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Re: [EnergyPlus_Support] Re: Fwd: Outside air mixer is recirculating return flow: Why?
Yes, that is certainly true. The program sums up all the zone exhaust
air flow rates for
a system, denotes this as the system exhaust air; the flow rate at the
mixed air node
(the supply air flow rate) is the sum of the system level exhaust flow
and the return
air flow. The outside air flow is whatever is set by the outside air
controller, but it
is constrained to be >= the system exhaust flow. All of this is based on
the assumbtion
that zone exhaust exits the system. Zone exhaust should not be used to
send air from
one zone to another.
Fred Buhl
Richard Raustad wrote:
> I think I know what's happening. The outside air mixer thinks the
> exhaust air from each zone (the air through the exhaust fans) is being
> sent to outdoors. So it subtracts that amount from the exhaust air steam
> at the outside air mixer. There's no way (currently) for the outside air
> mixer to know that the exhaust air is directed to another zone (even
> though you see it at the return node, that's a different calculation).
> So it sounds like you will not be able to mix air from one zone to the
> next using the exhaust fan approach (at least for now). You did get
> pretty close though.
>
> anarui.oliveira wrote:
>
>> Any breakthrough?...
>>
>> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx
>> <mailto:EnergyPlus_Support%40yahoogroups.com>, "anarui.oliveira"
>> <anarui.oliveira@...> wrote:
>>
>>> Yes, I mean't exchange in a one-way mode only ("push").
>>>
>>> I may have missed something, but those zones have "Controlled Zone
>>> Equip Configuration" objects, with return flow nodes (which are
>>>
>> being
>>
>>> reported, by the way) and there are additional Node Lists defining
>>> the outlets and inlets in source and target zones, corresponding to
>>> those "inter-zone" fans inlet/outlet nodes.
>>> If it simplifies the analysis, we can set the other fans, that are
>>> indeed exhausting flow directly to the outside, all to zero flow
>>> (there are already a few like this), and leave active only the
>>>
>> inter-
>>
>>> zone ones...
>>>
>>> I also checked the flow downstream of the Air Mixer, by Brent's
>>> suggestion, and another strange thing is that, at the outlet of the
>>> supply fan, the flow is again 2.4264 m3/s (Msg 10804).
>>>
>>> I have already uploaded a file to the Problem Submittal area:
>>> "UTAN2_Recirculating_flow.idf"
>>>
>>> Ana.
>>>
>>> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx
>>>
>> <mailto:EnergyPlus_Support%40yahoogroups.com>, Richard Raustad
>>
>>> <RRaustad@> wrote:
>>>
>>>> You are not actually exchanging air between zones. You
>>>>
>>> have "pushed" air
>>>
>>>> into unconditioned zones using exhaust fans. Do these
>>>>
>> unconditioned
>>
>>>> zones have return nodes connected to the air loop? If so, the
>>>>
>>> mixer's
>>>
>>>> return air flow rate should be the same as the mixer's mixed air
>>>>
>>> flow
>>>
>>>> rate. This is obviously not happening since I do not see this in
>>>>
>>> the
>>>
>>>> numbers you provided. The only way to get a return node in a zone
>>>>
>>> is to
>>>
>>>> include a Controlled Zone Equip Configuration object for that
>>>>
>> zone.
>>
>>> If
>>>
>>>> you have already done this, please upload your file to the
>>>>
>>> EnergyPlus
>>>
>>>> support group web site in the problem submittals folder.
>>>>
>>>>
>>>>
>> http://tech.groups.yahoo.com/group/EnergyPlus_Support/files/_Problem_S
>> <http://tech.groups.yahoo.com/group/EnergyPlus_Support/files/_Problem_S>
>>
>>> ubmittals/
>>>
>>>> anarui.oliveira wrote:
>>>>
>>>>> Inlet flow is as I manually set it to be: 2.4264 m3/s
>>>>> This inlet flow first passes the heat exhanger, and then enters
>>>>>
>>> the
>>>
>>>>> air mixer, and has the same flowrate there (I checked).
>>>>>
>>>>> The return air flow at the inlet of the air mixer equals the
>>>>>
>> sum
>>
>>> of
>>>
>>>>> all zones returns: 2.1623 m3/s (it's not equal to insufflated
>>>>>
>> flow
>>
>>>>> because I already extracted some airflow from some zones
>>>>>
>> directly
>>
>>> to
>>>
>>>>> the outside).
>>>>>
>>>>> The relief airflow (at the outlet of the air mixer and at the
>>>>>
>>> inlet
>>>
>>>>> of the heat exchanger) equals the return flow (2.1623 m3/s)
>>>>>
>> minus
>>
>>>>> 0,9029 m3/s, i.e. 1.2594 m3/s.
>>>>>
>>>>> Then (and here's the trick), the mixed airflow (the air loop
>>>>>
>>> supply
>>>
>>>>> flow) equals the outside airflow plus 0.9029 m3/s, i.e. 3.3293
>>>>>
>>> m3/s,
>>>
>>>>> so it would seem the air mixer is recirculating that amount of
>>>>> airflow, subtracting it from the relief flow to add it to the
>>>>>
>>> supply
>>>
>>>>> flow again...
>>>>>
>>>>> Now, coincidence or not, the 0.9029 value is exactly the amount
>>>>>
>> of
>>
>>>>> airflow I exchanged between zones using exhaust fans...
>>>>>
>>>>> Although it seems related, and I admit it may create some
>>>>>
>>> confusion
>>>
>>>>> on E+ algorithm if it wasn't ready to account for that use of
>>>>> exhaust fans, as Fred says, it isn't clear why it would decide
>>>>>
>> to
>>
>>>>> react like that (recirculating that airflow) to this posibly
>>>>> unexpected modelling situation?...
>>>>>
>>>>> What I want it to do is to send all return air to the heat
>>>>>
>>> exhanger
>>>
>>>>> and then to relief air, and do NOT recirculate any of it. Then
>>>>>
>> all
>>
>>>>> the rest would be just fine.
>>>>>
>>>>> I'm uploading the file (this situation relates to UTAN.2 air
>>>>>
>>> loop),
>>>
>>>>> in case you want to take a look, but it might be a little
>>>>>
>>> confusing,
>>>
>>>>> because it refers to an actual project, and not a test case, so
>>>>> there are quite a lot of zones, and exhaust fans, some of which
>>>>>
>> I
>>
>>>>> had to set to zero flow because of these errors that I was
>>>>>
>> getting
>>
>>>>> at the heat exchanger, and so on...
>>>>>
>>>>> Well, in any case, I appreciate the help.
>>>>> I'd like to start understanding how E+ "thinks".
>>>>>
>>>>> Regards,
>>>>> Ana.
>>>>>
>>>>> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx
>>>>>
>> <mailto:EnergyPlus_Support%40yahoogroups.com>
>>
>>>>> <mailto:EnergyPlus_Support%40yahoogroups.com>, "Richard Raustad"
>>>>> <RRaustad@> wrote:
>>>>>
>>>>>> Congratulations, I don't think anyone has tried this. It
>>>>>>
>> sounds
>>
>>>>> like
>>>>>
>>>>>> it is working. The air in the first zone that is moved to a
>>>>>>
>>> second
>>>
>>>>>> zone using an exhaust fan and is not included in the return
>>>>>>
>> air
>>
>>>>> for
>>>>>
>>>>>> the first zone (i.e., first zone supply air minus first zone
>>>>>>
>>>>> exhaust
>>>>>
>>>>>> air = first zone return air).
>>>>>>
>>>>>> This means that the mixed air from the mixing box is provided
>>>>>>
>> to
>>
>>>>> some
>>>>>
>>>>>> of the zones. Some of this air is moved to other zones using
>>>>>>
>>>>> exhaust
>>>>>
>>>>>> fans and pulled back to the mixing box through the return air
>>>>>>
>>>>> system.
>>>>>
>>>>>> So the return air at the mixing box should be the same flow
>>>>>>
>> rate
>>
>>>>> as
>>>>>
>>>>>> the mixed air. Then the outside air flow rate is set by you,
>>>>>>
>> and
>>
>>>>> the
>>>>>
>>>>>> exhaust air flow rate should be this same amount. Tell me
>>>>>>
>>> exactly
>>>
>>>>>> what is happening at the mixing box (describe all 4 air flow
>>>>>>
>>>>> paths:
>>>>>
>>>>>> return, exhaust, OA, and mixed air). Also describe what you
>>>>>>
>>> would
>>>
>>>>>> like to happen.
>>>>>>
>>>>>> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx
>>>>>>
>> <mailto:EnergyPlus_Support%40yahoogroups.com>
>>
>>>>> <mailto:EnergyPlus_Support%40yahoogroups.com>, "anarui.oliveira"
>>>>>
>>>>>> <anarui.oliveira@> wrote:
>>>>>>
>>>>>>> Thank you, but I checked the zones receiveing the exhaust
>>>>>>>
>> fans
>>
>>>>>> flow:
>>>>>>
>>>>>>> the corresponding inlets are registering that incoming
>>>>>>>
>> flow,
>>
>>> and
>>>
>>>>>> the
>>>>>>
>>>>>>> return flow from those zones is summing that airflow to the
>>>>>>>
>>>>> airflow
>>>>>
>>>>>>> directly insufflated to that zone. The return flow after
>>>>>>>
>> the
>>
>>> air
>>>
>>>>>>> mixer (after collecting all zones return flows) is correct.
>>>>>>> Everything checks out until the return flow reaches the Air
>>>>>>>
>>>>> Mixing
>>>>>
>>>>>>> Box; if E+ can recognize and properly account the passage of
>>>>>>>
>>>>>> airflow
>>>>>>
>>>>>>> introduced by the fan, why would it create a problem further
>>>>>>>
>>>>> ahead?
>>>>>
>>>>>>> I did what you suggest in previous versions of my model...
>>>>>>> I'm insisting to get it working this way for two reasons:
>>>>>>>
>>>>>>> 1- My buildings' HVAC project actually works like that; it
>>>>>>>
>>>>> collects
>>>>>
>>>>>>> all its return flow in uncontrolled zones, rather than in
>>>>>>>
>> the
>>
>>>>> zones
>>>>>
>>>>>>> that are being controlled and where the flow is being
>>>>>>>
>>>>> insufflated,
>>>>>
>>>>>>> and since I have a heat recovery unit, I thought it might
>>>>>>> influenciate the results; I don't really have enough
>>>>>>>
>>> experience
>>>
>>>>> to
>>>>>
>>>>>>> say if it makes that much difference, but...
>>>>>>>
>>>>>>> 2- The other reason is the 2:1 limit on the insufflated vs.
>>>>>>>
>>>>> return
>>>>>
>>>>>>> flow in the heat exchanger, since this and other projects
>>>>>>>
>>>>> actually
>>>>>
>>>>>>> have a higher ratio, so, in order not to overestimate the
>>>>>>>
>> heat
>>
>>>>>>> recovery potential, I would want to send the extra airflow
>>>>>>>
>> to
>>
>>> a
>>>
>>>>>> dummy
>>>>>>
>>>>>>> zone with (nearly) exterior conditions.
>>>>>>>
>>>>>>> Thanks again,
>>>>>>> Ana.
>>>>>>>
>>>>>>> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx
>>>>>>>
>> <mailto:EnergyPlus_Support%40yahoogroups.com>
>>
>>>>> <mailto:EnergyPlus_Support%40yahoogroups.com>, Fred Buhl
>>>>>
>> <wfbuhl@>
>>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> I suspect it is due to the exhaust fans. Have you tried
>>>>>>>>
>>> using
>>>
>>>>>> Cross
>>>>>>
>>>>>>>> Mixing objects instead?
>>>>>>>> EPlus will assume the exhaust fans exhaust the air - and
>>>>>>>>
>>> will
>>>
>>>>> not
>>>>>
>>>>>>> send
>>>>>>>
>>>>>>>> it to another zone.
>>>>>>>>
>>>>>>>> Fred Buhl
>>>>>>>>
>>>>>>>> anarui.oliveira wrote:
>>>>>>>>
>>>>>>>>> ... Apparently, it is recirculating the exact amount of
>>>>>>>>>
>>>>> airflow
>>>>>
>>>>>>> that
>>>>>>>
>>>>>>>>> I am trying to pass from some zones to adjacent zones
>>>>>>>>>
>> with
>>
>>>>>>> exhaust
>>>>>>>
>>>>>>>>> fans (the aim was precisely to force the return path to
>>>>>>>>>
>>>>> collect
>>>>>
>>>>>>> its
>>>>>>>
>>>>>>>>> air flow from those areas, which are not controlled).
>>>>>>>>>
>> Is
>>
>>> it
>>>
>>>>>>> related?
>>>>>>>
>>>>>>>>> I can't do that?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx
>>>>>>>>>
>> <mailto:EnergyPlus_Support%40yahoogroups.com>
>>
>>>>> <mailto:EnergyPlus_Support%40yahoogroups.com>, "anarui.oliveira"
>>>>>
>>>>>>>>> <anarui.oliveira@> wrote:
>>>>>>>>>
>>>>>>>>> Hello.
>>>>>>>>>
>>>>>>>>> I have set up a VAV system, with heat recovery, and
>>>>>>>>>
>>> manually
>>>
>>>>>>> entered
>>>>>>>
>>>>>>>>> the exact same value for for all outside airflow
>>>>>>>>>
>> variables
>>
>>>>> and
>>>>>
>>>>>>>>> airflow variables (the "air primary loop",
>>>>>>>>>
>>>>>>> the "controller:outside
>>>>>>>
>>>>>>>>> air", the fan, the heat exchanger,... all have the same
>>>>>>>>>
>>>>> airflow
>>>>>
>>>>>>>>> defined).
>>>>>>>>> I hoped, with this, to establish that the supply air
>>>>>>>>>
>>> should
>>>
>>>>> be
>>>>>
>>>>>>> 100%
>>>>>>>
>>>>>>>>> OA, and no return air should be recirculated, but rather
>>>>>>>>>
>>>>> become
>>>>>
>>>>>>> all
>>>>>>>
>>>>>>>>> reflief air, passing through the heat excahnger, as I
>>>>>>>>>
>> had
>>
>>>>>>> understood
>>>>>>>
>>>>>>>>> from previous analysis...
>>>>>>>>>
>>>>>>>>> Msg 9808:
>>>>>>>>>
>>>>>>>>> Yes. this will provide 100% outside air with heat
>>>>>>>>>
>> recovery
>>
>>>>> and
>>>>>
>>>>>> no
>>>>>>
>>>>>>>>> recirculation. However, the outside air flow will never
>>>>>>>>>
>> be
>>
>>>>> more
>>>>>
>>>>>>> than
>>>>>>>
>>>>>>>>> the system supply flow rate. So, when the VAV terminal
>>>>>>>>>
>>> units
>>>
>>>>>>> reduce
>>>>>>>
>>>>>>>>> the total system flow rate, the outside air flow rate
>>>>>>>>>
>> will
>>
>>>>> be
>>>>>
>>>>>>>>> reduced to match.
>>>>>>>>> Mike
>>>>>>>>>
>>>>>>>>> On 12 Jan 2008 at 1:44, anarui.oliveira wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> Hello again.
>>>>>>>>>> If in the Compact HVAC:System:VAV I set:
>>>>>>>>>> - Heat recovery to sensible
>>>>>>>>>> - both the Fan Max and Min Flow rate, and both the Max
>>>>>>>>>>
>>> and
>>>
>>>>> Min
>>>>>
>>>>>>>>> Outside Air Flow rate to the same value,
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> - and Outside Air Control Type to fixed minimum
>>>>>>>>>> do I get a system that exhausts to the outside all the
>>>>>>>>>>
>>>>> return
>>>>>
>>>>>>> air
>>>>>>>
>>>>>>>>> of the ATU, recirculating none to the inside and
>>>>>>>>>
>>>>> substituting
>>>>>
>>>>>> it
>>>>>>
>>>>>>> by
>>>>>>>
>>>>>>>>> outside air, at a fixed rate, but first using the
>>>>>>>>>
>> return
>>
>>> air
>>>
>>>>> to
>>>>>
>>>>>>> do
>>>>>>>
>>>>>>>>> sensible heat recovery?
>>>>>>>>>
>>>>>>>>> BUT, reporting the flow at the different nodes, I find
>>>>>>>>>
>>> that,
>>>
>>>>>>>>> although the zone return airflows add up correctly to
>>>>>>>>>
>>>>> the "air
>>>>>
>>>>>>> loop
>>>>>>>
>>>>>>>>> inlet" flow, the Outside Air Mixing Box is diverting
>>>>>>>>>
>> about
>>
>>>>> 37%
>>>>>
>>>>>> of
>>>>>>
>>>>>>>>> that return flow and adding it to the outside air flow
>>>>>>>>>
>>>>> supplied
>>>>>
>>>>>>> by
>>>>>>>
>>>>>>>>> the heat exchanger... which was causing the 2:1 limit
>>>>>>>>>
>>> error
>>>
>>>>> I
>>>>>
>>>>>> was
>>>>>>
>>>>>>>>> getting at the heat exchanger, altough the defined
>>>>>>>>>
>> exhaust
>>
>>>>> fans
>>>>>
>>>>>>> are
>>>>>>>
>>>>>>>>> not taking out nearly that much flow (in fact, I checked
>>>>>>>>>
>>>>> that
>>>>>
>>>>>> the
>>>>>>
>>>>>>>>> return flow is as expected, until it reaches the Air
>>>>>>>>>
>>>>> Mixer)...
>>>>>
>>>>>>>>> The zone air distribution equipment is Direct Air in all
>>>>>>>>>
>>>>> zones
>>>>>
>>>>>>>>> served by this Air Loop, so there's no change in the
>>>>>>>>>
>>> airflow
>>>
>>>>>>> there...
>>>>>>>
>>>>>>>>> And I can't figure out what is controlling this
>>>>>>>>>
>> behaviour
>>
>>> or
>>>
>>>>>> how
>>>>>>
>>>>>>> to
>>>>>>>
>>>>>>>>> change it to the desired model...
>>>>>>>>>
>>>>>>>>> Can anyone help me? I'd really like to get this one
>>>>>>>>>
>>> working.
>>>
>>>>>>>>> Best regards,
>>>>>>>>> Ana Rui.
>>>>>>>>>
>>>>>>>>> --- End forwarded message ---
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> ------------------------------------
>>>>>>>>>
>>>>>>>>> The primary EnergyPlus web site is found at:
>>>>>>>>> http://www.energyplus.gov <http://www.energyplus.gov>
>>>>>>>>>
>> <http://www.energyplus.gov <http://www.energyplus.gov>>
>>
>>>>>>>>> The group web site is:
>>>>>>>>> http://groups.yahoo.com/group/EnergyPlus_Support/
>>>>>>>>>
>> <http://groups.yahoo.com/group/EnergyPlus_Support/>
>>
>>>>> <http://groups.yahoo.com/group/EnergyPlus_Support/
>>>>>
>> <http://groups.yahoo.com/group/EnergyPlus_Support/>>
>>
>>>>>>>>> Attachments are not allowed -- please post any files to
>>>>>>>>>
>>> the
>>>
>>>>>>> appropriate folder in the Files area of the Support Web
>>>>>>>
>> Site.
>>
>>>>>>>>> EnergyPlus Documentation is searchable. Open
>>>>>>>>>
>>>>> EPlusMainMenu.pdf
>>>>>
>>>>>>> under the Documentation link and press the "search" button.
>>>>>>>
>>>>>>>>> Yahoo! Groups Links
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>> [Non-text portions of this message have been removed]
>>>>>>>>
>>>>>>>>
>>>>>
>>>> --
>>>> Richard A. Raustad
>>>> Senior Research Engineer
>>>> Florida Solar Energy Center
>>>> University of Central Florida
>>>> 1679 Clearlake Road
>>>> Cocoa, FL 32922-5703
>>>> Phone: (321) 638-1454
>>>> Fax: (321) 638-1439 or 1010
>>>> Visit our web site at: http://www.fsec.ucf.edu
>>>>
>> <http://www.fsec.ucf.edu>
>>
>>>> UCF - From Promise to Prominence: Celebrating 40 Years
>>>>
>>>>
>>
>>
>
>
[Non-text portions of this message have been removed]
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