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
Types of Ventilation Systems
Two-position CAV
A two-position CAV type of ventilation system is essentially the same as the one position
CAV system shown in Figure 1. The difference is that the two-position system has a means
to provide two separate levels of ventilation airflow. As was indicated previously, standards
and codes typically allow a lesser ventilation rate when a laboratory is unoccupied as
opposed to when the laboratory is occupied and in use. In order to take advantage of this and
save considerable air conditioning energy, a two position CAV system may be utilized.
The best way of attaining two separate levels of airflow in a two-position CAV system is by
adding a control device, typically a variable speed drive, on the fan motor power circuit. This
enables the fan to be run at a lower speed and thus deliver less air through the primary air
handling unit when less ventilation air is required. It must also be noted that along with
reducing the air supplied to the laboratory rooms, the exhaust system must also reduce the
exhaust air removed by a similar amount. In fact the actual amount by which the supply can
be reduced during the unoccupied time is mostly dependent upon the extent that the exhaust
air can be reduced.
Chemical fume hoods should maintain a minimum airflow even with their sash fully closed
2
and a minimum laboratory room ventilation rate must also be maintained. Therefore, the
room supply air during the unoccupied period must still be sufficient to make up for the total
amount of room exhaust that must be maintained.
VAV Systems
The variable air volume (VAV) ventilation system’s major characteristic is that it has the
ability to vary the amount of supply and exhaust air to closely match actual facility needs at
any given time. This enables achieving a substantial reduction in the amount of energy
necessary to transport and condition the ventilation air. Applying a VAV system to a facility
with chemical fume hoods also requires that the fume hoods be equipped with VAV exhaust
controls wherever possible
3
.
Thus, a VAV ventilation system with the appropriate control system can ensure that only the
required amount of air is conditioned, supplied and then exhausted from the laboratory
rooms. The specific amount of air supplied and exhausted will likely vary throughout the
occupied time as the needs and activities in a laboratory room vary. The specific airflow
needs of each room may be governed by such factors as meeting the required room
ventilation (ACH) rate, providing sufficient replacement air for air being exhausted by the
fume hoods, and the necessary amount of supply air to maintain the required room ambient
conditions. Any of these requirements can establish the specific airflow rate that the VAV
system must provide for each individual room at a given time.
2
Current standard recommend maintaining a minimum exhaust airflow of approximately 20% of the normal maximum fume hood
airflow even with the sash fully closed.
3
Perchloric acid fume hoods and radioisotope fume hoods are not normally suitable for the addition of VAV controls. Adding VAV
control devices in the exhaust of such hoods would likely interfere with effective perchloric acid hood washdown and might also
become contaminated with radioactive material making servicing difficult.
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