Laboratories are maintained at a lower pressure than surrounding areas, called negative pressure, to prevent contaminants from spreading through a building. In constant volume laboratories, the supply and exhaust airflows are balanced to always maintain a given airflow. In constant volume 2-position laboratories, the supply and exhaust airflows are controlled to maintain either full or reduced airflows. For either of these types of laboratories, either venturi valves or other HVAC controls are normally used. Room monitors also may be used to warn facilities maintenance staff and laboratory users if room airflows or pressure differentials are not maintained. 

In a VAV laboratory, airflows almost constantly change since fume hood sashes move and space temperature loads vary. Some sort of control system is needed to modulate supply and room exhaust volumes in order to maintain room pressure. Engineers choose direct pressure, flow tracking or flow tracking with pressure feedback controls. In order to determine which method is best for each situation, an understanding of each system’s limitations is needed.

①Direct Pressure Controls 

Direct pressure controls may be the simplest style of VAV room controls. With this control style, the room controls modulate the supply and exhaust dampers to maintain room pressure differential between the laboratory and its reference space. If the direct pressure changes for any reason, the controls take appropriate action. Direct pressure controls can therefore hold the room pressure differential closest to setpoint, which is important for rooms housing highly hazardous substances. 

Like any control system, direct pressure controls have limitations. Direct pressure controls will modulate supply and general exhaust dampers to any change in room pressure differential, whether the reaction is wanted or unwanted. Effectively applying direct pressure controls requires designing the building to keep the reference pressure stable. Key requirements include:

• Maintain stable pressure in the reference space. Pressure fluctuations in the reference space cause corresponding disturbances to the room pressure differential. 

• Keep laboratory doors closed. When the door is closed, the open area comprised of the cracks around the sides and top of the door plus any undercut is very small. When the door opens, the room pressures of the reference and laboratory spaces will almost immediately equalize, causing the direct pressure controller to minimize the room supply and maximize the room exhaust in order to return the room pressure differential to setpoint. This large difference between exhaust and supply flow rates could cause problems with room pressure differentials for adjacent spaces or even the building pressure.

Door switches, used to lock the supply and exhaust airflows or change to a reduced room pressure setpoint, can reduce this problem. 

• Avoid elevators opening directly to the reference space without a vestibule. As the elevator moves between floors, it pumps air into or out of the shaft, altering the pressure of adjacent spaces.

• Avoid doors opening directly to the outdoors. Gusts of wind impacting or blowing past the building can influence the reference pressure even when the door is closed. 

②Flow Tracking Controls 

 Flow tracking controls are designed to maintain larger exhaust flows than supply flows. Because more air is exhausted than supplied, the laboratory is negative. The extra air exhausted, commonly referred to as the “offset” air, is actually supplied to adjacent spaces, entering the laboratory through doors and other penetrations. Flow tracking controls modulate the supply and exhaust volumes to maintain a constant offset. 

To work correctly, room controls using flow tracking must measure all ducted supply and exhaust airflows. Room pressure is not part of flow tracking controls—room pressure changes will go undetected. Open doors, which can cause challenges for direct pressure controls, are ignored by flow tracking controls. While an open door causes the room pressure differential to drop to zero, it does not directly affect the supply or exhaust airflow so the room controller will not take action. Flow tracking controls can therefore be used with open architecture laboratories, designed and constructed without doors.

Some engineers prefer flow tracking controls because supply and exhaust airflows are easy to predict, simplifying the sizing of fans, air handlers, ductwork and other capital equipment. In practice when designing a laboratory with flow tracking controls, the offset is calculated based on the number of doors and other penetrations into the space. Once the laboratory is constructed, the actual offset is adjusted until the space is sufficiently negative. The room controls are then configured to maintain this offset.

Flow tracking controls should not be used in laboratories housing highly toxic substances, because they can only assume that the room pressure differential is maintained.

③Flow Tracking with Pressure Feedback Controls

Flow tracking with pressure feedback combines flow tracking control with a measurement of room pressure differential. This control combines benefits of both flow tracking and direct pressure control systems.

In a room using the flow tracking with pressure feedback control system, all ducted supply and exhaust airflows are measured like with any flow tracking system. Supply and exhaust flows are modulated to maintain a specific offset, maintaining negative laboratory balance. However, the hybrid control system also measures the room pressure differential. 

The room controls use the room pressure measurement to slowly adjust the offset, correcting long-term changes in room dynamics. Should the room pressure differential change too quickly, the room controls can alert facilities maintenance and laboratory personnel of the potentially unsafe conditions. 

Doors, therefore, do not cause the same level of challenge with flow tracking with pressure reset controls as they do with direct pressure controls. When a door is opened, it almost immediately causes the room pressure differential to drop to zero. The pressure feedback component of this control system can only slowly adjust the offset to maintain room pressure setpoint. In the short time that a door is typically opened, flow tracking with pressure feedback controls will not significantly adjust the offset so the supply and exhaust airflows will remain nearly constant. Should the door be propped open, however, flow tracking with pressure feedback controls will eventually increase the exhaust and decrease the supply, similar to direct pressure controls.

Temperature Control

Maintaining laboratory balance and pressure is not sufficient. The room controller must maintain the temperature at levels appropriate for laboratory personnel and processes. The VAV temperature control sequences are effective at maintaining laboratory space temperature.

In the standard VAV temperature control sequence, hot water in the reheat coil warms cool supply air. The warm supply air then enters the space. If the space is too cool, the temperature control system reduces the volume of supply air to its minimum setpoint and then uses more hot water to heat the air. If the space is too warm, the temperature control system decreases the amount of hot water heating the supply air, and then increases the volume of cool air delivered to the room. 

Space temperature control therefore changes the volume of supply air delivered to a laboratory. Similarly, room controls must adjust supply air volume in response to changing exhaust volumes from fume hoods and other VAV equipment. These changing supply air volumes will affect space temperature. 

Effective control of laboratory temperature therefore requires integrating temperature control functions into the room controller. Bolting a temperature control system to a room control system adds complexity and the potential for unfavorable interactions when compared to a laboratory system only control airflow or temperature.