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RE: [EnergyPlus_Support] Re: OutdoorAir:Mixer questions
Dear Jean,
As you and Dr. Li indicate, the air balance of a real building is very
complex. The fact that real air zones can exchange air in a multitude of
ways, and that actual "crack area" is impossible to quantify without
extensive physical testing, means that our efforts to model them are
doomed as far as any correlation between real life and the model goes.
For those reasons, I don't expect to have high accuracy regarding
infiltration. Rather, I try to prepare the model so that it's defensible
to another knowledgeable person and (hopefully) keep the warnings at a low
level.
The @#$%^& imbalanced airflow warning is an annoyance, especially when you
get a few million of them, as I am prone to do. I've recently begun trying
to improve my input process so as to minimize these warnings. Next steps
include understanding the ZoneMixing object better (as suggested by Dr.
Li), since it may help. Keep in mind that this is only to reduce the
number of warnings; I doubt that any accuracy improvement will be
provable.
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> Hi, Jean If the kitchen air is exhausted to the enveronment air tank, the
> kitchen zone return air is reduced, therefore there will be less air for
> the OA mixer relief node. This is another interesting point. All the OA
> mixer, infiltration and ventelation paths are also through mixer and
> splitter pair in the OA tank. With AFN, the pressure change (wind
> velosity) is added to the tank. If the exhaust air is larger than the OA
> inlet, the air must come from the leaks. In practice, you will hear the
> speed of the exhaust fan chaning, as you open and close the kitchen door.
> Un-balanced air between zone is handled by zone mixing, or plenum. That
> was the reason why I said you can say 100% fresh air from OA by making
> inlet flow rate the same as the fan rate. In practice, it is not easy to
> do. You can specify min. rate of OA equals to max flow rate of the fan.
> The return air amount is not known or specified, and the zone exhaust and
> inlet can be from other paths of the OA tank. Dr. Li
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: jeannieboef@xxxxxxxxx
> Date: Sat, 24 Sep 2011 11:53:04 +0000
> 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, that
> the relief air is equal to the ODA flowing into the ODA system. In real
> life as in the simulation more than 1 system may be present, e.g. AHU
> serves supply air to both office and kitchen zones, but the kitchen has
> 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, but
> the "environment tank" remains balanced as the kitchen also exhausts to
> it.
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> Now when kitchen makeup air is the combination of supply air and
> infiltrated via the office zones, it becomes tricky. As the demands of the
> kitchen and the offices are independant, it may occure that the flow is
> unbalanced at any possible timepoint. Should one not wish to use an AFN
> crack between kitchen and office and office and outdoors, is there a way
> 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
> from outdoors and may decrease the accuracy of results.
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