Environmental Engineering Reference
In-Depth Information
Use of Figure 8.2 to obtain ventilation rates requires the measurement
of peak CO
levels. Depending on the actual building air exchange rate, peak
2
CO
levels may occur within a few hours of early morning occupancy to
upwards of 4 to 5 hours later (the latter under poorly ventilated building
conditions).
2
ii. Thermal balance. Percent outdoor air can also be determined from
temperature measurements of outdoor air, return air, and mixed air mea-
sured in the AHU box where outdoor and recirculated air are mixed. In the
latter case, access to the mixing box is essential, and such access may not be
easily achieved in small AHUs.
The percent of outdoor air can be calculated from Equation 8.3.
T R
-
T M
outdoor air (%)
=
-------------------
(8.3)
T R
-
T O
where T
= temperature of return air
R
T
= temperature of mixed air
M
T
= temperature of outdoor air
O
Percent outdoor air can be converted to a volumetric flow rate using Equa-
tion 8.2.
The thermal balance method is limited to assessing airflow through the
HVAC system and does not take into account air exchange associated with
infiltration (as do CO
techniques).
2
iii. Tracer gases. Outdoor air ventilation rates can, in theory, be deter-
mined by the use of tracer gases such as sulfur hexafluoride (SF
). Their
effective use is limited to small buildings or building spaces where tracer
gases can be easily injected, mixed with building air, and sampled.
Tracer gas use is based on the mass balance assumption that the change
of tracer gas concentrations in building spaces is a function of the amount
of tracer gas introduced minus the amount removed by mechanical ventila-
tion, exhaust systems, and exfiltration.
After the introduction of a tracer gas and mixing with air in building
spaces, the decay or decrease of tracer gas levels with time is exponentially
related to the outdoor air ventilation rate. From measurements of tracer
gas levels over time, air exchange rates (ACH) can be calculated using
Equation 8.4.
6
C o
C t
ln
------
n
=
-----------
(8.4)
T
Search WWH ::




Custom Search