[Bldg-sim] Baseline Sizing

[Paul Raftery]

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
>>>
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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
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