Basic Documentation
Table Of Contents
- About this Application Guide
- Chapter 1–Introduction
- Chapter 2–Goals of the Laboratory Environment
- Chapter 3–Unique Ventilation Needs of a Laboratory Facility
- Chapter 4–Ventilation Systems Classification
- Chapter 5–Laboratory Facility Exhaust Systems
- Chapter 6–Laboratory Containment Units - Ventilation
- Chapter 7–Room Ventilation, Makeup Air, and Pressurization Control Systems
- Chapter 8–Laboratory Temperature and Humidity Control Systems
- Chapter 9–Laboratory Emergencies - Ventilation System Response
- Chapter 10–Laboratory Ventilation System - Validation
- Chapter 11–Laboratory Ventilation System - Commissioning
- Glossary
- Index
Chapter 5–Laboratory Facility Exhaust Systems
Exhaust System Discharge
Laboratory exhaust must be discharged sufficiently high above the roof so that the
exhaust will not re-enter the facility through fresh air intakes or be a problem to
nearby structures or property. The common way to attain sufficient discharge height
is to utilize a sufficiently high exhaust stack with an adequate exit velocity. As an
alternative to tall exhaust stacks, special laboratory axial exhaust fans designed to
entrain large amounts of outside air and discharge the combination (exhaust plume)
upwards at a high velocity may be utilized. These fans are not only effective at
eliminating the need for a tall stack but the exhaust plume created (combination of
exhaust air and locally entrained air) usually provides sufficient dilution to greatly
reduce the exhaust from potentially posing a hazard or nuisance to neighboring
properties or structures.
Exhaust System Stacks - Height, Location and Exit Velocity
It is beyond the scope of this publication to provide very detailed procedures for
determining the necessary stack or exhaust plume height for a specific application. It
is suggested that readers consult the data provided in the latest edition of American
Industrial Hygiene Association Standard ANSI/AIHA Z9.5 (Expected availability date
of the latest edition is approximately mid 1999). Additional details on exhaust stack
height and location may be found in the ASHRAE Fundamentals Handbooks in the
Airflow Around Buildings chapter.
Some general requirements regarding proper laboratory exhaust discharge are
provided here to help identify if existing situations may pose problems particularly
with respect to re-entrainment:
• Exhaust stacks should always be located downwind from (based upon the
prevailing wind direction) and as far distant as practical from ventilation
system air intakes.
• In general, exhaust stacks should be located on the highest portion of a
building's roof.
• Standards require that exhaust stacks should always have a minimum height
of 10 feet above the roof to protect workers who may be on the roof.
However, this 10 foot minimum height is very often insufficient as an
adequate stack height to eliminate the potential for exhaust air re-
entrainment due to wind caused turbulence on the roof and near the outer
walls of the structure.
• An adequate stack or discharge height may also need to be established with
respect to adjacent structures, topographical features, or other unique local
situations.
When adjacent to populated areas or in close proximity to other structures, it may be
prudent to have scale model wind tunnel testing done on various exhaust stack
heights and locations in order to more closely identify potential exhaust airflow
patterns as it leaves the stack. If it is determined that the exhaust system exit velocity
is very critical, especially during certain wind direction and speed, stack velocity
control and monitoring may be appropriate.
32 Siemens Building Technologies, Inc.