Hospital Air Handler, Discharge Airflow vs. Static Pressure

In summary: The design engineers and air balancing company are telling me that everything looks good during their checks. Can you explain to me in simple terms why static does not increase at the same rate as volume of delivered air?
  • #1
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?
Engineering news on
  • #2
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:

Do you have a way to speed up the fresh air fan, in order to increase the positive pressure in the lab?
  • #3
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?
Last edited by a moderator:
  • #4
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.
  • Like
Likes russ_watters
  • #5
  • #6
All of the dampers were sequenced into position as they are supposed to be during this failure.


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.
  • Informative
Likes berkeman
  • #7
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.
  • Informative
Likes berkeman

Related to Hospital Air Handler, Discharge Airflow vs. Static Pressure

1. What is a hospital air handler?

A hospital air handler is a mechanical unit that is responsible for circulating and filtering air throughout a hospital building. It is a crucial component of the heating, ventilation, and air conditioning (HVAC) system in a hospital, as it helps maintain a safe and comfortable indoor environment for patients, staff, and visitors.

2. What is discharge airflow?

Discharge airflow refers to the volume of air that is being pushed out of the air handler and into the hospital building. It is measured in cubic feet per minute (CFM) and is an important factor in maintaining proper air circulation and ventilation in a hospital setting.

3. What is static pressure?

Static pressure is the amount of resistance that air encounters as it moves through the ductwork and components of an HVAC system. In the case of a hospital air handler, it is the pressure that the fan must overcome in order to push air through the system. It is measured in inches of water gauge (in. wg) and is an important factor in determining the efficiency of an air handler.

4. How does discharge airflow affect static pressure in a hospital air handler?

The discharge airflow of a hospital air handler is directly related to the static pressure within the system. As the discharge airflow increases, the static pressure also increases because the fan must work harder to push more air through the system. This can lead to increased energy consumption and decreased efficiency if not properly monitored and maintained.

5. What are the ideal discharge airflow and static pressure levels for a hospital air handler?

The ideal discharge airflow and static pressure levels for a hospital air handler will depend on the specific design and needs of the hospital building. However, in general, the discharge airflow should be sufficient to maintain proper air circulation and ventilation, while the static pressure should be within the recommended range for the specific air handler model. It is important to regularly monitor and adjust these levels to ensure optimal performance and energy efficiency.

Similar threads

  • Mechanical Engineering
  • DIY Projects