Fault Detection and Diagnostics for Rooftop Air Conditioners

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5. 2 Pll
∆ deviates drastically from zero only when there is a restriction in the liquidline
and is a decoupling feature for liquid-line restriction.
In decoupling condenser fouling and evaporator fouling faults, a refrigerant mass
flow rate measurement m&
ref
is necessary. However, a mass flow rate meter is too
expensive for this application. So, the decoupling scheme shown in Figure 1-12 is called
ideal decoupling. An alternative way to obtain a refrigerant mass flow rate measurement
m&
ref
is to estimate it indirectly using compressor map data with some readily available
measurements. However, using a virtual sensor instead of a real sensor has a penalty.
Because the accuracy of real refrigerant mass flow rate measurement has nothing to do
with other faults such as a compressor leakage fault, an ideal decoupling among
component-level faults can be achieved if a real measurement is used. However, the
estimate from a virtual sensor is strongly dependent on the compressor performance. If
the compressor has a valve or other fault, the refrigerant mass flow rate could be
overestimated. Since condenser and evaporator air mass flow rates are also estimated by
virtual sensors, which use refrigerant mass flow rate as an input, overestimated
refrigerant mass flow rate would result in underestimated condenser and evaporator air
mass flow rate. Therefore, the couplings from compressor leakage to condenser and
evaporator fouling are not broken. In other words, use of a virtual sensor would result in
unilateral decoupling between a compressor leakage fault and condenser and evaporator
fouling faults.
Unlike control applications, in which bilateral decoupling is preferred, unilateral
decoupling is sufficient for an FDD application. Later sections will discuss how to do
FDD after decoupling. Figure 1-14 modifies the ideal decoupling scheme in Figure 1-13
to an actual decoupling scheme using a virtual sensor for refrigerant flow rate.
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