Environmental Engineering Reference
In-Depth Information
to typical indoor environment conditions. Testing is conducted by placing a
sample of a source material in the chamber and measuring the concentration
of individual substances at the chamber outlet or some well-mixed location.
An exception to this is the widely used field and laboratory emission cell
(FLEC) developed by Danish scientists. The stainless steel top of the FLEC
chamber is placed on the surface of a source which becomes the bottom of
the chamber. The size of the source sample is usually determined by expected
loading factors under conditions of real-world use. A loading factor is
described as the surface area of a source divided by the air volume of the
space in which it is used. Loading factors are typically expressed as m
2
/m
3
or, in the case of HCHO, ft
2
/ft
3
.
Concentration data are collected over a sufficient time interval to
describe the history of emission rates. Emission rates may be determined for
different chamber air exchange conditions or ventilation rates. They are often
determined under very high ventilation rates to ensure that contaminant
levels are above their LOD values.
b. Large chambers.
Small chambers, while very useful in determining
emission characteristics of many source materials, have obvious limitations.
They cannot be used to test large assemblages such as furniture, work sta-
tions, office equipment, etc. Large (15 to 30 m
3
) chambers are used to over-
come these limitations but are much more expensive to construct, house,
and operate. Nevertheless, they are the chamber of choice in emission and
concentration studies, and testing when sources need to meet (1) perfor-
mance criteria such as those required by the state of Washington for govern-
ment office buildings or (2) compliance with the Department of Housing
and Urban Development (HUD) HCHO product emission standards for
particle board and hardwood plywood paneling used in the construction of
manufactured housing.
B. Emission rates and rate modeling
Emission rates are expressed as mg/m
2
(hr) or
g/m
2
(hr). They are calcu-
lated from concentration data, the loading rate (source surface area divided
by chamber volume), and air exchange rate using the following equation:
µ
ER= C(N/L)
(9.4)
where ER = emission rate, mg/m
2
(hr)
C
= chamber concentration (mg/m
3
,
µ
g/m
3
)
N
= chamber air exchange rate, (hr
-1
)
L
= loading rate, m
2
/m
3
Emission rates decrease with time. These time-dependent changes in ER
are often modeled using a first-order decay equation.
