Fault Detection and Diagnostics for Rooftop Air Conditioners

56
Condenser Air Mass Flow Rate
(lbm/hr)
20000
17500
15000
12500
10000
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 4-4 Decoupling condenser fouling fault using measured refrigerant mass flow rate
A refrigerant mass flow rate meter is too expensive for this application, so it is
estimated using compressor map data. Figure 4-5 shows the condenser mass flow rate
estimated using a refrigerant mass flow rate estimate under different fault types with
different fault and load levels. It can be seen that the condenser mass flow rate estimate is
influenced simultaneously by condenser fouling and compressor valve leakage with
inverse directions. The dependence on compressor valve leakage is caused by errors in
refrigerant mass flow rate prediction, since the compressor map was built using normal
compressor data. When there is a compressor valve leakage fault, the compressor model
over-estimates the refrigerant mass flow rate and this results in an over-estimate of
condenser air mass flow rate. So, the coupling from compressor valve leakage to
condenser fouling is not broken if the refrigerant mass flow rate is estimated using the
compressor map. However, this would not impact the FDD application, because the
coupling from condenser fouling to compressor valve leakage has been broken already.
In other words, unilateral or partial decoupling can be achieved even if refrigerant mass
flow rate is estimated using a compressor map.