Pressurization for critical spaces

Pressurization is the highest priority HVAC function for lab room control. Ventilation and temperature control are subordinate to pressurization.

The pressurization feature controls supply airflow to the room and general exhaust from the room. Exposure control devices (fume hoods) are separately controlled, independent exhaust devices. If fume hood(s) are present, they can be coordinated with the pressurization feature but are not specifically directed by it. The presence of fume hood(s) can affect:

Airflow tracking

Airflow tracking is the measure of controlled airflows in and out of the room and the enforcement of a selected difference between them. The difference is made up by flow that is transferred through the envelope of the room. This flow transfer opposes the unwanted migration of air contaminants. The difference between supply and extract = the transfer flow between the spaces.

When the system operates as intended, the configured transfer flow value corresponds to a level of pressurization and contamination control. Conditions that can disrupt this correspondence either briefly or for a prolonged time include:

CAUTION

caution

Health and safety procedures rely on alarms configured in the BACS

To counter unplanned pressure deviations, consider
- Airflow transfer alarm
- Room pressure separate sensor and alarm
- Procedures for monitoring and maintenance to verify pressurization over time

Flow pressure/cascade

Flow pressure/cascade means the airflow transfer quantity is adjusted dynamically to create a selected pressure difference between the room and an adjacent space. This closed loop feedback eliminates some of the uncertainty with simple airflow tracking.

Airflow transfer values

Airflow limits and parameters (for 1 segment)

The transfer value is the difference between air supplied to the room and air extracted. The system defines this difference as a positive value regardless of positive or negative pressure in the room. The setpoint for transferred flow is always positive, regardless of whether intended pressurization for the space is positive or negative. SpAirFlTrn (Transferred air volume flow setpoint) is always set as a positive value.

AirFlTrnR (Room transferred air volume flow) will always therefore be a positive value when pressurization is working as intended. Operators monitoring pressurized rooms can check the value of AirFlTrnR. If positive, then the room is pressurized in the right direction; if negative, then transfer is opposite of the intended direction.

Special cases

The system selects the lowest flow rate that meets all requirements for each function: heating, cooling, ventilation, pressurization. When requirements conflict, the control priorities are:

  1. Maximum flow limits for terminals (and minimum flow limit, if they (min and max) are equal)
  2. Pressurization (SpAirFlTrn) in the desired direction, not less than required level
  3. Ventilation rate for room
  4. Temperature control (heating and cooling functions)
  5. Minimum flow limits for terminals (if not equal to maximum)
  6. Pressurization (AirFlTrnR) in the desired direction, not greater than required level

In some special cases, the controller sets flows below the minimum:

Supply cannot match exhaust in a positive room
If a room includes independent exhaust devices (like fume hoods) and the combined exhaust flow is high enough that the supply flow hits maximum, the maximum (1) is enforced. To achieve the desired outward transfer flow (2) the controller lowers the general exhaust flow of the extract terminal below the configured minimum (5).

Exhaust flow is too low in a negative room
If the general exhaust terminal is at maximum (1) and the inward transfer flow is too small (2) when the supply flow is at minimum (5), the controller lowers the supply flow below the minimum to maintain the transfer flow. Usually this situation is prevented by sizing the terminals and setting appropriate flow limits. However, it can arise if some separate exhaust device is shut off or is operating below the flow level anticipated when the terminals were sized.

In some special cases, flow limits are enforced while transfer flow may not match transfer setpoint:

Supply cannot match exhaust in a negative room
Sometimes the capacity of the supply terminal(s) is less than fume hood exhaust capacity. Terminals were sized with the assumption that the hoods should not all be open at once. If the hoods do all open together, the controller enforces the supply terminal maximum (1) and the extract terminal minimum (5), allowing the transfer flow to exceed the desired level (6) and make the room "too negative".

Constant Volume setup
Supply terminal – When a supply terminal is set up to run as constant volume (min and max flow limits are set equal), the setpoint does not go below the minimum to maintain pressurization (as it does in "Exhaust flow too low in negative room" above). Instead, the min and max flow limits are strictly enforced, and the setpoint is exactly the configured constant volume value. If this happens, the transfer flow rate can be less than the transfer setpoint (SpAirFlTrn).

Extract terminal – When an extract terminal is set up to run as constant volume (min and max flow limits are set equal), the setpoint does not go below the minimum to maintain pressurization (as it does in "Supply cannot match exhaust in positive room" above). Instead, the min and max flow limits are strictly enforced, and the setpoint is exactly the configured constant volume value. If this happens, the transfer flow rate can be less than the transfer setpoint (SpAirFlTrn).

Options for failure modes

When flow sensors fail, the airflow tracking system cannot work as intended. It is necessary to choose the failure response. This should be specified by the ventilation engineer. It is not intended that the default settings will be appropriate for any particular project or room. Furthermore, failures should be reported through the BACS so that they can be fixed without delay. Failure responses are not intended as a long-term remedy. They are intended to be the least harmful of the possible, compromised responses.

HVAC designers usually try to select the safest approach, considering the rest of the mechanical system. Considerations include the level of airflow and the effect on pressurization. They might choose to maximize flow or to minimize disruption from the normal state or to maximize the pressurization. Each has advantages and drawbacks.

For the local loop (the one with the failed flow sensor) the options are:

CAUTION

caution

Health and safety procedures rely on alarms configured in the BACS

Standards call for:
Air pressure indicators and alarms for some kinds of spaces, such as isolation rooms in hospitals and laboratories used for Biosafety Level 3.
Physical verification of pressurization; this may require periodic inspection with a means to visualize directional airflow.