Environmental Engineering Reference
In-Depth Information
lation, can be measured using tracer gas techniques. Tracer gases used in
such measurements are characteristically unreactive, nontoxic, and easily
measured at low concentrations. Sulfur hexafluoride and perfluorocarbons
are commonly used to measure air exchange rates because they can be
detected and quantitatively determined in the parts per billion range (ppbv).
On occasion, nitrous oxide (N
) are used. Carbon
dioxide has the advantage of being measured in real time on relatively
inexpensive continuous monitors. It has the disadvantage of being produced
by humans. Thus it cannot be used when building spaces are occupied.
Silicon hexafluoride, perfluorocarbons, and N
O) or carbon dioxide (CO
2
2
O are collected using one-time
sampling techniques and require analysis on sophisticated, expensive instru-
ments. Perfluorocarbon measurements are usually made using permeation
tubes as sources and passive samplers as collectors. As such, air exchange
measurements based on perfluorocarbons typically provide 7-day averages.
The concentration decay method (previously described in Chapter 8) is
the most widely used air exchange measuring technique. It involves initial
injection of a tracer gas into a space or building with the assumption that
the tracer gas is well mixed in building/space air. The decrease, or decay as
it is often described, of the tracer gas concentration is measured over time.
From these measurements the air exchange rate I in ACH can be determined
from the following exponential equations:
2
C
= C
e
-(Q/V) t
(11.2)
t
0
where C
= tracer gas concentration at the end of the time interval (
µ
g/m
3
,
t
ppmv, ppbv)
C
= tracer gas concentration at time t = 0 (
µ
g/m
3
, ppmv, ppbv)
0
V = volume of space (m
3
)
Q = ventilation rate (m
3
/hour)
e
= natural log base
t
= time (hours)
The ratio Q/V, considered in the context of hours, yields the air exchange
rate I in ACH.
C
= C
e
(11.3)
-It
t
0
Equation 11.4, used to determine I, the air exchange rate, can be derived
from Equation 11.2.
I = (ln C
/C
)/t
(11.4)
0
t
Let us assume that the initial tracer gas concentration was 100 ppbv and
at the end of 2 hours it was 25 ppbv. We could then calculate I as follows:
