DeLuga (1995) describes the use of air-flow tracking to maintain negative air pressure in a laboratory.
Another means to maintain a laboratory room at a negative pressure with respect to an adjoining non-laboratory area is by airflow tracking, commonly called flow tracking. ... airflow tracking is based upon always removing more air from a space than is supplied to the space. This will tend to create a slight vacuum condition within the space and make the space at a negative static pressure with reference to an adjacent area. Airflow tracking is applied to maintain the negative pressurization level of a space by ensuring that the amount of air being exhausted form the space exceeds the amount being supplied by a specific amount. Although at any given time the quantity of total room supply air and total room exhaust air will vary as the needs of the laboratory vary, the cfm difference between the total room supply air and the total room exhaust air will be controlled to remain at a constant value.
The essential difference [between the airflow tracking method and pressure sensing control] is that there is no differential pressure sensor in the airflow tracking method. However, an air flow sensor has been added to measure the total room supply air...the laboratory has a modulating damper in the supply air and room general exhaust which is controlled by the room controller. The laboratory room controller measures total room supply air cfm and also determines the total room exhaust air cfm. While maintaining the flow tracking differential airflow between the room supply air and the total room exhaust air, the room controller must also ensure that the laboratory is always being ventilated at the required minimum number of air changes per hour.
If the laboratory has VAV fume hoods, the individual fume hood controllers continue to vary the total amount of air leaving their respective fume hood face velocity. These variations in the fume hood exhausts will affect the total room exhaust cfm. As a result, the room controller must constantly monitor the total room exhaust cfm and modulate the room general exhaust damper to ensure that the minimum air change per hour rate is maintained. Each individual fume hood controller provides the room controller with its respective fume hood exhaust cfm value. The room controller adds all of the fume hood exhaust cfm values together and also adds the room general exhaust airflow rate to calculate the total room-exhaust cfm. As the fume hoods begin to exhaust more air and thus begin to increase the total room exhaust, the room controller must close off the room general exhaust air damper to reduce the total amount of air being exhausted from the room. As the fume hoods exhaust less air, the room controller must modulate the room general exhaust air damper open to increase the amount of air taken form the room by the general exhaust. As the total room-exhaust cfm changes, a specific amount of supply air is simultaneously required to maintain the flow tracking differential. As the fume hood sashes are opened, increasing the total room exhaust cfm beyond the amount of air needed to maintain the minimum ACH, the room controller must increase the amount of supply air to track with the increased total room exhaust cfm.
There may be times, particularly during the cooling season, when the laboratory's heat gain will require more conditioned supply air than the amount needed to provide the makeup air. In these instances, the room controller will increase the amount of supply air and maintain the tracking differential by opening the room general exhaust damper as necessary. Note that at anytime someone in the laboratory may reposition a fume hood sash and thus cause a change in the total room exhaust cfm. As this happens, the room controller will modulate the room general exhaust damper and the supply air damper to compensate for the increase or decrease in the fume hood exhaust, so that the minimum required room ventilation rate and flow tracking differential is maintained at the setpoint.
The specific difference between the total room supply and total room exhaust air is generally established at 100 to 200 cfm per laboratory door. The actual tracking value should be established when the ventilation system is in the process of being commissioned by adjusting the tracking value setpoint until the desired specific static pressure differential is obtained by an actual measurement. The actual tracking value will vary between laboratory rooms and is highly dependent upon construction and architectural factors such as the clearance around doors, "tightness" of the laboratory, wall construction and other unpredictable factors.