault pattern and the sensitivity is high enough over a very large range of
ambient and charge conditions. So, the non-condensable gas fault could be detected and
diagnosed independently. In a broad sense,
∆ could be considered as an
T
cond,
abnorm
independent feature to detect and diagnose a non-condensable gas fault and the coupling
between non-condensable gas fault and condenser fouling can be broken when the system
stops. Since it is not a real decoupling, it is termed pseudo-decoupling to distinguish it
from real decoupling.
1.2.3.2.2 Condenser Fouling Fault
A rooftop unit system typically uses a constant-speed fan to force ambient air to
cool condenser coils. Fouling mainly develops on the air side and affects heat transfer in
two ways. On the one hand, it increases the thermal resistance mainly by small-scale dirt
particles attached to the coils, and on the other hand, it reduces the air flow area by largescale
dirt and results in a reduction of air flow rate. Since a rooftop air conditioner is
normally installed outside, there can be a lot of large-scale dirt such as paper and leaves.
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