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
Laboratory Room Pressurization
Advantages and Disadvantages of Room Pressurization Control by
Pressure Sensing
Room static pressure control by pressure sensing enables precise closed loop
control of the desired room static pressure in applications where the room ventilation
airflow is constant (constant volume laboratories). It is also generally an appropriate
way to handle situations where multiple levels of pressurization might be desirable
between several rooms (+0.02 in. WC in one room, +0.01 in. WC in an adjacent
room, 0.00 in. WC in another, and so forth). Pressure sensing control also
compensates for varying laboratory conditions such as a door left slightly ajar or
changes in the static pressure of adjacent areas.
However, there are very distinct disadvantages associated with pressure sensing. A
rather costly differential pressure sensor is required in order to accurately sense the
very low static pressures involved in room pressure control. In addition, an
appreciable time period is needed to average out the normal pressure fluctuations
associated with such low pressure sensing. Therefore, the room control process will
have a slower response time in comparison to other methods of room pressure
control (that is, airflow tracking).
Pressure sensing control is not recommended for applications where the room
exhaust airflow can undergo rapid changes as is the situation with VAV fume
hoods.
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When a fume hood’s exhaust airflow rate changes, so does the room’s
pressure. In such instances, the slower response of pressure sensing control will
result in a longer periods where the room static pressure is not at the desired level.
When a fume hood sash is opened, the inability to quickly increase the supply
makeup air can result in an undesirably long fume hood face velocity response time.
Since a wall mounted differential pressure sensor is also susceptible to air currents
caused by persons passing by and it can be adversely affected by the placement of
laboratory furnishings, room air currents, heat-producing devices and other effects.
These can produce inaccuracies in the pressurization control process.
An additional complication with direct pressure sensing control is the effect that door
openings will have on room pressurization. Potential problems can occur if a door is
held open for an extended period of time. The large flow of air into the room through
the doorway will effectively prevent maintaining the desired level of room
pressurization. However the room controller will attempt to maintain the room’s
negative pressure by reducing the supply air. The reduction in supply air in turn can
adversely affect the room ventilation rate and prevent maintaining the desired room
temperature and humidity.
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If a VAV fume hood sash is suddenly opened after being fully closed, it would require perhaps 15 or 20 seconds before
a new stable room static pressure value could be used for controller response. This would thus delay a timely increase
in the level of supply makeup air required and could therefore affect the fume hood containment.
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