Active Chilled Beam/Fan Coil System

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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

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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.

James Hansen, PE, LEED AP

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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

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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

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Have you looked into SYSTEM TYPE = IU (Induction Unit). This may be a better path to try.

John Grando LEED AP

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My main comment, indirectly, was that in my opinion chilled beams are a
dry climate device as I noticed James was from Arlington VA. You will
be dealing with incoming air with a 57F dew point and James will have a
66F DP. You will have a latent load from the people in the room and
their outside air requirement. Phoenix for example would have a 49F
DP. Part of this will also depend on your chilled water temperature,
that will set the dewpoint you have to control to in the room.
Typically once you have to do any major dehumidification all the
capacity you require is already in the DOAS. You won't need a chilled
beam. You let the humidity float in a wider band to avoid having to
reheat your air stream. I would do it initially as a single zone air
handler with an economizer for the economics, capacities and a check on
air temperatures. You could then manually split off the capacity of the
chilled beam adding more fan energy and maybe pumping energy to run the
beam. The item to resolve is the tipping point for your area and system
adding the increased cost, energy and complexity of the chilled beam
system compared to its advantages. The main one being reducing the size
of the DOAS. One other item to consider is using your chilled water
return flow for the beam, with it being a higher temperature it might be
a more suitable flow for the chilled beam and allow an increase of the
humidity in the room.
Bruce

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Thanks John, chilled beams are also called induction diffusers.
Bruce

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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).

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Greg,

Three questions. Have you talked to Mitch in your Portland office? He's a
great resource but I don't know how things are at Glumac right now. Are
running your OA through a HRV, and lastly is your OA constant volume or are
you varying it?

Carol

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Maybe you are now running into what eQuest uses to prevent condensation
on the chilled beam coil. Condensation would be allowed in the FCU as
it could/would have a drain tray. Where the mixing takes place is
important on the chilled beam. eQuest should consider the mixing takes
place after the coil and the warm humid air is entering the coil. I
would think eQuest would be set up to prevent condensation on the coil,
you don't want water dripping out of the unit. What is the temperature
of your chilled water supply?
Bruce

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Thanks to everybody for the interest and feedback so far..

Chilled water supply to the beams is 56F.

I've attached my inp and pd2 files in case anybody wants a closer look.

Greg

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So apparently somebody's computer started doing some funky stuff after
download the files I attached to my previous email.

DO NOT DOWNLOAD THEM until it's figured out if they caused her problems.

Was anybody else able to successfully open them or have similar
problems?

Greg

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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

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Strange, I just got 2 McAfee GroupShield alerts on something coming from
onebuilding.org. time stamp 18:01:24. I don't run McAfee on my
systems. My system did scan your incoming email and files Greg, no
warnings. I sent a reply to your 3:00 pm email about 7:30 my time, did
you receive that? I haven't opened your files but if I get a chance I
will scan them and open them on an old computer. Thanks for the warning.
Bruce

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Re: the funky stuff

Hi Greg, all,

Here's what happened. I downloaded the .inp & .pd2 files. I clicked on
Greg's .pd2 file to open it and it opened an .inp file of the last project I
worked on, not his .inp file. I have no idea why this happened. I was not in
eQUEST at the time. I immediately called you Greg, and thanks for
broadcasting the info, then I began running my anti-virus program, in my
case AVG. No virus. I wonder if the .pd2 file was corrupted in some way. It
looked awfully small in comparison to the .inp file. Perhaps someone on the
development team will let us know, perhaps not.

I had a couple of scary hours because as a result of whatever happened I was
unable to open email, my caps lock key worked backwards, and I wasn't able
to restart my computer. The good news is that completely turning the
computer off and back on seems to have cleared up the situation.

Carol

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Greg,

what the error message should say is that you need MORE THAN 6 degrees. I simply changed your ECHWT to 55.9 so the approach was now 6.1 degrees and the simulation worked with the IU system. I also changed your CHW DT to 6 degrees instead of the default 10 but I'm not sure whether eQUEST forces you to keep it at that number or not..

John Grando LEED AP

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