
The recirculation airflow rate can then be calculated using:
Q
62
¼ Q
24
Q
12
ð2:66Þ
with
Q
62
¼ TðP; 1Þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q
2
24
þ Q
2
12
q
ffi TðP ; 1Þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þð1 RÞ
2
q
Q ð2:67Þ
assuming that the relative error Q=Q is the same for both airflow rates, and
taking into account that Q
12
¼ð1 RÞQ
24
. Note that, in this case, Q
62
decreases when R increases.
The extract airflow rate Q
46
cannot be assessed without injecting a tracer
gas in the extrac t duct. However, in air handling units that have no exhaust
duct (such as most units in Singapore and other tropical count ries), Q
60
¼ 0,
hence Q
46
¼ Q
62
, and Q
40
¼ Q
01
þ Q
04
.
The recirculation ratio is defined by:
R ¼
Q
62
Q
24
¼
Q
62
Q
62
þ Q
12
ð2:68Þ
Assuming that there is no leak in the air handling unit, it can be assessed using
three different methods:
Method A
R ¼
C
3k
C
1
0
k
C
6k
C
1
0
k
ð2:69Þ
the subscript k being for any tracer gas except the one inject ed in the inlet duct.
The confidence interval is:
R ¼
TðP; 1Þ
ðC
6k
C
1
0
k
Þ
2
ffiffiffiffiffi
f
R
p
ð2:70Þ
where
f
R
¼ðC
3k
C
6k
Þ
2
C
2
1
0
k
þðC
6k
C
1
0
k
Þ
2
C
2
3k
þðC
3k
C
1
0
k
Þ
2
C
2
6k
ð2:71Þ
If we assume that the relative error is the same for all concentrations, and taking
into account that, for tracers injected at locations 2 and 3, C
1
0
k
ffi 0 and therefore
C
3k
ffi RC
6k
, we can get a simpler expression for the confidence interval of the
recirculation ratio:
R ffi
TðP; 1ÞC
C
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2ðR
2
R þ 1Þ
q
ð2:72Þ
The recirculation ratio can also be calculated using:
Method B
R ¼
Q
62
Q
24
Airflow Rates in Air Handling Units 37
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