Hospital Air Handler, Discharge Airflow vs. Static Pressure

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Discussion Overview

The discussion revolves around the performance of an air handling unit (AHU) in a compounding pharmacy, specifically focusing on the relationship between discharge airflow and static pressure during a failure of one of the supply fans. Participants explore the implications of fan operation, damper positioning, and system design in maintaining the required static pressure in a clean environment.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes a failure where a single supply fan could not maintain the required static pressure despite increasing its speed to maximum.
  • Another participant notes that static total pressure does not increase at the same rate as the volume of delivered air when a fan is sped up, referencing fan affinity laws.
  • A participant explains the configuration of the fans and dampers, indicating that the fresh air fans pull a mix of fresh and return air, and that dampers modulate based on environmental conditions.
  • Concerns are raised about the potential misconfiguration of dampers affecting the air mixture ratio during single fan operation, which could lead to inadequate pressure maintenance.
  • Questions are posed regarding whether the initial balancing tests included single-fan operation and if those results align with current observations.

Areas of Agreement / Disagreement

Participants express uncertainty about the performance of the air handling system, with no consensus on whether the issue lies with the engineering design, air balancing, or fan capacity. Multiple competing views on the cause of the pressure maintenance failure remain unresolved.

Contextual Notes

Participants highlight the complexity of the system, including the interaction between fan speed, damper positions, and the mixture of fresh and recirculated air, which may not be fully understood or documented in the current discussion.

Who May Find This Useful

HVAC professionals, engineers involved in air handling system design, and those interested in the operational challenges of maintaining environmental controls in specialized settings like pharmacies.

steveldan
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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?
 
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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?
 
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?
 
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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.
 
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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.
 
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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.
 
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