Fume hoods and other similar Exposure Control Devices (ECDs) are not standalone devices and are a part of the overall Lab Ventilation System (LVS). Fume hoods are exhaust devices, which are drawing their supply air from the room HVAC system.
Despite this close interdependence, in most systems, the fume hood does not directly communicate with the room HVAC system in real time, to tell it of its supply air requirements.
When the fume hood’s supply air requirements are changing, there is no direct way for the HVAC system to know this and it will continue to supply the same volume of air to the room, until a related factor like room differential pressure changes. Until this happens, there is a high likelihood of a loss of containment.
This problem is more commonly visible in VAV based Fume Hoods.
VAV fume hoods reduced operating costs effectively while delivering the required level of safety, only if they are a part of a responsive LVS. Differential pressure-based controller systems are commonly used to regulate room supply when using VAV fume hoods.
Lab differential pressure changes when the change in fume hood exhaust volume is not correspondingly matched by a change in room supply air and vice versa.
Lab supply air and fume hood VAV system are in a constant state of flux, and to be stable and responsive in real-time is a challenge.A certain time lag is involved in this process of supply-exhaust coordination, during which, the room’s differential pressure is constantly shifting, leading to a potential loss of containment.
By the time the hoods and HVAC system are back in balance, another shift in the differential pressure may have occurred, again restarting the process of system stabilization between the fume hoods and HVAC system.
Many Fume Hood VAV Systems also have a significant time lag. VAV dampers adjust and regulate the exhaust volumes based on the sash opening, which determine the volume of incoming air. As the sash opening increases, the damper allows a larger volume of air to flow through it. However, commonly used VAV systems have a lag time before the damper opens to the required position. If the air flow rate is not re-established within 2 to 3 seconds after the sash is opened, fugitive contaminants may escape from the fume hood, leading to a loss of containment. VAV Systems that deliver near real-time responses are relatively more expensive, and are hence not commonly used.
Given the dynamic nature of both systems, susceptibility of sensors and the constant shifts in differential pressure, the labs can very quickly move out of balance leading to a loss of containment from the fume hoods.
