Fault Detection and Diagnostics for Rooftop Air Conditioners

110
temperature on the RTD is uniform and identical with the tube wall temperature, which is
equivalent to thermocouple (see Figure A2-2).
T
of
α
o
T
ow
α
i
δ
w
= d 2
− d 1 λ
T
if
T
iw
Figure A2-2 Thermocouple measuring scheme
Figure A2-3 sketches the equivalent thermal circuit of the thermocouple
measuring scheme and thermal resistance values are listed in Table A2-1. It can be seen
that the thermal resistance between refrigerant inside the tube and the thermocouple only
accounts for 1.6% of the total thermal resistance, which means that the measuring error
would be 1.6 F if there was a 100 F temperature difference between the refrigerant inside
the tube and ambient air temperature. So it can be concluded that the thermocouple can
measure the refrigerant temperature at reasonably good accuracy.
T
if
R
i
T
iw
R
tb
T
ow
R
in
T
in
R
o
T
of
1
α πd i
1
ln( d
2
d1)
2πλ
tb
ln( d3
d2)
2πλ
in
1
α o πd
1
Figure A2-3 Equivalent thermal circuit
110