[Equest-users] Active Chilled Beam/Fan Coil System

[Bruce Easterbrook]

     I think it has come to me, basically eQuest is saying you don't 
have enough cooling capacity at a certain point in your simulation. To 
get the cooling you need, the air has to come off the coil cooler than 
6F above the entering water temp.  In a FCU you just increase the water 
flow, coil size, you get the cooling you need, you would see unmet hours 
but the program won't necessarily kick out an error message.  You will 
get more condensation.  Because condensation is not allowed in a chilled 
beam you can't do this, the temperature of the coil has to stay above 
the dew point.  The only way for eQuest to do that is to limit water 
flow to the coil.  You get less cooling.  The program has a quandary and 
kicks out that error message.  You have to add more units, run them at a 
lower water flow or you drop the dewpoint of your room air so you have 
more capacity with the 6F eQuest restriction.
     The 56F CWT helps, your summer design (Long Beach) 84DB/66MWB, the 
DP is 56F.  eQuest is restricting you to 62F DB off the coil to prevent 
condensation.  In reality you can't drop your water temperature because 
that just increases condensation with a chilled beam.  With a FCU that 
is an option.  The options are to move more 62F air to meet the room 
cooling load with the chilled beams (use more) or shift more load to the 
DOAS (bigger).  Your tip point is changing.  Shifting load to the DOAS 
will also reduce your room DP though I don't know if eQuest will give 
you credit for that.  You will increase your costs because you are 
bringing in more outside air than you need.  More chilled beams also 
increases cost.  Moving more air increases costs.  To reduce outside 
air, you need return air.  The DOAS is now an AHU.  This can be good 
because you get an economizer, free cooling and you can do latent 
cooling on the return air.  You have a high CWT so I assume you have a 
ground loop or some other cheaper cooling source than mechanical 
cooling?  That being true then moving more air is an economical option.
     I'm sorry I don't have time right now to look at your files and see 
your exact situation.  All this stimulus and energy efficiency stuff has 
got me loaded down too.  Good time to be a mech.  Sucks to still be on 
the computer at 7:30 Friday night.  Hopefully I have given you some 
clues to resolve your situation.
Bruce

On 09/04/2010 04:32 PM, Greg Collins wrote:
>
> I have tried it, but I end up getting an error saying the leaving air 
> temp off the coil needs to be at least 6F higher than the entering 
> water temp to the coil, and for some reason I don't get the same error 
> for the FCU.  In an active chilled beam, the coil is actually just 
> cooling the room induction air, then mixing it with the primary air 
> from the DOAS.  I think that equest assumes the air mixes before 
> entering the coil, which fundamentally is the same thing, but the 
> numbers trip this error message.  I'll look into more why the FC 
> system doesn't make that happen, but the two seem to be essentially 
> the same system except for the fan (which I make 0.000001 kW/cfm anyway).
>
> *From:* Grando, John [mailto:JGrando at cosentini.com]
> *Sent:* Friday, April 09, 2010 1:00 PM
> *To:* Greg Collins; Bruce Easterbrook
> *Cc:* equest-users at lists.onebuilding.org
> *Subject:* RE: [Equest-users] Active Chilled Beam/Fan Coil System
>
> Have you looked into SYSTEM TYPE = IU (Induction Unit).  This may be a 
> better path to try.
>
>
> *John Grando** **LEED AP**| **Mechanical Engineer
> *
>
> ------------------------------------------------------------------------
>
> *From:* Greg Collins [mailto:GCollins at glumac.com]
> *Sent:* Friday, April 09, 2010 2:00 PM
> *To:* Bruce Easterbrook
> *Cc:* equest-users at lists.onebuilding.org
> *Subject:* Re: [Equest-users] Active Chilled Beam/Fan Coil System
>
> Bruce,
>
> I agree with your theory and the justification, but the question is 
> ultimately "how to model it?"  I don't think you can put in negative 
> heat for a baseboard system, hook it up to a chilled water loop and 
> get results you can count on.
>
> Greg
>
> *From:* Bruce Easterbrook [mailto:bruce5 at bellnet.ca]
> *Sent:* Friday, April 09, 2010 8:59 AM
> *Cc:* equest-users at lists.onebuilding.org
> *Subject:* Re: [Equest-users] Active Chilled Beam/Fan Coil System
>
>     I'm not totally up on using DOAS with eQuest or with chilled beams 
> being in Canada but it would seem to me the OA will have to be used to 
> control humidity in the space.  The chilled beam should not be used 
> for latent cooling, sensible only, to prevent condensation issues.  
> The DOAS will need to control the dewpoint of the room air and 
> maintain it below the operating temperature of the coil in the chilled 
> beam.  The chilled beam would only be dealing with the sensible load 
> from the space.  The DOAS would do everything else.  The question 
> would be will eQuest allow a single zone AHU c/w economizer and a 
> baseboard system with negative heat?  I have never tried that with eQuest.
> Bruce Easterbrook
> Abode Engineering
>
> On 08/04/2010 04:46 PM, James Hansen wrote:
>
> I'm interested in this as well -- I know that in DOAS systems like 
> active chilled beams, the primary air often takes on a fairly large 
> chunk of space latent cooling, and if this can't be passed on from the 
> DOAS system to the space, then I don't see how we can believe the results.
>
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> *From:* equest-users-bounces at lists.onebuilding.org 
> <mailto:equest-users-bounces at lists.onebuilding.org> 
> [mailto:equest-users-bounces at lists.onebuilding.org] *On Behalf Of 
> *Greg Collins
> *Sent:* Thursday, April 08, 2010 4:39 PM
> *To:* equest-users at lists.onebuilding.org 
> <mailto:equest-users at lists.onebuilding.org>
> *Subject:* [Equest-users] Active Chilled Beam/Fan Coil System
>
> Hi all,
>
> I'm working on modeling an active chilled beam system.  It consists of 
> the chilled beams and a dedicated unit to provide primary air (mainly 
> ventilation air) to the beams at 55 degrees.  I've modeled the chilled 
> beam system as a fan coil system, but am wondering about my options 
> for modeling the primary air.
>
> My question is this:  If I attach the primary/ventilation air unit to 
> the fan coil system in the FC's Outdoor Air tab under "Outside Air 
> from System" will the temperature of the air coming from that unit 
> have an impact on the load in the space?  I have a ton of hours at 
> very low load during the winter months, and I want eQuest to use the 
> 55 degree primary air as the first stage of cooling for the space and 
> not "double up" the cooling energy by cooling the 55 degree air and 
> then not use that for cooling in the zones.
>
> If anybody has some input, it would be appreciated.
>
> *Greg Collins*
> Design Engineer / Energy Analyst
>
> Mechanical Department
> LEED® Accredited Professional
>
> http://www.glumac.com/glumacsig.gif <http://www.glumac.com/>
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