Basic Documentation

Siemens Industry, Inc. Page 7 of 8
Document No. 149-978
and maximum airflows required for each airflow
control application in an appropriate airflow control
schedule. The turndown ratios required for each
individual airflow control application will be inherent
when these maximum and minimum airflows are
specified. This puts the responsibility for providing
the required airflow control turndown ratios on the
control system provider.
The control system specifications then only need to
state: "The turndown requirements for each
individual air terminal and airflow control application
shall be sufficient to provide the full range of
minimum to maximum airflows listed in the airflow
control schedules."
Conclusion
This paper provided an in-depth analysis of many
VAV laboratory airflow control situations in
conjunction with the applicable standards
requirements and good design practices to establish
the actual turndown ratios typically needed for
laboratory airflow control applications. In virtually
every instance, it has been shown that high
turndown ratios (more than 8-to-1) are not required.
The following statements summarize the results of
this analysis:
 Large turndown ratios do not reflect
proportionately larger control ranges. (For
instance 16-to-1 does not cover twice the range
of 8-to-1.)
 Larger turndown ratios do not equate to
superior control performance. Rather, optimum
performance will be attained when the control
elements and control system closely match
actual needs.
 VAV Laboratory rooms typically require airflow
control turndown ratios within 7-to-1.
 VAV fume hood turndown ratios are limited by
the need to always maintain a minimum
exhaust of 25 cfm per square foot of interior
work surface. This effectively limits VAV fume
hood turndown ratios to 5-to-1.
 A room general exhaust may be needed to
allow sufficient supply airflow to meet minimum
ventilation rates, ensure adequate room
negative pressurization, maintain room ambient
requirements, and to remove contaminants at
the ceiling level. Room general exhaust
turndown ratios will typically be 5-to-1.