Fault Detection and Diagnostics for Rooftop Air Conditioners

α , ρ,
τ
I t
t s
h o
A
t a
Figure 4.3 Solar radiation impact on the temperature sensor reading
Rearrange the above equation to get
I
= α
t
t
s
+
ho
t
a
αI
t
Note that
h
o
+ t
a
is also the sol-air temperature for any surface having the same α , I , h
t
o
and
t a
. The sol-air temperature of HVAC equipment surfaces is an equivalent outside air
temperature increased by an amount to account for the solar radiation when the heat
transfer is calculated between outside air with the opaque HVAC equipment surfaces.
However, only the form is the same, since different surfaces have different absorptances
and incident radiation. HVAC equipment have surface absorptances that are different from
the absorptance of temperature sensor and also different components of the HVAC
equipment have different absorptances. The condenser can be considered a black body
with an absorptance of 1. What’s more, in deriving the above so called sol-air temperature,
thermal balance is assumed and thermal storage is neglected, which is well satisfied by the
temperature sensor but not by the HVAC equipment whose thermal storage is more
significant. So the unshielded temperature sensor’s reading is not exactly the sol-air
temperature of the HVAC equipment.
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