[Bldg-sim] Baseline Sizing

[Paul Raftery]

Hi Vinay,

No problem. Would you mind describing the overall system that you would like
to model?

For example, is it a single zone system where the exhaust air flow varies
proportionally to the supply flow rate (as you mentioned before)? If so
model the exhaust fan as a return fan and set the outdoor air economiser to
NoEconomiser.

Regards,
Paul

On 13 July 2010 15:13, Vinay Devanathan <vinay.devanathan at gmail.com> wrote:

> Paul,
>
> Thanks for the clarification. But in my case, I am just using a
> AirTerminal:SingleDuct:Uncontrolled object. I am not able to define a
> minimum supply airflow with this object. Do you have any suggestions on how
> I can model in this situation?
>
> Thank you very much.
>
> Regards,
> Vinay
>
>
> On Tue, Jul 13, 2010 at 9:56 AM, Paul Raftery <p.raftery at gmail.com> wrote:
>
>> Hi Vinay,
>>
>> I presume by 'not higher always' you mean an object which varies airflow
>> at the zone level depending on load (say you are modelling a VAV box in that
>> zone). EnergyPlus doesn't look at the exhaust airflow requirements when
>> figuring out the supply airflow to the zone. It calculates supply airflow
>> based on what is needed to maintain space conditions assuming that this
>> airflow does not exceed the max or min design airflow (take a look at the
>> Engineering Reference included in EPlus documentation).
>>
>> So the minimum supply airflow must be greater than the exhaust airflow at
>> each timestep.
>>
>> You can do this easily by ensuring that the VAV box max design airflow
>> multiplied by the VAV box minimum airflow fraction is greater than the
>> exhaust fan airflow.
>>
>> Regards,
>> Paul
>>
>>
>> On 13 July 2010 14:30, Vinay Devanathan <vinay.devanathan at gmail.com>wrote:
>>
>>> Paul,
>>>
>>> Thanks for your reply. I understand that the supply airflow should be
>>> equal or greater than the exhaust airflow. But, the design airflow from
>>> auto-sized runs give me higher CFM than my exhaust fan CFM. But, the problem
>>> maybe, it is not higher always. Is there any method using which I can
>>> simulate my Zone Exhaust fan operation proportional to the supply fan
>>> operation so that the exhaust fan doesn't pull out a constant CFM?
>>>
>>> Thank you.
>>>
>>> Regards,
>>> Vinay
>>>
>>>
>>> On Tue, Jul 13, 2010 at 4:21 AM, Paul Raftery <p.raftery at gmail.com>wrote:
>>>
>>>> Hi Vinay,
>>>>
>>>> You are getting that error because EnergyPlus does not currently take
>>>> exhaust fans into account when autosizing zone airflows. In your case,
>>>> EnergyPlus sized the supply airflow for that zone lower than the exhaust
>>>> flow rate, and that's why you are getting that error.
>>>>
>>>> To fix this, you can 'force' the supply airflow to be equal to or larger
>>>> than the exhaust airflow at all timesteps. You can do this by setting the
>>>> autosize value in the airflow parameter for the air terminal unit object for
>>>> that zone equal to the exhaust airflow. For example, where you are using
>>>> constant volume reheat:
>>>>
>>>> !-   ===========  ALL OBJECTS IN CLASS:
>>>> AIRTERMINAL:SINGLEDUCT:CONSTANTVOLUME:REHEAT ===========
>>>>
>>>> AirTerminal:SingleDuct:ConstantVolume:Reheat,
>>>>     ObjectName,                        !- Name
>>>>     Schedule,                        !- Availability Schedule Name
>>>>     OutletNode,                        !- Air Outlet Node Name
>>>>     InletNode,                        !- Air Inlet Node Name
>>>>     autosize *(change this to exhaust airflow)*,
>>>>  !- Maximum Air Flow Rate {m3/s}
>>>>     HWInletNode,                        !- Hot Water or Steam Inlet Node
>>>> Name
>>>>     RHCoilType,                        !- Reheat Coil Object Type
>>>>     RHCoilName,                        !- Reheat Coil Name
>>>>     autosize,                        !- Maximum Hot Water or Steam Flow
>>>> Rate {m3/s}
>>>>     0,                        !- Minimum Hot Water or Steam Flow Rate
>>>> {m3/s}
>>>>     0.001;                   !- Convergence Tolerance
>>>>
>>>> Regards,
>>>> Paul
>>>>
>>>> On 12 July 2010 16:46, Vinay Devanathan <vinay.devanathan at gmail.com>wrote:
>>>>
>>>>> Hello All,
>>>>>
>>>>> I am using EnergyPlus to model a building according to ASHRAE App. G
>>>>> for baseline case. I included Zone exhaust fans in the baseline as advised,
>>>>> in a fashion similar to what is prescribed in the proposed design. But, when
>>>>> I auto-size the model with this Zone Exhaust Fan object, I get a warning
>>>>> message saying -
>>>>>
>>>>> **   ~~~   **   there is unbalanced exhaust air flow.
>>>>> **   ~~~   **   Unless there is balancing infiltration / ventilation
>>>>> air flow, this will result in
>>>>> **   ~~~   **   load due to induced outdoor air being neglected in the
>>>>> simulation.
>>>>>
>>>>> And, I think the design loads are increased due to this, which would
>>>>> probably cause over-sizing.
>>>>>
>>>>> Any help in solving this issue would be greatly appreciated.
>>>>>
>>>>> Thank you.
>>>>>
>>>>> Regards,
>>>>> Vinay
>>>>>
>>>>> _______________________________________________
>>>>> Bldg-sim mailing list
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>>>>> To unsubscribe from this mailing list send  a blank message to
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>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Regards,
>>>> Paul
>>>>
>>>>
>>>> =====================
>>>> Contact Details
>>>> =====================
>>>> Paul Raftery, BEng(Hons) (Mech), Fulbright Fellow, IRCSET Scholar
>>>> Informatics Research Unit for Sustainable Engineering (IRUSE) Galway,
>>>> Department of Civil Engineering,
>>>> National University of Ireland, Galway,
>>>> University Road,
>>>> Galway,
>>>> Ireland.
>>>>
>>>> Landline: +353 91 49 3086
>>>> Mobile: +353 85 125 4999
>>>> Skype: praftery
>>>>
>>>
>>>
>>
>>
>> --
>> Regards,
>> Paul
>>
>>
>> =====================
>> Contact Details
>> =====================
>> Paul Raftery, BEng(Hons) (Mech), Fulbright Fellow, IRCSET Scholar
>> Informatics Research Unit for Sustainable Engineering (IRUSE) Galway,
>> Department of Civil Engineering,
>> National University of Ireland, Galway,
>> University Road,
>> Galway,
>> Ireland.
>>
>> Landline: +353 91 49 3086
>> Mobile: +353 85 125 4999
>> Skype: praftery
>>
>
>


-- 
Regards,
Paul


=====================
Contact Details
=====================
Paul Raftery, BEng(Hons) (Mech), Fulbright Fellow, IRCSET Scholar
Informatics Research Unit for Sustainable Engineering (IRUSE) Galway,
Department of Civil Engineering,
National University of Ireland, Galway,
University Road,
Galway,
Ireland.

Landline: +353 91 49 3086
Mobile: +353 85 125 4999
Skype: praftery
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