AHU design for space with high exhaust air requirements

[Engineer Who]

I am trying to find solutions for a lab space with fume hoods of a total of 5000m³/h exhaust rate. The makeup air cannot come from adjacent spaces, it has to be outdoor air. Also the makeup air should not be supplied directly into the hoods. The diversity factor will range from 25% to 100%.

The space has to be maintained at 23^oC and 50%RH. I calculate the sensible heat gain from walls, windows and equipment to be around 18kW. Space latent cooling loads are negligible. The ventilation cooling loads are 22kW sensible + 29 kW latent.

One of the solutions I am evaluating is the installation of an AHU which will be supplying preconditioned, cooled and dehumidified, makeup air. The air has to be cooled to the space's dew point to completely remove the ventilation latent load. At this temperature the volume rate of the air can also remove the space heat gain, thus an indoor unit (VRF, split, fan coil) is not needed.

For the times when not all hoods are in use, air will have to return to the AHU and mix with the fresh makeup air such that the fresh air intake rate matches the exhaust rate from the hoods. Exhaust from the AHU is not needed.

My question is how can this be implemented? How can the intake be controlled to match the exhaust? Can the AHU be of a single fan (supply fan, without return or relief fan) circulating 5000m³/h, with modulating return and fresh air dampers to control the mixing ratio and thus intake rate?

(ciqa.net)

The dampers could be modulated by a PI/PID controller to maintain a set differential pressure between the room and the environment (either zero or slightly negative ≈-15Pa). Will this work? Are there problems in this idea?

 

[Compositepro]

It depends on the specific application. Generally, you cannot recirculate air from lab fume hoods. If the application is benign enough (no toxins or odors), then there are lab hoods with filters that recirculate directly back into the room. 100% outside exhaust results in very high heating and cooling loads. You mention a clean room. The air handlers in clean rooms with HEPA filters add a significant heat load.

You can recover heat from the exhaust with a runaround loop to the make-up air. (Cross-contamination between the air streams must be avoided).

 

[nuuvox000]

Laboratories are to be kept at a negative air pressure with respect to surrounding rooms. This is complicated due to doors opening/closing, fume hoods running, people moving around, etc. Usually for laboratories, phoenix valves should be used and wired to pressure sensors; phoenix valves can adjust rapidly to match air pressure changes in the room. Along with this, you should have the fan on your AHU supplied with a VFD, good turndown on your heating and cooling sections as well. Size it so it can handle the full load with everything exhausting as well as the minimum outside requirements when nothing is exhausting. If it can't do that entire range, you may need to split it into two smaller AHU's.