[Bldg-sim] Baseline Sizing

[Vinay Devanathan]

Paul,

Yes, it is a single zone system. As such, I do not have an OA Economizer.
But, if I already have a return fan, should I just give that fan the CFM of
Zone Exhaust Fan also? Then again, it would result in the same problem
right?

On a different note, if a proposed design has a return fan and is Multizone
system and the baseline is requiring a single zone system. Do I include a
return fan for a each single zone system?

Thank you so much.

Regards,
Vinay

On Tue, Jul 13, 2010 at 10:42 AM, Paul Raftery <p.raftery at gmail.com> wrote:

> 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
>>>>>>
>>>>>> _______________________________________________
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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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