[Bldg-sim] Direct Evaporative Coolers & the eQUEST Games

Xiang Liu xiang.liu at colorado.edu
Thu Jan 24 15:56:30 PST 2008

If you are using Packaged Single Zone system to simulate the evaporative
cooler (without mechanical cooling). Other tricks you have to play to get
the system running with no unmet cooling hours are:

1. Specify the mechanical cooling capacity to a very small number, like 0.01,
so mechanical cooling wont kick off. (As I recall, 0 wont work, because
eQUEST treats cooling capacity of 0 as autosizing, so you would still have
machanical cooling).

2. Find another keyword named EVAP-CL-LIMIT-T in the add-on evaporative
cooling tab, raise it from the default value (usually 90) to over the
maximum OA drybulb of your site. EVAP-CL-LIMIT-T specifies outside air
dry-bulb temperature above which the evaporative cooler will be turned
off. A low EVAP-CL-LIMIT-T will turn off the evaporative cooler when cooling
is still needed, and without mechanical cooling, your unmet cooling hours
will go up because nothing is air conditioning the space.

*Xiang (Shawn) Liu*


*Nexant, Inc.*

1401 Walnut St., Suite 400
Boulder , CO 80302

Ph:  (303) 998-2474

Fax: (303) 440-6644

xliu at nexant.com

On 1/24/08, Shawn <fwdbker at gmail.com> wrote:
> Here might be why:
> When you design an evaporative cooler, you wanna use the design wetbulb
> and the effectiveness to figure out the the design supply air T and then the
> design CFM.  Say the location is Salt Lake City, UT.  Because the design
> wetbulb is 68F, and the resulted supply air T is slightly higher than 68F,
> say 70F. Then the designed CFM would be much higher than the value you get
> when using 55F as the design supply T. However, eQUEST default supply T is
> usually 55F (specified by the keyword: MIN-SUPPLY-T).  If you change that
> to 68 or even higher, your cooling unmet hours are almost gone.
> With this change, you will see the CFM surges dramatically, which is
> probably because your high internal loads and higher supply T.  Therefore,
> the previous huge unmet hours is due to the lack of CFM. If by doing this,
> eQUEST designed CFM is found to be much higher than the actual design, it is
> because you and the mechanical guy use different internal loads (assuming
> the mechanical design is sound). You would have to talk to him/her to clear
> that up.
> Of course, you can imagine how much fan energy VFDs on the supply fan of
> the evaporative cooler can save if internal gains are huge. Most of the
> time, you can get a lower supply T because the OA wetbulb is lower than 68F.
> Therefore,  although to erase the unmet hours, your design supply T should
> be slightly higher than 68F, ideally you may want to run supply T as low
> as possible to save fan energy. So that MIN-SUPPLY-T should be as low as
> possible. But I just don't know if there is any other place in eQUEST to set
> the design supply T then. If we set the MIN-SUPPLY-T to about 68, supply T
> would never go below 68 even if it can. So a lot of fan energy would be
> wasted.
> So anybody knows another place to plug in the design supply air T?
> *Xiang (Shawn) Liu*
> ****
> *Nexant, Inc.*
> 1401 Walnut St., Suite 400
> Boulder , CO 80302
> Ph:  (303) 998-2474
> Fax: (303) 440-6644
> xliu at nexant.com
>  On 1/21/08, Kearns, Patrick <pkearns at nexant.com > wrote:
> >   Help!  I have been modeling a car dealership which uses several disparate
> > systems in combination to condition spaces.
> >
> > The baseline HVAC system is Packaged Single Zone with Gas Heat.
> >
> > The actual design for many of the service bays is:
> >
> > Direct Evaporative Cooler
> > Unit Heater(s)
> > Exhaust Fans
> >
> > For purposes of modeling the actual design, it seemed appropriate to
> > model the system as "Evaporative Cool' with "Furnace" selected for heat.
> > The OA fraction was set to 1.0 (more on this later).  Actual design
> > capacities were input into the model.  The unfortunate by-product of this
> > (as is often the case) was that there were a bunch of unmet hours (76),
> > mostly cooling.
> >
> > So after playing around with several settings, the only way to get the
> > unmet cooling hours down seemed to be to change the system type back to
> > "Package Single Zone" and set the unitary power to 0 for the 'compressor'
> > which really does not exist.  This makes the unmet hours go away, but seems
> > to create a higher heating load, despite the fact that the actual design has
> > higher thermal efficiency than the baseline.  The space heating almost
> > doubles from the baseline to the proposed.
> >
> > My suspicion has been that eQUEST does not realize that the evap.
> > coolers only run during the summer, and thinks it has to heat 100% OA
> > year-round.  Appendix G says that the minimum outdoor ventilation rates
> > should be the same (right or wrong) for the proposed and baseline designs.
> > So...my solution was to set a minimum OA schedule, with the summer being 1
> > and the winter at 0.  Well...this may be closer to reality, but now the
> > unmet hours spike to 423!
> >
> > So...it seems to be the recurring problem of fixing one problem, only to
> > create another one.  Any ideas on how better to simulate this?
> >
> > Thanks for any tips in advance.
> >
> > Regard,
> >
> > Patrick A. Kearns, P.E.
> > Nexant, Inc.
> > www.nexant.com
> > 303-998-2479 Office
> > 303-918-8731 Mobile
> > pkearns at nexant.com
> >
> >
> > _______________________________________________
> > Bldg-sim mailing list
> > Bldg-sim at lists.onebuilding.org
> > http://lists.onebuilding.org/listinfo.cgi/bldg-sim-onebuilding.org
> >
> >
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