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RE: [EnergyPlus_Support] Re: OutdoorAir:Mixer questions


Hi, Jim

It is good to have more discussion on similar topics.

Jean was proposing some strategy to compensate CO2 with control based on measurement.  Eplus does not use the feed back measurement in general, except for the temperature values.  However, the fresh air will come from the OA mixer.

OA inlet flow rate can be dependent on the number of people in the building.  However, the OA control is based on a schedule.  If Jean can link the CO2 measurement to this OA control, the job is done.   The level of CO2 is related to the people in the building.

Because EPlus is not designed to follow exact real time conditions, which is not required for energy usage comparison, the open loop system is implemented for most situations.
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In daylighting simulation, the glare control can close a blind at the window exposed to sun beam.  This is not done using the measured light intensity in the room.   It is done by calculation of the sun position and the slat angle of the blind.  The cloud cover is not taken into account directly. The cloud effect is included in the solar beam intensity in the .epw listing.  What I am trying to say is that the calculation is simplified to hourly average, rather than through real time measurement.

If the internal lights are turned on, the glare value would change.  Again, this is done by schedule and light intensity given to the simulation object as a constant.  If the room wall colour is different, the program will do some more calculation to provide a different value.  The input values does not change, until the scheduled values are changed.
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Jean's CO2 level control system has a maximum value.  If this number is based on the estimated number of people,  then we are back to the scheduled value model again.   Therefore, specifying the minimum intake per person is good enough as a simulation parameter.

You pointed out the sweat and odor of occupant should be removed.  This is done very effectively by using a high ceiling and exhaust from the ceiling. 

It is difficult to force air into a large space effectively.  Aircraft has an air jet per person.
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The room mixing of a bathroom is also a schedule event.  The mixing is stopped, when the door is closed, and the ventilation is turned on for the bathroom.  If the bathroom has a window leak, a small ventilation flow is very close to the real situation.

My concluding remark is that do not try  to over simulate and turn the model into one using a feedback control system of many variables.  EPlus is a  simulation system  to maintain a  comfortable indoor  condition using different HVAC equipment to compare annual  energy usage using TMY weather conditions and scheduled  
 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: <mailto:EnergyPlus_Support%40yahoogroups.com>EnergyPlus_Support@xxxxxxxxxxxxxxx
> From: <mailto:jim%40buildingperformanceteam.com>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
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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
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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
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>> inlet can be from other paths of the OA tank. Dr. Li
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>> To: <mailto:EnergyPlus_Support%40yahoogroups.com>EnergyPlus_Support@xxxxxxxxxxxxxxx
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>> From: <mailto:jeannieboef%40gmail.com>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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>> 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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