Pressure control in a lab by exhaust damper

[seanhkim]

A negative pressure is usually kept by modulating exhaust damper in a lab, instead of supply damper. Is there any reason for that? In my sense, increasing exhaust air volume has the same negative pressure effect in the lab compared to decreasing supply air volume.

If all of exhaust dampers in the floor are one AHU, which means one exhaust fan, and one of exhaust dampers just increases the volume. I think it will eventually affect pressure in other labs because it makes the exhaust fan run harder. So not only controling local exhaust dampers, controling the whole AHU is required. Am I correct?
Or is there any tolerance that the exhaust fan does not need to move within, but local exhuast damper still can modulate?

 

[RossABQ]

Supply responds to cooling load. Exhaust modulates to maintain pressure. The exhaust fan is on a VFD to maintain constant exhaust header pressure.

 

[Labuzan]

To control lab at negative space pressure, you would modulate the exhaust damper. The exhaust fan for a laboratory exhaust will generally be constant volume in order to ensure proper "throw" away from the building and ensure no capture of lab exhaust at the outside air intake. As the exhaust damper modulates and varies air volume leaving the lab space, an outside air bypass on the roof at the exhaust fan opens to ensure constant airflow. I think this damper could be barometric or actively controlled based on exhaust duct static.

You are correct that as the exhaust dampers modulate, they will affect exhaust duct static which will affect the space pressure of all labs. That is why good control loop tuning is necessary for space pressurization in multiple laboratories with a manifolded exhaust system. The exhaust fan bypass system needs to respond fairly quickly to maintain constant exhaust duct static.

 

[mauricestoker]

The reason why exhaust leads modulation is that laboratories, generally, need to be maintained under negative pressure or negative differential volume. If the room is to be "clean", then supply would be master. Also, most labs have an air change per hour requirement. For a negative environment, ACH is determined by exhaust. If you have to demonstrate ACH for a negative differential pressure/volume space, it is best to do it from the system that leads control.

Multiple labs can be supplied from the same AHU, but they might not always be exhausted by manifold. NFPA 45 and NFPA 30 make certain of that (while a good control engineer and nice differential volume control can help you around NFPA 45). Multiple and adjoining labs can be placed on the same AHU and easily maintain differential volume/pressure requirements, though I would not recommend using a manual volume damper to do so. Where manual dampers are what is available, flooding the outside corridor with supply in comparison to lab will provide for differential volume/pressure requirements. Always another way to skin a cat. However, multiple exhausts can and are used against a single AHU. A perchloric hood, five fume hoods, and six dry labs can be placed on separate lab exhausts with one supply; just an ulcer for the engineer or the O&M people.