Fault Detection and Diagnostics for Rooftop Air Conditioners

39
rncg
o
∆ Tcond, abnorm
< −34.14 C
(1-17)
χ
Suppose that there is 1% (in mole sense) non-condensable gas in the whole
system, and refrigerant quality is 0.03
1
r
ncg
= and χ
ref
= 0. 05
99
then,
y
ncg
1
=
χ
1+
r
ref
ncg
ref
1
= ≈ 0.17
0.05
1+
1 99
It is obvious that although the mole fraction of the non-condensable gas in the
whole system is very small, say 1%, the refrigerant partial pressure
P ,
is
ref vapor
considerably less than the total pressure P
dis
(83%). According to equation (1-17),
r
1
ncg
T
99
o
∆
cond, abnorm
< −34.14
= −34.14
= −6.90
C = −12.
41
χ
0.05
ref
F
So,
∆ is a very sensitive FDD feature. From equation (1-17), it is
T
cond,
abnorm
obvious that
∆ T , is inversely dependent on the refrigerant quality, χ
ref
cond
abnorm
. If the
refrigerant quality approaches to 1, the sensitivity would be low. However, the high
quality case is not possible when the system is stopped and immediately after the service.
Table 1-2 lists the refrigerant quality at different ambient temperature for a 3-ton rooftop
unit with the compressor off and nominal charge. From Table 1-2, it can be seen that
refrigerant quality is very low when the system is stopped and fully charged, so the
sensitivity would be very high.
Table 1-2 χref
and ∆ T
cond,
abnorm
with compressor off and nominal charge and 1% noncondensable
gas
T
amb( o C ) 20 30 40 30
χ 0.031 0.040 0.032 0.063
ref
∆ T
cond,
abnorm( o C ) -11.12 -8.62 -6.63 -3.31
39