The purpose of installing manual volume dampers in each branch and sub branch is necessary for multiple reasons.
Energy conservation by reducing unnecessary static to non-critical paths:
In a ducted air system, there is typically a critical path that is not getting the required airflow (path with the highest pressure drop). The fan pressure needs to be increased until air reaches that critical path. This requires more horsepower because increased flow causes more air down non-critical paths. The fan has to maintain this excess static to non-critical paths, resulting in energy loss, and often times running out of fan capacity. Branch and sub branch dampers are used to equalize the pressure drop among the branches and the terminals (return/exhaust grilles in this case), requiring the fan to produce only the static required for the critical path.
Controlling unwanted noise:
Branch dampers keep the sound in the insulated duct rather than at the sub branch damper feeding the terminal units. Without branch dampers, the dampers closest to the mains will need to be closed down considerably in order to direct the air to the critical paths. This considerable closing of dampers cause the opening that the air travels through to become smaller, and thus the air will move faster creating noise.
Flexibility during modification:
Branch dampers provide for flexibility during building modifications. When a portion of the hospital is remodeled or repurposed, any changes in the outlet volumes impacts the entire rest of the system. If branch dampers are not installed, there is no means to direct the air once modifications have been made. No matter how small the project may seem at the time, over the years, it ends up having a large impact on the system as a whole. Compound effects of this will show up as changes in building pressures room to room, building to connected buildings, and to the outside.
Controlling cost due to modifications:
If installed in the correct locations, branch and sub branch dampers can reduce cost of future balancing. Rather than balancing all outlets on an entire floor when small modifications are made, the air can be moved to where necessary using branch dampers. If none are installed, entire systems can be impacted with slight changes, which can become costly.
ASHRAE, SMACNA, NEBB recommended:
Dampers are recommended in publications by our governing organizations in all branches and sub branches. In short there is better control of the system with the least amount of wasted energy.