Fault Detection and Diagnostics for Rooftop Air Conditioners

4. Modeling field site data
Polynomial plus GRNN models had good performance when tested with laboratory data.
However, some factors, which will affect the actual field operations of rooftop units, were
not considered in the experimental tests. Examples include unmeasured ambient weather
conditions, such as solar radiation, rain, and strong wind on the condenser. Also the
damper position changes the air flow rate. The laboratory experimental data was collected
with a constant air flow rate. Since the mixing chamber is small, outdoor air and return air
are not mixed well and different damper positions also have some impact on mixing. So it
is necessary to investigate the impact on the model of these different factors.
4.1 Impact of unmeasured weather variables
4.1.1 Solar radiation impact on the system
Solar radiation has an impact on the whole system, which will absorb different amounts of
radiation when the intensity of sunshine is varied. The most significant and direct impact is
on the condenser. Fortunately, the solar radiation impact on the system is also reflected in
the temperature sensor readings exposed to the solar radiation. However, whether the solar
radiation impact on the whole system is properly reflected in the sensor’s readings needs to
be investigated. Solar radiation impact on the whole system will be analyzed inversely by
modeling. It is necessary to investigate the impact on the sensor’s reading before the data
are used to build a model.
There are two temperature sensors, condenser inlet and outlet air temperature sensors, that
are exposed to solar radiation. From the data gathered during August 13 to August 17 at
Purdue field site (figure 4.1) when the solar radiation was not very intense, it is apparent
that the solar radiation impact on the condenser inlet air temperature sensor is as much as
15F.
From figure 4.2, it is clear that the solar radiation impact on the condenser outlet air
temperature sensor readings depends on whether the condenser fan is on or off. When the
condenser fan is off, the solar radiation impact is up to 15 F. However, when the
condenser fan is on, the solar radiation impact on the condenser outlet air temperature
sensor readings is less than 1 F. Fortunately, the data when the condenser fan is off is not
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