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[EnergyPlus_Support] Re: Fwd: Outside air mixer is recirculating return flow: Why?
Thank you for the insight.
I'll study the plenum approach option... see if it may be applied
here.
I don't suppose there is some other way to exchange air between
zones in a way that the HVAC components will aknowledge it correctly?
Ana.
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Fred Buhl <wfbuhl@...>
wrote:
>
> Ana,
>
> 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?
> >
> The system air balance is done at the outside air mixer, since
this is
> where the decision has
> to be made on how much outside air to bring in. The system code
keeps
> track of the total
> air exhausted, but assumes that it all is really exhauste, not
going to
> another zone.
> > 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...
> >
> Could you make these uncontrolled return zones into plenum zones?
> Plenums don't have to be
> actual plenums - they are just uncontrolled zones with return air
> passing through.
>
> Fred Buhl
> > 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, 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, "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 ---
> >>>
> >>>
> >>>
> >>> ------------------------------------
> >>>
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> >>>
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> >>>
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> >
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> >>>
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> >>>
> >>>
> >> [Non-text portions of this message have been removed]
> >>
> >>
> >
> >
> >
> > ------------------------------------
> >
> > The primary EnergyPlus web site is found at:
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> >
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> >
> > Attachments are not allowed -- please post any files to the
appropriate folder in the Files area of the Support Web Site.
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>
>
> [Non-text portions of this message have been removed]
>
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