[Equest-users] Modeling DOAS-II Reply and Unexplained Fan Energy Increase

[Doebber, Ian]

Xavier

                          Modeling DOAS-II Reply

Thanks for addressing the issue with the thermal error for the Dummy
Zones.  My recommendation to avoid the issue with an oversized DOAS fan
is increase the "INFILTRATION ACH" instead of the "SUPPLY AIR ACH".
Equest then sizes the DOAS fan based on the Outside Air demand of the
attached AHUs downstream.  And of course still increase the Zone
Cooling/Heating Setpoint Temperature to something ridiculous like 150 F
/ 0 F so no fictitious load occur.

Unfortunately I have yet to enter ERV or ECO into the DOAS or AHU so can
not say whether it provides spurious results.  Yet, from digging into
the Air Flow Rates of the DOAS and AHUs, the process of a DOAS works.
Additionally, the DOAS systems show a pretreatment of the Outdoor Air
before entering the AHUs.  For example, my DOAS units with DX Cooling
accounts for ~25% of the total school's cooling energy with the rest
handled by the Chilled Water Coils in the AHU.

                       Unexplained Fan Energy Increase

Inevitably a warning in the .SIM file will state that the Air Flow Rate
of the DOAS system has been resized to meet the demand of the AHU
ventilation rates.  The warning then states that it is strongly
recommended that you enter this Flow Rate into your DOAS system. 

I have checked the DOAS and AHU flow rates without and with entering in
the DOAS Flow Rate.  Although the amount of Air Flow does not change AT
ALL, the fan energy increases.  This makes no sense.  

If anyone could explain the reason for the reduced fan energy, I would
be very appreciative.

Regards
Ian







Date: Fri, 12 Sep 2008 19:19:10 +0200
From: Xavier Garc?a Casals  <xavier.garcia at aiguasol.coop>
Subject: [Equest-users] modelling DOAS-II
To: equest-users at lists.onebuilding.org
Message-ID: <48CAA48E.1090901 at aiguasol.coop>
Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"

Dear eQuest users,
Let me add some more info and some conclusions to the former post on 
'modelling DOAS':
The thermal error introduced in the dummy zone by its difference of 
temperature with OA can be reduced by increasing ACH in the dummy zone 
to a very high value (and increasing COOL-TEMP-SCH to get rid of these 
fictious loads). But then eQuest oversizes the DOAS fan, which will work

at part load with its real flow.
When introducing ECO and ERV the results show increased errors in 
comparison with the reference case without DOAS (besides, there is a 
high variety of results depending on where ECO and ERV are placed).
These results are using PZS or HVSYS for the DOAS.
As a conclussion I think that there is no way to model the DOAS in 
eQuest keeping a minimum guaranty of accuracy on the results. I could be

missing something in the many trials I did (I would be thankful if 
someone else indicates me other possibilities I didn't take into 
account), but anyway, the fact of not having at hand the possibility of 
having a look on what is the software really doing in each case, and 
having to guess it from interpreting the detailed reports, it is by 
itself a very error prone procedure that doesn't give many guaranties on

modelling accuracy.
Regards,
-- 

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*Xavier Garc?a Casals*

 

*Sistemas** Solares, Eficiencia Energ?tica e Ingenier?a Bioclim?tica** *

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