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RE: [EnergyPlus_Support] Re: OutdoorAir:Mixer questions
Dr. Li,
Apparently I dopped into the middle of the conversation without reading
the previous discussion Sorry!
I was only addressing the issue of modeling infiltration.
>
> Hi, Jim What I was suggesting was not to over simulate. Many user had
> asked "how to simulate 100% fresh air intake?" and "How to feed a wanted
> amount of fresh air to each zone with a dual system? As we are not really
> accounting for every crack in the building, how a zone is dividing the air
> through the exhaust and return path, I am suggesting that all the air
> comes from this environment air tank and return to this air tank. With the
> mixer and spliter idea, temperature will change at the mixer output node
> and not at the splitter branches. This is where all these discussion
> started. Why the mixed air node temperature does not change, or follow
> the OA temperature. The actual problem was not in the OA mixer. The air
> loop was not used to cool and heat the zone, therefore the strange
> results.
>
> Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx
> Date: Sat, 24 Sep 2011 12:59:04 -0400
> Subject: RE: [EnergyPlus_Support] Re: OutdoorAir:Mixer questions
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> Dear Jean,
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> As you and Dr. Li indicate, the air balance of a real building is very
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> complex. The fact that real air zones can exchange air in a multitude of
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> ways, and that actual "crack area" is impossible to quantify without
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> extensive physical testing, means that our efforts to model them are
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> doomed as far as any correlation between real life and the model goes.
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> For those reasons, I don't expect to have high accuracy regarding
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> infiltration. Rather, I try to prepare the model so that it's defensible
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> to another knowledgeable person and (hopefully) keep the warnings at a low
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> level.
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> The @#$%^& imbalanced airflow warning is an annoyance, especially when you
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> get a few million of them, as I am prone to do. I've recently begun trying
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> to improve my input process so as to minimize these warnings. Next steps
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> include understanding the ZoneMixing object better (as suggested by Dr.
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> Li), since it may help. Keep in mind that this is only to reduce the
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> number of warnings; I doubt that any accuracy improvement will be
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> provable.
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>> Hi, Jean If the kitchen air is exhausted to the enveronment air tank,
>> the
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>> kitchen zone return air is reduced, therefore there will be less air for
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>> the OA mixer relief node. This is another interesting point. All the
>> OA
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>> mixer, infiltration and ventelation paths are also through mixer and
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>> splitter pair in the OA tank. With AFN, the pressure change (wind
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>> velosity) is added to the tank. If the exhaust air is larger than the
>> OA
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>> inlet, the air must come from the leaks. In practice, you will hear
>> the
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>> speed of the exhaust fan chaning, as you open and close the kitchen
>> door.
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>> Un-balanced air between zone is handled by zone mixing, or plenum. That
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>> was the reason why I said you can say 100% fresh air from OA by making
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>> inlet flow rate the same as the fan rate. In practice, it is not easy
>> to
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>> do. You can specify min. rate of OA equals to max flow rate of the fan.
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>> The return air amount is not known or specified, and the zone exhaust
>> and
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>> inlet can be from other paths of the OA tank. Dr. Li
>
>> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
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>> From: jeannieboef@xxxxxxxxx
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>> Date: Sat, 24 Sep 2011 11:53:04 +0000
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>> Subject: [EnergyPlus_Support] Re: OutdoorAir:Mixer questions
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>> Hi Dr. Li,
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>> I find the "environment tank" analogy very interesting. Particularly,
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>> the relief air is equal to the ODA flowing into the ODA system. In real
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>> life as in the simulation more than 1 system may be present, e.g. AHU
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>> serves supply air to both office and kitchen zones, but the kitchen has
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>> it's own fan:zoneexhaust to the outdoors.
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>> In this case the relief air of the AHU is not equal to the supply air,
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>> the "environment tank" remains balanced as the kitchen also exhausts to
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>> Now when kitchen makeup air is the combination of supply air and
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>> infiltrated via the office zones, it becomes tricky. As the demands of
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>> kitchen and the offices are independant, it may occure that the flow is
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>> unbalanced at any possible timepoint. Should one not wish to use an AFN
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>> crack between kitchen and office and office and outdoors, is there a way
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>> you would suggest to address the unbalanced flow?
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>> e+ simply issues the warning that the unbalanced air is not flowing in
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>> from outdoors and may decrease the accuracy of results.
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