Hospital Air Handler, Discharge Airflow vs. Static Pressure

[steveldan]

We have a new AHU installed for a Compounding Pharmacy. The unit has two return fans and two supply fans. They are installed in parallel and are powered by VFDs. In the event of one fan failing, the other fan is supposed to speed up to maintain static pressure in the clean environment. This morning, we experienced this type of failure. The remaining supply fan did speed up to its maximum HZ value but did not maintain static pressure. The static setpoint is 1.3" and one fan maintained .63".

The Pharmacy workspace pressure monitors stayed in alarm with only one fan running, due to the low static. The confusing part is that the difference in discharge airflow was nominal. Only about a 150 to 200 CFM difference. I am being told by our design engineers and air balancing company that everything was good and passed during their checks, but that is obviously not the case. Can anyone explain this?

 

[Lnewqban]

Welcome!
Static total pressure does not increase at the same rate as the volume of delivered air increases when that remaining fan is sped up to a maximum by the VFD (60 Hz).

Please, see:
https://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html

Do you have a way to speed up the fresh air fan, in order to increase the positive pressure in the lab?

 

[steveldan]

Thank you for the response. The "redundant" fans that I am asking questions about are essentially the fresh air fans. They pull a combination of fresh air and return air to the space. The return fans pull air out to exhaust or recirculate it. There is an exhaust damper, a return/mixed air damper, and an outside air damper. These will all change position based on temperatures and humidity both in the space and outdoors. The fans will also modulate slightly as needed when any of the dampers change position.

The drive that stayed energized this morning went from 65% speed to 100% speed automatically as it is supposed to. We just couldn't achieve the air pressure that is required. All of the dampers were sequenced into position as they are supposed to be during this failure. This space also has independent exhaust fume hoods where they compound the chemotherapy drugs.

I get the feeling that either the engineering or the air balancing company has something wrong, but I am struggling to determine which it is and struggling to understand why we cannot achieve pressure at max speed. Can you explain in a simple way (I am a dumb HVAC guy) why static does not increase at the same rate of volume of delivered air?

 

[Lnewqban]

Do you have a cutsheet for that airhandler that we could see?
Could you ask the test and balance company or the engineer of record about what may be not working as specified in approved drawings?
They will need to see what each automatic damper is doing during the fail mode.

 

[Lnewqban]

@steveldan
Any progress to report?

 

[Dullard]

All of the dampers were sequenced into position as they are supposed to be during this failure.

Reference: https://www.physicsforums.com/threa...discharge-airflow-vs-static-pressure.1015124/

That makes my spidey-sense tingle. I'm not sure how your dampers are plumbed to the fan inlets, but I can imagine several arrangements where the 'ratio' of outside air to recirc air would be different for single vs, dual fan operation. It's impossible to say anything for certain (given the limited information), but the general concern is: if duct velocity becomes high enough to produce significant (compared to the dampers) pressure drops, the actual ratio of 'fresh' to 'recirc' air may not be what you think it is. A 'too high' recirc/fresh ratio will cause the problem that you see.

 

[russ_watters]

Did the balancer test single-fan operation in the initial balancing? Does it match what is happening now? If no to either, get TAB readings and compare.

It sounds like the fans are too small for full redundancy.