Fault Detection and Diagnostics for Rooftop Air Conditioners

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Figure 1-9 illustrates the liquid-line restriction on a P-h diagram. Normally the refrigerant
at the condenser outlet (3) has about 15 F subcooling. For the conditions of Figure 1-9,
the pressure drop resulting from a restriction would need to be larger than 49.2 psi for the
temperature to drop and 14.9 psi more pressure drop would cause a 5 F temperature drop.
Consequently, if temperature drop is used to do FDD, any restriction lower than 49.2 psi
is not detectable and to get 5 F temperature drop, a total pressure drop of 64.1 psi is
necessary. In addition, when there is a liquid-line restriction, the actual subcooling would
become even larger (say 20 F at 3’) and more pressure drop (64.1 psi) is needed to trigger
the temperature drop.
400
R22
P [psia]
300
200
3' 3
∆P=49.2psi
3 flash
∆P=14.9psi
3' flash
120 F
105 F
100 F
0.2 0.4 0.6 0.8
150
70 90 110 130 150 170 190
h [Btu/lb m ]
Figure 1-9 Liquid-line restriction illustration
It also should be pointed out that a large restriction may be necessary to obtain a
phase change but once the phase change occurs the pressure drop rate would increase
significantly with the restriction size because two-phase flow is more sensitive to flow
area than pure liquid refrigerant flow. The dryer/filter with a restriction acts as an
expansion device, which can result in a significant pressure and temperature drop
between the inlet and outlet of the dryer/filter. Therefore, the temperature drop can serve
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