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



The outside airflow is exactly what I manually set it to be: 2.4264 
m3/s.
This airflow passes through the heat exchanger and enters the Air 
Mixer, with the exact same value (I checked).

The return airflow at the inlet of the Air Mixer equals the sum of 
all zones' return airflows, i.e., 2.1623 m3/s (it's lower than the 
insufflated flow because I already extracted some flow from some 
zones directly to the outside, also with exhaust fans, but this 
shouldn't be an issue here).

The relief airflow, that exits the Air Mixer an goes on to the Heat 
Exchanger, and then to the outside, is 1.2594 m3/s (i.e., it equals 
the return airflow minus 0.9029)

Then (and here's the trick), the Mixed Air airflow (at the exit of 
the Air Mixer, which is the supply flow to the air loop), equals the 
outside airflow plus 0.9029, i.e., 3.3293 m3/s...
.... so it seems the Air Mixer is recirculating those 0.9029 m3/s, 
diverting that flow from the return airfloe and adding it to the 
supply airflow again...

Now, coincidence or not, that .9029 value is exactly the amount of 
flow I tried to exchange between zones with exhaust fans (the sum of 
the various inter-zone fans defined).

Even if E+ wasn't prepared to account for this use of exhaust fans, 
like Fred says, it's still not clear why it would react like this, 
recirculating that amount of flow, instead of... anything else, or 
an error or warning message...

Could it have something to do with the Mixed Air Setpoint or Outside 
Air Controller?

What I'd like it to do is simply to let all the return flow fllow to 
be relief flow, out of the Air Mixer, through the heat Exchanger and 
to the outside.

I'm uploading the file, in case you want to look at it, though it 
may be confusing, because it's an actual project and not a test 
case, so there are a lot of zones, other fans that I added to 
exhaust more airflow according to the project... and then when these 
erros started showing up at the heat exchanger I set those exhaust 
flows to zero...

Well, in any case, I appreciate the help.
I'd like to star understanding how E+ "thinks".

Regards,
Ana.

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "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, "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, 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 ---
> > > >
> > > >
> > > >
> > > > ------------------------------------
> > > >
> > > > 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
> > > >
> > > >
> > > >
> > > >   
> > > 
> > > 
> > > [Non-text portions of this message have been removed]
> > >
> >
>



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

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