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
Centralized Exhaust Systems with Constant Volume Fans
Centralized Exhaust Systems with Constant
Volume Fans
Centralized exhaust systems for either constant air volume or variable air volume
ventilation systems are suitable for modest sized facilities. These types of exhaust
systems are configured using exhaust fans that run at a constant speed along with a
control arrangement that maintains a nearly constant volume of exhaust air through
the fans and exhaust stack. Figure 5 shows a rooftop centralized exhaust system
configuration based upon multiple fans running at a constant speed. A means to
control the static pressure as part of an overall exhaust system control arrangement
is illustrated.
In general, the exhaust system arrangement of Figure 5 is suitable for application in
a laboratory facility that utilize either CAV or VAV ventilation systems. With reference
to Figure 5, a modulating bypass damper is used to allow outside air to directly enter
the exhaust system plenum as a means to vary the exhaust airflow from the facility.
Along with this bypass damper, a static pressure sensor and controller make up the
exhaust system static pressure control system. This basic static pressure control
arrangement can be applied to most any centralized exhaust system regardless of
size and overall configuration. The static pressure sensor is generally located where
a distant branch duct connects to the main vertical exhaust riser. This ensures that
adequate static pressure will be maintained in the region of the exhaust system
farthest from the exhaust fan plenum.
In large or more complex exhaust system layouts, multiple static pressure sensors
can be utilized. In such arrangements, the inputs of all of the sensors can be
averaged by the control system, or the control system can be set up to utilize the
lowest static pressure input as the basis of control for the entire system. In operation,
the static pressure control arrangement modulates the outside air bypass damper to
add or decrease the amount of bypass air in order to maintain the system static
pressure at the desired level.
When there is a need for more exhaust airflow from within the facility, the system
static pressure tends to decrease (become less negative) and the control
arrangement will reduce the bypass damper opening to increase negative static
pressure (suction) throughout the system. Conversely, when there is less need for
exhaust airflow from within the facility, the system static pressure will tend to increase
(become more negative) and the control arrangement will increase the bypass
damper opening to reduce the negative static pressure (suction) throughout the
system. Because of the bypass damper arrangement, the airflow through the fans
remains relatively constant; thus, constant volume fans provide excellent functionality
in this arrangement.
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