Basic Documentation

Siemens Industry, Inc. Page 6 of 8
Document No. 149-978
Room Supply Airflow
The Room Supply Airflow Min. CFM equals the Total
Room Exhaust Min. CFM minus the Excess Room
Exhaust Airflow For Negative Pressure.
The Room Supply Airflow Max. CFM equals the
Total Room Exhaust Max. CFM minus the Excess
Room Exhaust Airflow For Negative Pressure.
Room Supply Turndown Ratio
The room examples in Table 1 show the supply
airflow turndown ratios (right most column) vary from
a high of 7-to-1 for Room 1 down to a low of 4.3-to-1
for Room 17. If a room general exhaust were used
(for the reasons described in the following
paragraph), the room supply airflow turndown ratio
would decrease since the Room Minimum Supply
Airflow would increase.
Room General Exhaust
A room general exhaust (typically located in the
ceiling) must be provided whenever there will not be
sufficient exhaust from the fume hoods and
miscellaneous exhaust provisions to maintain the
required minimum room ventilation (ACH) rate,
ensure negative room pressurization or to maintain
the required room ambient conditions. If Rooms 1
and 2 in Table 1 did not have the two 100 cfm
snorkels, the
minimum and maximum total room
exhaust values would only be 150 cfm and 200 cfm
respectively. This would not be sufficient to provide
the 250 cfm of Excess Room Exhaust Airflow For
Negative Pressurization. In such situations, a room
general exhaust provision must be added to remove
at least another 100 cfm to maintain the room's
negative pressurization and a sufficient minimum
room ventilation (ACH) rate.
Sometimes a room general exhaust is needed to
help increase the supply airflow in order to maintain
the required room ambient conditions, especially
cooling and dehumidification needs. Since
laboratory rooms typically have an expected heat
gain of between 5 and 25 Btu per square foot per
hour
13
, a laboratory room of 250 square feet could
have a heat load as high as 6250 Btu/Hour. If a
room ambient temperature of 72°F were desired with
55°F supply air, a supply airflow of 334 cfm would be
required
14
. The minimum total room exhaust in
13. ASHRAE, Applications Handbook, Chapter 13.
14. Cooling cfm = Btu/Hr. ÷ Air Temp. Rise × 1.10
Cooling cfm = 6250 ÷ (72 -55) × 1.10 = 334.2
Rooms 1 through 5 would not allow that much
supply airflow without the addition of a room general
exhaust of 250 cfm for Room 1 and 50 cfm for Room
4.
Room General Exhaust Maximum
Airflow
When desirable, a room general exhaust typically
only needs to provide a few hundred cfm
(approximately 1 cfm per square foot of lab floor
area). When a relatively small general exhaust is
required, it may be more economical to implement it
as a simple constant volume exhaust provision
rather than incorporating VAV control. When more
general exhaust capacity is required, perhaps 500
cfm or more, it may be implemented as a VAV
exhaust to enable reducing the ventilation energy
when maximum general exhaust airflow is not
required.
Room General Exhaust Minimum
Airflow
Since fume hoods and snorkels only exhaust air
from mid room height, buoyant fumes, gasses and
warm air will often accumulate at the ceiling level
and may not be adequately purged from the room.
Therefore, when a VAV room general exhaust is
used, it is good design practice to always maintain
sufficient minimum exhaust airflow to maintain 12
ACH for the room area that extends down to at least
1 foot below the ceiling. This equates to about 1 cfm
for every 5 square feet of laboratory room area.
Room General Exhaust Turndown
Ratio
Based upon a nominal maximum of 1 cfm per
square foot of laboratory room area and a nominal
minimum of 1 cfm per every 5 square feet of
laboratory room area, the turndown requirements for
a room general exhaust becomes approximately 5-
to-1.
Ensuring Proper VAV Airflow
Turndown Ratios
Designers of VAV laboratory ventilation systems do
not need to determine and specify every different
turndown ratio required or establish a "one size fits
all" turndown to cover all airflow control situations.
Rather, its only necessary to specify the minimum