Fault Detection and Diagnostics for Rooftop Air Conditioners

For the condenser outlet air temperature sensor, the air flow rate is very high when the
condenser fan is on, and thus
is large and
h o
αI
t
h
o
is small.
As a result,
ts
≈ t a
When condenser fan is off, the air flow rate is very low and h is very small and
o
αI
t
h
o
is
relatively large.. However, the data when unit is off is not used to build the model.
In order to decide whether the unshielded or shielded sensor’s reading should be used to
build the HVAC system models, both options were evaluated.
4.1.2 Rainfall impact on the system
Rainfall on the condenser will cause evaporative cooling and will increase energy transfer
rates. Rainfall will also influence the readings of exposed temperature sensor, Figure 4.4
shows that an unshielded sensor’s reading will be about 2 F lower than a shielded
temperature sensor in rainfall. The shielded sensor’s reading is the dry bulb temperature of
the ambient air and the unshielded sensor’s reading approaches the wet bulb temperature
of the ambient air, since the unshielded sensor is exposed to the rain and the rain water on
the sensor will evaporate and cool the rain water on the sensor. Although, this temperature
is not exactly the same as wet bulb temperature of the air, it is near to the wet bulb
temperature. So the difference between the shielded sensor and unshielded sensor is
dependent on the relative humidity of the air. However, normally the relative humidity of
air in rain will be high enough to make the difference within 2-3 F.
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