
where:
x
¼
_
mm
x
_
mm
x
þ
_
mm
exf
ð5:32Þ
is the extraction efficiency, i.e. that part of the air leaving the ventilated volume,
which is extr acted through the air handling unit, and
re
¼ 1 R
xs
¼
_
mm
re
_
mm
x
ð5:33Þ
is the air recovery efficiency, or that part of the extract air that passes through
the heat recovery unit.
Looking at Equation 5.32, it seems at first glance that the global heat
recovery efficiency depends only on extract and exfiltration airflow rates.
However, the purpose of ventilati on is to provide fresh, outdoor air in the venti-
lated volume. Let us see how Equation 5.32 is changed when the fresh airflow
rate taken at inlet grille is used as a reference.
Fresh air entering the air handling unit is
_
mm
o
. Because of external recircula-
tion, this air is mixed with exhaust air into the inlet duct. A part, R
ie
, of this mix
is recirculated to the exhaust duct. All the fresh, outdoor air that enters the
building through the air handling unit is found in
_
mm
rs
, which is, from the defi-
nition of R
ie
and using Equation 5.25:
_
mm
rs
¼ð1 R
ie
Þ
_
mm
i
¼ð1 R
ie
Þð
_
mm
o
þ R
e
_
mm
e
Þð5:34Þ
Since
_
mm
e
is no longer fresh, the only part of
_
mm
rs
that is fresh is
_
mm
o
ð1 R
ie
Þ.
Therefore the total fresh airflow rate entering the ventilated space in building
is:
_
mm ¼
_
mm
o
ð1 R
ie
Þþ
_
mm
inf
ð5:35Þ
which means that
_
mm
o
ð1 R
ie
Þ¼
_
mm
_
mm
inf
ð5:36Þ
replacing in Equation 5.32
_
mm
x
by its value given by Equation 5.29, and taking
into account the above relation, gives finally:
G
¼
b1
exf
R
e
R
ie
ð
inf
exf
Þcð1 R
xs
Þ
1 R
e
R
ie
ð
inf
exf
Þ
exf
½R
e
þ R
xs
ð1 R
e
Þ
"
HR
ð5:37Þ
where
inf
¼
_
mm
inf
_
mm
and
exf
¼
_
mm
exf
_
mm
ð5:38Þ
are respectively the infiltration and exfiltration ratios. In other terms, the
extraction efficiency is:
x
¼
1
exf
R
e
R
ie
ð
inf
exf
Þ
1 R
e
R
ie
ð
inf
exf
Þ
exf
½R
e
þ R
xs
ð1 R
e
Þ
ð5:39Þ
92 Ventilation and Airflow in Buildings
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