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[EnergyPlus_Support] Re: Fwd: Outside air mixer is recirculating return flow: Why?
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, 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
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>, "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:
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> > > > > >
> > > > > > The group web site is:
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> > > > > >
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the
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> > > > >
> > > > > [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
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------------------------------------
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