[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [EnergyPlus_Support] Re: Fwd: Outside air mixer is recirculating return flow: Why?



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_Submittals/
 
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>, "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>, "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>, 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>, "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>
> > > > >
> > > > > The group web site is:
> > > > > 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

UCF - From Promise to Prominence: Celebrating 40 Years


------------------------------------

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
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

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/EnergyPlus_Support/

<*> Your email settings:
    Individual Email | Traditional

<*> To change settings online go to:
    http://groups.yahoo.com/group/EnergyPlus_Support/join
    (Yahoo! ID required)

<*> To change settings via email:
    mailto:EnergyPlus_Support-digest@xxxxxxxxxxxxxxx 
    mailto:EnergyPlus_Support-fullfeatured@xxxxxxxxxxxxxxx

<*> To unsubscribe from this group, send an email to:
    EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/