Modulating exhaust air damper from supply fan inlet sp

[MechEngNCPE]

Ok, so I just wanted to post this to get a feel of what the community thinks on economizer relief/exhaust damper modulation when the ahu has both supply and return fans on vfd's, the supply vfd modulating off supply duct static pressure, and the return vfd modulating off space static. During normal operation the OA damper modulates off DCV control from a CO2 sensor in the return duct. During economizer operation the OA damper and RA damper modulate to provide 55 LAT. I have the exhaust air damper set to modulate to provide a -0.2" supply fan inlet pressure. We have airflow monitoring stations on OA, RA, and SA for the BMS to view and also set an OA airflow if they want. Does anyone see anything wrong with this setup, besides it being a controls nightmare? Oh, and this is VAV setup with about 40 DDC VAV's.

 

[ChasBean1]

Please do this and ignore all else that has been marketed:

Supply fan VFD should modulate to maintain supply duct static pressure – looks like this is the case so you’re all set. Reading the supply flow, the return fan should modulate its VFD to maintain the supply value minus the minimum outdoor air. That value should be the sum of all normally operating exhaust plus about five percent to pressurize the building. It should at least equal the ventilating needs for occupants on a typical workday.

For economizer control, when the outdoor air enthalpy is less than the return air enthalpy, free cooling is enabled. If the supply air temperature is above set point, the economizer dampers first modulate toward full outdoor air. The full outdoor air economizer damper position is: OA 100% open, return air 0% open, and relief/exhaust air 100% open. If this does not resolve the AHU DAT conditions, the chilled or hot water valve should be used (without overlap) to maintain the design conditions.

When minimum outdoor air conditions are required (maximum heating during winter or maximum cooling during summer), the economizer dampers modulate to minimum outdoor air. This is: outdoor air ~50% open, return 100% open, and relief/exhaust 100% closed.

Do NOT modulate the exhaust damper to maintain a -0.2” supply fan inlet pressure.

Do NOT do CO2 control. It’s a fallacy. It can be used to increase ventilation as needed but should not be used as sold, as a ventilation reduction (or energy savings) mechanism.

 

[MechEngNCPE]

"Do NOT do CO2 control. It's a fallacy." I disagree, in this situation (conferecing facilty where rooms are either full or empty) I think it can be used to reduce required OA levels.

 

[cry22]

Chasbean

True that CO2 are known to be problematic (especially calibration every 6 months or so), but they sure do give a good way to get around the ventilation code to downsize the equipment, especially the Chiller/boiler plants. Can you imagine a ballroom or a lecture hall with 2000 people occupied 4 hours in a day or in a week sucking all that Outside air for nothing?

I go with MechEng on this.

 

[ChasBean1]

I don't. Sorry. The minimum is the amount just exceeding exhaust to keep a slight positive. We shouldn't go below that just becuase CO2 is 450 ppm. No problem with using CO2 to detect occupancy and increase ventilation. Maybe 'fallacy' is a strong word, but to me CO2 control is used for IAQ and not for the purpose of saving energy (rather the opposite)...

 

[MechEngNCPE]

Do you have a specific application where DCV did not lower your minimum outside air? I believe in the case of this conferencing center, where the avereage room is occupied only 2 hours a day, that CO2 levels would allow us to operate well below the 7700cfm required by the 15-20CFM/person rule. Also, I set the min OA so that building pressure is still met.

 

[ChasBean1]

All of them. Look, I understand what DCV proponents say, but please hear me out: Why not run the systems at a default value of the minimum OA (the value that just exceeds exhaust)? If something goes wrong, the system operates at the 15% minimum outdoor air rather than the 70% CO2 increased minimum air.

If CO2 increases above set point, modulate the minimum outdoor air upward as necessary to mitigate it. Using your arguments, if the space is occupied 2-4 hours a day, why base the control on those 2-4 hours and not the 20-22 off-hours?

Coils SHOULD be sized for the increased OA capacity but should not normally operate at that value.

For the current DCV philosophy, if CO2 gets low, we can save energy.

For me, if CO2 gets high, we can improve IAQ. But the DEFAULT value is minimum outdoor air and low energy. CO2 control is used for improved IAQ and not energy savings.

 

[MechEngNCPE]

All of them. ha. I disagree. If people are not in the space, no CO2 is being introduced, then we can calibrate the CO2 sensor/OA damper to maintain less OA then would be required by the ASHRAE minimum.

 

[ChasBean1]

Can you clarify your statement: "If people are not in the space, no CO2 is being introduced?" I can't say I agree.

 

[MechEngNCPE]

People exhale CO2, so, the amount of people in the space has a direct effect on the concentration levels of CO2.

 

[MechEngNCPE]

"For economizer control, when the outdoor air enthalpy is less than the return air enthalpy, free cooling is enabled. If the supply air temperature is above set point, the economizer dampers first modulate toward full outdoor air. The full outdoor air economizer damper position is: OA 100% open, return air 0% open, and relief/exhaust air 100% open. If this does not resolve the AHU DAT conditions, the chilled or hot water valve should be used (without overlap) to maintain the design conditions."

Why not modulate the outside air and return air dampers to satisfy the LAT setpoint? Why waste energy on using chilled / hot water? I think you are too quick to provide the standard solution and don't think about ways to provide the BEST solution. In my opinion. The typical solution is not always the best.

 

[317069]

ChasBean1: I have questions:
- "When minimum outdoor air conditions are required (maximum heating during winter or maximum cooling during summer), the economizer dampers modulate to minimum outdoor air. This is: outdoor air ~50% open, return 100% open, and relief/exhaust 100% closed" do you mean 5% or 50%?
- "Do NOT do CO2 control. It's a fallacy. It can be used to increase ventilation as needed but should not be used as sold, as a ventilation reduction" if we use it to increase ventilation why don't we do for reduction?
- "If this does not resolve the AHU DAT conditions, the chilled or hot water valve should be used (without overlap) to maintain the design conditions."
Could you please expalin what do you mean by overlap,
for example let say we have a space needs to be at 75dbF and 50%RH, the out side air is 65F, its enthalpy is less than space enthalpy, economizer is enabled( enabeled doesn't mean started)could you please tell what are the positions of OA, RA, EXA dampers and chilled water valve would be?

Regards

 

[ChasBean1]

I’ve been away a few days and haven’t been able to respond…

The fan cfm tracking is your minimum outdoor air. If total building exhaust is 1700 cfm, you might set your return fan to track supply by 2,000 cfm, to make up for that exhaust volume and pressurize the building.

If you track the supply and return fans the right way, the position of the outdoor air dampers is negligible, as long as they are open sufficiently.

You keep this fan flow tracking and modulate the relief and return dampers to maintain your desired conditions, NOT your outdoor air damper.

You said “Why waste energy on using chilled / hot water?” You may have misinterpreted what I said; Read back. I’ll leave this as is…

I think you might be too quick to interpret what I’ve said so as to classify it as a “standard solution.” I am offering you the best solution. You don’t have to take it.

317069, for your questions. Responses to each are below:

"When minimum outdoor air conditions are required (maximum heating during winter or maximum cooling during summer), the economizer dampers modulate to minimum outdoor air. This is: outdoor air ~50% open, return 100% open, and relief/exhaust 100% closed" do you mean 5% or 50%?

I mean 50. It could be 100% or maybe 75% if you’d like.

- "Do NOT do CO2 control. It's a fallacy. It can be used to increase ventilation as needed but should not be used as sold, as a ventilation reduction" if we use it to increase ventilation why don't we do for reduction?

If you design a car, its idle speed should be 55 miles per hour, right? 55 mph might be needed at some point. But if you’re in residential neighborhoods or back roads, you should put on the brakes every now and then because the 55 mph might not be needed… Or should you have the car idle at 5 mph and increase toward the 55 value if conditions demand?

- "If this does not resolve the AHU DAT conditions, the chilled or hot water valve should be used (without overlap) to maintain the design conditions." Could you please expalin what do you mean by overlap, for example let say we have a space needs to be at 75dbF and 50%RH, the out side air is 65F, its enthalpy is less than space enthalpy, economizer is enabled( enabeled doesn't mean started)could you please tell what are the positions of OA, RA, EXA dampers and chilled water valve would be?”

Yes. If the AHU DAT set point is 55 while the condition you describe is in progress, first modulate the relief and mixing dampers toward full outdoor air. Because it’s 65 out and you can’t get enough free cooling, then slowly modulate the mechanical cooling. Ensure via programming that heating and cooling can’t happen at once. In pneumatics (old school) operation of the two were separated by a few psig control air pressure. It was mechanically assured that the two would not operate at once.

 

[317069]

- what about 125%
- I agree with your example but what makes driver knows that he has to drive at 5 mph at point A and at 55 mph at point B?
Co2 sensor in common return duct for a VAV system which has more than one zone will not give the real co2% in ech zone, but the average in hall system which is not a good idea.
- "Supply fan VFD should modulate to maintain supply duct static pressure – looks like this is the case so you're all set. Reading the supply flow, the return fan should modulate its VFD to maintain the supply value minus the minimum outdoor air. That value should be the sum of all normally operating exhaust plus about five percent to pressurize the building. It should at least equal the ventilating needs for occupants on a typical workday."
Do you mean by " that value" the minimum outdoor air?

-Then you said later "I don't. Sorry. The minimum is the amount just exceeding exhaust to keep a slight positive"
could you please explain litlle more?
let say again weh have space has 10 persons need 200 cfm of fresh air, let say by some code fore example, supply fan gives 1200 cfm. what the return fan cfm would be?
and what the minimum outdoor air amount would be?
- if we spend energy on 1000 cfm OA for example, then we reduce this amount to 500 cfm, do we save energy?
- each way has advantage and disadvantage and we are just talking about a subject to learn.

 

[ChasBean1]

317069 (Mechanical), I re-read your last post and lost all or most meaning... can you please boil things down to 2 or 3 questions? Promise I'll answer the best i can...

 

[ChasBean1]

Ok, my best replies as follows:

"- what about 125%"

OA dampers can be fully open. But not 125%, then they start to close again.

"- I agree with your example but what makes driver knows that he has to drive at 5 mph at point A and at 55 mph at point B? Co2 sensor in common return duct for a VAV system which has more than one zone will not give the real co2% in ech zone, but the average in hall system which is not a good idea."

I read this a few times but still don't really get it. But I'll say, systems should default to the minimum values to save energy and not the maximum values. Systems that do the opposite should not be sold as energy savers but rather performance enhancers based on needed conditions. Opinions differ.

"- "Supply fan VFD should modulate to maintain supply duct static pressure – looks like this is the case so you're all set. Reading the supply flow, the return fan should modulate its VFD to maintain the supply value minus the minimum outdoor air. That value should be the sum of all normally operating exhaust plus about five percent to pressurize the building. It should at least equal the ventilating needs for occupants on a typical workday."
Do you mean by " that value" the minimum outdoor air?"

By "that value" I meant the return air cfm value.

"-Then you said later "I don't. Sorry. The minimum is the amount just exceeding exhaust to keep a slight positive" could you please explain litlle more?"

Ok. The net mechanical airflow into a building should be slightly more than the mechanical exhaust. This helps avoid infiltration cold perimeter problems, plus mold issues during humid outdoor conditions. I'm not the first to come up with this notion, and it is not based on a whim.

"let say again weh have space has 10 persons need 200 cfm of fresh air, let say by some code fore example, supply fan gives 1200 cfm. what the return fan cfm would be? and what the minimum outdoor air amount would be?"

Return fan: track at a value slightly less than the exhaust from the space. I'd give you a value, but you didn't provide this parameter in your example. If exhaust is 150 cfm, make the return track the supply by about 180 cfm; maybe 200 cfm. Make sure the net outdoor air slightly exceeds exhaust. If CO2 elevates, let the return track supply by a lesser value. For example, this was originally 180 cfm, but now it modulates toward 500 cfm to keep space CO2 below 850 ppm.

"- if we spend energy on 1000 cfm OA for example, then we reduce this amount to 500 cfm, do we save energy?"

Yes.

"- each way has advantage and disadvantage and we are just talking about a subject to learn."

Ok.

 

[317069]

Thank you for your answers
- In your first post(July 25) you said” the return fan should modulate its VFD to maintain the supply value minus the minimum outdoor air. That value should be…"

then you said” By "that value" I meant the return air cfm value.”

- you gave an example of a car speed and I asked what makes driver to drive at 55mph at point A and drive at switch to 5 mph at point B.
- My “co2 sensor… comment” is a separate than driver example right above, “sorry for mixing”, it was a comment on using Co2 sensor in main return for multi zones space.
- For my example I understand that the return fan has to have 200 cfm for example as long as the co2 is acceptable, this amount would go up if the co2 level increase,
here I have some points
1- My example was about a supply fan, a return fan, exhaust and OA dampers, not about a supply fan, OA damper, and exhaust fan.
2- Forget about co2 for now, if return fan was 200 or 500 cfm, OA is 200 cfm , how can I provide supply fan with 1200 cfm?
3- in your answer you used co2 sensor to change return fan cfm (increase or decrease) which is translated to operation cost of this fan and energy issue.

As a conclusion, we have two scenarios of building control during a normal operation mode:
The first is:
- DCV system to control OA amount depend on the co2 level inside the space
- DCV control exhaust damper too.
- Supply fan depend on duct pressure
- Return fan depend on building pressure?

The second is
- OA damper is on minimum air requirement
- Supply cfm depent on duct pressure
- Exhaust damper is 100% closed

Now we come back to co2 sensor issue.
For DCV I have a question: when we use DCV, this control depend on Co2 level inside building and could use the Oxygen level inside the building before to star to modulate the OA and EX dampers (note: exhaust damper does not stay 100% on normal operation mode, DCV will modulate it with OA damper),
I mean during system operation there is a chance for OA/EX dampers to be 100% closed for considered time as long as the building has a enough Oxygen level and large volume compare to people number in it.
In this case what about the relative humidity level RH% inside this building?

 

[317069]

oops, I missed your answer about my example, sorry
you are right about your cfm numbers.

 

[ChasBean1]

Thanks for engaging me and it's been a fun back and forth. For pete's sake don't use oxygen% for control as it's sort of like using earth's gravitational acceleration as a control input. If oxygen drops from 21% to 19%, just run.