Hybrid Air Distribution System

[MCSEng]

We are considering an air conditioning air distribution control strategy as follows:
1) VAV boxes in some zones are being proposed to regulate the air flow and zone temperature.
2) Other zones will have no control damper. No damper or VAV boxes to regulate the air flow. It is expected that air flow will remain constant after system balancing.
3) The floor which comprises several zones is served by a chilled water central station air handling unit with variable frequency drive.
4) Static pressure sensor will be installed in the air conditioning ducting.
5) The required control hardware will be fitted and programmed (PID loop).

My colleague describes this as a hybrid system and said it will not work as more air will be forced through the diffusers that have no upstream actuator or VAV boxes. I thought that as the VAV dampers in some of the zones close, the static pressure in the duct will increase and the sensor and control system will detect this and signal speed reduction of the fan via the VFD; restoring the static pressure to the original setpoint.

Does anyone have any comment on this? Can you direct me to literature on advanced air conditioning control strategies?

 

[DRWeig]

I engineer the controls on these things often. In my experience, I find it all the time in large buildings. A single air handler in a hospital I'm working on right now has a mix of constant-volume terminal units and VAV terminal units. The mix is about half and half. These are pressure-independent terminal units, so air flow will not vary in the constant-volume versions as the pressure controller reacts to changes in static pressure.

Your system sounds like it's pressure-dependent (no airflow sensing or compensation), since you have no damper at all in the constant-volume terminal units. The airflow will vary there as the system pressure control reacts to pressure changes and gets the pressure back to setpoint. This will be especially true during startup after setback, or anytime the system comes back online after being turned off. Once everything gets stable after that startup, though, the variations will be less noticeable since they won't happen so quickly.

If your system is small, the pressure control response time can be fairly quick. If large, it can be really slow.

It would help if we knew the size of the equipment, size of the area served, expected duct static pressure, and total design airflow.

I would suggest that your constant-volume zones be made VAV as well, you may not have adequate temperature control with this system. Or, if they require constant volume, invest in pressure-independent terminals for these.

Let's see what some of the others say, as well. I have found that to be prudent. I've been in this business a long time, but I learn something new almost every time I visit eng-tips.

Best to you,

Goober Dave

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[DrRTU]

Good luck with that design. I believe the system will have the following issues.
1. You will oversize the duct to account the max flow condition.
2. The uncontrolled zones will have changes in flow due to the inability to regulate flow down to the accuracy to control flow. The fan drive needs a PI or PID loop with factors to dampen the pressure/flow oscillations. Dwell time and dead band are very important when sensing pressure. Without small swings in pressure, the system will hunt.
3. You lose the ability to operate a pressure reset sequence. PR may be required per the code.
4. The load may change in the fixed zones due to a change in space use or alteration of load profiles. Look at old terminal reheat designs that have the RH coils turned off, they have lots of tenant complaints.
5. You will need to utilize a low loss ring design of the main supply trunk to minimize branch pressure drop if the terminals open up.
6. SA reset will require the unregulated zones to have a flow change. I do not know which zone you are in, but typically SA can float all the way up to 60 F or more. This alone will require 25% more flow. SA reset is also a 90.1 requirement. Economizing is also a 90.1 requirement.
7. CFM measuring based systems have been implemented but error buildup causes flow issues.

 

[KiwiMace]

I agree, putting unregulated CV zones will destabilize your control particularly in these zones. The pressure control may not function either.