If vented outdoors, A2 cabinets are usually connected to the HVAC system by a single exhaust manifold. While it may seem reasonable to connect the B2 cabinet to the same manifold, there are significant consequences, including increased energy consumption and operating costs and a greater chance for airflow issues.
Type A2 cabinets require a negligible amount of static pressure to operate (-0.2” WG on average) and only a portion of the total flow is exhausted, whereas Type B2 cabinets have relatively high static pressure requirements (1.7” WG on average) and all of the flow is exhausted. Therefore, the exhaust system fan has to work harder to exhaust the B2 cabinet.
Now image you’re the exhaust system fan – and you have a straw that tee’s with one end going into a soda and the other into a milkshake. Think of the soda as an A2 cabinet – it’s easy to suck the soda through the straw. Think of the milkshake as a B2 cabinet – it’s much more resistant to flow and more energy is required to suck it through the straw.
How hard would you have to suck to get 3 times more milkshake than soda? It’s nearly impossible. The only way it would work is if you somehow hinder the flow of the soda. But you’d still have to suck really hard to get the necessary amount of milkshake. Much harder than you would have to suck if you were just trying drink the soda.
So to relate this back to the laboratory – if you manifold 50 A2 cabinets and one B2 cabinet, you would need to install dampers in the A2 exhaust ducts in order to impede the flow, just like with the soda. In essence you are letting the B2 cabinet drive the exhaust system flow. And it’s working a lot harder than it needs to if it only had A2 cabinets on the manifold duct.
As you can see, it is much more efficient for total exhaust biosafety cabinets to have a dedicated exhaust system. While it may add more costs during building construction, it will save money in operating costs, as well as system balance and maintenance throughout the life of the cabinet.